Measures taken against water pollution in the kraft pulp and paper industry

Abstract

Without reclamation of waste liquor and chemicals recovery, the kraft pulp and paper industry would constitute an enormous pollution problem. Already for economical reasons, substantial recovery is secured, however, and considerable further improvements are foreseen. Recovery is outlined in terms of cycles for sodium, calcium, sulphur, and—in a wider sense—carbon. In addition, the mechanical losses of bark fragments and pulp fibres must be observed, calling for a recycling of water and a limitation of the fresh water and effluent volumes. The trend towards higher pulp yields, particularly semichemical pulping, helps to limit the pollution, since less wood substance is dissolved. This refers to kraft pulping as well as to neutral sulphite pulping, which is often combined with the kraft recovery system. Recent machinery developments, such as digesters with countercurrent washing, continuous diffusers and closed screens allowing the screen room filter to be used as a final washer, have all aided in increasing the black liquor reclamation. Developments under way on high-capacity presses are likely to make it almost complete. This has shifted the attention to other pollution sources in bleaching and chemicals recovery. The elimination of bleach effluents remains to be developed. Oxygen bleaching may become a method for the removal of half the pollution. The considerable extra investment may be compensated by lower operating costs. The continuous diffuser may facilitate the elimination of bleach effluent. Chemicals recovery involves evaporation and combustion of the black liquor and causticizing of the dissolved soda smelt. In the evaporation, volatiles tend to leave the system in vent gases and condensates. Systems have been developed to destroy the vent gases and eliminate the condensate pollution to more than 90 per cent, not counting the occasional entrainment of black liquor droplets, which calls for further improvements in machine design. In the combustion, air pollution is controlled by electroprecipitators and wet scrubbers, but a proper use of the scrubber liquor must be observed. The combination of kraft and neutral sulphite pulping provides such a use, since the scrubber liquor forms the base for neutral sulphite cooking liquor preparation, whereas the kraft recovery system can use the corresponding waste liquor as a sodium make-up. The kraft papermaking operations lead to fibre losses, which are minimized by recirculation of water. Developments in systems and machine design are needed to improve the situation. Both here and in the recovery system, the temporary losses require increased attention for both pollutional and economical reasons. Improved machine reliability, better process control and production planning are essential factors. In addition to the efforts within the production processes, pollution abatement must involve effluent treatment. Condensate treatment and particularly fibre reclamation ponds are essential and practical, whereas biological effluent treatment at present is unsatisfactory from both economical and pollution aspects. The recent improvements are evaluated in examples. The current restructuration of the industry offers an ideal opportunity to make use of them, and further efforts are being jointly undertaken by the industry, the financing of which should be in the interest of both industry and society. © 1972, Walter de Gruyter. All rights reserved.