Table of Contents
Metal complexed cyanides in wastewaters form as a result of interactions of free cyanide with metals present in the wastewater and exhibit varying degrees of stability, toxicity, and treatability. Thiocyanate, a pollutant commonly found in cyanide bearing wastewaters, is formed through the interaction of free cyanide with sulfur containing species (i.e. pyrrhotite) both present in the wastewater.
In certain industrial processes, such as the beneficiation of gold and silver, cyanide is an essential reagent. Since free cyanide, complexed cyanides, and thiocyanate are potentially toxic to humans and aquatic organisms, these compounds must be removed from wastewaters prior to their discharge into surface or ground waters serving as potential potable water sources, marine or fresh water habitats.
Analytical and Toxicologial Testing
Comparison and evaluation of the various treatment processes required accurate and interference free analytical procedures. Concurrently with the pilot plant testing of the chemical and physical treatment processes, research was conducted to evaluate and develop reliable methods for cyanide analysis. Two analytical methods with modification were found dependable and were used extensively in the laboratory during pilot plant evaluations.
Emphasis on specific parameter removal could have resulted in toxic effluents due to pollutants associated with or produced through breakdown of the target pollutants, which were not analyzed for and removed through further treatment.
Evaluation of Chemical and Physical Treatment Processes
Several chemical and physical treatment processes were pilot plant evaluated either along or in combination (Table 2) . These processes included acidification/volatilization, ozonation, ion exchange, prussian blue oxidation/ precipitation, carbon adsorption, alkaline chlorination, and copper catalyzed hydrogen peroxide oxidation (DuPont Kastone Process).
As a result, research was directed toward evaluation and development of biological treatment process which was theoretically capable of overcoming the limitations associated with other treatment processes, while producing a high quality, low toxicity effluent simply and cost effectively.
Development Of The Biological Treatment Process
Although full scale biodegradation of cyanidation wastewater was not practiced in the mining industry, the literature indicated biological oxidation of free cyanide, metal complexed cyanides, thiocyanate, and their breakdown products was possible on a laboratory scale. To test the effectiveness of biological treatment both suspended growth and attached growth processes were evaluated with and without supplemental carbon sources (raw sewage). The results indicated supplemental carbon sources were not required for the biological degradation of free and complexed cyanides.
