Environment and Water Treatment-Cyanide Oxidation

Peroxygens are widely used throughout industry for the safe and effective treatment of cyanide in process and waste waters. Under alkaline conditions cyanide is efficiently oxidised to the, much less toxic, cyanate by peroxygens such as hydrogen peroxide, Caro’s acid and sodium percarbonate. For example with hydrogen peroxide :-

CN^- + H₂O₂ --> CNO^- + H₂O

The cyanate formed during oxidation of cyanide undergoes hydrolysis to carbonate and ammonia.

Peroxygen treated waste waters can usually be safely discharged following ammonia removal if necessary eg. via ammonia recovery or biological nitrification.

OXYPER® S, sodium carbonate peroxyhydrate, is particularly useful for emergency treatments eg. for spillages to natural waters, when solid oxidants can provide storage and handling advantages over liquid alternatives. Solvay supplies the OXYPER® S grades of sodium carbonate peroxyhydrate as granular solids which dissolve quickly in water to yield an alkaline solution of hydrogen peroxide and sodium carbonate.

For treatment of concentrated cyanide waste and process waters, hydrogen peroxide is a very efficient oxidant - reducing cyanide levels from several thousands of ppm to below 1 ppm. The rate of cyanide removal with hydrogen peoxide varies from hours down to minutes depending on the nature of the cyanide (whether complexed or free), the treatment conditions (pH and temperature) and the presence of other components in the waste water. In most cases, reaction rates and the extent of cyanide removal can be enhanced by addition of soluble metal catalysts eg. copper.

The use of sodium carbonate peroxyhydrate for cyanide destruction provide a source of alkali from the soda ash which helps to maintain a high pH and to prevent emission of toxic hydrogen cyanide during treatment.

	Percarbonate Sodium Carbonate Peroxyhydrate

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