How is gold mined? Gold mining process

1. Mining - Open pit and underground - To determine ore in waste rock, samples are taken and assayed. The assay results are used to delineate separately mined ore and waste rock areas. Some harder areas require blasting to loosen the rock before the hydraulic excavator digs. Dump trucks transport rock to primary crushers in open pit mines or from underground to ore piles.

2. Crushing - Primary crushers located in open pits receive ore and waste separately. They break down larger rocks into sizes suitable for transport on conveyor belts.

3. Transportation - Rubber belt conveyors transport ore and waste rock from the approximately two-kilometer open pit through a tunnel through Union Hill to the mill and waste rock embankment. Large electromagnets can scavenge any shards of steel excavated from the old Martha mine.

4. Grinding and grading - Ore from the open pit and underground is stored in separate mills and sent to a semi-autogenous (SAG) mill with lime, water and steel balls. The larger particles from this mill are returned to the SAG mill for more grinding. The finer particles are ground more in the ball mill and size sorted 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 a large steel leaching tank where gold and silver are dissolved. After this leaching process, the slurry is passed through six adsorption tanks containing carbon particles, adsorbing gold and silver. This process removes 93% gold and 70% silver.

6. Elution and Electrodeposition - The loaded carbon is sent to the elution column where the gold bars are washed away. Barren carbon is recycled. The washing solution (pregnancy electrolyte) is passed through an electrolytic deposition cell where gold and silver are extracted onto stainless steel cathodes.

7. Gold Bar Production - The loaded cathode is rinsed to produce a gold and silver containing sludge, which is dried, mixed with flux and placed in a furnace. After a few hours, the molten material is poured into a series of molds to produce Dore Gold bars.

8. Water Treatment – Part of the water from mine dewatering, under-embankment drainage and tailings pond decanting is recycled for use in the grinding circuit. The excess water is pumped to a water treatment plant and meets required standards before being discharged into the Ohinemuri River.

9. Tailings Disposal - Waste rock from open pit mines is used to build embankment structures. The levee retains the tailings slurry in a pond where solids settle and compact. The water is decanted and used in processing plants or treated before discharge.