Metallurgy
Naturally occurring nickel laterites are derived from peridotite rocks containing olivine and serpentine. Under favourable conditions and with abundant rainfall various acids (humic and others) are produced as a result of decaying organic matter. These acids leach out the magnesium and silica values, while enriching the residue with iron and nickel. Such weathering of peridotites leads to the formation of nickel laterites.
Laterite deposits can be divided into 3 zones at increasing depth from the surface.
These zones are:
1. Limonite Zone
2. Serpentinite Zone
3. Garnierite Zone
The zone of interest for Mt Thirsty Ores is the Limonite Zone. Typical of the Limonite zone is the presence of ASBOLITE type minerals e.g. Cryptomelane. At Mt Thirsty the mineral ASBOLANE has been identified.
Typically, until recently, laterites have been treated with various degrees of success utilising the High Pressure Acid Leach route.
The Mt Thirsty ores contain an abundance of Asbolane and the treatment methods for these ores and mineral types has been developed separately in the 1970’s.
The method of treatment was by the reduction of the mineral species by sulphur dioxide.
Ores were submitted to Murdoch University for evaluation with initial leach tests performed by various Ph.D. students under the supervision of Prof. Mike Nicol. Results show that a simple and inexpensive process route for leaching these ores is available through a process using sulphur dioxide as a reducing agent.
Asbolane laterite - Prony Bay, Mont-Dore Commune, Southern Province, New Caledonia
Leach test work also identified a remarkably low up take of iron. A low up take of iron species allows for a less capital intense infrastructure requirement and lower operating costs.
The work so far has highlighted that the process route is likely to be at atmospheric pressure and low temperature. This does not have a capital intense infrastructure requirement.
With some optimization, nickel recoveries can be improved substantially, particularly with grinding, to less than 100 microns. Leach recoveries for nickel could be maintained well above 70%, at the optimum grind size.
Cobalt recoveries approaching 90% or better given the optimised grinding conditions described above. The reduction process when applied to Mt Thirsty ore is highly successful predominantly due to the unique mineralogy (Asbolane) and minimal presence of clay contaminants. Representative samples of the Mt Thirsty ore body will be submitted to Murdoch University for pilot testing. The Murdoch University Mineral Processing facilities have treated similar manganiferous type ores from other locations in Australia. The use of sulphur dioxide as a reducing agent is of significant value to the venture and will be pursued in any future laboratory and pilot trials for the Mt Thirsty ores. Project economics have been calculated on the basis of producing a combined nickel hydroxycarbonate/cobalt - hydroxycarbonate product, for on selling to a nickel refinery. Recent work on the development of an Ion Exchange processing route for leach liquors similar to a proposed Mt Thirsty operation has demonstrated that a product of nickel sulphate hexahydrate and cobalt sulphate pentahydrate can be produced economically. This process option has not been evaluated at this stage however the revenue received for these products would exceed those of a combined nickel hydroxycarbonate/cobalt - hydroxycarbonate product.
