Cyanide process

Hydrogen Cyanide Process. This process, one of two used for the industrial production of malonates, is based on hydrogen cyanide [74-90-8] and chloroacetic acid [79-11-8]. The intermediate cyanoacetic acid [372-09-8] is esterified in the presence of a large excess of mineral acid and alcohol. 

The chlorination process, introduced in Europe in 1843, roasted ore with chlorides, followed by a hot brine leach and subsequent precipitation of the silver on copper. In 1887 it was discovered that gold and silver can be recovered by sodium cyanide, and this process displaced the dangerous chlorination process. By 1907 the cyanide process, where a cyanide solution is mixed with 2inc dust to precipitate the silver, was universally in use. 

Figure 5.41 Standard cyanide process Figure 5.42 Carbon-in-pulp process,...

Replace cyanide process baths with noncyanide process baths to simplify the treatment... 

Kiss An obsolete process for extracting silver from its ores. The ores were roasted with sodium chloride, producing silver chloride, and this was leached out with a solution of calcium thiosulfate. The process was replaced by the cyanide process. 

Merrill-Crowe An improvement on the cyanide process for extracting gold from rock. The solution of gold cyanide is reduced with zinc dust, thereby precipitating the gold as a fine powder which is filtered off and smelted. Operated in South Africa. 

PAL [peroxide assisted leach] An improved cyanide process for extracting gold from its ores. Addition of hydrogen peroxide to the system improves the yield of gold and reduces the usage of cyanide. First operated in South Africa in 1987. 

Salsigne A cyanide process for extracting gold from ores containing arsenic or antimony. Pre-treatment with a lime slurry reduces cyanide losses. 

In the cyanidation process the ore is crushed and roasted with sodium chloride and then treated with a solution of sodium cyanide. Silver forms a stable silver cyanide complex, [Ag(CN)2]. Adding metallic zinc to this complex solution precipitates sdver. 

The salt baths in some nitriding and cyaniding processes also constitute hazardous wastes when spent. Nitriding and cyaniding baths for example, contain sodium and potassium cyanide and cyanate. Implementation of gas nitriding eliminates this waste stream. Gas nitriding employs ammonia gas to supply the nitrogen, and produces no hazardous waste stream. 

It may be possible to improve the operation of zinc cyanide process solutions and effect enormous reductions in waste generation.[12][13] After improving operations, it may be further possible to capture and return "escaped" process solution from the rinsing system. However, even if cyanide plating systems could be "close-looped" and the process solutions successfully maintained using standard purification techniques, the trend is definitely towards the substitutes. It may be necessary to operate... 

Manufacture. The predominant manufacturing processes arc the hydrogen cyanide process and carbon monoxide process. 

Economic Aspects. Dimethyl and dieihyl malonaies are produced via the carbon monoxide process at Hiils (Germany i. Juzen (Japan), and Korean Fertilizers iS. Korea) they arc produced via ihe hydrogen cyanide process ai L.ouza (Switzerland) and Tateyama (Japan). Total capacity is estimated to be aboul 12,000 t/yr. Furthermore, producers are also reported in the People s Republic of China mid in Romania. 

Manufacture. Cyanoacciic acid and cyanoacetaies are industrially piodiicetl by the same route as the malonates using a modified hydrogen cyanide process, starting from a sodium chloroaeeiale solution v la a sodium cyanoacetale solution. 

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