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Mineralogy of primary phases in slags and mattes from the Tsumeb smelter (Namibia)

V. Ettler1*, Z. Johan2, B. Kříbek3, and H. Nolte4 1Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic

2BRGM, 3, av. Claude Guillemin, 45060 Orléans cedex 2, France 3Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic

4General Manager of the Ongopolo Smelter, Tsumeb, Namibia*Corresponding author (e-mail: [email protected])

The mineralogy of primary phases of historical and recent slags from smelters in the Tsumeb district (Namibia) has been studied. X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron microprobe (EPMA) techniques were used for identification of the individual slag phases. The 100-year old slags (slag I) are mainly composed of Pb-bearing feldpars, Cu-rich spi-nels, delafossite-like phases and complex Ca-Pb arsenates. In contrast, 30-40 year old and recent granulated slags (slags II and III) are mainly composed of high-temperature phases: Ca-Fe alumosilicates (olivines, melilite), Pb- and Zn-bearing silicate glass, spinel oxides and small metallic/sulphide inclusions trapped within the silicate glass. The differences in the mineralogical composition of the slags are related to the initial composition and the cooling regime of the melt. The slag I melts were probably low in alkalis, Ca and Si and strongly enriched in Cu, Pb and As and were cooled very slowly. In addition, the presence of Cu1+-bearing phases (delafossite CuFeO2 and mcconnelite CuCrO2) in these slags indicates highly variable redox conditions in the historical shaft furnaces. Furthermore, the Ca-Pb arsenates found in the matrix indicate that these slags have undergone an alteration process in the dumping environments. In contrast, more recent granulated slags (II and III) were cooled very rapidly (presence of silicate glass) and the slag melt was poorer in Cu and enriched in alkalis, Ca and Zn.

Introduction

Smelting slags are the most important mineral wastes resulting from metallurgical processes. They correspond to the silicate melt produced during the py-rometallurgical recovery of base metals by reducing fu-sion in a blast furnace. Slags are often enriched in toxic elements, in particular heavy metals (Pb, Cu, Zn) and in metalloids (As, Sb) (Ettler, 2002; Piatak et al., 2004). Mineralogical investigations of slags are essential to understand the binding of toxic elements in primary phases and represent the first step in the assessment of their environmental impact (Ettler et al., 2000, 2001; Lottermoser, 2002; Parsons et al., 2001). Furthermore, the textures and the chemical composition of primary phases in slags may also be used to estimate their condi-tions of formation, especially in relation to the advances in smelting technologies (Ettler et al.; 2000; Manasse et al., 2001; Manasse and Mellini, 2002a; Sáez et al., 2003). An attempt is made in this paper to describe the changes in the mineralogical composition of Tsumeb smelting slags resulting from historically different smelting operations. The extensive mining and ore processing activities in the Tsumeb district (Namibia) left huge amounts of various mining and smelting waste materials that can be considered to represent a serious problem in relation to environmental contamination (Ongopolo Mining and Processing Limited, 2001; Kříbek and Kamona, 2005).

Background Information

In 1907, two lead-copper blast furnaces were built in the Tsumeb area, supplemented by a third furnace in 1923, which were operated until the end of World War

II. After the cessation of smelting activities in the 1950s, new smelters were erected in 1963, consisting of a Cu smelter with reverbatory furnace and a Pb smelter with a shaft furnace. During the 1980s, a Slag Mill was built to re-process old copper reverbatory slags by converter techniques producing granulated slag, which is passed to the Pb smelter to recover Pb and also Cu. Since 1997, the Slag Mill has been used to re-treat converter slag tailings (Tsumeb Corporation Ltd., 1987; Schneider and Seeger, 1992; Wartha and Genis, 1992; Biswas and Davenport, 1976).

Materials and Methods

Slag and matte samples

Fifteen slag and matte samples were collected in the vicinity of the Tsumeb smelter (Figure 1), representing the available materials corresponding to the histori-cal evolution of smelting technologies. The following samples were collected: (i) Historical slags produced from 1907 to 1948 (denoted in this paper as “Slag I”) in lead-copper blast furnaces processing mainly carbon-ate and oxidic ores and fired by coke from Germany and South Africa; (ii) Historical slags and mattes pro-duced between 1963 and 1970 (denoted as “Slag II”) resulting from reverbatory furnaces (Cu smelting) and shaft furnaces (Pb smelting) fired with aerated pulver-ized coal. During this period, mainly sulphide ores were processed; (iii) Granulated slags from reverbatory fur-naces produced by a recent Ausmelt technology in the Cu smelter from 1980 to 2000 (denoted as “Slag III”). In this case, the furnaces were fired with black oil and the furnace charge was pelletized with pulverized coal and fluxes (lime and silicate hornfels) prior to melting. Macroscopically, historical slag samples (slag I and

Communs geol. Surv. Namibia, 14 (2009), 3-14

3

II) are high-density fragments of grey or black colour showing occasional alteration features (newly formed phases on the surface). One sample, which according to its mineralogical composition was considered a sul-phide-metallic matte, was covered by secondary Cu-phases of bluish colour. Recent slags resulting from the Ausmelt technology (slag III) occur as powders of black colour.

Methods of investigation

Slag samples were embedded into resin and pre-pared as polished thin sections. They were examined under a Leica DM LP polarizing microscope in trans-mitted and reflected light. Polished sections were then studied under a CamScan scanning electron microscope (SEM) equipped with an Oxford Link energy disper-sion spectrometer (EDS). This equipment was used for microphotographs in backscattered electrons (BSE), semi-quantitative spot analyses (EDS) and relative el-emental distributions in certain areas of polished thin sections of the slag samples (X-ray maps). Qualita-tive microanalyses were performed using a Cameca SX-100 electron microprobe (EPMA). For silicate and oxide phases, the analytical conditions were: accelerat-ing voltage 15 kV, beam current 10 nA and counting time 10 s. The following standards were used: jadeite (Na), diopside (Mg, Ca), synthetic SiO2 (Si), synthetic Al2O3 (Al), leucite (K), tugtupite (Cl), apatite (P), barite (S), rutile (Ti), magnetite (Fe), spinel (Cr, Mg), cuprite (Cu), willemite (Zn), synthetic CdS (Cd), crocoite (Pb) and synthetic GaAs (As). For metals and sulphides, analytical conditions were: accelerating voltage 20 kV, beam current 10 nA, and counting time 10 s for all the

elements. The following standard set was used for met-als and sulphides: pyrite (S, Fe), pure nickel (Ni), pure copper (Cu), sphalerite (Zn), synthetic GaAs (As), pure silver (Ag), pure tin (Sn), pure antimony (Sb) and ga-lena (Pb). An aliquot part of each sample (approximately 20 g) was crushed and pulverized in an agate mortar us-ing the Fritsch Pulverissette apparatus. About 0.5 g was used for the X-ray powder diffraction analysis (XRD), made on a PANanalytical X´Pert Pro diffractometer us-ing CuKa radiation, at 40 kV and 30 mA, over the range 5-80° 2 theta with a step of 0.02° and counting time of 150 s in each step (X´Celerator detector). The X´Pert HighScore, version 1.0d equipped with the JCPDS PDF-2 database (ICDD, 2003) was used for the qualita-tive analysis of the diffractograms.

Results

Mineralogy of slags

Significantly distinct phase compositions were ob-served for 100-year old slags compared to younger slags as revealed by XRD analysis (Table 1), indicat-ing both different conditions of the slag formation (e.g. chemical composition of the slag melt, cooling regime), and also possible alteration processes at the dumping site (presence of secondary phases). Interestingly, the slag I is mainly composed of feld-spars (with anorthite and Pb-feldspar structures), spinels, delafossite, an As-bearing matrix phase (corresponding to arsenate) and galena. In contrast, slags II and III are predominantly composed of typical high-temperature Ca-Fe silicates (olivine-type phases, melilite), amor-

Figure 1: Schematic sketch of the Tsumeb smelter area with indication of the slag dumps.

Ettler, Johan, Kříbek and Nolte

4

phous silicate glass, spinel-type oxides, wuestite, and also trace sulphides (e.g. galena, wurtzite/sphalerite, bornite), pure metals (Pb, Cu) and various intermetal-lic compounds (Table 1). The 30 to 40 year old mattes, initially sampled together with the slag material, are mainly composed of pure Pb, litharge (probably an al-teration product of Pb), digenite and domeykite.

Slag I Microscopic investigation shows that feldspars forming hundreds of μm large euhedral crystals are the dominant phase in these slags (Figure 2a). Oxides be-longing to the spinel and delafossite-mcconnelite series form euhedral crystals up to tens of μm in size, which are often optically zoned (Figure 2a). Another genera-tion of prismatic feldspar crystals (brighter on SEM im-ages than the previous ones) is associated with spinels and the slag matrix. The latter is composed of As-bear-

ing phases, often enriched in Pb, Cd, Cu and Ca (Fig-ure 3), probably in form of arsenates, indicating that the slag underwent some alteration process. In places, inclusions of galena (PbS) are observed in the slag ma-trix.

Slags II and III and mattes Slags II and III are composed of phases commonly reported in papers devoted to smelter slag mineralogy, i.e. spinel oxides, silicates, glass and sulphide-metallic inclusions (Ettler et al., 2001; Manasse and Mellini, 2002b; Piatak et al., 2004). In general, spinels are the first crystallising phases, forming small euhedral crys-tals (Figure 2b) or larger zoned crystals (Figure 2c). Oc-casionally, dendritic wuestite aggregates form at the be-ginning of the slag melt solidification (Figure 2d). The crystallisation sequence is followed by the formation of large euhedral melilites (Figure 2c) or lath-like ol-

Group Phase Chemistry Slag and matte types

Slag I 100 years old

Slag II 30-40 years old

Slag III recent

Matte 30-40 years old

Silicates fayalite Fe2SiO4 ** *

monticellite CaMgSiO4 ***

melilite Ca2(Mg,Fe,Zn)Si2O7 *** *

anorthite CaAl2Si2O8 ***

unnamed (PDF 087-1003) # PbAl2Si2O8 **

amorphous glass Si-Ca-Fe-Al ** ***

Oxides spinel series (Zn,Mg,Fe,Cu)(Fe,Al)2O4 *** * ***

wuestite FeO *

litharge PbO ***

delafossite-mcconnelite Cu1+(Fe3+,Cr3+)O2 **

Sulphides galena PbS * *

wurtzite ZnS *

sphalerite ZnS *

chalkopyrite CuFeS2 *

digenite Cu1-xS *

pyrrhotite Fe1-xS *

cubanite CuFe2S3 *

covellite CuS *

Elements lead Pb * ***

copper Cu *

Others domeykite α Cu3As *

Cu3(Sn,Sb) Cu3(Sn,Sb) *

Cu5Sb Cu5Sb *

Fe2As Fe2As *

FeAs FeAs *

unidentified Ca-Pb arse-nates

ideal formula ~ (Pb,Ca,Fe)3(AsO4)2 ·H2O

**

Table 1: Primary phases observed in the Tsumeb slags and mattes (normal – both XRD and EPMA data, italics – only EPMA).

Relative abundance: *** dominant phase, ** common phase, * trace phase; # unnamed phase („Pb feldspar“) according to Benna et al. (1996)


5

Figure 2: Scanning electron micrographs of slags and mattes from the Tsumeb smelter (in backscattered electrons). (a) slag I, large euhedral Pb-rich feldspar (feldspar, F) crystals, small grains of Cu-spinels (Spl) and matrix composed of various Ca-Pb arsenates (As, bright phases); (b) slag III, fragment composed of euhedral spinel (Spl) and olivine (Ol) crystals trapped within glass (Gl), milled glassy matrix with galena (Gn) grains; (c) slag II, zoned spinel (Spl) crystal associated with euhe-dral melilite (Mel) and glass (Gl) with inclusions of sulphide droplets (galena and bornite, Gn+Bn) (zoom indicates two chemically distinct glasses with trapped submicrometric galena inclusions); (d) slag II, large olivine (Ol) crystals in glassy (Gl) matrix associated with spinel (Spl) euhedral crystals, wuestite (W) dendrites and sulphide-metallic droplets composed of galena (Gn), digenite (Dg), wurtzite (Wz) and koutekite (Cu5As2 phase, Kt); (e) 30-40-year old matte, pure Pb crystals altered to lithargite (PbO) associated with primary digenite (Dg) and domeykite (Do); (f) matte droplet in slag II, “speiss” inclusion with pure Pb surrounded by koutekite (Kt) and myrmekite of digenite (Dg) and galena (Gn).


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ivines (Figures 2b,d). Glass solidifies last and contains small inclusions of sulphides (mainly galena, wurtzite, bornite, digenite) or intermetallic compounds (Figures 2b,c,d). Often two, chemically distinct glasses solidify in the symplectitic intergrowths as a result of rapid cool-ing of the slag melt (Figure 2c – zoom). Rare mattes (sulphide-rich materials) and speiss (arsenide-rich mate-rials) were present as massive symplectitic intergrowths of sulphides, metals and intermetallic compounds (Fig-ure 2e), or large droplets trapped within the silicate slag and composed also of the sulphide-metals-intermetallic

assemblages (Figure 2f). The presence of litharge (PbO) between the pure Pb crystals and in cavities probably indicates alteration of the Pb-rich matte (Figure 2e).

Chemistry of slag phases

Olivines The chemical composition of olivine-type phases varies from nearly pure fayalite (Fe2SiO4) to kirschstei-nite (CaFeSiO4) – monticellite (CaMgSiO4) solid solu-tion as revealed by EPMA (Table 2). They mainly occur

Phase Slag type

fayalite II

kirschsteinite II

melilite II

Pb-bearing anorthite I

Pb feldspar I

wt.%

SiO2 30.46 33.77 40.10 44.23 39.02

TiO2 0.02 0.03 - - 0.04

Al2O3 0.02 0.04 3.34 32.34 20.39

FeO 50.23 23.07 4.71 0.89 0.41

MnO 0.63 0.36 0.10 - -

MgO 6.55 11.06 5.26 - 0.04

CaO 4.04 26.48 36.44 16.21 1.39

Na2O 0.32 0.19 1.45 0.67 0.27

K2O 0.02 0.02 0.09 0.45 2.78

P2O5 0.06 0.14 0.04 0.05 0.06

Cr2O3 - - - 0.06 -

PbO - 0.09 0.06 3.66 34.00

ZnO 8.74 4.89 8.31 0.05 0.43

CuO - - - 0.39 0.49

CdO - - - 0.15 0.16

Total 101.06 100.12 99.90 99.14 99.47

apfu

Si 0.979 0.992 1.940 2.128 2.436

Al 0.001 0.001 0.060 (0.130)* 1.834 1.501

Ti 0.000 0.001 0.000 0.000 0.002

Fe 1.350 0.566 0.191 0.036 0.021

Mn 0.017 0.009 0.004 0.000 0.000

Mg 0.314 0.484 0.379 0.000 0.004

Zn 0.207 0.106 0.297 0.002 0.020

Cu 0.000 0.000 0.000 0.014 0.023

Cd 0.000 0.000 0.000 0.003 0.005

Cr 0.000 0.000 0.000 0.002 0.000

Ca 0.139 0.833 1.888 0.836 0.093

Na 0.020 0.011 0.136 0.063 0.033

K 0.001 0.001 0.005 0.028 0.221

Pb 0.000 0.001 0.001 0.047 0.572

P 0.002 0.003 0.002 0.002 0.003

* partitioning between Al (IV) and Al (VI) in the melilite structure– not detected

Table 2. Representative composition of selected silicate phases. Structural formulae were calculated on the basis of 4 (olivine), 7 (melilite) and 8 oxygens (feldspars).


7

as a dominant phase in slags II and III. Olivine crystals are slightly zoned (Figure 2d) with bright rims corre-sponding to Ca-poor olivine and dark cores correspond-ing to Ca-rich kirschsteinite (up to 26.48 wt.% CaO). All observed olivines are Zn-bearing; the fayalite end members are generally most enriched (up to 8.74 wt.%). Zinc can enter into octahedral sites of the olivine struc-ture and substitutes for Fe2+ (Chaudhuri and Newesely, 1993; Ettler et al., 2001). The previous work on smelt-ing slags showed that these phases are the most com-mon silicates that form large skeletal crystals or laths in slowly crystallizing slags, and dendrites in quenched slags (Ettler et al., 2001; Manasse et al., 2001). In con-

trast to Manasse et al. (2001), the two generations of olivine-type phases described by these authors (early crystallising fayalite and late dendritic kirschsteinite) were not found in the Namibian slags. Therefore, the crystallisation temperature estimate of the slag melt, us-ing the miscibility diagram from Mukhopadhyay and Lindsley (1983), cannot be made.

Melilite The composition of melilite varies significantly and often corresponds to a solid solution of predominant åkermanite (CaMgSi2O7) and hardystonite (CaZnSi2O7) with minor amounts of sodium melilite (NaCaAlSi2O7),

Figure 3: X-ray elemental mapping showing the relative distribution of Al, As, Ca, Cd, Cu, Fe, K, Pb, Si, Ti and Zn performed by scanning electron microscopy (SEM) equipped with energy-dispersion spectrometer (EDS) (slag I).


8

gehlenite (Ca2Al2SiO7) and ferro-åkermanite (CaFe-Si2O7) (Table 3). Thus, melilite appears to be an effi-cient Zn concentrator, exhibiting concentrations up to 8.31 wt.% of ZnO. Similar Zn contents in melilite were observed in other smelting slags resulting from similar technological processes (Ettler et al., 2001, 2002).

Feldspars Anorthite (CaAl2Si2O8) and an unnamed phase cor-responding to a “Pb-feldspar” (PbAl2Si2O8) studied by Benna et al. (1996), were observed only in slag I. Ac-cording to EPMA, their composition is rather variable and Pb concentration can reach up to 34 wt.% PbO. They certainly crystallized from the slag melt (large euhedral crystals) and represent (together with spinels) the first phases crystallizing in slag I. They were not detected in other Namibian slags studied. The presence of Pb in feldspar indicates that the slag melt must have been strongly enriched in Pb, Si and Al. Benna et al. (1996) show that the disordered structure of Pb-feldspar is a product of quenching. The presence of two generations of chemically different feldspars indicates two different stages of the slag melt cooling regime. The first gen-eration of large euhedral feldspars is Pb-depleted and was probably formed by slow crystallisation (consistent with the slow cooling in ladles) (Table 2). However, the second generation of smaller Pb-rich feldspars associat-ed with spinels is probably the product of a more rapid cooling regime (Benna et al., 1996; Table 2, Figure 3). It

should be recalled that the presence of feldspars in slags is rather scarce and was observed only by Sáez et al. (2003), who detected plagioclase in Cu-smelting slags from Spain. The feldspars detected by Sáez et al. (2003) could, however, be residual (from unmelted gangue), as was also observed in medieval smelting slags from the Czech Republic (Ettler and Červinka, unpublished data).

Silicate glass The presence of glass was observed mainly in slags II and III and confirms the rapid cooling of the slag melt (granulation). Its composition is rather variable and, similar to other silicates in slags, the glass is enriched in Ca and Fe (Table 3). Furthermore, it is a principal concentrator of contaminants such as Pb (up to 11.8 wt.% PbO), Zn (up to 16.2 wt.% ZnO), Cu (1.78 wt.% CuO) and As (1.56 wt.% As2O3). Low analytical totals of some EPMA indicate the presence of H2O or pos-sibly the presence of trivalent Fe in the glass structure (Table 3).

Oxides Oxides in Namibian slags are mainly represented by spinel-type compounds. The Fe partitioning into divalent and trivalent Fe was calculated according to the methodology described in Ettler (2002). EPMA re-vealed that their composition is rather variable (Table 4) and can be expressed by the general formula (Zn,Cu,

Slag type III III II II II

wt.%

SiO2 25.14 40.22 28.21 39.62 40.14

TiO2 0.25 0.12 0.28 - -

Al2O3 4.48 1.97 4.31 12.93 5.36

FeO 24.71 32.47 21.84 11.80 5.07

MnO 0.10 0.22 0.36 0.07 0.14

MgO 3.44 0.32 5.34 2.84 0.93

CaO 19.88 5.16 18.75 1.77 32.06

Na2O 0.96 1.01 1.31 15.18 3.92

K2O 0.07 0.21 0.64 0.14 0.10

P2O5 0.32 0.09 0.36 0.12 0.29

Cr2O3 - 0.09 0.36 0.04 0.07

PbO 7.85 11.80 1.30 0.06 0.14

ZnO 9.12 3.23 16.21 14.70 11.62

CuO 1.78 - 0.08 - -

CdO 0.05 0.03 0.10 - 0.07

As2O3 1.56 0.56 - 0.07 -

SO3 0.01 0.50 0.90 - 0.01

Cl 0.01 - 0.01 - 0.02

Total 99.73 97.99 100.36 99.36 99.95

– not detected.

Table 3: Representative composition of glass from slags of type II and III.


9

Mg,Fe)(Fe,Al,Cr,Si,Ti)2O4. Whereas the composition of spinel-type phases from slag III correspond either to Fe-Cr spinels or to magnetite (Fe3O4), those in slag II exhibit a composition close to gahnite (ZnAl2O4) or Cr-bearing spinels. Interestingly, the spinel-type phases from slag I are complex solid solutions of spinel group end-members, which are often enriched in Cu (up to 10.6 wt.% CuO). The presence of a delafossite- (CuFeO2) or mcconnelite-like (CuCrO2) phase corresponding to the stoichiometric formula (Cu1+

0.978Mg0.003)0.981(Al0.252Cr0.61

9Fe3+0.126Ti0.006)1.003O2 was observed in this type of slag

(Table 4, Figure 3). This phase was commonly found in Cu-bearing slags in Spain (Sáez et al., 2003) and its presence (with Cu1+ in the delafossite or mcconnelite

structures) indicates extreme reducing conditions in the slag melt. This is compatible with the historical fact that blast furnaces at the beginning of the 20th century were fired with first-class German or South African coke, in-ducing a highly reducing environment during smelting of the ore (Tsumeb Corporation Ltd., 1987).

Sulphides and intermetallic compounds The chemical composition of sulphides, intermetal-lic compounds and metals is given in Table 5. The most commonly observed sulphides are galena (PbS), wur-tzite (ZnS), pyrrhotite (Fe1-xS) and various Cu or Cu-Fe sulphides (digenite, cubanite, covellite, chalcocite). Wurtzite is a high-temperature phase (> 1020°C) (Ettler

Phase Slag type

ferrochromiteIII

magnetiteIII

gahniteII

chromiteII

gahniteI

mcconneliteI

delafossiteI

wt.%

SiO2 0.18 1.88 0.16 - 0.02 - 0.04

TiO2 0.25 0.26 0.36 0.16 0.23 0.35 4.56

Al2O3 9.30 0.78 48.77 14.08 39.37 9.21 5.38

Cr2O3 20.14 0.36 3.26 52.67 0.14 33.73 -

Fe2O3* 31.92 62.20 9.07 5.09 15.14 7.99 32.19

FeO* 15.37 33.91 6.88 8.40 7.29 - -

MgO 6.36 0.34 6.58 13.17 2.53 0.08 0.50

ZnO 15.63 1.26 25.41 6.17 20.77 0.51

CuO (Cu2O)# 0.20 0.06 - 0.10 10.62 50.19 50.03

CdO - - - - 0.15 - -

Na2O 0.42 0.07 0.67 0.10 0.58 0.00 -

K2O - - - 0.02 0.02 - -

CaO 0.17 0.08 0.03 0.02 0.02 - 0.02

PbO - - - - 0.10 - -

P2O5 - 0.03 - - - - -

SO2 - - - - 0.02 - -

Total 99.94 101.22 101.19 99.97 96.97 101.54 93.24

apfu

Si 0.008 0.084 0.005 0.000 0.001 0.000 0.001

Ti 0.007 0.007 0.008 0.004 0.006 0.006 0.091

Al 0.392 0.034 1.687 0.532 1.531 0.249 0.169

Cr 0.569 0.011 0.076 1.335 0.004 0.613 0.000

Fe(3) 0.860 1.751 0.201 0.123 0.377 0.138 0.646

Fe(2) 0.460 1.059 0.169 0.225 0.201 0.000 0.000

Zn 0.413 0.035 0.550 0.146 0.506 0.000 0.010

Mg 0.339 0.019 0.288 0.629 0.124 0.003 0.020

Cu 0.005 0.002 0.000 0.002 0.265 0.968 1.120

Na 0.029 0.005 0.038 0.006 0.037 0.000 0.000

Ca 0.007 0.003 0.001 0.001 0.001 0.000 0.004

* partitioning of Fe between Fe2O3 and FeO was calculated according to method described in Ettler (2002).# Cu expressed as Cu2O in the case of mcconnelite and delafossite. – not detected.

Table 4: Representative composition of oxides. Structural formulae were calculated on the basis of 4 (spinels) and 2 oxygens (mcconnelite-delafossite).


10

Phas

e C

ompo

sitio

n Sl

ag ty

pe

wur

tzite

(Zn,

Fe)S

II

wur

tzite

(Zn,

Fe)S

II

pyrr

hotit

e Fe

1-xS

III

gale

naPb

SII

gale

naPb

SII

I

dige

nite

(C

u,Fe

) 9S5

II

cuba

nite

C

uFe 2S

3 II

I

cove

llite

CuS III

chal

coci

teC

u 2S II

dom

eyki

te α

Cu 3A

sII

Cu 3(S

n,Sb

)C

u 3(Sn,

Sb)

II

Cu 5S

bC

u 5Sb

II

(Fe,

Cu)

2As

(Fe,

Cu)

2As

I

FeA

sFe

As

II

Cu

Cu II

Pb Pb II

wt.%

S Fe Ni

Cu

Zn As

Ag

Sn Sb Pb Tota

l

33.1

14.

05-

1.00

61.1

0 - - - - -99

.25

33.2

715

.41 -

0.67

49.7

4 -0.

040.

050.

05-

99.2

2

38.9

156

.71

0.06

2.07

0.03

- -0.

06- -

97.8

4

12.4

60.

070.

092.

980.

040.

160.

070.

501.

2182

.89

100.

46

13.7

6 - -0.

950.

080.

06-

0.07

0.10

87.1

710

2.18

21.0

12.

740.

0474

.92 -

0.22

0.07

- - -98

.99

35.3

638

.24 -

24.4

30.

37- -

0.05

0.04

-98

.48

30.2

42.

52-

63.3

60.

560.

110.

06- - -

96.8

5

19.3

7 - -78

.26

0.03

- - - -0.

3097

.96

0.96

0.48

-66

.65 -

27.8

90.

08-

0.89

1.82

98.7

5

- -0.

3160

.65 -

1.29

0.33

23.1

612

.93

0.06

98.7

2

-0.

310.

1968

.88 -

1.10

0.24

1.39

26.5

2 -98

.62

0.10

33.6

00.

5427

.06 -

38.8

5 - -0.

050.

0700

.25

0.12

39.0

51.

611.

30-

53.8

80.

050.

05- -

96.0

7

0.07

1.03

-96

.30 - -

0.26

- -0.

1697

.82

-0.

11 -1.

45-

0.06

0.06

- -98

.86

100.

54

at.%

S Fe Ni

Cu

Zn As

Ag

Sn Sb Pb

50.2

53.

530.

000.

7645

.47

0.00

0.00

0.00

0.00

0.00

49.7

413

.23

0.00

0.50

36.4

70.

000.

020.

020.

020.

00

53.6

144

.86

0.04

1.44

0.02

0.00

0.00

0.02

0.00

0.00

45.4

20.

150.

185.

480.

070.

240.

070.

491.

1646

.74

49.4

30.

000.

001.

720.

140.

090.

000.

070.

0948

.46

34.7

12.

600.

0462

.47

0.00

0.16

0.03

0.00

0.00

0.00

50.6

431

.43

0.00

17.6

50.

260.

000.

000.

020.

010.

00

47.2

52.

260.

0049

.96

0.43

0.08

0.03

0.00

0.00

0.00

32.8

80.

000.

0067

.02

0.02

0.00

0.00

0.00

0.00

0.08

2.02

0.58

0.00

71.0

50.

0025

.21

0.05

0.00

0.50

0.59

0.00

0.00

0.41

74.4

70.

001.

350.

2415

.22

8.29

0.02

0.00

0.41

0.24

80.9

50.

001.

100.

160.

8716

.26

0.00

0.20

38.5

90.

5927

.32

0.00

33.2

60.

000.

000.

030.

02

0.26

47.5

31.

871.

390.

0048

.89

0.03

0.03

0.00

0.00

0.15

1.20

0.00

98.7

80.

000.

000.

150.

000.

000.

05

0.00

0.39

0.00

4.54

0.00

0.16

0.10

0.00

0.00

97.8

1

apfu

S Fe Ni

Cu

Zn As

Ag

Sn Sb Pb

1.00

00.

070

0.00

00.

015

0.90

50.

000

0.00

00.

000

0.00

00.

000

1.00

00.

266

0.00

00.

010

0.73

30.

000

0.00

00.

000

0.00

00.

000

1.00

00.

837

0.00

10.

027

0.00

00.

000

0.00

00.

000

0.00

00.

000

1.00

00.

003

0.00

40.

121

0.00

10.

005

0.00

20.

011

0.02

51.

029

1.00

00.

000

0.00

00.

035

0.00

30.

002

0.00

00.

001

0.00

20.

980

5.00

00.

374

0.00

58.

998

0.00

00.

023

0.00

50.

000

0.00

00.

000

3.00

01.

862

0.00

01.

046

0.01

50.

000

0.00

00.

001

0.00

10.

000

1.00

00.

048

0.00

01.

057

0.00

90.

002

0.00

10.

000

0.00

00.

000

1.00

00.

000

0.00

02.

039

0.00

10.

000

0.00

00.

000

0.00

00.

002

0.07

30.

021

0.00

02.

562

0.00

00.

909

0.00

20.

000

0.01

80.

021

0.00

00.

000

0.01

72.

996

0.00

00.

054

0.01

00.

612

0.33

30.

001

0.00

00.

022

0.01

34.

439

0.00

00.

060

0.00

90.

048

0.89

20.

000

0.00

61.

153

0.01

70.

816

0.00

00.

993

0.00

00.

000

0.00

10.

001

0.00

50.

966

0.03

80.

028

0.00

00.

994

0.00

10.

001

0.00

00.

000

0.00

10.

012

0.00

00.

984

0.00

00.

000

0.00

20.

000

0.00

00.

001

0.00

00.

004

0.00

00.

045

0.00

00.

002

0.00

10.

000

0.00

00.

948

Tabl

e 5:

Rep

rese

ntat

ive

com

posi

tion

of su

lphi

des,

inte

rmet

allic

com

poun

ds a

nd p

ure

met

als (

- not

det

ecte

d).


11

and Johan, 2003) and contains significant amounts of Fe (up to 15.4 wt.%) and minor amounts of Cu (up to 1 wt.%). Other sulphides are also enriched in Cu. Up to 2 wt.% Cu was observed in pyrrhotite (Fe1-xS) and up to 2.98 wt.% Cu was detected in galena (PbS). Copper can be bound in the galena structure, and even higher Cu contents were observed in hydrothermal galenas (e.g. Clark and Sillitoe, 1971). Unlike other Pb-Zn slags (Ettler et al., 2001), those from Tsumeb contain higher amounts of Cu-rich sulphides, as the Cu-rich ores were treated in the Tsumeb smelter. The chemical composi-tions of representative Cu (and Cu-Fe) sulphides are given in Table 5. Intermetallic compounds mainly belonging to the Cu-As, Cu-Sb and Cu-Sn binary systems were also ob-served. The elemental substitutions in their structures (see Table 5) correspond to the substitutions observed by Ettler and Johan (2003) in matte phases from prima-ry Pb metallurgy. EPMA showed that the “pure” metals forming metallic droplets trapped within the slag glass or in larger chemically complex inclusions may contain substantial amounts of other elements (Table 5).

Arsenates According to EPMA (data not given) and X-ray elemental mapping (Figure 3), the As-bearing matrix phases observed in the oldest Tsumeb slags probably correspond to complex Ca-Pb arsenates, close to the ideal formula (Pb,Ca,Fe,Cu,Zn,Cd)3(AsO4)2·H2O. Such phases would probably be chemically similar to tsum-corite [FePbZn(AsO4)2·H2O] and other arsenates, gen-erally found in the oxidation zones of the Tsumeb ore deposit, and certainly indicate the alteration processes in slags.

Discussion

Phase formation in historically different slags

Mineralogical investigation of the Tsumeb slags in-dicates that two distinct groups of materials were pro-duced in this district, according to the differences in the smelting technology, primary ore (or concentrate) com-positions and secondary alteration processes. Not all the studied slag samples contain clinopyroxene, which is a good indicator of relatively slow cooling of the slag melt (Ettler et al., 2000, 2001). Nevertheless, the cool-ing regime is completely different for these two distinct slag groups. Furthermore, the initial compositions of the melt were probably significantly different. The oldest technology from the beginning of the 20th century produced a slag melt (slag I) that cooled very slowly in ladles and was highly enriched in Pb, Cu and As. The glass missing in these slags is also a good indicator of a very slow cooling regime. The crystallisation of Ca-Pb feldspars indicates that the slag melt was poor in alkalis and enriched in Al (Figure 3). In addition, the feldspar formation

completely removed silica from the melt. Further so-lidification continued through the formation of Cu-rich oxides (spinel and delafossite-family phases). Their presence shows that the redox conditions in the slag melt were locally variable, probably due to the use of first-class cokes for firing in the furnaces (Cu1+ ↔ Cu2+, Fe2+ ↔ Fe3+ redox pairs). Magnetite, generally formed by the reaction with oxygen-bearing shaft gases, is lacking in these old slags, also indicat-ing rather reducing conditions (Biswas and Davenport, 1976). The Fe3+ present in the melt is then trapped within the delafossite-mcconnelite phases. The matrix of the slag is composed of Ca-Pb arsenates, which are often enriched in Cu, Zn and other metallic elements (Figure 3). It is hardly possible that these phases repre-sent the last step in solidification of the slag melt. They probably correspond to alteration products of arsenides or arsenites, and derived from weathering of the slag over many decades. We have some analytical evidence that Cu-rich oxides (spinels or delafossite) can also be altered to form secondary products with a composition close to the stoichimetric Cu3Fe3+

8O15·3H2O. The low sums observed for some delafossite-like phases also in-dicate the presence of water and some degree of altera-tion (Table 4). The mineralogical compositions of 40-year old and more recent slags (slags II and III) exhibit a typical crys-tallisation sequence (see e.g. Ettler et al., 2001). These slags were granulated (quenched) as is also documented by the presence of silicate glass. The spinel-family ox-ides with compositions corresponding to gahnite-fer-rochromite-magnetite solid solutions are the first phas-es crystallising from the slag melt. In contrast to the 100-year old technology, the slag melt was more en-riched in alkalis and Ca (lime was probably used as a flux agent) and Ca-Fe alumosilicates, such as olivines and melilite formed as the major slag constituents. Melilite appears in silica-undersaturated and Ca-rich melts (Middlemost, 1997). Furthermore, the melt was significantly depleted in Cu (as indicated by the forma-tion of residual Cu-sulphide droplets trapped within the silicate phases) and enriched in Zn. Zinc is signifi-cantly concentrated in all oxides, silicates and silicate glass occurring in these slags. The initial concentration of Pb in the melts was also probably lower than in the melts produced at the beginning of the 20th century; it is concentrated only in the residual glass (solidifying at the end of crystallisation sequence) and in small galena droplets.

Environmental perspectives

The association of primary phases within the Tsumeb slags indicates a number of metal- and metal-loid-bearing phases whose environmental stability is rather variable. From the oldest slags (beginning of the 20th century), Cu-oxides of the delafossite family can release Cu during the alteration process. The presence


12

of complex arsenates in the slag matrix indicates a sig-nificant degree of weathering of these materials. In con-trast, granulated slags from 40-year old and more recent technologies are significantly more stable, and mainly Pb- and Zn-bearing glass and various metal sulphides and intermetallic compounds would be prone to altera-tion and release of inorganic contaminants into the en-vironment. Further studies are now ongoing in our labo-ratories to determine the slag alteration processes, and to evaluate possible leachability of toxic elements in the slag dump environments.

Conclusions

The investigation of slags from the Tsumeb district showed that, from the mineralogical point of view, the waste materials produced and dumped in this area form two distinct groups: (i) 100-year old slags with domi-nant Pb-bearing feldpars, Cu-rich spinels, delafossite-like phases and unidentified Ca-Pb arsenates and (ii) recent and 30-40 year old granulated slags, composed of high-temperature Ca-Fe alumosilicates (olivine fam-ily phases, melilite), Pb- and Zn-bearing silicate glass, spinel oxides and small metallic/sulphide inclusions trapped within the silicate glass. The differences in the mineralogical composition of the slags are strictly relat-ed to the initial composition and the cooling regime of the melt. The slag melts of the oldest technology were probably low in alkalis, Ca and Si and strongly enriched in Cu, Pb and As, and were cooled very slowly. In con-trast, granulated slags were rapidly cooled (presence of silicate glass) and the slag melt was poorer in Cu and enriched in alkalis, Ca and Zn. A detailed knowledge of the chemical composition of ores and concentrates historically used in the Tsumeb smelters would be use-ful for more accurate interpretation of the phase assem-blages and compositions found. This work would help to identify a large number of phases acting as signifi-cant traps of toxic metals and metalloids during the slag formation. The environmental implications of slag pro-duction/dumping, natural alteration of these materials and the release and mobility of inorganic contaminants under conditions simulating the dump milieu are cur-rently being investigated by our research group.

Acknowledgements

This work was supported by Project No. RP/20/2004 (“Impact Assessment of mining and processing of ores on the environment in mining districts of Namibia”) of the Development Cooperation of the Czech Republic. The institutional funding was provided by Ministry of Education of the Czech Republic (MSM 0021620855).

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mineralogy of primary phases in slags and mattes from … of primary phases in slags and mattes from...

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