Results from the study of chlorination filtrate recycle effects on (DC)(EO) prebleaching at varying levels of Active chlorine charge and chlorine dioxide substitution are reported. Laboratory simulation of filtrate cling was achieved by reusing one half of spent chlorination filtrate as dilution water for a subsequent chlorination. After repeating this recycling eight times in order to reach near steady-state conditions, effluent and pulp properties were compared with those obtained with open bleaching (i.e., 100% fresh water for dilution). Kraft-oxygen pulp of relatively low kappa number (<15) was chosen to represent the type of radiata pine pulp to be produced in the near future. Filtrate recycle decreased the extent of delignification during prebleaching and this was compensated for by a 10% increase in active chlorine charge. Discharges of COD and colour in prebleaching effluents were marginally increased by filtrate recycle (<10%) despite initial reductions during chlorination. Discharges of fatty and resin acids, on the other hand, were substantially decreased by filtrate recycle, presumably due to increased retention of these materials in the prebleached pulp. Effluent AOX was unchanged with filtrate recycle, except at high molecular chlorine where there was more potential for extensive chlorine substitution reactions to occur. In support of this, substantial increases in tri- and tetrachlorophenols were observed when filtrates were recycled at high molecular chlorine charge. Thus to minimise the potential for environmental harm from polychlorinated chlorophenols when recycling chlorination filtrate, molecular chlorine usage should be minimised through increased chlorine dioxide substitution and/or reduced active chlorine charge.
