Gold and Silver Refining
The gold and silver used in jewellery is rarely pure gold or silver as they are generally too soft for everyday wear. To improve the strength of these metals, they are alloyed with other metals making them much more suitable for use in jewellery.
Silver is alloyed with (7.5%) copper to produce sterling silver.
Gold is alloyed with some or all of these metals; silver, copper, zinc, rhodium and palladium, to create an alloy with the colour and purity required.
To create these alloys, pure gold or silver is required for mixing with the other metals in the finished alloy. This is how they are refined to close to 100% purity.
The output from a gold mine is typically an alloy of silver and gold containing roughly 70-80% gold (Au) and 10-15% silver (Ag), but this will vary between mines. This alloy is supplied to refiners in the form of ‘dore bars’ which require processing to separate the silver, gold and other materials.
Each dore bar is weighed and recorded before being melted and homogenised . This ensures that samples taken anywhere in the bar will yield the same results. As sample is taken at this time and assayed to establish an accurate measurement of the amount of gold and silver present.
The mine is provided with a document called an ‘outturn’ for each dore initially supplied. This outturn lists the weight and the percentage of silver and gold in the dore. This information allows a calculation of the total weight of pure gold and pure silver present for which the mine will be paid. The dore becomes the property of the refinery to be processed into bullion of 99.5% purity or 99.99% purity to suit the final application.
To create a gold bar of 99.5% purity, the Miller process is used. Chlorine gas is passed in bubbles through the molten dore where the silver and most of the other metals react. This process results in silver chloride being produced, occurring as a slag on top of the molten gold. The gold from this process is normally sold in wholesale markets, cast into 400 ounce (about 11kg) bars.
The slag created by the Miller process then goes through a further silver leaching step which removes any base metals, leaving mostly silver chloride (AgCl). After being reduced to metallic silver, electrolysis is employed to refine the silver to 99.9% purity. This is then granulated and cast into small silver bars, or the granulation process is skipped when casting 1000 ounce bars.
When a higher purity than 99.5% gold is required, additional processing is required. The bars created by the Miller process are cast into anodes and the Wohlwill process begins. An acid bath containing hydrochloric acid has the gold anode and a cathode immersed into the solution. Electricity is passed through the bath which causes gold to leave the anode and deposit onto the cathode. This creates a cathode covered in gold at a purity of 99.99%
The cathodes are then melted and formed into small granules. Gold bars ranging from one half an ounce up to one kilogram are created from an accurate weighing of these granules.
Dore bars originating from a silver mine usually contain around 90% silver and 10% gold. The initial process of weighing, melting, assaying and issuing an outturn is much the same as it is for gold refining.
Refining silver using electrolysis passes electrical current through
a bath of nitric acid and a silver anode to deposit 99.9% pure silver on
the cathode. The cathodes are then melted and granulated before being
cast into 1 kilogram silver bars.