Detoxification and clarification of effluents from the steel industry

A chemical detoxification procedure has been developed to remove cyanide and zinc from gas scrubbing effluents originating in the production of iron and steel. High levels of cyanide were reduced to below 10 mgl'^ by complexation and precipitation with ferrous iron. These residual levels were further reduced to approximately 0.1 mgl ^ by oxidation of free cyanide with Caro's acid (peroxymonosulphuric acid), The various parameters affecting the removal efficiency of cyanide, including pH, aeration, temperature and dose rate, were evaluated and optimum conditions established. Similarly the variables affecting zinc removal, by adsorption onto complexed cyanide and hydrous ferric oxide precipitates were evaluated and optimised. Conditions for optimum cyanide and zinc removal were determined in seawater spiked with cyanide and zinc, and actual effluent collected from a nearby steelworks. The final stage of this project was concerned with construction and operation of a pilot plant in order to establish that the batch cyanide and zinc removal process could be converted into a continuous detoxification procedure.