primaryandsecondaryphasesincopper-cobaltsmelting … · tonnes of cuper year. in 1994, an el...
TRANSCRIPT
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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