Insights from a number of INCO cyanide destruction process reviews and studies are presented and discussed. In particular, the performance of three plants are discussed in terms of reagent (sulfite and oxygen) stoichiometry and utilisation, and the impact of process design, control, reagent addition points and slurry properties. Discussed also is the effectiveness of hydrogen peroxide use (as a supplementary source of oxygen) in two of the plants reviewed. The impact of the ore on an INCO process is studied by conducting cyanide destruction testwork separately on slurry and filtrate. It has been found that for some ores, the mineral surfaces can catalyse the undesirable direct oxidation of sulfite by oxygen (to sulfate). High dissolved oxygen can also result in excess reagent demand/consumption. Oxygen mass transfer is often the limiting parameter of an INCO process and hydrogen peroxide can be used to improve the oxygen capacity of the process (though design improvements could provide a better economic outcome). Optimisation of the INCO process requires constant monitoring of feed and discharge weak acid dissociable cyanide (WAD CN) concentrations and maintaining low but measurable dissolved oxygen in the reactor (eg 25 mg L-1). The solids properties in a feed change over time which can change the discharge WAD CN concentration and require adjustment to the sodium metabisulfate-to-weak acid dissociable cyanide weight ratio used for the destruction process control.
CITATION:Breuer, P L and Hewitt, D M, 2019. INCO cyanide destruction insights from plant reviews and laboratory evaluations, in Proceedings World Gold 2019, pp 130142 (The Australasian Institute of Mining and Metallurgy: Melbourne).