CA1089162A

CA1089162A - Fluidized bed treatment of kraft black liquor with h.sub.2s absorption

Info

Publication number: CA1089162A
Application number: CA245,131A
Authority: CA
Inventors: Howard B. Lange, Jr.
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1975-02-21
Filing date: 1976-02-05
Publication date: 1980-11-11
Also published as: JPS51105401A; SE7601954L; IT1053639B; GB1544446A; IN144575B; MX3460E

Abstract

FLUIDIZED BED TREATMENT
OF

ABSTRACT OF THE DISCLOSURE

Disclosed is a process for treatment of Kraft Black Liquor for the purpose of recovery of valuable chemicals contained therein. This process comprises con-centrating the black liquor, gasifying in a fluidized bed reactor the organic components while allowing the in organic components to remain in the solid form, recover-ing a portion of the heat from the off gases, treating the gases to absorb H2S with absorbing solutions of the nonvolatized inorganic components of the black liquor, and returning the used absorbing solution for treatment to form cooking liquor for the digester.

Description

10~5~16~

The present invention relates to the processing and recovery of chemicals from the residual waste liquor discharged from a Kraft paper pulp digestion process.
A common method of chemical recovery used in ~;
the Kraft Paper Pulping Process starts with the concentrat-ing of what is known as the residual waste liquor. The re-sidual waste liquor is that stream which results from the washing of the pulp after the wood has been digested by ;
the cooking liquor. This stream is rich in valuable chemicals which if recovered may be used to produce the basic cooking liquor employed in the digester. This liquor also contains organic material picked up in the digestion . . .
process of the prepared woods. Generally this residual waste li~uor is concentrated to approximately 65 per cent ~-solids by weight and is sprayed or introduced into the bottom portion of a chemical recovery furnace where the organic portion of the liquor is gasified at temperatures ~-~ . .: .~,: ., in excess of 1800F and the inorganic portion becomes ~ ~
.
molten forming the smelt in the bottom of the furnace. ~-In such a process, hydrogen sulfide ~ S and sulfur dioxide S2 may be generated in amounts in excess of acceptable emission standards. Other air born contaminants such as vaporized sodium compounds in the form of a fine fume not only aggravate the pollution problem, but can also con-dense on the steam generating tubes causing fouling in many cases and a reduction in heat transfer ability. The smelt which is deposited in the bottom part of the furnace can be hazardous. Should leaks develop in the water tubes~
the probability of an explosion from the contact of the water and smelt is quite high. This flowing smelt will be .
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~08916Z

drawn off and treated so as to reconstitute the cooking liquorfor the digestion process.
In view of the foregoing, the desirability of alternate approaches to the recovery problem becomes obvious.
The present invention seeks to overcome the disadvantages of the presently used chemical recovery processes in Kraft pulping processes by the use of low temperature treatment of the residual waste liquor in a reactor so as to gasify the organic portion of the liquor while allowing the inorganic portion to remain in the solid phase. Such treatment eliminates the formation of a smelt and thus eliminates the possibility of a smelt water explosion while simultaneously doing away with the fouling of the heat transfer tubes and the fume pollution problem. The off gases from the gasification step are cooled and are then contacted in an absorbing device with an absorbing medium composed of a solution of the remaining inorganic solids from which any residual carbon has been separated. In such absorption, the H2S is removed from the gaseous phase. Any ;-remaining H2S can be oxidized to SO2 and be subjected to further treatment as necessary. The spent absorbing medium can then be treated to form the cooking liquor which is returned to the digestion process.
Thus, according to the present invention there is provided in a kraft pulping process of cellulosic materials employing compounds of sodium and sulfur in the coo~ing liquor, wherein a residual waste liquor containing digested ceIlulosic organic ~ ~
and sodium bearing inorganic solids is formed, a method of ~ ;
recovering chemicals useful in forming a kraft cooking liquor from the residual waste liquor while avoiding the hazard of smelt-water explosion comprising introducing the residual waste liquor into a lower section of a fluidized bed gasifying unit; gasifying the digested cellulosic organic solids IB

contained in the residual waste liquor in the bed at a tempera-ture not in excess of 1400F to produce a gas phase containing hydrogen sulfide, while maintaining the sodium bearing inorganic solids of the residual waste liquor in a solid state, reducing in the bed at least a portion of the sodium bearing inorganic solids to form sodium sulfide and sodium carbonate;
removing the sodium sulfide and sodium carbonate from the .
fluidized bed unit; and absorbing hydrogen sulfide from the gas phase with aqueous absorbing media containing sodium sulfide ~:
and sodium carbonate. - ~-BRIEF DESCRIPTION OF THE DRAWINGS ~ -Figure 1 depicts the relevant part of a chemical recovery ~:.
process for a pulping process and shows ~ow the present ~;-invention is integrated into this recovery process. This figure also contains a legend so as to better enable one to follow the various streams employed in using this :-~

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~ase 3980 ~~`` ~0891~Z
procev~.
Fl~urc 2 depicts a cutaway view Or a fluldized bed reactor which i5 employed in the preferred embodiment of the present invention.
~ igure 3 depicts a multi-stage absorption system and shows how the same fits into the present invention.
A more fully understandable description follows.

DESC~IPTION OF THE PREFERRED EMBODIMFNTS

Referring generally to Figures 1 and 2, wood, having been prepared for pulping is fed into the digester (not shown) to be digested by the cooking liquor. This liquor is rich in the sulfur-bearing compounds that produce the pulp by the delignification of the prepared wood. The Kraft cooking li~uor generally contains NaOH, Na2S, Na2C3, Na2S4, Na2S03 and Na2S203. This liquor will acquire ~
organic matter from its intimate contact with the wood in ~-the digester. The contents of the digester are transferred ;
to a blow tank (not shown) wherein gaseous products are ~ ented to a condenser (not shown). The pulp and spent cook- -~ ~ 20 ing liquor move on to a pulp washer (not shown) where the chemicals are washed ~rom the pulp. This operation usually takes place in a multiple drum rotary type washer where the chemicals are washed from the pulp. This operation usually ; takes place in a multiple drum rotary type washer where the fl~w of the pulp 1s counter-current to the flow of the wash water. The pulp and the wash liquor are now separated and the pulp goes on for further processing to make paper and other products. The wash liquor~ which is basically a dilute solution of the spent cook~ing liquor containing organic rem-0 nants of the wood moves on for chemical recovery. Because of the color o~ this wash liquor, it is referred to as the black liquor.
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10l~16Z c~ 39~0 The black liquor is introduced into a concentration system. qhis systern is usually in the form of a multlple effect evaporator 10. Such a system can have as many as 5 or 6 effects. A vacuum is usually applied to the evaporator wherein water is driven off by the counter-current flow of steam through the tubes thus concentrating the black liquor to about 50-55 per cent solids by weight. A further concen-tration step may be added though it is not necessary. In ;~
the present invention, black liquor concentrated in the range of 40-70 per cent by weight solids may be employed but better operating economy will be experienced with the use of at least a 50 per cent solids stre~m. It is preferable that the ;~
black liquor be concentrated in the range of 50-60 per cent by weight. The present lnvention can be operative with the solids in a completely dry form if they are so available.
These solidscontain the organic matter ~rom the wood and the sodillm salts of the spent cooking liquor.
In Figurel, the black liquor, stream 12 having been suitably`concentrated is discharged from the multiple ef~ect evaporator 10 and is now ready for the gasification step.
Salt cake Na2S04 may be added to stream 12 i~ so desired~
This would increase the concentration o~ the stream and pro- .
vlde additional recovery benefits. Such addition may be made in a mix tank (not shown). The gasification step will result in the organic content of the concentrated black liquor stream 12 being driven off in a gaseous state. The ~ ~ .
procedure is carried out so as to allow the inorganic con-stituents of stream 12 to remain in thé solid phase. In the preferred embodiment of thepresent invention, this gasification ``
step is carried out in a ~luid bed reactor 16 at a temperature of 1300-1400F. Reference to Figure 2 will aid in understand-ing the method of practlcing this step of the invention. The ,.. ,. ' `' ``

108916Z case 39~0 flui.d bed rcactor 16 baslcally conslsks of a brick-lined housin~ 17 (brick-lining not shown~ containing a bed of material 18 supportcd on a plate 28 with perforations 29.
The bed of material 18 can be composed of inert materials or inor~anic chemicals useful in the pulping process. Thus, bed 18 can be composed of sodiwn carbonate Na2C03, sodium sulfide Na2S, sodium sulfate Na2S04 and hot char. In the preferred method of practice of this invention, the black llquor stream 12 will be introduced into the lower region ~0 20 of this bed 18. Stream 12 will be introduced by nozzle means 22. In the actual practice of the invention a number ~ ~' of nozzle means 22 will be placed about the perimeter of -~ ' reactor 16. Nozzle means 22 may all be connected to a header pipe 24 which can surround reactor 16 and will distribute stream 12 to the various nozzle means 22. The nozzle means 22 should be arranged so as to introduce and to give even distribution of stream 12 over the cross section of the lower .. .
region 20 of bed 18. In practicing this invention stream 12 ahould be dispersed and atomized by nozzle means 22 so as to generate droplets of a size in the 500~1000 micron range.
Air a'nd gas, stream 53 is introduced into reactor 16 beneath '~
- support plate 28 and passed upwardly through perforations 29.
' It is desirable to minimize the amount of air used ~or gasi~
~; ri'cation so às to restrict the forma;tlon of C02. The C02 ' -interferës with the absorption'of the H2S. ;As such, it will ` ;~ be found beneficiai to limit the ~ontent'of stream 53 so tha~ it contains 1-2 lbs. dry air per lb o~ dry solid in stream 12; The air portion may be preheated to approxlmately ~ -. .
300-600F as shown ln Figure 1. Stream 53 provides the fluidiæing medium for bed 18 and conveys the thermal energy required for the gasification step. Fuel, stream 14 is combusted in a conventional type fuel burner (not shown) to pr~vide the he~t content necess~ry to ca-ly out the 10~il91~Z cAse 39~o gasification. l'he u~lard velocity of strcam 53 through bed 1~ i~ in the ordcr Or 1-10 f`eet per second depending on the size Or the dropleks generated by noz~le means 22.
Bed 18 should be of sufricient hei.ght to allow the required residence time for the liquid droplets so as to bring about the gasification of the organic material and allow the sodium sulfur-bearing compounds, including sodium sulfate Na2S04 to be reduced to sodium sulfide Na2S. The residence time of the liquid dropiets is estimated to be ~ -of the order of 30 minutes per 1 foot of bed height. Such residence time may vary depending on the concentration of ':.
stream 12 and the quantity of stream 53 relative to stream . 12. ~ -. As mentioned before, the fluidized b'ed 18 is ; operated at a temperature of 1300-1400F. Such an operat-ing temperature range is essential to the practice of this ., invention when processing residual waste li~uors from a Kraft . ' .
process since the lowest melting point of the sodium salts .' ~orm'ed is approximately 1400F. If higher temperatures are ~ :
20 ' used, a molten phase and the attendant vapors may.form re- ;
sulting in the formation of sodium fumes. If the fluid bed reactor 16 is operated under increased pressure.s one can '~
expect increased treatmént capàcity. Accordingly, other ad~ustments to the system.will.be''réqui'red shouid.this '' . . mode of operation be practiced. . :~
~ . . . . .
While the organic portion of stream 12 has been gasified, the inorganic portion has remained in the solid . phase in the form of a particulate. This particulate .. -. would typically contain sodium carbonate Na2C03 at about 3 90 per cent by weight, sodium sulfide Na2S at about 9 per cent, sodium sulfate Na2S04 at Iess than 1 per cent and ~:

unburned carbon at less than 1 per cent Such materials ' .
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cas e 39~0 can be use~ to rorm ~cd 18. There will be extra particulate in the form of a~glomerate, which will be removed by down-comer means (not shown), and fines which will be carried over in stream ~2 and separated in the cyclone 36. Removed particulate is sent to the dissolving tankll4 but the agglomerate may contain substantial unburned carbon,therefore it may be recycled to the feed liquor 12. Preferably, this recycled portion is passed through a comminuting device such as a crusher (not shown) prior to its introduction into stream 12. ' Stream 53,having passed through bed 18,will pass into a free board area 19 located within reactor 16 above bed 18 and then be exhausted through exhaust opening 30.
This exhaust stream which is now indicated in the drawings as stream 32 is expected to have the following approximate composition by volume due to the reactions having taken place in reactor 16: C0 - 9~; C02 ~ ; H20 - 23%;
H2 - 21%; N2 - 35% and H2S - 0 2 - o. 8~. Stream 32 will also contain an amount o~ particulate matter (i.e. the fines 20 ' mentioned above) which has been picked up from the bed 18.
The upper portion of housing l~ may be enlarged in cross ~ sectional area so as to lower the upward velocity below ' ` ~ the conveying velocity o~ the particulate causing part o*
the entrained solids to fall out.''The stream 32 temperature w1ll b~ somewhat less than 1400F. The'exhaust stream 32 '~
is ducted to a gas cleaning device 36 which is preferably ~-' a dry type cyclone. A multiclone or high efficiency cyclone is pre~erred but such e**iciency req~irements are llOt necessary for the practice of ~his invention. The particulate matter collected in and discharged from gas cleaning device 36 can be processed further along wlth ~hat portion of stream 33 not recycled to black liquor .~, . .
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Case 3980 '~08gl6Z

stream 12. Strc~m 34 on the drawines is thc result if such comblnation is desired. Stream 34 delivcrs the parti-culate which consists of mainly the inorganic components of stream 12 to a dissolving tankll4 wherefrom a portion of the later described absorbing solution may be drawn.
Exhaust stream 32, having been cleaned of particulate matter by air cleaning device 36,is exhausted from device 36 and is designated as stream 38. The thermal content of stream 38 is quite high and part of this may be recovered by a heat recovery unit 40. Conventional steam generating units or waste heat bo~lers can be used for this purpose, wherein part of the thermal content o~ stream 38 is transferred by pre~erably indirect means to a liquid phase 42 to gene,rate a steam or vapor phase 44. This re~
c~vered thermal value may be employed in other parts of ~he pulping process or the plant. It will be of interest -~
that due ~o the elimination of the sodium ~ume, the heat .
trans~er surfaces (not shown) of heat recovery unit 40 will require less maintenance since no fume can ~ondense on the transfer surfaces. The exhaust o~ the heat re~
covery unit, stream 46, has been cooled to 400-700F by the use of conventional heat recovery equipment 40 and may still be rich in thermal content. Such thermal content~ ;
may~ be recovered by a heat exchange device 4O Such device ;~
48 may be an air heater wherèin ambient air 50 may be heated to 300-600F. Such heated-air indicated as stream 52 may ultimately be used to form stream 53 which is introduced into a lower area 26 beneath support plate 28 ~` in reactor I6. Again the~heat exchange process is pre-~erably an ~ndirect one and should pre~erably cool ex-haust gas stream 46 to a temperature o~ about 350-400F.~ -is ~as str~am~indicated as stream 56 in the drawings ,, . . . ~, .- ~ :

.10~91~;2 case 39~30 i8 no~l rcady I'or the a~sorptlon process more ~ully herein-after described.
The absorption step is concerned with removal of the hydrogen sulfide ~2S gas from exhaust stream 56.
As indïcated by the above approximate flue gas analysis, the H2S contaminant will usually be present in an amount less than or equal to one (1) per cent by volume. Loss of the sul~ur-bearing cornpound is undesirable from the point o~ view of chemical recovery as well as air pollution conslderations. The gas stream leaves absorption system 58 as gas stream 60 and may go on for further treatment.
As shown in Figure 1, stream 60 enters condenser 68 wherein stream 60, which was at approximately 140-170F, and is expected to have the following approximate composition on '~ -a Yolume basis: C0-9%; CC~-10%; E~20-25%; H2~21%; N2-35~ ,~
and H2S-002-.06% may then be cooled to approximately 100F
using an indirect heat trans~er mechanism. The cooling M u1d is cold water stream 70 which results in hot water stream 72- and condensate stream 740 The gas stream exits 20 , condenser 68 as stream 76 and will have a considerably ' , -~
lower moisture content. Stream 76 may then be introduced lnto gas boiler 78 where, by the combustion of fuel 80 with air 82, the C0 will oxidize to C02, H2 to H20 and H2S
to S02. I~ater, 84 may be used for thermal rec~very re~
~, ~ sulting in stream 86. The gas 88 may then be ven~ed to ,~
~; atmosphere. ,~
~; ~ ' Most of the inorganic solids retrieved from reactor 16 have been,led to dissolving tank 114 wherein .
the inorganic solids preY~ously mentioned are dissolved in , 3o ~ an aqueous solukion along with make-up chemicals 110 and make-up water 111. The effluent liquld stream 116 (which is re~erred to as the green liquor)is led to a clarifier , It is here that any re~ain~na carbon and insolubLe :, .
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iO891~2 Case 39~0 wood ~sh 322 ~rc scparatcd from stream llG producing clarified strcam 120. Stream 120 enters storage tank - 124 from which may bc drawn as necessary, stream 126~
which contributes to the formation of the absorbent 62.
Absorption system 58 c~mprises a multi-stage contacting arrangement. It is to be understood that the term contacting stage refers to a zone wherein the gas ~-phase containing H2S contacts a liquid phase~ the result being mass transfer of the H2S from the gas to the con-tacting liquid phase. A typical arrangement of the absorption system and a corresponding liquid circuit is ~;
shown in Figure 3 ~or a multi-contacting stage approach. ~ ~ -Of course, it will be understood that the present invention ~ -can be practiced with more than two contacting stages but for convenience and simplicity of description, only two such stages are depicted in Figure 3 In Figure 1, stream 56 is ducted to the absorption ;
system 58, and liquid stream 62 is piped to the absorption ^~
system 58 wherein the two phases will be brought into intimate contact so as to allow the mass trans~er phenomenon , . , to proceed. The scrubbed gas stream emerges from absorption system ~8 as gas stream 60. The spent absorbing medium contalning primarily Na2C03, Na2S and NaHS exits the absorption system as stream 66 and is split into stream 66A which is piped to storage tank 67 and stream 66B which ~is~piped to storage tank 124. The liquid from tank 67 shown as stream 67A is piped to causticizer 90 wherein slaked lime 9? is added to form white liquor and CaC03. The whi~te liquor contains NaOH, Na2S-and Na2C03~ Part of the Na2C03 has been converted to NaOH which has converted a ~portion of the NaHS to Na2S. Stream 94 is transferred to filter drum 96 wherein the filtered solution 98 is .

10891~;Z c~g~ 39~0 scpa.rated from the CaC03 solids. A portion of stream 98 is rcturncd via stream 100 and piped to storage tank 67 ~la stream lOOA for odor control purposes and to storage tank 124 via stream lOOB wherein it combines with NaHS to produce additional Na2S. The remaining portion of stream 98 is ~ sedonfor disposal. Slurry 102 from filter drum 96 goes to a second filter drum 104 where the CaC03 is washed with water 106. The offcoming liquid phase 108 is known as weak wash and may be reused in the process. Accordingly, it is piped via stream108A to dissolving tank 114 and via stream 108B to storage tank 124. The CaC03, 112, is returned to the lime kiln (not shown) wherein lime CaO, is regenerated -~
for use in causticizer 90. ;~
Figure 3 depicts the preferable arrangement of ~ ~
absorption system 58 as being one with two (2) contacting ~ ~ ;
stages. It has been fou~ld that such a two-stage contact~
ing absorption system is most beneficial in the practice of this invention. It has been further found that Venturi-like contacting stages give greater effectiveness. It will be understood that the present invention may be practiced using other conventional mass transfer equipment in the absorption sy~tem. Such substitutions may include a packed or plate tower or cyclonic scrubbers. While in many absorption devices, the direction of flow of the absorb-ing medium is concurrent with that of the gas stream, `~
the present invention may be practiced USillg either con-current or counter-current flow. In employing a multi-stage arrangement as depicted in Figure 3, it is desirable to employ a counter-current flow pattern. It also should ~;
be understood that while two distinctly separate contact stages are depicted in Figure 3, a single piece of equip-ment which houses the two stages may or may not .. ... . . .

case 39~0 10891~Z

separa~! the two stages by a distinctly defined separationzone, may be used for the practice o~ the invention.
In Figure 3, ~as stream 56 enters the first absorption stage 58A wherein lt is accelerated and brought into intirnate contact with stream 62A which preferably has been shattercd into liquid droplets either by stream 56 shattering a liquid film or by nozzle means (not shown).
Stream 56 is cooled and picks up liquid droplets which can be separated from the gas stream in separating device 58B.
This separating device 58B may employ a cyclonic effect or a baffled gas path to separate the droplets from the gas.
The collected liquor exits the separator device 58B and is referred to as 65 in Figure 3. Gas stream 56 exits the separator 58B as stream 56A and is ducted to the second absorption stage 58C which may be substantially identical to 58A. Contact is made with absorbing medium 62 whereby an additional amount o~ HzS is absorbed. Gas stream 56A then passes into separating device 58D which may be substantially identical to 58B. The separated liquid exits separating device 58D as stream 64. It should be pointed out that substitution of a wet cyclone or cyclonic scrubber for separator 58B will prodùce substantially the same results as the system described above. Those familiar with the Art will recognize that slight modifications of the liquid circuit will be necessitated in the event that such substitution is made.
Although the absorption stages 58A and 58C may be substantially identical as regards the actual equipment, each~sta~e will operate somewhat di~ferently in that the second stage 58C will be expected to remove the H2S at a greatly reduced concentration in comparison with that as seen by absorption stage 58A. As such, in the preferable ~, ... . . . . . .
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c~s~ 39~0 ~08~6Z

mode Or practl(c, cach stagc will bc operated somewhat dlfferently, th~ rnaln dif`fcrence being in the relative amounts of the constituents in the absorbing media 62 and 62A.
~ s previously indicated, gas stream 56 will contain approximately 1~ H2S by volume or 10,000 ppm. It is well known to those skilled in the art that scrubbing such a stream ~lith alkaline green liquor, which is available from storage tank 124 will give respectable absorption e~cierAcies and can reduce the H2S concentration down to about 1000 ppm (.1%). As indicated by Figure 3, a portion of stream 126, 126B will combine with stream 64B to form 126C which will unite ~ith 65A to form absorbing medium 62A. Such com~
bination will give a partial recycle effect as well as a counter-current flow effect so as to use the recovered ~-chemicals most efficiently. A portion of the spent absorbing medium 65,which contains Na2C03, Na2S and NaHS
will be returned to storage tanks 67 and 124 via stream 66 which splits into 66A and 66B respectively. Exposure to white liquor will convert NaHS to Na2S.
Gas stream 56A will contain H2S at approximately .1% by volume. Absorption of H2S at~such low concentrations has proven to be a formidable problem when C02 is present.
~- Fortunately, a practical solution to the problem is avail~
able in the teachings of Markant et al in U.S. Patent No. 3,471,249, A System for Absorbing H2S Gases. Markant points out that critical ratios of certain constituents ~: .
of the absorbing media and the H2S in the gas must be malntained to bring about the desired resu-l~s. l!he 30 absorbent medi~ml should contain sodium sulfide Na2S, some sodium hydrosul~ide NaHS, and sodium carbonate Na2C03.

10~91~2 Case 3980 The flo~l ratio of the absorbing medium to the gas contain-ing ~l2S shou]~ be in the range o~ 6-10, on a weight basis.
Na2S should be present in such amounts so that the welght ration I~a2S/H2S is 35 or greater. Furthermore, Na2C03 should be controlled so that the weight ratio Na2C03/
H2S is 30 or greater. Also, the'molar concentration of Na2S is to be greater than 0.1 of the molar concentration of sodium hydrosulfide NaHS. Such ratios can be established and maintained by the intermixing and control of various streams of the recovery process. To further aid in main-taining'these ratios~ part or all of the makeup chemicals required by the rnill can be added to the absorber feed streams.
Markant recommends that the makeup chemicals such as MaOH, ~' Na2S or Na2C03 be added at the inlet side of the recirculation pump (not shown) and so may be the practice here. However, a separate mixing tank 130 may be provided whereby these constituents are added to maintain the critical ratios mentioned ab'ove, or one may employ dissolving tank 114 for such purpose. Na2C03 may be introduced into the system by addition to the causticizer 90. Absorbing med~m 62 having been contacted with gas stream 56A, leaves separator 58D as stream 64. A portion will be a recycle stream 64A, a portion 64B will combine with stream 65A and the remainder, which combines with 66B will return to storage tank 124.
The recycle stream 64A will unite with stream 126A to form stream 128. In the preferabIe form of the invention, stream 128 enters mix tank 130 wherein chemical additions ~32 are made tQ achieve the Markant ratios thus forming absorbing medium 62. As will be apparent to those skilled in the Art, proper employment of automatic or manual means for control and mixing of the varlous streams in various amounts will re-sult ln form~n~ nbsorbin~'medium 62 with mai.ntenance of the c~se 3980 108~1~;Z

proper ratios.
~ hi.le ln accordance with the provisions of th~
statutes there is :;llustrated and described herein a specific embodlment of` the invention, those skilled in the Art will understand that changes may be made in the form : :
of the invention covered by the claims, and that certain features of the invention may sometimes be used to .
advantage ~lithout a corresponding use of the other features.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a kraft pulping process of cellulosic materials employing compounds of sodium and sulfur in the cooking liquor, wherein a residual waste liquor containing digested cellulosic organic and sodium bearing inorganic solids is formed, a method of recovering chemicals useful in forming a kraft cooking liquor from the residual waste liquor while avoiding the hazard of smelt-water explosions comprising:
introducing the residual waste liquor into a lower section of a fluidized bed gasifying unit; gasifying the digested cellulosic organic solids contained in the residual waste liquor in the bed at a temperature not in excess of 1400°F to produce a gas phase containing hydrogen sulfide, while main-taining the sodium bearing inorganic solids of the residual waste liquor in a solid state, reducing in the bed at least a portion of the sodium bearing inorganic solids to form sodium sulfide and sodium carbonate; removing the sodium sulfide and sodium carbonate from the fluidized bed unit; and absorbing hydrogen sulfide from the gas phase with aqueous absorbing media containing sodium sulfide and sodium carbonate.

2. A method as in Claim 1 wherein the residual waste liquor is concentrated prior to introducing it into the gasifying unit.

3. A method as in Claim 1 wherein the gas phase contains no more than one percent hydrogen sulfide by volume.

4. A method as in Claim 1 wherein the absorption step comprises contacting the gas phase at least twice with absorbing media while accelerating the gas phase.

5. A method as in Claim 2 wherein the residual waste liquor is concentrated so as to have at least 40 percent solids by weight.

6. A method as in Claim 1 wherein the gas phase from the gasification unit is cooled prior to absorbing the H2S.

7. A method as in Claim 6 wherein the absorbing media leaving the absorption step are treated so as to form the kraft cooking liquor.

8. A method as in Claim 7 wherein the sodium sulfide and sodium carbonate removed from the fluidized bed unit are treated to form the aqueous absorbing media.

9. A method as in Claim 6 wherein the gas phase is cooled to below 400°F.

10. A method as in Claim 2 wherein the gas phase contains no more than one percent by volume of hydrogen sulfide.

11. A method as in Claim 2 wherein the absorption step comprises contacting the gas phase at least twice with absorbing media while accelerating the gas phase.

12. A method as in Claim 11 wherein the weight ratio of absorbing medium used in the second contacting step to the gas is in the range of 6-10:1; the weight of Na2S in the absorbing medium to the H2S in the gas is greater than 35:1;
the weight ratio of Na2CO3 in the absorbing medium to the H2S
in the gas is greater than 30:1; and NaHS is present in an amount such that the molar concentration of Na2S in the absorbing medium is greater than .1 times the concentration of the NaHS in the absorbing medium.