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Tuesday, February 14, 2012

Recovering Gold with Cyanide


A heap leach pile for extracting gold with cyanide in Elko, Nevada
Photo by USGS


How do you like that one of my relatives invented the MacArthur-Forrest process for recovering gold in the process that is more commonly known as the cyanide process.  This is one of the many techniques used to recover gold from low grade ore that works by dissolving gold and other metals is a solution of water and cyanide that causes gold to become water soluble.  Chemically this is called a water soluble coordination complex that is the most commonly used technique to recover gold.

Fifteen percent of the cyanide produced is used for gold recovery the balance is used in other industrial processes for such things as adhesives, plastics and insecticides.  Although both sodium and potassium cyanide are used the most commonly used is calcium cyanide.  Because of its extreme toxicity cyanide has been outlawed in many countries, but efforts are being made to replace its use in mining by less toxic chemicals.  One of these is sodium ferricynide that has been used under the name “hypo” in photography to dissolve silver in photographic materials.  Hypo also dissolves gold, but not as rapidly as cyanide.

Gold mining uses cyanide for both the MacArthur-Forrest Process for which it was originally developed and for the later heap leaching process that was developed for recovering gold from disseminated ore like that found in the Carlin Trend in Nevada.  In the MacArthur process the gold ore is first ground into a powder in a ball mill, a large tumbling barrel where the grinding action is provided by large steel balls.  The ore is then transferred to a vat where it is mixed with cyanide solution and agitated with compressed air.

The other place where cyanide is used is the Heap Leach Process here the ore is ground to ¼ minus and heaped up in huge piles usually covering several acres.  A solution of cyanide is sprinkled onto the pile with sprinklers where it seeps down through the heap dissolving the gold in the process.  This process goes on for a considerable time with the cyanide solution being recirculated several times until all the gold is dissolved.

In both processes the gold is precipitated from the pregnant solution with powdered zinc although in some cases it is recovered using activated charcoal filters.  The powder is black because it contains more then gold that is cast into a Dore bar and sent on for further refining to remove the impurities.

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