Leaching of polished sections : an integrated approach for studying the liberation of heavy metals from lead-zinc metallurgical slags

Abstract

Abstract Pb-Zn metallurgical slags are defined by European Community regulations either as waste (hazardous materials) or as secondary commercial substances. The knowledge of their stability is essential in order to assess strategies for their management. A scientific understanding of metal release requires (i) knowledge of the phases hosting the different hazardous metals (e.g., Pb) and (ii) knowledge of the relative solubility of these phases. This paper is devoted to a mineralogical characterisation of slags resulting from different processing technologies and to long-term static leaching experiments run under different pH-conditions. The main mineral phases of slags are spinel-family oxides, Ca-Fe aluminosilicates (clinopyroxene, olivine-type phases, melilite, garnet), silicate glass, sulphide (galena, wurtzite) and intermetallic droplets. Leaching experiments were conducted on polished sections, which were introduced into batch reactors and leached by the following solutions: (i) citric acid-sodium citrate buffer at pH ~ 3 (organic soil-simulating solution), (ii) deionised water without any pH control and (iii) calcium hydroxide saturated solution buffered at pH ~ 12.5 (concrete-simulating solution). Sulphide/metallic phases display an extremely low stability whatever the solution. Under organic-acid conditions, Ca-rich (melilite) and Fe-rich (fayalite) phases are preferentially dissolved. Spinel-family oxides are always extremely stable. SEM observations evidence the important role of citrate, which favours the extraction through chelation of metallic elements on the slag surface. The silicate glass and sulphide/metallic droplets are the most unstable phases under intermediate conditions (deionised water). However, under such conditions, the mobility of Pb, Zn and As is drastically limited by precipitation/adsorption processes, as shown by geochemical modelling (PHREEQC, EQ3NR). A preferential dissolution of glass, clinopyroxene and garnet occurs in alkaline environments. Additionally, lead is significantly released by the dissolution of galena and remains dissolved. On the contrary, zinc content is likely controlled by precipitation of secondary zincite (ZnO) or zinc hydroxide, as predicted by the EQ3NR calculations. The results of static leaching experiments under various conditions confirm that the disposal of slag in organic-rich environments (e.g., peat soil) or the use of slag in concrete technology should be avoided due to the high mobilisation of Pb and other toxic elements.