Hi, In general, it's to accelerate the chemical reactions that break down minerals, particularly for extracting metals like nickel and cobalt from laterite ores.

Hi Nazih Asoufar, thank you for the clarification. I was wondering how the high pressure accelerates the reaction when dealing with a slurry (something that is not a gas). Or are the reactions so sensitive to pressure that even liquid and solid components respond to it in the reaction kinetics?

In addition to increasing reaction rates, high temperature increases solubility of Ni2+ and Co2+ and precipitates Fe3+ as hematite. High pressure is required to keep acid and water in liquid phase at high temperature. In other words, the pressure increases the boiling temperature of the slurry.

Exactly mister Richard Sherritt, There is a great impact when we increase pressure about temperature, so even you work in liquid or solid phase, a high pressure bring an important temperature.

Nazih Asoufar Richard Sherritt Thank you both, this was extremely informative!

I agree generally that high pressure is needed to allow high temperature, which in turn is needed to break down the limonite solids and precipitate hematite. However, I am afraid that Richard Sherritt is wrong by saying that high temperature increases the solubility of Ni and Co. Cobalt sulphate maximum solubility is around 65C and nickel sulphate 110C. Higher temperatures result in a steep depression of the solubility.

One other important factor with HPAL is that the sulphuric acid is monoprotic at 240-270C, so that twice the expected amount of acid based on simple stoichiometry has to be added. At letdown, this then reverts to the normal diprotic state, and the excess acid then has to be neutralised, usually with lime/limestone, resulting in significantly more solids having to be disposed of than were treated originally..

Finally, HPAL is essentially limited to the limonite horizon (i.e. the low-magnesium part) of the laterite, since Mg consumes acid, and unlike the Fe and Al, it is not hydrolysed at pressure. This means that the other horizons (e.g. saprolite, transition zone, smectites) of the laterite cannot be economically processed, despite saprolite, for example, usually having a higher nickel content.

Bryn Harris I really appreciate the added context to Richard Sherritt's answer, your discussion is very valuable. I just wanted some clarification - if high temperatures (around that >200 degC range) do not improve solubility of Ni and Co, what is the purpose of going that high in HPAL?

I am currently trying to compare high pressure acid leaching with atmospheric pressure leaching, and the necessity for high temperatures and high pressures in the first process are confusing with regard to what you explained.

Alyssa Ishchenko - the nickel and cobalt sulphates will redissolve once the system reverts to atmospheric conditions, so it, in iself, is not an issue. The high temperatures are needed to control and precipitate the iron, and more especially the aluminium, as hematite and alunite respectively. The high temperature also markedly improves the kinetics. For example, with Turkish laterites, at 90C, the residence time is of the order of 12 hours, versus 90 minutes at 240C.

If you are going to review atmospheric leaching, then you should also include the hydbrid so-called EPAL process, which BHP (Neudorf and Huggins) developed. This process used high-Ni content saprolite to neutrlise the HPAL slurry. It was not very successful.