CA1104765A - Process for recovery of chemicals from pulping waste liquor - Google Patents
Process for recovery of chemicals from pulping waste liquorInfo
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- CA1104765A CA1104765A CA273,974A CA273974A CA1104765A CA 1104765 A CA1104765 A CA 1104765A CA 273974 A CA273974 A CA 273974A CA 1104765 A CA1104765 A CA 1104765A
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Abstract
PROCESS FOR RECOVERY OF CHEMICALS
FROM PULPING WASTE LIQUOR
Abstract of the Disclosure:
A process for recovery of chemicals from a sodium sulfite pulping waste: liquor is disclosed in which a smelt obtained from the waste liquor is introduced into an aqueous slurry containing solidified smelt while make up water and a weak aqueous slurry are supplied to effect incompletely dissolving the smelt into the aqueous slurry to maintain the content of total solid and the temperature of the slurry at constant levels, the resulting aqueous slurry is subjected to a solid-liquid separation to obtain a wet cake having the molar ratio of S/Na20 substantially equal to that of the smelt, the wet cake is mixed with hot particles of sodium carbonate and sodium sulfite while hot air is supplied to effect oxidation of sodium sulfide in the wet cake to sodium sulfite and then the oxidized product is dissolved in aqueous medium and sulfur dioxide-containing gas, preferably the exhaust gas from the recovery boiler, is contacted with the resulting aqueous solution to convert sodium carbonate into sodium sulfite, whereby overall process is carried out in a closed system and the sulfur component and the sodium component present in the waste cooking liquor are recovered and regenerated into a cooking liquor.
FROM PULPING WASTE LIQUOR
Abstract of the Disclosure:
A process for recovery of chemicals from a sodium sulfite pulping waste: liquor is disclosed in which a smelt obtained from the waste liquor is introduced into an aqueous slurry containing solidified smelt while make up water and a weak aqueous slurry are supplied to effect incompletely dissolving the smelt into the aqueous slurry to maintain the content of total solid and the temperature of the slurry at constant levels, the resulting aqueous slurry is subjected to a solid-liquid separation to obtain a wet cake having the molar ratio of S/Na20 substantially equal to that of the smelt, the wet cake is mixed with hot particles of sodium carbonate and sodium sulfite while hot air is supplied to effect oxidation of sodium sulfide in the wet cake to sodium sulfite and then the oxidized product is dissolved in aqueous medium and sulfur dioxide-containing gas, preferably the exhaust gas from the recovery boiler, is contacted with the resulting aqueous solution to convert sodium carbonate into sodium sulfite, whereby overall process is carried out in a closed system and the sulfur component and the sodium component present in the waste cooking liquor are recovered and regenerated into a cooking liquor.
Description
~47~s ~his invention relates to a process for recovery of chemicals from a pulping waste liquor, and more particularly to a process for recovery of chemicals from a smelt which is obtained by combustion of concentrated pulp cooking waste liquor containing a sulfite or a bisulfite.
Various processes for producing pulp from cellulosic material, for example, wood chips have been practiced.
Among them, more interest has recently been dra~n to a process in which the cooking chemicals are sodium sulfite or sodium bisulfite in combination with sodium carbonate, because cellulosic pulp is produced in high ~ield to make the process economical. However, in this process no effective commercial recovery ~ystem of chemicals from the waste liquor has been established.
~ 15 In Japanese Patent Publication 14401/74, we proposed ;~ a system for recover~ of chemicals comprising conce~trating C! a cooking waste liquor from sodium sulfite process, bur~ing the concentrate to obtain a smelt mainly comprising sodium j sulfide and sodium carbonate, oxidizing the smelt with air `~' 20 to convert Rodium sulfide into sodium sulfite, di~solvingthe oxidized ~aterial in water and treating the resultin~
aqueous solution with a sulfur dioxide-containing gas to convert sodium carbonate i~to sodium sulfite, thereby regenerating an aqueous solution which can be used as a cooking liquor in the pulping process.
Although this prior procsss deals with fundamental technical co~cept for recovering chemicals from sulfite pulping wa~te liquor, there are still many problems to be solved. For example, the smelt contains, in addition to sodium sulfide a~d sodium carbonate, a small amount of -- 2 ~
.. . - .. .. .
~4 ~S
sodium sulfate and slight amounts of sodium thiosulfate t sodium sulfite and sodium chloride some of which are hard to convert into the desired chemicals, and the proportion of various chemicals present in the smelt may var~ over a wide range depending upon the cooking conditions employed in the particular pulp mill; accordingly, it is difficult to standardize the conditions under which the recover~ of chemicals is practiced and to design an apparatus suitable for carrying out the process.
Accordingly, an object of thi~ invention is to provide -~; a proceæs for recovery of chemicals ~rom sulfite pulping waste liquor without causing environmental pollution and any appreciable 108s of chemicals.
; When a typical waste liquor from semichemical sulfite process is concentrated and burned, the resulting smelt has the following compositio~ r~nges (by weight):
Na2S 30 to 40%
Na2C3 45 to 60~o Na2S4 5 t O l O~o Na2S203 0 - 5~
others 2 - 4%
It ha~ alread~ been know~ that, when smelt is treated with a molecular ox~gen-contai~ing gas in the presence of water, the sodium ~ulfide and sodium sulfate are oxidized a¢cording to the following reaction formulae:
2Na2S + 32 2Na2$3 ............................ (1) Na2S + 202 ~ Na2S4 ........ (2) 347~:5 Na2SO~ + 1/2 2 ~ ~a2~4 ~ --- (3)
Various processes for producing pulp from cellulosic material, for example, wood chips have been practiced.
Among them, more interest has recently been dra~n to a process in which the cooking chemicals are sodium sulfite or sodium bisulfite in combination with sodium carbonate, because cellulosic pulp is produced in high ~ield to make the process economical. However, in this process no effective commercial recovery ~ystem of chemicals from the waste liquor has been established.
~ 15 In Japanese Patent Publication 14401/74, we proposed ;~ a system for recover~ of chemicals comprising conce~trating C! a cooking waste liquor from sodium sulfite process, bur~ing the concentrate to obtain a smelt mainly comprising sodium j sulfide and sodium carbonate, oxidizing the smelt with air `~' 20 to convert Rodium sulfide into sodium sulfite, di~solvingthe oxidized ~aterial in water and treating the resultin~
aqueous solution with a sulfur dioxide-containing gas to convert sodium carbonate i~to sodium sulfite, thereby regenerating an aqueous solution which can be used as a cooking liquor in the pulping process.
Although this prior procsss deals with fundamental technical co~cept for recovering chemicals from sulfite pulping wa~te liquor, there are still many problems to be solved. For example, the smelt contains, in addition to sodium sulfide a~d sodium carbonate, a small amount of -- 2 ~
.. . - .. .. .
~4 ~S
sodium sulfate and slight amounts of sodium thiosulfate t sodium sulfite and sodium chloride some of which are hard to convert into the desired chemicals, and the proportion of various chemicals present in the smelt may var~ over a wide range depending upon the cooking conditions employed in the particular pulp mill; accordingly, it is difficult to standardize the conditions under which the recover~ of chemicals is practiced and to design an apparatus suitable for carrying out the process.
Accordingly, an object of thi~ invention is to provide -~; a proceæs for recovery of chemicals ~rom sulfite pulping waste liquor without causing environmental pollution and any appreciable 108s of chemicals.
; When a typical waste liquor from semichemical sulfite process is concentrated and burned, the resulting smelt has the following compositio~ r~nges (by weight):
Na2S 30 to 40%
Na2C3 45 to 60~o Na2S4 5 t O l O~o Na2S203 0 - 5~
others 2 - 4%
It ha~ alread~ been know~ that, when smelt is treated with a molecular ox~gen-contai~ing gas in the presence of water, the sodium ~ulfide and sodium sulfate are oxidized a¢cording to the following reaction formulae:
2Na2S + 32 2Na2$3 ............................ (1) Na2S + 202 ~ Na2S4 ........ (2) 347~:5 Na2SO~ + 1/2 2 ~ ~a2~4 ~ --- (3)
2 H20 + 202 ~ Na2S203 + 2NaOH (4) ~eaction (1) occurs at a relativel~ low temperature a~d reactions (2) and (3) at relativel~ high temperature.
In addition to the reactions mentioned above, the following side reactions concurrently occur:
Na2S203 + 2~aOH ~ 3/4 Na2S03 + 2/3 Na2S + H20 Na2~203 + 2NaOH ~ 2 ~ 2~a2$03 + E20 .................... (6) On the other hand, the sodium carbonate which is one component of the smelt is unchanged during the oxidation treatment.
~hus, in general, such oxidation treatment i~volves 1 various reactio~s and the primar~ purpose is to convert ; 15 the sodium sulfide into sodium sulfite but to prevent the formation of sodiu~ sulfate and sodium thiosulfate which are inactive in the pulping process. Eowever, in practice, it is difficult to control the oxidation treatment to such an extent that onl~ the desired reaction (1) will occur.
Further, an agueous solution of the oxidized product is treated with a sulfur dioxide-containing gas to convert the sodiu~ c~rbonate into sodium sulfite. ~he source of said sulfur dioxide is, in ge~eral, an exhaust gas from the recovery boiler and substantiall~ all of the sulfur dioxide released durin~ the combustion of concentrated waste liquor is recovered b~ being absorbed in the aqueous solution to re~enerate a cooking liquor whereby the overall process can be operated as a closed system.
Accordingl~, in order to success~ully carry out ~0 the process, it is essential that, in the mi~ture to be .... . .
47~5 oxidized, the molar ratio of ~/Na20 be maintained sub-stantially equal to that of the smelt and the water content be kept at an appropriate level.
In general, the temperature at which the oxidation is effected is controlled by adjusting the amount of water co~tained in the reaction mixture, consequently, by adjusting the amount of heat removed from the mixture by evaporation of water, so that the reaction temperature is maintained within a range within which sodiu~ sulfide i8 effectively converted into sodium sulfite.
According to this invention7 the ~olar ratio of S/~a20 in the reaction mixture is readily controlled by adjusting the amount of aqueous slurry to be supplied to a solid-liquid separation step and the water content of the wet cake to be oxidized and by establishing the balance between the ~ake up water to be supplied to the aqueOu~
slurr~ and the a~ount of water lost from the s~stem, ~ai~l~
b~ evaporation in the s~elt hopper and the oxidizers.
According to this invention, there is provided a process for recover~ of chemicals from sodium sulfite pulping waste liquor compri~ing steps of:
(1) introducing a smelt obtained by burning a concentrated waste liquor into an aqueous slurr~
while suppl~ing make up water and a weak aqueous slurr~ rec~cled fro~ step (2) to effect inco~plete dissolving of the smelt into the aqueous slurry and suppl~ing a part of the resulting aqueous slurr~ to step (2) thereby ~aintaining the aqueous slurry to a total solid content of from about 35 to about 70%
b~ weight, a proportion of sodiu~ carbonate in the 7~5 total solid material lower than that of the s~elt and a te~perature of from about 55 to about 90C, (2) separating the slurr~ formed in step (1) into a wet cake containing water in a proportion of from about 10 to about 50% b~ weight and having a molar ratio of S/Na20 substantially equal to that of the smelt and a weak aqueous slurry, and recycling the weak slurr~ to step (1), and
In addition to the reactions mentioned above, the following side reactions concurrently occur:
Na2S203 + 2~aOH ~ 3/4 Na2S03 + 2/3 Na2S + H20 Na2~203 + 2NaOH ~ 2 ~ 2~a2$03 + E20 .................... (6) On the other hand, the sodium carbonate which is one component of the smelt is unchanged during the oxidation treatment.
~hus, in general, such oxidation treatment i~volves 1 various reactio~s and the primar~ purpose is to convert ; 15 the sodium sulfide into sodium sulfite but to prevent the formation of sodiu~ sulfate and sodium thiosulfate which are inactive in the pulping process. Eowever, in practice, it is difficult to control the oxidation treatment to such an extent that onl~ the desired reaction (1) will occur.
Further, an agueous solution of the oxidized product is treated with a sulfur dioxide-containing gas to convert the sodiu~ c~rbonate into sodium sulfite. ~he source of said sulfur dioxide is, in ge~eral, an exhaust gas from the recovery boiler and substantiall~ all of the sulfur dioxide released durin~ the combustion of concentrated waste liquor is recovered b~ being absorbed in the aqueous solution to re~enerate a cooking liquor whereby the overall process can be operated as a closed system.
Accordingl~, in order to success~ully carry out ~0 the process, it is essential that, in the mi~ture to be .... . .
47~5 oxidized, the molar ratio of ~/Na20 be maintained sub-stantially equal to that of the smelt and the water content be kept at an appropriate level.
In general, the temperature at which the oxidation is effected is controlled by adjusting the amount of water co~tained in the reaction mixture, consequently, by adjusting the amount of heat removed from the mixture by evaporation of water, so that the reaction temperature is maintained within a range within which sodiu~ sulfide i8 effectively converted into sodium sulfite.
According to this invention7 the ~olar ratio of S/~a20 in the reaction mixture is readily controlled by adjusting the amount of aqueous slurry to be supplied to a solid-liquid separation step and the water content of the wet cake to be oxidized and by establishing the balance between the ~ake up water to be supplied to the aqueOu~
slurr~ and the a~ount of water lost from the s~stem, ~ai~l~
b~ evaporation in the s~elt hopper and the oxidizers.
According to this invention, there is provided a process for recover~ of chemicals from sodium sulfite pulping waste liquor compri~ing steps of:
(1) introducing a smelt obtained by burning a concentrated waste liquor into an aqueous slurr~
while suppl~ing make up water and a weak aqueous slurr~ rec~cled fro~ step (2) to effect inco~plete dissolving of the smelt into the aqueous slurry and suppl~ing a part of the resulting aqueous slurr~ to step (2) thereby ~aintaining the aqueous slurry to a total solid content of from about 35 to about 70%
b~ weight, a proportion of sodiu~ carbonate in the 7~5 total solid material lower than that of the s~elt and a te~perature of from about 55 to about 90C, (2) separating the slurr~ formed in step (1) into a wet cake containing water in a proportion of from about 10 to about 50% b~ weight and having a molar ratio of S/Na20 substantially equal to that of the smelt and a weak aqueous slurry, and recycling the weak slurr~ to step (1), and
(3) mi~ing the wet cake with hot particles containing sodiu~ sulfite and sodium carbonate while supplying ~imultaneousl7 a mole~-ular ox~gen-contai~ing gas to effect oxidation of sodium sulfide present i~ the wet cake into sodium sulfite.
~his invention will be explained in detail.
~irst steP
A smelt which is formed in conventional recovery boiler by co~bustion of a concentrated pulping cooking waste liquor i8 discharged to smelt receiving means comprising a smelt hopper and a screen. ~gainst the s~elt strea~ directed into the hopper, a steam or air jet iæ impinged to cool and divide out the smelt into solid particles. An aqueous slurry rec~cled from a circulating tank is flowed downwardly on the internal surface of the hopper to re~ove ths particles attached thereto. ~he solid particles drop onto a conveying ~5 screen provided below the hopper to separate lumps or large particles and particles, the former being supplied to a ~reen liquor tank in which the lumps are dissolved in an aqueous medium introduced from a gas washer and the latter bein~ supplied to a slurry circulating tank.
The atmosphere of the smelt hopper is maintained _ ``
. . .
47~S
under a reduced pressure b~ means of air fan which discharges the air in the hopper to the atmosphere after being treated in a gas washer, and the air passinæ through the hopper effects oxidation of a part of the sodium sulfide present in the smelt and the rec~cled slurr~ into sodiu~ thiosulfate.
(Hereunder this is referred to a preliminary oxidation~) The circulating tank to which solidified smelt particles are supplied, contains a large amount of the aqueous slurr~ to which simultaneousl~ a weak green liquor and/or a weak aqueous slurry rec~cled from the second step are supplied thereb~ effecting partial or incomplete di~solving of the chemicals in the aqueous slurry. By "partial or inco~plete dissolving", we mean a state in which the final mixture of the chemi~als and the aqueous medium contains some undissolved solid ~aterial, so that the for~ation of a slurr~
by precipitati~g solid particle fro~ an aqueous solution with cooling or from an aqueous supersaturated solution is also within this meanlng.
The temperature of the aqueous slurr~ is ~aintai~ed at fro~ 55 to 90C, preferabl~ 75 to 83C b~ a cooling means, for exa~ple, a cooling coil or jacket provided in the tank.
~he substantial portion of the slurr~ is recycled to the smelt hopper and the remainder is supplied to the subse-~uent step of solid-liquid separation after which the separated liquid phase is recycled to the slurr~ circulating tank.
The a~ount of slurry to be recycled to the smelt hopper is fro~ 20 to 200 times, preferably about 100 times based on the weight o~ smelt introduced from the recovery boiler. Thus, the ~ount o~ a~ueous slurry hold in the 7~S
circulating tank is extremely large amou~t in c~mparison with the amount of the smelt introduced and the resident time of the aqueous slurry in the tank is usually ~ore than 20 hours.
The slurr~ i~ composed of a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is an aqueous sodium sulfide.
In consequence, the liquid phase rec~cled from the second step contains mainly sodium sulfide. ~herefore, at ~tead~
state, the proportion of sodium carbonate present in the slurry is ~aintained at a co~stant level lower than that ; of the smelt and the content of total solid in the slurry is from 35 to 70% preferably 45 to ~P~, by weight, the total solid being the sum of che~icals present in the slurry as solute and solid particles undissolved as well as in soluble materials.
Second E;teP
q'he slurr~ is pl~ped from the circulating tank to a solid-liquid separator, for example, a screw decantor at a constant flow rate and a constant feed pressure through a head tank. In the separator, the slurry is separated into a wet cake which comprises solid sodium carbonate and an aqueous sodiu~ sulfide and contains water of from 10 to 50%, pre~erably 15 to 3gh by weight, and a weak aqueous slurry which comprises the aqueous sodium sulfide and microparticles not separated. ~he wet cake i5 supplied to the subsequent oxidation step and the weak aqueous slurry is recycled to the circulati~g tank.
In order to carr~ out the overall process according to this invention satisfactorily, it is essential to control `76$
the molar ratio of ~/Na2~ in the wet cake to be substantially equal to that of the smelt, and this ca~ be done b~ adJusting appropriatel~ the a~ount of aqueous sodium sulfide to be present in the cake.
However, in some cases, depending upon the cooking conditions and properties of the pulp to be produced, the proportion of sodium carbonate in the cooking che~icals is ; small, for exa~ple, less than 10% b~ weight; then the sodiu~ sulfide content of the smelt increases to abo~e 40% b~ weight. Thus, it is necessar~ that the wet cake contains a higher proportio~ of ~odium sulfide in order to achieve the required level of the molar ratio of 8/~a20;
this naturall~ means that the liquid phase in the wet cake must a high concentration of sodium ~ulfide if the amount f the aqueou~ sodium sulfide i~ relatively small or that the proportion of the liquid phase must be increa~ed, if the concentration is relatively low.
If a high co~centration of sodium sulfide is available, the temperature of the aqueous slurry must be raised in order to undesirable precipitation of sodiu~
sulfide on various parts, especially on the cooling means, since the solubilit~ of sodium sulfide in water lowers toward to lower temperature. At a high temperature, especiall~ above 85C, aqueous sodium sulfide is extremely corrosive and at such a high te~perature even 18-8 stainless steel cPnnot resist and more expensive non-corrosive material will be required. On the other ha~d, if the liquid phase is of low co~centration, the wet cake must contain a relativel~ large a~ount of liquid phase in order to achieve re~uired ~olar ratio of S/~a20. However, the screw decantor 7~5 which is usuall~ employed in this invention can not perform ~3uccessful in the case where a relatively dilute wet cake is intended, accordingly, at a solid content at which a wet cake is readil~ available, the amount of sodium sulfide contained in the wet cake becomes shortage.
~ or the foregoing reasons, when the smelt has a higher proportion of sodium sulfide, it is difficult to increase the amount of liquid phase in the wet cake in order to achieve the required molar ratio of S/~a20 for smoothl~ proceeding the process and additio~al sodium sulfide must be supplied to the oxidation reaction mixture.
One feature of the process according to this invention is that solid material obtained b~ contacting a part of the aqueous slurr~ discharged from the first step and/or a part of the weak aqueous slurr~ recovered from the solid-liquid separator wi~h a cooling surface, is supplied to the oxidation step thereb~ ad~usting the molar ratio of S/~a20 of the reaction mixture to be oxidized to that of the smelt.
Such cooling surface is con~eniently a drum flaker comprising an aqueous slurr~ receiving vat, a rotary drum positioned in the vat and so arranged that the lower part thereof is immer~ed in the slurr~, a cooling mea~s to maintain the dru~ at a temperature below the solidif~ing temperature of the aqueous slurr~ &nd a doctor means for re~oving the solidified material from the drum surface.
Third SteP
The filter cake recovered in the 3econd step a~d, if necessar~, additional solid material obtained from the drum flaker are continuousl~ supplied to an oxidation reactor in which the feed is thoroughl~ admixed with hot particles of 7~5 ~odium carbo~ate and sodiu~ sulfide at a~ elevated te~perature lmder agitation while a ~olecular oxygen-contai~ing gas is passed therethrough to oxidi~e sodium sulfide present in the feed to sodium sulfite. ~he oxidation reactor is conveniently a doùble-s aft Z type kneader having a feeding port and an overflow chute for discharging the oxidized product.
In the kneader, the sha~t adjacent to the overflow chute is rotated at a speed of 10 to 50 r.p.m. and higher by 10 to 20h than that of the other shaft to facilitate the discharge of the oxidized product. ~he oxidized product i5 obtained in the for~ of particles having a dia~eter of, for example, from 200 to 400~ depending upon the resident ti~e.
Depending upon the oxidation conditions and the composition of the feed to be oxidized, a relatively small amount of unoxidized sodium sulfide ma~ be present in the oxidation product. In such a case, the product is subjected to additio~al oxidation in which the product is passed through a ~onfined space in a piston flow while a molecular ox~gen-co~taining gas i~ supplied æimultaneousl~.
During the ~ain oxldation and the suboxidation, sodiu~ sulfide and sodium thiosulfate are oxidized to sodiu~ sulfite. The molecular oxygen-containing gas is o~ygen or air, the latter being preferred.
The amou~t of the molecular oxygen--containin~ gas to be supplied to the main oxidatio~ step is from 2 to 20 times, preferabl~ 10 times, in ter~s of oxygen required for co~pletely oxidize the sodium sulfide the a~ount of which i~ based on the a~ount in the smelt, since so~e of the sodium sulfide in the s~elt has been converted into other P~L~476S
c~mpounds, for example, such as sodium thiosulfate, and the exact amount thereof in the reaction mixture is hardl~
determined. ~he temperature of the reaction mixture rises by the heat generated in the oxidation reaction. At a hlgh temperature, for example, above about 300C, undesirably large amount of sodiu~ sulfate will be formed; on the other hand, at a lower te~perature, for e~ample, below 100C, sodium sulfide is ~ot completel~ converted into sodiu~
sulfite rather the formation of sodium thiosulfate increases.
The heat of reaction in converting sodium sulfide into sodiu~ sulfite is 171 E cal. per one ~ole of sodium sulfide and that of into sodium thiosulfate is 112 E cal.
per one mole of ~odium sulfide. ~hus, it is beneficial to effect the preoxidation of the smelt in order to prevent the temperature from rising undul~ during the oxidation reaction. ~he temperature of the reaction mixture to be oxidized is held to an appropriate level by adjusting the water content of the wet cake, and additional water ma~ be supplied to the oxidation reactor, if necessary.
~he oxidation reaction is carried out at a temperature of from 100 to 300C, preferabl~ 150 to 250C, for 2 to 15 houx~, preferably 3 to 6 hours with the supply of a molecular ox~gen-containing gas at 100 to 200C, preferabl~ 150 to 180C.
In the case where the suboxidation reaction is effected, the molecular ox~gen-co~taini~g gas i8 supplied i~ an a~ount of from 10 to 30~ used for the mai~ oxidation.
In the reactions (5) and (6) ~entioned above, there are sodium h~droxide is required; this will be supplied from the reaction (4~ in the stoichiometric quantit~. If any 7~5 shortage of sodium hydroxide is observed, a~ additional amount may be supplemented. Residual sodiu~ sulfide is often observed in the oxidation product due to the shortage of water, especially in the suboxidation reactor; in this case water may be added to the reaction mixture.
~ine particles which are entrained in the exhaust gas fro~ the main oxidation and the suboxidation reactors are collected in a dust collector æuch as cyclone and recycled to any of the reactors. The exhaust gas thus treated is further cleaned in a scrubber, if necessary, to completely remove the fine particles entrai~ed.
Fourth Step ~he oxidized product is continuously supplied to a dissolving tank to which washed water discharged from the scrubber is simult~neously supplied or fresh water to for~
an aqueous solution containing sodium carbonate and sodium sulfite at a co~centration of from about 15 to about 25%
by weight and ha~ing a temperature of above about 30C.
The aqueous solution is allowed to stand to effect sedi~entation of insoluble materials including carbon and iron compound, which are then removed b~, for example, a thickener. ~ince such insoluble materials catalytically promote the oxidation of sodium sulfite into undesirable sodium sulfate in the subsequent step.
Into the aqueous solution thus clarified, the exhaust gas discharged from the recovery boiler which contains sulfur dioxide formed by combustion of the black liquor is blown to form sodium sulfite by the reaction of sodium carbonate and sulfur dioxide. If necessary, the exhaust gas from an auxiliary boiler is used together with - 13 - ~
s the recover~ boiler exhaust gas. Such exhaust gas often contains a relatively large amount of sulfur trioxide which reacts with sodium carbonate to form undesirable sodium sulfate and therefore, in such a case, the precaution must be taken to remove sulfur trioxide b~ washing with water.
~ he resulting aqueous solution contains sodium sulfite and sodium carbonate in a proportion and at concentrations suitable for use in a pulp making process.
Fifth ~tep Since the aqueous solution obtained contains solid particles which are accompanied with the exhaust gas, the aqueous solution is treated with a thickener to remove sludge.
~ his sludge and the sludge recovered in the fourth step contain a relatively large amount of aqueous solution of sodium salts~ Both sludges are combined and mixed with water. From the resulting mixture, the aqueous solution containing useful chemicals is separated b~ a thickener and a filter and is recycled to the first step and/or the fourth step to use for dissolving the smelt and/or the oxidation product.
As mentioned above, according to this invention the overall process is operated as a closed system in which the sulfur dioxide generated in the recover~ boiler and present in the exhaust gas is absorbed in the aqueous solution which contains substantiall~ all of the sodium component and sulfur component present in the smelt as soaium carbonate and sodium sulfite, and little or no loss of chemicals will occur.
~igure 1 illustrates the process according to this invention.
76,5 Concentrated black liquor is burned in recovery boiler 1 to form a smelt which is supplied continuously in the form of stream 2 to smelt hopper 3. ~team or high pressure air stream 4 is directed to the smelt stream to effect cooling and dividing out of the smelt into fine particles. An aqueous ~lurry recycled from circulating ta~k 6 is flowed downwardl~ on the inner surface of the hopper to prevent accumulation of smelt thereon. ~he rate of the slurr~ to be supplied to the hopper is 100 times tha~ of the smelt, by weight~ l'he smelt particles drop on conveying screen 5 by which fine particles and large parti-cles are separated, the former being supplied to the slurry circulating tank 6 and the latter to green liquor tank 7.
In the green liquor tank, the large particles are dissolved under agitation i~ water, which is supplied from scrubber 8 for washing exhaust ~as from the hopper, to form a weak gree~ liquor which is pumped to the slurr~ circulatin~
tank.
Though the concentration of the green liquor produced varies depending upon the amount of water to be supplied, which is determined taking in account the total water balance throughout the overall operation, the concentration is a factor in determi~ing the concentration of slurry to be formed in the slurr~ circulating tank and is usually maintained at about 10~ by weight.
Under stead~ operation conditionæ, a well established balance of water between (1) the sum of the supply to oxi-dation step and the water di~charged together with air from the smelt hopper ~nd (2) water supplied ~rom the green liquor tank is maintained to make the solid content in the green 3L1~34~S
liquor tank at a constant level.
~he green liquor is mixed in the slurr~ circulating tPnk with the aqueous slurry, the solidified smelt particles and a weak aqueous slurry recycled from the second step under agitation, to form a slurry comprising a multi-component aqueous phase co~taining mainly sodium sul~ide as well as sodium h~droxide, sodium polysulfides and other sodium salts such as sodium thiosulfate, sodium sulfate, sodium carbonate and sodium chloride and a solid phase cont~i~ing mainly sodium carbonate and other undissolved components above and various derivatives therefrom.
~he molar ratio of S/~a20 of total solid in the slurry is considerably higher than that of the smelt, for example, the ratio of ~melt being about 0.5 the ratio of slurry being from 0.8 to 1Ø ~he temperature of the slurry is maintained at from 55 to 90C, preferably 75 to 83C, for example, by means of cooling water passing through a cooling coil or jacket and maintained at a temperature from 5 to 40C, preferably 10 to 20C, lower than that of the slurry.
The use of cooling water of too low temperature results in the precipitation of solid on the cooling surface to impede cooling efficiency. lf desired, cooling by passing cold air through the slurry may be used in addition to such water cooling. In this case, some of the sodium sulfide is o~idi~ed.
'~he total content of the chemicals in the slurry is maintained within a ra~ge of from 35 to 70%, preferably 45 to 60~o by weight.
At a concentration below 35yo, thoug~ cooling efficiency ' 47~5 is improved, the wet cake obtained in the second step contains more sodium carbonate than required for maintaining the desired molar ratio of S/Na20. Further, if the slurr~
or the filtrate is cooled on a drum flaker, there is insufficient solidification, or the resulted flakes contain much water which lowers the oxidation temperature in the oxidation reactor to which the flakes are supplied.
On the other hand, at a concentration above 70%, there are disadvantages in that it becomes more difficult to maint~in the slurry temperature below 90C and there is clogging of the pipe lines.
~he slurry is pumped via head tank 9 to screw decantor 10. With the provision of the head tank, the slurr~ can be supplied continuously at a constant pressure to the decantor. ~y the decantor, a part of the solid phase is separated and removed as a wet cake from the aqueous slurr~ and the wet cake i8 supplied to oxidation reactor 11 while the reminder is recycled to the slurry circulating tank as a weak aqueous slurry.
In the case where the smelt has a proportion of sodium sulfite less tha~ 40~, especially less than 30%, b~ weight, it is easy to adjust the molar ratio of S/Na20 of the wet cake to that of the smelt.
On the other hand, the proportion of sodium sulfide i~ the smelt increases to more than 30~, especiall~ more than 4~h by weight, it is naturall~ necessar~ to increase the proportion of sulfur component in the aqueous slurry in order to adjust the ~/Na20 ratio of the wet cake to that of the smelt. If the concentration of sodium sulfide ; 30 in the aqueous slurry increases, the cooling efficiency of
~his invention will be explained in detail.
~irst steP
A smelt which is formed in conventional recovery boiler by co~bustion of a concentrated pulping cooking waste liquor i8 discharged to smelt receiving means comprising a smelt hopper and a screen. ~gainst the s~elt strea~ directed into the hopper, a steam or air jet iæ impinged to cool and divide out the smelt into solid particles. An aqueous slurry rec~cled from a circulating tank is flowed downwardly on the internal surface of the hopper to re~ove ths particles attached thereto. ~he solid particles drop onto a conveying ~5 screen provided below the hopper to separate lumps or large particles and particles, the former being supplied to a ~reen liquor tank in which the lumps are dissolved in an aqueous medium introduced from a gas washer and the latter bein~ supplied to a slurry circulating tank.
The atmosphere of the smelt hopper is maintained _ ``
. . .
47~S
under a reduced pressure b~ means of air fan which discharges the air in the hopper to the atmosphere after being treated in a gas washer, and the air passinæ through the hopper effects oxidation of a part of the sodium sulfide present in the smelt and the rec~cled slurr~ into sodiu~ thiosulfate.
(Hereunder this is referred to a preliminary oxidation~) The circulating tank to which solidified smelt particles are supplied, contains a large amount of the aqueous slurr~ to which simultaneousl~ a weak green liquor and/or a weak aqueous slurry rec~cled from the second step are supplied thereb~ effecting partial or incomplete di~solving of the chemicals in the aqueous slurry. By "partial or inco~plete dissolving", we mean a state in which the final mixture of the chemi~als and the aqueous medium contains some undissolved solid ~aterial, so that the for~ation of a slurr~
by precipitati~g solid particle fro~ an aqueous solution with cooling or from an aqueous supersaturated solution is also within this meanlng.
The temperature of the aqueous slurr~ is ~aintai~ed at fro~ 55 to 90C, preferabl~ 75 to 83C b~ a cooling means, for exa~ple, a cooling coil or jacket provided in the tank.
~he substantial portion of the slurr~ is recycled to the smelt hopper and the remainder is supplied to the subse-~uent step of solid-liquid separation after which the separated liquid phase is recycled to the slurr~ circulating tank.
The a~ount of slurry to be recycled to the smelt hopper is fro~ 20 to 200 times, preferably about 100 times based on the weight o~ smelt introduced from the recovery boiler. Thus, the ~ount o~ a~ueous slurry hold in the 7~S
circulating tank is extremely large amou~t in c~mparison with the amount of the smelt introduced and the resident time of the aqueous slurry in the tank is usually ~ore than 20 hours.
The slurr~ i~ composed of a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is an aqueous sodium sulfide.
In consequence, the liquid phase rec~cled from the second step contains mainly sodium sulfide. ~herefore, at ~tead~
state, the proportion of sodium carbonate present in the slurry is ~aintained at a co~stant level lower than that ; of the smelt and the content of total solid in the slurry is from 35 to 70% preferably 45 to ~P~, by weight, the total solid being the sum of che~icals present in the slurry as solute and solid particles undissolved as well as in soluble materials.
Second E;teP
q'he slurr~ is pl~ped from the circulating tank to a solid-liquid separator, for example, a screw decantor at a constant flow rate and a constant feed pressure through a head tank. In the separator, the slurry is separated into a wet cake which comprises solid sodium carbonate and an aqueous sodiu~ sulfide and contains water of from 10 to 50%, pre~erably 15 to 3gh by weight, and a weak aqueous slurry which comprises the aqueous sodium sulfide and microparticles not separated. ~he wet cake i5 supplied to the subsequent oxidation step and the weak aqueous slurry is recycled to the circulati~g tank.
In order to carr~ out the overall process according to this invention satisfactorily, it is essential to control `76$
the molar ratio of ~/Na2~ in the wet cake to be substantially equal to that of the smelt, and this ca~ be done b~ adJusting appropriatel~ the a~ount of aqueous sodium sulfide to be present in the cake.
However, in some cases, depending upon the cooking conditions and properties of the pulp to be produced, the proportion of sodium carbonate in the cooking che~icals is ; small, for exa~ple, less than 10% b~ weight; then the sodiu~ sulfide content of the smelt increases to abo~e 40% b~ weight. Thus, it is necessar~ that the wet cake contains a higher proportio~ of ~odium sulfide in order to achieve the required level of the molar ratio of 8/~a20;
this naturall~ means that the liquid phase in the wet cake must a high concentration of sodium ~ulfide if the amount f the aqueou~ sodium sulfide i~ relatively small or that the proportion of the liquid phase must be increa~ed, if the concentration is relatively low.
If a high co~centration of sodium sulfide is available, the temperature of the aqueous slurry must be raised in order to undesirable precipitation of sodiu~
sulfide on various parts, especially on the cooling means, since the solubilit~ of sodium sulfide in water lowers toward to lower temperature. At a high temperature, especiall~ above 85C, aqueous sodium sulfide is extremely corrosive and at such a high te~perature even 18-8 stainless steel cPnnot resist and more expensive non-corrosive material will be required. On the other ha~d, if the liquid phase is of low co~centration, the wet cake must contain a relativel~ large a~ount of liquid phase in order to achieve re~uired ~olar ratio of S/~a20. However, the screw decantor 7~5 which is usuall~ employed in this invention can not perform ~3uccessful in the case where a relatively dilute wet cake is intended, accordingly, at a solid content at which a wet cake is readil~ available, the amount of sodium sulfide contained in the wet cake becomes shortage.
~ or the foregoing reasons, when the smelt has a higher proportion of sodium sulfide, it is difficult to increase the amount of liquid phase in the wet cake in order to achieve the required molar ratio of S/~a20 for smoothl~ proceeding the process and additio~al sodium sulfide must be supplied to the oxidation reaction mixture.
One feature of the process according to this invention is that solid material obtained b~ contacting a part of the aqueous slurr~ discharged from the first step and/or a part of the weak aqueous slurr~ recovered from the solid-liquid separator wi~h a cooling surface, is supplied to the oxidation step thereb~ ad~usting the molar ratio of S/~a20 of the reaction mixture to be oxidized to that of the smelt.
Such cooling surface is con~eniently a drum flaker comprising an aqueous slurr~ receiving vat, a rotary drum positioned in the vat and so arranged that the lower part thereof is immer~ed in the slurr~, a cooling mea~s to maintain the dru~ at a temperature below the solidif~ing temperature of the aqueous slurr~ &nd a doctor means for re~oving the solidified material from the drum surface.
Third SteP
The filter cake recovered in the 3econd step a~d, if necessar~, additional solid material obtained from the drum flaker are continuousl~ supplied to an oxidation reactor in which the feed is thoroughl~ admixed with hot particles of 7~5 ~odium carbo~ate and sodiu~ sulfide at a~ elevated te~perature lmder agitation while a ~olecular oxygen-contai~ing gas is passed therethrough to oxidi~e sodium sulfide present in the feed to sodium sulfite. ~he oxidation reactor is conveniently a doùble-s aft Z type kneader having a feeding port and an overflow chute for discharging the oxidized product.
In the kneader, the sha~t adjacent to the overflow chute is rotated at a speed of 10 to 50 r.p.m. and higher by 10 to 20h than that of the other shaft to facilitate the discharge of the oxidized product. ~he oxidized product i5 obtained in the for~ of particles having a dia~eter of, for example, from 200 to 400~ depending upon the resident ti~e.
Depending upon the oxidation conditions and the composition of the feed to be oxidized, a relatively small amount of unoxidized sodium sulfide ma~ be present in the oxidation product. In such a case, the product is subjected to additio~al oxidation in which the product is passed through a ~onfined space in a piston flow while a molecular ox~gen-co~taining gas i~ supplied æimultaneousl~.
During the ~ain oxldation and the suboxidation, sodiu~ sulfide and sodium thiosulfate are oxidized to sodiu~ sulfite. The molecular oxygen-containing gas is o~ygen or air, the latter being preferred.
The amou~t of the molecular oxygen--containin~ gas to be supplied to the main oxidatio~ step is from 2 to 20 times, preferabl~ 10 times, in ter~s of oxygen required for co~pletely oxidize the sodium sulfide the a~ount of which i~ based on the a~ount in the smelt, since so~e of the sodium sulfide in the s~elt has been converted into other P~L~476S
c~mpounds, for example, such as sodium thiosulfate, and the exact amount thereof in the reaction mixture is hardl~
determined. ~he temperature of the reaction mixture rises by the heat generated in the oxidation reaction. At a hlgh temperature, for example, above about 300C, undesirably large amount of sodiu~ sulfate will be formed; on the other hand, at a lower te~perature, for e~ample, below 100C, sodium sulfide is ~ot completel~ converted into sodiu~
sulfite rather the formation of sodium thiosulfate increases.
The heat of reaction in converting sodium sulfide into sodiu~ sulfite is 171 E cal. per one ~ole of sodium sulfide and that of into sodium thiosulfate is 112 E cal.
per one mole of ~odium sulfide. ~hus, it is beneficial to effect the preoxidation of the smelt in order to prevent the temperature from rising undul~ during the oxidation reaction. ~he temperature of the reaction mixture to be oxidized is held to an appropriate level by adjusting the water content of the wet cake, and additional water ma~ be supplied to the oxidation reactor, if necessary.
~he oxidation reaction is carried out at a temperature of from 100 to 300C, preferabl~ 150 to 250C, for 2 to 15 houx~, preferably 3 to 6 hours with the supply of a molecular ox~gen-containing gas at 100 to 200C, preferabl~ 150 to 180C.
In the case where the suboxidation reaction is effected, the molecular ox~gen-co~taini~g gas i8 supplied i~ an a~ount of from 10 to 30~ used for the mai~ oxidation.
In the reactions (5) and (6) ~entioned above, there are sodium h~droxide is required; this will be supplied from the reaction (4~ in the stoichiometric quantit~. If any 7~5 shortage of sodium hydroxide is observed, a~ additional amount may be supplemented. Residual sodiu~ sulfide is often observed in the oxidation product due to the shortage of water, especially in the suboxidation reactor; in this case water may be added to the reaction mixture.
~ine particles which are entrained in the exhaust gas fro~ the main oxidation and the suboxidation reactors are collected in a dust collector æuch as cyclone and recycled to any of the reactors. The exhaust gas thus treated is further cleaned in a scrubber, if necessary, to completely remove the fine particles entrai~ed.
Fourth Step ~he oxidized product is continuously supplied to a dissolving tank to which washed water discharged from the scrubber is simult~neously supplied or fresh water to for~
an aqueous solution containing sodium carbonate and sodium sulfite at a co~centration of from about 15 to about 25%
by weight and ha~ing a temperature of above about 30C.
The aqueous solution is allowed to stand to effect sedi~entation of insoluble materials including carbon and iron compound, which are then removed b~, for example, a thickener. ~ince such insoluble materials catalytically promote the oxidation of sodium sulfite into undesirable sodium sulfate in the subsequent step.
Into the aqueous solution thus clarified, the exhaust gas discharged from the recovery boiler which contains sulfur dioxide formed by combustion of the black liquor is blown to form sodium sulfite by the reaction of sodium carbonate and sulfur dioxide. If necessary, the exhaust gas from an auxiliary boiler is used together with - 13 - ~
s the recover~ boiler exhaust gas. Such exhaust gas often contains a relatively large amount of sulfur trioxide which reacts with sodium carbonate to form undesirable sodium sulfate and therefore, in such a case, the precaution must be taken to remove sulfur trioxide b~ washing with water.
~ he resulting aqueous solution contains sodium sulfite and sodium carbonate in a proportion and at concentrations suitable for use in a pulp making process.
Fifth ~tep Since the aqueous solution obtained contains solid particles which are accompanied with the exhaust gas, the aqueous solution is treated with a thickener to remove sludge.
~ his sludge and the sludge recovered in the fourth step contain a relatively large amount of aqueous solution of sodium salts~ Both sludges are combined and mixed with water. From the resulting mixture, the aqueous solution containing useful chemicals is separated b~ a thickener and a filter and is recycled to the first step and/or the fourth step to use for dissolving the smelt and/or the oxidation product.
As mentioned above, according to this invention the overall process is operated as a closed system in which the sulfur dioxide generated in the recover~ boiler and present in the exhaust gas is absorbed in the aqueous solution which contains substantiall~ all of the sodium component and sulfur component present in the smelt as soaium carbonate and sodium sulfite, and little or no loss of chemicals will occur.
~igure 1 illustrates the process according to this invention.
76,5 Concentrated black liquor is burned in recovery boiler 1 to form a smelt which is supplied continuously in the form of stream 2 to smelt hopper 3. ~team or high pressure air stream 4 is directed to the smelt stream to effect cooling and dividing out of the smelt into fine particles. An aqueous ~lurry recycled from circulating ta~k 6 is flowed downwardl~ on the inner surface of the hopper to prevent accumulation of smelt thereon. ~he rate of the slurr~ to be supplied to the hopper is 100 times tha~ of the smelt, by weight~ l'he smelt particles drop on conveying screen 5 by which fine particles and large parti-cles are separated, the former being supplied to the slurry circulating tank 6 and the latter to green liquor tank 7.
In the green liquor tank, the large particles are dissolved under agitation i~ water, which is supplied from scrubber 8 for washing exhaust ~as from the hopper, to form a weak gree~ liquor which is pumped to the slurr~ circulatin~
tank.
Though the concentration of the green liquor produced varies depending upon the amount of water to be supplied, which is determined taking in account the total water balance throughout the overall operation, the concentration is a factor in determi~ing the concentration of slurry to be formed in the slurr~ circulating tank and is usually maintained at about 10~ by weight.
Under stead~ operation conditionæ, a well established balance of water between (1) the sum of the supply to oxi-dation step and the water di~charged together with air from the smelt hopper ~nd (2) water supplied ~rom the green liquor tank is maintained to make the solid content in the green 3L1~34~S
liquor tank at a constant level.
~he green liquor is mixed in the slurr~ circulating tPnk with the aqueous slurry, the solidified smelt particles and a weak aqueous slurry recycled from the second step under agitation, to form a slurry comprising a multi-component aqueous phase co~taining mainly sodium sul~ide as well as sodium h~droxide, sodium polysulfides and other sodium salts such as sodium thiosulfate, sodium sulfate, sodium carbonate and sodium chloride and a solid phase cont~i~ing mainly sodium carbonate and other undissolved components above and various derivatives therefrom.
~he molar ratio of S/~a20 of total solid in the slurry is considerably higher than that of the smelt, for example, the ratio of ~melt being about 0.5 the ratio of slurry being from 0.8 to 1Ø ~he temperature of the slurry is maintained at from 55 to 90C, preferably 75 to 83C, for example, by means of cooling water passing through a cooling coil or jacket and maintained at a temperature from 5 to 40C, preferably 10 to 20C, lower than that of the slurry.
The use of cooling water of too low temperature results in the precipitation of solid on the cooling surface to impede cooling efficiency. lf desired, cooling by passing cold air through the slurry may be used in addition to such water cooling. In this case, some of the sodium sulfide is o~idi~ed.
'~he total content of the chemicals in the slurry is maintained within a ra~ge of from 35 to 70%, preferably 45 to 60~o by weight.
At a concentration below 35yo, thoug~ cooling efficiency ' 47~5 is improved, the wet cake obtained in the second step contains more sodium carbonate than required for maintaining the desired molar ratio of S/Na20. Further, if the slurr~
or the filtrate is cooled on a drum flaker, there is insufficient solidification, or the resulted flakes contain much water which lowers the oxidation temperature in the oxidation reactor to which the flakes are supplied.
On the other hand, at a concentration above 70%, there are disadvantages in that it becomes more difficult to maint~in the slurry temperature below 90C and there is clogging of the pipe lines.
~he slurry is pumped via head tank 9 to screw decantor 10. With the provision of the head tank, the slurr~ can be supplied continuously at a constant pressure to the decantor. ~y the decantor, a part of the solid phase is separated and removed as a wet cake from the aqueous slurr~ and the wet cake i8 supplied to oxidation reactor 11 while the reminder is recycled to the slurry circulating tank as a weak aqueous slurry.
In the case where the smelt has a proportion of sodium sulfite less tha~ 40~, especially less than 30%, b~ weight, it is easy to adjust the molar ratio of S/Na20 of the wet cake to that of the smelt.
On the other hand, the proportion of sodium sulfide i~ the smelt increases to more than 30~, especiall~ more than 4~h by weight, it is naturall~ necessar~ to increase the proportion of sulfur component in the aqueous slurry in order to adjust the ~/Na20 ratio of the wet cake to that of the smelt. If the concentration of sodium sulfide ; 30 in the aqueous slurry increases, the cooling efficiency of
4 ~ ~5 the slurr~ lowers due to the precipitation of sodium sulfide on the cooling surface, then, it is difficult to lower the temperature of the aqueous slurry to the required level.
In such a case, part of the slurry discharged from the head tank is divided out a~d directed to drum flaker 12; alterna-tively a part of the filtrate from the decantor i8 directed to the drum flaker.
~ y the drum flaker, the chemicals present in the slurry or the filtrate are solidified on the drum which iæ
usually cooled to a temperature below 70C, preferably 30 to 50C and the flakes formed are removed and supplied to the main oxidation reactor.
The flakes have a composition similar to that of the a~ueous slurry supplied. ~or easy operation of the drum flaker, it is preferred to solidify only a part of the aqueous slurr~ and to rec~cle the remaining aqueous slurr~ to the circulating tank. Thus, by controlling the amou~t of flakes æupplied to the reactor, the molar ratio of S/Na20 of the combined filter cake a~d flake is readily adjusted to that of the smelt.
~ he main oxidation reactor is a double-shaft Z type kneader the lower portion of which contains a large amount of solid particles of the oxidized product containing a small am~u~t of water of, for example, a few percents and 25 up to 5% by weight. As the wet cake a~d the flake are supplied to the reactor, the~ are immediately mixed with the oxidized product particles, while hot air (lO0 to 200C, preferabl~ 150 - 185C) is simulta~eousl~ supplied. Si~ce the exhaust gas from the reactor contains a considerable amount of moisture, if too small an amount of air i~ supplied, 471~5 the exhaust gas becomes to have a higher moisture content hich results in water droplets condensing on the surface of various parts, for example, cyclone separator and duct.
Such droplets catch fine particles entrained in the exhaust gas a~d cause clo~ging. The supplied air also facilitates agitation and mixing of the reaction mixture in the reactor; therefore, an adequate air suppl~ is desirable.
However, too much air requires more energy for heating and supplying air, and in additio~ causes the escape of a large amount of particles from the reactor. ~hus, the amOuQt of air to be supplied is from 2 to 20 times, preferably 10 times th~t required to completely o~idize the total sodium sulfide present in the smelt.
The exhaust gas from the reactor i~ fed to cyclone 14 in which particles e~trai~ed are recovered and recycled to the reactor.
The oxidized product which contains uno~ dized chemicals and oxmdation intermediates is discharged from the reactor via the overflow chute a~d is supplied to suboxidation reactor 13.
~ he purpose of the suboxidation reactor is to effect as complete an oxidation reaction as possible, if such u~oxidized chemicals and intermediates are introduced in the ~ubsequent step, they react with sulfur dioxide to form h~drogen sulfide which is a pollutant gas and sodium thio-sulfate which is undesirable for pulp making.
To the suboxidation reactor, air is introduced to effect additio~al oxidation at a temperature of from 100 to 300C, preferably 150 to 250C. The water, up to about
In such a case, part of the slurry discharged from the head tank is divided out a~d directed to drum flaker 12; alterna-tively a part of the filtrate from the decantor i8 directed to the drum flaker.
~ y the drum flaker, the chemicals present in the slurry or the filtrate are solidified on the drum which iæ
usually cooled to a temperature below 70C, preferably 30 to 50C and the flakes formed are removed and supplied to the main oxidation reactor.
The flakes have a composition similar to that of the a~ueous slurry supplied. ~or easy operation of the drum flaker, it is preferred to solidify only a part of the aqueous slurr~ and to rec~cle the remaining aqueous slurr~ to the circulating tank. Thus, by controlling the amou~t of flakes æupplied to the reactor, the molar ratio of S/Na20 of the combined filter cake a~d flake is readily adjusted to that of the smelt.
~ he main oxidation reactor is a double-shaft Z type kneader the lower portion of which contains a large amount of solid particles of the oxidized product containing a small am~u~t of water of, for example, a few percents and 25 up to 5% by weight. As the wet cake a~d the flake are supplied to the reactor, the~ are immediately mixed with the oxidized product particles, while hot air (lO0 to 200C, preferabl~ 150 - 185C) is simulta~eousl~ supplied. Si~ce the exhaust gas from the reactor contains a considerable amount of moisture, if too small an amount of air i~ supplied, 471~5 the exhaust gas becomes to have a higher moisture content hich results in water droplets condensing on the surface of various parts, for example, cyclone separator and duct.
Such droplets catch fine particles entrained in the exhaust gas a~d cause clo~ging. The supplied air also facilitates agitation and mixing of the reaction mixture in the reactor; therefore, an adequate air suppl~ is desirable.
However, too much air requires more energy for heating and supplying air, and in additio~ causes the escape of a large amount of particles from the reactor. ~hus, the amOuQt of air to be supplied is from 2 to 20 times, preferably 10 times th~t required to completely o~idize the total sodium sulfide present in the smelt.
The exhaust gas from the reactor i~ fed to cyclone 14 in which particles e~trai~ed are recovered and recycled to the reactor.
The oxidized product which contains uno~ dized chemicals and oxmdation intermediates is discharged from the reactor via the overflow chute a~d is supplied to suboxidation reactor 13.
~ he purpose of the suboxidation reactor is to effect as complete an oxidation reaction as possible, if such u~oxidized chemicals and intermediates are introduced in the ~ubsequent step, they react with sulfur dioxide to form h~drogen sulfide which is a pollutant gas and sodium thio-sulfate which is undesirable for pulp making.
To the suboxidation reactor, air is introduced to effect additio~al oxidation at a temperature of from 100 to 300C, preferably 150 to 250C. The water, up to about
5% by weight, pre~ent in the ~olid particles discharged 7~5 from the main reactor is enough for performing the conversion of residual sodium sulfide i~to sodium sulfite. However, if less amount of water is presentl an additional water may b~
fed in order to facilitate the oxidation reaction. ~he e~haust gas from the suboxidation reactor is supplied to the cyclone 14 in which e~trained solid particles are recovered.
If the e~trained particles are not completel~ separated in the cyclone, then the exhaust gas is further treated in scrubber 14 to which fresh water or a dilute aqueous chemical solution is supplied to dissolve solid particles a~ completely as possible.
The oxidized product is discharged from the suboxi-dation reactor and supplied to dissolving tank 15 to which the aqueous solution discharged from the scrubber 16 is supplied to form an aqueous solution containing sodium carbonate and sodium sulfite~
The amount of aqueous solution to be supplied to the dissolving tank is controlled so that the con¢entration of the resulting aqueous solution is about 20~o by weight. ~t a concentration above 20yo~ there is encountered difficulty in treating the aqueous solution with sulfur dioxide for producing a pulp cooking liquor; on the other hand too dilute a~ aqueous solution cannot give a liquor having a co~centra-tion suitable for cooking.
The aqueous solution is supplied to ediment tank17 in which insoluble material is separated as sludge, and the clarified liquor is supplied to absorber 18.
In the absorber, the clarified liquor and the exhaust gas discharged from the recovery boiler are intimatedly Lr7~ 5 contacted to effect conversion of sodium carbonate into ~odium sulfite to the extent required for the desired cooking liquor composition. The exhaust gas from the absorber does not contain sulfur dioxide and is vented to atmosphere.
The cooking liquor thus produced is clari~ied in thickener 19 and used ~or pulp making. The sludge recovered from the thickener is combi~ed with the sludge from the sediment tank, washed with water, filtered and removed from the processing æystem. The washing and the filtrate are recycled to the scru~ber and/or the dissol~ing tank.
~ his invention will be explained by means of Examples.
However, it should be understood that this invention is in uo way limited b~ these Examples.
Example 1:
Smelt recovered from a recovery furna~e was treated according to the procedures explained above ~nd using the apparatus illustrated b~ referring to ~ig. l to produce a cooking liquor. ~he temperature and the content o~ total chemicals in the slurry circulating tank 6 were maintained at 83.5C and 59.6~ b~ weight, respectivel~. In the seco~d step, the screw decantor 9 wa~ used but ~ot the drum flaker 11. The temperatures of the reaction mixtures in the main oxidation reactor 10 and the ~uboxidation reactor 1~ were maintained at 190C a~d 150a, respectivelyO
The smelt was introduced in the process at a rate of 2.0 tons per hour and fresh water was supplied at 24 tons per hour.
The composition in each step is given in Table 1.
- 21 _ 7~5 . ~.
~
~ ,~
~:: h ~ ~1 ~ 1`
O ~ o ~r ~r) tY) I` I ,1 o o X
O-,l ~o ~r ~ ~
-- C: ._ ._ .,1 h ^ ~`1 ~ po~ ~
. ~ _ __ .
O
.~ ~) ~ ~ C~ ~D O ~ ~ ~
I ~ . . . . . I .
R-~l O o ~t co ~1~D.I~1 o :1 X h ~ ~) U~ O ~
_ _ I
. ~~ ~ o ~ ~r ~1 ~~ ~ ~ . . . . . .
rl rl O C~ ~ r) ~tD ~ O
~ X h L~ ~
. ~r a) ." co ~1~r ~X . . I .. . . .
Q) ~ ~r ~D I ~lD ~ ~ t~ O
~~ ~ ~ ~D ~_ _ h~ C~ D . ~
r-l ~ ~ o ~n ~1 ~1 U~
E~ ~ 1 o o ~ 1 O
U~ ~ ~D
_ O O O ~ O
o\ V~ N O ~1 ~d N
N N N U~ rl 1~
d ~ rd N NI ~~ ,1 Z
:~ Z Z Z ~~ X ~ O O
ZZ u~ Z E~ u) U~
.
. ~. ~ ,,.
1 Example 2:
Procedures similar to those of Example 1 were repeated.
The slurry in the circulating tank had a temperature of 78C and total chemical of 55.1%. In the second step, the scre~w decantor and the drum flaker were used. The temperature of the main oxidation reactor and the suboxidation reactor were 210 C and 160C, respectively. The amount of water introduced to the suboxidation reactor was 50 liters per hour.
The compositions in each step is given in Table 2.
7~S
____ . . ._ r~ -~ ~o~ O ~ O O ~
__. ~
h Go ~ o\i~
C~
_ X~ c~ U:~
Q~ C~ , o ~n~ Q
_ 0~ ~D
a ~ ~ ~ In ~ r u~
. . . . . I .
'~0 ~ ~ 1 O
~ r X ~ ~
~ ~r o I ~ ~ r o N ¦ ~ 1~1 Ltl E~ ~ X O C~> ~ I` O N 00 ~r ) ~ i O
..
~D
h ~1 o~ ~ ~
~ N ~ I t~ r) ~ It ) O
. ~ ......
~1 ~D
0~ O 1~ ~
U~ r~ I ~ O
_ ~ ~ O ~;r O
o\~ O O ~I O ~ ~1 ~a ~, J,~ N N N N N I C ) ~ ,1 Z
~ æ Z Z Z ~nX Z O O ~
~ _ _ _ 24 --r~
fed in order to facilitate the oxidation reaction. ~he e~haust gas from the suboxidation reactor is supplied to the cyclone 14 in which e~trained solid particles are recovered.
If the e~trained particles are not completel~ separated in the cyclone, then the exhaust gas is further treated in scrubber 14 to which fresh water or a dilute aqueous chemical solution is supplied to dissolve solid particles a~ completely as possible.
The oxidized product is discharged from the suboxi-dation reactor and supplied to dissolving tank 15 to which the aqueous solution discharged from the scrubber 16 is supplied to form an aqueous solution containing sodium carbonate and sodium sulfite~
The amount of aqueous solution to be supplied to the dissolving tank is controlled so that the con¢entration of the resulting aqueous solution is about 20~o by weight. ~t a concentration above 20yo~ there is encountered difficulty in treating the aqueous solution with sulfur dioxide for producing a pulp cooking liquor; on the other hand too dilute a~ aqueous solution cannot give a liquor having a co~centra-tion suitable for cooking.
The aqueous solution is supplied to ediment tank17 in which insoluble material is separated as sludge, and the clarified liquor is supplied to absorber 18.
In the absorber, the clarified liquor and the exhaust gas discharged from the recovery boiler are intimatedly Lr7~ 5 contacted to effect conversion of sodium carbonate into ~odium sulfite to the extent required for the desired cooking liquor composition. The exhaust gas from the absorber does not contain sulfur dioxide and is vented to atmosphere.
The cooking liquor thus produced is clari~ied in thickener 19 and used ~or pulp making. The sludge recovered from the thickener is combi~ed with the sludge from the sediment tank, washed with water, filtered and removed from the processing æystem. The washing and the filtrate are recycled to the scru~ber and/or the dissol~ing tank.
~ his invention will be explained by means of Examples.
However, it should be understood that this invention is in uo way limited b~ these Examples.
Example 1:
Smelt recovered from a recovery furna~e was treated according to the procedures explained above ~nd using the apparatus illustrated b~ referring to ~ig. l to produce a cooking liquor. ~he temperature and the content o~ total chemicals in the slurry circulating tank 6 were maintained at 83.5C and 59.6~ b~ weight, respectivel~. In the seco~d step, the screw decantor 9 wa~ used but ~ot the drum flaker 11. The temperatures of the reaction mixtures in the main oxidation reactor 10 and the ~uboxidation reactor 1~ were maintained at 190C a~d 150a, respectivelyO
The smelt was introduced in the process at a rate of 2.0 tons per hour and fresh water was supplied at 24 tons per hour.
The composition in each step is given in Table 1.
- 21 _ 7~5 . ~.
~
~ ,~
~:: h ~ ~1 ~ 1`
O ~ o ~r ~r) tY) I` I ,1 o o X
O-,l ~o ~r ~ ~
-- C: ._ ._ .,1 h ^ ~`1 ~ po~ ~
. ~ _ __ .
O
.~ ~) ~ ~ C~ ~D O ~ ~ ~
I ~ . . . . . I .
R-~l O o ~t co ~1~D.I~1 o :1 X h ~ ~) U~ O ~
_ _ I
. ~~ ~ o ~ ~r ~1 ~~ ~ ~ . . . . . .
rl rl O C~ ~ r) ~tD ~ O
~ X h L~ ~
. ~r a) ." co ~1~r ~X . . I .. . . .
Q) ~ ~r ~D I ~lD ~ ~ t~ O
~~ ~ ~ ~D ~_ _ h~ C~ D . ~
r-l ~ ~ o ~n ~1 ~1 U~
E~ ~ 1 o o ~ 1 O
U~ ~ ~D
_ O O O ~ O
o\ V~ N O ~1 ~d N
N N N U~ rl 1~
d ~ rd N NI ~~ ,1 Z
:~ Z Z Z ~~ X ~ O O
ZZ u~ Z E~ u) U~
.
. ~. ~ ,,.
1 Example 2:
Procedures similar to those of Example 1 were repeated.
The slurry in the circulating tank had a temperature of 78C and total chemical of 55.1%. In the second step, the scre~w decantor and the drum flaker were used. The temperature of the main oxidation reactor and the suboxidation reactor were 210 C and 160C, respectively. The amount of water introduced to the suboxidation reactor was 50 liters per hour.
The compositions in each step is given in Table 2.
7~S
____ . . ._ r~ -~ ~o~ O ~ O O ~
__. ~
h Go ~ o\i~
C~
_ X~ c~ U:~
Q~ C~ , o ~n~ Q
_ 0~ ~D
a ~ ~ ~ In ~ r u~
. . . . . I .
'~0 ~ ~ 1 O
~ r X ~ ~
~ ~r o I ~ ~ r o N ¦ ~ 1~1 Ltl E~ ~ X O C~> ~ I` O N 00 ~r ) ~ i O
..
~D
h ~1 o~ ~ ~
~ N ~ I t~ r) ~ It ) O
. ~ ......
~1 ~D
0~ O 1~ ~
U~ r~ I ~ O
_ ~ ~ O ~;r O
o\~ O O ~I O ~ ~1 ~a ~, J,~ N N N N N I C ) ~ ,1 Z
~ æ Z Z Z ~nX Z O O ~
~ _ _ _ 24 --r~
Claims (31)
1. A process for recovery of chemicals from sodium sulfite pulping waste liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry, which comprises a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55° to about 90°C, (2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3), and (3) mixing a feed consisting of said wet cake (a) and sufficient additional solid material to adjust the total molar ratio of S/Na20 of said feed to that of said smelt with hot particles containing sodium sulfite and sodium carbonate while supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium sulfide present in said feed into sodium sulfite, wherein said additional solid material is obtained by contacting at least one of a portion of said slurry supplied to step (2) and a portion of said weak aqueous slurry (b) obtained in step (2) with a cooled surface to obtain said solid material.
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry, which comprises a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55° to about 90°C, (2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3), and (3) mixing a feed consisting of said wet cake (a) and sufficient additional solid material to adjust the total molar ratio of S/Na20 of said feed to that of said smelt with hot particles containing sodium sulfite and sodium carbonate while supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium sulfide present in said feed into sodium sulfite, wherein said additional solid material is obtained by contacting at least one of a portion of said slurry supplied to step (2) and a portion of said weak aqueous slurry (b) obtained in step (2) with a cooled surface to obtain said solid material.
2. A process for recovery of chemicals according to claim 1, wherein in the step (1) said body of aqueous slurry is circu-lated between a smelt receiving means and a smelt dissolving means, and said smelt is introduced into said aqueous slurry in said smelt receiving means.
3. A process for recovery of chemicals according to claim 2, wherein the amount of the circulating slurry is from 20 to 200 times by weight that of the smelt introduced in the receiving means.
4. A process for recovery of chemicals according to claim 2, wherein said weak aqueous slurry recycled from step (2), and said make up water are added to said body in said smelt dis-solving means.
5. A process for recovery of chemicals according to claim 1, wherein said smelt is impinged with an air or steam stream to effect cooling and dividing out into particles.
6. A process for recovery of chemicals according to claim 2, wherein said smelt receiving means comprises a smelt hopper and a screening means, and wherein lumps remaining after said smelt is introduced into said smelt hopper through which is circulating said body of aqueous slurry are separated from smaller solidified smelt particles by said screening means and said separated lumps are separately dissolved in make up water to form a weak green liquor.
7. A process for recovery of chemicals according to claim 1, wherein said make up water is a weak green liquor.
8. A process for recovery of chemicals according to claim 1, wherein said make up water is wash water recovered from a means for washing an exhaust gas from the smelt receiving means.
9. A process for recovery of chemicals according to claim 1, wherein said aqueous slurry is cooled by means of cooling water having a temperature by 5° to 40°C lower than that of the slurry.
10. A process for recovery of chemicals according to claim 1, wherein the amount of said molecular oxygen-containing gas to be supplied to the oxidation reaction is from 2 to 20 times that required to completely oxidize the sodium sulfide present in the smelt into sodium sulfite.
11. A process for recovery of chemicals according to claim 1, wherein the temperature of said molecular oxygen-containing gas to be supplied to the oxidation reaction is from 100° to 200°
C.
C.
12. A process for recovery of chemicals according to claim 1, wherein the temperature of the reaction mixture to be oxidized is from 100° to 300°C.
13. A process for recovery of chemicals according to claim 1, wherein sodium hydroxide is added to the reaction mixture to be oxidized.
14. A process for recovery of chemicals according to claim 1, wherein at least one of a part of said aqueous slurry and a part of said weak aqueous slurry are supplied to the reaction mixture to be oxidized.
15. A process for recovery of chemicals according to claim 1, wherein said oxidation reaction is effected in two stages, the first stage being mixing said wet cake with said hot particles while supplying said molecular oxygen-containing gas to form an-oxidized product in the form of particles, and a second stage being contact of said oxidized product particles and a molecular oxygen-containing gas within a confined space.
16. A process for recovery of chemicals according to claim 1, wherein at least one of said smelt, and said aqueous slurry formed in step (1) are contacted with a molecular oxygen-.
containing gas to effect preliminary oxidation of the sodium sulfide.
17. A process for recovery of chemicals from sodium sulfite pulping waste liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry, which comprises a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2) maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55° to about 90°
C, (2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and (b) a weak aqueous slurry, recycling said
containing gas to effect preliminary oxidation of the sodium sulfide.
17. A process for recovery of chemicals from sodium sulfite pulping waste liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry, which comprises a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2) maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55° to about 90°
C, (2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and (b) a weak aqueous slurry, recycling said
Claim 17 continued weak slurry (b) to step (1), and supplying said wet cake (a) to step (3), and (3) mixing a feed consisting of said wet cake (a) and sufficient additional solid material to adjust the total molar ratio of S/Na20 of said feed to that of said smelt with hot particles containing sodium sulfite and sodium carbonate while supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium sulfide present in said feed into sodium sulfite to obtain a product mixture of sodium sulfite and sodium carbonate, wherein said additional solid material is obtained by contacting at least one of a portion of said slurry supplied to step (2) and a portion of said weak aqueous slurry (b) obtained in step (2) with a cooled surface to obtain said solid material, (4) dissolving the product mixture in water, separating insoluble material from the resulting aqueous solution to obtain a clarified aqueous solution, contacting the clarified solution with a sulfur dioxide-containing gas to effect conversion of sodium carbonate present into sodium sulfite and separating insoluble material from the final aqueous solution, and (5) combining both insoluble materials, washing with water and recycling the wash water to step (1) for dissolving the smelt and/or step (4) for dissolving the oxidation product.
18. A process for recovery of chemicals from sodium sulfite pulping waste liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of Claim 18 continued aqueous slurry which is is circulated between a smelt receiving means and a smelt dissolving means, said said smelt is introduced into said aqueous slurry in said smelt receiving means, and wherein the amount of said circulating slurry is from 20 to 200 times by weight that of the smelt introduced in said smelt receiving means, and wherein said smelt receiving means comprises a smelt hopper and a screening means, and wherein lumps remaining after said smelt is introduced into said smelt hopper through which is circulated said body of aqueous slurry are separated from from smaller solidified smelt particles by said screening means and said separated lumps are separately dissolved in make up water to form a weak green liquor, said large body of aqueous slurry comprising a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55 to about 90°C;
(2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and having a molar ratio of S/Na2O substan-tially equal to that of said smelt, and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3); and (3) mixing a feed consisting of said wet cake (a) with hot particles containing sodium sulfite and sodium carbonate while
18. A process for recovery of chemicals from sodium sulfite pulping waste liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of Claim 18 continued aqueous slurry which is is circulated between a smelt receiving means and a smelt dissolving means, said said smelt is introduced into said aqueous slurry in said smelt receiving means, and wherein the amount of said circulating slurry is from 20 to 200 times by weight that of the smelt introduced in said smelt receiving means, and wherein said smelt receiving means comprises a smelt hopper and a screening means, and wherein lumps remaining after said smelt is introduced into said smelt hopper through which is circulated said body of aqueous slurry are separated from from smaller solidified smelt particles by said screening means and said separated lumps are separately dissolved in make up water to form a weak green liquor, said large body of aqueous slurry comprising a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbonate in the total solid material at lower than that of the smelt, and the temperature at from about 55 to about 90°C;
(2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and having a molar ratio of S/Na2O substan-tially equal to that of said smelt, and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3); and (3) mixing a feed consisting of said wet cake (a) with hot particles containing sodium sulfite and sodium carbonate while
Claim 18 continued supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium present in said feed into sodium sulfite.
19. The process of claim 18, wherein said weak aqueous slurry recycled from step (2), and said make up water are added to said body in smelt dissolving means.
20. The process of claim 18, wherein said smelt is impinged with air or steam stream to effect cooling and dividing out into particles.
21. The process of claim 18, wherein said make up water is weak green liquor.
22. The process of claim 18, wherein said make up water is wash water recovered from a means for washing an exhaust gas from said smelt receiving means.
23. The process of claim 18, wherein said body of aqueous slurry is cooled by means of cooling water having a temperature from 5 to 40°C lower than that of said slurry.
24. The process of claim 18, wherein the amount of said molecular oxygen-containing gas to be supplied to the oxidation reaction is from 2 to 20 times that required to effect oxidation of all the sodium sulfide present in said feed into sodium sulfite.
25. The process of claim 18, wherein the temperature of said molecular oxygen-containing gas to be supplied to the oxi-dation reaction is from 100 to 200°C.
26. The process of claim 18, wherein the temperature of the reaction mixture to be oxidized is from 100 to 300°C.
27. The process of claim 18, wherein sodium hydroxide is added to the reaction mixture to be oxidized.
28. The process of claim 18, wherein at least one of a part of said aqueous slurry and a part of said weak aqueous slurry is supplied to the reaction mixture to be oxidized.
29. The process of claim 18, wherein said oxidation reaction is effected in two stages, the first stage being mixing said feed with said hot particles while supplying said molecular oxygen-containing gas to form an oxidized product in the form of particles, and a second stage being contact of said oxidized product particles and a molecular oxygen-containing gas within a confined space.
30. The process of claim 18, wherein at least one of said smelt and said aqueous slurry formed in step (1) is contacted with a molecular oxygen-containing gas to effect preliminary oxi-dation of the sodium sulfide contained therein.
31. A process for recovery of chemicals from sodium sulfite pulping wast liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry which is circulated between a smelt receiving means and a smelt dissolving means, and said smelt is introduced into said aqueous slurry in said smelt receiving means, and wherein the amount of said circulating slurry is from 20 to 200 times by weight that of the smelt introduced in said smelt Claim 31 continued receiving means, and wherein said smelt receiving means comprises a smelt hopper and a screening means, and wherein lumps remaining after said smelt is introduced into said smelt hopper through which is circulated said body of aqueous slurry are separated from smaller solidified smelt particles by said screening means and said separated lumps are separately dissolved in make up water to form a weak green liquor, said large body of aqueous slurry comprising a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbon-ate in the total solid material at lower than that of the smelt, and the temperature at from about 55 to about 90°C;
(2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and having a molar ratio of S/Na20 substan-tially equal to that of said smelt, and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3);
(3) mixing a feed consisting of said wet cake (a) with hot particles containing sodium sulfite and sodium carbonate while supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium sulfide present in said feed into sodium sulfite to obtain a product mixture of sodium sulfite and sodium carbonate;
31. A process for recovery of chemicals from sodium sulfite pulping wast liquor comprising the steps of:
(1) introducing and incompletely dissolving a smelt, obtained by burning a concentrated waste liquor and containing mainly sodium sulfide and sodium carbonate, in a large body of aqueous slurry which is circulated between a smelt receiving means and a smelt dissolving means, and said smelt is introduced into said aqueous slurry in said smelt receiving means, and wherein the amount of said circulating slurry is from 20 to 200 times by weight that of the smelt introduced in said smelt Claim 31 continued receiving means, and wherein said smelt receiving means comprises a smelt hopper and a screening means, and wherein lumps remaining after said smelt is introduced into said smelt hopper through which is circulated said body of aqueous slurry are separated from smaller solidified smelt particles by said screening means and said separated lumps are separately dissolved in make up water to form a weak green liquor, said large body of aqueous slurry comprising a solid phase the main component of which is sodium carbonate and a liquid phase the main component of which is aqueous sodium sulfide, adding to said body make up water and a weak aqueous slurry recycled from step (2), and supplying a portion of the resulting aqueous slurry to step (2), maintaining the total solid content of said body of aqueous slurry at from about 35 to about 70% by weight, the proportion of sodium carbon-ate in the total solid material at lower than that of the smelt, and the temperature at from about 55 to about 90°C;
(2) separating the slurry supplied from step (1) into (a) a wet cake containing water in a proportion of from about 10 to about 50% by weight and having a molar ratio of S/Na20 substan-tially equal to that of said smelt, and (b) a weak aqueous slurry, recycling said weak slurry (b) to step (1), and supplying said wet cake (a) to step (3);
(3) mixing a feed consisting of said wet cake (a) with hot particles containing sodium sulfite and sodium carbonate while supplying simultaneously a molecular oxygen-containing gas to effect oxidation of sodium sulfide present in said feed into sodium sulfite to obtain a product mixture of sodium sulfite and sodium carbonate;
Claim 31 continued (4) dissolving said product mixture in water, separating insoluble material from the resulting aqueous solution to obtain a clarified aqueous solution, contacting the clarified solution with a sulfur dioxide-containing gas to effect conversion of sodium carbonate present into sodium sulfite and separating insoluble material from the final aqueous solution; and (5) combining both insoluble materials, washing with water and recycling the wash water to step (1) for dissolving the smelt and/or step (4) for dissolving the oxidation product.
Applications Claiming Priority (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP28988/1976 | 1976-03-17 | ||
| JP2898776A JPS52114705A (en) | 1976-03-17 | 1976-03-17 | Process for recovering pulp digesting reagents |
| JP28987/1976 | 1976-03-17 | ||
| JP2898876A JPS52114706A (en) | 1976-03-17 | 1976-03-17 | Process for treating smelt |
| JP57135/1976 | 1976-05-18 | ||
| JP5713576A JPS52140601A (en) | 1976-05-18 | 1976-05-18 | Process for recovering digestion reagents by sulphite pulp |
| JP156345/1976 | 1976-12-27 | ||
| JP15634676A JPS5380390A (en) | 1976-12-27 | 1976-12-27 | Production of flake like material |
| JP156346/1976 | 1976-12-27 | ||
| JP51156345A JPS591835B2 (en) | 1976-12-27 | 1976-12-27 | Recovery method for sodium sulfite cooking chemicals |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1104765A true CA1104765A (en) | 1981-07-14 |
Family
ID=27521091
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA273,974A Expired CA1104765A (en) | 1976-03-17 | 1977-03-15 | Process for recovery of chemicals from pulping waste liquor |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1104765A (en) |
-
1977
- 1977-03-15 CA CA273,974A patent/CA1104765A/en not_active Expired
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