Mineralogical Magazine, August 2010, Vol. 74(4), pp. 581–600
Primary and secondary phases in copper-cobalt smelting slags
from the Copperbelt Province, Zambia
M. VÍTKOVÁ1,*, V. ETTLER1, Z. JOHAN2, B. KŘÍBEK3, O. ŠEBEK4
1
2
3
4
AND
M. MIHALJEVIČ1
Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Faculty of Science,
Albertov 6, 128 43 Prague 2, Czech Republic
Bureau des Recherches Géologiques et Minières (BRGM), av. Claude Guillemin, 45060 Orléans, cedex 2,
France
Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic
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]
ABSTR ACT
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)2As) and alloys. Weathering features corresponding to the
presence 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 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.
K EY WORDS : slags, smelting, Copperbelt, Zambia, copper, cobalt, clinopyroxene, leucite.
Introduction
THE environmental assessment of smelting dumpsites and their vicinity has been the subject of
numerous studies (Bril et al., 2008; Ettler et al.,
2001, 2003a, 2009a; Ganne et al., 2006; Křı́bek et
al., 2006, 2010; Parsons et al., 2001; Piatak et al.,
2004). Metallurgical slags represent the most
* E-mail: [email protected]
DOI: 10.1180/minmag.2010.074.4.581
# 2010 The Mineralogical Society
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 weathering processes. The chemical and mineralogical
characterization of the waste materials is essential
for their environmental assessment prior to the
application of suitable environment-friendly
s t a b i l i z a t i o n / s o l i d i fi c a t i o n te c h n o l o g i e s
(Piantone, 2004), dumping scenarios (Ettler et
M. VÍTKOVÁ ET AL.
arkoses and sandstones (e.g. Chambishi SE
deposits); and (4) vein-type quartz-carbonatesulphide 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).
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 district
The Zambian Copperbelt forms the southeastern
part of the Neoproterozoic Lufilian Arc, a PanAfrican orogenic belt consisting of metasedimentary 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 dunebedded 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 mineralization in footwall arkoses and conglomerates
that are referred to as ‘footwall arenite ore bodies’
(e.g. Mufulira deposit); (2) stratabound disseminated mineralization in black shales referred to as
‘Ore Shale ore bodies’ (Nkana and Chingola
deposits); (3) stratabound massive sulphide mineralization between black shales and hangingwall
History of smelting technologies
582
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 facilities. 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
MINERALOGY OF SLAGS, ZAMBIA
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 commissioned 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 approximately 850,000 tonnes of Cu concentrate per year.
The discarded slag contains up to 1 wt.% Cu
(T. Gonzáles, Mufulira smelter, pers. comm.).
The Chambishi smelter is currently reprocessing 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 reduction 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 sampling
The 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.
583
M. VÍTKOVÁ ET AL.
(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’’. These
slags occur as black, glassy fragments up to 10 cm
in size and a few mm thick (n = 2).
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 microanalysis
Polished 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 microscope (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: accelerating 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.
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 procedure 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 gravimetric 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).
Results
Bulk chemical composition
The slags studied are mainly composed of SiO2
(15 60 wt.%), FeO + Fe2O3 (7.1 48 wt.%), CaO
584
39.87
0.40
5.68
9.50
19.21
0.06
3.33
13.65
0.09
2.07
0.18
0.66
184
<0.3
11221
7622
12
<0.22
<6
6.2
237
416
199
193
<15
619
30
96.79
SiO2 (wt.%)
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
Stot
LOI
Cr (ppm)
Ag
Cu
Co
Ni
Cd
Sb
Pb
Zn
Ba
Sr
Mo
Sn
As
Bi
Totalb (wt.%)
98.45
254
<0.3
3492
3834
12
<0.22
<6
<1.5
44
623
443
322
<15
872
0.5
48.60
0.63
8.96
1.50
5.60
0.04
6.45
21.87
0.13
3.17
0.05
0.46
A1
Slag I
Z1B
100.84
169
0.5
13947
8405
36
<0.22
<6
10
123
546
413
208
<15
603
6.5
47.11
0.58
7.79
6.62
13.35
0.05
3.55
16.14
0.09
2.25
0.30
0.55
A1
Slag I
Z1C
96.37
544
2.1
8548
14684
24
<0.22
<6
8.0
256
453
155
182
<15
628
3.6
41.74
0.50
5.58
5.09
25.34
0.07
3.21
7.76
0.09
2.17
0.13
2.14
A1
Slag I
Z10B
94.67
a
760
<0.3
15553
12454
51
<0.22
<6
24
286
385
181
153
19
947
14
39.13
0.58
6.16
6.68
21.23
0.07
3.48
10.56
0.08
2.11
0.07
1.45
A1
Slag I
Z10C
* Slag I, Nkana; Slag II, Mufulira; Slag III, Chambishi;
A1
Slag I
Z1A
Assemblage
Type*
Sample
98.28
246
<0.3
4625
720
9.0
<0.22
<6
26
247
349
467
103
<15
750
3.0
41.12
0.63
5.09
8.74
19.37
0.06
2.25
18.03
0.19
1.84
0.19
<0.01
A1
Slag II
Z8B
98.78
233
0.3
18165
6251
24
<0.22
<6
32
99
477
386
145
<15
683
4.2
51.64
0.63
9.92
3.99
10.21
0.04
3.25
13.39
0.12
2.45
0.36
0.12
A2
Slag I
Z4
98.14
3841
0.4
8226
7605
15
<0.22
20
106
229
556
347
167
<15
3050
5.4
44.26
0.53
11.39
4.74
14.97
0.08
3.12
12.79
0.17
3.14
0.35
0.18
A2
Slag I
Z13D
mixture of silicate slag and matte;
95.93
65
<0.3
7958
7980
22
<0.22
<6
48
299
442
295
220
<15
1031
14
37.46
0.40
7.44
11.57
16.12
0.07
2.99
15.05
0.07
1.85
0.15
0.91
A1
Slag I
Z13A
b
98.58
163
0.7
19963
4386
16
<0.22
<6
30
177
469
305
147
<15
715
8.1
54.72
0.40
7.58
4.26
12.20
0.05
1.89
9.00
0.14
4.83
0.46
0.42
A3
Slag I
Z9
98.16
7510
12
86314
24104
935
<0.22
39
1134
2287
259
226
162
<15
3642
140
15.53
0.37
3.90
18.79
29.52
0.19
4.80
7.41
0.03
1.13
2.62
1.18
A3
Slag I
Z11a
97.95
89
<0.3
10898
10965
27
<0.22
<6
29
437
518
408
245
<15
1013
5.5
37.56
0.35
8.33
10.52
17.84
0.10
2.56
14.06
0.11
3.57
0.07
0.42
A3
Slag I
Z12
99.70
148
11
353580
7117
149
5.2
<6
658
1487
246
235
118
82
987
1695
22.14
0.75
12.13
5.22
15.77
0.04
0.64
3.18
0.06
1.56
0.09
1.46
A4
Slag I
Z13Ba
102.88
339
0.4
12094
922
15
<0.22
<6
30
168
564
391
61
<15
855
16
60.18
0.73
12.60
0.68
7.42
0.04
3.09
12.08
0.10
4.15
0.11
0.16
A5
Slag I
Z13C
97.87
380
0.6
9349
2716
46
<0.22
<6
26
496
445
341
145
<15
627
2.4
36.88
0.43
5.84
6.53
27.50
0.08
2.91
12.91
0.19
2.57
0.55
<0.01
99.15
883
1.9
3724
2178
8.2
<0.22
<6
39
80
667
370
30
<15
621
<0.1
51.38
0.63
9.66
1.76
11.30
0.10
6.37
13.65
0.15
3.06
0.24
<0.01
99.36
853
<0.3
2050
2414
5.5
<0.22
<6
<1.5
123
731
392
36
17
1191
0.2
48.24
0.58
11.03
1.43
15.12
0.10
4.75
14.29
0.14
2.73
0.17
<0.01
A5
A5
A5
Slag II Slag III Slag III
Z7A
Z5
Z14
Total = sum of oxides + Stot + LOI + metals in elemental form
97.32
89
<0.3
5263
17638
13
<0.22
<6
<1.5
48
1297
406
422
<15
652
1.9
38.40
0.43
7.11
5.96
20.70
0.10
1.88
14.31
0.30
4.62
0.27
0.66
A3
Slag I
Z3
TABLE 1. Bulk chemical compositions of selected slags.
M. VÍTKOVÁ ET AL.
(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 concentrations (>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 compositions 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,
leucite, ol
Cu
metallic copper, CuS
copper sulphide corresponding to Cu9S5 Cu2S solid solution, le
olivine, CoS cobalt sulphide, bn bornite, gl glass, qz quartz, Cr-sp Fe-Cr spinel.
586
MINERALOGY OF SLAGS, ZAMBIA
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 assemblages 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):
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)
A1: (spinel) ? clinopyroxene ? glass
(n = 9, Fig. 2a,b)
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 bornitedigenite 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 CuSbornite 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.
587
M. VÍTKOVÁ ET AL.
TABLE 2. Phases determined using EPMA and XRD, occurring in the slags studied and their relative
abundances in the samplesa.
Group
Name
Composition
Silicates
Diopside-hedenbergite s.s.
Fayalite–forsterite s.s.
Kirschsteinite-monticellite s.s.
Leucite
Anorthite
Quartz
Ca(Fe,Mg)Si2O6
(Fe,Mg)2SiO4
Ca(Fe,Mg)SiO4
KAlSi2O6
CaAl2Si2O8
SiO2
+++
+
++
++
A4
+
+++
++
Oxides
Magnetite
Cr-spinel
Rutile
Cuprite
Delafossite
Fe3O4
Fe2+(Cr,Al,Fe3+)2O4
TiO2
Cu2O
FeCuO2
+++
+
tr
tr
A4
+++
Sulphides
Bornite
Digenite
Chalcocite
Troilite
Chalcopyrite
Co-pentlandite
Co4S3
CoS
Galena
Cu5FeS4
Cu9S5
Cu2S
FeS
CuFeS2
Co9S8
Co4S3
CoS
PbS
+
+
+
+
tr
+
+
+
+
+
+
Elements
Copper
Lead
Bismuth
Cu
Pb
Bi
+
tr
tr
Others
Alloys
Intermetallic compounds
(Fe-Co-As-Cu-Ni)
(Fe,Co)2As
tr
Silicate glass
———— Type* ————
Slag I
Slag II
Slag III
+
+
+
tr
+
tr
++
+
+
+
++
+++
* Slag I, Nkana; Slag II, Mufulira; Slag III, Chambishi
Relative abundance: +++ dominant phase, ++ common phase, + minor phase, tr traces, A4 – dominant in
assemblage A4, (according to observations in polished sections)
s.s. = solid solution
a
only). The predominant crystalline phases (clinopyroxene, olivine) commonly show textures with
skeletal, herring-bone, harissitic, dendritic or
irregular lath-like crystals indicating rapid crystallization (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
(CaMgSi2 O 6-CaFeSi 2O 6) 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 Å): Co 0.75, Cu 0.77,
588
MINERALOGY OF SLAGS, ZAMBIA
TABLE 3. Selected microprobe analyses of clinopyroxene, olivine and leucite.
Assemblage
Type*
Sample
Phase
A1
A1
A1
A3
Slag I
Slag I Slag II Slag I
Z1A
Z1C
Z8B
Z3
——— Clinopyroxene ———
A3
A3
Slag I
Slag I
Z3
Z9
– Olivine –
A2
A3
Slag I
Slag I
Z4
Z3
– Leucite –
SiO2 (wt.%)
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
CoO
CuO
ZnO
Na2O
K2O
SO2
Total
47.20
0.18
3.24
48.75
0.25
3.43
32.17
30.23
0.06
0.05
55.76
54.68
22.95
22.33
20.05
0.21
6.79
21.17
1.22
13.48
60.24
0.21
1.52
6.46
1.90
1.23
0.80
0.08
0.04
0.08
0.29
20.52
0.94
20.76
99.57
100.79
100.88
99.65
Si (a.p.f.u.**)
Ti
Al
Fe
Mg
Ca
Co
Na
K
Scat.
0.10
0.07
0.03
100.26
1.876
0.005
0.152
0.666
0.402
0.902
0.039
4.042
10.26
22.35
1.24
0.12
0.12
0.06
0.03
100.09
1.882
0.007
0.156
0.435
0.590
0.924
0.038
4.032
Proportion of end-members (mol.%)
Wo
46
48
En
20
30
Fs
34
22
43.03
0.76
5.83
0.09
25.51
0.08
3.49
21.07
43.82
0.53
5.72
0.14
24.97
0.17
1.43
21.32
1.59
0.19
0.03
0.06
0.07
36.30
0.16
4.23
22.47
3.66
0.19
0.18
0.17
0.03
0.02
99.91
100.08
1.760
0.023
0.281
0.873
0.213
0.924
0.002
1.804
0.016
0.278
0.860
0.088
0.941
0.053
4.076
4.040
46
11
43
50
5
45
1.005
0.995
0.949
0.197
0.752
0.092
1.658
0.074
0.228
0.050
2.995
Fa
Fo
La
50
10
40
3.005
0.04
0.13
0.10
2.009
2.005
0.975
0.037
0.965
0.024
0.020
0.943
3.984
0.067
0.971
4.032
84
4
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
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 concentrations of Cu and Zn were detected in clinopyroxene (up to 0.2 wt.% of CuO and ZnO).
Chemical compositions of olivine-type phases
vary from fayalite (Fe 2 SiO 4 ) forsterite
(Mg2SiO4) to kirschsteinite (CaFeSiO4)–monticellite (CaMgSiO4) solid solutions: Fo3Fa64La33
589
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.%).
M. VÍTKOVÁ ET AL.
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 calculation 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)
Anorthite (CaAl2Si2O8) was observed exclusively 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
TABLE 4. Selected microprobe analyses of spinels.
Assemblage
Type*
Sample
A1
Slag I
Z1C
A1
Slag I
Z1Aa
A1
Slag I
Z1Ab
A2
Slag I
Z13Da
A2
Slag I
Z13Db
A5
Slag II
Z7A
SiO2 (wt.%)
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
CoO
CuO
ZnO
Na2O
K2O
SO2
Total
0.16
1.29
2.08
1.89
57.72
27.31
0.05
0.19
14.16
53.01
3.57
7.35
0.41
0.83
3.56
10.74
51.52
29.85
0.03
0.15
14.21
57.01
5.36
2.59
0.20
0.75
5.43
19.35
43.53
27.54
0.19
1.33
0.31
3.69
2.56
11.73
0.10
5.12
4.11
0.52
0.21
1.28
21.24
0.05
1.76
0.03
2.85
Si (a.p.f.u.**)
Ti
Al
Cr
Fe3+
Fe2+
Mn
Mg
Co
Cu
Zn
Scat.
0.09
0.05
98.48
99.39
0.08
0.41
0.03
0.03
0.35
99.33
101.13
101.44
0.002
0.005
0.542
1.361
0.087
0.200
0.016
0.024
0.158
0.320
1.463
0.942
0.001
0.003
0.503
1.353
0.121
0.065
0.007
0.020
0.230
0.551
1.179
0.829
0.077
0.115
0.075
0.568
0.133
0.101
0.029
0.039
0.950
0.094
0.082
3.037
2.999
2.991
* Slag I, Nkana; Slag II, Mufulira
core of the crystal
b
rim of the crystal
not detected
a.p.f.u.: atoms per formula unit
** structural formulae calculated on the basis of 4 oxygens
a
590
0.03
3.54
0.09
0.006
0.038
0.095
0.058
1.686
0.887
0.002
0.009
3.007
65.98
29.92
0.14
2.992
0.47
100.36
0.007
1.933
0.974
0.004
0.104
0.003
3.025
MINERALOGY OF SLAGS, ZAMBIA
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).
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; Sáez 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 insufficient (kinetics effect) (Ettler et al., 2009b); or (3)
the furnace charge was not defined correctly and
the SiO2 flux was overestimated.
Sulphides and metallic inclusions
Cu-Fe and Co-Fe sulphides were detected in
all the studied samples (Figs 2, 3). The
stoichiometry of some Cu-Fe sulphides varies
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.
Type
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
CoO
CuO
ZnO
Na2O
K2O
SO2
Total
—————— Interstitial glassa ——————
Slag I n = 20
Slag II n = 3
min
max
mean
min
max
mean
———— Vitreous slagb ————
Slag I Slag II Slag III n = 60
Z13C
Z7A
min
max
mean
51.73
0.08
8.12
61.79
0.67
10.38
0.12
7.29
0.06
2.77
11.15
0.09
0.20
0.80
1.36
0.08
2.68
95.88
76.41
3.48
18.31
0.18
19.31
0.14
3.27
10.64
0.91
6.90
0.28
0.70
10.39
0.09
99.98
64.15
0.87
11.73
0.06
6.78
38.07
0.58
5.58
1.42
5.27
0.22
0.77
1.98
14.03
0.06
0.27
6.64
0.02
98.28
29.74
0.36
1.95
0.04
98.85
39.28 38.77
0.68
0.61
5.96
5.73
0.07
35.25 31.85
0.07
3.12
2.48
18.54 16.45
0.17
0.12
0.25
0.08
0.20
0.07
0.61
0.49
4.15
3.25
0.06
0.05
101.30 100.02
* Slag I, Nkana; Slag II, Mufulira; Slag III, Chambishi
interstitial glass filling the space between crystals
b
glass from vitreous slag samples
not detected; n number of analyses
a
591
0.10
4.26
98.86
36.35
0.52
5.10
0.08
37.58
0.10
2.94
12.62
0.29
0.16
0.28
2.33
0.36
98.70
48.02
0.50
8.90
12.76
4.37
12.67
2.88
0.03
98.90
53.62 51.01
0.71
0.60
9.92
9.31
0.34
0.16
18.54 14.60
0.29
0.10
6.51
5.49
15.91 14.53
0.48
0.27
2.54
0.32
0.44
0.33
0.21
3.64
3.21
0.56
0.18
102.74 100.03
592
S (a.p.f.u.**)
Fe
Ni
Cu
Zn
As
Pb
Co
Bi
Scat.
S (at.%)
Fe
Ni
Cu
Zn
As
Pb
Co
Bi
25.31
10.34
0.16
63.60
S (wt.%)
Fe
Ni
Cu
Zn
As
Pb
Co
Bi
Total
6.024
4.000
0.938
0.014
5.072
39.90
9.36
0.14
50.60
99.41
A1
Slag I
Z1B
Bornite
Cu5FeS4
Assemblage
Type*
Sample
Phase
Formulae
1.972
1.922
8.867
0.005
9.076
1.000
0.050
64.66
62.99
0.04
5.000
0.204
33.65
1.70
35.52
1.45
99.86
77.67
77.64
0.05
101.35
20.40
1.79
A3
Slag I
Z9
Cu9S5-Cu2Sa
22.09
1.57
A1
Slag I
Z1B
Digenite
Cu9S5
1.998
1.996
1.000
0.002
66.57
33.35
0.08
100.54
80.19
20.27
0.08
A1
Slag I
Z10C
Chalcocite
Cu2S
1.003
0.048
0.014
1.000
0.941
2.38
0.69
49.94
46.99
98.36
3.13
0.98
35.71
58.54
A2
Slag I
Z13D
Troilite
FeS
0.001
1.984
0.989
2.000
0.994
0.02
24.83
50.20
24.95
0.08
97.03
33.40
34.06
29.49
A3
Slag I
Z11
Chalcopyrite
CuFeS2
1.256
0.892
0.002
0.002
0.001
1.000
0.361
39.50
0.10
0.10
0.03
44.30
15.97
97.62
50.59
0.14
0.15
0.13
28.63
17.98
A2
Slag I
Z13D
Cu-sulphideb
8.000
2.910
0.052
0.558
0.010
0.017
0.004
5.500
0.004
9.038
46.91
17.06
0.31
3.27
0.06
0.10
0.02
32.25
0.03
32.91
20.86
0.39
4.55
0.08
0.16
0.10
41.59
0.12
100.76
2.143
1.000
0.065
1.008
0.087
0.037
0.011
1.000
31.17
2.02
31.42
2.71
1.16
0.34
31.17
100.56
29.57
1.04
28.25
2.56
1.19
0.36
37.59
A2
A5
Slag I
Slag III
Z13D
Z14
(Co,Fe,Ni)9S8c (Co,Fe)2As
TABLE 6. Selected microprobe analyses of sulphides and metallic phases.
0.998
0.367
0.047
0.569
0.005
0.010
36.72
4.69
0.06
56.94
0.54
1.04
100.88
37.67
6.11
0.03
55.36
0.56
1.15
A5
Slag III
Z14
Alloyd
M. VÍTKOVÁ ET AL.
* Slag I, Nkana; Slag III, Chambishi
a
solid solution between Cu2S and (Cu,Fe)9S5
b
not corresponding to any structural formula, possible replacement of bornite Cu5FeS4 by idaite Cu5FeS6
c
solid solution between (Fe,Ni)9S8 and Co9S8
d
complex Fe-Co-As-Cu-Ni alloy
not detected
a.p.f.u.: atoms per formula unit
**sulphides calculated on the basis of the following numbers of S atom per formula unit: 4 for bornite, 5 for digenite, 1 for Cu2S-(Cu,Fe)9S5, 1 for chalcocite, 1 for
troilite, 2 for chalcopyrite, 1 for undetermined Cu-Fe sulphide, 8 for cobaltpentlandite; the formula of (Co,Fe)2As was calculated on the basis of 1 As and that of alloy
for Scat = 1
MINERALOGY OF SLAGS, ZAMBIA
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 (CaCO 3) occur locally in
assemblage A1 (Fig. 5b). Globular pink Ca,Mnbearing 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 concentrations 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 slags
The SiO2-FeO-CaO ternary diagram (Osborn and
Muan, 1960) was used to estimate the temperatures 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
593
M. VÍTKOVÁ ET AL.
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).
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 spineltype 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 crystallization of other silicates (olivine, clinopyroxene)
594
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
Přı́bram (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; Sáez 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).
MINERALOGY OF SLAGS, ZAMBIA
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.
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
595
(Raghavan, 2006; Tsujimura and Kitakaze,
2004; Fig. 7), indicating that these ternary
phases yield compositions between the intermediate solid solution (iss) and bornite.
According to Picot and Johan (1982), bornite
can be replaced by idaite (Cu 5 FeS 6 ); the
M. VÍTKOVÁ ET AL.
FIG. 6. Plot of bulk-slag analyses and glass analyses in the ternary SiO2 FeO CaO diagram (Osborn and Muan,
1960).
replacement starts as tiny lamellae along cleavages. 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
596
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
MINERALOGY OF SLAGS, ZAMBIA
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).
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 contamination 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 (Šráček 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. Vı́tková.
Conclusions
597
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 (spinelseries 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 intermetallic 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/
M. VÍTKOVÁ ET AL.
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 speciation-solubility modelling is currently in progress
in the laboratory of M. Vı́tková 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 (GAČR 205/08/03211) and the
Ministry of Education, Youth and Sports of the
Czech Republic (MSM 0021620855). The student
project carried out by Martina Vı́tková 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. Gonzáles from the
Mufulira smelter for discussion of the smelting
technologies. Laboratory and technical assistance
were provided by a number of colleagues: Marie
Fayadová and Věra Vonásková (chemical
analyses), Radek Procházka (SEM/EDS), Petr
Drahota (XRD) and Anna Langrová (EPMA).
Thorough reviews by Prof. Jacek Puziewicz
(University of Wrocław), Dr Nadine M. Piatak
(USGS) and Prof. Hubert Bril (Université de
Limoges) helped to significantly improve the
manuscript.
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