EFFECTS OF CHLORINATION CONDITIONS ON EFFLUENTS FROM BLEACHING OF RADIATA PINE KRAFT PULP .2. CHLORINATION FILTRATE RECYCLE

Results from the study of chlorination filtrate recycle effects on (DC )(EO) prebleaching at varying levels of Active chlorine charge and chl orine dioxide substitution are reported. Laboratory simulation of filt rate cling was achieved by reusing one half of spent chlorination filt rate as dilution water for a subsequent chlorination. After repeating this recycling eight times in order to reach near steady-state conditi ons, effluent and pulp properties were compared with those obtained wi th 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 an d this was compensated for by a 10% increase in active chlorine charge . Discharges of COD and colour in prebleaching effluents were marginal ly increased by filtrate recycle (<10%) despite initial reductions dur ing chlorination. Discharges of fatty and resin acids, on the other ha nd, were substantially decreased by filtrate recycle, presumably due t o increased retention of these materials in the prebleached pulp. Effl uent AOX was unchanged with filtrate recycle, except at high molecular chlorine where there was more potential for extensive chlorine substi tution reactions to occur. In support of this, substantial increases i n tri- and tetrachlorophenols were observed when filtrates were recycl ed at high molecular chlorine charge. Thus to minimise the potential f or environmental harm from polychlorinated chlorophenols when recyclin g chlorination filtrate, molecular chlorine usage should be minimised through increased chlorine dioxide substitution and/or reduced active chlorine charge.