Processing
Copper
The Sepon copper operation produces approximately 80,000 tonnes of copper cathode (copper sheets) per year from the nearby Khanong and Thengkham ore bodies.
Crushing and milling
Run-of-mine (ROM) ore is crushed and then milled, which reduces the diameter of the pieces of rock from about 500mm to about 0.106mm (106 microns).
The milling process at Sepon is unusual as it uses acidic solution instead of water to help grind and dissolve some of the copper. The mixture of ore and acidic solution resembles a thick liquid or paste, and is called slurry. 90% of the copper in the slurry can be extracted by Sepon’s copper processing plant.
Leaching and thickening
The slurry is heated and leached at 80°C for five hours. It is then cooled and thickened, before being washed in a counter decantation circuit to remove soluble copper. The pregnant leach solution (a copper rich solution) from the counter decantation circuit is then clarified to remove the finer solids, before being pumped to a solvent extraction and electrowinning circuit.
Neutralisation circuit
After the copper has been extracted from the slurry, the slurry is mixed with limestone and lime to neutralise the acidic solution. Contaminants in the slurry are also made safe. Slurry from both the gold and copper plants are mixed together before being pumped to a tailings storage facility.
Autoclave circuit
A number of reagents (substances used to bring about a chemical reaction) are used in the leaching circuit. The reagents are expensive and required in large quantities, so instead of buying them and transporting them to Sepon, they are produced on site in an autoclave.
Water use
In order to minimise the amount of fresh water taken from the Namkok river system, a large amount of water from the tailings dam is pumped back to the gold and copper plant and reused.
Cell room where the sheets of Sepon copper are produced.
Gold
The Sepon gold processing facility has been in operation since 2002, and to date has produced in excess of one million ounces of gold. The facility is designed to process up to 2.5 million tonnes of ore per year, through a standard carbon in leach processing plant. Oxide ore processed at Sepon contains both gold and silver, and both are recovered in the gold process plant.
Ore is crushed and then milled, before being introduced into the carbon-in-leach circuit. Sodium cyanide is used to leach the gold from the ore into solution, and recovers on average about 80% of the gold. Small pellets of activated carbon within each tank absorb the leached gold before being sent to the stripping circuits, where desorption of the loaded gold takes place.
The pregnant solution (called ‘eluate’) is sent to the gold room where electrowinning cells recover gold and silver onto stainless steel cathodes. Once the cells are sufficiently loaded they are cleaned to produce a gold sludge, which is then smelted in a special furnace on site and poured into moulds to produce gold bars called ‘ingots’.
The ingots are known as gold doré as they contain both gold and silver. They are sent off-site for further refinement.
Gold pour – the last stage of gold production at Sepon mine.