Investigation on the cyanide leaching optimization for the treatment of KCGM gold flotation concentrate—phase 1

Abstract

Kalgoorlie Consolidated Gold Mines (KCGM), the largest gold producer in Australia, has spent in the last 4 years considerable efforts to develop a parallel processing alternative to roasting primarily because of the increasing sulphide content of the ore in the Superpit and the long term tendency for more stringent environmental regulations. Ultra-fine grinding (UFG) was the technology selected to process part of the increased concentrate tonnage. A laboratory investigation was then initiated, to improve the leaching conditions of the flotation concentrate with the objective of maximizing gold recovery while minimizing processing costs. The concentrate used for the laboratory investigation had the following composition: 41.4 g/t Au, 59.7% pyrite, 0.3% chalcopyrite, 38.2% gangue, and a P 80 of 114 μm. The gold in the concentrate is refractory and contains free gold, gold tellurides and gold associated with pyrite. Deslime concentrate and the ultra-fine grind fraction of the concentrate were also used in the study. A high concentration of lime in conjunction with lead nitrate proved more effective to dissolve gold tellurides compared to pH 11 used by the plant. The new leaching conditions also reduced significantly cyanide consumption from 12–15 kg/t to 3.5–4.0 kg/t. At a P 80 of 21 μm, the cyanidation with 4.5 g/L NaCN, 25 ppm DO, 1 g/L lime yielded solid residues with 8.6 g/t Au. Addition of lead nitrate to the leach produced a leach residue at 7.8 g/t Au. Using a finer grind, a P 80 of 11 μm, an additional gold extraction of 1.6 g/t was obtained, which was similar to the gold extraction of the UFG processed on site (leach residue of Gidji). The reduction of cyanide consumed was attributed to passivation of pyrite and chalcopyrite. Scanning Electron Microscopy (SEM) examination of the leach residue indicated that the majority of remaining gold was primarily encapsulated within pyrite; indicating that the refractoriness of the tellurides has been effectively mitigated by the approach. The high oxygen demand of the conditioning stage was reduced by adding lead nitrate. The leaching of the sulphide concentrate at a coarser grind is not an economical option. Lead nitrate addition showed a more beneficial effect on the leaching of UFG than leaching of the bulk concentrate. A plant trial on the effect of high lime concentration and lead nitrate addition, as well as a pilot plant trial, are scheduled to test the laboratory results.