The following factors have lead to the joint venture partner to take the decision to develop a processing facility to serve the Angostura mine but also with potential to generate its own income from other small mines in the area.
- Opportunity of freehold purchase.
- Moratorium / amnesty of 2 years to apply for license.
- Location; close to mine, highway and electricity grid, distant from any population and downstream from water supplies.
- Grid supplied electricity rather than a diesel generator reduces electricity cost dramatically.
- Security of processing capacity and quality.
Reduced processing cost to approximately US $30.00 per tonne and reduced transportation cost to under $10 per tonne as opposed to the $40 previously charged. Control of development will enhance the scalability and profitability of the mine.
The plant will be an end-to-end processing solution, except for the final gold extraction from carbon and smelting - a decision in part governed by capital restrictions, practicality and security. The process uses cyanide leaching and carbon capture as follows:
- The process involves several stages, milling, agitation tanks followed by smaller carbon tanks.
- Mineral from ore bins is run through a crusher thereby reducing the ore to half inch pieces. From there the ore is mixed with cyanide water solution and pumped into a ball mill where it is further reduced to a powder of sub-200µm to -100 µm particles.
- The powder is then pumped into agitation tanks with a water cyanide solution. Once thickened leaching reagents in the form of cyanide and an oxidant such as air or oxygen are added. Leaching takes place as the pH solution is adjusted to a value of around 9.5–11 to ensure minimum loss of cyanide as hydrogen cyanide and the mixture is agitated in the leach reactors either mechanically (rotation stirrer) or by air.
- Typical residence times in the leach section may range from 20–40 hours depending on the head grade and nature of the ore and the number of agitation tanks.
- In the Carbon in Pulp process (CIP) the carbon of concentration 10–25 grams of carbon per litre of pulp (0.5 to 1.2% by volume carbon) is added directly to the leach solution to save on processing time and capital investment for a slight reduction in efficiency yields. The carbon is retained in each reactor by means of screens having an aperture of 0.6–0.8 mm which allows pulp to flow through and out of the reactor whilst retaining the carbon in the reactor. The gold aurocyanide complex in the aqueous phase is readily adsorbed onto the activated carbon.
The whole process is scalable with base infrastructure being installed from the beginning to allow a final capacity of 300 tonnes of ore per day. Capacity however will start at 35 tonnes a day in line with the expected output of the Angostura mine and as both the mine and the processing plant are scalable, production of both is expected to increase, funding permitting, to 70 tonnes a day within 60 days or by the end of quarter 1, 2013.
The design takes advantage of the natural topography of the land with the process starting at a higher gradient and almost rolling down hill to the bottom.