US2032437A

US2032437A - Fiber-liberating process

Info

Publication number: US2032437A
Application number: US747922A
Authority: US
Inventors: George A Richter
Original assignee: Brown Co
Current assignee: Brown Co
Priority date: 1934-10-11
Filing date: 1934-10-11
Publication date: 1936-03-03
Anticipated expiration: 1953-03-03

Description

Patented Mar. 3, 1936 UNITED STATES FlBER-LIBERATING PROCESS George A. Richter, Berlin, N. H., assignor 'to Brown Company, Berlin, N. H., a corporation of Maine Application October 11,

21 Claims.

This invention relates to a liber-liberating process, being more particularly concerned Withthe liberation of fiber from Wood or other raw cellulosic material by the action of a sulphite or acid sulphite cooking liquor. While not limited thereto, the process of the present invention will be described more specifically as applied to the pulping of wood.

'- It has been proposed to subject chipped Woody to the liber-liberating action of sulphite and/0r acid sulphite cooking liquors, wherein ammonia in the form of the ammonium radical constitutes the basic element. sulphite wood pulp produced by cooking under suitable conditions of temperature,

pressure, time, and chemical concentration in such liquors are of a quality even excelling'pulps prepared under comparable conditions in sulphite liquors containing calcium and/or sodium as the basic element. To be sure, ammonia is a more expensive base than either calcium or sodium, but it is possible to operate commercially with an ammonia base sulphite liquor particularly when provision is made for recovering the ammonia constituent present in the spent ammonia base sulphite liquors. For instance, the recovery of the ammonia constituent present in spent ammonia base sulphite cooking liquors may be correlated economically With the recovery of sodium constituent from spent sodium base alkaline cooking liquors, such as so-called kraft and/or soda liquors, as disclosed inl my application Serial lNo. 745,924, iiled September 28, 1934.

The present invention devolves more particularly about the impregnation of the chipped wood or other raw cellulosic material with fiber-liberating chemical in the form of ammonium sulphite and/or ammonium bisulphite preparatory to the cooking or fiber-liberating operation. The object of the present invention is to make possible the resolution of the chipped Wood or other raw cellulosic material into a high grade cellulose pulp in a minimum period of time and with a minimum usage of fiber-liberating chemical in the form of ammonium sulphite and/or ammonium bisulphite. Another objective is the use of a composite base sulphite ancl/ or bisulphite liquor comprising both ammonia and sodium as the basic elements and yielding spent cooking yliquors from which the valuable ammonia and sodium constituents may be easily recovered for reuse in the pulping of other raw cellulosic material.

In accordance with the present invention, the chipped wood or other raw cellulosic material to be resolved into cellulose pulp is impregnated or permeated with ammonia fumes or vapors 1934, Serial No. 747,922

solution is added to the confined ammoniated mass 10.

to produce the ammonium sulphite and/or ammonium bisulphite cooking liquor of -the appropriate combined SO2 content in situ therein. Upon the addition of the sulphurous acid solution to the ammoniated mass, lfor instance, a charge 15 of wood chips conned in an ordinary sulphite digester, the sulphurous acid solution diffuses promptly into the pores and interstices of the wood wherein it immediatelyundergoes reaction with the ammonia already present therein to form 20 ammonium sulphite and/or bisulphite in situ therein. This means that fiber-liberating chemical has been introduced rapidly and uniformly into and throughout the wood structure and, accordingly, that it is unnecessary to bring the 25 charge slowly to ber liberating or cooking temperatures for the purpose of distributing fiber-liberating chemical substantially uniformly throughout the wood structure, as is customary practice. On the contrary, once the 30 sulphurous acid solution has been added to the ammoniated charge, it is possible to heat the charge rapidly to pulpingV or fiber-liberating temperatures while at the same time attaining substantially uniform pulping or ber- 35 -f liberating action throughout the chip structure and producing substantially uniform pulp. By the practice of the present invention, it is hence possible to prepare a high grade sulphite pulp, such as is suitable for making papers and cellu- 40 lose derivatives, in a period oftime shorter than that ordinarily required and/or with a liquor of a combined SO2 content lower than that of the usual sulphite liquor. The expression combined SO2 content, as used herein means sulphur di- 45 oxide constituent existing in combination with the l base as the straight or neutral sulphite. In order words, the quoted expression is used herein in the sense customarily understood in the sulphite pulp-making industry, namely, as indicating sul- 50 phur dioxide constituent that is unavailable for neutralization with caustic soda or other alkali. There are various Ways in which the initial treatment'v o f the woods chips with ammonia fumes may be perfumed accordant with my in- 55 vention, the wood employed being of such varieties as spruce, birch, pine, r, etc. Thus, gaseous ammonia under super-atmospheric pressure may be delivered from a suitable source of supply into a closedsulphite digester charged with the wood chips, the amount of ammonia thus added being controlled so as to yield ultimately in the digester a cooking liquor of the appropriate composition. If desired, the air in the digester may be evacuated before the introduction of the ammonia gas thereinto under atmospheric or super-atmospheric pressure, thereby avoiding dilution of the ammonia by air and also promoting a more rapid filling of the pores and interstices of the wood chips with the Aamrnonia gas. The wood chips may be in air-dry condition and thus contain, as ordinarily, about 30% of moisture, based on the bone-dry weight of the wood. r The4 presence of moisture in the chips fosters a diffusion of the ammonia fumes into, and an absorption of the ammonia b-y, the chips. If desired, the chips may be steamed or otherwise moistened prior to bringing the ammonia vapors into contact therewith, although, as already indicated, it is possible to work with chips in air-dry or even drier condition. In some instances, the ammonia vapor and/or the chips to be treated therewith may be heated, as by the use of live steam, in which case, the steam may be introduced along with the ammonia vapor and/or be applied di-A rectly to' the chips.

An advantageous way of getting the ammonia gas into the pores and interstices of the chips is by injecting liquid ammonia into the closed digester charged with the chips, the interior of the digester being under atmospheric or subatmospheric pressure. The liquid ammonia thus introduced into the closed digester immediately flashes into ammonia fumes which penetrate into and throughout the mass of chips and chip body. The liquid ammonia may be introduced into the digester in amount and under temperature conditions to develop a pressure of, say, 20 to 100 pounds, in the digester. The liquid ammonia may be added in amount greater than that necessary to yield a sulphite cooking liquor of the desired composition, whereupon, after the liquid ammonia has flashed into vapor, such excess ammonia. as is present in the closeddigester may be released therefrom and recovered. Thus, the excess ammonia vapor may be permitted to escape from the digester into a suitable cooling and condensing system designed to re-liquefy the ammonia vapor for reuse. n

Once the chips have been'ammoniated to the desired degree, a sulphurous acid solution may be added to the closed digester containing the ammoniated chips. The amount of ammonia used in pretreating the chips and the amount of sulphurous acid solution added thereto may be controlled to yield a cooking liquor containing in solution, say, about 0.5% to about 1.0% combined SO2 and about 3% to 5% or more of free SO2. In this connection, it is to be observed that the combined SO2 values given for the cooking liquor are calculated ones that include combined SO2 constituent that exists both in situ in the wood structure and in the free solution' surrounding the chips. Indeed, if samples of the cooking liquor were actually taken from the digester and tested for combined SO2 content, the test would show a. combined SO2 content less than that given for the liquor. In other words, although the calculated combined SO2 content' existing in the digester is given as ranging from, Say, about 0.5%

wherein it exercises the desired ber-liberating action, so that only a comparatively small total combined SO2 content, based on wood, maybe relied upon to promote the desired pulping or fiber-liberating action. When combined SO2 is present in the liquor surrounding the chips to the same degree Yor concentration as exists in the chips, as in ordinary cooking practice, a much higher precentage of combined SO2, based on wood, must be present in the digester to produce pulping or fiber-liberation by reason of the fact that the combined SO2 in the liquor does not actually do any work of promoting pulping or fiberliberation, simply serving as a reservoir for combined SO2 from which the combined SO2 necessary for pulping or liber-liberating action diffuses into and is absorbed by the chips. 'Ihe digester contents may then be heated quickly to tempera- `tures of about 240 to 300 F.; and cooking at such temperatures may be continued until complete ber-liberation has been eiected. Whereas cooking by the usual sulphite process must be carried on for about ten hours to produce satisfactory pulp, it is necessary to cook wood chips in accordance with my invention for only about six hours in producing highly satisfactory pulp. Thus, when cooking is performed at a temperature of about 275 F. or greater, the cooking period need be only about six hours. Of course, the cooking period may be longer when performed at lowe"r temperatures, for instance, at temperatures below about 245 F. In such latter instance, cooking may be continued at a temperature not exceeding 245 F.' for a period of about 10 to 11 hours to produce pulp of unusually good qualities and with greater yield of pulp than that ordinarily realized.

An important advantage attainable by the practice of the present invention is, as already indicated, the production of satisfactory pulp in a sulphite cooking liquor containing less combined S02 than that present in the usual sulphite cooking liquor. Indeed. it is possible to effect satisfactory pulping of chipped wood in a .liquor containing only about 2% to 4% combined SO2, based on the bone-dryV weight of the wood, when the sulphur dioxide has been combined with the ammonia. in situ in the chip body, as hereinbefore described, despite the fact that in usual sulphite cooking it is necessary to use a cooking liquor containing as much as 8% or more combined SO2, based on the bone-dry weight of the wood. Itis, of course, possible to practice my invention while employing liquors of a combined SO2 content, based on wood, greater than that just cited. For instance, it may be desirable to 'practice my invention in the production of a uniform but undercooked pulp of the variety that lends itself admirably to the manufacture of so-called glassine papers. may be practiced while employing a liquor containing, say, 6% or more combined SO2, based on the bone-dry weight of wood, but less free SO2 content than that present in the usual acid sulphite cooking liquors. Depending on such fac- In such latter instance, my invention tors as the concentration of the ammonia gas used in the pretreatment of the Wood chips, the temperature and the pressure under which ammoniation is performed, the moisture content of the chips during ammoniation, the time of the pretreatment with the ammonia gas may vary from, say, ten minutes to four hours. Ordinarily, however, when liquid ammonia is introduced into the digester charged with the wood chips and caused to ilash into ammonia vapor, as hereinbefore described, the period of ammoniation necessary to effect the desired impregnation of the chips is about 1 to 2 hours. To be sure, basic element in the form of ammonia is more expensive than basic element in the form of, say, sodium, but one molecular Weight of sulphur dioxide, that is, 64 parts by weight of sulphur dioxide, requires for chemical combination therewith 106 parts by Weight of .sodium constituent in the 'form of soda ash but only 34 parts by weight of liquid ammonia. Inasmuch as it is possible, as already indicated, to pulp wood satisfactorily with much less ammonia base than with sodium base and inasmuch as a given combined SO2 content in the cooking liquor is realized with much less weight of ammonia base than of sodium base, it is obvious that it may be economical to Work with ammonia base even though such base is far more expensive than sodium base. For example, assuming that the ammonia base cooking liquor employed in accordance with my invention has a combined SO2 content of 4%, Whereas the ordinary sodium base cooking liquor has a combined SO2 content of 8%, and assuming further that sodium constituent is available on the market in the form ,of soda ash at one cent per pound, calculation indicates the commercial feasibility of using liquid ammonia at five cents per pound. This calculation is made on the assumption that both the spent ammonia base and sodium base sulphite liquors are to be discarded or Wasted. As already mentioned, however, it is possible to recover economically the ammonia constituent of spent ammonia base sulphite cooking liquor, particularly by a practice such as has been outlined in my aforementioned patent application.

In some instances, the cooking liquor employed according to the present invention may contain some sodium base as well as ammonia base. Thus, the sulphurous acid solution added to the ammoniated wood chips may contain dissolved therein sodium sulphite and/or sodium sulphate (salt cake) and/or sodium acid sulphate (niter cake) and, if desired, the sulphurous acid solution may contain some free sulphuric acid to form ammonium sulphate as Well as ammonium sulphite in the cooking liquor. Rather than adding the sulphur dioxide to the ammoniated wood chips in the form of a sulphurous acid solution, it may be introduced into the closed digester charged with the ammoniated chips in the form of a gas or liquid in much the same manner as the ammonia is introduced. If desired, gaseous sulphur dioxide may be introduced into the digester in amount only sulicient to react with the ammonia existing largely inside the chips to form ammonium bisulphite in situ therein, whereupon such additional or free SO2 as is desired in the cooking liquor during the cooking operation may be added to the digester in the form of a sulphurous acid solution which serves to cover partly or completely the chips during the cooking operation. It is thus seen that part of the sulphur dioxide may be added in gaseous form and part of sodium carbonate and sodium sulphate.

as a sulphurous acid solution. After the ammonia and sulphur dioxide have reacted in the digester to form ammonium sulphite or while such reaction is taking place, Water may be added to the digester in sucient amount to cover partly or completely the chips and thus to permit their uniform cooking or pulping. The use of gaseous orl liquid sulphur dioxide does away with the need for an absorption system such as is necessary for preparing a .sulphurous acid solution and may, in some instances, be preferred practice.V

In those instances when cooking is performed in a composite basesulphite liquor comprising sodium as well as ammonia base, the recovery of the valuable sodium and ammonium 'constituent from the spent sulphite cooking liquor may be eiected to good advantage as illustrated by the iioW-sheet on the accompanying drawing. As depicted in the flow-sheet, the chips may be pulped in a cooking liquor comprising a solution of ammonium acid sulphite and sodium acid sulphite and having the desired combined and free SO2 contents. After cooking or pulping has been completed, the digester contents may, as ordinarily, be discharged or blown into a blow-pit wherein the spent cooking liquor may be separated, as by draining, from the pulp. The spent cooking liquor may then be treated with suiiicient sodium base alkaline compound to be neutralized thereby. Neutralization of the liquor tends to liberate its ammonia content; and substantially all of the ammonia content of the neutralized liquor may be liberated and expelled therefrom by delivering the liquor downwardly through a tower .into countercurrent contact with steam and/or hot air. The ammonia thus liberated may be recovered in compressed or liquid form vand be employed in the pretreatment of the chips, as already described. The spent liquor delivered from the tower may be concentrated, as by multiple-eiect evaporation, to a condition where its organic content may be readily burned and its inorganic content smelted. In such concentrated condition the liquor may be delivered into a furnace operated under oxidizing conditions wherein its organic content is consumed and its inorganic content smelted to produce a mixture The mixed sodium carbonate and sodium sulphate smelt being substantially completelywater-solumingled with the sulphurous acid solution that is added to the ammoniated Wood chips in the digester. The ammonia liberated and recovered from the spent cooking liquor may, as previously stated, be used in the pretreatment of the wood chips. If desired, however, it may be caused to react with a sulphurous acid solution to f orm an ammonia base sulphite cooking liquor. The ammonia base sulphite cooking liquor maybe commingled with the sodium base sulphite liquor to formv the composite base cookingliquor delivered to the digester. 'If desired, both the recovered ammonia and the solution of smelted compounds may be caused to react simultaneously with sulphur dioxide to produce the composite base sulphite cooking liquor.

Inasmuch as there will be a tendency for the accretion of sodium constituent in the composite base sulphite cooking liquor on account of the use of sodium base alkaline compounds for the neutralization of the spent acid cooking liquor, provision may be made for the removal of sodium carbonate from the solution of the smelted compounds. Thus, sodium carbonate may be removed from such solution as by selective crystallization or by precipitating it with carbon dioxide as sodium bicarbonate, and the sodium carbonate or sodium bicarbonate thus removed used as the neutralizing agent for the spent composite base cooking liquor. However, it is preferable to causticize the sodium carbonate or sodium bicarbonate before neutralizing the spent composite base cooking liquor so as to avoid the dilution of the liberated ammonia gas with the simultaneously liberated carbon dioxide gas. 'I'he sodium sulphate and any residual sodium carbonate in the mother liquor may, if desired, be causticized preparatory to its reaction with sulphur dioxide. Caustcization should be effected in those instances when it is preferable that the sodium base constituent be present in the cooking liquor in the form of sodium sulphite rather than sodium sulphate. Of course', only part of the sodium sulphate present in the mother liquor need be causticized, in which case the cooking liquor will contain ammonium sulphite and sodium constituent in the form of both sodium sulphite and sodium sulphate.

In order to ensure a substantially uniform distribution of the ammonia fumes as promptly as possible throughout a charge of wood chips in the digester, particularly a large-sized commercial unit, it may be desirable to introduce the ammonia, fumes or the liquid ammonia into .the digester at a number of properly spaced localities and/or to provide for positive circulation in the digester, as by a suction fan or pump that serves to withdraw gas from vone end of the digester, i. e., the top or bottom, and discharge or inject it into the other end. When liquid ammonia is added to the digester, the heating of the digester contents, as by the introduction of live steam thereinto, may be desirable, as this promotes the vaporization of the ammonia and its diffusion throughout the charge of wood chips and the chip structure. A

Inasmuch as acid sulphite liquors containing highly soluble sulphites, such as ammonium sulphite and sodium sulphite, are more severe in their chemical or erosive action on the brick 1inings of the usual commercial digesters than lcalcium base acid sulphite liquors, it may be desirable to add to the acid sulphite cooking liquors of the present invention containing the highly soluble ammonium sulphite such comparatively insoluble salts as calcium sulphate, barium sulphate, barium sulphite, or the like, in such amount that they occur in` suspension in the cooking liquor surrounding the chips during the cooking operation. Such salts are usually objectionable in the cooking liquor during the cooking operatien, as they tend to plug up the surface pores of the wood chips and thus to interfere with the desired penetration of combined SO2 into the interior of the chips wherein combined S01, that is, sulphite, is desired for the purpose of promoting fiber-liberation as quickly and uniformly as possible. In this connection, it might be observed that if wood chips were first soaked for a considerable period of time in an ordinary cooking liquor, such as a calcium bisulphite liquor, dissolved calcium bisulphite, i. e., combined SO2, would penetrate into the wood structure, but it is impossible to introduce into the wood structure by 'any commercially feasible practice ol' which I am aware substantially the entire quantity of combined SO2 that makes for substantially uniform and complete pulping or fiber-liberating action on the wood. Accordingly, in such latter instance, the suspension of insoluble salts, such as calcium sulphate, in the cooking liquor is attended by a retarded pulping or ber-liberating action, as the insoluble salt in suspension interferes, as already indicated, with a diffusion into the chips of the combined SO2 that is required for complete pulping or liber-liberation from the cooking liquor serving as the reservoir for such combined SO2 as is necessary for complete pulping or liber-liberating action. In the practice of the present invention, on the other hand, substantially all of the combined SO2 necessary for complete pulping or liber-liberating action'is introduced quickly and uniformly throughout the Wood structure, as already stressed, wherefore, the concentration of combined SO2 within the wood structure is appreciably higher, particularly during the initial stages of the cooking operation, than that of the cooking liquor surrounding the Wood. When, however, combined SO2 in the form of ammonium sulphite has been introduced in situ in and throughout the chip structure accordant with my invention, the presence of insoluble` salt in suspension in the liquor surrounding the chips is not detrimental. The presence of insoluble alkaline earth metal salt, and more especially the sulphate, in suspension in the liquor surrounding the chips is of value in that such salt tends to deposit during the cooking operation as a scale or cake on the digesterl lining and thus to oiset the chemical or erosive action that `the acid sulphite liquor containing ammonium sulphite would otherwise have on the lining. In some instances, the cooking liquor of the present invention may be one containing besides ammonium bisulphite suilicient other bisulphite, such as calcium, and/or barium and/or strontium bisulphite, in solution therein, to precipitate the less soluble calcium sulphite and/or barium sulphite and/or strontium sulphite at the end of the cooking operation, that is, when free SO2 is being released from the digester, as

cooking operation but wherein insoluble salt is precipitated at the end stages of the cooking operation so as to deposit the desired protective layer of insoluble material on the digester lining.

For the sake of simplicity, I havedescribed the cooking liquors of -the present invention as having particular calculated combined SO: contentsdespite the fact that most or practically all of the combined SO2, particularly during the initial stages of the cooking operation, may be present in the wood structure rather than in the liquor surrounding the wood chips. The appended claims have also been drafted in terms of such calculated combined SO2 contents,` that is, total combined SO2 content present in the digester, irrespective of what fraction of the total combined SO2 content exists in the wood structure and what fraction exists at any time in the cooking liquor surrounding the wood.

I claim:

l. A process of pulping raw cellulosic material which comprises impregnating a mass of such material while under confinement with ammonia fumes, adding sulphur dioxide and water to the ammoniated mass to form ammonium sulphite rpulping liquor in situ therein, and cooking the mass in the resulting liquor to form pulp.

2. A process of pulping raw cellulosic material which comprises premeating a mass of such ma.- terial while under confinement withammonia fumes, adding a sulphurous acid solution to the ammoniated sulphite pulp liquor in situ therein, and cooking the mass in the resulting liquor to form pulp. i

8. A process of pulping raw cellulosic material which comprises annnoniating a mass of such material While under confinement with ammonia fumes at super-atmospheric pressure, adding a sulphurous acid solution to the ammoniated mass to form ammonium bisulphite pulping liquor in situ therein, and cooking the mass in the resulting liquor to form pulp.

4. A process of pulping raw cellulosic material which comprises adding liquid ammonia to a mass of such material while under confinement under conditions to cause the ammonia to ash into fumes and thereby to permeate such material, adding a sulphurous acid solution to the ammoniated mass to form ammonium sulphite cooking liquor in situ therein, and cooking the mass in the resulting liquor to form pulp.

5. A process of pulping chipped wood which comprises impregnating a mass of such wood while under connement with ammonia fumes in such amount as to be capable of yielding a bisulphite cooking liquor of a combined SO2 content not exceeding about 4%, based on the bonedry weight of the wood, adding a sulphurous acid solution to the ammoniated wood to form amfmonium bisulphite cooking liquor in situ therein, and cooking the wood in the resulting liquor to form pulp. I

6. A process of pulping chipped wood which comprises impregnating a mass of such wood while under connement with ammonia fumes in such amount as to be capable of yielding a bisulphite cooking liquor of a combined SO2 content not exceeding about 4%, based'on the bonedry weight of the wood, adding a'sulphurous acid solution to the ammoniated wood to form ammonium bisulphite cooking liquor of about 0.5% to 1.0% strength in situ therein, and cooking the Wood in the resulting liquor to form pulp.

7. A process which comprises impregnating a mass of raw cellulosic material while under confinement with ammonia fumes, adding a sulphurous acid solution and sodium sulphite to the ammoniated mass to form acomposite base sulphite pulping liquor containing amonium sulphite and sodium sulphite as liber-liberating chemicals, and cooking the mass in the resulting liquor to form pulp.

8. A process which comprises cooking raw cellulosic material in a chemical pulping liquor containing ammonium sulphite and sodium sulphite as ber-liberating chemicals, separating the pulp from the resulting spent pulping liquor, adding sodium base alkaline compound to the spent pulping liquor under conditions to liberate its ammonia content, recovering the liberated ammonia, and recovering as a substantially completely water-soluble smelt the sodium constituent remainingy in the spent pulping liquor.

9. A process which comprises impregnating a' mass of raw cellulosic material while under confinement with ammonia fumes, adding a sulphurous acid solution and sodium sulphite to the ammoniated mass to form a composite base sulphite pulping liquor containing ammonium sulphite and sodium sulphite as fiber-liberating chemicals,

cooking the mass in the resulting liquor to form pulp, separating the pulp from the resulting spent pulping liquor, adding sodium basealkaline com.-

pound to the spent pulping liquor under conditions to liberate its `ammonia content, and recovering the liberated`ammonia for use in the impregnation of other raw cellulosic material.

l0. A process which comprises impregnating a mass of raw cellulosic material while under connement with ammonia fumes, adding a sulphurous acid solution and sodium sulphite to the ammoniated mass to form a'composite base sulphite pulping liquor containing ammonium sulphite and sodium sulphite as fiber-liberating chemicals, cooking the mass in the resulting liquor to form pulp, separating the-pulp from the resulting spent pulping liquor, adding sodium base alkaline compound to the spent pulping liquor under conditions to liberate its ammonia content, recovering the liberated ammonia for use in the impregnation of other raw cellulosic material, and recovering the sodium constituent remaining in the spent pulping liquor at least partly as a compound convertible by reaction with sulphurous acid into sodium sulphite, whereby sodium sulphite may be formed from said recovered sodium constituent for addition along with a sulphurous acid solution to` otherv ammoniated raw cellulosic material to reproduce a fresh composite base sulphite pulping liquor.

'11. A process which comprises impregnating a mass of raw cellulosic material while under connement with ammonia fumes, adding a sulphurous acid solution and sodium sulphite to the ammoniated mass to form a. composite base sulphite pulping liquor containing ammonium sulphite and sodium sulphite as fiber-liberating chemicals, cooking the mass in the resulting liquor to form pulp, separating the pulp from the resulting spent'pulping liquor, adding sodium base alkaline compound to the spent pulping liquor under conditions to liberate its ammonia content, recovering the liberated ammonia for use in the impregnation of other' raw cellulosic material, smelting the sodium constituent remaining in the spent pulping liquor under oxidizing conditions, and treating the smelted compounds with a sulphurous acid solution to produce a solution containing sodium sulphite for addition to Aother ammoniated raw cellulosic material and thereby to reproduce a fresh composite base sulphite pulping liquor. 1

12. A process which comprises cooking raw ce1- lulosic material in a composite base sulphite pulping liquor containing ammonium sulphite and sodium sulphite as` fiber-liberating' chemicals, separating the pulp from the resulting spent pulping liquor, adding sodium base alkaline compound to the spent pulping liquor under conditions to liberate its ammonia content, recovering the liberated ammonia, recovering the sodium constitutent remaining in the spent pulping liquor pulping liquor, adding sodium base alkaline 'Il compound to the spent pulping liquor, under conditions to liberate its ammonia. content, recovering the liberated ammonia, recovering the sodium constituent remaining in the spent pulping liquor as a substantially completely water-soluble Land sodium sulphite as'iiber-liberating chemicals, separating the pulp from the resulting spent pulping liquor, adding sodium base alkaline compound'to the spent pulping liquor under conditions to liberate its ammonia content, recovering the liberated ammonia, recovering the sodium constituent remaining in the spent pulping liquor as a substantially completely water-soluble smelt containing substantially only sodium base alkaline compound, using part of said recovered sodium base alkaline compound in treating spent pulping liquor to liberate its ammonia content, and causing the rest of said recovered sodium base alkaline compound and said recovered ammonia to react with a sulphurous acid solution to reproduce a fresh composite base sulphite pulping liquor.

15. A process which comprises pulping raw cellulosic material in an ammonium bisulphite cooking liquor containing in suspension therein alkaline earth metal salt of much lower solubility than calcium bisulphite.

16. A process which comprises impregnating wood chips with ammonium sulphite as a berliberating chemical and then pulping such impregnated chips in a sulphurous acid solution containing alkaline earth metal salt in suspension therein.

17. A process which comprises ammoniating wood chips substantially uniformly throughout their structure, adding sulphur dioxide and water to the ammoniated wood chips to form ammoni- .pounds, adding a sulphurous acid solution to um bisulphite in situ therein, and pulping the wood chips in the resulting solution in the presence of alkaline earth metal sulphate suspended in such solution.

18. A process of pulping chipped wood which 5 comprises adding liquid ammonia to a massv of such wood while under confinement under conditions to cause the ammonia to flash into fumes and to develop a pressure of about 20 to 100 the ammoniated mass to form in situ therein ammonium sulphite cooking liquor, and cooking the mass in the resulting liquor to form pulp.

19. A process of pulping chipped wood which comprises adding liquid ammonia to a mass of such wood while under confinement under conditions to cause the ammonia to ilash into fumes and thereby to permeate the mass, adding a sulphurous acid solution to the ammoniated mass to form in situ therein a sulphite cooking liquor of about 0.5% to 1.0% combined SO2 content, and cooking the mass in such liquor to form pulp.

20. A processof pulping chipped Wood which comprises adding liquid ammonia to a mass of such wood while under confinement under conditions to cause the ammonia to flash into fumes and thereby to permeate the mass, adding a sulphurous acid solution to the ammoniated mass to form in situ therein a bisulphite cooking liquor of about'0.5% to 1.0% combined SO: content and of at least about 3% free SO2 content, and cooking the'mass in such liquor to form pulp.

2l. A process of pulping chipped wood which comprises adding liquid ammonia to a masso! such wood while under coniinement under conditions to cause the ammonia to flash into fumes and to develop a pressure of about 20 to 100 pounds, adding a sulphurous acid solution to the ammoniated mass to form in situ therein a bisulphite cooking liquor of about 0.5% to 1.0% combined SO2 content and of at least about 3% free S02 content, and cooking the mass in such liquor to form pulp.

GEORGE A. RICHTER. g