The Fosterville Goldmine in Victoria employs bacterial oxidation (BIOX Process) to liberate gold from its flotation concentrate. Gold occurrence is primarily as submicron particles in pyrite and arsenopyrite and has been described as being in solid solution' and therefore not amenable to direct cyanidation even with fine grinding. Once liberated by the oxidation process, the gold is then available for cyanide leaching._x000D_
While the oxidation process is very effective, it has been found that a portion of the liberated gold can become suspended in the liquid phase in what appears to be a colloidal state. This liquid becomes a waste stream during washing of the oxidised solids and the suspended gold is therefore lost._x000D_
This paper discusses investigations into the causes of the loss and also describes the successful use of quebracho tannin in reducing colloidal gold loss. Laboratory and subsequent plant trials, showed a gain of 2.6 per cent in overall gold recovery based on mill feed and waste stream data.