Three main objectives have underpinned this investigation. The first being to characterise a refractory gold calcine CIL tailing obtained from Kalgoorlie Consolidated Gold Mines’ Gidji Roaster plant, which is partly owned by Newmont Australia Limited. The second objective is to assess the feasibility of using aqueous solutions of SO2 to reductively dissolve iron oxide mineral phases in the calcine CIL tailings though to be responsible for its refractory behaviour. The final objective is to assess the efficiency of thiocyanate (SCN-), a gold lixiviant stable under acidic conditions, to extract gold exposed/liberated by the SO2 leaching.
Sample characterisation displayed that iron oxides/hydroxides were the most abundant mineral phases (>53 w%t) in the calcine. Gold concentrations in the calcine were within the order of 2.2ppm to 2.61ppm. Gold was not however detected via MLA at a 5μm detection limit. Based on these studies, minerals such as hematite were thought primarily responsible for the refractory nature of the calcine, either via surface passivation or occlusion of gold within hematite crystal lattices.
By injecting SO2 into deionised water, the gas is initially hydrated and then undergoes deprotonation to form species including the bisulfite ion (HSO3-). This anion can to bind to Fe3+ of iron bearing mineral phases and reenter solution as iron bisulfite (FeHSO3+) complex. Fe2+ introduced into solution by this process is stable cation under the acidic and reducing conditions created by SO2 sparging. Having removed iron bearing phases responsible for calcine refractoriness, gold exposure and extraction by to SCN-, was expected to proceed with increased effectiveness. Although displayed to occur, the efficiency of these processes was limited.
Iron extraction via SO2 leaching was largely proportional to treatment duration, but only produced maximum iron extractions of 6.34% after 24 hours of leaching. In the attempt to enhance this process, additions of Na2S2O5 and Cu2+ ions were made to SO2 leaching systems. Na2S2O5 had adverse impacts upon iron extraction compared to leaching in its absence, but did stabilise solution Eh and pH. Cu2+ is a known catalyst for the reduction ferric iron, increasing iron extractions to 10.07% in the presence of 16g/L of Cu2+. Based upon increases in iron extraction created by Cu2+, this reagent was added to all SO2 leaching procedures used to prepare calcine for SCN- treatment.
Leaching of gold with variable concentrations of SCN- was performed in oxidised SO2 leaching solutions containing SO2 treated calcine. Oxidation was achieved via ferric sulfate addition, which also provided a suitable oxidant (Fe3+) for gold dissolution, but decreased solution pH. This system extracted 17.89% of the gold contained in SO2 treated materials, upon leaching with 0.5M SCN- solutions. Conversely extraction of gold occurred during SO2 leaching procedures and from calcine only treated with SCN- and not SO2.
A number of occurrences were identified as possibly contributing to limited gold recoveries with SCN-. These include reduced SCN- availability due to protonation, SCN- removal from solution via precipitation due to reaction with Cu2+ and also reduced availability of Fe3+ due to excessive complexation with sulfur bearing species created by SO2 sparging. However, the limited efficiency of SO2 solutions to extract iron from the sample and therefore reduce refractoriness, is thought to be the main cause of limited gold recovery. Hence, SO2 treatment followed by thiocyanate leaching of calcine CIL tailings to extract refractory gold has not proven to be a particularly efficient process.