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Processing

Once the ore rocks have been mined they are transported from the mine to the processing plant. Here, they undergo a series of physical and chemical treatments to extract the valuable minerals.

Extraction processes vary according to the type of mineral being processed and the way in which it is caught up with other minerals in the ore rock. This relates to the chemistry of the minerals and the ore rocks that are to be processed.

Here is a general overview of how gold can be extracted from ore rocks:

Crushing

Large ore rocks are broken down to smaller pieces (usually < 20cm) by a machine called a jaw crusher. These smaller rocks are then crushed in two "cone crushers" until they are only about 2cm each.

Screening

These rocks are passed through a vibrating screen to ensure that only smallest ones are transported by conveyor belts to the grinding mills. Large rocks are sent back to the crushers to be broken down further.

Grinding

At the mill, the fine ore rocks are mixed with water to make "slurry". This sloppy mix is passed through a cyclone to separate the different materials.

The finer, lighter particles are screened and sent on to the next stage of processing. The heavier, coarser ones go back to the mill where they are ground smaller.

Chemical Processing

The slurry containing the finest particles is treated with cyanide to dissolve the gold in it. Lime is added to make the slurry alkaline enough (ie. high pH) to allow the gold to dissolve.

Very little gas escapes into the environment during this process because the cyanide is consumed in this reaction.

Carbon in Leach

At the "carbon in leach plant" carbon granules are added to the slurry to soak up the gold as it passes through several large tanks.

Each tank is pumped full of oxygen to encourage the gold to stick to the highly porous carbon granules. By the time it reaches the last tank, most or all of the gold should have migrated from the solution to within the carbon granules.

Elution

Gold is stripped out of the carbon in a pressurised "carbon elution chamber". Inside this chamber water is removed and caustic soda and cyanide are added.

The carbon is heated to very high temperatures so it can be reused back at the carbon leach plant.

Meanwhile, the remaining gold is plated or adheres to stainless steel electrodes in the final chamber.

The gold is washed off the electrodes, heated in a furnace to > 1 000° C (ie. smelted) and poured into moulds to make gold bars.

Investigate the resource From Exploration to Rehabilitation - the Life of a Gold Mine for further information about processing gold.

Did you know?

Gold is not just used for jewellery. It is also widely used to make electrical circuits, dental fillings, pacemakers, glass tinting, paints, glues and even anti-lock braking systems in cars.