Pressure leaching: HPAL and friends

The autoclave appeared in Module 3 as a way to oxidise a refractory sulfide before leaching. The same vessel can do the leaching itself. Pressure leaching dissolves the target metal directly under temperature and pressure, and its defining case — high-pressure acid leaching of nickel laterites — is one of the major hydrometallurgical flowsheets, so it earns its own topic in the leaching family.

The leach as the severe step

High-pressure acid leaching, HPAL, treats nickel laterite ore — an oxide ore that an atmospheric acid leach handles poorly — by leaching it with sulfuric acid in an autoclave at high temperature and pressure. Under those conditions the nickel and cobalt dissolve quickly and the iron and aluminium that dissolve with them largely re-precipitate in the vessel, so the autoclave both leaches the value and rejects much of the gangue in one severe step. The discharge is a liquor carrying nickel and cobalt forward to neutralisation and precipitation. The distinguishing feature of the family is that the leach itself is the demanding unit operation, rather than a mild contacting step downstream of a pre-treatment.

The autoclave, again

The reason for the pressure vessel is the same as in pressure oxidation: the chemistry runs at a useful rate only at a temperature above the atmospheric boiling point of the liquor, and holding water liquid there requires pressure. So pressure leaching and pressure oxidation are the same machine put to two ends — in oxidation the autoclave opens a sulfide ahead of a separate leach; in pressure leaching the autoclave is the leach. Recognising that shared vessel is part of reading the autoclave on a flowsheet correctly: the block looks identical, and what tells you which job it is doing is what enters and leaves it.

The tie to pressure oxidation, and to steam

Because the conditions are the same, the steam and heat balance is the same too. A high-temperature, high-pressure aqueous vessel lives on the saturation relationship between water and steam, with heat into the autoclave to reach temperature and flash steam off as the discharge lets down — the same property tie the pressure-oxidation topic drew. This page lands on the committed water and steam hub for that relationship and connects back to Module 3’s autoclave and forward to Module 8’s heat duties. It describes the process and its place in the flowsheet; it gives no operating set-points or safety procedure, which belong to the engineered design of a specific autoclave.