1. Mining – Open Pit and Underground — 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 in the open pit or to the ore
stockpiles from underground.
2. Crushing — The primary crushers located at the open pit mine
site, receive ore and waste at separate times. They break the
larger rocks down to a size suitable for transport on the conveyor
belt.
3. Transport — A rubber belted conveyor transports the ore and
waste rock from the open pit, approximately two kilometres, via a
tunnel through Union Hill, to the mill and waste rock embankments.
Large electromagnets remove any steel debris excavated from the old
Martha Mine workings.
4. Grinding and Sizing — Ore from both open pit and underground is
stockpiled separately at the mill before being fed into a semi
autogenous grinding (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%
< microns.
5. Leaching and 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.
6. Elution and 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 and silver is won onto
stainless steel cathodes.
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.
8. Water Treatment — Some water from dewatering the mine, from the
embankment under-drains and decant water 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.
9. Tailings Disposal — Waste rock from the open pit mine is used to
build the embankment structures. The embankments retain 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.
