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The process traces to 1887, though the original discovery was made in 1783 by the chemist Carl Wilhelm Scheele, the discoverer of cyanide.
The modern cyanide process was developed in Glasgow, Scotland in 1887 by J. S. MacArthur and others.
The chemical reaction is called the Elsner Reaction; its stoichiometry is:
The ore is ground to a fine powder in a revolving cylinder with steel balls, and may be further concentrated by flotation, given the presence of certain impurities. It is combined with a dilute solution of sodium cyanide or potassium cyanidePotassium cyanide is the salt of potassium and hydrogen cyanide, a colorless compound with smell of almonds, similar in appearance to sugar and highly soluble in water. Despite being highly toxic, it is one of the few substances that form soluble compound while bubbling air through it, forming a solution called "slime".
The negatively charged cyanide anions release the gold cations from the ore as a metal complex. The gold oxidizes to form the soluble aurocyanide metallic complex, NaAu(CN)2.
The solution is separated from the ore by methods such as filtration, and then the gold is displaced by adding zincZinc is a chemical element in the periodic table that has the symbol Zn and atomic number 30. Notable characteristics Zinc is a moderately reactive metal that will combine with oxygen and other non-metals, and will react with dilute acids to release hydro dust, which precipitates the gold: zinc has a higher affinity for the cyanide ion than gold.
SilverThis page is about Silver the element. For the color, see Silver Silver is a chemical element in the periodic table that has the symbol Ag ( L. Argentum and atomic number 47. A soft white lustrous transition metal, silver has the highest electrical and th may also precipitate, and unreacted zinc. The precipitate is further refined, e.g., by smelting, to remove the zinc and by treating with nitric acid to dissolve the silver. Mines formerly thought played out may be revived via the cyanide process.The process is controversial, due to the highly toxic nature of cyanide; see also Summitville mine, the worst environmental mining disaster in United States history, which rendered 17 miles of a Colorado river devoid of life. The American state of Montana and several countries have banned cyanide mining.