US3210236A - Combined acid sulfite and modified sulfate pulping process with recovery cycle - Google Patents

Description

Oct. 5, 1965 Filed April 8, 1963 S. COPPICK ET AL COMBINDED ACID SULFITE AND MODIFIED SULFATE PULPING PROCESS WITH RECOVERY CYCLE sets-Sheet 1 IMPROVED SODIUM BASE SULFATE AOIO SULFITE PULPlNG Recycle Recycle\ PULPING S lf -f Spent Sulfure Spem Sulfire Sulfhe p Liquor Liquor Pulp EVAPORATING 7 ZONE l l SMELT Self Coke DISSOLVING Make Up 5 ZONE ZONE CO2 HZS lo SMELT II CLARIFIER I T 8 soZ REACTOR E CARBONATING 2 NO 25 ZONE No Go Sulfur 9 z 3 Make Up 23 z s LIQUOR I l2 NQHCO3 STORAGE 24 ZONE SULFUR CARBONATED m'g SMELT RECEIVING AND STORING l6 2? ZONE H O cAusTIcIzING E ZONE 25 l4 l3 ABSORPTION G I CMOH) ZONE 2 I W\ CLARIFYING H2303 To ZONE Stock STSOZ G is lPPlN suEEITING so ZONE ZONE 1 CO1 20 IMPROVED 8mm I 2| SULFATE 2e COOKING DECARBONATING LIQUOR so ZONE 22 REGOvEREO SULFITE q- COOKING |QUOR INVENTORS.

SYDNEY COPPICK JAMES J. FERRlGAN,JR.

k wi/L AGENT Get. 5, 1965 5 o c ETAL 3,210,236

COMBINDED ACID SULFITE AND MODIFIED SULFATE PULPING PROCESS WITH RECOVERY CYCLE Filed April 8, 1963 2 Sheets-Sheet 2 y 2 I 7.5 NCLS \8.8 so1 |8.0 NuOH Z 3.86 NuOH 3.5 s I I I Y'ELD SULFOFATE I SULFITE IOOlbs.

55.5lbs. WOOD WOOD Recycle Recycle 2.22m 10.35 No 3.08 s 5.65 s

6.I6 so 1 3.86 NuOH 4 a 6 or EVAPORATOR E & Sr1|r Coke Make Up FURNACE (if needed) Sulfur Make Up (if needed) I I257 No I 8.73 s 5.25 N018 V |2.6N0OH CHEMICAL I 5.5 s CONVERSION I65 Nu,

CQIOI-n "i 5.55 I 5.25 N028 I65NcI CO CAUST'C'ZER 12.6 NuOH INVENTORS. SYDNEY COPPICK JAMES J. FERRIGAN, JR.

AGENT United States Patent C 3,210,236 COMBINED ACID SULFITE AND MODIFIED SUL- FATE PULPING PROCESS WITH RECOVERY CYCLE Sydney Coppick, Ridley Park, and James J. Ferrigan, Jr.,

Chester, Pa., assignors to Scott Paper Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 8, 1963, Ser. No. 271,358 3 Claims. (Cl. 162-33) This invention relates to an improved pulping process dependent upon a novel chemical recovery cycle. More specifically, this invention relates to an improved pulping process wherein the liquor recovery phase of an improved sulphate pulping process in combination with a sodium base acid sulfite pulping process provides a novel combined process having increased yields and having an improved acid sulfite and improved sulfate cooking liquor recovery cycle.

The present invention overcomes some of the prior art shortcomings found in the conventional pulping processes. For example, the present process allows the alternative processes to be run within Wide limits of amount of pulp recovered or cooked in one process per amount of pulp recovered or cooked in the other process, i.e., the present combined process embodies processes that are proportion independent. In other words, if one process is run at given conditions, the other process will be confined to a certain condition; however, the various proportions in each process have a wide range of corresponding proportions in the other process. Some of the integrated prior art processes that embody entirely different systems, such as, neutral sulfite-sulfate pulping system, are pro portion dependent. The proportion independent feature of the present invention allows a mill to be run in an optimum manner.

Another advantage which the novel process enjoys over prior art is the liquor recovery cycle. Entirely different pulping systems are serviced by the novel liquor recovery cycle, yet the pulping systems may be varied to run Within wide limits. Furthermore, the prior art thus far has failed to disclose an integrated system having a novel elemental sulfur pulping (sulfofate) system operating in conjunction with another process; and the art has also failed to disclose a conjoint process which allows novel elemental sulfur recovery.

The above-mentioned sulfofate in general may be defined as a modified sulfate pulping process in which the wood is reduced to pulp by the employment of cooking liquors composed essentially of a solution of the three main ingredients: sodium hydroxide, sodium sulfide and elemental sulfur. In particular, as in this specification, the sulfofate process refers to an improved sulfate pulping process in which the sulfide is present in the liquors in amounts of 28 to 30% sulfidity, and in excess of this, the elemental sulfur is added in quantities equivalent to from 3% to 5% of the dry weight of the wood. Furthermore, in the cyclic sulfofate process the spent liquors from the pulping operation are recovered and chemically processed to produce a white liquor with 28% to 30% sulfidity as well as elemental sulfur, which on combining, result in a cooking liquor to complete the cycle of the sulfofate pulping process. The sulfofate process is a modification of the conventional kraft process. The sulfofate process is disclosed in our copending application, Serial No. 271,- 357, filed April 8, 1963.

In the present operation of the sulfofate pulping process, variably integrated with the operation of a one or two stage sodium base acid sulfite process, the improvement resides in the pulping process as well as in the cyclic recovery process. Briefly, the liquor recovery part of 3,210,236 Patented Oct. 5, 1965 'ice the invention is accomplished by the following procedural steps.

Combining the spent liquors from the two processes, evaporating them to a concentration of above 50% solids, burning the evaporated solutions in a reducing furnace to obtain a smelt of high sulfidity, dissolving and clarifying this smelt and dividing it into two portions and proceeding with its chemical conversion and recovery in the following manner.

Carbonating one portion of the clarified smelt solution and catalytically converting the released hydrogen sulfide to free sulfur via treatment with sulfur dioxide, dividing the carbonated smelt solution into two portions and combining one of these portions with the residual portion of the non-carbonated clarified smelt solution, treating the combined solution with hydrated lime and clarifying thus to convert it in a solution of sodium sulfide and caustic soda as normally used as a sulfate or kraft cooking liquor, modifying this liquor by the addition of some of the free sulfur recovered via the above catalytic process to produce an improved sulfate cooking liquor;

Sulfiting the residual portion of the carbonated smelt solution to produce an acid sulfite cooking liquor for reuse in the sulfite cooking process and releasing carbon di oxide for re-use in the carbonating process, and proceeding with the cyclic process of the pulping of wood in an improved kraft (sulfofate) pulping operation in conjunction with a oneor two-stage sodium base acid sulfite pulping process. 1

Further, the invention is accomplished by operating a combined novel sulfate and sulfite cellulose pulping and process cycle in such a manner that higher yields of pulp are obtained by means of a novel combined, continuous liquor recovery, and in such a manner that the preparing and the utilizing of fresh liquors for each individual process is achieved, the combined process comprising the steps of: introducing together with normal sulfate liquor and cellulose-containing material an amount of free sulfur into digesting zone of the sulfate pulping process, combining the spent sulfur containing sulfate digesting liquor with a spent sulfite process digesting liquor, making green liquor from the combined streams, separating the green liquor into two streams, adding to the first stream of green liquor a stream from a carbonation zone and adding calcium hydroxide to produce white liquor, adding to the white liquor elemental sulfur and re-introducing the combined stream into the sulfate pulping zone; taking the second stream of the green liquor and carbonating it in the presence of carbon dioxide, driving off in the carbonating step hydrogen sulfide and carbon dioxide laden gases, converting the hydrogen sulfide in presence of sulfur dioxide into elemental sulfur stream, separating the elemental sulfur stream into two streams, taking the first I elemental sulfur stream and introducing it into the white liquor stream; taking the second stream of the elemental sulfur and making sulfur dioxide out of it, separating the sulfur dioxide into three streams; taking the first purified stream and introducing it into the elemental sulfur producing zone, taking the second purified stream and introducing it into a decarbonation zone, and taking the third sulfur dioxide stream and introducing it into a sulfiting zone, taking the liquid efiluent stream from the carbonation zone and separating it into two streams, taking the first stream and introducing it into the green liquor, taking the second stream and decarbonating it, taking the decarbonated effluent from the decarbonation step and introducing it into the sulfiting tower in presence of sulfur dioxide and introducing, together with the cellulose-containing material, the recovered sulfite cooking liquor into the acid sulfite pulping zone.

With the foregoing in mind, the present invention can be comprehended more readily by reference to the attached drawings wherein the same reference characters are used to indicate the corresponding zones and streams and wherein:

FIGURE 1 illustrates as an example of the invention the diagrammatic flow sheet of the disclosed operation;

FIGURE 2 illustrates as an example of the invention the diagrammatic material balance and the proportion relationship of sodium base acid sulfite processes at a certain proportion relationship in respect to sulfofate (improved kraft) process.

In reference to FIGURE 1, the novel process is carried out .as follows:

Wood chips or cellulose-containing material are introduced into the sulfofate pulping zone 1 and the cooking liquor added in admixture with elemental sulfur or the elemental sulfur introduced after the liquor has been run into the pulping zone. The chips are then cooked for a time sufficient to effect the desired pulping. In practicing the invention, conventional apparatuses are used which are readily suggested to a man skilled in the art. The same pertains to the actual cooking conditions. The conditions such as time and temperature are generally those which are found in the art. There are innumerable references to the conventional kraft processes and it is readily apparent to a man skilled in the art that the process is directed only to those variables which have heretofore inherently limited the pulping operations.

The sodium base acid sulfite pulping is carried out in the conventional manner in the sulfite pulping zone 2.

After the two pulping processes are concluded according to the respective practices, the liquor from cook is drained, combined and the combined stream 3 introduced into an evaporating zone 4. If the process requires salt cake make-up 5, it is introduced into stream 3 after the stream is evaporated. Generally, the make-up is necessary to compensate for normal losses in the pulping and recovery cycle. The evaporated liquor stream, rich in organic matter containing from about 50-60% solids, is introduced into reducing zone 6 where the burning of the organic matter produces useful process steam. The recovered smelt is dissolved in a smelt dissolver 7 and clarified. The dissolved smelt is now separated in two streams 8 and 9. Stream 8 is introduced in a carbonating zone 10 where in the presence of carbon dioxide the hydrogen sulfide together with excess carbon dioxide is liberated from the green liquor. The operating conditions of zone 10 are as follows: the temperature is at about 30 C.; the smelt contains about 70 grams/liter Na O equivalent; the effluent is at a pH of about 9. The carbon dioxide may be obtained from the burning zone 6, decarbonating zone 19 and it may be recycled from elemental sulfur recovery zone 11 wherein the gaseous eflluents from zone 10 are introduced. The gaseous effluents may be treated to remove undesirable impurities. In some instances for more eifective operation of elemental sulfur zone, the gaseous eflluent stream from zone 10 is purified to remove all entrained materials plus water vapor.

After the carbonation of green liquor, it is received in a storing zone 12 and divided in two streams according to the process demands. The first stream 13 is recombined with the green liquor from stream 9 in a liquor storage zone 15 from where it is causticized with lime in zone 16 and clarified in zone 17. The liquor is combined in zone 18 with the elemental sulfur from hydrogen sulfide reducing zone 11 and is then ready to be introduced in the sulfofate pulping zone 1.

The other carbonated stream 14 from the carbonating zone 10 is introduced in the decarbonating zone 19 in presence of sulfur dioxide thereby reconverting the carbonated smelt into sodium sulfite and carbon dioxide. The operating conditions of zone 19 are as follows: the pH is at about 6.0, and the temperature is of from about Carbon dioxide liberated from zone 19 is combined with carbon dioxide from zone 11 to be introduced into zone 10. The decarbonated stream is introduced into sulfiting zone in presence of sulfur dioxide to convert the decarbonated stream into a sodium base acid cooking liquor 22 which is used as cooking liquor in pulping zone 2.

The cooking liquor coming from the sulfiting zone contains the following proportions of constituents: 1.25% combined S0 7.50% total S0 1.56% Na calculated as NaOH.

The operating conditions of the sulfiting zone are as follows: the temperature is of from about 25-35 C.; and the pressure is of from about 5-10 p.s.i. (gauge).

As it was mentioned above, the hydrogen sulfide stream is converted in presence of sulfur dioxide into elemental sulfur in zone 11. Generally, a Claus type of reactor is used to effect the conversion to sulfur. It operates at about 350 C. The reaction is sustained by bauxite, hydrated alumina, or Al O -2H O acting as catalysts.

The free sulfur is recovered in a molten form and is divided in two streams; one stream is combined with white liquor in zone 18. To the other stream is added make-up sulfur 23 and the combined streams are introduced into sulfur burning zone 24 where the free sulfur is converted to sulfur dioxide. The sulfur dioxide is introduced into a sulfur dioxide absorption zone and absorbed into water to form H 50 The absorption zone operates at about 30 C. and at about 15 p.s.i. (gauge). The overhead efiluent contains about 8% sulfur dioxide and the remainder in water. Part of the sulfur dioxide is introduced in sulfiting zone 21. The other part on the form of H is introduced in a stripping zone, where in the presence of steam the H SO is decomposed to S0 and water. This zone operates free from air at about C. The sulfur dioxide is split into two streams. The first stream 27 is introduced into the hydrogen sulfide reducing zone 11. The second stream is introduced into the decarbonating tower 19.

The above-described process illustrates the integrated recovery system used in the novel process. The specific sulfofate pulping subcombination process has been more amply illustrated in applicants own application, Serial No. 271,357, filed on the same date with this application.

In reference to FIGURE 2, the novel process is represented by a self-explanatory material balance flow sheet which can be readily understood by reference to FIGURE 1. More specifically, the sulfofate zone 1 corresponds to 1 in FIGURE 1. In the same manner( 2 refers to the sodium base acid sulfite pulping zone, while 4 and 6 combined represent the corresponding evaporating and reducing furnace zone depicted in FIGURE 1. The cau sticizing zone 16 is the same as zone 16 in FIGURE 1, while the chemical conversion zone represents the combined hydrogen sulfide and sulfur dioxide operations best understood by reference to FIGURE 1. Although the material balance flow sheet represents no make-up, it is readily understood that the material balance is calculated after the first completed batch and the make-up streams, as previously mentioned for sake of clarity have been disregarded.

The following examples as embodied in FIGURE 2 and in Table I show the material balances in the various process streams. Thus, the examples more particularly supplement the inventive concept as set out above and aid in understanding the operating conditions.

EXAMPLES In reference to FIGURE 2, the following process conditions as represented by examples are tabulated to more amply illustrate the various streams. These examples presume no sulfur recovery from stack gases, and sulfur recovery in smelt losses is not taken into account, nor are the losses taken into account in calculating make-up.

Table 1 Example 1 Example 2 Example 3 Example 4 Example 5 100# wood in sul- 100# wood in 100# wood in "sul- 100# wood in sul- 100# wood in sulfofate process; normal kraft fofate process; fofate process; fofate process; b 3.5% added S process; 0% added added S; 4.0% added S 3.5% S added;

para mg 3515 (based on wood); S; 100% sulfidity 100% sulfidity (based on wood); wood in sodium 100% sulfidity from furnace 100% sulfidity base acid sulfite from furnace from furnace Fresh sulfofate liquor (white liquor+free i Nms gig sulfur) (pounds). g 3.5 S. I Sulfofate pulping liquor in (white liquor ig ig gfj sulfur+recycle liquor) (pounds). S S Fresh sodium base acid sulfite liquor (pounds). i f So(dium)z;se acid sulfite liquor in fresh liquor NaO ;.;3Sl OIaOH poun s 2. Sodium base acid sulfite liquor in fresh liquor-l- {3.8 1.73 NaOH recycle (pounds). 8613?]? Spent sulfofate liquor (pounds) 5 s 5 9 Spent sodium base acid sulfite liquor (pounds). {3:3 11123 5 a. 7 05 S. 9 Green liquor from furnace (pounds) fig i 17 .2 ({Gazs 2 17 N aOH Liquor to be worked up into sodium base acid 4 2 4 S 4 1195 N323. gullfite riplplngliiqpor and free sulfur (stream 16.05 NazS 2 .55 NazS 1 5 N82 1 .85 NaiS L51 NaOH- 1g. oun s Carbonated rnelt returned to be combined 16.5 NazCOa 16.5 NazCO; 16.5 NagCOa 16.5 Nl3003 15.8 NagCOz.

with green liquor (stream 13, Fig. 1) (pounds). Sulfur from recovery zone (zone 11, Fig. 1) to 3.5 S.. O S 5 S 4.0 S... 3.5 S.

be combined with white liquor (pounds). Sulfidity from fnrn ace 100% 100% 100% 100% 90%. Amount of wood that is pulped in sodium base 55.5 180 1.75 37 25.

acid sulfite process (pounds). Percent of increase in yield for sulfofate 2.9..- 0 5.1.. 4.0.- 1.9.

process. Percelnt pg increase in yield for sodium base 3.4-4 2 3.44 2 3.4-4 2 3.4-4 2 3.4-4.2.

aci su te process. Weighted percent increase in yield for both 3.1-3.4 2.2-2.7 5.0-5 1 3.8-4.0 3.0-3.2.

rocesses.

Although the disclosure of the present invention is EXAMPLE 7 generally applicable to sodium base acid sulfite pulping in conjunction with an improved sulfate (sulfofate) pulping process, in the preferred embodiment thereof, it is especially advantageous to employ either a two stage sodium base sulfite process or a high combined single stage acid sulfite cooking procedure, in conjunction with the improved sulfate operations. By such combinations, an increased yield in both the sulfite and the sulfate processes is realized as a practical result of the cyclic recovery features of the present invention.

Examples are as follows:

EXAMPLE 6 Wood was pulped in the usual acid sulfite process as follows: a softwood-hardwood blend of chips was charged to the digester in the ratio of 80% softwood to 20% hardwood. The softwood was an equal parts mixture of northeastern spruce and balsam fir, while the hardwood was a mixture of northeastern beech, birch and maple. The wood was digested in an acid sulfite liquor prepared from dolomitic limestone with the base in the ratio of 3/2 CaO to MgO. The cooking liquor composition was 0.93% combined S0 and 7.5% total S0 This was added to the digester in proportions of 4.5 parts of liquor per 1.0 part equivalent of dry wood.

The temperature of the digester was raised slowly via steam by circulation of the liquor in an indirect heat exchange over a period of 4.5 hours to 138 C. Then, for 1.75 hours the contents of the digester were maintained at 138 C. with a maximum pressure of 90 psi. (gauge). The pressure was relieved over a period of 1.25 hours and the contents of the digester dumped, washed and screened. The final unbleached pulp was analyzed as follows:

Yield of screened pulp percent-- 45.0 Yield of screenings --do 2.4 Permanganate number 15.4

In a high combined sodium base acid sulfite single stage process, exactly the same type wood was digested under the same conditions as in Example 6, but with the following exceptions. The liquor was sodium base and had a combined S0 of 1.30% and a total S0 of 7.5 The maximum temperature was 143 C. to compensate for the otherwise reduced rate of pulping due to the higher com-' bined S0 The liquor was separated and retained for the recovery cycle. The pulp was analyzed as follows:

Yield of screened pulp percent 48.4 Yield of screenings do 0.7 Permanganate number 15.7

It will be noted that at substantially the same bleachability (permanganate number), a 3.4% increase in screened yield over Example 6 is achieved on a dry wood basis, and the screenings are appreciably reduced.

EXAMPLE 8 In a two stage sodium base sulfite operation the same type of wood was digested as in Examples 6 and 7. The first stage sulfite cooking liquor was composed of a blend of recycle first stage liquor from a previous cook, and fresh liquor from the recovery system in the ratio of 60% recycle liquor and 40% fresh liquor. The liquor was added to the digester as 5 parts of liquor per 1.0 part equivalent of dry wood, and had a composition such that it contained an amount of sodium equal to 12.9% based on the dry wood and the S0 content was adjusted to result in a pH of 6.0. The digester and it contents was heated rapidly via indirect steam to 130 C. over a period of 1.5 hours, maintaining a constant pressure of p.s.i. (gauge). The temperature was maintained at C. for 0.5 hours, then 3 parts out of the original 5 parts (based on dry wood) of this spent first stage sulfite liquor were withdrawn from the digester and retained for a subsequent cook. Sufiicient water was injected into the digester to maintain circulation together with 20% liquid S based on wood, and the digester contents heated to 135 C. These draw-down, injection and reheating operations occupied 1 hour. The second stage digestion was continued for 2.5 hours at 135 C. and 95 p.s.i. (gauge) after which the pressure was relieved over a period of 1.5

hours. The liquor was separated and retained for recovery, while the pulp was analyzed as follows:

Yield of screened pulp "percent" 49.2 Yield of screenings do 0.9 Permanganate number 15.3

It will be noted that at substantially the same bleachability (permanganate number), a 4.2% increase in screened yield over Example 6 is achieved on a dry wood basis, and the screenings are appreciably reduced.

EXAMPLE 9 A chip blend comprised of 80% northeastern softwood (i.e., spruce and balsam fir) and 20% northeastern hardwood (i.e., beech, birch and maple), was prepared having a chip length of A, and 1.0 part equivalent of dry wood was charged to the digester in the amount of 3.5 parts. The cooking liquor was a normal sulfate pulping liquor containing 2021% chemical (as Na O) on basis Yield of screened pulp "percent" 45.1

Yield of screenings do 0.1

Permanganate number 12.6

EXAMPLE 10 A chip blend similar to that employed in Example 9 was digested as in Example 9 with the exception that 3.5% elemental sulfur base on dry wood was added to the starting liquor. The washed and screened pulp gave the following results:

Yield of screened pulp percent 48.0 Yield of screenings do 0.3 Permanganate number 12.5

It will be noted that an increased pulp yield of 2.9% on dry wood was obtained over that achieved via the normal sulfate pulping procedure of Example 9.

EXAMPLE 11 A similar chip blend was digested as in Example 9 with the exception that 5.0% elemental sulfur, based on dry wood, was added to the starting liquor. The Washed and screened pulp gave the following results:

Yield of screened pulp percent 50.5 Yield of screenings do 0.5 Permanganate number 14.2

It will be noted that an increased pulp yield of 5.1% on dry wood was obtained over that achieved via the normal sulfate pulping procedure of Example 9.

EXAMPLE 12 A chip blend similar to that of Example 9 was pulped in a manner identical to that of Example 9 with the exception that 4.0% elemental sulfur, based on dry wood, was added to the cooking liquor. The liquor was separated from the pulp, and retained for recovery, while the pulp was analyzed as follows:

Yield of screened pulp percent 49.1 Yield of screenings do 0.3 Permanganate number 13.6

The above pulp had an increased yield of 4.0%, based on dry wood, over and above that realized from the usual sulfate pulping operations of Example 9.

In FIGURE 2 there is illustrated a situated a situation for a combined recovery operation wherein material balances are such that production of the sulfofate" pulp at 3.5% added sulfur is about twice that of the high yield sulfite pulp. This situation is summarized in Example 1 of Table I.

A similar type operation is depicted in Example 2 of Table I. In Example 2, however, a normal sulfate process is carried out without added sulfur in conjunction with the high yield sulfite operation, and the material balances are such that the production rates of the sulfite are about 1.8 times those of the sulfate process.

Again in Example 3, the versatility of the recovery system is demonstrated where essentially all of the production, at 5% added sulfur, is from the sulfofate operation.

Still further in Example 4, at 4% added sulfur, the materials balance results in the operation of the sulfofate process at production rates approximately three times those of the high yield sulfite operation.

In Example 5, with 3.5% added sulfur and restricting the smelt to sulfidity, the material balances arrived at allow for a sulfofate production about four times that of the sultite production.

It is thus apparent that by appropriate choice of conditions, the combined recovery system has such versatility as to permit dominant operation of either the sulfite or sulfate sides of the pulping system. This is of great advantage in the manufacture of various paper products wherein the quantities of these two pulps may be altered at will to accommodate the varied quality demands of papers suitable for differing purposes and end uses.

Examples 6 to 12 illustrate the pulping procedures which demonstrate the added yields of pulp which are obtained via the pulping processes used most advantageously in conjunction with the combined cyclic recovery method of this invention.

In the light of these increased yields in both the sulfite and sulfate operations, they have been weighted on the basis of the various production rates of Table I, and the overall yield increases are illustrated in the final tabulations of Table I.

In the normal sulfate cyclic process it is usual to compensate for the loses of chemical that occur by the addition of sodium sulfate (salt cake to the furnace). In some cases it is preferred to supply this make-up chemical as sulfur and sodium carbonate or caustic. These chemicals are presently added to supply quantities of Na and sulfur in amounts to compensate for loses and in proportions one to the other so as to maintain the sulfidity at the usual 30%. However, in the present invention Where smelt sulfidities are much higher, the preferred embodiment visualizes adding both salt cake to the furnace and sulfur to the burner to maintain sulfidities at the desired levels.

The preceding flow charts, as represented by FIGURE 1, FIGURE 2 and the examples, illustrate the pulping of Wood by the two processes. These include: first, the digestion of the wood chips by an improved sulfate process utilizing a cooking liquor composed of sodium hydroxide and sodium sulfide together with an amount of native sulfur in amounts which exceed those normally regarded as make-up sulfur to compensate for losses during the recovery cycle. In such a process, the wellknown sulfate recovery would result in excessive buildup of soditun sulfide in the system, while the requirement for successful operation is that there should be an equilibrium state attained to result in liquors with about from 20-30% sulfidity together with a recovery of free sulfur to complete the cyclic process.

Second, the flow charts and examples also depict the pulping of wood chips in liquors composed of a sodium sulfite or bisulfite together With an excess of sulfur dioxide. To complete the cyclic recovery process for this system, there is required an operation to produce alkali or alkaline carbonates together with free sulfur which may be burned to sulfur dioxide and used to convert the latter sodium compounds to sulfites and bisulfites as well as to provide the excess sulfur dioxide for the sulfite cooking liquors.

What we claim is:

1. A combined acid sulfite and a modified sulfate pulping process, and a recovery cycle fully integrated with said combined process allowing liquor recovery for each process at various ratios, comprising the steps of: introducing free sulfur, cellulose-containing material and sulfate pulping liquor into a digesting zone of a modified sulfate pulping process, said sulfur being in excess of from about 20% to about 30% sulfidity of the pulping liquor by about 3.5% to about 5.0% based on the cellulosic material; introducing a high combined base acid sulfite liquor having in excess of 1.2% combined S and cellulose-containing materials into an acid sulfite digesting zone; coking the cellulose-containing materials in the respective digesting zones to obtain pulp; combining spent liquors from said modified sulfate and sulfite digesting zones; obtaining green liquor by evaporating combined spent liquors from both processes after draining the liquors from the pulp, and combining and burning in a reducing furnace the same and, thereafter dissolving and clarifying a high-sulfidity smelt from said reducing furnace; dividing said green liquor into a first and a second portion; recovering by carbonation from the first portion of said green liquor hydrogen sulfide and converting said hydogen sulfide to elemental sulfur by reduction and sulfur dioxide by oxidation of sulfur; combining the element sulfur with said second portion of said green liquor which has been first treated with hydrated lime before being used with the elemental sulfur to obtain a modified sulfate liquor; converting the first treated portion of the green liquor into an acid sulfite cooking liquor of high combined basicity by contacting said first treated portion of the green liquor with the recovered sulfur dioxide; and reintroducing said acid sulfite liquor and said modified sulfate liquor in their respective digesting zones.

2. A combined sulfite and a modified sulfate process of pulping cellulose-containing materials including the continuous recovery of combined liquors from both processes for preparing fresh cyclic liquors for each process, said combined process comprising the steps of:

(a) introducing free sulfur together with sulfate pulping liquor into a digesting zone containing cellulosic materials, said free sulfur being in excess of from about 28% to about 30% sulfidity of the liquor by about 3.5% to about 5.0% based on the bone dry cellulosic material;

(b) introducing a high combined base acid sulfite liquor and cellulosic materials into an acid sulfite digesting zone, said high combined base liquor having in excess of 1.2% combined 80 100 ml. of solution;

(c) cooking the cellulosic material in the respective digesting zones and thereafter combining spent liquors from said digesting zones;

((1) obtaining green liquor from the combined spent pulping liquor from sulfite and modified sulfate digesting zones by burning, dissolving and settling said combined spent pulping liquor;

(e) separating the green liquor into a first and a second stream;

(f) adding to the first stream of green liquor a portion of a carbon dioxide carbonated green liquor stream from the carbonation zone of step (g) and calcium oxide to produce white liquor for reintroduction into the modified sulfate digesting zone;

(g) carbonating the second stream of the green liquor in a carbonation zone in the presence of carbon dioxide and driving off in the carbonating step hydrogen sulfide and carbon dioxide laden gases;

(h) converting the hydrogen sulfide obtained from step (g) in presence of sulfur dioxide into element sulfur;

(i) separating the elemental sulfur into a first stream and a second stream and reintroducing the first stream into the modified sulfate digesting zone;

(j) oxidizing the second elemental sulfur stream to sulfur dioxide and dividing the sulfur dioxide into first, second and third streams;

(k) purifying and introducing the first sulfur dioxide stream into the elemental sulfur producing zone;

(1) purifying and introducing the second sulfur dioxide stream into a decarbonation zone of step (0);

(m) introducing the third sulfur dioxide stream into a sulfiting zone of step (p);

(n) separting the liquid efiluent stream from the carbonation zone of steps (g) into a first and a second stream and introducing the first stream into the first green liquor stream of step (f);

(o) decarbonating the second stream from the carbonation zone of step (g);

(p) introducing the decarbonated effluent from the decarbonation step into the sulfiting zone in presence of sulfur dioxide and;

(q) reintroducing the recovered sulfite cooking liquor into the acid sulfite pulping zone.

3. A combined sulfite and sulfur modified sulfate process of pulping cellulose-containing materials including the recovery of liquor from each process and preparation of fresh liquor at various ratios for each process, comp-rising the steps of: combining a spent modified sulfate liquor and a spent sulfite liquor, said sulfate liquor being moditied for pulping by including from about 3.5% to about 5% of free sulfur in a sulfate liquor having of from about 28% to about 30% sulfidity; evaporating the spent liquors, adding sodium sulfate salt cake to the evaporated liquors to make up for losses occasioned by pulping and liquor recovery and burning the evaporated liquors in a burning zone; separating the burned liquors into a first stream and a second stream after said liquors have been dissolved and clarified to obtain green liquor; combining the first green liquor stream with a carbonated stream derived from part of the second green liquor stream and which has been carbonated with carbon dioxide, a portion of said carbon dioxide being derived from the burning zone flue gases; causticizing with calcium hydroxide said first green liquor stream and said part derived from second green liquor stream which has been carbonized; clarifying the causticized stream; combining the clarified stream with elemental sulfur and introducing these streams int-o the sulfur-modified sulfate pulping process; introducing the second green liquor stream from the burning step into a carbonating zone in the presence of carbon dioxide to effect carbonation of said green liquor and driving off therefrom excess carbon dioxide and hydrogen sulfide; collecting a carbonated smelt from the carbonation zone and dividing it into first stream and second stream, the said first stream being combined with said first green liquor stream from the burning step, the said second carbonated smelt stream being introduced into a decarbonation zone in the presence of sulfur dioxide to drive off carbon dioxide from said smelt and to obtain a decarbonated liquor; combining the carbon dioxide removed in the decarbonation zone with carbon dioxide derived from an elemental sulfur-producing zone and introducing the combined carbon dioxide into said carbonating zone; introducing the decarbonated liquor from the decarbonation zone into a sulfiting zone in the presence of sulfur dioxide; introducing the carbon dioxide and hydrogen sulfide from said carbonation zone in the presence of sulfur dioxide into said elemental sulfur-producing zone; adding make-up elemental sulfur to the elemental sulfur from the sulfur-producing zone and splitting the elemental sulfur stream into first and second streams; combining for pulping the first elemental sulfur stream with the said clarified stream coming from the causticizing zoneghurning the second elemental sulfur stream in presenceof air, and separating the sulfur dioxide into first, second and third streams; introducing the first sulfur dioxide into the sulfiting zone; introducing the second purified sulfur dioxide stream into the elemental sulfur producing zone, introducing the third purified sulfur dioxide stream into the decarbonation zone; and combining the first sulfur dioxide stream with the decarbonated liquor in a sulfiting zone and introducing the resulting sulfite liquor into the sulfite pulping zone.

References Cited by the Examiner UNITED STATES PATENTS 1,602,553 10/26 Richter 162-30 5 1,915,315 6/33 Hoffman 1 62'-33 1,970,258 8/ 34 Textor 16233 2,841,561 7/58 Gray et al. 16233 2,944,928 7/60 Kibrick et a1. 16282 10 DONALL H. SYLVESTER, Primary Examiner.

MORRIS o. WOLK, Examiner.

Claims (1)

1. A COMBINED ACID SULFITE AND A MODIFIED SULFATE PULPING PROCESS, AND A RECOVERY CYCLE FULLY INTEGRATED WITH SAID COMBINED PROCESS ALLOWING LIQUOR RECOVERY FOR EACH PROCESS AT VARIOUS RATIOS, COMPRISING THE STEPS OF: INTRODUCING FREE SULFUR, CELLULOSE-CONTAINING MATERIAL AND SULFATE PULPING LIQUOR INTO A DIGESTING ZONE OF A MODIFIED SULFATE PULPING PROCESS, SAID SULFUR BEING IN EXCESS OF FROM ABOUT 20% TO ABOUT 30% SULFIDITY OF THE PULPING LIQUOR BY ABOUT 3.5% TO ABOUT 5.0% BASED ON THE CELLULOSIC MATERIAL; INTRODUCING A HIGH COMBINED BASED ACID SULFITE LIQUOR HAVING IN EXCESS OF 1.2% COMBINED SO2 AND CELLULOSE-CONTAINING MATERIALS INTO AN ACID SULFITE DIGESTING ZONE; COKING THE CELLOLOSE-CONTAINING MATERIALS IN THE RESPECTIVE DIGESTING ZONES TO OBTAIN PULP; COMBINING SPENT LIQUORS FROM SAID MODIFIED SULFATE AND SULFITE DIGESTING ZONES; OBTAINING GREEN LIQUOR BY EVAPORATING COMBINED SPENT LIQUORS FROM BOTH PROCESSES AFTER DRAINING THE LIQUORS FROM THE PULP, AND COMBINING AND BURNING IN A REDUCING FURNACE THE SAME AND, THEREAFTER DISSOLVING AND CLARIFYING A HIGH-SULDIFITY SMELT FROM SAID REDUCING FURNACE; DIVIDING SAID GREEN LIQUOR INTO A FIRST AND A SECOND PORTION; RECOVERING BY CARBONATION FROM THE FIRST PORTION OF SAID GREEN LIQUOR HYDROGEN SULFIDE AND CONVERTING SAID HYDROGEN SULFIDE TO ELEMENTAL SULFUR BY REDUCTION AND SULFUR DIOXIDE BY OXIDATION OF SULFUR; COMBINING THE ELEMENT SULFUR WITH SAID SECOND PORTION OF SAID GREEN LIQUOR WHICH HAS BEEN FIRST TREATED WITH HYDRATED LIME BEFORE BEING USED WITH THE ELEMENTAL SULFUR TO OBTAIN A MODIFIED SULFATE LIQUOR; CONVERTING THE FIRST TREATED PORTION OF THE GREEN LIQUOR INTO AN ACID SULFITE COOKING LIQUOR OF HIGH COMBINED BASICITY HY CONTACTING SAID FIRST TREATED PORTION OF THE GREEN LIQUOR WITH THE RECOVERED SULFUR DIOXIDE; AND REINTRODUCING SAID ACID SULFITE LIQUOR AND SAID MODIFIED SULFATE LIQUOR IN THEIR RESPECTIVE DIGESTING ZONES.

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