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LOUGHBOROUGH UNIVERSITY

THE INVESTIGATION OF SCALE FORMATION

IN THE BA YER PROCESS

A Thesis submitted for the Degree of Master of Philosophy

by

Jennifer Ann Armstrong

Department of Chemistry Loughborough

October 1999

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Acknowlegements

Firstly, I would like to thank Or. Sandra Dann for taking me on and giving me

the opportunity to study for this Mphil. I am indebted for all her help,

encouragement, support and patience throughout this last year.

Importantly, I would also like to thank AIcan International for their financial

support.

Finally, I would like to thank Mum, Dad and Michael, and ALL my friends,

for their support, love and affection.

Abstract Declaration Nomenclature

CONTENTS

CHAPTER 1: Introduction

1. I The Bayer Process

1.2 'The Scale Problem'

1.3 A Brief History of Zeolites

1.4 The Structure of Sodium Aluminosilicates

1.5 The Scope of this Investigation

1.6 References

CHAPTER 2: Experimental Techniques

2.1 Preparation of Synthetic Scale

2.2 Analytical Techniques

2.2.1 Powder X-ray Diffraction (PXD)

2.2.1.1 Theory of X-ray Diffraction

2.2.1.2 The Diffraction Experiment

2.2.2 Powder Neutron Diffraction (PND)

2.2.2.1 Instrumentation

2.2.3 Thermogavimetric Analysis (TGNDT A)

2.2.4 Infra-Red Spectroscopy (IR)

2.3 References

Page

11

I1I

2

6

9

\0

\3

13

16

16

16

17

17

19

19

21

22

23

CHAPTER 3: Analysis of Bayer Plant Scale

3.1 Introduction 25

3.2 Experimental 27

3.3 Results and Discussion 29

3.3.1 Characterisation of High Temperature Scale using FTIR 30 Spectroscopy, X-ray Diffraction and EDAX Analysis

3.3.2 Characterisation of Low Temperature Scale using FTIR 38 Spectroscopy and Powder X-ray Diffraction

3.3.3 Scanning Electron Microscopy Study of Low and High 41 Temperature Scale

3.3.4 Thermal Analysis of Low and High Temperature Scale 41

3.4 Conclusions 45

3.5 References 45

CHAPTER 4: Synthesis of Scale

4.1 Introduction 46

4.2 Experimental 48

4.2.1 The Effect of Anion Type, Symmetry and Competition 49 on Phase Formation

4.2.1.1 Anion Type and Symmetry 49

4.2.2.2 Competition of Anions 49

4.2.2 Effect of Sodium Hydroxide Concentration 50

4.2.3 Effect of Reaction Time and Ageing 50

4.2.4 The Effect of Temperature 51


4.4 Conclusions 65

4.5 References 66

CHAPTER 5: The Use of Lime in the Bayer Process and Inhibition of the Formation of Synthetic Sodiwn Alwninosilicates

5.1 Introduction 69

5.2 Experimental 71


5.4 Concl usi ons 74

5.5 References 74

LOUGHBOROUGH UNIVERSITY

Abstract

FACULTY OF SCIENCE CHEMISTRY

Master of Philosophy

The Investigation of Scale Formation in the Bayer Process by lennifer Ann Armstrong

This thesis reports an investigation of the formation of zeolite scales in the industrial extraction of alumina from bauxite ore. These materials have been characterised by powder x-ray and neutron diffraction, infra-red spectroscopy, scanning electron microscopy and thermogravimetric analysis.

Samples of zeolite scale were removed from both low temperature (c.a. 135°C) and high temperature (c.a. 220°C) Bayer plants and analysed. At low temperature the scale crystallises predominantly as natrodavyne with minority phases of sodalite and cancrinite whilst at high temperature the cancrinite phase is exclusively formed. Although a variety of different anions are present in both the scale and the liquor in small amounts (e.g sulphate, hydroxide, vanadate, and oxalate) the majority anions, with apparently controlling influence, are the carbonate and hydroxide anions.

Laboratory experiments using the representative anions in Bayer liquor (hydroxide, carbonate, oxalate, sulphate and chloride) as a function of temperature (90° - 225°C) and anion concentration (10-3

- lO-IM) were performed using hydrothermal methods. Sulphate formed cancrinite at all temperatures and concentrations and sodalite was consistently formed for both oxalate and chloride anions. However, both temperature and concentration were critical in the case of carbonate where sodalite and intermediate phase dominated at low temperatures and concentrations but at high temperature and concentration, the cancrinite structure was exclusively formed.

Experiments with sodium hydroxide liquors of different concentrations and standing times have shown that the cancrinite structure only forms after standing, where carbon dioxide is readily absorbed from the air, indicating a pure hydroxycancrinite is unlikely to form. In the absence of standing time, hydroxysodalite is formed at all temperatures and all concentrations.

The addition of concentrations of lime used extensively to remove carbonate from Bayer liquor between 0.013 M and O.13M was also investigated. At high concentrations the addition oflime results in no zeolite scale being formed.

Nomenclature

i) Description of Metal-Oxide Polyhedra

Vertex-linked metal-oxide polyhedra are encountered frequently in this thesis and are described as MOx. For example an aluminium-oxygen tetrahedron would be assigned the terminology Al04., tetrahedral.

ii) Oxidation States

A number of different terms have been used for designation of oxidation state in this work. Oxidation state is denoted by superscripted Arabic Numerals e.g. AlJ

+ or by Roman Numerals in parentheses e,g Si (IV).

iii

CHAPTER 1

INTRODUCTION

Chapter 1 Introductiou

1.1 THE BA YER PROCESS

The Bayer Process is the mam industrial process for extracting alurnina

(aluminium oxide, Ah03) from bauxite ore, it has been used for over one hundred

years. The majority of the refined alumina is then used to produce aluminium.

Aluminium is the most common metallic element in the Earth's crust and at

8% is the third most common element with only silicon and oxygen present in greater

proportions. It is present in the ore as very stable oxides- gibbsite (AI(OR)3),

boehmite (AIO(OH)) and diaspore (Alz03.RZO). Gibbsite and boehmite are

commonly referred to as trihydrate (or just hydrate) and monohydrate, respectively,

even though they are not hydrates. The largest bauxite deposits are found in Australia,

North and South America, Africa and Asia. Commercial-grade bauxite contains 40-

60% alumina along with oxides of silicon, iron and titanium as well as other minor

trace impurities. Analysis of bauxite ore used in the Bayer plant at Aughinish, Eire, is

given in Table 1.1 [I]. During the seven-year period the quality of the bauxite has

deteriomted. In particular the iron oxide impurities have almost doubled.

Table 1.1 Analysis of bauxite ore used in the Bayer plant at Aughinish, Eire

Alz03 SiOz Fez03 TiOz LOM* 1991 58.1 1.92 7.53 3.95 27.87 1998 53.4 1.92 14.0 3.27 26.93

*LOM = loss of mass on ignition, i.e. organic material.

Twenty million tonnes of aluminium are consumed each year and this requires over

100 million tonnes of bauxite ore; between four-ta-five tonnes of bauxite are needed

to produce two tonnes of alumina, which in turn yields one tonne of aluminium.

Aluminium, because of its mechanical, physical and chemical properties, is the

material of choice across many products and industry sectors, i.e. transportation,

building and construction, and packaging. It is light but strong, can be fabricated and

cast into complex shapes, has excellent corrosion resistance, good thermal and

electrical conductivity and can be recycled without loss in metal quality [2-4].

Montreal-based Alcan Aluminium Limited, financial sponsors of this research, is the

second largest aluminium producer in the world- primary production 1,490,000

2

Chapter I Iotroduction

tonnes. Last year it signed a lO-year multi-billion dollar agreement to supply

aluminium to General Motors [5].

The Bayer process is summarised in Figure 1.1 [6]. The cyclic nature and the

continuos flow of the process solution (termed liquor) through the plant should be

emphasised.

The bauxite ore is crushed up into a fine powder and mixed with liquid caustic soda

(i.e. sodium hydroxide solution). It is pumped as a slurry into high

pressure/temperature digesters. The temperature at which these digesters operate

depends on the form in which the aluminium oxide is present in the bauxite. The

solubility of gibbsite is higher than that of boehrnite, hence the conditions for

digesting boehmitic bauxite must be more severe, i.e. the temperature and/or the

casutic concentration must be increased [7]. Boehmite-rich ore requires digestion

temperatures of -255°C, whereas if the bauxite is mainly gibbsitic, it can be reduced

considerably to -140°C [8]. The aluminium oxide dissolves in the caustic soda to

form sodium aluminate (Na20.Ah03) and the impurities, mainly oxides of iron,

silicon and titanium, remain undissolved:

2NaOH + Ah03.3H20 --+ Na20.Ah03 + 4H20 + Impurities J.. [1)

( for gibbsite digestion)

These impurities, red in colour due to the iron oxide and hence termed 'red mud', are

allowed to settle and then removed by filtration, a procedure termed clarification. The

red mud is washed to remove chemicals and is then discarded. The red mud waste

site can be clearly seen in Figure 1.2.

After clarification, the now clear sodium aluminate solution is pumped into

precipitator tanks. This solution is termed green or pregnant liquor, i.e. liquor from

which gibbsite has not yet precipitated. Very fme and pure alumina trihydrate is

added as 'seed' and under agitation with compressed air and with gradual cooling,

alumina trihydrate precipitous out of the caustic solution:

3

Chapter 1

F" 19ure 1.1 The Bayer P rocess

4

Introduction

OF .llJLIIS

PRECIPITATION

ALUMINA

ChlPter 1 Introduction

Figure 1.2 Aerial View of the Alumina Plant at Aughinish, Limerick, Eire

5

Chapter 1 Introduction

Na20.AhOJ + 4H20 s<.d) AhOJ.3H20.J, + 2NaOH [2]

This is a reversal of equation I.

The trihydate is allowed to separate from the solution by settling. To remove the

chemically combined water, it is passed through rotary kilns at approximately

IOOO°C:

[3]

The resulting product, aluminium oxide, is a white powder that is known as calcined

alumina. The caustic soda solution left behind in the precipitator (spent liquor) is

recycled and used again with fresh bauxite. The calcined alumina from the Bayer

process is smelted, i.e. reduced in electrolytic cells, to produce aluminium metal in the

Hall-Heroult process [3].

1.2 'THE SCALE PROBLEM'

In reality the Bayer process is not as 'perfect' as described above. Unfortunately,

as well as aluminium oxide, silica impurities in the ore also dissolve in the caustic

soda. The dissolved silica then reacts with soda and alumina in solution and

precipitates as sodium aluminium silicate hydrate desilication product (DSP)

throughout the plant. Kaolin (Ah~.2Si02.2H20) is the major reactive silica mineral

in bauxite. Quartz is not significantly attacked during the extraction process and is

removed in the red mud waste. The scaling reaction is generally thought to proceed

by two steps; dissolution of kaolin:

6

Chapter I Introduction

and subsequent precipitation ofDSP:

6SiOt + 6Al(OH)' + 6Na+ + 2NaX ~ Nag[AlSiO.102.nH20 (s) + (6-n)H20 + 120H" [6]

sodium aluminosilicate scale

where X could be Y. cot, y. sol', cr, Off, etc. [9].

As equation 6 shows, anion impurities play a key role in the precipitation of scale. It

has been shown in this investigation that the critical anion controlling the exact nature

of scale formed is carbonate. Sodium carbonate is present in Bayer liquors due to the

digestion of organic matter in bauxite and the absorption of CO2 from the atmosphere

[8]. Sodium carbonate concentration in the liquor is typically 30-40g/l [10].

When the spent liquor is recycled it is reheated in a series of low and then

high temperature heat exchangers. It is in the tube walls of these high temperature

heaters that the most severe scaling problem occurs, Figure 1.3. Scale formation adds

significantly to process costs. For example:

• It impairs the efficiency of high temperature heaters and removal leads to

corrosIon

• Loss of caustic and alumina

• Descaling, which involves ceasing liquor flow and flushing sulphuric acid

through the circuit to dissolve the scale, results in loss of production time

and incurs extra costs (manpower and chemical cleaning reagents). In the

most severe cases desca1ing is required every six days, i.e. one-day a week

the plant is out of operation.

It is reported the costs of dealing with the scaling and related energy-consumption

problems in Queensland AIumina Limited (QAL) refinery were estimated at $10

million in 1995 [11]. After peaking in 1995 aluminium prices have remained

generally depressed for the past four years, putting pressure on all the big producers to

economise and [rod cheaper ways of making aluminium [5]. It is hoped that with

results and conclusions obtained in this investigation and future research, the problem

of scaling may be alleviated leading to a substantial reduction in production costs.

7

Chaoter 1 Introduction

Figure 1.3 Severe scaling within the tube walls of a high temperature heat exchanger

8

Cbapter I Introduction

1.3 A BRIEF mSTORY OF ZEOLITES

Zeolites is the term given to crystalline, hydrated aluminosilicates, formed under

hydrothermal conditions. The Swedish mineralogist Cronstedt who observed that the

mineral stilbite frothed and gave off steam when heated coined the term zeolite in

1756. It comes from the Greek words zeo- to boil and lithos- stone. Today over 200

aluminosilicate framework structures are known. Approximately 40 of these are

naturally occurring minerals and the others are synthetic structures developed for use

as dehydrating agents, ion-exchangers, adsorbents, molecular sieves and catalysts.

R.M. Barrer is regarded as the founding father of zeolite chemistry [12]. In the

1940s and 50s he demonstrated the synthesis of zeolitic materials under hydrothermal

conditions. Syntheses involved the formation of a gel from a mixture composed of an

alumina component, a silica component and inorganic base(s). The gel is allowed to

crystallise, usually under autogenous pressure, for a period of between a few hours to

several weeks at temperatures between -60°C and 200°C. Factors affecting the

product obtained include temperature, time, pH and degree of agitation. Barrer et al.,

1970, report synthesising sodalities [13] and cancrinites [14] by hydrothermal

reactions of kaolinite. It was reported that nitrate, chromate, and molybdate promoted

the formation of cancrinite. In the absence of these salts, or in the presence of salts of

cr, Br", CI03-, and CI04-, sodalite formed. It was suggested that salts act as a

template catalysing formation.

Zeolites synthesised in the 1940s and 50s, such as zeolites A and X, had low

silica to aluminium ratios. High silica zeolites, such as ZSM-5 (a catalyst now used

widely in the industrial world) and ZSM-12, were synthesised in the late 1960s and

70s.

The 1980s witnessed a huge increase in the number of zeolites and related

crystalline materials. For example, Al04 and Si04 in aluminosilicate zeolites have

been substituted for other T04 groups including T= P, Be, Ga, Ge, and Zn.

Techniques commonly used today to characterise zeolites include neutron diffraction,

29Si MAS NMR, Raman spectroscopy, extended X-ray absorption fine structure

(EXAFS) and MOssbauer spectroscopy.

9

Chapter I Introduction

1.4 THE STRUCTURE OF SODIUM ALUMINOSILICATES

The general fonnula for aluminosilicate zeolites is:

Mm+xhn [Sil_.AI.Oz] _ nHzO.

There are three components to the structure: the framework ([Sil_.AIxOZ]), non

framework cations ~+xIm) and the sorbed phase (which in the above fonnula is

water but can also be other small molecules such as OH", sol·, cr, cot, etc). The

important structural feature of zeolites is the system of channels and cavities of

molecular dimensions throughout the framework structure. The framework has a net

negative charge that is balanced by incorporating exchangeable cations into these

channels and cavities. Anions and/or molecules of water of crystallisation coordinate

with the cations. Some zeolites can be dehydrated by heating under vacuum without

destruction of the a1uminosilicate framework.

This investigation focuses on three distinct sodium a1uminosilicate phases:

sodalite, cancrinite and an intennediate phase. Fourier transfonn infra-red (FTIR), x

ray diffraction (XRD) and scanning electron microscopy (SEM) were used to

distinguish between them. Sodalite and cancrinite are both observed in nature, they

are typical feldspathoid minerals, Table 1.2. Other feldspthoid minerals include

nepheline, scapolite and ultramarine [Table 2.3, 13]. Sodalite and cancrinite both have

highly ordered three-dimensional framework structures, whereas the intennediate

phase shows a strong one-dimensional stacking disorder.

Table 1.2 Sodalite and cancrinite - typical feldspathoid minerals

Mineral Unit Cell Contents Crystallograpbic

Data SodaIite N8lj[(AIOz)6(SiOz)6]CIz

Cubic, a= 8.87A SpacegroupP-43n

Cancrinite (Na, Ca, K)6-8 [(AIOz)6(SiOZ)6] Hexagonal,

a = 12.6 and c = 5.18A (COl, S04, CI)I-Z Space group P6l

[Si04]4- and [A104t tetrahedra are the primary building units of the sodalite

and cancrinite frameworks. Four- and six-membered rings of alternating corner

sharing T04 tetrahedra fonn secondary building units. Lowenstein's rule of

aluminium avoidance forbids the presence of an AI-O-AI linkage under mild

10

------

Cbapter 1 Introductiou

hydrothermal conditions, hence Si and Al atoms alternate throughout the structure and

the Si/Al ratio is 1. (There are a few contraindications to Lowenstein's rule, for

example CILt[A4012]S04 [16] and CILt[SjzAl40 12](OH)4 [17] which both have sodalite

frameworks but are formed at high temperature or under pressure.) The individual

T04 tetrahedra are generally close to regular, but the shared oxygen linkage can

accommodate a wide range of T-O-T bond angles from 120-180°. The sodalite and

cancrinite structures differ in the connectivity of these rings.

The sodalite structure was determined by Pauling (1930) [18] following a

single-crystal x-ray diffraction study of a natural sample. It was refined by L6ns and

Schulz (1967) [19]. The rings are connected to form a cubic truncated octahedron

(sodalite or ~- cage), Figure 1.4. These polyhedra are arranged forming a central

cage. Occupying the centre of the cage is the anionic species tetrahedrally coordinated

to four cations balancing the negative charge of the framework. For example, in

natural chloride soda1ite [NILtCl]3+, each cr is tetrahedrally coordinated to Na +. The

cations also interact with framework oxygen atoms. The framework can collapse via

tilting and deformation of the T04 tetrahedra to accommodate the wide range of

cation and anion sizes. Sodalite has an aperture dimension of 2.6A. Hence, small

species (e.g. Na+ [dwo,., 2.oA] and H20 [dmolec ,., 1.9A]) can slowly diffuse in and out

of the cages but anions such as cr ([d;on ,., 3.6 A]) and CI03- ([d;on ,., 4.0 A]) are

firmly trapped once inside the structure.

The cancrinite structure was determined by Jarchow (1965) [20]. It comprises

of small II-hedra1 E- cages which are linked to give a wide channel system, Figure

1.5. The internal free diameter of the cancrinite channel is 5.9A.

The structures may also be considered as consisting of identical hexagonal

layers formed by single rings of 6 tetrahedra. These are stacked in a way analogous to

closest packed spheres yielding the cubic symmetry of sodalite by an ABCABC ...

sequence and the hexagonal symmetry of cancrinite by an ABAB ... sequence.

11

Chapter 1

Figure 1.4 Sodahte or j3-cage

Figure 1.5 Cancrinite framework (anion positions not sbown)

12

Introduction

• AI! Si atoms

oxygen bridge between

an AI and Si atom

• Ai atoms

.. SI atoms

• o atoms

Naatoms

Chapter 1 Introduction

1.5 THE SCOPE OF THIS INVESTIGATION

The aim of this work is to:

• Fully characterise samples of scale from the Bayer process

1.6 REFERENCES

• Replicate its formation in the laboratory

• Understand the factors which control its formation

• Investigate the effect of calcium on the crystallisation of

sodium aluminosilicates and hence modify scale growth.

1 Personal communication

2 Aluminium for future generations "The UK Aluminium Industry and sustainable

development", booklet produced by Alfed Aluminium Federation Ltd, October

1998

3 The Aluminium Industry Worldwide, booklet produce by British Alcan

Aluminium Plc, March 1990

4 Alcan ... aluminium to the world, booklet produced by Alcan Aluminium Ltd.

5 The Sunday Times, 15 August 1999

6 Raw materials, booklet produced by Alcan Raw Materials Grp, Canada

7 Chapter 3- Alumina Production, L. K. Hudson, in Critial Reports on Applied

Chemistry Volume 20- Production of Aluminium and Alumina, edited by AR

Burkin

8 AR Gerson, K. Zheng. Journal o/Crystal Growth 171 (1997) 197-208

9 B.I. Whittington, B.L. Fletcher, C. Talbot. Hydrometallugy 49 (1998) 1-22


11 J. Addai-Mensah, AR Gerson, K. Zheng, A O'Dea, R St.C Smart. Light Metals

1997,23-28

12 http://www.iza.ethz.ch/izalRMB

13 RM. Barrer, J.F. Cole . .I. Chem. Soc. (A) (1970) 1516-1523

14 RM. Barrer, J.F. Cole, H. Villiger. .I. Chem. Soc. (A) (1970) 1523-1531

13

Chaoter 1

15 D.W. Breck. Zeolite Molecular Sieves, John Wiley & Sons, Inc, 1974

16 W. Depmeir. Acta Crystallogr. B44 (1988) 201

17 S.E. Dann, P.J. Mead, M.T. Weller. Inorg. Chem 35 (1996) 1427-30

18 L. Pauling. Z Krist. 74 (1930) 213

19 J. LOns, H. Schulz. Acta Cryst. 23 (1967) 434

20 O. Jarchow. ZKrist. 122 (1965) 407

14

Introdurtion

CHAPTER 2

EXPERIMENTAL TECHNIQUES

15

Chapter 2 Experimental Techniques

2.1 PREPARATION OF SYNTHETIC SCALE

This thesis describes the preparation and characterisation of zeolite scales.

Zeolites can be prepared at low tempemture and pressure using a strongly basic

aqueous solution containing tetmhedral building units, such as NaAl02 and Si02

together with a tern plating ion. The shape of the templating ion directs the

crystallisation of the aluminate and silicate tetmhedra and determines the structure of

the zeolite product.

However, the crystallisation process is extremely slow under low tempemture

conditions and can be speeded up using hydrothermal methods. This methodology

uses a sealed vessel to heat up the solution above its boiling point. During the course

of this work many experiments were performed using both test solutions and Bayer

test liquor in hydrothermal autoclaves. This more closely replicates the conditions of

the Bayer process plant which opemtes between tempemture of 90 -240°C.

The autoclave consists of a stainless steel jacket which encases a Teflon liner

containing the reaction solution. The Teflon liner has a capacity of 28 ml but can

only be filled to 16 ml for safety reasons. The stainless steel jacket is sealed around

the liner and the whole autoclave is heated in an autoclave oven. The oven has a

tempemture stability of ±2°C.

2.2 ANALYTICAL TECHNIOUES

2.2.1 Powder X-ray Diffraction (PXD)

X-ray diffraction is one of the principal techniques available to the solid state

chemist. PXD [1,2] can be employed in a variety of applications including crystal

structure determination, measurement of particle size, detection of crystal defects and

disorder and determination of phase tmnsitions. The main application of PXD in the

work was phase identification of samples in powder form and determination of cell

parameters.

16

Chapter 2 EIoerimental Techniques

2.2.1.1 Theory of X-ray Diffraction

A crystal may be divided into layers by sets of planes passing through lattice

points. Each of the planes are described by h,k,l (Miller Indices) which describe a full

set of planes running through a crystal structure separated by a perpendicular distance

called the d-spacing described by dw. The d-spacing, dhkl, can be related to both the

diffraction angle and the wavelength in a diffraction experiment using Bragg's Law.

In the simplest analogy, a crystal should display diffraction data from each lattice

plane which gives rise to an observed 29 value on the diffraction pattern.

However, since reflection conditions and systematic absences resulting from

the symmetry of the system can cause interference effects, intensity is not always

observed for all planes. In addition to absences arising from a non-primitive lattice

type, a number of space symmetry elements can lead to systematic absences. These

include glide planes and screw axes which apply to two-dimensional and one

dimensional sets of reflections respectively. These absences can be useful when

attempting to designate a space group for a new material.

2.2.1.2 The Diffraction Experiment

Powder x-ray diffraction (PXD) has been used generally throughout the course

of this work to assess sample purity. PXD data have been used to calculate refined

unit cell parameters for systems which were shown to be single phase. The powder

diffraction data were collected on a Phillips X50 diffractometer.

An x-ray tube fitted with nickel filter provides copper Ka radiation which is

collimated through an aperture onto the sample. The sample is mounted on grease on

a microscope slide. The diffracted x-rays are detected by a standard scintillation

counter. The sample rotates with constant angular velocity such that the angle of

incidence of the primary beam changes, while the detector rotates at double the

angular velocity around the sample. The diffraction angle is hence twice the glancing

angle. Collection times for diffraction patterns gathered to ascertain sample purity

were in the order of 30-60 minutes.

17

Chapter 2 Experimental Teehniques

Lattice parameters are calculated using the CELL program which minimises

the expression

using an iterative least squares procedure where ffij is a weighting factor proportional

to tan9.

The diffractometer operates in continuous scan mode which moves between

points without pausing, scanning continuously. A schematic diagram of the

diffractometer is shown in Figure 2.1.

Figure 2.1 Schematic Diagram of the Diffractometer

X-my tube

K~ Filter

/ ......................... 76 ........

Detector

Sample

18

Chapter 2 Experimental Tec:hnigues

2.2.2 Powder Neutron Diffraction (PND)

Powder neutron diffraction [3] is a powerful technique for the study of

complex materials. PND can allow the accurate determination of light atoms in the

presence of heavy ones and, since it is not affected by a form factor, has significant

advantages over the analogous x-ray method.

The applicability of neutrons to the diffraction technique is a result of a

number of properties including a wavelength comparable to atomic separation and an

intrinsic magnetic moment. The intrinsic spin of the neutron can interact with an

ordered spin arrangement and give rise to magnetic scattering, in addition to the

scattered intensity from the nuclear unit cell. This allows neutron diffraction to probe

local magnetic order in matter, but since the effect is based on the interaction between

the intrinsic moment of the neutron with the spin oriented electrons, it is affected, in a

similar way to x-rays by a form factor.

2.2.2.1 Instrumentation

Powder neutron diffraction measurements were carried out on the fixed

wavelength diffractometer TAS3 at RIS0, Denmark [4]. TAS3 is on a thermal

neutron beam line using a soller collimator assembly which allows a take off angle

from the monochromator crystal to vary between 36 and 110°. The monochromator

table can house two monochromators placed on top of one another. A motorised

vertical translation (elevator) is installed for selection of the particular

monochromator. For the experiments described in this thesis, a pyrolytic (002)

graphite monochromator composed of nine I cm high single crystal slabs, was used

which was vertically focussing. The monochromator elevator is mounted on a

motorised turn-table and each monochromator is placed in a RIS0 made holder. An

optical bench is provided at the monchromatic beam exit for accessories such as beam

monitors and apertures. The sample table is also RIS0 designed with a 500 mm

diameter turn table, an xy goniometer (340 x 340 mm, ± 15 tilt) and an xy translation

(340 x 340 mm, ± 15 tilt). This allows bulky and heavy equipment such as the

cryocooler to be supported.

19

Chapter 2 Experimental Techniques

The powder multi-detector arm is composed of two units. A sol\er col\imator

(lOO) with high transmission mounted on the sample table ofTAS3. This provides a

well-defined incident beam approximately 3 cm2. The second unit consists of an

additional, but simpler, table connected to the collimated detector bank of the powder

ann with 20 col\imator/detectors spanning 105°. The incident beam col\imator moves

with a precision of 0.001°. The detector bank moves with a precision of 0.01°

(heavier and more bulky). The resolution is "'did of 0.5% for scattering angles up to

115°. The twenty detectors are 2 x 10 cm2 side window helium detectors at a pressure

of 5 atm and an efficiency of 62% at 1.3A.

The low temperature experiments were perfonned using a cryocooler which

continuously circulates helium vapour in the evacuated chamber containing the

sample. This al\ows cooling of the sample to 10 K in c.a. 2 hrs. Samples were

mounted in both high and low temperature experiments in vanadium sample

containers. The sample containers were run as a blank and subtracted from each

experimental diffraction pattern. A schematic diagram of TAS3 is shown in Figure

2.2.

Figure 2.2 Schematic Diagram ofTAS3

Neutron Guide Tube

MonodJromating graphite crystal >-----,--

122"

/ 60"

Sample

20

Chapter 2 Experimental T""hniques

2.2.3 Tbermogravimetric Analysis (TGAlDTA) [5]

Studies of thermal stability of scale and laboratory samples were carried out

on a Stanton Redcroft STA 1500 at Alcan International Ltd.

A direct plot of weight versus temperature for any sample over the

temperature range, room temperature to 1500°C can be produced. The electronic

microbalance has a reproducibility of 0.5!lg with a maximum load of 100 mg. The

sample is contained with an a1umina crucible and heated with reference to a standard.

The samples were in all cases heated in alT. Typically 20 mg were used and

experiments were repeated in duplicate.

Figure 2.3 Schematic Diagram of the Thermogravimetric Analyser

~

Fum."" Windiog

" ( l ...

I,\~

;: "-. ::: r:: ~ ~ II

~ ,...

"

r;:W f-'

t:: :;r---::::

,/

~ ....

+0- G •• in r

ELECTRONIC

MICROBALANCE

Counler Pan

Sample Crucible

FURNACE

Thermocouple

21

Chsoter2 EIoerimental Techniques

2.2.4 Infra-Red Spectroscopy (IR)

Infra-red [6,7] and Raman spectroscopy are both concerned with the change in

vibrational state of a molecule. The atoms in molecules and solids are held together

by bonds_ The frequency at which the bond vibrates is related to the mass of the

atoms and the strength of the bond between them. Hence heavy atoms held together

by weak bonds vibrate at lower frequency than lighter atoms or those held together by

multiple bonds. For many bonds, the frequency of these vibrations occur in the infra

red region of the electromagnetic spectrum 300-4000 cm-I

For a vibration to be infra-red active (and observed in the infra-red spectrum)

there must be a change in the electric dipole of the molecule. For example, the

symmetric stretch of nitrogen would be infra-red inactive but the same stretch for

carbon monoxide would be active. In fact, the latter is used diagnostically in the

characterisation of metal carbonyls.

In this thesis, two major groups of zeolites have been studied by infra-red

spectroscopy using small samples dispersed in potassium bromide disks. All zeolites

have a number of stretching and deformation modes in the region 400 - 1200 cm-!

which correspond to framework absorptions: stretch 600-1000 cm-! and

bend/deformation 300-600 cm-I.

The infra-red spectra of the aluminosilicate sodalites have been modelled

extensively by Creighton et al. [8]. These authors showed that of the fourteen

expected IR active modes only eight had detectable intensity and only four of these

were experimentally observed in the active region. The frequency of these bands

have been shown to linearly correlate with cell parameter, which, in turn, is close1y

related to the size of the species incorporated into the sodalite cage. The sodalite cage

is very flexible allowing incorporation of a wide range of different anions with

different shapes from simple monoatomic ions, e.g. Cl, to tetahedral units such as

molybdate. The IR absorptions of the occluded anions can also be used to distinguish

different types of sodalite species.

22

Cbapter2 Experimental Tec:bniques

In contrast, the infra-red spectra of the cancrinite cage is more complex, with

eight expected IR modes. Although the sodalite and cancrinite structures are related,

where both are constructed from aluminium and silicon tetrahedra, the three-fold

symmetry of the framework is reflected in the symmetry of the occluded ions in the

case of the cancrinite. For example, the most common cancrinite structures contain

carbonate, nitrate and sulphate.

The different IR absorptions can be used to used to differentiate between the

two structures. A summary of the different expected absorptions is given in Table

2.1.

Table 2.1 IR of Sodalite and Cancrinite Asymmetric Stretcb of Symmetric Stretch of

cm·1 framework I cm·1

2.3 REFERENCES

1 C. Whiston. X-Ray Methods, John Wiley & Sons, Inc, 1987

2 EAV. Ebsworth, D.w.H. Rankin and S. Cradock. Structural Methods In

Inorganic Chemistry, B1ackwell Scientific Publications, 1991

3 G.E. Bacon. Neutron Diffraction, Clarendon Press, 1975

4 http://www.risoe.dk

5 R.B. Braun. Introduction to Instrumental Analysis,

6 J.H. van der Maas. Basic Infrared Spectroscopy, Heyden & Son Ltd, 1972

7 A.K. Brisdon. Inorganic Spectroscopy Methods, OUP, 1998

8 JA Creighton, H.W. Deckman and J.M. Newsarn. J. Phys. Chem. 95 (1991)

2099

23

CHAPTER 3

ANALYSIS OF BAYER PLANT SCALE

24

Chaoter Three Analysis of Bayer Plant S!:ale

3.1 INTRODUCTION

This chapter is concerned with the analysis and full characterisation of scale

formed in Bayer process plants.

Although the formation of scale throughout Bayer plant equipment causes

serious problems that are very costly, there is very little in the literature examining the

exact nature of plant scale samples and rationalising the phases observed with the

conditions of formation. Previously due to the lack of phase identification, all sodium

aluminosilicate scale was simply given the general term 'Bayer sodalite'.

Gerson et al. investigated scale formed in a Bayer Process plant using powder x-ray

diffraction [1,2]. The Bayer plant was divided into three regions in terms of the scale

examined: digesters, low temperature heat exchangers and high temperature heat

exchangers. Scale precipitated during digestion of bauxite was found to be mainly

cafetite and a small amount of haematite. Analysis of a sample of low temperature

heat exchanger scale sample revealed boehmite was the main phase. Three sodium

aluminosilicate phases: sodalite\, sodalite2 and cancrinite were observed in the high

temperature heat exchanger scale. Haematite (Fe20l) in the form of banding was also

observed. It was reported that the relative concentrations of sodaliteI, sodalite2 and

cancrinite was dependent on both temperature and in situ ageing. The proportion of

cancrinite in the scale samples was observed to increase with increasing temperatures

of formation. The following ageing mechanism was proposed:

Sodalite\

(high COl" concentration)

a= 9.077(2)A

Sodalite2

(Iow COl2" concentration)

a= 8.988(1)A

~ Cancrinite

The initial sodalite phase (sodalite\) contains a high concentration of carbonate.

Carbonate diffuses out of the sodalite lattice to give a distinct second sodalite phase

(sodalite2) which then transforms to cancrinite. This loss of carbonate from the scale

could not be verified. The transformation from sodalite2 to cancrinite was shown to

be the rate-detennining step.

25

Chapter Three Analysis of Bayer Plant SgIe

An internal report examined various scale samples obtained from the San Ciprian

Bayer Plant, Spain by scanning electron microscopy [3]. The majority of the samples

analysed were digester scale which is not within the scope of this investigation. The

major phases which crystallised during digestion were not sodium aluminosilicates.

However, there was an analysis of a sample of scale from the heat exchangers in an

organic evaporator. This was identified as cancrinite. It is reported the crystal forms

of cancrinite were individual rods, aggregates or bundles of rods, blocky prisms and

possibly irregular shaped plates. There was no sodaIite phase present. The report

identifies the need for studies to be carried out to establish the conditions favouring

formation of sodaIite and cancrinite and under what conditions will sodaIite transform

into cancrinite.

Roach describes two basic mechanisms for scale formation- growth scale and settled

scale [4]:

Growth scales form by nucleation of the supersaturated phase on pipe or tank

walls and subsequent growth of those nuclei. For nucleation, the two critical factors

are the degree of supersaturation and the form of the surface. A higher degree of

supersaturation and surfaces of similar crystal structure favour nucleation. After

cleaning there is often sufficient scale remaining to act as nuclei for growth. Also,

acid used in chemical cleaning can attack the pipes causing pitting which provides

ideal nucleation sites. The higher the temperature the faster the rate of growth of these

nuclei even though the supersaturation is less; hence high temperature heat

exchangers scale most rapidly.

Settled scale forms in slurries; the slurry particles settle out and are cemented

by the supersaturated liquor. It occurs in low velocity flow regions or during

shutdown.

26

------------------

Chapter Three Analysis of Bayer Plant Scale

3.2 EXPERIMENTAL

Scale samples fonned in the Bayer process were obtained and analysed using

powder x-ray diffraction, Fourier transfonned infrared spectroscopy, SEM imaging

and thennal analysis. The acquisition of the samples was not ideal:

• They were collected randomly so there is not a representative sample of scale

fonned at every stage within the process;

• The precise origin and hence the conditions under which the scale fonned is

unknown in some cases;

• The in situ age of the samples is unknown;

• Some of the samples were collected many years ago and how they were stored

is unknown.

The scale samples were classified into two categories based on the temperature at

which they were fonned, i.e. scale fonned at approximately 135°C is referred to as

low temperature scale whilst high temperature scale refers to scale fonned above

170°C. Tables 3.1 and 3.2 give the origin of the low temperature and high

temperature scale samples, respectively. The high temperature scale samples, with

the exception of # 12, all originate from high temperature heat exchangers.

Table 3.1 Identification of Low Temperature Scale Samples.

Sample Origin of Scale

Identification Vaudreuil 22 April 1999

#4B From a tube which had failed due to corrosion #4C (no details known)

LT Pipe Section of a scaled pipe from a region of the plant operating at -135°C.

27

Chapter Three Analysis of Barer Plant Scale

Table 3.2 Identification of High Temperature Scale Samples

Sample Origin of Scale

Identification #1 Heater 78, November 1987, from the heater header (l85°C) #2 Heater 98, November 1987, from the heater outlet (220°C) #3 Heater 98 November 1987, from the inlet header (205°C) #4 Heater 6A, July 1988, from the top of heater header (1 70°C) #6 From spent liquor line into digesterNo.1 October 1991, (220°C) #7 Heater 98, November 1992, from blocked tubes on the top of the

return heater (220°C) #8 Heater tube, unspecified location #9 Heater tube, unspecified location #10 AAL heater 9L Scale built up in heater header 12/11198 #ll High temperature heater scale AAL heater head (6C) return

24/11198 #12 AAL digester (2) scale 24/11198

The scale samples were finely ground for analysis. A homogeneous powdered

sample of specimens that clearly showed banding was used in the analysis.

FTIR spectra of the scale samples were recorded using a Perkin-Elmer 2000

Fourier-Transform spectrometer over the range 4000-220cm-1 using pressed

potassium bromide pellets.

Powder x-ray diffraction patterns were recorded usmg a Phi lips X50

diffractometer operating with CuKa radiation with a scan rate of 1° 29/minute

between 10-60° 29.

The elemental composition and morphology were examined using EDAX and

SEM, respectively.

28

Chanter Three AnalYsis of Bayer Plant Scale

3.3 RESULTS AND DISCUSSION

3.3.1 Cbaracterisation ofHigb Temperature Scale using FTIR Spectroscopy, X-ray Diffraction and EDAX Analysis

A typical ITIR spectrum is shown in Figure 3.1a. Table 3.3 summarises the FTIR

data of all the samples. Comparing the data with samples of synthetic sodium

aluminosilicates prepared in the laboratory (Table 3.4) at 220°C shows that these high

temperature scale samples are primarily cancrinite based. The samples showed strong

absorptions at 1450cm-1 and 850cm-l, which correspond to an asymmetric stretching

vibration and a bending mode of the carbonate anion, respectively. All the samples

were hydrated which was indicated ~y board bands in the 3440-3530cm-1stretching

region and 1640cm-1 in the bending region. The water molecules have a strong

influence on the position of the carbonate asymmetrical vibration in the 1410-

1450cm-1 range. The band shifts towards higher wavenumber and develops a clear

shoulder at 1410cm -I

Table 3.3 FTIR Absorptions for High Temperature Scale

Sample Asymmetric Stretcb of Symmetric Stretcb of framework C032- anioD framework I cm-1 I cm-1 I cm-!

#1 1108 1036 1007 954 762 683 622 573 1458 867 #2 1108 1032 1005 960 759 682 622 573 1455 850 #3 1108 1036 1008 957 762 681 622 573 1458 866 #4 1107 1033 1003 966 761 684 622 572 1459 -#6 1107 1033 1003 963 761 683 622 571 1461 850 #7 1107 1035 1007 958 761 681 622 573 1457 849 #8 1110 1033 1008 962 761 683 623 572 1458 847 #9 1111 1033 1007 963 761 684 623 572 1458 850 #10 1109 1031 1007 960 761 683 622 573 1454 -#11 1113 1031 1005 965 684 621 572 #12 1113 1026 1009 965 679 618 560 1449

29

Chapter Three Analysis of Bayer Plant !kale

Figure 3.1a A typical FTIR Spectrum of High Temperature Scale (Scale Sample #3)

4000 3000 2000 1500 -I cm

1000

Figure 3.1b FTIR spectrum of Synthetic Carbonate Cancrinite

4000 3000 2000 1500 1000 cm-I

30

500

500

Cbapter Three Analysis of Bayer Plant Scale

Table 3.4 FTlR Absorptions for Laboratory Prepared Phases

Asymmetric of Symmetric

1101 1031 1004 965 761 684 622 572 ll48

1117 1036 991 762 685 622 575 1424

Powder x-ray diffraction of high temperature scale gave the expected pattern

for cancrinite [Ref: JCPDS 20-0257]. A typical PXD pattern is shown in Figure 3.2a

and the data for that pattern given in Table 3.5. There is a high degree of overlap

between the sodalite and cancrinite x-ray diffraction patterns. The presence of

cancrinite is identified by the 101 diffraction peak, at 19° when using CuK, radiation,

sodalite does not have a diffraction peak at this particular d spacing. The PXD

patterns of all the samples of high temperature scale analysed had this characteristic

cancrinite 101 diffraction peak present.

Table 3.5 Powder X-Ray Diffracton Data for High Temperature Scale Sample #6

31

'" IV

Figure 3.2a Typical Powder X-ray Diffraction Pattern of High Temperature Scale (Scale Sample #6)

c

J • u

" t • 6 J

400~

2OOi, ._J-J~Jl",~l~jl __ jl_,. __ tU __ uL._~JLG,-'-_J\'-°ITI''"TT1--r-r--n-rn--l"Tr''~!--''TTI''''''''''''''-Tl. j. i i i i i i! nr-T":-:-r:,rT!I'T-r:-rr;-"Trrnrnr'l~-:rn:

10 1S :!o 25 30 3S 40 .. 5 so 55 eo

Figure 3.2b Powder X-ray Diffraction Pattern of Synthetic Carbonate Cancrinite

c o u

" t

•

"""'I J

6OO,~

j """-

200.]

o·t 10 15 20 25 30 :'5 40 45 !O 55 !SO

OltgNth 2· Th.1I

> "

..

." ;r la I~ , ..

Chanter Three Analysis of Bayer Plant Stale

Using PXD data lattice parameters of the scale samples were calculated using the

computer program 'CELL' and are given in Table 3.6. The calculated cell parameters

are consistent between samples and compare favourably with the literature a = 12.70

and c = 5.18A [2]. The cancrinite structure can accommodate a considerable amount

of water and differences in the degree of hydration between samples would explain

the slight variation in lattice parameters seen.

Table 3.6 Calculated Lattice Parameters of High Temperature Scale Samples

SAMPLE a (A) ceA) 1 12.643(2) 5.153(2) 2 12.682(20) 5.170(10) 3 12.693(4) 5.177(4) 4 12.598(21) 5.148(13) 6 12.681(6) 5.176(4) 7 12.660(1.5) 5.168(10) 8 12.715(18} 5.198(9) 9 12.700(20) 5.161(12) 10 12.647(21) 5.115(24} 11 12.631(20) 5.145(17) 12 12.609(21) 5.140(24)

The results ofEDAX analysis of the high temperature scale samples are given

in Table 3.7. A disadvantage of EDAX is elements below sodium cannot be

determined. Therefore the amount of carbon in the samples could not be measured.

Table 3.7 EDAX Analysis of High Temperature Scale

Na AI Si S Ca Ti V Fe Cu K #1 8.59 35.88 48.09 0.98 1.79 1.57 0.17 1.69 0.82 0.43

#2 8.14 38.94 47.59 0.11 1.44 0.63 0.03 2.10 0.75 0.29

#3 11.03 30.82 30.57 0.41 3.30 3.96 0.02 19.18 0.28 0.40

#4 7.26 34.39 52.80 2.00 1.22 0.77 0.04 0.83 0.33 0.36

#7 10.04 34.61 41.26 3.02 2.94 2.54 0.21 3.72 0.64 1.02

#8 7.96 35.78 52.19 0.00 0.35 0.05 0.06 1.88 0.49 1.23

#9 6.63 36.32 54.53 0.00 0.34 0.09 0.06 0.47 0.66 0.89

#10 8.50 32.79 47.87 7.95 0.68 0.42 0.04 0.71 0.05 1.00

#11 7.97 33.98 45.35 6.64 0.66 0.63 0.04 4.00 0.10 0.64

#12 7.17 38.17 12.60 1.25 7.86 8.12 0.02 24.05 0.10 0.56

33

Chanter Three Analysis of Daver Plant Scale

The natural effect of dehydration was very apparent in scale sample # 11. The

sample was collected more recently than the majority of the other samples, and hence,

when it was analysed, contained a lot more water. It was kept on a bench in the

laboratory, i.e. -25°C and under normal atmospheric pressure and conditions, and

over a period of a few months it began to dehydrate. The scale was initially a stratified

lump, approximately 45mm thick, which faulted along definite planes and separated

into three individual layers upon dehydration. The layers, which have a rippled

surface caused by caustic flow, represent scale growth between acid washings. There

was a white powder on the surface of the layers. This powder was analysed using

FTIR, PXD and thermal analysis. The PXD analysis, Figure 3.3 and Table 3.8,

revealed that it was crystalline and composed mostly of gibbsite- aluminium oxide

hydrate, Ah03.3H20 [JCPDS reference number 01-0264]. There was no evidence of

any gibbsite in the reddish brown bulk scale material from FTIR and PXD data.

However, thermal analysis detected a small gibbsite impurity, indicated by the peak

just below 300°C, Figures 3.4 and 3.5. A sample of this bulk scale material was

exposed to sulphuric acid (conc) for 20 minutes and then a PXD pattern recorded.

Although it was not as crystalline as the white powder, indicated by the high

background and low peak intensities, it was still identified as aluminium oxide.

Therefore, it can be assumed that during the acid washings the acid only penetrated

the surface of the carbonate cancrinite scale that had built up. The surface scale

reacted with the acid to deposit aluminium oxide which was not all washed away with

some remaining at the surface on which new scale growth occurred after washing had

ceased.

34

w v.

Figure 3.3 Powder X-ray Diffraction Pattern of White Powder found between Faults in Scale Sample #11

c o u n t s

800-

600-

400··

i

I 1

200

· r:~;0::) Vb~~-:;i~~:-:c;~~, 10 15 20 25 30 35 40 45 SO 55 60

Degre" 2-Thl!lta

19 ..

'> =

,,, ~

.."

if ,~

f('

~

'" '"

Figure 3.4 Thermal Analysis of White Powder found between Faults in Scale Sample #11

105

~OO

95

!Ill

<J C Cl u B5

'-Q ~ eo

'5

70

65

60

~ 0 1.00 200 .700 300 400 500 600

;leg C

I 800

s

0

-5

-iO

-!5

-20

-25

-30

-35

··40

-45 .~

~ RESIDUE 62.95 ill

-50 I I

BOO 1000 HOO

(J) .... rl 0 ;, 0 C-u ~.

:E

1(") ,or , .. 1;-., ';I I~

> = ,i!l.

Iq ." I;; la 'I(' I.

Figure 3.5 Thennal Analysis of Scale Sample # 11 'n !Q" :D'

!02 0

"2

100

-4

98 -6 D) ..

.' ~I

c Cl 96 u (. Cl n.

94

-8 rl

0 ;,. 0

-!O t.. u ... X

-12

92 -14

-~6

90 -1B

as

RESIDUE 89.36 :l

o 200 300 400 500 800 Jeg C

700 800 900 1000 100 ; -20

~ 1!00

,~ '",

Chapter Three Analysis of Bayer Plant !kale

Table 3.8 Powder X-ray Diffraction Data for White Powder between Faults in Scale Sample #11

29 Angle d-space Relative Intensity

18.46 4.806 100 20.48 4.337 55 36.66 2.451 15 37.82 2.379 18 41.84 2.159 9 44.36 2.042 14 45.60 1.989 11 47.58 1.911 7 50.72 1.800 9 52.36 1.747 13 54.62 1.680 8

3.3.2 Characterisation of Low Temperature Scale using FTIR Spectroscopy and Powder X-ray Diffraction

Firstly, a note on the Vaudreuil sample of low temperature scale. X-ray

diffraction analysis identified it was very crystalline alwninium oxide (gibbsite). It

cannot be established with certainty whether this phase was precipitated during the

process or is a result of chemical cleaning, as discussed above. It is believed the latter

is the case and hence this sample is not a true representation of low temperature scale

and therefore will not be referred to again.

Analysis of the scale taken from the section of pipe, identified a phase that has

a structure intermediate between that of sodalite and cancrinite [5,6]. Sodalite and

cancrinite both have highly ordered three dimensional framework structures whereas

the intermediate phase has a strong one-dimensional stacking disorder. The structure

of cancrinite exhibits a system of wide channels, which in the intermediate phase

appear to be blocked forming cages of various sizes. The FTIR spectrum is shown in

Figure 3.6. All the vibrations in the intermediate phase coincide with cancrinite with

the exception of the symmetric framework stretch at 76Icm-1, which is missing in the

intermediate phase. The powder x-ray diffraction pattern is given in Figure 3.7. It

shows only reflections that are common to both soda1ite and cancrinite, Table 3.9.

This is not a mixture of sodalite and cancrinite otherwise the 10 I reflection used to

38

Chapter Three Analysis of Bayer Plant Scale

identify cancrinite would be present; it is definitely a distinct phase from both

sodalite and cancrinite.

Figure 3.6 ITIR Spectrum of Scale inside section of Pipe at Low Temperature

4000 3000 2000 1500 1000 500

Table 3.9 Powder X·ray Diffraction Data for Scale inside section of Pipe at Low Temperature

29 Angle d-space Relative

h kl Intensity 13.94 6.353 53 100 24.22 3.675 100 110 34.48 2.601 45 001 42.62 2.121 34 111/300

39

~

Figure 3.7 Powder X-ray Diffraction Pattern of Scale inside section of Pipe at Low Temperature

~ 500-1 n . t . • 1

400~ j j

300-]

j j

2OQ-.!

~ I I

1001. 1 . \ rl"lW ... .Jo'.~~'""~.,'~\ yJ ';IW·:.I·tJ,",,,,,,\...y.I~f~' '~''''~'_...A,.....""A''''~

o-f-r-r--r-....--7-r--r-T'"""'I'f-r-r-:--,-....,.....,...-- i I Ti-T,-r-, , I I -.--. I I T ........-y-. I I

5 15 25 35 45 55 65 Degrees 2~ Theta

I~

> Ig

!;;'

Chaoter Three Analysis of Bayer Plant Sc:aJe

FTIR spectroscopy and powder x-ray diffraction of low temperature scale

samples #48 and #4C revealed there was some cancrinite in the samples which made

it impossible to identil'y the presence of the intermediate phase.

3.3.3 Scanning Electron Microscopy Study of Low and High Temperature Scale

SEM imaging of samples of low and high temperature scale, Figure 3.8,

showed pure cancrinite and the intermediate phase have different morphologies. Pure

cancrinite forms hexagonal rods, Figure 3. 8b, whereas the intermediate forms plate

like crystals, Figure 3.8a.

3.3.4 Thermal Analysis of Low and High Temperature Scale

Low temperature scale sample #4C and high temperature scale sample #9 were

heated to IOOO°C at a rate of lOoC/min in air. In both cases there was a gradual loss

of water, Figures 3.9 and 3.10, respectively. There was a 19% weight loss associated

with the high temperature scale and only 8% with the low temperature scale. The

peak at -2S0°C in the high temperature scale plot is due to a small amount of gibbsite

present in the sample, which was not detected by either FTIR spectroscopy or powder

x-ray diffraction. Due to this gibbsite impurity the number of water molecules

associated with the formula cannot be calculated.

Powder Neutron Diffraction Study

Powder neutron diffraction data for dehydrated and deuterated scale samples

were collected on T AS3. Data analysis is in progress.

41

.... IV

Figure 3.8a SEM Images of Low Temperature Scale

X5,OOO X10,OOO

Figure 3.8b SEM Images of High Temperature Scale

X5,OOOO X7,OOO

:n ,'" .. Iq ;l I~

> =

... w

Figure 3.9 Thermal Analysis of Low Temperature Scale Sample #4C

101

100

99

98

.' c: III tJ

97 Co III n.

98

95

94 -l \ . ../- '"V\u. If' ...... "--

93 '-...I'

92

0 100 200 300 400 SOD SOD iJeg C

I~

2

0

-2

-4 U)

" rl 0

-6 > 0 L U ...

-8 :E:

-:'0

~ -!2

f- -!,4

RESIDUE 92.38 " -~6

" 0 BOO 900 1000 HOO

Figure 3.10 Thennal Analysis of High Temperature Scale Sample #9

102

~oc

98

96

94 .... c

tl <:I 92 u t. Cl n. 90

BB

BS

84

B:!

80 ~ 0 100 200 300 400 500 600 neg c

2

0

-2

-.\

-6

-a

-10

-!2

RESIDUE as. 01 x -14

700 BOO 900 1000 1100

Ul .... M

0 ;, 0 t. u ... l:

19 Itt ;l li

> =

,a :~ "

Chapter Three Analysis of Baver Plant Scale

3.4 CONCLUSIONS

An extensive analysis on scale formed in the Bayer process has been

performed. It has been round that scale formed in low temperature regions of the

plant is mainly intermediate phase whereas in high temperature regions it is mainly

cancrinite. It has been shown from FTIR spectroscopy data that carbonate anions are

enclathrated into the structures. No transformation of sodalite to cancrinite was

observed as suggested in the literature. This investigation concludes that the

aluminosilicate phase observed is totally dependant on the temperature of formation

and not the period of in situ ageing.

3.5 REFERENCES

1 AR. Gerson and K. Zheng. Journal o/Crystal Growth 171 (1997) 209-218

2 l Addai-Mensah, AR. Gerson, K. Zheng, AO'Dea and R. st. Smart. Light Metals

(1997) 23-28


4 G.LD. Roach. J.B.J. Journal 12 (1994) 22-34

5 G. Hermeler, l-Ch. Buhl and W. Hoffmann. Catalysis Today 8 (1991) 415-426

6 K. Hackbarth, M. Fechtelkord and 1. --Ch. Buh!. React. Kinet. Catal. Lett. Vo!. 65

No.1 (1998) 33-39

45

CHAPTER 4

SYNTHESIS OF SCALE

46

Chapter 4 Synthesis of Scale

4.1 INTRODUCTION

The previous chapter was concerned with the detailed characterisation of samples

of scale fonned within the Bayer process, three sodium aluminosilicate phases:

sodalite, cancrinite and an intennediate phase, were clearly identified. The next stage

was to replicate the synthesis of these phases in the laboratory under similar

conditions to those in the Bayer process. In order to achieve this, both pure sodium

hydroxide and spent Bayer liquor were used as the reaction solution. The reaction

parameters which strongly influence the phase fonned, such as temperature, reaction

concentration and time, were also investigated.

Several authors have investigated the effect of reaction conditions on the

solubility of silica in Bayer liquors. However, there are many inconsistencies between

the results obtained by these authors.

The effect of alumina concentration on the solubility of silica and precipitation of

scale has been investigated by many authors. Experiments [1-4] suggested that the

solubility of silica increased with increasing dissolved alumina concentration.

However, Kotte [5] and Pevzner et al. [6] contradicted these experiments, concluding

that the solubility of silica decreased with increasing alumina concentration. In

contrast, Cresswell [7] found that a1umina concentration had a negligible effect on the

solubility of silica. Cresswell's results were partially confinned by Jamialahmadi and

Miiller-Steinhagen [8]; they reported silica solubility increased with increasing

alumina concentrations at a1umina concentrations above 50 kg/m3, but at

concentrations less than this the effect was negligible.

The effect of sodium hydroxide (caustic) concentration has also been extensively

studied. Solubility of silica was shown by Ostap [4] to increase with increasing

sodium hydroxide concentration. The results of Jamialahrnadi and Miiller-Steinhagen

[8] were in agreement with Ostap, except at sodium hydroxide concentrations of less

than 50 kg/m3 where the low sodium hydroxide concentration had a deleterious effect

on the solubility of silica in Bayer liquor.

47

-------- - - --

Cbaoter4 Synthesis of Scale

Similarly, the effects of temperature on silica solubility are also under debate. Ostap

[4] and Breuer et al. [9] reported silica solubility increased with increasing

temperature. However, Oku and Yamada [1] reported no temperature dependence,

up to 150°C, in the desilication rate of Bayer liquors.

The purpose of this chapter is to examine the effect of the different anions on the

type of zeolite phase formed under conditions analogous to those generated during the

processing of alumina in the Bayer process.

4.2 EXPERIMENTAL

The conditions of the Bayer process were mimicked as closely as possible in the

laboratory experiments. Four sets of experiments were performed to investigate the

effects of:

• type, symmetry and competition of anions;

• sodium hydroxide concentration;

• reaction time;

• reaction temperature.

In all these experiments a number of factors were constant. The major silica

contaminant in bauxite, kaolinite- Ah03.2Si02.2H20 (A1drich), was used as the silica

source. All the reactions were carried out hydrothermally using parr® Teflon-lined

steel autoclaves (23mL volume). In all the experiments the microcrystalline powders

obtained were washed with distilled water and dried at approximately 100°C. The

products were then characterised using FTIR spectroscopy and powder x-ray

diffraction. Unit cell parameters were calculated from XRD data using the computer

programme 'CELL'.

48

Chapter 4

4.2.1 THE EFFECT OF ANION TYPE. SYMMETRY AND COMPETITION ON PHASE FORMATION

4.2.1.1 Anion Type and Symmetry

Synthesis or Scale

Bayer liquor contains a variety of different anIOns which originate from

bauxite ore and cleaning processes. In addition, organic anions such as oxalate and

formate accumulate in the liquor by the oxidation of organic material present in the

bauxite. These anions interact with the dissolved silica and alumina species, by acting

as templates for the formation of aluminosilicates. In order to determine the crucial

anions responsible for formation of scale, the major contaminant anions were used in

laboratory based experiments to mimic the formation of scale.

A typical experiment involved placing kaolinite (1.5 x 10-3 moles) and the

sodium salt of the anion (1.24 x 10-2 moles) in the hydrothermal bomb, adding the

sodium hydroxide solution (8M) and placing the bomb in a preheated oven at 175°C

for 48 hours. N.B. a 8M solution of sodium hydroxide was initially chosen as this is

the molarity used in the Hund method [10].

4.2.1.2 Corn petition of Anions

Bayer liquor is a complex mixture of cations and anions. The previous

experiments were based on pure solutions of one anion dissolved in sodium

hydroxide. In order to mimic the competition provided by mixtures of anions, as in

. Bayer liquor, experiments were also performed with mixed anion solutions. For

example, equimolar quantitieS of sodium carbonate and sodium acetate were placed in

the reaction mixture.

49

Chaoter4 Synthesis of Scale

4.2.2 EFFECT OF SODIUM HYDROXIDE CONCENTRATION

The sodium hydroxide concentration in the Bayer process is variable due to

carbon dioxide absorption from the atmosphere, according to the equation:

Hence during the circulation of the liquor sodium hydroxide concentration varies

between 2-5M. In section 4.2.1 a sodium hydroxide concentration of 8M was used.

This set of experiments investigates the effect of varying sodium hydroxide

concentration on phase formation. 2, 4, 6, 10, 12, 14, 16 M solutions of sodium

hydroxide were used in a typical hydrothermal bomb experiment.

4.2.3 EFFECT OF REACTION TIME AND AGEING

Gerson et al. [11] proposed that over a period of time sodalite transforms to

cancrinite in-situ. This occurs according to the following equation:

Sodalite}

(high CO/- concentration)

a= 9.077(2)A

Sodalite2

(low CO/- concentration)

a= 8.988(I)A

~ Cancrinite

This implies a decrease in carbonate concentration encapsulated in the structure as

time elapses and a change structural morphology. In order to investigate these

findings, two different sorts of experiments were performed:

• the reactions were conducted over a range of time periods from 1-168 hours;

• products were aged in different solutions.

so


4.2.4 THE EFFECT OF TEMPERATURE

The effect of temperature on the type of phase formed from carbonate,

sulphate, oxalate and chloride- rich solutions was examined. The temperature range

studied was 90-225°C.


The effect of anion on phase formation is summarised in Table 4.1.

Table 4.1 Effect of Anion Type on Phase Formation

The symmetry of the anions exhibited a controlling influence on the phase formed.

Zeolite structures all form in a similar way, where the cage is formed around an anion

or molecular species using the shape of the anion as a template [12,13]. For example,

anions with three-fold symmetry are known to template the cancrinite structure

(Figure 4.1) such as nitrate, carbonate and sulphate due to the P63 symmetry of the

structure. Such restrictions are not imposed on the sodalite cage as it has a cubic

structure. As can be observed from Table 4.1, anions with three-fold symmetry form

the cancrinite structure under these conditions. Ions with three-fold symmetry have

only been shown reside in the sodalite structure when generated by post synthesis

51


reaction [14-16]. For example, nitrate sodalite can be generated by oxidation of nitrite

using oxygen gas at 400°C.

Figure 4.1 Cancrinite Structure (showing carbonate anions enclathrated into the framework)

52


IR absorptions of these phases are summarised in Table 4.2. Example spectra are given in Figures 4.2 and 4.3 for sodalite and cancrinite, respectively.

Table 4.2 IR Absorptions of Sodalite and Cancrinite Phases

ASYMMETRIC STRECB OF SYMMETRIC STRETCH OF ALUMINOSILICATE FRAMEWORK ALUMINOSILICA TE

I cm-' FRAMEWORK I cm-' HYDROXY 982 736 710 666 CHLORIDE 979 737 712 669 ACETATE 991 735 708 664 FORMATE 993 734 708 664 OXALATE 990 736 710 665 ~.;;w;~~~~~~~~~~~ CARBONATE 1113 1036 996 970 688 625 573 SULPHATE 1102 1031 1002 965 685 622 572 NITRATE 1117 1036 991 685 622 575 PHOSPHATE 1004 684 624 564 VANADATE (V) 1110 1003 684 623 566 MOLYBDATE 1113 1031 1008 965 681 623 571

The framework bands of sodalite are more sensitive to the occluded species than those

of cancrinite. The bands shift to lower frequency as the size of the occluded anion

increases. This is due to the conformational flexibility of the sodalite framework

where different size anions can he occluded by rotation about the T -0-T bond angle to

allow better coordination of the framework oxygens to the cations. In contrast, the

cancrinite bands show no variation with anion-type due to the much larger size of the

channels in relation to the occluded species.

The synthesis and structural characterisation of sodalites with acetate and formate

guest anions is has already been reported in the literature [14]. The unit cell

parameters reported are a = 9.077(2) A and 8.960(2) A, respectively which are in

good agreement with the values quoted in Table 4.1. These authors also report that

carbonate sodalite cannot be formed directly, but can be generated by annealing

acetate and formate sodalites in air or oxygen at 1123 K and 923 K, respectively.

In general, all the organic specIes resulted in the formation of sodalite phases.

Experiments to investigate the competition between inorganic (carbonate) and organic

(acetate) anions using different concentrations of the two anions showed the inorganic

53

'" ..

Figure 4.2 A typical FTIR Spectrum of a Sodalite (Formate Sodalite)

%T

4000 3000 2000 1500 cm-I

I~ l~ ..

'Cl)

1000 500

'" '"

Figure 4.3 A typical FTIR Spectrum of a Cancrinite (Molybdate Cancrinite)

16T

4000 3000 2000 1500 cm-!

1000

.. ,~ ...

I'"

500

'!. ..

Chaoter4 Synthesis of Scale

species was dominant. This could be observed by the formation of the cancrinite

phase rather than the sodalite phase down to concentrations of 10-4 M.

The effect of changing the sodium hydroxide concentration on phase formed with

constant sodium carbonate anion as the templating agent is given in Table 4.3.

Table 4.3 Effect of Sodium Hydroxide Concentration of Phase Formed

NaOH Cone. (M) Phase Formed 2 Cancrinite 4 Cancrinite 6 Cancrinite IO Cancrinite 12 Cancrinite 14 Cancrinite 16 Sodalite

Hydroxy (or basic) cancrinite is reported in the literature [l7]. However, totally

carbonate-free hydroxycancrinite has not been observed in these or any of the other

experiments, and it is questionable whether it can be synthesised. Sodalite

consistently formed, irrespective of temperature, whenever there was only hydroxy

anions in solution acting as template. It has been demonstrated that the age of the

sodium hydroxide solution used can cause a phase change from sodalite to cancrinite.

If the solution used is freshly prepared, hydroxysodalite is formed from a mixture of

just kaolin and sodium hydroxide solution. Prolonged exposure of the solution to the

atmosphere before use results in the uptake of carbon dioxide and the formation of

cancrinite. This implies that to form cancrinite, some carbonate is needed.

Elemental analysis of the two Bayer liquors used is shown in Table 4.4. Experiments

using Bayer liquor instead of pure sodium hydroxide solution and the salts of the

anions gave identical results to that of pure sodium hydroxide of similar

concentration.

56

Cbapter4 Syntbesis of Scale

Table 4 4 C . oncentratton 0 specIes ID fS 'B L' r ayer ,IqUO Sample I Sample 2

Total Caustic (gII) 334.5 216.9 AhO,l(gII) \35.7 72.6 Na2C03 (gII) 36.0 28.1 NaF(W)) 1.8 1.7 NaCI (W)) 2.4 5.6 Na2S04 (W)) 0.4 2.9 Na2C20 4 (g/l) 1.4 2.56 Si02 (g/I) 1.62 0.39 V20s (g/I) 0.21 0.37 TOC (gII) 5.4 9.3 Free caustic (gII) 193.5 141 AlC ratio 0.335 0.406 Fe203 (mgll) 0.8 1.0

There is a notable inconsistency between the two papers written by Gerson et al.

regarding the source of the FTIR band at II 02cm- l. This band is originally identified

as a cancrinite framework band [18] but is later reassigncd as the sulphate absorption

[I9]. Comparing these data with the samples of synthetic aluminosilicates prepared

in the laboratory confirm this band is a framework absorption as it is present in both

synthetic nitrate and carbonate cancrinites where no sulphate was present. The true

sulphate absorption is at 1150 cm-I which is in agreement with Whittington [20] and

Nakamoto [21]. In order to confirm this, the synthetic sulphate and carbonate

cancrinites and scale samples were transformed by heating at 1000°C to sodalite

phases. The transformation method for producing sodalite from cancrinite is well

known but is only possible for occluded ions which can stabilise both structures; the

sulphate ion with tetrahedral symmetry is suitable for both, although templates

cancrinite under hydrothermal conditions. On heating the thermodynamically more

stable sulphate sodalite forms with loss of the cage bands at J1 00, 1035, 1008, 762,

690, 630 and 560 cm-I and appearance of bands at 727, 699 and 657cm- I The latter

bands are indicative of the sodalite cage. The band at 1150 cm-I remained proving

that this band belonged to the sulphate anion (Figure 4.4). Transformation

cxperiments on carbonate cancrinite resulted in formation of a non-zeolitic phase

(nepheline) which is the characteristic decomposition product of collapsed zeolites.

Similar heating experiments on scale samples largely resulted in decomposition of the

scale to nepheline implying that the sulphate concentration was certainly low and not

the key anion involved in phase formation.

57

Chapter 4 . Synthesis of Scale

Figure 4.48 Sulphate Cancrinite

b Sulphate Sodalite

4000 3000 2000 1500 1000 500 cm-I

58


Carbonate, sulphate, chloride and oxalate, which are the four anions with the highest

concentrations in Bayer liquor, were used in the investigation into the effects of

temperature and anion concentration on phase fonnation. Four temperatures: 90, 135,

175 and 220·C and four anion concentrations: 5 x 104 ,9 X 104 ,3 X 10-3 and 7 x 10-3

M were studied. In all cases kaolin (2.2 x 10-3 moles) and sodium salt of the anion

were treated hydrothennally in aqueous sodium hydroxide solution (8M) for 48 hours.

The results of these experiments are summarised in Table 4.5.

Table 4.5 The Effect of Temperature and Anion Concentration on Phase Fonnation

ANION TYPE & CONC." 90

TEMPERATURE I·C 135 175

* Concentration 1= 3.57 x 10-2, 2= 6.43 X 10-2

, 3= 0.214 and 4= 0.5M.

C= cancrinite phase, 1= intennediate phase and S= sodalite phase; the phase in bold type represents the major phase in the mixture.

220

Sulphate, oxalate and chloride consistently gave the same phase irrespective of

concentration and temperature. In the oxalate experiments there was a small amount

of carbonate cancrinite fonned due to the decomposition of oxalate to carbonate. The

previous chapter clearly showed that there was a difference in morphology between

59


scale formed in low temperature and high temperature Bayer plants. It can be

assumed, therefore, that these anions have little effect in the determination of the type

of scale formed. If these anions did control the type of scale formed then low and

high temperature scale should be identical which is not the case.

For the carbonate experiments, at high temperature (220°C) the phase formed is

exclusively cancrinite based with strong absorptions indicative of a carbonate

containing phase, as observed for the Bayer plant scale. This hypothesis is confirmed

by the SEM of the samples (Figure 4 .5c) where the crystals exhibit the characteristic

rod-shaped morphology of cancrinite crystals. At low temperature the phase formed

is mostly sodalitic with a cell parameter of9A which is indicative of hydroxy soda lite,

although a small amount of plate-like crystals are apparent in the SEM (Figure 4.5a)

which are distinct from the cubeoctahedral sodalite crystals. The weak: carbonate

absorption in the infra-red spectrum could imply the inclusion of carbonate at a low

level in the sodalite phase or more probably in the plate-like crystals, since significant

levels of carbonate inside a sodalite cage have only been observed by decomposition

of organic species inside a pre-formed sodalite cage and not by direct synthesis.

The most interesting phases are formed at intermediate temperatures which have IR

absorptions indicative of cancrinite except the band at 762 cm-1 is absent and the XRD

patterns only show reflections common to both sodalite and cancrinite with a high

background indicative of a disordered phase, as previously seen in the low

temperature scale (Figures 4.6 and 4.7). Pure sodalite and cancrinite have very

similar XRD patterns and the two have previously been distinguished by the presence

of the 101 reflection at c.a. 19° (copper radiation). If this phase was just a mixture of

cancrinite and soda1ite the 10 1 reflection should be present. In addition, SEM studies

of these materials show crystals of the plate-like morphology (Figure 4.5b) as seen in

small quantities in the low temperature experiment and low temperature scale which

are clearly different from the pure phases; sodalite fonns cubeoctahedral crystals with

typical particle size 50-100 micrometres and cancrinite fonns rods of similar size.

These results are entirely consistent with the work of Buhl [22,23] who has shown

that carbonate concentration is the controlling factor in the kinetics and type of phase

60

Figure 4.58 SEM Images of Synthetic Scale formed at 90°C: a mixture of Sodalite and Intermediate Phase

X 5,000

b SEM Images of Synthetic Scale formed at 135°C: mainly a mixture of Sodalite and Intermediate Phase

X 5,000 X 10,000

c SEM Images of Synthetic Scale formed at 220°C: pure Carbonate Cancrinite

X 5,000 X 5,000


Figure 4.6 FTIR spectra of three phases formed at 90,135 and 220°C using a carbonate-rich solution, top, middle and bottom, respectively.

, 1 &00 1600 1400 1200 lOOO 800 600 400 250

cm- l

63

'll

Figure 4.7 Powder XRD patterns of the three phases formed at 90, 135 and 220·C in carbonate-rich solutions

c o u n t s

C 0 u n I 8

C 0 u n I •

4 (a) 90·C: Sodalite

300

(I 200-

100

~ 1\",

400- (b) 135·C: Intennedlate Phau

3

200

100 ~

..:1=::"""""'" o --

~I I~~'v~~~~

800 (c) 220"C: Cancrinite

800

400

25

L~~~ 200

'1"" 'Wi

• . ",11. ~'--30 ,.111111'1' A 0" 35 "jijil~~""""""\"""""--Deg' ...... 2-Th 40 I "",. IT"T1""TT -~ eta 4S _,.rrTl"Il!ii 50 55 lIiiil'!

60 10 15 20

,/,,) il:r'

:;-., ....

(Il

,,,

Chapter 4 Synthesis of Sale

fonned from kaolinite solutions at different temperatures. The weak carbonate bands

in the low temperature phase are due to low level inclusion of carbonate (up to 0.25%)

in the sodalite cages (where the rest of the cages are filled with hydroxide) and

inclusion of carbonate in an intennediate phase which is a ID intergrowth of sodalite

and cancrinite which fonns at temperatures below 175°C.

Over the range of temperatures and concentrations investigated, all three phases:

sodalite, the intennediate and cancrinite were observed when carbonate was used as

the templating anion. The phase fonned was dependent upon the temperature. A

mixture of sodalite and the intennediate phase was fonned at low temperatures which

was replaced by pure cancrinite at high temperatures.

4.4 CONCLUSIONS

The carbonate anion had the greatest effect on the type of phase fonned with respect

to both temperature and concentration. These experiments closely mirror the

fonnation of different type of scale in the Sayer process plant. Despite the mixture of

anions in the Sayer liquor, the carbonate concentration appears to be the crucial factor

in detennining the phase fonned. As well as highly crystalline sodalite and cancrinite,

a third phase with much poorer crystallinity, which is intennediate between sodalite

and cancrinite was shown to fonn at low temperatures and carbonate concentrations.

The morphology of the three different phases are definitive and allows the phases to

be distinguished by SEM measurements.

Powder Neutron Diffraction Study

Powder neutron diffiaction data for deuterated carbonate cancrinite,

dehydrated carbonate cancrinite, deuteroxysodalite, as well as cancrinites templated

from the more unusual tellurite and vanandate ions, were collected on TAS3. Data

analysis is in progress.

65


4.5 REFERENCES

1 T. Oku and K. Yamada. Light Metals 1971 31-45

2 AN. Adamson, E.J. Bloore and AR Carr. J. Extr. Met. of Aluminium 1 (1963)

23-58

3 A. Duncan. Paper presented at the 1994 TMS Annual Meeting, San Francisco,

CA, February 27-March 3,1994

4 S. Ostap. Control of silica in the Bayer process used for alumina production,

Impurity Control Disposal, Proc. of the 15th ClM Annual Hydrometallurgy

Meeting (1985) 14

5 J.J. Kotte. Light Metals 1989 46-81

6 l.Z. Pevzner et at. TSV Met. 16 (1975) 53-57

7 PJ. Cresswell. 12th Austrialian Chem. Eng. Conf., Australia, 1 (1984) 285

8 M. Jamialahmadi and H. Miiller-Steinhagen. JOM November 1998 44

9 RG. Breuer, L.R Barsotti and AC. Kelly Behaviour of Silica in Sodium

Solutions (Interscience, New York, 1963) p.133

10 F. Hund. Z Anorg. Allg. Chem 511 (1984) 225

11 A.R. Gerson and K.Zheng. Journal of Crystal Growth 171 (1997) 209-218

12 RM. Barrer and J.F. Cole. J. Chem Soc. (A)(1970) 1516-1523

13 RM. Barrer, J.F. Cole and H. Villiger. J. Chem. Soc. (A) (1970) 1523-1531

14 P. Sieger, AM. Schneider, M. Wiebcke, P. Behrens and J. Felsche. Chem. Mater.

7(1995) 163-170

15 J.C. Buhl, D. Reich, C. Mundus and W. MliIler-Warmuth. Reaction Kinetics and

Catalysis Lellers 58(1) 1996 13-18

16 M.RM. Jiang and M.T. Weller. Senors and Activators 30(1) 19963-6

17 K. Katsumi, K. Majima and Y. Yashina. Nippon Kagaku Kaishi 3 (1989) 313-317

18 K. Zheng, AR Gerson, J. Addai-Mensah and R. S1. C. Smart. Journal of Crystal

Growth 171 (1997) 197-208

19 K. Zheng, R S1.C Smart, J. Addai-Mensah and A Gerson. J. Chem. Eng. Data 43

(1998) 312-317

20 B.l. Whittington, B.L. F1etcher and C. Talbo1. Hydrometallurgy 49 (1998) 1-22

21 K. Nakamoto, Infrared and rarnan spectra of inorganic and coordination

compounds, Third edition., John Wiley & Sons

66

Cbapter4 Synthesis of Scale

22 G. Henneler,1. -Ch. Bubl and W. Hoffmann. Catalysis Today 8 (1991) 415-426

23 K. Hackbarth, M. Fechtelkord and J-Ch Buhl. React. Kinet. Catal. Left. 65 I

(1998) 33-39

67

-----~~~~

CHAPTER 5

THE USE OF LIME IN THE BAYER PROCESS

AND

INHIBITION OF THE FORMATION OF SYNTHETIC SODIUM ALUMINOSILICATES

68

Chanter 5 Lime

5.1 INTRODUCTION

This chapter discusses the use of lime in the 8ayer process and reports on the effect of

adding lime on the formation of synthetic sodium aluminosilicates. Lime is used

extensively within the 8ayer process, it is often described as the "aspirin of the

process" because it alleviates a wide range of problems.

Useful functions oflime within the 8ayer process [I]:

• Enhancement of Alumina Extraction

Enhancement of boehmite and diaspore dissolution

The extraction of boehmite and diaspore from bauxite ore requires high digestion

temperatures (>2S0°C) and caustic concentrations. The dissolution can be enhanced

by the addition of CaO to the bauxite slurry and hence allow for a reduction in the

digestion temperature. Mal'ts et al. [2] suggest the enhanced dissolution occurs by

the initial formation of a calcium aluminate which decomposes in the alkaline liquor:

Ca(OH)z + AIOOH B CaO.AI(OHh ( H2O. OH- ) Ca(OHh + Al(OHk

Enhancement of the aluminogoethite to hematite transformation

Aluminogoethite, stoichiometry Fe(l_xAlxOOH (x = 0-0.33), may also be present in

bauxite. Its conversion to hematite, FeZ03, in sodium aluminate liquors at -240°C

offers a means of extracting the goethitic alumina. It is suggested to occur via an iron

hydogamet intermediate.

Minimising the inhibiting effect of sodium titanates on aluminium extraction

Reaction between sodium hydroxide solution and titanium impurities in bauxite ore

results in the undesirable formation of a sodium titanates. The reaction of CaO with

TiOz to form calcium titanates CaTi03 may eliminate the formation of sodium

titanates.

69

Chapter 5 Lime

• Control of Carbonate, Silica and Phosphorus Impurities

Causticisation

Carbon dioxide is readily absorbed from the air into the 8ayer liquor, equation 7.

7

The causticity (i.e. hydroxide concentration) of the liquor gradually decreases with

time as a result of the formation of carbonate. Carbonate also accumulates in liquor

from the breakdown of humic substances present in the bauxite ore. Lime precipitates

the carbonate ions as calcium carbonate and restores the hydroxide concentration, this

process is known as causticization, equation 8.

CaO (s) + H20 + CO/" ~ CaCO) (s) + 20H" 8

Xu et al. [3] suggest that the calcium oxide reacts with water to give calcium

hydroxide, i.e. lime slacking, equation 9a. The calcium hydroxide then dissolves to

give calcium ions, equation 9b, which combine with the carbonate ions present in

solution to precipitate calcium carbonate, equation 9c.

CaO (s) + H20 ~ Ca(OH)2 (s)

Ca(OH)2 (s) ~ Ca2+ + 20H"

Ca2+ + col- ~ CaCO) (s)

Desilication

9a

9b

9c

Lime addition enhances desilication and results in the production of high-purity

gibbsite. Desilication is thought to proceed via the formation of a calcium aluminate

hydrate, possibly Ca)Ah(OH)12, and the subsequent incorporation of silica into the

structure to give hydrogarnet, Ca)Ah(Si04),,(OH)(I2-4n), where n gives an indication of

the amount of silica incorporated within the hydrogarnet structure. Carbonate anions

remove some of the calcium as calcium carbonate and, hence, reduces the desilication

efficiency.

70

Chapter 5 Lime

Phosphorus control

Lime is used to control the phosphorus concentration in liquor through the formation

of an insoluble carbonate apatite .

• Liquor Polishing (Filter Aid)

Filtration of the pregnant liquor reduces the liquor impurity levels and thereby

increase~ the purity of the resultant gibbsite. Addition of lime, i.e. a filter aid, assists

with this impurity removal and also enhances the filtration rate .

• Minimisation of Soda Losses

Addition of lime results in the formation of alternative desilication products:

hydrogamet and calcium-containing cancrinite. Both have Na20/Si02 values lower

than hydroxysodalite and therefore minimise the soda lost in the red mud.

S.2 EXPERIMENTAL

As in the previous experiments, kaolin and sodium hydroxide solution were used as

silica and caustic sources, respectively. Kaolin (9.76 x 10-4 moles), sodium carbonate

(3.77 x 10-3 moles) and calcium oxide (1.78 x 10-4,5.34 X 10-4, 8.92 X 10-4, 1.24 X 10-3

and 1.78 x 10-3 moles, respectively). were placed in a hydrothermal bomb. Sodium

hydroxide solution (8M) was added and the bomb was placed in a preheated oven at

220°C for 48 hours. The products, microcrystalline powders, were washed with

distilled water and dried at 100°C. They were characterised using FTIR spectroscopy

and powder x-ray diffraction. Note, the sodium carbonate concentration can be

expressed as 30gll which is typical of the carbonate concentration in Bayer liquor.

The calcium oxide concentrations expressed in gIl are 0.71, 2.14, 3.57, 5.00 and 7.14

gIl, respectively.

71

ChapterS Lime


The results are given in Table 5.1.

Table 5.1 The Effect of Lime Addition

Concentration of CaO addedll!f L Phase Formed 0.00 Carbonate cancrinite 0.71 Carbonate cancrinite 2.14 Carbonate cancrinite 3.57 Carbonate cancrinite 5.00 Carbonate cancrinite 7.14 Sodium calcium hydrogen silicate

Without the addition of any calcium oxide a carbonate cancrinite fonns. The

fonnation of carbonate cancrinite continues with the addition of lime at all

concentrations below 7.14g11. The cell parameters of the cancrinite formed when

5.00gll calcium oxide was added are a= 12.66 and c= 5.17 A, identical to the cell

parameters of the cancrinite fonned when no calcium oxide was added. There may be

exchange of some of the sodium ions in the cancrinite channels with calcium ions in

the solution.

When 7.14g/1 calcium oxide was added carbonate cancrinite formation was inhibited,

XRD analysis revealed that a non-zeolitic phase- sodium calcium hydrogen silicate

had fonned (JCPDS reference number 25-1319), Figure 5.1 and Table 5.2.

Table 5.2 XRD Data d-spacing Relative Intensity hkl

Product from the 2.842 100 addition of 7.14g/1 2.722 47 calcium carbonate 2.683 24

. ... ,<<<,' <-''''''''''', . . ...

Sodium calcium 2.85 100 -120 hydrogen silicate 2.73 45 -102 (JCPDS 25-1319) 2.70 25 10 1

Grinding by hand with a pestle and mortar the sodium calcium hydrogen silicate was

noticeably much finer than cancrinite. This could suggest that if it was to fonn in the

pipes instead of carbonate cancrinite it would be easier to remove.

72

Figure 5.1 Powder XRD pattern sodium calcium hydrogen silicate 'n I; ~ U1

C 400 0 u n t

•

JOO I 1

j J

~I ;lOo-! ~

"

~ I ,oo~ ~ ~ I!

j ! I 11i' /1 'I'll 'il ,\ In

. A ,A I I i !', ' I" !:_" !v /1 1\ A ,i)1

() :~;;~":""~~~'" ~;:;;~~";'~;:~'~.;~ ~ I ,v ,:,::.:~J~,~~< : ':-7; ~~.~ ,y, ~~t'" ,'>~~ I :1 ,~ ~\;-1~ 15 20 2S 30 35 40 45 50 55 60

Oegrees 2-Thola

t'" e' ..

Chapter 5 Lime

5.4 CONCLUSIONS

The many useful functions lime performs in the Bayer process have been discussed.

The effect of adding calcium oxide to a reaction which would normally precipitate

pure carbonate cancrinite was studied. It was found carbonate cancrinite formation

was inhibited by the addition of calcium oxide above concentrations of 7.14g/1,

sodium calcium hydrogen silicate formed instead.

5.5 REFERENCES

1 8.1. Whittington. Hydrometallurgy 43 (1996) 13-35

2 N.S. Mal'ts, Y.I. Komeev, A.G. Suss, S.G. Sennikov and 1.8. Firfarova. Tsvet.

Metal. 10 (1983) 47-49 (English Edition)

3 8. -A. Xu, D.E. Giles and 1.M. Ritchie. Hydrometallurgy 48 (1998) 205-224

74

the investigation of scale formation in the bayer … in the bayer process ... the investigation of...

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