US2656244A - Process of recovering chemicals from smelts obtained in pulping operation - Google Patents

Description

Oct. 20, 1953 K. R. GRAY ET AL 2,656,244

} PROCESS OF RECOVERING CHEMICALS FROM SMELTS OBTAINED IN PULPING OPERATION Filed July 15, 1950 SODIUM BASE S.W.L.

SMELT ING FURNACE SOLUTION SMELT 3\ DISSOLVING MAKE-UP E 4 TANK GREEN E uouo. CLARIF R ABSORBENT CLARIFIED v GREEN UQUOR H a: INERT, GAS y K g (002) 112i 4- BURNER 2 Q P00 (e0 H2503 I WATER, FREE 2 l 7 WATER SODIUM ggggM 0N RESIN RE5|N 5QD|UM RESIN RE5|N ON RES, WASH DESORPTION WASH LEAKAGE SODIUM BISULFITE T0 COOKING ACID INVENTORS KENNETH RUSSELL GRAY 8 HARTZELL LANCE CROSBY ATTORNEYS ing a minimum quantity of, objectionable by-- products.

This invention is based on our discovery of the treatment of soda salts of weakly acidic volatile acids with certain cation exchange resins for the adsorption of sodium on the resin, the release of the weak acid, and the regeneration of sodium bisulfite by treating the resin with sulfurous acid. The general applicability of our invention is de scribed and claimed in our copending application Serial Number 174,102, filed July 15, 1950.

Our present process utilizes the important conjoint steps which we have discovered comprising treating aqueous solutions of soda salts of weakly acidic volatile acids such as CO2 and H28 with a cation exchange resin having active exchange centers in hydrogen form which are substantially less acidic than sulfurous acid, resulting in the adsorption of the sodium on the resin with the liberation of the weak volatile acid, and the treatment of the resin with sulfurous acid to form sodium bisulfite. The resin is accordingly regenerated for further use.

In one preferred application, the sulfur dioxide used for treating the resin containing sodium is obtained at least in part by the oxidation of the hydrogen sulfide evolved in the above treatment.

Where the solution containing the sodium salt of weakly acidic volatile acids also contains the sodium salts of relatively nonvolatile, strong acids, the resin treatment will adsorb the sodium component of only the weakly acidic volatile acids. Thus, since the weakly acidic volatile acids are liberated from the solution in gaseous form and the sodium is adsorbed on the resin, a complete separation of the sodium salts of the weakly acidic volatile acids from the sodium salts of the relatively nonvolatile, strong acids can be efiected.

Among the resins which we have found particularly applicable in our process are those containing carboxylic acid or phenolic hydroxyl exchange centers, or both, and in particular, the newer synthetic organic exchange resins contain ing these groups exclusively and which are characterized by high adsorptive capacities. A number of resins of this type are at present available.

. The carboxylic acid type resins are especially desirable. One such resin which we have found particularly suitable is sold under the trade name of Amberlite IRC-50.

We have found that the process may very advantageously be conducted at elevated temperature, as for example, 50100 0., whereby the effective capacity of the resin is not only materially increased due to the temperature eifect but due to the reduced solubility of the evolved volatile acids. Thus, one reaction product is effectively removed from the solution enabling very rapid approach to equilibrium conditions and facilitating adsorption of the soda. Removal of the volatile acid may also be further facilitated by carrying out this phase under vacuum.

Following the adsorption step, the resin is washed in a conventional manner and the adsorbed soda removed by a treatment with sulfurous acid solution. In place of sulfurous acid solution, the resin containing sodium may be treated with sulfurous acid formed in situ by passing sulfur dioxide containing gases in contact with the wet resin.

It is understood that, dependin on the characteristics of the installation, the sodiu adsorption and/or desorption may be carried out in more than one step.

It is a desirable feature of our process that many impurities present in the original solution (e. g. chlorides, sulfates, thiosulfates, etc.) remain in the liquor drained from the resin following the adsorption step and therefore the regenerated solution or product liquor is recovered as a high quality material.

The sulfide content of the influent solution is largely released as hydrogen sulfide during the soda adsorption stage. With influence solutions of substantial sulfide content the hydrogen sulfide is evolved in substantial concentration and may be readily and practically burned to sulfur dioxide free from undesirable by-products, Whereupon it may be used in the process if desired for removal of the sodium from the resin as sodium bisulfite.

The solution, from which all or a portion of the soda has been removed, is drained from the resin at the completion of the adsorption step. This said efiluent solution has been termed by us the leakage and the term leakage is hereinafter used in the specification in this sense.

In one method of applying our process, where only salts of weakly acidic volatile acids are present, the ratio of the amount of resin used in the treatment to the amount of soda salts of the Weakly acidic volatile acids is such that substantially all of the soda of the influent solution is adsorbed on the resin and substantially all of the volatile acids are liberated in gaseous form. The leakage from this process, containing substantially no sodium ions, may be readily stripped of any remaining traces of volatile hydrogen sulfide and may therefore be reused as water in the process or discarded with no attendant waste disposal problem.

In another method of applying our process, the sodium containing solution influent to the resin treatment contains, in addition to the soda salt of weakly acidic volatile acids, other soda salts, such as soda salts of strong acids, which are not decomposed in the exchange reaction and therefore remain in the leakage.

By limiting the ratio of influent liquor to resin, relatively complete removal of the soda from the salts of the weakly acidic volatile acids may be achieved, in which case the leakage contains largely the sodium salts of the relatively nonvolatile acids and may be either recycled in the process, treated for recovery of the inactive soda values, or discarded.

Where the liquor infiuent to the resin contains sodium salts of the weakly acidic volatile acids substantially in excess of the effective capacity of the resin, and in addition sodium salts of relatively nonvolatile acids, the leakage will contain substantially the total amount of the sodium salts of the relatively nonvolatile acids as well as the unadsorbed portion of the soda salts of the weakly acidic volatile acids. Operation in this manner provides a very high soda ion concentration in the influent solution throughout the reaction stage, thereby resulting in exceptionally high effective capacities and eiiicient use of the resin. The leakage from such operation would contain considerable soda values and could be recycled, treated for further soda recovery, or used elsewhere in the process.

The accompanying drawing illustrates diagrammatically by flow sheet an operation embodying the invention.

Soda-base sulfite waste liquor recovered as an animus fitment 1mm thezdies i n ct cellulosicsubs ances in base cidulfite cook ng illllQH-lsfiQIl c nt a doatl s 5 totalsol s ontent i conventional;multiplerefiect evaporators and-fired to..a smelting type recovery furnace, (1),. This -ffurnace may Joe-similar .to th conventional units .now used in the kraft and soda zpulp .mills throughout the industry. When operated under reducing conditions in thesmelting-zone (e. g, withhigh temperatures and limited excess air), the .furnace will generate steam and ,yield .a lsmelt ,of fused soda salts. Thissmelt flows from the rurnace and may, for example, be .quenchedor otherwise dissolved \(2) in water or some suitable effluent solution from the process toyield ,acharacteristic fgreen liquor contain- .ing essentially sodium sulfide with smaller :amounts of sodium carbonate, sodium sulfate and sodium thiosulfate. Following clarification ,in' conventional clarification equipment, the green liquor is ready for treatment for the recovery of soda and production of sulfite cooking vacid.

Inpne adaptation-of the invention, a suitable resin -(for example, the weakly-acidic vcarboxylieacid type) in hydrogen form is contacted in a bed with the proper amount of smelt solution. .Thismay be accomplished in anumber of ways, as .for example bypassing the solution through a bed of theresin or by adding the resin to the solution either batchwise or in-a continuous manner, laterseparating the resin from the solution by mechanical means. The conditions of this treatment are not-critical in any way with regard to time, temperature, or solution'concentration;

however a high solution concentration (100 grams equivalent NazO per liter) speeds the reaction and permits a high resin -capacity. Since adsorption of the soda (4) is accompanied by the spontaneous evolution of the corresponding free acids, His and H2803 (5), the reaction may conveniently and advantageously be carried out at an elevated temperature of SO-90 C., which approximates the temperature of th reen liquor from the clarifiers. This elevated temperature reduces the solubility of the volatile product gases, thereby accelerating the reaction and improving the effective capacity of the resin.

Where the ratio of soda to resin is such that the capacity of the resin is not exceeded, substantially complete removal of the soda present as sulfide and carbonate can .be accomplished and this represents the preferred method of operation.

However, it may be noted that use of green liquor containing sodium in excess of the capacity of the resin increases the driving force and the ei iective capacity of the resin. Under some conditions, it may be desirable to operate in this manner, returning the leakage containing unreacted sulfide and carbonate to the process for dissolving additional smelt.

The resin containing the adsorbed sodium is washed by displacement (6) to remove excess smelt solution. The washed resin is then treated with an excess of sulfurous acid solution which e cts a complete re e eration o the resin and desorption of the soda (7) to form an efliuent containing sodium bisulfite and free sulfurous acid (8). This solution may be used directly as c king ac d or itmey be i ted if desired o f rti d t addi ional ee 502-.

u jec n the res n a br e W sh to r mo residual acid (9) completes the cycle and leaves the resin ready tor reu e Steps (5). (6),!

;sorn ons ep 1 (-1 s ar ely d c ated-b i sicem -p0s t n hich, in, t m i a inactio (ii the pulping ,process, th furnace operation, and the ion ex hange cycle. n; general, 'the main om.-

ponents will be sodium ome, sodium thioslll :fate, and tra es 0i dissolved eas s- Ifthe --mi1 uses salt water ffloatedlogs, chlorid Will 5115 -118 a m jor-constituent, thedissowed eases maybe very v readily removed, by stripping, if desir d;

bo everrdu t hellim ted-solub lity, the amount t eas pr se t in 'the' l akaee is "ver "small-v the inactive salts in the leakage rep sent an appreciable soda value, economic considerations will necessitate their being retained in the system. Wherethe nature and-amount of the salts are such that no detrimental effects willbe ine curred in the-cooking cycle, the leakage maybe used to. make up sulfurous acid solution andus'ed asregenerant for the resin or maybe simply added to the cookingacid as a diluent. In such a case sulfur dioxide gas, either pure or diluted (e. g., burner gas), maybe passed directly into .the mass of resin and smelt solution from which thesulfide has been eliminated, thereby forming sulfurous acid in situ, effecting regeneration of .the resin by desorption of the sodium, andforme ing a sodium bisulfitesolution which is simply drained from the resin and used in'theprepa-ration of cooking acid.

The sulfate and traces of thiosulfate carried into the cooking acid wouldfbe returned to the furnace in the waste liquor irom the cook, where they would be largely reduced to sulfide during the smelting operation, thereby preventing an ac cumulation of these salts in the system.

Recovery of the soda values in the leakage, consisting .in large part of sodium salts of strong and non-volatile acids, may also be accomplished either by evaporation of the excess water and return of the concentrated solution to the furnace or by treatment with anion exchange resin containing strongly acidic groups and capable of ad:- sorbing the sodium from the neutral salts.

Returning the leakage to the smelt dissolving tank is practical, especially where the residual salt concentration is low. However, this would result in an accumulation of the inactive salts in the cycle and would necessitate provision for the removalof a portion or these salts each cycle in order to prevent precipitation of soda compo nd u t o centrations exceeding the solubility limits.

The gas evolved during the soda adsorption step (5 consists of HeSand CO2 in mol ratio substantially equal to the mol ratio of sulfide to carb na e n the g een iquor Due to the high ratio of sulfur to soda the digestion liquor used in the sulfite process, the ratio of sulfide to carbonate in the sm alt may be on the order of 3:1

.to 8:1 depending on furnaceoperation. It can be seen that the evolved gases are preponderant- 1y Has with the practical advantage that they are i ifi nfl h concen r tio o be read- 1y burned (11), resulting in the production of $02 which may be adsorbed (12) to produce a portion of the sulfurous acid required for removal of sodium from the resin ('7) s The ab o ben fo this S 2 e water, or more nvenien y neef the effl ent item th ex hange cyc e ma b used in order t ma ntain a substantially closed system, thereby reducing so or sulfur lo I'hus the absorbent soluprepared for various tion might be the leakage, or one of the wash effluents.

Washing may be carried out in a manner familiar to the art whereby a strong and a weak fraction is recovered, the strong fraction being added to the efiluent from the preceding stage and the weak fraction being stored for the first wash liquor of the subsequent cycle.

The example outlined above is only intended as an illustration of one application and it is obvious that the invention may be applied to a variety of pulping operations. Many such operations proposed in the past have not heretofore been practical but are now rendered economically feasible by use of the present invention.

As an example of use in another pulping operation, the invention may be applied to a pulping sequence wherein lignocellulosic material is treated in one stage with a sodium base acid sulfite solution and in another stage with an alkaline solution. The waste liquors from these treatments, containing organic matter, sodium and sulfur, could be combined, evaporated to 45% to 75% total solids and burned in a furnace similar to that used in the alkaline pulping industry, to generate steam and recover a fused mass of soda chemicals. This smelt flows from the furnace and is dissolved in water or a process efiluent to produce a characteristic green liquor containing sodium sulfide and sodium carbonate with smaller amounts of other soda chemicals.

Clarification of the green liquor is carried out in conventional clarification equipment, whereupon all or a portion of the liquor is diverted to the ion exchange plant. This plant is operated according to the process disclosed by us to remove sufficient sodium from the green liquor to supply the soda base requirements of the acid sulfite treatment step. Hydrogen sulfide and carbon dioxide are evolved during the resin treatment, and sodium is adsorbed on the resin. The leakage, plus the balance of the liquor if only a portion was treated in the ion exchange plant, is treated by a conventional causticizing process to create the desired alkaline liquor for pulp preparation. The resinis washed, and treated with a sulfurous acid solution to regenerate the resin and to produce sodium bisulfite for use in the acid sulfite treatment of lignocellulose. As has been mentioned previously in this specification, the sulfur dioxide to prepare said sulfurous acid solution may be obtained at least in part from oxidation of the hydrogen sulfide evolved in the process. Where the concentration of sulfur dioxide in the product gases from the oxidation is low, a relatively concentrated gas can be obtained by absorbing the sulfur dioxide in a suitable solvent (e. g., water, ammonium sulfite solution or an allranolamine) and subsequently liberating it in concentrated form by heat.

Certain pulping applications require use of neutral sodium sulfite rather than sodium bisulfite. These include the so-called neutral sulfite process as well as processes using strongly alkaline cooking liquors. Examples of such liquors would include mixtures essentially consisting of and other combinations.

Neutral sodium sulfite can be conveniently uses by reacting sodium 'bisulfite recovered in the manner heretofore described with sodium carbonate or bicarbonate at an elevated temperature. Caustic soda may also obviously be used for reacting with the sodium bisulfite to produce sodium sulfite.

Sodium bicarbonate or caustic soda for such use may be obtained in a modification of the invention. In such modification the resin containing adsorbed sodium is regenerated at least in part with carbonic acid, whereupon sodium salts of carbonic acid would be formed which could be used as such or converted to caustic soda in a causticizing operation. Depending upon the products desired a, split regeneration first with carbonic acid to the desired degree, then with sulfurous acid to complete the regeneration may be used. By controlling the regeneration to give sodium bicarbonate and sodium bisulfite in substantially equimolecular quantitles, the resulting product solutions may be mixed at an elevated temperature whereupon carbon dioxide is evolved and a solution of normal sodium sulfite substantially free from bicarbonate is formed.

In order to utilize the maximum effective capacity of the resin, it may be desirable to regenerate with solutions containing carbon dioxide or sulfur dioxide in excess of the theoretically required amount. Following regeneration the resulting efliuents may be stripped of the excess volatile constituent to produce solutions substantially free of excess regenerant.

In one modification our process may be advantageously applied to the preparation of pure chemicals from naturally occurring mineral deposits or brines containing sodium salts of weakly acidic volatile acids. An analysis of one such deposit is shown below.

Analysis of natural brine from Soap Lake, Washington Weight per cent of T. S. 33.9 16.2

Sodium carbonate Sodium bicarbonate Sodium sulfate 26.6 Sodium chloride 20.1

Minor constituents::::::::III: 3.2

Total 100.0

The process of the invention is not limited to change resins us alkali-soluble polymers jtroduced into nat lf any'particuiar manner of reparation or the carboxylic acid or phenolic' hydrox'yl type ion ex- Some methods" whereby satisfactory" weakly acidic resins maybe'prepared renew; ,1

Carboxylio acid typeresinsmay befprepa'red by"polymeriz'ing or polymerizin'g unsaturated organicfacids 'or't r anhydride's under conditions whereby cross linked' polymers are formed. Alternatively, esters of unsaturated organic acids i'iiay b'e polymerizedto form a cross linked resin and later sap'oni'fied Again, non-'crcss linkea 1 containing" carboxyl gr ups may be subjected to a cr'os's linkin'g reaction to prepar insoluble ion exchange re'ini' Agaimcarboxy I acidgroup'sniay bein "p'olym'er's not aireauycen tainin'g""these' groups. In such cases; where necessary to obtaininsolubility, a" prior, can

current; or "subsequentTerese-linking treatment would be effected. Introduction of carboxyl groups could be effected by such means as substit tion of j" carboxy aikyygroups or my partial 'diidanbndr the'origih'al struoturel' Phenolic hydroxyl type ion exchange resins may" be "prepared' by polymerizing phenols (pref erablypolyphenol's) to'give a porous cross-linked polymer, as, for example, by use of suitable amounts of formaldehyde. Naturally occurring provide an" economically attractive source of polypheno ls for this purpose. Alterfiati've1y, phenolic Thydroxyl groups may be produced in Qn'atura l'y. occurring polymeric materials not containir'igappreciable amounts of this group; by: such means as hydrolysis of phenolether; or; estengroups in the original molecule. Again, in such cases where necessary to obtain insolubility, prior, concurrent, or subsequent cross-linking will be effected.

Ion exchange-resinsv containing both carboxyl and phenolic hydroxyl groups. may be prepared by polymerizing simple molecules containing both carboxylicacid" and phenolic hydroxyl groups. Alternatively,"- such resins may 'be'p're- "paredby-subjecting polymeric materials containing phenolic hydroxyls as the" only "exchange groups to procedures whereby carboxylic acid groups are introdiioed'in addition to the phenolic hydroxyl groups. Herefalso, where necessary to obtain insolubility, prior; concurrent, or subsequent cross linking will be effected.

Examples illustrating the process of the invention as wellasthe-preparation of some weakly'iacidi'c resinsiare' given below. EXAMPLE I Ninety ml. of styrene, 60 ml. of a divinylbenzene solution containing -25% divinylbenzene dissolved in other aromatic hydrocarbons, 100 gms. of maleic anhydride, and ml. of acetone were heated on a steam bath for a period of two hours. Temperature in the mixture rose to a maximum of 107 C. and dropped to 90 C. at the end of the two hour period. The product was then heated in an oven at 135 C. for three hours. It was then washed thoroughly with acetone, soaked for 18 hours in 5% NaOH and then thoroughly washed with water and dried. Yield of product was 121 grams.

Three-hundred ml. of this wet resin in hydrogen form was slurried with 100 ml. of solution containing 76.5 gm. sodium per liter, 80% of which was present as sulfide and carbonate; the balance being present as sulfate, chloride, thiosulfate and sulfite. The reaction was alioweie tbproceea for 15 minutes at atmospheric pressureand at C., during which time hydrog'e'i'i" sulfide'a'nd carbon dioxide were spontaneously evolved from the solution and sodium was adsorbedon the'resin. The resin was then separated from the solution; washed, and treated. with 500 ml. of sulfurous acid containing 30 gramssulfur dioxide for 15 minutes at 25 C. and atmospheric pressure. The product solution containing all 'the so'di'um formerly adsorbed on the resin, was drained'ofi, and the resin was washed, whereupon it was ready for re-use. In this experiment 6.9 gm. sodium was adsorbed by the 300 ml. of resin and was regenerated by means of sulfurous acid.

EXAMPLE II Using, automatic ion exchange equipment, 500 repeated :cycles wereoarried out with the following-average r.esults:.,4000 ml. wet carboxylic acid type cation exchange resin (sold commercially under the trade name of Amberlite IRC-50) in hydrogen state. was. treated with 3.78 liters of solution containing gm. NazO perv liter and having the following composition (expressed as mol per cent NazO): NazS 65%, NazCoa 15%, Na2SO4 15%, NaCl 3%, NazSzOs 1.6%, NazsOs 0.4%. This treatment, wasconducted at 50 C. for 15 minutes at atmospheric pressure during which period HzS and C02 were spontaneously evolved and 2-10 gm. sodium was adsorbed on the resin. (This represents adsorption of 68% of the total infiuent-sodiumor 85% of the sodium initially present as salts of weak volatile acids.)

Following a brief washpthe: resin was treated with .suli'urous .acidcontaining 25 gm. $.02 per literto produce-a :solution of .sodium bisulfite at a concentration of 7.4 grams sodium .per liter and containing all of -thexsodium formerly adsorbed on the resin (210? grams).

capacity had occurred and no measurable decrease'in resin 'volumeor appreciable change in screen -analysis was found;

EXAMPLEIII' l500igrams :of: quebracho tannin were dissolved in: 1500mm of water=in;an .autoc1ave and 609 m1. of formaiinand 151111; of concentrated hydrochloric acid were added: The mixture was then heated with a steamrjacketiwith "a steam pressure of lbs/sq. in. in the jacket. Heating was continued for three hours, during which time the temperature reached C. in the autoclave. The mass was then removed from the autoclave, dried at low temperature and heated at 105 C. for a total of 36 hours. The product Was then ground and screened to remove fines.

200 ml. of this Wet resin in hydrogen form was slurried with 200 ml. of solution containing 100 gm. sodium per liter, 85% of which was present as sulfide and carbonate; the balance being present as sulfate, chloride, thiosulfate and sulfite. The reaction was allowed to proceed for 15 minutes at atmospheric pressure and at the desired temperature, during which time hydrogen sulfide and carbon dioxide were spontaneously evolved from the solution and sodium Was adsorbed on the resin. The resin was then separated from the solution, washed, and treated with 500 ml. of sulfurous acid containing 30 grams sulfur 11 dioxide for 15 minutes at 25 C. and atmospheric pressure. The product solution, containing all the sodium formerly adsorbed on the resin, was drained oif and the resin was washed, whereupon it was ready for re-use. Typical results of such an experiment are as follows:

Sodium Tempera- Adsorbed igg ga ture of by 100 ml. Regeneration Resin ioldium Wet Itch with Sul sorpgenera e tion, "0. Resin, gf f gms.

Phenolic Hydroxyl 26 2. 1 2. 16 Resin (prepared as described above) 82 2. 8 2. 92

We claim:

1. In the preparation of wood pulps, the process for recovering chemicals which comprises combusting waste liquor from the alkaline treatment of sulfite digested pulp to yield a smelt containing a sodium salt of a weakly acidic volatile acid of the group consisting of hydrogen sulfide and carbonic acid, treating a solution of said smelt with a cation exchange resin having active exchange centers in hydrogen form Which are substantially less acidic than sulfur dioxide and evolving a substantial portion of said volatile acids in gaseous form and adsorbing sodium on the resin, treating said resin containing adsorbed sodium with sulfurous acid to form a solution containing sodium bisulfite suitable for use in the acid sulfite digestion of wood, and returning the sodium bisulfite to the pulping operation.

2. In the process of claim 1, treating the resin containing the adsorbed sodium with sulfurous acid to form a solution containing sodium bisulfite suitable for use in the sulfite digestion of wood.

3. In the preparation of Wood pulp, the process for recovering chemicals which comprises combusting combined waste liquors from successive sulfite and alkaline treatments of wood to yield a smelt containing a sodium salt of a Weakly acidic volatile acid of the group consisting of hydrogen sulfide and carbonic acid, treating a solution of said smelt with a cation exchange resin having active exchange centers in hydrogen form which are substantially less acidic than sulfur dioxide and evolving a substantial portion of said voltaile acids in gaseous form and adsorbing sodium on the resin, treating said resin containing adsorbed sodium with sulfurous acid to form a solution containing sodium bisulfite suitable for use in the sulfite treatment, and

returning the sodium bisulfite to the pulping operation.

4. In the preparation of wood pulp, the process for recovering chemicals which comprises combusting waste liquor from a sulfite digestion of wood to yield a smelt containing sodium salts of weakly acidic volatile acids including hydrogen sulfide, treating a solution of said smelt with a cation exchange resin having active exchange centers in hydrogen form which are substantially less acidic than sulfur dioxide and evolving a substantial portion of said volatile acids as a gas and adsorbing sodium on the resin, oxidizing the hydrogen sulfide content of said gas to form sulfur dioxide, treating said resin containing adsorbed sodium with sulfurous acid derived at least in part from the oxidation of hydrogen sulfide to form a solution containing sodium bisulfite suitable for use in the sulfite digestion of wood, and returning the sodium bisulfite to the pulping operation.

5. The process of claim 4 in which the waste liquor is from an alkaline treatment of sulfite digested pulp.

6. The process of claim 4 in which the waste liquor is from successive sulfite and alkaline treatments of wood.

7. The process of claim 4 in which the resin has carboxylic acid-type exchange centers in hydrogen form.

I 8. The process of claim 4 in which the resin has phenolic hydroxyl exchange centers in hydrogen form.

9. The process of claim 4 in which the resin has in hydrogen form both carboxylic acid-type and phenolic hydroxyl exchange centers.

KENNETH RUSSELL GRAY. HARTZELL LANCE CROSBY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,545,522 Richter July 14, 1925 1,6 8,492 Laury Aug. 9, 1927 2,104,501 Adams et a1. Jan. 4, 1938 2,409,861 Hunter et al. Oct. 22, 1946 FOREIGN PATENTS Number Country Date 118,486 Australia May 4, 1944 521,207 Great Britain May 15, 1940 OTHER REFERENCES Ion Exchange in Waste Treatment, Ind. and Eng. Chem. vol. 41, No. 3 (Mar. 1949), pages 448 to 451.

Claims (1)

1. IN THE PREPARATION OF WOOD PULP, THE PROCESS FOR RECOVERING CHEMICALS WHICH COMPRISES COMBUSTING WASTE LIQUOR FROM THE ALKALINE TREATMENT OF SULFITE DIGESTED PULP TO YIELD A SMELT CONTAINING A SODIUM SALT OF A WEAKLY ACIDIC VOLATILE ACID OF THE GROUP CONSISTING OF HYDROGEN SULFIDE AND CARBONIC ACID, TREATING A SOLUTION OF SAID SMELT WITH A CATION EXCHANGE RESIN HAVING ACTIVE EXCHANGE CENTERS IN HYDROGEN FORM WHICH ARE SUBSANTIALLY LESS ACIDIC THAN SULFUR DIOXIDE AND EVOLVING A SUBSTANTIAL PORTION OF SAID VOLATILE

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