The behaviour of carbonaceous matter in cyanide leaching of gold

Carbonaceous matter was extracted from ore obtained from Stawell Gold Mine and characterised by microscopic methods and XRD. The carbonaceous matter was predominantly hexagonal crystalline graphite mixed with some minor semi-crystalline graphite or amorphous carbon. It was found that the natural carbonaceous matter had a low gold adsorption capability, dependent on cyanide concentration. No adsorption occurred with > 250 mg/L KCN. Auger studies demonstrated that during grinding, the carbonaceous matter preferentially smeared on iron sulphide rather than alumino-silicate surfaces present in the gold ore. The carbonaceous matter coated unevenly, preferentially at the edges and the defect sites. XPS analysis indicated that when gold foil was ground with the ore, the carbonaceous matter coated on gold surfaces mainly in the form of elemental carbon or graphite. Very limited amounts of organic carbon compounds smeared on the gold foils during grinding. Sulphur species were also present on the gold surfaces after grinding with sulphide ores. rrent work shows that the carbonaceous coating had a significant detrimental effect on gold dissolution when gold was pre-ground with the carbonaceous ore. Sulphide coating on gold surfaces appeared to hinder gold dissolution to a lesser extent. The dissolution of gold foils artificially coated with the natural carbonaceous matter extracted from Stawell ore was also substantially retarded. In comparison, graphite had a much lower coating ability on gold. The addition of free carbonaceous matter to pre-ground non-carbonaceous sulphide gold ores did not have much effect on gold cyanidation. However, the carbonaceous matter ground with the non-carbonaceous sulphide ores significantly reduced gold leaching kinetics and recoveries.