thermodynamics of selenium and tellurium in …...thermodynamics of selenium and tellurium in molten...

Thermodynamics of Selenium and Tellurium in Molten Metallurgical Slags

and Alloys

Murray D. Johnston B.Sc. (Hons)

This thesis is presented for the degree of Doctor of Philosophy of the University of Western Australia, School of Biomedical, Biomolecular and Chemical Sciences,

Discipline of Chemistry.

11 April 2007

DECLARATION FOR THESES CONTAINING PUBLISHED WORK AND/OR WORK PREPARED FOR PUBLICATION

This thesis does not contain work that I have published, nor work under consideration for publication. The thesis is completely the result of my own work, and was substantially conducted during the period of candidature, unless otherwise stated in the thesis. Signature……………………………….

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2 This thesis contains sole-authored published work and/or work prepared for publication. The bibliographic details of the work and where it appears in the thesis is outlined below. Signature………………………………

2

3 This thesis contains published work and/or work prepared for publication, some of which has been co-

authored. The bibliographic details of the works and where they appear in the thesis are set out below. (The candidate must attach to this declaration a statement detailing the percentage contribution of each author to the work. This must been signed by all authors. Where this is not possible, the statement detailing the percentage contribution of authors should be signed by the candidate’s Coordinating Supervisor). 1. M.D. Johnston, S. Jahanshahi, and F.J. Lincoln: “Thermodynamics of Selenium and Tellurium in Calcium Ferrite Slags”

Metall. Mat. Trans. B, 2007, vol. 38B (3), pp. 433-442. The results described in this paper also appear in section 4.1, 5.1, 5.2 and 5.3 of the thesis. The percentage contribution of each of the authors is estimated as follows: MDJ 90%, SJ 5%, FJL 5%.

2. M.D. Johnston, S. Jahanshahi, and F.J. Lincoln: “Effect of Slag Basicity on Phase Equilibria and Minor Element

Distribution of Calcium Ferrite Based Slags” Manuscript in preparation.

The results described in this paper also appear in section 4.1, 4.3, 5.2, 5.3 and 5.5 of the thesis. The percentage contribution of each of the authors is estimated as follows: MDJ 90%, SJ 5%, FJL 5%.

Signature………………………………

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ABSTRACT

There are a number of impurity elements present in sulphide ores that can have a

deleterious effect on the properties of the final copper metal product. In this thesis, an

equilibrium distribution technique was used to determine the thermodynamic behaviour

of selenium and tellurium in molten slags used in copper production. Calcium ferrite

based slags and copper or silver alloy were equilibrated in magnesia crucibles at

temperatures of 1200 to 1400 °C and oxygen partial pressures of 10-11 to 10-0.68 atm.

Under conditions typical of those employed during copper converting, the minor

elements were found to enter the slag as negatively charged species. The partitioning of

selenium and tellurium to the slag was greatest at high temperature, low oxygen partial

pressure and at highest concentration of basic oxide (CaO or BaO). The experimentally

derived data were combined with published information to calculate the selenide and

telluride capacities of the slag, and also to generate fundamental thermodynamic activity

data for selenium and tellurium in the slag phase. It was found that the activity

coefficients of selenium and tellurium were independent of their concentration in the

slag over the range studied, but were strongly dependent on the temperature, slag

chemistry and oxidation state of the slag.

Experiments were also designed and carried out to determine what effect the presence

of iron oxide and its oxidation state has on the behaviour of selenium in the slag. A

series of experiments involving iron oxide additions to a calcium aluminate slag was

conducted under increasingly oxidising conditions to assess the effect of total iron on

the selenide capacity as the dominant oxidation state of iron in the slag changed. It was

shown that at a constant ratio of CaO:Al2O3, the selenide capacity increased with total

iron in the slag. However, the effect on the selenide capacity did not appear any more

significant as the Fe3+:Fe2+ ratio changed in a particular direction.

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Another series of experiments was carried out with iron calcium silicate slags to

determine the stability of phases within the slag, and how this affected the equilibrium

distribution and activity coefficient of selenium in the slag. A number of solid phases

were identified and their composition determined by scanning electron microscopy,

energy dispersive spectroscopy and electron microprobe analysis. The composition and

minor element content of the remaining liquid was calculated using a thermodynamic

model. From this it was found that the capacity of the liquid slag has a region of

independence against slag chemistry, before increasing strongly with increasing lime

content to the calcium ferrite composition.

Some of the implications of this work are discussed with reference to the practicality of

adjusting the process variables in a large-scale industrial process for the purpose of

managing minor element content of the molten phases. Considerations include the

effect on copper recovery and rate of wear of furnace refractory materials.

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Contents

Acknowledgements ...........................................................................................................8

List of Figures .................................................................................................................10

List of Tables ..................................................................................................................15

1. Introduction.............................................................................................................16

2. Literature Review....................................................................................................20

2.1. The Structure of Molten Slags ........................................................................20

2.2. Slag Basicity and Capacity..............................................................................21

2.3. Calcium Ferrite Slags......................................................................................24

2.3.1. Advantages..............................................................................................25

2.3.2. Characteristics .........................................................................................26

2.3.3. Applications ............................................................................................27

2.3.4. Minor Elements in Calcium Ferrite Slag ................................................31

2.4. Iron Calcium Silicate Slag ..............................................................................32

2.5. Behaviour of Se and Te...................................................................................33

2.5.1. Sodium Carbonate Slag...........................................................................33

2.5.2. Matte Systems .........................................................................................35

2.5.3. Fayalite Slag............................................................................................35

2.6. Summary .........................................................................................................38

2.7. Scope of Current Work ...................................................................................39

3. Experimental ...........................................................................................................40

3.1. Materials..........................................................................................................40

3.2. Equipment .......................................................................................................42

3.3. Procedure ........................................................................................................44

3.3.1. Equilibrium Experiments ........................................................................44

3.3.1.1. High-Iron Slags ...............................................................................44

3.3.1.2. Low-Iron Slags................................................................................48

3.3.2. Slag Phase Stability Experiments ...........................................................50

3.4. Analysis...........................................................................................................53

3.4.1. Chemical .................................................................................................53

3.4.2. Microstructural........................................................................................54

4. Results.....................................................................................................................55

4.1. Ferrite Slag......................................................................................................55

4.1.1. Oxygen Partial Pressure ..........................................................................55

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4.1.1.1. Copper-Slag ....................................................................................55

4.1.1.2. Silver-Slag.......................................................................................59

4.1.2. Temperature ............................................................................................61

4.1.2.1. Constant Oxygen Partial Pressure...................................................61

4.1.2.2. Varying Oxygen Partial Pressure....................................................63

4.1.3. Slag Composition....................................................................................68

4.1.3.1. Varying Lime ..................................................................................68

4.1.3.2. Barium Ferrites ...............................................................................69

4.2. Low Iron Slags ................................................................................................71

4.3. Iron Calcium Silicate Slag ..............................................................................72

4.3.1. Cu/Slag Equilibrium ...............................................................................72

4.3.2. Drop-Quench Experiments......................................................................75

4.3.2.1. Minor Element Distribution ............................................................75

4.3.2.2. Liquid Chemistry ............................................................................76

4.3.2.3. Identification of Solid Phases..........................................................78

4.4. Summary .........................................................................................................87

5. Discussion ...............................................................................................................88

5.1. Oxidation state of Se and Te ...........................................................................89

5.1.1. Oxygen Partial Pressure Dependence .....................................................89

5.1.2. Composition of the Ferrite Slag ..............................................................92

5.2. Capacities ........................................................................................................94

5.2.1. Slag Basicity ...........................................................................................94

5.2.2. Temperature Dependence......................................................................103

5.3. Activity Relations .........................................................................................108

5.3.1. Concentration ........................................................................................108

5.3.2. Slag Basicity .........................................................................................111

5.3.3. Temperature ..........................................................................................115

5.4. Influence of Iron............................................................................................120

5.5. Iron Calcium Silicate Slags...........................................................................123

5.5.1. Bulk Distribution...................................................................................123

5.5.2. Liquid Distribution................................................................................125

5.6. Applications ..................................................................................................130

5.7. Summary .......................................................................................................135

6. Conclusions...........................................................................................................136

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6.1. Future Work ..................................................................................................140

References .....................................................................................................................142

Appendix .......................................................................................................................149

A. Analytical Results .................................................................................................150

B. Activity of Slag Components ................................................................................157

C. Published Work.....................................................................................................162

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ACKNOWLEDGEMENTS

This work was made possible through the award of the CSIRO Metallurgical Processes

Scholarship. This work has also been a part of the Centre for Sustainable Resource

Processing (CSRP) foundation project “Control of Minor Elements”. The continued

support for this work and the opportunities afforded to me by the CSRP, including the

award of a research scholarship, is gratefully appreciated.

Many thanks must first go to my supervisors, Dr Frank Lincoln at UWA, and Dr Sharif

Jahanshahi at CSIRO Minerals, for their help and advice on the direction of the project,

analysis of the data and preparation of this thesis.

The assistance of many people at CSIRO Minerals, Clayton, where the experimental

work was carried out, is greatly appreciated. Thanks to Rowan Davidson and Mandie

Matheson for help setting up the high temperature experiments; to the Analytical

Services team, especially Steve Peacock and Fuping Hao, for the chemical analyses; to

Nicki Agron-Olshina for her expert XRD work; to Cameron Davidson, Colin Macrae

and Nick Wilson for help with the SEM; and to the HTP group for general advice and

encouragement.

The assistance of Sharon Platten and Dr Janet Muhling at the Centre for Microscopy

and Microanalysis at UWA is also appreciated.

Special thanks also to my Dad, for his many helpful comments and suggestions on the

thesis, and to the rest of my family for their encouragement.

And finally, thanks again to Kelvin, for joining in the long Rally between Melbourne

and Perth, and to Andy, Kostia, Josh, Michael, Shaun, Mushy, Nathan, and every other

PhD student for their moral support over the years.

9

The activities for which I was solely responsible, under the guidance of my supervisors,

(Dr. Frank Lincoln and Dr. Sharif Jahanshahi) were:

• Planning and conducting the high temperature experiments, and the preparation of

samples. Dr Jahanshahi and other CSIRO staff assisted in selecting appropriate

techniques and training me in the use of laboratory equipment.

• The interpretation and presentation of results. Dr Jahanshahi assisted with the

prediction of some compositional data by computer simulations.

• Reviewing the relevant literature, and writing the thesis.

The experimental work and chemical analyses were carried out on site at CSIRO

Minerals, Clayton, Victoria. Some SEM work was also carried out at the Centre for

Microscopy and Microanalysis at UWA. Funding was provided by CSIRO Minerals,

The University of Western Australia, and The Cooperative Research Centre for

Sustainable Resource Processing.

One journal paper and two conference papers based on this work have so far been

published. A second journal paper is in preparation at the time of submission of this

thesis. These are listed in Appendix C. All papers were contributed to by multiple

authors, and permission has been obtained from all co-authors to include these papers in

the thesis. Some parts of these papers have been reproduced or adapted in the thesis,

and represent my own work.

List of Figures

Page Figure 2.1 Correlation of sulphide capacities of multicomponent slags against optical

basicity ....................................................................................................................23

Figure 2.2 Phase relations in the CaO-FeO-Fe2O3 system at 1300 °C ...........................26

Figure 2.3 Copper concentrations of slags in matte-slag equilibria................................29

Figure 2.4 Distribution coefficients of Se and Te between sodium carbonate slag and

copper as a function of 2Op ...................................................................................34

Figure 2.5 Distribution coefficients of Se (left) and Te (right) between fayalitic slag and

copper as a function of 2Op ....................................................................................36

Figure 2.6 Effect of slag additives on distribution coefficients of Se (left) and Te (right)

between fayalite slag and copper ............................................................................38

Figure 3.1 Schematic diagram of the furnace setup used in the equilibrium experiments

.................................................................................................................................43

Figure 3.2 Liquidus isotherm and oxygen isobars in the CaO-FeO-Fe2O3 system at 1300

°C. ...........................................................................................................................44

Figure 3.3 Liquidus isotherms in the CaO-FeO-Fe2O3 system at 1200 and 1300 °C .....45

Figure 3.4 Distribution coefficients of selenium and tellurium between calcium ferrite

slag and Cu as a function of time at 1300 °C and 10-9 atm 2Op .............................48

Figure 3.5 The CaO-Al2O3-MgO system........................................................................49

Figure 3.6 Schematic diagram of the crucible array used in the drop-quench

experiments. ............................................................................................................51

Figure 3.7 The CaO-FeOx-SiO2 system ..........................................................................52

Figure 4.1 Distribution coefficients of Se and Te (DM) between calcium ferrite slag (20-

25 wt% CaO) and Cu as a function of 2Op at 1300 °C ..........................................56

Figure 4.2 Copper content of equilibrated calcium ferrite slags (20-25 wt% CaO) as a

function of 2Op at 1300 °C.....................................................................................58


26 wt% CaO) and Ag as a function of 2Op at 1300 °C ..........................................60

Figure 4.4 Distribution coefficients of Se and Te (DM) between calcium ferrite slag (25

wt% CaO) and Cu as a function of temperature at 10-10 atm 2Op ..........................61

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Figure 4.5 Copper content of equilibrated calcium ferrite slags (25 wt% CaO) as a

function of temperature at 10-10 atm 2Op ................................................................62


27 wt% CaO) and Cu as a function of temperature at constant CO2:CO................64


function of temperature at constant CO2:CO ..........................................................65


wt% CaO) and Cu as a function of temperature at constant aCaO ...........................66

Figure 4.9 Copper content of equilibrated calcium ferrite slags (~25 wt% CaO) as a

function of temperature ...........................................................................................67

Figure 4.10 Distribution coefficients of Se and Te (DM) between calcium ferrite slag and

Cu as a function of lime content at 1300 °C and 10-9 atm 2Op ..............................68

Figure 4.11 Distribution coefficients of Se and Te (DM) between Ba-containing slags (8-

32 wt% CaO, 5-34 wt% BaO) as a function of slag basicity (β) and Cu at 1300 °C

and 10-9 atm 2Op .....................................................................................................70

Figure 4.12 Distribution coefficients of Se in Fe-containing calcium aluminate slags

(34-42 wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying 2Op ........................71

Figure 4.13 Distribution coefficients of Se and Te between mixed FCS slags (2-25 wt%

CaO, 2-32 wt% SiO2) and Cu as a function of slag basicity (Q) at 1300 °C and 10-9

atm 2Op ...................................................................................................................73

Figure 4.14 Copper contents of mixed FCS slags (2-25 wt% CaO, 2-32 wt% SiO2) as a

function of slag basicity (Q) at 1300 °C and 10-9 atm 2Op ....................................73

Figure 4.15 Distribution coefficients of Se and Te between liquid FCS slags (3-30 wt%

CaO, 2-28 wt% SiO2) and Cu as a function of the basicity of the liquid slag (Ql) at

1300 °C and 10-9 atm 2Op ......................................................................................76

Figure 4.16 Copper concentration of liquid FCS slags as a function of the basicity of the

liquid slag (Ql) at 1300 °C and 10-9 atm 2Op ..........................................................78

Figure 4.17 SEM micrographs of the 1:9 FCS slag showing the iron magnesium oxide

(O) and iron magnesium silicate (FS) solid phases.................................................79

Figure 4.18 SEM micrographs of FCS slags containing the iron magnesium oxide (O)

and iron magnesium silicate (FS) solid phases together .........................................80

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Figure 4.19 SEM micrographs of FCS slags showing the iron magnesium oxide (O) and

calcium iron magnesium silicate (FCS) solid phases..............................................80


calcium silicate (CS) solid phases together.............................................................81

Figure 4.21 SEM micrographs showing the iron magnesium oxide (O) solid phase in a)

the 5:2 and b) the 10:1 FCS slags ...........................................................................81

Figure 4.22 SEM micrographs of the 1:1 FCS slag a) after equilibration with Cu alloy

and b) after quenching.............................................................................................82

Figure 4.23 XRD pattern of the 2:1 mixed FCS slag......................................................85

Figure 4.24 XRD pattern of the 13:2 mixed FCS slag....................................................85



Figure 5.1 Selenide and telluride capacities (CM) of calcium ferrite slag (20-25 wt%

CaO) under varying 2Op as a function of the activity of lime at 1300 °C .............95

Figure 5.2 Selenide and telluride capacities of varying-lime calcium ferrite slags (16-32

wt% CaO) as a function of the activity of lime at 1300 °C and 10-9 atm 2Op .......98

Figure 5.3 Selenide and telluride capacities of calcium ferrite slags as a function of the

activity of lime at 1300 °C ......................................................................................99

Figure 5.4 Selenide and telluride capacities of Ba-containing slags (8-32 wt% CaO, 5-

34 wt% BaO) as a function of slag basicity (β) at 1300 °C and 10-9 atm 2Op .....101

Figure 5.5 Selenide and telluride capacities of calcium ferrite slag (25 wt% CaO) as a

function of temperature at 10-10 atm 2Op ..............................................................104

Fig 5.6 Selenide and telluride capacities of calcium ferrite slag (23-27 wt% CaO) as a

function of temperature at constant CO2:CO ratio................................................104


function of temperature at constant activity of lime (aCaO = 0.432) .....................106

Figure 5.8 Selenide and telluride capacities of calcium ferrite slag (22-26 wt% CaO) as

a function of temperature ......................................................................................107

Figure 5.9 Activities of Se and Te (aM) in calcium ferrite slag (24 wt% CaO) as a

function of mole fractions of Se and Te in the slag (χM) at 1300 °C and 10-10 atm

2Op ........................................................................................................................108

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Figure 5.10 Activity coefficients of Se and Te (γM) in calcium ferrite slag (24 wt% CaO)

as a function of mole fractions of Se and Te in the slag (χM) at 1300 °C and 10-10

atm 2Op .................................................................................................................110

Figure 5.11 Activity coefficients of Se and Te (γM) in calcium ferrite slag (20-25 wt%

CaO) under varying 2Op as a function of the activity of lime at 1300 °C ...........111

Figure 5.12 Activity coefficients of Se and Te (γM) in varying-lime calcium ferrite slags

(16-32 wt% CaO) as a function of the activity of lime at 1300 °C and 10-9 atm 2Op

...............................................................................................................................112

Figure 5.13 Activity coefficients of Se and Te (γM) in barium-containing slags (8-32

wt% CaO, 5-34 wt% BaO) as a function of slag basicity at 1300 °C and 10-9 atm

2Op ........................................................................................................................114


as a function of temperature at 10-10 atm 2Op ......................................................115


CaO) as a function of temperature at constant CO2:CO .......................................117

Figure 5.16 Activity coefficients of selenium (γSe) in calcium ferrite slag (23-27 wt%

CaO) as a function of the activity of lime at 1200 to 1400 °C..............................118

Figure 5.17 Activity coefficients of tellurium (γTe) in calcium ferrite slag (23-27 wt%

CaO) as a function of the activity of lime at 1200 to 1400 °C..............................118


as a function of temperature at constant activity of lime (aCaO)............................119

Figure 5.19 Selenide capacities (CSe) of Fe-containing calcium aluminate slags (34-42

wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying 2Op .................................120

Figure 5.20 Activity coefficients of Se (γSe) in Fe-containing calcium aluminate slags

(34-42 wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying 2Op ......................121

Figure 5.21 Distribution coefficients of Se and Te (DM) and composition ranges of the

different solid phase assemblages in mixed FCS slags (2-25 wt% CaO, 2-32 wt%

SiO2) at 1300 °C and 10-9 atm 2Op .......................................................................123

Figure 5.22 Selenide capacities (CSe) of FCS (3-30 wt% CaO, 2-28 wt% SiO2) and

calcium ferrite (16-32 wt% CaO) slags as a function of the activity of lime at 1300

°C ..........................................................................................................................127

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Figure 5.23 Telluride capacities (CTe) of FCS (3-30 wt% CaO, 2-28 wt% SiO2) and

calcium ferrite (16-32 wt% CaO) slags as a function of the activity of lime at 1300

°C ..........................................................................................................................127

Figure 5.24 Activity coefficient of selenium (γSe) in liquid FCS slags (3-30 wt% CaO, 2-

28 wt% SiO2) as a function of the activity of lime at 1300 °C and 10-9 atm 2Op 129

Figure 5.25 Activity coefficient of tellurium (γTe) in liquid FCS slags (3-30 wt% CaO,

2-28 wt% SiO2) as a function of the activity of lime at 1300 °C and 10-9 atm 2Op

...............................................................................................................................129

Figure 5.26 Distribution coefficients of Se (DSe) between calcium ferrite slag (20-26

wt% CaO) and Cu at 1200 to 1400 °C and 10-11 to 10-6 atm 2Op ........................131

Figure 5.27 Distribution coefficients of Te (DTe) between calcium ferrite slag (20-26

wt% CaO) and Cu at 1200 to 1400 °C and 10-11 to 10-6 atm 2Op ........................132

Figure B.1 Activities of oxides in varying lime magnesia saturated calcium ferrite slags

at 10-9 atm 2Op and 1300 °C.................................................................................157

Figure B.2 Activities of oxides in magnesia saturated calcium ferrite slag under varying

2Op at 1300 °C......................................................................................................158


temperature at 10-10 atm 2Op ................................................................................159


temperature at constant CO2:CO ratio...................................................................160

Figure B.5 Activity of components of liquid FCS slags with varying basicity of the

liquid slag (Ql) at 1300 °C and 10-9 atm 2Op as predicted by the MPE simulations

...............................................................................................................................161

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List of Tables

Page Table 3.1 Estimated uncertainties in XRF analyses........................................................54

Table 4.1 Measured and calculated ferric to ferrous ratios of calcium ferrite slags after

equilibration with Cu alloy at 1300 °C. ..................................................................57

Table 4.2 Variation of the copper content of calcium ferrite slag (in wt%) with varying

oxygen partial pressure at 1300 °C. ........................................................................59

Table 4.3 Composition of solid phases in mixed FCS slags. ..........................................83

Table 5.1 Activity coefficients of Se and Te in dilute Cu alloys. .................................109

Table A.1 Analysis of equilibrated slags and alloys from varying 2Op experiments at

1300 °C. ................................................................................................................150

Table A.2 Analysis of equilibrated slags and alloys from varying temperature

experiments at a 2Op of 10-10 atm.........................................................................151


experiments at constant ratio of CO2:CO..............................................................151


experiments at constant aCaO. ................................................................................152

Table A.5 Analysis of equilibrated slags and alloys from experiments with slags of

varying lime content at 10-9 atm 2Op and 1300 °C. .............................................152

Table A.6 Analysis of equilibrated slags and alloys from experiments with barium-

containing slags at 10-9 atm 2Op and 1300 °C......................................................153

Table A.7 Analysis of equilibrated slags and alloys from experiments with different Cu

alloys at 10-10 atm 2Op and 1300 °C.....................................................................153

Table A.8 Analysis of equilibrated slags and alloys from experiments with varying-iron

calcium aluminate slags at 1500 °C. .....................................................................154

Table A.9 Analysis of equilibrated slags and alloys from experiments with silicate slags

at 10-9 atm 2Op and 1300 °C.................................................................................155

Table A.10 Composition of liquid FCS slags in equilibrium with Cu alloy from MPE

simulations. ...........................................................................................................156

15

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1. INTRODUCTION

The pyrometallurgical extraction of copper from sulphide ore can be separated into

three basic steps:

1. Smelting, where ore and fluxes are melted to separate a matte and slag phase

2. Converting, where matte is oxidised to blister copper, slag and SO2

3. Electro-refining, where the impure copper is dissolved under an applied current,

then redeposited at a pure copper cathode.

The presence of trace amounts of minor elements in a metal product can have a

significant adverse affect on its electrical and mechanical properties. Selenium (Se) and

tellurium (Te) are common minor components found in sulphide ore, and as such must

be managed during refining of metals such as copper and nickel. Selenium, for

example, has a high affinity for copper, and small quantities can cause failure in finely

drawn copper wire. While Se and Te do have value in that they are used in electronic

components and as a colorant in glass and ceramics, they are not in great demand.

These minor elements are becoming increasingly problematic as high-grade ore reserves

become depleted and low-grade deposits containing higher concentrations of impurities

are exploited. Typically, Se and Te are retained in the matte or metal phases, to be

removed in the final refining step. They can, however, be removed earlier by

partitioning into the slag by the addition of a suitable flux. However, there arises the

issue that waste slag containing toxic elements such as Se and Te may pose an

environmental hazard if not handled appropriately. Knowledge of the thermodynamic

behaviour of such minor elements is therefore necessary to enable them to be managed

appropriately during the refining processes. The thermodynamic behaviour of Se and

Te between the various molten phases is reasonably well documented for systems

making use of the traditional fayalite slag (iron silicate, Fe2SiO4) for copper processing,

17

as well as others such as those based on sodium oxide or sodium carbonate. In the

converting step, calcium ferrite slag (CaO-FeOx) is a recently adopted alternative that

can also be used. While data exist for the behaviour of some minor elements in this

system, Se and Te have not been studied.

The overall aim of this work was to comprehensively define the thermodynamic

behaviour of Se and Te under copper converting conditions with calcium ferrite slag in

terms of variables such as temperature, oxygen partial pressure and slag composition.

The data generated were used to determine how Se and Te contaminate this new slag

during copper converting, and to predict the occurrence of the minor elements in the

bulk metal. This could also potentially allow for the design of practical procedures to

minimise dispersion of Se and Te, or to remove any excess impurities if necessary. For

comparison, the behaviour of Se and Te was also investigated in iron calcium silicate

slags, as the slag most appropriate for a particular starting concentrate depends on a

number of factors such as cost and availability of fluxes required, physical properties,

refractory wear and valuable metal losses.

It was decided to investigate the behaviour of the minor elements by an equilibrium

distribution technique. This involved reacting molten slag and metal under controlled

oxygen partial pressure and temperature, and measuring the final minor element content

of each phase. The thermodynamic behaviour of Se and Te is known for the Cu-Se and

Cu-Te systems, so in a Cu-slag system at equilibrium, the activities of the minor

elements can also be calculated easily. There are methods for the direct measurement of

the activity, such as the Knudsen cell-mass spectrometer and isopiestic techniques, but

these require sensitive equipment and complex setups that were not readily available.

18

As a supplement to the determination of the equilibrium distribution and activities of Se

and Te, the influence of the oxidation state of iron in the slag on minor element

behaviour was also investigated. Since iron is the major component of the calcium

ferrite slag, it is expected to interact with the minor elements to some degree, either

through a contribution to the free oxygen in the slag, or by direct associations of ions.

However, changing the temperature and oxygen partial pressure of the system also has

an effect on the activity of the slag components, including FeO and Fe2O3, which are

expected to interact with the minor elements differently.

Phase equilibria studies on the silicate slags were also conducted, using techniques such

as scanning electron microscopy (SEM) and X-ray diffraction (XRD) to identify

mineral phases within the slag, as the usefulness of a slag is limited by the conditions

under which it remains liquid. This has consequences in the performance of the

extraction process as well as in the take up of impurity elements.

This thesis is presented in chapters as follows:

• Chapter 2 is a survey of the literature on the nature and applicability of various

metallurgical slags, and of the behaviour of common minor elements during the

extraction of valuable metals from sulphide ore.

• Chapter 3 describes the high temperature experimental setup and analytical

techniques used.

• Chapter 4 describes the results of the equilibrium experiments and microstructural

investigation.

• Chapter 5 describes the mechanisms by which the minor elements enter the slag,

and the thermodynamic data generated from the results. Some practical implications

19

of the results in regards to their application in a large-scale industrial process are

also presented in this chapter.

• Chapter 6 is a summary of the main findings of this work.

20

2. LITERATURE REVIEW

2.1. The Structure of Molten Slags

Liquid slags are generally considered to be ionic in nature. In this interpretation, metal

oxides (eg, CaO, FeO, MnO) dissociate, donating oxygen to the melt, i.e. act as a base.

This oxygen is accepted by components that act as an acid (eg, SiO2, Al2O3, Fe2O3),

which form anion complexes, and at sufficient concentration, polymerise the melt (Ban-

ya and Hino, 1991). For example, in calcium silicate slag:

CaO ⇌ Ca2+ + O2- [2-1]

SiO2 + 2O2- ⇌ SiO44- [2-2]

SiO44- + SiO4

4- ⇌ Si2O76- + O2- [2-3]

The polymerisation continues to form ring and chain structures. Richardson (1955)

suggested that in binary silicate slags, there would be some distribution of the various

ions present depending on chemical equilibria. Toop and Samis (1962) expressed this

distribution in term of an equilibrium constant and showed that the equilibrium constant

was characteristic of the cation. Masson (1965) subsequently put forward the

hypothesis that the equilibrium ratio for the reaction of the reaction of two silicate ions

to yield a higher member of the series and an oxygen ion tend to approach constancy as

the size of the silicate ions increases, and may have any value depending on the nature

of the cation.

The network formed by SiO4 tetrahedra in the liquid, however, is irregular, or distorted,

compared to crystalline SiO2. The network is broken up by the addition of basic metal

oxides (MeO), such that, at above a certain MeO content, only high-polymer anion

21

complexes exist (Schlackenatlas-Slag Atlas, 1981). The breakdown of the silica

network occurs at a higher MeO concentration when the cation is divalent, compared to

univalent, due to the ability of Me2+ ions to bridge oxygen atoms of neighbouring anion

complexes (Turkdogan, 1983).

In iron containing slags, both Fe2+ and Fe3+ are octahedrally coordinated with oxygen at

low total iron concentration. As the concentration of total iron increases, the proportion

of tetrahedral Fe3+ increases, which then takes part in network forming reactions. As

the temperature is increased, the ratio of tetrahedral to octahedral Fe3+ decreases,

indicating a breakdown of the network (Turkdogan, 1983).

2.2. Slag Basicity and Capacity

As the concept of an acidic or basic oxide is well known, it is common practice for

metallurgists to classify a slag as acidic or basic as an indicator of its chemistry. This

can be from a simple ratio or difference of concentrations of basic and acidic

components, but will not always be an accurate indication if the relative strength as an

acid or base is not taken into account. However, many slags commonly make use of

aluminium and iron oxides, which are considered amphoteric. These will donate

oxygen to the melt to some degree as well as contribute to polymerising the slag. As

such, attempts have been made to define a measure and thereby quantify the slag

basicity. A commonly applied term is the optical basicity for oxides, as defined by

Duffy and Ingram (1971). The bulk optical basicity (Λ) of a multi-component oxide

slag is calculated from the expression

Λ = XAΛA + XBΛB + … [2-4]

where XI is the equivalent cation fraction of each oxide, as described by Sommerville

and Sosinsky (1984), and ΛI is the optical basicity of that oxide. Oxides generally

considered basic are those of the alkali and alkaline earth group elements, while acidic

oxides are generally those of non-metals such as Si and P. Since Λ are calculated from

the Pauling electronegativity, values for transition metal oxides cannot be determined.

This is an important consideration, as higher oxidation state oxides of the same element

are expected to be less basic in character, and many slags contain iron. Some authors

(Nakamura et al., 1986) have estimated values by use of average electron density

instead. A recent review of various methods for expressing slag basicity is presented by

Sommerville and Yang (2000).

The term capacity in reference to a slag is a measure of its ability to take up a particular

element or species. Commonly used in metallurgical systems is the sulphide capacity,

as first defined by Fincham and Richardson (1954). For the gas-slag equilibrium

½S2(g) + O2-(sl) ⇌ S2-(sl) + ½O2(g) [2-5]

22

Swhere (sl) indicates the molten slag phase, the sulphide capacity (C ) becomes

2/1

S

Oslag

-2S

2

2][S ⎟⎟⎠

⎞⎜⎜⎝

⎛=

pp

C [2-6]

where and are the partial pressures of oxygen and sulphur respectively and the

concentration of sulphide is in wt%. From their experimental work with fayalite based

slags, Simeonov et al. (1995) concluded that C

2Op2Sp

S increases with increasing temperature,

and while the concentration of sulphur in the slag varies, CS is independent of both 2Op

and . The sulphide capacity of a slag is often measured against slag basicity (Figure

2.1), and Sosinsky and Sommerville (1986) used existing sulphide capacity data and the

correlation of Duffy et al. (1978) to determine Λ values for some transition metal

oxides. However, there is a danger of misassigning values by this method, as the total

contribution to capacity by a certain species can be due partly to its donation of oxygen

to the melt, and partly through direct interaction with the element in question. For

example, Sosinsky and Sommerville (1986) give an unexpectedly high value for FeO

(Λ

2Sp

FeO = 1.03), when compared to ΛCaO = 1.0.

23

igure 2.1 Correlation of sulphide capacities of multicomponent slags against optical F

basicity (after Sosinsky and Sommerville, 1986).

24

espite difficulties in predicting absolute values of basicity, the effect that changing the

o

nd

.3. Calcium Ferrite Slags

onsidered as a possible replacement for iron silicate slags

e

an

of

t

ing

refractories.

D

chemistry, and hence the basicity of the slag might have on capacities can be predicted.

Model calculations for several binary slags at 1500 and 1600 °C (Pelton et al., 1993)

show an increase of CS with increasing basicity of the slag, which is partly attributed t

the acidic nature of sulphur. Increasing optical basicity implies that the amount of free

oxygen in the melt is increasing. As such, sulphide capacity should indeed increase

with basicity as per equation 2-5, as seen is the case in Figure 2.1 above. Selenium a

tellurium are in the same group as sulphur in the periodic table, hence it is expected that

they would show very similar behaviour in this regard. An expression for capacities of

many other species including selenide and telluride were presented by Reddy (2003).

2

Calcium ferrite slag has been c

in copper production. It was first used commercially by the Mitsubishi Materials

Corporation in 1974 in the converting of matte to blister copper. Early work on th

stability of phases in the CaO-FeO-Fe2O3 system was carried out by Phillips and Mu

(1960), with activities of the oxides calculated by Turkdogan (1961). Studies of the

thermodynamics and physico-chemical properties of the CaO-FeO-Fe2O3 system by

Takeda et al. (1980), Yazawa et al. (1981) have been used to determine the feasibility

this type of slag in certain situations. It was the pioneering work of Prof Akira Yazawa

of Tohoku University, Japan, that led to the use of the term calcium ferrite slag.

Recently, Jahanshahi & Sun (2003) reviewed the experimental work carried out a

CSIRO Minerals on the physico-chemical properties of calcium ferrite slags, includ

minor element behaviour, copper solubility, transport properties and reactions with

2.3.1. Advantages

A major drawback of fayalite slag is that it cannot dissolve large amounts of ferric

problematic precipitation of magnetite during sulphide smelting

c

sm lesser

ity, which can lead to the loss of valuable metals to

lag by mechanical entrainment as well as dissolution. The absence of polymerising

f the

er

impurities

uch as P, As and Sb from the metal. The activity of CaO is much higher in calcium

ite

oxide, leading to the

under oxidising conditions. Calcium ferrite slags, however, do not suffer magnetite

precipitation problems, even at high 2Op , provided that they have sufficient lime

content. Furthermore, the efficiency of calcium ferrite over fayalite in taking up ferri

oxide during converting means that a aller volume of slag is needed, and hence

amounts of flux need to be added.

Fayalitic slags also have high viscos

s

SiO2 means that calcium ferrite slags are much more fluid, and thus easier to work with.

The experiments of Yazawa and Takeda (1982) with slags equilibrated with liquid

copper showed that copper losses to the calcium ferrite slag are about 70% of those

observed in the iron silicate system. It was also concluded that the copper content o

ferrite slag increases with 2Op but is not dependent on the CaO content of the slag.

Increasing temperature on the other hand, has the effect of lowering the copper content

at fixed 2Op (Tanaka et al. 03). The effect of

2Op , 2Sp and silica content on copp

in calcium ferrite slag has been investigated in detail by Somerville (2000).

Since calcium ferrite is also a very basic slag, it is useful for taking up acidic

, 20

s

ferrite slags than in silicate-based slag, due to its tendency to combine with SiO2 in the

latter type. Thus, CaO is free to interact with such minor elements in the calcium ferr

slag.

25

2.3.2. Characteristics

Calcium ferrite slags with approximately 20 wt% CaO were found to be stable over a

00 °C (Takeda et al., 1980). X-ray diffraction studies by

lexes

e2+

et al.,

980).

eparation was observed to occur by Font et al. (2000) in calcium ferrite slags

quilibrated with Ni matte at grades in the range 45 to73.3 wt% Ni at a 2SO of 10.1

wide range of 2Οp at 13

Yazawa et al. (1981) in CaO-Fe2O3 systems showed that simple ionic pairs such as

FeO+ were expected to exist in the dilute CaO region, while the formation of comp

such as FeO45- and Fe2O5

4- was a realistic assumption at >50 mol% CaO. The Fe3+:F

ratio is around 10 times higher in ferrite slag than in silicate slag, and is increased by

increasing 2Οp , decreasing the temperature and increasing the CaO content of the slag

(Yazawa and Takeda, 1982). The experimental results of Takeda et al. (1980) showed

that the slag region is bordered by iron, wustite, magnetite, dicalcium ferrite and lime

phases (Figure 2.2).

Figure 2.2 Phase relations in the CaO-FeO-Fe2O3 system at 1300 °C (after Takeda

26

1

Phase s

pe

27

y

ts

en in

kPa (0.1 atm*). Observations of the separated phases by scanning electron microscop

showed that slag only penetrates into the matte phase at the boundary, and that matte

components are not obvious in the slag phase. Heterogeneous mixing was noted at

lower grades, where there was no phase separation, by the observation of large amoun

of matte inclusions in the slag. Magnetite behaviour in calcium ferrite slag was

analysed by Tanaka et al. (2003), and it was found that the activity of magnetite

decreased as the Fe:CaO ratio was decreased. That is, at a given temperature and

solid magnetite is precipitated in slags with lower lime content. Transfer of oxyg

calcium ferrite slags is reported to be fast (Sayad-Yaghoubi et al., 1997).

2.3.3.

2Οp ,

Applications

One area of particular interest for the use of calcium ferrite slag is in the copper

e Mitsubishi process, limestone is added as a flux at the

ite

tion in

i

converting step. In th

converting stage, collecting the iron oxidised from the matte to form calcium ferr

slag. The thermodynamics of calcium ferrite slag in regard to its direct applica

the Mitsubishi process is discussed by Tanaka et al. (2003). Currently, the Mitsubish

process is used in copper production at plants in Naoshima, Japan; Timmins, Canada;

Onsan, South Korea; and Gresik, Indonesia. This process was also used at a plant in

Australia at Port Kembla.∗ Studies have also been carried out at CSIRO to evaluate the

potential benefits of using calcium ferrite slag in single, and two-stage copper making

processes by the SIROSMELT technology (Jahanshahi et al., 1995, Somerville et al.,

1999).

* 1 atm = 101.3 kPa = 1.013 bar

∗ The smelter at Port Kembla was closed in July, 2003

28

have been performed in metal-slag (Yazawa and Takeda, 1982) and metal-

atte-slag systems (Acuña and Yazawa, 1986, Park et al., 1984, Roghani et al., 1996).

n the

ni et

lower matte grades, and dominant as an oxidic species at higher matte

rades (Roghani et al., 1996). This was also shown by Acuña and Yazawa (1986),

he

984)

g

e

To assess the practicality of calcium ferrite slags in copper converting, equilibration

studies

m

In systems involving a copper matte phase, the dissolution of sulphur into calcium

ferrite slag is reported to decrease as the matte grade increases, more sharply than i

case of iron silicate slag, becoming very small at a matte grade of around 80% (Park et

al., 1984). The solubility of sulphur also appears to be independent of 2SOp (Rogha

al., 1996).

Copper dissolution into calcium ferrite slag as a sulphidic species is expected to be

dominant at

g

whose distribution data showed that both copper and sulphur have a preference for t

matte over ferrite slag, which becomes stronger as [Cu]matte increases. Park et al. (1

reported the copper content of slag to be closely related to the sulphur content, findin

[Cu]slag to be at a minimum at a matte grade of 78% Cu, and to also decrease sharply as

matte grade increases. Roghani et al. (1996) reported similar behaviour of copper in

slag to a matte grade of 72.5% Cu, with a subsequent increase in [Cu]slag above this

point. While their results also showed [Cu]slag to be independent of 2SOp , Park et al.

(1984) found the copper content of calcium ferrite slag to be dependent on 2Οp (Figur

2.3).

Figure 2.3 Copper concentrations of slags in matte-slag equilibria (after Park et al.,

1984).

29

2

distribution ratios between matte and slag tending to unity in the lower matte grade

Their results showed [Cu]slag to decrease with increasing across the range 102Οp -11 to

10-9 atm, where it is considered to exist as a sulphidic species. A further increase in

from 10Οp -8 to 10-6 atm led to an increase in copper partitioning into the slag phase,

where it is considered to exist as an oxidic species.

It was also noted by Park et al. (1984) that copper matte and calcium ferrite slag form a

homogeneous melt at matte grades <40%. The matte grade at which this occurs

increased to 50% when Cu metal is also present in the system, in which case, the metal

exists with sulphur-containing slag. This mutual dissolution behaviour of matte and

slag exists to a greater extent at low matte grades, which accounts for the observation of

30

atte

e of a

.

he application of calcium ferrite slags in Ni-Fe matte systems has also been

s found

ferrite

dly

l.,

region (Acuña and Yazawa, 1986). Such behaviour is also reflected in the calcium

content of the matte phase, which was found to gradually increase with decreasing m

grade (Park et al., 1984). It was also reported by Acuña and Yazawa (1986) that

considerable amounts of alkaline earth sulphides exist in ferrite slag in the absenc

strong acid oxide such as SiO2. The replacement of SiO2 with an alkaline earth oxide,

such as CaO, will produce a slag with a weaker ionic character, such that there will be

less difference in fundamental character between the matte and slag phases. Much

higher mutual dissolution is then expected in the ferrite slag-matte system than with

silicate slag, as alkaline and alkaline earth metals exist in both oxidic and sulphidic

forms, which mix with each other in the absence of SiO2 (Acuña and Yazawa, 1986)

T

investigated (Font et al., 2000). Nickel dissolution into calcium ferrite slag wa

to decrease with increasing matte grade, and to increase with increasing 2SOp at a given

matte grade. The partitioning of copper into calcium ferrite slag over ma ecreased

with increasing nickel content of the matte, and was found to be independent of 2SOp .

At high matte grades, the distribution of both copper and nickel between calcium

slag and matte became comparable to that in the iron silicate slag system. Sulphidic

dissolution of nickel in slag was found to prevail with negligibly small oxidic

dissolution across all matte grades. Nickel distribution however, is not reporte

significantly different between the calcium ferrite and fayalite slag types (Yazawa et a

1981).

tte d

2.3.4. Minor Elements in Calcium Ferrite Slag

Takeda et al. (1983) studied in detail the distribution behaviour of many minor elements

between the copper and calcium ferrite slag phases, and determined the predominant

oxide form of each in the slag. They also showed that as the CaO content of the slag

increased, the partitioning of As and Sb to the slag increased, while that of Pb

decreased. By comparing activity coefficient data, Eerola et al. (1984) concluded that

calcium ferrite slags are more favourable than silicate slags for the removal of Sb and

As to slag during copper fire-refining. However, Kim and Sohn (1996) reported that

even though calcium ferrite slags are able to remove large amounts of As and Sb by

slagging, overall elimination of these elements is less than with fayalite slag, as they are

volatilised in the smelting stage when fayalite slag is used. They also found that the

total elimination of Pb and Bi does not significantly differ between the two slag types,

with calcium ferrite being more efficient for volatilisation and fayalite more efficient for

slagging of these elements (Kim and Sohn, 1996). Distribution ratios for Pb between

calcium ferrite slag and metallic Cu (Park et al., 1984) were found to be around 10

times smaller than in the iron silicate slag, which shows that ferrite slag is much less

efficient for removing Pb from metallic Cu. The distribution of Pb into calcium ferrite

slag over matte increased with increasing matte grade and with increasing in the

high oxygen partial pressure range, where it is assumed that PbO is the predominant Pb

species in calcium ferrite slag.

2Οp

The behaviour of minor elements in other lime and calcium fluoride based slags has

also been studied. Sano (1992) found that under reducing conditions, BaO-BaF2 slag

was more effective at taking up As and Sb than CaO-CaF2 slag from a silver alloy.

These elements, like sulphur, are considered acidic, so it is expected that they should be

31

32

2 2

taken up by the more basic BaO-containing slag to a greater degree. Wakasugi and

Sano (1989) found that Bi and Pb could be removed to CaO-CaF2 slag from iron, but

under extremely reducing conditions. As these elements are considered basic, such

conditions would be necessary for them to be taken up by this type of basic slag.

Tavera et al. (2000) later determined distributions of a number of minor elements, not

including Se and Te, between metal, matte and CaO-CaF2-SiO2 slag in copper-slag

equilibria under various temperature, and conditions. Οp SΟp

2.4. Iron Calcium Silicate Slag

The use of calcium ferrite slag also has deficiencies, such as associated refractory wear

and the low Pb solubility. For this reason, slags of the type CaO-FeOx-SiO2 have been

discussed in recent years for application in copper converting. The phase equilibria of

the CaO-FeOx-SiO2 system was investigated by Muan and Osborn (1965), and more

extensively by Timucin and Morris (1970). Its application in the copper converting step

was later discussed by Yazawa et al. (1999). This type of slag is unsuitable for the

matte-making step, however, due to the high dissolution of copper as Cu2S (Ojima et

al., 2003), whereas copper enters the slag as an oxide in the converting step. It has also

been shown that the addition of SiO2 to a calcium ferrite slag results in a decrease in the

solubility of Cu2O (Somerville et al., 1995, Yazawa, 2005). However, as highlighted by

Vartiainen and Kytö (2002), problems persist in that large amounts of fluxes are

required, and therefore a large quantity of slag is produced in reaching the minimum

copper solubility slag composition in the processing of normal concentrates. Phase

equilibria of iron calcium silicate slags at copper saturation have been determined by

Nikolic et al. (2006) for the purpose of optimising process parameters. The sulphide

capacity of iron calcium silicate slags at 1500 °C has been estimated by model

calculations by Chen et al. (1989) and later by Reddy et al. (1992).

2.5. Behaviour of Se and Te

2.5.1. Sodium Carbonate Slag

Measurements of the distribution of Se and Te between slag and copper were made by

Alvear et al. (1994), who found that the distribution coefficients have a different

dependency on in the relatively oxidising (> 102Op -6 atm) and more reducing (< 10-7

atm) regions. Under more reducing conditions, distribution coefficients decreased as

the was increased to ~102Op -2 Pa for Te and ~10-1 Pa for Se. Distribution coefficients

then increased with a further increase in to more oxidising conditions. From this it

was concluded that Se and Te form an intermetallic sodium compound at lower oxygen

partial pressures, and enter the slag by reaction with Na

2Op

2CO3 at higher oxygen partial

pressures. This confirmed the earlier work of Kojo et al. (1985), who concluded that

Na2Se and Na2Te form in slag under reducing conditions, while tetravalent selenate and

tellurate species form in oxidising conditions. It was also concluded that this type of

slag was effective for removing Se and Te from copper under reducing conditions,

where distribution coefficients for slag over metal were around 1000 for both minor

elements (Figure 2.4). From the equilibrium distribution data, it was concluded that

formation of a selenium oxide is favoured at higher , whereas at lower the

formation of Na

2O 2Op p

2Se is favoured. It was further concluded that other additions to the slag

were necessary since slag/metal distribution ratios for Se were still too low for industrial

application.

33

34

2O

Figure 2.4 Distribution coefficients of Se and Te between sodium carbonate slag and

copper as a function of (after Kojo et al., 1985). p

The use of sodium carbonate slag has associated problems however, in that the Na2O

formed is corrosive, affecting the life of furnace refractories. Tandon et al. (1991)

attempted to alleviate this problem by diluting the system with B2O3, which also helps

to dissolve sulphur, thereby preventing the release of hazardous Na2S. The effect of

CaO additions to this type of slag on Se distribution was studied by Tandon et al.

(1992). Removal of Se to slag was improved as increasing amounts of CaO were added

to the slag mixture, which was attributed to the increasing basicity of the slag. Addition

of CaO was also found to have the effect of increasing the viscosity of the slag, but not

to an extent that affected its applicability to industrial processes.

2.5.2. Matte Systems

Activities of Se and Te were measured in the Cu-Cu2S two phase region by Hino and

Zakeri (2000). Both minor elements were found to distribute preferentially into the

matte phase over Cu metal. Activities were reported to show quite large deviations

from the ideal behaviour, and were higher in the Cu phase. In the Cu phase, values for

Te increase rapidly to 8×10-4 at mole fraction (χM) around 0.01, and more gradually in

the Cu2S phase, with the same value reached at χTe ~0.09. Activities of Se were found

to follow a similar pattern, with a value of 1.3×10-4 reached at χSe ~0.01 in the Cu phase

and χSe ~0.12 in the Cu2S phase. Activity coefficients were generally found to show

little variation with changing composition of the phases.

In Cu-Fe mattes, distribution studies by Zakeri et al. (1999) established the preference

of Se and Te for the Cu-Fe matte phase over Cu metal. Increasing the iron content of

the matte was found to increase Se but decrease Te partitioning into the matte phase.

They found that the activities of both minor elements showed extreme negative

deviation from ideality in the matte. The activity of Te in the matte phase was shown to

increase with the addition of iron, while values for Se are unaffected by the changing

matte composition. In the matte-saturated copper phase, the activities of both minor

elements were found to increase with the addition of iron to the system.

2.5.3. Fayalite Slag

Distribution experiments by Nagamori and Mackey (1977) with alumina-containing

fayalitic slags and copper showed Se and Te partition preferentially into the metallic

phase. They found that partitioning of these elements to the slag decreased with

decreasing temperature, and was strongly dependent on in the relatively more 2Op

35

reducing region, as the increased to about 102Op -8 atm. Then, as conditions became

more oxidising, the distribution coefficients were found to be independent of the

across the range 10

2Op

-8 to 10-6.5 atm (Figure 2.5). In the more oxidising region where

distribution is independent of , Nagamori and Mackey (1977) reported dissolution

to be in a monatomic form, whereas in the more reducing region where distribution is

dependent on , it was suggested that Se and Te entered the slag as an iron complex.

This so-called molecular dissolution of minor elements in slag, as characterised by

increasing solubility with decreasing , can occur provided the compounds of the

minor elements with iron are stable under copper smelting conditions, i.e., are as stable

as iron sulphide (Nagamori and Mackey, 1978).

2Op

2Op

2Op

36

2O

2

Figure 2.5 Distribution coefficients of Se (left) and Te (right) between fayalitic slag and

copper as a function of (after Nagamori and Mackey, 1977). p

Fang and Lynch (1987) observed similar behaviour against , as indicated by their

determination of Fe

2Op

3+:Fe2+ ratios of the slag. They also showed that the fraction of

neutral Se in the slag can be calculated as a function of the product χSe Op , where χSe is

the mole fraction Se in copper. From this they concluded that Se existed in slag as a

neutral species and as a complex with iron, but also proposed the existence of an oxidic

species at higher oxygen partial pressures. The range of investigated by both

Nagamori and Mackey (1977) and Fang and Lynch (1987) was limited, however,

covering only about 3 orders of magnitude to a maximum of about 10

2Op

-6.5 atm.

37

2

It has also been established (Choi and Cho, 1996, Choi and Cho, 1997, Celmer and

Toguri, 1988) that Se and Te distribute preferentially into the matte phase over fayalite

slag phase. The dependence of the partitioning behaviour of Se and Te on oxygen

partial pressure observed by Choi and Cho (1997) supports the dissolution of molecular

(i.e. selenide and telluride) forms of Se and Te in slag. Similarly, in the matte phase, Se

and Te react preferentially with copper to form the selenide and telluride respectively.

The distribution was enhanced by increases in matte grade, temperature and in the

range 10

2Op

-9 to 6×10-8 atm (Choi and Cho, 1996). However, as the relative proportion of

nickel in Fe-Ni matte (Celmer and Toguri, 1988) and Cu-Ni matte (Choi and Cho,

1996) decreases, the distribution of Se into the matte phase also decreases. By

combining expressions for the partial pressure of Se and Te in matte and slag, Choi and

Cho (1997) derived equations for the distribution of both elements as a function of . Op

The effect of additives CaO, MgO and Al2O3 to the fayalite slag on Se and Te

partitioning was also studied by Choi and Cho (1997). All three additives were found to

increase Te partitioning into matte at a given matte grade. For Se, partitioning was

similarly increased by additions of CaO and MgO. However, Se partitioning only

increased with Al2O3 additions at matte grades < ~55% (Figure 2.6). They reported the

opposite effect with cobalt distribution however, and attributed this to competition from

added cations to fill positions in the silicate slag network structure as it is broken down

by the basic oxides.

Figure 2.6 Effect of slag additives on distribution coefficients of Se (left) and Te (right)

between fayalite slag and copper (after Choi and Cho, 1997).

2.6. Summary

The review of the literature has shown that while the behaviour of many minor

elements, such as As and Sb, has been studied in the newly developed calcium ferrite

slag, no data is available for Se and Te. Due to the difference in acid-base character of

calcium ferrite slag compared to fayalite, the behaviour of Se and Te is expected to be

different between the two common types. While the behaviour in other basic slags may

act as a guide, the exact trends and absolute values of distribution coefficients need to

be determined experimentally under representative conditions to properly evaluate the

applicability of this new slag in high temperature copper production.

38

39

2.7. Scope of Current Work

The main purpose of this study is to fill in gaps in the knowledge of the behaviour of Se

and Te in calcium ferrite and iron calcium silicate slags. This was done by conducting

equilibrium distribution experiments that involved reacting slags and metals under

controlled conditions, varying key operating variables such as the temperature and

oxygen partial pressure as in a typical copper converting process. As the activities of Se

and Te in copper are known, they can be applied to the determination of the activities of

these elements in the equilibrated slag. The nature of the interactions between the slag

components and the minor elements and how these vary with the changing experimental

conditions is also examined. This work also extends to the examination of the

microstructure of the iron calcium silicate slags and the determination of any further

partitioning of the minor elements between separated phases within the slag.

The specific research questions to be addressed in this work are:

1. What is the dependence of the oxidation state of Se and Te in the slag on the oxygen

partial pressure and the composition of the slag?

2. What are the effects of oxygen partial pressure, temperature and slag chemistry on

the capacity of the slag to take up Se and Te?

3. How do the activities of Se and Te in the slag phase vary with the temperature,

oxidation state and composition of the slags?

4. Do Se and Te show strong interactions with iron oxides in the slag in a similar

manner as they do with strongly basic oxides?

40

3. EXPERIMENTAL

In this chapter, the materials used and the experimental and analytical procedures

applied are described. A Cu alloy was chosen as the reservoir of Se and Te in the

experiments, as the Cu-Se and Cu-Te systems have been previously characterised.

Thus, the alloy is used as a kind of ‘probe’ to enable the determination of the activities

of Se and Te in the system at equilibrium. The equilibrium distribution technique was

chosen based on the availability of a suitable apparatus, and also as it allows for precise

control of the temperature and atmospheric conditions. Before commencing the

experiments, the reaction time necessary for the system to reach equilibrium was

determined. Equilibrium between the gas, slag and metal phases was achieved by

heating samples placed in magnesia crucibles in a controlled atmosphere at between

1200 and 1500 °C. The alloy was doped with Se and Te such that the loss of volatile

species was not a problem. Samples were then quenched in water and analysed by a

number of techniques. For the phase equilibria studies, samples were heated in

platinum capsules under a controlled atmosphere at 1300 °C and allowed to equilibrate

before being rapidly quenched in water. Mineral phases were identified and their

composition quantified by electron microscopy techniques.

3.1. Materials

Calcium ferrite master slag was synthesised from high purity (>99%) ferric oxide and

AR grade calcium carbonate powders. Preparation on the kilogram scale was carried

out by melting the dried powders in a magnesia crucible in air using an induction

furnace. Nitrogen was bubbled through the molten slag to assist mixing. For

compositional variation studies, calcium carbonate, ferric oxide or technical grade

barium oxide were usually added to portions of the calcium ferrite master slag.

41

Fayalite master slag was prepared by melting wustite and >99% silica powders in a

platinum crucible in a muffle furnace. The wustite used in the preparation of slags was

synthesised from ferric oxide by reduction in the solid state in a gas-tight glass tube

under CO2/CO gas flow at 800 °C for 2 hours. Several iron calcium silicate (FCS) slags

were prepared by mixing, in various ratios, a calcium ferrite and a fayalite master slag,

then melting the mixture in a platinum crucible inside a muffle furnace.

Pre-reduction of high-iron slag to be used in experiments was carried out in order to

reduce the time to reach equilibrium with the metal phase. Pre-reduction also helped to

prevent the precipitation of fine copper prills that form as copper oxide, taken up by the

slag at the beginning of an experiment, is reduced as the experiment progresses. The

presence of these prills can lead to an anomalous apparent copper content of the

equilibrated slag. Contamination by these prills could possibly also alter the apparent

minor element content of the slag, but due to their size and starting concentration, this

effect is expected to be negligible. Pre-reduction of the master slag was carried out in a

gas-tight vertical tube furnace. In this step, around 50 g of slag at a time was weighed

out into a small magnesia crucible and pre-reduced at 1300 ºC for a period of 24 hours

under CO2/CO gas flow. For the mixed FCS slags where there was a smaller amount of

slag prepared, pre-reduction was carried out in the solid state by the same method that

wustite was prepared.

To investigate the influence of iron on minor element behaviour, calcium aluminate

(CA) master slags were prepared by melting calcium carbonate and alumina powders in

a platinum crucible in a muffle furnace. These slags were also pre-reduced for 24 hours

in the tube furnace. Up to 20 wt% wustite was added to portions of this slag for use in

the equilibrium experiments.

42

Master copper alloy with approximately 1% Se and Te was synthesised under flowing

N2 in an induction furnace. Powdered Se and Te were wrapped in 99.96% purity Cu

foil, then this packet was added to molten copper at >1100 ºC. Master silver alloy with

approximately 2.5% Se and Te was synthesised under Ar flow in a vertical tube furnace.

Powdered Se and Te were melted with 99.99% purity Ag at >1000 ºC in an alumina

crucible inside the furnace. In both cases, the molten alloy was extracted from the

crucible while it was still inside the furnace by suction through silica glass tubing with

an inside diameter of 4 mm. The Se and Te content of the alloys were designed to be

low such that there would be no significant effect of one minor element on the activity

coefficient of the other in the alloy.

Gases used in the experimental work were food grade CO2 (BOC) and 99.5% purity CO

(Linde). The argon (BOC) used to flush out the furnace was ultra-high purity

(99.999%). The gases were dried by passing them through a column of silica gel, and,

except for CO, deoxidised by passing them through Cu turnings at 500 ºC.

3.2. Equipment

The pre-reduction of slag and all experiments were carried out in a gas-tight vertical

tube furnace. The alumina work tube inside the furnace was fitted with water-cooled

brass end caps, sealed with rubber “O” rings. The temperature profile inside the furnace

was determined to locate the position of the hot zone, where the temperature remained

constant to within ±1 °C over the course of an experiment. Inside the tube, crucibles

were positioned so that the base of the crucible was in the middle of the hot zone. An

R-type (Pt/Pt-13% Rh) thermocouple, inserted inside an alumina lance, closed at one

end, was used to measure the temperature at the top of the platform. Gas flow into the

furnace was regulated by MKS Instruments and Brooks mass-flow controllers, which

were calibrated regularly by a rising soap bubble technique. A diagram of the furnace

setup for these experiments is shown in Figure 3.1.

To exhaust line

Alumina work tube Alumina

lance

Heat shield

Pin

SlagMetal button

Magnesia crucible

Gas inlet

Brass end cap Alumina lance Thermocouple

Figure 3.1 Schematic diagram of the furnace setup used in the equilibrium experiments.

43

3.3. Procedure

3.3.1. Equilibrium Experiments

3.3.1.1. High-Iron Slags

A calcium ferrite slag with 20 wt% CaO was used in the majority of experiments. This

composition was chosen because it is at this composition that a single liquid is stable

over the widest range of in the CaO-FeO-Fe2Op 2O3 system as shown in Figure 3.2.

Figure 3.2 Liquidus isotherm and oxygen isobars in the CaO-FeO-Fe2O3 system at 1300

°C (after Yazawa et al., 1981).

For the slag/metal equilibration experiments, equal amounts of approximately 4-5 g of

slag and alloy were used. The alloy was cut into pieces and placed in the bottom of a

small, recrystallised magnesia crucible, then covered with slag. Magnesia crucibles

were chosen because of the low solubility of MgO in calcium ferrite slags. The tapered

egg-cup shape crucibles (supplied by Fuji-Sho Inc. of Japan) were lowered into place

onto a cast ceramic platform that was cemented to the end of an alumina lance fixed to

the bottom end cap. The crucibles were attached to an alumina lance fixed to the top

end cap by means of an alumina pin. A length of platinum wire was threaded through

44

the pin and wrapped around the lance to provide additional support in case of pin

failure. A ceramic heat shield rested on the pin, fitting inside the rim of the crucible.

The furnace was set initially to 800 ºC, then flushed with Ar for around half an hour.

After the flushing period, the crucible was slowly lowered into the hot zone of the

furnace, and once in position, the temperature of the furnace was raised to the desired

level at a rate of 300 °C per hour.

For most of the experiments, the temperature was set to 1300 °C. At this temperature,

the region of single liquid stability in the CaO-FeO-Fe2O3 system is sufficiently large to

allow studies to be carried out over an extensive range of and CaO content,

compared to lower temperature, as shown in Figure 3.3.

2Op

Figure 3.3 Liquidus isotherms in the CaO-FeO-Fe2O3 system at 1200 and 1300 °C (after

Takeda et al., 1980).

Once the furnace reached the set temperature, the CO2 and CO gases were introduced at

predetermined flow rates and left to react with the slag and metal phases for the desired

45

length of time. The necessary flow rate of the gases required to achieve the desired

was calculated by the relation

2Op

05.929510log2logCO

COO

2

2+−⎟⎟

⎠

⎞⎜⎜⎝

⎛=

Tpp

p [3-1]

as determined by Yazawa and Takeda (1982) from the thermodynamic data for the

reaction

2CO(g) + O2(g) ⇌ 2CO2(g) [3-2]

A total gas flow rate of 500 mL/min was used throughout. The gases were mixed

before entering the furnace through the bottom end cap, then flowed up the work tube

and over the molten slag. Exhaust gas exited through the top lance, where a bubbler

seal on the exhaust line imparted a slight positive pressure on the work tube.

At the completion of an experiment, the crucible assembly was quickly removed from

the furnace through the top and immersed in water to the level of molten material inside

the crucible. While doing this, care was taken to avoid disturbing the contents so that

mixing of the slag and metal phases did not occur. Cooling the slag by this method

typically took several seconds. After quenching, the slag was broken away from the

crucible, taking care to exclude any pieces of the magnesia crucible from the sample. A

dark reaction layer, visible to the naked eye, was also observed at the slag-crucible

interface. Yan et al. (2005) showed that this layer was either a magnesiowustite or

magnesioferrite spinel, depending on the . Further care was taken to exclude this

material from the sample taken by filing away the outer edge of the slag. Slag samples

2Op

46

were then ground to a fine powder for analysis. The alloy settled to the bottom of the

crucible and was easily broken away from the bulk slag, then cleaned by filing. Any

visible metallic pieces within the bulk slag were also excluded from the slag sample.

The time for the system to reach equilibrium was determined by a series of experiments

in which the Cu alloy was allowed to react with calcium ferrite slag for reaction times

between 4 and 30 hours at a temperature of 1300 °C and a of 102Op -9 atm. The

concentration of Se and Te was measured in the slag and metal phases, and it was

apparent that equilibrium between the molten phases was reached fairly rapidly, as a

levelling off in both Se and Te content of the slag was observed. This is shown in

Figure 3.4. The Fe3+:Fe2+ ratio of slags after 4 and 16 hours were roughly equal at

around a value of 0.78, indicating that equilibrium between the gas and slag phases was

also reached rapidly. A reaction time of 24 hours was therefore chosen for subsequent

experiments.

The rapid equilibration of the phases can also be attributed to the good exposure of the

alloy, covered only by a thin layer of slag, to the gas flow inside the crucible. However,

the direct contact between the metal and gas phases may also contribute to the loss of

the minor elements and copper to the exhaust due to their volatility. Typically the

difference in the total mass of the reactants from before to after equilibration was about

0.05 g (approx 1%), although was as high as 0.2 g at 1500 °C. However, it is possible

that there is also some degree of volatilisation of the copper and the slag components, so

it cannot be said for certain that the difference in masses before and after equilibration is

due solely to the minor elements.

47

t (hrs)

0 5 10 15 20 25 30 35

log

DM

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2SeTe

1300 οC, 10-9 atm

Figure 3.4 Distribution coefficients of selenium and tellurium between calcium ferrite

slag and Cu as a function of time at 1300 °C and 10-9 atm . 2Op

3.3.1.2. Low-Iron Slags

Slags used in these experiments were prepared so that their composition was within the

CaO.Al2O3 field of the Al2O3-CaO-MgO system where they would remain liquid at

1500 °C (Figure 3.5). Magnesia crucibles were used so that the CaO:Al2O3 ratio of the

slag would remain constant despite the possibility of interactions between the crucible

and the components of the slag. The same experimental setup and apparatus as

described above was used for equilibrium distribution experiments with CA based slags

and copper alloy. Experiments with varying amounts of iron in the slag were conducted

at different oxygen partial pressures to determine if the effect of iron additions on minor

element distribution differed depending on the oxidation state of the iron in the slag.

48

Figure 3.5 The CaO-Al2O3-MgO system (after Schlackenatlas - Slag Atlas, 1981). The

CaO.Al2O3 field is boxed.

The time to reach equilibrium in the Cu-CA system was expected to be longer than with

the high-iron slags due to the low diffusivity of species in (Amini et al., 2006), and

higher viscosity of, the iron free slag. Similar experiments were conducted with the

higher alumina slag (molar ratio CaO:Al2O3 = 1.24) at 1500 °C and 10-8 atm with

reaction times up to 72 hours. The tellurium concentration in the slag could not be

detected, but distribution coefficients of Se did appear to level off to a value of about

2Op

49

7.08×10-3 above 24 hours. A reaction time of 30 hours was chosen for subsequent

experiments with this slag type.

3.3.2. Slag Phase Stability Experiments

These experiments were designed to melt a small amount of slag that had already been

equilibrated with metal, then quench it rapidly in water so that any phases formed

within the slag are preserved and crystal growth during cooling is inhibited. A sub-

sample of around 200 mg of equilibrated slag was placed in a small platinum foil

capsule and hooked onto a platinum wire array, suspended in the hot zone of the

furnace. The temperature and conditions were re-created from the original

experiment. The reaction time in each case was 2 hours. To allow for rapid quenching,

the seal on the bottom and cap of the furnace was removed, and the opening covered

with plastic wrap. Pieces of ceramic were attached to the crucible array below the

crucibles to act as ballast, so that when the wire was released, these pierced the plastic

wrap, allowing everything above to pass through unobstructed. The samples fell

directly into a bucket of water, raised so that the water level was at the opening of the

furnace. This ensured that the samples were not exposed to air. A diagram of the

furnace setup for these experiments is shown in Figure 3.6. The material resulting from

each run was mounted in resin and made into a polished section for analysis by

scanning electron microscopy (SEM).

2Op

50

Exhaust Clamp

Pt wire Thermocouple

Wire array Pt capsule

Ceramic ballast

Gas inlet

Plastic film

Water

Figure 3.6 Schematic diagram of the crucible array used in the drop-quench

experiments.

51

The drop-quench experiments were carried out on samples of previously equilibrated

slag from the Cu/FCS slag experiments. Their purpose was to enable the observation of

differences in textural characteristics of the slag as the composition changes from

calcium ferrite to iron silicate. The line AB (bold) in Figure 3.7 indicates

approximately the range within which the compositions of slags used in experiments

were located within the CaO-FeOx-SiO2 ternary system. As the slag composition is

changed along this path, phase boundaries may be crossed, possibly leading to the

precipitation of solid phases during an experiment. This diagram can be used as a guide

only as it is for magnesia-free slags at iron saturation, which is more reducing than the

at which these experiments were conducted. 2Op

B

52Figure 3.7 The CaO-FeOx-SiO2 system (after Schlackenatlas - Slag atlas, 1981).

A

53

3.4. Analysis

3.4.1. Chemical

All chemical analyses were carried out on site at CSIRO Minerals Clayton laboratories.

Master slags were analysed for their major element content by X-ray fluorescence

spectroscopy (XRF). Slags that had been equilibrated with a metal phase were analysed

for their major and minor element content by inductively coupled plasma (ICP)

techniques. Normally ICP-AES (Atomic Emission Spectrometry) was used, but in

some cases detection limits were not low enough to be able to measure the Se or Te

concentration. This was overcome by repeating the analysis using ICP-MS (Mass

Spectrometry) for these elements. Because of their relatively high concentrations,

alloys were analysed for the minor elements by ICP-AES.

Samples for analysis by ICP were prepared by acid digestion, with nitric acid added to

stabilise any volatile compounds of Se and Te in solution. Slags containing silver were

digested in concentrated nitric acid only to avoid the formation of insoluble Ag

compounds.

The uncertainty associated with the ICP-AES and ICP-MS analyses is estimated to be ±

2 % relative to the measured value. For the analysis of Si by XRF, a sub-sample of the

slag was first dissolved in a 12:22 lithium metaborate/lithium tetraborate flux. XRF

was also used for the equilibrated low-iron slags. XRF is generally not an ideal

technique for the analysis of equilibrated slags, however, because of the tendency of

copper and silver to form an alloy with the platinum crucibles used to hold the samples

in the XRF spectrometer. The uncertainty associated with the XRF analysis is expected

to vary with concentration, as shown in Table 3.1.

54

Table 3.1 Estimated uncertainties in XRF analyses

Concentration range (wt%)

0.5 – 1 1 – 10 10 – 50 50 – 100

Relative uncertainty

± 2 % ± 1 % ± 0.6 % ± 0.3 %

The Fe2+ content of slags was determined by dichromate titration under a nitrogen

blanket using a Metrohm 798 MPT Titrino auto-titrator. The uncertainty associated

with the ferrous analysis is estimated to be ± 2 % relative to the measured value.

3.4.2. Microstructural

SEM work was performed using the Zeiss SUPRA 55VP SEM at the Centre for

Microscopy and Microanalysis at the University of Western Australia, and the Quanta

400F FEG ESEM and JEOL JXA-8900R instruments at CSIRO Minerals Clayton

laboratories. Qualitative analysis was performed by energy dispersive spectroscopy

(EDS) on the Zeiss and Quanta instruments. Quantitative analysis of major and minor

components by electron microprobe analysis (EMPA) was performed on the JEOL

instrument.

XRD analysis of slags was performed using a Philips X’Pert Diffractometer equipped

with a Co target tube at CSIRO Minerals Clayton laboratories.

4. RESULTS

The experimental results are presented in this chapter in three sections. The first section

covers the equilibrium distribution experiments with the calcium ferrite based slags.

Results are generally expressed in the form of distribution coefficients (DM), defined as

the ratio of concentration in wt% of minor element M in the slag phase to that in the

metal phase.

metal

slagM [M]

M][=D [4-1]

The second section covers the investigation into the effect of iron in the slag on the

equilibrium behaviour of the minor elements. This is presented in terms of the effect of

total iron, and the oxidation state of iron, in the slag.

The third section covers the experiments with FCS slags. This comprises both the

equilibrium distribution experiments and the slag phase equilibria/microstructural

investigation.

The results of the chemical analyses for each series of experiments are tabulated

separately in Appendix A.

4.1. Ferrite Slag

4.1.1. Oxygen Partial Pressure

4.1.1.1. Copper-Slag

Distribution coefficients of Se and Te calculated from the experiments with Cu alloy

and calcium ferrite slag across the range of oxygen partial pressures from 10-11 to 10-6

55

56

2O

2O 2O

atm are plotted in Figure 4.1. A good trend was established for Se, with DSe decreasing

from 5 to around 0.01 with increasing . As [Te]2Op slag was typically around an order of

magnitude less than [Se]slag, and below detection limits in the more oxidising region,

DTe could not be calculated from all experiments. Values of DTe decrease from 0.3 to

0.02 with increasing , and the apparent trend was the same as that in the Se results.

However the fewer data points available for Te may make the observed trend less

reliable than that for Se.

p

log pO2 (atm)

-12 -11 -10 -9 -8 -7 -6 -5

log

DM

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0SeTe

-0.5

1300 οC


25 wt% CaO) and Cu as a function of at 1300 °C. Two points at a single

indicate where repeat experiments were conducted.

p p

The Fe3+:Fe2+ ratios of the slag determined from the analytical results match very well

with those determined for each experiment by equation [4-2] (Takeda et al., 1980),

despite the slags in the present work containing copper and being magnesia saturated.

52.2T

5500CaO) wt%(018.0log17.0FeFelog

2O2

3

−++=+

+

p [4-2]

The actual and calculated value for each experiment are shown for comparison in Table

4.1, using the [CaO]slag from the analytical results in the calculation. While this is

useful, it is not perfectly reliable, as the iron redox equilibrium is subject to artefacts

from Cu in the slag (Somerville, 2000) and exposure of the slag to air as it is removed

from the furnace.

Table 4.1 Measured and calculated ferric to ferrous ratios of calcium ferrite slags after

equilibration with Cu alloy at 1300 °C.

2Op (atm) 10-11 10-10 10-9 10-9 10-8 10-7 10-6 10-6

Fe3+:Fe2+ (measured) 0.41 0.64 0.79 0.73 1.11 1.51 1.90 2.32

Fe3+:Fe2+ (calculated) 0.36 0.53 0.75 0.74 1.10 1.52 2.27 2.13

Copper contents of the slags from these experiments are shown in Figure 4.2.

Experiments were not continued to higher , as the increasing levels of [Cu]2Op slag under

increasingly oxidising conditions would significantly alter the bulk composition of the

slag. Further to this, copper in the slag can alter the apparent Fe3+:Fe2+ ratio of the slag.

The redox reaction

Cu+ + Fe3+ → Cu2+ + Fe2+ [4-3]

57

58

2O

2O

2O

takes place upon acid digestion prior to chemical analysis, so it will increasingly affect

results at higher where both [Cup +] and [Fe3+] are high.

log pO2 (atm)

-12 -11 -10 -9 -8 -7 -6 -5

log

(wt%

Cu)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1300 οC

0.25


function of at 1300 °C. p

The solubility of copper in calcium ferrite slag has been studied extensively, and the

present results agree well with the existing data of Takeda (1994) and Somerville

(2000). At 10-6 atm there is some discrepancy, but this may be due to the degree to

which entrained Cu metal prills contaminated the slag in each experiment. The

agreement with the results of Palacios and Gaskell (1993) is not as good, with the

present results being higher by 0.2 to 0.5 wt%. Yazawa and Takeda (1982) derived an

equation for [Cu]

2Op

slag as a function of temperature and from their experimental p

results, which, except at 10-6 atm, also agrees very well with the present results, as

shown in Table 4.2.

Table 4.2 Variation of the copper content of calcium ferrite slag (in wt%) with varying

oxygen partial pressure at 1300 °C.

2Op (atm) 10-11 10-10 10-9 10-8 10-7 10-6

This work 0.5 0.73 1.15 1.89 3.1 5.75

Yazawa & Takeda (1982) 0.4 0.67 1.12 1.86 3.10 5.15

Palacios & Gaskell (1993) - 0.53 0.85 1.48 2.61 -

Takeda (1994) 0.44 0.62 1 1.8 3 6

Somerville (2000) 0.48 0.69 1.18 1.94 3.31 6.85

The effect of entrained metal on the slag minor element assays is expected to be

negligible, however. Entrained metal particles are very fine, so they would only ever

make up a very small proportion of the total sample mass. As such, even 1 or 2 wt% Se

and Te in the alloy will add up to only a small additional amount in the slag.

4.1.1.2. Silver-Slag

Distribution coefficients of Se and Te calculated from the experiments with the silver

alloy and calcium ferrite slag across the range of from 102Op -11 to 10-0.68 atm are

shown in Figure 4.3. These experiments were facilitated by silver’s tendency to remain

unoxidised, and hence not enter the slag and alter its composition. The starting Ag alloy

was doped to approximately 2.5 wt% of each of Se and Te to overcome detection limit

restrictions in the ICP analysis for the minor elements in the slag anticipated at higher

. 2Op

59

log pO2 (atm)

-12 -10 -8 -6 -4 -2 0

log

DM

-4

-3

-2

-1

0

1

2Se Te

-0.5 0.5

1300 οC

60

2O

2


26 wt% CaO) and Ag as a function of at 1300 °C. p

The distribution coefficients determined from the Ag/slag experiments match well with

those from the Cu/slag experiments, indicating that the activity coefficient of each

minor element (γSe and γTe) in the Ag alloy is similar to the corresponding value in the

Cu alloy. The negative dependence on in the reducing region is again observed,

but as conditions become increasingly more oxidising (>10

2Op

-6 atm), Se and Te behave

differently to each other. Distribution coefficients of Te begin to increase with

increasing above 10Op -6 atm, while DSe appears to level off above 10-5 atm. The final

[Ag]slag recorded did not follow any trend, and were generally <0.5 wt% (Table A.1).

Values around 2 wt% were recorded in a small number of samples, but this is likely to

be due to entrained metal.

4.1.2. Temperature

4.1.2.1. Constant Oxygen Partial Pressure

Distribution coefficients of Se and Te calculated from the experiments with Cu alloy

and calcium ferrite slag at varying temperatures at constant are shown in Figure

4.4. For these experiments, the CO

2Op

2:CO gas ratio was adjusted for each experiment so

that a of 102 2

2

Op -10 atm was maintained throughout the series. This was chosen so

that if [M]

Op

slag was to decrease, levels would hopefully still be detectable and therefore be

less susceptible to uncertainty. The distribution coefficients of both Se and Te

increased with increasing temperature from 1200 to 1400 °C, while [Cu]slag decreased

significantly over the same interval (Figure 4.5).

T (οC)

1150 1200 1250 1300 1350 1400 1450

log

DM

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0SeTe

10-10 atm


wt% CaO) and Cu as a function of temperature at 10-10 atm . Op

61

T (οC)

1150 1200 1250 1300 1350 1400 1450

[Cu]

slag

(wt %

)

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

10-10 atm

62

2O

Figure 4.5 Copper content of equilibrated calcium ferrite slags (25 wt% CaO) as a

function of temperature at 10-10 atm . p

The influence of temperature is seen to be slightly stronger on Te, with DTe increasing

32 times from the lowest to highest temperatures while DSe increased 15 times over the

same interval. The concentration of copper in the slag decreases with increasing

temperature from 1.31 to 0.69 wt % Cu. In this series, the values from the present work

are all higher than expected from existing data (Yazawa and Takeda, 1982) by 0.1 to 0.5

wt %. It is possible that this is due to the effect of entrained metal in the slag in the

present work. The group VIb elements are known to be highly surface active in liquid

copper, which means that their presence reduces the surface tension of the liquid metal

alloy (Monma and Suto, 1961). This effect facilitates the mixing of molten phases and

subsequent entrainment of the metal within the slag, and occurs to a greater extent with

decreasing temperature (Sakai et al., 1997). It is therefore notable that it was at the

lowest temperature studied (1200 °C) where there was the largest discrepancy in [Cu]slag

between the present results and those of the Cu-calcium ferrite equilibrium study of

Yazawa and Takeda (1982), who did not have S, Se or Te in their system.

4.1.2.2. Varying Oxygen Partial Pressure

63

2O

The experiments with varying temperature were repeated with the CO2:CO ratio held

constant instead of maintaining a constant . A ratio of 2.26 was chosen, which

resulted in an increase in with increasing temperature from 10

2Op

2Op -10.3 atm at 1200 °C

to 10-7.87 atm at 1400 °C. In this case, the temperature and terms in Equation [4-2]

should counteract each other, resulting in near constant Fe

p

3+:Fe2+ ratios across this

series of experiments. As a consequence, calculated activity coefficients should not be

distorted by any difference in the strength of any interactions between the minor

elements and the different oxidation states of iron. Distribution coefficients of Se and

Te from these experiments are shown in Figure 4.6.

T (οC)

1150 1200 1250 1300 1350 1400 1450

log

DM

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0SeTe

CO2:CO = 2.265


27 wt% CaO) and Cu as a function of temperature at constant CO2:CO.

64

2O

In this case increasing the temperature has the opposite effect to the original constant

series of experiments, with distribution coefficients now decreasing as the

temperature increases. However the influence of temperature here is much less

significant than previously observed (Fig 4.4). Copper contents of the equilibrated slags

from this series are shown in Figure 4.7.

p

T (οC)

1150 1200 1250 1300 1350 1400 1450

[Cu]

slag

(wt%

)

1.0

1.2

1.4

1.6

1.8

2.0CO2:CO = 2.265


function of temperature at constant CO2:CO.

65

2O

Similarly, [Cu]slag now increases with increasing temperature, where previously they

decreased over the same temperature interval (Fig 4.5). This suggests that in this

situation, has the stronger influence, with an increase of about 2.5 orders of

magnitude enough to completely counter the effects of increasing the temperature by

200 °C. The Fe

p

3+:Fe2+ ratios in this series of experiments were not constant as

expected, but did not follow any particular trend with temperature. Values instead

varied in the range 0.71 to 0.87 (Table A.3).

Another series of experiments with varying temperature was carried out to eliminate the

influence of slag chemistry. In this case the CO2:CO ratio was adjusted so that the

activity of lime in the slag in each case would be the same. Any variations in the

66

2O

2O

activity coefficients of Se and Te calculated for the slag phase should then be due to the

temperature only, provided any direct interactions between iron and the minor elements

also remain the same. An activity of lime of 0.432 for a 20 wt% CaO slag at 1300 °C

and 10-9.2 atm was estimated using the CSIRO Multi-phase Equilibria (MPE)

programme (Zhang et al., 2002). The programme was then used to estimate the

CO

p

2:CO ratio required for that same aCaO to result at 1200, 1225, 1250 and 1275 °C.

Distribution coefficients of Se and Te from these experiments are shown in Figure 4.8.

T (οC)

1180 1200 1220 1240 1260 1280 1300 1320

log

DM

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2SeTe


wt% CaO) and Cu as a function of temperature at constant aCaO.

In this case, DM values increase with increasing temperature, despite an increase in

from 10

p

-9.82 atm at 1200 °C to 10-9.21 atm at 1300 °C. This is the opposite of what was

observed when the CO2:CO ratio was kept constant (Fig 4.6), although in that series of

experiments the increase in from 1200 to 1300 °C was over one order of

magnitude. The increase in D

2Op

M is much steeper from 1250 to 1300 °C than it is from

1200 to 1250 °C. However the overall increase in DM is only about 2 times, whereas in

the original series, DM increased 5 times over the same temperature range.

Copper levels in the slag from this series of experiments increase with increasing

temperature to a maximum at around 1225-1250 °C, before decreasing with a further

increase in temperature to 1300 °C. The values are very similar to those in both

previous temperature variation series from this work over the range 1200 to 1300 °C, as

shown in Figure 4.9. There is only slight variation between 1 and 1.25 wt% Cu, so this

could be due to scatter around a straight line, rather than a real trend.

T (οC)

1180 1200 1220 1240 1260 1280 1300 1320

[Cu]

slag

(wt%

)

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

const aCaO

const pO2

const CO2:CO

Figure 4.9 Copper content of equilibrated calcium ferrite slags (~25 wt% CaO) as a

function of temperature.

67

4.1.3. Slag Composition

4.1.3.1. Varying Lime

Distribution coefficients of Se and Te calculated from experiments with Cu alloy and

calcium ferrite slags with lime varying from 10 to 30 wt% CaO are shown in Figure

4.10. For all experiments, the temperature was set to 1300 °C and the to 102O

2

p -9 atm,

to allow for the greatest possible variation in the lime content of the slag while staying

within the single liquid region of the CaO-FeO-Fe2O3 system. The partitioning of both

minor elements is seen to be influenced only slightly by the lime content, with DSe

increasing from 0.26 to 0.47, and DTe increasing from 0.019 to 0.023, between the

lowest and highest lime slags.

CaO in slag (wt %)

15 20 25 30 35

log

DM

-2.0

-1.5

-1.0

-0.5

0.0SeTe

1300 οC, 10-9 atm

Figure 4.10 Distribution coefficients of Se and Te (DM) between calcium ferrite slag and

Cu as a function of lime content at 1300 °C and 10-9 atm . Op

68

69

2

2

Copper levels in the slag do not show a clear trend, with values in the range 1.05 to 1.52

wt% Cu. This supports the findings of Yazawa and Takeda (1982) who concluded that

[Cu]slag was dependent on and temperature only, and Sakai et al. (1997) who

concluded that the amount of CaO in the slag did not effect the entrainment of Cu metal

in the slag. Magnesia levels in the slag were found to increase from 3.4 to 7.8 wt%

MgO with increasing lime content (Table A.5). This is becoming a significant

proportion in the slag, and so may begin to make some contribution to the ability of the

slag to take up the minor elements.

Op

4.1.3.2. Barium Ferrites

Distribution coefficients of Se and Te from experiments with Cu alloy and barium-

containing slags at 1300 °C and 10-9 atm are shown in Figure 4.11. Each barium

ferrite slag was prepared so that the sum of CaO and BaO was around 25 wt%.

Although BaO is not in common usage as a flux to produce metallurgical slags, these

experiments were intended to further examine the effect of slag basicity on minor

element distribution. Barium, in the same group as calcium on the periodic table, has

similar chemistry, but is expected to interact with minor elements differently. This is

due to its oxide being more basic than lime, i.e. it should have greater ability to donate

oxygen to the melt. Since the molecular weight of BaO is much larger than that of CaO,

adding the same mass of each will lead to quite different molar amounts actually in the

slag. To account for this, the results are plotted against the mole fraction of BaO in

terms of the basic components, defined as

Op

CaOBaO

BaO

nnn

+=β [4-4]

where n is the number of moles of the indicated species in a 100 g sample of the slag.

This is a simplified indicator of relative slag basicity, such that a calcium ferrite slag has

β = 0, and for a barium ferrite slag, β = 1.

β

0.0 0.2 0.4 0.6 0.8 1.0

log

DM

-2.0

-1.5

-1.0

-0.5

0.0

0.5SeTe

1300 οC, 10-9 atm

70

2O

Figure 4.11 Distribution coefficients of Se and Te (DM) between Ba-containing slags (8-

32 wt% CaO, 5-34 wt% BaO) as a function of slag basicity (β) and Cu at 1300 °C and

10-9 atm . p

The influence of BaO is reasonably significant, with DM for Se and Te increasing 4 to 5

times as β increases from 0 to 1. Similar results were obtained by Acuña and Yazawa

(1987), who found distribution coefficients of As and Sb to be higher in barium ferrite

slag than in calcium ferrite slag. Final [Cu]slag and [Mg]slag of the barium ferrite slags in

the present work were found to be at similar levels to those typically recorded in the

calcium ferrite slags. The β = 1 slag after pre-reduction had a larger than usual reaction

layer between the slag and crucible, but this broke away smoothly from the bulk slag.

XRF analysis of this layer showed that it to concentrate the MgO. This layer was also

present in the equilibrium experiment with this slag, although not as prominently, but

was equally as easy to separate and exclude from the prepared slag sample.

4.2. Low Iron Slags

71

2

2O

To determine the effect of iron on minor element behaviour, wustite was added in

increasing amounts to portions of a calcium aluminate slag with a molar CaO:Al2O3

ratio of 1.56. Equilibrium distribution experiments were carried out as described

previously with this slag and Cu alloy at a temperature of 1500 °C and a of 10Op -10 to

10-8 atm. Distribution coefficients of selenium are shown in Figure 4.12. Values of DTe

were not calculated as [Te] could not reliably be determined due to their low levels.

The concentration of FeOx on the plot is the total iron in the slag from the XRF analysis

expressed as the concentration of Fe2O3 in wt%.

FeOx (wt%)

0 2 4 6 8 10 12 14 16 18

log

DS

e

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.410-8

10-9

10-10

1500 οC

Figure 4.12 Distribution coefficients of Se in Fe-containing calcium aluminate slags

(34-42 wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying . p

72

2O

The distribution coefficients of selenium were found to increase with increasing

additions of iron to the slag. This confirms that iron does have some contribution to the

slag’s ability to take up the minor elements. The large difference in DSe at zero iron

between data sets in this series also indicates that there is also an influence of in

these results. The copper content of the slag increased with increasing as expected,

but did not show any trend against total iron in the slag (Table A.8).

2Op

p

4.3. Iron Calcium Silicate Slag

4.3.1. Cu/Slag Equilibrium

Distribution coefficients of Se and Te from experiments with Cu alloy and FCS slags

prepared by mixing a calcium ferrite slag with approximately 20 wt% CaO and a

fayalite slag with approximately 35 wt% SiO2 in various ratios are shown in Figure

4.13. Conditions for each experiment were fixed at 1300 °C and 10-9 atm to allow

for direct comparisons with the results of the experiments with variations in the

composition of the ferrite slags. The D

2Op

M are plotted against the basicity of the slag,

which was measured by the composition quotient Q, defined as

]SiO[[CaO][CaO]

2+=Q [4-5]

as used by Takeda (1994). Here, the concentrations of the slag components are in wt%.

This is another simplified measure of relative slag basicity such that for acidic fayalite

slag Q = 0 and for basic calcium ferrite slag Q = 1. Copper contents of slags with

varying Q are shown in Figure 4.14. Included on the plots are the data for the calcium

ferrite and iron silicate end members.

Q

0.0 0.2 0.4 0.6 0.8 1

log

DM

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0SeTe

1300 οC, 10-9 atm

.0

73

2O

2

Figure 4.13 Distribution coefficients of Se and Te between mixed FCS slags (2-25 wt%

CaO, 2-32 wt% SiO2) and Cu as a function of slag basicity (Q) at 1300 °C and 10-9 atm

. p

Q

0.0 0.2 0.4 0.6 0.8 1.0

log

(wt%

Cu)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.41300 οC, 10-9 atm

Figure 4.14 Copper contents of mixed FCS slags (2-25 wt% CaO, 2-32 wt% SiO2) as a

function of slag basicity (Q) at 1300 °C and 10-9 atm . Op

Both DM follow the same trend, passing through a local maximum around Q = 0.2, then

a minimum around Q = 0.5, before increasing again to an absolute maximum as Q → 1.

This behaviour is somewhat unusual, as DM were expected to simply increase with

increasing lime content of the slag as found previously. Copper in the slag was lowest

at Q = 0.52, which is in agreement with the results of Takeda (1994). The ratio of

[Cu]slag in the fayalite slag to the calcium ferrite slag is 1.62, which is in excellent

agreement with the results of Yazawa et al. (1981).

74

2O

The quotient Q has its limitations, however, in that it does not incorporate the influence

of Fe. For example, a slag 10 % in each of CaO and SiO2 will have the same Q as a

slag 25 % in each, yet total iron will be different. The Fe3+:Fe2+ ratio of the slag could

be used as an indirect measure of slag basicity in this system, as at constant temperature

and , the iron redox equilibria is influenced by the other components. Adding CaO

to slag is known to stabilise Fe

p

3+, while addition of silica has the opposite effect

(Timucin and Morris, 1970), so a higher Fe3+:Fe2+ will indicate a more basic slag.

Jahanshahi and Wright (1993) calculated activity coefficients of FeO and FeO1.5 and

found that the ratio of these was independent of the total iron content of the slag above

about 6 wt% Fe.

A number of FCS slags were found to spontaneously disintegrate on cooling, which

meant that the experiment had to be abandoned because a proper sample could not be

recovered. This so-called “dusting” effect is attributed to the expansion of dicalcium

silicate (Yazawa et al, 1999), and was observed in the present work around Q = 0.6.

One advantage of slag dusting is that it eliminates the need for milling of the slag prior

to further processing, such as for the recovery of copper by flotation.

75

4.3.2. Drop-Quench Experiments

4.3.2.1. Minor Element Distribution

Sub-samples of equilibrated slag from the mixed FCS slag series were used in the drop-

quench runs to investigate whether solid phases appeared in these slags over the course

of the equilibration run, and whether Se and Te entered these solid phases. In each

sample, the EDS analysis showed that Se was concentrated in the liquid, and was not

present in any of the solid phases. Tellurium could not be determined due to the overlap

of its characteristic peaks with those of calcium, but it is expected to similarly

concentrate in the remaining liquid over the solid phases within the slag.

In some slags there were only very small patches of the liquid phase in between solid

particles, so there is the possibility of overlap where the beam struck the sample for the

analysis. Also, dendrites were quite extensive in some liquids, and like the main solid

phases, these were depleted in the minor elements. As a result, the likelihood remains

that the quantitative analyses of the liquid phase showed these elements to be less

concentrated than they actually are. Also, iron may alloy with the platinum foil used to

hold the sample under a reducing atmosphere. A significant change in the bulk

composition of the slag as a result of this could have the effect of raising the liquidus

temperature, leading to the precipitation of the observed solid phases.

To overcome these uncertainties, CSIRO’s Multi-Phase Equilibria model (Zhang et al.,

2002) was used, with the slag composition as determined from the ICP analyses as the

starting point, to predict the proportion of the slag that should remain liquid and the

composition of this liquid portion in the original equilibrium experiments with the

molten Cu alloy. The Se and Te concentrations within the liquid portions were then

calculated from the ICP analyses from the equilibration experiments and the estimated

proportion of liquid slag. These were then used to calculate distribution coefficients of

Se and Te between the remaining liquid and the Cu alloy, the so-called “liquid

distribution”. The results of these simulations are shown in Table A.10. Figure 4.15

shows the variation of DM (liquid) with the basicity of the liquid slag (Ql) as calculated

from the MPE simulations.

Ql

0.0 0.2 0.4 0.6 0.8 1.0

DM

( liqu

id)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35SeTe 1300 οC, 10-9 atm

iron silicate calcium ferrite

76

2O

Figure 4.15 Distribution coefficients of Se and Te between liquid FCS slags (3-30 wt%

CaO, 2-28 wt% SiO2) and Cu as a function of the basicity of the liquid slag (Ql) at 1300

°C and 10-9 atm . p

4.3.2.2. Liquid Chemistry

The magnesium content of the liquid slag varied between 1 to 10 wt% MgO, and was

generally less than that of the bulk slag. As expected, calcium and silicon were highest

77

in the liquid at the calcium ferrite and fayalite ends of the series respectively. The

observation of solid magnesio-wustite in the majority of slags in these experiments

indicates that the presence of MgO greatly stabilises wustite. In the present work,

magnesio-wustite precipitated from the slag at 1300 °C at lower [FeO]slag than expected

for wustite to precipitate from the magnesia-free system (Fig 3.7). The expansion effect

on the wustite field was also observed by Henao et al. (2006) for Al2O3 as well as MgO

additions to iron calcium silicate slag.

Copper, like Se, was found to concentrate in the liquid FCS slags, as shown by the

results of the EDS analysis. However, it is possible that, like iron, the copper in the

sample alloyed with the platinum foil holding the sample. This, combined with the

limited area within the samples from which data could be obtained for the liquid phase,

would again give a distorted apparent concentration by EMPA. The copper

concentration of the liquid FCS slags as predicted by the MPE simulations are shown in

Figure 4.16. The results show that [Cu]liquid is independent of composition over the

range 0.2 < Ql < 0.8 at about 0.7 wt% Cu2O, which is less than was recorded in both the

calcium ferrite and iron silicate slags.

Ql

0.0 0.2 0.4 0.6 0.8 1.0

[Cu 2O

] liqui

d (w

t%)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1300 οC, 10-9 atm

78

2O

Figure 4.16 Copper concentration of liquid FCS slags as a function of the basicity of the

liquid slag (Ql) at 1300 °C and 10-9 atm . p

This implies that achieving significantly lower copper losses than those associated with

iron silicate or calcium ferrite will be much easier if the FCS slag can be made to

remain molten. That is, a small amount of silica could be added to the calcium ferrite

slag for a large reduction in [Cu]slag without bringing about significant increase in DM.

This is in accordance with the findings of Somerville (2000).

4.3.2.3. Identification of Solid Phases

Four different solid phases were identified in the samples by SEM and characterised by

EDS analysis. These were, in order of appearance with increasing Q of the particular

slag being examined, iron magnesium silicate, iron magnesium oxide, calcium iron

magnesium silicate and calcium silicate. In any one sample there was a maximum of

two solid phases, these being the oxide and any one of the silicates. Only the iron

magnesium oxide and the iron magnesium silicate appeared as a single solid phase. A

dendritic crystal phase was also commonly observed, although this is a result of the

liquid cooling upon quenching, not separation at high temperature (Ashby and Jones,

1986). Representative images of the solid phases are shown in Figures 4.17-4.21.

dendrites

FS

liquid

O

Figure 4.17 SEM micrographs of the 1:9 FCS slag showing the iron magnesium oxide

(O) and iron magnesium silicate (FS) solid phases.

79

FS

O

O

FS

Figure 4.18 SEM micrographs of FCS slags containing the iron magnesium oxide (O)

and iron magnesium silicate (FS) solid phases together.

O

FCS

O


calcium iron magnesium silicate (FCS) solid phases.

80

CS

O

O

CS


calcium silicate (CS) solid phases together.

O

a)

O

b)

Figure 4.21 SEM micrographs showing the iron magnesium oxide (O) solid phase in a)

the 5:2 and b) the 10:1 FCS slags.

Figure 4.22 shows images of the 1:1 slag after equilibration with Cu alloy, and after the

subsequent drop-quench experiment. Specks of entrained Cu are visible in the

equilibrated slag, but were not observed in any of the drop-quench experiments.

81

Cu

a) b)

Figure 4.22 SEM micrographs of the 1:1 FCS slag a) after equilibration with Cu alloy

and b) after quenching.

To identify the mineral phases present in the samples, the compositions of the solid

phases were quantified by EMPA. The compositions of the solid phases observed in

each sample are summarised in Table 4.3. The slag column indicates the ratio of

calcium ferrite to fayalite slag in the mixture. The ratios of the elements are in terms of

moles. The regions A to G were differentiated according to the observation of new

phase assemblages from one slag to the next in terms of number and composition of the

solids present.

82

Table 4.3 Composition of solid phases in mixed FCS slags. Region Slag Solid Fe:Mg Ca:Fe+Mg Ca:Si Fe:Si Mg:Si

A 1:9 oxide

silicate

33

1.42

<0.01

<0.01

-

-

-

1.18

-

0.83

B 2:13 silicate 0.99 <0.01 - 1.02 1.03

1:4 silicate 0.73 0.01 - 0.87 1.19

1:2 oxide

silicate

8.6

0.65

<0.01

0.02

-

0.04

-

0.81

-

1.24

C 2:3 oxide

silicate

5.3

0.44

<0.01

0.03

-

0.06

-

0.61

-

1.39

1:1 oxide

silicate

3.1

0.30

<0.01

0.07

-

0.14

-

0.45

-

1.48

D 2:1 oxide

silicate

4.4

0.61

<0.01

1.50

-

1.23

-

0.31

-

0.51

E 5:2 oxide 3.8 <0.01 - - -

7:2 oxide

silicate

2.1

-

0.01

63*

-

2.0

-

-

-

-

4:1 oxide

silicate

2.2

-

0.02

42*

-

2.0

-

-

-

-

F 13:2 oxide

silicate

1.15

-

0.02

65*

-

2.0

-

-

-

-

9:1 oxide

silicate

0.83

-

0.02

>100*

-

2.0

-

-

-

-

G 10:1 oxide 0.33 <0.01 - - -

*Mg not included

With changing composition from fayalite to calcium ferrite, the oxide phase changed

from being Fe rich to Mg rich. According to the phase diagram of Wallace (1999), the

oxide phase is expected to be a magnesio-wustite, and not a magnesio-ferrite spinel at

the temperature and in this study. Also, the iron magnesium silicate and calcium

iron magnesium silicate phases observed changed from being Fe rich to Mg rich with

increasing Q within a particular region. In regions A to D, the sum of the Fe:Si, Mg:Si

2Op

83

84

and Ca:Si ratios of 2 indicates an olivine phase ((Ca,Fe,Mg)2SiO4) and not wollastonite

((Ca,Fe,Mg)SiO3). Similarly, in region F, the Ca:Si ratio of 2 indicates dicalcium

silicate (Ca2SiO4) and not pseudowollastonite (CaSiO3). Lime and wustite form an

incomplete solid solution, however calcium in the oxide phase was found to be always

<1.5 wt% Ca. Regions E and G are differentiated because of the different Fe:Mg ratios

of the oxide phase. Similarly, regions A and C are differentiated because of the

different Fe:Mg ratios of the silicate phase.

The identity of the solid phases were confirmed by X-ray diffraction (XRD) analysis of

the equilibrated slags. Because of the complete solid solution between wustite and

periclase (MgO), and fayalite and forsterite (Mg2SiO4), the resulting patterns of iron-

containing phases did not always match perfectly with the pattern returned by the

database*. Figure 4.23 shows the XRD pattern of the 2:1 FCS slag (region D),

confirming the presence of wustite and kirschsteinite (CaFeSiO4). Figure 4.24 shows

the XRD pattern of the 13:2 FCS slag (region F), confirming the presence of wustite

and larnite (β-Ca2SiO4). In the FeOx-CaO-SiO2 ternary, α-Ca2SiO4 and β-Ca2SiO4 are

separate phases. However, in the XRD pattern of the 4:1 FCS slag, peaks were

identified for larnite and γ-Ca2SiO4, as shown in Figure 4.25. Dicalcium silicate

undergoes a transformation from the γ to α structure at 870 °C, so it is possible that the

latter did exist in this slag but underwent transformation to the former as the slag

cooled. For simplicity, dicalcium silicate is discussed as a single solid phase in this

study. Figure 4.26 shows the XRD pattern of the 1:9 FCS slag (region A), indicating

the presence of wustite and olivine. The overlap of the fayalite and forsterite peaks here

* International Centre for Diffraction Data Powder Diffraction File Version 2.0602

highlights the solid solution between Fe and Mg, and as such, the iron magnesium

silicates will also be discussed as a single solid phase in this study.

85

Figure 4.24 XRD pattern of the 13:2 mixed FCS slag.


46- 1312 WUSTITE 34- 98 KIRSCHSTEINITE, SYN

File Name: h:\...\hv76a\hv76a02.xpt

C2F1 - 9CS

2-Theta Angle (deg)10.00 20.00 30.00 40.00 50.00 60.00 70.00

1

2

3

4

5

6

7

Inte

nsity

(Cou

nts)

X 1

00

5.569

4.214

3.896

3.665

3.3473.2183.157

2.956 2.783

2.752

2.674

2.612

2.609

2.5542.531

2.467

2.423

2.3452.286

2.230

2.138

2.088

2.021

1.927

1.857

1.835

1.761

1.610

1.591 1.537

1.511

1.508

46- 1312 WUSTITE 47- 1744 SREBRODOLSKITE, SYN 6- 615 WUSTITE, SYN 33- 302 LARNITE, SYN


C13F2-9CS

2-Theta Angle (deg)10.00 20.00 30.00 40.00 50.00 60.00 70.00

1

2

3

4

5

6

7

8

Inte

nsity

(Cou

nts)

X 1

00

7.394

3.677

3.378

3.014

2.877

2.793

2.787

2.743

2.705

2.673

2.609

2.544

2.481

2.473

2.405 2.281

2.190

2.148

2.143

2.070

2.0492.022

1.984

1.940

1.893

1.8781.807 1.707

1.634

1.608

1.587

1.574

1.554

1.522

1.516

1.373


46- 1312 WUSTITE 49- 1672 CALCIUM SILICATE 33- 302 LARNITE, SYN


C4F1-9CS

2-Theta Angle (deg)10.00 20.00 30.00 40.00 50.00 60.00 70.00

1

2

3

4

5

6

7In

tens

ity (C

ount

s) X

100

7.427

5.604 4.308

4.057

3.815

3.761

3.663

3.3783.294

3.008

2.900

2.788

2.744

2.731

2.696

2.663

2.610

2.591

2.539

2.512

2.476

2.458

2.4002.324

2.284

2.187

2.140

2.0852.0672.0492.023

1.984

1.938

1.909

1.882

1.8131.802

1.756

1.706

1.6901.674

1.6351.626

1.6051.5841.543

1.527

1.513

1.504

1.4601.373

46- 1312 WUSTITE 19- 629 MAGNETITE, SYN 31- 633 FAYALITE, MAGNESIAN 31- 795 FORSTERITE, FERROAN


C1F9-9CS

2-Theta Angle (deg)10.00 20.00 30.00 40.00 50.00 60.00 70.00

5

10

15

20

25

30

35

40

45

Inte

nsity

(Cou

nts)

X 1

0

5.1905.125

4.8094.334

3.941

3.904

3.736

3.535

3.522

3.011

2.945

2.789

2.618

2.594

2.534

2.519

2.477

2.395

2.374

2.337

2.299

2.288

2.270

2.1712.138

2.080

1.9761.907

1.840

1.830

1.762

1.756

1.702

1.684

1.655

1.631

1.587

1.5121.508

1.4971.478

1.447

1.430

1.411

1.361

86


87

Magnetite (Fe3O4) peaks occasionally appear also, and this is likely to be due to

contamination of the slag sample by the spinel layer that forms between the slag and the

magnesia crucible entering the sample prepared from the equilibrated slag.

Srebrodolskite (Ca2Fe2O5) peaks also appear occasionally, and this is likely to be due to

the remaining liquid crystallising on cooling. Selenium and Te phases are not expected

to appear on the XRD patterns, as they would be in too low concentrations to form their

own mineral phases in appreciable quantities.

4.4. Summary

In this chapter the results of the equilibrium distribution experiments were presented. It

e taken up by the calcium ferrite

slag is dependent on all the variables examined. The highest distribution coefficients

were recorded at low 2O , high temperature, and at the highest concentration of basic

oxide. It is also confirmed that iron in the slag makes a contribution to the ability of the

slag to take up these elements. It is also shown that solid phases precipitate from the

FCS slag at high temperature, reducing the volume of molten slag available to take up

the minor elements.

is demonstrated that the degree to which Se and Te ar

p

88

n the

or the calcium ferrite slag and liquid

opper equilibrium experiments are expressed in terms of capacities, a common

rgy, of the level of contamination of the slag by a particular species.

lph

5. DISCUSSION

The discussion of the thermodynamics of Se and Te in slags is presented in this chapter

in six sections. The first section deals with the oxidation state of Se and Te i

calcium ferrite slag and how this is influenced by the atmospheric conditions and slag

basicity.

In the second section, the experimental results f

c

indicator in metallu

The capacities of Se and Te in the slag (CM) are calculated in the present work in a

similar way to the sulphide capacity using Equation [5-1], with the sulphur partial

pressure term replaced with the partial pressure of the respective minor elements, and

the concentration of su ur with the concentration of the minor element in wt%.

2/1

M

OslagM

2[M] ⎟⎟

⎜⎜=

pp

C [5-

In the third section, a

2 ⎠

⎞

⎝

⎛1]

ctivity data is generated for the calcium ferrite slag. Since the

molten slag and alloy phases are at equilibrium, the chemical potential of each minor

element will be the same in one phase as it is in the other. Considering that the same

standard state that was nominated for the minor elements in the Cu alloy (vapour at 1

atm) is applied to the slag, it follows that the activity of each minor element will also be

the same in both phases. This then allows for the determination of the activity

coefficients in the slag by Equation [5-2].

)(χ)(γ

M

M M slag

aslag = [5-2]

89

section, the equilibrium distribution and phase equilibria results of the

xperiments with FCS slags are discussed. Capacities and activity coefficients of Se

In the fourth section, the influence of iron in the slag on the behaviour of Se and Te in

the slag in terms of capacities and activities is discussed.

In the fifth

e

and Te are also presented for the liquid silicate slag.

In the sixth and final section, some practical implications of the findings from this study

are presented.

5.1. Oxidation state of Se and Te

5.1.1. Oxygen Partial Pressure Dependence

The results of the Cu/slag experiments with varying showed that the partitioning of

Se and Te were strongly dependent on the atmosphe

e results of the Cu/slag experiments (Fig 4.1) that DM have a negative dependence on

⇌ Se2-(sl) + ½O2(g) [5-3]

2Op

ric conditions. It can be seen from

th

2Op , that is, partitioning to the slag phase is favoured more as the 2Op is decreased

(conditions become more reducing). This indicates that across the range of 2Op

studied, both minor elements enter the slag as a negatively charged species. The data

for both Se and Te fit a slope close to -0.5, indicating that the dominant oxidation state

is –2. In the case of Se, the selenide forms in the slag according to

½Se2(g) + O2-(sl)

90

here (sl) indicates the slag and (g) the gas phase. The diatomic gas is assumed to

work (Mompean et al.,

005). Similarly, tellurium exists as the telluride, Te2-, forming according to

-4]

this reducing region, the DM values are higher in the present work than observed in

te slag (Kojo et al., 1985). This suggests that the activity coefficients of

e minor elements (γM) in the slag are significantly lower in a more basic slag.

e original Cu/slag ones,

incr m,

the ag. In

-5

2. The presence

f such an oxidised Se species was not confirmed in this study, but could possibly be

investigated with further

w

participate in the reaction, as although Se forms a number of polyatomic gaseous

species, Se2 is dominant at the temperatures studied in this

2

½Te2(g) + O2-(sl) ⇌ Te2-(sl) + ½O2(g) [5

In

fayalite slag (Nagamori and Mackey, 1977), but are much lower than those observed in

sodium carbona

th

The results from the Ag/slag experiments match well with th

indicating that the activity coefficients of Se and Te in the Ag alloy are similar to the

corresponding values in copper. However, both minor elements exhibit a major change

in behaviour beyond a certain point in the more oxidising region. With a further

ease in 2Op , there is a subsequent increase in DTe above 10-6 at indicating that

there is a transition to an oxidised species being the dominant form of Te in sl

the case of Se, although the trend is not very reliable, DSe appears to level off above 10

atm. This may indicate that Se co-exists in various oxidation states at 2Op between 10-5

and 10-4 atm before an oxidised species becomes dominant at higher Op

o

experiments.

91

(Fig

,

½Te2(g) + ½O2(g) ⇌ Te2+(sl) + O2-(sl) [5-5]

comparison of the equilibrium constants for the Te oxides, shown in equations [5-6]

[5-6]

-7]

et al., 1977) are -5.33×105 and -4.48×105 Jmol-1

spectively, indicating that CaSe is relatively more stable than CaTe. Also, CaSe is

A line of slope close to 0.5 can be fitted to the Te data in this more oxidising region

4.3), indicating that the +2 oxidation state, i.e. the divalent oxide, is dominant here

forming according to

A

and [5-7] at 1300 °C (Nagamori and Mackey, 1977), suggests that the tetravalent oxide

should be the dominant Te species.

½Te2(g) + ½O2(g) ⇌ TeO(g); K = 5.13

½Te2(g) + O2(g) ⇌ TeO2(g); K = 37.58 [5

Such a transition to the +4 oxidation state was observed for both Se and Te in sodium

carbonate slags by Alvear et al. (1994) and Kojo et al. (1985) under similar

experimental conditions to this work. Both authors also found that the Se transition

occurred at a higher 2Op than with Te, and that DSe reached a lower minimum value

than DTe, although this is not the case in the present work. Since the Gibbs energies of

the monoxides of Se and Te at 1327 °C are very similar, -3.95×105 and -3.58×105 Jmol-1

respectively (Barin et al. 1977), the absence of the Se transition in the present work

appears to be somewhat unexpected. Values of the Gibbs energy of calcium selenide

and calcium telluride at 1227 °C (Barin

re

more stable compared to SeO to a greater extent than CaTe is compared to TeO. This

may account for the observation that Te underwent a transition and Se did not.

92

he transition from a reduced to an oxidised species was also observed for sulphur in

calcium ferrite slag by St

961). However, in both these cases, the transition was to the +6 oxidation state. As

d have been observed, indicating a transition to a

igher oxidation state becoming dominant. However, the levelling-off behaviour of Se

suggested in the present

monatomic and molecular dissolution as proposed by Nagamori and Mackey (1978).

.1.2. Composition of the Ferrite Slag

T

. Pierre and Chipman (1956) and Turkdogan and Darken

(1

the reaction [5-7] requires a higher degree of oxygen enrichment of the input gas than

[5-6], it may be that if experiments were continued to even higher 2Op , another change

in slope of the distribution data woul

h

work is consistent with the composite mechanism combining

5

to be

ts

ith f

8]

difference in distribution coefficients observed for Se and Te under reducing conditions

Since Se and Te are considered to be acidic, they are expected to associate mainly with

the most basic component of the slag. In calcium ferrite slag, this is expected

CaO. This is confirmed by the observed increase in partitioning of the minor elemen

into the slag phase w urther addition of lime to the calcium ferrite slag. A likely

general mechanism representing the dominant slag-minor element interactions under

reducing conditions is then

½M2(g) + CaO(sl) ⇌ CaM(sl) + ½O2(g) [5-

As CaSe is relatively more stable than CaTe, it follows that there will be a large

93

ee

xygen in the system from the other major slag components, namely FeO and Fe2O3,

t.

d

ver the experiments reported here, up to 31 wt% CaO, which resulted in an increase in

DTe of only 1.2 times.

an

ows that replacing CaO with BaO is a much more effective

ethod of enhancing partitioning of the minor elements to the slag than simply

increasing the amount o

free oxygen to the slag for the minor elements to replace. It is interesting to note that in

er with

(Fig 4.1), and the addition of lime to the slag would have less of an influence on Te

partitioning. However, there will also be contributions to the total amount of fr

o

and as such, they will also stabilise the selenide and telluride in the slag to some exten

For Te under oxidising conditions, a possible overall mechanism would be

½Te2(g) + ½O2(g) + CaO(sl) ⇌ CaTeO2(sl) [5-9]

Overall, the increase in partitioning of Se and Te to the slag achieved by increasing the

amount of lime is not all that appreciable (Fig 4.10), indeed it is far less than what was

achieved by increasing the temperature (Fig 4.4). The lime content was almost double

o

DSe of about 2 times, and an increase in

With barium ferrite slag, DSe was 2.6 times higher while DTe was 5.6 times higher th

with the highest lime calcium ferrite slag. The highest lime and highest barium slags

had roughly the same concentration of CaO and BaO in terms of wt%, but the mole

fraction of BaO in the barium ferrite slag was half that of CaO in the highest lime

calcium ferrite slag. This sh

m

f lime. This is attributed to BaO having greater ability to donate

this series of experiments, the gap between DSe and DTe appears to become small

94

aO

.2.1. Slag Basicity

increasing basicity of the slag, the opposite of what was observed with the varying C

experiments (Fig 4.10).

5.2. Capacities

5

capacities,

as calculated using the equation of Nagamori and Mackey (1977) for the vapour

pressure of the pure su

the alloy. The partial pressures of Se and Te in the system at 1300 °C were

As such, it is assumed

at the activity coefficients of the minor elements in copper will be the same in each

c

of

a more useful parameter than simply the concentration, as at a given [CaO]slag, aCaO

The partial pressure of each minor element, needed for the determination of

w

bstance at the appropriate temperature, and the calculated activity

in

calculated to be 1.15×10-3 and 1.03×10-3 atm respectively. The activities of the minor

elements in the alloy were calculated from the analytical results and the known activity

coefficient in copper (Sigworth and Elliott, 1974). Here, the assumption is made that

there are no interactions between any components of the alloy, including any oxygen

and iron that may be taken up over the course of an experiment.

th

series of experiments and does not to be adjusted according to slight changes in the

composition of the alloy. The capacity is further subject to significant uncertainty, in

the calculation of 2Mp , and from the

2Op term, which arises from the thermodynami

data from which the CO2:CO ratio is determined, and in the calibration and setting

the gas flow rates.

Capacities determined from the Cu/slag experiments with varying 2Op are shown in

Figure 5.1. The activity of lime (aCaO) in the equilibrated calcium ferrite slags was

estimated from the iso-activity diagram of Takeda et al. (1980). The activity of lime is

95

(2CaO.Fe2O3) forms. This would lead to a reduction in the

xtent that CaO is able to donate oxygen to the melt and the volume of liquid available

elements.

g

r2O .

is a

ss

decreases with increasing [Fe3+]slag, to the extent that under highly oxidising conditions

the dicalcium ferrite phase

e

to take up the minor

0.1 0.2 0.3 0.4 0.5 0.6 0.7

log

MC

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0SeTe

1300 οC

aCaO

Figure 5.1 Selenide and telluride capacities (CM) of calcium ferrite slag (20-25 wt%

CaO) under varyin as a function of the activity of lime at 1300 °C. The Te data

does not extend over the same range of a

2Op

CaO as for Se, as [Te]slag could not be

determined at highe p

As expected from the final minor element content of the slags, CSe lways much

higher than CTe. However, both CSe and CTe appear to be constant against aCaO acro

the range studied, which is somewhat unexpected. From its derivation, the capacity

should be dependent on the basicity of the slag. This can be seen from the reaction

96

]

nt is

½M2(g) + O2-(sl) ⇌ M2-(sl) + ½O2(g) [5-10

for which the equilibrium consta

2/1MO

2/1OM

2-2

2-2

pa

paK = [5-11]

From this, the capacity, in terms of the activity coefficient of the minor element, is

expressed as

-2

-2

M

OM γ

aKC = [5-12]

So if a dilute solution, and hence constant γM, is assumed, the capacity should increase

with increasing free oxygen in the melt, (i.e. basicity), the majority of which is expected

to be from CaO. It is possible that where aCaO is low and the activity of ferric oxide

) is high, the capacity is elevated by a contribution to [M]slag through direct

3+ 2-

n b

, and therefore these direct interactions, become more

ignificant. This then results in elevated values of the capacity at the high end.

e

of

(32OFea

interactions with ferric iron. This effect has been observed with sulphur in glasses

through the formation of the Fe S amber chromophore (Douglas and Zaman, 1969).

Then, as 2Op increases, while the reactio etween the minor elements and free oxygen

is less significant, Fea32O

2Ops

Alternatively, if γM did not remain constant, an increase in γM in conjunction with th

increase in free oxygen in the slag (as brought about by changes in the activity of slag

components with decreasing the 2Op ) would need to occur for CM to be independent

97

CaO as shown. Activity coefficients of Se and Te in the slag are calculated and this

possibility discussed in s

latively unstable and decompose at low

mperature (<400 °C). As such, it is suggested that it is unlikely that copper taken up

by the slag contributes to CM through direct interact

opper should therefore not have an effect on the activity coefficients of the minor

tal

ved in the slag, so it should not be contributing to the ability of the slag to

ke up the minor elements.

ecause selenium is considered to be more acidic than tellurium, it is to be expected

,

ents

a

ection 5.3 below.

The compounds CuSe and CuTe are re

te

ions with the minor elements.

C

elements. Most of the silver in the slag analysis is expected to be from entrained me

and not dissol

ta

B

that further additions of CaO to the slag would have a greater effect on the ability of the

slag to take up Se than Te. Selenide and telluride capacities calculated from the

experiments with varying lime content of the slag are shown in Figure 5.2. However

the corresponding increase in CM with increasing aCaO across this series of experim

is only slight, and in the case of Te is essentially independent of aCaO.

aCaO

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

log

CM

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0SeTe

1300 οC, 10-9 atm

98

Figure 5.2 Selenide and telluride capacities of varying-lime calcium ferrite slags (16-32

wt% CaO) as a function of the activity of lime at 1300 °C and 10 atm Op .

In this series of experiments, the increase in aCaO is brought about by the addition of

lime to the slag. Conversely, the activity of ferric oxide is then highest at low [CaO]slag.

So, where aCaO is low, additional contributions to [M]slag, and the apparent CM, through

direct interactions between ferric iron and the other group VIb elements could be

compensating for a lack of free oxygen in the slag at lower [CaO]slag. The CM values

from the varying lime and varying 2O series with the copper alloy match quite well, as

shown in Figure 5.3.

-92

p

aCaO

0.0 0.2 0.4 0.6 0.8 1

χCaO

0.0 0.2 0.4 0.6 0.8 1

log

CM

-5

-4

-3

-2

-1

Se varying limeTe varying limeSe varying pO2

Te varying pO2

16-32 % CaO, 10-11 - 10-6 atm

S, 1620 οC (Turkdogan and Darken, 1961)1300 οC

.0

.0

99

urk an and

arken (1961), plotted on the top abscissa against the mole fraction of CaO (χCaO) in the

e

mp

7 °C is -6.22×105 Jmol-1 (Barin et al., 1977), which is more

egative than for CaSe and CaTe. It follows that CaS is the most stable, and therefore is

more readily taken up by the calcium ferrite slag. This is reflected by the increase in

capacity of the Group VIb elements up the periodic table.

Figure 5.3 Selenide and telluride capacities of calcium ferrite slags as a function of the

activity of lime at 1300 °C. Included are the sulphide capacity data of T dog

D

calcium ferrite slag.

As with the selenide and telluride capacities, the CS data are also constant over much of

the composition range, before showing a slight increase at high χCaO. The sulphid

capacities are again greater than CSe by about 2 orders of magnitude, and were also

shown to increase with increasing te erature. The value of the Gibbs energy of

calcium sulphide at 122

n

100

Figure 5.3 also shows that CM are essentially independent of aCaO. However, there is

some scatter in both sets of data from the present work, especially for Se. The capacity

is generally interpreted to be an indication of the changing nature (basicity) of the slag,

and in the calcium ferrite slag there will be some contribution from ferrous and ferric

oxides to the total amount of free oxygen in the slag.

The activity of the slag components were predicted by the CSIRO MPE programme

(Zhang et al., 2002), and are plotted in Appendix B. The activity of ferrous oxide (aFeO)

was found to be higher, and more sensitive to changes in , temperature and

[CaO]slag in the CaO-FeO-Fe2O3 system than 32OFe over the ranges studied in this work.

Hence, it is likely that any contribution to the free oxygen in the slag is actually from

FeO and not Fe2O3. At constant temperature, the activity of FeO is highest at low

2+

aFeO end. The overall contribution from FeO

creasing interactions with CaO as [CaO]slag increases, resulting

um

2Op

a

[CaO]slag, (Turkdogan, 1961, Takeda et al., 1980) so there may be significantly more

free oxygen in the slag here than the aCaO would suggest. Any direct interactions

between the minor elements and iron are then more likely to be through Fe , which

would also be more significant at the high

is cancelled out by the in

in the constant values of CM.

The results of Nzotta et al. (1999) do show the capacity to have a dependence on slag

composition however, with CS increasing with increasing χCaO. They go on to predict

values by model calculations, which strangely show CS going through a local minim

at about χCaO = 0.3. It must be noted however that the sulphur content of the slag in

their measurements was quite high, reaching about 13 wt%. However in this situation,

the slag itself cannot be considered simply as calcium ferrite, and the variation in the

101

capacities are again greater than

e telluride capacities, with both increasing as BaO replaces CaO.

e capacities of Ba-containing slags (8-32 wt% CaO, 5-

4 wt% BaO) as a function of slag basicity (β) at 1300 °C and 10-9 atm2O

activity coefficient of sulphur with its concentration in the slag would need to be taken

into account.

Selenide and telluride capacities calculated from the experiments with barium-

containing slags are shown in Figure 5.4. The selenide

th

β

0.0 0.2 0.4 0.6 0.8 1.0

log

MC

-5.5

-5.0

-4.0

-3.5

-4.5

-3.0SeTe

1300 οC, 10-9 atm

Figure 5.4 Selenide and tellurid

p . 3

In this case the increase in CTe is about 8 times while CSe increases only 3.5 times from

the least to most basic slag. This is an interesting result, since in the previous

experiments, CaO additions had less of an effect on Te than on Se, and capacities

appeared essentially constant. Since barium and calcium are chemically similar, it is

expected that the activities of the oxide slag components, including FeO, vary in a

102

e and CaTe at 1227 °C are -4.77×105 and -4.48×105 Jmol-1 respectively

arin et al.). While this does suggest that the former should be relatively more stable,

lation.

a and the minor

elments is such that the contribution of FeO is not as significant, allowing the expected

dependence on basicity to be observed.

It must also be noted that magnesia becomes a significant component of the slag in

some experiments, but does not always follow a simple trend across a single series.

Magnesium, chemically similar to calcium, would be expected to interact with Se and

Te in the slag in a similar manner, possibly causing a distortion of the distribution

results. The Gibbs energy of MgSe and MgTe at 1227 °C are -4.30×105 and -3.74×105

Jmol-1 respectively (Barin et al., 1977), so they are both relatively less stable than the

corresponding calcium and barium compounds. This seems reasonable as magnesium,

above calcium on the periodic table, is expected to be the least “basic” of these

d b he more

CaO and BaO in the slag. This is also important to consider from a

similar manner in the barium containing slags as they do in calcium ferrite. The Gibbs

energies of BaT

(B

the difference between the values is not large, so this remains a matter of specu

Since data is not available for BaSe, no direct comparison can be made to CaSe.

However, it is possible that the strength of the interactions between B

elements, and therefore have less affinity for Se and Te. Any influence that MgO does

have on the selenide and telluride capacities would also be overshadowe y t

concentrated

process point of view, as the rate of wear of the refractories due to chemical degradation

is an important consideration when designing any pyrometallurgical process.

5.2.2. Temperature Dependence

At constant 2Op , the temperature was shown to have a significant effect on the

partitioning of Se and Te between the molten phases (Fig 4.4). Both minor elements

followed essentially the same trend, with the D recorded at the highest temperature

(1400 °C) over one order of magnitude greater compared to the those at the lowes

temperature (1200 °C). Selenium partitioning favoured the slag phase at 1300 °C and

above, but D remained <1 throughout this series of experiments.

M

t

elenide and telluride capacities for each temperature, at constant 2, were calculated

is

ct

from

as

ose to

ose determined from the original series of temperature variation experiments (Fig

5.5), despite this, and the decrease in [M]slag in this case.

Te

OpS

in the same manner as above and are shown in Figure 5.5. Again, CSe > CTe, which

expected from the higher final levels of Se in the slag. However in this case, the effe

of temperature on CTe is slightly stronger than on CSe. Values for Te increased almost 9

times, while values for Se increased only 3 times as the temperature was increased

1200 to 1400 °C.

The selenide and telluride capacities of the calcium ferrite slag for the series of

experiments where the CO2:CO ratio was held constant while the temperature w

varied are shown in Figure 5.6. In this case, the values for both Se and Te again

increase with increasing temperature (and hence increasing 2Op ), with the same slope

fitting the data for each minor element. The increase in 2Op across this series of

experiments leads to a decrease in aCaO, whereas previously increasing the temperature

(at constant 2Op ) led to an increase in aCaO. The values of CM here are very cl

th

103

10 /T (K)

104


function of temperature at 10 atm Op .

Fig 5.6 Selenide and telluride capacities of calcium ferrite slag (23-27 wt% CaO) as a

function of temperature at constant CO2:CO ratio.

-102

4

5.8 6.0 6.2 6.4 6.6 6 8 7.0.

log

CM

-5.5

-4.5

-3.0

-6.0

-5.0

-4.0

-3.5

SeTe

10-10 atm

10 /T (K)4

5.8 6.0 6.2 6.4 6.6 6.8 7.0

log

MC

-6.0

-4.0

-3.5

-3.0SeTeCO2:CO = 2.265

-5.5

-5.0

-4.5

105

As the temperature is being varied, the equilibrium constant will not be the same across

a series of experiments, introducing a third variable into the calculation of capacity in

Equation [5-12] above. Furthermore, as iron is the major component of these calcium

ferrite slags, it is likely that part of the CM value determined is due to its presence. It is

possible that even though aCaO decreased with increasing temperature in the constant

CO2:CO series, the total free oxygen in the system increased due to increasing

contributions from the iron oxides. Ferrous oxide is expected to be more basic than

ferric oxide, and therefore is able to donate more oxygen to the slag for the minor

elements to replace.

It is also possible that direct interactions between Fe in the slag and Se and Te also

account for part of the total [M] recorded. The associated species FeSe and FeTe are

considered likely to form as they are as stable as FeS, and are discussed as the main

minor element containing species present in fayalitic slags by a number of authors

(Nagamori and Mackey, 1977, Fang and Lynch, 1987, Choi and Cho, 1997). This then

is in contradiction with the observations of Douglas and Zaman (1969) where

interactions with Fe3+ appeared to contribute to CS in silicate glasses.

When aCaO was kept constant while varying the temperature, CM again increased with

increasing temperature. This is shown in Figure 5.7. The values themselves for each

minor element are a close match with those from the previous two temperature variation

studies (Fig 5.2 and 5.3) in the range 1200 to 1300 °C, despite some variation in the

activity of lime between the three series at each temperature.

slag

104/T (K)

6.3 6.4 6.5 6.6 6.7 6.8 6.9

log

CM

-5.5

-5.0

-4.5

-4.0

-3.5SeTe

aCaO = 0.432

106

aCaO = 0.432).

ags

at 1400 to 1600 °C. Like Se and Te, their results show CS to

crease with increasing temperature, and with increasing CaO in the slag. These are

ent


function of temperature at constant activity of lime (

The Fe3+:Fe2+ ratio of the slag decreases with increasing temperature, more steeply from

1250 to 1300 °C (Table A.4), but again the slope through CM for each minor element is

essentially constant. This would suggest that any direct interactions between ferrous

iron and the minor elements and their influence on capacities might be negligible

compared to the effect of temperature.

Sulphide capacity measurements have been made on a number of calcium ferrite sl

by Nzotta et al. (1999)

in

shown along with the results of the temperature variation experiments from the pres

work in Figure 5.8. The composition of Nzotta et al.’s (1999) slags all lie in the range

0.24 < χCaO < 0.35, where χCaO is the mole fraction of lime in the slag.

104/T (K)

5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0

log

CM

-6

-5

-4

-3

-2

0

-1

SeTeS (Nzotta et al., 1999)

107

Figure 5.8 Selenide and telluride capacities of calcium ferrite slag (22-26 wt% CaO) as

a function of temperature.

The slope through the data is similar for each element, which should be expected since

these elements, being in the same group of the periodic table, are chemically very

similar. Sulphide capacities are greater than those for Se, which are in turn greater than

CTe, reflecting the expected ‘acidity’ of each of these elements. Sosinsky and

Sommerville (1986) derived a correlation expressing CS in terms of optical basicity and

temperature, which could possibly be used to estimate CS at the temperatures in the

present work. However, caution must be exercised with calculations involving optical

cations, eg, Mn2+,

e2+, Fe3+, and S2- anions, instead assigning all effects to the optical basicity of the

respective oxides.

basicities due to the difficulties in assigning values to transition metal oxides. The

weakness of this approach is that it ignores any interactions between

F

5.3. Activity Relations

5.3.1. Concentration

Further experiments were conducted using calcium ferrite slag and Cu alloys of varyin

minor element concentration. This was to allow for the examination of minor elem

behaviour in terms of changing concentra

g

ent

tion in the slag at a single temperature and

2. Analytical results from these experiments are presented in the appendix in Table

Figure 5.9 Activities of Se and Te (aM) in calcium ferrite slag (24 wt% CaO) as a

2O

pO

A.7. As was observed in the vast majority of ferrite slag experiments, DSe was greater

than DTe by around one order of magnitude. The DM were independent of the initial

amount of the minor elements present, thus allowing the determination of the activity-

concentration relationships for Se and Te shown in Figure 5.9.

χM(slag)

0.000 0.001 0.002 0.003 0.004

a M

0.00000

0.00005

0.00010

0.00015Se Te

1300 οC, 10-10 atm

function of mole fractions of Se and Te in the slag (χM) at 1300 °C and 10-10 atm p .

108

109

he Te data fit a slope of 1.2, indicating behaviour very close to Raoultian ideal, which

the

are

2211

T

should give a slope of 1. The Se data on the other hand, fit a slope of 4.6×10-3,

indicating strong negative deviation from the ideal behaviour. As the trend through

Se data is also a straight line, over the composition range studied, Se appears to follow

Henry’s Law behaviour.

The values of γM (slag) calculated from Equation [5-2] for this series of experiments

shown in Figure 5.10. Sigworth and Elliott (1974) list activity coefficients of Se and Te

in dilute Cu solutions at 1200 °C. The standard state given for both is the elemental

vapour at 1 atm pressure. For the experiments at temperatures other than this, the

regular solution approximation

γlnTγlnT = [5-13]

calc

° 1200 1225 1250 1275 1300 1350 1400 1500

was applied to determine the appropriate values of γM(Cu) to be used in the calculation

of the activity of the minor elements from the Cu alloy data. It must be stressed that this

is a rough approximation only as applied to the γM(Cu) as the base values used in the

ulations show significant negative deviation from the ideal. The values obtained

and used in subsequent calculations in this thesis are listed below in Table 5.1.

Table 5.1 Activity coefficients of Se and Te in dilute Cu alloys. T C

γSe 0.002 0.0022 0.0024 0.0027 0.003 0.0035 0.0042 0.0057

γTe 0.0328 0.0347 0.0367 0.0387 0.0408 0.045 0.0493 0.0585

log χM (slag)

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0

log

γ M (s

lag)

-3

-2

-1

0

1SeTe

1300 οC, 10-10 atm

110


M

2O

M slag

M

γSe are much less than γTe. This reflects

ractions between the slag components and

taken to be the limiting activity coefficient in the slag. These limiting activity

as a function of mole fractions of Se and Te in the slag (χ ) at 1300 °C and 10-10 atm

p .

This confirms that γ (slag) are independent of [M] , which implies that any variation

in γ (slag) is due only to changes in the experimental conditions or slag chemistry.

Since the activities of Se were lower than that of Te, and since Se was much more

concentrated in the slag than Te, the resulting

the difference in strength of the overall inte

each minor element; that is, the interactions are stronger with Se than with Te. Since

the activity coefficients for both elements are essentially constant over the composition

range studied, and at quite low concentrations in the slag, the resulting values can be

111

tivity

5.3.2. Slag Basicity

coefficients are 4.8×10-3 for Se and 1.2 for Te, confirming the slope through the ac

data in Figure 5.9 above.

he activity coefficients of Se and Te in the slag from the series of experiments with

re


CaO) under varying 2 as a function of the activity of lime at 1300 °C.

T

calcium ferrite slag and copper at varying 2Op are shown in Figure 5.11. Activity

coefficients of Te are greater than those of Se by about 2 orders of magnitude. A

reasonable correlation can be determined for log γSe in terms of aCaO, and although the

is more limited data, a similar slope can be fitted to the Te results.

aCaO

0.0 0.2 0.4 0.6 0.8 1.0

loM (s

l

-4

-3

-2

-1

0

1SeTe

1300 οC

ag)

g γ

Op

112

This shows that γM(slag) are influenced by the reaction conditions, and that the

influence of on the minor elements is quite significant. The lowest γM(slag) were

record2 (i.e. at the highest aCaO), confirming that it is where the

2

is lower that conditions are more favourable for the slag to retain Se and Te.

The activity coefficients of Se and Te in the slag from the series of experiments with

varying-lime calcium ferrite slags are shown in Figure 5.12. The values of γM again

decrease with increasing aCaO, although here the effect is much less than was observed

previously with varying2. In this case, γSe are all <1, while γTe are all >1, and appear

to be almost independent of aCaO.

2Op

ed at the lowest Op Op

Op

Figure 5.12 Activity coefficients of Se and Te (γM) in varying-lime calcium ferrite slags

(16-32 wt% CaO) as a function of the activity of lime at 1300 °C and 10-9 atm 2Op .

aCaO

0.0 0.2 0.4 0.6 0.8 1.0

log

γ M (s

lag)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0SeTe

1300 C, 10-9 atm

ο

113

decreases over the same interval (Figure B.1). This again

uggests that there are interactions between ferrous iron and the minor elements that

M(slag) than recorded previously at the low aCaO end. This effect

om

,

to the overall strength of interactions between the slag and the minor elem is

e effect of ferric iron was similar to that of ferrous iron, it would be

γM(slag) in Figure 5.11 would show a shallower slope in the lower aCaO

(higher 2O ) region.

The activity coefficients of Se and Te in the slag from the series of experiments with

barium-containing slags are shown in Figure 5.13. The values of γM in the barium

ferrite slag are lower than in the highest lime calcium ferrite slag, indicating it retains

the minor elements to a greater extent. This would be expected, since the interactions

between barium and the minor elements are stronger than those between calcium and

the minor elements. As such, it is suggested that the associated species BaSe and BaTe

are the dominant minor element containing species in CaO-BaO-FeOx slags under

In these experiments, the increasing [CaO]slag leads to an increase in aCaO, while aFeO is

highest at low [CaO]slag and

s

result in much lower γ

would become less significant as lime is added to the slag, resulting in the shallow

dependence on aCaO observed in this case c pared with that in the varying 2Op

experiments. When the2Op increases at constant [CaO]slag, aCaO and aFeO decrease

while 32OFea increases gradually in the range 10-11 to 10-8 atm, and more rapidly from

10-8 to 10-6 atm. (Figure B.2). This suggests that ferric iron makes a lesser contribution

ents. In th

regard, if th

expected that

p

reducing conditions.

114

Figure 5.13 Activity coefficients of Se and Te (γM) in barium-containing slags (8-32

wt% CaO, 5-34 wt% BaO) as a function of slag basicity at 1300 °C and 10-9 atm2O .

In this case there is now some significant dependence of γTe on the basicity of the slag.

Activity coefficients of the minor elements decrease as β increases, that is, as BaO

replaces CaO. In fact, the influence on Te appears stronger than on Se, with γTe

decreasing 11 times from β = 0 to β = 1, while γSe only decreases 5 times over the same

interval. However the dependence on basicity here is still not as significant as in the

varying 2O experiments. Thus it is likely that there is the same influence of ferrous

iron on γM as described previously for the varying lime experiments.

β

0.0 0.2 0.4 0.6 0.8 1.0-3

-2

p

p

log

M (s

la γ

g)

-1

0

1SeTe

1300 οC, 10-9 atm

5.3.3. Temperature

The activity coefficients of Se and Te in the slag from the series of experiments where

the temperature was varied at constant 2Op are shown in Figure 5.14.

5.8 6.0 6.2 6.4 6.6 6.8 7.0

log

(sl

-3.0

1.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

SeTe

10-10 atm

γ M

ag)

104/T (K)

Figure 5.14 Activity coefficients of Se and Te (γM) in calcium ferrite slag (25 wt% C

as a function of temperature at 10

aO)

2

2O as varied, the decrease in γM with increasing temperature from

200 to 1400 °C corresponds to the interval over which aCaO is increasing (Figure B.3).

Although aFeO remains close to constant, it is possible that the observed decrease in γM

with increasing temperature is actually brought about by the changing nature of the slag.

-10 atm p .

As observed previously, γ

O

Te > γSe, and both decrease with increasing temperature.

However, the values do not show an approach to the ideal (Raoultian) behaviour (γ→1),

as is normally expected with increasing temperature. In these experiments, as with

those where the p w

1

115

116

That is, the overall degree of interaction between the slag and the minor elements

become greater as the temperature increases.

Where the CO2:CO ratio was held constant with the temperature variations, the opposite

effect is observed. In this case, the activity coefficients of Se and Te increase with

increasing temperature, although the influence does not seem as strong as where 2

was constant. As shown in Figure 5.15, the values of γSe are all <1, so in this case the

deviation from ideality is reduced as the temperature is increased. However for Te, the

values are all >1, so the deviation from ideality actually becomes greater as the

temperature is increased. This also suggests that γM actually vary in response to the

changing nature of the slag, and this is masking any direct effect of temperature. In this

case, both aCaO and aFeO decrease with increasing temperature (Figure B.4), and again

is apparent from these results that the chemistry of the slag (i.e. activity of CaO and

r of

tant,

Op

γM are lowest where both aCaO and aFeO are highest.

It

FeO) has a more significant influence on the behaviour of Se and Te than does the

temperature. This is also further evidence for any influence of iron on the behaviou

Se and Te being due to ferrous rather than ferric iron, as 32OFea remains at a cons

very low value (Figure B.4). However it is not clear whether the influence of the

ferrous iron is due to direct interactions between Fe2+ and the minor elements, or

through the contribution of FeO to free oxygen in the slag.

117


CaO) as a function of temperature at constant CO2:CO.

igures 5.16 and 5.17 combine the activity coefficients for Se and Te respectively, as

2

p ce aCaO, with values decreasing with

creasing aCaO at a given temperature. The influence of temperature itself appears

alues

104/T (K)

5.8 6.0 6.2 6.4 6.6 6.8 7.0

log

γ M (s

lag)

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

SeTe

CO2:CO = 2.265

F

determined from the varying temperature and varying Op at 1300 °C Cu/slag results.

It is evident that γM has a strong de enden on

in

much less significant, especially for Se, with very little difference between the γM v

at a particular aCaO over a temperature range of 200 °C. Note that although these figures

contain five series of points, a trend line is only drawn through one series on each

figure. This is done for clarity, and all points are not meant to fit this line.

118

aCaO

0.1 0.2 0.3 0.4 0.5 0.6 0.7

log

γ Se(

slag

)

-4

-3

-1

-2

1

0

1200 οC1250 Cο

1300 οC1350 οC1400 οC

Figure 5.16 Activity coefficients of selenium (γSe) in calcium ferrite slag (23-27 wt%

CaO) as a function of the activity of lime at 1200 to 1400 °C.

aCaO

0.2 0.3 0.4 0.5 0.6 0.7

log

γ(s

laTe

g)

-0.8

-0.2

0.0

0.2

0.4

0.6

-0.6

-0.4

0.8

1.01200 οC1250 οC1300 οC1350 οC1400 Cο

Figure 5.17 Activity coefficients of tellurium (γTe) in calcium ferrite slag (23-27 wt%

CaO) as a function of the activity of lime at 1200 to 1400 °C.

119

Activity coefficients of Se and Te calculated from the temperature variation

experiments where the activity of lime is kept constant are shown in Figure 5.18. These

confirm that there is only very slight dependence of γM on temperature in the range 1200

to 1300 °C.

)

aC

1225

riments

show variations with aFeO as observed previously, providing further evidence for the

existence of direct interactions between ferrous iron and the minor elements.

104/T (K)

6.3 6.4 6.5 6.6 6.7 6.8 6.9

log

γ M(s

lag)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

SeTe

aCaO = 0.432

Figure 5.18 Activity coefficients of Se and Te (γM) in calcium ferrite slag (23 wt% CaO

as a function of temperature at constant activity of lime ( aO).

It appears that γM increase only very slightly, then decrease again with an increase in

temperature from 1250 to 1300 °C. However, in the case of Se, this again indicates that

the departure from ideality is actually becoming greater at higher temperature. Under

these conditions, the MPE simulations also predict a decrease in aFeO from 1200 to

°C, then a slight increase up to 1300 °C. The γM calculated from these expe

120

5.4. Influence of Iron

With the calcium aluminate slags, DSe increased with increasing additions of iron to the

slag. However, since DM decreased as the iron in the calcium ferrite slag was diluted

(i.e. as lime was added), the interactions between iron and the minor elements would

appear to be weaker than those between lime and the minor elements. That is, lime is

more basic than the iron oxides.

Selenide capacities determined from the experiments with low-iron calcium aluminate

slags (molar CaO:Al2O3 = 1.56) are shown in Figure 5.19. As expected from the

distribution coefficients, CSe increase with increasing iron at each 2O studied. Note

that although this figures contains three series of points, a trend line is only drawn

p

through one. This is done for clarity and all points are not meant to fit this line

FeOx (wt%)

0 2 4 6 8 10 12 14 16 18

log

CS

e

-5.2

-5.1

-5.0

-4.9

-4.8

-4.7

-4.6

-4.510-8

10-9

10-10

1500 οC

Figure 5.19 Selenide capacities (CSe) of Fe-containing calcium aluminate slags (34-42

wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying 2Op .

121

es, the

h increasing iron in the slag. This suggests that over the range studied,

e effect of iron in the slag on capacity is the same regardless of its oxidation state.

me

studied, it cannot be confirmed whether these interactions are stronger

between Se and iron in a particular oxidation state.

Figure 5.20 Activity coefficients of Se (γSe) in Fe-containing calcium aluminate slags

(34-42 wt% CaO, 40-50 wt% Al2O3) at 1500 °C and varying 2O .

While there is some scatter in the Fe3+:Fe2+ ratio of the slags within each seri

values generally decrease with decreasing 2Op . However, the values of CSe show little

difference between the three series. Instead, it appears that the values fit a single trend,

increasing wit

th

Activity coefficients of selenium in the slag determined from the iron-addition

experiments are shown in Figure 5.20. The activity coefficient of Se is shown to

decrease with increasing iron in the slag, confirming the presence of interactions

between Fe and the minor elements. However, since the trend in the data was the sa

at each 2Op

log

γ -0.2

0.4

0.6

p

FeOx (wt%)

0 2 4 6 8 10 12 14 16 18

10-8

10-9

10-10

1500 οC

0.0

0.2

Se

-1.0

-0.8

-0.6

-0.4

122

e slag

l2O3 ratio of the slag remains unchanged in these

e

ve the effect of reducing the activity coefficient

f oxygen (γO) in the alloy. It is possible then, that there is a similar effect of oxygen on

y will

is also possible that the contamination of the metal phase with iron during

equilibration with the iron-containing calcium aluminate slags has an effect on the

activity coefficient of Se in the copper alloy. Iron has been shown to have quite a

strong influence on the activity coefficient of oxygen in liquid copper, in that γO(Cu)

decreases with increasing [Fe]Cu (Turkdogan, 1980). Assuming that there is a similar

effect on the other group VIb elements, the increased capacity due to the addition of

iron to the slag will be offset to some degree. That is, Se is retained by the slag as iron

in the slag increases, but Se is also increasingly retained in the alloy as a result of the

reduction in γSe(Cu) as iron is taken up by the alloy. In the present results, iron in the

alloy increased with decreasing (Table A.8), and as a consequence, this effect will

be most significant at the lowest 2O (10-10 atm). This suggests that the increase in CSe

The large drop in the γSe values with decreasing 2Op at zero iron suggests that there is

actually an influence of 2Op that leads to stronger interactions overall between th

and Se. Although the CaO:A

experiments, it is possible that the 2Op has an influence on the structure, and hence the

activities of the slag components. As a consequence, the strength of the interactions

between the slag and Se may also be affected. Alternatively, it has been shown that th

presence of S (Turkdogan, 1980), Se (Seetharaman and Staffansson, 1988) and Te

(Seetharaman, 1992) in copper each ha

o

the activity coefficients of the other group VIb elements. That is, as the 2Op and hence

[O]Cu increases in the present work, γSe(Cu) decreases. As a consequence, the allo

retain Se in preference to the slag, resulting in an increase in γSe(slag) with increasing

2Op .

It

2Op

p

with increasing FeOx may actually be greater at lower 2Op , if these effects could be

accounted for in the determination of CSe.

5.5. Iron Calcium Silicate Slags

5.5.1. Bulk Distribution

The sequence of separation of solid phases seen in the SEM can be used to provide an

explanation for the partitioning behaviour of the minor elements observed from the

whole-equilibrated slag assays, the so-called “bulk distribution”. Figure 5.21 is

reproduction of Figure 4.13, with the boundaries between the regions described

previously in Table 4.3, marked on the figure. The observed trend will be discuss

below as Q increases from 0 to 1, i.e. as CaO replaces SiO

a

ed

Figure 5.21 Distribution coefficients of Se and Te (DM) and composition ranges of the

different solid phase assemblages in mixed FCS slags (2-25 wt% CaO, 2-32 wt% SiO2)

at 1300 °C and 10-9 atm2O .

2 in the slag.

Q

0.0 0.2 0.4 0 0.8 1.0

log

DM

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

SeTe

A B C D E F G

.6

p

123

124

nce ated in the liquid,

.

two solid phases, which make up a large proportion of the total slag volume. As there is

nly a small amount of liquid, the slag takes up less of the minor elements in total, and

was

ll

Region A: The first addition of CaO results in an increase in DM (bulk) as expected.

Even though two solid phases precipitate, calcium remains co ntr

with levels in the solid all <0.5 wt% Ca. The solid was not a significant portion of the

slag and was not spread evenly throughout, so there was not a large reduction in liquid

volume available to take up the minor elements.

Region B: The slags in this region show only a single sparsely distributed solid phase

As in region A, calcium does not enter the solid in appreciable amounts. As a result, the

addition of CaO continues the expected increasing trend in DM (bulk) through this

region.

Region C: Across this region, the drop in DM (bulk) is quite large. While the silicate

phase does take up more calcium as Q increases, calcium is still concentrated in the

liquid, so should still readily take up the minor elements. However, these slags contain

o

as such, their concentrations appear reduced in the analysis of the bulk slag.

Region D: In this slag, the amount of liquid available to take up the minor elements

even smaller than previously, with only small interstitial patches present. It is also in

this region where calcium is more concentrated in the silicate phase than the liquid,

which means that the ability of the remaining liquid to take up the minor elements wi

be further reduced. As a consequence, it is at about this value of Q where DM (bulk) are

at a minimum.

125

of

gnificantly across region F, even though this region

sence of the calcium silicate phase. While this phase does

uced

th Se

me

nt dilution

ffect is not as significant.

Region G: The calcium silicate phase is no longer present in region G, which means that

all calcium and silicon partition to the large fraction of liquid phase. The DM (bulk) for

both Se and Te increase linearly from the last slag of region F to the calcium ferrite

composition, reaching an absolute maximum at Q = 1.

5.5.2. Liquid Distribution

Region E: The DM (bulk) begin to increase in this region, but are still low despite the

absence of a solid phase containing calcium. The remaining solid oxide phase is still

taking up most of the slag volume, so the overall ability the slag to take up the minor

elements is again being limited.

Region F: The DM (bulk) increase si

is characterised by the pre

take up a more significant proportion of the calcium in the system, it also concentrates

the silicon, leaving the liquid much more depleted in silicon than in any other slag

containing a solid silicate phase. With the interactions between lime and silica red

further as Q increases across this region, the remaining CaO is free to interact wi

and Te, thereby increasing their capacity. Further to this, the reduction in liquid volu

is not as great in this region compared to regions D and E, so the appare

e

In an industrial process it is the overall result that is of practical significance, but it is

the behaviour in the liquid that properly establishes the effect of slag basicity. Even

though all the phases (solid and liquid) are theoretically in equilibrium, it is shown that

it is the liquid phases that control the distribution of the minor elements. In the solid

126

inor elements have had a chance to diffuse

r

he distribution coefficients of selenium between the remaining liquid slag portion and

on that would normally be expected. A similar effect was observed for Te,

lthough the increase in DTe (liquid) is not as significant as with Se. The capacities of

, match

ery well with the calcium ferrite data. The activity of lime for the liquid FCS slag was

phases that precipitated from the FCS slags, very low levels of Se were consistently

recorded. This indicates that the solid phases separate from the liquid early on in the

course of each experiment, before the m

throughout the entire slag volume, and that there is no further partitioning of the mino

elements between the liquid and these solids. Since the minor elements are

concentrating in the liquid, it is these measurements that give the proper indication of

the effect of slag basicity on minor element behaviour.

T

the Cu alloy as determined from the MPE simulations (“liquid distribution”), followed

an interesting trend, in that the values appear to be independent of basicity, up to Ql =

0.5. Above this, however, there is an increase in DSe (liquid) to the calcium ferrite

compositi

a

the liquid slags follow a different trend to the distribution coefficients, in that values

initially increase rapidly with increasing activity of lime up to about aCaO = 0.2, then

level off above this point. This is shown in Figure 5.22 for Se and Figure 5.23 for Te.

Included in both of these figures are the capacity data for the varying 2Op and varying

lime Cu/calcium ferrite slag experiments at 1300 °C. It can be seen that for both minor

elements, the CM (liquid) values reached where there is independence of aCaO

v

determined from the MPE simulations, and is plotted along with the activities of the

other slag components in Appendix B (Figure B.5).

a

127

CaO

0.0 0.2 0.4 0.6 0.8 1.0

log

Se

C(li

quid

)-3.6

-3.4

liquid FCS-4.8

-3.8

-5.0

-4.6

-4.4

-4.2

-4.0

Ca ferrite

1300 οC

Figure 5.22 Selenide capacities (CSe) of FCS (3-30 wt% CaO, 2-28 wt% SiO2) and

Values of a

calcium ferrite (16-32 wt% CaO) slags as a function of the activity of lime at 1300 °C.

CaO for the liquid FCS slags were calculated by MPE simulations.

aCaO

0.0 0.2 0.4 0.6 0.8

-5.0

-4.8

-4.6

-4.4

liquid FCSCa ferrite

)Te

(liqu

id

-5.2

log

C

-6.0

-5.8

-5.6

-5.4

1300 οC

Figure 5.23 Telluride capacities (CTe) of FCS (3-30 wt% CaO, 2-28 wt% SiO2) and

calcium ferrite (16-32 wt% CaO) slags as a function of the activity of lime at 1300 °C.

Values of aCaO for the liquid FCS slags were calculated by MPE simulations.

128

errite results, CSe > CTe by around an order of magnitude.

to

the

h

iO2 system

gher silica contents (35 to 63

t% SiO2 sequence, some CS values are actually lower than the CSe values

°C

igure 5.12), again

As was seen in the calcium f

While there is some scatter in both sets of data, it is apparent that the addition of lime

the slag does in fact have a strong influence on CM (liquid), but only when the slag is

still relatively acidic. That is, once the activity of lime has reached a certain point,

further additions of lime have no effect on capacity, regardless of the slag type. It

follows that only a small addition of lime to a fayalite slag could be used to improve

slag’s physical properties before the capacity of the slag for Se and Te becomes as hig

as in a calcium ferrite slag.

Saint-Jours and Allibert (1988) also found CS to increase in the CaO-FeOx-S

as CaO replaced SiO2 at 1400 °C and 7.38×10-8 atm 2Op , as well as at 1480 °C and

1.77×10-3 atm 2Op . However, their work covers much hi

), and as a conw

determined in the present work. Also, the separation of solid phases from the slag is

still also possible at high temperatures, especially given the higher 2Op in the work of

Saint-Jours and Allibert (1988). Simeonov et al. (1995) however, found CS to be

independent of [CaO]slag in the magnesia saturated CaO-FeOx-SiO2 system at 1250

and 10-10 atm 2Op , although their data only extends up to 5.1 wt% CaO.

Activity coefficients of the minor elements in the liquid slag were calculated by the

same method as previously, using the compositional data from the MPE simulations.

The values are shown in Figure 5.24 for Se and Figure 5.25 for Te. Values for Te are

much higher than those of Se at a given aCaO as observed previously (F

indicating that Se is retained by the slag more readily than Te.

aCaO

0.0 0.1 0.2 0.3 0.4 0.5

γ Se

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1300 οC, 10-9 atm

Figure 5.24 Activity coefficient of selenium (γSe) in liquid FCS slags (3-30 wt% CaO, 2-

28 wt% SiO2) as a function of the activity of lime at 1300 °C and 10-9 atm O . p2

aCaO

0.0 0.1 0.2 0.3 0.4 0.5

γ Te (l

iqui

d)

0

10

20

30

40

50

60

70

1300 οC, 10-9 atm

Figure 5.25 Activity coefficient of tellurium (γTe) in liquid FCS slags (3-30 wt% CaO,

2-28 wt% SiO2) as a function of the activity of lime at 1300 °C and 10-9 atm2O . p

129

130

Since there are strong interactions between basic CaO and acidic SiO2 in these slags,

many have a very low aCaO as a result. Because of this, the values of γM (liquid) for the

FCS slags are generally higher than in the calcium ferrite slags (Fig 5.12). The values

of γM (liquid) show a sharp decrease with increasing aCaO up to about aCaO = 0.1, which

is in line with the increase in the CM (liquid) over the same interval. This is further

indication that the overall interactions between the slag and the minor elements become

much stronger with only a small addition of lime to an acidic slag. This is due to the

interactions between lime and the minor elements being stronger than the interactions

between silica and the minor elements. However, the activity coefficient of both minor

elements in the liquid slag appears to be independent of aCaO across the range 0.1 < aCaO

<0.5, as did the capacity, confirming that further additions of lime to the slag past a

nt e of iron

in the FCS slags as described previously. According to the MPE simulations, aFeO

initially increases with increasing Ql, then decreases over approximately the same

region as the CM(liquid) and γM(liquid) values were found to be constant (Figure B.5).

5.6. Applications

Typically, Se and Te are extracted in a separate process from the anode slimes

remaining from the electrorefining of the impure copper. The benefit of having these

elements retained by the matte or metal until the final step is that the slag, free from

these elements, does not require special handling, and there is no danger of toxic

components becoming dispersed in the environment. While it has been demonstrated

ent

certain point do not affect the overall strength of interactions between the slag and the

minor eleme s. It is possible that there is a similar counterbalancing influenc

that the Se and Te content of the metal phase can be controlled through the managem

of the reaction conditions and slag composition, the optimum practice will depend on

131

20

inimised by operating

at lower temperature.

m ferrite slag (20-26

wt% CaO) and Cu at 1200 to 1400 °C and 10-11 to 10-6 atm2O

economic and other factors. These include the cost of fluxes to produce the desired

slag, the energy required to calcine and fuse the fluxes, the rate of wear of refractories

by the slag, the amount of valuable metal losses, and health and safety concerns.

The distribution coefficients of Se and Te from all the Cu/slag experiments with the

wt% CaO calcium ferrite are shown in Figures 5.26 and 5.27 respectively. From a

practical standpoint, two important conclusions can be drawn from this:

1. At constant 2Op , contamination of the slag by Se and Te is m

2. A certain concentration of Se or Te in the slag can be maintained by simultaneously

increasing the temperature and the 2Op .

-0.5

0.0

0.5

1.0

Figure 5.26 Distribution coefficients of Se (DSe) between calciu

p .

log pO2 (atm)

-12 -11 -10 -9 -8 -7 -6 -5

1200 οC1250 oC1300 Co

1350 οC1400 Cο

log

DS

e

-2.5

-2.0

-1.5

-1.0

-3.0

132

Figure 5.27 Distribution coefficients of Te (DTe) between calcium ferrite slag (20-26

wt% CaO) and Cu at 1200 to 1400 °C and 10-11 to 10-6 atm2O

The distribution coefficients of both minor elements are almost always less than unity,

which means that, in general, partitioning of Se and Te to the metal phase is favoured.

Copper converting systems typically operate at a temperature of 1250 °C and a 2O of

10-6 to 10-7 atm, where the selenide and telluride are the species entering the slag. These

experiments show that under these conditions, the calcium ferrite slag will take up very

little Se and Te, so its use should not complicate the current practice of retaining these

elements in the metal phase.

It was also demonstrated in this work that the capacity of the calcium ferrite slag to t

up Se and Te could be enhanced by adjusting the key operating variables. This may be

log pO2 (atm)

-12 -11 -10 -9 -8 -7

log

DTe

-2.5

-2.0

-1.5

-1.0

-0.5

0.01200 οC1250 οC1300 οC1350 οC1400 οC

p .

p

ake

133

ure

is is that while copper recovery is high under such conditions, the slag will be

and Te, creating the potential for the dispersion of these toxic

calcium

,

nd

g

y

of benefit if the ore being processed is particularly high in these impurities and there

was a need to remove any excess to prevent residual amounts remaining in the metal

after refining. The highest DM in the present work were recorded under strongly

reducing conditions. Such conditions are unlikely to be employed on a large scale

because of cost and safety issues associated with using CO. Extremely oxidising

conditions may also eventually lead to a further increase in DM such that they are

greater than unity, but would not be employed in the converting step due to the

associated losses of copper to the slag.

The recovery of valuable metal during processing is of paramount concern to producers,

and like Se and Te, [Cu]slag is strongly dependent on the reaction conditions. While

keeping [Cu]slag to a minimum is necessary for profitability, the costs of energy input

and materials needed for successful copper recovery must be taken into consideration

when a process is put into practice on a large scale. The effect of both the temperat

and 2Op on [Cu]slag is opposite to that on the minor elements, that is, [Cu]slag is

minimised by decreasing the 2Op and increasing the temperature. The consequence of

th

contaminated with Se

elements into the environment. If necessary, copper can be recovered from the

ferrite slag at the completion of converting, by flotation, for example (Somerville et al.

2000), although the benefit of this in terms of cost would need to be evaluated. If fou

to be practical, this would increase the usefulness of this slag type in the convertin

step. However, with too large an increase in temperature, copper can actually be lost b

volatilisation. This is indicated by the large increase in the vapour pressure of Cu from

4.0 × 10-6 atm at 1200 °C to 9.0 × 10-5 atm at 1400 °C (Kubaschewski and Evans,

134

s.

the

und 50%, but

unlikely to be practical in a large-scale industrial process. More favourable results were

BaO that would need to be assessed. It is apparent, therefore, that the temperature, 2O

nd slag composition will need to be adjusted in tandem in order to appropriately

partition the minor elements between the slag and metal, and this will also depend on

the composition of the concentrate being processed.

If an iron calcium silicate slag was to be used, the precipitation of large amounts of

solid phases would severely hamper the copper production process by causing foaming,

and difficulties in tapping the furnace. Only small amounts of SiO2 could be added to

the calcium ferrite slag before it is likely that problems would be encountered. The

process would be further slowed, as iron present in the matte that is normally removed

to the slag by oxidation would have a reduced liquid slag volume to enter into, once

these phases precipitate. However the addition of SiO2 would help with keeping Se and

Te in the metal phase until the final refining step. The precipitation of solids from FCS

of Se and Te in the slag.

1958). Furthermore, the highest magnesia content of the slag was recorded at the

highest temperature studied. The accelerated rate of wear of magnesia-based

refractories at higher temperature would then add even more to the production cost

The cost of fluxes required to make a high-lime or high-BaO slag also needs to be

considered if the slag was to be used to remove some of the minor elements early in

process. The increase in CSe from the lowest to highest lime slag was aro

the increased cost associated with doubling the amount of flux used to make the slag is

achieved with BaO as an additive, but there are also health issues associated with using

p

a

slags could then possibly be overcome by increasing the temperature, while

simultaneously adjusting the 2Op to maintain suitable levels

135

Copper losses are also lower in the FCS slag than in calcium ferrite, making it possibly

the most attractive slag for use in the converting step. A slag of the type CaO-FeOx-

SiO2 would only be useful however if the composition of the concentrate was such that

the amount of flux required to produce the slag with the minimum copper solubility did

not lead to an excessively large slag volume.

5.7. Summary

In this chapter, the mechanisms by which Se and Te enter the calcium ferrite slag under

both oxidising and reducing conditions have been proposed, and the capacity of the slag

to hold these elements has been determined. Capacities in the calcium ferrite slag were

dependent on the temperature, but not, it appeared, on the basicity of the slag, as the

distribution data may have suggested. The capacities were greatest in the most basic

f d a thought to be

ependent on the activity of the slag components and not the reaction conditions

and the minor elements was not quantified for low-iron

lags. In the calcium ferrite slag, however, FeO appears to interact more strongly with

slag (barium ferrite), and lowest in the most acidic (fayalite). The activity coefficients

of Se and Te were determined for the calcium errite slag, an re

d

directly. The studies with iron calcium silicate slags, however, showed that the selenide

and telluride capacities of the liquid slag are strongly dependent on the activity of lime,

but only at aCaO < 0.2. In relatively more basic slags, the corresponding activity

coefficients of Se and Te in the liquid slag were unaffected as CaO further replaced

SiO2. It was also shown the iron content of the slag has an influence on the activity

coefficients of Se and Te in the slag. However, the relative strength of interactions

between ferrous and ferric iron

s

Se and Te than does Fe2O3.

136

2 and

e

to

xidation

rimental data were combined with

ublished data to determine capacities and activity coefficients of Se and Te in the

ay

d

It has

lag

ing

6. CONCLUSIONS

This thesis provides experimentally derived thermodynamic data on selenium and

tellurium in magnesia saturated slags of the type CaO-BaO-FeOx, CaO-FeOx-SiO

CaO-Al2O3-FeOx, over a broad temperature and oxygen partial pressure range. The

classical gas-metal-slag equilibrium distribution technique was used to examine th

partitioning behaviour of Se and Te between the slag and metal phases in response

changes in the reaction conditions. From this, it was possible to determine the o

state of these elements in the slag. The expe

p

various slags. Further experiments were carried out in an attempt to determine the

relative effects of ferric and ferrous iron in the slag on minor element behaviour. X-r

diffraction and scanning electron microscopy techniques were used to characterise soli

phases within the iron calcium silicate slag, while the composition of the remaining

liquid slag phase was predicted by model calculations. The results indicate that the

copper converting process can be optimised in terms of copper recovery, or,

contamination of the slag by Se and Te, by means of the careful control of the main

operating variables (oxygen partial pressure, temperature and slag composition).

also been revealed that the exact mechanism by which the minor elements enter the s

is quite complex, in that there are direct interactions with the major slag components, as

well as reactions to replace free oxygen in the slag.

In the calcium ferrite based slag, the following conclusions as to the general partition

behaviour of Se and Te can be drawn:

• Except under strongly reducing conditions (2Op >10-9 atm), Se and Te are both

much more concentrated in the metal phase than in the slag

137

o

10-11 atm, and by increased additions of CaO to the calcium ferrite slag

s

ure

he selenide capacity of the calcium ferrite slag as determined from the copper/slag

t

ibution to

• Selenium is much more readily taken up by the slag than Te

• Distribution coefficients of Se and Te can be increased by increasing the

temperature from 1200 to 1400 °C, decreasing oxygen partial pressure from 10-6 t

• Increasing the basicity of the slag by replacing lime with barium oxide enhance

partitioning of Se and Te to the slag phase

• The dominant minor element containing species in the slag under these conditions

are CaSe and CaTe

• Distribution coefficients of Te begin to increase with increasing oxygen partial

pressure above 10-6 atm, indicating a transition to the divalent oxide becoming the

dominant species in the slag

• Distribution coefficients of Se appear to level off above an oxygen partial press

of 10-5 atm

T

experiments was around an order of magnitude greater than the telluride capacity. A

1300 °C, capacities appeared to remain independent of aCaO as the lime content of the

slag or 2Op were varied, even though the capacity theoretically is dependent on the

basicity of the slag. Where BaO was substituted for CaO in the slag, there was,

however, a significant increase in the capacities of both minor elements. Capacities also

increased as the temperature of the slag was increased regardless of whether the 2Op

remained constant or was varied, and also when the activity of lime was held constant.

This suggests that the variation in the capacities is due to the variations of the activity of

FeO, as well as CaO, with the experimental conditions, either through a contr

‘free’ oxygen in the slag or direct interactions with the minor elements.

138

Se

showed strong negative deviation from

e ideal (Raoultian) behaviour, whereas Te showed slight positive deviation from

ts

s of the

on in

f the minor elements were again close to constant. In this

case, the predicted decrease in aFeO counterbalances the increase in aCaO from additions

The activities of the minor elements in the slag were determined for the copper/slag

experiments using the compositional analyses and the known activity coefficients of

and Te in liquid copper. At 1300 °C, the activity coefficients of the minor elements in

the slag were found to be independent of their concentration over the range studied, i.e.

Henrian behaviour was demonstrated. Selenium

th

ideality.

It was also found that the activity coefficients of the minor elements in the slag phase

were dependent on the slag chemistry, and not the other imposed experimental

conditions directly. Where the 2Op or temperature was varied, the activity coefficien

of both minor elements in the slag phase decreased with increasing activity of lime.

This is due to the interactions between the slag and each minor element becoming

stronger as the activity of lime is increased, thus lowering the activity coefficient

minor elements. This accounts for the different behaviour observed between the

temperature variation experiments at constant 2Op and constant CO2:CO ratio, as aCaO

varies in a different direction with temperature in these two cases. It also indicates that

this dependence on the activity of lime is strong enough that the expected reducti

deviations from the ideal behaviour with increasing temperature does not occur.

However, where the activity of lime was kept constant as the temperature was

increased, activity coefficients of Se and Te remained close to constant, further

suggesting that there must be some degree of interaction between iron in the slag and

the minor elements. This was also evident in the results with varying lime content, in

which activity coefficients o

139

ll interactions between the slag and the minor elements

e

n the

alues

ate

the bulk slag

Distribution coefficients of Se and Te calculated for the remaining liquid slags show

e composition of the slag changes from iron

of lime, to the extent that overa

remain constant.

Equilibrium experiments with calcium aluminate slags and copper alloy showed that

adding increasing amounts of iron oxide to the slag leads to an increase in the selenid

capacity. This effect was the same regardless of the 2Op , and therefore Fe3+:Fe2+ ratio

of the slag. Because of this, the influence of the individual oxidation states of iron on

minor element behaviour could not be confirmed. Likewise, the effect of iron o

activity coefficient of Se in the slag was the same at each 2Op , despite ferrous iron

becoming dominant under more reducing conditions. The difference between the v

of the activity coefficient of selenium in the slag between these series of experiments at

a given iron content confirmed that there was still an influence of 2Op in this system.

For the iron calcium silicate slag, the following conclusions as to the general

partitioning behaviour of the minor elements can be drawn:

• Se and Te are more readily taken up by the calcium ferrite slag than the iron silic

slag

• The precipitation of solid phases from the slag at high temperature greatly reduces

the volume of liquid available to take up minor elements, resulting in an apparent

reduction of the minor element concentration of

•

a region of independence from the basicity of the slag, followed by a region over

which values increase sharply as th

silicate to calcium ferrite

140

lising the wustite field

ate solid

pha

to b ncrease

wh has no effect. The activity coefficient of

cap level off in a similar manner. This is further evidence that the

be

inc

calc same value. However, the values of the activity

.1. Future Work

e

ditions

e

nts similar to

The presence of magnesia in the FCS slags had the effect of stabi

of the CaO-FeOx-SiO2 system. Magnesio-wustite, olivine and dicalcium silic

ses were identified in these slags by SEM and XRD techniques, and were all found

e devoid of Se. In the liquid silicate slags, the minor element capacities i

sharply with increasing basicity in the relatively acidic slags, until a point is reached

ere the further substitution of lime for silica

Se and Te calculated for the liquid slag decrease sharply over the same interval as the

acities increase, then

overall strength of the interactions between the slag and the minor elements can only

reased to a certain point. For both minor elements, the capacities of the silicate and

ium ferrite slags reached the

coefficient were generally higher in the FCS slag than in calcium ferrite.

6

While the experiments reported in this thesis have yielded much useful fundamental

information, there is almost no limit to the ways in which the variables examined can b

modified. Some recommendations as to particular areas that could be covered in future

work include:

• The varying 2Op experiments could be extended to cover more oxidising con

to determine whether Se eventually undergoes a transition to an oxidised species in

the slag. This could be further extended for both minor elements, to determin

where this transition occurs at temperatures other than 1300 °C.

• The effect of magnesia on minor element behaviour was not determined, as although

it was present in the slag in every series of experiments, its concentration was

always significantly lower than the other slag components. Experime

141

n

of

ditions, allowing better evaluations of the differences between data sets.

Further investigation is also required to determine the cause of the observed

a m

sign

the varying lime series in this work could be designed to examine this, although a

alternative to the magnesia crucibles, such as alumina, would be necessary.

• While trends were established for the calcium aluminate slag data, there is scope for

a more thorough investigation by studying a wider range of both iron contents and

2Op . It would also be more worthwhile if the Fe3+:Fe2+ ratios were more precisely

controlled, so that they were less subject to scatter within each individual series

iron ad

dependence of CSe and γSe on 2Op , and to establish what interactions exist between

elements dissolved in the alloy.

• Similarly, there is scope for uch more thorough investigation into the behaviour

of the minor elements in the CaO-FeOx-SiO2 system, particularly in the region

between the CaSiO3 and FeSiO4 compositions. It would also be beneficial to de

the experiments to eliminate the stabilising effect magnesia has on solid phases, for

ease of analysis and interpretation of the data.

142

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149

Appendix

A. Analytical Results

150

equilibrated slags and alloys from varying 2O experiments at

Slag (wt %)

pTable A.1 Analysis of

1300 °C. Metal (wt%)

COp Ag CO2

alloy Se Te Ca Fe Fe2+ Mg Cu/ Se Te p

0.226 Cu 0.17 0.5 18.1 53.7 38.1 2.0 0.5 0.86 0.16

0.716 Cu 0.43 0.65 17.8 53.4 32.5 2.3 0.73 0.57 0.04

Cu 0.72 0.73 16.7 49.3 28.5 3.1 1.2 0.25 0.01

.16 Cu 1.02 0.77 16.9 50.4 23.9 3.0 1.9 0.08 -

-

16.5 55.0 43.1 2.7 0.11 2.2 0.26

.716 Ag 0.73 2.2 16.7 53.2 35.8 3.5 0.15 1.8 0.06

8 2.0 17.3 49.2 25.9 2.8 0.35 0.49 0.01

.16 Ag 2.1 2.0 16.7 47.4 23.5 3.4 3.2 0.29 0.01

2.6 2.4 16.5 46.6 17.6 3.4 1.8 0.08 0.004

1.6 Ag 2.4 2.3 17.7 48.8 13.2 2.5 0.81 0.02 0.001

1.6 2.1 14.7 52.4 7.2 2.0 0.15 0.002 0.01

16 Ag 0.97 1.9 15.6 53.6 7.1 2.1 0.35 0.006 0.02

0.03

O2 Ag 1.0 2.1 13.2 52.8 5.7 1.9 0.30 0.002 0.03

3

2.26 Cu 0.85 0.63 17.1 49.1 27.4 3.8 1.0 0.27 0.02

2.26

7

22.6 Cu 1.06 0.8 15.7 48.3 19.2 4.0 3.1 0.03

71.6 Cu 1.1 0.75 15.9 47.6 16.4 3.3 4.8 0.02 -

71.6 Cu 0.94 0.75 14.8 47.8 14.4 2.6 6.7 0.002 -

0.226 Ag 0.23 1.9

0

2.26 Ag 1.5 2.0 18.6 51.9 29.1 3.1 2.2 0.89 0.02

4.03 Ag 1.

7

12.7 Ag 2.1 2.0 16.8 48.2 20.0 3.2 0.6 0.15 0.01

22.6 Ag

7

226 Ag 2.3 2.2 15.3 49.4 8.2 3.2 0.37 0.007 0.02

716 Ag

7

CO2 Ag 0.92 1.9 14.1 52.2 5.4 1.8 0.24 -

C

Air Ag - 0.06 12.4 52.5 1.4 2.0 2.0 - 0.0

151

equilibrated slags and alloys from varying temperature

t%) Slag (wt%)

Table A.2 Analysis of

experiments at a 2Op of 10-10 atm.

Metal (w

CO

CO2p

p Se Te Ca Fe Fe2+ Mg Cu Se Te T °C

1200 3.104 0.73 0.81 17.8 54.4 27.5 2.0 1.3 0.20 0.008

1250 1.46 0.58 0.71 17.2 53.5 31.9 3.2 0.97 0.34 0.01

300 0.716 0.43 0.65 17.8 53.4 32.5 2.3 0.73 0.57 0.04

0.06

400 0.197 0.17 0.26 18.3 52.9 40.1 3.4 0.69 0.69 0.08

1

1350 0.368 0.26 0.42 17.8 53.9 38.8 2.9 0.81 0.63

1

Table A.3 Analysis of equilibrated slags and allo

Metal (wt%) Slag (wt%)

ys from varying temperature

experiments at constant ratio of CO2:CO.

T °C Se Te Ca Fe Fe2+ Mg Cu Se Te

1200 0.70 0.85 18.9 51.6 29.0 2.1 1.1 0.28 0.02

1250 0.73 0.80 18.7 51.7 27.6 2.2 1.2 0.26 0.01

1300 0.72 0.73 16.7 49.3 28.5 3.1 1.2 0.25 -

350 0.74 0.57 17.3 50.9 29.7 3.6 1.7 0.21 0.01

07

1

1400 0.74 0.45 17.2 50.0 28.6 4.6 1.8 0.18 0.0

152


Slag (wt%) experiments at constant aCaO. Metal (wt%)

T °C CO

CO2

pp

Se Te Ca Fe Fe2+ Mg Cu Se Te

1200 3.78 0.76 0.80 15.7 48.8 * 4.5 1.1 0.16 0.01

1225 3.72 0.76 0.82 16.6 49.6 26.4 3.4 1.24 0.16 0.011

1250 2.91 0.71 0.77 16.4 49.7 26.4 3.5 1.23 0.16 0.012

1275 2.28 0.71 0.75 16.0 49.3 28.7 4.1 1.14 0.23 0.013

1300 1.79 0.65 0.70 16.4 49.7 29.7 3.7 0.967 0.27 0.015

*Not determined

Table A.5 Analysis of equilibrated slags and alloys from experiments with slags of

varying lime content at 10-9 atm2O and 1300 °C.

Metal (wt%) Slag (wt%) p

Se Te Ca Fe Fe2+ Mg Cu Se Te

0.91 0.89 11.9 53.7 35.0 4.7 1.5 0.23 0.017

0.845 0.9 15.1 50.8 33.3 3.2 1.5 0.27 0.017

0.72 0.73 16.7 49.3 28.5 3.1 1.2 0.25 0.011

0.85 0.63 17.1 49.1 27.4 3.8 1.0 0.27 0.017

0.58 0.68 22.6 46.6 21.0 2.0 1.3 0.27 0.015

153

nts

slags at 10-9 atm2 and 1300 °C.

l (wt% Sla

Table A.6 Analysis of equilibrated slags and alloys from experime with barium-

containing OpMeta ) g (wt%)

Se g u Se e Te Ca Ba Fe Fe2+ M C T

0.58 0.68 22.6 0 46.6 21.0 2.0 .3 0.27 0.015 1

0.70 0.72 4 6 .7 27 19

7 30 21

6 .2 40 26

3 .7 59 87

12. 4.6 48.5 27.0 4. 1 0. 0.0

0.65 0.71 9.0 10.2 46.9 29.4 4. 1.5 0. 0.0

0.62 0.71 5.6 16.7 49.3 28.0 3. 1 0. 0.0

0.48 0.68 0 30.0 46.1 24.9 2. 1 0. 0.0

Table A.7 Analysis of eq rated slags and allo rom fe

s at 0 a2O 13

tal (w

uilib ys f experiments with dif rent Cu

alloy 10-1 tm p and 00 °C. Me wt% Slag t%)

Alloy 2+ g Se Se Te Ca Fe Fe M Cu Te

1 0.43 0.65 .5 3 3 7 4 17.8 53.4 32 2. 0.7 0.5 0.0

2 0.43 0.69 .3 2 .90 0.50 5

0.14 0.22 16.5 50.8 34.8 2.3 0.74 0.18 0.02

0.29 0.43 16.9 50.6 34.5 2.1 0.91 0.38 0.03

16.4 50.3 33 2. 0 0.0

3

4

154

ing-iron

. Met

Table A.8 Analysis of equilibrated slags and alloys from experiments with vary

calcium aluminate slags at 1500 °Cal (wt%) Slag (wt%)

CO

CO2

pp

Se Fe A 3 FeOx O O e CaO l2O * Fe2+ Mg Cu S

0.627 0.72 - 41.8 4 0 8 8.8 - 9.5 0.56 0.00

0.627 0.8 46.4 4.8 1

44.2 8.6 15

39.5 16.5 6.0 19

4 0 33

.198 0.71 1.1 38.2 44.9 7.7 3.0 9.3 0.30 0.04

.198 0.65 2.1 35.7 41.6 14.9 6.7 9.2 0.35 0.08

.063 0.59 - 42.4 49.4 0 - 8.9 0.15 0.10

.063 0.64 1.6 41.0 47.6 3.1 0.65 9.3 0.12 0.11

.063 0.62 3.2 39.7 46.1 6.0 2.6 9.2 0.13 0.13

.063 0.61 7.0 37.2 43.5 9.6 4.6 11.0 0.14 0.16

The concentration of total iron from the XRF analysis expressed as Fe2O3

0.16 39.9 1.7 9.1 0.64 0.0

0.627 0.7 0.03 38.2 3.2 9.5 0.59 0.0

0.627 0.74 0.06 34.3 10.1 0.73 0.0

0.198 0.76 - 42.4 9.4 - 8.7 0.36 0.0

0

0

0

0

0

0

*

155

Table A.9 Analysis of equilibrated slags and alloys from experiments with silicate slags

2O

Slag (wt%) (ppm)at 10-9 atm p and 1300 °C. Metal (wt%)

Mix Se T a Si Fe Fe2+ u Se Te e C Mg C

F* 0.82 0.70 0 15.2 43.5 41.0 5.1 1.7 0.03 46

1:9 0.88 0.69 1.5 .9

0.70 2.7

0.72 4.1

0.91 4.2

0.67 5.6

2 7.4 7.4 48.8 33.4 6.0 0.41 0.02 21

2:1 0.84 0.65 7.8 4.6 50.9 35.5 5.1 0.27 0.02 90

5:2 0.97 0.81 7.8 3.6 49.8 38.8 4.2 0.40 0.03 40

7:2 0.97 0.73 7.7 2.2 49.6 38.2 6.0 0.60 0.04 53

4:1 0.85 0.68 14.5 3.4 42.8 36.5 4.4 1.2 0.08 105

0.87 0.77 17.4 2.6 48.1 25.4 4.4 0.92 0.16 68

9:1 0.79 0.70 16.3 2.2 44.6 29.3 4.5 1.2 0.17 84

0.78 0.73 18.0 1.0 51.3 24.3 3.9 1.3 0.21 120

Fayalite master slag

13.6 42.5 38 5.1 1.9 0.04 61

2:13 0.80 13.1 46.0 40.5 3.9 1.4 0.06 70

1:4 0.88 10.7 45.3 39.6 4.0 1.0 0.06 45

1:2 1.1 9.3 47.2 39.7 2.8 0.70 0.06 110

2:3 0.87 8.1 44.9 39.9 4.3 0.65 0.04 30

1:1 1.1 0.8

13:2

10:1

*

156

Table A.10 Composition of liquid FCS slags in equilibrium with Cu alloy from MPE

simulations. Liquid slag (wt%)

Mix % liq CaO SiO2 FeO Fe Cu2O Se Te 2O3 MgO

1:9 3 8 73 2.8 27.3 58.9 4.3 5.4 1.2 0.05 0.00

2:13 0

14.1

2:1 49 22.3 20.6 38.4 11.6 6.5 0.61 0.033 0.002

5:2 48 23.1 16.7 39.5 15.6 4.1 0.87 0.054 0.008

7:2 15 25.2 8.8 34.5 28.9 1.8 0.86 0.27 0.035

4:1 53 27.6 8.1 31.6 30.3 1.7 0.68 0.15 0.020

77 30.2 7.0 27.9 32.8 1.5 0.81 0.21 0.009

9:1 74 27.1 5.7 32.4 32.5 1.5 0.79 0.23 0.011

83 28.3 2.4 33.1 33.8 1.4 0.92 0.25 0.014

100 4.2 28. 56.5 5.0 5.5 0.69 0.058 0.007

1:4 99 6.2 24.0 56.0 6.5 6.7 0.64 0.063 0.004

1:2 100 6.4 21.6 58.1 8.2 5.1 0.70 0.065 0.011

2:3 100 8.7 19.4 54.5 8.9 7.9 0.67 0.038 0.003

1:1 72 21.6 46.2 7.7 9.7 0.67 0.022 0.003

13:2

10:1

157

B. Activity of Slag Components

CaO (wt%)

14 16 28 30 3218 20 22 24 26

activ

ity

0.0

0.2

0.4

0.6

0.8

1 οC, -9 atm300 10

F

Fe

igure B.1 Activities of oxides in varying lime magnesia saturated calcium ferrite slags

-9 atm2O and 1300 °C.

eO

CaO

2O3

F

pat 10

log pO2

-12 -11 -1 8 -50 -9 - -7 -6

activ

ity

0.0

1

2

.3

4

.5

0.6

0.7

0.

0.

0

0.

0

1300 οC20 wt% CaO

FeO

CaO

e O3

B t o oxides in mag a s e ium rit u ary

t C

F 2

Figure .2 Ac ivities f nesi aturat d calc fer e slag nder v ing

2Op a 1300 ° .

158

159

T (οC)

115 1 0 13 14

activ

ity0.6

0.7

0 1200 250 130 50 1400 500.0

.1

.2

.3

4

5

0

0

0

0.

0.

10 atm20 wt% CaO

FeO

CaO

2O

Figure B.3 Activities of s i gne tur alc err g u a

temperature at 10-1 atm2.

-10

Fe 3

oxide n ma sia sa ated c ium f ite sla nder v rying

0 Op

T (οC)

1150 1200 1250 1300 1350 1400 1450

activ

ity

0.0

0.1

0.2

0.3

0.4

0.5

0.6

CO2:CO = 2.2620 wt% CaO

FeO

CaO

Fe2O3


temperature at constant CO2:CO ratio.

160

Ql

0.0 0.2 0.4 0.6 0.8 1.0

activ

ity

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

FeO

SiO2

Fe2O3

CaO

1300 οC10-9 atm

Figure B.5 Activity of components of liquid FCS slags with varying basicity of the

quid slag (Ql) at 1300 °C and 10-9 atm2 as predicted by the MPE simulations. Opli

161

162

C. Published Work

The following papers and conference abstracts have resulted from this work:

Reviewed Papers

M.D. Johnston, S. Jahanshahi, and F.J. Lincoln: “Thermodynamics of Selenium and

Tellurium in Calcium Ferrite Slags” Metall. Mat. Trans. B, 2007, vol. 38B (3), pp. 433-

442.

M.D. Johnston, S. Jahanshahi, and F.J. Lincoln: “Effect of Slag Basicity on Phase

Equilibria and Minor Element Distribution of Calcium Ferrite Based Slags” Manuscript

in preparation.

Conference Proceedings

M.D. Johnston, S. Jahanshahi, and F.J. Lincoln (2004) “A Thermodynamic Study of

Selenium in Calcium Ferrite Slag and Copper”, Chemeca 2004 Sustainable Processes,

extended abstract, Sydney, 27-29 September 2004.

etw en Calcium Ferrite Slag and Alloys”, Sohn International

ymposium on Advanced Processing of Metals and Materials, Volume 1: Thermo and

Physicochemical Principles: Non-Ferrous High-Temperature Processing, pp 779-790,

San Diego, 27-31 August 2006.

M.D. Johnston, S. Jahanshahi, and F.J. Lincoln (2006) “Equilibrium Distribution of

Selenium and Tellurium B e

S

thermodynamics of selenium and tellurium in …...thermodynamics of selenium and tellurium in molten...

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