New Technologies In Refining Gold
The process of refining gold is aimed at obtaining the highest purity gold, from a variety of sources including primary precious metals producers, industry, recyclers banks, dealers, mints and financial institutions.
It is necessary, because the gold obtained by ore dressing processes is often contaminated with silver and trace impurities. It may be refined, another name for purification, by several technologically advanced methods.
Some of the largest USA - based refiners have a production capacity of 10 million plus troy ounces per year. The standard process of refining gold had the initial target of 9999 fine purity in the mid-1960's. In 1982, some of the best American refineries were able to produce 9999 bullion coins. Currently, the most advanced technological processes deliver a granulation gold that is 99999 fine.
The refining stages are as follows:
- Feed material is refined to 995 fine through the Miller chlorination process.
- The gold is melted and cast into anodes which are purified to 9999 fine, utilizing the Wohlwill electrolytic process.
These processes are explained in details, below.
Electrolysis is the most common method at all of the United States mints. Anodes of crude gold from the mines and cathodes made of pure gold are suspended in a chemical bath made of gold chloride and hydrochloric acid. When electric current flows through the bath (electrolyte), gold dissolves from the anodes and deposits on the cathodes, with impurities remaining "stuck" on the anodes. Even though it is a slow process, the electrolysis process yields gold that is nearly pure; almost 999.9 fine.
Later, in the chlorination process, chlorine gas is passed through a specially designed furnace, filled with molten crude gold. The chlorine gases combine with impurities, and they rise to the top. The reaction continues till all impurities are removed. Then the process has to be terminated, as the chlorine begins to react with pure gold.
In another popular refining process, gold is treated with acids that react only with impurities but not with gold itself. Sulphuric, hydrochloric and nitric acids are used.
At the last stage, refined gold is melted and cast into bricks or bars. Typical weight is about 28 pounds.
Cupellation is the technique forming the first part of the fire assay process. In short, lead is added to the gold undergoing the refining process. The mixture is then heated in air to between 1,832°F - 2,011°F (1,000°C - 1,100°C). At this temperature, the gold-containing metal dissolves in the lead. As a consequence of the high temperature, all base metals including the lead, are oxidized and form a lead oxide slag.
This method is recommended only for very small quantities (up to 10 grams). The reason being the emission of large quantities of poisonous lead oxide fumes. From the environmental perspective, larger quantities should be refined using more eco-friendly technologies, like inquartation and parting. In this process, the metal undergoing the process of refination is melted with additional silver or copper to produce an alloy containing about 25 percent or less gold. The dilution enables the base metals to be dissolved in nitric acid. The alloy is then grained and attacked with nitric acid to dissolve out all the base metals and silver. This leaves behind a gold sludge which is washed, cleaned and drained.
Any other precious metals, like platinum and palladium will also be dissolved.
The inquartation and parting process is capable of producing gold of up to 99.99 percent pure. The process is ideal for treatment of low karat gold scrap, since the additions of copper or silver are already present in the material being refined.
These refining methods are limited to a highly specialized industrial environment. Apart from the above mentioned large scale systems, refining gold can be carried out even at home. There are several chemical kits available on the market that require minimum equipment, usually only a battery charger supplying DC current and some vessels.
The process is claimed to be entirely safe and easy to run. Search the online sources for detailed information about suppliers. 
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