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Primary and secondary phases in copper-cobalt smelting slags from the Copperbelt Province, Zambia M. VI ´ TKOVA ´ 1, *, V. ETTLER 1 , Z. JOHAN 2 , B. KR ˇ I ´ BEK 3 , O. S ˇ EBEK 4 AND M. MIHALJEVIC ˇ 1 1 Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic 2 Bureau des Recherches Ge ´ologiques et Minie `res (BRGM), av. Claude Guillemin, 45060 Orle ´ans, cedex 2, France 3 Czech Geological Survey, Geologicka ´ 6, 152 00 Prague 5, Czech Republic 4 Laboratories of the Geological Institutes, Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic [Received 16 December 2009; Accepted 2 August 2010] ABSTRACT Pyrometallurgical slags from three Cu-Co smelters (Nkana, Mufulira, Chambishi) in the Copperbelt Province, Zambia, were studied from mineralogical and chemical points of view. The slags were enriched in metals and metalloids, mainly Cu (up to 35 wt.%), Co (up to 2.4 wt.%) and As (up to 3650 ppm). The following primary phases were observed in slags: Ca-Fe silicates (clinopyroxene, olivine) and leucite, oxides (spinel-series phases), ubiquitous silicate glass and sulphide/metallic droplets of various sizes. The presence of glass and skeletal/dendritic crystal shapes indicated rapid cooling of the slag melt. Copper and cobalt were found in low concentrations in the majority of silicates (olivine, clinopyroxene) and oxides, substituting for Fe in their structures (up to 7.15 wt.% CoO in olivine, 4.11 wt.% CuO in spinel). Similarly, up to 0.91 wt.% CoO and 6.90 wt.% CuO were observed in the interstitial glass. Nevertheless, the main carriers of these metals in the slags studied were Cu sulphides (digenite, chalcocite, bornite, chalcopyrite), Co-Fe sulphides (cobaltpentlandite), Co-bearing intermetallic phases ((Fe,Co) 2 As) and alloys. Weathering features corresponding to the presence of secondary metal-bearing phases, such as malachite (Cu 2 (CO 3 )(OH) 2 ), brochantite (Cu 4 SO 4 (OH) 6 ) and sphaerocobaltite (CoCO 3 ), were observed on the slag surfaces. They indicate that the slags studied are reactive on contact with water/atmosphere and that their environmental stability and release of potentially harmful metals and metalloids must be evaluated further. KEYWORDS: slags, smelting, Copperbelt, Zambia, copper, cobalt, clinopyroxene, leucite. Introduction THE environmental assessment of smelting dump- sites and their vicinity has been the subject of numerous studies (Bril et al., 2008; Ettler et al., 2001, 2003a, 2009a; Ganne et al., 2006; Kr ˇı ´bek et al., 2006, 2010; Parsons et al., 2001; Piatak et al., 2004). Metallurgical slags represent the most important mineral waste produced during the pyrometallurgical extraction of metals from ores. Smelting slags are wastes containing silicates, oxides, glass and sulphide/metallic inclusions, often enriched in contaminants such as Pb, Zn, Cu or As (Ettler et al., 2001, 2003a, 2009a; Gorai et al., 2003; Piatak and Seal, 2010), which represent a potential environmental risk due to the weath- ering processes. The chemical and mineralogical characterization of the waste materials is essential for their environmental assessment prior to the application of suitable environment-friendly stabilization/solidification technologies (Piantone, 2004), dumping scenarios (Ettler et * E-mail: [email protected] DOI: 10.1180/minmag.2010.074.4.581 Mineralogical Magazine, August 2010, Vol. 74(4), pp. 581–600 # 2010 The Mineralogical Society

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Primary and secondary phases in copper-cobalt smelting slagsfrom the Copperbelt Province, Zambia

M. VITKOVA1,*, V. ETTLER

1, Z. JOHAN2, B. KRIBEK3, O. SEBEK4 AND M. MIHALJEVIC

1

1 Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Faculty of Science,

Albertov 6, 128 43 Prague 2, Czech Republic2 Bureau des Recherches Geologiques et Minieres (BRGM), av. Claude Guillemin, 45060 Orleans, cedex 2,

France3 Czech Geological Survey, Geologicka 6, 152 00 Prague 5, Czech Republic4 Laboratories of the Geological Institutes, Charles University in Prague, Faculty of Science, Albertov 6, 128 43

Prague 2, Czech Republic

[Received 16 December 2009; Accepted 2 August 2010]

ABSTRACT

Pyrometallurgical slags from three Cu-Co smelters (Nkana, Mufulira, Chambishi) in the CopperbeltProvince, Zambia, were studied from mineralogical and chemical points of view. The slags wereenriched in metals and metalloids, mainly Cu (up to 35 wt.%), Co (up to 2.4 wt.%) and As (up to3650 ppm). The following primary phases were observed in slags: Ca-Fe silicates (clinopyroxene,olivine) and leucite, oxides (spinel-series phases), ubiquitous silicate glass and sulphide/metallicdroplets of various sizes. The presence of glass and skeletal/dendritic crystal shapes indicated rapidcooling of the slag melt. Copper and cobalt were found in low concentrations in the majority ofsilicates (olivine, clinopyroxene) and oxides, substituting for Fe in their structures (up to 7.15 wt.%CoO in olivine, 4.11 wt.% CuO in spinel). Similarly, up to 0.91 wt.% CoO and 6.90 wt.% CuO wereobserved in the interstitial glass. Nevertheless, the main carriers of these metals in the slags studiedwere Cu sulphides (digenite, chalcocite, bornite, chalcopyrite), Co-Fe sulphides (cobaltpentlandite),Co-bearing intermetallic phases ((Fe,Co)2As) and alloys. Weathering features corresponding to thepresence of secondary metal-bearing phases, such as malachite (Cu2(CO3)(OH)2), brochantite(Cu4SO4(OH)6) and sphaerocobaltite (CoCO3), were observed on the slag surfaces. They indicatethat the slags studied are reactive on contact with water/atmosphere and that their environmentalstability and release of potentially harmful metals and metalloids must be evaluated further.

KEYWORDS: slags, smelting, Copperbelt, Zambia, copper, cobalt, clinopyroxene, leucite.

Introduction

THE environmental assessment of smelting dump-

sites and their vicinity has been the subject of

numerous studies (Bril et al., 2008; Ettler et al.,

2001, 2003a, 2009a; Ganne et al., 2006; Krıbek et

al., 2006, 2010; Parsons et al., 2001; Piatak et al.,

2004). Metallurgical slags represent the most

important mineral waste produced during the

pyrometallurgical extraction of metals from ores.

Smelting slags are wastes containing silicates,

oxides, glass and sulphide/metallic inclusions,

often enriched in contaminants such as Pb, Zn, Cu

or As (Ettler et al., 2001, 2003a, 2009a; Gorai et

al., 2003; Piatak and Seal, 2010), which represent

a potential environmental risk due to the weath-

ering processes. The chemical and mineralogical

characterization of the waste materials is essential

for their environmental assessment prior to the

application of suitable environment-friendly

stabi l izat ion/sol idificat ion technologies

(Piantone, 2004), dumping scenarios (Ettler et* E-mail: [email protected]: 10.1180/minmag.2010.074.4.581

Mineralogical Magazine, August 2010, Vol. 74(4), pp. 581–600

# 2010 The Mineralogical Society

al., 2003a) or direct industrial applications (Gorai

et al., 2003; Lim and Chu, 2006).

Extensive mining and smelting activities in the

Zambian Copperbelt have resulted in huge

amounts of various waste products, usually

dumped without any pre-treatment. As a first

step in the project on the environmental impacts

of wastes from Cu-Co smelting in Zambia, this

paper is focused on detailed mineralogical

investigation of Cu- and Co-bearing slags from

three different smelters in this province in order to

describe binding of metal/metalloid contaminants

in primary phases and secondary weathering

products.

Background information

Geology of the Zambian copper-mining districtThe Zambian Copperbelt forms the southeastern

part of the Neoproterozoic Lufilian Arc, a Pan-

African orogenic belt consisting of metasedimen-

tary rocks of the Katanga Supergroup, and

represents one of the world’s most important

sediment-hosted stratiform Cu- and Co-bearing

sulphide deposits (Porada and Berhorst, 2000;

Unrug, 1983). The Katanga Supergroup rocks are

believed to have accumulated in a fault-bounded

intracratonic rift zone (Annels, 1984), which

closed during a protracted period of folding and

thrusting (the Lufilian Orogeny) near the end of

the Precambrian. The Roan group of the Katanga

Supergroup consists of carbonate and siliciclastic

units, including dolomites, arenites, argillites,

schists and shales. Within the Katanga

Supergroup, copper deposits are essentially

restricted to its lowermost part, the Lower Roan

Group, which consists of footwall conglomerates,

coarse arkoses, argillaceous sandstones and dune-

bedded quartzites. Ore shale consists of organic

carbon- and pyrite-rich laminated argillaceous

siltstones. The hanging wall of ore deposits is

composed of quartzites, siltstones and dolomitic

sandstones (McGowan et al., 2006; Porada and

Berhorst, 2000).

The thickness of the mineralized sections varies

from 4 to 35 m. Four types of ore bodies were

distinguished: (1) stratabound disseminated miner-

alization in footwall arkoses and conglomerates

that are referred to as ‘footwall arenite ore bodies’

(e.g. Mufulira deposit); (2) stratabound dissemi-

nated mineralization in black shales referred to as

‘Ore Shale ore bodies’ (Nkana and Chingola

deposits); (3) stratabound massive sulphide miner-

alization between black shales and hangingwall

arkoses and sandstones (e.g. Chambishi SE

deposits); and (4) vein-type quartz-carbonate-

sulphide mineralization controlled by shear zones

that cut arkoses, sandstones and black shales in the

low-grade metamorphosed parts of the Lower

Roan Group (Kamona and Nyambe, 2002).

The most abundant sulphide minerals of the

Zambian Copperbelt are bornite (Cu5FeS4),

followed by chalcocite (Cu2S), chalcopyrite

(CuFeS2), covellite (CuS) and cobaltiferous

pyrite (Fleischer, 1984; Garlick, 1961).

Carrollite (Cu(Co,Ni)2S4) occurs as an accessory

phase of economic significance (up to 0.5 wt.%

Co) in the SW part of the Zambian Copperbelt

(McGowan et al., 2006).

On the Zambian side of the Copperbelt, it is

estimated that 30 million tonnes of copper metal

have been produced since mining began on a full

scale in 1930. The average ore grade is 3 wt.% Cu

and 0.18 wt.% Co. Small amounts of Au, Pt and

Ag have been recovered from the Cu-slimes

(Kamona and Nyambe, 2002).

History of smelting technologies

Copper-extraction technologies used in the

Zambian Copperbelt consist of the following

steps: sulphide ore concentrate smelting, Cu

extraction from matte and Cu refining. The ore

concentrate is composed mainly of chalcopyrite,

bornite and chalcocite. The first step consists of

smelting in a furnace (historically, reverbatory

furnaces were used; electric furnaces are largely

used nowadays in Zambia) with coke and silica in

order to produce Cu matte and remove Fe into the

silicate slag. The second step is generally

performed in Peirce-Smith (PS) converters,

where Cu matte (mainly composed of Cu2S) is

melted and oxidized to Cu2O, which reacts with

the remaining Cu2S to give molten Cu, referred to

as ‘blister copper’ due to the SO2 inclusions

trapped within the Cu. Blister copper (purity

between 96 and 99%) is further refined in anode

furnaces by electrolysis to obtain Cu of high purity

(Cutler et al., 2006; Davenport et al., 2002).

The oldest Cu smelter in the area was located at

Nkana near Kitwe (commissioned in 1931, closed

in 2009). It consisted of reverbatory furnaces,

Peirce-Smith converters and blister casting facil-

ities. The blister copper production was 6000

tonnes during the first year of operation. At the

production apogee in 1971, 330,000 tonnes of Cu

were produced; between 1993 and 2006, the

production was between 100,000 and 125,000

582

M. VITKOVA ET AL.

tonnes of Cu per year. In 1994, an El Teniente

converter (CT) was installed to upgrade the

reverbatory furnace matte to white metal

(S-poor Cu2S, with composition Cu2S1�x), prior

to its refinement in conventional PS converters.

The slag discarded generally contained <1 wt.%

Cu (Cutler et al., 2006).

The Mufulira smelter was initially commis-

sioned in 1937 with two reverbatory furnaces and

four PS converters. The smelter was upgraded in

1952 with two anode furnaces and in 1956 with

the construction of a third reverbatory furnace and

a fifth PS converter. The electric furnace was

commissioned in 1971, operating with one

reverbatory furnace to provide 230,000 tonnes

of Cu per year. This upgrade was followed by the

installation of a sixth PS converter in 1972. Due

to operational failures, the electric furnace was

first rebuilt in 1977 and the reverbatory furnace

was put offline. In 1991, the electric furnace was

upgraded, operating with four PS converters and

anode processing (Ross and de Vries, 2005). The

last upgrade of the furnace took place in 2006,

when ISASMELT technology was commissioned.

Currently, the Mufulira smelter treats approxi-

mately 850,000 tonnes of Cu concentrate per year.

The discarded slag contains up to 1 wt.% Cu

(T. Gonzales, Mufulira smelter, pers. comm.).

The Chambishi smelter is currently reproces-

sing the old Nkana dump slags in order to recover

Co. The dumps located near Kitwe, which result

from six decades of mining and smelting activities

in the area, consist of about 20 Mt of slag grading

between 0.3 and 2.6 wt.% Co (average 0.76 wt.%

Co). The Nkana slag is crushed to a particle size of

15 mm and mixed with fluxes (lime, coal, rutile) to

prepare the furnace charge. The re-smelting

facility consists of the electric direct current

(DC) arc furnace where the carbothermic reduc-

tion of desirable metals (Co, Ni, Cu) occurs, while

the maximum possible quantity of Fe is retained in

the slag. The alloy containing from 5 to 14 wt.%

Co is tapped, superheated using a plasma torch at

1650ºC and prepared as fine particles, <100 mm in

diameter by the water atomization technique

(injection of high-pressure water into the molten

alloy). The molten slag is tapped from the electric

arc furnace into the 60-ton slag pots and evacuated

to the dump (Jones et al., 2002).

Materials and methods

Slag samplingThe slag samples were collected at three smelting

sites (Fig. 1). In total, 22 slag samples were

investigated in this study.

FIG. 1. Study area, including locations of the Kitwe (Nkana), Mufulira and Chambishi smelters and the Nchanga and

Konkola mines.

MINERALOGY OF SLAGS, ZAMBIA

583

(1) Slag I corresponds to historical slags from

the Nkana smelter. The GPS position of the dump

is S 12º50’20’’, E 28º12’40’’. They generally occur

as fragments, 7 cm in size (with local megascopic

pores) of black to grey colour with green, pink,

rusty or red crusts of secondary phases coating the

surface (number of samples, n = 16). White grains

of unmelted quartz gangue (several mm in size)

were observed locally in the silicate matrix.

(2) Slag II corresponds to slags produced by the

Mufulira smelter. The GPS position of the dump

is S 12º32’09’’, E 28º13’45’’. This slag type is

represented either by granulated black vitreous

material or by grey fragments up to 6 cm in size

with a greenish coating of secondary phases

(n = 4).

(3) Slag III corresponds to vitreous slags

produced during the reprocessing of old Nkana

slags in the Chambishi smelter. The GPS position

of the dump is S 12º38’35’’, E 28º02’16’’. Theseslags occur as black, glassy fragments up to 10 cm

in size and a few mm thick (n = 2).

Bulk chemical analyses

An aliquot of each slag sample (~20 g) was

crushed and pulverized in an agate mortar using

the Fritsch Pulverisette apparatus, dried at 40ºC

and then used for bulk chemical analyses and

X-ray diffraction (XRD). The analytical proce-

dure includes loss on ignition (LOI) after heating

to 1000ºC. Due to the large FeO content, the LOI

value obtained was corrected taking into account

the weight increase due to the iron oxidation. The

bulk chemical composition of the pulverized

samples was determined after digestion in acids

(HClO4, HF, HNO3) and/or sintering. Subsequent

chemical analysis was performed using gravi-

metric and volumetric analysis and photometry to

determine the major elements. Trace elements

were determined by flame atomic absorption

spectrometry (FAAS, Varian SpectrAA 280FS,

Australia) and inductively coupled plasma mass

spectrometry (ICP-MS, THERMO XSeries II,

USA with AS 520 Cetac autosampler). Detailed

descriptions of dissolution/analytical procedures

were given by Ettler et al. (2009a). The total S

content was determined using an ELTRA CS 530

analyser (Germany). Procedural blanks were run

simultaneously for all the determinations. The

analyses were controlled by the USGS standard

reference material G2 with accuracy of better

than 10% of the relative standard deviation

(RSD).

Mineralogical analyses of primary and secondary phases

XRD analysis

About 0.5 g of pulverized sample and chips of

coatings were used for the identification of

primary and secondary phases, using XRD

(PANalytical X’Pert PRO diffractometer with

X’Celerator detector) with Cu-Ka radiation, at

40 kV and 30 mA, over the range 2�80º2y with a

step size of 0.02º2y and counting time of 300 s per

step. X’Pert HighScore 1.0 software equipped

with the JCPDS PDF-2 database (ICDD, 2002)

was used for the qualitative analysis.

Microscopy and electron probe microanalysisPolished thin sections of slags were used for

microscopic examination (transmitted and

reflected light) and electron probe microanalysis

(EPMA). The weathering products on the surface

of the slags were studied under a binocular

microscope and sampled using a separation

needle. A CamScan S4 scanning electron micro-

scope (SEM) equipped with an Oxford Link

energy dispersion spectrometer (EDS) and a Link

ISIS 300 microanalytical system was used for

subsequent imaging and semi-quantitative

chemical analyses of both primary and secondary

phases. Quantitative microanalyses were

performed using a Cameca SX-100 electron

microprobe. The analytical conditions for silicates

and oxides were: accelerating voltage 15 kV,

beam current 4 nA, counting time 10 s. The

following standards were used: jadeite (Na),

quartz (Si), synthetic Al2O3 (Al), leucite (K),

diopside (Ca), synthetic TiO2 (Ti), synthetic

Fe2O3 (Fe), Mn-Cr spinel (Cr, Mn), synthetic

MgO (Mg), baryte (S), cobalt metal (Co), cuprite

(Cu) and willemite (Zn). For metals and

sulphides, the analytical conditions were: accel-

erating voltage 20 kV, beam current 10 nA,

counting time 10 s. The following set of standards

was used: synthetic ZnS (S, Zn), FeS2 (Fe),

copper metal (Cu), Cr2O3 (Cr), cobalt metal (Co),

GaAs (As), galena (Pb) and pentlandite (Ni). The

Fe2O3-FeO content in spinels was calculated

using the methodology described in detail by

Ettler (2002). In total, ~300 analyses were

performed by EPMA.

Results

Bulk chemical compositionThe slags studied are mainly composed of SiO2

(15�60 wt.%), FeO + Fe2O3 (7.1�48 wt.%), CaO

584

M. VITKOVA ET AL.

TABLE1.Bulk

chem

ical

compositionsofselected

slags.

Assem

blage

A1

A1

A1

A1

A1

A1

A1

A2

A2

A3

A3

A3

A3

A4

A5

A5

A5

A5

Type*

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagII

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagII

SlagIII

SlagIII

Sam

ple

Z1A

Z1B

Z1C

Z10B

Z10C

Z13A

Z8B

Z4

Z13D

Z3

Z9

Z11a

Z12

Z13Ba

Z13C

Z7A

Z5

Z14

SiO

2(w

t.%)

39.87

48.60

47.11

41.74

39.13

37.46

41.12

51.64

44.26

38.40

54.72

15.53

37.56

22.14

60.18

36.88

51.38

48.24

TiO

20.40

0.63

0.58

0.50

0.58

0.40

0.63

0.63

0.53

0.43

0.40

0.37

0.35

0.75

0.73

0.43

0.63

0.58

Al 2O3

5.68

8.96

7.79

5.58

6.16

7.44

5.09

9.92

11.39

7.11

7.58

3.90

8.33

12.13

12.60

5.84

9.66

11.03

Fe 2O3

9.50

1.50

6.62

5.09

6.68

11.57

8.74

3.99

4.74

5.96

4.26

18.79

10.52

5.22

0.68

6.53

1.76

1.43

FeO

19.21

5.60

13.35

25.34

21.23

16.12

19.37

10.21

14.97

20.70

12.20

29.52

17.84

15.77

7.42

27.50

11.30

15.12

MnO

0.06

0.04

0.05

0.07

0.07

0.07

0.06

0.04

0.08

0.10

0.05

0.19

0.10

0.04

0.04

0.08

0.10

0.10

MgO

3.33

6.45

3.55

3.21

3.48

2.99

2.25

3.25

3.12

1.88

1.89

4.80

2.56

0.64

3.09

2.91

6.37

4.75

CaO

13.65

21.87

16.14

7.76

10.56

15.05

18.03

13.39

12.79

14.31

9.00

7.41

14.06

3.18

12.08

12.91

13.65

14.29

Na 2O

0.09

0.13

0.09

0.09

0.08

0.07

0.19

0.12

0.17

0.30

0.14

0.03

0.11

0.06

0.10

0.19

0.15

0.14

K2O

2.07

3.17

2.25

2.17

2.11

1.85

1.84

2.45

3.14

4.62

4.83

1.13

3.57

1.56

4.15

2.57

3.06

2.73

Stot

0.18

0.05

0.30

0.13

0.07

0.15

0.19

0.36

0.35

0.27

0.46

2.62

0.07

0.09

0.11

0.55

0.24

0.17

LOI

0.66

0.46

0.55

2.14

1.45

0.91

<0.01

0.12

0.18

0.66

0.42

1.18

0.42

1.46

0.16

<0.01

<0.01

<0.01

Cr(ppm)

184

254

169

544

760

65

246

233

3841

89

163

7510

89

148

339

380

883

853

Ag

<0.3

<0.3

0.5

2.1

<0.3

<0.3

<0.3

0.3

0.4

<0.3

0.7

12

<0.3

11

0.4

0.6

1.9

<0.3

Cu

11221

3492

13947

8548

15553

7958

4625

18165

8226

5263

19963

86314

10898

353580

12094

9349

3724

2050

Co

7622

3834

8405

14684

12454

7980

720

6251

7605

17638

4386

24104

10965

7117

922

2716

2178

2414

Ni

12

12

36

24

51

22

9.0

24

15

13

16

935

27

149

15

46

8.2

5.5

Cd

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

<0.22

5.2

<0.22

<0.22

<0.22

<0.22

Sb

<6

<6

<6

<6

<6

<6

<6

<6

20

<6

<6

39

<6

<6

<6

<6

<6

<6

Pb

6.2

<1.5

10

8.0

24

48

26

32

106

<1.5

30

1134

29

658

30

26

39

<1.5

Zn

237

44

123

256

286

299

247

99

229

48

177

2287

437

1487

168

496

80

123

Ba

416

623

546

453

385

442

349

477

556

1297

469

259

518

246

564

445

667

731

Sr

199

443

413

155

181

295

467

386

347

406

305

226

408

235

391

341

370

392

Mo

193

322

208

182

153

220

103

145

167

422

147

162

245

118

61

145

30

36

Sn

<15

<15

<15

<15

19

<15

<15

<15

<15

<15

<15

<15

<15

82

<15

<15

<15

17

As

619

872

603

628

947

1031

750

683

3050

652

715

3642

1013

987

855

627

621

1191

Bi

30

0.5

6.5

3.6

14

14

3.0

4.2

5.4

1.9

8.1

140

5.5

1695

16

2.4

<0.1

0.2

Totalb

(wt.%)

96.79

98.45

100.84

96.37

94.67

95.93

98.28

98.78

98.14

97.32

98.58

98.16

97.95

99.70

102.88

97.87

99.15

99.36

*SlagI,Nkana;

SlagII,Mufulira;SlagIII,Cham

bishi;

amixture

ofsilicate

slag

andmatte;bTotal=sum

ofoxides

+Stot+LOI+metalsin

elem

entalform

(3.2�22 wt.%) and Al2O3 (3.9�13 wt.%; Table 1).

Furthermore, the slags contain significant amounts

of Cu, Co and As, concentrations of which are, in

general, <2 wt.% except for samples Z11 and

Z13B (mixture of slag and matte), where the Cu

contents are 8.6 and 35 wt.%, respectively, and that

of Co reaches 2.4 wt.% (Z11). Large concentra-

tions (>1000 ppm) of some other contaminants

(Pb, Zn, Bi) were detected locally (Table 1). The

small analytical totals may be due to the presence

of metals listed in elemental form in Table 1 which

may be, in fact, present as oxides. A large number

of samples collected at the dump sites in the

vicinity of the Nkana smelter show variable

chemical compositions, probably reflecting varia-

tions in the furnace charge and smelting conditions

over time. Cutler et al. (2006) stated that large

variations in chemical and mineralogical composi-

tions also occurred for concentrates processed at

the Nkana smelter. For example, the concentrates

from the Nchanga and Konkola mines (Fig. 1) are

deficient in S and Fe, being composed mainly of

chalcocite, in contrast to the Nkana concentrates,

which are composed mainly of chalcopyrite (Cutler

et al., 2006). Large concentrations of K2O were

observed in the slags studied (1.1�4.8 wt.%, mean:

2.7 wt.%; Table 1); in contrast, other studies of

slags generally indicate ~1 wt.% K2O (Ettler et al.,

2009a,b; Kierczak et al., 2009; Puziewicz et al.,

2007). These large K2O values are probably related

FIG. 2. Silicate and oxide phases in backscattered electrons (SEM). Nkana slags: (a) rounded Cu and CuS inclusions

and dendritic spinels associated with skeletal clinopyroxene trapped within the glassy matrix (Z1C, A1);

(b) framboidal Cr-spinel crystals, skeletal clinopyroxene and micrometer-sized Cu inclusions trapped within the

glass matrix and associated with the residual quartz grain from the unmelted gangue (A1); (c) leucite-clinopyroxene

assemblage with rare sulphide inclusions (Z4, A2); (d) sulphide prill (CoS and bornite) and leucite crystals

associated with lath-like olivine and late clinopyroxene (Z3, A3). Abbreviations: px � clinopyroxene, sp � spinel,

Cu � metallic copper, CuS � copper sulphide corresponding to Cu9S5�Cu2S solid solution, le � leucite, ol �olivine, CoS � cobalt sulphide, bn � bornite, gl � glass, qz � quartz, Cr-sp � Fe-Cr spinel.

586

M. VITKOVA ET AL.

to the composition of the processed concentrates,

which contain up to 13.5 vol.% of microcline

(KAlSi3O8) (Cutler et al., 2006).

Slag petrography

Representative slag textures and phase assem-

blages are given in Figs 2 and 3 and the identified

phases are listed in Table 2. The following

assemblages and crystallization sequences were

recognized in the studied slags (n = number of

samples, phases in parentheses do not occur in all

the samples):

A1: (spinel) ? clinopyroxene ? glass(n = 9, Fig. 2a,b)

A2: (spinel) ? clinopyroxene ? leucite ? glass(n = 3, Fig. 2c)

A3: spinel ? leucite ? olivine ? clinopyroxene ?glass (n = 4, Fig. 2d)

A4: spinel ? plagioclase ? glass (n = 1, not shown)A5: (spinel) ? glass (n = 4, not shown)A6: spinel ? olivine ? glass (n = 1, not shown)

Unmelted quartz observed locally is not

included in these assemblages. Sulphides and

metallic inclusions (Table 2) are ubiquitous.

Assemblages A1 to A5 occur in the Nkana slags

(type I), those of A1, A5 and A6 occur in the

Mufulira slags (type II). The Chambishi slags (type

III) are characterized by assemblage A5 (glass

FIG. 3. Sulphide phases in backscattered electrons (SEM). Nkana slags: (a) irregular inclusion composed of bornite-

digenite symplectitic intergrowth with enclosed CoS crystals (Z1B, A1); (b) rounded inclusion composed of Cu and

Co sulphides, native Cu, Bi, and Fe-Co alloy (Z9, A3); (c) complex matte fragment composed of predominant CuS-

bornite intergrowth with inclusions of Cu2S, PbS, CoS and metals (Pb, Bi, Cu) (Z11, A3); (d) troilite and CoS

crystals associated with CuS-bornite intergrowth with trace digenite and galena (Z13D, A2). Abbreviations: px �clinopyroxene, gl � glass, bn � bornite, dg � digenite, CoS � cobalt sulphide, qz � quartz, Bi � metallic Bi, Fe-Co

� Fe-Co alloy, Cu � metallic Cu, CuS � copper sulphide, Cu2S � chalcocite, PbS � lead sulphide, Pb � metallic

Pb, mgt � magnetite, tr � troilite.

MINERALOGY OF SLAGS, ZAMBIA

587

only). The predominant crystalline phases (clino-

pyroxene, olivine) commonly show textures with

skeletal, herring-bone, harissitic, dendritic or

irregular lath-like crystals indicating rapid crystal-

lization (Fig. 2). Leucite forms two types of

crystals: (1) globular crystals associated with

olivine, spinel and clinopyroxene (Fig. 2d); and

(2) nodular crystals associated with clinopyroxene

and sometimes spinel (Fig. 2c). Spinels (magnetite,

Cr-spinel) were observed in all the assemblages,

except the slag type III. Coloured interstitial glass

was systematically observed in assemblages A1 to

A4 and in A6. Sulphides associated with metallic

phases generally form inclusions ranging from <1

mm to 500 mm in size within the silicate matrix

(Fig. 2a,d; Fig. 3a,b). Symplectitic intergrowths,

generally composed of bornite and digenite, were

observed in larger sulphide inclusions (Fig. 3).

Mineralogy and crystal chemistry of primary phases

Silicates, oxides and glass

The composition of Ca-Fe clinopyroxenes in

slags corresponds to the diopside-hedenbergite

(CaMgSi2O6-CaFeSi2O6) solid solution, the

extent of which ranges from En1Fs49Wo50 to

En31Fs20Wo49 (Table 3). Taking into account the

ionic radii, we suggest the replacement (Co, Cu,

Zn) > (Fe, Mg); ionic radii in octahedral

coordination are (in A): Co 0.75, Cu 0.77,

TABLE 2. Phases determined using EPMA and XRD, occurring in the slags studied and their relativeabundances in the samplesa.

Group Name Composition ———— Type* ————Slag I Slag II Slag III

Silicates Diopside-hedenbergite s.s. Ca(Fe,Mg)Si2O6 +++ +++Fayalite–forsterite s.s. (Fe,Mg)2SiO4 + ++Kirschsteinite-monticellite s.s. Ca(Fe,Mg)SiO4 ++Leucite KAlSi2O6 ++Anorthite CaAl2Si2O8 A4Quartz SiO2 + +

Oxides Magnetite Fe3O4 +++ +++Cr-spinel Fe2+(Cr,Al,Fe3+)2O4 +Rutile TiO2 trCuprite Cu2O trDelafossite FeCuO2 A4

Sulphides Bornite Cu5FeS4 + + +Digenite Cu9S5 + +Chalcocite Cu2S + +Troilite FeS + + +Chalcopyrite CuFeS2 tr trCo-pentlandite Co9S8 +Co4S3 Co4S3 +CoS CoS +Galena PbS tr

Elements Copper Cu + +Lead Pb trBismuth Bi tr

Others Alloys (Fe-Co-As-Cu-Ni) tr +Intermetallic compounds (Fe,Co)2As +

Silicate glass ++ ++ +++

* Slag I, Nkana; Slag II, Mufulira; Slag III, Chambishia Relative abundance: +++ dominant phase, ++ common phase, + minor phase, tr traces, A4 – dominant inassemblage A4, (according to observations in polished sections)s.s. = solid solution

588

M. VITKOVA ET AL.

Fe 0.78, Mg 0.72, Zn 0.74 (Shannon, 1976). The

results of EPMA showed that Co might substitute

for Fe (or Mg) in the clinopyroxene structure up

to 1.8 wt.% CoO (0.059 a.p.f.u., atoms per

formula unit). In contrast, only small concentra-

tions of Cu and Zn were detected in clino-

pyroxene (up to 0.2 wt.% of CuO and ZnO).

Chemical compositions of olivine-type phases

vary from fayalite (Fe2SiO4)�forsterite

(Mg2SiO4) to kirschsteinite (CaFeSiO4)–monti-

cellite (CaMgSiO4) solid solutions: Fo3Fa64La33

to Fo27Fa69La4 (Table 3). According to EPMA,

up to 7.15 wt.% CoO, 0.19 wt.% CuO and 0.44

wt.% ZnO were observed in this solid solution.

The greatest Co concentrations correspond to

0.178 a.p.f.u., indicating an efficient substitution

of Co for Fe in the olivine structure.

Leucite (KAlSi2O6) was identified in slag

samples with the greatest K2O content. It forms

euhedral crystals in association with olivine, spinel

and clinopyroxene (Fig. 2d). According to EPMA,

leucite contains minor FeO (up to 2.35 wt.%).

TABLE 3. Selected microprobe analyses of clinopyroxene, olivine and leucite.

Assemblage A1 A1 A1 A3 A3 A3 A2 A3Type* Slag I Slag I Slag II Slag I Slag I Slag I Slag I Slag ISample Z1A Z1C Z8B Z3 Z3 Z9 Z4 Z3Phase ——— Clinopyroxene ——— – Olivine – – Leucite –

SiO2 (wt.%) 47.20 48.75 43.03 43.82 32.17 30.23 55.76 54.68TiO2 0.18 0.25 0.76 0.53 � 0.06 � �Al2O3 3.24 3.43 5.83 5.72 0.06 0.05 22.95 22.33Cr2O3 � � 0.09 0.14 0.07 � � �FeO 20.05 13.48 25.51 24.97 36.30 60.24 1.23 0.80MnO 0.21 � 0.08 0.17 0.16 0.21 � �MgO 6.79 10.26 3.49 1.43 4.23 1.52 � 0.04CaO 21.17 22.35 21.07 21.32 22.47 6.46 0.13 �CoO 1.22 1.24 � 1.59 3.66 1.90 � 0.10CuO � 0.12 � � 0.19 � � �ZnO � 0.12 0.19 � 0.18 � � �Na2O 0.10 � 0.03 0.17 � 0.04 0.29 0.94K2O 0.07 0.06 � 0.03 0.08 0.08 20.52 20.76SO2 0.03 0.03 � 0.02 � � � �Total 100.26 100.09 100.08 99.91 99.57 100.79 100.88 99.65

Si (a.p.f.u.**) 1.876 1.882 1.760 1.804 1.005 0.995 2.009 2.005Ti 0.005 0.007 0.023 0.016 � � � �Al 0.152 0.156 0.281 0.278 � � 0.975 0.965Fe 0.666 0.435 0.873 0.860 0.949 1.658 0.037 0.024Mg 0.402 0.590 0.213 0.088 0.197 0.074 � �Ca 0.902 0.924 0.924 0.941 0.752 0.228 � �Co 0.039 0.038 0.002 0.053 0.092 0.050 � �Na � � � � � � 0.020 0.067K � � � � � � 0.943 0.971Scat. 4.042 4.032 4.076 4.040 2.995 3.005 3.984 4.032

Proportion of end-members (mol.%)Wo 46 48 46 50 Fa 50 84En 20 30 11 5 Fo 10 4Fs 34 22 43 45 La 40 12

* Slag I, Nkana; Slag II, Mufulira� not detected; Wo � wollastonite (Ca2Si2O6), En � clinoenstatite (Mg2Si2O6), Fs � clinoferrosilite (Fe2Si2O6), Fa� fayalite (Fe2SiO4), Fo � forsterite (Mg2SiO4), La � larnite (Ca2SiO4)a.p.f.u.: atoms per formula unit** structural formulae calculated on the basis of 6 (clinopyroxene), 4 (olivine) and 6 (leucite) oxygens

MINERALOGY OF SLAGS, ZAMBIA

589

Anorthite (CaAl2Si2O8) was observed exclu-

sively in A4 forming laths included in K-rich

interstitial glass. The composition corresponds to

An87 according to EPMA.

Spinels occur as minute dendrites (Fig. 2a)

included in silicate matrix, as euhedral crystals up

to 50 mm in size (magnetite, Fe3O4, not shown) or

as larger, zoned crystals >100 mm (Cr-spinels,

Fig. 2b). The zoned Fe-Cr spinels observed in the

slag I type have Cr-poor rims (Table 4). Up to

5.12 wt.% of CoO (0.133 a.p.f.u.) was detected

for Fe in the spinel structure (Table 4). Spinels

also concentrate Cu (up to 4.11 wt.% CuO, i.e.

0.101 a.p.f.u.) and to a lesser extent Zn (up to

0.99 wt.% ZnO, i.e. 0.028 a.p.f.u.) (Table 4). The

Ca, Na and K concentrations are considered to be

impurities and were not included in the calcula-

tion of the chemical formula.

Silicate glass is a ubiquitous phase in slags,

indicating the rapid cooling of the slag melt. Two

types of glasses were observed: (1) glass solidifying

at the end of crystallization sequences, filling spaces

between the earlier precipitated phases (referred to

hereafter as ‘interstitial glass’) (Fig. 2a,b,d); (2)

TABLE 4. Selected microprobe analyses of spinels.

Assemblage A1 A1 A1 A2 A2 A5Type* Slag I Slag I Slag I Slag I Slag I Slag IISample Z1C Z1Aa Z1Ab Z13Da Z13Db Z7A

SiO2 (wt.%) 0.16 0.05 0.41 0.03 0.20 0.19TiO2 1.29 0.19 0.83 0.15 0.75 �Al2O3 2.08 14.16 3.56 14.21 5.43 �Cr2O3 1.89 53.01 10.74 57.01 19.35 �Fe2O3 57.72 3.57 51.52 5.36 43.53 65.98FeO 27.31 7.35 29.85 2.59 27.54 29.92MnO � � � � � 0.14MgO 1.33 11.73 0.52 21.24 1.76 �CaO 0.31 0.10 0.21 0.05 0.03 0.03CoO 3.69 5.12 1.28 � 2.85 �CuO 2.56 4.11 � 0.08 � 3.54ZnO � � � 0.41 � 0.09Na2O 0.09 � 0.03 � � �K2O 0.05 � 0.03 � � �SO2 � � 0.35 � � 0.47

Total 98.48 99.39 99.33 101.13 101.44 100.36

Si (a.p.f.u.**) 0.006 0.002 0.016 0.001 0.007 0.007Ti 0.038 0.005 0.024 0.003 0.020 �Al 0.095 0.542 0.158 0.503 0.230 �Cr 0.058 1.361 0.320 1.353 0.551 �Fe3+ 1.686 0.087 1.463 0.121 1.179 1.933Fe2+ 0.887 0.200 0.942 0.065 0.829 0.974Mn � � � � � 0.004Mg 0.077 0.568 0.029 0.950 0.094 �Co 0.115 0.133 0.039 � 0.082 �Cu 0.075 0.101 � 0.002 � 0.104Zn � � � 0.009 � 0.003Scat. 3.037 2.999 2.991 3.007 2.992 3.025

* Slag I, Nkana; Slag II, Mufuliraa core of the crystalb rim of the crystal� not detecteda.p.f.u.: atoms per formula unit** structural formulae calculated on the basis of 4 oxygens

590

M. VITKOVA ET AL.

glass from vitreous slags produced by granulation

(not shown). Analytical results by EPMA of both

glass types are given in Table 5. Greater Cu

concentrations (up to 6.90 wt.% CuO, average

0.77 wt.%) were observed in the interstitial glass of

the Nkana slags (type I). The mean CoO values are

slightly greater in the glass from Chambishi vitreous

slags (0.27 wt.%) in contrast to the interstitial glass

(Nkana 0.22 wt.%, Mufulira 0.12 wt.%).

Quartz corresponds exclusively to unmelted

relics of the furnace charge. The presence of

unmelted gangue (or flux) grains has been

reported in many investigations devoted to

historical pyrometallurgical slags (Ettler et al.,

2009b; Saez et al., 2003). When quartz is present

in the slag (Figs 2b, 3b), we speculate that: (1) the

temperature in the furnace was not high enough to

melt completely the furnace charge (Ettler et al.,

2009b); (2) the duration of melting was insuffi-

cient (kinetics effect) (Ettler et al., 2009b); or (3)

the furnace charge was not defined correctly and

the SiO2 flux was overestimated.

Sulphides and metallic inclusionsCu-Fe and Co-Fe sulphides were detected in

all the studied samples (Figs 2, 3). The

stoichiometry of some Cu-Fe sulphides varies

according to the compositional field of bornite,

which depends heavily on temperature (Cabri,

1973; Fleet, 2006). The EPMA revealed that Cu-

(Fe) sulphides are mainly represented by bornite

(Cu5FeS4) and digenite (Cu9S5) or by a solid

solution between Cu9S5 and chalcocite (Cu2S)

(Table 6). Digenite systematically contains small

amounts of Fe (Table 6). This feature was

observed by Morimoto and Koto (1970), who

suggested that Fe is the digenite structure

stabilizer. Troilite (FeS) and chalcopyrite

(CuFeS2) were observed locally. Nearly 6% of

Fe sites in the troilite structure are occupied by

(Cu+Co) (Table 6).

The Co-bearing phases are mainly represented

by cobaltpentlandite ((Co,Ni,Fe)9S8) (Table 6).

Galena (PbS) as an accessory phase was observed

in sample Z11. Although metallic Cu is frequently

present, metallic Pb and Bi are rare. In addition to

the (Fe,Co)2As compound, a disordered Fe-Co

alloy containing ~60 at.% Fe and enriched in Cu

and As, was observed (Table 6).

Secondary phases

Secondary phases of dark green to yellow green,

pink, rusty, red and white colours were observed

TABLE 5. Average compositions of glasses according to electron probe microanalyses.

—————— Interstitial glassa —————— ———— Vitreous slagb ————Type Slag I n = 20 Slag II n = 3 Slag I Slag II Slag III n = 60

min max mean min max mean Z13C Z7A min max mean

SiO2 51.73 76.41 64.15 38.07 39.28 38.77 61.79 36.35 48.02 53.62 51.01TiO2 0.08 3.48 0.87 0.58 0.68 0.61 0.67 0.52 0.50 0.71 0.60Al2O3 8.12 18.31 11.73 5.58 5.96 5.73 10.38 5.10 8.90 9.92 9.31Cr2O3 � 0.18 0.06 � 0.07 � 0.12 0.08 � 0.34 0.16FeO 0.80 19.31 6.78 29.74 35.25 31.85 7.29 37.58 12.76 18.54 14.60MnO � 0.14 � � 0.07 � 0.06 0.10 � 0.29 0.10MgO � 3.27 1.42 1.98 3.12 2.48 2.77 2.94 4.37 6.51 5.49CaO 1.36 10.64 5.27 14.03 18.54 16.45 11.15 12.62 12.67 15.91 14.53CoO � 0.91 0.22 0.06 0.17 0.12 0.09 0.29 � 0.48 0.27CuO � 6.90 0.77 � 0.25 0.08 0.20 � � 2.54 0.32ZnO � 0.28 � - 0.20 0.07 � 0.16 � 0.44 �Na2O 0.08 0.70 0.27 0.36 0.61 0.49 0.10 0.28 � 0.33 0.21K2O 2.68 10.39 6.64 1.95 4.15 3.25 4.26 2.33 2.88 3.64 3.21SO2 � 0.09 0.02 0.04 0.06 0.05 � 0.36 0.03 0.56 0.18

Total 95.88 99.98 98.28 98.85 101.30 100.02 98.86 98.70 98.90 102.74 100.03

* Slag I, Nkana; Slag II, Mufulira; Slag III, Chambishia interstitial glass filling the space between crystalsb glass from vitreous slag samples� not detected; n number of analyses

MINERALOGY OF SLAGS, ZAMBIA

591

TABLE6.Selectedmicroprobeanalysesofsulphides

andmetallicphases.

Assem

blage

A1

A1

A3

A1

A2

A3

A2

A2

A5

A5

Type*

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagI

SlagIII

SlagIII

Sam

ple

Z1B

Z1B

Z9

Z10C

Z13D

Z11

Z13D

Z13D

Z14

Z14

Phase

Bornite

Digenite

Cu9S5-Cu2Sa

Chalcocite

Troilite

Chalcopyrite

Cu-sulphideb

(Co,Fe,Ni)9S8c

(Co,Fe)

2As

Alloyd

Form

ulae

Cu5FeS

4Cu9S5

Cu2S

FeS

CuFeS

2

S(w

t.%)

25.31

22.09

20.40

20.27

35.71

34.06

28.63

32.91

1.04

0.03

Fe

10.34

1.57

1.79

0.08

58.54

29.49

17.98

20.86

28.25

55.36

Ni

0.16

��

��

��

0.39

2.56

0.56

Cu

63.60

77.64

77.67

80.19

0.98

33.40

50.59

4.55

1.19

1.15

Zn

�0.05

��

��

0.14

0.08

0.36

�As

��

��

��

0.15

0.16

37.59

6.11

Pb

��

��

��

0.13

0.10

��

Co

��

��

3.13

��

41.59

29.57

37.67

Bi

��

��

�0.08

�0.12

��

Total

99.41

101.35

99.86

100.54

98.36

97.03

97.62

100.76

100.56

100.88

S(at.%)

39.90

35.52

33.65

33.35

49.94

50.20

44.30

46.91

2.02

0.06

Fe

9.36

1.45

1.70

0.08

46.99

24.95

15.97

17.06

31.42

56.94

Ni

0.14

��

��

��

0.31

2.71

0.54

Cu

50.60

62.99

64.66

66.57

0.69

24.83

39.50

3.27

1.16

1.04

Zn

�0.04

��

��

0.10

0.06

0.34

�As

��

��

��

0.10

0.10

31.17

4.69

Pb

��

��

��

0.03

0.02

��

Co

��

��

2.38

��

32.25

31.17

36.72

Bi

��

��

�0.02

�0.03

��

S(a.p.f.u.**)

4.000

5.000

1.000

1.000

1.000

2.000

1.000

8.000

0.065

�Fe

0.938

0.204

0.050

0.002

0.941

0.994

0.361

2.910

1.008

0.569

Ni

0.014

��

��

��

0.052

0.087

0.005

Cu

5.072

8.867

1.922

1.996

0.014

0.989

0.892

0.558

0.037

0.010

Zn

�0.005

��

��

0.002

0.010

0.011

�As

��

��

��

0.002

0.017

1.000

0.047

Pb

��

��

��

0.001

0.004

��

Co

��

��

0.048

��

5.500

1.000

0.367

Bi

��

��

�0.001

�0.004

��

Scat.

6.024

9.076

1.972

1.998

1.003

1.984

1.256

9.038

2.143

0.998

592

M. VITKOVA ET AL.

on the Nkana slag surfaces (type I, A1�A4,

Fig. 4), occurring as coatings or stains or filling

slag pores. Weathering features of the Mufulira

slags (type II) were only rarely observed and are

not included in the results presented here. The

main phases identified by SEM/EDS and

confirmed by XRD (Fig. 5) comprise sulphates,

carbonates and oxides. The formation of

secondary phases depends mainly on the element

speciation and does not directly reflect the bulk

chemical composition of slags (Table 1).

Green coatings or stains of brochantite

(Cu4SO4(OH)6), often associated with malachite

(Cu2(CO3)(OH)2), were typically observed on

surfaces (A1�A4; Fig. 5a). Compact crusts of

Cu-rich secondary phases generally occur on

samples with large Cu bulk concentrations (e.g.

Z13B, A4; Fig. 4a). White nodules, needles or

fibres of calcite (CaCO3) occur locally in

assemblage A1 (Fig. 5b). Globular pink Ca,Mn-

bearing sphaerocobaltite (CoCO3) coats Co-rich

samples of assemblage A1 (Figs 4b, 5c; see also

Table 1) and is associated with CaCO3 fibres.

Sphaerocobaltite was not detected on Co-rich

samples of assemblage A3 as Co substitutes in the

olivine structure. A red-coloured coating of

hematite (Fe2O3) was observed on the surface of

sample Z13A (A1; Fig. 5d) and on some other

samples.

Weathering products were not observed on

sample Z11 (A3), which contains large concentra-

tions of Cu (8.6 wt.%) and Co (2.4 wt.%) and is

essentially composed of spinels (chromite and

magnetite) and sulphides.

Discussion

Chemical compositions of slagsThe SiO2-FeO-CaO ternary diagram (Osborn and

Muan, 1960) was used to estimate the tempera-

tures of slag formation (Fig. 6) (Ettler et al.,

2009b). Two sets of data were used: (1) bulk

chemical compositions (Table 1); and (2) glass

composition of vitreous slags (Table 5). This was

because some samples given in Table 1 contain

unmelted quartz gangue. Consequently, we

consider that vitreous slags, which represent

silicate liquids, can improve the temperature

estimation obtained from the bulk compositions.

The plot of these data in the SiO2-FeO-CaO

ternary diagram indicates temperatures ranging

from 1150 to 1400ºC, except for a few anomalous

samples. These estimates correspond well with

the furnace temperatures of Cu smelting*SlagI,Nkana;

SlagIII,Cham

bishi

asolidsolutionbetweenCu2Sand(Cu,Fe)

9S5

bnotcorrespondingto

anystructuralform

ula,possible

replacementofborniteCu5FeS

4byidaite

Cu5FeS

6csolidsolutionbetween(Fe,Ni)9S8andCo9S8

dcomplexFe-Co-A

s-Cu-N

ialloy

�notdetected

a.p.f.u.:atomsper

form

ula

unit

**sulphides

calculatedonthebasisofthefollowingnumbersofSatom

per

form

ula

unit:4forbornite,5fordigenite,1forCu2S-(Cu,Fe)

9S5,1forchalcocite,1for

troilite,2forchalcopyrite,1forundetermined

Cu-Fesulphide,8forcobaltpentlandite;theform

ulaof(Co,Fe)

2Aswas

calculatedonthebasisof1Asandthatofalloy

forScat=1

MINERALOGY OF SLAGS, ZAMBIA

593

according to Davenport et al. (2002). Glasses

from Chambishi vitreous slags yield apparently

anomalous melting temperatures close to 1600ºC.

However, Jones et al. (2002) reported slag tapping

temperatures at the Chambishi smelter of

~1500ºC. The temperature overestimation for

samples located close to the SiO2 pole may be

due to the presence of unmelted grains of quartz

gangue or to the presence of other elements (cf.

Ettler et al., 2009b).

Phase formation and binding of metals in slags

The mineralogical investigation of the Zambian

slags reveals the presence of phases comparable

to those reported from other smelting slags (Ettler

et al., 2001; Kierczak et al., 2009; Piatak and

Seal, 2010; Puziewicz et al., 2007). The spinel-

type phase is generally the first one crystallizing

from the slag melt, as in other slags (Ettler et al.,

2001; Piatak and Seal, 2010; Puziewicz et al.,

2007). Spinel crystals are zoned with cores

enriched in MgCr2O4 end-member and rims

have the composition of magnetite. This zoning

is related to the variation of the oxygen fugacity,

as the formation of magnetite-rich rims requires

more oxidizing conditions (Czamanske et al.,

1976). The copper and cobalt concentrations are

highly variable, and are not correlated with the

spinel composition. Ettler et al. (2009a) reported

small concentrations of Cu in spinel from slags

from Tsumeb, Namibia. Magnetite rims are

enriched in Si (Table 4), indicating a high

temperature of precipitation (Berger et al., 1982).

According to microscopic observations

(Fig. 2), slag solidification continued either by

the crystallization of leucite or of clinopyroxene.

The precipitation sequence depends essentially on

the K concentration in the slags and the crystal-

lization of other silicates (olivine, clinopyroxene)

follows an increase in the Si activity. The

solidification process of studied slags ends

systematically with the formation of interstitial

glass. Except for leucite, similar crystallization

sequences were observed in Pb-Zn slags from

Prıbram (Ettler et al., 2001; Ettler et al., 2009b)

and Zn slags from Illinois (Piatak and Seal, 2010).

The presence of leucite was also noted in Zn slags

from Poland (Puziewicz et al., 2007) and reflects

an initially large K concentration in the slag melt.

Melilite was not detected in slags from Zambia,

probably due to the different composition of melts

(smaller Ca content) compared to slags from other

sites (e.g. Ettler et al., 2009a; Kierczak et al.,

2009). The presence of corroded quartz crystals is

an indicator of an insufficiently molten furnace

charge, as was also observed at other sites (Ettler

et al., 2009b; Saez et al., 2003).

Some Co, Cu and Zn can also be incorporated

into the crystal structures of silicates and in glass,

where these metals replace Fe or Mg due to their

similar ion radii. The greatest Co contents were

observed in olivine (up to 7.15 wt.% CoO),

whereas much smaller concentrations were

observed in clinopyroxene (1.8 wt.% CoO) and

glass (0.91 wt.% CoO). Despite the amount of

published data, no information is available

concerning Co contents in silicates from slags.

However, large Co concentrations in olivine and

clinopyroxene were observed in natural and

experimental systems (e.g. Mukhopadhyay and

Jacob, 1996; Sugawara and Akaogi, 2003). In the

slags studied here, Cu and Zn are present in

silicates in small concentrations. In contrast, Zn

was commonly found in silicates from slags at

numerous smelting sites and it has even been

suggested that spinels are the most important Zn

carriers in slags (Ettler et al., 2001, 2009a,b;

Lottermoser, 2002; Manasse and Mellini, 2002;

Piatak and Seal, 2010; Puziewicz et al., 2007).

FIG. 4. Coatings of secondary phases on the Nkana slags: (a) green crust of malachite with brochantite (Z13B, A4);

(b) pink crust of sphaerocobaltite (Z10C, A1).

594

M. VITKOVA ET AL.

The EPMA results showed that the main

carriers of metals (Cu, Co, Zn, Pb) are sulphides

and intermetallic compounds within the silicate

matrix (Fig. 3; Table 6). The compositional

variability of the Cu-Fe sulphides can be depicted

from a Cu-Fe-S isothermal section at 800ºC

(Raghavan, 2006; Tsujimura and Kitakaze,

2004; Fig. 7), indicating that these ternary

phases yield compositions between the inter-

mediate solid solution (iss) and bornite.

According to Picot and Johan (1982), bornite

can be replaced by idaite (Cu5FeS6); the

FIG. 5. Secondary phases developed on slag surfaces (Nkana slags), micrographs in secondary electrons (SEM) with

corresponding EDS spectra and interpretations based on EDS and XRD results.

MINERALOGY OF SLAGS, ZAMBIA

595

replacement starts as tiny lamellae along clea-

vages. The bornite stability field is larger at higher

temperatures (>600ºC). Consequently, the

presence of bornite is systematically observed in

quenched samples. At lower temperatures, the

bornite stability field shrinks and chalcocite/

digenite and bornite can coexist at 300ºC (Cabri,

1973). The Cu-Fe-S phases quenched from high

temperatures (>900ºC) are far beyond the ideal

bornite or iss (cubanite, CuFe2S3) compositions,

leading to large variations in the Cu and Fe

contents (Fleet, 2006). Troilite has nearly

stoichiometric composition with a very limited

replacement of Fe by Cu and Co (Fig. 7).

Weathering features and potential environmental impacts

The presence of secondary phases on the slag

surfaces indicates that slags can undergo the

weathering processes on the dumps. The majority

of the secondary phases were observed on the

Nkana slag dumps, in particular on slag fragments

sampled in the close-to-surface layers of the

dumps. The prevailing presence of carbonates

(sphaerocobaltite, calcite, malachite) and Fe

oxides indicates near-neutral to alkaline alteration

conditions. Ettler et al. (2003b) showed that

malachite is stable at pH >5, whereas brochantite

can form in slightly more acidic environments

(pH >4) and needs a large sulphate supply. At

numerous mining and smelting sites, the

secondary phases were considered to be a risk

for release of metallic elements. The two key

factors influencing contaminant release from

secondary efflorescence minerals are rain and

changes in the pH of percolating/alteration waters

(Bril et al., 2008; Lottermoser, 2005; Piatak and

Seal, 2010). Seasonal variations between dry and

wet periods may lead to the formation of

secondary phases and their rapid dissolution

during the rain (Ettler et al. , 2009a ;

Lottermoser, 2005). During our sampling

campaigns between May and September, the

dump environments were rather dry with no

visible percolating water. Similarly to the

Tsumeb smelting site in Namibia (Ettler et al.,

2009a), thunderstorm and floods occurring in

Zambia between October and March may be

responsible for Co and Cu mobilization from the

secondary phases on the slag dumps. Another

mechanism enhancing the mobilization of toxic

elements is acidification, which can be either

FIG. 6. Plot of bulk-slag analyses and glass analyses in the ternary SiO2�FeO�CaO diagram (Osborn and Muan,

1960).

596

M. VITKOVA ET AL.

related to the long-term dissolution of primary

sulphide phases in slags (generating acidity by

oxidation) or to direct dissolution of sulphates

producing leachates with low pH (Bril et al.,

2008; Ettler et al., 2003b; Lottermoser, 2005;

Piatak and Seal, 2010). This process will mainly

affect the stability of carbonates and, at extremely

low pH, also Fe oxides, which both commonly

coat the surfaces of the Zambian slags. So far, no

detailed investigation of metal/metalloid contam-

ination of groundwater in the vicinity of the

Zambian mining/smelting sites has been carried

out. However, according to our field observations

and investigations of mine tailing ponds from the

Zambian Copperbelt (Sracek et al., 2010), slag

dumps can be assumed to be less important

sources of pollution of groundwater and seepage

water in the area. The kinetics of release of metals

from the Cu-Co slags and the metal mobilities

related to the alteration of primary and the

formation of secondary phases must be further

evaluated and work on this aspect is currently in

progress in the laboratory of M. Vıtkova.

Conclusions

Slags from three important Cu-Co smelters in the

Copperbelt Province, Zambia, were studied using

multi-method mineralogical and chemical

approaches (XRD, SEM/EDS, EPMA, bulk

chemistry). These results represent the first step

of a larger project concerning the environmental

assessment of slags from the Zambian smelting

areas. The slags are composed of Ca-Fe silicates

(clinopyroxene, olivine), leucite, oxides (spinel-

series phases), ubiquitous silicate glass and

sulphide/metallic droplets. Although Cu and Co

substituting for Fe or Mg in the structure of

silicates (olivine, clinopyroxene), spinels and in

glass were observed, the main carriers of Cu and

Co are sulphides (bornite, digenite, chalcocite,

cobaltpentlandite). Furthermore, Co-bearing inter-

metallic phases and alloys form droplets in the

silicate slag matrix. The presence of secondary

phases, such as malachite, brochantite and

sphaerocobaltite, indicate that slags can be

partially reactive with respect to the water/

FIG. 7. Plot of the chemical compositions of Cu-Fe sulphides in the Cu-Fe-S isothermal section at 800ºC.

Abbreviations: bn � bornite, iss � intermediate solid solution, L � liquid, po � pyrrhotite (diagram compiled from

Raghavan, 2006; Tsujimura and Kitakaze, 2004).

MINERALOGY OF SLAGS, ZAMBIA

597

atmosphere contact, and toxic metals and

metalloids can be released from the slag during

weathering on the dumps. A whole battery of

leaching tests coupled to thermodynamic specia-

tion-solubility modelling is currently in progress

in the laboratory of M. Vıtkova in order to

provide information on the kinetics and controls

on the release of Cu and Co from these materials

under simulated dump conditions.

Acknowledgements

This study was supported by the Czech Science

Foundation (GACR 205/08/03211) and the

Ministry of Education, Youth and Sports of the

Czech Republic (MSM 0021620855). The student

project carried out by Martina Vıtkova was

supported by the Grant Agency of Charles

University (GAUK 53009) and University

Student Project No. SVV261203. The authors

are grateful to the technical staff of the Chambishi

Smelter for help with slag sampling at the

Chambishi site and T. Gonzales from the

Mufulira smelter for discussion of the smelting

technologies. Laboratory and technical assistance

were provided by a number of colleagues: Marie

Fayadova and Vera Vonaskova (chemical

analyses), Radek Prochazka (SEM/EDS), Petr

Drahota (XRD) and Anna Langrova (EPMA).

Thorough reviews by Prof. Jacek Puziewicz

(University of Wrocław), Dr Nadine M. Piatak

(USGS) and Prof. Hubert Bril (Universite de

Limoges) helped to significantly improve the

manuscript.

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