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.
Positive beneficiation test work results demonstrate a high potential for improvements to the Dutwa Project’s capital cost and operating costs.
Following a significant and targeted laboratory metallurgical test campaign on the Dutwa ores, the results demonstrate that they are amenable to beneficiation prior to processing and that the beneficiated ore has improved leach characteristics when compared to the run-of-mine (“RoM”) ore. The results exceed the promise identified in the 2011 mineralogical studies, conducted by the Natural History Museum, while also characterising the significantly improved leach performance of the beneficiated ores, an outcome that is unexpected.
As a result, beneficiation offers the potential to significantly reduce both the operating and capital costs of the Project for the equivalent metal output.
Test work highlights:
The test work indicates that between one half and two thirds of the FeSi mined RoM ore will be rejected from the beneficiation process and that the nickel grade of this rejected material will be below economic levels. FeSi ore comprises approximately 70% of the total resource of the Project.
Strong potential exists to increase the Ni grade of the FeSi ore that is fed to the plant from up to 1% to 2%. Furthermore, the test work also indicates that the leach response of the beneficiated ore, compared to the RoM ore, has improved characteristics in terms of a reduction in acid consumption, reduction in leach residence time and a reduction in the quantity of limestone required for neutralisation with the potential consequence of a reduction in operating costs and infrastructure requirements.
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 neutralised 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).
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