The Method on Diffusion Washing of Chemigroundwood Pulp under Pressure in its Digester

Abstract

On the process of chemigroundwood pulp manufacturing by neutral sodium sulfite, small part of black liquor, after cooking, has been usually recovered and great part has followed with cooked chips which are passed to fiberizing, screening in low consistency and washing processes. The majority of waste black liquor has been difficult to be recovered for its low concentration.Nowaday, many methods have been used…separating out black liquor from cooked chips by squeezing equipment, screw press etc. before fiberization…pressing out black liquor from fiberized chips by screw press and washing out dilute black liquor by vacuum drum washer to separate it from fiber…and so on.These methods above have many troubles which are low yield recovery of black lipuor if kept in high concentration, on the other hand low concentration with expensive recovery if kept in good washing.Recently the method of counter current washing chips through washing tower has been developed and gave high attention. Black liquor is recovered as relatively high yield and concentration by this method.At Futatsuka Mill, Tonami Paper Co., Ltd., chips are washed in global digester which is used for cooking, in counter currency to chips with two grades of black liquor in middle and low concentration and hot water on three stages by diffusion under pressure.We have succeeded to get black liquor in high concentration to be able to be mixed to kraft recovery process from chemigroundwood process and good washing.