Extraction of platinum, palladium and gold by cyanidation of Coronation Hill ore

Abstract

The effects of cyanidation conditions (pH, time, particle size of the ore, additives to the leach, and oxidative pre-treatments) at ambient temperatures on the extraction of platinum, palladium and gold from samples from the Coronation Hill deposit, Australia, were studied. Three ore lithologies were investigated: a diorite, a green tuffaceous siltstone, and a quartz-feldspar porphyry. Coarse and fine native gold, native platinum and palladium, stibiopalladinite and platinum-palladium selenides were identified in the samples. Coarse gold, but no platinum or palladium, was recovered by amalgamation. Cyanidation at pH 11.5 for 48 h of ore ground to 80% − 74 μm (standard cyanidation conditions) extracted 6–18% of the platinum, 20–50% of the palladium (the values depending on the ore type), and over 95% of the gold. Lowering the cyanidation pH to 9.5 resulted in extraction of up to 85% of the palladium, although the platinum extractions were usually less than 20%. Final grinding increased the palladium extractions but had little effect on platinum extractions, and decreased gold extractions by a few percent. The high gold extractions were obtained by 24 h cyanidation, but longer times were required for extraction of platinum and palladium. Addition of thallium salts at pH 9.5 increased the platinum extraction values to 30–35%. Addition of sodium chloride, sodium sulphite, sodium thiosulphate, lead salts and hydrogen peroxide had little effect on the extraction of the precious metals. Pre-treatment with sodium chloride/hydrogen peroxide solution at pH 1, and filtration before the subsequent cyanidation at pH 9.5, increased the platinum extraction from 20% to 37%. However, the palladium extraction was reduced to 25%.