Treatment of Cyanide Effluent

A recognized treatment for eyanide effluents, for the purpose of detoxification before discharge to a public sewer, is by reaetion with NaOQ (bleaeh liquor), which converts toxic 

At pH 10 and above, no further hypochlorite is reacted and the cyanogen chloride hydrolyses to the cyanate  

Cyanogen chloride is volatile and almost as toxic as cyanide, so the reaction should be carried out in a closed vessel and the temperature optimized at 20°C. The second reaction involving the hydrolysis of the CNCl is rather slow and is the rate determining step. 

The vessel should be well agitated during the treatment and after the reaction, any excess NaOCl is treated with Na2S203 after determining the amount by titration. 

A sample of effluent is checked with phenolphthalein to ensure that the liquor is alkaline if not, then excess NaOH must be added. The effluent should be treated in batches to ensure so sufficient availability of hypochlorite and the requisite period of contact. About 10 1 of NaOCl (14% available chlorine), diluted with 2 3 volumes of water is added per kg of NaCN, which will provide a 20% excess that would require about 0.6 kg of sodium thiosulphate crystals (Na2S203.5H20) for dechlorination, equivalent in its simplest form to  

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