US1841551A - Multiple-step process of fiber liberation - Google Patents

Description

' duce a pulp which at thetime of liberation components, inc uding pentosans, and which' first .liquor employed is a SuIphurousacid- Ho Drawing.

ent invention, three chemical liquors of difmaterial and in so modifying the constitution Patented Jan. 19,1932

UNITED STATES PATENT OFFICE enonen a. Brennan, or nnanm, rmw nssronoa 'ro :Bnowrr CODA-NY, or

- BERLIN, NEW a coarom'rron or we v111ml:Iran-swan rn'ocnss or FIBER manner I museum .153? e, 1980. Serial I'm 450,314.

This invention relatestothe liberation of necessary to complete fiber liberation, but

fiber from raw cellulosic materials to proalso a higher yieldof pulp, basedon wood, {without impairing thebleachability of the is practically free from non-alpha cellulose pulp. The intermediate treatment with the bisulphite liquor is efiected under conditions designed not to isolate the fiber content of thecellulosic material. The combined S0 content des'ired in'the bisulphite liquor may is characterized by a low so-called solution viscosity, which renders it especially valuable for conversion into derivatives to be treated with solvents to form solutions of low. viscosity.

- soda, sodium sulphite, or sodium sulphate to In accordance with the process of the presthe sulphurous acid solution remaining after the first treatment. These chemicals may be ferent compositions are employed in the treatment of the cellulosic material before its fiber content is isolated as pulp freed from the associated organic materials. Each liquor exerts a chemical action on'the cellulosic material, and thus contributes to the results secured, but there is a preferable se quence in which the liquors are used. The

in amount calcillated to produce the desired combined SO content. When such aproce- [dure is followed, heat consumed in raising the temperature of the sulphurous acid solution of the first treatment is conserved for the secof the first treating solution is also conserved used in the second treatmentyand the time lost when the first treating solution is drained 'from the chips and replaced by a previously prepared bisulphite solution is avoided. 1 g The process of the present invention is applicable to various raw cellulosic materials solution which is especially effective for removing pentosans from the raw cellulosic of the cellulose fibers asto lead to a final product of low solution viscosity. The s ec 1 0nd liquor is a bisulphite solution which may be addedto the cellulosic material after the sulphurous acid solution has been drained therefrom, or which may be prepared from the sulphurous acid solution by adding suitable chemicals thereto. The bisulphite solution is an active agency in the removal of ligneous matter contained in the cellulosic materialand in conditioning the cellulosic material for the treatment with the third chemical liquor, which brings about complete fiber liberation. The third chemical liquor is a solution of alkali, such as that used in the soda or kraft process, which; even at low concentrations, readily completes the ,removal of residual ligneous matter contained in the cellulosic material to effect complete fiber liberation. I f

The treatment of the cellulosic material with a bisulphite liquor between the first treatment with a sulphurous acid solution and the treatment with a solution of alkali is advantageous, in that it not only makes possible a reduction in the quantity of alkali in material such as hardwoods, hemp, bamboo, and bagasse. It is also applicable to wood such as spruce, which is" an excellent 1 raw material for the production of pulps of high alpha cellulose content and suitable both for papermaking and for the preparation of cellulose derivatives. A specific example of procedure which may be practised with eminently successful results in the case of spruce, may be substantially as follows. The chipped wood, which may be in the form of. chips of the normal size, or of a size larger or smaller than that usually employed in a pulp mill, is first treated with a sulphurous acid solution containing about 2% to 5% S0 This treatmaintained for about two hours while the pressure is ordinarily not allowed to exceed ondtreatment. The sulphur dioxide content ment may be efiected in adigester in which. the charge-of wood and liquor may be confined under pressure and at elevated temperature. The digester contents are heated to atemperature of 230 to 260 F which is be obtained by adding alkali, such as caustic added in the form of concentrated solutions 7 for the preparation of the bisulphite solution which a-re of high pentosan content as found 7 about to 75 pounds; but in special cases higher pressure may be reached. At the end of this time, the solution may be drained from the chips and a suitable bisulphite liquor added to the digester, but it is preferable to form the bisulphite liquor from the sulphurous acid solution of the first treatment. Ac-

cordingly, a concentrated solution of caustic soda, sodium sulphite, or sodium sulphate may be added to the sulphurous acid solution until the desired combined SO content is produced therein, say, 1% to 2%, or greater. When sodium sulphate is the added chemical, only a relatively small portion will react with the sulphurous acid solution to form bisulphite, but as the sulphite present in the liquor is consumed by reaction on the cellulosic material, more bisulphite is formed from the excess sulphate present in the liquor. By injecting the added chemicals in the sulphurous acid solution in the digester, the liquor and chips may be held at a temperature of 250 to 260 F., under which temperature conditions it is desired to effect the treatment with the resulting bisulphite liquor. If the combined SO content of the liquor is greater than, say, 1% to 2%, the temperature of cooking may be advantageously raised to, say, 285 to 300 F. The treatment with the bisulphite liquor may be for one to two hours, at the end of which time examination of the cellulosic material indicates that it is still in the form of firm chips, even though some softening has taken place. The bisulphite liquor is then withdrawn from the digester and the chips are preferably washed in order to avoid loss of alkali from the alkaline cooking liquor by reaction with free acid carried by the chips.

The washed or unwashed chips are then digested in an alkaline liquor the alkalinity of which is afforded by the hydroxide, sulphide, or sulphite of sodium, or by a combination of these chemicals. The alkaline liquor need contain only about one pound of active alkali per cubic foot, calculated as Na O, to comlete fiber liberation, suchan alkalinity bemg about half that required in the alkaline liquor used for the production of kraft pulp. Preferably, the liquor contains alkali in the form of caustic soda and sodium sulphide,

temperature and pressure conditions used in the kraft process, say, 300 to 335 F. and 67 to 110 pounds pressure, it is found that comlete fiber liberation is effected in about two ours. The resulting pulp, after being washed free of spent cooking liquor and analyzed, shows an alpha cellulose content of 96% to 98%, a pentosan content of less than 1%, and a viscosity of about 1." Such a product compares favorably with wood pulps which have been refined by subjecting chemical wood pulps such as sulphite or kraft, to chemical refining treatments; and in fact it is of higher alpha cellulose but of lower pentosan content and has a lower solution viscosity. The solution viscosity value of the pulp is considerably lower than that of the usual commercial wood pulps, such as sulphite or kraft, or refined pulps prepared therefrom. The raw pulp obtained by a process such as described may be bleached to whiteness in the usual bleach liquors, but it is preferable to employ a bleach liquor containing sufiicient free alkali to maintain a condition of distinct alkalinity in the liquor throughout the bleaching operation, in order to preserve the alpha cellulose content of the pulp while coloring matters are being removed.

The broad principles of the process hereinbefore described may be applied by treat ing the raw cellulosic material first with the bisulphite liquor and then with the sulphurous acid solution. The bisulphite treatment, if practised under the conditions previously described, will not be accompanied by a substantial isolation of fiber, but the subsequent treatment should be carried out in a sulphurous acid solution containin only about 1% to 2% S0 in order to avoi substantial fiber liberation. Such a sequence of ste s does not, however, permit the use of the liquor together with its heat and chemicalsin the second treatment, as is the case when a sulphurous acid solution is employed in {the initial treatment.

It is important that the completion of fiber liberation during the first two treatments be avoidedas much as possible in producing the product of the present invention, as the action'of an acid solution, particularly of a hot sulphurous acid solution, on a liberated pulp is to injure the pulp seriously for pa makingpurposes, as well asto diminish gh e yield of product obtained after digestion in the alkaline liquor. By permitting the acid solution to act on the cellulosic material while it is associated with non-fibrous organic ma.- terial on which the acid solution may exert a selective chemical action, the fiber content 3 of the cellulosic material may be preserved to a considerably greater degree.

, I claim:

1. A multiple-step process of fiber liberation, in which the fiber content of raw cellulosic material is liberated in the last step, which comprises subjecting the raw oelluloslc material to the independent action of a sulphurous acid solution and of a bisulphite solution, and then cooking the material so treated in an alkaline liquor to'complete fiber liberation.

2. A multiple-step process of-fiber liberation,'which comprises cooking raw cellulosic material successively in a sulphurous acid solution, a bisulphite solution, and an alkaline liquor, while keeping such material short of complete fiber liberation until the alkaline liquor exerts its action thereon.

3. A process of fiber liberation, which comprises cooking raw cellulosic material in a, sulphurous acid solution, converting the hot sulphurous acid solution into'a bisulphite liquor, cooking in the bisulphite liquor short of fiber liberation, and finally cooking in an alkaline liquor to complete fiber liberation.

In testimony whereof I have afiixed my signature. I

GEORGE A. RICHTER.