Technology
To the right is a brief description of the nickel production process utilised at Murrin Murrin. Click the image to download the complete flowchart.
At nameplate capacity, the Murrin Murrin plant has the potential to produce 40,000 tonnes of nickel and 5,000 tonnes of cobalt per annum, making Minara one of the world's largest producers of LME grade nickel and cobalt metal.
After mining from nearby open pits, ore is dumped on 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 pressure acid leach circuit consists of four giant titanium lined autoclaves, each the size of a small submarine, which is not surprising considering they were built by the Australian Submarine Corporation.
The nickel and cobalt is leached out of the ore slurry, grading approximately 1.35% nickel and 0.09% cobalt, and into solution, by raising the pressure up to 44 atmospheres and spraying it with highly concentrated sulphuric acid at a temperature of 255 degrees celsius. This generates substantial quantities of heat and acid which are later recycled throughout the plant.
The ore is leached, with the valuable nickel and cobalt in a soluble form which must be separated from the residue waste material. This solution is then "washed" to remove waste materials and is recycled back into the slurry ore preparation before eventually being pumped out to the tailings dam as neutralised and 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 cobalt sulphide.
The mixed nickel cobalt sulphide enters another autoclave where pure oxygen converts the solution from a mixed sulphide into a metal sulphate. At this point the nickel and cobalt molecules remain attached to each other. Impurities such as iron and zinc are removed, before the cobalt is separated using an organic reagent.
The nickel sulphate solution then enters five parallel autoclaves, known as the hydrogen reduction circuit, where the hydrogen is added, liquids are separated and the remaining solids are converted into a dry powder of pure nickel.
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.
Heap Leach
Minara's Murrin Murrin nickel operations have been accumulating stockpiles of low grade ore and a waste material known as scats since the plant commenced operation. During the first quarter of 2007, Minara Resources commissioned the heap leach demonstration plant, and stacking of scats and ore continued from this time.
As at January 2008, over 300,000 dry tonnes of scats and ore had been stacked. Scats nickel recovery of 78% was achieved after 180 days, exceeding recoveries based on test work.
In 2007, the company invested a further $32 million in expanding and integrating the heap leach demonstration plant. Heap leach technology will allow the processing of existing stockpiles of waste material and would add a low-risk production stream delivering a year-in-year-out source of revenue not previously available to Minara.
The low capital cost of the heap leaching technology will also allow it to be applied to the lower grade nickel deposits held, but not currently mined, by the company. The technology will therefore provide a second processing stream independent from HPAL.
The demonstration plant continues to operate to capacity and remains on track to deliver over 2000 tonnes of nickel metal and 150 tonnes of cobalt by the end of 2008.