Recovery of sodium sulphate in production of iron oxide black pigments

In the production of ferric oxide black pigments with the simultaneous recovery of sodium sulphate wherein ferrous sulphate is reacted with an alkaline sodium compound at a pH above about 6 and a temperature above about 50 DEG C., and the pigment formed is separated from the mother liquor containing sodium sulphate, the improvement which comprises conducting the reaction at a sodium sulphate concentration of at least about 20% by weight, after separation of the pigment cooling at least part of the sodium sulphate-containing mother liquor to a temperature of at most about 30 DEG C., thereby to crystallize sodium sulphate decahydrate, and separating the crystals. Air is preferably passed through the reaction solution to oxidize the precipitated pigment and, advantageously, make-up water is added to compensate for water carried off by the air. The end product mother liquor, containing an appreciable amount of sodium sulphate, is employed to dissolve ferrous sulphate for a subsequent cycle.