US2778714A - Method for recovering base ions from waste sulfite liquor and producing sulfite cooking liquor - Google Patents

Description

Jan.' 22, 1957 A, A, KASPER ETAL 2,778,714 E: suLFITE METHOD FOR RECOVERING BASE IONS FROM WAST LIQUOR AND PRODUCING SULFITE COOKING LIQUOR 4 Sheds-Sheet 1 Filed April 26. 1952 ATTORNFY Jan. 22, 1957 A- A, KASPER ET AL 2,778,714 METHOD FOR RECOVERING BASE IoNs FROM WASTE sum-ITE LIQUOR AND PRODUCING SULFITE COOKING LIQUOR Filed April 26. 1952 4 Sheets-Sheet 2 DOQ:

AT TORNF v Jan. 22, 1957 A. A. KASPER ET AL. 2,778,714

METHOD FOR REQOVERIHG BASE IONS FROM WASTE SULFITE LIQUOR AND PRODUCING SULFITE COOKING LIQUOR Filed April 2e. 1952 4 sheets-sheet s CARBONATE LIME K ILN CLARIF IER CAUST- ICIZER lLEACHING TANK SMELTER F URNACE coNTAcT EvAPoRAToR F IG. 3

IN VEN TORS. ANDREW A. KASPER WILLIAM E. KATZ ATTORNEY BLACK LIQUOR Jan. 22, 1957 A. A. KAsPER ET AL 2,778,714 STE SULFITE METHOD FOR RECOVERING BASE IONS FROM WA LIQUOR AND PRODUCI Filed April 26, 1952 NG SULFITE CGOKING LIQUOR 4 Sheets-Sheet 4 METHD FR RECVERNG BASE INS FRM WASTE SUIjFlT'E LIQUR AND PRQDUCiNG SULFITE COOKING LilQUR Andrew A. Kasper, Watertown, and William E. Katz,

Boston, Mass., assignors to llonics incorporated, Cambridge, Mass, a corporation .oi Massachusetts Application April 26, .11952, Sferial No. 284,604

4 Claims. (Cl. 23--1'29) This invention relates -to methods and apparatus for producing cooking `liquor for 'the yproduction of paper pulp. It has reference lto the Vproduction of -so-called neutral sulte cookingliquors of the type employed in making either semi-'chemical suliite pulp, lfull-chemical sullite pulp frequently referred to as full monosul'iite-pulp, andalso tothe production 'of acidk-suliitecooking liquors used in the product-ioneof acid sulte pulp. The neutral sultite cooking liquors,-which`areusually usedinthe pulping of hard woods, areproduce'd Afrom normal'sullites, as distinguishedfrom acidvsultes, while the'acid sulte cooking liquors 'arelargelysolutions of acid sulites usually with an excess of-sulfurdioxidepresent. `rl`he invention is particularly concerned with those cooking liquors of both types, which have tas their metallic base component, sodium or potassium. Actually'the so-called neutral sulte liquors are-not usuallylstrictly'neutral but ya're'more nearly so than otlrertypes of cooking liquors which are distinctly acid or alkaline. Neutral'suliite cookingliquors frequently have afpH Well abovev 77.0 and provide `a-waste liquor having a pH'las high as `810.

The invention relates also toithe cyclic production and useof such cooking liquors by a-method'in'lwliich the valuable-ingredients are continuously'recovered and-reused. in the pulping of successive quantities orl charges'of wood or other cellulosic raw' material.

It has long been appreciated thatneutral suliite cooking liquors, such as those consisting primarily of'solutions of vsodium sullite'with a certain lamount ofl the corresponding carbonate, arel desirable' from the standpoint of relatively high yields ofl pulp from the Wood lor other cellulosic source material -an'd the production of-a good grade of pulp. They are particularly'usefulV in thevpulpingof hard Woods. However, neutral sulte 1cooking processes as heretofore practiced, particularly in mills' not equipped tofcarry on kraftv or sulfate pulp producing operations in conjunction with the suliite pulping operations, have involved such'heavy losses of thev'alua'ble yconstituents, such as the sodium and sulfur as to be substantial-ly prohibitive from the standpoint of cost. The use of sodium or potassium based acidsuliite lcookingliquors has -been restricted for similar reasons. One method of reducing 'the -loss of valuable constituents'from suliite cooking operations is disclosed-in the copendingfpatent application of Gerald Haywood, Serial No. 235,614, led luly 7, 1951, now Patent YNo. 2,736,f635,granted February 28, 1956.

Aprimary object of the present invention has been to provide -an improved method of'recovering and -producing sulte cooking liquor for neutral or acid suliite pulp producing processes, of the semichemical or fullchemicalv types indicated above, which Will-enable such processes to be operatedwith suflicienteconomy ofthe valuable components of the cooking liquor as to becommercially competitivecwith, and even less expensive-than, other methods or" producing suliite pulp. it' `vill'be understood that'thefterm semi-'chemical' suliiterpulp refers -to thattypeof pulp which is produced by'removingthe nited States Patent O Patented Jan. 22, 1957 lignins ofthe -raw material, Without removing most of the pentosans and similar constituents, by cooking with a suliite liquor and then forming the ypartially cooked material into a usable pulp by physical means, such as mechanicalfdisintegrators and the'like. The terms full chemical sullite pulp and "ffull lmonosulfite pulp are commonly used to designate pulps vwhich are produced fromIwoodlorlthe'likein a-form usable/for-the production of paper or paperzproductsfthroughthe use ofa -neutral sulte cooking liquor/without :the ynecessity Y of subsequent mechanical disintegration. Thecharacter ofthe cooking liquor, the quantity employed 1in-the pnl-ping of a given amount of the raw material landthe length lof time required for the fcookingstep dider with'thetypevof processfemployed.

VIn accordance wit-h the present invention the suliite cooking 'liquor is formed, Iatleast in large lpart, by recoveringsodium or potassium, together with'the desired acid y'or active-component ifrom-a'- prior cooking liquor-by a ysuitable lion exchange procedure. rThis involves ,contacting an -appropriate strongly' acid cation exchangeresin partially in-thesodiumfor potassium form with an acid sulfte'solutionsuch-as -sulfurous acid or a solution of sulfur dioxide in Water, lor the'like `to -bring about an exchange of hydrogen Afrom lthe yacid for atleast a part of the sodium or'potassiumof 'thefresinto producethereby alsolution containing bisultite, atleast inepart, and lsubsequently contaetingthissolutionwith a weakly acid cation exchange resin'primarily in thefsodium or potassium form to bring about an Aadditional exchange l of hydrogen v ions forfat least a part of'theI-sodum-offtheresin. The-resulting solutionis inaform suitable for use, with or without subsequent concentration, dilution, additionsor other treatment, as a neutralerI acid sullite cooking' liquor depending' upon i the conditions under which the exchange is carriedcout. 'While thevinvention willlbe describedvin connection with sodium as the'basefcomponenitis to be understood thatpotassium may be substitutedtherefnor.

The acidic material used .1 inthe foregoing procedure' lis in'part derived'fromthewasteliquor of-a prior sulfite cooking operation. A certain :aniount of sulfur `dioxide is recovered orproduced 'from such liquor fand is dissolved in Water to form at'fleastta part of the-acidfrequired. The cation exchange resins,rwhichhave` been converted 'from the sodium formf toi the hydrogenf form,V by: the foregoing procedure,;are` recharged-.with sodium byfcontac-ting with an alkaline liquor,:or.components thereof, derived from the smelting of a prior neutral or acid suliite cooking liquor. :inl this Way .a cyclicfprocess is provided in ywhich a continuous ysupply-of cooking liquor? is availabley for the pulping of successive -batches ofwoo'd or other cel-lulosic material withfthe :addition: of onlysuflicientffresh liquor,

orconstituentsthereof,llasmay*be required to make up for' losses: in thezcyclic operations.

As Awill appear from: the detailed descriptionhereinafter,the Apresent invention not only contemplates the recovery. ofmost of thessodiumfor other base constituent and sulfur' fromfthewaste liquors, but also ycertain-other constituents are ,separated -andmay advantageously be burned 'fwith the production of heat required in the operations.

Certain ionI exchange resins`1 havefbeen found to `be more eilicientthan'lothersain: the absorption of sodium from solutionin exchangefor-hydrogenybut atthe same time, the resins .whichware rn'orefefficient in this step have' been found to befl vless efiicient =in the reverse -operation, ipe., in thelex'changeeof fthe sodiumfor the hydrogen of sulfurous .acidi linlthe course-of :producing :the vdesired fresh cooking liquor. .Strongly acid` resins, such as lthose ofthe sulfonic'typehavetbeen found to=be 1of this character. Other'resinspo.then-weakly\acidA type-such as the carboxylic yresins;ihavefbeen: -tfound :to rpresent 'fthe' :reverse situation. Thus, they Ihave been found to be readily convertible, with good efficiency from the sodium form to the hydrogen form upon contact with sulfurous acid, but they kare less ecient in the reverse exchange. Accordingly, when a sodium-containing solution is passed through a column containing such a weakly acidic resin, there is danger of substantial leakage of the sodium ion which will remain in the eiiiuent from the column. Eicient recovery of sodium ions from alkaline waste liquors derived from the smelting of sulfite cooking liquors and economical production of fresh sulte cooking liquor can be provided by the employment of two columns in series, one column containing a weakly acid resin and the other a strongly acid resin. The alkaline liquor is first passed through a column containing the weakly acid exchanger which will remove a substantial portion of the sodium ions. Hydrogen sulfide is evolved and sodium is absorbed by the column. The eluent from. the first column is then passed through a second column containing an exchanger of the strongly acid type which will effectively remove a high percentage of the remaining sodium ions and liberate hydrogen sulfide and some carbon dioxide. In the subsequent recovery step, the acidic agent, such as sulfurous acid, is rst passed through the column containing the strongly acid exchanger wherein, by exchange of some of the hydrogen ions, it will remove a substantial part of the sodium ions picked up by the column in the preceding stage. The effluent is then passed through the column containing the weakly Vacid resin where it will efciently remove a high percentage of the sodium ions previously absorbed in that column. It will be understood, of course, that the recovery of the sodium ions will not necessarily be complete, but by the two-stage process described a quite high percentage of recovery will be effected. Furthermore, the eiuent from the second column may readily be made of a suitable character for use as a fresh neutral or acid sulte cooking liquor for a succeeding cooking operation. By the use of sulfurous acid of appropriate strength in the regeneration of the beds of the two columns, the effluent from the inal column may be satisfactory as a cooking liquor without material modification. However, if desired, the process may be so carried out that the effluent will require some modification either by way of concentration, dilution, or addition of other active ingredients prior to the use thereof as a cooking liquor. Certain unavoidable losses of valuable ingredients are encountered inthe pulp and in the subsequent preparation of the alkaline sodium liquor. These losses must be made up by addition of appropriate sodium and sulfur compounds.

Because of the composition desired for the fresh cook ing liquor, namely, that it have a particular ratio of sultite to carbonate and a particular quantity and concentration of sodium and sulfur, it is desirable to use in the re generation of the ion exchange columns, toward the hydrogen form, a quantity of sulfurous acid which will produce in the effluent an amount of free and combined sulfur dioxide at least equivalent to that which must be present in the fresh cooking liquor. Any bisulte or sulfurous acid present in the efuent in excess of that desired may be readily neutralized with sodium carbonate, sodium bicarbonate or sodium hydroxide. To avoid the necessity of concentrating the fresh cooking liquor, it is preferable to use a concentration of sulfurous acid which will produce an effluent having a total sulfur dioxide concentration equal to or greater than that which must be present in the cooking liquor. It has been found, however, that efficient use of regenerant increases with increasing dilution so that it is preferable to use the lowest concentration of sulfurous acid consistent with the required concentration in the cooking liquor.

It is known to the ion exchange art that exchange reactions are less eiicient in concentrated solutions. This ineciency is due largely to an etect described as Donnan absorption, the absorption of salt, at high concentrations of solutions, over and above the ions absorbed by the Xchange reaction proper. The effect is manifested, for example, when a cation exchange resin in the sodium form is immersed in a concentrated solution of a sodium salt. Salt from the solution is found to difuse into the exchanger until an equilibrium is attained in which the concentration of the external solution has been reduced and the exchanger contains substantial quantities of salt. Consequently in ion exchange processes in concentrated solutions the average eiuent solution is less concentrated than the influent, and the resin, especially in the case where a reaction is possible between the exhausting solution and the regenerating solution, may have to be washed free of diffused salts to prevent unwanted side reactions. Because of this effect, in the method of the invention, a certain loss of sodium and sulfur occurs in absorptionV of green or white liquor and in regeneration with sul` turous acid. it is therefore necessary to increase the amount of sulfurous acid used in order to obtain a fresh cooking liquor containing the requisite amount of sulte ion. The extent of this correction will be illustrated in the examples.

The green or white liquor may be concentrated or diluted before absorption and the effluent from the rcgeneration may be concentrated or diluted, these treat* ments depending upon economic and other requirements of operation, the efficacy of any of these treatments being subject to the phenomena described above.

It is common in the neutral sulte pump industry to use a cooking liquor based on sodium and containing carbonate and sulte in such proprotions as to have a pH of 10.5 to 11.5. Neutral sultite liquors are commonly used in scmiand full-chemical reduction of the wood. A typical liquor for semi-chemical pulping comprises about 16 percent sodium suliite and about 3 to 4 percent of sodium carbonate on the basis of the weight of the wood. The sodium suliite concentration may be 4 to 5 percent of the weight of the water. About SGO to 700 pounds of chemicals per ton of pulp are required. If the digestion with this liquor is conducted under proper conditions and the pulp iinally dc-bered by mechanical disintegration, only about 30 percent of the wood is dissolved and the resulting pulp comprising about 79% of the original wood, has desirable properties. About 200 to 300 pounds more pulp are obtained per cord of wood than when full chemical digestion is employed.

A typical liquor for full mono-sulte pulping comprises about 45 percent sodium sulte and about 1.3% sodium carbonate on the basis of the weight of the wood. The sodium sulite concentration may be lO-l?. percent of the weight of the water. About 1530 pounds of Na2CO3 and 455 pounds of sulfur, per ton of pulp, are required.

After the reduction of the Wood, the delibered cellulose is separated from the cooking liquor and washed to remove most of the soluble and colloidal content. The spent cooking liquor and concentrated wash water are combined and ordinarily contain most of the cooking chemicals (the remainder having been lost in washing), solubilized lignin, cellulose and other wood components. This solution with its suspended materials is known in the trade as black liquor and, because of its organic content, a substantial portion thereof is not reusable.

in accordance with the present invention, part of the cooking chemicals in the black liquor is recovered in a condition suitable for reuse by evaporating the waste liquor to obtain the solid content thereof; smelting the solid content to obtain sodium and other non-volatile sulides and carbonates and to destroy organic constituents; leaching the resulting smelt with water to obtain a solution of soluble salts of the suldes and carbonatos (conn monly referred to as green liquor); clarifying the green liquor to remove suspended matter; contacting a solution of the green liquor with a weakly acid hydrogen ex amm change resin and witha stronglyV acid hydrogen exchangeA bicarbonate. As will be explained more fully hereinafter,

white liquor may be used in place of -green liquor in the exchange absorption step.

By the methodof this invention, the economicalrecoveryA of the cation base of the green or whiteliquor can be-increased above that'possible with a single exchange bed.- rl`he evaporation ofthe black-'liquori may be carried outfin conventional evaporators suitableforthisl purpose, for example, in multiple effect evaporators. Ordinarily the final stages of evaporation are carriedout by meansy of contact evaporators, for-example, disoevaporators utilizing waste heat.

The solid content may be'- smeltedinv any ofthose furnaces-normally used by the industry, thosel havingprovisions for waste heat recovery being especially satisfactory. TheV ash from the smelting-furnacemay-con-l tainfragments of furnace brick, unburned carbon, insol-v uble sulides, carbonates and sulfates'and 'other materials; Considerable amounts of solid-'material may leave the smelter with the'` gases generated and.` may be recovered if desirediby means of al1-electrostatic-precipitator or by sonic agglomeration (the material so yrecovered being preferably added to the evaporated'liquor inuent to the smelting furnace).

The smelt from the furnace maybe dissolvedl inwater or inl a solution obtainedfrom other sources, forV example, as shownbelow. Suspended insoluble. material is removed by clariers to avoidzsedimentationzinthe resin bedsy and consequent interference with the exchange reaction.

In the absorption step, hydrogen sulfide and carbon dioxide are evolved by the exchangexof thehydrogen of. the resins for the sodium of the green or white liquor.`

The gases frequently cause channelling of the liquor'and consequent inefficient use of the exchangers. Some of the exchange resins suitable for the method of this invention exhibit a considerableincrease in volume upon passing from the hydrogen form to the sodium form. This swelling may so compact the resin as toresult in excessive hydraulic pressure drop. and excessiveichanneling of the liquor. In some cases the swelling may be so greatfas to result in rupture ofthe resin container. Itv

has been found that the gases formed during absorption may be freely evolved and resins which have a high-swelling tendency may be utilized by passing they green or white liquor solutionupwardly through the. resin .beds at suchl a velocity as to rsuspendfthe resin particles, i. e., a linear velocity at which the resin particlesare UdiZed, The granules are thereby mutuallyl separated from each other by the solution. The gases. liberatedreadily-pass through the beds without channellingofV theliquid, entrapment of the gas, or compacting of the` resin.v Linear velocities suitable for this purpose are thosevin which the buoying force of thefupwardly flowing. solution on` in a varietyof shapes and sizesy and, even in ygraded resins,r there is considerable variation among the.r granules. l

Hence it may be necessary to vary-the .velocityoffupow density of the, green or whiteliquor solution-Y must be adjusted tobe' less than'the density ofthe exchange` resin:

granules to avoid oatingf the resin.

The ion exchange resins useful in thisinvention are solid f insoluble materials ,which are cap-able ofA absorbing metallic ions from solutions thereof. and replacing them with hydrogen'ions and are also capable o f being recon-- Among` verted, atleast partia1ly,.with acidsolutions. the suitable strongly acid exchangers for use in this invention are synthetic cationexchange resins based on sulfonic acid groupssuch as .those described in U. S. Patents to DAlelio No. 2,366,007, grantedDecember 26, 1944 and Bauman No; 2,466,675, granted April 12, 1949. Various resins ofthischaracter are sold under the trade designation Dowex-SO" (Dow Chemical 0o.), ,Pem1utit Co.), Nalcite HCR (National'Aluminate Co.), Duolite CS-ZO (ChemicalProcess C0.) and..Ionac C-240.

(American Zeolite (20.). Suitable weaklyvv acid type resins are usually carboxylictype resins.` Suitable resinsof this characterv Iare those based on polyacrylates such as those-made in accordance, with'the teachings 0f DAlelio Patent No. 2,340,111; granted' January 25,. 194.4. A

typical `resin yoff'this character is ythat sold under the trade..

cluded generally under the term available sulfurous acid.

Inithe noimal'use of the method of this invention, absorption andregeneration are repeated, and in a continuous operation, the quantity of sodium eluted in the` regeneration step4 is-substantially equal to that taken up in the absorption step.

In the rusual manner ofoperation, the hydrogenrof thev exchanger is not completely exhaustedor regenerated; not-all of the'sodium'isabsorbed and not-'all ofthe acidity ofthe regenerant is utilized. The separated spent regenerant may be subsequently neutralized by the addition of` sodium hydroxide', carbonate or bicarbonate to bring'the sodium-content to the desired point for cookingliquor-purposes.

With the foregoing purposes, features, and advantages inview, the invention-willy now'be describedin greater detail iny conjunction with the accompanying drawings in which:

Fig. l isna diagrammaticview'illustrating a simple processL forproducingI neutr-al or acid sulte cooking liquorin accordance -with the-invention;-

Fig. 2 is a diagrammatic view, in the nature-of=a flow sheet, illustrating f a lprocessby 'whieh fresh cooking liquor maybe produced and utilized'in-tlte--,pulping of' cellulosicmaterialand by which-a substantial'portion of they valuable constituents may be recoveredl from'the-wasteliquor and utilized in. the production. of newy cooking liquor.

Fig. 2a is a ow sheet illustrating a modification of a.

portion of the process of Fig. 2;

Fig. 3 is a diagrammatic view of.a columnsuitable for carrying out themethod lof thisrinvention; and

Fig. 4 is a diagrammatic view illustrating preferred recycling steps in the process of Fig. l. v

Referring now'to Fig. l `of the drawings, there is illustrated schematically a simple arrangement of apparatus forV carrying out' the improved method of producing fresh cooking liquor. A- tank 1 is provided for the storage of green-or white :liquor derived from the smeltingfof a previous .sulite cookiugfoperation or a derivative from suchliquors. Azsolution of green or white liquorhavinga suitable sodiumfioncontentis taken fromthe wenn tank 1 and led to a weak cation exchange column 2.l The latter is filled to an appropriate height with a weakly acid catlon exchange resin such as a carboxylic resin (of the character sold by Rohm & Haas Co. under the trade designation Amberlite IRC -50). Prior to the introduction of the liquor, the resin should be placed primarily in the hydrogen form as distinguished from the sodium form and the resin voids `should be filled with water. The solution of liquor from the tank 1 is passed upwardly through the resin bed and is discharged and led to a strong cation exchange column 3. The gases liberated in column 2 are separated from the eiuent liquor and sent to a hydrogen sulfide storage tank 5.

Column 3 is filled to an appropriate height with a strongly acid cation exchange resin, such as a sulfonic resin (of the character sold by Dow Chemical Company under the trade designation Dowex-SO). Prior to the introduction of the liquor, the resin should be placed primarily in the hydrogen form as distinguished from the sodium form and the resin voids should be filled with water. The liquor effluent from column 2 is passed upwardly through the resin bed in column 3 and is discharged and led to a treated waste liquor storage tank 4. The gases liberated in column 3 are separated from the effluent liouor and sent to the hydrogen sulfide storage tank 5. The rate of flow of liquor through the columns 2 and 3 should be such as to allow fluidization of the resins in the columns so as to permit the free passage of the liberated gases from the columns.

As a result of the ion exchange process within the columns 2 and 3, a solution containing some carbon dioxide and hydrogen sulfide will be delivered to tank 4. lf, however, the character of the green liquor solution and the quantity of exchange resins in the column are appropriately selected, the liquor delivered to tank 4 will be substantially free of sodium ions. Ultimate disposition of this liquor will depend upon its composition. It may, for example, be used to make up fresh green or white liquor solution.

When the exchange of hydrogen ions of the resins for the sodium ions of the liquor drops below a point which is commercially practicable and beyond which the effluent from the column 3 contains an unsatisfactorily high concentration of sodium ions. the flow of liquor through the columns is discontinued. The columns are then washed with water preferably by passing water from a tank S upwardly through the column 2, then upwardly through column 3, subsequently discharging the wash Water to a tank 9. By passing the wash water through the columns in the same direction as the liquor solution was passed. unabsorbed sodium retained in column 2 bv Dorman absorption and contained in the voids of the resin bed may be washed ont of column 2 and absorbed in column 3 constituting thereby an additional recovery of sodium ions. i

The hydrogen sulfide collected in tank 5 is led to a burner 6, forming dilute sulfur dioxide gas which is led to an absorber 7 where it is absorbed in water. forming sulfurous acid which is lcd to a storage tank lll. ln absorber 7, water is preferably passed downilow and sulfur dioxide upliow. Since some of the sulfur from the cooking liquor is lost in pulp washing in the recovery step and in other ways, it is necessary to add sulfur dioxide to make a sulfurous acid solution suitable for regenerating the ion exchange resins to the hydrogen form and for preparing fresh cooking liquor.

To regenerate the ion exchange resins, sulfur-ous acid is led from the tank .lli to the column 3 passing preferably downwardly through the column and is discharged and led to column 2, preferably passing downwardly through column 2. If the concentration and quantity of sulfurous acid are controlled properly, the eiuent solution from column 2 will be satisfactory for use as fresh cooking liquor with or without modifications. Suitable modiof approximately lO-12`minutes.

exchange columns should be within the range of approxiflcations of the effluent may include the addition of an appropriate amount of one or more of the following: sodium carbonate, bicarbonate or hydroxide or ammonium hydroxide. Particularly in the production of acid cooking liquor, it has been found desirable to modify the effluent with addition of ammonia in solution. The quantity of ammonia used will vary up to about 10% of the amount of sodium in the liquor. The ammonia will be lost in the subsequent smelting operation. The added base will react with any excess acidity to produce more sodium sulfite or bisulte (or ammonium sulfite or bisulfite) and is preferably added in sufficient mnount to give the liquor thc desired pH.

When the exchange of sodium ions of the resin for hydrogen ions of the acid drops below a point which is commercially practicable and beyond which the eiiluent from column 2 no longer forms a satisfactory cooking liquor, with or without economically feasible modifications, the flow of acid solution through the columns is cliscontinued.

After the regeneration step, the columns are washed, preferably by passing water from tank 12 downllow through column 3 and thence downflow through column 2. collecting the water in a tank i3. By this means, acidity not utilized in column 3 can be used in column 2 to effect further regeneration. The wash is continued until the wash waters no longer contain appreciable acidity. The columns are then in condition for a repetition of the cycle. The wash of the ion exchange resins in each half of the exchange cycle is provided to suppress the formation of free sulfur by the interaction of sulfur-ous acid and sodium sulfide. Free sulfur interferes with the efficient operation of the exchange resins, for example, if free Sulfur is present or formed in the regeneration cycle. thio-sulfate formed by the interaction of sulfurous acid and sulfur will be present in lthe fresh cooking liquor.

In the absorption step, hydrogen sulfide and carbon dioxide are evolved by the exchange of the hydrogen of the resins for the cation of the green liquor. ln the case of white liquor, hydrogen sulfide only is evo-lved. These gases are evolved in such quantities as to cause channelling of the liquor and generally result in inefficient use of the exchange resins. The weakly acid exchange resins most suitable for the method of this invention generally yexhibit an undesirably large increase in volume upon passing from the hydrogen form to the sodium form. The swelling may so compact the resin as to result in excessive hydraulic pressure drop and in some cases the swelling may result in rupture of the resin column. This may be overcome, as indicated hereinbefore, by providing a uidizing flow of the green or white liquor solution upwardly through the resin beds.

Certain parameters are used in the ion exchange industry to describe the operation of an exchange column. The Contact time of an exchange reaction is defined as the ratio of the apparent packed volume ot' resin contained in the column to the volume of solution passing through the resin per minute and is a measure of the time available for exchange. The linear velocity is defined as the ratio of the volume of solution per minute passing through the column multiplied by the volume percent of the resin containing region of the column not occupied by the resin granules to the area of cross section of the column. It has been found that for many exchange reactions ciiioient operation is accomplished in a narrow range of contact time of solution. lt is therefore possible to use a column of such cross section as to provide the desired Contact time of solution consistent with the necessary linear ilow so as to ensure both sulicient time for exchange and free evolution of gases. lt has been found that the contact time for both regeneration and absorption should lie within the range of approximately 5 to 2O minutes and preferably Within the range The height of the ion were.

mately, 15 to 5G" high and is' .preferablyabout 30,2' high... Withheights below 15 ,f dilicultyinsecurmg suitable ex#l change is..eXperienced,.- While with heights ofqoverSOf',` hydraulic diculties are encountered'due to the necessity of `maintaining a relatively high velocity ofzow.

During the .absorption of sodium ions, the resin beds. are preferably iluidized as, explained above. The resins. are therefore substantially. uniformly exhaustedlduring` the absorption, thatl is,` an ,exchange gradient ris not established in which the. degree of iexhaustion of the resin bed-varies substantially from' one .portionot the bed to another. Consequently, it isV not' possible. to take.full"' advantage of the countercurrentprinciple. in .one pass of the greenV liquor solution or regenerating solution through the columns.; Toincrease `the overall recovery 'of sodium and sulfur and 'to increase the eiiencyofthe process. oneor more expedientsmay beA adopted, among which' are:

(a) The latter portionsof the, green liquor solution. passedftnrough the columns may Vbev saved;and sentagain through the 'columns after 'a succeedingregeneration step,

(b') Partof the green-liquor-solutiongwhichhas been contacted withV the ion exchange resins; andnwhich"v contains substantial-sodium and sulfuricontent/may be'used to make up fresh green liquor solution-for Aa later absorp tion step.

(c) Part oftherwashlwaterfollowing'the absorption-1 step may be used-to make upfresh green'liquor solution or may be processed for'sulfurrecovery;

(d) Part of the wash water"follcwving;theregeneration step may be lused to make'up'partof'th'esulfllrous acid-regeneration solution forr alater regeneration, for' example, by' recyclingto the SO'z absorber. This washl water, or a portion thereof, may also be recycled tothe pulp washers.

(e) The latter portionsy of 'the `sulfurous acid-solutions passed vthrough the columns. may rbe-sentagain L- through the'columns after a succeeding absorptionstep:`

(f) Aregenerating solutioncontaining excessf'sulfurousv acid may be passedthroughthe-columnsand the excess sulfurousacid subsequently stripped-r from the-eluent regenerating solution: by heating, evacuation or other means. The eilluent regenerantfmay, forexample; be passed'through abcd of`weaklybasic" anion exchange resin in the basevform wherebyV the excess sulfurI dioxide will be absorbed. Subsequentlysteam is passed'upwardly through the bed as described ,in-D'utchlatent"55;207`,'y releasingv the sulfur dioxide-in substantiallyl concentrated' form. The sulfur dioxide'recoveredv in' this mannermayv be reused in'regeneration.

(g) The water usedto iluidizethebeds beforel the absorption step, i. e. the water sent through'thefcolumns before passing'the green liquor therethrough; maybe reused for thesarne purpose: This water-contains small amountsof sodiurnand sultite: Portions ofthe waterfmay;` be sentfto the SO2 absorber-and tothe pulpawashe, the latter being preferred.

(h) The latter portionsof theewash'watervrnaybereused in a'tsucceedingcycle asa-rstportion ofia similar wash.

The choice of. expedients depends, offcourse; upon theI value of thechemicals saved compared'totheadditional* cost of adoptingthe-expedient. 'The-expedientscan,` of-' course, be used'in connecticmwith.white-liquorr as-well` as green liquor.

ln- Fig. 2, there isschematically shown, in the 'form of'av ow sheet, a procedureand suitable apparatusfor the" such .conditions of temperature andpressureas are. nonnalf,

1y Aemployed Ain neutral 'or acidjsuliite pulpingoperations.. The character; of the liquoremployed. will depend upon Whether theprocess is ofV the semi-'chemical type or fullv chemical type and whether ofthe neutral or acid type.

After the digestion has been completed, the contents ofv the. digester are blowninto a tank 32, from which they are fed, in `the case of semi-chemical pulp, to a disintegrator 33 and then to a knotter 34; In the case of fllchemical pulp the.di'sintegrato.r is eliminated since the reduction of the wood 'to the pulp form is completed .iny

the digester. From the knotter any incompletely digested portions of the raw materials .are separated from the pulp.

above.` This constitutes the waste liquor of the process and is passed'to a wasteor black liquor tank 37. Inthe secondsection of the washer the pulpis.washed with a. weaker Vfiltrate--taken.from. a tank 3S.' l This in passing. through the washer, forms. the strongltrate which isv delivered tothe tankv 36. The partially washed-pulp passingfrom the washer 35. is thenA passed to a second washer 39%in theA rst section of which it is .washed with a.

still weaker ltrate Vtaken from a .tankdtlfand which, after passing through the' Washer 39; is dischargedy to. the tank 33'; Inth'e .second section ofwasher 39the pulp. is washedv with-plain water'and the ltrate from this operation is discharged'ito the Weak iiltrate tank 40.' The clean pulp emerging'from the second washer is` deliveredto a raw pulp storage tank'41.

A portionof thewaste liquor may be delivered from. the'tank 37 directly to the fresh liquor storage tank 31.

for reuse' alongywith the fresh liquor in a subsequent digestion operation. This is for the purpose of reducing the'load'on'the; ion exchange columns to be described..

However, the major portion of the Waste liquor is delivered from thettank` 37 toanevaporator 42: which may be of.

themultiple eifect type and then to a contact evaporator I3-which may be of the disc type .using waste heat.. The. residuelfromthe latter is passed to a smelter furnacel 44 tain'frag-ments of fu-rnaceV brick, unburned carbon, iron,

insoluble 'suliides,\ carbonates, sulfates andother materials which-mustA be removed inthefsucceeding steps. ConsiderablefamountsA ofsolid material may leave the smelter withth'e flue-gases and may be-recovered if desired by. fmeansfoffelectrostatic orfsonicprecipitators (the material' so.recovered1beingadded, if desired, to the evaporated' liquor in-uenb to the smelting furnace). A certain amount =ofsulfur dioxide may be liberated in the furnace, especially. inthe caseof acid liquors, and this may be passed .to anfabsorber SZfin which it is takenupin cold Water introducedf'into-this member; Tne ash fromthe smelter-isidischarged toa leachingtank 45vwhere it is-for the most parttdissolvedfin water or efuent froman'exchange. absorption stepy to form asolution containing primarily' sodium carbonate and sulfide (greenliquor): The.; green liquor isthensent to a clarier 46 wherein suspendedi insoluble matter is removed. This step is desirable. toz prevent clogging of the exchange columns.

The claried'diquor is thendiluted in a tank 47' with water: or with :effluent 'from'. the exchange absorption L step. The dilution increases: the efciency of= absorption of sodium-to `some extent `by reducing the amount of -Donnan1 absorption.v The: concentration ishere adjusted to thaty which isoptimumv .i for the. exchange.

Increased'gy recovery of sodium= and,A sulfur mayy be TheV smelten furnace may be any of those normally used byl they industry, thosehavingA provisions for waste heat. recoverybeingespeciallysatisfactory. The ash maycon.-

achieved by converting the sodium carbonate content of the green liquor to sodium hydroxide so that white liquor rather than green liquor is supplied to the exchangers.

Conversion of the sodium carbonate may be effected by addition of lime which reacts with the sodium carbonate to yield sodium hydroxide and calcium carbonate, the latter being insoluble in water may be separated by settling, tilteiing or the like.

Lime may be added to the green liquor, after smelting, yielding a white liquor with about 50% sulfidity. This process is desirable if calcium scale formation is troublesome when lime is present during the smelting operation. As shown in Fig. 2a, which is a iiow sheet illustrating a modification of a portion of Fig. 2, calcium carbonate is burned in a lime kiln 45A to yield lime. The lime therefrom is added to the dissolved smelt from the leaching tank i5 in a causticizing tank 45B. The lime reacts with the sodium carbonate of thc green liquor to produce sodium` hydroxide and calcium carbonate, the latter being insoluble is precipitated out in clarifier 46. The liquor from clarifier 46, which contains primarily sodium sulfide and sodium hydroxide (white liquor), is passed to diluting tank 47 where it is diluted with water or with eiliuent from the exchange absorption step. The calcium carbonate recovered in clarifier 456 may be supplied to lime kiln 45A for use in preparing additional lime.

Lime may be added before the smelting step, as shown by the dashed line in Fig. 2a, yielding a white liquor having a sulidity of about 85%. Lime from kiln 45A is added to the evaporated black liquor influent to the smelting furnace 44. The smelting furnace ash contains primarily sodium sulfide, sodium carbonate and lime. The ash is discharged to the leaching tank 45 where it is for the most part dissolved in water` or effluent from an exchange absorption step. In the leaching tank 45 the calcium hydroxide formed as the lime is dissolved in water reacts with the sodium carbonate to produce calcium carbonate and sodium hydroxide. The calcium carbonate and other insoluble matter is removed from the liquor in clarifier 46, leaving a white liquor composed primarily of sodium sulfide and sodium hydroxide. If desired, lime may be added both to the smelter influent and to the green liquor.

It has been found that iron present in the green or white liquor will be absorbed on the weakly acid exchanger in column 49 and will for the most part be removed by the regenerating solution in a continuous operation. iron present in the fresh cooking liquor interferes with the production of high quality bleached pulp. By

passing the green liquor solution through a weakly acid ion exchanger 48, which is initially in the sodium form, the iron will be effectively removed, the deep green color of the liquor then changing to a pale green. The eiciency of removal of iron in absorber 48 increases somewhat with the age of the green liquor solution. It has been found that, for a green liquor having a sodium concentration of 1.7 N, approximately 50() volumes may be passed through l volume of Amberlite IRC-50 before the resin is exhausted. To regenerate absorber 48, l N hy` drochloric acid may be passed through the resin, the column then being returned to the sodium form by passing l N sodium carbonate solution therethrough.

The green liquor solution is then passed to a weakly acid exchange column 49, where sodium is absorbed and hydrogen sulfide is liberated, and thence to a strongly acid exchange column 50 where additional sodium is obsorbed and hydrogen sulfide and carbon dioxide are evolved. In the case of white liquor, no carbon dioxide is evolved. The effluent liquor from column 50 may be discarded or returned in part to the leaching tank 45 and/or the diluting tank 47 to prepare fresh green or white liquor solution for subsequent absorption steps. The columns are then washed with water as explained above in connection with Fig. l. The gases liberated in equipment, which, while not shown in the drawing, are

the exchangers are sent to a hydrogen sulfide burner 51 where sulfur dioxide is formed by combustion of the hydrogen sulfide with the oxygen Iof the air. The dilute sulfur dioxide gas formed may be sent upiiow through the absorber 52 wherein it is absorbed in cold water. Nitrogen, carbon dioxide and other unabsorbed gases are discharged from the absorber through the vent line indicated. The absorber may suitably be a tower packed with Raschig rings through which the gases are passed upwardly and cold water is passed downwardly. On the other hand, if it is desirable for the regeneration to use a sulfurous acid more concentrated than that which can be prepared in this manner, the dilute sulfur dioxide gas may be passed upwardly through a bed 54 of weakly basic anion exchange resin in the basic form as described in Dutch. Patent 45,425 whereby the sulfur dioxide is absorbed. Subsequently, steam is passed upwardly through the bed 54, as described in Dutch Patent 55,207, releasing the sulfur dioxide in substantially pure and concentrated form. Inevitably some sulfur is lost in the pulp washing, smelting and exchange operations requiring therefore some make-up. This may be accomplished by means of the sulfur burner 53, wherein sulfur dioxide gas is formed by burning sulfur in air.

The resins in the columns 49 and 50, which have been substantially exhausted of their hydrogen, are restored to the hydrogen form by passing the solution of sulfur-ous acid from exchanger and/or absorber 52 through the strong exchange column 50 and then through the weak column 49 as described above. The columns are then washed with water as described above in connection with Fig. l. The waste wash water from this step may be supplied to absorber 52.

The regeneration of the resins to the hydrogen form may be only partially completed, but they are restored sufficiently to serve effectively in a repetition of the absorption operations. The eiuent from the regeneration step is passed to the fresh liquor storage tank 31. Apparatus for adding sodium carbonate or hydroxide or the like to the regenerant efiluent to adjust the fresh cooking liquor to the desired pH may be provided, although it has not been shown in thedrawing.

By appropriate conduct of the operations, i. e. by control of the extent of exhaustion of the hydrogen of the resin during contact with the green liquor and the extent of regeneration during contact of the resin with the sulfurous acid; and by the employment of sulfurous acid of appropriate concentration in the regeneration step, the efliuent will provide a fresh liquor of suitable character for use in a subsequent digestion operation. it will be understood, however, that if the effluent is not of the character ultimately desired it may suitably be modified by concentration or dilution with or without the addition of other chemicals such as sodium carbonate and the like. Usually it will be found necessary or desirable to add sodium carbonate or sodium hydroxide to the efiluent for the purpose of neutralizing the residual acidity of the sulfurous acid employed in regeneration and to make up for unavoidable losses of sodium. Therefore sufficient sodium carbonate or the like is preferably added to the regeneration effluent for this purpose before it is passed to the storage tank 31. Moreover, the cation base content of the liquor passed to the storage tank should be sufiiciently above that actually desired for cooking purposes to offset the diluting effect of the portion of the waste liquor passed directly from the black liquor tank 37 to the tank 31.

It will be understood that theapparatus employed in the process .above described and schematically indicated in Figs. 2 and 2a includes such items as pumps, valves, vents, condensers, coolers, boilers, precipitators, agitators, motors, blowers, storage vessels, and similar auxiliary employed to enable the various procedures explained to be properly carried out. The direction of .ow of the.

various gases and liquids shouldv be sucl as-.t'o facilitate the particular. operations involved.. Thus, in the opera-V tion of the ion exchange columnit. has been foundidesirable to pass the liquids .upwardly,through the column when gases are evolved as a .result ofthe ion exchange.. Even in the absence ofv such gasevolutionsit is. often de. sirable to provideupward. flow fofv the liquids to. overcome the. tendency of the resinxo. become. compacted due toy swelling' in the course of the reaction..

In a specific example. of anoperatlon lnaccordance with thel foregoing, ony a `laboratory scale.the columns.

were formed. of Pyrex. glass havingan internal. diameter of 3.7 cm. The weak-cation exchange column had a height ofv115 cm.. and-thestrongcation:exchange'column hada height of 105 cm.

inside diameter 15 cm. Asuitablecolumn. offthistype, for use in the methodY of the-invention, is :illustrated'iinv Fig. 3. The weakexchange. column vcontained-.500; ig... of surface dried Amberlite IRC-50. (a weaklyacid.- eX-.

changer of the carboxylic type based-on polyacrylic acid:v

and l manufactured by Rohm.. andfHaas- Company inaccordance with the teaching; of. DAlelio, Patent No. 2,340,111 grantedl January 25., 1944),. inathehydrogen form containing. 50 percent waterby weight. The strong exchange column contained-500 g. yof surface dried Dowex 50 (a-.strongly acid exchanger ofthe sulfonicfacid' type based on sul-fonated cross-linked polystyrene and-manu. factured by Dow Chemical Company in :accordance with the teaching of DAlelio,.Patent Nof..-2,366,007y granted December 26, 1944, Bauman, Patent No. 2,466,675, granted April 12,' 1949, and Boyer, Patent No. 2,500,149 granted March 14,- 1950)- containing40 percent water.

by. weight. The Arnberlite IRC-60 had-.a capacity'of.

4 milliequivalentsper surface dried .gram andtheDoweX 50 had a'capacity of 3.0 milliequivalentsl per4 surface dried gram.

Recovery. of chemicals from the green liquor. inthe form of fresh sodium sullite solution-.was accomplished.

in a tive-step; two-hour, semi-continuous cyclefof the type described above. The. plant operatedthesecycles continuously twelve perday.. In a.1.2`cycle day,.there was a total throughput of.5.6 gallons of-dilutedgreenliquor containing 1.17 lbs. of sodium and.0.42l lb. of.

fied and purified of iron and whichcontained NazS and.` NazCOg in a suldity ratio of about 57 percent was .v diluted to a total sodium concentrationof 1.67y N and.

passed upwardly through the weakly acid IRC-50. Sodium was exchanged for hydrogen on the resin, andhydrogen sulfide was liberated. The green liquor, partly stripped of sodium and sulfur, was. withdrawnfrom thev IRC-50 and passed upwardly. through the strongly acid Dowex-50. More sodium was absorbed by thisresin and both hydrogen sulfide andsome carbon dioxide were liberated. The stripped green liquor was passed to waste.

(2) The columns were washed with water as described hereinbefore to remove absorbed sulfide.

(3) Sulfurous acid solution (3.7 percent), was passed downwardly, irst throughthe Dowex-SO where it picked up sodium and then through the IRC-50 where it eluted more sodium, producing thereby a mixture of sodium bisulte and neutral sodium sulfte. This liquor was neutralized and became cookingliquor for the next cycle.

(4) The residual acid was washed from the resin.

(5) The resin was uidizedwwith water upow Ito remove air and achieve an evenseparationof'resin particles in preparation for the gas evolutionl intheV green Each. column. had a conical.. disengaging section of. inside.1height..22l ctrl..v andJargest liquor run. That is, the air in the resin,voids was re.

placed'witli.waterbeforetheow vof green, liquor was.

begun in order that' the resin would :be suspended 'at the. beginning of therun and the gases formed would' be 'readily evolved;v

Theresia beds are supportediat the bottom. byu two double' thicknesses of. 28-mesh. polyviuylidine. chloride screening,..such`.as that soldiunder the trade name of. Lumite (equivalent to No; 4.01U. S.` standard .sieve) No passage of resin through this system` Was observed.` Above the resin levels each column containedia plastic filter box covered 'with 28`mesh Lumite screening. The,y ltenbox serves as a screen .to prevent the lossof resin through the exit pipe. Resin fines and sulfur, whichmay be formed` during. the absorption. or regenerationl steps, obstruct liquidigow throughy the resin.. beds.V The green. liquor generally contains some polysuldes...which.clecom:

pose to give free sulfur when the pH`is reducedlduring..

theabsorption cycle. The resiniines and. sulfur. are Withdrawn throughv these .filters by passing... most. ofltheab: sorption eluents Vthrough them.. The remaining portion.

of the effluent passes out through connections at the bote, tom of the column. The` portion ofthe liquid .withdrawn through the filters was-passed through-a.series.ofgsettling,v

tanks or traps Whereinthe resin nes sett1e..out and ...aref subsequently. returnedr to the. columna.

The laboratory size ion. exchange column...sho.wn in.

Fig. 3'comprises a base section .60 having, an inlet .and .outa

let connection 61'in the bottom thereof for the admission.

and removal offluidsduring variousfstages ofthe, cycle.

A screen -62lis also provided in ,section titlfor supporting,v the resin bed; Attached to section .60 bya. coupling-,63. is. a cylindrical body member.64. whichcontainsmost of'` ingo veffluent vaccumulated .in conical -section .65.. during..

the absorption portion, of. thepcycle. Connection.. 67vis.

providedlwitha plastic .flterboir as described. abovetopreventloss of resin therethrough. Connections. 68..'and:

69 areprovidedin.theclosed upper endof .coniealseci tion 65 for the removal of gases and .theadmission offluids. such, assulfurous acid solutionvandregenerating wash water. at.. appropriate.-times. during. the. cycle..

The.. resins. and gases are separatedfrornl thesolution in the. conical section GS-ofthe column .shown-infie. 3. In this. sectionV thelinear velocity ofthe solution decreases 'owingtofthe enlarging cross section, thereby' permittingthe resin .to separatefromthe solution and gasesand to. The gases also are l -separatedfrom the. solution in this section of the column returnto. the. body ofv the column.

without.substantial entrainmentfof the solution. lnthe.

operation. described,l .the liquor passing. throughv the weakY exchanger was accumulated in the conical section of column, 21.(Fig.. l) before .passing to column Sand theliquor passingthrough the strongy exchanger was .accumulated in theconicalsection of column 3 beforepassing on tol the treated wasteliquor tank 4.

Most of the green liquor which` had passed through column .2- andaccumulated in theconical `section thereof was led directly `to column 3. After completionof the.

was .removed'at the completion of the absorptionr cycle in .that column. Complete draining of the solutions fromy the columns may be facilitated by passing `air throughthe columns before andv after passing wash Water there.-

through.r Passing air through the columns beforewash Water. will aid-.in removingliquor or sulfurous acidfrom the resin, -thusrequiring less wash water. Passing air In a similar way the. liquor which had passed into column` sesso-f therethrough after the Wash Water will remove most of the Wash water retained on the resin, thus reducing dilution in the next step.

In a typical cycle in continuous operation at a steady state, the green liquor inuent to column 2 had a concentration of 1.65 milliequivalents of sodium per milliliter and contained 1820 milliequivalents of sodium. After the exchange reaction 206 milliequivalents of sodium were in the treated liquor eluent from the system; 78 milliequivalents of sodium were in the Wash Water effluent after the absorption step. In washing, 2000 cc. of Water were used.

The regenerating solution had a concentration of 1.15 milliequivalents of sulfurous acid per milliliter.

The regeneration eiuent contained 1320 milliequivalents of sodium and 549 milliequivalents of hydrogen, of concentration 0.537 milliequivalents of hydrogen per milliliter.

The Wash Water following regeneration contained 170 milliequivalents of sodium and 878 milliequivalents of hydrogen. A total of 2400 milliliters of wash water were used.

The sodium recovery was 62 percent of the total amount of sodium in the cooking liquor. 30 percent of the total sulfur in the cooking liquor was recovered. However, percent of the sodium in the cooking liquor had been lost in the pulp and in the smelter. The overall recovery of sodium from the green liquor was 73 percent.

The effluent resulting from the passage of green liquor through the columns was not further used. Similarly the absorption and regeneration wash Waters were discarded.

As explained above, increased recovery is possible byv using these solutions in the process.

In carrying out the five step process on a typical White liquor from a neutral semi-chemical process smelt, it has been found possible to operate the process using a large column of carboxylic resin and a smaller column of sulfonic resin in series. In this case, a more dilute SO2 solution may be used as compared with that required for `the green liquor process. The decreased concentration of the SO2 solution results in substantially decreased Donnan absorption.

In a typical cycle in continuous operation similar to that above at a steady state, a white liquor (of 52 percent sulidity, prepared by causticization of green liquor as shown in Fig. 2o) of concentration of 1.65 milliequivalents of sodium per milliliter and containing 1820 milliequivalents'is passed into column 2. The weak cation exchange column contains 475 grams of surfacedried Amberlite IRC-50 (containinsy in the hydrogen form 50 percent Water by weight) and the strong exchange column contains 110 grams of surface-dried Dowex-SO (containing in the hydrogen form 40 percent water by weight) The regenerating solution is 2.8 percent SO2. The sodium recovery is 82 percent of the sodium in the white liquor While the sulfur `recovery is 79 percent of that in the white liquor. In a similar procedure, using a White liquor of 85 percent suldity (prepared by adding lime to the influe ent to the smelter, a sodium recovery of 82 percent and a sulfur recovery of 78 percent of that in the white liquor has been attained.

Sodium .recovery in the green liquor ion-exchange process may be greatly increased through the use of various ecycling steps, as pointed out hereinbefore. An ion exchange process employing a preferred combination of recycling steps is illustrated in Fig. 4. The rst recycling step comprises employing `the eilluent from the exchange absorption step as a solvent for the smelted black liquor. As shown in 4, the green liquor from tank l is passed upiiow firstly through weak cation exchanger 2 and secondly through strong cation exchanger 3, the etliuent therefrom being passed to treated waste liquor tank 4. This absorption step eh'uent may be passed to lea-ching tank 451:01 use as a solvent for the smelted black liquor.

CTI

, 16 All or part of the liquor from tank 4 may, if desired, be supplied to diluting tank 47. In either event, Vthe absorption step eluent will have been used in making up the green liquor solution supplied to tank 1.

The second recycling step comprises employing the regeneration wash Water in preparing fresh sulfurous acid solution for use in succeeding regeneration steps. After ysulfurous acid from tank 10 has been passed through exchangers 3 and 2, Water from tank 12, is passed downow firstly through exchanger 3 and secondly through exchanger 2 and is collected in a tank 13. The waste regeneration Wash water from `tank 13 is supplied to sulfur dioxide absorber 7 for use in absorbing the sulfur dioxide gas generated in hydrogen sultide burner 6.

The `third recycling step comprises supplying the fluidizing water to the pulp washers. Before the absorption step', water from a tank 75 is sent upliow rstly through exchanger 2 and secondly through exchanger 3 to fluidize the resins, i. e., to remove air and to achieve an even separation of resin particles in preparation for gas evolution when the green liquor is passed therethrough. The uidization Water efliuent from exchanger 3 is sent to `the pulp washers and may be used as part of the wash water supplied to the second section of washer 39 (Fig. 2).

These three steps constitute a preferred recycling procedure. In one form of the ion exchange process without recycling, a 54% recovery level of sodium from green liquor is achieved. By employing the preferred combination of recycling steps, the recovery level is increased to 87%. The recovered sulfur as HzS gas is reduced from 48% of the green liquor sulde content to 31%. This reduction in the volume of evolved I-IsS is due to the altered method of operation of the process; considerable free H28 remains dissolved in the absorption wash stream because the increased sodium recovery requires about a two-fold increase in the amount of strongly acidic resin used. The volume of green liquor used in each cycle remains largely at the bottom of the resin bed in the void spaces. Exchange of the dissolved sodium is incomplete until the green liquor is washed through the exchanger with Water. The resulting diluting does not aiect the sodium absorption by the strongly acidic resin but it does have the ettect of retaining considerable sulfur in solution as dissolved H28. This sulfur content is removable by heat or by processing the eiuent streams from the absorption step with CO2. As shown in Fig. 4, the waste absorption wash Water collected in tank 9 may be passed to a suitable sulfur recovery process 76.

In a laboratory operation of a preferred recycle system, the green liquor passes upiiow in series through two resin beds containing, respectively, a weakly acidic resin such as Amberlite IRC-50 and a strongly acidic resin such as Dowex-50. As compared with no recycle, the volume of Amberlite IRC-50 is slightly reduced (from 19.4 liters to 18.1 liters) while the volume ot Dowex-50 is substantially increased (from 17.8 liters to 34.4 liters) in order to increase the quantity of absorbed sodium. The residual resin green liquor content is next swept through the system by chasing the green liquor immediately with water wash, thus permitting the absorption of desired sodium content by the DoWeX-SO column. In this manner, percent of the green liquor sodium content is removed by the resins.

The resins are next regenerated with 4.2 percent aqueous SO2 containing 258 grams sulfur and 152 grams sodium as soda ash. (The optimum SO2 concentration under non-'recycle conditions of low sodium recovery is about 3.7 percent.) In the regeneration eiuent stream which contains the recovered sodium, there are 425 grams of sodium as soda ash (87.5 percent of the green liquor sodium content) and 147 grams of sulfur. r)the residual acid is next Washed from the resins with Water. The effluent Wash Water contains 152 grams of sodium as soda ash and 108 grams of sulfur. The entire stream is recycled for SO2 makeup for use asjregenerant in a succeeding cycle. The

S02 makeup which is requiredto raise the available acid i 17 to 1.31 N (corresponding to 4.2 percent SO2) amounts to 147 grams of sulfur.

A number of features which do not form a part of the present invention have been described herein for purposes of clarity. These features form a part of an invention of William Potter and Edgardo Parsi disclosed and claimed in their copending patent application filed concurrently herewith.

While various embodiments of the present invention have been described in considerable detail, it will be understood that other embodiments or modifications than those specifically suggested are within the scope of the invention as defined by the appended claims. The apparatus and procedures specified for the various steps of the several processes disclosed are to be understood as being merely illustrative and not restrictive. A variety of different cation exchange resins of the character of those specifically mentioned herein may be employed in the various columns disclosed. Where specific resins have been mentioned, resins of the same strongly or weakly acid types may be substituted therefor.

What is claimed is:

1. A method of recovering valuable constituents from waste liquor from a pulping operation and producing fresh cooking liquor which comprises passing water upwardly through at least one substantially vertically disposed bed of synthetic organic cation exchange resin which is` at least partially in the hydrogen form disposed on a pervious support, substantially to fill the void spaces in said bed with water and to provide a layer of water Within and below said pervious support, passing an aqueous alkaline solution having sodium and sulfide constituents derived from the smelting of said waste liquor upwardly into said layer of water and through said bed of cation exchange resin whereupon at least a portion of the exchangeable hydrogen of said cation exchange resin is replaced with sodium and gaseous hydrogen sulfide is evolved, maintaining the flow of said aqueous alkaline solution ata fluidizing velocity for said bed, and subsequently passing an aqueous solution of available sulfurous acid through said bed of cation exchange resin and collecting the efliuent from the later.

2. An ion exchange method of recovering sodium from a sodium containing waste liquor derived from a pulping operation which comprises passing water upwardly through at least one substantially vertically disposed bed of synthetic organic cation exchange resin which is at least partially in the hydrogen form disposed on a pervious support substantially to fill the void spaces in said bed with Water and provide a layer of water within and below said pervious support, passing an aqueous solution of sodiumcontaining compounds derived from said waste liquor into said layer of water and upwardly through said bed, said compounds being such as to produce a gas upon exchange of the sodium thereof for the hydrogen of said resin, maintaining said bed in an enclosed space, continuously removing from a point adjacent the top of said enclosed space the gas produced by the exchange of hydrogen from said resin for the sodium of said solution, and passing said aqueous solution through said bed at a uidizing velocity y for said bed.

3. A method according to claim 2 wherein said enclosed space ares outwardly toward its top to permit the resin to spread laterally adjacent the upper end of the bed to facilitate separation of said solution, resin and gas during the upward passage of said solution.

4. A method of recovering valuable constituents from sodium and sulfur bearing waste liquor from a pulping operation and producing fresh cooking liquor which comprises evaporating the waste liquor to obtain the solid content thereof, smelting the solid content, leaching the solid content with aqueous solutions to produce an aqueous solution having sodium and sulfide ions, causticizing the thus modified waste liquor in at least one of the preceding steps, clarifying said causticized solution to remove therefrom suspended insoluble matter, passing water upwardly through a plurality of substantially vertically disposed beds of synthetic organic cation exchange resins at least partially in the hydrogen form disposed on pervious supports substantially to fill the void spaces in said beds with water and provide a layer of water within and below each said pervious support, the resin in at least one of said beds being of the weakly acid type and that in at least another of said beds being of the strongly acid type, passing said clarified solution successively through said beds of cation exchange resin, maintaining the fiow of said clarified solution at a uidizing velocity for said beds, said clarified solution being passed firstly into said layer of water in said weakly acid type resin and upwardly through said bed of weakly acid type resin and the efuent therefrom being passed into said layer of water in said bed of strongly acid type resin and upwardly through said bed of strongly acid type resin, whereupon at least a portion of the exchangeable hydrogen of said cation exchange resins is replaced with sodium ions and gaseous hydrogen sulfide is evolved, subsequently passing an aqueous solution of available sulfurous acid firstly through said bed of weakly acid type resin and collecting the effluent from the latter.

References Cited in the le of this patent UNITED STATES PATENTS 1,605,926 Drewsen Nov. 9, 1926 1,605,927 Drewsen Nov. 9, 1926 1,892,100 Bradley Dec. 27, 1932 1,934,655 Bradley et al Nov. 7, 1933 1,935,381 Wang Nov. 14, 1933 2,029,616 Haglund Feb. 4, 1936 2,104,501 Adams et al Ian. 4, 1938 2,191,853 Holmes Feb. 27, 1940 2,385,955 Tomlinson Oct. 2, 1945 2,393,249 Holmes Jan. 22, 1946 2,656,244 Grey et al Oct. 20, 1953 2,656,245 Grey et al Oct. 20, 1953 2,656,249 Grey et al Oct. 20, 1953 2,736,635 Haywood Feb. 28, 1956 FOREIGN PATENTS 519,848 Great Britain Apr. 8, 1940 859,205 France June 3, 1940 OTHER REFERENCES Ion exchange, Chemical Engineering, July 1927, pages 12S-127.

Ind. and Eng. Chemistry, February 1943, pages 186- 192.

Wilhelm et al.: Chemical Engineering Progress, vol. 44, No. 3 (pages 201-218).

Selke: Continuous countercurrent ion exchanges, Chemical Engineering Progress, vol. 74, No. 10, pages 529, 533, October 1951.

Claims (1)

1. A METHOD OF RECOVERING VALUABLE CONSTITUENTS FROM WASTE LIQUOR FROM PULPING OPERATION AND PRODUCING FRESH COOKING LIQUOR WHICH COMPRISES PASSING WATER UPWARDLY THROUGH AT LEAST ONE SUBSTANTIALLY VERTICALLY DISPOSED BED OF SYNTHETIC ORGANIC CATION EXCHANGE RESIN WHICH IS AT LEAST PARTIALLY IN THE HYDROGEN FORM DISPOSED ON A PERVIOUS SUPPORT, SUBSTANTIALLY TO FILL THE VOID SPACES IN AID BED WITH WATER AND TO PROVIDE A LAYER OF WATER WITHIN AND BELOW SAID PERVIOUS SUPPORT, PASSING AN AQUEOUS ALKALINE SOLUTION HAVING SODIUM AND SULFIDE CONSTITUENTS DERIVED FROM THE SMELTING OF SAID WASTE LIQUOR UPWARDLY INTO SAID LAYER OF WATER AND THROUGH SAID BED OF CATION EXCHANGE RESIN WHEREUPON AT LEAST A PORTION OF THE EXCHANGEABLE HYDROGEN OF SAID CATION EXCHANGE RESIN IS REPLACED WITH SODIUM AND GASEOUS HYROGEN SULFIDE IS EVOLVED, MAINTANING THE FLOW OF SAID AQUEOUS ALKALINE SOLUTION AT A FLUIDIZING VELOCITY FOR SAID BED, AND SUBSEQUENTLY PASSING AN AQUEOUS SOLUTION OF AVAILABLE SULFUROUS ACID THROUGH SAID BED OF CATION EXCHANGE RESIN AND COLLECTING THE EFFLUENT FROM THE LATER.

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