2,750,290 United States Patent Office Patented June 12, 1956 2 the pulping phase of the cycle, thereby obviating the need for substantial storage facilities to contain the re 2,750,290 generated semi-chemical pulping liquor. RECOVERY OF COOKING LIQUOR FROM Another object of the present invention is to provide a SPENT SEM -CHEMICAL PULPNG LIQUORS method for the regeneration of semi-chemical pulping Etagege W. Schoefai, Kronenwetter, Wis, assignor, by liquor, wherein the calcium alkalies recovered with the Kaesare assig nets, to Stering. Drug ac, New York, sodium sulfite can be re-introduced and utilized in the N.Y., a corporation of New York regeneration procedure. No Drawing. Application February 21, 1952, An additional object of the present invention is to pro 10 vide a method for the regeneration of cooking liquor in Serial No. 272,719 high yield from spent semi-chemical pulp liquor, wherein 5 Clains. (C. 92-2) the organic and inorganic materials are substantially com pletely oxidized and converted to usable cooking liquor without the necessity of employing smelting or coking The present invention relates to the semi-chemical l5. procedures. pulping of wood and is more particularly concerned with A further object of the present invention is to provide an improved cycle for the regeneration of sodium sulfite a method for the recovery of semi-chemical pulping (Na2SO3) cooking liquor from spent semi-chemical pulp liquor that insures a continuous supply of regenerated liquors. cooking liquor that can immediately be utilized in the The semi-chemical procedures customarily involve the 20 pulping phase of the semi-chemical pulping cycle. puping with buffered solutions of sodium sulfite. Acid, Other objects of the present invention will be apparent neutral, or alkaline sodium sulfite solutions buffered with to one skilled in the art to which this invention pertains. sodium carbonate, sodium hydroxide or sodium bicarbon The novel process of the present invention is directed ate are representative semi-chemical pulping liquors. The to the recovery of sodium sulfite from spent semi-chemical semi-chemical pulping process results in superior yields pulp liquor and comprises the steps of oxidizing Sub of wood pulp of improved quality and finds applicability stantially completely the said liquor while maintained to a variety of wood materials which cannot be success under the vapor pressure of the reaction mixture and fully cooked by the conventional calcium bisulfite cooking substantially in the liquid phase in a reaction Zone, in acid or other sulfite or sulfate procedure. troducing sulfur dioxide (SO2) and a calcium alkali into However, no technically operative means have been 30 the sodium-containing concentrate from said reaction discovered for the economic reclamation of the sodium zone, separating the calcium sulfate thus precipitated, sulfite cooking liquor from the spent semi-chemical pulp treating the resulting filtrate with a calcium alkali, and liquor. Conventionally the alkali regeneration has been returning the sodium sulfite (Na2SO3) so-formed to the accomplished by concentration of the waste liquor, fol pulping process and the calcium sulfite so-formed to the lowed by a smelting or cooking process in which the 35 said sodium-containing concentrate. In the aqueous, sodium sulfate and sodium sulfite are converted primarily liquid-phase, flameless oxidation of the spent pulp liquor, to sodiuim sulfide (Na2S) and sodium carbonate the inorganic salts are changed to their highest states of (Na2CO3). The generated hydrogen sulfide is expelled oxidation and the sulfurous materials are substantially with carbon dioxide and subsequent treatment with a completely converted into sodium sulfate (Na2SO4), mixture of carbon dioxide, and sulfur dioxide is employed 40 virtually all of the remaining sodium ion being converted to depress the formation of elemental sulfur and thio to sodium carbonate (Na2CO3). The concentration of sulfate or ploythionate salts. The hydrogen sulfide (H2S) the liquor during the oxidation step enhances the Subse is then converted to sulfur dioxide and the reclaimed quent successive conversion of the sodium sulfate to cooking liquor formed from the sodium carbonate and sodium bisulfite and thence to the desired sodium Sufite. sulfur dioxide, eliminating sodium, thiosulfate (Na2S2O3) The amounts of sulfur lost from the system as calcium formation. Maximum yields of the recovered sodium sulfate are at the expense of economically reclaiming Sub ion of slightly over seventy percent have been reported, stantially all of the sodium ion. Careful regulation of arge soda, losses being incurred during the Smelting or the addition of calcium alkalies in the conversion of coking phase of the recovery process. To date, no simple sodium bisulfite to sodium sulfite is productive of an procedure for the regeneration of the semi-chemical pulp 50 already buffered regenerated cooking liquor having the ing liquor (Na2SO3) has been demonstrated to be eco desired pH in accord with the precise type of the semi nomically feasible. chemical pulping process being conducted. Therefore, it is an object of the present invention to The process of the present invention has utility in provide a simple and economical method for the recovery the economical regeneration of the valuable sodium Sul of cooking liquor from the semi-chemical pulp waste fite pulping liquor from the spent semi-chemical pulp liquors in good yields. liquor by a procedure wherein the recovered cooking It is another object of the present invention to provide liquor is immediately recycled to the pulping process, a method for the substantially complete recovery and thereby permitting a virtually continuous pulping opera regeneration of the sodium from spent semi-chemical tion in which the overall sodium loss is small. Recovery pulp liquor. 60 of a small amount of the sodium ion as sodium hydroxide, It is still a further object of the present invention to associated with the sodium sulfite, enables the buffering provide a process for the recovery of sodium sulfite cook action of the sodium hydroxide to be advantageously ing liquor from spent semi-chemical pulp liquor by employed in the regenerated cooking liquor. Useful aqueous, liquid phase, flameless oxidation of the spent amounts of the calcium sulfite (CaSO3) formed in the semi-chemical pulp liquor and conversion of the thus 65 liming of the sodium bisulfite are returned to the phase formed sodium salts in good yield to sodium sulfite of the cycle wherein the sodium sulfate (Na2SO4) is (Na2SO3) and sodium hydroxide (NaOH). reacted with calcium bisulfite (Ca(HSO3)2), produced Another object of the present invention is to provide a in situ. The SO4 ion is eliminated as calcium sulfate, method for the regeneration of the spent semi-chemical which under, controlled roasting conditions yields the pulp liquor wherein the cooking liquor is continuously necessary sulfur dioxide employed in the treatment of recovered from the waste pulp liquor in a properly sodium-containing concentrate obtained by oxidation of buffered sodium sulfite 'solution ready to be returned to the waste liquors. 2,750,290 3 4 In carrying out the process of the present invention, a to the sodium-containing concentrate or added by form starting spent liquor or effluent, resulting from the semi ing calcium bisulfite in situ by reacting sulfur dioxide chemical pulping of soft or hard Woods, or mixtures with a calcium alkali such as calcium carbonate, calcium thereof, with acid, neutral, or alkaline sodium sulfite in oxide, calcium hydroxide, calcium sulfite (CaSO3) and the presence of sodium carbonate, sodium hydroxide or the like in the presence of water. The in situ prepara sodium bicarbonate, and even in the presence of Some tion of the calcium bisulfite is preferred since the calcium sodium sulfate and sodium thiosulfate, is oxidized. Any sulfite produced as a by-product in the Subsequent liming waste liquor or effluent wherein substantial amounts of of the sodium bisulfite can be continuously recycled into sodium sulfite have been employed and are present per the system at this point and admixed with the continu se or in the form of sodium sulfate or sodium thiosulfate 10 ous flow of sulfur dioxide to form the requisite calcium is an appropriate starting material from which the desired bisulfite reagent. The calcium bisulfite reaction step is sodium sulfite can be regenerated according to the process generally conducted in a closed system in order to re of the present invention. duce corrosion of the equipment to a minimum. The In substantially completely oxidizing the spent semi reaction mixture comprising the sodium-containing con chemical pulp liquor, the necessary apparatus comprises 5 centrate and the added calcium alkali is maintained at a pump for continuously charging a reactor with Spent about room temperature or slightly above, temperatures semi-chemical pulp liquor, an air compressor, a tower of 12-25 degrees centigrade being preferred, and the reactor provided with means therein to remove periodi preferred sulfur dioxide gas introduced into the system cally any precipitate formed therein, and a flash chamber over a period of one or more hours. The course of the to receive the oxidized residual spent semi-chemical pulp 20 reaction and degree of completion thereof can be gauged liquor from the reactor. by employing a sufficient analytical control, e. g., a in initiating the oxidation phase, the spent semi-chemi carbon dioxide absorption tube, to avoid the use of an cal pulp liquor is pumped into the reactor under pressure excess of sulfur dioxide. The pH of the sodium bi of from about 200 to 2000 pounds per square inch, the sulfite solution is controlled in part by the amount of preferred pressure being that which is sufficient to main 25 sulfur dioxide utilized in the conversion of sodium sulfate tain substantially all of the waste effluent in the liquid to sodium bisulfite and is preferably less than five, the phase. The waste liquor charge is then heated by means pH value of between about three and 4.5 being regarded of an oil ring to a temperature of between above 100 as useful. Rapid conversion of the sodium sulfate to degrees and 330 degrees centigrade, temperatures of from sodium bisulfite is achieved by the employment of vigor 235 to 250 degrees centigrade being useful for sodium 30 ous, continual stirring of the reaction admixture. Sub sulfate recovery. Compressed air, or other compressed stantially in excess of ninety percent of the sodium sul gaseous materials capable of furnishing free oxygen, under fate is converted to sodium bisulfite and a like percentage a pressure slightly greater than the pressure in the re of calcium alkali is converted to calcium sulfate. If action zone is then introduced into the reactor contain desired, the calcium sulfate precipitate can be filtered ing the spent semi-chemical pulp liquor through a dis 35 and removed from the cycle at this point, or, alternatively, persion head to initiate the oxidation of the combustible it can be kept in admixture with the acidic, aqueous, so materials. The rate of charging the reactor with spent dium bisulfite sclution and removed at a subsequent semi-chemical pulp liquor and the rate of introduction point in the regeneration cycle. of the compressed air or other oxidation media is Syn The aqueous, acidic sodium bisulfite (NaHSO3) solu chronized to insure the substantially complete oxidation 40 tion is next treated with a calcium alkali, resulting in the of all the inorganic and organic constituents. formation of the desired, regenerated sodium sulfite Once initiated, the oxidation proceeds exothermically (Na2SO3) together with a minor amount of sodium hy so that no external heating is required and, in fact, the droxide (NaOH). Control of the added alkali regulates spent pulp liquor can be introduced into the reactor at the final pH of the regenerated sodium sulfite cooking a temperature of as low as twenty degrees centigrade, liquor, thereby facilitating the recovery of a cooking the exothermic heat liberated from the oxidation being liquor that is thus buffered to the desired degree and which sufficient to raise the temperature of the carbonaceous can immediately be recycled and utilized in the pulping material in the incoming spent chemical liquor to a point of fresh quantities of wood. Calcium carbonate is the where substantially complete oxidation will ensue. The preferred alkali embodiment, although calcium oxide quantity of oxidizing agent supplied is preferably that 50 (lime), calcium hydroxide and the like are entirely op theoretically required to convert all of the organic com erative in the aqueous liquid medium. The conversion of bustible material to its ultimate end products, viz., carbon the sodium bisulfite to sodium sulfite is conducted at dioxide (CO2) and water (H2O), and in general suffi slightly elevated temperatures, seventy degrees centigrade cient oxygen to produce substantially complete oxidation being preferred. The calcium alkali may be introduced of all other oxidizable materials in the spent pulp liquor 55 into the reaction mixture in a single addition, or, alterna should be employed in the oxidation reactor. tively, may be added in small portions during the course The oxidized liquor from the reactor is then continu of the reaction, e.g., from about fifteen minutes to about ously passed through a flash chamber, from which the three hours or more. Sufficient quantities of alkali are fixed gases, i. e., nitrogen, carbon dioxide, and excess air added to insure the formation of an alkaline admixture, are vented off with steam under pressure. The liquid 60 pH values of from seven to twelve being generally satis effluent is then obtained from the reactor. factory. When lime (calcium oxide) is reacted with The liquid effluent (sodium-containing concentrate) sodium bisulfite solution, a pH of about twelve of the from the flash chamber contained primarily sodium sul aqueous sodium sulfite reaction product is obtainable. fate (Na2SO4), a smaller amount of sodium carbonate Employment of a comparable amount of calcium carbon (NaCO3) and a certain amount of sodium acetate and 65 ate (CaCO3) instead of calcium oxide may result in the acetic materials. In this concentrate, virtually all (greater formation of an admixture having a pH of about seven than 99 percent) of the sulfur materials and most of the to nine. If necessary, nitrogen may be continuously cir sodium are present as sodium sulfate, the remaining culated through the reaction system to prevent oxidation minor amounts of sodium ion being contained in the and resultant formation of undesirable oxidized inorganic sodium carbonate and sodium acetate. 70 products. Conversion of sodium bisulfite (NaHSO3) to The thus-oxidized, spent semi-chemical pulp liquor is sodium sulfite (NaSO3) in amounts substantially above next reacted with calcium bisulfite (Ca(SO3H)2) to yield ninety percent is consistently achieved, a varying amount chiefly a precipitated calcium sulfate (CaSO4) and an of sodium hydroxide, e.g., one to eight or nine percent, aqueous acidic sodium bisulfite (NaHSO3) solution. In can be formed in the reaction product. The sodium hy such reaction, the calcium bisulfite can be added per se droxide exhibits a desirable buffering effect on the re 2,750,390 5 6 generated sodium sulfite cooking liquor. Upon comple 95.0 percent calcium oxide (CaO) added in small por tion of the reaction, the admixture is filtered, the filter tions. The resulting mixture was stirred for three hours, cake comprising primarily the precipitated calcium sulfite at the end of which time the pH was 12.4. Analysis of (CaSO3) which is subsequently returned to the sodium the completed reaction product showed that 69.2 per containing concentrate obtained from the oxidation of the cent of the original sodium from the starting sodium-con spent semi-chemical pulp liquor and is reacted with sulfur taining concentrate was present in final form as sodium dioxide in the presence of water to form in situ the cal sulfite (Na2SO3), 7.1 percent of the sodium as sodium hy cium bisulfite reagent. The aqueous, alkaline filtrate con droxide (NaOH), which together with the approximately tains chiefly the desired sodium sulfite cooking liquor and three percent sodium contained in the sodium bisulfite the desired amount of sodium hydroxide. Since the pH O solution removed for analysis earlier represented a total of the filtrate has been controlled by the alkali addition conversion yield of about eighty percent. An additional to the sodium bisulfite, the regenerated cooking liquor is sixteen percent of the original sodium ion was present thus adequately buffered for the precise type of semi as sodium sulfate (Na2SO4), apparently having been chemical pulping process being conducted and can be formed either from the oxidation of the sodium bisulfite continually recycled into the fresh pulping apparatus. 5 or the sodium sulfite (Na2SO3). Thus, about 96 percent The following examples illustrate the practice of the of the original sodium ion was recovered. The reaction present invention but are not to be construed as limiting mixture was filtered, the filtrate containing the desired the same. regenerated sodium sulfite and sodium hydroxide cook Example 1 ing liquor, whereas the filter cake comprised a mixture of 20 Spent semi-chemical pulp liquor from the semi-chemical calcium sulfate and calcium sulfite. The residue con pulping of a mixture of soft, and hard woods with a tained 52.2 percent calcium sulfite (CaSO3). buffered sodium sulfite cooking liquor was oxidized sub Example, 2 stantially completely in a system comprising a pump for Employing the apparatus for converting sodium sulfate continuously charging the reactor with spent pulp liquor, 25 to sodium bisulfite, illustrated in Example 1, the residual an air compressor, a tower reactor and a flash chamber calcium sulfite (CaSO3) obtained upon completion of to receive the oxidized residual waste liquor from the the reactions in Example 1 was introduced into the re reactor and from which the effluent gases are vented. The action flask along with another aliquot of oxidized semi spent semi-chemical pulp liquor was pumped into the chemical liquor, prepared according to the procedure of tower reactor under a pressure of about 800 pounds per 30 Example 1 and containing one-half mole of sodium sul square inch and the waste liquor charge heated by means fate, and sulfur dioxide gas admitted to the system at a of an oil ring at a temperature of 235 degrees centigrade. temperature of seventy degrees centigrade. A stream Compressed air under pressure slightly greater than that of nitrogen was introduced to forcibly expel the air from in the reaction zone was then introduced into the reactor the reaction system. After an excess of sulfur dioxide through a dispersion head to initiate the oxidation of 35 had been taken up in the absorption tube, the mixture was the combustible materials, and thereafter the aqueous, filtered and a sample of the filtrate assayed as in Example liquid phase, flameless oxidation was self-sustaining, re 1. Direct conversion of the sodium in the sodium-con quiring no further external heat, the rate of charging the taining concentrate to sodium bisulfite was greater than reactor with spent semi-chemical pulp liquor and the rate ninety percent. The aqueous sodium bisulfite (NaHSO3) of introduction of compressed air being regulated to 40 filtrate was returned to the reaction flask, excess calcium insure substantially complete oxidation of all of the oxide (CaO) introduced at seventy degrees centigrade organic and inorganic constituents. The oxidized liquor and the contents of the flask vigorously agitated for two from the tower reactor was continuously passed through hours, nitrogen being continuously circulated through a flash chamber from which nitrogen, carbon dioxide and the system to prevent oxidation. At the end of the two excess air were vented off with steam at about 800 pounds 45 hour period, the pH of the reaction admixture was 11.1. per square inch pressure. The liquid effluent was con Filtering of the flask contents yielded 83 percent of the tinuously drained from the flash chamber. Analysis of starting sodium ion as sodium sulfite (Na2SO3), 1.6 an appropriate sample of the resulting sodium-contain percent as sodium hydroxide (NaOH), which together ing concentrate showed the entire sulfur content and the with the three percent removed for bisulfite analysis in major portion of the sodium ion to be present as sodium 50 the first step represented about 87 percent conversion to sulfate (Na2SO4), the concentrate also containing minor regenerated semi-chemical pulping liquor. amounts of Sodium carbonate, sodium acetate and acidic materials. An aliquot portion of the thus-oxidized spent Example 3 semi-chemical pulp liquor containing five-tenths mole of Spent semi-chemical pulp liquor was oxidized under sodium sulfate was admixed with five-tenths mole of 55 aqueous, liquid phase conditions at a temperature of 250 calcium carbonate and the resulting mixture diluted to a degrees centigrade and pressure of 400 pounds, employ volume of about 500 milliliters with water, placed in a ing the apparatus and procedure described in Example three-necked, one-liter round bottom flask, equipped with 1, the oxidized liquor containing primarily sodium sulfate a sealed stirrer, a gas inlet tube leading to the bottom of (Na2SO4), sodium carbonate (Na2CO3) and a small the flask, an outlet tube and reflux condenser connected 60 amount of sodium acetate. An aliquot portion, con with a carbon dioxide absorption vessel. While the tem taining 14.2 parts sodium sulfate and 10.0 parts of cal perature of the reaction mixture in the flask, was main cium carbonate were placed in a closed reaction flask, tained at 23 degrees centigrade, sulfur dioxide (SO2) gas the temperature of the mixture being maintained at was introduced into the system over a period of about one fifteen degrees centigrade, and 12.4 parts of sulfur di hour. When the carbon dioxide tube had absorbed 26.0 65 oxide (SO2) introduced into the system and the resulting grams (theoretical 22.0 grams) of carbon dioxide, the admixture. Stirred vigorously for a period of one hour. apparatus was disconnected, the resulting mixture filtered The resulting pH was 4.5. The contents of the reaction and the filter cake washed with five successive 200-milli flask were filtered, the precipitate comprising 17.8 parts liter portions of water. Analysis of an appropriate sample of calcium sulfate (virtually a quantitative yield), where indicated that over ninety percent of the sodium sulfate as the sodium bisulfite (NaHSO3) filtrate analyzed at was converted to sodium bisulfite (NaHSO3) and that 6.4 parts sulfur dioxide content (corresponding to a yield over ninety percent of the calcium carbonate was con of approximately 10.4 parts of NaHSO3). To the aque verted to calcium sulfate (CaSO4). The aqueous, acidic ous sodium bisulfite solution was added 5.6 parts of cal sodium bisulfite filtrate was returned to the reaction flask, cium oxide and the resulting mixture heated by means of warmed to seventy degrees centigrade and 36 grams of 5 a water bath, for one hour with vigorous stirring, the 2,750,290 7 8 pH of the admixture at the end of this period being an aqueous solution of sodium sulfate; separating calcium twelve. The admixture was filtered, the desired filtrate sulfate thus precipitated and treating the resulting filtrate containing primarily the sodium sulfite (Na2SO3) cook with a calcium-containing material which will form so ing liquor with a small amount of sodium hydroxide. dium sulfite and calcium sulfite with said filtrate; separat The filtered residue comprised calcium sulfite and a ing the calcium sulfite so-formed and returning it to said minor amount of calcium sulfate. The sulfur dioxide Sodium-containing concentrate; and returning the sodium (SO2) content of the recovered semi-chemical sodium sulfite so-formed to the pulping step. sulfite cooking liquor was 3.6 grams per 100 milliliters, 2. In a process for the recovery of sodium sulfite from indicating that one-half of the sulfur dioxide had been spent semi-chemical pulp liquor, the steps comprising: converted into the precipitated calcium sulfite. The O pulping wood chips with a buffered solution of sodium total sodium recovery based upon the soda content of the sulfite; separating pulp and a spent semi-chemical pulp starting oxidized, spent semi-chemical pulp liquor was liquor; oxidizing substantially completely the organic and about 93 percent. inorganic constituents of said liquor while maintained under the vapor pressure of the reaction mixture and Example 4 5 substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing Sulfur Following the procedure of Example 2, another aliquot dioxide and calcium carbonate into said sodium-contain of sodium-containing concentrate obtained by the oxida ing concentrate from said reaction Zone; Separating the tion procedure of Example 1 and having 14.2 parts of calcium sulfate precipitated and treating the resulting sodium sulfate, and the calcium sulfite residue from the 20 filtrate with an excess of calcium oxide to form sodiunil process of Example 3, were admixed and vigorously agi sulfite and calcium sulfite; separating the calcium sulfite tated with sulfur dioxide in the manner of Example 3. so-formed and returning it to said sodium-containing con Subsequent filtration removed the precipitated calcium centrate; and returning the sodium sulfite So-formed to sulfate and the residual aqueous sodium bisulfite filtrate the pulping step. was stirred in a closed system for a period of about two 25 3. In a process for the recovery of sodium sulfite from hours with an excess of calcium carbonate, instead of spent semi-chemical pulp liquor, the steps comprising: the calcium oxide employed in Example 3. A final pH pulping wood chips with a buffered solution of sodium of 9.2 was obtained. The product was filtered, yielding sulfite; separating pulp and a spent semi-chemical pulp the precipitated calcium sulfite and the aqueous sodium liquor; oxidizing substantially completely the organic and sulfite cooking liquor. Overall recovery of the sodium 30 inorganic constituents of said liquor while maintained un ion was about 91 percent based upon the sodium sulfate der the vapor pressure of the reaction mixture and Sub input. The precipitated calcium sulfite was again re stantially in the liquid phase in a reaction Zone to obtain cycled into the oxidized spent pulp liquor and the regen a sodium-containing concentrate; introducing calcium erated semi-chemical cooking liquor employed in pulping bisulfite into said sodium-containing concentrate from fresh quantities of wood. 35 said reaction zone; separating the calcium Sulfate thus The novel process of the present invention achieves precipitated and treating the resulting filtrate with an ex an overall regeneration and recovery yield of the sodium cess of a calcium-containing material which will form ion, based upon the soda content of the completely sodium sulfite and calcium sulfite; separating the calcium oxidized spent semi-chemical pulp liquor, of from about sulfite; and returning the sodium sulfite so-formed to the 85 to 98 percent. The overall calcium and sulfate ion 40 pulping step. recovery is generally in excess of 98 percent. The re 4. In a process for the recovery of Sodium Suite from action by-products from the various individual steps of spent semi-chemical pulp liquor, the steps comprising: the regeneration procedure are re-cycled and employed pulping wood chips with a buffered solution of sodium in other phases of the recovery process. The precipitated sulfite; separating pulp and a spent semi-chemical pulp calcium sulfate, either separated after completion of the 45 liquor; oxidizing substantially completely the organic reaction in which the sodium bisulfite (NaHSO3) is and inorganic constituents of said liquor while maintained formed or separated after final conversion of the sodium under the vapor pressure of the reaction mixture and Sub bisulfite to the regenerated cooking liquor, can be roasted stantially in the liquid phase in a reaction Zone to obtain at a temperature of approximately 1600 degrees centi a sodium-containing concentrate; introducing sulfur di. grade to convert a minimum of 90 percent of the total 50 oxide and a soluble calcium-containing material which will sulfur content into the desired sulfur dioxide, which in form insoluble calcium sulfate when contacted with an turn is recycled into the agitating sodium-containing aqueous solution of sodium sulfate; treating the resulting concentrate of the oxidized spent semi-chemical pulp filtrate with a calcium-containing material which will liquor to form the calcium bisulfite reagent and to control form sodium sulfite and calcium sulfite; separating the the acidity of the resulting sodium bisulfite solution. The 55 calcium sulfite so-formed and returning it to said Sodium remaining calcium oxide (lime) from the roasting pro containing concentrate; and returning the Sodium sulfite cedure can then be conveniently employed in the sub so-formed to the pulping step. sequent reaction wherein the sodium bisulfite is con 5. In a process for the recovery of Sodium sulfite from verted to the regenerated sodium sulfite cooking liquor. spent semi-chemical pulp liquor, the steps comprising: Various modifications may be made in the invention 60 pulping wood chips with a buffered solution of sodium without departing from the spirit or scope thereof and sulfite; separating pulp and a spent semi-chemical pulp it is to be understood that I limit myself only as defined liquor; oxidizing substantially completely the organic and in the appended claims. inorganic constituents of said liquor while maintained claim: under the vapor pressure of the reaction mixture and 1. In a process for the recovery of sodium sulfite 65 from spent semi-chemical pulp liquor, the steps compris substantially in the liquid phase in a reaction Zone to ob ing: pulping wood chips with a buffered solution of so tain a sodium-containing concentrate; introducing sulfur diurn Sulfite; separating pulp and a spent semi-chemical dioxide and a calcium-containing material selected from pulp liquor; oxidizing substantially completely the organic the group consisting of calcium oxide, calcium carbonate, and inorganic constituents of said liquor while maintained. calcium hydroxide, calcium bisulfite and calcium sulfite; under the vapor pressure of the reaction mixture and separating calcium sulfate thus-formed and treating the Substantially in the liquid phase in a reaction zone to ob resulting percipitate with a calcium-containing material tain a sodium-containing concentrate; introducing sulfur selected from the group consisting of calcium carbonate, dioxide and a solube calcium-containing material which calcium oxide, and calcium hydroxide, separating the cal will form insoluble calcium sulfate when contacted with 75 cium sulfite thus-formed and returning it to said sodium 2,750,290 10 containing concentrate; and returning the sodium sulfite 2,516,827 Marshall et al. ------July 25, 1950 So-formed to the pulping step. 2,528,350 Farber ------Oct. 31, 1950 2547,913 Lagally ------Apr. 3, 1951 References Cited in the file of this patent 2,598,311 Schoefel ------May 27, 1952 UNITED STATES PATENTS 5 2,692,291 Bryan ------Oct. 19, 1954 1,765,560 Barbou ------June 24, 1930 FOREIGN PATENTS 1,815,328 Richter ------July 21, 1931 1983,789 Bradley et al. ------Dec. 11, 1934 27,733 Denmark ------June 6, 1921 2,010.077 Haglund ------Aug. 6, 1935 10 OTHER REFERENCES 2,029,616 Haglund ------Feb. 4, 1936 "Paper Trade Journal," pages 30-34; October 12, 1950, 2,354,553 Sherk ------July 25, 1944 “Oxidation of Sulphate Black Liquor,” T. T. Collins. 2,482,594 Pearl et al. ------Sept. 20, 1949