The hydrometallurgy of nickel sulfides with elemental sulfur formation is a neglected area of research although it is the only route to solve the pollution problems associated with smelters. A review of the early attempts to apply this technology, the advances recently made in pressure hydrometallurgy, and an evaluation of the present situation is given. Attention is drawn to the difference in behaviour between nickel and iron sulfides in hydrometallurgical systems using acid medium. While both sulfides liberate elemental sulfur, nickel sulfide consumes acid and the ni~(2+) ion goes into solution, while ferrous sulfide does not consume acid and the iron precipitates as Fe_2O_3. This is a great advantage when treating pentlandite - pyrrhotite type concentrates where pyrrhotite is many times more than pentlandite.
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