Clearly this is a development from the use of ferric oxide in the solid phase.

Gas passes upward through a descending flow of an alkaline suspension of ferric hydroxide in ammonia.

Oxide regenerated by air blowing and sulphur was recovered as a filter cake mixed with iron oxides.

It worked well when the hydrogen cyanide content was very low but in the presence of cyanide, iron/cyanogen/sulphur complexes formed. This caused odour problems when the filter cake was autoclaved to recover the sulphur. This technique is now obsolete but several more recent processes involving oxidation by iron compounds:

• Konex process(Japan)-sodium ferrate.

• Sulfint process(Austria) - iron EDTA complex.