Abstract:
Biooxidation is an attractive process for unlocking of gold from refractory ores, but could be further improved by reducing the rather high cyanide consumption during the cyanidation of the biooxidation residues. The high cyanide demand is typically ascribed to the presence of metastable intermediate sulfur species formed during the oxidation of the sulfide, but it is also conceivable that jarosite formation during the acidic biooxidation process and subsequent leaching during the alkaline cyanidation could also account for significant cyanide consumption. The present work was done to establish if jarosite would readily form under typical biooxidation conditions and, if so, if it would then leach during cyanidation. Methods to stabilise or remove the jarosite were also investigated. To this end crystalline potassium jarosite was synthesized simulating conditions for the mesophile BIOX? process and its decomposition in alkaline media at pH 10.5 to 11 at 25?C investigated. It was found that it transformed into another amorphous iron compound as indicated by a change in colour from yellow to reddish, the presence of potassium and sulfate ions in solution, as well as the disappearance of the characteristic XRD pattern of jarosite. The jarosite also leached in aqueous alkaline cyanide as indicated by the presence of iron in solution, with increased leaching at higher cyanide concentrations. The jarosite could not be totally passified by aging, but removing it by leaching with an iron-complexing agent like oxalic acid was found to be possible. The consumption of cyanide by jarosite during gold leaching could thus be significant, but would obviously depend on how much jarosite is formed during the bioleaching process.
Jarosite was observed to be present in the mesophile and thermophile BIOX? products from Fairview Mine with more jarosite present in the products of the thermophile process. The process jarosite was significantly more stable than that produced in the laboratory during alkaline cyanidation with 17 mass percent of the jarosite converted to ferric hydroxide for the process jarosite compared to complete conversion for the laboratory jarosite for a 24 hours leaching period. However, it still accounted for a significant amount of the cyanide consumed.
