The Cyanide Process is a mining technique for extracting gold from low-grade ore via the use of cyanide compounds. Due to the highly toxic nature of cyanide, the process is controversial.

Table of contents
1 History
2 The reaction
3 The process and its uses
4 Controversy
5 External links and references


The process traces to 1887, though the original discovery was made in 1783 by the chemist Carl Wilhelm Scheele, the original discover of cyanide.

The modern cyanide process was developed in Glasgow, Scotland in 1887 by J. S. MacArthur and others. It is at present the most important and most-often used process for extracting gold from ore, particularly low-grade ores.

The reaction

The reaction is called the Elsner Reaction; it's stoichiometry is:

4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH

The process and its uses

Notably, mines formerly thought played out may be revived via the cyanide process.

The ore is ground in a revolving cylinder with steel balls to a fine powder, and may be further concentrated by flotation, be there certain impurities. It is combined with a dilute solution of sodium or potassium cyanide while bubbling air through it, forming a solution called "slime." The cyanide releases the gold 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 the gold is displaced by adding zinc dust, which precipitates the gold.

Silver 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.


The process is controversial, due to the highly toxic 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.

External links and references