CA1059271A - Removal of sodium chloride from pulp mill systems - Google Patents
Removal of sodium chloride from pulp mill systemsInfo
- Publication number
- CA1059271A CA1059271A CA220,692A CA220692A CA1059271A CA 1059271 A CA1059271 A CA 1059271A CA 220692 A CA220692 A CA 220692A CA 1059271 A CA1059271 A CA 1059271A
- Authority
- CA
- Canada
- Prior art keywords
- sodium
- sodium chloride
- liquor
- aqueous phase
- line
- 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.)
- Expired
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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
- D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
- D21C11/063—Treatment of gas streams comprising solid matter, e.g. the ashes resulting from the combustion of black liquor
- D21C11/066—Separation of solid compounds from these gases; further treatment of recovered products
Abstract
REMOVAL OF SODIUM CHLORIDE FROM PULP MILL SYSTEMS
ABSTRACT OF THE DISCLOSURE
Sodium chloride present in precipitator catch of a Kraft pulp mill is separated in pure solid form without loss of active components by selective leaching of the solids and recovery of solid sodium chloride from the aqueous phase.
ABSTRACT OF THE DISCLOSURE
Sodium chloride present in precipitator catch of a Kraft pulp mill is separated in pure solid form without loss of active components by selective leaching of the solids and recovery of solid sodium chloride from the aqueous phase.
Description
~(~S9Z71 This invention relates to the removal of sodium chloride from pulp mill systems, especially from Kraft mill operations.
In the Kraft system, cellulosic fibrous material, generally wood chips, is subjected to pulping with a pulping liquor containing sodium hydroxide and sodium sulphide, known as white liquor, and the resulting pulp is separated from the spent pulping liquox, known as black liquor. Usually the pulp is washed and subjected to brightening and purification steps to provide a pulp having the required brightness.
The black liquor is subjected to recovery and re-generation steps to form white liquor for use in contacting further pulp. The recovery and regeneration steps involve concentration of the black liquor and furnacing of the con-centrated black liquor ~o form a smelt containing sodium sulphide and sodium carbona~e. After the smelt is dissolved in water to form green liquor, the sodium carbonate values thereof are subjected to causticization to sodium hydroxide, thereby forming white liquor for recycle to the pulping step.
To make up sodium and sulphur values lost from the system during the recovery and regeneration operation, a sodium-and sulphur- containing material generally is added to the black liquor in the required quantity. Generally, the sodium-and sulphur- compound used is sodium sulphate, which may be provided from any convenient source.
The furnacing operation results in the sublimation of solid materials from the furnace. The solid materials are removed, usually by electrostatic precipitators, from the flue gases, the removed material being known as precipitator dust or fume deposit. The precipitator dust may contain a variety of materials depending on the constitution of the black liquor
In the Kraft system, cellulosic fibrous material, generally wood chips, is subjected to pulping with a pulping liquor containing sodium hydroxide and sodium sulphide, known as white liquor, and the resulting pulp is separated from the spent pulping liquox, known as black liquor. Usually the pulp is washed and subjected to brightening and purification steps to provide a pulp having the required brightness.
The black liquor is subjected to recovery and re-generation steps to form white liquor for use in contacting further pulp. The recovery and regeneration steps involve concentration of the black liquor and furnacing of the con-centrated black liquor ~o form a smelt containing sodium sulphide and sodium carbona~e. After the smelt is dissolved in water to form green liquor, the sodium carbonate values thereof are subjected to causticization to sodium hydroxide, thereby forming white liquor for recycle to the pulping step.
To make up sodium and sulphur values lost from the system during the recovery and regeneration operation, a sodium-and sulphur- containing material generally is added to the black liquor in the required quantity. Generally, the sodium-and sulphur- compound used is sodium sulphate, which may be provided from any convenient source.
The furnacing operation results in the sublimation of solid materials from the furnace. The solid materials are removed, usually by electrostatic precipitators, from the flue gases, the removed material being known as precipitator dust or fume deposit. The precipitator dust may contain a variety of materials depending on the constitution of the black liquor
- 2 --` L05~
and the conditions of furnacing. Usually, the precipitator dust consists mainly of sodium sulphate and is returned to the black liquor.
In coastal regions where logs are floated in sea water, sodium chloride is introduced to the mill with the wood chips.
The presence of sodium chloride in the balck liquor results in the presence of sodium chloride in the precipitator dust.
If the precipitator dust therefore is recycled directly to the black liquor, the sodium chloride, which is unaffected chemically by the furnacing operation, will build up in the furnace as a dead load, causing considerable problems in the furnacing operation.
The precipitator dust usually contains small amounts of sodium carbonate, in addition to sodium sulphate and sodium chloride. It has previously been suggested in Canadian Patent No. 958,158 to purge sodium chloride from the precipitator dust in an amount approximately equal to the amount present therein, followed by recycle of the purified sodium sulphate to the black liquor. This prior art procedure involves preferentially leaching the sodium chloride from the precipitator dust with water or an acid medium including recycled leach liquor con-taining dissolved sodium sulphate and sodium chloride to obtain a dilute readily filtrable slurry. The undissolved portion of the precipitator dust, which is mostly sodium sulphate, is recovered by filtration and forwarded to the black liquor.
The aqueous solution containing dissolved sodium chloride par-tially is recycled as leaching liquor and partially is removed from the system by sewering.
Operation in this manner has the drawback that quantities of sodium sulphate, and hence soda and sulphur values, are lost by the sewering steps. The aqueous phase - 105~'~71 contains dissolved quantities of sodium sulphate along with the sodium chloride, and hence, when the sodium chloride solution is sewered, the sodium sulphate values present in the solution also are sewered.
In addition, since the sodium chloride is sewered as an aqueous solution thereof, the potentially valuable sodium chloride is not recovered. Further, in this prior art system, in order to avoid loss of the sodium carbonate values con-tained in the precipitator dust and which normally would be present in the aqueous phase recycled and sewered, although some sodium carbonate values may remain in the solid phase, the sodium carbonate values are converted to sodium sulphate by the use of the acid leach liquor or by addition of acid to the slurry. Spent sulphuric acid from a chlorine dioxide generator, wherein sodium chlorate is reduced with sodium chloride in the presence of sulphuric acid typically is used to provide the acidity.
In certain chlorine dioxide producing systems, such as described in Canadian Patents Nos. 825,084 and 826,577 liquid effluents containing spent sulphuric acid are not formed.
Therefore, in a mill using one of the latter chlorine dioxide-forming processes to furnish its chlorine dioxide requirement, in order to utilize the prior art proposal of Canadian Patent No. 958,158 for the n~val of sodium chloride, an additional sulphuric acid requirement is necessary if loss of the sodium carbonate values is to be avoided.
In accordance with the present invention, the loss of sodium sulphate values experienced by the prior art process through sewering is eliminated, sodium chloride is recovered in substantially pure solid form for use in a number of ways, liquid effluents are avoided and the necessity for neutraliza-~0~ '71 tion of sodium carbonate is eliminated.
The process of the present invention utilizes the leaching procedure used in the above described prior art process with the optional exception of the addition of acid.
Thus, the precipitator dust is slurried with water and recycled mother liquor to dissolve all the sodium chloride values therein to provide a slurry of workable consistency, typically containing about 10 to 20% solids, allowing readily separation of the solid and liquid phases by filtration. Some sodium sulphate also is dissolved, along with any sodium carbonate present.
After filtration, the solid phase, consisting of sodium sulphate substantially free from sodium chloride, possibly together with some sodium carbonate, is recycled to the black liquor. In the prior art system of Canadian Patent No. 958,158 prior to filtration, the sodium carbonate values are converted to sodium sulphate values using spent sulphuric acid contained in the Ieach liquor and are precipitated from the aqueous phase and hence, after filtration, the recycled solid contains sodium sulphate from the sodium carbonate values.
In the process of the present invention this step may be omitted since the sodium carbonate values are not lost as will become apparent hereinafter. However, it may be utilized, if desired. Since the neutralization of the sodium carbonate step adds to the processing steps and is unnecessary, it is preferred in the present invention to omit the step.
In order to increase the proportion of sodium sulphate in the solid phase and hence reduce the quantity in the aqueous phase, the leaching operations may be carried out at an elevated temperature, up to 212 F, but typically about 140 to ~(~5~Z7~
The mother liquor resulting from the filtration is a saturated solution of sodium chloride, sodium sulphate and usually sodium carbonate. As mentioned above, in the prior art system, this solution partially is recycled to dilute slurry and partially is discharged to sewer.
In accordance with the present invention, the aqueous solution of sodium chloride, sodium sulphate and sodium car-bonate is manipulated to deposit sodium chloride thereform and substantially pure sodium chloride is recovered from the deposited materials. The sodium sulphate and sodium carbonate values par~ially are recycled as dilutent solution and, partially are returned to the recovery and regeneration steps, after concentration, if desired, to recover the same or solids.
Hence, liquid effluents from the- system are eliminated and no sodium sulphate or sodium carbonate values are lost from the system. Alternatively, all the mother liquor may be recycled to the leaching step.
The invention is described further with reference to the accompanying drawing, which is a schematic flow sheet of one embodiment of the invention.
~ In the drawing, wood chips, or other cellulosic fibrous material, are fed by line 10 to a digester 12 wherein the wood chips are digested with a pulping liquor fed by line 14 and containing sodium sulphide and sodium hydroxide as the active pulping chemicals, and hence utilizing the Kraft process.
The resulting pulp and ~lack liquor or spent pulping liquor are separated and the pulp is washed in a brown stock washer 16. The pulp may be washed with water fed by line 18 or "contaminated condensate", or aqueous bleach plant effluent.
The washed and unbleached pulp is fed by line 20 to a bleach plant 22 wherein the pulp is subjected to a series of -lOS9Z71 bleaching and purification processes, at least one of the bleaching steps generally involving the use of one or more chlorine-containing bleaching agents. Generally, the bleaching and purification processes involve bleaching with chlorine dioxide or mixtures thereof, and purification by caustic -extraction, using aqueous sodium hydroxide solution, typically in the well known CEDED sequence. The pulp is washed during the bleach plant operations, typically after each bleaching or caustic extraction operation, by fresh water or white water.
The spent wash water from the bleach plant washing operations together with the spent chemicals from the bleaching and caustic extraction steps proviae ~leach plant effluent. Preferably, the washing operations involve countercurrent flow of pulp and wash water. The bleach plant effluent contains sodium chloride values resulting from reactions between the soda and chlorine materials used in the bleaching and purification steps. The bleached and purified pulp of required brightness is recovered from t~e bleach plant 22 ~y line 24 and is sold as such or passed to paper-making procedures.
The black liquor from the brown stock washer 16 in line 26 is passed to a recovery and regeneration zone 28 for recovery of spent pulping chemicals and regeneration of pulping liquor. In the recovery and regeneration zone 28 the black liquor first is evaporated in an evaporator 30, prior to passage by lines 32 and 34 to a furnace 36 of any convenient construction. The water recovered from the evaporator 30 by line 38 may be used to provide at least part of the water required of the system, for example, as at least part of the water fed to the bleach plant after suitable cleaning, if required.
Sodium sulp~ate or other source of sodium and sulphur values, such as, spent sulphuric acid, is added to the recovery ~ 05~;~'7~
and regeneration zone, typically by addition to the black liquor, either in solid form or as an aqueous solution thereof by line 40, for example, after evaporation in the evaporator 30. The sodium sulphate fed by line 40 is used to make up the sodium and sulphur values lost from the system in the chemical recovery and regeneration processing steps.
The black liquor forms in the furnace 36 a smelt containing sodium sulphide and sodium carbonate. The smelt is dissolved in a smelt dissolver 42 by water, generally weak wash water from the washing of calcium carbonate mud, fed by line 44, to form a green liquor.
If desired, the smelt may be fractionated to remove the sodium sulphide values therefrom and thereby provide a sulphide-lean green liquor, substantially free from sodium sulphide values.
The green liquor after clarification to remove dregs, is passed by line 46 to a causticizer 48 of conventional con- !
struction wherein the sodium carbonate values in the green liquor are to a large extent converted to sodium hydroxide by lime fed by line 50 from a lime kiln 52. The calcium carbonate precipitated from the resulting white liquor is separated and after washing to remove entrained white liquor (not shown) is returned to the lime kiln 52 by line 54. The aqueous solution resulting from this washing is known as "weak wash water" which may be used, as mentioned above, to form the green liquor.
The white liquor resulting from the causticization step in line 56 represents the effluent from the recovery and re-generation zone 28 and contains the active pulping chemicals sodium sulphide and sodium hydroxide. The white liquor is recycled by line 56 to provide at least part of the pulping liquor in line 14.
The furnace 36 has a flue gas stream 58 which contains ~)5S~i~7~
ent~ained solids. Where the wood chips fed by line 10 contain entrained quantities of sodium chloride, for example, when the logs from which they are formed are sea-water borne, the black liquor fed to the recovery and regeneration zone 28 by line 26 contains dissolved quantities of sodium chloride. The presence of sodium chloride in the black liquor in line 26 may arise from other sources, such as the bleach plant effluent.
The presence of sodium chloride in the black liquor in line 26 gives rise to quantities of sodium chloride in the flue gas in line 58. The bulk of the entrained solid matter in line 58 is sodium sulphate, together with some sodium car-bonate.
Where the quantity of sodium chloride introduced to the system is low, i.e. less than 60 lbs/ton of pulp, the quantity of sodiu~ chloride removed from the system with the pulp in line 20 and in the flue gas in line 58 maintains the sodium chloride content of the recycled white liqucr in line 56 at a steady state value. Where larger amounts of sodium chloride are involved, it is necessary to utilize the procedures outlined in Canadian Patent No. 915,361, copending Canadian application Serial No. 187,714 or copending Canadian application Serial No. 206,076 in order to remove sufficient sodium chloride to prevent build up in the white liquor in line 56.
The flue gas in line 58 is passed to a precipitator 60 wherein at least part, preferably substantially all, the en-trained solids are removed from t~e flue gas stream, the remain-ing gas being vented by line 62 to atmosphere, after scrubbing to remove noxious gases, if required. The precipitator dust collected in the precipitator 60 consists mainly of a mixture of sodium sulphate, sodium car~onate and sodium chloride the bulk of the solid being provided by sodium sulphate.
The precipitator dust in line 64 is slurried in a leach g _ - lOS~71 tank 67 with water and recycled mother liquor fed by lines 66 and 68 to form a slurry of consistency about 10 to 20% solids in line 70.
The resulting slurry in line 70 is passed to a filter 72 wherein the solid phase, consisting substantially of sodium sulphate, is separated. The separated sodium sulphate is passed by line 74 to form at least part of the sodium sulphate fed to the black liquor by line 40. The remainder of the sodium sulphate fed by line 40 to the recovery and regeneration zone 28 may be fed by line 76 from any convenient source thereof.
The sodium sulphate in line 74 may be forwarded to the black liquor in line 32 in any other convenient manner. For example, the sodium sulphate may be forwarded to the green liquor in line 46 or to the white liquor in line 56.
Preferably, the slurrying operation is carried out at an elevated temperature, preferably above about 150F, so that the mother liquor in line 78 resulting from the filtration is a hot aqueous solution of sodium chloride, sodium carbonate and sodium sulphate.
The mother liquor is cooled in a crystallizer 80, with evaporation, if desired, to result in precipitation of sub-stantially pure sodium chloride which is recovered by line R2.
After separation of the pure sodium chloride, the resulting saturated solution in line 84 is divicled into two streams, one constituting the recycle stream in line 68, typic-ally heated to the leach temperature prior to feed to the leach tank 67, and the other passing by line 86 to the recovery and regeneration zone 28. The passage of the saturated solution to the zone 28 may be to any convenient location. For example, the saturated solution in line 86 may be included in the green liquor in line 46 by direct addition thereto by line 88.
Alternatively, the solution may be added to the white liquor in 1~5~Z71 line 56 by line 90. Further, the saturated solution may be added to the concentrated black liquor in line 32 by line 92.
If desired, the saturated solution in line 86 may be partially fed to two or more different locations.
The solution in line 86 contains the sodium sulphate values which otherwise would be discarded in the prior art process of Canadian Patent No. 958,158. In addition, the solution in line 86 contains sodium carbonate values, preferably in an amount substantially equal to that contained in the precipitator dust and hence these values are recycled within the system.
Other manners of manipulation of the mother liquor are pos~ible to deposit sodium chloride. For example, the mother liquor may be evaporated to deposit pure sodium chIoride which is recovered by line 82.
Alternatively, the sodium chloride may coprecipitate with sodium carbonate and sodium sulphate by evaporatlon of the mother liquor. After separation of the deposited solids, the resulting solution is forwarded as the saturated solution of line 84.
The quantity of sodium chloride precipitated from the mother liquor and recovered by line 82, whether deposited in pure form or in admixture with other salts, preferably is substantially equal to the quantity of sodium chloride in the precipitator dust in line 64, so that a balanced system is provided.
Where the sodium chloride is precipitated and recovered as a mixture, the sodium chloride is recovered in substantially pure form and the sodium sulphate and sodium carbonate is returned to the mill. The separation may be achieved in any desired manner. For example, the mixture may be leached to dissolve the sodium sulphate and sodium carbonate valu~s from 1~)5g'~7~
the mixture together with some of the sodium chloride values, leaving substantially pure sodi~m chloride. The aqueous solution of sodium sulphate, sodium carbonate and sodium chlor-ide obtained in this leaching may be forwarded to the system.
Therefore, it is possible by the present invention to achieve separation of substantially pure sodium chloride from precipitator dust without discard of sodium sulphate or sodium carbonate values, and hence, the required removal of sodium chloride is achieved without the necessity of sewering liquids.
The sodium chloride, recovered in substantially pure form in line 82, may be sold as such or put to a variety of uses in the mill. For example, it may be used to form bleach plant chemicals, such as, chlorine dioxide and chlorine by use as a reducing agent for sodium chlorate, or sodium hydroxide for use as caustic extraction liquor by electrol~ysis of an aqueous solution thereof. Alternatively, the sodium chloride may be used to form sodium chlorate by electrolysis of an aqueous solution thereof, the sodium chlorate being used to form chlorine dioxide.
Modifications are possible within the scope of the invention.
and the conditions of furnacing. Usually, the precipitator dust consists mainly of sodium sulphate and is returned to the black liquor.
In coastal regions where logs are floated in sea water, sodium chloride is introduced to the mill with the wood chips.
The presence of sodium chloride in the balck liquor results in the presence of sodium chloride in the precipitator dust.
If the precipitator dust therefore is recycled directly to the black liquor, the sodium chloride, which is unaffected chemically by the furnacing operation, will build up in the furnace as a dead load, causing considerable problems in the furnacing operation.
The precipitator dust usually contains small amounts of sodium carbonate, in addition to sodium sulphate and sodium chloride. It has previously been suggested in Canadian Patent No. 958,158 to purge sodium chloride from the precipitator dust in an amount approximately equal to the amount present therein, followed by recycle of the purified sodium sulphate to the black liquor. This prior art procedure involves preferentially leaching the sodium chloride from the precipitator dust with water or an acid medium including recycled leach liquor con-taining dissolved sodium sulphate and sodium chloride to obtain a dilute readily filtrable slurry. The undissolved portion of the precipitator dust, which is mostly sodium sulphate, is recovered by filtration and forwarded to the black liquor.
The aqueous solution containing dissolved sodium chloride par-tially is recycled as leaching liquor and partially is removed from the system by sewering.
Operation in this manner has the drawback that quantities of sodium sulphate, and hence soda and sulphur values, are lost by the sewering steps. The aqueous phase - 105~'~71 contains dissolved quantities of sodium sulphate along with the sodium chloride, and hence, when the sodium chloride solution is sewered, the sodium sulphate values present in the solution also are sewered.
In addition, since the sodium chloride is sewered as an aqueous solution thereof, the potentially valuable sodium chloride is not recovered. Further, in this prior art system, in order to avoid loss of the sodium carbonate values con-tained in the precipitator dust and which normally would be present in the aqueous phase recycled and sewered, although some sodium carbonate values may remain in the solid phase, the sodium carbonate values are converted to sodium sulphate by the use of the acid leach liquor or by addition of acid to the slurry. Spent sulphuric acid from a chlorine dioxide generator, wherein sodium chlorate is reduced with sodium chloride in the presence of sulphuric acid typically is used to provide the acidity.
In certain chlorine dioxide producing systems, such as described in Canadian Patents Nos. 825,084 and 826,577 liquid effluents containing spent sulphuric acid are not formed.
Therefore, in a mill using one of the latter chlorine dioxide-forming processes to furnish its chlorine dioxide requirement, in order to utilize the prior art proposal of Canadian Patent No. 958,158 for the n~val of sodium chloride, an additional sulphuric acid requirement is necessary if loss of the sodium carbonate values is to be avoided.
In accordance with the present invention, the loss of sodium sulphate values experienced by the prior art process through sewering is eliminated, sodium chloride is recovered in substantially pure solid form for use in a number of ways, liquid effluents are avoided and the necessity for neutraliza-~0~ '71 tion of sodium carbonate is eliminated.
The process of the present invention utilizes the leaching procedure used in the above described prior art process with the optional exception of the addition of acid.
Thus, the precipitator dust is slurried with water and recycled mother liquor to dissolve all the sodium chloride values therein to provide a slurry of workable consistency, typically containing about 10 to 20% solids, allowing readily separation of the solid and liquid phases by filtration. Some sodium sulphate also is dissolved, along with any sodium carbonate present.
After filtration, the solid phase, consisting of sodium sulphate substantially free from sodium chloride, possibly together with some sodium carbonate, is recycled to the black liquor. In the prior art system of Canadian Patent No. 958,158 prior to filtration, the sodium carbonate values are converted to sodium sulphate values using spent sulphuric acid contained in the Ieach liquor and are precipitated from the aqueous phase and hence, after filtration, the recycled solid contains sodium sulphate from the sodium carbonate values.
In the process of the present invention this step may be omitted since the sodium carbonate values are not lost as will become apparent hereinafter. However, it may be utilized, if desired. Since the neutralization of the sodium carbonate step adds to the processing steps and is unnecessary, it is preferred in the present invention to omit the step.
In order to increase the proportion of sodium sulphate in the solid phase and hence reduce the quantity in the aqueous phase, the leaching operations may be carried out at an elevated temperature, up to 212 F, but typically about 140 to ~(~5~Z7~
The mother liquor resulting from the filtration is a saturated solution of sodium chloride, sodium sulphate and usually sodium carbonate. As mentioned above, in the prior art system, this solution partially is recycled to dilute slurry and partially is discharged to sewer.
In accordance with the present invention, the aqueous solution of sodium chloride, sodium sulphate and sodium car-bonate is manipulated to deposit sodium chloride thereform and substantially pure sodium chloride is recovered from the deposited materials. The sodium sulphate and sodium carbonate values par~ially are recycled as dilutent solution and, partially are returned to the recovery and regeneration steps, after concentration, if desired, to recover the same or solids.
Hence, liquid effluents from the- system are eliminated and no sodium sulphate or sodium carbonate values are lost from the system. Alternatively, all the mother liquor may be recycled to the leaching step.
The invention is described further with reference to the accompanying drawing, which is a schematic flow sheet of one embodiment of the invention.
~ In the drawing, wood chips, or other cellulosic fibrous material, are fed by line 10 to a digester 12 wherein the wood chips are digested with a pulping liquor fed by line 14 and containing sodium sulphide and sodium hydroxide as the active pulping chemicals, and hence utilizing the Kraft process.
The resulting pulp and ~lack liquor or spent pulping liquor are separated and the pulp is washed in a brown stock washer 16. The pulp may be washed with water fed by line 18 or "contaminated condensate", or aqueous bleach plant effluent.
The washed and unbleached pulp is fed by line 20 to a bleach plant 22 wherein the pulp is subjected to a series of -lOS9Z71 bleaching and purification processes, at least one of the bleaching steps generally involving the use of one or more chlorine-containing bleaching agents. Generally, the bleaching and purification processes involve bleaching with chlorine dioxide or mixtures thereof, and purification by caustic -extraction, using aqueous sodium hydroxide solution, typically in the well known CEDED sequence. The pulp is washed during the bleach plant operations, typically after each bleaching or caustic extraction operation, by fresh water or white water.
The spent wash water from the bleach plant washing operations together with the spent chemicals from the bleaching and caustic extraction steps proviae ~leach plant effluent. Preferably, the washing operations involve countercurrent flow of pulp and wash water. The bleach plant effluent contains sodium chloride values resulting from reactions between the soda and chlorine materials used in the bleaching and purification steps. The bleached and purified pulp of required brightness is recovered from t~e bleach plant 22 ~y line 24 and is sold as such or passed to paper-making procedures.
The black liquor from the brown stock washer 16 in line 26 is passed to a recovery and regeneration zone 28 for recovery of spent pulping chemicals and regeneration of pulping liquor. In the recovery and regeneration zone 28 the black liquor first is evaporated in an evaporator 30, prior to passage by lines 32 and 34 to a furnace 36 of any convenient construction. The water recovered from the evaporator 30 by line 38 may be used to provide at least part of the water required of the system, for example, as at least part of the water fed to the bleach plant after suitable cleaning, if required.
Sodium sulp~ate or other source of sodium and sulphur values, such as, spent sulphuric acid, is added to the recovery ~ 05~;~'7~
and regeneration zone, typically by addition to the black liquor, either in solid form or as an aqueous solution thereof by line 40, for example, after evaporation in the evaporator 30. The sodium sulphate fed by line 40 is used to make up the sodium and sulphur values lost from the system in the chemical recovery and regeneration processing steps.
The black liquor forms in the furnace 36 a smelt containing sodium sulphide and sodium carbonate. The smelt is dissolved in a smelt dissolver 42 by water, generally weak wash water from the washing of calcium carbonate mud, fed by line 44, to form a green liquor.
If desired, the smelt may be fractionated to remove the sodium sulphide values therefrom and thereby provide a sulphide-lean green liquor, substantially free from sodium sulphide values.
The green liquor after clarification to remove dregs, is passed by line 46 to a causticizer 48 of conventional con- !
struction wherein the sodium carbonate values in the green liquor are to a large extent converted to sodium hydroxide by lime fed by line 50 from a lime kiln 52. The calcium carbonate precipitated from the resulting white liquor is separated and after washing to remove entrained white liquor (not shown) is returned to the lime kiln 52 by line 54. The aqueous solution resulting from this washing is known as "weak wash water" which may be used, as mentioned above, to form the green liquor.
The white liquor resulting from the causticization step in line 56 represents the effluent from the recovery and re-generation zone 28 and contains the active pulping chemicals sodium sulphide and sodium hydroxide. The white liquor is recycled by line 56 to provide at least part of the pulping liquor in line 14.
The furnace 36 has a flue gas stream 58 which contains ~)5S~i~7~
ent~ained solids. Where the wood chips fed by line 10 contain entrained quantities of sodium chloride, for example, when the logs from which they are formed are sea-water borne, the black liquor fed to the recovery and regeneration zone 28 by line 26 contains dissolved quantities of sodium chloride. The presence of sodium chloride in the black liquor in line 26 may arise from other sources, such as the bleach plant effluent.
The presence of sodium chloride in the black liquor in line 26 gives rise to quantities of sodium chloride in the flue gas in line 58. The bulk of the entrained solid matter in line 58 is sodium sulphate, together with some sodium car-bonate.
Where the quantity of sodium chloride introduced to the system is low, i.e. less than 60 lbs/ton of pulp, the quantity of sodiu~ chloride removed from the system with the pulp in line 20 and in the flue gas in line 58 maintains the sodium chloride content of the recycled white liqucr in line 56 at a steady state value. Where larger amounts of sodium chloride are involved, it is necessary to utilize the procedures outlined in Canadian Patent No. 915,361, copending Canadian application Serial No. 187,714 or copending Canadian application Serial No. 206,076 in order to remove sufficient sodium chloride to prevent build up in the white liquor in line 56.
The flue gas in line 58 is passed to a precipitator 60 wherein at least part, preferably substantially all, the en-trained solids are removed from t~e flue gas stream, the remain-ing gas being vented by line 62 to atmosphere, after scrubbing to remove noxious gases, if required. The precipitator dust collected in the precipitator 60 consists mainly of a mixture of sodium sulphate, sodium car~onate and sodium chloride the bulk of the solid being provided by sodium sulphate.
The precipitator dust in line 64 is slurried in a leach g _ - lOS~71 tank 67 with water and recycled mother liquor fed by lines 66 and 68 to form a slurry of consistency about 10 to 20% solids in line 70.
The resulting slurry in line 70 is passed to a filter 72 wherein the solid phase, consisting substantially of sodium sulphate, is separated. The separated sodium sulphate is passed by line 74 to form at least part of the sodium sulphate fed to the black liquor by line 40. The remainder of the sodium sulphate fed by line 40 to the recovery and regeneration zone 28 may be fed by line 76 from any convenient source thereof.
The sodium sulphate in line 74 may be forwarded to the black liquor in line 32 in any other convenient manner. For example, the sodium sulphate may be forwarded to the green liquor in line 46 or to the white liquor in line 56.
Preferably, the slurrying operation is carried out at an elevated temperature, preferably above about 150F, so that the mother liquor in line 78 resulting from the filtration is a hot aqueous solution of sodium chloride, sodium carbonate and sodium sulphate.
The mother liquor is cooled in a crystallizer 80, with evaporation, if desired, to result in precipitation of sub-stantially pure sodium chloride which is recovered by line R2.
After separation of the pure sodium chloride, the resulting saturated solution in line 84 is divicled into two streams, one constituting the recycle stream in line 68, typic-ally heated to the leach temperature prior to feed to the leach tank 67, and the other passing by line 86 to the recovery and regeneration zone 28. The passage of the saturated solution to the zone 28 may be to any convenient location. For example, the saturated solution in line 86 may be included in the green liquor in line 46 by direct addition thereto by line 88.
Alternatively, the solution may be added to the white liquor in 1~5~Z71 line 56 by line 90. Further, the saturated solution may be added to the concentrated black liquor in line 32 by line 92.
If desired, the saturated solution in line 86 may be partially fed to two or more different locations.
The solution in line 86 contains the sodium sulphate values which otherwise would be discarded in the prior art process of Canadian Patent No. 958,158. In addition, the solution in line 86 contains sodium carbonate values, preferably in an amount substantially equal to that contained in the precipitator dust and hence these values are recycled within the system.
Other manners of manipulation of the mother liquor are pos~ible to deposit sodium chloride. For example, the mother liquor may be evaporated to deposit pure sodium chIoride which is recovered by line 82.
Alternatively, the sodium chloride may coprecipitate with sodium carbonate and sodium sulphate by evaporatlon of the mother liquor. After separation of the deposited solids, the resulting solution is forwarded as the saturated solution of line 84.
The quantity of sodium chloride precipitated from the mother liquor and recovered by line 82, whether deposited in pure form or in admixture with other salts, preferably is substantially equal to the quantity of sodium chloride in the precipitator dust in line 64, so that a balanced system is provided.
Where the sodium chloride is precipitated and recovered as a mixture, the sodium chloride is recovered in substantially pure form and the sodium sulphate and sodium carbonate is returned to the mill. The separation may be achieved in any desired manner. For example, the mixture may be leached to dissolve the sodium sulphate and sodium carbonate valu~s from 1~)5g'~7~
the mixture together with some of the sodium chloride values, leaving substantially pure sodi~m chloride. The aqueous solution of sodium sulphate, sodium carbonate and sodium chlor-ide obtained in this leaching may be forwarded to the system.
Therefore, it is possible by the present invention to achieve separation of substantially pure sodium chloride from precipitator dust without discard of sodium sulphate or sodium carbonate values, and hence, the required removal of sodium chloride is achieved without the necessity of sewering liquids.
The sodium chloride, recovered in substantially pure form in line 82, may be sold as such or put to a variety of uses in the mill. For example, it may be used to form bleach plant chemicals, such as, chlorine dioxide and chlorine by use as a reducing agent for sodium chlorate, or sodium hydroxide for use as caustic extraction liquor by electrol~ysis of an aqueous solution thereof. Alternatively, the sodium chloride may be used to form sodium chlorate by electrolysis of an aqueous solution thereof, the sodium chlorate being used to form chlorine dioxide.
Modifications are possible within the scope of the invention.
Claims (8)
1. In a Kraft mill system comprising digesting wood chips in a pulping liquor containing sodium hydroxide and sodium sulphide as active pulping chemicals, separating spent pulping liquor, providing dissolved quantities of sodium chloride in said spent pulping liquor, subjecting said spent pulping liquor to recovery and regeneration operations to recover white liquor which is recycled as at least part of said pulping liquor, and introducing make up quantities of sodium sulphate to said spent pulping liquor, said recovery and regeneration operations including furnacing the spent pulping liquor, forming a gaseous product of the furnacing containing entrained solids and remov-ing and recovering from said gaseous product said entrained solids containing sodium sulphate and sodium chloride, the improvement which comprises leaching said solids with an aqueous medium to dissolve substantially all the sodium chloride values of said solids in said aqueous medium, to provide a slurry containing solids in a state readily separable from the aqueous phase, separating the solid phase from the aqueous phase of the slurry, forwarding said separated solid phase as at least part of said make up sodium sulphate, manipulating the aqueous phase to deposit sodium chloride therefrom, separating said sodium chloride from the resulting aqueous phase and recycling at least part of said resulting aqueous phase as part of said aqueous medium.
2. The process of claim 1 wherein said entrained solids consist essentially of sodium sulphate, sodium carbonate and sodium chloride.
3. The process of claim 2 wherein said resulting aqueous phase is recycled partly as said part of said aqueous medium and partly is recycled to said recovery and regeneration operations.
4. The process of claim 1 wherein said entrained solids consist essentially of sodium sulphate, sodium carbonate and sodium chloride, the aqueous phase of said slurry is acidified prior to said separation of solid phase from aqueous phase with sulphuric acid to form sodium sulphate from said sodium carbonate which substan-tially precipitates from the aqueous phase.
5. The process of claim 1 wherein said slurry has a consistency of about 10 to about 20% solids.
6. The process of claim 1 wherein said leaching is carried out at an elevated temperature and said manipulation is achieved by cooling the aqueous phase from said separation to deposit substantially pure solid sodium chloride there-from, said deposited sodium chloride is removed from the mother liquor and the mother liquor partially is recycled as said recycled aqueous phase and partially is fed to said recovery and regeneration steps.
7. The process of claim 6 wherein said mother liquor partially is fed to said spent pulping liquor.
8. The process of claim 6 wherein said elevated temperature is at least 150°F.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB792474 | 1974-02-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1059271A true CA1059271A (en) | 1979-07-31 |
Family
ID=9842408
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA220,692A Expired CA1059271A (en) | 1974-02-21 | 1975-02-20 | Removal of sodium chloride from pulp mill systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1059271A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU688683B2 (en) * | 1995-07-12 | 1998-03-12 | Eka Chemicals Ab | Leaching process |
| WO1998019003A1 (en) * | 1996-10-31 | 1998-05-07 | Kvaerner Pulping Ab | Process for purifying ash which principally consists of sodium sulphate from a recovery boiler |
| US5961803A (en) * | 1995-07-12 | 1999-10-05 | Eka Chemicals Ab | Leaching process |
-
1975
- 1975-02-20 CA CA220,692A patent/CA1059271A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU688683B2 (en) * | 1995-07-12 | 1998-03-12 | Eka Chemicals Ab | Leaching process |
| US5961803A (en) * | 1995-07-12 | 1999-10-05 | Eka Chemicals Ab | Leaching process |
| WO1998019003A1 (en) * | 1996-10-31 | 1998-05-07 | Kvaerner Pulping Ab | Process for purifying ash which principally consists of sodium sulphate from a recovery boiler |
| US6162408A (en) * | 1996-10-31 | 2000-12-19 | Kvaerner Pulping Ab | Process for purifying ash which principally consists of sodium sulphate from a recovery boiler |
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