Method for controlling batch alkaline pulp digestion in combination with continuous alkaline oxygen delignification

摘要

A method for batch pulp digestion with hydroxide-bearing cooking liquors in combination with continuous alkaline oxygen delignification is described. The brown stock washing plant and the connected oxygen stage are freed from oxygen-consuming black liquid by withdrawing black liquor from a pre-cook wherein cooking liquor is reacted with a surplus of hydroxide-consuming chips. Moreover, spent cooking liquor in the brown stock washing plant is displaced with spent liquor from the oxygen delignification stage (oxy-liquor) and/or alkalization spent liquor (bleaching spent liquor worth recovering because of its sodium content). A mixture of said liquors is prepared in a pressurized cooking liquor accumulator and charged to the digestion plant liquor circulation system. The cooking liquor's content of hydroxide for lignin release is continuously replenished by means of white liquor dosage which also compensates the dilution which is caused by the supply of oxy-liquor and other bleaching spent liquors whose hydroxide content reflects variations in the preceding digesting conditions pertaining to the cooking liquor and resulting Kappa number fluctuations.