Measuring potential in a pressure oxidation autoclave

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Pressure oxidation (POX) is used to treat the increasing number of gold ores that result in poor gold recovery when subjected to whole-ore direct cyanidation. Typically these "refractory" ores consist of sulphides, such as pyrite (FeS2) and arsenopyrite (FeAsS), which encapsulate the small gold particles and render them inaccessible to cyanide. The function of POX then is to break down the sulphide, thus liberating the gold for future cyanidation. POX is typically operated at high oxygen partial pressures (150 to 700 kPa) and high temperature (ca. 220°C) in a pressure vessel known as an autoclave. The conditions prevalent in the POX reactor are critical to the success of the downstream processing and ultimate gold recovery. In particular, it is very important that sulphide oxidation be controlled to ensure adequate break down of the sulphide minerals and high gold extraction during cyanidation. One variable that is often used for process control is the redox potential (ORP) of the oxidized slurry after discharge from the POX reactor. However, this method has the drawback of not reflecting the actual conditions in the reactor, as the measurement is also a reflection of the significant thermo-chemical changes associated with the cooling process.