GB834006A - Improvements in or relating to the pulping of cellulosic material - Google Patents

Abstract

Cellulosic material is pulped by subjecting the material to a first digestion at a temperature below the boiling point and at atmospheric pressure with a liquor containing 0.5-7% weight per volume of sodium hydroxide (or its equivalent in terms of Na2O), with or without sulphidity, for a period of time depending inversely upon the temperature, the liquor to material ratio being not less than 3:1, diluting if necessary, fiberizing the digested material by a rubbing action, and subjecting at least a portion of the resulting material to a second digestion at a temperature below the boiling point and at atmospheric pressure with liquor similar to that used in the first digestion step, the liquor being removed from the pulp produced by the first and second digestion steps or, in the case where all the material is subjected to a second digestion, from the pump produced by the second digestion step. "Sulphidity" is defined as <FORM:0834006/IV (a)/1> per cent, both Na2S and NaOH being expressed as Na2O, and "fiberizing" is the mechanical treatment applied to resolve the softened aggregates into a fibrous pulp. The liquor to material ratio is the ratio by weight of the sum of moisture in the raw material and the added cooking liquor to the oven-dry weight of the raw material. After the first digestion the material may be fiberized, washed and screened, only the rejects from the screening step being re-cycled for a second digestion at the location of the first digestion. Alternatively, after the first digestion, the material may be washed, fresh liquor added, and all the material fiberized and then digested in a second digester, the liquor removed after the second digestion being concentrated and returned to the first digester. The process is particularly applicable to the pulping of graminaceous cellulosic material, e.g. esparto grass, juncus maritimus, wheat straw, rice straw, sabai grass, bamboo and bagasse obtained from sugar cane, but it may also be applied to hardwoods, e.g. birch or aspen, and bast and leaf fibres. After the second digestion the pulp may be washed, screened, bleached and cleaned and used for the manufacture of fine papers, but if the bleaching is omitted, pulp suitable for the manufacture of board is obtained.