US1915953A - Chemical pulping process - Google Patents

Description

Patented June 27, 1933 UNITED STATES PATENT- OFFICE cnoncn a. mourns, or BERLIN, nnw mrsnmn, ASSIGNOB ro nnown comm,

or BERLIN, rmw mrsmn, A conroanrron or MAINE cnmncar. rcrrme raocnss No Drawing.

This invention relates to a chemical pulping process, and more particularly to one in which sulphites are used as fiber liberating agents.

2 5 In the production of soca'lled sulphite pulps, it is usual practice to employ as the cooking liquor a sulphurous acid solution of a sulphite in which sulphurous acid is present in amount materially beyond that neces 1c sary to form the bisulphite. The sulphurous acid or sulphur dioxide content which is neutralizable when an alkali such as caustic soda is added to the liquor, is sometimes known as free S whereas the sulphur dioxide which is combined in the form ofthe neutral salt or monosulphite is termed combined S0 The acid sulphite liquors customarily employed for the production of sulphite pulps, e. g., calcium base liquors, have a relativel low combined S0 content and a high free 0 content, say, 1% to 2% combined and 5% or more free S0 In accordance with the present invention, I depart, sharply from the usual sulphite high combined 2 content and a relatively low free S0 content. In fact, the sulphite liquors which I employ have a free SO content less than one-half of their combined S0 content In order to obtain liquors of this type in which the combined S0 exists in solution, it is necessary that the sulphitebe one of the alkali metal group,sodium sulphite being preferable because of its general fiber liberation in as short a period of time as that required for the usual sulphite process, but further that the yield and quality of pulp Application filed Kay 29,

process by usin sulphite liquors having a availability and relatively low cost. The

1930. Serial No. 457,541.

do not suffer thereby. For instance, as hereinafter described, cooking may be practised advantageously at temperatures even higher than about 325 F. This is especially true when the free SO content of the liquor is less than 1% and the combined S0 content is 5% or greater, as in such case cooking at 335 to 375 makes possible the complete pulping of chipped wood in from six to eight hours. Even when the free SO content of the liquor is as high as 1% to 2%, if the combined S0 content is, in accordance with my invention, more than double this amount, for instance, 5% or greater, I preferably practise cooking at temperatures as high as 335 F. \All this will appear hereinafter in greater detail when I give specific examples of procedure. This is in marked contrast to the scorching of the fiber and the low yields of pulp resulting when cooking is effected 70 above temperatures of about 250 to 275 F.,

in the usual acid sulphite liquors. Evidently a high free S0 content relative to combined S0 content in a sulphite cooking liquor means that the liquor is highly active in the liberation of fiber from raw cellulosic material,so much so that the temperature and time or cooking must be carefully controlled. L The use of sulphate liquors in which the combined SO content preponderates materially over the free S0 content as hereinbefore described makes possible the production of pulps having characteristics quite diiferent from those associated withthe usual sulphite pulps. Thus, a pulp liberated in accordance with the process of the present invention has notably higher strength and tear resistance than that of the usual sulphite pulp. These superior characteristics are doubtless attributable to the fact that a high 9 free S0 content in the liquor causes a hydrolysis and weakening of the cellulose fibers, as well as a more extensive removal of pentosans and ligneous matter, from the raw cellulosic material than the liquors of the present invention. The fact is that the prod-- not of the process ofthe present invention is much higher in its content of pntosans and ligneous matter than the usual sulphite pulp; and this matter evidently serves as a binder 100 "sulphite pulp.

- ing withground-wood pulp in the manufacture of a good quality newsprint. It is also especially serviceable as a raw material in the manufacture of papers of high Mullen strength, for, instance, glassine papers, wherein its high pentosan content tends to enhance the transparency of the paper. It may further be used as a raw fiber for refinement into a final product of high alpha cellulose content by a series of suitable chemical treatments, including chlorination, digestion'in a liquor of suitable akalinity in either hot or cold condition, and a final bleaching operation.

While there may be various sulphite cooking liquors falling within the purview of my invention, it is distinctly preferable that the combined SO content of the liquor be about 4% or greater, and that the free SO content be about 2% or smaller. The use of liquors having such a high combined S0 content gives rise to a high rate of penetration of chemical into the raw'cellulosic material, so that in a comparatively short period of time the raw cellulosic material is uniformly impregnated with the liquor and the end of the cooking operation marks a substantially uniform resolution of the cellulosic material into pulp. The process maybe carried out substantlally in accordance with the standard sulphite mill practice. That is to say, chipped wood, e. g., spruce, is placed in an acid-resistant digester along with the desired cooking liquor, say, one containing 5% combined S0 in the form of sodium sulphite and 1% free S0 The digester charge is then confined and is heated to about 300 to 335 E, which temperature is maintained for from six to eight hours, in order to ensure complete fiber The digester contents are then liquor, which may be subjected to suitable processing for the recovery and reuse of its valuable sodium constituent, when a sodium base liquor has been employed. The pulp may be washed, screened, and subjected to such further treatments as may be desired. In unbleached condition it has a Mullen strength 'of about 170 and a tear resistance of about 200, which characteristics are superior to those possessed by the usual unbleached sulphite pulp, which has a Mullen strength of about 120 and a tear resistance of about 150. I present herein a table giving various important characteristics of the new fiber produced as hereinbeforedescribed as compared with those possessed by the usual commercial unbleached kraft and sulphite pulps Un- Kraft bleached fg sulphite 1 Beaten to same slowness Slowness 6. 00 6,00 6.00 Shrinkage on drying sheet 12. 00 I 14. 00 12. ()0 Strength of sheet dried without ten sion. 120 170 Tear 150 200 Pentosans- 3-3. a 7-9 Color Cream Cream Lignin 23% 10-15% Yield on cooking 45% 60-75% If the free S0 content of the liquor is reduced .from 1% to, say, 5%, the cooking temperature necessary for the realization of pulp in from six to eight hours ranges from 335 to 375 F., but if it is increased to, say, 2%, the cooking temperature range for the liberation of pulp in the same length of time is from 300 to 335 F. In other words, the higher the free SO content of the cooking liquor, the lower the temperature necessary to produce within a given period of time a spruce-wood pulp answering approximately the characteristics of the pulp hereinbefore described. The temperature of cooking may further vary with the types of raw material employed,-hardwoods, for example, generally requiring somewhat lower temperatures than soft woods. In any event, however, cooking is eifected in a liquor in which the free S0 hate to the combined S0 content, and in content is far 'subordiproportion relative to the combined S0 content of the usual sulphite liquors.

If desired, the cooking liquor may be prepared by forming a solution of sodium sulphite'of a strength corresponding to both thecombined and free S0 content desired in the liquor and adding sulphuric acid and/or niter cake in amount to produce in situ the desired free S0 content. The preparation of the sodium sulphite may be carried out in an apparatus constructed entirely of wood and iron, which is less expensive than the acid-proof apparatus necessary for the production of an'acid sulphite liquor. The sodium sulphite solution may then be treated in anacid-proof tank with the desired amount of sulphuric acid and/or niter cake. Such a procedure, which dispenses with the use of sulphurburner gas, is partiularly advantageous when niter cake, which is an inexpensive chemical by-prodnot, is used, since, aside from the function of the sulphuric acid content of the niter cake in reacting with the sodiumsulphite to produce sulphurous acid and sodium sulphate, the sodium sulphate content of the niter cake and the sodium sulphate reaction product are active chemicals in the cooking liquor in the sense that as sodium sulphite is exhausted by reaction on the raw cellulosic material, the sodium sulphate undergoes a metathetical reaction with the free S0 content of the liquor to generate sodium sulphite. The same metathetical reaction takes place when the sodium sulphate is formed by the addition of only sulphuric acid to the sodium sulphite solution or when sodium sulphate is added in prepared condition, i. e., as salt cake, to a liquor containing free as well as combined S0 The sulphite cooking liquor of the present invention may therefore contain sodium sulphate as well as sodium sulphite and free S If the temperature of cooking is sufiiciently high, say, 375 to 400 F., the fiber liberation or pulping of wood may be accomplished in from six to eight hours in a straight monosulphite liquor, for instance, in a liquor containing 5% sodium sulphite. Such a liquor removes less ligneous matter and pentosans from the wood than a liquor containing free S0 but because it is on the alkaline rather than on the acid side,

it may be used in steel digesters similar to those used in the soda and sulphate processes of fiber liberation.

The specific examples of cooking hereinbefore given, in which the raw cellulosic material in converted into a pulp, may be modified considerably, especially as regards time and temperature conditions, when the cooking operation is merely one of a multiplicity of cooking operations or other treatments designed to liberate pulp from the raw cellulosic material. For instance, when the cooking operation is carried out upon raw wood, such as chipped spruce, for about six hours at 300 F., in a liquor containing 5% combined S0 and free S0 the cooked product is a mass of softened chips containing from 10% to 15% lignin. The yield of this product is to based on the dry weight of Wood used as a raw material. The product may be subjected to mechanical treatment, as in a heater engine or in a Kollergang, to produce pulp; or it may be subjected to further chemical treatment, for

2. In the production of pulp from cellulosic material, the step which comprises cooking such material in an acid sulphite liquor containing sodium sulphate and in which combined S0 is present greatly in excess of free S05.

3. In the production of pulp from raw cellulosic materials, including wood, the step which comprises cooking such materials at a temperature of not less than about 325 F. in a sulphite liquor whose free S0 content is less than one-half of its combined SO content.

4. A process of producing pulp from raw cellulosic materials, including wood, which comprises cooking such materials at a temperature of not less than about 325 F. in an acid sulphite liquor whose combined S0 content is not less than 4% and whose free S0 content is less than one-half its combined S0 content.

5. A process of producing pulp from raw cellulosic materials, including Wood, which comprises cooking such materials at a temperature of not less than about 325 F. in a sulphurous acid solution of an alkali metal sulphite of the nature of sodium sulphite,

whose combined S0 content is not less than 4% and Whose free S0 content is less than one-half of its combined SO content.

In testimony whereof I have aflixed my signature.

GEORGE A. RICHTER.

CERTIFICATE OF CORRECTION.

Patent No. 1,915,953. June 27, 1933.

GEORGE A. RICHTER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction asfollows: Page 1, line 79, for "sulphate" read sulphite; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this l2th day of December, A. D. 1933.

" F. M. Hopkins (Seal) Acting Commissioner of Patents.