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Cyanide is found
primarily in plating shop rinse water, spent plating solutions, and metal
stripping solutions. It is used to complex a wide range of metals including
gold, silver, copper, zinc, cadmium, and nickel.
Cyanide is after referred to as free cyanide or amenable cyanide and complexed
cyanide or non-amenable.
The free cyanide can be easily treated by oxidizing with chlorine or sodium
hypochlorite, however, the complexed cyanide is virtually inert to this type of
treatment.
The complexed cyanide treatment usually involves precipitation with ferrous ion
to form the insoluble iron Ferro-cyanide.
Because hydrogen cyanide gas can be formed at a pH of less than nine, most
cyanide is treated at pH values greater than 10. However, if the cyanide
concentration is low, less than 5 ppm, the pH restriction is less critical.
The treatment of free cyanide with chlorine or hypochlorite is very fast. Ninety
percent reduction in 15 seconds. The product of this reaction is cyanate which
then decomposes to nitrogen and carbonate ion. This second reaction is much
slower and is often done at a pH of 8 to 9 to increase the reaction rate.
Usually, the cyanate ion is not regulated so treatment of cyanate to low
concentrations of less than 15 mg/l is omitted.
The precipitation on non-amenable cyanide is done at a pH of 8.5 to 9.5 to keep
enough soluable iron in solution for the reaction to proceed.
When combining the treatment of amenable and non-amenable cyanides, the residual
total cyanide concentration of less than 1 mg/l is easily met with the typical
value being about .4 mg/l for the initial treatment. If lower concentrations of
cyanide are required, secondary treatment with iron gives a residual cyanide
concentration of less than .01 mg/l.
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