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Gold And Silver Refining Explained

Topics [ gold refining gold refinery ]

EDUCATION

The Perth Mint has refined gold since its establishment in 1899. Today, we refine most of Australia’s gold production, gold from surrounding regions, silver, and scrap gold jewellery mainly from Asian countries. Typically, we process between 300 and 400 tonnes of precious metals each year, which makes The Perth Mint refinery one of the largest in the world.

Ever wondered how gold and silver refining is carried out?

Gold Refining

Gold mines process ore using various techniques to produce an alloy composed primarily of gold and silver, which is called a doré bar. The composition of doré can vary significantly between mines, but generally the gold doré bars The Perth Mint processes are composed of between 70-80% gold and 10-15% silver.

Doré bars are first weighed and melted to ensure the metal is homogenous, that is, that there are no pockets of high or low purity within the bar. A sample is taken from this melted doré and assayed to determine the exact amount of gold and silver present.

The miner then receives an outturn, which is a statement indicating the weight of the doré bar, the percentage of gold and silver in the bar, and from these two, a calculated amount of pure gold and silver. Miners will then either sell this pure gold and silver for cash, or request a loco swap. The doré bar then becomes the property of the refiner.

The doré bar first goes through a chlorine refining process, also known as the Miller process – originally developed at the Sydney Mint by Francis Bowyer Miller (see Australian Dictionary of Biography). It involves bubbling chlorine gas through the molten doré metal. Silver (and most other metals) react with the chlorine to form silver chloride as a slag on top. This process produces gold to a purity of 99.5%, which is usually cast directly into odd weight 400oz bars used in the wholesale markets.

The key output of the chlorine refining process is silver chloride, which enters a silver leaching process to remove the base metals. The silver chloride is then reduced to metallic silver and is then refined by electrolysis.

If there is demand in the market for higher purity gold, then the 99.5% pure gold from the chlorine refining process is cast into anodes to be used in the electrolytic refining process, also known as the Wohlwill process. The anodes are placed into a bath of hydrochloric acid and an electric current is passed through it, which causes the gold to dissolve and then deposit on a cathode at a purity of 99.99%.

The resulting cathodes are melted, granulated and the granules are then used to measure out exact weights of gold for casting into bar sizes from kilo (32.15 ounces) down to half ounce.

Silver Refining

The composition of silver doré bars The Perth Mint processes are composed of around 90% silver and 10% gold. As with gold doré, the silver doré is weighed, melted, assayed and outturned.

The electrolytic silver refining process is similar to gold, except that the silver anodes are dissolved in a bath of nitric acid. The resulting cathodes are 99.9% pure silver. As with small gold bar production, these cathodes are melted, granulated and cast into retail sized bars (most common being 100oz or kilo).

Gold and silver refining process

The diagram gives an overview of the key processes involved in the refining of gold and silver. Please note that this is highly simplified and does not detail the many sub-processes involved within each key process, the chemical inputs involved in each process, as well how the waste outputs from each process are handled.

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