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

Three types of smelting slags were studied from historically different smelting technologies, the processing of carbonate/oxide ore in a Cu-Pb smelter (1907-1948), Cu and Pb smelting of sulphide ores (1963-1970) and granulated Cu smelting slags (1980-2000). The slags are significantly enriched in metals and metalloids. 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 the slags underwent long-term alteration. More recent slags are composed of high-temperature phases including Ca-Fe aluminosilicates, Pb- and Zn-rich glass, spinel oxides and small sulphide/metallic inclusions embedded in glass. Secondary alteration products that developed on the surface of slags exposed to weathering included highly soluble complex Cu-Pb-(Ca) arsenates associated with litharge and hydrocerussite. Short-term batch leaching tests and speciation solubility modelling showed that As in the leachate exceeded the EU regulatory limit for hazardous waste materials and the toxicity limits defined by USEPA were exceeded for Cd, Pb and As. Complex arsenates are the most important solubility controls for metals and metalloids and these phases can readily dissolve during the rainy season and flush significant amounts of As, Pb and Cu into the environment in the vicinity of the slag dumps.
Three types of smelting slags were studied from historically different smelting technologies, the processing of carbonate/oxide ore in a Cu-Pb smelter (1907-1948), Cu and Pb smelting of sulphide ores (1963-1970) and granulated Cu smelting slags (1980-2000). The slags are significantly enriched in metals and metalloids. 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 the slags underwent long-term alteration. More recent slags are composed of high-temperature phases including Ca-Fe aluminosilicates, Pb- and Zn-rich glass, spinel oxides and small sulphide/metallic inclusions embedded in glass. Secondary alteration products that developed on the surface of slags exposed to weathering included highly soluble complex Cu-Pb-(Ca) arsenates associated with litharge and hydrocerussite. Short-term batch leaching tests and speciation solubility modelling showed that As in the leachate exceeded the EU regulatory limit for hazardous waste materials and the toxicity limits defined by USEPA were exceeded for Cd, Pb and As. Complex arsenates are the most important solubility controls for metals and metalloids and these phases can readily dissolve during the rainy season and flush significant amounts of As, Pb and Cu into the environment in the vicinity of the slag dumps.