Pressure oxidation: the autoclave route

When a sulfide is too refractory for an atmospheric treatment and a roaster is unwanted — for its emissions, its capital, or its chemistry — the aqueous answer is to oxidise the sulfide under pressure. Pressure oxidation, or POX, holds the feed slurry in an autoclave with oxygen at a temperature well above the atmospheric boiling point, where the sulfide oxidises fast and completely and the value behind it is set free for the leach that follows.

What pressure oxidation does

POX is an oxidation step, not a recovery step. It takes a sulfide that shields or stabilises the target metal and converts it, with oxygen and water at temperature, to an oxidised form — sulfate in solution and an iron oxide residue — so the gold, or the base metal, is no longer locked behind unreacted sulfide. The autoclave discharge is an oxidised slurry that goes forward to the actual leach; the value is exposed, not yet won. In a gold flowsheet the POX product feeds the cyanide leach; in a base-metal flowsheet it feeds an acid leach, sometimes in the same vessel.

Why an autoclave

The reason for the pressure vessel is rate. The oxidation that would crawl at atmospheric boiling point runs in a manageable time only at higher temperature, and to keep water liquid above its atmospheric boiling point the vessel has to be held above atmospheric pressure. The autoclave exists to sustain that temperature-and-pressure window, and to hold the oxygen partial pressure the oxidation needs in contact with the slurry. The chemistry sets the conditions; the vessel is what makes those conditions possible.

The steam and heat tie

The oxidation of sulfides is strongly exothermic, so a running autoclave generates heat, and the heat balance of the vessel — start-up steam in, flash steam out as the discharge lets down to atmospheric pressure — is central to how it operates. Water and steam are the working substance of that balance: the saturation relationship between temperature and pressure is the same steam behaviour tabulated for any pressurised hot-water system. This page lands on the committed water and steam hub for that property tie, and connects forward to the heat-and-cooling duties of Module 8. It describes the role of steam and heat in the process; it gives no operating set-points or safety procedure, which belong to the engineered design of a specific autoclave.