Strategies for High Brightness, Low Effluent TCF Bleaching of Sulphite Pulps, 1994 Pulping Conference Proceedings

J. Devenyns, N. Troughton, F. Desprez

With TCF practice being state of the art, the remaining issues in sulphite pulp bleaching are higher brightness and mill closure.

The recovery of COD-rich liquors from the sulfite prebleaching stages is a significant step towards the closed mill concept. Such recovery is considerably simplified if the alkali used in the prebleaching stages is the same as the base used for the preparation of the sulfite cooking liquor.

However, changing from NaOH to MgO in Magnefite pulp prebleaching reduces the delignifying efficiency of the prebleaching stages. Moreover, undissolved MgO can be conveyed through the washing devices together with pulp and may therefore impair further process steps in the final brightening and sheet formation areas.

A complementary treatment with Caro’s acid can close the kappa gap between the MgO based prebleaching stages and final brightening, thus allowing a high and stable final brightness. Meanwhile, the Caro’s acid stage acts as an acid wash, effectively dissolving and removing MgO in excess from prebleaching, ready to serve as make-up source for Magnesium and Sulphur upon recovery.

OP(MgO) CA P sequences therefore open interesting perspectives for high brightness, low effluent TCF bleaching of sulphite pulps.