US1761544A - Process for separating or extracting cellulose or paper pulp - Google Patents

Description

' cellulose an Patented June 3, 1930 PATENT OFFICE EDIONDSON SPENCER, 0F CALCUTTA, BRITISH INDIA PROCESS FOR SEPARATING 0B EXTBACTING GELLULOSE 0R PAPER IPULP Io Drawing. Application filed February 10, 1828, Serial Ko.,2 5 8,311, and in British India Kay 28, 1827.

This invention relates to processes for the separation or extraction of cellulose or paperpulp from vegetable matter containing the same, and has for its object modifications of 5 the processes which are calculated to increase the ield of cellulose and the quality of the pro uct.

In the di estion of materials to produce paper pulp it is usual to increase the activity 0 the digesting liquors towards the final stages of the digestion in cases where the digestion is carried out on the semi-continuous, progressive, step-by-step or the cascade s stem; whereas in the ordinary or overhea process, the activity cannot increase as the digestion proceeds since the proportion of activejreagent is continually diminishing. In the; overhead rocess a given amount of caustic isadde and the strength of active caustic progressively diminishes towards the end of the di estion. This together with the fact that all t e pro ducts of decomposition are present to the end of the process constitutes the main weakness of the over-head process. To surmount these two difficulties the cascade process aims at removing the products of decomposition in successive stages and in replac ng the liquors with fresh solutions relatively stronger in active alkali and relativel weaker in products of decomposition. this means the'digestion time is shortened very considerably. The lar ely increased activity of free caustic in t e latter sta s may however result in a partial destruction of'a portion of the cellulose fibre and to guard against this. it may be advisa 1e to have the greatest strength of active alkali not at the last digestion stages but at some stage earlier in the digestion at a lower pressure and with more of the non-cellulosic material still to be removed, or, alternately, when using mixed liquors (such as caustic and sulphites and/or su phides, which latter exert a protective action a similar result is obtained by proportioning the liquors added in such a way that the hydrolysis is prevented b having a relatively greater proportion of t e more protective or lesssev ere reagents in the final stage of the digestion.

The invention relates mainly to processes wherein mixed digestor liquors are used in at least part of the di estion "period and wherein the rocess of digestion is carried out in successive stages, but it is not restricted to use with mixed digestor liquors alone.

It is known that sodium mono-sulphite Na SO is not so severe in its action on cellulose fibres at raised pulping temperatures as ordinary caustic is.

Also it is known that sulphide liquors are somewhat similar to sulphite liquors in this respect that they are not so active as caustic in attacking the more easily resolvable celluoses.

hen using a mixed digestor liquor, such as a mixture of caustic soda, sulphite and/or sulphideit may be advantageous to have certain of these chemicals in solution in separate tanks and add certain limited quantities to the digestors according to a preconcerted plan. For example, sodium sulphite and sodium sulphide being known to exert a pro tecting action on the cellulose fibre in reducing the hydrolyzing effect it ma accordingly be advantageous to have a relhtively large roportion of these reagents in the final drastic) stages of the treatment and a relatively larger roportion of the caustic soda in the preceding stages of the process.

According to one part of this invention the composition of the digestin liquors is altered as the digestion procee s. The alteration may take place at any stage but more particularly towards the final stages of the digestion.

According to this invention a certain amount of protection may be afforded to the fibre reducing hydrolysis, by manipulating and even by actually reducing the digestion pressures in the final stage of the process. By manipulating the composition of the, digesting liquors and the pressure at which the rocess operates, by raising the pressure gra ually or in steps in the earlier stages and by holding the pressure constant towards the end of the cook combined with a variation in the liquor constitution or proportions, and even by reducing the final pressures below the maximum ones used when found desirin steps or at some point or points in the process.

It is however more particularly applicable to processes of the type described and claimed in application Serial No. 253,310, filed February 10, 1928, and Patent No.- 1,690,954, dated November 6, 1928, wherein the pulp forming material is subjected in a digester to successive treatments at increasing pressures and with increasing active liquor concentrations in a step-up pressure-free-caustic-concentration gradient.

The invention accordingly comprises a process for separating or extracting cellulose or paper-pulp from the material (such as bamboo, wood, grasses, reeds or other vegetable matter of fibrous nature) containing the same, wherein the extraction of the solubles from the material fed into any digestor is effected by digestion in a semi-continuous, progressive, step-by-step or cascade system,

ut more particularly wherein the digestion is effected in a generally'increasing pressure and free caustic or digesting liquor concentration gradient, or in a series of steps or stages, the digesting liquor concentration and the pressure enerally being raised at each stage, from the first treatment of the fresh stock until the digestion is nearly complete, characterized by the digestor liquor activity or concentration in the last stage of digestion, or the composition of the digesting liquor, and, if desired, the pressure also, in the final stage or sta es of digestion being modified with the ob ect of avoiding hydrolysis of the cellulose fibre in said last stage or stages.

The invention also includes a process for separating or extracting cellulose or paper pulp from the material containing the same wherein the extraction of the solubles from the material fed into any digestor is etfected in stages by digestion on a semi-continuous, progressive, step-by-step system or on a cascade system, wherein a modification is introduced in that the greatest strength of active alkali is not at the last digestion stage but such greatest strength is at some stage earlier in the digestion, preferably at a lower pressure, and when more of the non-cellulosic material still remains to be removed, in order to prevent or reduce the hydrolysis of the cellulose fibre in the final stages of digestion.

It may comprise processes such as those referred to above wherein mixed dlgesting liquors are used, such as caustic and sulphites and/or sulphides (which latter exert a protective action reducing the tendency towards hydrolysis of the cellulose fibres taking place), and the liquors added are proportioned in such a way that hydrolysis is prevented or reduced by having a greater roportion of the protective reagents in the nal stage or stages of the digestion.

The invention may comprise such a process wherein mixed digesting liquors are used and wherein the liquor or liquors, of those used, which have the greater retarding effect on the hydrolysis of the cellulose at raised pressures is or are in excess, or form the whole of the liquor, in the final stage or stages of the digestion, the liquor or liquors having a stronger effect in causing hydrolysis bein added at a point in the digestion process efore the final stage in the digestion is reached and while the fibre still contains a considerable or appreciable proportion of non-cellulosic matter.

It also includes a process wherein mixed caustic liquor and sulphide and/or sul hite liquor are used, but wherein the sulphide or sulphite liquor or liquors is or are used alone or in excess in the final stage or stages of the digestion owing to their retarding effect on hydrolysis of the fibre, but wherein the canstic is added at a point in the digestion before the completion stage, is reached.

The invention may further include a process as described, or as claimed in any of the claims of my application Serial No. 253,310, filed February 10, 1928, and Patent No. 1,690,- 954, dated November 6, 1928, wherein a mixed digestor liquor is used in a portion at least of the digestion process, and wherein the different digestion liquors are supplied from separate sources with the object that the composition of the said liquor in the different digestion stages may be varied as desired.

In this last process the modification may be introduced in that the whole of the fresh digestion liquor is not added to that digestor which is about to undergo its final digestion, but instead further fresh digestion liquor is added to a digestor in a subsequent stage of the general step-down pressure-liquor-concentration series.

The invention further includes a process wherein the digestor undergoing final (liges' tion at the highest steam pressures has sodium sulphite, or sodium sulphide, or both a sul phite and sulphide, as the digesting liquor, or in which the digesting liquor for the final digestion has a considerable proportion of sulphite or sulphide therein, and wherein into the next digester in the step-down pressure series, or alternatively, at some other step or period of the digestion, caustic soda is added to make upwthe strength of the digesting liquor.

The invention also includes within its scope the modification whereby in addition to the variations in the digesting liquor composition the pressurein the final stage or portion of the digestion is held steady for a period or is even reduced to below the maximum pressure used in said digesting for the purposes explained.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is 1. A cascade process for the extraction of cellulose pulp from fibre yielding vegetable material, which comprises treating the material with digester liquor and under pressure in stages in-digesters, progressively increasing the pressure and active liquor concentra-' tion at each stage until the di estion is nearly complete,an d then varying the active liquor concentration in the final stages sufliciently to substantially prevent the hydrolysis of the cellulose fibre in the final sta es.

2. A process according to c aim 1, wherein pressure and caustic liquor concentration at each stage until the digestion is nearly com- 'plete, and then reducing the proportion of caustic liquor in the final stages to a value sufficient to substantially prevent the hydrolysis of the cellulose fibre in the final stages.

4. A cascade rocess for the extraction of cellulose pulp fire yielding vegetable material, which comprises treating the material in stages 1n digesters under pressure and with a mixed liquor composed of caustic liquor and a liquor having a retardin influence on the hydrolysis of cellulose bre at raised pressures, progressively increasing the 7 pressure .and the caustic liquor concentration until the digestion is-nearly complete, and then increasing the proportion of retarding liquor in the final stages until it is in excess of the caustic liquor.

5. A cascade process for the extractionof cellulose pulp from fibre yielding vegetable material, which comprises treating the material in stages in digesters under pressure and with a liquor composed of a mixture'ofcaustic and other liquors supplied from sep, arate sources, progressively increasing. the pressure and the concentration of caustic in said liquorat each stage until the digestion is nearly complete, and then varying the value sutficient to prevent the hydrolysis of the cellulose fibre in the final stages.

6. A cascade process for the extraction of cellulose pulp from fibre yielding vegetable material, which comprises treating the material in stages in digesters under pressure and with a mixed liquor composed of caustic soda and sodium sulphite, progressively increasing the pressure and the caustic .concentration at each stage until the digestion is nearly complete, and then treating the material in the final stages with a liquor composed substantially entirely of sodium sulphite.

7. A cascade process for the extraction of cellulose pulp from fibre yielding vegetable material, which comprises treating the material in stages in digesters under pressure and with a mixed liquor including sodium sulphite, progressively increasing the pressure and adding caustic soda until the digestion is nearly complete, and then increasing the pro portion of sodium sulphite until the liquor in the final'stages iscomposed substantially entirely of sodium sulphite.

'8. A cascade process for the extraction of cellulose pulp from fibre yielding vegetable materiahwhich comprises treating the mate rial with a mixed liquor and under pressure in stages in digesters, progressively increasing the pressure and. active liquor concentration at each stage until the digestion is nearly complete, and then maintaining the pressure constant and reducing the active liquor concentration in the final stages sufiiciently tosubstantially prevent the hydrolysis of the cellulose fibre in the final stages.

In testimony whereof I hereto aflix my signature this' 11th day of January, 1928.

EDMONDSON SPENCER