PERSULPHATE PREBLEACHING OF CHEMICAL PULPS AND THEIR ENVIRONMENTAL IMPACT

Abstract

Environment concerns about the use of chlorine and chlorine containing reagents for pulp bleaching have led to implementation of a number of modified pulping and bleaching practices. Pretreatment of pulp so as to reduce incoming kappa number is one of such option which is expected to reduce bleach chemical demand and pollution loads. Persulphate (Px) and persulphate-peroxide (PxP) as a predelignifying stage has been tried with conventional (CEH) and an ECF (DED) bleaching sequences. The studies have been performed on three different pulps, wheat straw soda, bagasse kraft and mixed hardwood kraft pulp. The pulp properties and environmental parameters for the bleaching process are evaluated and results are compared with oxygen as a prebleaching agent. The conditions are optimized for persulphate (Px), persulphate-peroxide (PxP) and oxygen stages and the pretreated pulps are characterized for residual lignin content (kappa number) and pulp viscosity (CED). The bleaching charge is optimized in the three raw materials to arrive at 80% ISO brightness in wheat straw and mixed hardwood kraft pulps while 85% ISO brightness in bagasse pulp through different bleaching routes. The bleached pulp was characterized for brightness, viscosity and mechanical strength properties. The combined effluent from different sequences is characterized for COD, BOD, color and AOX. Pattern of chlorophenolics generated was studied on wheat straw soda pulp.