US2987432A

US2987432A - Treatment of spent sulfite liquor

Info

Publication number: US2987432A
Application number: US676533A
Authority: US
Inventors: Roy P Whitney; Han Shu-Tang; James F Bakken; Richard B Kesler
Original assignee: Paper Chemistry Inst
Current assignee: Institute of Paper Chemistry
Priority date: 1957-08-06
Filing date: 1957-08-06
Publication date: 1961-06-06
Anticipated expiration: 1978-06-06

Description

June 6, 1961 R. P. WHITNEY ETAL 2,987,432

TREATMENT OF SPENT SULFITE LIQUOR Filed Aug. 6, 1957 7'0 sun-m? EMQWEE SMELT WHTFK PZ/%I%IZTORS. 2 2 A y;

sou/770A! Patented June 6, 1961 2,987,432 TREATMENT OF SPENT SULFITE LIQUOR Roy P. Whitney, Appleton, Wis., Shu-Tang Han, Roscoe,

Ill., and James F. Bakken and Richard B. Kesler, Appleton, Wis., assignors to The Institute of Paper Chemistry, Appleton, Wis., a corporation of Wisconsin Filed Aug. 6, 1957, Ser. No. 676,533 7 Claims. (Cl. 162-36) The present invention relates generally to the preparation of wood pulp by a sulfite process employing a sodium base liquor. It is more particularly related to improvements in the preparation of cooking liquor for the sulfite process from spent liquor.

Paper and paper products are manufactured from wood which has been converted into pulp. Wood pulp may be prepared by one of a number of processes. Generally speaking, wood pulp is either prepared by a mechanical process, or a chemical process. Chemical processes include the acid sulfite, neutral sulfite, sulfate and soda processes. A more recently employed process is the semichemical pulping process which has achieved substantial importance because of the high yield of pulp obtainable therefrom. Since in the semichemical process, the wood is subjected to a milder treatment than in the chemical processes, it is readily understood how higher yields result.

Wood treated in accordance with the semichemical pulping process may be digested under substantially neutral conditions with a sodium compound. A well known process utilizes sodium sulfite in the cooking liquor in connection with digestion of the pulp. This invention primarily relates to the recovery of the spent liquor, i.e. the liquor remaining after digestion and removal of the pulp.

While various processes have been developed for recovering sodium base, spent sulfite liquor, they have not all been economical to practice nor have they provided a cooking liquor of high quality. Furthermore, previously known processes have required substantial capital investment for recovery of the liquor.

The pulp produced by the neutral sulfite semichemical pulping processes possesses strength and good bleachability characteristics. These and other characteristics, in view of the high yield, have established neutral sulfite semichemical pulp for use in the manufacture of paper and paperboard products. The semichemical pulp has been combined with other pulps in the manufacture of high grade paper, such as book paper.

In order to provide an overall process which is commercially practicable, various attempts have been made to recover the spent liquor by concentration and burning to provide a smelt. The smelt is dissolved to furnish an aqueous solution which is then treated with sulfur dioxide to form sodium sulfite. However, this technique as usually practiced in the past has resulted in the formation of excessive amounts of thiosulfates and other polythionic compounds which are not effective in digestion and result in a dead load in the system.

Attempts have also been made to oxidize smelt solutions in order to prepare a cooking liquor. These attempts have not resulted in a commercially practicable process for the paper industry.

Similarly, carbonation processes for recovering spent liquor have not been adopted.

A suitable recovery method for spent liquor from a sodium base, sulfite pulping process has been discovered and it is set forth in United States patent application, Serial No. 382,678, filed September 28, 1953, now United States Patent No. 2,802,791, issued August 3, 1957, assigned to the assignee of this invention. This process involves the treatment of smelt recovered from spent sulfite liquor. The smelt is dissolved and treated under particular conditions to minimize the contact between hydrogen sulfide and sulfur dioxide.

We have now discovered a new method whereby a cooking liquor of high quality for a sodium base, sulfite pulping process can be prepared from spent liquor in a simple and economical manner without substantial formation of thiosulfates and other polythionic compounds so that a high quality liquor results. In addition, by this method, hydrogen sulfide from a sulfitation operation may be recovered in a concentrated form. The recovered hydrogen sulfide is converted to sulfur dioxide and then to sodium bisulfite, the sulfitation agent of the method. The method may be carried out in a continuous or batch operation and may be utilized in the preparation of acid sulfite liquor or neutral sulfite liquor.

In the method, sodium bisulfite, in aqueous solution, is reacted with an aqueous smelt solution under particular conditions to provide a high quality cooking liquor.

A main object of the present invention is to provide an improved method for handling the spent liquor from a sodium base, sulfite pulping process.

Another object of this invention is to provide a high quality cooking liquor from spent liquor from a sodium base, sulfite pulping process by direct sulfitation with sodium bisulfite.

It is also an object of this invention to provide a cooking liquor, substantially free from sodium thiosulfate or other polythionic compounds, for the sodium base, sulfite semichemical pulping process or for the sodium base acid sulfite pulping process, through the use of sodium bisulfite.

Further objects of this invention will be obvious from a study of the accompanying drawing and following disclosure. In the drawing:

FIGURE 1 is a schematic drawing of a portion of a recovery system which may be employed in the recovery of spent sulfite liquor.

FIGURE 2 is a schematic drawing of a conversion tower included in the system shown in FIGURE 1.

In accordance with the invention, spent liquor from a sodium base, sulfite pulping process is concentrated to form about 55 to about 70 percent solids content in a suitable manner. The concentrated liquor is then burned under reducing conditions so as to limit the removal of sulfur as much as possible. In this connection, a major portion of the sulfur should be retained in the resulting smelt. The smelt formed by burning primarily comprises sodium sulfide and sodium carbonate. The smelt is then dissolved to form an aqueous solution of the sodium sulfide and sodium carbonate.

The sodium sulfide and sodium carbonate are then converted to the desired sodium sulfide. This conversion is effected by direct sulfitation of the aqueous smelt solution with sodium bisulfite. The reaction occurring during the sulfitation may be represented by the following equations:

Thus, not only is the sodium sulfide converted to sodium sulfite, but so also is sodium carbonate.

Since an equilibrium exists between sodium bisulfite and sulfur dioxide-sodium sulfite and since hydrogen sulfide reduces sulfur dioxide, a condition exists which results in the formation of sodium thiosulfate. This may be represented by the following equations:

Accordingly, it appears that the preseuce of hydrogen Sincesulfur dioxide is the common sulfitation agent 1 in the conventional commercial process, formation of sodium thiosulfate during direct sulfitation has been generally accepted.

From the foregoing, however, thefo rmation ofthiosulfate depends upon the initial reaction between the hydrogen sulfide and the sulfur dioxide, with the resulting formation of elemental sulfur. Consequently, when the contact bctween'these'two agents is minimized, the

formation of thiosulfate is accordingly minimized; It

has been found that sulfur dioxide'does not react with sodium' sulfide, and will not react with'sodium hydrosulfide to'form thiosulfate.

The smelt from the furnace should have'a sulfidity of between about percent and about 70' percent. The term sulfidity refers to the molar ratio of sodium sulfide in the smelt to the sum of'the sodium sulfide and the sodium carbonate in the smelt; Of course,- this ratio is multiplied by 100 to provide the percent.

'-In addition, the reduction of the smelt must be greater 3 than 85 percent, the percent reduction being the molar ratio of sodium sulfide to sodium sulfide plussodium sulfate, times 100.

'It is within the skill of the art to provide burning conditions in arecovery furnace so as to give a smelt having the desired degree of reduction and-the desired sulfidity.

As has been previously indicated; this invention is based upon the establishment of certain conditions for minimizing the contact'between the hydrogen sulfide and sulfur dioxide, when a smelt solution from spent sulfide 40 liquor is reacted with a sodium bisulfi'desolution so as to provide aliquor of high quality. For purposes of com-' mercialoperation, the gross quality of the liquor should beinexcess' of 85 percent and the net quality should exceed '80'percent. At' lower liquor qualities," the w coveryprocess is not commercially'feasible and the digestion of the pulp is not as satisfactorily carried out;

I Th termfgross'quality refers'to the molar ratio of the amount-of sodium sulfiteto-the-amount of sodium sulfite plus-theamount of sodium thiosulfate-formediu the reaction. The term net quality" refers to the amountofsodiurn sulfite recovered -from-the sodium sulfidein the smelt to the amount of such sodium sulfite plus -'theamount of sodium thiosulfate.- e

The smelt-solution-should be at a boiling temperatureand the contact between thesolution and sodium bisulfiteshould be under conditions which establish "alarge surface area and immediate stripping of any formed-hydrogensulfide gas. In this connection, in a batch operationythe boiling rate of the smeltsolutiofishould exceed 40 grams of vapor per minute'per square foot; of surface; area to 6 obtain 100 percent-conversion'bf-the sodium sulfide in the smelt solution. While 'lower'boiling rates can be-tolerated at-lower degrees of conversion to provide'the desired net quality, i.e. quality in excess of 80 percent, a boiling rate of less than about '15 grams of vapor per minute per squarefoot of surface area'is not satisfactory.

Because of the inherent difficulties in carryingout the batch operation, the preferred method for practicing this invention involvesmixing-of thesmelt solution, at high temperature, for a short period and then spraying the solution into an open chamber. In this connection,'the

e time of contact betwecn the'solutions should'not exceed five seconds, and the time is preferably less-than second. The solution; is; sprayed; as indicated, and stripping steam is flowed through the spray at a rate in excess of 0 per minute per gallon of liquor sprayed.

25 pound moles of steam per pound mole of sodium sulfide in the smelt solution, i.e. the total amount of stripping steam employed should be at least 25 pound moles of steam per pound mole of sodium sulfide in the smelt solution. For the most satisfactory operation, the stripping steam should exceed 40 pound moles of steam per pound mole of sodium sulfide. .In order that the steam can act effectively, the spray should establish a surface area on the drops in. excess of 100 square feet The area of the droplets and the amount of stripping steam should be adjusted so that the hydrogen sulfide in the resulting liquor is retained at a ratio of less than .14 mole per mole of sodium sulfide introduced in spray. form and preferably less than .12 mole per mole of sodium sulfide introduced. If the retained hydrogen sulfide is present in a ratio of less than .12 mole per'mole of sodium sulfide in: the liquor sprayed; and otherconditions are maintained, the net quality will exceed. about: percent.

The concentration of sodium sulfide in the smelt solution should. be no more than about 2 gram moles per liter, themaximum concentration of sodium sulfide being controlled: by the solubility limit of sodium sulfite in the resulting liquor;

Inorderto provider satisfactory continuous operation, themole ratio of the amount of sodium bisulfite to the amount. of sodium sulfite'plus sodium bisulfite in the solutionv whichis mixed with the. smelt solution should not be greater than about .90. The use of higher ratios 0 results in a lower liquor quality. Of course, too low a ratioresults ininefficient operation. 7 At a ratio above .7 excessive sulfur losses occur in condensation of the exhausted gases.

.In order to effect most satisfactory treatment of the 5 smelt: solution, the pH of the aqueous smelt solution Compound: Lbs. Nagsog Naacog; NaHCO 51 N32830:; Na- SO -2. 14

Total 470 accordance with this invention, isthen concentrated.

After concentration, theliquor is a combustible concentrate and will ordinarily contain-aboutbipercent solids. Itisthempassedto-andburned iniafurnacqsuch as a Babcock and Wilcox furnace, to providetheidesired reductionand a smelt having the required sulfidity.

, The'smelt from the-furnace -isdissolved ,inwater in tank 5 t o provide the dilutegaqueoussolution: which :is to be sulfited. This SOllltlOIllS passed;toclarifier IIlfllB clarifier, the solution is separated from 'anyiundis'solved solida; It=is-1then pumped to a storagejtanktlh An analysis {of a: typical smelt solution; or: this. in-

vention, leaving the dissolving mask 5.. isggivembelow. The analysis on the basis of one ton of pulp.

Compound: Lbs. Na s 1'47 NaHS 30 NaOH 22 Ma oo 114 Na SO 14 Total 327 The smelt solution is then pumped to a conversion tower 11 in which the solution is converted to the desired treating liquor. Upon suitable conversion, the liquor is then stored in a tank 13 for use in the digester.

Conversion tower 11 may be of the type shown in FIGURE 2 for continuous operation. The regeneration system of FIGURE 2 may be used in the preparation of either neutral sulfite or acid sulfite liquor.

As shown in FIGURE 1, the smelt solution, or green liquor, is introduced into a pump 15 and is mixed with steam to heat it, whereupon it is introduced into the conversion tower 11. Sodium bisulfite liquor (as previously pointed out, this liquor includes sodium sulfite), which has been heated by steam, is also pumped from a pump 17 into the conversion tower 11. The gases which leave the conversion tower are introduced into a cyclone separator 19, which returns any liquid to the tower 11. The gases leave the cyclone 19 and enter a condenser 20 and the remaining gas, referred to as sour gas, is passed through a fan 21 to a sulfur burner (not shown). A portion of the liquor which has been recovered as cooking liquor leaves the regeneration system 11 through the pump 23, and another portion of the liquor is mixed with the green liquor and introduced into the pump 15. Thus, some of the liquor is recycled.

The conversion tower :11 basically comprises three sections, they being a spray chamber 31, a packed section 33 and a sump 35. The lowermost section is the sump 35. Superimposed thereon is the packed section 33, and above the packed section is the spray chamber 31. The packed section 33 may be filled with any of several commercially available packing materials, such as partition rings, spiral tiles, Raschig rings, Berl saddles, wire-mesh packings, or other suitable types of conventional absorption tower packing material.

The smelt solution enters conversion tower 11 through pipe 37. The sodium bisulfite solution enters the conversion tower '11 through pipe 39. The

pipes

37 and 39 each connect to nozzles 41 which spray the mixed liquor and bisulfite solution into the spray chamber. The sprayed liquor then passes through the packed section 33 to the sump 35. Y

The tower is operated at the boiling point of the combined spray, that is, at least about 212 F. to 214 F. under atmospheric pressure conditions. Preferably, suflicient extra heat is continuously added in the liquor and thereby to the spray to provide an increased boiling rate.

During passage of the boiling spray down through the packing, it countercurrently contacts steam which has entered the sump 35 of the conversion tower 11. Steam entering through line 40 passes up through the packed section 33 of the tower 11. The steam is at a temperature which is a function of the pressure conditions in the tower and the steam aids in stripping hydrogen sulfide formed during the reaction between the sodium'bisulfite and the sodium sulfide in the spray. The steam has the additional eifect of aiding in maintaining the downfiowing spray solution at the desired temperature.

Hydrogen sulfide and other efiiuent gasses pass from the tower through stack 43 after flowing around the baffle 45. Battle 45 is provided so as to substantially eliminate exiting of inflowing bisulfite solution spray and smelt solution spray before their passage down through the packed section to the sump 35.

When the liquor reaches the sump 35, it passes from the tower 11 through pipes 47 and 49, pipe 47 conducting 6 the liquor to the storage tank 13 and pipe 49 conducting the liquor to the pump 15 for recycling.

It will be understood that the tower shown in FIGURE 2 may be operated on a batch basis by filling the sump 35 melt solution.

EXAMPLE I An eight inch stainless-steel conversion tower of the type shown in FIGURE 2 was operated on a batch basis in accordance with the conditions set forth in Table I below.

Table I BATCH BISULFITE PROCESS [Spray method] Run. I 11 Smelt Solution:

olume, litels 7. 0 5.0

N82320:, g. mo1es 0.035 0 040 Nflgsog, g. moles..- 0. 042 0.040

N aIS, g. moles 0. 588 2. 323

NazOO g. moles 0.721 2 315 Bisulfite solution:

Volume, liters 12. 2 6. 1

N a180,, g. moles 0. 037 2.075

NaHS0 g. molt- 2. 365 9. 250 Reaction solution:

Reacting time, min 120 80 Volume, liters 35. 7 15. 2

N M810 g. moles 0.071 0. 639

Na1SO3, g. moles 2. 538 10. 60

NaHOO;, g. mo1es 0.350 Recycling rate, gal/min 1 1 Nags conversion, mole percent 100 Na CO; conversion mole percent. 100 92. 5 Gross quality, mole percent 98. 6 93. 5 Net quality, mole percent 96. 5 87. 8

1 Estimated volume based on sodium balance.

sary to achieve minimal boiling of the combined solution.

EXAMPLE II In a commercial unit, the smelt includes the following material in the specified amounts:

Moles per ton Sodium sulfide 2.90

Sodium sulfate .35

Sodium carbonate 1.78

About 488 pounds of this smelt is introduced into the dissolving tank 5 at about 1800 F. In clarification, about one percent of the chemicals is lost. Water is added, about 4940 pounds being added to the smelt at F. The clarified green-liquor is discharged at about 194 F. and conducted to the conversion tower 11 which, in this case, is operated under atmospheric pressure. The flow rate is 46 gallons per minute.

The spray nozzle pressure in the conversion tower is 50 psi. and the smelt solution is heated to a temperature of 250 F. The cooking liquor is recycled from the sump 35 so that the recycled liquor has a ratio to the entering green liquor of 3 to 1, by volume.

Bisulfite solution is introduced at a ratio of bisulfite to sodium sulfite of 7 to 3, on a molar basis. The bisulfite liquor is at a temperature of 205 F. at the pulp 17. The liquor has the following composition:

Steam is introduced through pipe 40 at the rate of Exit liquor Bisul Green 'fite Liquor Liquor for for for Recy- Bisul- Cookcling tite ing Naisleonversionjmole percent. N810 O; reacted, moles/ton. NazG conversion, mole percent; Liquor quality, mole percent NmSO; formed, moles/ton.. NfizSgOz formed, moles/ton" Gross quality, moles percent NmSO; formed trom Nags,

moles/ton Net Quality, mole percent-.-

.The overall data on the operation. of .the conversion.

tower 11 for a commercial operation is set forth below:

Lb./ton P l Green liquor 5, 790 Bisulfite liquor (205 F. 13, 240

N S0 485. 935. 193. 99. 11,530. 7:3 by mole. 1,485 gaL/ton or 103 gaL/min. 17, 680

39. Water 15,830. Recycling-green liquor ratio--. 3:1 by volume. Flow 1,985 gaL/tou or r 138 gaL/mm 7 Steam at atmospheric pressure M 2, 600

V Total input 39, 310 Exit liquor (215 F.) 36,830

NBQSO:

Spray chamber dimeusions.. ..14 it. D. 4 it.

7 Packed section 4.5 it. D. x 10 it.

6ft.'D.x5ft. Sour gas exit. l0-in. Packing 2-111. glazed porcelain Rasehig rings;

Steam rate, packed section. 280 lb./ (hr.) (sq. ft). Liquor rate, spray chamber. 1,000 lb./(hr.) (sq. ft.). Liquor rate, .packedsection- -..10,000. lb./(hr.). (sq. in). Spray rate 350,000 sq. ttJmln.

varied within the specified limits, in relation to one another, to providethe desired results of thisinvent'ion. While the ep'e'rsfia eenaifians can "he vanes; conditions appear to be' at aisstannal importance. In

1 this connectionfthe reads-embarrass the bisulfitev solu- $51 9... a cient sodium bisulfitc to react with substantially all of merit of. the high boiling conditions, 10'

tion and the -snie'lt selfitien'snsnddeenrnnaernign boiling rate conditions Thetiiiie of es-hue: between bisulfite solution and the smelt solution should be minimized which, bf'dufs, "is"pi'oniotedby the establish- To aid in minimizing this contact time, the spray area, i.e. the droplets discharged trom the nozzles, is preferably increased as much as possible and stripping steam is employed over this area in a particular amount. The concentration of sodium sulfide should be maintained at a relatively low level in the smelt solution and the concentration of sodium ion should be also maintained at a comparatively low level. If the proper smelt has been provided, a liquor having high net quality and high gross quality can be provided. Of course, the pH of the solutions should be above 7.0 and the pH of the initial smelt solution is desirably above 11.0. 7

As before indicated, the present invention may be practiced by utilizing spent liquors from either the sodiurn base, neutralsulfite pulping process orthe sodium base acid sulfi-te pulping process. In the vent that the batch or continuous method of recovery is utilized with spent liquor from the acid sulfite pulping process, the aqueous liquor recovered from the conversion tower 11 may be subjectedto a further treatment with sulfur dioxide to increase its acidity. This may be done in another conversion tower by furt-her sulfitation with sulfur dioxide gas. a

In the foregoing discussion, continuous and batch processes have been described for recovering spent liquors from neutral sulfite and acid sulfite pulping processes in an easy and economical manner. The processes of this invention are independent of other processes.

It will be evident that various modifications in the procedural steps and/or equipment for carrying them out canbe made inthe processes described without departing firom the scope of the present invention.

The various features of this invention whichare believed to be new are set forth in the following claims.

We claim:

1. In "a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the'pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 30 percent and about percent, the reduction of said smelt being in excess of 8.0 percent, dissolving said smclt to provide an aqueous solution and boiling said solution having a sulfidity between about 30percent and about 70 percent at a rate in excess of .15 grams of vapor per minute per square foot of surface area during addition of sodium bisulfite solution to said aqueous solution in an amount sufiicient to provide suificient sodiurn bisulfite to react with substantially all of the sodium sulfide in ,said aqueous solution, whereby direct-sulfitation occurs, and rapidly removing'the hydrogen sulfide from the resulting boilingsolution, whereby formation of sodium thiosulfate and associated substances is substantially avoided. f

2. In a sodium base, sulfite pulpingv process, the. improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 45 percent and about 7 0 percent, the reduction of said smelt being in excess of percent, dissolving .said sineltto provide aqueous solution and boiling saidsolution having a sulfidityof between about 45 percent and about 70 percent ata rate in excess of 40 grams of vapor per minute per square foot of; surface a t es. s iti ois s i 'bi u fi u i n tgsaid olution in a amount suflicient to providesuflithe sodium sulfide in said aqueous solution, whereby direct sulfitation occurs, and rapidly removing the hydro gen sulfide from the resulting boiling solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

3. in a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 30 percent and about 70 percent, the reduction of said smelt being in excess of 80 percent, dissolving said smelt to provide an aqueous solution and mixing said aqueous solution having a sulfidity of be tween about 30 percent and about 70 percent with a sodium bisulfite solution in an amount sufiicient to provide sufiicient sodium bisulfite to react with substantially all of the sodium sulfide in said aqueous solution for a total contact time of less than 5 seconds at a boiling temperature, whereby direct sulfitation occurs, spraying the mixed solution, and passing stripping gas through the sprayed mixed solution, the amount of said gas exceeding 25 pound moles per pound mole of sodium sulfide in said aqueous solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

4. In a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 30 percent and about 70 percent, the reduction of said smelt being in excess of 85 percent, dissolving said smelt to provide an aqueous solution having a sulfidity of between about 30 percent and about 7 percent, the concentration of sodium siulfide in said smelt solution being less than about 2.0 gram moles per liter and mixing said aqueous solution with a sodium bisulfite solution in an amount sufiicient to provide sufficient sodium bisulfite to react with substantially all of the sodium sulfide in said aqueous solution for a total contact time of less than .1 second at a boiling temperature, whereby direct sulfitation occurs, spraying the mixed solution, and passing stripping steam through the sprayed solution, the amount of steam exceeding 40 pound moles per pound mole of sodium sulfide in said aqueous solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

5. In a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 45 percent and about 70 percent, the reduction of said smelt being in excess of 85 percent, dissolving said smelt to provide an aqueous solution having a sulfidity of between about 45 percent and about 70 percent and mixing said aqueous solution with a sodium bisulfite solution in an amount suflicient to provide suflicient sodium bisulfite to react with substantially all of the sodium sulfide in said aqueous solution for a total contact time of less than seconds at a boiling temperature, whereby direct sulfitation occurs, spraying the mixed solution, said spray establishing a surface area at a rate in excess of 100 square feet per minute per gallon of solution sprayed, and passing stripping steam through the sprayed solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

6. In a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the con centrated liquor to provide a smelt having a sulfidity of between about 45 percent and about percent, the reduction of said smelt being in excess of percent, dissolving said smelt to provide an aqueous solution having a sulfidity of between about 45 percent and about 70 per cent, the concentration of sodium sulfide in said smelt solution being less than about 2.0 gram moles per liter and mixing said aqueous solution with a sodium bisulfite solution in an amount sufficient to provide sufiicient sodium bisulfite to react with substantially all of the sodium sulfide in said aqueous solution for a total contact time of less than 5 seconds at a boiling temperature, whereby direct sulfitation occurs, said bisulfite solution including sodium sulfite, the molar ratio of sodium bisulfite to so dium siulfiite plus sodium bisulfite being less than .9, spraying the mixed solution, and passing stripping steam through the sprayed solution, the amount of steam exceeding 40 pound moles per pound mole of sodium sulfide in said aqueous solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

In a sodium base, sulfite pulping process, the improvement which comprises the steps of concentrating the spent liquor from the pulping process, burning the concentrated liquor to provide a smelt having a sulfidity of between about 45 percent and about 70 percent, the reduction of said smelt being in excess of 85 percent, dissolving said smelt to provide an aqueous solution having a sulfidity of between about 45 percent and about 70 percent and having a pH in excess of 11.0 and mixing said aqueous solution with a sodium bisulfite solution in an amount suflicient to provide sufficient sodium bisulfite to react with substantially all of the sodium sulfide in said aqueous solution for a total contact time of less than .1 second at a boiling temperature, whereby direct sulfitation occurs, spraying the mixed solution, and passing stripping steam through the sprayed solution, the amount of steam exceeding 40 pound moles per pound mole of sodium sulfide in said aqueous solution, whereby formation of sodium thiosulfate and associated substances is substantially avoided.

References Cited in the file of this patent UNITED STATES PATENTS 2,800,388 Ahlborg July 23, 1957 2,802,791 Whitney Aug. 13, 1957 2,864,669 Ahlborg Dec. 16, 1958 FOREIGN PATENTS 534,769 Canada Dec. 25, 1956 OTHER REFERENCES Sodium Base Sulfite Recovery as Related to the Semichemical Process; article by T. T. Collins et al.; Southern Pulp and Paper Manufacturer (Journal); January 1956, Atlanta, Ga., 12 pp.; pp. 10-12 relied on.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No; 2 987 432 June 6 1961 Roy Po Whitney et ale It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2. line 41 for "form" read from ====g column 4 line 74 for mask read tank ==g column 6 Table I line 11 thereof for 'Reaction" read Reacted column 8 line 25 for "vent read event column 9 line 33 for "'siulfide" read sulfide column 10 line 2O for "siulfiite" read sulfite Signed and sealed this 24th day of October 19610 SEAL) Lttest:

ERNEST W. SWIDER Ittesting Officer DAVID L. LADD Commissioner of Patents USCOMM-DC

Claims

1. IN A SODIUM BASE, SULFITE PULPING PROCESS, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF CONCENTRATING THE SPENT LIQUOR FROM THE PULPING PROCESS, BURNING THE CONCENTRATED LIQUOR TO PROVIDE A SMELT HAVING A SULFIDITY OF BETWEEN ABOUT 30 PERCENT AND ABOUT 70 PERCENT, THE REDUCTION OF SAID SMELT BEING IN EXCESS OF 80 PERCENT, DISSOLVING SAID SMELT TO PROVIDE AN AQUEOUS SOLUTION AND BOILING SAID SOLUTION HAVING A SULFIDITY BETWEEN ABOUT 30 PERCENT AND ABOUT 70 PERCENT AT A RATE IN EXCESS OF 15 GRAMS OF VAPOR PER MINUTE PER SQUARE FOOT OF SURFACE AREA DURING ADDITION OF SODIUM BISULFITE SOLUTION TO SAID AQUEOUS SOLUTION IN AN AMOUNT SUFFICIENT TO PROVIDE SUFFICIENT SODIUM BISULFITE TO REACT WITH SUBSTANTIALLY ALL OF THE SODIUM SULFIDE IN SAID AQUEOUS SOLUTION, WHEREBY DIRECT SULFITATION OCCURS, AND RAPIDLY REMOVING THE HYDROGEN SULFIDE FROM THE RESULTING BOILING SOLUTION, WHEREBY FORMATION OF SODIUM THIOSULFATE AND ASSOCIATED SUBSTANCES IS SUBSTANTIALLY AVOIDED.