Cyanide is a very toxic chemical but has properties that make it useful in many industries, which include the mining and electroplating industries. Waste streams containing cyanide from these processes have high cyanide concentrations and are unacceptable to current standards due to the potential toxicity to humans and animals. Australia has no set regulations relating to cyanide discharges in industry but the Department of Environment states that wastewater should have a cyanide concentration less than or equal to 50mg/L to be safe for wildlife.
In order to lower cyanide concentrations in waste streams, various cyanide remediation methods are practiced by industries which can be either separation or destruction processes. Separation processes are used to recover and recycle cyanide and can be performed by physical, adsorption or complexation methods. The main method for separation is the Acidification-Volitilisation-Regeneration (AVR) complexation method, more commonly referred to the CyanisorbTM process. Destruction processes are used to sever the bond between the carbon and nitrogen atoms. Chemical methods, which include the alkaline chlorination, H2O2 and INCO SO2/Air processes, are the destruction methods that have been used with the INCO process currently being adopted due to the high reagent costs of the others. Other destruction processes are biological, catalytic, electrolytic and photolytic methods. A relatively new technique that has been trialled in laboratories is using ferrate(VI) to oxidise cyanide.
Ferrate(VI) is iron in its +6 valence state, in the form FeO42-. It has been used previously for water purification and disinfection, for coagulation and for oxidation of pyrite, arsenopyrite and manganese ions. Ferrate(VI) is a very strong oxidising agent, destroys cyanide in a matter of minutes, has no dangerous by products, is relatively cheap and can be easily prepared by electrochemical methods, all of which make it desirable for oxidising cyanide.
The objective of this thesis was to test the efficiency of ferrate(VI) as an oxidiser for cyanide. Four parameters were studied in the tests conducted, ferrate(VI) concentration, cyanide concentration, ferrate(VI)/cyanide solution volume ratios and mixing time. Cyanide solutions were also prepared to simulate Western Australia’s hypersaline bore field water used in process plants to see the effect of ferrate(VI) on hypersaline solutions..
Ferrate(VI) was produced by the electrochemical method which involved two stainless steel cathodes, a cast iron anode and a divided cell. The concentration of ferrate(VI) was determined using the chromite method and was potentiometrically titrated. Cyanide solutions of 1000, 500 and 100ppm were prepared to pH 12 with the analysis of cyanide performed using the picric acid method, provided by Gympie Eldorado Gold Mines Pty Ltd, together with the use of a UV-vis spectrophotometer.
The experimental work conducted showed that high cyanide destruction efficiencies were obtained when cyanide was mixed with ferrate(VI). An improvement in cyanide destruction was observed with increasing ferrate(VI)/cyanide solution volume ratios and a gradual improvement in cyanide destruction was observed with time.
The results obtained clearly show that ferrate(VI) can be used efficiently as an oxidising agent for cyanide. The method used is simple to perform using minimal equipment. However, further testwork is needed to determine if this method is viable for slurry applications. The picric acid method to analyse cyanide is of concern as it suffers from interferences and can be unreliable at lower cyanide concentrations.