GB720004A - Improvements in or relating to a process in the production of pulp by the digestion of cellulosic fibrous vegetable materials by circulation of lye through the materials - Google Patents

Abstract

In the digestion of cellulosic fibrous materials by circulating through a digester containing the materials a lye consisting of an aqueous solution or a suspension containing a relatively small proportion of one or more chemical reagents (e.g. calcium hydroxide) which are sparingly soluble in water, a reservoir forming the lower part of the digester is charged with water or previously prepared lye and a fresh aqueous saturated solution or a suspension of the chemical reagent or reagents is then added, the level of the liquid in the reservoir not being allowed to rise sufficiently to contact the fibrous material. Circulation of the lye through the fibrous mass is then started, the liquid being withdrawn from the reservoir at a point such that the withdrawn liquid is relatively free from suspended chemical reagent and being pumped up (optionally through a heater) into the digester above the top of the fibrous material and allowed to percolate down through the material by gravity and thus return to the reservoir, from which it is again pumped up. During the circulation of the lye, fresh lye (optionally preheated) is continuously or intermittently bled into the reservoir, the level of the liquid in the reservoir not being allowed to rise substantially above the bottom of the mass of fibrous material so that the liquid, while in the reservoir, is maintained substantially out of contact with the material. The proportion by weight of lye to cellulosic material may be 1 : 10 or less. As shown, (in Fig. 1), an aqueous solution or suspension of a chemical reagent is supplied to a removable or hingedly attached reservoir b by means of a pipe e from a container (not shown). The liquid, before entering pipe e, passes through the shell k of a heater in which it is heated by means of a steam coil l. A lye circulating pipe f leads from the reservoir <FORM:0720004/IV(a)/1> <FORM:0720004/IV(a)/2> b to a distributer ring g via a circulation pump (not shown). The upper part of the digester consists of a chamber a holding the fibrous material and provided with toothed racks c holding an upper sieve member d which is provided with locking pawls and is pressed down on to the material (not shown). In the upper part of the reservoir b is mounted a lower sieve member h. As shown in Fig. 2, the reservoir b is charged by introducing water or lye through an inlet pipe i and leading it through a conduit j, which keeps the incoming liquid out of contact with the fibrous material and also carries the sieve members d and h.