There are two patented versions of the sulphur dioxide cyanide destruction process. The first
patented process and most widely applied is marketed by INCO Ltd. The INCO process is based
upon conversion of WAD cyanides to cyanate using a mixture of SO2 and air in the presence of a
soluble copper catalyst at a controlled pH. In the INCO process, the forms of cyanide are
removed by different processes. One process involves the conversion of WAD cyanides to
cyanate. Iron complexed cyanides are reduced to the ferrous state and precipitated as insoluble
copper-iron-cyanide complexes. Residual metals liberated from the WAD cyanide complexes
are precipitated as their hydroxides.