An acidic pressure oxidation pre-treatment of refractory gold concentrates from the KweKwe roasting plant, Zimbabwe

Roasting is the only commercialised process in Zimbabwe for the pre-treatment of refractory gold feedstocks for the subsequent cyanidation leaching. This process results in only about 75% gold recovery and is characterised by high gold values in the residue. A pressure oxidation process in an autoclave has been evaluated as an alternative. The process resulted in substantially higher gold recoveries (>90%) over shorter periods. The results also show that a near quantitative silver recovery (62%) over even shorter periods of 12 minutes is possible. The dependence of these recoveries on temperature, retention time, pulp density, oxygen partial pressure, particle size and matrix of the sulphide host minerals is described. The effect of jarosite formation on the silver recoveries is also discussed.