Precious Metal Refining Process

 

The diagram below 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.

Gold Refining

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

Dore 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 dore 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 dore 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 dore bar then becomes the property of the refiner.

The dore bar first goes through a chlorine refining process, also known as the Miller process. This involves bubbling chlorine gas through the molten dore metal in which the 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 a 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 purity of 99.99%.

The resulting cathodes are melted, granulated and the then granules are 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 dore bars the Perth Mint processes are composed of around 90% silver and 10% gold. As with gold dore, the silver dore 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).