Mineralogy and Environmental Stability of Slags from the Tsumeb Smelter, Namibia
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Applied Geochemistry 24 (2009) 1–15 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem Mineralogy and environmental stability of slags from the Tsumeb smelter, Namibia Vojteˇch Ettler a,*, Zdenek Johan b, Bohdan Krˇíbek c, Ondrˇej Šebek d, Martin Mihaljevicˇ a a Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic b Bureau des Recherches Géologiques et Minières (BRGM), av. Claude Guillemin, 45060 Orléans, cedex 2, France c Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic d Laboratories of the Geological Institutes, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic article info abstract Article history: Three types of smelting slags originating from historically different smelting technologies in the Tsumeb Received 27 June 2008 area (Namibia) were studied: (i) slags from processing of carbonate/oxide ore in a Cu–Pb smelter (1907– Accepted 22 October 2008 1948), (ii) slags from Cu and Pb smelting of sulphide ores (1963–1970) and (iii) granulated Cu smelting Available online 30 October 2008 slags (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 electron Editorial handling by R. Fuge microprobe (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. The presence of arsenates indicates that these slags underwent long-term alteration. More recent slags are composed 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 to study secondary alteration products developed on the surface of slags exposed for decades to weathering on 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 standardized short-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 test were exceeded for Cd, Pb and As. The PHREEQC-2 calculation predicted that complex arsenates are the most 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 the environment in the vicinity of slag dumps. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction investigations of slags are essential for understanding the position of potentially toxic elements in primary solid phases and represent Slags are the most important mineral wastes resulting from the first step in assessing their environmental impact (Ettler et al., pyrometallurgy. They correspond to the silicate melt produced 2000, 2001; Parsons et al., 2001; Lottermoser, 2002; Puziewicz during the pyrometallurgical recovery of base metals by fusion in et al., 2007). Furthermore, the textures and the chemical composi- a blast furnace and are produced in large amounts. Slags are gener- tion of primary phases in slags may also be used to estimate the ally deposited on dumps or, if considered to be unreactive materi- conditions of their formation, especially in relation to advances als, used for civil-engineering purposes such as road construction in smelting technologies (Ettler et al., 2000, in press; Manasse (Ettler et al., 2002, 2003a). However, these waste materials are of- et al., 2001; Manasse and Mellini, 2002a; Sáez et al., 2003; Haupt- ten enriched in toxic elements, in particular metals (Cu,Pb,Zn) and mann, 2007). in metalloids (As,Sb) that can be released into the environment Some recent monitoring studies have shown that the extensive through alteration processes and leaching (Parsons et al., 2001; Et- mining and ore processing activities in the Tsumeb district (Nami- tler, 2002; Piatak et al., 2004; Ettler et al., 2004, 2005; Lottermoser, bia) left important amounts of various mining and smelting waste 2005; Navarro et al., 2008; Costagliola et al., 2008). Mineralogical materials that can be considered as a serious problem in relation to environmental contamination (Ongopolo Mining and Processing * Corresponding author. Tel.: +420 221 951 493; fax: +420 221 951 496. Limited, 2001; Krˇíbek and Kamona, 2005). Approximately E-mail address: [email protected] (V. Ettler). 24,550,280 t of ore were mined out during the modern history of 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.10.003 2 V. Ettler et al. / Applied Geochemistry 24 (2009) 1–15 the Tsumeb Mine and it has been estimated that millions of tons of to produce granulated slag (Tsumeb Corporation Ltd., 1987; War- mining/processing wastes are stored in this area (200,000 t of tha and Genis, 1992). slag on dumps and at least 10 million t of material in tailing ponds) (Krˇíbek and Kamona, 2005). Because this has never been studied, 2.2. Slag sampling and processing this paper is focused on the Tsumeb smelter slags resulting from historically different smelting operations. Bulk chemical analyses Numerous slag samples were collected during the mission and phase/mineralogical analyses of slags were coupled with guided by the Czech Geological Survey in 2004 (Project of the experimental leaching and thermodynamic modelling, in order to Development Cooperation of the Czech Republic No. RP/20/2004). provide information on the solid speciation of pollutants in slags, Eleven slag samples were studied in this detailed investigation. weathering products and possible environmental impacts. They represent the available materials corresponding to the histor- ical evolution of smelting technologies in Tsumeb. The following 2. Materials and methods three groups of slags were collected (the detailed locations are gi- ven in Fig. 1): 2.1. Smelting site history (i) Slag type-I corresponds to historical slags produced from The Tsumeb deposit belongs to the northern Namibia sulphidic 1907 to 1948 in Pb–Cu blast furnaces processing mainly car- metallogenic province. The deposit lies in the upper part of the bonate and oxide ores and fired by coke from Germany and Otavi group, which consists of limestones and dolomites of Neo- South Africa. They are found as 20-cm large fragments of proterozoic age (Miller, 1983). The mineralization is characterised massive and heavy material of black and grey colour with by large-scale alteration (calcification and silicification of host a green crust of secondary phases (number of samples, rocks) and common hydrothermal carbonate veins. The deposit n = 1). contains a great diversity of ore minerals of Pb, Cu, Zn, Ag, As, Sb, (ii) Slag type-II corresponds to historical slags and mattes pro- Cd, Co, Ge, Ga, Au, Fe, Hg, Mo, Ni, Sn and W, as well as V, containing duced between 1963 and 1970 resulting from reverbatory about 11% Pb, 5% Cu and 4.3% Zn, with economic concentrations of furnaces (Cu smelting) and shaft furnaces (Pb smelting) fired Ag, Cd, Ge and As. The deposit was mined by a large open pit and with aerated pulverized coal. During this period, mainly sul- by several shafts. It was once the foremost producer of Pb in Africa phide ores were processed. The slags are found as up to 10- and, over its lifetime, has produced over 2 million t of Pb, some cm large fragments of black, vitreous material, often covered 500,000 t of Zn and over 1 million t of Cu (Frimmel et al., 1996; by white and blue secondary phases (n = 8). Chetty and Frimmel, 2000). (iii) Slag type-III corresponds to granulated slags from reverba- In 1907, two Pb–Cu blast furnaces were built in the Tsumeb tory furnaces produced by recent Ausmelt technology in area by the Otavi Minen- und Eisenbahn-Gesellschaft (OMEG) the Cu smelter from 1980 to 2000. In this case, the furnaces Company to smelt local ores. These furnaces were supplemented were fired with black oil and the furnace charge was pellet- by a third furnace in 1923; the smelter was then operated until ized with pulverized coal and fluxes (lime and quartz chert) the end of World War II, first processing the carbonate ore from prior to melting. The slags are found as granulated black the upper part of the Tsumeb ore body. After interruption of the material with fragments up to 3 cm in size or as milled pow- smelting activities in the 1950s, new smelters were constructed der (n = 2). in 1963, consisting of a Cu smelter with a reverbatory furnace and a Pb smelter with a shaft furnace. In this period, deeper parts 2.3. Bulk chemical analyses of the Tsumeb ore body composed of sulphide ores were processed (main minerals: chalcocite, enargite, galena, sphalerite). During the An aliquot of each sample (approximately 20 g) was crushed early 1980s, the Slag Mill was built to re-process old Cu reverba- and pulverized in an agate mortar using the Fritsch Pulverisette tory slags, which are milled and treated by flotation. The resulting apparatus and was used for bulk chemical analyses and phase concentrate is then passed to the smelters to recover Pb and Cu and identification using X-ray diffraction analysis.