CA1103412A - Kraft mill recycle process - Google Patents
Kraft mill recycle processInfo
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- CA1103412A CA1103412A CA294,604A CA294604A CA1103412A CA 1103412 A CA1103412 A CA 1103412A CA 294604 A CA294604 A CA 294604A CA 1103412 A CA1103412 A CA 1103412A
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Abstract
ABSTRACT
A cellulosic pulp and bleaching process is described. The process integrates two cycles, one is a digestor liquor cycle and the other is a sulfur cycle, The digestor liquor cycle comprises the steps of digesting a fibrous cellulosic feed to form a pulp, separating the pulp and bleaching it in at least one stage with chlorine dioxide, evaporating and burning the remaining waste liquor to form a smelt, dissolving the smelt to form a green liquor and utilizing the green liquor, after as at least a portion of the feed stock for the digestor step.
The sulfur cycle of the present invention utilizes a portion of the green liquor of the digestor liquor cycle. The green liquor is carbonated to form a bicarbonate which is then stripped of H2S. The H2S is converted into H2SO4 and utilized as a feed stock to a chlorine dioxide generator.
The chlorine dioxide generator utilizes H2SO4 as feed stock and produces Na2SO4 as a by-product. The Na2SO4 by-product from the chlorine dioxide generator is furnaced in common with the evaporated waste pulping liquor from the digestor liquor cycle to form a common smelt, which is dissolved to form a green liquor, a portion of which is used in the sulfur cycle and a portion of which is used in the digestor cycle.
A cellulosic pulp and bleaching process is described. The process integrates two cycles, one is a digestor liquor cycle and the other is a sulfur cycle, The digestor liquor cycle comprises the steps of digesting a fibrous cellulosic feed to form a pulp, separating the pulp and bleaching it in at least one stage with chlorine dioxide, evaporating and burning the remaining waste liquor to form a smelt, dissolving the smelt to form a green liquor and utilizing the green liquor, after as at least a portion of the feed stock for the digestor step.
The sulfur cycle of the present invention utilizes a portion of the green liquor of the digestor liquor cycle. The green liquor is carbonated to form a bicarbonate which is then stripped of H2S. The H2S is converted into H2SO4 and utilized as a feed stock to a chlorine dioxide generator.
The chlorine dioxide generator utilizes H2SO4 as feed stock and produces Na2SO4 as a by-product. The Na2SO4 by-product from the chlorine dioxide generator is furnaced in common with the evaporated waste pulping liquor from the digestor liquor cycle to form a common smelt, which is dissolved to form a green liquor, a portion of which is used in the sulfur cycle and a portion of which is used in the digestor cycle.
Description
The present invention relates to the production of bleached cellulosic fibrous pulp. More particularly, the present invention relates to a method of producing bleached fibrous cellulosic pulp in which the chemical compounds used and formed in the various steps and stages are utilized to maximum efficiency with a minimum of loss and resulting pol-lution by the recovery and reuse of products previously passed as waste from pulp bleaching processes.
In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heat-ing,in a pulping liquor, (white liquor) which contains sodium sulfide and sodium hydroxide as the active pulping chemicals.
The digestion provides a pulp and spent pulping liquor (black liquor). The black liquor is separated from the pulp by washing in a brown stock washer and the pulp utilized as feedstock in a bleach plant for brightening and purification operations.
The black liquor is then concentrated, usually by evaporating, and the concentrated black liquor burned in a reducing furnace to yield a smelt containing primarily sodium carbonate and sodium sulfide.
The smelt is then dissolved in water to yield a raw green liquor which may be clari~ied to remove undissolved solids. The green liquor is then causticized, usually by treatment with Ca~ to convert sodium carbonate to sodium hydroxide. The resulting liquor is white liquor and is use-ful in the initial digestion step to provide at least a part of the pulping liquor.
The foregoing sequence or cycle is well known and 30 i5 referred to herein as the digestor liquor cycle.
- 1 - ~,",~
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Bleach plant operations generally involve a sequence of briyhtening and purification steps which may be combined with washing steps. The brightening steps generally involve the use of bleaching agents, such as chlorine or chlorine di-oxide. The purification steps invol~e washings and treatment with sodium hydroxide solution, caustic extraction.
A particular bleaching sequence which finds use in one mode of the present invention involves an initial bleach-ing of the pulp with an aqueous solution containing chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using aqueous sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing.
These are the so~called DCEDED or D/C EDED sequences.
A common source of chlorine dioxide for the bleach-ing operation is a chlorine dioxide generator which produces chlorine dioxide, usually as an aqueous solution of chlorine dioxide and chlorine by the reduction of a chlorate salt, e.g., sodium chlorate. Such chlorine dioxide generators utilize a feed of H2S04 which is reacted with a mixture of sodium chlorate with some sodium chloride to produce a chlor-ine dioxide bleaching product and a sodium sulfate (saltcake) by-product.
The present invention utilizes the digestor liquor cycle, as described in the foregoing, in tandem with a sulfur cycle. The sulfur cycle utilizes a portion of the green liquor of the digestor cycle and has in common the furnacing and smelt dissolving steps of the digestor liquor cycleO The green liquor utilized in the sulfur cycle is carbonated, using stack gas, or other suitable sources of carbon dioxide, producing three phases. One is a gas phase of substantially pure hydrogen sul~ide. One is a solid phase of sodium bi-carbonate. ~he remaining phase is an aqueous liquid phase containing the soluble impurities of the green liquor, mainly chlorides. The hydrogen sulfide phase is converted into H2SO4 by oxidation and the product utilized as feedstock into a chlorine dioxide generator. The solid phase of NaHCO3 may be converted by causticizing to yield a relatively pure feed-stock for caustic extraction steps in a bleaching sequence ormay be furnaced to yield additional smelt. The aqueous phase, high in chlorides, is suitably used as feed into the chlorine dioxide generator as a source of chlorides or may be fed to a salt extractor, in either case, removing chlorides from the operationO
The alkali metal suifate ~y-product o~ the chlorine dioxide generator, for example sodium sulfate, is furnaced in common with the evaporated or concentrated black liquor to form a common smelt and subsequently additional green liquor, this completing the sulfur cycle.
It is contemplated that the carbonation reactions of the present invention proceed as follows; however, it is to be understood that the reactions are shown for clarity of understanding and are not limiting to the invention as it is to be considered that the green liquor and flue gas reactants are highly impure products.
Na2S + H20 + CO2-~ NaHS + ~aHC03~
Na2S + 2H20 + C2-~ 2~aHC03~ -t H2S
~a2S + H2S-~ 2NaHS
In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heat-ing,in a pulping liquor, (white liquor) which contains sodium sulfide and sodium hydroxide as the active pulping chemicals.
The digestion provides a pulp and spent pulping liquor (black liquor). The black liquor is separated from the pulp by washing in a brown stock washer and the pulp utilized as feedstock in a bleach plant for brightening and purification operations.
The black liquor is then concentrated, usually by evaporating, and the concentrated black liquor burned in a reducing furnace to yield a smelt containing primarily sodium carbonate and sodium sulfide.
The smelt is then dissolved in water to yield a raw green liquor which may be clari~ied to remove undissolved solids. The green liquor is then causticized, usually by treatment with Ca~ to convert sodium carbonate to sodium hydroxide. The resulting liquor is white liquor and is use-ful in the initial digestion step to provide at least a part of the pulping liquor.
The foregoing sequence or cycle is well known and 30 i5 referred to herein as the digestor liquor cycle.
- 1 - ~,",~
~ ... .
.~i ' .
4~
Bleach plant operations generally involve a sequence of briyhtening and purification steps which may be combined with washing steps. The brightening steps generally involve the use of bleaching agents, such as chlorine or chlorine di-oxide. The purification steps invol~e washings and treatment with sodium hydroxide solution, caustic extraction.
A particular bleaching sequence which finds use in one mode of the present invention involves an initial bleach-ing of the pulp with an aqueous solution containing chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using aqueous sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing.
These are the so~called DCEDED or D/C EDED sequences.
A common source of chlorine dioxide for the bleach-ing operation is a chlorine dioxide generator which produces chlorine dioxide, usually as an aqueous solution of chlorine dioxide and chlorine by the reduction of a chlorate salt, e.g., sodium chlorate. Such chlorine dioxide generators utilize a feed of H2S04 which is reacted with a mixture of sodium chlorate with some sodium chloride to produce a chlor-ine dioxide bleaching product and a sodium sulfate (saltcake) by-product.
The present invention utilizes the digestor liquor cycle, as described in the foregoing, in tandem with a sulfur cycle. The sulfur cycle utilizes a portion of the green liquor of the digestor cycle and has in common the furnacing and smelt dissolving steps of the digestor liquor cycleO The green liquor utilized in the sulfur cycle is carbonated, using stack gas, or other suitable sources of carbon dioxide, producing three phases. One is a gas phase of substantially pure hydrogen sul~ide. One is a solid phase of sodium bi-carbonate. ~he remaining phase is an aqueous liquid phase containing the soluble impurities of the green liquor, mainly chlorides. The hydrogen sulfide phase is converted into H2SO4 by oxidation and the product utilized as feedstock into a chlorine dioxide generator. The solid phase of NaHCO3 may be converted by causticizing to yield a relatively pure feed-stock for caustic extraction steps in a bleaching sequence ormay be furnaced to yield additional smelt. The aqueous phase, high in chlorides, is suitably used as feed into the chlorine dioxide generator as a source of chlorides or may be fed to a salt extractor, in either case, removing chlorides from the operationO
The alkali metal suifate ~y-product o~ the chlorine dioxide generator, for example sodium sulfate, is furnaced in common with the evaporated or concentrated black liquor to form a common smelt and subsequently additional green liquor, this completing the sulfur cycle.
It is contemplated that the carbonation reactions of the present invention proceed as follows; however, it is to be understood that the reactions are shown for clarity of understanding and are not limiting to the invention as it is to be considered that the green liquor and flue gas reactants are highly impure products.
Na2S + H20 + CO2-~ NaHS + ~aHC03~
Na2S + 2H20 + C2-~ 2~aHC03~ -t H2S
~a2S + H2S-~ 2NaHS
2~3 + H20 ~ C02~2NaHC0
3~2 The present invention also contemplates a balanced mill operation utilizing the digestor liquor and sulfur cycles in tandem.
The present inv~ntion will be described more in detail by reference to and discussion of the drawings~
Figure 1 is a schematic flow sheet illustrating a typical pulp mill operation utilizing a digestor liquor cycle, a bleach plant, and a chlorine di~xide generator to produce bleaching solutions.
Figure 2 is a schematic flow sheet illustrating a pulp mill operation utilizing a digestor liquor cycle in combination with a sulfur cycle, and a bleach plant. In this mode a portion of the sodium bicarbonate produced is causti- -cized and utilized in the bleaching operation and effluent from the carbonating tower is utilized as feed to the chlorine dioxide generator.
The chemlcal consumptions and requirements given in the following descriptions are given in pound equivalents required to produce one ton of air-dried pulp.
Looking now to Figure 1, the basic digestor liquor cycle is shown in combination with a chlorine dioxide genera-tor. Thus, fibrous cellulosic material, e~g., wood chips, are fed by line 10 into the digestor 12 wherein the wood chips are digested with a pulp liquor fed by line 14 contain-ing NaOH and Na2S as the main pulping chemicals. Miscel-laneous losses in the digestor amount to about 24~5 pounds of ~a20 equivalent, about 4.5 pounds of sulfur and about 0.6 pounds of sodium hydroxide, based upon one ton of air-dried pulp product from the plant. The pulp liquor fed by line 14 contains about 286 pounds sodium sulfide, about 684 pounds sodium hydroxide, about 7 pounds sodium sulfate, about ~34~;~
70 pounds sodium carbonate, about 0.1 pound sodium chloride and about 5,700 pounds of water.
The resulting pulp and spent pulping liquor are fed through line 15, and the pulp washed and separated from the waste pulping liquor in brown stock washer 16. The washed but unbleached pulp is fed by line 20 to bleach plant 22 wherein the pulp is subjected to a series of bleaching and purification processes invol~ing, in at least one stage, the use of chlorine dioxide. These processes may include ad-ditonal bleaching or purification steps utilizing a causticextraction wherein aqueous solutions of sodium hydroxide are fed through line 26. Generally, the pulp is washed during the ble~ch plant operation, typically after each bleaching or caustic extraction step by water fed through line 28. The spent wash water from the bleach plant washing operations, together ~ith the spent chemicals from the bleaching and caustic extraction steps, provide the plant bleaching ef~l-uent in line 18.
The bleach plant effluent in line 18, a potential loss andpollutant,might be considered to be added to the spent pulp liquor in line 36; however, this procedure is not feasible because substantial amounts of chlorides would be added to the pulp digestion system which are a cause of e~uipment erosion and, since chlorides do not react in this cycle, they accumulate as large amounts of non-useful mater-ials in the cycle. As shown in the remaining Figures, ~he present invention alleviates the problem of build~up of unusable chlorides in the digestor liquor cycle and further produces a sodium bicarbonate product which may readily be converted into sodium hydroxide and returned as a useful component to either the digestor liquor cycle or to the bleach plant operation.
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~ ~3~2 A chlorine dioxide generating unit 24 utilizes H2S0~, NaC103 and NaCl as feedstock and produces chlorine dioxide which is fed through line 30 into bleach plant 22.
Typically, about 24 pounds of NaCl, about 40.7 pounds of NaC103 and about 95 pounds of H3S04 are utilized to produce about 24.5 pounds of chlorine dioxide and about 15.2 pounds of chlorine. About 56.3 pounds of salt cake, Na2S04 are produced and are fed through line 32 to furnace 34. Although furnace 34 is equipped with precipitator 36, there is stack loss of about 2.90 pounds Na20 equivalent, 3.70 pounds of sulfur and about 0.3 pound NaCl.
The waste pulp liquor is fed by line 36 to evapora-~or 38, and the evaporator materials fed through line 40 to furnace 34. The smelt produced by furnace 34 is fed into smelt dissolver 42 to produce green liquor which is butted by the addition of soda ash, in the amount of about 116 pounds, and recausticized in recausticizer 48~ The recausti-cized solution is fed by line 46 into digestor 12, thus completing the digestor liquor cycle.
Looking now to Figure 2, this figure shows the digestor liquor cycle of Figure 1 in tandem with a sulfur cycle. A quantity of green liquor containing ~a2S equivalent to about ~4 pounds H2S04 (the requirement for chlorine dioxide generator 24 based upon one ton of air-dried pulp mill pro-duct) is withdrawn from smelt dissolver 42 through line 50 and fed into precarbonation vessel 52. The precarbonation vessel receives feed of C02 gas through line 54 from bicar-bonate tower 56. The precarbonated material is fed through line 58 into ~2S stripping tower 60 where additional bicar~
bonate solution cr slurry is fed through line 62 and exhausted solution is returned through line 64 to bicarbonate ~ 39~
tower 56. Substantially pure H2S in gaseous form is fed by line 66 into oxidizer 68 to produce M2SO4 or H2SO3. The suIfuric acid thus produced is fed through line 70 as feed-stock into chlorine dioxide generator 24, thus completing the sulfur cycle.
Looking now in more particular to bicarbonating tower 56, an integral part of the present invention, this tower, as a source of CO2, receives stack gas through line 72, although it will be understood that CO2 from any source may be used. The carbonation reaction precipitates a rela-tively pure NaHC03 product which may be easily separated by elutriation and recausticized by treatment with CaO to yield a substantially pure NaOH product for use in bleach plant 22 or to butt or produce additional pulping liquor. A por-tion of the carbonated product is utilized H2S stripper 60 -to liberate H2S. The mother liquor from elutriation, mainly chlorides, is utilized at leaat in part as a source of chlorides for chlorine dioxide generator 24.
It may be noted at this point that bleach plant e~fluent, especially extraction effluent containing large amounts of chlorides may now be returned to the digestor liquor cycle as the chlorides are removed in the sulfur cycle and utilized as feed into the chlorine dioxide genera-tor. Thus extraction effluent is returned from bleach plant 22 through line 19 to brown stock washer 16 and utilized as wash make-up.
The stack gases fed into bicarbonating tower 56 by line 72 precipitator 36 contain about 15 percent by weight of CO2. The entering CO2 reacts with Na2S and NaCO3 to produce NaHCO3. Although bicarbonating tower 56 may operate at pressures from atmosphere to about 100 p.s.i.a.
'~1 . .
~3~
and at temperatures of from about 25~ to about 100 C, a more practical range is atmospheric pressure to about 30 p.s.i.a. and about 50 to about 80r C. The main products of bicarbonating tower 56 are ~aHC03 and a chloride feedstock for chlorine dioxide generator 24. In precarbonization vessel 52, green liquor from line 50 is treated with a solution of ~aHC03 containing some C02 from bicarbonating tower 56. The desired reaction is to convert ~a2S to HaHS without libera-tion of H2S. This treatment is preferably carried out in a closed vessel at atmospheric pressure to minimize formation and loss of H2S. The treated solution from precarbonator vessel 52 is fed by line 58 to stripping column 60 where the mixture is stripped, preferably by use of steam to release H2S in concentrated and substantially pure form. H2S is removed from the solution preferably by flash or vacuum stripping in which is fed solution containing NaHC03 by line 62 and exhausted solution containing Na2C03 returned to bicarbonating tower 56 through line 64. Steam is preferabl~
used as the stripping agent since a recovery of concentrated ~2S gas can be effected simply by condensing the steam from the mixture of steam and H2S.
The bicarbonating and stripping operations may be carried out in any type of equipment conventionally employed for gas absorptiorl or stripping operatiorls. Thus pacXed columns, plate columns, spray columns and continuous liquid phase columns are aptly suited to use. Agitated gas disper-sion equipment rnay be advantageously used in the bicarbon-ating tower.
The liberated H2S is fed by line 66 to sulfuric acid pla~t 68 where the H2S is oxidized to produce S03 which is dissolved in a H20 solution to yield a H2SO4 product.
, , The H2SO~ produced is fed into chlorine dioxide generator 24 as feedstock. The salt-cake produced as a by~
product from chlorine dio~ide generator 24 is fed by line 32 to furnace _ where it is furnaced in common with evaporated waste pulping liquor from evaporator 38.
In this mode of the invention, the excess ~aHCO3, about 200 pounds, is fed to a recausticizer 76 which converts the ~a2CO3 into about 95 pounds of ~aOH which is suitably fed through line 78 to bleach plant 22 and utilized in the pulp bleaching operation, although, if required, can be utilized to butt or produce pulp liquorO
Because of losses in the various stages of a Kraft mill operation, additonal alkali soda (~aOH) and, in some cases, additional sulfur make up are required. As much as 1 possible, this is done by the utilizatlon of salt cake, but, because of its sulfur content, salt cake usually cannot be fully utilized, and additional alkali soda will need to be acquired. It will be noted that, in the process of Figure 2, the alkali soda in excess is produced from salt cake without the addition of excessive amounts of sul-Eur being added to the digestor liquor.
It will be understood that many modifications to the present invention may be made, for example, various bleach sequences will yield effluents of varying compositions, or other paper making activities which have chemical require-ments or by-products, which may be provided for by modifica-tion of the present process.
Economic or environmental factors may also be-pro-vided for by modifications, for example, in one mode of operation of the present invention a pulp bleaching sequence of DCEDED is utilized in bleach plant 22. ~hen this sequence _ g _ is used, the entire effluent from the bleach plant may be returned to brown stock washer 16, thus utilizing and there-by eliminating all effluent from the bleach plant operation.
If desired, a salt crystallizer may be installed in connection with bicarbonating tower 56 and sodium chloride in substan-tially pure form recovered.
The following table gives comparative.chemical requirements and credit balances for the process of the present invention when operated under the conditions of Figure 2 (Column A) and under a total recycle of bleach plant effluent (Column B) as compared with a plant operating under a normal digestor liquor cycle (Column C). The requirements and credits are given in pounds of chemicals required to produce one ton of air-dried pulp.
Requirements A B C
H2SO4 ~ - 38,8 NaClO3 96.8 96.8 40,7 NaCl - - 24.0 Na2C3 94.2 - 116.0 40.8 NaOH _ _ 95 0 Na2S4 46.'7 43.9 Credits Na2SO~ - - 6.3 ~aCl - 39.0 ~,l
The present inv~ntion will be described more in detail by reference to and discussion of the drawings~
Figure 1 is a schematic flow sheet illustrating a typical pulp mill operation utilizing a digestor liquor cycle, a bleach plant, and a chlorine di~xide generator to produce bleaching solutions.
Figure 2 is a schematic flow sheet illustrating a pulp mill operation utilizing a digestor liquor cycle in combination with a sulfur cycle, and a bleach plant. In this mode a portion of the sodium bicarbonate produced is causti- -cized and utilized in the bleaching operation and effluent from the carbonating tower is utilized as feed to the chlorine dioxide generator.
The chemlcal consumptions and requirements given in the following descriptions are given in pound equivalents required to produce one ton of air-dried pulp.
Looking now to Figure 1, the basic digestor liquor cycle is shown in combination with a chlorine dioxide genera-tor. Thus, fibrous cellulosic material, e~g., wood chips, are fed by line 10 into the digestor 12 wherein the wood chips are digested with a pulp liquor fed by line 14 contain-ing NaOH and Na2S as the main pulping chemicals. Miscel-laneous losses in the digestor amount to about 24~5 pounds of ~a20 equivalent, about 4.5 pounds of sulfur and about 0.6 pounds of sodium hydroxide, based upon one ton of air-dried pulp product from the plant. The pulp liquor fed by line 14 contains about 286 pounds sodium sulfide, about 684 pounds sodium hydroxide, about 7 pounds sodium sulfate, about ~34~;~
70 pounds sodium carbonate, about 0.1 pound sodium chloride and about 5,700 pounds of water.
The resulting pulp and spent pulping liquor are fed through line 15, and the pulp washed and separated from the waste pulping liquor in brown stock washer 16. The washed but unbleached pulp is fed by line 20 to bleach plant 22 wherein the pulp is subjected to a series of bleaching and purification processes invol~ing, in at least one stage, the use of chlorine dioxide. These processes may include ad-ditonal bleaching or purification steps utilizing a causticextraction wherein aqueous solutions of sodium hydroxide are fed through line 26. Generally, the pulp is washed during the ble~ch plant operation, typically after each bleaching or caustic extraction step by water fed through line 28. The spent wash water from the bleach plant washing operations, together ~ith the spent chemicals from the bleaching and caustic extraction steps, provide the plant bleaching ef~l-uent in line 18.
The bleach plant effluent in line 18, a potential loss andpollutant,might be considered to be added to the spent pulp liquor in line 36; however, this procedure is not feasible because substantial amounts of chlorides would be added to the pulp digestion system which are a cause of e~uipment erosion and, since chlorides do not react in this cycle, they accumulate as large amounts of non-useful mater-ials in the cycle. As shown in the remaining Figures, ~he present invention alleviates the problem of build~up of unusable chlorides in the digestor liquor cycle and further produces a sodium bicarbonate product which may readily be converted into sodium hydroxide and returned as a useful component to either the digestor liquor cycle or to the bleach plant operation.
~ ~ " ~
~ ~3~2 A chlorine dioxide generating unit 24 utilizes H2S0~, NaC103 and NaCl as feedstock and produces chlorine dioxide which is fed through line 30 into bleach plant 22.
Typically, about 24 pounds of NaCl, about 40.7 pounds of NaC103 and about 95 pounds of H3S04 are utilized to produce about 24.5 pounds of chlorine dioxide and about 15.2 pounds of chlorine. About 56.3 pounds of salt cake, Na2S04 are produced and are fed through line 32 to furnace 34. Although furnace 34 is equipped with precipitator 36, there is stack loss of about 2.90 pounds Na20 equivalent, 3.70 pounds of sulfur and about 0.3 pound NaCl.
The waste pulp liquor is fed by line 36 to evapora-~or 38, and the evaporator materials fed through line 40 to furnace 34. The smelt produced by furnace 34 is fed into smelt dissolver 42 to produce green liquor which is butted by the addition of soda ash, in the amount of about 116 pounds, and recausticized in recausticizer 48~ The recausti-cized solution is fed by line 46 into digestor 12, thus completing the digestor liquor cycle.
Looking now to Figure 2, this figure shows the digestor liquor cycle of Figure 1 in tandem with a sulfur cycle. A quantity of green liquor containing ~a2S equivalent to about ~4 pounds H2S04 (the requirement for chlorine dioxide generator 24 based upon one ton of air-dried pulp mill pro-duct) is withdrawn from smelt dissolver 42 through line 50 and fed into precarbonation vessel 52. The precarbonation vessel receives feed of C02 gas through line 54 from bicar-bonate tower 56. The precarbonated material is fed through line 58 into ~2S stripping tower 60 where additional bicar~
bonate solution cr slurry is fed through line 62 and exhausted solution is returned through line 64 to bicarbonate ~ 39~
tower 56. Substantially pure H2S in gaseous form is fed by line 66 into oxidizer 68 to produce M2SO4 or H2SO3. The suIfuric acid thus produced is fed through line 70 as feed-stock into chlorine dioxide generator 24, thus completing the sulfur cycle.
Looking now in more particular to bicarbonating tower 56, an integral part of the present invention, this tower, as a source of CO2, receives stack gas through line 72, although it will be understood that CO2 from any source may be used. The carbonation reaction precipitates a rela-tively pure NaHC03 product which may be easily separated by elutriation and recausticized by treatment with CaO to yield a substantially pure NaOH product for use in bleach plant 22 or to butt or produce additional pulping liquor. A por-tion of the carbonated product is utilized H2S stripper 60 -to liberate H2S. The mother liquor from elutriation, mainly chlorides, is utilized at leaat in part as a source of chlorides for chlorine dioxide generator 24.
It may be noted at this point that bleach plant e~fluent, especially extraction effluent containing large amounts of chlorides may now be returned to the digestor liquor cycle as the chlorides are removed in the sulfur cycle and utilized as feed into the chlorine dioxide genera-tor. Thus extraction effluent is returned from bleach plant 22 through line 19 to brown stock washer 16 and utilized as wash make-up.
The stack gases fed into bicarbonating tower 56 by line 72 precipitator 36 contain about 15 percent by weight of CO2. The entering CO2 reacts with Na2S and NaCO3 to produce NaHCO3. Although bicarbonating tower 56 may operate at pressures from atmosphere to about 100 p.s.i.a.
'~1 . .
~3~
and at temperatures of from about 25~ to about 100 C, a more practical range is atmospheric pressure to about 30 p.s.i.a. and about 50 to about 80r C. The main products of bicarbonating tower 56 are ~aHC03 and a chloride feedstock for chlorine dioxide generator 24. In precarbonization vessel 52, green liquor from line 50 is treated with a solution of ~aHC03 containing some C02 from bicarbonating tower 56. The desired reaction is to convert ~a2S to HaHS without libera-tion of H2S. This treatment is preferably carried out in a closed vessel at atmospheric pressure to minimize formation and loss of H2S. The treated solution from precarbonator vessel 52 is fed by line 58 to stripping column 60 where the mixture is stripped, preferably by use of steam to release H2S in concentrated and substantially pure form. H2S is removed from the solution preferably by flash or vacuum stripping in which is fed solution containing NaHC03 by line 62 and exhausted solution containing Na2C03 returned to bicarbonating tower 56 through line 64. Steam is preferabl~
used as the stripping agent since a recovery of concentrated ~2S gas can be effected simply by condensing the steam from the mixture of steam and H2S.
The bicarbonating and stripping operations may be carried out in any type of equipment conventionally employed for gas absorptiorl or stripping operatiorls. Thus pacXed columns, plate columns, spray columns and continuous liquid phase columns are aptly suited to use. Agitated gas disper-sion equipment rnay be advantageously used in the bicarbon-ating tower.
The liberated H2S is fed by line 66 to sulfuric acid pla~t 68 where the H2S is oxidized to produce S03 which is dissolved in a H20 solution to yield a H2SO4 product.
, , The H2SO~ produced is fed into chlorine dioxide generator 24 as feedstock. The salt-cake produced as a by~
product from chlorine dio~ide generator 24 is fed by line 32 to furnace _ where it is furnaced in common with evaporated waste pulping liquor from evaporator 38.
In this mode of the invention, the excess ~aHCO3, about 200 pounds, is fed to a recausticizer 76 which converts the ~a2CO3 into about 95 pounds of ~aOH which is suitably fed through line 78 to bleach plant 22 and utilized in the pulp bleaching operation, although, if required, can be utilized to butt or produce pulp liquorO
Because of losses in the various stages of a Kraft mill operation, additonal alkali soda (~aOH) and, in some cases, additional sulfur make up are required. As much as 1 possible, this is done by the utilizatlon of salt cake, but, because of its sulfur content, salt cake usually cannot be fully utilized, and additional alkali soda will need to be acquired. It will be noted that, in the process of Figure 2, the alkali soda in excess is produced from salt cake without the addition of excessive amounts of sul-Eur being added to the digestor liquor.
It will be understood that many modifications to the present invention may be made, for example, various bleach sequences will yield effluents of varying compositions, or other paper making activities which have chemical require-ments or by-products, which may be provided for by modifica-tion of the present process.
Economic or environmental factors may also be-pro-vided for by modifications, for example, in one mode of operation of the present invention a pulp bleaching sequence of DCEDED is utilized in bleach plant 22. ~hen this sequence _ g _ is used, the entire effluent from the bleach plant may be returned to brown stock washer 16, thus utilizing and there-by eliminating all effluent from the bleach plant operation.
If desired, a salt crystallizer may be installed in connection with bicarbonating tower 56 and sodium chloride in substan-tially pure form recovered.
The following table gives comparative.chemical requirements and credit balances for the process of the present invention when operated under the conditions of Figure 2 (Column A) and under a total recycle of bleach plant effluent (Column B) as compared with a plant operating under a normal digestor liquor cycle (Column C). The requirements and credits are given in pounds of chemicals required to produce one ton of air-dried pulp.
Requirements A B C
H2SO4 ~ - 38,8 NaClO3 96.8 96.8 40,7 NaCl - - 24.0 Na2C3 94.2 - 116.0 40.8 NaOH _ _ 95 0 Na2S4 46.'7 43.9 Credits Na2SO~ - - 6.3 ~aCl - 39.0 ~,l
Claims (14)
1. A cellulosic pulp and bleaching process comprising the steps of:
a) digesting fibrous cellulosic material, b) separating the fibrous pulp and waste liquor produced by the said digesting step, c) evaporating and burning said waste liquor in a furnace to form a smelt, d) dissolving said smelt to form a green liquor, causticizing a portion of said green liquor and utilizing said causticized green liquor in said digesting step, e) treating a portion of said green liquor to produce NaHCO3, and utilizing said NaHCO3 for the purpose of, f) treating a portion of said green liquor to produce hydrogen sulfide, g) oxidizing at least a portion of said hydrogen sulfide to produce H2SO4, h) utilizing at least a portion of said H2SO4 as feedstock in a chlorine dioxide generator to react with an alkali metal chlorate and an alkali metal chloride to produce C1O2 and an alkali metal sulfate, and i) utilizing said alkali metal sulfate as a feed into said furnace.
a) digesting fibrous cellulosic material, b) separating the fibrous pulp and waste liquor produced by the said digesting step, c) evaporating and burning said waste liquor in a furnace to form a smelt, d) dissolving said smelt to form a green liquor, causticizing a portion of said green liquor and utilizing said causticized green liquor in said digesting step, e) treating a portion of said green liquor to produce NaHCO3, and utilizing said NaHCO3 for the purpose of, f) treating a portion of said green liquor to produce hydrogen sulfide, g) oxidizing at least a portion of said hydrogen sulfide to produce H2SO4, h) utilizing at least a portion of said H2SO4 as feedstock in a chlorine dioxide generator to react with an alkali metal chlorate and an alkali metal chloride to produce C1O2 and an alkali metal sulfate, and i) utilizing said alkali metal sulfate as a feed into said furnace.
2. The process of Claim 1 wherein the fibrous cellu-losic material is wood,
3. The process of Claim 1 wherein the alkali metal is sodium.
4. The process of Claim 1, 2 or 3, wherein the flue gas from said furnace in (c) is utilized in (e) to supply CO2 to treat liquor to produce NaHCO3.
5. The process of Claim 1, 2 or 3 wherein the C1O2 product from (h) is used to bleach pulp.
6. The process of Claim 1, 2 or 3 wherein the C1O2 product is an aqueous solution of C1O2.
7. The process of Claim 1, 2 or 3 wherein the mother liquor from (e) after NaHCO3 removal is utilized as a feed into the said chlorine dioxide generator in (h).
8. The process of Claim 1, 2 or 3 wherein a conden-sate is produced in (f) and H2S is separated, utilizing said condensate in (d) to dissolve smelt.
9. The process of claim 1, 2 or 3 wherein (b) is carried out in a brown stock washer and involves a wash.
10. The process of Claim 1, 2 or 3 wherein (b) is carried out in a brown stock washer and involves a wash and bleach plant effluent is utilized as at least a portion of said wash.
11. The process of Claim 1, 2 or 3 wherein at least a portion of the NaHCO3 produced in (e) is recausticized and utilized in a pulp bleaching sequence.
12. The process of Claim 1, 2 or 3 wherein at least a portion of the NaHCO3 produced in (e) is recausticized and utilized as digestor liquor in (a).
13. The process of Claim 1, 2 or 3 wherein NaHCO3 from (e) is elutriated to obtain a substantially pure NaHCO3 product.
14. The process of Claim 1, 2 or 3 wherein the green liquor in (f) is initially precarbonated by treatment with NaHCO3 and CO2.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA294,604A CA1103412A (en) | 1978-01-09 | 1978-01-09 | Kraft mill recycle process |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA294,604A CA1103412A (en) | 1978-01-09 | 1978-01-09 | Kraft mill recycle process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103412A true CA1103412A (en) | 1981-06-23 |
Family
ID=4110491
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA294,604A Expired CA1103412A (en) | 1978-01-09 | 1978-01-09 | Kraft mill recycle process |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1103412A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5607549A (en) * | 1994-06-17 | 1997-03-04 | Ahlstrom Machinery Corporation | Method for the manufacture of cooking liquors of different sulphidity by green liquor crystallization |
-
1978
- 1978-01-09 CA CA294,604A patent/CA1103412A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5607549A (en) * | 1994-06-17 | 1997-03-04 | Ahlstrom Machinery Corporation | Method for the manufacture of cooking liquors of different sulphidity by green liquor crystallization |
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