Modeling and Comparison of Cyanides Elimination from Wastewater of an Industrial Unit by Chemical Treatment and Physical Absorption Methods

Cyanides are chemical compounds containing the cyano group CN-. These compounds are strongly toxic for human and other living species. Cyanides are used in many industries specially the mining of metal ores and hence are found in wastewaters produced by these industries. Without treatment and reducing the cyanide levels in these wastes, they grossly harm the environment. Based on experimental results of chemical treatment of ammonium cyanide in wastewater of coking unit in Isfahan Steel Company (CUISC) in Iran with sodium sulfide which converts the volatile ammonium cyanide to stabilized form of sodium cyanide a mathematical model is proposed. As there are many different chemicals in the wastewater of coking unit such as phenols, ammonia, cyanide compounds etc., in order to estimate the concentration of hydrogen cyanide in the air, modified Raoult’s law is used. Activity coefficient of hydrocyanic acid was calculated from liquid-vapor equilibrium data. Based on available data in literature for physical absorption of cyanide ions on three type of absorbers (activated carbon AC, and activated carbon impregnated with cupper and silver, AC-Cu and AC-Ag), a mathematical model was proposed for physical absorption of cyanide on these absorbers, and by using the calculated activity coefficient from first part, these models were used for predicting the reduction in cyanide content of wastewater of CUISC for three scenarios of using AC, AC-Cu, and AC-Ag absorbers. Comparing the results of models for chemical treatment and physical absorptions show that AC-Ag absorber has the best ability to remove the cyanide from both wastewater and air above it. But in areas with average ambient temperatures less than 36 °C where cyanide of coking unit is mostly in form of ammonium cyanide, chemical treatment with sodium sulfide is more practical. Also in case of chemical treatment capacity of cyanide removal is higher and process can continue for longer time.