Dutwa Metallurgy

PHASE 2 METALLURGICAL TESTWORK, DUTWA NICKEL PROJECT, TANZANIA

African Eagle has now received the final results for the second phase of metallurgical testwork carried out as part of African Eagle’s pre-feasibility study on Dutwa. The tests were performed by Mintek Laboratories in South Africa on 250 kg of sample taken from seven drill cores from the main Dutwa nickel deposit, Wamangola Hill. They included column and tank leach tests, preliminary agglomeration tests and preliminary ore characterisation tests.

The work was undertaken as part of African Eagle’s pre-feasibility study on Dutwa, to select the best extraction process and to optimise leach extraction parameters such as acid addition, residence time and temperature.

Column tests

Column leach tests are used to investigate the likely behaviour of the ore, and to predict metal recoveries and acid consumptions under heap leach conditions. Three tests were performed at ambient temperature, using columns 1 metre high and 200mm diameter, filled with ore crushed to 25mm or less in size, and agglomerated using sulphuric acid solutions of 150 or 500 g/L concentration.

Each of the three columns was continuously leached for 156 days with 100 g/L acid at a rate of 10 L/m²/h, then rinsed for 12 days with very weak acid (3g/L) to wash out the nickel-rich solution remaining.

Chart 1 below shows nickel extraction and net acid consumption (including the acid used in agglomeration) against time. Two samples agglomerated with 150 g/L acid are shown in red and green, and the sample agglomerated with 500 g/L acid in blue. Assays were made using inductively coupled plasma (ICP) analysis. Prior to day 33, samples were taken daily and analysed by atomic absorption spectrometry (AAS). These results showed a very fast initial reaction, with approximately 60% of the nickel extracted within two weeks.

Chart 2 shows cobalt extraction over time, from the ICP assays.

Initial leaching was extremely fast, with 60 to 65% of the nickel being extracted within two weeks. The rate of leaching then slowed, reaching 73 to 82% after one month and 90 to 94% over the full five month test. The acid consumed to achieve these extractions was low, averaging 199, 273and 360 kg/tonne respectively.

The fast initial leaching of nickel is especially encouraging if heap leaching is used. Extraction of 65% of the contained nickel in two weeks using less than 200kg/t of acid means that working capital will not be tied up for a long period while the heap reaches full productivity. Whilst the remainder of the nickel leaches more slowly, extractions of up to 92% were demonstrated in the comparatively short leach time of 90 days. Such a short leach cycle extends the options for heap construction and offers the potential to construct dynamic heaps (on/off leach pads).

The results indicate that more than half of the nickel in Dutwa ore is contained in a readily leachable mineral phase. The rapid leach kinetics are due to the unique mineralogy of the Dutwa nickel deposit, which comprises more than 50% silica (quartz) and has a very low iron, magnesium and clay content. Mineralogical analyses previously undertaken by Mintek indicated that a major proportion of the nickel and cobalt is associated with manganese “wad” minerals. We believe that the initial fast leach rate is likely to be attributed to this component, with the remaining slower leach rate associated with nickel in limonite. Alternatively, the initial fast leach rate may be attributed to leaching of a fines fraction. Further work will be undertaken to define the relationship between mineralogy and leach extractions.

The ore samples also showed good mechanical behaviour in all three column tests, with compaction averaging 13%, and no indications of percolation or permeability problems.

Tank leach tests

Five atmospheric leach tests were carried out, with ore samples pulverised to a typical particle size of 80% passing 75 micron, and leached as slurry (25% w/w solids) for 24 hours in agitated tanks, using sulphuric acid of various concentrations. The tests were performed at 90°C and atmospheric pressure.

In all five tests, the reaction was very fast, most of the nickel leaching within the first hour. Chart 3 shows the results for the two tests which gave the greatest nickel extraction.

The first three tests, using a relatively low initial acid addition of 98g/L, gave only moderate nickel extraction. The other two tests used higher initial acid concentrations of 200 and 250g/L and gave nickel extraction of about 70% in the first hour, with 250 kg/t acid consumption. After 8hrs, 80 to 85% nickel extraction was achieved with 360-400kg/t acid consumption and final nickel extractions of up to 96% were reached after the 24hrs, with around 410 to 440kg/t acid.

Further tests will be required to determine the optimum tank leach conditions, and process engineering design parameters, but the fast reaction kinetics and comparatively low acid consumptions are very encouraging, especially for unbeneficiated ore.

Phase 3 tests

A geochemical domain model of the deposit has been completed and will be used to design a programme of bulk sampling to provide a large representative ore sample across the Dutwa nickel deposit for the next phase of metallurgical testwork. This will include: large scale column tests, with columns 6-8m high to replicate typical heap leach stack heights, mineralogical analyses, beneficiation tests to assess the potential to beneficiate (upgrade) the ore prior to tank leaching, and further tank leach and agglomeration tests to optimise these processes. Results of this work will be used to develop further the optimum processing route for Dutwa ore.

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