Golden Mile ore is of two main types: weathered oxide ore from near the surface which is free-milling with the gold easy to extract, and sulphide ore in deeper ore bodies where the majority of the gold is locked as inclusions in tellurides and pyrites and is difficult to extract. In the sulphide or telluride ore, the gold is chemically combined with tellurium, an element related to sulphur, and the complex extraction process involves several stages.
The sulphide ore from the Open Pit is refractory and requires a complex series of processes to extract the gold. The free-milling Mt Charlotte ore, which is easier to process, was formerly direct-leached after grinding but is now combined in small batches with the Fimiston sulphide process. The ore from the Fimiston Open Pit and Mt Charlotte Underground Mine is processed at KCGM’s Fimiston and Gidji Processing Plants.
Fimiston and Gidji Processing Plants
The Fimiston and Gidji Processing Plants were commissioned in 1989, with Fimiston undergoing two expansion stages to become one of the largest milling complexes in Australia. The sites treat more than 12 million tonnes of ore per year to produce up to 800,000 ounces of gold.
The process of extracting gold from sulphide ore begins when the trucks deliver the ore from the blast site to the ROM (run of mine) pad at the Fimiston Plant. Here the crusher reduces it to chunks of rock about the size of a fist. The ore is then ground into very fine particles less than one-fifth of one millimetre in diameter in large rotating SAG (Semi Autogenous Grinding) and Ball Mills which look like huge steel drums.
Water is added to the process, creating mud-like slurry which is pumped to large tanks called flotation cells. Air is added to the bottom of the agitating flotation cells and the resulting froth which gathers at the top contains nearly all the gold. This combination of free gold, gold-bearing pyrite and telluride particles is known as sulphide concentrate.
Ultra Fine Grinding
The concentrate is dried by filtering before it is then either treated in the 10 tonne per hour (tph) Ultra Fine Grinding (UFG) Mill at Fimiston or is trucked to the Gidji Processing Plant, 20 kilometres north of Kalgoorlie-Boulder. At Gidji, the concentrate is treated in the either the 10tph UFG Mill or the new 30tph UFG Mill which was opened in June 2015.
KCGM was one of the earliest commercial users in Australia of the IsaMill ultra-fine grinding technology. The first IsaMill was commissioned at the Gidji Plant in 2001 and a second IsaMill was installed at the Fimiston Plant in 2002. UFG Mills use very small ceramic balls (2.5mm) to grind the concentrate and expose the trapped gold to the downstream cyanide-leaching process.
Prior to this concentrate trucked to Gidji was also treated in one of two circulating fluid bed roasters at 650°C. Commissioning of the 30tph UFG Mill completely replaced roasting, eliminating atmospheric emissions from the Gidji Processing Plant.
Extracting the Gold
The Carbon In Pulp (CIP) process is the next stage in the recovery circuit. In a series of large, mechanically agitated tanks, a 30% cyanide solution, lime, oxygen and lead nitrate are added to the slurry, or “pulp”, as it is usually called, to leach the gold out.
The slurry moves into the adsorption tanks which contain granules of activated carbon to adsorb the gold which has leached into solution. Carbon for use in gold processing is usually made from coconut husks and is activated by heating the husks to about 900ºC without air and in the presence of steam to drive off chemicals.
Adsorption is the technical term used to describe the attraction of chemical compounds to the surface of the carbon, known as “loading”. The loaded carbon is removed from the circuit and the gold cyanide is stripped from it. When the process is complete the pulp has had almost all of its gold removed. The substance which remains, known as the “tail”, is pumped onto a tailings storage facility where the slurry particles settle and the water is recycled.
Download KCGM Water and Tailings Information Sheet
From Solution to Gold Bars
All the gold is then removed from the carbon by the elution process which leaves a concentrated solution of gold. In this process the loaded carbon is sealed in a tall metal pressure vessel called a column. It is washed first with hydrochloric acid followed by sodium cyanide and a caustic soda solution and then finally a hot water rinse under pressure.
The resultant concentrated or “pregnant” solution is subjected to electrolysis to remove the gold. In this process, known as electrowinning, stainless steel wool is used as one electrode and stainless steel plates as the other. An electric current is passed through the solution and the gold is electrochemically plated onto the stainless steel wool.
Two electrowinning circuits are used at the Fimiston Plant for the recovery of gold from the high-grade solutions produced by the elution circuits. One circuit is dedicated to electrowinning the gold from the concentrate leach circuits, and the other electrowins the gold from the flotation tails leach circuit.
When all the gold has been recovered from the solution, the “barren” solution is pumped back into the leach circuit. The gold sludge is removed periodically from the cathodes by high pressure water spraying. The collected sludge is filtered and dried. The “cake” that remains is mixed with fluxes and heated in a ceramic crucible in a gas-fired furnace.
When molten, the fluxes combine chemically with the impurities in the mixture to form a slag of impurities which settles on top of the gold. When the molten mixture has reached more than 1,000°C the metal and slag are tipped into an ingot mould. The slag hardens into glass, leaving the gold ingot of 300 to 500 ounces in the bottom of the mould.
These gold bars, known as doré, are about 60% to 80% pure gold, with the rest consisting mainly of silver. Each bar is stamped with its own unique number, then the bars are transported to the Australian Gold Refinery at the Perth Mint where they are further refined to 99.9% pure gold.