US1795755A - Production of chemical pulp - Google Patents

Description

Patented Mar. 10, 1931 the same sulfur bein compound in each of the two types of cook- UNITED STATES PATENT orrlca LINN BRADLEY, OF MONTCLAIR, NEW JERSEY, AND EDWARD I. MCKEEFE, OF PLATTS- BURG, NEW YORK, ASSIGNORS TO BRADLEY-MCKEEFE CORPORATION, OF NEW YORK,

N. Y., A CORPORATION or NEW YORK raonucrrou or cHEmrcAL PULP No Drawing. Application filed September 2, 1926, Serial No. 183,292. Renewed January 23, 1931.

This invention relates to the production of chemical pulp, and to the treatment of residual liquors for the regeneration of cooking liquors. j.

More particularly the invention relates to cyclic and interrelated operations in which two different kinds ,or types I of sulfur-containing cooking liquors are employed, of which soda is the base, some .of

stance, in one type 0% cooking liquor as a sulfite and being present, in another instance, in another type of cooking liquor as sodium sulfid, and with successive employment of some of the same soda as an active ing liquors. The two types of cooking liquors may be generally described asan acid sulfite of sodium cooking liquor and as a sodium hydroxide-sodium sulfid cooking liquor. Some of the soda and some of the sulfur may in art be present as other compounds.

1 mongthe particular improvements which the invention presents are operations or steps in which cooking liquor for one type of cooking operation is regenerated (at least in part) from constituents of residual liquor from "the other type of cooking operation. The acid sulfite of sodium cooking hquors, to

which the invention part cularly relates, are those which contain both sodium bisulfite and sulfurous acid when charged into the digester or which contain these ingredients after the introduction of sulfur dioxide into the liquor .contained within the digester, for example by relieving sulfur dioxide from one digester into another. The other type of cooking liquors, to which the invention articularly relates, are cooking liquors w ich contain at least as much sodium hydroxide as sodium sulfid, and usually with-the amount of sodium hydroxide considerably in excess of the amount of sodium sulfid'.

According to one embodiment of the 1nvention, wood chips are cooked with cooking liquor which contains a suitable amount of sodium bisulfite and of sulfurous acid, the resulting residual liquor is separated from the pulp, another lot of wood is cooked with a cooking liquor which contains a suitable present, in one in amountof sodium hydroxide and a considerable amount of sodium sulfid, the sodium hydroxide being em loyed in larger amount than the sodium sul d, the resulting residual liquor is separated from, the pulp, and the residual liquors are subjected to concentrating :and furnacing steps to form recovered soda products which are employed in the production-of further cooking liquor.

The residual liquor from the acid cook is advantageously-admixed with residual liquor from the alkaline cook, to combine with and i x reactive acid constituents thereof and to term. an admixed residual liquor which is either mildly acid, neutral or alkaline to litmus, and the mixing step is regulated-for this result. Considerable organic" matter usually separates out from solution as a result of such mixing treatment. This liquor is subjected to a concentrating step in any suitable manner. In order -to facilitate the subsequent stepsof the residual liquor treatment, the mixed liquor may be further treated. For example, the liquor may be subjected to concentration in a suitable multiple-1 effect evaporator in which the liquor is subected to rapid and forced circulation adjacent the heatlng elements so as to prevent,

or so as to minimize the efi'ect of, organic deconcentrated, and, if desired, a small and regulated amount of a strong reactive sodium compound, e. g. NaOH, maybe added to the somewhat clarified liquor so as to redissolve remaining suspended organic matter andthus further prepare the liquor so that it may be readily concentrated in a multiple-efiect evaporator, or any other suitable device. The liquor may be sprayed or atomized into hot gases as a part of the concentratingtreatment, or all of such concentration can be thus effected. The organic matter separated from the liquor prior to concentration thereof may be advantageously returned to the liquor, or

,to a selected portion thereof, subsequent to the step in which the liquor is subjected to.

a regulated concentration, for example by returning it to theportion of the concentrated liquor which is to undergo a reducing furnace treatment, orto the portion of the concentrated liquor-which is to be treated to form a product containing the soda mainly iii the form of sodium carbonate. By adding it to the portion last mentioned, the ratio of organic compounds to sodium-sulfur compounds therein is changed in an advantageous manner.

Any unused portion of residual liquor from the alkaline cook, i. e. thattwhich is not employed for admixing with residual liquor from the acid cook, may be concentrated and constituents thereof subjected to a suitablefurnacing treatment whereby a considerable portion of the sulfur content thereof may be driven ofl'from its association with soda, and residuary sulfur is largely present as an oxidized sulfur compound, such as sulfate or sulfite, in the recovered soda product, which contains, as a result of the previous treatment, most of its soda in the form of sodium carbonate, and which is substantiallyfree from sodium-sulfid and from sodium thiosulfate. A solution is prepared, which contains the soda thus recovered, and in case there is an amount of sodium sulfidor other sulfur ingredient therein which would preventior render-difficult the production of a suitable cooking liquor for the acid, cook of the cycle, by treatment with sulfur'dioxide in adequateamount, such solution is subjected to a'tre'atment to remove such material suitable oxidizing treatment, such as heating it. after thoroughly mixing therewith a regulated amount of sodium eroxide; or sodium hypochlorite may be emp oyed as the oxidizing agent, especially when only: a small amount of such added reagent is required, or the solution maybe subjected to an aerating treatment to remove or oxidize objectionable material. A regulated amount of sodium manganate, or of sodium permanganate, maybe employed in such an oxidizing treatment. A moderate amount of lime may be employed-subsequently for assisting in the clarification of such. oxidized liquor and the clarified liquor separated from the solids. Sodium manganate therefore may be prepared by a suitable furnacing of a mixture of soda and manganese dioxide and such product can be advantageously employed directly without further treatment thereof. Some of the manganese compound required may be obtained by regenerating it from the sediment removed from the liquor subsequent to the oxidizing treatment, for example prior to adding the lime if any. Some of the sulfid sulfur may even be removed by treating the solution with a regulated amount of sulfate of iron, for example, to form iron sulfid, and a small amount of lime may be added to facilitate in such purification step, and the treated liquor separated from the solids.

The purified solution, which may be pre-. pared in the manner outlined above, is sulfited by means of sulfur dioxide to form cooking liquor of the desired composition which is later employed in a cooking operation of the cycle. The solution may be partially sulfited and the partially sulfited liquor may be further sulfited, for example by absor ng therein sulfur dioxide relieved from a digester. Such furthersulfiting may be accomplished either prior to charging the liquor into the digester or subsequent thereto.

In all cases, however, sufiicient sodium bisul-' fite and sulfurous acid will be employed in the cooking liquorsef the acid type, to perform the required cooking operation.

In carrying out the furnacing operation which produces the soda product which contains mainly sodium carbonate and which is substantially free from sodium sulfid, more air may be employed therein than is required for burnin the combustibles obtained fromthe resid iial liquor. Thus most of the carbon may be burned and removed,

thereby not only aiding in the driving off (if sulfur but also minimizing the reducing tendency whereby sodium sulfid might be formed. Theexcess of air may even reoxidize some N a- S'incase it should be present in objectionable amount.

A re ulated portion or all of the admixture of residual liquors, which may be prepared in the manner heretofore described, is subjected to a further treatment, subsequent to the concentrating step described above, whereby constituents thereof are subjected to a heat treatment of a reducing type whereby a soda product is obtained which contains sodium carbonate and sodium sulfid, generally with the sodium carbonate considerably greaterin amount than the sodium sulfid-but neverthe less containing arelatively large amount of sodium 'sulfid as compared with the soda product which is'to be sulfited to form the acid cooking liquor. This heating operatain in the recovered soda product a considerable portion of the sulfur content of the constituents of the liquor which are thus treated, preferably in the form of sodium sulfid, so that therecovered soda product can advantageously serve as a basis for the production of cooking liquor of the alkaline type of the cycle. Thus the amount of air em ployed is so regulated that the operation is of a highly reducing nature, and care is taken able manner.

. to avoid driving off from association with soda, too large an amount of sulfur, and to avoid the formation of an undue amount of sodium sulfate in the recovered soda product. Some sodium sulfite may be present in the product and as a consequence in'the cooking liquor produced therefrom, thusmodifying the cookingliquor of this ty e.

When sodium sulfate is added to the cycle to replenish the soda, on account of losses thereof, it is preferably added to the constituents which are to undergo such a reducing furnace treatment, thu-sforming sodium sulfid therefrom and augmenting that which is otherwise formed.

The sodium carbonate and the sodium sulfid in such recovered soda product-are dissolved in water and the solution is subjected to a causticizing treatment with sufficient lime to convert the greater part of the sodium carbonate into sodium hydroxide. In case there is notsuflicie'nt sodium hydroxide in the resulting liquor, such compound may be added to or produced therein in any suit- For instance, some sodium carbonate may be added to the li uor which is to be causticized so that liquor o the required composition can be preparedtherefrom by a suitable causticizing treatment. The liquid is separated from the lime mud and is em ployed in a cooking operation of the alkaline type, of tl e cycle. a

In case some of the concentrated mixed residual liquors is to be treated to form a soda productfwhic-h can'serve as a suitable base for the production of the acid cooking liquor, the required portion ofv such mixed liquors is treated so that constituents thereof are heated and substantially all of the combustibles are burned; at least the heating operation is carried out in such a manner that the resulting soda product contains mainly sodium carbonateand is free from such relatively large amounts of sodium sulfid as are contained in' the soda product which is to be causticized to form the alkalinetype of cooking liquor. By em loying an excess of air, in thc manner I iorc stated, a soda product can be obtained thereby which is free fronian objectionable amount of sodium sulfid, L although it may contain a small amount. Nevertheless, a method has been disclosed above which will permit the production of suitable acid cook.

ing liquor for the cycle of the present invention. A M

, Ordinarily ,we prefer to so regulate the operations that the amount of alkaline residual liquor produced is adequate to provide asufiicient amount for admixing with the acid residual liquor and a further sufficient, amount to provide the sodalwhich serves the'basis for the acid cooking liquor, as this ffiCllktates the production ofa soda product which contains mainlysodium carbonate and is substantially free from sodium sulfid, even when not all of the carbon from organic constitucuts of the residual liquor is consumed in the furnacing operation. This is especially the case when the alkaline cooking operation has been so carried out that a large portion of the sodium sulfid of such cooking liquor has been caused to react with organic material derived from wood, as such residual liquors appear to very readily part with sulfur when the constituents are heated to the required temperature' for a suflicient time.

Residual liquor from the alkaline cook, especially that portion which is to be utilized for producing the high sodium carbonate and low sodium sulfid product, may be employed may thenbe removed to such extent as may be readily withdrawn therefrom, and leaving the residue with the partially treated chips to admix with the main supply of alkaline cookingliq'uor which is to be subsequently addedthereto. .The portion of the treated residual liquor which has been-removed, may

carry a fair amount of suspended organic,

material, due perhaps to interaction between sodium-organic constituents present in the residual liquor so employed andsome ofthe readily dissolved organic constituents removed from the wood by the partial treatment. However, such suspended organic material is generally of such a fine state of division that it may not be readily separated therefrom, and such liquor may be subjected to the concentrating" step with the suspended matter contained therein. Forcing the liquor at high velocity ad-jacentthe heating surfaces of the. evaporator maywbe resorted to so as to facilitate the ready concentration, of such liquors. Thepartially treated chips generallymay be properly*cooked with a lesser amount of-freshly supplied active reagents than where the woodiclii'ps' have not been given the preliminary partial treatment, and

thus effect, a further economy since less total chemical is required and a lesser amount of lime may be employed for the causticizing step for preparing cooking liquor: Such 5 treatment of wood chips with alkaline residual l quor presents, other advantages, including the ready production of the high sodium carbonate-low sodium sulfid'sodaproduct re b ferred to above. I v I The hotgases from the furnace, operating under the highly reducing conditions, may

carry sulfur dioxide or sulfur trioxide, and I may carrysome sulfur or some combustible sulfur compounds. The recovery ofthesulfur content of the latter may be facilitated suitable manner.

burning operation, the resulting gases may be suitable for generating steam for the cycle, e. g. by passing them through a boiler. These gases generally contain a considerable amount of sodium compound. Passingthese gases through an electrical precipitator, preferably at a temperature above the dew point, such solids may be recovered, generally with a high Na SO content. advantageously added to that which'is to undergo a reducing furnace treatment to form the soda product for the alkaline cooking liquor. Sulfur dioxide may be regained from such gases, when contained therein in amount adequate to justify the step, for example by scrubbing them with a solution which contains sodium carbonate, which may be a solution obtained by dissolving the soda compounds from the product obtained by the furnace treatment which employs an excess of air. Some sodium sulfite may be thus formed in the liquor which is to be employed subsequently in the acid cooking operation. Such sodium carbonate solution is generally purified or freed from sodium sulfid and sodium thiosulfate such scrubbing step.

The acid cooking liquor may be prepared by first partially sulfiting the liquor and subsequently completing the sulfiting to the requlred degree. Such sulfur dioxide as is'relieved from the digester during or at the end of the cooking operation, for example, may be employed for the completion-of the sulfitiug of suclr liquor. charged into the digester generally contains a considerable amount of sodiumbisulfite, that is it contains a considerable amount of uncombined- S0 A further amount of S0 may be introduced-I into the liquor within the digester by, any suitable means. For example, some $0: may be liberated from liquor in one digester which may be almost ready to be blown, into 1i uor contained in a dlgester which has consi erable time to go before the cookin operation has been completed therein. T us, instead of relieving S0 into an,

outside vessel or an absorbing tower, for

fortifying the partially sulfited liquor, SO,

may be relieved from one digester into another, along with steam, absorbedin the latter digester. -By using a relatively high with the Such material is prior to employing it in The liquor which is relieved from one digester into another.

When sodium carbonate is relied upon for replenishing the soda of the cycle, due to losses, it is preferably added to the liquor which is to be sulfited so as to produce liquor for use in the acid cooking operation. Sulfur required for the operations may be supplied in any suitable manner, for example 'by burning sulfur in air and absorbing the sulfur dioxide inthe liquor or liquors described. When a considerable amount of the sulfur of the cycle is recovered or regained as a sulfite of sodium, the amount of sulfur to be freshly sup'plied may be reduced accord ingly.

The composition of the cooking liquors and the amounts of pulp produced by the two typesof cooking liquors, res ectivel may vary somewhat; and different kinds 9 woods may be cooked by either of the cooking liquors. When highly resinous woods, such as certain pines, are cooked by the alkaline liquor and the resulting residual liquor is admixed with the residual liquor from the acid sulfite type of cooking operation of the cycle, for exampleby-cooking gum wood with an acid liquor. which contains a fairly high content of combined S0 and a relatively low content of uncombined S0 the mixing being regulated so. as to roduce an admixtureof residual liquorswhlch is mildly acid, neutral or moderately alkaline to litmus, a considerable amountof organic matter. separates out. By separating the liquor from such material, the subsequent steps are improved, viz: the concentration step whenmultiple-efiect evaporators are employed therefor. By regulating the amount of acid liquor added to the alkaline residual liquor, somewhat different grades of products may be separated therefrom, thusthe liquor may be added in regulated steps with separation of organic material between the successive additions of liquor. Material suitable for a soap-stock ma 'be thus produced. v

s bmewhat different amounts of the alkaline residual liquor ma be admixed with the acid residual liquor. or example, a sufficient amount of acid residual liquor may be admixed with alkaline residual liquor to separate most of the 'precipitatible organic matter, leaving the mixture slightly acid if desired, then separating the liquor from suspended organic imaterial. e separated liquor, which may still be slightlyacid to litmus, may then be treated with a regulated amount of alkaline soda compound to leave the liquor moderately alkaline to litmus. For thislresult, either some sodium hydroxide may be employed, or some of the sodium carbonatesodium sulfid liquor may be added for this purpose, this treatment redissolving small amounts of organic material which may remain in the separated liquor. By regulating the mixing step orsteps, described above, a large amount of organic material may be separated and removed from the liquor, and for example by employing only a moderate amount of alkaline compound added there after, the mixture of the residual liquors may be readily subjected to the concentrating step of the cycle. Instead of employing liquors which contain a suitable alkaline sodium compound which are free from organic matter. for such treatment of the separated and slightly acid mixed liquor, some of the alkaline residual liquor may. be thus employed.

By conducting the operations in a manner outlined above, the organic constituents removed from the wood may be made use of for the production of steam for use either in a digester or in a multiple-effect evaporator of the cycle. a

When a mixture of steam and S0 is relieved from a digester, during or at the end or final stage of the digest-ingoperation, and this is not introduced directly into liquor contained in another digester, such mixture of steam and S0 may be utilized by passing it into a hot solution of sodium sulfite or carbonate, for example, contained within a suitable vessel of the closed type, absorbin the S0 therein and permitting steam an uncondensed gasesor vapors to ass therefrom in a regulated manner and su sequently employing the-steam in another operation'of the cycle. For instance, the steam-may be permitted to escape from the accumulator under regulated conditions and thence into the steam belt of a multiple-eifect evaporator of suitable type and construction, i. e. of suitable acid-resisting material when the steam thus employed contains any constituents of the acid type; or some of the steam may be employed in'the heating of a digester charge during. the early stages of such heating. Such CO as the steam may contain may be permitted to escape from the condensed steam in any suitable way. Such lsulfite of sodium as is thus formed by reaction between the soda in the liquor within the accumulator and the S0 absorbed from the steam, may be employed in the cooking liquor of the acid type. For example, the liquor' may be removed from the accumulator and later subjected to a further regulated sulfiting treatment with suitable vapors or gases which contain S0 and such su'lfiting continued until a cooking liquor is produced which contains the desired amounts and ratio of combined to uncombined S0 Where some sulfite of calcium or magnesium or both is desired, calcium hydroxide or magnesium hydroxide or both may be suspended in water. and

' utilized in the accumulator to absorb SO,

from the steam, thereby forming the sulfite. Ca SO or MgSO' or mixtures thereof may be further sulfited to form solutions of these in sulfurous acid, or they may be employed to react with sodium carbonate in solution forming sodium sulfite thereinand undissolved CaCO-d and/or MgCO Where such suspensions of hydroxide are thus employed, means should be provided toadequately keep them in suspension and thereby facilitate the step.

It is advantageous to employ some alkaline .of sodium carbonate and sodium sulfid.

The lime mud which results from the step of treatmg a solution of sodium carbonate containing sodium sulfid with lime to produce the alkaline cooking liquor, usually contains some calcium hydroxide. When such product has been suitably washed to free it from objectionable amounts of sodium sulfid, some of this washed lime mud may be employed to assist in clarifying the sodium carbonate liquor which is free from or contains but a relatively small amount of sodinm sulfid. Such sodium hydroxide as may result from such treament is present in the liquor which is to be sulfited, and liquor so produced 1s usually substantially free from iron compounds. Any required oxidizing treatment to advantageously alter sulfur compounds therein maybe conducted prior to such clarification step, especially when the lime mud so employed s free from objectionable compounds, or it may be employedcoincident 'Withsuchtreatment with the lime mud, or even subsequent thereto, depending upon the circumstances andthe requirements, so long as the liquor which is being sulfited is of the requlred composition so that suitable acid sulfite liquor may be readily produced therefrom.

We claim:

1. In a cyclic process for producing chemical pulp, the steps which include cooking one lot ofwood in a cooking liquor which contains sulfunous acid and sodium bisulfite,

thereby producing one type of residual liquor, cooking another lot of wood in a cooking liquor which contains sodium hydroxide and a lesser amount of sodium sulfid, thereby producing another type of residual liquor,

mixing residual liquor of the one type with residual liquor 'ofthe other type, separating suspended matter therefrom, adding a regubisulfite and sulfurous acid and being free lated amount of sodium compound capable of redissolving remaining suspended organic material, concentrating the resulting liquor to produce a concentrated liquor, subjecting constituents of one portion of the concentrated liquor to a reducing furnace treatment to form a product containing sodium carbonate and a considerable amount of sodiumsulfid, subjecting constituents of another portion of the concentrated liquor to a furnace treatment to form a product containing the soda mainly as sodium carbonate and free from any considerable amount of sodium sulfid, forming separate liquors from soda compounds in each of the respective products, subjecting the-liquor which contains the greater proportion of sodium sulfid to a causticin g treatment to form a cooking liquor which contains sodium hydroxide and sodium sulfid, subjecting the other liquor to a regulated treatment 'to form a cooking liquorcontaining sodium from an objectionable amount of sodium thiosulfate.

2. In claim 1,the regulated treatment for obtaining the acid sulfite cooking liquor including the step of oxidizing any sodium sulfid and any sodium thiosulfate in the liquor prior to the sulfiting step.

3.. In claim'l, the further step which comprises adding the separated organic material to concentrated liquor produced by the concentrating step.

4. In claim 1, the further step which includes feeding one portion of the concentrated liquor into a disc-evaporator and subsequently subjecting constituents thereof to a reducing furnace treatment. I

5. In claim 1, the further step which comprises mixing gases from the furnaces and employing the mixture for the generation of steam by passing it through a boiler.

6. In claim 1, the further step which comprises subjecting the furnace gases to an electrical precipitation treatment to remove solids therefrom. j I

7 In claim 1,' the further improvement characterized by subjecting the furnace gases to an electrical precipitation'treatment at a temperature above the dew point.

8; In claim 1, the further improvement which includes subjecting the furnace gases to an electrical precipitation, recovering the cludes absorbing an oxide of sulfur from fur-,

nace gases. and employing such compound in a cooking operation of the cycle.

10. In claim 1, the improvement which includes absorbing sulfur ,dioxide from furnace gases-in an alkaline soda liquor, and em ploying the sodium sulfite thus formed in the acid sulfite type of digestion of the cycle.

11. In claim 1,-the further improvement which includes employing the sulfite radicle from the furnace gases so as to sulfite sodiumit in heating one of the liquors of the cyclic operation.

14. In claim 1, the further improvement which comprises relieving steam and sulfur dioxide from acid liquor contained within a digester and absorbing the sulfur dioxide in asoda-containing liquor'within a vessel of the closed type, further sulfitin the liquor after removal from the vessel and employing the same in the acid cooking step of the cycle.

15. In claim 1, the further improvement wh1ch comprises relieving the steam and sul-- fur dioxide from acid liquor contained within a digester, absorbing some of the relieved steam and sulfur dioxide in liquor contained within another digester to increase the amount of uncombined sulfurous acid therein.

16-. In claim 1, the further improvement which comprises producing an adequate amount of residualliquor of the alkaline type to serve for neutralizing the acid ingredients of the residual liquor of the acid type and to serve for producing the soda product which contains mainly sodium carbonate and little or no sodium sulfid which is to be suliited to'produce the cooking liquor of the acid includes cooking wood by an acid sulfite cooking liquor, the steps which include'relleving sulfur dioxide and steam from the digester, absorbing the S0 in a liquor containing a reactive alkaline compound where- 17: In a process for producing pulp which .by a sulfite is formed, further sulfiting the liquor to form the required amount and ratio of combined to u'ncombined S0 and employmg the steam for heating liquor employed in the process.

18. A cyclicprocess for producing-chemi- I cal pulp which comprises cooking one lot of wood n a cooking liquor which contains sulfurpus acid and sodium bisulfite, thereby producing one type of residual liquor, cooking another lot of wood'in. a liquor which contains sodium hydroxide and a lesser amount of sodium sulfide, thereby producing another type of residual liquor, mixing residual liquor.

form a product containing the soda mainlyas sodium carbonate and free from any considerable amount of sodium sulfide, forming separate liquors from the soda compounds of each of the respective products, subjecting the liquor which contains the greater proportion of sodium sulfide to a causticizing treatment to form a cooking liquor which con tains'sodium hydroxide and sodium sulfide, subjecting the other liquor to a regulated treatment to form a cooking liquor containing sodium bisulfite and sulfurous acid free from an objectionable amount of sodium thio-sulfate, mixing gases from" the furnace, supplying a regulated amount of air to burn combustible material of'said gases and em ploying the hot mixture of gases for the generation of steam by passing it thru a boiler.

19. The cyclic process'of producing pulp which comprises cooking one lot of wood in an acid liquor which conta'ins sodium bisultite, thereby producing one type of residual liquor. cooking another lot of wood in a cooking liquor which contains sodium hydroxide and a lesser amount of sodium sulfide. thercs by producing another type of residual liquor mixing residual liquor of the firsttype with a portion of the residual liquor of the second type, treating the mixture of liquor so formed to produce a White liquor suitable for use in-the second named cooking treatment, utilizing another portion of the second type of residual liquor in a preliminary partial cooking treatment. separating residual liquor from the partially cooked wood, treating the last named residual liquor to recover its sodium content, principally as sodlum carbonate free from objectionable amounts of sodium sulfide, and treating this soda product to produce an acid cooking liquor.

20. The cyclic process as defined in claim 19 in which white liquor is added to the partially cooked wood and adhering residual liquor and the cooking process completed therewith.

2.1. The cyclic process according to claim 19 in which the acid liquor is utilized in the first named cooking process.

22. The cyclic process according to claim 19 in which the white liquor is utilized to complete the production of pulp from the partially cooked wood and the acid liquor is utilized for cooking further amounts of wood according to the first named cooking process.

and the acid cooking liquor and-the alkaline cooking liquor are used in such proportion that the residual liquor of the second type will be sufficient in amount to neutralize the residual liquor of the first type and also to supply sodium compounds suflicient to regenerate the necessary amount of acid cooking liquor.

23. The cyclic process for producing pulp which comprises cooking one lot of wood in a cooking liquor of one type, cooking another lot of wood in a cooking-liquor of a diflerent type, separating residualliquors from the pulp resulting from each cooking treatment, mixing said residual liquors, furnacing the solid constituents of the mixture, recovering the valuable constituents of, the resulting furnace product, treating a portion oi, said constituents to regenerate a cooking liquor of the first type and treating another portion of said constituents to regenerate a liquor of the second type.

' 24. The cyclic processfor producing pulp which comprises cooking one lot of wood in an acid cooking liquor, cooking anotherlot of wood in an alkaline cooking liquor, separating residual liquors fro'm the pulp produced in each cooking treatment, mixing the residual liquors so as to substantiallyneiu tralize each of them'and precipitate organic matter therefrom, separating the precipitated organlc matter, concentrating the residual liquor, subjecting the concentrated liquor to a suitable furnacing operation, recovering valuable constituents from the resulting furnace product, treating a portion of the said constituents to produce an acid cooking liquor suitable for the first named cooking process and treating another portion of said constituents to produce an alkaline cooking liquor suitable for the second named cooking treatment.

25. The cyclic process for producing pulp which comprises cooking one lot of wood in "a. cooking liquor which contains a sulfite of sodium, cooking another lot of wood in a cook ng liquor which contains sodium hydroxide, separatmg resldual liquors from the pulp resulting from each cooking treatment,

mixing said residual liquors, subjecting solid constituents of said mixture to a furnac'ing treatment, recovering sodium compounds from the resulting furnace product, treating a portion of said sodium compounds to produce a sulfite liquor adapted for use in the first named cooking treatment, and treating another portion of the sodium compounds to produce a caustic liquor suitable for use in the second named cooking treatment.

26. The process according to claim 25 in which the furnacing treatment is maintained under reducing conditions, the portion of the sodium compounds which is, to be used for producing a sulfite liquor is subjected to a regulated oxidizing treatment until substantially all the sodium sulfide contained therein has been oxidized.

27. A cyclic process for producing pulp which comprises cooking one lot of wood in a cooking liquor which contains sodium bisulfite, cooking another lot of wood in a cooking liquor which contains sodium hydroxide and sodium sulfide, separating residual liquors from the pulp resulting from each cooking treatment, mixing said residual liquors whereby organic matter is precipitated therefrom, separating organic matter, concentrating the resultant liquor, subjecting solid constituents of the liquor to a furnacing treatment under reducing conditions, leaching the resulting furnace product to produce a solution which contains sodium carbonate and sodium sulfide, subjecting a portion of said solution to a regulated oxidizing treatment until substantially all the sodium sulfide has been oxidized, sulfiting the resulting solution to produce a cooking liquor suitable for use in the first named cooking treatment, causticizing another portion of the solution containing sodium carbonate and sodium sulfide,'and returning the causticized solution for use in the second named cooking treatment.

28. The process according to claim 27 in Y which the regulated oxidizing treatment comprises adding to the solution an oxidizing sodium salt.

29. The process according to claim 27 in which the regulated oxidizing treatment comprises adding to the solution sodium peroxide.

30. The cyclic process for producing pulp which comprises digesting fibrous material with an acid cooking liquor containing an active sodium compound, digesting fibrous material in a cooking liquor containing alkaline sodium compounds, mixing residual liquor from the acid digestion with residual liquor from the alkaline digestion in amount suiticient to form a substantially neutral mixture, thereby precipitating organic matter, evaporating the mixture, subjecting a portion of the evaporated mixture to a reducing furnacing treatment and the furnace product to a treatment adapted to regenerate a cooking liquor of the second type above mentioned, subjecting another portion toan oxidizing furnacing treatment and the furnace product to a treatment adapted to regenerate a cooking liquor of the first mentioned type, and repeating the process.

In testimony whereof we aflix our signatures.

LTNN BRADLEY. EDWARD P. MOKEEFE.