THE EFFECTS OF BLEACH PLANT EFFLUENT RECYCLE IN KRAFT MILL GREEN LIQUORS .1. SLAKING AND CAUSTICIZING

Laboratory experiments show that with current levels of water use, eff luents from D-c, D-100, E(O) or Z Stage bleaching of softwood kraft pu lp can be recycled to the recausticizing area if a loss (5-18%) in cau sticizing efficiency is tolerable. High levels of sodium chloride (30 g/L) did not consistently reduce causticizing efficiency. The losses o bserved in causticizing efficiency due to the recycle of bleach plant filtrates to recausticization were recouped by recycling lime mud. Ble ach plant water usage was very high in the mills studied. Fifty percen t replacement of fresh water use in recaust would permit only 5 to 25% of the bleach plant effluent to be used in recausticization. Novel re cycle schemes to minimize water use in the bleach plant are necessary if a significant degree of closure is to be accomplished by the relati vely simple procedures investigated in this study.