With a nameplate capacity of 40,000 tonnes of nickel and 5,000 tonnes of cobalt per annum at the Murrin Murrin plant, Minara is a significant producer of LME grade nickel and cobalt metal.
After mining from nearby open pits, ore is delivered to the run of mine (ROM) pad, where it is sorted according to grade and blended to ensure consistent feed to the feed preparation circuit. The ore is then mixed with water to form a slurry for processing in the High Pressure Acid Leach (HPAL) circuit.
The HPAL circuit consists of four titanium lined autoclaves, each the size of a small submarine.
The nickel and cobalt is leached out of the ore slurry, grading approximately 1.3% nickel and 0.09% cobalt, and into solution, by raising the pressure up to 44 atmospheres and mixing it with highly concentrated sulphuric acid at a temperature of 255 degrees celsius. This generates substantial quantities of heat which is recovered and then used to pre-heat incoming slurry. Now that the nickel and cobalt have been leached and are subsequently in soluble form, they must be separated from the residue waste material. This solution is "washed" to remove the waste residue, allowing the valuable nickel and cobalt solution to move forward for further processing, and the residue to be neutralised for tailings disposal as inert waste.
Leached ore solution now passes into the neutralisation circuit where calcrete is added to neutralise the acid. The solution is then passed into the mixed sulphides precipitation circuit, where hydrogen sulphide gas is added to convert the solution into a mixed nickel and cobalt sulphide precipitate.
The mixed nickel and cobalt sulphide enters another autoclave where pure oxygen converts the solids from a mixed sulphide into a metal sulphate solution. Impurities such as iron and zinc are then removed, before the cobalt and nickel are separated using solvent extraction.
The nickel sulphate solution then enters six parallel autoclaves, known as the hydrogen reduction circuit, where the hydrogen is added, nickel metal is precipitated and then separated from the nickel free liquid stream. The nickel powder is then dried before entering the final processing stage.
The powder is formed into a pillow-shaped briquette, sintered in a furnace and then packaged for transportation. The entire process from beginning to end takes approximately 14 days. Murrin Murrin produces nickel and cobalt briquettes that easily meet the highest A Grade standards of the London Metals Exchange and are highly regarded and in strong demand from consumers around the world.
The cobalt sulphate solution from solvent extraction follows a separate processing path which is similar to the nickel processing path but on a smaller scale.