MONUMENT Mining continues to consider various options to treat transitional and sulphide material at the Selinsing Gold Project as part of the proposed phase IV development. In the meantime, the Selinsing plant is being fed with low grade oxide ore and stockpiled materials discharged from the mill in the early stage of production.
Independent metallurgical testing by Inspectorate Exploration and Mining Services in Canada has been performed on refractory sulphide ore samples from the Selinsing Deeps and Buffalo Reef projects. This work encompassed a variety of methods including pressure oxidation, ultrafine grinding, roasting and acid leaching techniques.
The best recoveries for sulphide ore were achieved in the mid to upper 80% range with a biological oxidation approach through a combined flotation and tank bioleach treatment approach. The processing facilities would require a capital investment of about US$50 million, according to an engineering study carried out by Lycopodium of Australia, and an additional estimated cost of US$10 million for a new power line being built with an estimated completion time of 2 to 3 years.
A follow-up engineering study by Tetratech in Canada shows that some savings could be realized by incorporating a modified design. Monument’s management is continuing its research and development work in evaluating less capital intensive alternatives.
One such option is biological heap leaching, which aims to use acid produced from sulphide minerals in the ore to stimulate the natural occurrence of soil microbes in dissolving sulphides contained in the ore materials. Once sulphides are dissolved in the leach pads, the oxidized sulphide ore would be further leached in the same pads or in the existing comminution and CIL circuit. This would be followed by smelting into dore bars for shipping to a refinery as is done for oxide ore.
This method has the advantage of low capital investment. A sulphide concentration plant and alterations to crush ore feed would not be required. Potentially crushed ore from the present crushing circuit can be directly treated though permanent heap leach pads. These permanent re-usable heap leach pads can be built and operated in-house by the present staffing at Selinsing with low capital expenditure that require multiple cells, suitably lined liquor ponds and a network of reticulation piping, pumps and metal recovery systems.
While recoveries of gold product can be lower than bioleach treatment plant, potential heap leach issues such as heap blinding and slumping can be minimized by methods such as whole ore stacking, de-sliming, eliminating agglomeration and other approaches being considered.
There is an adequate area of suitable land to construct heap leach cells and ponds on site and a plentiful supply of limestone is available for neutralizing the solutions prior to discharge to the tailing facility.
Research and development programs on various alternate oxidation processes will continue to completion in parallel with the heap leach trials before a final commercial decision is made.