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Thursday, February 9, 2012

The Miller Process for Refining Gold


Gold Bars   Agnico-Eagle


Ever wonder how most gold is refined?  No for the most part they don’t use the Wolhwill Process rather they use the Miller Process.  Unlike the Wolhwill Process described earlier in this series the Miller Process doesn’t use electricity but rather uses a stream of chlorine gas that is passed through a crucible of molten gold that produces gold that is 99.95 Fine.  It works to remove all the impurities because gold is least affected by chlorine so all the other impurities are removed as chloride slag floating on the surface of the molten gold where it is skimmed off.

In this process the dore bar is melted in an induction furnace where the chlorine gas is introduced into the molten metal by a rotating lance.  The lance is controlled in a very precise manner in a way there is no break through of the chlorine gas to the surface of the molten metal.  This removes what are termed “By-metals” like silver, copper, nickel along with any other metals.  The process also removes amphoteric elements like sulfur, selenium, tellurium, arsenic and bismuth.  These elements are all removed from the molten metal as chloride slag that floats to the surface of the charge where they are skimmed off for further treatment.

The process takes from 2 to 4 hours to complete with the chloride fumes being removed and filtered by a two stage wet filter or a two stage dry scrubbing system that maximizes the recovery of the chloride as well as the cleaning of the exhaust air from the process.  In the process slag is poured off the top of the melt into a crucible and allowed to slowly cool so any gold that has been caught up with the chlorides will settle to the bottom where it can be recovered as a button of gold when the crucible has cooled completely; it is then added to the remaining molten gold.  In this process the particles of gold remaining in the crucible are carefully removed by hand as well as the separation molds when they have cooled.

Once the gold has cooled it is poured through a ceramic filter directly into the granulation equipment or anode molds if the Wolhwill process is used to further refine the gold.

The main advantages of this process are its quick turnaround time, size of the charge that can be from 20 kg to as large as you like.  The rotating lances are made from temperature resisting ceramics for long life and evenly introduce chlorine to the melt.  The use of modern induction furnaces keep the consumption low using far less kilowatts then coil type furnaces for the same size of melted product.

In this process the gold produced is 99.95 Fine that is good enough for most commercial purposes.  Most of the metals remaining in the gold are mostly silver and copper that if desired can be removed using the Wolhwill Process producing gold bullion that is 99.999 Fine.   

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