Metallurgy

The company has chosen to employ straightforward and proven atmospheric acid leach, in agitated tanks, to dissolve the ores (with the addition of sulphuric acid) and recover the nickel. This atmospheric tank leaching (ATL) is a well proven technology for processing other metals, such as cobalt, copper and gold, and uses standard processes and equipment.

Unlike many other nickel laterites, Dutwa’s low iron and high silica content enables its ores to be amenable to leaching at atmospheric pressure (i.e. 95°C). The silica remains essentially inert and does not react with the acid at atmospheric pressure, lowering the overall acid consumption, which when combined with the "FeSi" ores low (2-3%) magnesium content, further reduces the acid demand.

Detailed Metallurgical Test Work To De-Risk the Process

A rigorous and detailed metallurgical test work campaign to define the preliminary process flowsheet for the Bankable Feasibility Study has been undertaken and this will be validated in a continuous pilot plant test programme in the future.

It is envisaged that a similar pilot plant will be commissioned that was used for First Quantum’s Ravensthorpe test work which reported early success in re-establishing the ATL plant at Ravenshorpe and validated the scaling from pilot plant to operation.

The Process – Simplified

Whilst the preliminary, detailed Dutwa process flowsheet is being validated, the simplified flowsheet comprises three stages, each straightforward and proven.

Firstly is ore preparation, where the ore is ground and thickened to form an ore slurry. This is pumped into a series of agitated tanks for the leaching process. The conditions are at 1 atmosphere and 95°C. The process uses sulphuric acid (which will be made in a sulphur-burning acid plant at Dutwa) and steam, which generates the conditions necessary for the dissolution of the minerals in the ore to take place. Each tank is well agitated to ensure each particle of ore has the maximum continuous contact with the sulphuric acid to fully dissolve it. The exact time of the leach process is still to be determined but is expected to be between 16 and 24 hours.

 

After the nickel has been leached, the leach slurry passes through a neutralisation stage where residual acid is destroyed and most of the iron dissolved in leaching is removed. The partly neutralised slurry (with pH ~3) is pumped to a Counter Current Decantation (CCD) circuit where the solids are washed to recover soluble nickel and thickened so that the waste material (tailings) can be safely sent to the tailings treatment plant and then on to the tailings storage facility. The process takes place in a counter current fashion (with nickel leach solution and solids moving in opposite directions) using conventional equipment called thickeners. The nickel leach solution is then purified to remove the aluminium and residual iron and then mixed with a reagent to produce a nickel intermediary product, which will either be a mixed nickel cobalt sulphide (MSP) or a mixed nickel cobalt hydroxide (MHP).

Images

Crushed Dutwa ore

Wet vibratory screening of Dutwa FeSi ore after drum scrubbing

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