The Super Pit
Wednesday 15 October 2014
Australia 2014 - (16) The Super Pit
Kalgoorlies Super Pit has to be one of the modern wonders of the world. Of course it is a huge scar on the face of the earth and uses a massive amount of energy to power the mining operations, but if you try and ignore that for a moment and just look at the engineering involved, its quite spectacular.
Back at the start of the nineteenth century when gold was first discovered in Kalgoorlie, men with pick axes and shovels would dig shafts by hand to extract the precious metal. By the 1980s Kalgoorlie was riddled with thousands of shafts going from the surface to 1.5 km underground and most of the easily accessible gold had been extracted.
Alan Bond, famous businessman and entrepreneur, saw an opportunity to get at the less accessible ore between these shafts by creating one big pit from which ore could be extracted at far less cost and started buying up all the individual land leases. Bonds company failed to complete the takeover but in 1989 all the leases were combined and Kalgoorlie Consolidated Gold Mines (KCGM) was formed to operate a gigantic, ever-decreasing pit in which massive equipment could tear out the old underground workings and access the unmined gold in between.
Currently the Super Pit is 4 km long, 2 km wide and 600 m deep. They are in the process of increasing the depth to 1.5 km, the level of the original hand dug mines, by expanding the width of the mine. You cant just dig a hole with straight walls as this will collapse into itself, so they have to go down in steps. Its taken 3 years to get it to its current width and will take another 4 years to take it down to the target depth of 1.5 km.
In terms of gold recovery, geologists determine where the gold is located and explosives are set to blow this out. Huge dumper trucks are then filled with the rubble formed. The high grade rubble goes to the ore crusher, the low grade to the stick picking areas where the old mine shaft workings are removed and recycled before it is taken to the waste rock dump.
The ore is deposited into one of two stock piles where it is transferred into huge steel barrels filled with steel balls that crush it into a powder. This is mixed with water to form a slurry. This is then pumped into flotation cells where air is added and the gold floats to the top where it is removed to storage tanks and dried. When it is dry it is then sent for roasting. This converts the concentrate into calcine which is then leached and absorbed into carbon. The gold is then removed from the carbon by an elution process which leaves a concentrated solution of gold. Electrolysis is then used to plate the liquid gold onto steel cathodes. Once all the gold is plated, high pressure water jets are used to remove the gold. The gold is collected and dried in large ovens. The dried cake is then put in crucibles and put in a furnace of over 1000 degrees C to melt the gold before it is finally poured into gold bullion.
About four gold bars are produced each day, each fetching about AU$690,000, thats AU$2.5 - $3 million a day. Considering it takes 200,000 tonnes of rock to be milled every day to produce this, you have to wonder, is it really worth it?