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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.
The reaction for cyanide treatment often is shown as:
CN- + H2O è CNO- + 2 H+ + 2 e-
ClO- + 2 H+ + 2e- è Cl- + H2O
2 CNO- + 2 H2O è N2 + 2 CO2 + 4 H+ + 5e-
2.5 ClO- + 5 H+ + 5 e- è 2.5 Cl- + 2.5 H2O
2 Fe++ + Fe(CN)6---- è Fe2Fe(CN)6 insoluble.
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