STUDIES ON THE RECOVERY OF SILVER FROM LEAD-ZINC FLOTATION TAILINGS

摘要

A lead-zinc flotation tailings sample containing silver from BHP Billiton's Cannington Mine was characterised to understand occurrences of silver in the tailings before hydrometallurgical silver recovery testing was commenced. Conventional elemental, X-ray diffraction (XRD) and size analyses were followed by mineralogicai analysis with a scanning electron microscope (SEM) equipped with mineral liberation analysis (MLA) software and the use of an adapted diagnostic leach process originally developed for gold containing materials. Cyanide, thiourea, thiosulphate and ferric chloride leach systems were investigated as options to recover silver from the tailings.The tailings sample contained 67 g/t Ag, 1.8% Pb and 0.52% Zn. The XRD analysis showed that the sample consisted of 74% silicates, 9% sulphides, 11% magnetite and small amounts of other minerals such as fluorite, fluorapatite and rutile. The silver minerals freibergite, acanthite, jamesonite. pyrargyrite and native silver were observed in the mineralogicai analysis of the sample by SEM/MLA. The associations of the minerals were intricate and the occurrences of silver were complex. There were finely disseminated silver minerals and also coarser grains of silver minerals in composites with the host minerals. It was estimated that almost 50% of the silver was contained in pyrargyrite. around 14% as native silver and contained in acanthite, 12.5% in galena and around 10% in freibergite and jamesonite. The silver leach recoveries under optimum test conditions were 50%, 95%, 82% and 60% for cyanide, thiourea, ferric chloride and thiosulphate systems, respectively. The composite particles and the type of the silver minerals present in the tailings limited leaching efficiencies of the silver. The silver recoveries increased with the use of more aggressive acidic thiourea and ferric chloride systems. Major increase in silver recoveries for cyanide and thiosulphate leach systems were obtained only by size reduction to very fine product sizes by wet grinding in a bead mill.