Chapter 21 – Pressure Oxidation Overview

An overview of the pressure oxidation of refractory gold ores is provided in this chapter. Often there are advantages to be gained by operating at temperatures above the normal boiling point of the solution and, therefore, at pressures above atmospheric. In such cases, the term pressure hydrometallurgy is used or applied. Pressure oxidation refers to the oxidation of sulfides, such as pyrite [FeS2] and marcasite [FeS2], at elevated temperature and pressure. Oxidation releases encapsulated gold grains from the sulfides and makes the oxidation residue of the ore or concentrate more amenable for gold recovery by cyanidation in a subsequent leaching step. Pressure oxidation of gold ores is therefore a pretreatment leaching step to enhance gold recovery. Although pressure oxidation of gold ores and concentrates does not directly recover gold in a strict sense of the above definition, it is an essential step to enable the gold to be recovered. The two major considerations in hydrometallurgical processes are: • the extent to which a reaction can proceed (thermodynamic property) and • the rate at which the reaction will proceed (kinetic property). To understand pressure oxidation of gold ores, it is important to consider two aspects: the thermodynamic properties of the chemical system, which determine the overall reaction driving force, and the reaction kinetics, which depends on a combination of physical, chemical and mass transport factors and can be influenced to some extent by appropriate plant design. Acid and alkaline pressure oxidation of gold ores and concentrates are considered in terms of chemistry, process flowsheets, development history, materials of construction and other factors.