Fibre shortages and a move towards closed cycle operation have resulted in a renewed interest in polysulphide kraft pulping, Recently, a process has been developed whereby polysulphide is formed directly in the kraft mills causticizers by the addition of air or oxygen. The purpose of this work was to identify materials of construction for polysulphide enriched kraft liquors and to characterize the effect of polysulphide on existing carbon steel equipment. Weight loss and D.C electrochemical experiments were carried out in white, been and weak-wash liquors, with and without polysulphide. It was found that304L and 316Lstainless steels cm experience corrosion rates of approximately 0.004 mm.y1 in the liquors tested. Carbon steel (ASTM A-285 Gr.C) exhibited distinct active/passive behavior in all three liquors. It was possible to passivity the carbon steel in green and weak-wash liquor even when the concentration of polysulphide in the liquors was below 2 grammes per litre (g, L-1). Carbon steel was more difficult to passivity irr white liquor but 2 g,L-1polysulphide was often sufficient, The steel nearly always passivated when the liquor concentration was 5 g,L-1nr greater. In a 14 day corrosion test in white liquor with a nominal polysulphide concentration of 8 g.L-1, carbon steel coupons were active and corroded at a rate of 0.5 mm.y-1 while other carbon steel coupons in the same environment passivated.
Fibre shortages and a move towards closed cycle effluent operation have resulted in a renewed interest in pnlysulphide kraft pulping. As kraft mills reduce effluent volumes that are sent to receiving waters, and move toward closed cycle operation, more material will be burned in the recovery furnace. Since marry mills are already recovery limited, recovery off-loading technologies will become increasingly important as means of allowing combustion of additional material in the burned without reducing the amount of cooking chemicals that can be recovered. Polysulphide Sx2- cooking is one such technology because it increases the yield of the kraft cook by up to 6% for a given pulp kappa number, and correspondingly reduces the amount of cellulose-rich material that must be combusted in the boiler compared to Sx2-freecooking, An alternative justification for the implementation of Sx2- cooking is to produce a lower kappa pulp at a given yield. A lower kappa pulp requires less bleaching chemical to achieve market brightness. Either one of the above driving forces make the implementation of Sx2- pulping attractive for existing mills.
Commercial processes which require proprietary catalysts for the production of Sx2-containing liquors have been used in the kraft pulping industry for more than a decade [1,2]. Recently, a new method of in-situ generation of Sx2- has been developed by Paprican whereby the Sx2- is formed by air or oxygen addition to the kraft mill causticizers (Figure 1). This renewed interest in S.’” pulping has been accompanied by concern about the possible corrosive effects of the liquor on recaustisizing and pulping equipment in the mill.