Cyanidation

(also cyanide process), in hydrometallurgy, a method of extracting metals, chiefly gold and silver, from ores and concentrates by dissolving them selectively in solutions of alkali-metal cyanides. Selective dissolution is achieved with a weak concentration of the solution (0.03 to 0.3 percent cyanide), in order that there be little interaction with other constituents in the ore. Gold and silver dissolve in a cyanide solution when the water contains dissolved oxygen; increasing the solution’s concentration intensifies the process. To avoid decomposition of the cyanides, 0.005 to 0.02 percent of a protective alkali, such as lime or caustic soda, is added to the solution.

The theory of cyanidation processes is based on the laws of the kinetics of dissolution at inhomogeneous surfaces (with cathode depolarization by oxygen) and the kinetics of the diffusion of metals (with the simultaneous diffusion of the cyanide and oxygen). Of great importance are the regularities of the interaction of reagents with minerals that take into account the minerals’ composition and structure.

Two cyanidation methods are used in industry. The first involves the percolation of solutions through a layer of finely crushed ore or sand and the agitation of the pulp along with intensive aeration. Gold and silver are often precipitated from solution with zinc dust. The second method, which is currently being developed, is sorption cyanidation, which combines the processes of leaching out and recovering the dissolved gold and silver from a pulp by sorption with anionites or activated charcoals. This method is effective for processing ore slimes that are difficult to filter.

The recovery rate of gold by the cyanidation of pulp ranges between 90 and 96 percent, using 0.25 to 3 kg of sodium cyanide per ton and 0.5 to 5 kg of protective alkali per ton.

The dissolution of gold and silver in cyanide solutions was first investigated by P. R. Bagration (1843) and was further studied by F. Eisner (Germany, 1846) and M. Faraday (1856). It was introduced into industrial practice in the early 1890’s (patents by J. S. MacArthur and the brothers R. W. Forrest and W. Forrest of Great Britain in 1887 and 1888).