US3248169A

US3248169A - Method of regenerating sodium sulfite waste liquors

Info

Publication number: US3248169A
Authority: US
Publication date: 1966-04-26
Anticipated expiration: 1983-04-26

Description

United States Patent 3,248,169 METHQD L REGENERZZ'ATING SGDIUM SULFITE WASTE LIQUORS Nils Viktor Mannhro, Box 349, ll iarrnaverlren, weden No Drawing. Filed Aug. 12, 1957, Ser. No. 677,755 Claims priority, application Sweden, Aug. 15, 1956, 7,546/56 13 @lairns. (Cl. 23-48) F or the manufacture of sulfite cellulose pulp from cellulosic materials, a number of pulping processes utilizing sodium sulfite cooking liquors have been proposed. Mainly it has been a question of an equivalent of the conventional sulfite pulping with calcium sulfite cooking liquor in such a manner that the calcium base has been replaced by sodium base. Due to the higher solubility of the sodium sulfites, a greater freedom of choice as regards the composition of the cooking acid has been obtainable and in addition to cooking acids comprising NaI-ISO and S0 it has been possible with advantage to use only NaHSO or Na SO Furthermore pulping to a high yield or semichemical pulp has been carried out with such cooking liquors or pulping has been carried out as a neutral sulfite pulping with cooking liquors comprising Na SO and Na CO o1 NaHCO Advantageously, the pulping has been carried out in several stages.

Pulping with sodium suffite cooking liquors enhances the yield as Well as the quality of the pulp. Compared, e.g., to calcium sulfite pulps, the sodium sulfite pulps are more readily bleached. In spite of these advantages the sodium sulfite pulping has not attained widespread use. This is due to the fact that the economically necessary regeneration of the spent cooking liquor chemicals has proved to be too complicated. Only in some mills, neutral sulfite pulping t-o semichemical pulp has achieved a more extensive use, since under these special circumstances it has been considered possible to let the relatively small amount of cooking chemicals go to waste.

In recent years, several sodium sulfite liquor regeneration processes have been described in the technical literature. Most of them involve combustion of the sodium sulfite spent liquor in liquor combustion furnaces or socalled soda house units in a manner similar to the treatment of the black liquor of the sulfate pulping process. In the combustion of the sodium sulfite liquor there is obtained a soda smelt which consists mainly of Na CO Na S and smaller amounts of other sodium compounds such as Na SO Na sO and Na S O It is usually suggested that a solution of the soda smelt be carbonated as a Whole with CO or cO -containing gases. Thereby, sodium sulfide (Na s) is converted into hydrogen sulfide and sodium bicarbonate (NaHCO By burning the hydrogen sulfide to sulfur dioxide and reacting it with sodium carbonate, sodium sulfite is obtained. This quantity of sodium sulfide, however, is only about half of the total amount of sulfur supplied to the soda house unit with the sulfite spent liquor. The remainder of the sulfur leaves with the combustion gases, mainly as sulfur dioxide. Recovery of the sulfur dioxide content of the combustion gases, therefore, would be important to the economy of the process. A circumstance preventing a complete sulfur recovery is the fact that sodium sulfide and sulfur dioxide or bisulfite together form sodium thiosulfate (Na S O which may become enriched to the point where the pulping is jeopardized or rendered wholly impossible. This will be the case, if carbonation is not complete and carbonated solution is used to scrub out sulfur dioxide from the flue gases. Complicated systems are described according to which carbonation is rendered complete by using carbon dioxide gas produced in the process. These processes also involve driving off hydrogen sulfide gas 3,248,169 Patented Apr. 26, 1966 from the carbonation as so concentrated a gas that it can be burnt to sulfur dioxide in a special furnace. These processes involving the handling of carbon dioxide and hydrogen sulfide gases necessitate a complex equipment.

In other processes, it has been suggested to allow sodium carbonate to crystallize out of the solution of soda smelt and then oxidize sodium sulfide to sodium sulfite. In one process, a slurry of concentrated sodium sulfide solution and sodium carbonate crystals is oxidized with air under specific conditions which require a meticulous control. The reaction product comprising mainly sodium sulfite and sodium carbonate is dissolved and utilized to scrub out sulfur dioxide from the liquor combustion gases. A similar crystallization of sodium carbonate in combination with carbonation has also been suggested in connection with a combination of the sodium sulfite and sulfate processes (N. Mannbro, Svensk Papperstidning, 5 8, 1956, 525,571). According to a recent publication (R. A. Nugent and R. Q. Boyer, Paper Trade Journal, 140, 1956, Nos. 18 and 19) this crystallization of sodium carbonate with simultaneous addition of a soda smelt comprising Na S and Na CO requires a special procedure. It is possible to crystallize out a pure soda from a solution of molten soda of normal concentration, if the solution is cooled under certain specified conditions. This process, however, is not economically attractive, wherefore it has been attempted to crystallize out the soda from a solution saturated in Na CO by adding further quantities of Na S and Na CO as a soda smelt. However, the soda smelt solidifies in the solution to lumps and Na CO cannot be recovered without entraining too much sodium sulfide. According to the last-cited publication, this problem has been solved by atomizing the soda smelt, before dissolution thereof, by means of a steam jet similar to the production of mineral wool. The atomized soda smelt then forms sodium carbonate crystals which after suitable separation may be washed entirely or substantially free of sodium sulfide in a centrifugal apparatus or on a filter. In accordance with a process developed in this connection, the Na s-enriched mother liquor is admixed with sulfite liquor recovered from the digesters. Thereby, the waste liquor is made alkaline and the Na CO added precipitates calcium derived inter alia from wood and water as calcium carbonate which is removed. The alkalized sulfite liquor is oxidized by passing air therethrough, whereby the sulfide sulfur added is converted into Na S O Thereby, release of sulfidic sulfur as hydrogen sulfide during evaporation is prevented. In the liquor combustion the sulfur of the sulfite liquor is reduced so that approximately one-half thereof is removed from the soda house unit as sodium sulfide in the soda smelt. Furthermore, about one-half of the sulfur leaves with the combustion gases as sulfur dioxide and is recovered by scrubbing the flue gases with a solution prepared from the crystallized soda, The same process has also been modified so that the soda is crystallized by evaporating the solution. The evaporation equipment used, which is of the salt evaporator type, may be connected, with respect to the vapor driven off from the solution, to the sulfite liquor evaporation system. This embodiment may therefore be found more favorable than the crystallization process above described.

The present invention involves the advantage that the sulfidic sulfur is not subjected to oxidation to sodium thiosulfate before introduction into the liquor combustion furnace. Apart from the considerable simplification that the oxidation equipment can be omitted, this has the advantage that the sulfur balance is improved. This is explained in the following manner. In the combustion of the liquor, there arises in the resultant smelt an equilibrium which may be determined by thermodynamic principles of calculation. In this case it is desirable that the smelt contains more Na CO and less Na S and other sulfur compounds and the flue gas as much S as possible. If the sulfidic sulfur is oxidized to thiosulfate prior to the liquor combustion, this sulfur will be bound to a greater extent to the soda smelt and once more leave the liquor combustion furnace in the form of Na s. The greater the amounts of sulfur compounds circulated within the system of liquor combustion-soda smelt-sulfide oxidationevaporation and back to the liquor combustion, the less satisfactory are the results achieved both as regards cheruical and heat economy. If the sulfidic sulfur as such is directly burnt to S0 this undesirable reconversion into Na s is prevented in the part of the liquor combustion furnace where a reducing combustion takes place. In a liquor combustion furnace it is possible schematically to distinguish a zone where an oxidizing combustion takes place and where the liquor is carbonated, dried and drydistilled to a material which is transferred to a zone for reducing combustion. is determined by the material supplied to the reduction zone and the conditions of combustion therein. After hydrogen sulfide has been released, it is burnt to S0 under more favorable conditions than those prev-ailing in the reduction zone. From the flue gas, sulfur dioxide is recovered by known methods which involve separation of sodium carbonate or sodium bicarbonate from the soda smelt or a solution thereof and the use of such sodium carbonate or bicarbonate for scrubbing the flue gas.

A simple manner of converting the sulfidic sulfur into sulfur dioxide comprises admixing the Na S-enriched mother liquor with the evaporated liquor passed to the liquor combustion furnace. In order not to burden the sulfite liquor regeneration with the additional quantity of Water present in said mother liquor, it may be of advantage to dissolve the soda smelt in sulfite liquor of a suitable concentration. In order that is bisulfite content shall not react with the sulfide to form inter alia thiosulfate this sulfite liquor may initially be neutralized. Such neutralization may be made with soda recovered in the process, and in this connection it may be advisable also to wash the Na CO crystals free of mother liquor with such sulfite liquor. It was found that crystallization of Na CO is readily achieved by the use of this process. If the temperature conditions are suitable, a certain amount of water may be removed as water of crystallization with the soda crystals.

To minimize the quantities of sodium and water carried to the liquor combustion with the sulfidic sulfur, this may to a greater part or to the desired extent be converted into hydrogen sulfide. This may be done in a manner known per se by the use of ion exchangers so that the liquor is acidified thereby. However, it appears more simple to carbonize the Na S-enriched mother liquor with CO or CU -containing gases, most preferably with combustion gas from the liquor combustion. In this case, combustion gas should first be purified to remove sulfur dioxide. The procedure herein described may be carried out in a simple manner Without the need of a special combustion furnace for H 3. H 5 may be burnt at a low concentration and Without hazardous handling, nor will gasometers or the like be required. Since only a minor part of the combustion gas is required therefor, the liquor combustion will not be interfered with, if hydrogen sulfide is driven off in admixture with flue gas which is returned to the furnace Where combustion to sulfur dioxide takes place. carbonation will not need to be made complete in this process, nor will any partial oxidation of the sulfide by the oxygen of the flue gas to thiosulfate have any disturbing effect. This is due to the fact that tie residual solution may be renewed progressively to the extent desired and a continuous conversion of it and its content of indifferent sodium compounds may take place by admixture thereof to the sulfite liquor before its combustion.

The composition of the soda smelt ssures In this manner, sodium will be recovered as crystals of sodium carbonate and/or sodium bicarbonate. By keeping the amount of sodium compounds circulating through the liquor combustion furnace at a minimum, but maintaining the S compound, a relatively greater amount of S compounds may be removed with the flue gas as sulfur dioxide to be made use of. Methods have also been developed to recover hydrogen sulfide in a concentrated form in spite of the use of combustion gases for carbonation (Swedish Patent No. 151,066). Hydrogen sulfide recovered by such a method may be burnt separately to sulfur dioxide and in accordance with the present invention be absorbed together with sulfur dioxide in the liquor combustion gases or at a laterstage in a subsequent separate absorption tower.

From the sodium carbonate which has been crystallized out, a soda solution of suitable concentration is prepared. For this purpose, water or suitably a fraction of the dilute sulfite liquor obtained in the washing of the sulfite pulp may be used as solvent, since recycling of spent sulphite liquor to the digestcr can be carried out with satisfactory results in sodium bisulphite pulping in contrast to calcium bisulfite pulping. The latter procedure, unless pulping is carried out at an extremely low temperature, results in black cooks after a number of recycling steps. Thus, in a sodium sulfite process the solids content of the waste liquor can be increased in this manner, which is of a great importance as regards heat economy. It has also been found that the sulfite liquor content in the absorption liquor acts an an inhibitor and prevents oxidation of sulfite, e.g., to sulfate.

Depending inter alia on the temperature conditions, sulfur trioxide will also be present in the flue gas in a state of equilibrium with sulfur dioxide. In the sulfur dioxide scrubber, sulfate will therefore be formed and Will be carried over to the cooking liquor. Sulfate may be permitted to be present in the cooking liquor, but if it is considered that it is more useful to have the corresponding amount of chemicals as sulfite, it is advisable first to absorb sulfur trioxide from the flue gas. Simultaneously the flue gas is cooled rapidly to 'a range of temperature where the formation of sulfur trioxide is insignificant. By recirculating scrubber liquid, it Will be acidified with sulphuric acid and absorb sulfur trioxide but not sulfur dioxide. The acidic scrubber liquid recovered can be passed to the unevaporated sulfite liquor so that ultimately the sulfate sulfur is reduced to sulfide sulfur in the liquor combustion furnace. By heat exchange of the scrubber liquid, the heat content of the flue gas may be utilized to heat water or to preheat sulfite cooking liquor. Equipment similar to that used at present in some sulfate mills may be used. As the scrubber liquid, sulfite liquor of a suitable concentration may be used in order not to burden the process unnecessarily with added Water.

In a practically closed recovery process, an accumulation of chlorides supplied with the wood, the Water or the chemicals may cause corrosion. By scrubbing the flue gases with an aqueous solution of sulfuric acid and bydrochloric acid recirculating to the desired extent, absorption of sulfur dioxide in the scrubber liquid will be prevented to such an extent that the part of the scrubber liquid that must be removed from the process will contain little sulfur dioxide. To be able to heat-exchange the scrubber liquid without undue corrosion it is necessary to keep the concentration of chlorides below a certain level. In this connection, one might study the possibility of supplying to this sodium sulfite liquor regeneration process suitable alkaline spent liquors obtained by bleaching or refining pulp. By fractional recovery of such alkaline spent liquors from a bleaching plant, it would be possible to discharge the alkaline spent liquor containing the greatest proportion of chlorine compounds separately as a waste product and to utilize the remainder. Further, the possibility may be mentioned of carrying out alkaline treatments of pulp in connection with bleaching or other refining with a suitable soda solution obtained from this process. In some methods of preparing chlorine dioxide for bleaching pulp, sulphur dioxide is employed, which can be recovered from this process, and it may also be possible to utilize the scrubber liquid in chlorine dioxide production or acidification of pulp. A combination of such processes in the opposite direction can also be suggested. Thus, in some methods of producing chlorine dioxide, products are formed which may be utilized in a sodium sulfite liquor regeneration process. Examples are acidic sodium sulfate solutions. In the production of chlorine dioxide, it has moreover been found suitable to add sodium sulfite liquor to the reaction mixture to increase the yield of chlorine dioxide.

The liquor combustion gases also carry with them an alkali dust consisting mainly of sodium sulfate. This is usually recovered with the aid of a dust separator, e.g., an electrostatic filter prior to scrubbing. But scrubbing to recover sulfur trioxide may also be carried out in a known apparatus in such a manner that sulfur trioxide and alkali dust are recovered together from the flue dust. The simplest form of this part of the process consists in passing the combustion gases directly from an electrostatic filter to the absorption device where sulfur dioxide is absorbed in soda solution. In this case, the temperature of the gas will influence the conditions of absorption. The usual procedure in the sulfate process consisting in cooling the combustion gases with simultaneous heating of air for the liquor combustion, would be applicable.

The crystallized soda is dissolved in the manner set forth above. When the soda crystals are separated from the mother liquor, some impurities derived from the soda smelt will accompany them, e.g., carbon particles, iron sulfide, calcium carbonate, etc. These impurities are caused to settle from the soda liquor and/ or are removed by centrifuging or filtration. For this purpose, it is suitable to employ the equipment used in the sulfate process for purification of white liquor. To prevent losses of chemicals in the removal of the sludge, this may be washed in connection with centrifuging or the like. The Washing liquid may be returned to the process.

The crystallized soda will contain a minor quantity of sulfide. The tower for absorption of sulfur dioxide should therefore be constructed so that carbon dioxide in the combustion gas passing through the solution is permitted by carbonation of the solution to displace a major portion of this sulfide sulfur as hydrogen sulfide. Since the soda contains only about a few percent of sodium sulfide and the removal of sulfide may easily be rendered practically complete (about 90%), the quantity of sodium thiosulfate formed will be insignificant. It will probably be found unnecessary to contemplate recovery of the small amount of sulfide sulfur driven off as hydrogen sulfide. However, the possibility exists of carrying out such a carbonation in a special tower, using only a part of the SO -free combustion gas, and returning hydrogen sulfide to the liquor combustion furnace or to a scrubber with bisulfite-containing sulfite liquor.

The composition of the sulfite solution obtained from the absorption of sulfur dioxide depends on the composition of the sulfite liquor subjected to combustion which in its turn depends on the pulping method used. In normal sodium bisulfite pulping, additional amounts of sulfur dioxide must be added to obtain a sodium sulfite cooking acid. Since the loss of sulfur dioxide in such a process is relatively greater than that of sodium, it is necessary, even if the losses in chemicals were to be convered with sodium sulfate or sodium sulfite instead of sodium carbonate or sodium hydroxide, to introduce additional sulfur dioxide in the process. Usually, pyrite or sulfur will be burnt for this purpose. The sulfur dioxide gas obtained is absorbed in the sulfite solution first obtained to prepare a so-called crude acid. The crude acid is introduced into the system where cooking acid is .or sodium bisulfite and sulfur dioxide.

prepared by utilizing sulfur dioxide gas and heat derived from the digesters. In order not to introduce carbon dioxide into this system, care must be taken that a complete decarbonation of the solutions introduced has taken place. Such a decarbonation can be achieved with a bisulfite solution, whereby also concentrated carbon dioxide gas is developed.

If it is desired to make use of the heat of the flue gases to the greatest extent possible, the sulfur dioxide absorption in warm sulfite solution or crude acid may be conducted under pressure according to known processes.

The spent liquor obtained from the pulping step will contain minor amounts of calcium derived from the wood and the water, which together with the sulfate content of waste liquor may cause formation of incrustat-ions or scale during evaporation of the waste liquor. Evaporation equipment has been developed, which overcomes the incrustation problem in the evaporation of conventional calcium sulfite waste liquor, and such equipment will be suitable, if it is desired to evaporate the acidic waste liquor and possibly make use of the sulfur dioxide containing condensate to prepare fresh cooking liquor. For other reasons it may be advantageous to evaporate a neutralized 0r alkalized waste liquor. If for this purpose a solution derived from the process and containing carbonate or crystallized soda is added to the spent liquor, the calcium content of the spent liquor may be precipitated as calcium carbonate and/or calcium sulfite prior to evaporation. Such a neutralization may also take place in connection with fermentation processes. It was mentioned above, that sodium sulfite pulping may be carried out in one or more stages. In the regeneration process herein described, sodium sulfite, sodium bis'ulfite, sodium carbonate and sodium bicarbonate may be recovered. Furthermore, concentrated sulfur dioxide may be recovered from the systems used in the present sulfite mills for gassing sodium sulfite to sodium bisulfite In sulfite pulping processes, where sulfur dioxide gas is present in the digesters or in the cooking acid tanks, this sulfur dioxide may be withdrawn and if desired be liquefied to form liquid sulfur dioxide which may be injected into the digester charge, as desired. 7

In such processes, the cellulosic material is first sulfonated with a cooking liquor containing sodium sulfite or sodium bisulfite, whereupon the cooking liquor is drawn off and in a subsequent hydrolysis stage is replaced by a cooking liquor containing sulfur dioxide or sodium bisulfite and sulfur dioxide. An injection of sulfur dioxide gas in the hydrolysis stage is also used. Such multistage pulping processes are used, e.g., for pulping barkdamaged spruce Wood and for pulping pine wood.

Among other pulping processes, the process may be mentioned in which the wood is first sulfonated with a solution of sodium sulfite or sodium bisulfite, to which solution, sometimes after withdrawal of some cooking liquior, sulfur dioxide is added by injection into the digester charge. After the wood has been subjected to further pulping to the extent desired and if desired a part of the sulfur dioxide has been removed, a solution of soda smelt or a soda solution is injected into the digester charge. Pulps suitable for the preparation of cellulose derivatives may easily be prepared in this manner, or the lightness, bleachability and yield of sulfite pulpmay be combined with the strength and low content of extractives of sulfate pulp. In accordance with the regeneration process herein described, all the chemicals required for such a process are available, and as an example, the last pulping stage may be effected with a solution of soda smelt obtained by dissolving the soda smelt in two separate dissolving vessels or with a portion of the sulfide-containing mother liquor from the crystallization system. It is also to be noted that for pulping and bleaching processes it is possible to prepare sodium hydroxide by causticizing a solution of soda smelt or carbonate.

A single regeneration process may be utilized simultaneously for several pulping processes. Thus, it has been suggested to prepare cooking liquor for neutral sulfite or so-called NSSC pulping from a sulfite spent liquor obtained from a normal sodium bisulfite pulping process (N. Mannbro, Svensk Papperstidning, 55, 1952, 665). Otherwise, the NSSC method has the disadvantage that since the pulp is produced in a high yield (semichemical pulp), the amount of combustible substances in the spent liquor is so low that difliculties arise, if the chemicals are to be regenerated by combustion of the spent liquor. According to this process, it is possible for example to produce at the same time a common sulfite pulp from spruce and NSSC pulp from birch or wood waste. NSSC pulp has obtained a very widespread use for manufacturing wrapping material. It may also be bleached and used as a substitute for pulps produced by normal pulping processes.

Besides the woods above referred to, a great number of hardwoods and softwoods as well as other cellulosic materials, such as straw, bamboo, etc., may be utilized in these pulping methods.

By known methods, by-products can be recovered from the sodium sulfite liquor. Examples of such methods include various fermentation processes, methods of recovering lignosulfonic acid products, e.g., with the aid of ion exchanges and for the production of vanillin or furfural. In some cases, one or more of the chemicals available in the regeneration process herein described may be employed in such methods or conversely chemicals obtained in the by-product recovery may be utilized in this process.

The invention is illustrated by the following example.

Example With the sulfite spent liquor to the recovery furnace 1162 kilograms of solids were supplied per metric ton of pulp. Besides water and wood substance dissolved during digestion, this sulfite liquor has the following composition:

Kilogram-mols Na O 2.84 S as sulfite and lignosulfonic acids 2.04 Na S 1.11 Na SO 0.30

The amount of Na S indicated consists of Na S separated from the soda smelt from the furnace and returned to the sulfite liquor according to this invention.

Due to the fact that this Na S has been added to the sulfite liquor without oxidation to Na S O this sulfidic sulfur is driven off in the CO -container atmosphere in the furnace to about 90% as hydrogen sulfide which is burnt to sulfur dioxide which is mixed with the combustion gases. Thereby, 1.0 kilogram-mol of S is formed.

In the carbonizatio-n of the liquor, additionally 1.02 kilogram-mols of S0 are driven off from sulfite and lignosulfonic acids. Together with the 80; formed according to the thermodynamically established reaction equilibrium in the soda smelt, totally 2.11 kilogrammols of S0 leave with the combustion gas.

The combustion gas has the following composition:

Percent S0 0 65 CO 11.2 H O 23.6 N 62.0 0 2.6

After passage through a heat recovery and dust separation system, the combustion gas is passed to an absorption tower filled with packing to a height of 4 meters. The absorption liquid consists of 0.5 molar soda solution with a temperature of about 60 C. In the lower portion of the tower, the absorption liquid is recirculated and to the upper portion thereof soda solution is supplied.

8 More than 96% of the sulfur dioxide in the combustion gas is absorbed in this tower.

The soda smelt from the furnace has the following composition Mol-percent Na CO 52.8 N328 41.3 Na SO and other substances calculated as Na SO 5.9

The soda smelt is dissolved in a concentrated solution of sodium sulfide at a temperature of 05 C. This solution contains about 40 g. of Na C0 per 1000 g. of water and 580 g. of Na S per 1000 g. of water and some Na SO In the dissolution of smelt soda in this solution carbonate crystals are for-med. The solution is maintained at the concentration referred to by continuously drawing off mother liquid from the carbonate crystallization. With this mother liquor, the total amount of Na S of the smelt is returned to the recovery furnace by admixture to sulfite spent liquor which is evaporated so that the mixture contains 56% of solids with the chemicals contents indicated above.

Carbonate crystals in an amount corresponding to 1.46 kilogram-mols of Na CO per metric ton of pulp are recovered, accompanied by about 0.04 kilogram-mol of Na SO Thus, there are obtained from the absorption tower for the combustion gas per metric ton of pulp:

Kilogram-mols S0 2.0 Na O 1.46

Kilogram-mols S0 3 .5 N320 1 .46

To compensate for the losses of chemicals, 24 kg. of Na SO and 16 kg. of sulfur are added per metric ton of pulp. The sulfate is added to the evaporated sulfite liquor before combustion thereof. The sulfur is burnt to S0 which is absorbed in the solution from the absorption tower.

I claim:

1. A method of regenerating chemicals from spent sulfite liquor obtained in pulping of oellulosic materials and producing a spent liquor containing sulfite compounds which are substantially preserved as such and which by subsequent combustion give off sulfur dioxide to the combustion gases which comprises evaporating the sulfite spent liquor in the presence of its original sulfite sulfur compounds and burning said sulfite liquor in a liquor combustion furnace to form soda smelt and sulfur dioxde-containing combustion gases, preparing a solution of said soda smelt, containing predominantly sodium carbonate and sodium sulfide, causing sodium carbonate to crystallize out from said solution by using such a concentration of soda smelt that the solubility of the sodium carbonate is exceeded, separating said sodium carbonate from the mother liquor, passing the sulfidic sulfur in the Na senriched mother liquor at original oxidation state to the liquor combustion furnace where the major portion of said sulfidic sulfur as intermediate hydrogen sulfide is burnt to sulfur dioxide, and absorbing said sulfur dioxide as well as sulfur dioxide present in said liquor combustion gases in a solution prepared from the crystallized sodium carbonate to form a fresh sulfite cooking liquor.

2. A method as in claim 1, in which said soda smelt is dissolved in one fraction of the sulfite spent liquor, separating sodium carbonate and passing the mother liquor with sulfidic sulfur to the liquor combustion furnace.

.3. A method as in claim 13., in which the Na s-enriched mother liquor is added to suliite liquor, passing the mixed liquor with the sulfide and sulfite compounds at substantially their original state of oxidation to the liquor combustion furnace.

4. A method as in claim 3, in which the Na s-enriched mother liquor is added to evaporated sulfitc liquor and is passed to the liquor combustion furnace.

5. A method as in claim 1, in which the Na S-enriched mother liquor is acidified by carbonating it with a member of the group consisting of carbon dioxide and carbon dioxide-containing gases so that hydrogen sulfide is lib-erated and said hydrogen sulfide is burnt to sulfur dioxide in the liquor combustion furnace.

6. A method as in ciai-m 5 in which sodium bicarbonate formed in the carbonation is crystallized out and separated from the mother liquor.

7. A method as in claim 5, which comprises passing to the liquor combustion a portion of the mother liquor required for a continuous removal of indifferent sodium compounds enriched in the mother liquor.

8. A method as in claim 1 in which a compound from the group consisting of sodium hydroxide, odium suliite, and sodium carbonate is added to the solution of recovered sodium carbonate.

9. A method as in claim 8, in which the combustion gases, prior to the absorption of sulfur dioxide, are scrubbed With an aqueous solution suitably sulfite liquor to be burnt, to cool the combustion gase and remove sulfur trioxide therefrom leaving the sulfur dioxide for said absorption.

iii. A method as in claim 9 in which alkali dust in the combustion gases is separated dry prior to the scrubbing step and is returned to the process.

11. A method as in claim 9, in which a minor portion of the scrubber liquid is passed to the liquor combustion stage and the major portion is recycled to the scrubbing stage.

1.2. A method as in claim 9 in which the circulating liquor is cooled by heat exchange with a liquid stream to the pulping process.

13. A method of treating spent sulfite liquor obtained from cooking cellulosic materials with a suliite cooking liquor which comprises the steps of:

evaporating Water from said spent liquor to form a concentrated liquor;

burning the concentrated liquor in a combustion furnace to form a sulfur dioxide containing combustion gas a smelt containing sodium and sulfur;

dissolving the smelt to obtain a solution containing sodium suliide and sodium carbonate; precipitating the sodium carbonate and separating the sodium carbonate from the solution of sodium sulfide;

passing the sulfide sulfur content of said solution to the combustion furnace Without appreciable reaction or": said sulfur content with the sulfur in the spent liquor wherein a portion of the sulfur so passed is converted to sulfur dioxide;

and absorbing the sulfur dioxide so produced in a solution prepared from the precipitated sodium carbonate.

llieierences Cited by the Examiner UNiTED STATES PATENTS MAURICE A. BRENDISI, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

Claims

13. A METHOD OF TREATING SPENT SULFITE LIQUOR OBTAINED FROM COOKING CELLULOSE MATERIALS WITH A SULFITE COOKING LIQUOR WHICH COMPRISES THE STEPS OF: EVAPORATING WATER FROM SAID SPENT LIQUOR TO FORM A CONCENTRATING LIQUOR; BURNING THE CONCENTRATED LIQUOR IN A COMBUSTION FURNACE TO FORM A SULFUR DIOXIDE CONTAINING COMBUSTION GAS AND A SMELT CONTAINING SODIUM AND SULFUR; DISSOLVING THE SMELT TO OBTAIN A SOLUTION CONTAINING SODIUM SULFIDE AND SODIUM CARBONATE; PRECIPITATING THE SODIUM CARBONATE AND SEPARATING THE SODIUM CARABONATE FROM THE SOLUTION OF SODIUM SULFIDE; PASSING THE SULFIDE SULFUR CONTENT OF SAID SOLUTION TO THE COMBUSTION FURNACE WITHOUT APPRECIABLE REACTION OF SAID SULFUR CONTENT WITH THE SULFUR IN THE SPENT LIQUOR WHEREIN A PORTION OF THE SULFUR SO PASSED IS CONVERTED TO SULFUR DIOXIDE; AND ABSORBING THE SULFUR DIOXIDE SO PRODUCED IN A SOLUTION PREPARED FROM THE PRECIPITATED SODIUM CARBONATE.