Cyanidation


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cyanidation

[‚si·ə·nə′dā·shən]
(chemistry)
Joining of cyanide to an atom or molecule.
(metallurgy)

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).

REFERENCES

Maslenitskii, I. N., and L. V. Chugaev. Metallurgiia blagorodnykh metallov. Moscow, 1972.
Osnovy metallurgii, vol. 5. Moscow, 1968.
References in periodicals archive ?
In Malaysia, limited published research exists on the health impacts of cyanidation on communities near gold mining operations.
Field observation of local trees was carried out in September 2013 at areas adjacent to the tailing disposal area of a cyanidation plant at Sekotong District of West Lombok, Indonesia (115[degrees].46'-116[degrees].20'E and 8[degrees].25'-8[degrees].55'S).
One promising alternative to using cyanide in gold mines is the Haber Gold Process, a non-toxic extraction system that tests have shown can result in more gold recovery over a shorter period than cyanidation. Another alternative is YES Technologies' biocatalyzed leaching process which proponents say is 200 times less toxic than cyanide.
Cyanidation as a chemical method for leaching of gold was studied in 1880s by John Steward MacArthur and due to instant good results, it eventually replaced chlorination processing.
The majority of the samples were sieved in the laboratory into six size fractions and the coarser fractions were analysed for gold by cyanidation which should extract all the oxidised gold in the sample and by fire assay which analyses total gold.
It conducted a pilot testing of the ore from the Taseevskoye deposit, and finally decided to use pressure oxidation technology with a cyanidation route.
The results are based on cyanidation of porphyry and stockwork style gold mineralized material, and include gold recoveries of 90 percent for oxide, 80.6 percent for partially oxidized, and 50 percent for unoxidized coarse reject samples.
Under autoclave conditions, the iron sulfide (FeS, Fe[S.sub.2]) and arsenopyrite (FeAsS) minerals are oxidized to liberate gold which can be efficiently recovered in the subsequent cyanidation stage.
Amalgamation remained the primary method of gold extraction worldwide until 1900 and the introduction of cyanidation, a far cheaper process that used cyanide-salt solutions to dissolve gold from its ores.
Gold recoveries following cyanidation of the bacterial oxidation residue are running in excess of 97% using the current stockpiled concentrate feed.