Patent GB-749542-A (GB)
Title: Improved method for producing cellulose
IPC: D21C3/22
Filed: 1953-10-21
Abstract: <PICT:0749542/IV(a)/1> <PICT:0749542/IV(a)/2> <PICT:0749542/IV(a)/3> Cellulose is produced from cellulosic material such as wood, straw, bagasse, esparto, or the like by digesting the material in one or more vessels under heat and pressure with a total proportion of more than 12 cubic metres of cooking liquid per 1000 kilograms of raw material (calculated as dry), the liquid being caused to flow in such manner that it acts firstly on material which is already largely disintegrated, and then on less disintegrated material, and finally on fresh material.  Liquid is drawn off each vessel while the contents are maintained in heated condition.  If a cellulose of high purity is desired, the raw material may first be subjected to a treatment with water or to a weak acid hydrolysis solution so as to remove pentosans before the material is subjected to the main digestion treatment. Preferably, the cooking liquor is passed through the material in several separate quanta or batches.  When treating wood, the volume of each batch of liquor should be at least 1, and preferably 1.3-3.0 cu. m. per 1000 kg. of raw material, and when treating straw, bagasse, or esparto, the proportion should be at least 1.5, and preferably 2-5 cu. m. per 1000 kg.  The pressure cooking may be carried out in at least two stages.  In the first stage the concentration of active inorganic substances in the cooking solution is allowed to rise as the digestion progresses, but in the second stage the concentration of the cooking chemicals is progressively reduced.  There may be an intermediate stage of cooking in which the concentration of active chemical is maintained substantially constant.  The cooking liquor may contain caustic soda, sodium sulphide and sodium carbonate.  For treating straw and bagasse, there may be used 9-14 kg. of active alkali (NaOH) per 100 kg. of raw material (dry weight) and for pinewood sawdust 14-18 kg. of alkali per 100 kg. of raw material may be used.  Water or wash liquor may be passed through the cooked and disintegrated material under pressure and then, after it has absorbed inorganic and organic substances and after it has been mixed with active inorganic chemicals, the resulting liquor may be passed into the first cooking stage.  After the major part of an individual batch of liquor has drained out of a digester through a screen filter, a residual run-off may be taken, gases such as carbon dioxide as well as liquor and water-vapour being removed. This procedure is particularly suitable when sodium carbonate is used.  The reaction temperature may be reduced as digestion progresses.  Before the cooking solution is admitted, the material may be freed from air and brought to a temperature just above the cooking temperature by passing steam through the digester.  Cooking liquors containing lignin may be used. As shown in Fig. 1, the raw material passes through a cylindrical vessel in the direction of the arrows 1, 4, while the cooking liquor flows through the vessel in the opposite direction.  Water enters at position 3 and waste liquor leaves at position 2. Liquor is drawn off at 5, enriched with chemicals at position 6, causticized if necessary at 7, and re-admitted to the treatment vessel at 8.  As shown in Fig. 3, a battery of six digesters is used.  Water enters at position 11 and spent waste liquor leaves at 16.  Between the two digestion stages, liquor which has been enriched with organic and inorganic substances is drawn off and conveyed via the intermediate container 12 to the dissolving station for black ash 13 and then to the causticizing station 14, whereupon the liquor which has been enriched in active inorganic substances is conveyed to the digester III via an intermediate chamber 15.  After two batches of liquor have passed through the digester with the most digested material, this digester VI is disconnected and emptied, and another, freshly filled, container is connected at the other end of the battery.  Each charge may at first receive 5-6 batches of liquor with increasing concentration of inorganic active chemicals, and then a further four batches of liquor with decreasing concentration, and final two batches of water.  As shown in Fig. 4, a single digester, equipped as a percolator, is used.  The first two batches of liquor are passed through the material contained in the digester and the waste liquors a, b, are collected and then withdrawn from the system. The subsequent batches of liquor are passed through the digester and the waste liquors c to m are separately collected in storage tanks.  They are used again during the next percolation as batches a1 to k1.  The liquors f and g are treated with alkali, if desired by the addition of black ash, and they may be cauticized, before being re-used.  The subsequent two water batches may be taken from pipe 23.  From the digester 17, the outflowing batches of liquor pass through a filter into a pressure release vessel 18 and are there cooled by the release of pressure. They are then delivered into the appropriate storage containers. In this method of operation, the concentration of active inorganic chemicals increases continuously in the dregs a/b to f and decreases continuously in dregs h to m.