Refining generally simply means the process of purification, from a certain percentage of desirable material to a higher percentage. Metal refining is made up of two types: primary and secondary. Primary refining is the refining of gold ore from the ground; the melting of scrap down to a fine gold product is considered secondary refining. There are many techniques from which gold can be extracted, in both primary and secondary refining.
The Miller Process from Scrap
The Miller process is one popular method to refine gold to a high degree of purity, from scrap metal with appreciable gold content. First, the scrap is melted down and granulated, to form cornflake shaped bits. Workers fill crucibles with these cornflake scrap pieces, which is then heated until melting point. Once the metals become molten, chlorine is bubbled through the mixture. This chlorine gas purifies the ingredients, because virtually all other precious metals combine with chlorine before gold does.
Purity and the Wohlwill Process
The Wohlwill process is another common process that can be used to purify metal ore with gold content into fine gold. Gold percentages on the Miller process are sometimes only 98 to 99 per cent pure in some industry locations, so occasionally industrial refining locations use the Miller process followed by the Wohlwill process. The Wohlwill process obtains the highest degree of purity--out to 99.999 per cent gold ingots.
Mechanics of the Wohlwill Process
The Wohlwill process procedure is a form of electrolysis. The gold, considered the anode, is suspended in chloroauric acid in an electrolytic system. Current is applied, and the electricity forces pure gold to be plated to the cathode of pure 24k gold sheeting. The anode dissolves completely under the influence of acid and current, and the cathode is removed and melted or grained. The Wohlwill process delivers purer results than the Miller process, but requires a large gold inventory (chloroauric acid), longer turnaround time, and because of its requirement of complex electrolysis systems, is more expensive initially for companies to install.