AbstractThe reaction between hydrogen and pyrites under suitable conditions gives good yields of hydrogen sulphide; practically the whole of the labile sulphur of pyrites can be removed from the ore, leaving ferrous sulphide. Under the conditions described in this paper, the rate of formation of hydrogen sulphide from pyrites increases to a maximum and then falls off according to the exponential law. The optimum conditions for the reaction are: (a) temperature of reaction should be 625‐650°, (b) space velocity of hydrogen should be 40‐50 volumes hydrogen per volume of pyrites at kiln temperatures of 625‐650°. Corrosion of plant materials is greatly diminished by using mild steel “servarized” by aluminium. Size of pyrites has little effect on reaction time if the lumps of ore are not greater than 1 1/2 in. More than 90% of the arsenic in pyrites is removed as arsenious sulphide, which passes over suspended in the