US2007024A - Chemical pulping process - Google Patents

Description

Patented July 2, 1935 PATENT OFFICE CHEMICAL PULPING PROCESS George A. Richter, Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporation of Maine No Drawing. Application December 29, 1933, Serial No. 704,513

I 8 Claims.

This invention relates to a chemical pulping process wherein an alkaline liquor serves to dissolve the ligneous, resinous, or other encrusting components of wood chips and other fiber-bearing raw cellulosic material and thus to liberate the utlimate fiber as pulp suitable for paper-making and other purposes. An objective of the present invention is the production of a pulp whose physical or paper-making qualities are good as, or even'excel, those-oi so-called kraft pulp and which contains few er' shives and is distinctly easier to bleach than kraft pulp, so that there will be less loss of pulp in the form of socalled screenings when it is screened and less physical and chemical damage done to the pulp when it is bleached to whiteness. Another objective is the attainment of such improved pulp with no greater consumption of fiber-liberating chemical than that entailed in the production of the usual kraft pulp and with no greater difliculty in regenerating fiber-liberating chemical from the spent cooking or fiber-liberating liquor. Indeed, still another objective is to reduce the consumption of chemical in both the fiber-liberating and chemical-regenerating operations.

In accordance with the present invention, I practice first the step of impregnating wood chips with an alkaline liquor available as an intermediate liquor in the chemical-regenerating treatment and second the step of pulping the alkaliimpregnated chips in a finished alkaline cooking liquor prepared from the intermediate liquor. The spent cooking or pulping liquor resulting from such practice is one that can be and is put through the chemical-regenerating treatments of the usual kraft process. That is to say, its organic content is consumed by combustion and its inorganic content is smelted to produce a smelt consisting essentially of sodium carbonate and sodium sulphide. The smelt is dissolved in water to form an alkaline liquor which serves as the medium for impregnating the wood chips before they are subjected to pulping or cooking in the finished cooking liquor prepared from the solution of the smelt. The finished cooking liq-'- uor is prepared as in the kraft process by causticizing the sodium carbonate content of the smelt solution with lime. The smelt solution may go partly into the impregnation of wood chips and partly into the preparation of finished cooking liquor. It is, however, possible to use all of the smelt solution once or repeatedly as impregnating liquor for the wood chips and to put liquor drained from the chips through the step of causticization to produce the finished cooking liquor.

The smelt solution containng sodium sulphide and sodium carbonate and used as the impregnating medium for the Wood chips may contain about 3 to 5 unds of NazO equivalent per; cubic foot and have a sulphidity ranging from about 25% to 45%. impregnation of the chips may be effected under various conditions, for instance, under atmospheric or superatmospheric 10 pressure, at room or at elevated temperature, by exhausting the air from the chips before the impregnating liquor is brought in contact therewith, by forcing the liquor into the chips under hydrostatic pressure, etc., but, in any event, the 15 temperature at which impregnation is conducted is distinctly below that at which pulping or fiberliberation can take place. Most of the alkali for pulping or fiber-liberation may thus be introduced into the chips so that when they are sub- 20 sequently cooked at pulping or fiber-liberating temperatures in the finished cooking liquor, uniform pulping action can be accomplished in a shorter period'of time and/or with considerably less total alkali associated with the chips than 25 that ordinarily required, despite the fact that some of the alkali is in the form of sodium carbonate, which is less effective as a fiber-liberating agent than sodium sulphide or caustic soda. The

sodium carbonate is, however, of value in dissolving some of the more solublewood gums and resins, particularly as it exists in the interior of the chips where it is in intimate contact with those encrusting materials that it'is able to act upon and dissolve.

The process of the present invention can be practiced not only with economy of fiber-liberating chemical but also with economy of lime over the usual kraft process, as the use of part of the smelt solution for impregnating the wood chips means that there is less of this solution upon which to expend lime in causticization. Economy or fiber-liberating chemical arises irom the fact that less total chemical need be associated with the chips while they are being cooked in the digesters, wherefore, there is less "chemical in the entire system, including the digesters and stracted by the chips before the solution goes through the causticizing step.

Specifically, the pulp-making process of the present invention may be practiced as follows. Wood of any suitable pulp-making character, for instance, not only such coniferous woods as spruce, but also pine and other resinous woods, hardwoods, etc., may be cut into chips of the usual size or even larger. After the chips have been impregnated with a smelt solution under conditions such as have already been indicated, they may be introduced into the usual alkaline digester and causticized smelt solution of an alkalinity added to provide in the digester about 0.15 pounds of alkali in all forms, but calculated as Nazo equivalent, per pound of dry wood. Mostof this alkali, say, to 75%, may advantageously constitute the smelt solution introduced into the chips as a. liquor of the composition already described, whereas the rest of this alkali may be furnished in the form of dilute causticized smelt solution and be added along with the alkali-impregnated chips to the digester. The charged digester may be closed and its contents heated for about 3 to 4, hours at a temperature of about 335 to 365 F., at the end of which time the wood is satisfactorily pulped. The digester may hence be blown and the resulting pulp washed free of spent cooking liquor. The washed pulp is found to be more easily bleachable and much freer from shives than kraft pulp produced as ordinarily in a cooking liquor containing 0.2 pounds of alkali essentially only in the form of caustic soda and sodium sulphide, but calculated as NazO equivalent per pound of dry wood. The physicalor paper-making characteristics of the'pulp are also superior to those of kraft pulp and they may be substantially preserved when the pulp is bleached to whiteness.

The advantageous results secured by my process are attributable to the fact that cooking-at fiber-liberating temperatures is performed only after the wood chips have been impregnated substantially throughout with most of the chemical requisite to fiber liberation. Unless cooking is carried out for such a long period of time as to be of little, if any, practicability, these results cannot be had by cooking the raw or unimpregnated chips to fiber-liberating temperatures in a single'cooking liquor whose alkalinity is furnished by the same chemicals and in the same amounts as those of both the liquor that I put into the chips as an impregnant and the liquor that I use as the cooking medium for the impregnated chips, since in cooking the unimpregnated chips in such a single cooking liquor, considerable cooking time is lost in penetrating the interior.

of the chips with the fiber-liberating chemicals; and during such time, when the interior of the chips is being severely heated in the absnce of fiber-liberating chemicals, the ligneous, resinous, and other encrusting components of the wood become more resistant to being acted upon by the fiber-liberating chemicals.

The spent cooking liquor separated by washing the pulp as ordinarily after the digester contents have been discharged is put through the recovery steps customarily performed in a kraft mill. The spent cooking liquor can be considerered as having resulted from a single cookingliquor containing sodium sulphide, sodium carbonate, and caustic soda even though these chemicals are not all commingled in a single solution to begin with but are used in such a way from the impregnated chips.

that most of the sodium sulphide and practically all of the sodium carbonate are present throughout the chips at the very beginning of the cook ing operation and the rest of the sodium sulphide and practically all of the caustic soda occur in dilute solution in the cooking medium surrounding the chips. Irrespective of whether such spent cooking liquor is first put through a rotary kiln and calciner to produce so-called black ash or whether the liquor is first concentrated as in multiple effect evaporators and the concentrated liquor delivered into a smelting furnace, the net result is that as in the ordinary kraft recovery process, the organic content of the spent liquor is consumed and its inorganic content smelted in a reducing atmosphere to produce a smelt containing essentially sodium sulphide and sodium carbonate, usually about 25% to 45% of the sulphide. As in the ordinary kraft process, chemical losses may be made up by adding salt cake (sodium sulphate) to the concentrated spent cooking liquor immediately before it undergoes the furnacing operation. Or, they may be made up partly or completely by adding sodium carbonate and free sulphur to the smelt solution and/or by adding caustic soda and free sulphur to the causticized smelt solution. The smelt is dissolved in water to form the solution which serves as impregnating liquor for the chips. Most of the solution may be consumed in impregnating the chips and the rest may be treated with suificient lime to causticize its sodium carbonate content and thus to give the medium in which the impregnated chips are cooked. In impregnating the chips, it is, of course, necessary to use an excess of solution and to drain the excess This excess may be repeatedly re-used after fortification with fresh smelt solution. It is possible to use all of the smelt solution for the purpose of impregnating the chips and to causticize for cooking liquor purposes the requisite amount of liquorv drained from the chips. As in ordinary kraft cooking, it may be desirable'to use some hot spent or so-called black liquor along with the causticized smelt solution or so-called white liquor as the cooking medium, thereby conserving such heat and eifective chemical value as may exist in the black liquor and increasing the organic content of the spent liquor so that better reducing conditions are created during the furnacing. operation.

The impregnating liquor and the cooking liquor employed in accordance with my invention have a feature in common and that is that they are of essentially the same sulphidity. In other words, in view of the fact that the cooking liquor is simply an impregnating liquor whose sodium carbonate content has been causticized, both these liquors have essentially the same sulphidlty even though the impregnating liquor may be used .at much higher chemical concentration in order to get most of the chemical necessary for pulping into the wood chips. The sulphidity of these liquors depends upon the conditions under which the inorganic content of the spent cooking liquor is smelted. I prefer to maintain the sulphidity at about 25% to 45%, which is that of a usual kraft smelt.

I claim:----

1. In a process of pulping wood chips involving the smelting of the inorganic content of spent cooking liquor to produce a smelt containing sodium sulphide and sodium carbonate, dissolving the smelt in water, and causticizing the sodium carbonate content of the resulting solu-= tion, those steps which comprise impregnating the chips with uncausticized smelt solution, and pulping the impregnated chips in causticized smelt solution.

2. In a cyclic process of pulping wood chips involving the smelting of the inorganic content of spent cooking liquor to produce a smelt containing sodium sulphide and sodium carbonate, dls= solving the smelt in water, and causticizing the sodium carbonate content of the resulting solu= tion, those steps which comprise impregnating the chips with uncausticized smelt solution, pulping the impregnated chips in causticized smelt solution, separating the spent cooking liquor from the resulting pulp, and smelting the inorganic content of the spent cooking liquor to produce a smelt containing sodium sulphide and sodium carbonate for the repetition of the cycle.

3. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with most of the pulping chemicals in the form of a solution of essentially only sodium sulphide and sodium carbonate in substantially unspent condition only, and then pulping the impregnated chips in a solu= tion of the rest of the pulping chemicals essentially in the form of sodium sulphide and caustic soda l. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with a. solution of essentially only sodium sulphide and sodium carbonate in substantially unspent condition only, and then pulping the impregnated chips in a solution of essentially sodium sulphide and caustic soda.

5. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with a solution of essentially only sodium sulphide and sodium carbonate in substantially unspent condition only, and then pulping the impregnated chips in a solution of essentially sodium sulphide and caustic soda whose total alkali content calculated as New equivalent is less than that put as impregnant into the chips.

6. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with a solution of essentially only sodium sulphide and sodi carbonate in substantially unspent condition only, and then pulping the impregnated chips in a solu tion of essentially sodium sulphide and caustic soda whose sulphidity is substantially the same as that of the impregnating solution.

"I. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with a solution of essentially only sodium sulphide and sodium carbonate in substantially unspent condition only, and then pulping the impregnated chips in a solution of essentially sodium sulphide and caustic soda whose sulphidity is substantially the same as that of the impregnating solution but whose total alkali content calculated as NazO equivalent is less than that present as impregnant in the chips.

,3. In the pulping of wood chips by the action of alkaline chemicals, those steps which comprise first impregnating the chips with a solution of essentially only sodium sulphide and sodium can bonate in substantially unspent condition only, and then pulping the impregnated chips in a solu= tion of essentially sodium sulphide and caustic soda whose sulphidity is substantially the same as that of the impregnating solution, the sulphidity of both said solutions being approximately 25% to 45%. a

GEORGE A. RICH.