Options to Close the Water Cycle of Pulp and Paper Mills by Using Evaporation and Condensate Reuse, 1996 Minimum Effluent Mills Symposium Proceedings

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Markku Pekkanen & Erkki Kiiskilä
Ahlstrom machinery

The present status of usage of water in bleached kraft mills, about 18-20m 3 /ADt, prerequisites segregation, stripping and reuse of the black liquor evaporator secondary condensates. Part of the condensates can be reused in the recausticizing plant. The other part which traditionally was used in brown stock washing, has now to be used in bleaching plant to substitute clean hot water, as the alkaline bleaching filtrate will be recycled into the recovery system. Bleaching plant sets new quality requirements for secondary condensate. The remaining pulp mill process effluent mainly consists of acid bleaching effluent and woodroom effluent. The pilot trials conducted at a Finnish mill show that filtrates from TCF-bleaching can be evaporated to about 30 % dry solids. The produced condensate is clean, so that it can be used as dilution water in the drying machine water cycles or in the last washing stages in bleaching. The modern wood room concept is normally dry, but the logs have to be de-iced with hot water in winter. In order to make this system effluent free the filtrate from the bark presses can be evaporated and the condensate can be reused as make up for the system. Paper mills must also apply new solutions to compete with the emission levels of the pulp mills. Woodcontaining paper mills integrated with mechanical pulping with CTMP, TMP and PGW or deinking processes produce higher liquid effluent emissions than modern chemical pulp mills. The water cycle can be closed by using only about 7 m 3 /ton in the paper machine showers and using the white water countercurrently in the mechanical pulp plant. The effluent can be treated by evaporation to produce clean water for reuse in the paper machine and the concentrated can be disposed of by burning with bark or black liquor.