Batch process for preparing pulp
According to the invention, polysulfide is used in the manufacture of kraft pulp in the initial stage of a displacement batch process in order to improve the yield. Polysulfide-containing white liquor is added inter-mixed with the first liquor charged to the digester, or as a separate charge before the impregnation liquor is introduced. Preferably, a cooking catalyst, e.g. antraquinone, is added to the white liquor charge as the cooking stage commences.
1 . A batch process for the preparation of kraft pulp with improved yield from lignin-containing cellulosic material, comprising the steps of
filling and pressurizing a vessel containing cellulosic material with impregnation liquor collected from a previous batch, said impregnation liquor admixed with, or preceded by an addition of, a volume of polysulfide white liquor;
impregnating the cellulosic material with the resulting mixture of impregnation liquor;
displacing the impregnation liquor with hot spent cooking liquor;
reacting the impregnated cellulosic material with said hot spent liquor;
displacing hot spent cooking liquor with a volume of hot white liquor and cooking the cellulosic material with said white liquor to a desired degree of delignification;
displacing the liquor used for cooking.
2 . The process of claim 1 , wherein the amount of polysulfide white liquor added corresponds to 25-90% of the total active alkali dosage for the batch.
3 . The process of claim 1 , wherein the amount of polysulfide white liquor added corresponds to 50-75% of the total active alkali dosage for the batch.
4 . The process of claim 1 , wherein the polysulfide dosage corresponds to 0.5-5% relative to abs. dry wood.
5 . The process of claim 1 , wherein the volume of polysulfide white liquor is added separately to the digester before the impregnation liquor.
6 . The process of claim 1 , wherein an amount of cooking catalyst is added to the hot white liquor.
7 . The process of claim 6 , wherein the cooking catalyst is anthraquinone, anthraquinone sulfonate, hydroantraquinone or a redox-catalyst derivative thereof.
8 . The process of claim 1 , wherein hot spent liquor displaced from the digester is collected in a single hot liquor tank.
9 . The process of claim 1 , wherein after the cook is finished, a volume of impregnation liquor is introduced into the digester as the first volume of displacement liquor.
10 . The process of claim 1 , wherein a volume of impregnation liquor is transferred from an impregnation liquor tank to a hot spent cooking liquor tank.
11 . The process of claim 10 , wherein the digester system comprises two hot spent liquor tanks.