US1880042A - Acid process of fiber liberation - Google Patents

Description

atente fiept. .1

GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR T0 BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE ACID PROCESS OF FIBER LIJBERA'I'ION No Drawing.

' This invention relates to the production from wood or other raw cellulosic materlals of a high grade chemical pulp characterized not only by excellent physical characteristics,

5 is thencooked in a solution of'sul-phurous 5 including strength and tear resistance, but

also by easy bleachability, so that it may be converted into a white product suitable for high grade papers, with low bleach consumption and without .substantial injury to its ph sical or chemical characteristics.

'lhe so-called acid sulphite process of fiber liberation is employed for the production of light-colored pulps which are bleachable to the whiteness desired in high grade papers, without excessive degradation of the cellulose or injur to the fibers. In practising this process of her liberation, chipped wood or other raw cellulosic material is cooked under 'conditions of high temperature and pressure in a liquor consisting of a sulphurous acid solution of a sulphite of calcium, magnesium, sodium, or the like, containing about 4% to 7% free S0 which is gradually released from the liquor during the cooking opera- 5 tion, while the temperature of cooking is increased.

in accordance with the process of the pres ent invention, fiber liberation is completed in a liquor of lower combined S0 content than that of the fresh liquor in which cooking is started, as I have found that such a process makes possible the production of a pulp of improved characteristics, including higher strength and tear resistance and easier bleachability. In carrying out my process, the raw cellulosic material is initially cooked in the usual acid sulphite liquors or in acid sulphite liquors in which chemicals such as sodium sulphate sodium bisulphate, borax, or the like, undergo progressive metathetical reaction with sulphurous acid to produce sodium bisulphite. "This initial cooking operation, however, is carried short of complete fiber liberation, whereupon the cellulosic material acid containing little, if any, S0 in combined condition. The second cooking operation may be carried out at lower temperature than the initial cooking operation, for under such lower temperature conditions a sulphur- Application filed octoberae, 1929. seri itcueparer.

ntaining little, if any, comb ned S0 ismore reactive and more selective in its actionupon hgneous constituents int cellulosic material than the usual acid su iquor. The'improved qualities of the ltiii'g' product overthe usual sulphite pulp may be attributed to the greater hydrolysis of fiber occurring during the usual acid sulphite cook under the progressively increasing temperatures employed to compensate for the removal of free SO from the cooking liquor.

The following specific example of procedure will serve to illustrate conditions under which the process of the present invention may be practised to produce a pulp havin markedly better characteristics than the usua sulphite pulp. Spruce or other wood chips are initially cooked while confined in a digester in an acid sulphite liquor containing a substantial proportion of combined S0 The acid sulphite liquor may be of thesodium base or the usual calcium base type, and contain 1% S0 combined in the form of sulphite, and 5% S0 in free condition. I

When a sodium base is employed, it may be preferable to dissolve suflicient sodium base chemical in the liquor to produce a much higher combined S0 concentration. therein than that of the less soluble calcium base, for instance 3% to 4%, as a more rapid and "uniform penetration of the wood by combined S0 is ensured. Such higher combined S0 concentration permits higher tem erature of cooking, as the combined S0 in t e chips retards hydrolyzing action on the fibers by the free S0 The temperature of cooking in an acid sulphite liquor containing 1% combined S0 and 5% free S0 shouldnot be higher than 300 F., a temperature of 275 F. preferably being employed, but when an acid sulphite cooking liquor containing, say, 4% combined and 4% free S0 is employed, cooking may be carried out at 300 to 325 F, and these higher temperatures may be reached more quickly without danger of a non-uniform action and injury to the fibers. After the chips and liquor have been brought to maximum cooking temperature, such temperature may be maintained for from two to till four hours, depending upon the composition of the particular liquor employed and the maximum temperature of cooking. At the end of this period, the chips are lighter in color than they were originally, but they are still hard because they contain considerable non-fibrous cementitious material, and hence cannot be reduced to pulp unless they are subjected to a severe grinding operation, such as practiced in making ground-wood pulp. The acid liquor is of light color and contains a. large proportion of unspent chemical, so that it may be separated, as by draining, from the cooked chips and again used for the cooking of raw wood, additional SO being dissolved therein, if desired, to compensate for such loss of free S0 as took place during cooking and separation of the cooked chips. In order to minimize the loss of free SO from the acid cooking liquor, it may be separated from the chips under pressure by pumping it from the confined charge in the digester to another closed digester in which the pulping of other raw wood is to be effected.

The cooked chips, while still hot and confined in the digester, may be treated with the second cooking liquor consisting of a solution of practically only sulphurous acid, but it is preferable to wash them before adding such cooking liquor to remove substantially all the combined S0 associated therewith after cooking. The sulphurous acid solution employed may contain 3% to 6% S0 it being preferable to employ liquors of such free S0 asan excessive period of time is required for completing fiber liberation in more dilute solutions. When using a liquor containing, say, 5% free S0 cooking of the chips may be carried out at a maximum temperature of 250 F. In order to avoid pressures above 7 5 to 80 pounds, free S02 may be released from the liquor as cooking proceeds, the released gas being recovered in suitable systems, as ordinarily. If cooking is effected at a temperature of 250 F. for about three to five hours, the chips will be completely pulped, so that the digester contents may then be blown into a pit, where the pulp may be drained or washed free of cooking liquor. The free S0 arising from the pit during the blowing of the digester may be recovered for use in the production of fresh acid liquor, and the separated spent liquor may be mixed with spent liquor resulting from the initial cooking step, when recovery of their valuable constituents is practiced, as for instance when a sodium. base cooking liquorjs employed in the initial cooking step.

A pulp produced as hereinbefore described is of high strength and tear resistance and has a high cooking stain value, indicating easy bleachability. I present herein a table showing certain characteristics of a pulp produced as hereinbefore described, and the usual commercial sulphite Product resulting from the Usual comprocess of the mercial sulpresent invenphite pulp tion Strength 150 125 Tear 180 150 Cooking stain 120 100 The cooking stain value of 120 indicates that the ulp resulting from the process herein descri ed will require only about 8% calcium hypochlorite bleach to be converted into a white product, whereas the sulphite pulp having a cooking stain of 100 will. consume about 15% of such bleach in acquiring the same degree of whiteness.

My process may be practised in a period of time not in excess of the cooking period of the usual acid sulphite process. Were it attempted in such a period of time to produce by the usual acid sulphite process a product having the easy bleachability of the product resulting from my process, it would be necessary to complete fiber liberation at such high temperatures as would cause serious degredation of cellulose and a marked lowering of the strength of the product. On the other hand, were it attempted to produce pulp by cooking in sulphurous acid solutions alone, it would be necessary to cook for exceedingly long periods of time and at a temperature of about 220 F., or lower, owing to the severe hydrolysis of cellulose and injury to the fibers taking place if fiber liberation is hastened by raising the temperature of the cooking liquor. Even when fiber liberation is effected at comparatively low temperature by cooking in a liquor consisting of only sulphurous acid solution, the resulting pulp is so dark that it requires an excessive amount of bleaching agent and undergoes such marked degradation during bleaching as to be of little value for papermaking or other purposes. Apparently the combined S0 content of the usual acid sulphite liquor reacts upon encrustations of the wood in a different way than a sulphurous acid solution alone, this being borne out by the fact that after wood has been cooked with a sulphurous acid solution alone, it cannot readily be converted into fiber by the usual acid sulphite cook. Evidently a sulphurous acid solution causes a resinification or polymerization of certain constituents present in the raw Wood to produce reaction products difiicultly soluble in the usual acid sulphite liquors, even under high temperature and pressure conditions. By first cooking the wood in an acid sulphite liquor containing combined S0 for instance the usualacid sulphite liquor, the production of such undesirable reaction products is evidently avoided, so that the fibers of the cooked chips may be readily liberated when fiber liberation is completed in a sulphurous acid solution containing little, if any, combined SO to produce a finished product havin the highly desirable characteristics herein efore noted. 5 I claim:

1. A process which comprises cooking raw cellulosic material short of fiber liberation in an acid sulphite liquor containing at least about1% combined S0 and then completing fiber liberation at lower temperature in. a liquor containing substantially only free S0 2. A process which comprises cooking raw cellulosic material short of fiber liberation in rating the liquor from the cooked material,

peratures in a solution of substantially only sulphurous acid until fiber liberation is complete. 2o 3. A process which comprises cooking raw cellulosic material short of fiber liberation in an acid sulphite' liquor, separating the liquor from the cooked material, washing the material, and cooking the material at lower temperature to complete fiber liberation in a solution of substantially only sul phurous acid.

4. A process which comprises cooking raw cellulosic material short of fiber liberation in an acid sulphite solution containing at least 1% combined S0 and at temperatures above 250 F., separating the liquor from the cooked material, and then cooking the material to complete fiber liberation in an as acid liquor containing less than 1% combined 0 and more than 3% free S0 and at temperatures below 250 F.

5. A process of fiber liberation which con1- prises cooking chipped wood in an acid sulphite liquor suificiently short of fiber liberation so that it contains considerable nonbrous cementitiousmaterial, and then cooking at lower temperature in a solution of substantially only sulphurous acid to complete fiber liberation into a pulp.

In testimony whereof I have aifixed my signature.

GEORGE A. RICHTER.