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 by 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. Depending on the composition of the ore, an option is available to process the finely ground ore through the flotation plant prior to, or after treatment by the agitation leaching plant. However, since installation of the second crusher line for the flotation plant in 2018, the two plants have been operating independently. 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. A synthetic ion exchange resin, in the form of small spherical plastic beads designed to absorb gold selectively over copper and silver, is mixed with the leach slurry or “pulp”. After separation from the pulp, the gold-loaded resin is treated with a second solution, which “strips” (i.e. desorbs) the gold, plus the small amounts of absorbed copper and silver, transferring the metals from the resin back into solution. The gold and silver dissolved in this final solution are recovered by electrolysis and are then smelted to produce the doré metal, comprising 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 metal 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. This precipitates the copper from the solution in the form of a finely divided copper sulphide concentrate containing silver and minor amounts of gold. The process also recovers cyanide from the solution, which is recycled back to leaching.
Flotation is carried out in a separate flotation plant. Feedstock, which can be either tailings from the agitation leaching plant or freshly crushed and milled ore, is mixed with water to produce a slurry called “pulp” and other reagents are then added. This pulp is processed in flotation cells (tanks). The flotation cells are agitated 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.
Initially, gold doré was produced at Gedabek only by heap leaching crushed and agglomerated ore. Heap leaching is a low capital cost method of production commonly used by mines when they first move into production. Ore at Gedabek is being crushed to less than 25mm in size and the resultant gold recovery is approximately 60 per cent. to 70 per cent. of the contained gold over leaching cycles which extend typically beyond one year.
To increase gold recoveries and production, in 2013 the Group constructed an agitation leaching plant. Compared to heap leaching, agitation leaching can deliver higher recoveries of gold without long leaching cycles. Heap leach pads also require considerable space for their construction and due to the topography of the Gedabek site, this is a constraint. The capacity of the agitation leaching plant was increased in 2016 by the installation of a second semi-autogenous grinding (“SAG”) mill.
The ore at Gedabek is polymetallic containing significant amounts of copper. Initially, the SART processing plant was constructed to recover some of the copper as a copper and precious metal chemical concentrate. However, to further exploit the high copper content of the Group’s ore reserves, the Group constructed a flotation plant whose function is primarily to produce a copper-rich mineral concentrate, containing gold and silver as by-products. The flotation plant commenced production in November 2015. The flotation plant has the flexibility to be configured for various methods of operation.
In 2018, a second crusher line was installed for the flotation plant. This has a budgeted capacity of 95 tonnes per hour compared to the original crusher of up to 120 tonnes per hour. This removed a large bottleneck and enabled independent operation of the agitation leaching and flotation plants from separate sources of feedstock. The addition of this second crusher not only significantly increases the capacity of our processing plants, but also their flexibility.