In-Situ Production of Polysulphide Liquor in a Kraft Mill's Causticizers, 1996 Pulping Conference Proceedings

V. Uloth, G. Dorris, R. Thring, R. Hogikyan, J. Wearing, L. Tench, J. Ayton

Batch and continuous pilot plant trials at two kraft mills confirmed the technical feasibility of partial and selective oxidation of a portion of the sulphide in white liquor to polysulphide, using lime mud as the primary oxidation catalyst. Use of a disc impeller to maximize gas-liquid mixing and the addition of small amounts of manganese dioxide (MnO2 ) were found necessary to improve reaction selectivity and reduce retention time requirements. Using oxygen, with MnO2 addition, a cooking liquor with polysulphide concentrations > 6 g/L (as sulphur) and with selectivities >70%, suitable for pulping, was produced with reaction times of 70 to 90 minutes. Using air, reaction times of about 130 minutes were required. Full scale systems for a 1000 tpd kraft mill were designed using engineering data from the pilot plant trials. Capital costs for a system utilizing oxygen were found to be significantly lower than for a system utilizing air or for commercial systems utilizing activated carbon catalysts. Assuming a 2% increase in pulp yield in comparison to conventional kraft pulping, payback periods of 1.9 to 5.2 months were estimated.