The gold in Kalgoorlie ore occurs in several different forms, each requiring its own special treatment route to enable the precious metal to be recovered and refined.
Ore delivered from the Superpit is processed through two treatment plants at KCGM called the Fimiston Plant and Gidji Roaster.
The treatment process involves many steps:
1. Crushing (Fimiston)
2. Grinding (Fimiston)
3. Froth flotation (Fimiston)
4. Roasting (Gidji)
5. Carbon In Leach - CIL (Fimiston and Gidji)
6. Gold recovery (Fimiston)
At Fimiston there are two circuits that treat different ore types. The sulphide ore from the Super Pit is the most difficult to treat and this is the process we describe here.
Crushing involves tipping ore from the Superpit into a large crushing machine. This machine is just a large chamber that contains a swinging weight.
Usually large dump trucks tip the ore directly into the chamber. The swinging weight compresses and fractures any large or small rocks into smaller rocks. 50% of the rocks from this process are smaller than your fist in size.
The smaller rocks are dropped onto a large travelling rubber belt. This belt is called a conveyor. Conveyor systems are used all over the world in the mining industry. Conveyors are good at deliveries rocks quickly to large stockpiles. The stockpile at Fimiston contains 105 000 tonnes of rocks ready for treatment.
Grinding is the process that breaks fist size rocks to very fine particle sizes. This is done in large rotating mills that look like a huge steel drum. These mills are called SAG mills or ball mills. These mills contain various amounts of small or large steel balls inside that are rotated to cause collisions with the rock fed to the mill. The balls break the rock fragments into extremely fine rock particles. Water is added to this process to ensure that all the fine particles are flushed from the mill to the next process.
The size of rock particles after milling is less than 1/5 of one millimetre. To break rock to this very fine size involves using a lot of electrical power. The Fimiston SAG mill alone uses 12 megawatts of power (enough to operate all the homes in 2 small towns).
All rock particles formed during grinding now travel with water added to the mills. This mixture is called a slurry. The slurry from milling is transported around the treatment plant by special pumps.
This process uses chemicals to collect both Gold and Pyrite (fools gold) into a concentrated form.
Special chemicals are added to the slurry from grinding. These attach themselves to certain minerals in the slurry. All the slurry is pumped to large tanks called flotation cells. Air is added to the bottom of flotation cells and rises through the slurry. The chemicals added earlier are hydrophobic (don’t like being in water) so they attach themselves to the passing air bubbles and float to the top of the cell. When the air bubbles reach the top of the cell they form a froth much like the top of a cappuccino. This froth contains nearly all the gold and the fool’s gold.
When the froth becomes too full it spills over the edge of the flotation cell and is collected into a launder. The launder contents are discharged and pumped into storage tanks. The concentrate grade contained in these storage tanks is 2 gold ounces per tonne of ore treated.
The contents of these tanks are eventually passed over a vacuum filter and the water is removed for reuse in the process. The high-grade solids that are left (filtered concentrates) are virtually dry so they can be conveyed to a stockpile then loaded into trucks and sent to the Gidji roaster site for further treatment.
All the material that did not float is called tailings. Tailings is the word used to describe slurry with low gold content.
The flotation tailings are passed through another process called CIL before being sent to a large storage dam.
All the concentrates delivered to Gidji from Fimiston are remixed with water to make a new slurry. This is pumped into 2 large roasters.
Roasting is the process used to convert the concentrates from flotation into a red calcine. It does this by heating the concentrate slurry to over 600oC. The water from the slurry and sulphur dioxide gas from the concentrates rises through a large stack and is emitted to the atmosphere at Gidji. This stack is 178m tall.
The material roasted undergoes a change in colour from black to red as the sulphur dioxide is removed. The red colour remaining is called calcine.
Calcine is leached and adsorbed onto carbon in a process called CIL.
All the flotation tailings and the calcine are processed by CIL at Fimiston and Gidji respectively. CIL means carbon in leach.
This process involves addition of chemicals to dissolve the gold then collection of the gold into a super rich grade on carbon. Carbon is used as it has a large surface area and can adsorb large amounts of gold from the slurry. Carbon has a very large surface area (1 gram of carbon has the same surface area as a tennis court), and one of the best basic ingredients for carbon is coconut shells!. Carbon is formed in a special kiln operated at over 1000oC in Sri Lanka after breaking the coconut shells into little fragments.
The carbon used in the CIL process can be reused many times by reactivating it in a kiln operated at Fimiston.
All the gold captured on the carbon is then removed from the carbon by the elution process.
The first step of gold recovery from carbon is called elution.
Elution involves feeding the carbon into a special pressure vessel called a column. The carbon is pumped to the column and the water is drained. A mixture of caustic soda and cyanide is pumped through the column to contact the carbon. This mixture extracts the gold from the carbon to make a concentrated solution of gold. This solution is transported to tanks located at the gold room
The second recovery phase is called electrowinning. Electrowinning takes place in a secure building called the gold room.
Electrowinning uses the science of electrolysis to plate the gold from solution onto steel cathodes. Electrolysis uses electrical current to ensure that steel is dissolved and gold is removed from solution. Electrolysis experiments are often completed at schools during science classes.
Once all the gold has been plated on the cathodes the cathodes are removed and washed with a water pressure cleaner. The gold is collected and water is removed in a filter press. The remainder is dried in large ovens.
The dried cake removed from these ovens are put in large crucibles and placed in a furnace to melt the gold. These furnaces operate at over 1000oC.
The molten material is later poured into ingots to make gold bars called bullion.
The gold grade of KCGM bullion varies from 65 – 80% gold. The remainder of the bullion is mostly silver.
All the gold bars are stamped and then collected by security guards. The guards transport the KCGM gold to the Australian Gold Refinery in Perth. The refinery increases the bar grade to above 99.5% gold.