How Is Gold Produced?

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From the Archives : Originally published July 10th, 2015
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Category : Gold University

PRECIOUS METAL SERIES - 7

The days when miners could dig or pan for nuggets of gold are largely gone. Today, gold is generally extracted from the Earth's crust as microscopic particles. 

Estimates put the amount of gold in the Earth's crust at just 11 parts per billion!To recover a single ounce of gold, many tonnes of material must therefore be blasted and processed.

Gold mines sell unrefined gold in the form of doré bars. Australian doré bars are usually composed of between 70-80% gold and 10-15% silver. The lion's share of these rough bars are processed into fine gold at The Perth Mint, which operates the largest LBMA-accredited gold refinery in the Southern Hemisphere.

Each doré bar first goes through a chlorine refining process, also known as the Miller Process. Originally conceived by Francis Bowyer Miller in Sydney during the 1860s, the Miller Process involves bubbling chlorine gas through molten doré gold so that silver (and most other metals) react with the chlorine to form silver chloride as slag on the top. The resulting gold is 99.5% pure and typically it's cast into bars weighing about 400oz for use in wholesale markets.

The Wohlwill process is used to increase purity further. A casting of 99.5% pure gold is lowered into a bath of hydrochloric acid and then has an electric current passed through it. Acting as an anode in this electrolytic refining process, the casting dissolves and then deposits on a cathode with a purity of 99.99%.

The resulting cathodes are melted, granulated and then the granules are used to measure out exact weights of gold for casting into bar sizes from 1oz up to 50oz for retail investors.

24hGold - How Is Gold Produced...

 

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