PARTIAL CLOSURE IN MODERN BLEACHING SEQUENCES

Despite the progress being made in research aimed at achieving a zero- liquid-effluent mill, there is justification for investigated the more gradual approach of partial recirculation of bleaching effluents (par tial closure). Ecka Chemicals, in cooperation with the University of I daho, is carrying out research using mill trials, laboratory testing, and computer simulation to evaluate combinations of bleaching stages, operating conditions, washing, and filtrate recycling and treatment. O ne idea being investigated for ECF bleaching is that of keeping the al kaline and acidic effluent streams separate and evaluating acid efflue nt treatment and alkaline filtrate recycling The laboratory potential of this approach on conventional southern pine softwood kraft pulp, O- 2-predelignified Scandinavian pulp, and conventional and O-2-predelign ified eucalyptus pulp is reported in this paper. For fully bleached EC F southern pine kraft pulp, comparisons of the results of external eff luent treatment (anerobic + activated sludge) on the total effluents, and on the acidic effluent, show that the acidic effluent is more easi ly treatable in terms of AOX and COD. Under well-controlled laboratory conditions with efficient washing, partial closure is promising. The extent to which these results will be influenced by more realistic mil l conditions (i.e, carryover) and the degree of compatibility of the a lkaline effluents with the recovery system are being evaluated through extensive simulation studies and mill trials.