Gold and copper extraction processes
Ore is processed at Gedabek to produce either gold doré (an alloy of gold and silver with small amounts of impurities, mainly copper) or a copper and precious metal concentrate. Gold doré is produced by cyanide leaching.
Initial processing is to leach (i.e. dissolve) the precious metal (and some copper) in a cyanide solution. This is done by various methods:
- Heap leaching of crushed ore
Crushed ore is heaped into permeable “pads” onto which is sprayed a solution of cyanide. The solution dissolves the metals as it percolates through the ore by gravity and it is then collected on the impervious base under the pad. - Heap leaching of run of mine (“ROM”) ore
The process is similar to heap leaching for crushed ore, except the ore is not crushed, instead it is heaped into pads as received from the mine (ROM) without further treatment or crushing. This process is used for very low grade ores. - Agitation leaching
Ore is crushed and then milled in a grinding circuit. The finely ground ore is placed in stirred (agitation) tanks containing cyanide solution and the contained metal is dissolved in the solution. Any coarse, free gold is separated using a centrifugal-type Knelson concentrator.
Slurries produced by the above processes with dissolved metal in solution are then transferred to a resin-in-pulp (“RIP”) plant. In this plant, a synthetic resin is used to selectively absorb the gold and silver from the slurry. The metal-loaded resin is then “stripped” of its gold and silver by desorption into another solution, from which the metals are recovered by electrolysis, followed by smelting to produce the doré metal, which comprises an alloy of gold and silver.
Copper and precious metal concentrates are produced by two processes, SART processing and flotation.
- Sulphidisation, Acidification, Recycling and Thickening (“SART”)
The cyanide solution after gold absorption by resin-in-pulp processing is transferred to the SART plant. The pH of the solution is then changed by the addition of reagents which precipitates the copper and any remaining silver from the solution. The process also recovers cyanide from the solution, which is recycled back to leaching. - Flotation
Finely ground ore is mixed with water to produce a slurry called “pulp” and reagents are then added. This pulp is processed in flotation cells (tanks), where the pulp is stirred and air introduced as small bubbles. The sulphide mineral particles attach to the air bubbles and float to the surface where they form a froth which is collected. This froth is dewatered to form a mineral concentrate containing copper, gold and silver.
Plant maintenance and upgrades
The Group’s processing plants underwent extensive maintenance in 2023 and 2024 during the period when agitation leaching and flotation processing was suspended. Extensive refurbishment of the agitation and flotation plants was carried out, including installing a new hopper and redesigned pipework for the agitation leach plant to improve ore feed.
The ball mills were relined and refurbished. Much of the work has improved safe working such as repairing minor leaks, installing new floors and improving ladders and gantries. Roof repairs have also been carried out where necessary. A substantial proportion of the exterior of the plant has been cleaned by shot blasting and repainted. Exterior pipework has also been cleaned or replaced as necessary.