Patent GB-727191-A (GB)
Title: Improvements in or relating to the production of cellulose pulp from plant materials
Inventor: ROSS JAMES HAMILTON
IPC: D21C1/10
Filed: 1953-01-27
Abstract: <PICT:0727191/IV (a)/1> Cellulose pulp is obtained from fibrous plant material by a process which includes the steps of introducing the material in subdivided condition into a pressure vessel, introducing steam under pressure into the vessel to displace air and to establish within the capillary interstices of the plant material a mixture of air and steam, rapidly withdrawing the mixture of air and steam from the vessel and plant material (while preventing additional air from entering the vessel) until a pressure is reached within the range of from atmospheric pressure to a gauge pressure half that which existed when the introduction of steam was discontinued, and immersing the pretreated plant material in a cooking liquor while continuing to exclude air from the material. The fibrous plant material may be wood chips, straw, bamboo, or other ligno-cellulosic material and the process may be a sulphate, sulphite or other type of chemical pulping process.  Preferably, the maximum value of the steam pressure in the pretreatment is within the range of 5 to 50 lbs. per square inch above atmospheric pressure.  If desired, the pretreatment with steam may be repeated one or more times before the material is introduced into the cooking liquor.  The pretreatment increases the moisture content of wood chips and other plant materials which originally have a low moisture content but decreases that of materials which originally have a high moisture content.  The final moisture content in most cases is close to the fibre saturation point, e.g. 40 per cent, at which the fibres exhibit their maximum ability to absorb defibrating reagents which are brought in contact with the material in aqueous solution.  In an example, Jack pine chips are packed into a digester (Fig. 2) through opening 6 by means of a steam packer, aided by introduction of steam into the bottom of the digester through valve 8.  The cover 11 is then closed with vent valve 14 open.  The supply of steam is continued until a steam discharge temperature of 212 DEG  F. is registered at vent valve 14.  This valve is then closed and the supply of steam is continued till the pressure in the digester is raised to 20 lbs. per square inch.  Valve 8 is immediately closed and valves 14, 16 are opened to reduce the steam pressure to 10 lbs. per square inch as rapidly as possible. This steam treatment is repeated. Cooking liquor is then pumped into the digester through valve 20 until the digester is filled.  The supply of liquor is then cut off and pressure is applied by steam which is introduced into the top of the digester, a pressure of 100 lbs. per square inch being reached in a few seconds.  A large proportion of the surplus liquor is then withdrawn and returned to the supply tank.  Steam is introduced to bring the contents of the digester to 250 DEG  F. and the cooking at this temperature is continued for 2 hours.  The cooling liquor is a kraft liquor containing 16.5 per cent active alkali of about 30 per cent sulphidity.  In another example, a uniform pulp is obtained from a mixture of Jack pine and poplar wood chips by a similar treatment. In another example, hemlock chips are subjected to the pretreatment with steam in a sulphite digester and then treated with an acid sulphite cooking liquor.