GB893334A - Improvements in the treatment of cellulosic fibrous materials for the production of purified cellulose or partially-purified cellulose - Google Patents

Abstract

In a continuous process for extracting non-cellulosic constituents from natural fibrous cellulosic material to produce purified cellulose, the cellulosic material to be purified, for example wood chips, is continuously introduced into a charging hopper 31, the lower end of which extends into digesting liquor contained in a feed tank 10 and into the mouth 16 of a pipe 15. The material, together with digesting liquor is drawn through the pipe 15 into the vacuum chamber 1 provided with a vacuum pump 7. Liquor is <PICT:0893334/IV (a)/1> withdrawn from the chamber 7 through the pipe 21 by the centrifugal pump 4 driven by the turbine 22 and electric motor 23, and discharged into the impregnating tower 22 at a rate sufficiently high to cause the concentration of cellulosic material in the liquor entering the chamber 1 to be relatively low and to maintain the height of the liquor in the chamber 1 at the level 8. In order that the period of resilience of the material in the chamber 1 may be relatively long the material entering the chamber is discharged into a receiver 32, the upper part of which is provided with apertures so that surplus liquor can escape into the chamber 1 and flow into the pipe 21. The fibrous material is continuously withdrawn from the bottom of the receiver 32 by a mechanically driven stirrer 36 and worm conveyed 37 at such a rate that the height of the material in the receiver remains about constant at the level 35. The material together with digesting liquor is pumped into the impregnation tower 22 in which the liquor is maintained at the level 9, liquor being continuously withdrawn from the tower 22 at an outlet 19 through the duct 18 into the feed tank 10. The fibrous material saturated with liquor is continuously raised by the mechanically driven worm conveyer 38 above the liquor level 9 and discharged into the pipe 6 down which the material falls into the digester 3, from which the digested fibrous material is continuously discharged through the delivery pipe 43 provided with gate valve 42. The height of the material in the digester is maintained at the level 40 and the material is brought to the required digesting temperature by the admission of steam through the pipe 41. Any condensate collecting above the level 9 of the digesting liquor in the tower 22 overflows through the outlet 44, pipe 45 and valve 46. Digesting liquor may be tapped off from the digesting liquor return pipe 18 to a tank 13 through a pipe 25 branching from the pipe 18 and provided with a valve 26. Fresh digesting liquor or a mixture thereof with liquor recycled from the impregnation tower 22 is fed through the control valve 11 and pipe 12 into the feed tank 10 to maintain the height of liquor in the latter at the level 14. Liquor from the tower 22 may be allowed to return to the tank 10 through the pipe 30 provided with a control valve 29 in order to prevent the temperature of the liquor in tank 10 from becoming too high.