At the very outset of the Leinster project it was believed that a conventional preliminary flotation would be needed to concentrate tailings prior to atmospheric oxidation and pressure leaching. However, preliminary metallurgical testing of the tailings in Finland by Outotec (formerly Outokumpo Technologies) and in Perth by AMMTEC showed that the envisaged front-end flotation would not be needed and that atmospheric leaching might be feasible.
The sulphide tailings can be oxidised and air leached directly, obviating the need for a costly pre-flotation circuit and, perhaps, removing the need for pressure vessel oxidation and leaching of residues after atmospheric leaching. The upshot is a far simpler processing flow sheet than first envisaged, with consequent lower capital and operating costs.
Leaching tests were conducted over a range of conditions including those where iron remains in solution and those where iron is precipitated. The atmospheric leaching tests were conducted in a titanium reactor with oxygen feed, reflux condenser, heating plate and mixer. Samples were taken for testing at incremental intervals once targeted temperature was attained. Leaching yields for nickel, magnesium and iron were calculated from assays - with nickel attaining significant yields as part of the engineering study, leading to a definitive feasibility study.
Sulphuric acid consumption is related to leaching and formation of sulphates. Magnesium is the highest consumer of acid while nickel represents only a few percent of total acid consumption. The testwork on direct atmospheric leaching by Outotec in Finland reported high nickel yields and dissolution rates from both sulphide and non-sulphide (silicate) nickel bearing minerals achieved. This significant development resulted in nickel yields and dissolution rates of 88% to 95.6% in eight to 24 hours, with the best result being 92.9% of nickel in solution after eight hours.
This is a significant improvement over earlier findings from which 72% nickel yield was selected as the base case assumption for feasibility studies. The implications are that the faster dissolution and higher nickel yields will translate into smaller-capacity leach tanks and hence a smaller footprint for the plant and therefore lower capital expenditure. The positive results have led to continuous pilot plant test work of the atmospheric leach at Outotec’s facilities in Finland.
Further test work to optimise the leaching conditions and acid consumption has been completed. Preliminary techno-economic evaluation is underway to finalise the process flowsheet and re-estimate capital and operating costs for the process.
The techno-economic study of the Leinster Nickel Sulphide Tailings project is due for completion by Outotec early in 2008, as part of the engineering study and definitive feasibility study under the Tailings Supply Agreement with BHP Billiton.
Braemore has exclusive rights to run pilot plant tests on nickel tailings at BHP Billiton’s three Western Australian mines – Leinster, Kambalda and Mount Keith