Polyminerallic, sulphidic mine wastes were treated with KH2PO4-H2O2 solutions to determine whether the formation of solid phosphate coatings inhibits sulphide oxidation and metal and metalloid mobility. The waste rocks were metal (PbZnCu) and metalloid (AsSb) rich and consisted of major quartz, dickite, illite and sulphide minerals (e.g. galena, chalcopyrite, tetrahedrite, sphalerite, pyrite, arsenopyrite) as well as minor to trace amounts of pre- and post-mining oxidation products (e.g. oxides, hydroxides, arsenates and sulphates). Scanning electron microscopy observations of the waste material treated with KH2PO4-H2O2 solutions showed that metal, metal-alkali and alkali phosphate precipitates formed and coatings developed on all sulphides (with the exception of tetrahedrite). The abundance of phosphate phases was dependant on the availability of metal and alkali cations in solution. In turn, the release of cations was dependent on the amount of sulphide oxidation induced during the experiment or the presence of soluble oxidation products. Lead and to a lesser degree Cu and Zn phosphate coatings remained stable during H2O2 leaching, preventing acid generation and metal release. In contrast, the lack of phosphate coating on tetrahedrite and arsenopyrite allowed oxidation and leaching of As and Sb to proceed and mobilized As and Sb did not form phosphate phases. As a result, As and Sb displayed the greatest release from the coated waste. Thus, the application of KH2PO4-H2O2 solutions to partly oxidized, polyminerallic mine wastes suppresses sulphide oxidation and is most effective in inhibiting Pb (Cu and Zn) release. However, the technique appears ineffective in preventing metalloid (As, Sb) leaching from tetrahedrite- and arsenopyrite-bearing wastes.

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