CA2171595A1 - Use of acid-stage filtrate in cooking liquor preparation - Google Patents
Use of acid-stage filtrate in cooking liquor preparationInfo
- Publication number
- CA2171595A1 CA2171595A1 CA002171595A CA2171595A CA2171595A1 CA 2171595 A1 CA2171595 A1 CA 2171595A1 CA 002171595 A CA002171595 A CA 002171595A CA 2171595 A CA2171595 A CA 2171595A CA 2171595 A1 CA2171595 A1 CA 2171595A1
- Authority
- CA
- Canada
- Prior art keywords
- filtrate
- liquor
- alkaline
- stage
- produce
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
- D21C11/0028—Effluents derived from the washing or bleaching plants
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P40/00—Technologies relating to the processing of minerals
- Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills
Abstract
Acidic bleach plant effluents from a cellulose pulp mill utilizing a sodium-based cooking process are effectively utilized without contamination of pulp mill processes with undesirable metal ions. The bleach plant effluent, in the form of filtrate, is treated with an alkaline stream to precipitate solids containingmetals from the filtrate, producing a reduced metal content filtrate. The reduced metal content filtrate is used to wash or dilute lime mud which is in turn used to produce lime for causticizing green liquor to produce white liquor.Acid filtrate which is not treated to remove metals may be used to dissolve smelt from the recovery boiler to make green liquor which is clarified to produce clarified green liquor and dregs containing metals. The alkaline stream used to precipitate solids from the acidic filtrate is preferably clarified green liquor or white liquor, although an alkaline filtrate from an alkaline bleaching or delignification stage may be used.
Description
- USE OF ACID-STAGE FILTRATE IN COOKING LIQUOR PREPARATION
BACKGROUND AND SUMMARY OF THE INVENTION
One of the most significant objectives of the pulp and paper industry in attempting to limit the environmental impact of pulp and paper mills5 is the minimization of liquid discharge from the mill, in particular from the bleach plant. Commonly referred to as "closing the bleach plant", this objective usually entails some way of collecting and re-using all the liquid effluents from the bleach plant with minimal discharge to the surrounding environment. In the past, this objective was partially achieved in practice by 10 recirculating alkaline bleach plant effluent to the brownstock washing stage and ultimately to the alkaline recovery system.
The recirculation of acidic bleach plant effluents is much more difficult. These acidic effluents, for example, from an acid wash stage (A), an acidic or neutral chelation stage (Q), an acidic ozone stage (Z), or other acidic 15 treatment stage, cannot be returned directly to the conventional recovery system. [Throughout this application, for simplicity, the expression "acidic treatment" or "acid treatment" should be interpreted to include a chelation treatment which may be of approximately neutral pH, as well as an acid pH (6 or below) treatment.] These acidic streams can cause excessive foam 20 generation if mixed with the alkaline streams, can overload the recovery system by the sheer volume of the liquid, and these streams contain undesirable metal ions which can negatively impact the processes and equipment.
One highly desirable destination for these acidic streams is 25 cooking liquor preparation, or what is commonly known as the recausticizationplant. This is the area in the pulp mill where cooking liquor is generated from the chemicals recovered in the recovery process. For example, in kraft pulping, white liquor is regenerated from the sodium salts in the recovery boiler smelt.
It is desirable to introduce the metal-laden acidic effluents to the 30 recausticization plant because this method provides the most efficient removal of metals from the fiberline. For example, even metals that pass through the recausticization plant and to the digester, with the white liquor, are eventually 217159~
BACKGROUND AND SUMMARY OF THE INVENTION
One of the most significant objectives of the pulp and paper industry in attempting to limit the environmental impact of pulp and paper mills5 is the minimization of liquid discharge from the mill, in particular from the bleach plant. Commonly referred to as "closing the bleach plant", this objective usually entails some way of collecting and re-using all the liquid effluents from the bleach plant with minimal discharge to the surrounding environment. In the past, this objective was partially achieved in practice by 10 recirculating alkaline bleach plant effluent to the brownstock washing stage and ultimately to the alkaline recovery system.
The recirculation of acidic bleach plant effluents is much more difficult. These acidic effluents, for example, from an acid wash stage (A), an acidic or neutral chelation stage (Q), an acidic ozone stage (Z), or other acidic 15 treatment stage, cannot be returned directly to the conventional recovery system. [Throughout this application, for simplicity, the expression "acidic treatment" or "acid treatment" should be interpreted to include a chelation treatment which may be of approximately neutral pH, as well as an acid pH (6 or below) treatment.] These acidic streams can cause excessive foam 20 generation if mixed with the alkaline streams, can overload the recovery system by the sheer volume of the liquid, and these streams contain undesirable metal ions which can negatively impact the processes and equipment.
One highly desirable destination for these acidic streams is 25 cooking liquor preparation, or what is commonly known as the recausticizationplant. This is the area in the pulp mill where cooking liquor is generated from the chemicals recovered in the recovery process. For example, in kraft pulping, white liquor is regenerated from the sodium salts in the recovery boiler smelt.
It is desirable to introduce the metal-laden acidic effluents to the 30 recausticization plant because this method provides the most efficient removal of metals from the fiberline. For example, even metals that pass through the recausticization plant and to the digester, with the white liquor, are eventually 217159~
returned to the recovery system in black liquor and removed in the green liquor dregs during green liquor clarification. Thus, the removal of metals in an acidic stage of the bleach plant and passing them to the liquor recovery system minimizes the introduction of any metals to the rest of the fiberline, e.g.
brownstock washing or oxygen delignification, where they are more difficult to remove and can accumulate.
The production of white liquor requires various dilution and washing steps that the acidic effluents can be used for. Typically, the water used for dilution and washing in the recausticization plant eventually ends up in the white liquor produced. However, as noted above, the acidic bleach plant effluents contain dissolved metal ions which can also affect the efficiency of the liquor manufacturing processes and the associated equipment. For example, in the manufacture of kraft white liquor, metal ions, e.g., manganese, iron, magnesium, etc., can accumulate within the lime cycle and detrimentally impact the quality of the lime produced. Lime quality is typically indicated by what is called its "activity", that is, its ability to effectively react with sodium carbonate to form sodium hydroxide. The presence of metals reduces this activity. Conventionally, the accumulation of metals in the lime is addressed by purging some of the metal-laden lime and replacing it with fresh lime.
However, this has the disadvantage of introducing the additional cost for the fresh chemical and also creates the potential for introducing to the system further undesirable substances, such as silicon and aluminum, from the purchased lime.
In addition to adversely affecting the activity of the lime, the presence of metals in the lime mud can severely impair the sedimentation and dewatering properties of the lime mud. In particular, the presence of magnesium in the lime mud should be avoided for this reason.
Thus, acidic bleach plant effluents cannot be used in the recausticization plant without some form of treatment or consideration of the effect of the presence of metal ions upon the chemical processes involved.
According to the present invention a method and apparatus are provided for overcoming problems created by the introduction of a metal-laden acidic bleach plant effluent stream to the recausticization plant. According to the invention the metal-laden acidic bleach plant effluent is properly treated and/or handled so that it can be effectively used in a manner that will facilitate the closing of the bleach plant without adverse consequences on the recovery system.
According to one aspect of the present invention a method of 5 handling acidic waste liquids from a bleach plant of a sodium-based (e.g. kraft, sodium-based sulfite, or soda) cellulose pulp mill having a recausticization plant in which lime mud is removed from causticized green liquor to produce white liquor and then reutilized to causticize green liquor, is provided. The method comprises the following steps: (a) Acid treating cellulose pulp produced by a 10 sodium based cooking process. (b) Washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals. (c) Treating at least part of the filtrate from step (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate.(d) Using the reduced metal content filtrate from step (c) to wash or dilute lime 15 mud. And, (e) using the washed or diluted lime mud from step (d) to produce lime used to causticize green liquor to produce white liquor.
Typically the pulp mill has a recovery boiler which produces smelt used to make green liquor. The method then comprises the further steps of: (f) using part of the filtrate from step (b) that is not treated in step (c) to 20 dissolve smelt from the recovery boiler to make green liquor, and then (9) clarifying the green liquor to produce clarified green liquor and dregs containing metals. Preferably at least part of the alkaline stream used to practice step (c) is clarified or unclarified green liquor from step (g) or clarified or unclarified white liquor from step (e), although an alkaline filtrate from an alkaline 25 bleaching or delignification stage of the bleach plant may also be used. As is conventional step (e) is typically practiced by dewatering the washed or dilutedlime mud, burning the dewatered lime mud in a lime kiln to produce lime, and using the lime to causticize clarified green liquor to produce white liquor.
The pulp mill may also include an alkaline bleaching stage. The 30 above method may further comprise the following further steps: (a1 ) treatingthe pulp in an alkaline stage; (a2) washing or thickening the pulp from step (a1 ) to produce an alkaline filtrate; and (a3) combining at least some of the alkaline filtrate with the filtrate from step (b) prior to step (c).
The invention also relates to a method comprising the following steps: (a) Acid treating cellulose pulp produced by a sodium based cooking process. (b) Washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals. (c) Treating at least part of the filtrate from step 5 (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate. (d) Using part of the filtrate from step (b) that is not treated in step (c) to dissolve smelt from the recovery boiler to make green liquor, and then (e) clarifying the green liquor to produceclarified green liquor and dregs containing metals.
Apparatus is also provided according to the present invention in the form of a pulp mill system for producing cellulose pulp produced from a sodium-based cooking process. The pulp mill system comprises the following components: A digester including at least one white liquor inlet, at least one black liquor outlet, and a pulp outlet. A pulp acid treatment stage downstream 15 of the pulp outlet from said digester. A wash or thickening stage after the acid treatment stage, the wash or thickening stage comprising a filtrate outlet.
Means for removing metals from at least some of the filtrate passing out of the wash stage filtrate outlet to produce a reduced metal content filtrate. A
recovery boiler connected to the digester black liquor outlet, and including a 20 smelt discharge. A smelt dissolver connected to the smelt discharge. A green liquor clarifier connected to the smelt dissolver. A causticizer connected to the green liquor clarifier. A lime mud separator connected to the causticizer.
A lime mud washing or dilution stage connected to the lime mud separator to produced washed or diluted lime mud. Means for directing the 25 reduced metal content filtrate to the lime mud washing or diluting stage. And, means for using the washed or diluted lime mud to produce lime used in the causticizer to produce white liquor from green liquor.
The system preferably further comprises a conduit directing some of the filtrate from the filtrate outlet to the smelt dissolver to dissolve30 smelt to produce green liquor. The means for removing metals from at least some of the filtrate passing out of the washing stage filtrate outlet preferablycomprises means for adding an alkaline stream of liquor from the pulp mill to the filtrate. The means for adding the alkaline stream preferably comprises means for adding at least one of white liquor and green liquor to the filtrate, although a bleach plant alkaline stage alkaline filtrate may alternatively be used.
The system may also include a pulp alkaline treatment stage, a washing or thickening stage after the alkaline treatment stage which includes 5 an alkaline filtrate outlet, and a means for combining at least some of the alkaline filtrate with the acidic filtrate prior to passing the filtrate to the metal removing means.
It is the primary object of the present invention to facilitate closing of a bleach plant of a cellulose pulp mill utilizing a sodium-based 10 cooking process by proper treatment and handling of acidic bleach plant effluents, such as from the acid wash stage, acidic or neutral chelation stage, or an acidic ozone stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.
FIGURE 1 is a schematic illustration of an exemplary cellulose pulp mill system according to the invention for practicing exemplary methods of handling acidic waste liquids from a bleach plant, according to the present invention; and 20 FIGURE 2 is a schematic detail view of one embodiment of the metals removal stage of FIGURE 1.
DETAILED DESCRIPTION
The system illustrated generally by reference numeral 10 in FIGURE 1 includes a digester 11 ~continuous digesters are preferred although 25 batch digesters may be utilized) which produces cellulose pulp utilizing a sodium-based cooking process, e.g. kraft (sulfate), sodium-based sulfite, or soda cooking processes. Slurried wood chips or other cellulose material is fed into the digester 1 1 as indicated by line 12, and white liquor (used substantially generically herein to cover sodium-based cooking liquors) is added30 to the digester by line 13, white liquor typically also being added at other points in the digester 11 and with the wood chips introduced in line 12. The pulp produced in the digester 11 is discharged in line 14 typically to a brownstock washing stage 15, and then to a bleach plant, shown generally by reference numeral 16. The bleach plant typically includes a number of different 5 stages, such as an alkaline bleaching or delignification stage or stages 17 (e.g.
an oxygen delignification stage and/or alkaline extraction stage, or other conventional alkaline stages) followed by a wash stage 18, and one or more acid bleach/treatment stages 19 followed by a wash thickening stage 20.
Typical processes utilized for the stage 19 comprise acid washing (A), acidic 10 or neutral chelation (Q), acidic ozone (Z), acidic peroxide (Pa), or peracetic acid.
Other bleaching stages are indicated schematically at 21. Bleached pulp is produced as indicated in line 22.
Filtrate from the acidic wash or thickening stage 20 is represented by line 23 in FIGURES 1 and 2. As shown in the international 15 patent application PCT/US94/012373, the disclosures of which are hereby incorporated by reference herein, metals are removed from the filtrate in line 23 utilizing a metals removal stage 24. The metals removal stage 24 may include any of the configurations illustrated in the international application, or the like, one exemplary such system being illustrated in FIGURE
20 2. For example as indicated in FIGURE 2, the filtrate in line 23 may be oxidized as indicated at 25, and passed to a reactor 26 where an alkaline stream of material (typically liquid) is added as indicated at 27. The alkaline stream causes solids containing metals to precipitate from the filtrate, the filtrate with precipitated solids then passing first to clarification and then to the stage 28, 25 or directly to stage 28. At stage 28 the solids that have been precipitated are removed, as by filtration, sedimentation, flotation, or any other suitable conventional technique. The metal-laden stream may take the form of solids and be discharged as sludge, as indicated at line 29 or a metal-laden liquid stream, the retentate 29', may be used, for example, for smelt dissolution 37, 30 or burned in the recovery boiler 35. The metal depleted stream is discharged in line 30.
The digester 11 has a black liquor (waste liquor containing dissolved lignin) outlet conduit 32 associated therewith. The black liquor is evaporated and/or concentrated as is conventional and as indicated by box 33 in FIGURE 1, and may be subjected to other treatments ~such as pressure heated treatment to reduce the viscosity and produce organic sulfur containing gases) as indicated at 34 in FIGURE 1. Ultimately the black liquor passes to a conventional recovery boiler 35 where it is burned producing a smelt which is discharged from the recovery boiler in line 36. The smelt in line 36 passes to a smelt dissolver 37 where liquid is added to the smelt to produce green liquor discharged in line 38. The green liquor is clarified in a clarifier or filter 39, to separate solids, i.e., dregs (containing metals), which dregs are discharged in line 40 for environmentally suitable disposal, while clarified green liquor passes in line 41 to a causticizer 42. In the causticizer 42, lime is added in line 43, the lime typically being from a lime kiln 44. From the causticizer 42 the causticized green liquor passes to a lime mud separation stage 45 which is typically a filtration or sedimentation means. The end product produced in line 46 is white liquor (or like cooking liquor) which of course is ultimately used in line 13 associated with the digester 11. The lime mud that is separated in stage 45 passes to a washing or dilution 48, and then typically is dewatered at stage 49 before being burned in the lime kiln 44 to regenerate lime from calcium carbonate ~a suspension of calcium carbonate is what is known as "lime mud").
According to the present invention the acidic filtrate in line 23 is used in an optimum manner in the recausticization plant -- the entire structure 50 illustrated in FIGURE 1. The metal depleted stream in line 30 is ideal for washing or diluting the lime mud in stage 48, as illustrated in FIGURE1. The liquid in line 30 has a minimum potential to introduce undesirable metals into the lime cycle and therefore the activity of the lime is not adversely affected. Also sedimentation and dewatering are optimized because they can be hindered by the presence of metals, few of which are present in the metal depleted stream 30.
Though not as desirable as using the metal-laden stream for smelt dissolution, stream 23 may also be used for lime mud washing in 48.
However, if this is done, some accommodation must be made for the potential to accumulate metals in the lime cycle.
In the international patent application PCT/US94/012373 it was suggested that the metal depleted stream could be utilized for green liquor ~ 9 production, that is by utilizing it as the liquor in the smelt dissolver 37 to produce green liquor in line 38. However it has been recognized according to the present invention that it is not necessary to effect removal of the metals from the acidic stream 23 for this purpose. Rather a part of the acidic bleach 5 plant effluent - that part in line 52 -- may be fed directly to the smelt dissolver 37 to be used alone (or with other liquid from conventional sources) to produce a green liquor. This use of the acidic bleach plant effluent is innocuous because after 10 dissolution of the smelt and the formation of the green liquor in line 38 thegreen liquor is clarified in clarifier 39. Thus metal containing solids are removed in the clarifier 39 in the form of the dregs 40 which can be disposed of in a conventional environmentally friendly manner without introducing metals to the processes in the liquor manufacturing cycle which they could be 15 detrimental to.
It will thus be seen that according to the present invention a novel and desirable process and apparatus are provided for closing a bleach plant. Metal-laden acidic bleach plant effluent is reused in the preparation of cooking liquor without adversely affecting the efficiency of the chemical 20 processes or the operation of the equipment. Again these techniques are applicable to any sodium-based cooking process including kraft, sodium-based sulfite, and soda cooking processes.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment 25 thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent systems and processes.
brownstock washing or oxygen delignification, where they are more difficult to remove and can accumulate.
The production of white liquor requires various dilution and washing steps that the acidic effluents can be used for. Typically, the water used for dilution and washing in the recausticization plant eventually ends up in the white liquor produced. However, as noted above, the acidic bleach plant effluents contain dissolved metal ions which can also affect the efficiency of the liquor manufacturing processes and the associated equipment. For example, in the manufacture of kraft white liquor, metal ions, e.g., manganese, iron, magnesium, etc., can accumulate within the lime cycle and detrimentally impact the quality of the lime produced. Lime quality is typically indicated by what is called its "activity", that is, its ability to effectively react with sodium carbonate to form sodium hydroxide. The presence of metals reduces this activity. Conventionally, the accumulation of metals in the lime is addressed by purging some of the metal-laden lime and replacing it with fresh lime.
However, this has the disadvantage of introducing the additional cost for the fresh chemical and also creates the potential for introducing to the system further undesirable substances, such as silicon and aluminum, from the purchased lime.
In addition to adversely affecting the activity of the lime, the presence of metals in the lime mud can severely impair the sedimentation and dewatering properties of the lime mud. In particular, the presence of magnesium in the lime mud should be avoided for this reason.
Thus, acidic bleach plant effluents cannot be used in the recausticization plant without some form of treatment or consideration of the effect of the presence of metal ions upon the chemical processes involved.
According to the present invention a method and apparatus are provided for overcoming problems created by the introduction of a metal-laden acidic bleach plant effluent stream to the recausticization plant. According to the invention the metal-laden acidic bleach plant effluent is properly treated and/or handled so that it can be effectively used in a manner that will facilitate the closing of the bleach plant without adverse consequences on the recovery system.
According to one aspect of the present invention a method of 5 handling acidic waste liquids from a bleach plant of a sodium-based (e.g. kraft, sodium-based sulfite, or soda) cellulose pulp mill having a recausticization plant in which lime mud is removed from causticized green liquor to produce white liquor and then reutilized to causticize green liquor, is provided. The method comprises the following steps: (a) Acid treating cellulose pulp produced by a 10 sodium based cooking process. (b) Washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals. (c) Treating at least part of the filtrate from step (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate.(d) Using the reduced metal content filtrate from step (c) to wash or dilute lime 15 mud. And, (e) using the washed or diluted lime mud from step (d) to produce lime used to causticize green liquor to produce white liquor.
Typically the pulp mill has a recovery boiler which produces smelt used to make green liquor. The method then comprises the further steps of: (f) using part of the filtrate from step (b) that is not treated in step (c) to 20 dissolve smelt from the recovery boiler to make green liquor, and then (9) clarifying the green liquor to produce clarified green liquor and dregs containing metals. Preferably at least part of the alkaline stream used to practice step (c) is clarified or unclarified green liquor from step (g) or clarified or unclarified white liquor from step (e), although an alkaline filtrate from an alkaline 25 bleaching or delignification stage of the bleach plant may also be used. As is conventional step (e) is typically practiced by dewatering the washed or dilutedlime mud, burning the dewatered lime mud in a lime kiln to produce lime, and using the lime to causticize clarified green liquor to produce white liquor.
The pulp mill may also include an alkaline bleaching stage. The 30 above method may further comprise the following further steps: (a1 ) treatingthe pulp in an alkaline stage; (a2) washing or thickening the pulp from step (a1 ) to produce an alkaline filtrate; and (a3) combining at least some of the alkaline filtrate with the filtrate from step (b) prior to step (c).
The invention also relates to a method comprising the following steps: (a) Acid treating cellulose pulp produced by a sodium based cooking process. (b) Washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals. (c) Treating at least part of the filtrate from step 5 (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate. (d) Using part of the filtrate from step (b) that is not treated in step (c) to dissolve smelt from the recovery boiler to make green liquor, and then (e) clarifying the green liquor to produceclarified green liquor and dregs containing metals.
Apparatus is also provided according to the present invention in the form of a pulp mill system for producing cellulose pulp produced from a sodium-based cooking process. The pulp mill system comprises the following components: A digester including at least one white liquor inlet, at least one black liquor outlet, and a pulp outlet. A pulp acid treatment stage downstream 15 of the pulp outlet from said digester. A wash or thickening stage after the acid treatment stage, the wash or thickening stage comprising a filtrate outlet.
Means for removing metals from at least some of the filtrate passing out of the wash stage filtrate outlet to produce a reduced metal content filtrate. A
recovery boiler connected to the digester black liquor outlet, and including a 20 smelt discharge. A smelt dissolver connected to the smelt discharge. A green liquor clarifier connected to the smelt dissolver. A causticizer connected to the green liquor clarifier. A lime mud separator connected to the causticizer.
A lime mud washing or dilution stage connected to the lime mud separator to produced washed or diluted lime mud. Means for directing the 25 reduced metal content filtrate to the lime mud washing or diluting stage. And, means for using the washed or diluted lime mud to produce lime used in the causticizer to produce white liquor from green liquor.
The system preferably further comprises a conduit directing some of the filtrate from the filtrate outlet to the smelt dissolver to dissolve30 smelt to produce green liquor. The means for removing metals from at least some of the filtrate passing out of the washing stage filtrate outlet preferablycomprises means for adding an alkaline stream of liquor from the pulp mill to the filtrate. The means for adding the alkaline stream preferably comprises means for adding at least one of white liquor and green liquor to the filtrate, although a bleach plant alkaline stage alkaline filtrate may alternatively be used.
The system may also include a pulp alkaline treatment stage, a washing or thickening stage after the alkaline treatment stage which includes 5 an alkaline filtrate outlet, and a means for combining at least some of the alkaline filtrate with the acidic filtrate prior to passing the filtrate to the metal removing means.
It is the primary object of the present invention to facilitate closing of a bleach plant of a cellulose pulp mill utilizing a sodium-based 10 cooking process by proper treatment and handling of acidic bleach plant effluents, such as from the acid wash stage, acidic or neutral chelation stage, or an acidic ozone stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.
FIGURE 1 is a schematic illustration of an exemplary cellulose pulp mill system according to the invention for practicing exemplary methods of handling acidic waste liquids from a bleach plant, according to the present invention; and 20 FIGURE 2 is a schematic detail view of one embodiment of the metals removal stage of FIGURE 1.
DETAILED DESCRIPTION
The system illustrated generally by reference numeral 10 in FIGURE 1 includes a digester 11 ~continuous digesters are preferred although 25 batch digesters may be utilized) which produces cellulose pulp utilizing a sodium-based cooking process, e.g. kraft (sulfate), sodium-based sulfite, or soda cooking processes. Slurried wood chips or other cellulose material is fed into the digester 1 1 as indicated by line 12, and white liquor (used substantially generically herein to cover sodium-based cooking liquors) is added30 to the digester by line 13, white liquor typically also being added at other points in the digester 11 and with the wood chips introduced in line 12. The pulp produced in the digester 11 is discharged in line 14 typically to a brownstock washing stage 15, and then to a bleach plant, shown generally by reference numeral 16. The bleach plant typically includes a number of different 5 stages, such as an alkaline bleaching or delignification stage or stages 17 (e.g.
an oxygen delignification stage and/or alkaline extraction stage, or other conventional alkaline stages) followed by a wash stage 18, and one or more acid bleach/treatment stages 19 followed by a wash thickening stage 20.
Typical processes utilized for the stage 19 comprise acid washing (A), acidic 10 or neutral chelation (Q), acidic ozone (Z), acidic peroxide (Pa), or peracetic acid.
Other bleaching stages are indicated schematically at 21. Bleached pulp is produced as indicated in line 22.
Filtrate from the acidic wash or thickening stage 20 is represented by line 23 in FIGURES 1 and 2. As shown in the international 15 patent application PCT/US94/012373, the disclosures of which are hereby incorporated by reference herein, metals are removed from the filtrate in line 23 utilizing a metals removal stage 24. The metals removal stage 24 may include any of the configurations illustrated in the international application, or the like, one exemplary such system being illustrated in FIGURE
20 2. For example as indicated in FIGURE 2, the filtrate in line 23 may be oxidized as indicated at 25, and passed to a reactor 26 where an alkaline stream of material (typically liquid) is added as indicated at 27. The alkaline stream causes solids containing metals to precipitate from the filtrate, the filtrate with precipitated solids then passing first to clarification and then to the stage 28, 25 or directly to stage 28. At stage 28 the solids that have been precipitated are removed, as by filtration, sedimentation, flotation, or any other suitable conventional technique. The metal-laden stream may take the form of solids and be discharged as sludge, as indicated at line 29 or a metal-laden liquid stream, the retentate 29', may be used, for example, for smelt dissolution 37, 30 or burned in the recovery boiler 35. The metal depleted stream is discharged in line 30.
The digester 11 has a black liquor (waste liquor containing dissolved lignin) outlet conduit 32 associated therewith. The black liquor is evaporated and/or concentrated as is conventional and as indicated by box 33 in FIGURE 1, and may be subjected to other treatments ~such as pressure heated treatment to reduce the viscosity and produce organic sulfur containing gases) as indicated at 34 in FIGURE 1. Ultimately the black liquor passes to a conventional recovery boiler 35 where it is burned producing a smelt which is discharged from the recovery boiler in line 36. The smelt in line 36 passes to a smelt dissolver 37 where liquid is added to the smelt to produce green liquor discharged in line 38. The green liquor is clarified in a clarifier or filter 39, to separate solids, i.e., dregs (containing metals), which dregs are discharged in line 40 for environmentally suitable disposal, while clarified green liquor passes in line 41 to a causticizer 42. In the causticizer 42, lime is added in line 43, the lime typically being from a lime kiln 44. From the causticizer 42 the causticized green liquor passes to a lime mud separation stage 45 which is typically a filtration or sedimentation means. The end product produced in line 46 is white liquor (or like cooking liquor) which of course is ultimately used in line 13 associated with the digester 11. The lime mud that is separated in stage 45 passes to a washing or dilution 48, and then typically is dewatered at stage 49 before being burned in the lime kiln 44 to regenerate lime from calcium carbonate ~a suspension of calcium carbonate is what is known as "lime mud").
According to the present invention the acidic filtrate in line 23 is used in an optimum manner in the recausticization plant -- the entire structure 50 illustrated in FIGURE 1. The metal depleted stream in line 30 is ideal for washing or diluting the lime mud in stage 48, as illustrated in FIGURE1. The liquid in line 30 has a minimum potential to introduce undesirable metals into the lime cycle and therefore the activity of the lime is not adversely affected. Also sedimentation and dewatering are optimized because they can be hindered by the presence of metals, few of which are present in the metal depleted stream 30.
Though not as desirable as using the metal-laden stream for smelt dissolution, stream 23 may also be used for lime mud washing in 48.
However, if this is done, some accommodation must be made for the potential to accumulate metals in the lime cycle.
In the international patent application PCT/US94/012373 it was suggested that the metal depleted stream could be utilized for green liquor ~ 9 production, that is by utilizing it as the liquor in the smelt dissolver 37 to produce green liquor in line 38. However it has been recognized according to the present invention that it is not necessary to effect removal of the metals from the acidic stream 23 for this purpose. Rather a part of the acidic bleach 5 plant effluent - that part in line 52 -- may be fed directly to the smelt dissolver 37 to be used alone (or with other liquid from conventional sources) to produce a green liquor. This use of the acidic bleach plant effluent is innocuous because after 10 dissolution of the smelt and the formation of the green liquor in line 38 thegreen liquor is clarified in clarifier 39. Thus metal containing solids are removed in the clarifier 39 in the form of the dregs 40 which can be disposed of in a conventional environmentally friendly manner without introducing metals to the processes in the liquor manufacturing cycle which they could be 15 detrimental to.
It will thus be seen that according to the present invention a novel and desirable process and apparatus are provided for closing a bleach plant. Metal-laden acidic bleach plant effluent is reused in the preparation of cooking liquor without adversely affecting the efficiency of the chemical 20 processes or the operation of the equipment. Again these techniques are applicable to any sodium-based cooking process including kraft, sodium-based sulfite, and soda cooking processes.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment 25 thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent systems and processes.
Claims (14)
1. A method of handling acidic waste liquids from a bleach plant of a sodium based cellulose pulp mill having a recausticization plant in which lime mud is removed from causticized green liquor to produce white liquor and then reutilized to causticize green liquor, characterized in that it comprises the steps of:
(a) acid treating cellulose pulp produced by a sodium based cooking process;
(b) washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals;
(c) treating at least part of the filtrate from step (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate;
(d) using the reduced metal content filtrate from step (c) to wash or dilute lime mud; and (e) using the washed or diluted lime mud from step (d) to produce lime used to causticize green liquor to produce white liquor.
(a) acid treating cellulose pulp produced by a sodium based cooking process;
(b) washing or thickening the pulp from step (a) to produce a filtrate containing dissolved metals;
(c) treating at least part of the filtrate from step (b) with an alkaline stream to precipitate solids containing metals from the filtrate, producing a reduced metal content filtrate;
(d) using the reduced metal content filtrate from step (c) to wash or dilute lime mud; and (e) using the washed or diluted lime mud from step (d) to produce lime used to causticize green liquor to produce white liquor.
2. A method as recited in claim 1, characterized in that the pulp mill has a recovery boiler which produces smelt used to make green liquor, and in that it comprises the further steps of: (f) using part of the filtrate from step (b) that is not treated in step (c) to dissolve smelt from the recovery boiler to make green liquor, and then (g) clarifying the green liquor to produce clarifiedgreen liquor and dregs containing metals.
3. A method as recited in claim 2, characterized in that at least part of the alkaline stream used to practice step (c) is clarified green liquor from step (g).
4. A method as recited in claim 1 or 2, characterized in that at least part of the alkaline stream used to practice step (c) is white liquor fromstep (e).
5. A method as recited in claim 1, characterized in that the bleach plant contains at least one alkaline bleaching or delignification stage and a wash or thickening stage after the alkaline bleaching or delignification stagewhich produces alkaline filtrate; and in that at least part of the alkaline stream used to practice step (c) is alkaline fiitrate from the wash or thickening stageafter the alkaline bleaching or delignification stage.
6. A method as recited in claim 1, characterized in that step (e) is practiced by dewatering the washed or diluted lime mud, burning the dewatered lime mud to produce lime, and using the lime to causticize clarified green liquor to produce white liquor.
7. A method as recited in claim 1, characterized in that it comprises the following further steps: (a1) treating the pulp in an alkaline stage; (a2) washing or thickening the pulp from step (a1) to produce an alkaline filtrate; and (a3) combining at least some of the alkaline filtrate with the filtrate from step (b) prior to step (c).
8. A method as recited in claim 1 characterized in that step (a) is practiced by acid washing, acidic or neutral chelation, acidic ozone, acidic peroxide, or peracetic acid bleaching.
9. A method as recited in claim 1 characterized in that the sodium based cooking process from step (a) is kraft cooking.
10. A method as recited in claim 1, characterized in that the sodium based cooking process from step (a) is sodium- based sulfite or soda cooking .
11. A pulp mill system for producing cellulose pulp produced from a sodium based cooking process, characterized in that it comprises:
a digester including at least one white liquor inlet, at least one black liquor outlet, and a pulp outlet;
a pulp acid treatment stage connected downstream of said pulp outlet from said digester;
a wash or thickening stage after said acid treatment stage, said wash or thickening stage comprising a filtrate outlet;
means for removing metals from at least some of the filtrate passing out of said wash stage filtrate outlet to produce a reduced metal content filtrate;
a recovery boiler connected to said digester black liquor outlet, and including a smelt discharge;
a smelt dissolver connected to said smelt discharge;
a green liquor clarifier connected to said smelt dissolver;
a causticizer connected to said green liquor clarifier;
a lime mud separator connected to said causticizer;
a lime mud washing or dilution stage connected to said lime mud separator to produced washed or diluted lime mud;
means for directing the reduced metal content filtrate to the lime mud washing or diluting stage; and means for using the washed or diluted lime mud to produce lime used in the causticizer to produce white liquor from green liquor.
a digester including at least one white liquor inlet, at least one black liquor outlet, and a pulp outlet;
a pulp acid treatment stage connected downstream of said pulp outlet from said digester;
a wash or thickening stage after said acid treatment stage, said wash or thickening stage comprising a filtrate outlet;
means for removing metals from at least some of the filtrate passing out of said wash stage filtrate outlet to produce a reduced metal content filtrate;
a recovery boiler connected to said digester black liquor outlet, and including a smelt discharge;
a smelt dissolver connected to said smelt discharge;
a green liquor clarifier connected to said smelt dissolver;
a causticizer connected to said green liquor clarifier;
a lime mud separator connected to said causticizer;
a lime mud washing or dilution stage connected to said lime mud separator to produced washed or diluted lime mud;
means for directing the reduced metal content filtrate to the lime mud washing or diluting stage; and means for using the washed or diluted lime mud to produce lime used in the causticizer to produce white liquor from green liquor.
12. A system as recited in claim 11, characterized in that it further comprises a conduit directing some of the filtrate from said filtrate outlet to said smelt dissolver to dissolve smelt to produce green liquor.
13. A system as recited in claim 11, characterized in that said means for removing metals from at least some of the filtrate passing out of said washing stage filtrate outlet comprises means for adding an alkaline stream of liquor from the pulp mill to the filtrate.
14. A system as recited in claim 13, characterized in that said means for adding an alkaline stream comprises means for adding at least one of white liquor and green liquor to the filtrate.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US40524395A | 1995-03-17 | 1995-03-17 | |
| US08/405,243 | 1995-03-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2171595A1 true CA2171595A1 (en) | 1996-09-18 |
Family
ID=23602876
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002171595A Abandoned CA2171595A1 (en) | 1995-03-17 | 1996-03-12 | Use of acid-stage filtrate in cooking liquor preparation |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH0921088A (en) |
| CA (1) | CA2171595A1 (en) |
| FI (1) | FI961191A (en) |
| SE (1) | SE9600977L (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114667269A (en) * | 2019-08-23 | 2022-06-24 | 芬欧汇川集团 | Method for reducing phosphorus in effluent from pulping process |
-
1996
- 1996-03-12 CA CA002171595A patent/CA2171595A1/en not_active Abandoned
- 1996-03-14 FI FI961191A patent/FI961191A/en unknown
- 1996-03-14 SE SE9600977A patent/SE9600977L/en not_active Application Discontinuation
- 1996-03-15 JP JP8100722A patent/JPH0921088A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114667269A (en) * | 2019-08-23 | 2022-06-24 | 芬欧汇川集团 | Method for reducing phosphorus in effluent from pulping process |
Also Published As
| Publication number | Publication date |
|---|---|
| JPH0921088A (en) | 1997-01-21 |
| SE9600977L (en) | 1996-09-18 |
| FI961191A0 (en) | 1996-03-14 |
| FI961191A (en) | 1996-09-18 |
| SE9600977D0 (en) | 1996-03-14 |
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