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Metallurgy | African Eagle skip to Main Content

African Eagle 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, otherwise known as atmospheric tank leaching (ATL). 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, it is the low iron and high silica content that enables the Dutwa ores to be amenable to leaching at atmospheric pressure (i.e. 95°C) as there is less that needs to be leached and subsequently removed from solution when it is purified prior to producing the concentrate. The silica remains essentially inert and does not react with the acid at atmospheric pressure, lowering the overall acid consumption. The Dutwa “FeSi” ores also have a low (2-3%) magnesium content, which further reduces their acid demand.

This follows a comprehensive metallurgical test work campaign, employing conservative conditions, that demonstrated that the Dutwa principal ores were amenable to ATL and that this process was more favourable to the overall project economics and operating environment, and possessed a low risk profile than either high pressure acid leach (HPAL) or heap leaching.

Detailed Metallurgical Test Work To De-Risk the Process

African Eagle is undertaking a systematic, rigorous and detailed metallurgical test work campaign to define the preliminary process flowsheet for the Bankable Feasibility Study. This will be validated in a continuous pilot plant test programme at the SGS Minerals laboratory in Perth, Australia and will process approximately 50 tonnes of ore, treating ore samples representative of the planned mining schedule at Dutwa. The programme is scheduled to commence in mid 2012 and will result in the confirmation of the optimal final process flowsheet and plant design for the Project.

The campaign will be conducted at the same laboratory, SGS Lakefield Oretest in Perth (Western Australia) and in the same pilot plant that was used for the Ravensthorpe test work. First Quantum’s reported early success in re-establishing the ATL plant at Ravenshorpe validates 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.

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). CRU are conducting a marketing study to evaluate industry demand for each product, both are already being produced at other nickel laterite facilities.

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