Applied Geochemistry 24 (2009) 1–15

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Applied Geochemistry

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Mineralogy and environmental stability of slags from the Tsumeb smelter, Namibia

Vojtech Ettler a,*, Zdenek Johan b, Bohdan Kríbek c, Ondrej Šebek d, Martin Mihaljevic a

a Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43 Prague 2, Czech Republicb Bureau des Recherches Géologiques et Minières (BRGM), av. Claude Guillemin, 45060 Orléans, cedex 2, Francec Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republicd Laboratories of the Geological Institutes, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic

a r t i c l e i n f o

Article history:Received 27 June 2008Accepted 22 October 2008Available online 30 October 2008

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0883-2927/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.apgeochem.2008.10.003

* Corresponding author. Tel.: +420 221 951 493; faE-mail address: ettler@natur.cuni.cz (V. Ettler).

a b s t r a c t

Three types of smelting slags originating from historically different smelting technologies in the Tsumebarea (Namibia) were studied: (i) slags from processing of carbonate/oxide ore in a Cu–Pb smelter (1907–1948), (ii) slags from Cu and Pb smelting of sulphide ores (1963–1970) and (iii) granulated Cu smeltingslags (1980–2000). Bulk chemical analyses of slags were combined with detailed mineralogical investi-gation using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM/EDS) and electronmicroprobe (EPMA). The slags are significantly enriched in metals and metalloids: Pb (0.97–18.4 wt.%),Cu (0.49–12.2 wt.%), Zn (2.82–12.09 wt.%), Cd (12–6940 mg/kg), As (930–75,870 mg/kg) and Sb (67–2175 mg/kg). Slags from the oldest technology are composed of primary Ca- and Pb-bearing feldspars,spinels, complex Cu–Fe and Cu–Cr oxides, delafossite–mcconnellite phases and Ca–Pb arsenates. Thepresence of arsenates indicates that these slags underwent long-term alteration. More recent slags arecomposed of high-temperature phases: Ca–Fe alumosilicates (olivine, melilite), Pb- and Zn-rich glass, spi-nel oxides and small sulphide/metallic inclusions embedded in glass. XRD and SEM/EDS were used tostudy secondary alteration products developed on the surface of slags exposed for decades to weatheringon the dumps. Highly soluble complex Cu–Pb–(Ca) arsenates (bayldonite, lammerite, olivenite, lavendu-lan) associated with litharge and hydrocerussite were detected. To determine the mineralogical and geo-chemical parameters governing the release of inorganic contaminants from slags, two standardizedshort-term batch leaching tests (European norm EN 12457 and USEPA TCLP), coupled with speciation-solubility modelling using PHREEQC-2 were performed. Arsenic in the leachate exceeded the EU regula-tory limit for hazardous waste materials (2.5 mg/L). The toxicity limits defined by USEPA for the TCLP testwere exceeded for Cd, Pb and As. The PHREEQC-2 calculation predicted that complex arsenates are themost important solubility controls for metals and metalloids. Furthermore, these phases can readily dis-solve during the rainy season (October to March) and flush significant amounts of As, Pb and Cu into theenvironment in the vicinity of slag dumps.

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1. Introduction

Slags are the most important mineral wastes resulting frompyrometallurgy. They correspond to the silicate melt producedduring the pyrometallurgical recovery of base metals by fusion ina blast furnace and are produced in large amounts. Slags are gener-ally deposited on dumps or, if considered to be unreactive materi-als, used for civil-engineering purposes such as road construction(Ettler et al., 2002, 2003a). However, these waste materials are of-ten enriched in toxic elements, in particular metals (Cu,Pb,Zn) andin metalloids (As,Sb) that can be released into the environmentthrough alteration processes and leaching (Parsons et al., 2001; Et-tler, 2002; Piatak et al., 2004; Ettler et al., 2004, 2005; Lottermoser,2005; Navarro et al., 2008; Costagliola et al., 2008). Mineralogical

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investigations of slags are essential for understanding the positionof potentially toxic elements in primary solid phases and representthe first step in assessing their environmental impact (Ettler et al.,2000, 2001; Parsons et al., 2001; Lottermoser, 2002; Puziewiczet al., 2007). Furthermore, the textures and the chemical composi-tion of primary phases in slags may also be used to estimate theconditions of their formation, especially in relation to advancesin smelting technologies (Ettler et al., 2000, in press; Manasseet al., 2001; Manasse and Mellini, 2002a; Sáez et al., 2003; Haupt-mann, 2007).

Some recent monitoring studies have shown that the extensivemining and ore processing activities in the Tsumeb district (Nami-bia) left important amounts of various mining and smelting wastematerials that can be considered as a serious problem in relation toenvironmental contamination (Ongopolo Mining and ProcessingLimited, 2001; Kríbek and Kamona, 2005). Approximately24,550,280 t of ore were mined out during the modern history of

2 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

the Tsumeb Mine and it has been estimated that millions of tons ofmining/processing wastes are stored in this area (�200,000 t ofslag on dumps and at least 10 million t of material in tailing ponds)(Kríbek and Kamona, 2005). Because this has never been studied,this paper is focused on the Tsumeb smelter slags resulting fromhistorically different smelting operations. Bulk chemical analysesand phase/mineralogical analyses of slags were coupled withexperimental leaching and thermodynamic modelling, in order toprovide information on the solid speciation of pollutants in slags,weathering products and possible environmental impacts.

2. Materials and methods

2.1. Smelting site history

The Tsumeb deposit belongs to the northern Namibia sulphidicmetallogenic province. The deposit lies in the upper part of theOtavi group, which consists of limestones and dolomites of Neo-proterozoic age (Miller, 1983). The mineralization is characterisedby large-scale alteration (calcification and silicification of hostrocks) and common hydrothermal carbonate veins. The depositcontains a great diversity of ore minerals of Pb, Cu, Zn, Ag, As, Sb,Cd, Co, Ge, Ga, Au, Fe, Hg, Mo, Ni, Sn and W, as well as V, containingabout 11% Pb, 5% Cu and 4.3% Zn, with economic concentrations ofAg, Cd, Ge and As. The deposit was mined by a large open pit andby several shafts. It was once the foremost producer of Pb in Africaand, over its lifetime, has produced over 2 million t of Pb, some500,000 t of Zn and over 1 million t of Cu (Frimmel et al., 1996;Chetty and Frimmel, 2000).

In 1907, two Pb–Cu blast furnaces were built in the Tsumebarea by the Otavi Minen- und Eisenbahn-Gesellschaft (OMEG)Company to smelt local ores. These furnaces were supplementedby a third furnace in 1923; the smelter was then operated untilthe end of World War II, first processing the carbonate ore fromthe upper part of the Tsumeb ore body. After interruption of thesmelting activities in the 1950s, new smelters were constructedin 1963, consisting of a Cu smelter with a reverbatory furnaceand a Pb smelter with a shaft furnace. In this period, deeper partsof the Tsumeb ore body composed of sulphide ores were processed(main minerals: chalcocite, enargite, galena, sphalerite). During theearly 1980s, the Slag Mill was built to re-process old Cu reverba-tory slags, which are milled and treated by flotation. The resultingconcentrate is then passed to the smelters to recover Pb and Cu and

Fig. 1. Schematic sketch of the Tsumeb sm

to produce granulated slag (Tsumeb Corporation Ltd., 1987; War-tha and Genis, 1992).

2.2. Slag sampling and processing

Numerous slag samples were collected during the missionguided by the Czech Geological Survey in 2004 (Project of theDevelopment Cooperation of the Czech Republic No. RP/20/2004).Eleven slag samples were studied in this detailed investigation.They represent the available materials corresponding to the histor-ical evolution of smelting technologies in Tsumeb. The followingthree groups of slags were collected (the detailed locations are gi-ven in Fig. 1):

(i) Slag type-I corresponds to historical slags produced from1907 to 1948 in Pb–Cu blast furnaces processing mainly car-bonate and oxide ores and fired by coke from Germany andSouth Africa. They are found as 20-cm large fragments ofmassive and heavy material of black and grey colour witha green crust of secondary phases (number of samples,n = 1).

(ii) Slag type-II corresponds to historical slags and mattes pro-duced between 1963 and 1970 resulting from reverbatoryfurnaces (Cu smelting) and shaft furnaces (Pb smelting) firedwith aerated pulverized coal. During this period, mainly sul-phide ores were processed. The slags are found as up to 10-cm large fragments of black, vitreous material, often coveredby white and blue secondary phases (n = 8).

(iii) Slag type-III corresponds to granulated slags from reverba-tory furnaces produced by recent Ausmelt technology inthe Cu smelter from 1980 to 2000. In this case, the furnaceswere fired with black oil and the furnace charge was pellet-ized with pulverized coal and fluxes (lime and quartz chert)prior to melting. The slags are found as granulated blackmaterial with fragments up to 3 cm in size or as milled pow-der (n = 2).

2.3. Bulk chemical analyses

An aliquot of each sample (approximately 20 g) was crushedand pulverized in an agate mortar using the Fritsch Pulverisetteapparatus and was used for bulk chemical analyses and phaseidentification using X-ray diffraction analysis. The bulk chemical

elter and location of smelting slags.

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 3

composition was determined on pulverized slag samples usingdigestion in mineral acids and/or sintering and subsequent chem-ical analysis. Two types of dissolution techniques were employedprior to the analytical procedures. Dissolution procedure I (modifiedfrom CSN 72 0100, 1984 and Šulcek and Povondra, 1989) was usedfor the analysis of Ag, As, Ba, Co, Cr, Cu, K, Mn, Na, Ni, Pb, Zn (VarianSpectrAA 280FS flame atomic absorption spectroscopy, FAAS), P(spectrophotometry) and Sb, Bi, Ga, Ge (VG Elemental PQ3 induc-tively coupled plasma mass spectrometer, ICP-MS). A mass of0.5 g was dissolved in a mixture of 10 mL HF (38%) and 2 mL HClO4

(70%) and was evaporated to near dryness. This procedure was re-peated and the residue was dissolved in 2% HCl or HNO3 (v/v), di-luted to 100 mL and stored in polypropylene (PP) Azlon� bottlesuntil analysis. Dissolution procedure II (again modified from CSN72 0100, 1984 and Šulcek and Povondra, 1989) was based on thesintering of 0.2 g of sample with 0.6 g Na2CO3 and 0.05 g NaNO3

overlapped with a thin layer of supplementary Na2CO3 in a Pt cru-cible at 750–820 �C for 3 h. The sintered sample was dissolved in amixture of 10 mL H2O and 8 mL HCl (37%) at 90 �C. This solutionwas used for the determination of SiO2 (gravimetric analysis) andfurther determinations of Al2O3, Fe2O3, FeO, MgO, CaO (volumetricanalysis) and TiO2 (spectrophotometry). Detailed methodologies ofthese analytical procedures are given in Johnson and Maxwell(1981). The standard solutions used were prepared with an ade-quate amount of the sintering mixture in order to minimize thematrix effect. All the laboratory glassware was acid-washed andall the chemicals used were reagent grade (Lach-ner, Czech Repub-lic and Merck, Germany). Procedural blanks were run simulta-neously. The accuracy of the dissolution/analytical procedure wascontrolled by G2 (granite) reference material certified by the USGeological Survey and was generally better than <10% relativestandard deviation (RSD) for all the elements.

2.4. Mineralogical analysis of primary phases

2.4.1. X-ray diffraction analysisAbout 0.5 g of pulverized sample was used for the X-ray powder

diffraction analysis (XRD), performed on a PANanalytical X’Pert Prodiffractometer using CuKa radiation, at 40 kV and 30 mA, over therange 5–80� 2h with a step of 0.02� and counting time of 150 s ineach step (X’Celerator detector). The X’Pert HighScore, version1.0d equipped with the JCPDS PDF-2 database (ICDD, 2003) wasused for qualitative analysis of the diffractograms.

2.4.2. Microscopy and electron microprobe analysisSlag samples were embedded in resin and prepared as polished

thin sections for microscopic observation. The polished thin sec-tions were examined under a Leica DM LP polarizing microscopein transmitted and reflected light and subsequently studied undera CamScan scanning electron microscope (SEM) equipped with anOxford Link energy dispersion spectrometer (EDS). Quantitativemicroanalyses were performed using a Cameca SX-100 electronmicroprobe (EPMA). For the silicate and oxide phases, the analyti-cal conditions were: accelerating voltage 15 kV, beam current10 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), syn-thetic CdS (Cd), crocoite (Pb) and synthetic GaAs (As). For metalsand sulphides, the analytical conditions were: accelerating voltage20 kV, beam current 10 nA and counting time 10 s for all the ele-ments. The following standard set was used for metals and sulp-hides: pyrite (S, Fe), pure nickel (Ni), pure copper (Cu), sphalerite(Zn), synthetic GaAs (As), pure silver (Ag), pure tin (Sn), pure anti-mony (Sb) and galena (Pb). The calculation of Fe2O3/FeO ratio inspinels was performed using the methodology described in detail

in Ettler (2002). A total set of 240 spot analyses was performedby EPMA.

2.5. Mineralogical analysis of secondary weathering products

The secondary weathering products were sampled on the sur-face of the slags using a preparation needle under a binocularmicroscope. They were analyzed by XRD (same analytical condi-tions as for bulk slag samples) and were examined using a JEOLJSM 6400 scanning electron microscope equipped with a KevexDelta energy-dispersion spectrometer (SEM/EDS).

2.6. Leaching experiments

To assess the leaching characteristics of these materials, leach-ing tests were performed on two slag samples from historically dif-ferent technologies: sample N2 (slag III, recent slag) and sample N6(slag I, �100-a-old material). Two standardized leaching tests wereperformed: European norm EN 12457 and USEPA Toxicity Charac-teristic Leaching Procedure (TCLP). All the experiments were per-formed in duplicate and with a procedural blank. The EN 12457and TCLP procedures necessitate a reduction of the grain size ofthe waste material to <4 mm and <9.5 mm, respectively. The effectof the particle size, in particular the dust fraction, was found to becrucial when interpreting the results, even for standardized leach-ing tests (Zandi et al., 2007). However, granulated slag N2, as sam-pled directly in the tailing ponds, was already crushed and milledwith a grain size of <0.2 mm (Fig. 2c). For the sake of consistency,sample N6 was crushed and milled prior to the leaching test,although it was known that this sample preparation would resultin a significantly higher reactivity. Both samples were subse-quently manually homogenized with a pestle in an agate mortarto obtain approximately the same grain size. The granulometryof both samples was measured by the low-angle laser light scatter-ing (LALLS) method using a Fritsch Analyzette 22 NanoTec laserparticle sizer, equipped with a He–Ne laser (k = 632.8 nm).Granulometric investigations of sample N2 yielded the followingsize distributions: <5 lm (30%), 5–10 lm (23%), 10–20 lm (33%),20–100 lm (14%). The granulometry of sample N6 was as follows:<5 lm (45%), 5–10 lm (24%), 10–20 lm (26%), 20–100 lm (5%).The obtained specific surfaces for N2 and N6 were 1.6 and2.2 m2/g, respectively. The results of the leaching test with thesefine-grained slags correspond to the ‘‘worst” leaching scenario, ascould be expected in the tailing ponds filled with granulated andcrushed/milled slag. The significantly lower specific surface areaof �20-cm-large slag fragments (sample N6) in the dumps will re-sult in slower release rates of contaminants than obtained by theleaching test.

The leaching experiment denoted as ‘‘EN 12457” was carriedout according to the experimental protocol of the static leachingtest described in detail by the European standard EN 12457 (part2) adopted recently by the Czech legislation (EN 12457, 2002). Amass of 10 g of solid was placed in the high-density polyethylene(HDPE) reactor and 100 mL of MilliQ+ deionised water was addedto maintain a L/S (liquid/solid) ratio of 10. The leaching test wasperformed at 22 ± 3 �C for 24 h and the reactors were gently agi-tated (60 rpm). After the experiments, the reactors were centri-fuged in order to settle fine slag particles at the bottom beforethe filtration and measurements. After opening the reactor, thepH, Eh and specific conductivity were measured in the leachate.The pH and Eh values were determined using a Schott Handylab1 pH meter equipped with a Schott L 7137 A combined electrodeand a Schott PT 737 A (Pt–Ag/AgCl) redox electrode, respectively.The temperature and specific conductivity were measured usinga Schott Handylab LF 1 conductometer equipped with a LF 513 Tmeasuring cell and a temperature detector. The supernatant was

Fig. 2. Scanning electron micrographs in backscattered electrons of slags with emphasis on silicate and oxide phases: (a) zoned Cu-bearing oxides (Cu–Fe oxide, Cu–Cr oxide,mcconnelite, see corresponding analyses in Table 4) and feldspar crystals within the Pb-arsenate matrix (sample N6, Slag I); (b) glass inclusion with zoned spinel oxides(granulated sample N1, Slag III); (c) milled slag N2 (Slag III) composed of glassy fragments with small sulphide/metallic droplets and/or dendritic spinels and olivine laths; (d)cross-like wuestite dendrites and euhedral spinels within the melilite crystal associated with olivine (sample N9-2, Slag II); (e) olivine crystals within the glassy matrix withsmall wurtzite/galena droplets (sample N9-1, Slag II); (f) euhedral melilite within the matrix composed of olivine laths with trapped spinel crystals, residual glass and smallmetallic/sulphide inclusions (sample N9-2, Slag II). Abbreviations: mcc – mcconnelite, spl – spinel, ol – olivine, as – arsenate, f – feldspar, Pb-f – Pb feldspar, mel – melilite, gl –glass, w – wuestite, gn – galena, wz – wurtzite.

4 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

then filtered to 0.45 lm (Millipore�) using a Sartorius polycarbon-ate filtering device and split into two aliquot parts, for cation andtrace element analysis (diluted and acidified to pH <2 by HNO3),and for anion analysis and alkalinity measurements. The obtainedresults were compared with regulatory levels for leachates definedfor non-hazardous and hazardous waste by the EU legislation(compiled in Van Gerven et al., 2005).

The second leaching experiment denoted as ‘‘TCLP” was con-ducted according to the toxicity characteristic leaching procedure

experimental protocol defined by USEPA (1994). With respect tothe alkalinity and buffering capacity of slags, measured accordingto the TCLP procedure, solution #1 with pH 4.93 ± 0.05 was used.The leaching solution was prepared by adding 5.7 mL of acetic acid(reagent grade, Merck, Germany) to 500 mL of deionised water(MilliQ+), subsequent addition of 64.3 mL of 1 mol L�1 NaOH (re-agent grade, Lach-ner, CZ) and dilution to 1 L. An amount of 5 gwas placed in the reactor and 100 mL of leaching solution wereadded to attain an L/S ratio of 20. The experiment was conducted

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 5

for 18 h at 22 ± 3 �C and the reactors were agitated at 30 rpm. Themeasurement of physico-chemical parameters and leachate pro-cessing (filtration, acidification) for analyses were analogous tothose of the previously described leaching test. The obtained re-sults were compared with the maximum permissible concentra-tions for toxicity characteristics defined by USEPA (2005).

Major cations were analysed by FAAS (Varian, SpectrAA 280 FS)and trace elements (Al, As, Cd, Cr, Cu, Fe, Mn, Sb, Pb, Zn) weredetermined by ICP-MS (VG Elemental PQ3). Quality control/qualityassurance (QC/QA) of the analytical measurements was controlledby a NIST 1640 standard reference material (Trace Elements inWater) and a Merck IV solution (ICP multielement standard IV,Merck). The accuracy of the measurement was generally betterthan 10% relative standard deviation (RSD). The alkalinity of thesamples was measured by back titration using the 0.05 M HCl (re-agent grade Lach-ner, CZ) with a Schott TitroLine Easy automatictitrator. The concentrations of Cl� and SO2�

4 in the leachates weremeasured by a Dionex ICS-2000 liquid chromatography instrument(HPLC).

2.7. Thermodynamic modelling

The PHREEQC-2 geochemical speciation-solubility code, version2.13.2 for Windows (Parkhurst and Appelo, 1999) was used todetermine the speciation of contaminants and the degree of satu-ration of leachates with respect to the mineral phases. The Min-teq.v4.dat database (derived from MINTEQA2, version 4 releasedby USEPA in 1999) with the thermodynamic data of acetic com-plexes was used for model calculations. The thermodynamicdatabase was supplemented by the solubility products of Ca- andPb-arsenates and antimonates recently compiled by Corneliset al. (2008). In addition, the solubility products of secondaryCu-, Zn- and Pb-arsenate minerals determined by Magalhãeset al. (1988) were included in the PHREEQC-2 calculations.

Table 1Bulk chemistry of selected slag samples.

Sample N1 N2 N6 N7 N8type Slag III Slag III Slag I Slag II Slag II

wt.%SiO2 14.68 35.50 27.68 20.86 9.82TiO2 0.25 0.20 0.70 0.15 0.15Al2O3 4.70 3.85 18.50 2.82 3.92Fe2O3 tot.a 32.47 28.81 8.42 25.25 64.22FeO 9.49 17.56 4.43 13.98 21.22MnO 0.08 0.28 0.05 0.16 0.10MgO 1.43 4.65 0.69 2.34 1.55CaO 4.50 11.53 3.85 8.48 2.29Na2O 0.26 1.87 0.25 0.74 4.31K2O 0.30 0.35 0.93 0.27 0.10P2O5 0.59 0.42 0.56 0.39 0.25S tot. 6.51 0.24 <0.01 3.85 0.65

mg/kgAs 24,060 17,445 28,750 75,865 13,135Ba 690 1275 1280 825 240Bi 56 0.54 78 3.5 0.36Cd 1364 65 6939 412 34Co 149 216 27 129 4244Cr 368 826 232 789 1351Cu 27,488 7938 106,200 121,850 47,650Ga 3.1 48 3.1 19 0.97Ge 9.3 123 15 33 2.4Ni 56 116 32 168 252Pb 103,900 40,300 183,800 55,900 62,750Sb 2175 1071 2110 821 729Zn 28,188 60,275 33,125 45,925 77,550

a Total Fe (including FeO + Fe2O3).

3. Results

3.1. Bulk chemical composition

The chemical compositions of the slag samples are reported inTable 1. These materials are mainly composed of SiO2 (9.82–35.50 wt.%), Fe2O3 tot. (8.42–64.22 wt.%), CaO (3.85–17.44 wt.%)and Al2O3 (2.82–18.50 wt.%). The slag samples are also significantlyenriched in metals and metalloids: Pb (0.97–18.38 wt.%), Cu (0.49–12.18 wt.%), Zn (2.8–12.09 wt.%), As (0.09–7.59 wt.%), Sb (67–2175 mg/kg). As some of metallic elements are present in variousphases (silicates, glass, oxides, sulphides, metallic compounds),the sums of oxides are generally low and are not given in Table 1.

Differences in the chemical compositions between the histori-cally different slag groups reflect the variation in furnace chargesand smelting conditions over time. In contrast to other samples,the oldest slag (Slag I, sample N6) is significantly enriched in Al(18.5 wt.% Al2O3), impoverished in Fe and Ca and also has high con-centrations of Pb, Cu, Zn and As (Table 1). This chemical composi-tion indicates relatively low efficiency of metal recovery andprobably also the fact that carbonate ore was processed withoutCa additives routinely used for sulphide ore processing in modernpyrometallurgy (Ettler et al., 2000, 2001). A low concentration of S(<0.01 wt.%) is consistent with the processed carbonate and oxideores. With the exception of sample N8 (particularly enriched inFe and impoverished in Si), slags of group II exhibit similar chem-ical compositions to Pb and Cu slags from other smelting sites (Et-tler et al., 2000; Manasse et al., 2001; Lottermoser, 2002). The Sconcentrations ranging from 0.7 to 3.85 wt.% indicate that metalsdissolved in slag melt are present not only in silicate matrix andpure metallic phases, but can be also associated with S in the formof sulphides (PbS, ZnS, CuS) (Ettler et al., 2001; Manasse and Mel-lini, 2002a). This was also confirmed by the mineralogical investi-gation of slags (see below). Particularly high S (and metallic

N9-1 N9-2 N10 N11 N12 N13Slag II Slag II Slag II Slag II Slag II Slag II

28.52 29.12 23.24 21.60 23.20 24.300.20 0.16 0.20 0.15 0.15 0.133.86 4.11 3.75 3.60 4.10 4.1835.94 30.24 37.90 42.32 39.43 42.9821.49 22.33 28.69 28.78 29.65 31.080.24 0.25 0.26 0.25 0.26 0.255.01 5.10 3.25 3.23 3.23 3.4917.44 17.34 12.98 12.26 12.99 13.361.57 1.57 0.81 0.80 0.80 0.840.61 0.61 0.63 0.57 0.58 0.610.22 0.23 0.28 0.27 0.25 0.270.77 0.70 1.99 1.82 1.98 1.68

2055 930 3420 3165 3210 9353150 3075 1065 1395 1965 10600.37 <0.1 0.55 0.85 0.88 0.3119 12 26 15 16 14130 119 142 169 168 127252 267 201 1576 238 24712,763 4963 23,263 19,613 19,513 11,16312 13 11 7.0 7.2 128.6 9.0 7.8 10 11 9.232 8.0 49 33 45 1237,213 12,575 24,713 43,163 10,263 9763341 109 174 219 214 6796,250 74,750 93,000 110,500 96,350 120,850

Table 2Primary phases from the Tsumeb slags obtained by XRD and EPMA (normal – both methods, italics – only EPMA)a.

Group Phase Chemistry Slag type

Slag I Slag II Slag III

Silicates Fayalite Fe2SiO4 ++ +Monticellite CaMgSiO4 +++Melilite Ca2(Mg,Fe,Zn)Si2O7 +++ +Anorthite CaAl2Si2O8 +++Unnamed (PDF 087-1003)b PbAl2Si2O8 ++Amorphous glass Si–Ca–Fe–Al ++ +++

Oxides Spinel series (Zn,Mg,Fe,Cu)(Fe,Al)2O4 +++ + +++Wuestite FeO +Delafossite–mcconnelite Cu1+(Fe3+,Cr3+)O2 ++

Sulphides Galena PbS + +Wurtzite ZnS +Sphalerite ZnS +Chalcopyrite CuFeS2 +Pyrrhotite Fe1�xS +Cubanite CuFe2S3 +Covellite CuS +

Elements Lead Pb +Copper Cu +

Others Domeykite a Cu3As +Cu5Sb Cu5Sb +Cu3(Sn,Sb) Cu3(Sn,Sb) +Fe2As Fe2As +FeAs FeAs +Unidentified Ca–Pb arsenates Ideal formula � (Pb,Ca,Fe)3(AsO4)2�H2O ++

a Relative abundance: +++ dominant phase, ++ common phase, + trace phase.b Unnamed phase (Pb feldspar) according to Benna et al. (1996).

6 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

element) contents were observed for sample N1 (slag III) (Table 1),indicating the lower efficiency of sulphide-bound metal recoverydue to quenching of the slag melt by granulation.

3.2. Mineralogy

3.2.1. Slag petrographyIn the present paper, mineral names are given for phases having

natural equivalents. Significantly distinct phase compositions wereobserved for slag I compared to younger slags as revealed by XRDanalysis (Table 2), indicating both different conditions of slag for-mation (e.g., the chemical composition of the slag melt, the coolingregime), and also possible alteration processes at the dumping site(the presence of secondary phases). Interestingly, slag I is mainlycomposed of feldspars (with anorthite and Pb-feldspar structures),Cu-bearing spinels, delafossite, an As-bearing matrix phase (corre-sponding to arsenate) and traces of galena (Table 2). In contrast,slags II and III are predominantly composed of typical high-tem-perature Ca–Fe silicates (olivine-type phases, melilite), amorphoussilicate glass, spinel-type oxides, wuestite and also trace sulphides(e.g., galena, wurtzite/sphalerite, bornite), pure metals (Pb, Cu) andvarious intermetallic compounds (Table 2).

Slag I. Microscopic investigation shows that feldspars forming50- to 300-lm large euhedral crystals are the dominant phase inthese slags (Fig. 2a). Oxides belonging to the Cu-spinel and delafos-site–mcconnelite series form euhedral crystals up to 80 lm in sizeand are often zoned (Fig. 2a). Another generation of prismatic feld-spar crystals (brighter on SEM images than the previous ones) isassociated with Cu-spinels and the slag matrix. The latter is com-posed of As-bearing phases, often enriched in Pb, Cd, Cu and Ca(Fig. 2a), probably in the form of arsenates, indicating that the slagunderwent some alteration process. Occasional inclusions of gale-na (PbS) are observed in the slag matrix (Fig. 2a).

Slags II and III are composed of phases commonly reported inpapers devoted to smelter slag mineralogy: spinel oxides, silicates,

glass and sulphide-metallic inclusions (Ettler et al., 2001; Manasseand Mellini, 2002b; Piatak et al., 2004). In general, spinels are thefirst crystallizing phases, forming large zoned crystals (up to50 lm across; Fig. 2b) or small dendrites (Fig. 2c) or euhedral crys-tals (Fig. 2d). Occasionally, dendritic wuestite aggregates form atthe beginning of the slag melt solidification (Fig. 2d). The crystalli-sation sequence follows with the formation of large euhedral meli-lites (Fig. 2d and f) or skeletal to lath-like olivines (Fig. 2e,f). Glasssolidifies last and contains small inclusions of sulphides (mainlygalena, wurtzite, bornite, digenite) or intermetallic compounds(Fig. 2c, e, and f). Mattes (sulphide-rich materials) and speiss (arse-nide-rich materials) were present as droplets and fragments of var-ious sizes composed of symplectitic intergrowths of sulphides,metals and intermetallic compounds, and embedded within thesilicate slag (Fig. 3a–c). The rare occurrence of massive mattematerial associated with silicate slags (sample N7) was also ob-served in one case (Fig. 3d): this heavy material is composed of sec-ondary litharge (PbO) filling the cavities between the pure Pbcrystals and intermetallic compounds (Cu3As).

3.2.2. Crystal chemistry3.2.2.1. Olivines. The chemical composition of olivine-type phasesvaries from the nearly pure fayalite (Fe2SiO4) to kirschsteinite (CaF-eSiO4) – monticellite (CaMgSiO4) solid solution as revealed byEPMA (Table 3). They occur as a dominant phase in slags II andIII. Olivine crystals are slightly zoned (Fig. 2e and f) with brightrims corresponding to Ca-poor olivine and dark cores correspond-ing to Ca-rich kirschsteinite (up to 28.51 wt.% CaO). All the ob-served olivines are Zn-bearing; the fayalite end-members aregenerally most enriched (up to 8.74 wt.%). Zinc can enter into octa-hedral sites of the olivine structure and substitutes for Fe2+ (Ettleret al., 2000, 2001). Previous works on smelting slags showed thatthese phases are the most common silicates that form large skele-tal crystals or laths in slowly crystallizing slags, and dendrites inquenched slags (Ettler et al., 2001, in press; Manasse et al., 2001).

Fig. 3. Scanning electron micrographs in backscattered electrons of sulphide phases: (a) Cu sulphide inclusions associated with sphalerite and overgrown with galena inmyrmekites (sample N7, Slag II); (b) inclusion composed of bornite with inclusions of wurtzite and pure Pb droplets (sample N11, Slag II); (c) large sulphide/metallic inclusioncomposed of galena-CuS myrmekite, crystals of covellite, sphalerite, Cu3As and pure Pb droplets (sample N9-1, Slag II); (d) metallic Pb altered to litharge (PbO) with residualcrystals of chalcocite and Cu3As intermetallic phase (matte associated with slag sample N7, Slag II). Abbreviations: gl – glass, CuS – copper sulphide, gn – galena, Pb – metallicPb, wz – wurtzite, bn – bornite, sph – sphalerite, co – covellite, cc – chalcocite.

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 7

3.2.2.2. Melilite. The composition of melilite varies significantly andoften corresponds to a solid solution of predominant åkermanite(CaMgSi2O7) and hardystonite (CaZnSi2O7) with minor amountsof Na-melilite (NaCaAlSi2O7), gehlenite (Ca2Al2SiO7) and ferro-åkermanite (CaFeSi2O7) (Table 3). Thus, melilite seems to effi-ciently accumulate Zn, exhibiting ZnO concentrations up to11.64 wt.%. Similar Zn contents in melilite were observed in othersmelting slags resulting from similar technological processes (Et-tler et al., 2001, 2002).

3.2.2.3. Feldspars. Anorthite (CaAl2Si2O8) and an unnamed phasecorresponding to a ‘‘Pb-feldspar” (PbAl2Si2O8), as reported by Ben-na et al. (1996), were observed only in slag I. According to EPMA,their composition is rather variable and the Pb concentration canbe as high as 34 wt.% PbO (Table 3). They certainly crystallizedfrom the slag melt (large euhedral crystals) and represent (to-gether with spinels) the early phases crystallizing in the slag I.The presence of two generations of chemically different feldsparsindicates significant changes in the slag melt during solidification.The first generation of large euhedral feldspars is Pb-depleted(analyses 18/1 and 28/1 in Table 3) and the second generation ofsmaller Pb-rich feldspars associated with spinels is probably theproduct of crystallisation from the melt strongly enriched in Pb,Si and Al (analyses 17/1, 20/1, 24/1 and 30/1 in Table 3). Bennaet al. (1996) showed that the disordered structure of syntheticPb-feldspar is a product of rapid cooling (quenching), as it can oc-cur during the slag melt solidification. It should be recalled that the

presence of feldspars in slags is rather scarce and was observedonly by Sáez et al. (2003), who detected plagioclase in Cu-smeltingslags from Spain, and by Puziewicz et al. (2007), who described Ca,Ba and Pb feldspars in Zn-slags from Poland. The shapes of feldsparcrystals from Namibian slags seem to indicate that these phasescrystallized directly from the melt. In other cases, feldspars canbe residual, i.e. from unmelted gangue, especially in medieval slagssuch as those studied by Sáez et al. (2003). The presence of angulargrains of residual feldspars indicate that (i) the temperatures in thesmelting furnaces were not sufficiently high and/or (ii) the dura-tion of melting was not sufficient to completely melt the furnacecharge containing gangue minerals (Hauptmann, 2007; Ettleret al., in press).

3.2.2.4. Silicate glass. The presence of glass was observed mainly inslags II and III and confirms the rapid cooling of the slag melt (gran-ulation). Its composition is rather variable and, similar to other sil-icates in slags; the glass is enriched in Ca and Fe (Table 4).Furthermore, it is the principal carrier of contaminants such asPb (up to 11.8 wt.% PbO), Zn (up to 16.2 wt.% ZnO), Cu (up to1.78 wt.% CuO) and As (up to 2.99 wt.% As2O3). Low analytical to-tals of some EPMA indicate the presence of H2O or possibly thepresence of trivalent Fe in the glass structure (Table 4).

3.2.2.5. Oxides. Oxides in Namibian slags are mainly represented byspinel-type compounds for slags II and III. EPMA revealed that theircomposition is rather variable (Table 5). Whereas the compositions

Table 3Selected microprobe analyses of primary silicates (olivines, melilites and feldspars). Structural formulae (apfu – atoms per formula unit) were calculated on the basis of four(olivine), seven (melilite) and eight oxygens (feldspars).

Spot analysis 58/1 59/1 67/1 46/1 49/1 57/1 17/1 18/1 20/1 24/1 30/1 28/1Sample N9-2 N9-2 N9-2 N7 N8 N9-2 N6 N6 N6 N6 N6 N6Slag type Slag II Slag II Slag II Slag II Slag II Slag II Slag I Slag I Slag I Slag I Slag I Slag IPhase Olivine Olivine Olivine Melilite Melilite Melilite Feldspar Feldspar Feldspar Feldspar Feldspar Feldspar

SiO2 wt.% 34.07 30.46 32.11 SiO2 40.72 40.14 40.36 SiO2 47.67 44.23 44.09 44.04 39.02 44.33TiO2 – 0.02 – TiO2 – – – TiO2 0.02 – 0.05 0.14 0.04 –Al2O3 – 0.02 0.11 Al2O3 1.48 2.58 3.62 Al2O3 22.58 32.34 23.96 18.78 20.39 31.22FeO 21.68 50.23 42.10 FeO 2.99 3.91 4.75 FeO 1.03 0.89 1.24 0.78 0.41 0.70MnO 0.48 0.63 0.79 MnO – 0.06 0.04 MnO – – – – 0.03 0.07MgO 10.74 6.55 13.19 MgO 5.98 3.01 3.90 MgO – – 0.04 0.11 0.04 –CaO 28.51 4.04 4.13 CaO 37.14 33.95 34.90 CaO 3.56 16.21 5.93 2.38 1.39 14.83Na2O 0.17 0.32 0.23 Na2O 0.63 2.91 2.15 Na2O 0.62 0.67 0.86 0.43 0.27 0.84K2O 0.01 0.02 0.01 K2O 0.06 – 0.11 K2O 6.41 0.45 2.56 2.26 2.78 0.38P2O5 0.01 0.06 0.14 P2O5 0.01 0.18 0.05 P2O5 0.03 0.05 0.18 0.15 0.06 0.03Cr2O3 0.06 – 0.05 Cr2O3 0.07 – 0.12 Cr2O3 – 0.06 0.13 0.07 – 0.11PbO – – – PbO 0.14 0.10 0.05 PbO 16.54 3.66 19.33 29.54 34.00 5.68ZnO 4.87 8.74 7.62 ZnO 10.37 11.64 8.94 ZnO – 0.05 0.13 0.27 0.43 –CuO – – – CuO – – – CuO 0.30 0.39 0.71 0.91 0.49 0.55CdO 0.10 – 0.04 CdO – – 0.07 CdO 0.05 0.15 0.14 0.14 0.16 0.27As2O3 – – – As2O3 – – – As2O3 – – 0.32 0.61 0.03 –SO2 – – – SO2 0.02 – – SO2 – – – – – –Cl – – – Cl – – – Cl 0.02 – 0.02 – 0.03 –Total 100.70 101.06 100.54 Total 99.61 98.48 99.06 Total 98.83 99.14 99.67 100.58 99.55 99.00

Si apfu 0.996 0.983 0.989 Si 1.985 2.000 1.974 Si 2.554 2.131 2.411 2.605 2.441 2.170Fe 0.530 1.355 1.084 Al 0.085 0.152 0.209 Al 1.426 1.836 1.544 1.310 1.504 1.801Mn 0.012 0.017 0.021 Fe 0.122 0.163 0.194 Fe 0.046 0.036 0.057 0.038 0.021 0.029Mg 0.468 0.315 0.606 Mn 0.000 0.003 0.002 Ca 0.204 0.837 0.347 0.151 0.093 0.777Ca 0.893 0.140 0.136 Mg 0.435 0.224 0.284 Na 0.065 0.063 0.091 0.049 0.033 0.080Cr 0.001 0.000 0.001 Ca 1.940 1.813 1.829 K 0.438 0.028 0.179 0.170 0.222 0.024Zn 0.105 0.208 0.173 Na 0.060 0.281 0.204 Pb 0.239 0.047 0.285 0.470 0.573 0.075

K 0.004 0.000 0.007 Zn 0.000 0.002 0.005 0.012 0.020 0.000Proportion of end-members (mol.%) Pb 0.002 0.001 0.001 Cu 0.012 0.041 0.029 0.041 0.023 0.020

La 47 8 7 Zn 0.373 0.428 0.323 Cd 0.001 0.003 0.003 0.004 0.005 0.006Fo 25 17 33Fa + Te 28 75 60 Proportion of end members (mol.%) Proportion of end members (mol.%)

Ha 37 40 31 Ab–Or 50 9 27 24 26 10Ak 43 21 27 An 50 91 73 76 74 90FeAk 12 15 19Gh 4 9 10SM 5 14 14

Symbols used: – not detected; La larnite (Ca2SiO4), Fo forsterite (Mg2SiO4), Fa fayalite (Fe2SiO4), Te tephroite (Mn2SiO4), Ha hardystonite (Ca2ZnSi2O7), Ak åkermanite(Ca2MgSi2O7), FeAk ferro-åkermanite (Ca2FeSi2O7), Gh gehlenite (Ca2Al2SiO7), SM soda-melilite ((Ca,Na)AlSi2O7), Ab albite (NaAlSi3O8), Or orthoclase (KAlSi3O8), An anorthite(CaAl2Si2O8).

Table 4Selected microprobe analyses and average compositions of all the studied glasses.

Spot analysis 1/1 8/1 38/1 43/1 50/1 53/1 54/1 62/1 73/1 n = 16

Sample N1 N2 N7 N7 N8 N8 N8 N9-2 N9-2 Min Max MeanSlag type Slag III Slag III Slag II Slag II Slag II Slag II Slag II Slag II Slag II

SiO2 25.14 40.22 38.35 28.21 39.62 35.72 37.05 40.14 39.57 25.14 45.49 37.50TiO2 0.25 0.12 – 0.28 – 0.10 0.03 – – – 0.28 0.07Al2O3 4.48 1.97 5.97 4.31 12.93 0.77 6.67 5.36 7.27 0.65 12.93 5.55FeO 24.71 32.47 21.86 21.84 11.80 18.50 20.96 5.07 5.02 5.02 58.48 20.96MnO 0.10 0.22 1.32 0.36 0.07 0.14 0.16 0.14 – – 1.32 0.24MgO 3.44 0.32 0.27 5.34 2.84 1.31 2.16 0.93 0.65 0.03 5.34 1.51CaO 19.88 5.16 8.30 18.75 1.77 15.95 2.32 32.06 30.70 0.81 32.06 12.01Na2O 0.96 1.01 3.53 1.31 15.18 12.25 10.08 3.92 5.00 0.40 15.18 5.50K2O 0.07 0.21 1.37 0.64 0.14 0.27 0.13 0.10 0.14 0.07 6.74 1.04Cr2O3 – 0.09 0.06 0.36 0.04 – – 0.07 – – 0.36 0.04P2O5 0.32 0.09 1.48 0.36 0.12 1.05 0.33 0.29 0.71 0.08 1.48 0.55PbO 7.85 11.80 5.26 1.30 0.06 0.32 0.25 0.14 0.08 – 11.80 2.29ZnO 9.12 3.23 11.42 16.21 14.70 9.19 10.94 11.62 10.51 2.42 16.21 9.29CuO 1.78 – – 0.08 – 0.09 – – – – 1.78 0.15CdO 0.05 0.03 – 0.10 – – 0.03 0.07 – – 0.10 0.03As2O3 1.56 0.56 0.58 – 0.07 1.69 0.16 – – – 2.99 0.54SO2 0.01 0.50 0.05 0.90 – 0.38 0.05 0.01 0.01 – 0.90 0.24Cl 0.01 – 0.02 0.01 – 0.02 – 0.02 0.01 – 0.20 0.03Total 99.73 97.99 99.83 100.36 99.35 97.71 91.31 99.95 99.66 90.39 102.79 97.55

Symbols used: – not detected.

8 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

Table 5Selected microprobe analyses of primary oxides.

Spotanalysis

2/1. 3/1. 7/1. 42/1. 44/1. 61/1. 68/1. 66/1. 13/1. 35/1.c 11/1.d 3/2.e 4/2.f 13/2.g

Sample N1 N1 N2 N7 N7 N9-2 N9-2 N9-2 N6 N6 N6 N6 N6 N6Slag type Slag III Slag III Slag III Slag II Slag II Slag II Slag II Slag II Slag I Slag I Slag I Slag I Slag I Slag IPhase Spinel Spinel Spinel Spinel Spinel Spinel Spinel Wuestite Mcconnelite Cu–Fe

oxideCu–Croxide

Cu–Feoxide

Zn–Cu–Al–Feoxide

Cu–Feoxide

SiO2 wt.% 0.18 0.09 1.88 0.16 – 0.39 0.26 0.38 – 0.05 – 0.06 0.54 0.14TiO2 0.25 0.14 0.26 0.36 0.16 0.88 2.00 0.41 0.35 5.98 0.55 4.89 0.23 0.63Al2O3 9.30 5.74 0.78 48.77 14.08 36.97 25.94 0.67 9.21 3.40 13.41 4.90 38.57 9.66Cr2O3 20.14 1.02 0.36 3.26 52.67 0.08 0.68 – 33.73 – 45.56 0.12 0.08 3.61Fe2O3 31.98 50.28 62.86 9.13 5.09 21.58 32.77 – 7.99 27.15 8.09 32.03 13.25 52.79FeO 15.31 23.13 33.31 6.83 8.40 12.84 18.98 87.09 – – 5.42 – 7.68 –MnO – 0.29 0.04 0.04 – 0.06 0.21 0.40 – – – – 0.11 0.14MgO 6.36 5.39 0.34 6.58 13.17 3.68 4.18 0.35 0.08 0.71 6.28 0.49 2.46 0.99ZnO 15.63 13.93 1.26 25.41 6.17 21.31 15.69 8.42 – 0.63 3.65 0.29 20.20 4.90CuO (Cu2O) 0.20 0.25 0.06 – 0.10 0.05 – – (50.19) 55.20 13.20 54.40 10.05 20.92CdO – – – – – – – – – – 0.08 0.06 – 0.24Na2O 0.42 0.37 0.07 0.67 0.10 0.72 0.43 0.19 – – 0.14 – 0.49 0.10K2O – – – – 0.02 0.01 – 0.01 – – – – 0.03 0.04CaO 0.17 0.29 0.08 0.03 0.02 0.36 0.18 0.42 – 0.05 0.02 0.02 0.03 0.03PbO – – – – – 0.07 – – – 0.35 0.08 0.12 0.12 –P2O5 – – 0.03 – – – 0.01 – – 0.01 – 0.02 0.01 –SO2 – – – – – 0.02 – – – 0.02 – – – –As2O3 – – – – – 0.04 – – – 0.15 – 0.12 – –Total 99.94 100.92 101.33 101.23 99.97 99.06 101.32 98.33 101.54 93.68 96.47 97.51 93.84 94.16H2Oa 6.32 3.53 2.49 6.16 5.84

Si apfu 0.007 0.003 0.071 0.005 0.000 0.012 0.008 0.009 0.000 0.004 0.000 0.004 0.019 0.020Ti 0.007 0.004 0.007 0.008 0.004 0.021 0.050 0.007 0.006 0.303 0.014 0.234 0.006 0.069Al 0.394 0.254 0.034 1.696 0.533 1.406 1.011 0.019 0.249 0.270 0.553 0.367 1.602 1.655Cr 0.573 0.030 0.011 0.076 1.336 0.002 0.018 0.000 0.613 0.000 1.261 0.006 0.002 0.414Fe(3) 0.867 1.424 1.782 0.203 0.123 0.525 0.817 0.000 0.139 1.379 0.214 1.532 0.352 5.787Fe(2) 0.460 0.727 1.047 0.169 0.225 0.347 0.525 1.774 0.000 0.000 0.159 0.000 0.226 0.000Zn 0.415 0.386 0.035 0.554 0.146 0.508 0.383 0.151 0.000 0.031 0.094 0.014 0.526 0.526Mg 0.341 0.302 0.019 0.290 0.630 0.177 0.206 0.013 0.003 0.071 0.327 0.046 0.129 0.214Cu 0.006 0.007 0.002 0.000 0.002 0.001 0.000 0.000 0.969 2.811 0.349 2.608 0.267 2.298Ab 1.222 1.422 1.103 1.012 1.004 1.033 1.114 1.938 0.971 2.913 0.929 2.668 1.148 3.037Bb 1.847 1.716 1.905 1.988 1.996 1.967 1.904 0.036 1.007 1.956 2.042 2.142 1.982 7.946sum 3.070 3.138 3.008 3.000 3.000 3.000 3.019 1.974 1.978 4.869 2.972 4.810 3.130 10.983oxygens 4 4 4 4 4 4 4 1 2 6 4 6 4 15H2O 2.84 0.82 1.05 1.45 5.66

Symbols used: – not detected.a H2O by difference.b A and B correspond to structural positions in the oxide structure (e.g., AB2O4 for the spinel structure).c Chemical interpretation: Cu3Fe2O6�3H2O (3CuO�Fe2O3�3H2O).d Chemical interpretation: (Cu,Mg)Cr2O4�H2O (CuO�Cr2O3�H2O).e Chemical interpretation: Cu3Fe2O6�H2O (3CuO�Fe2O3�H2O).f Chemical interpretation: (Zn,Cu,Mg)(Al,Fe)2O4�1.5H2O.g Chemical interpretation: Cu3Fe8O15�6H2O (3CuO�4Fe2O3�6H2O).

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 9

of spinel-type phases from slag III correspond either to Fe–Cr spi-nels or to magnetite (Fe3O4), those in slag II exhibit a compositionclose to gahnite (ZnAl2O4) or Cr-bearing spinels (Table 5). The den-drites of Zn-rich wuestite (FeO) are also reported for slag II (Table5; Fig. 2d). Spinel-type oxides from slag I are complex solid solu-tions of Cu–Fe–Cr-end-members. The presence of a phase of thedelafossite (CuFeO2) – mcconnellite (CuCrO2) system, correspond-ing to the stoichiometric formula ðCu1þ

0:969Mg0:003Þ0:972ðAl0:249Cr0:613-Fe3þ

0:139Ti0:006Þ1:007O2 was observed in this type of slag (analysis 13/1in Table 5, Fig. 2a). This phase has been commonly found in Cu-bearing slags in Spain (Sáez et al., 2003) and its presence (withCu1+ in the delafossite or mcconnellite structures) indicates ex-treme reducing conditions in the slag melt. This is compatible withthe historical fact that blast furnaces at the beginning of the 20thcentury were fired with German or South African coke, inducinga highly reducing environment during smelting of the ore (TsumebCorporation Ltd., 1987). Other complex Cu-bearing oxides are di-rectly associated with mcconnellite-like phases (Fig. 2a). Lower to-tals of their EPMA indicate the presence water in these compounds(1–6 moles per formula unit). The EPMA of these compounds exhi-bit the following structural formulae (see also Table 5):

3CuO�Fe2O3�3H2O, 3CuO�Fe2O3�H2O, 3CuO�4Fe2O3�6H2O, CuO�Cr2O3�H2O. A more complex spinel-like structure ideally corre-sponding to (Zn,Cu,Mg)(Al,Fe)2O4�1.5H2O was also detected in slagI. With respect to the stable spinel-type phases without Cu, the Cu-bearing oxides seem to be more susceptible to hydration, subse-quent weathering and also probable release of Cu.

3.2.2.6. Sulphides and intermetallic compounds. The most commonlyobserved sulphides are galena (PbS), wurtzite/sphalerite (ZnS),pyrrhotite (Fe1�xS) and various Cu or Cu–Fe sulphides (digenite(Cu,Fe)9S5, cubanite CuFe2S3, covellite CuS, chalcocite Cu2S).Wurtzite is a high-temperature phase (>1020 �C) (Ettler and Johan,2003) and contains significant amounts of Fe (up to 15.4 wt.%) andminor amounts of Cu (up to 1 wt.%). Other sulphides are also en-riched 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) (data notshown). Unlike the other Pb–Zn slags (Ettler et al., 2001), thosefrom Tsumeb contain higher amounts of Cu-rich sulphides, asCu-rich ores were treated in the Tsumeb smelter. Intermetalliccompounds mainly belonging to the Cu–As, Cu–Sb, Cu–Sn andFe–As binary systems were also observed: domeykite a (Cu3As),

10 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

Cu3(Sn,Sb), Cu5Sb (Fe,Cu)2As, FeAs. The elemental substitutions intheir structures correspond to the substitutions observed by Ettlerand Johan (2003) in matte phases from primary Pb metallurgicalprocesses.

3.2.2.7. Arsenates. According to EPMA, the As-bearing matrix phasesobserved in the oldest Tsumeb slag (slag I) probably correspond tocomplex Ca–Pb arsenates. An example of EPMA analysis of such aphase is (Pb1.273Ca1.056Cu0.382Cd0.171Zn0.112K0.050Fe0.016Na0.010)3.070

[(As1.641P0.311Si0.029)1.981O4]�H2O, corresponding to the ideal for-mula close to (Pb,Ca,Fe,Cu,Zn,Cd)3(AsO4)2�H2O. Such phases wouldprobably be chemically similar to tsumcorite [FePbZn(AsO4)2�H2O]and other arsenates, generally found in the oxidation zones of theTsumeb ore deposit, and certainly indicate alteration processes inthe slags (see further section on weathering products).

3.3. Natural weathering products

In addition to alteration phases observed in polished sections ofslag I (sample N6) and described above (matrix arsenates and hy-drated Cu oxides), the investigation of slag surfaces under a binoc-ular microscope showed the development of secondary phases ofdark green to yellow green, blue, red and white colours. The pres-

Fig. 4. Scanning electron micrographs of secondary alteration products developedmicroanalysis and XRD results.

ence of the majority of SEM-observed phases was also confirmedby XRD as indicated in Fig. 4. In some places, slag I was coveredby green-coloured phases corresponding to Cu and Cu–Pb arse-nates (lammerite Cu3(AsO4)2 and bayldonite Cu3Pb(AsO4)2(OH)2;Fig. 4a and b). Other Cu-bearing secondary alteration productswere observed for the most of the Cu-rich slags of type II (N7,see also bulk chemical composition in Table 1): green olivenite(Cu2AsO4OH) aggregates covering the altered Cu-rich parts of theslags (probably droplets composed of Cu sulphides and/or Cu-arse-nides) (Fig. 4c) or a blue continuous crust formed by lavendulan(NaCaCu5(AsO4)4Cl�5H2O) associated with white crystals of gyp-sum (CaSO4�2H2O). Other slags from group II contain significantlyless Cu and As and their alteration products appear mainly onPb-rich parts of slags forming platy white crystals of hydrocerus-site (Pb3(CO3)2(OH)) (Fig. 4e). In addition, the presence of litharge(PbO) associated with minor ZnO and Cu-arsenates was also de-tected in these samples (Fig. 4f and g). The presence of secondaryPbO was previously confirmed by EPMA in a polished section frommatte-rich parts of slag N7 (Fig. 3d). With the exception of rarerims of anglesite (PbSO4, micrograph not shown) developed onPb droplets in slags III, no other alteration products were detected,probably reflecting the fact that these slags were exposed toweathering for a short period of time.

on slag surfaces with corresponding EDS spectra and interpretations based on

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 11

3.4. Leaching behaviour

3.4.1. Leachability and speciation of contaminantsThe results of leaching of selected metals (Cd,Cu,Pb,Zn) and

metalloids (As, Sb) from two slag samples using both leaching testsand comparison with regulatory levels are reported in Fig. 5. Otherpossible metallic contaminants were present in low concentra-tions; for example Cr, although found in some slags in relativelyhigh concentrations (Table 1), yielded concentrations in leachatesclose to the detection limit of the analytical method used(0.07 lg/L). This fact is probably related to the tight binding of Crin less soluble phases such as spinels. In general, the concentra-tions of metals in leachates were higher for the TCLP leaching testthan for the EN 12457 tests due to the lower equilibrium pH of thesolution (5.71 for N2 and 4.93 for N6) and probably also the effectof enhanced extraction by acetate. A higher equilibrium pH wasobserved for the EN 12457 leachates (6.78 for N2 and 6.54 forN6). The concentration levels of As and Sb were similar for bothtests, probably due to the lower efficiency of the TCLP methodcompared to the EN 12457 protocol in extracting oxyanions fromwastes reported in the literature (e.g., Ghosh et al., 2004).

Comparison of the EN 12457 leaching results with the EU regu-latory levels shows that the concentrations of the metals are gen-erally below the limits imposed for both non-hazardous and

EN 12457

Cd Cu Pb Zn As Sb

conc

entra

tion

(mg/

L)

TCLP

element

conc

entra

tion

(mg/

L)

N2N6

Cd Cu Pb Zn As Sb0.01

0.1

1

10

100

1000

1 mg/L

5 mg/L 5 mg/L

nd

nd

nd

0.1 mg/L

5 mg/L10 mg/L

hazardous wastenon-hazardous waste

0.001

0.01

0.1

1

10

100

0.5 mg/L

5 mg/L

1 mg/L

20 mg/L

5 mg/L

0.2 mg/L

2.5 mg/L

0.07 mg/L

0.5 mg/L

a

b

Fig. 5. Results of experimental leaching procedures (EN 12457 and TCLP) andcomparison with regulatory concentration levels (EU regulatory levels for non-hazardous and hazardous waste materials are taken from Van Gerven et al. (2005)and TCLP maximum concentration levels are defined by USEPA (2005)). Sample N2corresponds to recent slag and sample N6 corresponds to �100-a-old slag.

hazardous wastes (Fig. 5a). However, As exceeds the limit fornon-hazardous waste (0.2 mg/L) for both slags and, in the case ofsample N2, even the limit for hazardous waste (2.5 mg/L). For slagN2, Sb leaching was higher than the limit for non-hazardous waste(0.07 mg/L) (Fig. 5a).

The TCLP method showed that concentrations of some contam-inants, in particular for slag N6, exceeded the regulatory levels gi-ven by USEPA for Cd, Pb and As (1 mg/L, 5 mg/L and 5 mg/L,respectively). Unfortunately, these maximum levels are not de-fined for Cu, Zn and Sb. As an alternative method for evaluationof contaminant leaching from slags, Piatak et al. (2004) comparedthe concentrations in the obtained leachates with the levels foracute and chronic toxicity given by USEPA in the National Recom-mended Water Quality Criteria (2006). If the same criteria are ap-plied to the present leachates, the concentrations in the TCLPleachates (and also in some EN 12457 leachates) are significantlyhigher than the limit values in the acute toxicity guidelines foraquatic habitats defined by this USEPA regulation (in lg/L: As340, Cd 2, Cu 13, Pb 65, Zn 120, Sb not determined).

PHREEQC-2 speciation modelling indicates that the metal speci-ation varies according to the pH and concentrations of other ligands(acetate, sulphate, carbonate). The prevailing species were the freeionic forms (Cd: 77–98%; Cu: 40–99%, Pb: 47–96%, Zn: of total speci-ation). Sulphate complexes accounted for up to 22 (Cd), 15 (Cu), 29(Pb) and 20 (Zn)% of the total speciation and were important onlyin the EN 12457 leachates, i.e. at higher pH. Carbonate complexescorresponding to the sum of the MeCO0

3 and MeHCOþ3 species wereimportant only in Cu and Pb speciation with up to 35% and 44% ofthe total speciation. A very low fraction of acetate complexes wasobserved (only 0.1–1.9% of the total speciation, with Me-acetate+

as the main species). The PHREEQC-2 predictions indicate that theprevailing As species are H2AsO�4 or HAsO2�

4 according to the pH va-lue of the leachate and Sb is mostly present in its oxidized form (SbV)as a the SbO�3 complex (i.e., SbðOHÞ�6 ).

Waters sampled in situ in the smelter complex during the rainyseason (Table 6) are slightly alkaline (pH � 8) and exhibit particu-

Table 6Chemical compositions of surface waters sampled in the Tsumeb smelter area duringthe rainy season (data from Walmsley Environmental Consultants, 2001).

Parameter Units Run-off from the smelterand slag deposits area

Water from evaporationdams in the smelter

pH Standardunits

8.0 8.2

ECa mS/m 75 507Alkalinityb mg/L 227 160As mg/L 6.0 16Al mg/L <0.1 <0.1Ca mg/L 91 469Cd mg/L 1.8 0.13Cl� mg/L 16 634Co mg/L <0.025 <0.025Cr mg/L <0.025 0.048Cu mg/L 0.60 0.14F� mg/L 1.5 2.2Fe mg/L 0.21 0.13Hg mg/L <0.002 <0.002K mg/L 3.4 19Mg mg/L 33 205Mn mg/L 0.16 0.56Na mg/L 23 620Ni mg/L <0.025 <0.025NO�3 mg/L <0.2 <0.2Pb mg/L 1.7 0.18Se mg/L 0.010 0.022SO2�

4 mg/L 157 1930Zn mg/L 2.2 0.25

a EC – electrical conductivity.b Expressed as CaCO3.

12 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

larly high concentrations of As (6–16 mg/L). Other toxic elements(Cd, Cu, Pb, Zn) also exceed the acute toxicity guidelines for aquatichabitats defined by the USEPA National Recommended WaterQuality Criteria (USEPA, 2006).

3.4.2. Solubility controlsThe results of the PHREEQC-2 calculation of the saturation in-

dex for possible solubility-controlling phases are given in Table7. All the leachates and surface waters from the vicinity of the Tsu-meb smelter are strongly oversaturated with respect to numerousCu-, CuPb- or Pb-bearing arsenates, some of which are also ob-served as secondary alteration products on the slag surfaces: bay-ldonite (PbCu3(AsO4)2(OH)2), conichalcite (CaCuAsO4(OH)), duftite(PbCuAsO4(OH)) and mimetite (Pb5(AsO4)3Cl). Schultenite exhibitssaturation indices close to 0 (EN 12457 leachates) or >0 (TCLPleachates) in agreement with Magalhães and Silva (2003), indicat-ing its predominance at low pH values and in Cl-free environ-ments. Zinc-bearing arsenates (adamite, Zn2AsO4(OH); austinite,CaZnAsO4(OH); legrandite, Zn2AsO4(OH)�H2O) tend to be oversatu-rated only in leachates with high concentrations of Zn (TCLP, sam-ple N2; Fig. 5, Table 7). Similarly, only leachates from the TCLPleaching test are oversaturated with respect to the other Cu arse-nates (clinoclase, Cu3AsO4(OH)3; cornubite, Cu5(AsO4)2�2H2O;Cu3(AsO4)2�2H2O, euchroite, Cu2AsO4(OH)�3H2O), probably due tothe high concentrations of Cu and As. In contrast to the investiga-tion of secondary alteration products, the leachates are not over-saturated with respect to more common Pb-controlling phases,such hydrocerussite (Pb3(CO3)2(OH)), cerussite (PbCO3), litharge(PbO) or anglesite (PbSO4). Although the pH and bulk chemicalcomposition of leachates are favourable for precipitation of car-bonates, oxides and sulphates, to efficiently control the Pb concen-trations in solution, these phases would be formed only underconditions of longer-term slag/water interaction (Ettler et al.,2003a). Due to the higher pH, waters from the smelter area areoversaturated with respect to metallic carbonates, such as(hydro)cerussite (Table 7), typically found as newly formed

Table 7Saturation index of selected solubility-controlling phases as calculated by PHREEQC-2Oversaturation of the solutions with respect to the solids is indicated in bold.

Solution EN 12457leachateN2 (Slag III) N6 (Slag I)

pH 6.78 6.54Phase CompositionAdamite Zn2AsO4(OH) �4.92 �5.35Anglesite PbSO4 �2.54 �3.85Austinite CaZnAsO4(OH) �2.13 �3.51Bayldonite PbCu3(AsO4)2(OH)2 3.66 2.59Brochantite Cu4(OH)6SO4 �4.31 �5.02Cerussite PbCO3 �1.21 �1.24Clinoclase Cu3AsO4(OH)3 �1.04 �1.33Conichalcite CaCuAsO4(OH) 4.19 2.83Cornubite Cu5(AsO4)2(OH)4 �1.10 �1.77Cu3(AsO4)2�2H2O Cu3(AsO4)2�2H2O �3.96 �4.82Duftite PbCuAsO4(OH) 3.42 2.73Euchroite Cu2AsO4(OH)�3H2O �1.04 �1.42Ferrihydrite Fe(OH)3 1.45 1.24Gypsum CaSO4�2H2O �1.68 �3.67Hydrocerrusite Pb3(CO3)2(OH) �3.90 �4.15Legrandite Zn2AsO4(OH)�H2O �5.18 �5.61Litharge PbO �6.67 �6.88Mimetite Pb5(AsO4)3Cl 12.40 10.72Pb3(AsO4)2 Pb3(AsO4)2 �6.04 �7.81SbO2 SbO2 �1.62 �2.23Schultenite PbHAsO4 0.04 �0.74Tenorite CuO �0.83 �0.73Zincite ZnO �2.88 �5.74

Symbols used: – not determined.a Calculated from analyses reported in Table 6 (data from Walmsley Environmental C

products on slag surfaces (Fig. 4). Typical phases observed in slagsand acting as efficient sorbents (hydrous ferric oxides, HFO and hy-drous Al oxides, HAlO) also yield positive saturation indices,although the concentrations of Fe and Al were low in the leachates(<0.07 mg/L and <0.17 mg/L, respectively). However, their forma-tion would require prolonged release of Fe from the slag, as ob-served in other studies (Ettler et al., 2005). Especially in very oldslags (slag I), no HFO or HAlO were observed as secondary alter-ation products, because of the low Fe contents in the original slagmaterials. In slags II and III, Fe is mainly bound in insoluble spinelsand is not released as extensively.

4. Discussion

4.1. Phase formation in slags and solid speciation of metals andmetalloids

Mineralogical investigation of the Tsumeb slags indicates thattwo distinct groups of materials were produced in this district,according to the differences in the smelting technology, primaryore (or concentrate) compositions and secondary alteration pro-cesses. The studied slag samples do not contain clinopyroxene,which is an indicator of relatively slow cooling of the slag melt (Et-tler et al., 2000, 2001). Nevertheless, the cooling regime is differentfor these two distinct slag groups, indicating that the initial com-positions of the melt were probably also significantly different.

The oldest technology used at the beginning of the 20th centuryproduced a slag melt that cooled very slowly in ladles and washighly enriched in Pb, Cu and As (slag I). The crystallisation ofCa–Pb feldspars indicates that the slag melt was poor in alkalisand enriched in Al. In addition, feldspar formation completely re-moved silica from the melt and prevented the formation of residualglass. Further solidification of the slag melt continued through theformation of Cu-rich oxides (spinel and mcconnellite-familyphases). Their presence shows that the redox conditions in the slagmelt were locally variable, probably due to incomplete mixing of

for the leachates and the surface waters sampled in the Tsumeb smelter area.

TCLPleachate

Surfacerun-off a

Evaporationdama

N2 (Slag III) N6 (Slag I) (Smelter and slag area) (Smelter area)

5.71 4.93 8.0 8.2

1.22 �3.49 3.07 1.17�0.98 �0.64 �2.10 �2.27

0.64 �3.91 3.91 3.618.91 9.31 9.39 7.510.33 �1.44 1.69 1.18

– – 1.47 0.412.73 2.14 3.51 2.525.38 3.16 7.42 7.685.65 5.01 5.83 4.071.23 1.65 �1.39 �2.695.68 6.14 6.78 5.661.95 1.89 1.34 0.570.38 0.36 3.61 3.45�1.20 �3.54 �1.38 �0.17– – 4.65 1.95

0.96 �3.75 2.81 0.91�6.61 �6.62 �3.50 �4.0819.56 23.33 22.55 20.07�3.02 �1.06 �0.51 �2.87�1.41 0.08 – –

1.52 2.51 1.22 0.33�0.04 �0.57 1.35 1.13�5.24 �5.17 �0.25 �1.05

onsultants, 2001).

V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 13

first-class cokes used for firing in the furnaces and ore concentrate(Cu1+

M Cu2+, Fe2+M Fe3+ redox pairs). Magnetite, generally

formed by the reaction with O2-bearing shaft gases, is lacking inthese old slags, also indicating quite reducing conditions (Biswasand Davenport, 1976). The Fe3+ present in the melt is then en-trapped in the delafossite–mcconnellite phases and other Cu-bear-ing oxides. The matrix of the slag is composed of Ca–Pb arsenates,which are often enriched in Cu, Zn and other metallic elements. Itdoes not seem to be possible that these phases would be the laststep in the solidification of the slag melt, because of the reducingconditions in the shaft furnace. They probably correspond to alter-ation products of arsenides or arsenites and result from weatheringof the slag over many decades. There is also some analytical evi-dence that Cu-rich oxides can also be partly altered and form hy-drated phases (indicated by low totals of EPMA; Table 5).

The mineralogical compositions of �40-a-old and more recentslags (slags II and III) exhibit a typical crystallisation sequence (seee.g., Ettler et al., 2001; Lottermoser, 2002; Puziewicz et al., 2007)and indicate the predominance of silicate glass. The spinel-familyoxides with compositions corresponding to gahnite–ferrochrom-ite–magnetite solid solutions are the first phases crystallizing fromthe slag melt. In contrast to the�100-a-old technology, the slag meltwas enriched in alkalis and Ca (lime was probably used as a fluxagent) and the Ca–Fe alumosilicates, such as olivine and melilite,formed as the major slag constituents. Furthermore, the melt was,in general, poorer in Cu (as indicated by the formation of residualCu-sulphide droplets embedded in the silicate phases and the ab-sence of Cu oxides) and enriched in Zn. Zinc is significantly concen-trated in all the oxides, silicates and silicate glass occurring in theseslags. The initial concentration of Pb in the melts was probably alsolower than for melts produced at the beginning of the 20th century;it is concentrated only in the residual glass (solidifying at the end ofthe crystallisation sequence) and in small galena droplets.

4.2. Environmental implications

The presence of secondary phases indicates that slags from Tsu-meb, especially those exposed to weathering on dumps for severaldecades, may be important sources of potentially toxic elements(metals and metalloids) that can be released into the environment.The Ca–Pb arsenates forming the matrix of the oldest slags revealthat the material underwent significant alteration processes(Fig. 2a). The spinel phases observed in polymetallic slags fromother smelting sites are generally considered to be the mostweathering-resistant phases (Ettler et al., 2002, 2003a; Seignezet al., 2007). In contrast, the Cu-bearing spinels and delafossite-likephases from the oldest Tsumeb slags are often highly weathered,forming various Cu–Fe or Cu–Cr hydrated oxide compounds (Table5). This alteration may be partly responsible for the small release ofCu into the environment during the long-term weathering of slagsin the dumps, although the observed Cu concentrations in leach-ates and waters from the smelter area were relatively low (Fig. 5,Table 6).

The investigation of secondary weathering products revealed apredominance of complex Cu- and Pb-arsenates locally developedon the slag surfaces. The large variety of these species indicates lo-cal differences in the chemical microenvironments on the weath-ered slag surfaces. Whereas lammerite is generally formed underhighly acidic conditions (pH <3), olivenite and bayldonite precipi-tate under slightly acidic to circum-neutral conditions (up to pH6 or 7) (Magalhães et al., 1988; Inegbenebor et al., 1989; Magalhãesand Silva, 2003). These complex Cu- and Pb-bearing arsenates werealso predicted by PHREEQC-2 calculations for the slag leachates tobe the main solubility-controlling phases for Cu, Pb and As. TheSEM and XRD study of weathered slag surfaces revealed that, inaddition to bayldonite, Pb is controlled by the precipitation of

hydrocerussite and litharge. Oxides and carbonates were com-monly observed as important secondary alteration products inslags and mattes (Ettler et al., 2003a,b; Seignez et al., 2007) andcan precipitate from solution at pH >5. PHREEQC-2 predicts thatmimetite (Pb5(AsO4)3Cl) and schultenite (PbHAsO4) precipitatefrom the obtained leachates (Table 7). Although it is suggested thatthey are important solubility-controlling phases in Pb- and As-richsystems (Magalhães and Silva, 2003), the high activity of CO2 orig-inating from the atmosphere probably causes the preferred precip-itation of Pb carbonates and the absence of mimetite andschultenite at the slag surfaces. Carbonates commonly form inthe vicinity of Pb-rich inclusions composed of galena and/or metal-lic Pb and efficiently immobilize Pb (Ettler et al., 2003a; Seignezet al., 2007). However, the short-term leachates obtained usingEN 12457 and TCLP normalized leaching tests were not oversatu-rated with respect to these Pb-bearing phases. Longer-term slag/water interaction resulting in higher Pb concentrations in theleachate would be necessary to precipitate these phases (Ettleret al., 2003a).

It has been suggested that secondary HFO and HAlO are themost important phases controlling the release and mobility ofpotentially toxic elements from slags through sorption and/orcoprecipitation (Parsons et al., 2001; Ettler et al., 2003a, 2005; Pia-tak et al., 2004). Although precipitation of these phases was pre-dicted by PHREEQC-2, they were not observed by SEM, as thesecondary alteration products developed directly on the slag sur-faces. According to Ettler et al. (2005), the key factors influencingthe precipitation of HFO are (i) the effective dissolution of primaryFe sulphides (pyrrhotite, Fe-bearing sphalerite/wurtzite) and, to alesser extent, of primary silicates, oxides and glass accompaniedby a release of Fe2+ into the solution and (ii) the time necessaryfor oxidation of dissolved Fe2+. However, relatively few Fe-bearingsulphides were observed in the slags from Tsumeb, preventing anysignificant release of Fe during the weathering. With the exceptionof Cu–Fe oxides, the spinels, which are the most important Fe car-riers, are not significantly weathered. The Fe concentrations in theleachates obtained by EN 12457 and TCLP methods were also verylow, ranging from 0.012 to 0.07 mg/L, thus preventing massive pre-cipitation of HFO. Furthermore, recent experimental investigationof Pb slag alteration by Seignez et al. (2007) did not revealed anyHFO and suggested that carbonates only are the key Pb-controllingphases.

The leaching results indicated that only As may be considered tobe a serious problem exceeding the EU regulatory limits for haz-ardous waste. According to the TCLP test, the regulatory levelswere exceeded for Cd, Pb and As for the oldest slag sample (N6).In comparison with the TCLP test, the EN 12457 leaching testseems to be more appropriate for quick evaluation of the hazard-ous properties of the slags, because no organic complexing agents(such as acetate) can be expected in the environment of slagdumps and tailing ponds. The leaching results are, however,strongly influenced by the presence of fine dust fractions (Zandiet al., 2007) and the release of contaminants from slags will bemuch slower in reality. In particular, metal-bearing sulphides can-not be dissolved in the short-term (24-h) regulatory leaching testsused in this study. For this purpose, long-term leaching with thecoarse-grained slag fraction (e.g., 2–5 mm, as performed by Ettleret al., 2003a) could be useful for estimation of more realistic con-taminant release from the slag dumps.

Seasonal variations between the dry and wet periods can lead tothe dissolution of primary slag phases and formation of secondaryefflorescence minerals, which are highly soluble, as documented atother smelting sites (Lottermoser, 2005). As the secondary weath-ering products of the Tsumeb slags are composed of soluble arse-nates (see the high values of the solubility products in Magalhãeset al., 1988), some rain events during the rainy season in Namibia

14 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15

can flush out a significant amount of As from the slag dumps. InTsumeb, the annual average rainfall is 470 mm and flash floodsoccasionally occur from October to March with up to 50 mm rain-fall. This phenomenon is also confirmed by high concentrations ofAs in run-off from the smelter area and slag dumps and in waterfrom evaporation dams in the Tsumeb smelter, also indicating thatarsenate phases can be more soluble under slightly alkaline condi-tions (pH � 8; Table 6). To better understand the cycling of toxicelements in the vicinity of the dumps during the rainy season, itwould be necessary to perform in situ long-term monitoring stud-ies in the Tsumeb area using the groundwater and surface watersampling methodologies reported from other smelting sites (Lot-termoser, 2002; Parsons et al., 2001; Navarro et al., 2008). It isimportant to point out that finely ground, reprocessed slags depos-ited in tailing ponds – if exposed to weathering – can be expectedto release large amounts of potentially toxic elements due to theirhigher reactive surface and can be considered to be a serious prob-lem for the environment in the Tsumeb area.

5. Conclusions

The smelting slags from the Tsumeb area resulting from histor-ically different technologies are chemically and mineralogicallycomplex waste materials. Up to 100-a-old slags are composed ofanorthite and Pb-feldspars, Cu-spinels and other Cu–Cr–Fe oxidesand matrix Ca–Pb arsenates. More recent granulated slags aremostly composed of glass, high-temperature Ca–Fe alumosilicates,spinel-family oxides and sulphide/metallic inclusions. Numerousnatural weathering features were observed at the slag surface,indicating that these materials underwent alteration during theirlong-term disposal on the dumps. The secondary phases identifiedby SEM and XRD were mainly complex Cu–Pb–(Ca) arsenates.According to batch leaching experiments coupled with speciationmodelling, these phases were also suggested to be the main solu-bility controls on the mobility of metals and metalloids in slagleachates. The presence of these highly soluble phases might beresponsible for the significant release of related potentially toxicelements (As, Cu, Pb) from slag dumps, through their rapid dissolu-tion during thunderstorm events occurring in the Tsumeb area be-tween October and March.

Acknowledgements

This study was supported by Project of the Development Coop-eration of the Czech Republic No. RP/20/2004 (‘‘Assessment of min-ing and processing of ores on the environment in the miningdistricts of Namibia”) and was partly financed by a Czech ScienceFoundation project (GACR 205/06/0298). The institutional fundingwas provided by Ministry of Education, Youth and Sports of theCzech Republic (MSM 0021620855). Dr. Ladislav Strnad (CharlesUniversity) assisted with the ICP-MS measurements of trace ele-ments in digests, Radek Procházka (Charles University), AnnickGenty (Université d’Orléans) with SEM/EDS measurement andAnna Langrová (Geological Institute, Czech Academy of Science)with EPMA measurements. Dr. Eva Gregorová (Institute of Chemi-cal Technology, Czech Republic) is thanked for the LALLS granulo-metric analyses. The thorough reviews of Dr. Nadine M. Piatak (USGeological Survey) and two anonymous referees helped to improvethe final version of the manuscript.

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