HPAL Process

High Pressure Acid Leaching (HPAL)

The technology of Pressure Acid Leaching (PAL) which was developed in Cuba (Moa Bay) in the 1950’s using a process now owned by Sherritt. It was developed for laterite ores which are difficult to concentrate.

Process Challenges

Slurried, crushed ore from the mine is conveyed to the processing plant to be pressure leached with sulphuric acid. The Pressure Acid Leaching area receives the slurried feed, heats it to 225°C and mixes it with hot concentrated sulphuric acid inside four titanium-lined autoclaves. Nickel and cobalt are leached into the solution.

The autoclaves are served by large preheating and pressure let down systems. In this part of the process SchuF pressure let down control valves are used.

Conditions in this part of the plant for valves are severe due to the high velocity of the abrasive and corrosive slurry.

The materials used for the autoclaves and associated with piping and valves in the pre-heating and let down systems include titanium, brick lining, and high nickel alloys.

Counter Current Decantation

The autoclaves discharge leached slurry to the counter-current decantation (CCD) washing area, where it is mixed with flash-steam heated water, producing a clear solution of nickel and cobalt in preparation for hydrogen sulphide precipitation of mixed nickel/cobalt in the sulphide precipitation plant. Here nickel/cobalt sulphide is leached in an autoclave by pure oxygen at approximately 12 bar, 120° C, producing a metal sulphate solution.

Valves for Flashing Slurry

Before further processing this liquor,  the pressure is let down with the help of an Angle Control Valve to a sulphide precipitation flash pressure let down drum. The challenges for the SchuF angle control valve are the effects of cavitation on the valve trim common when handling flashing slurry. This damage can be minimized using SchuF’s flow to open design Type 74 control valves designed especially for controlling slurries in three phase flow conditions.

Impurity Removal

Iron, copper and zinc impurities are removed, cobalt is separated and a concentrated cobalt solution feeds the cobalt hydrogen reduction area. The remaining nickel solution feeds the nickel hydrogen reduction area. The metal sulphate solutions are hydrogen-reduced in parallel autoclaves. The resulting powders are dewatered, dried and packaged for sale.

Other SchuF valves used by the mining industry include Ram Valves for various injection, sampling and draining applications as well as Isolation Valves for exotic alloys.

Gold is also processed using acid leaching autoclaves and similarly requires Angle Control Valves for pressure let down.