To define the ore from the waste rock, samples are taken and assayed. Assay results are used to mark out areas of ore and waste rock, which are mined separately. Some of the harder areas require blasting to loosen the rock prior to excavation by hydraulic diggers. Dump trucks haul the rock to the primary crushers. For more detail on mining click here.
2. CRUSHING
The primary crushers - two Stamlers and a jaw crusher - located at the mine site, receive ore and waste at separate times. They break the larger rocks down to a size suitable for transport on the conveyor. For more detail on crushing click here.
3. TRANSPORT
A rubber belted conveyor transports the ore and waste rock approximately two kilometres, via a tunnel through Union Hill, to the mill and waste disposal area. Large electromagnets remove any steel debris excavated from the old workings. For more detail on conveying click here.
4. GRINDING AND SIZING
Ore is stockpiled at the mill before being fed into a S.A.G. mill with lime, water and steel balls. The larger particles from this mill are returned to the S.A.G. mill for more grinding. The finer particles receive more grinding in a ball mill, and are size classified to give a final product of 80% <70 microns. For more detail on grinding and sizing click here.
5. LEACHING & ADSORPTION
A slurry of ground ore, water and a weak cyanide solution is fed into large steel leach tanks where the gold and silver are dissolved. Following this leaching process the slurry passes through six adsorption tanks containing carbon granules which adsorb the gold and silver. This process removes 93% of the gold and 70% of the silver. For more detail on leaching and adsorption click here.
6. ELUTION & ELECTROWINNING
The loaded carbon is fed into an elution column where the bullion is washed off. The barren carbon is recycled. The wash solution—pregnant electrolyte—is passed through electrowinning cells where gold & silver is won onto stainless steel cathodes. For more detail on elution and electrowinning click here.
7. BULLION PRODUCTION
The loaded cathodes are rinsed to yield a gold and silver bearing sludge which is dried, mixed with fluxes and put into the furnace. After several hours the molten material is poured into a cascade of moulds producing bars of doré bullion. For more detail on bullion production click here.
8. WATER TREATMENT
Some water from dewatering the mine, from the embankment underdrains and decantation from the tailings pond is recycled for use in the grinding circuit. Excess water is pumped to the Water Treatment Plant and treated to the required standards before discharge into the Ohinemuri River. For more detail on water treatment click here.
9. TAILINGS DISPOSAL
Waste rock from the mine is used to build the embankment structure. The embankment retains the tailings slurry in a pond where solids settle and compact. Water is decanted off and used in the process plant or treated before it is discharged. For more detail on tailings disposal click here.
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