Detoxification of effluents with cyanide. Application on a cyanided gossan ore

Abstract

The cyanidation of minerals is carried out for recovering their gold and silver content. As a consequence of this process, effluents and solid wastes are generated, which will contain free cyanide as the most toxic form. Cyanides in solution can be transformed to less toxic forms through the application of a degradation processes. The objective of this work is to study the degradation of cyanide effluents by using sodium hypochlorite (NaClO) or ozone (O3), determining the best conditions for the degradation of the total cyanide, without generating any other type of contaminants. The optimum conditions will be used in the cyanidation process applied over a Gossan ore during its gold and silver recovery. Gossan ore of Rio Tinto (Huelva, Spain) is used for the application of the detoxification of effluents with cyanide and using the techniques of characterization to determining the main elements: iron (25.0%) and silicium (21.2%), the amount of gold and silver is 2.00 ppm and 87.4 ppm respectively, and the main mineralogical species: quartz, barite, goethite, hematite, beudantite and jarosite. With the application of techniques such as alkaline chlorination and ozonation it is possible to remove cyanides to achieve percentages higher than 98% leaving less than 40 ppm cyanide in 15 minutes, reducing the effluent toxicity and solid waste. The degradation of cyanide with ozone can generate carbonates that are much friendlier to the environment compared to hypochlorite that forms cyanates. However ozone is very little used in mining because it has some drawbacks: its high price and high consumption.