US2789883A - Process for monosulfite recovery - Google Patents

Description

April 23, 1957 w. s. cooK PRocEss RoR MoNosuLFITE RECOVERY 2 Sheets-Sheet 1 Filed July 2 9, 1952 www nur

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April 23,v 1957 w. s. cooK PROCESS FOR MONOSULITE RECOVERY 2 Sheets-Sheet 2 Filed July 29, 1952 INVENTOR Wil/iam .5. Cao/r@ ATTORNEYS United States Patent My presentf inventioni` relatesto r an.- improved method of converting sodium'suliide to:sodium".suliite.` More` particularly it relates to an improved methodf of'con-a verting so-called vsmelt liquor*containing:essentiallyVI sodium sulfide and sodium carbonate to liquor containing essentially .sodiuml suliite suitable: for cooking wood by the so-called monosultite method..`

Thismethod of pulpinglhas 'long beeni proposed but its use has been limited because ofdifculties inthe recovery of sodium and sulfur values in the waste liquor. Y

ln most instances the waste liquor has been discarded, thereby creating a: streamv lpollution problem, or yutilized inflthe recovery system of `a kraftmilljwhere 'such mill;

happens to be convenientlyzadjacent. When'the-fwaste liquor is concentrated and burned, the sulfur presentis reduced to the sulfide, thereby" posing lthe problemgofrv converting the sulfide tothe swulite.V AMany proposals Ahave been made --to thisend.v These may be divided into at least `tive general fcategoriesfviz., precipitation of in.. soluble fstilides; direct oxidation, .of `the smelt solutions;- electrolysis;.ionexchangeg and, finally, `liberation of the suifurras hydrogen sulfideand its subsequent disposition either by `oxidation or otherwise.

Except yfor some of thetmethods involving electrolysis,

and ion exchange, those ,named require concentration and burning' ofthe Waste liquor..

My presentmethod relates to the-last named, i. e., that involving `theliberation andsubsequent oxidation of hydro-v lt also involves an initial carbonation off-y gen sulfide. the smeltzliquor ,whichroperates to release .the sulfur, combined v as sodiumsulfitle; as hydrogen sulfide., Most of the heretofore proposed methods which involve the formation ofhydrogen sulfiderequire ythe :burning of itto sulfur dioxide. l it will `be observed thatinfcarrying-.out suchy methods an excessfof ycarbon dioxide must lbe employed in'r order-for it to act as a strippingagentto-carry fromtheu solution the formed hydrogen sulde.- Now whenfthef mixture-of carbondioxideand hydrogen'sulde Ais burned,

aa'gaseousfmixture,is obtained consisting essentiallytof l the carbondioxide v originally presenu: they lsulfur dioxideq formedzby" the air oxidation of the hydrogen suldeand thev largevolume ofv nitrogen remaining from the air oxidation. Thereby a mixture is had` whichbecause ofl its great volume t frequiresfequipment of l extreme size and of'high` .cost -tohandle it. Furthermore vthe AAcorrosive'i quality of the gaseous mixture addszto the difficulty.

Nevertheless for the processto becommercially feasible,;.z thessulfurvalues must be :recovered .to :a substantial de-:A

gree..

My invention is predicated upon myy discovery -thatfv the above type of recovery :method may be greatly simpliiied and .improved by establishing-a closed gas cycle in which a recycled stream of carbon dioxide `willperf0rm Patented Apr; n 23, ,1957* ice..

21;' sulfide' with elimination off substantiallyy ,all ofy the sulfide sulfur from ithecarbonate solution;

(3 After the removal of .the hydrogen sulfide', ,.aseleg mental fsulfur; by reactionwith sulfur dioxide,'prefereA ably .in the form ofrsulfurous-acid, of actingnas a strip-rl.

ping or desorbing,agenty for a sufcient quantity-ofsulfffur dioxide from its-solution in water 4whereby the mix-` required.

My improved method involves the following known Advantages which' will be readily apparent arethat the major equations involved,i. e., Nos. 2 and 5, readily l go to completion with the result that no appreciablequantity'of sulfide sulfurescapes reaction and no appreciable quantity of sulfur dioxide is admitted to the carbonating step.v Oifhand it might be supposed that the presence of sulfur'dioxide in-the carbonating column would merely anticipatel the reaction which is carried out in the suliting column `and hence would beI without significance. However, to sullite in the carbonatingcolumn would have the result of giving riserto large quantities of thiosulfates which`represent a complete loss of useful sulfur. Such thiosulfates are formed according to the following reaction which `easily goes to completion:

2Na2S+2Na2CO3+4SO2+H20 3Na2S203--2N3HCO3 1 Substantially' nofsodium salts are lost and under the conditions obtaining for the oxidation of the HzS formed,

. carried outy under atmospheric pressure and with a minimum of such steps asevaporatio'n,,precipitation and ltration. Since no airis admitted to the sulfiting column, there is no tendency to form sulfates. Other advantages will ,be apparent as the description proceeds and the features of novelty4 will be pointed out in the appended claims.

Thepinvention will` be best understood by reference to the following Ydetailed description taken with the annexedv drawingsin `which:l

Fig. l is a process tlow diagram of a preferred embodiment;

Fig.A 2 is a somewhat diagrammatic showing of afplant for carrying out the process shown in Fig. l.

For sake of brevity, the following lchemical symbols' will be used in lieu of the names of the more frequently referred to chemicalcompounds involved in the process: for carbon'idioxide, CO2; for sulfur dioxide, SO2; for hydrogenksulide, Has; lfor sulfurous acid, H2503; for `sodium sulde, Na2S;' for sodium sulte, NazSOa; forse dium monocarbonate, NazCOa; for sodium bicarbonate, NaHCOs.

A brief description of the method will iirst be given followed by specic examples of quantities of materials used, reference being had to Fig. 2. Smelt liquor is introduced through pipe 9 into the carbonating column 10 countercurrent to a stream of substantially pure CO2 introduced at the bottom thereof through the pipe 11. The carbonation as represented by Equations 1 and 2 takes place and the excess CO2 sweeps with it the formed H28 out the top of the column through pipe 12 through which it is conveyed to the bottom of column 13 which is the sulfur oxidizing column and stripper. Normally the HzS formed will be about 10% of the mixture of CO2 and HzS, i. e., 7 moles of-COz to one mole of HzS. There the HzS meets the HzSOs introduced into the top thereof through pipe 14 and in the column the oxidation of the H2S to elemental sulfur (Equation 3) takes place. The slurry of sulfur and water which is formed leaves the bottom of the column through pipe 15 and, aided by pump 16, is passed into the sulfur settling tank 16a. By having the HzSOa as nearly spent as possible at the bottom of the tower where it meets the incoming CO2 and HzS mixture, any polythionic acids formed further up the tower or elsewhere will be largely reduced to sulfur.

The CO2 stream in column 13 desorbs some of the SO2 and this mixture of CO2 and SO2 leaves the top of the column through pipe 17, and aided by the blower 18 is introduced into the bottom of sulting column 19 in which it meets the sodium bicarbonate solution from column 10 which ilows through pipe 20, pump 21, to the top of said column 19. Therein the sodium bicarbonate or monocarbonate present is converted to sodium sulte substantially in accordance with Equation 5. An agitator 21a in column 19 serves the purpose of releasing dissolved CO2 from the sulfited liquor; otherwise the dissolved gas may interfere with the subsequent pumping operation. The resulting sodium sulte solution is the product of the system and leaves the column 19 through the pipe 22. The sulfur from the settling tank 16a is removed therefrom through pipe 23 to the dewatering press 24 and a more concentrated slurry leaves through pipe 25 by which it is conveyed to the sulfur melt tank 25a and thence to sulfur burner 26. Make-up sulfur is added to tank 25a through pipe 27. The products of the air oxidation of the sul-fur pass through the column 28, the SO2 removed by the action of water from tank 16a introduced into the column through pipe 29. The HzSOa formed leaves the bottom of column 28 by pipe 30, and aided by pump 31, passes through pipe 14 to the column 13. The amount of CO2 used will in general be determined by that needed in the stripping column 13 since for the most part that required for stripping in said column will be little more than twice that required for carbonation. That needed in the stripping column is governed by the composition of the equilibrium mixture of the two gases, i. e., of CO2 and SO2, which is dependent on the parti-al pressure of SO2 over the sulfurous acid. In this manner the CO2 required is related to the concentration and temperature of the sulfurous acid, regard being had to the fact that as the temperature is raised the equilibrium partial pressure of the SO2 is also raised so that when the SO2 from the sulfur burner 26 is absorbed at the higher temperature a weaker acid is produced; on the other hand, a higher temperature will cause a greater amount of SO2 to be desorbed by the CO2 in the stripping operation. Lower temperatures are advantageous in avoiding vent losses.

The amount of SO2 desorbed in sulting column 19 will bear a substantially stoichiometric relationship to the amount of sodium salts of carbonio acid formed in the carbonating step, it being noted from Equations and 6 that one mole of SO2 is needed for each two mols of sodium present regardless of the amount of the carbonic acid radical present. The excess CO2 mayif desired be carried along through the carbonator provided the column will accept so large a gas ilow. Preferably a valved by-pass 32 is employed for the purpose of bypassing a portion of the CO2. Where the amount of CO2 increases beyond that needed as make-up for losses, as may happen through the operation of Equation 5, the excess is bled olf through valved by-pass 34 to disposition not shown.

yIt will be noted that the concentration of the H2803 is determined by the fact that it is obtained by passing the products of the -air oxidation of sulfur through a scrubbing tower under atmospheric pressure. Under these conditions, the partial pressure of the SO2 in the resulting solution is such as to limit the amount of dissolved SOz to a Value of from 1 to 2% depending upon the temperature as above pointed out. Greater concentrations than this would result in an undesirably large amount of SO2 being stripped from the solution and added to the gas cycle. Furthermore, by circulating a larger -amount of water in the so-called acid cycle (see Fig. 1), the concentration of acid is lower whereas if a smaller amount of water is circulated, the concentration of acid ishigher.

The use of the dewatering press 24 is optional; also it has been found of advantage to add a occulating agent to ,the sulfur slurry to aid in its precipitation. Of such agents, alum (aluminum sulfate) has been used satisfactorily.

Example l In this example the smelt liquor from the pulping of one ton of wood is introduced to the column 10. rlhis liquor has the following composition:

To run this amount of liquor, 2000 lbs. of CO2 is admitted through pipe 11. When the reaction is complete, 74 lbs. of H2S is formed and added to the 2000 lbs. of CO2 leaving the top of the carbonating column through pipe 12. 332 lbs. of SO2 in 33,600 lbs. of water is introduced into the column 13 (pipe 14), producing 95 lbs. of sulfur by reaction with the 74 lbs. of HzS. It may be noted at this point that counter-current ow of gas to liquor in the column 13 is important in order that, as already pointed out, the spent acid may contact the incoming HzS so as to minimize the formation of polythionic acids or to reduce any such that are formed to sulfur. It is therefore desirable to provide the stoichiometric quantity of SO2 to oxidize the HzS plus what is to be desorbed by the stream of CO2. In order to provide for 256 lbs. ofSOz to be desorbed by the CO2, a total quantity of 332 lbs. of SO2 is provided in the 33,600 lbs. of water. This leaves the exiting liquor substantially free of sulfurous acid. Some polythionic acids are present in the liquor and are recirculated.

The lbs. of sulfur is allowed to settle and the thickened slurry passed to the press 24. The sulfur now in the form of a still thicker slurry is conveyed to the melt tank 25a and molten sulfur is passed to the sulfur burner 26 as has already been described and additioned by 60 lbs. of make-up sulfur.

672 lbs. of carbonates expressed as sodium bicarbonate is formed in the column 10 and is passed in solution in 10,000 lbs. of water to the sultiting column 19 where it is reacted with 256 lbs. of SO2 already mentioned, thereby producing 504 lbs. of sodium monosulte and 352 additional pounds of CO2 which most conveniently is vented through pipe 34.

Example 2 In this example a liquor low in sodium sulte is used. Its composition is:

and producing 15 lbs. of sulfur. The sulfuris recovered" and burned as previously described; 1,19 lbs. of makeup sulfur is required, the same beingl admitted through pipe 27 as heretofore.

Example 3 In this example a smelt liquor richin NazS is usedjwhich has the following composition:

Na2S, lbs 311 NazCOs, lbs Water, lbs 10,000

Upon carbonation, 136 lbs. ofi-.H28 is formed in the same 2000 lbs. of C02, it being noted that more CO2 is consumed because of the greater amount of H2S formed. The carbonates. formed, expressed as NaHCOsare approximately 672 lbs. as in the preceding examples. In order to oxidize. the 136 lbs. of H28', 140 lbs. 0f S02 is required, sufcientadditionalSOz being desorbed to bring the S02 content of the exiting CO2 to the same value of4 256 lbs. as in the precedingexamples. 170 lbs. of sulfur is formed which is fed to the sulfur burner 26 as before. 24 lbs. of make-up sulfur is needed in this example.

In the above examples, the temperature within the column is maintained at the ordinary ambient temperatures, i. e., l0-80 F. for the region of Charleston, S. C. Elevating the temperature of the liquor during carbonation results in a shift of the equilibrium to give less bicarbonate formation. At about 160 F. a solu tion having a maximum of about sodium monosulte is produced. The proportion of the monocarbonate to bicarbonate is not critical provided only that the removal of the sulfide sulfur is complete. However, by thus throwing the equilibrium in the direction of the monocarbonate formation, a higher concentration of smelt liquor may be employed without precipitation of NaHCOs and consequently a higher concentration of sodium sulte solution may be obtained as the nal product of the system.

If desired, instead of carrying out both the operations of stripping the S02 from the H2803 solution and oxidizing the H2S in the same tower, these operations may be carried out separately. Thus the gaseous mixture from the carbonator may be led to a first or oxidizing column and there contacted with a solution of H2S0a in suicient amount to oxidize the H2S to elemental sulfur substantially quantitatively and the resulting sulfur slurry passed to a settling tank as heretofore described. The oxidation of H28 can if desired be effected by use of gaseous SOz instead of H2803, as is apparent to those skilled in the art. Hence the use of the expression SO2 as the reagent in the oxidation of HzS to elemental sulfur should be taken to embrace both the gaseous S02 and its equivalent solution form, H2803. Then the gas stream deprived of its H2S but containing some acquired S02 is passed to a second or stripping column to which a further supply of H2503 is admitted and in which the required amount of S02 is picked up by the gaseous stream. Spent acid from the stripping column is passed directly to the SO2 adsorber. By so conducting the oxidizing and stripping operations in two steps instead of one, the concentration of H2803 may be varied in the two columns best to suit the conditions there obtaining. Furthermore the sulfur slurry obtained in the oxidizing column will be advantageously somewhat more concentrated.

In the foregoing examples the sulfur lost has amounted to approximately 15% chiefly as polythionic acids; in larger scale operations it would be expected that such loss would be considerably less.

Also inlieufof theiratherdilute solution of smelt liquor as set forth in the above specific examples, a stronger,

smeltsolution may be used in the, initial-carbonationstep, i. e., onecontaini-ngl in the neighborhood of 20% solids. Then duringl the carbonation, the relatively insoluble sodium bicarbonate formed will precipitate out. At the completion ofvvthecarbonation, the precipitate of sodiumbicarbonate may-.beliiltered off and washed, thereby giving a sodium bicarbonate of high purity. The mother liquor filtered off may then be sent back to the evaporators to be added to the concentrate which is about to be-burnedto form a, new smelt. The recovered sodium bicarbonate may then be dissolved in suicient water and sulited vin the. manner already described.

Various modifications will occur to those skilled in the .art within the. spirit of the present invention. For. example, if, desired the vcarbonating column may be superimposed upon the suliting column in such a manner that there will be created' a lower sulfiting zonel andan upper carbonating zone and the same result had.

It will be understood that if desired thesuliiting operation instead ofbeing carried to thecomplete formation of monosulte may be carried merely to a partially sulfted stage consistingfofa mixture of-sodium suliite andthe sodium salts of carbonic acid, in which case a gaseous mixture weaker in S02 will be used.

Likewise the suliting operation may be carried out to produce sodiurnbisuliite bythe use of a gaseous mixture containing a greater portion of S02. 0r, if desired, a product may be obtained comprising a mixture of the monosulfite and bisullite salts. By using even greater portions of SO2 mixtures of sodium bisulfite and free sulfurous acid can be produced. By so proceeding the process will be adatped for furnishing cooking liquod suitable for the soda base acid suliite process provided only that the waste liquor from such process be satisfactorily concentrated and burned to produce a smelt liquor.

When wood pulp is manufactured by digesting the woo-d in sulfurous acid which is wholly or partially combined with an alkali metal (sodium) and the .resultant black liquor collected and burned in a smelter furnace, a smelt results which when dissolved in water is termed a smelt liquor. Smelt liquor is frequently called green liquor because impurities are often present which impart a green color to the solution. The smelt liquor is essentially a solution of sodium sulfide and sodium carl bonate. The present process provides a method whereby the smelt liquor can be converted to fresh pulping liquor.

I claim:

1. The method of converting sodium sulfide, together with any sodium carbonate present in smelt liquor obtained from the pulping of wood, to sodium suliite which comprises passing a stream of C02 into Contact with said liquor to carbonate same with the formation of sodium salts of carbonio acid and H28 and continuing such passage of such stream until such H28 is stripped substantially completely from said liquor, passing the H2S bearing stream of C02 to an oxidizing zone wherein said stream is brought into contact with SO2 whereby to oxidize said HzS to elemental sulfur, removing said sulfur from said stream, enrich-ing said stream of CO2 with SO2, utilizing said S02 enriched Stream to sulte the already carbonated smelt liquor to convert the carbonates pres.- ent to suliites and to obtain substantially pure CO2, and utilizing said CO2 so `obtained to carbonate a fresh supply of smelt liquor, the amount of CO2 used in the carbonating step being at least tive times the stoichiometric requirement.

2. The method of converting sodium sulfide, together with any sodium carbonate present in smelt liquor obtained from the pulping olf wood, to sodium sulte, which comprises passing a stream of C02 in intim-ate contact with said liquor to carbonate same with the formation of sodium salts of carbonic acid and H2S and simultaneously to strip from said liquor said HzS, the quantity of CO2 utilized being in substantial excess of the stoichio metric iamount for carbonating said liquor to effect a substantially complete stripping of said H2S from said liquor by CO2 sweeping, passing the Hzs-bearing stream of CO2 to Ian oxidizing zone wherein said stream is brought into contact with SO2 whereby to oxidize said H28 to ielemental sulphur, removing said sulphur from said stream, enriching said stream of CO2 with SO2, and utilizing said SO2-enriched stream to sulte 4the already carbonated smelt liquor to convert the carbonates present to sultes and to obtain substantially pure CO2.

3. rlhe method as defined in claim 2, wherein said stream of CO2 is caused to be enriched with SO2 simultaneously with sai-d oxidizing step.

4. The method as dened in claim 2, wherein the oxidation of the H2S is brought about by contacting the CO2 stream bearing the same with SO2 in aqueous solution in coun-tercurrent manner, whereby said solution of SO2 is substantially spent upon initial Contact with said stream.

5. The method as dened in claim 2, where-in the elemental sulfur `obtained and removed in the oxidation of H2S is burned to SO2 and such SO2 utilized for the oxidation of H28 and enrichment of the CO2 stream.

6.4 The method as defined in claim 2, wherein the amount of SO2 incorporated in the stream of CO2 as a result of the enriohment step bears a substantially s ttoichiometric relationship to the amount of sodium salts of carbonic acid formed in the carbonating step.

7. The method of converting a solution of sodium sulde to sodium sulte, which comprises passing a stream of CO2 in intimate contact with said solution to carbonate same with the formation of sodium salts of carbonio acid and H2S and simultaneously to strip from said solution said H2S, the quantity of CO2 utilized being in substantial excess of the stoichiometric amount for carbonating said solution to effect a substantially complete stripping of said H2S from said solution by CO2 sweeping, passing the E28-bearing stream of CO2 to an oxidizing zone wherein said stream is brought into contact with SO2 whereby to oxidize said H28 to elemental sulphur, removing said sulphur from said stream, enriching said stream of CO2 with SO2, and utilizing said SO2-enriched stream to isulte the already carbonated solution to convert the carbonates present to sulfltes and to obtain substantially lpure CO2.

References Cited in the le of this patent UNITED STATES PATENTS 1,728,252 Rawling Sept. 17, 1929 1,983,789 Bradley et al Dec. 11, 1934 2,163,554 Gaither June 20, 1939 2,177,707 Gaither Oct. 31, 1939 2,496,550 Larson et al. Feb. 7, 1950 2,675,297 Gray et al Apr. 13, 1954

Claims (1)

1. 2. THE METHOD OF CONVERTING SODIUM SULFIDE, TOGETHER WITH ANY SODIUM CARBONATE PRESENT IN SMELT LIQUOR OBTAINED FROM THE PULPING OF WOOD, TO SODIUM SULFITE, WHICH COMPRISES PASSING A STREAM OF CO2 IN INTIMATE CONTACT WITH SAID LOQUOR OF CARBONATE SAME WITH THE FORMATION OF SODIUM SALTS OF CARBONIC ACID AND H2S AND SIMULTANEOUSLY TO STRIP FROM SAID LIQUOR SAID H2S, THE QUANTITY OF CO2 UTILIZED BEING SUBSTANTIAL EXCESS OF THE STOICHIOMETRIC AMOUNT FOR CARBONATING SAID LIQUOR TO EFFECT A SUBSTANTIALLY COMPLETE STRIPPING OF SAID H2S FROM SAID LIQUOR BY CO2 SWEEPING, PASSING THE H2S-BEARING STREAM OF CO2 TO AN OXIDIZING ZONE WHEREIN SAID STREAM IS BROUGHT INTO CONTACT WITH SO2 WHEREBY TO OXIDIZE SAID H2S TO ELEMENTAL SULPHUR, REMOVING SAID SULPHUR FROM AND STREAM, ENRICHING SAID STREAM OF CO2 WITH SO2, AND UTILIZING SAID SO2-ENRICHED STREAM TO SULFITE THE ALREADY CARBONATED SMELT LIQUOR TO CONVERT THE CARBONATES PRESENT TO SULFITES AND TO OBTAIN SUBSTANTIALLY PURE CO2.

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