INCO Cyanide destruction insights from plant reviews and laboratory evaluations

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 (supplementary oxygen source) in two of the plants reviewed. Oxygen mass transfer often limits the capacity of an INCO process. Hydrogen peroxide addition can improve the capacity, though process design improvements could provide a better economic outcome. Optimisation of the INCO process requires constant monitoring of feed and discharge weak acid dissociable cyanide concentrations and maintaining low but measurable dissolved oxygen in the reactor (e.g. 2-5 mg L-1). Changing solids properties over time requires adjustment to the sodium metabisulfate-to-weak acid dissociable cyanide weight ratio used for the destruction process control.