GB1434232A - Method of producing cellulose pulp by means of oxygen gas delignification - Google Patents

Abstract

1434232 Producing cellulose pulp MO OCH DOMSJO AB 4 July 1973 [5 July 1972] 31941/73 Heading D2W A method of producing cellulose pulp comprises pretreating a lignocellulosic material with a liquor containing an alkali at 60-200‹C until 1-30% by weight of the dry weight of the lignocellulosic material has passed into solution, and then digesting the lignocellulosic material with oxygen in the presence of an alkali containing aqueous solution at 100- 180‹C and an oxygen partial pressure of 3-200 bars, the pH being maintained at 6-9 during the major part of the oxygen digestion. The Dre-treatment may be carried out at a pH of 7-14 and a pressure of 1-20 bars, and may be a continuous or a discontinuous process, the alkali being added in one or more batches or added continuously. The alkali material may may be sodium hydroxide, sodium carbonate and/or sodium bicarbonate, and the corresponding compounds of calcium and magnesium may also be present. Sodium carbonate and bicarbonate for use in the pretreatment may be obtained by combustion of the waste liquors from either the pretreatment or the digestion, and absorption of carbon dioxide produced in the digestion. The pretreatment liquor may contain waste liquors from both steps, and may also contain waste sulphide liquor from other cooking processes. To produce a low bernicellulose content in the final product, the pretreatment may be combined with an acid pre-hydrolysis which may be a separate step or achieved by allowing the pH to fall temporarily in the pretreatment. Air may be removed during the pretreatment, and a surface tension reducing agent, a peroxide brightener, a reducing agent such as dithionite and/or hydridoborate, and metal complexing agent may be added, the latter during or after the pretreatment. Suitable complexing agents are polyphosphates, aminocarboxylic acids, ethylenediamine tetraacetic acid, nitrilotriacetic acid, diethylenetriamine pentaacetic acid, gluconic acid, saccharic acid and ethylenediamine. The product of the pretreatment may be drained or washed with waste liquor from the digestion step, roll pressed and peg shredded before passing to the digestion. The alkali used in the digestion may be sodium carbonate and/or sodium bicarbonate, optionally replenished with sodium hydroxide. The carbon dioxide pressure may be 0.2 to 5 bars and excess carbon dioxide may be removed by venting, cooling or absorption in sodium carbonate solution, optionally containing sodium hydroxide. A surface tension reducing agent, a foam prevention agent and an inhibitor against carbohydrate decomposition may be present during the digestion. The digestion may be carried out with the lignocellulosic material immersed in or sprayed with the solution. Alternatively, the lignocellulosic material may be impregnated with the solution, drained, and then contacted with the oxygen. Suitable lignocellulosic materials are sawdust, wood shavings, wood chips (e.g. birch chips) straw and bagasse. The wood chips may be split in a shredder.