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.
The second sulphur dioxide process was developed at Heath Steel Mines Ltd. and the patent
assigned to Noranda Incorporated (Ferguson and Walker, 1985). In the Noranda process, pure
sulphur dioxide is fed into a solution or slurry to lower the pH into the range of 7.0 to 9.0. A
copper sulphate solution is then added at such a rate to yield an effluent containing the desired
cyanide concentration.
The INCO process has been used at over 80 mining operations worldwide and is the process
addressed in this section. A primary application of the sulfur dioxide and air process is in
treatment of tailings slurries, but it is also effective for the treatment of solutions for the
oxidation of free and WAD cyanides.