Organic Carry-over in Kraft Pulp and Bleaching Discharges, 1990 Pulping Conference Proceedings

Pulp samples from seven pulping lines, with or without oxygen delignification and representing both softwood and hardwood pulping, were studied. In order to characterize the organic carry-over in unbleached kraft pulp, each mill sample was purified by treating the pulp with a mild alkaline solution. The carry-over was characterized by the COD value of the liquor from the purification step and by the kappa-number difference between unpurified and purified pulp samples, The effect of carry-over on the discharges from pulp bleaching was determined as the difference in discharges from the bleaching of each pulp sample, in purified and unpurified states.

The results show that a significant amount of AOX is formed when the organic carry-over is present in the pulp, somewhat more than would be expected on the basis of the additional chlorine chemical charge introduced by the carry-over substances. With oxygen delignified softwood pulps, the contribution was higher than with other types of pulps. At high levels, the carry-over promoted the formation of chlorinated phenolics and the toxicity of bleaching filtrates in the case of softwood pulps. At a low level, and in the case of hardwood pulps, it had no significant effect. In no case did the carry-over contribute to the formation of chlorinated acetones; nor did it contribute significantly to the mutagenicity of filtrates. In these experiments the carry-over had no significant effect on the dioxin content of pulp.