IS EXTENDED COOKING NECESSARY FOR THE CLOSURE OF BLEACH PLANT? - A STRATEGIC SHIFT IN PULPING AND BLEACHING

摘要

Elemental Chlorine Free (ECF), Totally Chlorine Free (TCF), and Totally Effluent Free (TEF) bleaching of pulps requires low Kappa number pulps to bleach plant. Lowering of kappa number is usually achieved by extended delignification in digesters; reduced yield and increased wood cost per ton of pulp are the consequences. With the increasing cost of wood, the basic premise of extended cooking (EC) to lower the pulp kappa number is questioned. A strategic shift in pulping and bleaching is discussed to decrease wood use per ton of pulp (or more pulp from a ton of wood used) while decreasing the kappa number of pulp to bleach plant. Such a shift would allow the mills to lower bleaching cost, comply with the environmental requirements under Cluster Rules but most importantly, allow the closure of bleach plants cost effectively. The approach involves high kappa pulping and selective delignification of softwood pulps by oxygen before ECF or TCF bleaching. To avoid the loss of yield gained from high kappa pulping to screen rejects, pulping could be carried out using combination of digester additives like anthraquinone (AQ), polysulfide (PS) and surfactant based chip penetrants (SURF). Other benefits include savings in Active Alkali (AA), H-Factor, and decreased solids and heat loads to recovery boiler. Using oxygen, the pulps can be selectively delignified with little loss of yield and strength but with strong resolution of dirt and shives. This approach requires capital investment and some operating costs; attractive cost-benefit is mandated to justify the capital investment. A detailed cost-benefit analysis (CBA) has run to outline the benefits of high Kappa pulping, oxygen delignification and chlorine free bleaching (both ECF and TCF bleaching) to 88-89 %ISO brightness over conventional extended pulping and bleaching. High rate of return on the investment (%ROI) and shorter pay-back on the expended capital do justify this shift in pulping and bleaching.