CA1057909A - Production of chlorine dioxide - Google Patents
Production of chlorine dioxideInfo
- Publication number
- CA1057909A CA1057909A CA209,010A CA209010A CA1057909A CA 1057909 A CA1057909 A CA 1057909A CA 209010 A CA209010 A CA 209010A CA 1057909 A CA1057909 A CA 1057909A
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- Canada
- Prior art keywords
- sodium
- sodium sulphate
- sodium chloride
- solution
- chlorine dioxide
- Prior art date
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Abstract
Abstract of the Disclosure A Kraft pulp mill and a chlorine dioxide generator are integrated so that the stoichiometric requirements of the pulp mill with respect to sodium sulphate and chlorine dioxide are satisfied without waste of chemical efficiency. Sodium sulphate and sodium chloride mixture removed from the chlorine dioxide generator are separated into pure salts, so that the feed of the sodium sulphate produced in the chlorine dioxide generator to the pulp mill does not introduce sodium chloride contamination thereto.
Description
1057~09 This invention relates to the production of chlorine dioxide, more particularly to the integration of a chlorine dioxide producing procedure with a pulp mill oFeration.
A Kraft pulp mill operation, which is the pulp mill operation to which the present invention is particularly directed, although any other convenient pulp mill operation may be employed, involves digestion of wood chips, or other . ~
cellulosic- fibrous material, with white liquor containing æodium sulphide and sodium hydroxide as the active pulping . ~., . . :
-~ chemicals to form a wood pulp which is separated from spent pulping liquor, otherwise known as black liquor.
The pulp thereafter usually is subjected to bleaching - and purification operations in a bleach plant to form a pulp of the desired brightness. The black liquor is subjected to recovery and regeneration operations to form fresh white liquor which then is recycled to the digestion-~stage to provide at least part of the white liquor digesting further wood chips.
The recovery and regeneration operations generally involve an initial concentration of the black liquor followed : .
by burning in a furnace to provide a solid mass, otherwise known as smelt, containing sodium sulphide and sodium carbonate.
The smelt is dissolved in water to form an aqueous solution thereof, known as green liquor, which, after clarification to remove undissolved solids, is causticized with slaked lime resulting in the conversion of carbonate to hydroxide, with consequent deposition of calcium carbonate. The calcium carbonate is recovered and usually is used to form further slaked lime. The aqueous solution resulting from separation
A Kraft pulp mill operation, which is the pulp mill operation to which the present invention is particularly directed, although any other convenient pulp mill operation may be employed, involves digestion of wood chips, or other . ~
cellulosic- fibrous material, with white liquor containing æodium sulphide and sodium hydroxide as the active pulping . ~., . . :
-~ chemicals to form a wood pulp which is separated from spent pulping liquor, otherwise known as black liquor.
The pulp thereafter usually is subjected to bleaching - and purification operations in a bleach plant to form a pulp of the desired brightness. The black liquor is subjected to recovery and regeneration operations to form fresh white liquor which then is recycled to the digestion-~stage to provide at least part of the white liquor digesting further wood chips.
The recovery and regeneration operations generally involve an initial concentration of the black liquor followed : .
by burning in a furnace to provide a solid mass, otherwise known as smelt, containing sodium sulphide and sodium carbonate.
The smelt is dissolved in water to form an aqueous solution thereof, known as green liquor, which, after clarification to remove undissolved solids, is causticized with slaked lime resulting in the conversion of carbonate to hydroxide, with consequent deposition of calcium carbonate. The calcium carbonate is recovered and usually is used to form further slaked lime. The aqueous solution resulting from separation
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105790~
of the calcium carbonate therefrom is the recycled white liquor.
Sodium sulphate commonly is used as a source of make-up chemical for the system, to- make-up losses of soda and sulphur values. Various sources of sodium sulphate may be used and typically, the sodium sulphate may be formed in the production of chlorine dioxide and chlorine for use in the bleaching operations in the bleach plant.
A typical operation for the production of chlorine dioxide involves reduction of sodium chlorate with chloride ions in an acid medium containing sulphuric acid. Sodium sulphate is recovered as a by-product of this process. In such a process the basic reaction involves the production of chlorine dioxide, chlorine and water in accordance with the equation:
C103 + Cl + 2H+ - ? C102 + ~Cl2 + H20 -(1) The quantity of make-up chemical required for the mill recovery system has been falling with tightening up of the recovery system while the demand for chlorine dioxide and chlorine has remained at the same level. The quantity of sodium sulphate formed in a chlorine dioxide generation operation may be lowered while maintaining the chlorine dioxide output at the same level by utilizing mixtures of hydrochloric acid and sulphuric acid to provide the acid requirement. The hydrochloric acid at the same time may provide part or all of the chloride reducing agent, the remainder of the reducing agent, where required, being provided by sodium chloride, The quantity of sodium sulphate formed depends on the relative concentrations of the hydrochloric acid and
105790~
of the calcium carbonate therefrom is the recycled white liquor.
Sodium sulphate commonly is used as a source of make-up chemical for the system, to- make-up losses of soda and sulphur values. Various sources of sodium sulphate may be used and typically, the sodium sulphate may be formed in the production of chlorine dioxide and chlorine for use in the bleaching operations in the bleach plant.
A typical operation for the production of chlorine dioxide involves reduction of sodium chlorate with chloride ions in an acid medium containing sulphuric acid. Sodium sulphate is recovered as a by-product of this process. In such a process the basic reaction involves the production of chlorine dioxide, chlorine and water in accordance with the equation:
C103 + Cl + 2H+ - ? C102 + ~Cl2 + H20 -(1) The quantity of make-up chemical required for the mill recovery system has been falling with tightening up of the recovery system while the demand for chlorine dioxide and chlorine has remained at the same level. The quantity of sodium sulphate formed in a chlorine dioxide generation operation may be lowered while maintaining the chlorine dioxide output at the same level by utilizing mixtures of hydrochloric acid and sulphuric acid to provide the acid requirement. The hydrochloric acid at the same time may provide part or all of the chloride reducing agent, the remainder of the reducing agent, where required, being provided by sodium chloride, The quantity of sodium sulphate formed depends on the relative concentrations of the hydrochloric acid and
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10575~9 sulphuric acid in the feed. Up to the ~oint where the hydr~oric acid provides half the acid requirement, sodium sulphate is recoverable in substantially pure form; however, when the quantity of hydrochloric acid exceeds half the acid require-ment, the sodium sulphate is recoverable only in admixture with sodium chloride.
The processes may be represented by the following equation:
NaC103 + (l-x) NaCl + xHCl + (2-x) H2S0 C12 + ~Cl2 + H20 + (2-x) Na2So4 -(2) in the case where x is 0 to 1, and , NaC103 + x~Cl + (22x) 2 4 2 ~ 2 + H20 + (2-x) ~a2S04 + (x-l) ~aC1 -(3) in the case where x is 1 to 2.
Thus, the mill requirement for sodium sulphate and ; chlorine dioxide may be such that the chlorine dioxide generator must be operated in a manner which produces mixtures of sodium sulphate and sodium chloride if stoichiometry is to be maintained.
It has been considered undesirable to introduce sodium chloride to pulp mill recovery and regeneration systems since this material is not affected by the recovery and regeneration operations and hence builds up as a dead load.
Hence, where the quantity of pure sodium sulphate recoverable from the chlorine dioxide generation has exceeded the stoichiometric requirements, the excess must be disposed of, with consequential loss of soda and sulphur values.
10575~9 sulphuric acid in the feed. Up to the ~oint where the hydr~oric acid provides half the acid requirement, sodium sulphate is recoverable in substantially pure form; however, when the quantity of hydrochloric acid exceeds half the acid require-ment, the sodium sulphate is recoverable only in admixture with sodium chloride.
The processes may be represented by the following equation:
NaC103 + (l-x) NaCl + xHCl + (2-x) H2S0 C12 + ~Cl2 + H20 + (2-x) Na2So4 -(2) in the case where x is 0 to 1, and , NaC103 + x~Cl + (22x) 2 4 2 ~ 2 + H20 + (2-x) ~a2S04 + (x-l) ~aC1 -(3) in the case where x is 1 to 2.
Thus, the mill requirement for sodium sulphate and ; chlorine dioxide may be such that the chlorine dioxide generator must be operated in a manner which produces mixtures of sodium sulphate and sodium chloride if stoichiometry is to be maintained.
It has been considered undesirable to introduce sodium chloride to pulp mill recovery and regeneration systems since this material is not affected by the recovery and regeneration operations and hence builds up as a dead load.
Hence, where the quantity of pure sodium sulphate recoverable from the chlorine dioxide generation has exceeded the stoichiometric requirements, the excess must be disposed of, with consequential loss of soda and sulphur values.
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579(~
In accordance with the present invention, there is provided an improvement in a pulp mill process including the steps of contacting a cellulosic fibrous material with a pulping liquor containing sodium sulphide and sodium hydroxide, separating pulped material from spent pulping liquor, sub-jecting the spent pulping liquor to recovery and regeneration steps to provide a white liquor, recycling the white liquor as at least part of the pulping liquor, providing sodium sulphate in the spent pulping liquor to make up soda- and sulphur-losses from the process, and subjecting the pulp to a series of brightening and purification steps to provide a bleached pulp, the improvement comprising forming chlorine dioxide by reaction of sodium chlorate with hydrochloric acid in the presence of sulphuric acid, recovering a mixture of sodium chloride and sodium sulphate from the reaction, separating substantially pure sodium sulphate from the mixture, utilizing the sodium sulphate as the make-up sodium sulphate and utilizing the chlorine dioxide in at least one - of the brightening steps.
In acco~dance with the present invention,-therefore, the sodium sulphate obtained in admixture with sodium chloride ~m thechlorine dioxide generating process is separated to provide a feed of substantially pure sodium sulphate for . . .
the pulp mill.
The invention is described further, by way of .. - . .:
illustration, with reference to the accompanying drawings, in which Figure 1 is a phase diagram for the system Na2SO4 - NaCl - H2O at temperatures of 50C, 75 C, and 100 C;
Figure 2 is a schematic flow sheet of one embodiment of one aspect of the invention; and Figure 3 is a schematic flow sheet of a secon~
embodiment of the one aspect of the invention.
.
- ."~ , , 10579(~9 It is known that the solubility characteristics of sodium chloride and sodium sulphate vary with temperature, the solubility of sodium chloride increasing with increasing temperature while the solubility of sodium sulphate falls at certain relative concentrations of sodium sulphate to sodium chloride, while the reverse is true at other relative concentrations, namely, that the solubility of sodium chloride decreases with temperature while the solubility of sodium sulphate increases. These facts may be utilized in the present invention to ~eve the separation of the sodium sulphate and sodium chloride.
The phase diagramfor the system Na2S04 - NaCl - H20 is shown as Figure 1 of the accompanying drawings, the dat~ ~or the phase diagram being-computed from:
.. . .. . . . . .
Chretien A, Annalen der Chemie 10 serie 11-12, pp 9 to 155 (1929) for the 50 C and 75C data; and Pylkova,E.V. and Akhimov,E.I., Russian Journal of Inorg.Chem. 7(1), 104 (1962).
,. . . . .
, ... .. , , . . .. .. . . , . , . . . -- -- -- -- -- -- -- ----In accordance with one aspect of the present invent-ion, chlorine dioxide and chlorine are formed by reduction of an aqueous solution of sodium chlorate using chloride ions in the presence of a mixture of hydrochloric acid and sulphuric acid in accordance with equation 3 above, a mixture of sodium sulphate and sodium chloride is recovered from the reaction medium, the mixture is dissolved in water, sodium chloride and sodium sulphate are crystallized separately from the solution, their order of separation depending on the solubility relationships which are, in turn, dependent on the initial weight ratio of sodium sulphate to sodium chloride , - ' :, 1057~09 in the recovered mixture, and the sodium sulphate is used as make-up chemical in the recovery and regeneration of spent pulping chemical in a pulp mill system.
In accordance with one embodiment of this aspect of the invention, the separation of the salts independently t from the aqueous solution may be achieved by concentrating the aqueous solution to deposit sodium chloride therefrom until the substantial saturation point of sodium sulphate is reached. The aqueous solution thereafter is heated to render the solutlon undersaturated with respect to sodium chloride, whereupon sodium chloride is added thereto to deposit sub-stantially pure sodium sulphate from the solution. This procedure may be used at high relative concentrations of sodium chloride, as will be apparent from the phase diagram shown in Figure 1. At lower relative concentrations, however, an alternative procedure may be adopted.
In accordance with a second embodiment of this - aspect of the present invention, the separation of the salts independently from the aqueous solution may be achieved by concentrating by evaporation the aqueous solution to deposit sodium sulphate therefrom until the substantial saturation point of sodium chloride is reached. The aqueous solution thereafter is cooled to deposit some sodium chloride therefrom.
Sodium sulphate may be added to-tne cooled solution to deposit further quantities of sodium chloride. ;
In a second aspect of the present invention, the separation of the sodium sulphate and sodium chloride re-covered from the chlorine dioxide generator is achieved using a classification of the crystal sizes. Thus, the 10575~09 sodium sulphate and sodium chloride mixture may be slurried in an aqueous solution which is saturated with respect to sodium sulphate and sodium chloride and then passed through a classifier, which may take the form of a vertical tube through which the slurry is upwardly passed at a flow rate which allows the heavier crystals to settle in the tube while the less heavy crystals pass upwardly and out of the tube to be separated from the aqueous phase in any convenient manner. Alternatively, the slurry may be passed through a sieve in which the openings allow one of the crystal sizes to pass therethrough but prevent the other crystal size from passing therethrough.
The sodium chloride crystals are larger and heavier than those of sodium sulphate and hence separation of the components by the above-described physical separation methods may be used in the present invention.
- Separation of the sodium sulphate and sodium chloride may be achieved in accordance with a third aspect of the present invention. me solid mixture of sodium sulphate and sodium chloride obtalned from the chlorine dioxide gener-ator is contacted with an aqueous solution of sodium sulphate and sodium chloride in a ratio that upon saturation of the aqueous solution, sodium sulphate will be the only solid phase present. In order to minimize the quantity of sodium sulphate leached from the mixture, the ratio should be as close to the eutecticcomposition, as seen in Figure 1, as possible. In practising this aspect of the invention, it is preferred to carry out the leaching at higher temperatures, to decrease further the quantity of sodium sulphate dissolved lVS~9(~
from the mixture. Following the leaching, the solid sodium sulphate is forwarded to the furnace, while the mother liquor is partly recycled, after dilution, to leach further mixture.
This leaching procedure to recover pure sodium sulphate is utilizable when the weight ratio of sodium sulphate to sodium chloride is less than that at the eutectic defined by a set operating temperature. For example, at 100C, the limiting weight ratio of sodium sulphate to sodium chloride is 0.17.
In accordance with a further aspect of the invention, the separation is achievable by a flotation technique. For example, the mixture of sodium sulphate and sodium chloride may be ground to fine particle size, such as 36 to 170 mesh and formed into a pulp with a brine saturated with sodium chloride and sodium sulphate. A solution of sodium octyl sulphate is added to the slurry and stirred, after which the pulp is diluted with further brine until the weight ratio of solids to brine is about 20 to 30:100.
Thereafter, the slurry is introduced to a flotation cell of any convenient construction and air is passed through the cell as a fine stream of bubbles. Sodium sulphate then separates out and the crystals may be collected~
. . .
Referring to Figure 2 of the accompanying drawings, 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.
-` lOS790~
The resulting pulp and black liquor or spent pulp-ing liquor are separated and the pulp is washed in a brown stock washer 16 with water fed by line 18. The wash water used in line 18 may be fresh water or "contaminated condensate".
The washed and unbleached pulp is fed by line 20 to a bleach plant 22 wherein the pulp is subjected to a series of bleaching and purification processes. Generally the bleaching and purification processes involve bleaching with chlorine, chlorine dioxide or mixtures thereof. The chlorine dioxide is fed as a solution by line 24 and the chlorine is fed by line 26. The purification generally is _ carried out by caustic extraction, using aqueous sodium hydr-oxide solution fed by line 28. Any desired sequence of steps may be used in the bleach plant 22 to achieve the desired pulp brightness. For example, the so-called CEDED, C/DEDED or DEDED sequences may be used, wherein C represents bleaching with chlorine, C/D represents bleaching with mixtures of chlorine and chlorine dioxide, D represents bleaching with chlorine dioxide and E represents caustic extraction. In the DEDED sequence, chlorine is not required and hence feed line 26 may be omitted. - -The pulp is washed during the bleach plant operations, typically after each bleaching or caustic extraction operation, by water fed by line 30. The washing operations may involve countercurrent flow of pulp and wash water through the bleach plant 22. The bleached and purified pulp of required brightness is recovered from the bleach plant 22 by line 32 and is sold as such or is passed to further operation to be converted into paper or other pulp products.
The chlorine dioxide which is used in the bleach plant 22 is formed in a chlorine dioxide generator 34 by reduction of sodium chlorate fed by line 36 with hydrogen chloride fed by line 38 in the presence of hydrochloric acid provided by the hydrogen chloride and sulphuric acid fed by line 40.
The generation of chlorine dioxide may take place in any convenient manner, typically by carrying out the process at the boiling point of the reaction medium, typically with the generator 34 being maintained under a reduced pressure.
- When the generator 34 is operated in this manner, the water vapour evaporated from the aqueous reaction medium , ~
dilutes the chlorine dioxide and chlorine and a gaseous mixture of steam, chlorine dioxide and chlorine is removed from the generator 34. t - The gaseous mixture removed from the generator generally is forwarded by line 42 to a separator 44 to condense the steam and recover an aqueous solution of chlorine dioxide containing some dissolved chlorine which then is forwarded to the bleach plant 22 by line 24~for use in the bleaching steps designated D. The chlorine separated from the gaseous mixture may be used in the bleaching steps designated C in the bleach plant 22 as at least part of the chlorine feed in line 26.
Alternatively, the separated chlorine is forwarded by line 46 to form part of a chlorine feed 48 to a hydrogen chloride reactor 50 wherein the chlorine reacts with hydrogen fed by line 52 to form hydrogen chloride, which is forwarded ~ . .
by line 38 to the chlorine dioxide generator 34 as such or as hydrochloric acid.
-- 11 -- .
. .
.
The quantity of chlorine fed by line 48 and hydrogen fed by line 52 should be substantially the stoichiometric quantities to form the amount of hydrogen chloride required in the generator 34, so that a balance of chemicals may be achieved in a continuous operation.
In addition, operation of the generator 34 at the substantial boiling point of the reaction medium causes the deposition of product sodium salt in the generator 34. The generator 34 is operated with the hydrochloric acid providing more than half the acid requirement, i.e. in accordance with equation 3 above, so that a mixture of sodium chIoride and .. .... . ..
sodium sulphate is deposited in the generator 34 and is removed therefrom by line 54.
The form of the sodium sulphate present in the mixture in line 54 depends on the acidity of the reaction mixture in the generator 34, and may be sodium bisulphate if the reaction is carried out at high acidity typically at ~-above 9N, sodium sesquisulphate at acidities around 5 to 7 and neutral sodium sulphate at acidities below about 4N.
The sodium chlorate solution fed to the generator 34 by line 36 may be provided by electrolysis in a suitable chlorate cell 56 of an aqueous sodium chloride solution fed thereto by line 58. The by-product hydrogen formed in the latter electrolysis is removed by line 60 from the cell 56 and is used partially to provide the hydrogen feed in line ; 52 to the hydrogen chloride reactor 50 and partially is discharged by line 62. The discarded hydrogen in line 62 - may be used to form part of the water requirement of the system.
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The black liquor in line 64 is evaporated in an evaporator 66, prior ~o passage by lines 68 and 70 to a furnace 72 of any convenient construction. The water re-covered from the evaporator 66 by line 74 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 22 in line 30, after suitable cleaning, if required.
Sodium sulphate is added to the black liquor, either in solid or slurry form or as an aqueous solution thereof, either directly, or indirectly, typically by line 76 to the concentrated black liquor in line 68. The sodium sulphate 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 72 a smelt containing sodium sulphide and sodium carbonate. The smelt is dissolved in a smelt dissolver 78 by water, generally weak wash water from the washing of calcium carbonate mud, fed by line 80, to form a green liquor in line 82.
2~ The green liquor, after clarification to remove dregs, is passed to a causticizer 84 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 86 from a lime kiln 88. 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 88 by line 90. 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.
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lOS79(19 The white liquor resulting from the causticization step in line 92 is recycled to provide at least part of the pulping liquor in line 14.
The mixture of sodium chloride and sodium sulphate removed from the chlorine dioxide generator 34 by line 54 is passed to a dissolver 94 wherein the mixture is dissolved in water fed by line 96.
The relative proportions of the sodium chloride and sodium sulphate in the mixture depends on the relative pro-portions of hydrochloric acid and sulphuric acid present in ... . ~ , . . .
the reaction medium.
The quantity of chlorine dioxide and quantity of sodium sulphate produced in the generator 34 depend on the individual requirements of the mill for these products. The present invention is concerned with a pulp mill operation where the requirements of chlorine dioxide and sodium sulphate are such that the generator 34 must be operated with an acid mixture of hydrochloric and sulphuric acids such that it is possible only to produce the sodium sulphate in admixture with sodium chloride.
In those instances where the sodium sulphate require-mentis such that equation 2 above applies, then the sodium sulphate is recovered in substantially pure form and may be forwarded directly by line 76 to the black liquor.
In thls embodiment, the weight ratio of sodium sulphate to sodium chloride is less than 0.17 and hence the phase diagram of Figure l to the left of the eutectic points applies.
.
1~)57~V9 The aqueous solution of sodium sulphate and sodium chloride resulting from the dissolver 94 is forwarded by line 98 to a first evaporator 100, which is the first stage of three multiple effect evaporators 100, 102 and 104. Water is evaporated from the solution in each of the evaporators 100, 102 and 104, sodium chloride being deposited from the solution in each evaporator and being removed by lines 106, 108, 110 respectively, to provide a combined sodi~m chloride product stream 112.
The concentration of the solution in the evaporators 100, 102 and 104 continues until the solution is substantially saturated with sodium sulphate. The evaporators 100, 102 and 104 operate at successively lower temperatures, typically about 100 C, about 75C and about 50C, the evaporators being indirectly heated by steam, the steam for evaporators 102 and 104 being provided by the steam formed in evaporators 100 and 102 respectively.
The aqueous solution resulting from evaporator 104 has a reduced sodium chloride content as compared to the solution in line 98 and is substantially saturated with respect both to sodium sulphate and sodium chloride.
The sodium chloride recovered from the evaporators 100, 102 and 104 in line 112 may be put to a variety of uses. For example, it may be used to provide part of the sodium chloride solution fed by line 58 to the chlorate cell 56 and/or part of the sodium chloride solution which is fed by line 114 to a caustic-chlorine cell 116 wherein there is formed the sodium hydroxide feed solution for the bleach plant 22 in line 28. The caustic-chlorine cell 116 also produces .,: .
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chlorine which is removed by line 118. The chlorine gas in line 118 may provide the chlorine requirement of the bleach plant in line 26, with any excess chlorine being recovered by line 120. The excess chlorine in line 120 may be used to provide part of the chlorine feed in line 48 to the hydrogen chloride reactor 50. Alternatively, the sodium chloride may be sold as such.
The aqueous solution o sodium chloride and sodium sulphate resulting from the third evaporator 104 is passed by line 122 through a heater 124 wherein the solution is heated, typically to about 100C, so that it becomes under-saturated with respect to sodium chloride and by line 126 to a precipitator 128.
,, , To the heated solution in the precipitator 128 is added solid sodium chloride by line 130 causing precipitation of sodium sulphate from the solution. The sodium chloride fed by line 130 may be provided by part of the sodium chloride recovered from the evaporators 100, 102 and 104 in line 112.
The sodium sulphate which is precipitated in the precipitator 128 is forwarded by line 76 to the concentrated black liquor in line 68. The mother liquor is recycled by line 132 to the evaporators 100, 102 and 104, typically by feeding, as illustrated, to the liquid entering the third evaporator 104.
The manner of concentration of the aqueous solution of sodium sulphate and sodium chloride in line 98 to the substantial saturation point of the solution with sodium sulphate and precipitating sodium chloride therefrom may be varied from that illustrated.
, ~057909 Similarly, the manner of achieving deposition of ~26 sodium sulphate from the heated solution in line ~ may be varied, as desired. For example, the heated solution in line 126 may be evaporated to render it more concentrated with respect to sodium chloride, thereby resulting in pre-cipitation of sodium sulphate from the solution.
Figure 3 illustrates an alternative embodiment to the embodiment outlined in Figure 2, applicable when the weight ratio of sodium sulphate to sodium chloride in the mixture formed in the chlorine dioxide generator is greater than 0.17, and hence the portion of the phase diagram of Figure 1 to the right of the eutectic point applies.
In the embodiment of Figure 3, a mixture of sodium sulphate and sodium chloride, recovered from a chlorine dioxide generator operating as described above with reference to chlorine dioxide generator 34, but with a lesser proportion of the acid requirement being provided by hydrochloric acid, -is passed by line 200 to a dissolver 202 which the mixture P is dissolved in water-, preferably the minimum quantity, fed by line 204.
The resulting solution is passed to a first evaporator 206, which is the first stage of three multiple effect evaporat-ors 206, 208 and 210. Water is evaporated from the solution in each of the evaporators 206, 208 and 210, sodium sulphate being deposited from the solution in each evaporator and being removed by llnes 212, 214 and 216 respectively, to provide a combined sodium sulphate product stream 218.
The concentration of the solution in the evaporators 206, 208 and 210 is continued until the solution is substantial-lOS7gO5~ .
ly saturated with sodium chloride. The evaporators 206, 208 and 210 operate at successively higher temperatures, typically about 50C, about 75C and about 100C, the evaporators being indirectly heated by steam, the steam for evaporators 206 and 208 being provided by the steam formed in the evaporators 208 and 210 respectively. Ifdesired, all the evaporators may be carried out in a single evaporator, typically at about 100 C.
The aqueous solution resulting from the evaporator 210 has a reduced sodium sulphate content as compared to the solution in line 205 and is substantially saturated with respect both to sodium sulphate and sodium chloride.
The sodium sulphate recovered from the evaporators 206, 208 and 210 in line 218 is forwarded as make-up chemical to the concentrated black liquor prior to passage thereof to the furnace, in analogous manner to the passage of sodium sulphate by line 76 to the concentrated black liquor in line 68 as described above with reference to Figure 2.
The aqueous solution of sodium chloride and sodium sulphate resulting from the third evaporator 210 is passed by line 220 through a cooler 222, wherein the solution is cooled, typically to a temperature of about 50 C, so that it becomes undersaturated with respect to sodium sulphate, and by line 224 to a precipitator 226. Some precipitation of sodium chloride may occur during the cooling operation, and this precipitated sodium chloride may be separated from the cooled solution.
To the cooled solution in the precipitator 226 is added solid sodium sulphate by line 228 causing precipitation of sodium chloride from the solution. The sodium sulphate fed by line 228 may be provided by part of the sodium sulphate recovered from the evaporators 206, 20~ and 210 in line 218.
The sodium chloride which is precipi-tated in the precipitator 226 is recovered by line 230 and may be put to a variety of uses, as discussed above in connection with the sodium chloride recovered in line 112 in Figure 2. The mother liauor is recycled by line 232 to the evaporators 206, 208 and 210, typically by feeding, as illustrated, to the first evaporator 206.
The manner of concentration of the aqueous solution of sodium sulphate and sodium chloride in line 205 to the ~ -substantial saturation point of the solution with sodium chloride and precipitating sodium sulphate therefrom may be varied from that illustrated.
Similarly, the manner of achieving deposition of sodium chloride from the cooled solution in line 224 may be varied as desired. For example, the cooled solution in line 224 may be evaporated to render it more concentrated with respect to sodium sulphate, thereby resulting in precipitation of sodium chloride from the solution.
The invention is illustrated further by the following Example:
EX~MPLE
A chlorine dioxide generator, maintained under a reduced pressure, is operated at the boiling point of an aaueous solution containing 1.4 moles of hydrochloric acid and 0.3 moles of sulphuric acid per mole of sodium chlorate.
- This results in the deposition of a mixture of 0.3 mole of sodium sulphate and 0.4 mole of sodium chloride per mole of chlorine dioxide.
A mixture of 23.38g of sodium chloride and 42.6g of sodium sulphate is leached with water at 100C and operat-ing at the eutectic point. The amount of water required per gram of sodium chloride leached is 2 69g, i.e. a total of 62.89g for 23.38g of sodium chloride and the quantity of sodium sulphate leached per gram of sodium chloride is 0.169g, and hence 3.95g of sodium sulphate is leached from the mixture 38,~:r along with the 23.38g of sodium chloride, leaving 19.13g of pure sodium sulphate for forwarding to the mill per mole of chlorine dioxide producedO
,.... ..
The present invention, therefore, is able to provide an integrated pulp mill system in which the sodium sulphate and chlorine dioxide requirements are provided while avoiding the feeding of sodium chloride contamination to the pulp mill.
Modifications are possible within the scope of the inventionO
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.
579(~
In accordance with the present invention, there is provided an improvement in a pulp mill process including the steps of contacting a cellulosic fibrous material with a pulping liquor containing sodium sulphide and sodium hydroxide, separating pulped material from spent pulping liquor, sub-jecting the spent pulping liquor to recovery and regeneration steps to provide a white liquor, recycling the white liquor as at least part of the pulping liquor, providing sodium sulphate in the spent pulping liquor to make up soda- and sulphur-losses from the process, and subjecting the pulp to a series of brightening and purification steps to provide a bleached pulp, the improvement comprising forming chlorine dioxide by reaction of sodium chlorate with hydrochloric acid in the presence of sulphuric acid, recovering a mixture of sodium chloride and sodium sulphate from the reaction, separating substantially pure sodium sulphate from the mixture, utilizing the sodium sulphate as the make-up sodium sulphate and utilizing the chlorine dioxide in at least one - of the brightening steps.
In acco~dance with the present invention,-therefore, the sodium sulphate obtained in admixture with sodium chloride ~m thechlorine dioxide generating process is separated to provide a feed of substantially pure sodium sulphate for . . .
the pulp mill.
The invention is described further, by way of .. - . .:
illustration, with reference to the accompanying drawings, in which Figure 1 is a phase diagram for the system Na2SO4 - NaCl - H2O at temperatures of 50C, 75 C, and 100 C;
Figure 2 is a schematic flow sheet of one embodiment of one aspect of the invention; and Figure 3 is a schematic flow sheet of a secon~
embodiment of the one aspect of the invention.
.
- ."~ , , 10579(~9 It is known that the solubility characteristics of sodium chloride and sodium sulphate vary with temperature, the solubility of sodium chloride increasing with increasing temperature while the solubility of sodium sulphate falls at certain relative concentrations of sodium sulphate to sodium chloride, while the reverse is true at other relative concentrations, namely, that the solubility of sodium chloride decreases with temperature while the solubility of sodium sulphate increases. These facts may be utilized in the present invention to ~eve the separation of the sodium sulphate and sodium chloride.
The phase diagramfor the system Na2S04 - NaCl - H20 is shown as Figure 1 of the accompanying drawings, the dat~ ~or the phase diagram being-computed from:
.. . .. . . . . .
Chretien A, Annalen der Chemie 10 serie 11-12, pp 9 to 155 (1929) for the 50 C and 75C data; and Pylkova,E.V. and Akhimov,E.I., Russian Journal of Inorg.Chem. 7(1), 104 (1962).
,. . . . .
, ... .. , , . . .. .. . . , . , . . . -- -- -- -- -- -- -- ----In accordance with one aspect of the present invent-ion, chlorine dioxide and chlorine are formed by reduction of an aqueous solution of sodium chlorate using chloride ions in the presence of a mixture of hydrochloric acid and sulphuric acid in accordance with equation 3 above, a mixture of sodium sulphate and sodium chloride is recovered from the reaction medium, the mixture is dissolved in water, sodium chloride and sodium sulphate are crystallized separately from the solution, their order of separation depending on the solubility relationships which are, in turn, dependent on the initial weight ratio of sodium sulphate to sodium chloride , - ' :, 1057~09 in the recovered mixture, and the sodium sulphate is used as make-up chemical in the recovery and regeneration of spent pulping chemical in a pulp mill system.
In accordance with one embodiment of this aspect of the invention, the separation of the salts independently t from the aqueous solution may be achieved by concentrating the aqueous solution to deposit sodium chloride therefrom until the substantial saturation point of sodium sulphate is reached. The aqueous solution thereafter is heated to render the solutlon undersaturated with respect to sodium chloride, whereupon sodium chloride is added thereto to deposit sub-stantially pure sodium sulphate from the solution. This procedure may be used at high relative concentrations of sodium chloride, as will be apparent from the phase diagram shown in Figure 1. At lower relative concentrations, however, an alternative procedure may be adopted.
In accordance with a second embodiment of this - aspect of the present invention, the separation of the salts independently from the aqueous solution may be achieved by concentrating by evaporation the aqueous solution to deposit sodium sulphate therefrom until the substantial saturation point of sodium chloride is reached. The aqueous solution thereafter is cooled to deposit some sodium chloride therefrom.
Sodium sulphate may be added to-tne cooled solution to deposit further quantities of sodium chloride. ;
In a second aspect of the present invention, the separation of the sodium sulphate and sodium chloride re-covered from the chlorine dioxide generator is achieved using a classification of the crystal sizes. Thus, the 10575~09 sodium sulphate and sodium chloride mixture may be slurried in an aqueous solution which is saturated with respect to sodium sulphate and sodium chloride and then passed through a classifier, which may take the form of a vertical tube through which the slurry is upwardly passed at a flow rate which allows the heavier crystals to settle in the tube while the less heavy crystals pass upwardly and out of the tube to be separated from the aqueous phase in any convenient manner. Alternatively, the slurry may be passed through a sieve in which the openings allow one of the crystal sizes to pass therethrough but prevent the other crystal size from passing therethrough.
The sodium chloride crystals are larger and heavier than those of sodium sulphate and hence separation of the components by the above-described physical separation methods may be used in the present invention.
- Separation of the sodium sulphate and sodium chloride may be achieved in accordance with a third aspect of the present invention. me solid mixture of sodium sulphate and sodium chloride obtalned from the chlorine dioxide gener-ator is contacted with an aqueous solution of sodium sulphate and sodium chloride in a ratio that upon saturation of the aqueous solution, sodium sulphate will be the only solid phase present. In order to minimize the quantity of sodium sulphate leached from the mixture, the ratio should be as close to the eutecticcomposition, as seen in Figure 1, as possible. In practising this aspect of the invention, it is preferred to carry out the leaching at higher temperatures, to decrease further the quantity of sodium sulphate dissolved lVS~9(~
from the mixture. Following the leaching, the solid sodium sulphate is forwarded to the furnace, while the mother liquor is partly recycled, after dilution, to leach further mixture.
This leaching procedure to recover pure sodium sulphate is utilizable when the weight ratio of sodium sulphate to sodium chloride is less than that at the eutectic defined by a set operating temperature. For example, at 100C, the limiting weight ratio of sodium sulphate to sodium chloride is 0.17.
In accordance with a further aspect of the invention, the separation is achievable by a flotation technique. For example, the mixture of sodium sulphate and sodium chloride may be ground to fine particle size, such as 36 to 170 mesh and formed into a pulp with a brine saturated with sodium chloride and sodium sulphate. A solution of sodium octyl sulphate is added to the slurry and stirred, after which the pulp is diluted with further brine until the weight ratio of solids to brine is about 20 to 30:100.
Thereafter, the slurry is introduced to a flotation cell of any convenient construction and air is passed through the cell as a fine stream of bubbles. Sodium sulphate then separates out and the crystals may be collected~
. . .
Referring to Figure 2 of the accompanying drawings, 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.
-` lOS790~
The resulting pulp and black liquor or spent pulp-ing liquor are separated and the pulp is washed in a brown stock washer 16 with water fed by line 18. The wash water used in line 18 may be fresh water or "contaminated condensate".
The washed and unbleached pulp is fed by line 20 to a bleach plant 22 wherein the pulp is subjected to a series of bleaching and purification processes. Generally the bleaching and purification processes involve bleaching with chlorine, chlorine dioxide or mixtures thereof. The chlorine dioxide is fed as a solution by line 24 and the chlorine is fed by line 26. The purification generally is _ carried out by caustic extraction, using aqueous sodium hydr-oxide solution fed by line 28. Any desired sequence of steps may be used in the bleach plant 22 to achieve the desired pulp brightness. For example, the so-called CEDED, C/DEDED or DEDED sequences may be used, wherein C represents bleaching with chlorine, C/D represents bleaching with mixtures of chlorine and chlorine dioxide, D represents bleaching with chlorine dioxide and E represents caustic extraction. In the DEDED sequence, chlorine is not required and hence feed line 26 may be omitted. - -The pulp is washed during the bleach plant operations, typically after each bleaching or caustic extraction operation, by water fed by line 30. The washing operations may involve countercurrent flow of pulp and wash water through the bleach plant 22. The bleached and purified pulp of required brightness is recovered from the bleach plant 22 by line 32 and is sold as such or is passed to further operation to be converted into paper or other pulp products.
The chlorine dioxide which is used in the bleach plant 22 is formed in a chlorine dioxide generator 34 by reduction of sodium chlorate fed by line 36 with hydrogen chloride fed by line 38 in the presence of hydrochloric acid provided by the hydrogen chloride and sulphuric acid fed by line 40.
The generation of chlorine dioxide may take place in any convenient manner, typically by carrying out the process at the boiling point of the reaction medium, typically with the generator 34 being maintained under a reduced pressure.
- When the generator 34 is operated in this manner, the water vapour evaporated from the aqueous reaction medium , ~
dilutes the chlorine dioxide and chlorine and a gaseous mixture of steam, chlorine dioxide and chlorine is removed from the generator 34. t - The gaseous mixture removed from the generator generally is forwarded by line 42 to a separator 44 to condense the steam and recover an aqueous solution of chlorine dioxide containing some dissolved chlorine which then is forwarded to the bleach plant 22 by line 24~for use in the bleaching steps designated D. The chlorine separated from the gaseous mixture may be used in the bleaching steps designated C in the bleach plant 22 as at least part of the chlorine feed in line 26.
Alternatively, the separated chlorine is forwarded by line 46 to form part of a chlorine feed 48 to a hydrogen chloride reactor 50 wherein the chlorine reacts with hydrogen fed by line 52 to form hydrogen chloride, which is forwarded ~ . .
by line 38 to the chlorine dioxide generator 34 as such or as hydrochloric acid.
-- 11 -- .
. .
.
The quantity of chlorine fed by line 48 and hydrogen fed by line 52 should be substantially the stoichiometric quantities to form the amount of hydrogen chloride required in the generator 34, so that a balance of chemicals may be achieved in a continuous operation.
In addition, operation of the generator 34 at the substantial boiling point of the reaction medium causes the deposition of product sodium salt in the generator 34. The generator 34 is operated with the hydrochloric acid providing more than half the acid requirement, i.e. in accordance with equation 3 above, so that a mixture of sodium chIoride and .. .... . ..
sodium sulphate is deposited in the generator 34 and is removed therefrom by line 54.
The form of the sodium sulphate present in the mixture in line 54 depends on the acidity of the reaction mixture in the generator 34, and may be sodium bisulphate if the reaction is carried out at high acidity typically at ~-above 9N, sodium sesquisulphate at acidities around 5 to 7 and neutral sodium sulphate at acidities below about 4N.
The sodium chlorate solution fed to the generator 34 by line 36 may be provided by electrolysis in a suitable chlorate cell 56 of an aqueous sodium chloride solution fed thereto by line 58. The by-product hydrogen formed in the latter electrolysis is removed by line 60 from the cell 56 and is used partially to provide the hydrogen feed in line ; 52 to the hydrogen chloride reactor 50 and partially is discharged by line 62. The discarded hydrogen in line 62 - may be used to form part of the water requirement of the system.
-` lQ57~
The black liquor in line 64 is evaporated in an evaporator 66, prior ~o passage by lines 68 and 70 to a furnace 72 of any convenient construction. The water re-covered from the evaporator 66 by line 74 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 22 in line 30, after suitable cleaning, if required.
Sodium sulphate is added to the black liquor, either in solid or slurry form or as an aqueous solution thereof, either directly, or indirectly, typically by line 76 to the concentrated black liquor in line 68. The sodium sulphate 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 72 a smelt containing sodium sulphide and sodium carbonate. The smelt is dissolved in a smelt dissolver 78 by water, generally weak wash water from the washing of calcium carbonate mud, fed by line 80, to form a green liquor in line 82.
2~ The green liquor, after clarification to remove dregs, is passed to a causticizer 84 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 86 from a lime kiln 88. 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 88 by line 90. 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.
., , ~ :
- , . ., : :
lOS79(19 The white liquor resulting from the causticization step in line 92 is recycled to provide at least part of the pulping liquor in line 14.
The mixture of sodium chloride and sodium sulphate removed from the chlorine dioxide generator 34 by line 54 is passed to a dissolver 94 wherein the mixture is dissolved in water fed by line 96.
The relative proportions of the sodium chloride and sodium sulphate in the mixture depends on the relative pro-portions of hydrochloric acid and sulphuric acid present in ... . ~ , . . .
the reaction medium.
The quantity of chlorine dioxide and quantity of sodium sulphate produced in the generator 34 depend on the individual requirements of the mill for these products. The present invention is concerned with a pulp mill operation where the requirements of chlorine dioxide and sodium sulphate are such that the generator 34 must be operated with an acid mixture of hydrochloric and sulphuric acids such that it is possible only to produce the sodium sulphate in admixture with sodium chloride.
In those instances where the sodium sulphate require-mentis such that equation 2 above applies, then the sodium sulphate is recovered in substantially pure form and may be forwarded directly by line 76 to the black liquor.
In thls embodiment, the weight ratio of sodium sulphate to sodium chloride is less than 0.17 and hence the phase diagram of Figure l to the left of the eutectic points applies.
.
1~)57~V9 The aqueous solution of sodium sulphate and sodium chloride resulting from the dissolver 94 is forwarded by line 98 to a first evaporator 100, which is the first stage of three multiple effect evaporators 100, 102 and 104. Water is evaporated from the solution in each of the evaporators 100, 102 and 104, sodium chloride being deposited from the solution in each evaporator and being removed by lines 106, 108, 110 respectively, to provide a combined sodi~m chloride product stream 112.
The concentration of the solution in the evaporators 100, 102 and 104 continues until the solution is substantially saturated with sodium sulphate. The evaporators 100, 102 and 104 operate at successively lower temperatures, typically about 100 C, about 75C and about 50C, the evaporators being indirectly heated by steam, the steam for evaporators 102 and 104 being provided by the steam formed in evaporators 100 and 102 respectively.
The aqueous solution resulting from evaporator 104 has a reduced sodium chloride content as compared to the solution in line 98 and is substantially saturated with respect both to sodium sulphate and sodium chloride.
The sodium chloride recovered from the evaporators 100, 102 and 104 in line 112 may be put to a variety of uses. For example, it may be used to provide part of the sodium chloride solution fed by line 58 to the chlorate cell 56 and/or part of the sodium chloride solution which is fed by line 114 to a caustic-chlorine cell 116 wherein there is formed the sodium hydroxide feed solution for the bleach plant 22 in line 28. The caustic-chlorine cell 116 also produces .,: .
~ .
chlorine which is removed by line 118. The chlorine gas in line 118 may provide the chlorine requirement of the bleach plant in line 26, with any excess chlorine being recovered by line 120. The excess chlorine in line 120 may be used to provide part of the chlorine feed in line 48 to the hydrogen chloride reactor 50. Alternatively, the sodium chloride may be sold as such.
The aqueous solution o sodium chloride and sodium sulphate resulting from the third evaporator 104 is passed by line 122 through a heater 124 wherein the solution is heated, typically to about 100C, so that it becomes under-saturated with respect to sodium chloride and by line 126 to a precipitator 128.
,, , To the heated solution in the precipitator 128 is added solid sodium chloride by line 130 causing precipitation of sodium sulphate from the solution. The sodium chloride fed by line 130 may be provided by part of the sodium chloride recovered from the evaporators 100, 102 and 104 in line 112.
The sodium sulphate which is precipitated in the precipitator 128 is forwarded by line 76 to the concentrated black liquor in line 68. The mother liquor is recycled by line 132 to the evaporators 100, 102 and 104, typically by feeding, as illustrated, to the liquid entering the third evaporator 104.
The manner of concentration of the aqueous solution of sodium sulphate and sodium chloride in line 98 to the substantial saturation point of the solution with sodium sulphate and precipitating sodium chloride therefrom may be varied from that illustrated.
, ~057909 Similarly, the manner of achieving deposition of ~26 sodium sulphate from the heated solution in line ~ may be varied, as desired. For example, the heated solution in line 126 may be evaporated to render it more concentrated with respect to sodium chloride, thereby resulting in pre-cipitation of sodium sulphate from the solution.
Figure 3 illustrates an alternative embodiment to the embodiment outlined in Figure 2, applicable when the weight ratio of sodium sulphate to sodium chloride in the mixture formed in the chlorine dioxide generator is greater than 0.17, and hence the portion of the phase diagram of Figure 1 to the right of the eutectic point applies.
In the embodiment of Figure 3, a mixture of sodium sulphate and sodium chloride, recovered from a chlorine dioxide generator operating as described above with reference to chlorine dioxide generator 34, but with a lesser proportion of the acid requirement being provided by hydrochloric acid, -is passed by line 200 to a dissolver 202 which the mixture P is dissolved in water-, preferably the minimum quantity, fed by line 204.
The resulting solution is passed to a first evaporator 206, which is the first stage of three multiple effect evaporat-ors 206, 208 and 210. Water is evaporated from the solution in each of the evaporators 206, 208 and 210, sodium sulphate being deposited from the solution in each evaporator and being removed by llnes 212, 214 and 216 respectively, to provide a combined sodium sulphate product stream 218.
The concentration of the solution in the evaporators 206, 208 and 210 is continued until the solution is substantial-lOS7gO5~ .
ly saturated with sodium chloride. The evaporators 206, 208 and 210 operate at successively higher temperatures, typically about 50C, about 75C and about 100C, the evaporators being indirectly heated by steam, the steam for evaporators 206 and 208 being provided by the steam formed in the evaporators 208 and 210 respectively. Ifdesired, all the evaporators may be carried out in a single evaporator, typically at about 100 C.
The aqueous solution resulting from the evaporator 210 has a reduced sodium sulphate content as compared to the solution in line 205 and is substantially saturated with respect both to sodium sulphate and sodium chloride.
The sodium sulphate recovered from the evaporators 206, 208 and 210 in line 218 is forwarded as make-up chemical to the concentrated black liquor prior to passage thereof to the furnace, in analogous manner to the passage of sodium sulphate by line 76 to the concentrated black liquor in line 68 as described above with reference to Figure 2.
The aqueous solution of sodium chloride and sodium sulphate resulting from the third evaporator 210 is passed by line 220 through a cooler 222, wherein the solution is cooled, typically to a temperature of about 50 C, so that it becomes undersaturated with respect to sodium sulphate, and by line 224 to a precipitator 226. Some precipitation of sodium chloride may occur during the cooling operation, and this precipitated sodium chloride may be separated from the cooled solution.
To the cooled solution in the precipitator 226 is added solid sodium sulphate by line 228 causing precipitation of sodium chloride from the solution. The sodium sulphate fed by line 228 may be provided by part of the sodium sulphate recovered from the evaporators 206, 20~ and 210 in line 218.
The sodium chloride which is precipi-tated in the precipitator 226 is recovered by line 230 and may be put to a variety of uses, as discussed above in connection with the sodium chloride recovered in line 112 in Figure 2. The mother liauor is recycled by line 232 to the evaporators 206, 208 and 210, typically by feeding, as illustrated, to the first evaporator 206.
The manner of concentration of the aqueous solution of sodium sulphate and sodium chloride in line 205 to the ~ -substantial saturation point of the solution with sodium chloride and precipitating sodium sulphate therefrom may be varied from that illustrated.
Similarly, the manner of achieving deposition of sodium chloride from the cooled solution in line 224 may be varied as desired. For example, the cooled solution in line 224 may be evaporated to render it more concentrated with respect to sodium sulphate, thereby resulting in precipitation of sodium chloride from the solution.
The invention is illustrated further by the following Example:
EX~MPLE
A chlorine dioxide generator, maintained under a reduced pressure, is operated at the boiling point of an aaueous solution containing 1.4 moles of hydrochloric acid and 0.3 moles of sulphuric acid per mole of sodium chlorate.
- This results in the deposition of a mixture of 0.3 mole of sodium sulphate and 0.4 mole of sodium chloride per mole of chlorine dioxide.
A mixture of 23.38g of sodium chloride and 42.6g of sodium sulphate is leached with water at 100C and operat-ing at the eutectic point. The amount of water required per gram of sodium chloride leached is 2 69g, i.e. a total of 62.89g for 23.38g of sodium chloride and the quantity of sodium sulphate leached per gram of sodium chloride is 0.169g, and hence 3.95g of sodium sulphate is leached from the mixture 38,~:r along with the 23.38g of sodium chloride, leaving 19.13g of pure sodium sulphate for forwarding to the mill per mole of chlorine dioxide producedO
,.... ..
The present invention, therefore, is able to provide an integrated pulp mill system in which the sodium sulphate and chlorine dioxide requirements are provided while avoiding the feeding of sodium chloride contamination to the pulp mill.
Modifications are possible within the scope of the inventionO
~ . .
Claims (20)
1. In a pulp mill process including the steps of contacting a cellulosic fibrous material with a pulping liquor containing sodium sulphide and sodium hydroxide, separating pulped material from spent pulping liquor, subject-ing said spent pulping liquor to recovery and regeneration steps to provide a white liquor, recycling said white liquor as at least part of said pulping liquor, providing sodium sulphate in said spent pulping liquor to make-up soda and sulphur losses from the process, and subjecting said pulp to a series of brightening and purification steps to provide a bleached pulp, the improvement which comprises forming chlorine dioxide by reaction of sodium chlorate with hydro-chloric acid in the presence of sulphuric acid, recovering a mixture of sodium chloride and sodium sulphate from said reaction, separating substantially pure sodium sulphate from said mixture, utilizing said separated sodium sulphate as said make up sodium sulphate and utilizing said chlorine dioxide in at least one of said brightening steps.
2. The process of claim 1 wherein the weight ratio of sodium sulphate to sodium chloride in said recovered mixture is less than about 0.17 and said recovered mixture of sodium chloride and sodium sulphate is separated by forming an aqueous solution containing said sodium sulphate and said sodium chloride, concentrating said aqueous solution to deposit sodium chloride therefrom to the substantial saturation of said aqueous solution with sodium sulphate, separating said deposited sodium chloride from the concentrated solution, undersaturating said concentrated solution with respect to sodium chloride while maintaining said concentrated solution saturated with respect to sodium sulphate, increasing the concentration of the resulting solution with respect to sodium chloride and thereby depositing sodium sulphate from the solution, and removing the deposited sodium sulphate from the resulting mother liquor.
3. The process of claim 2 wherein said aqueous solution of sodium sulphate and sodium chloride is concentrated by evaporating water therefrom.
4. The process of claim 3 wherein said undersaturat-ing of said concentrated solution is carried out by heating said concentrated solution to a higher temperature than that of the concentrated solution at the end of said evaporation.
5. The process of claim 4 wherein the concentration of sodium chloride in said resulting solution is increased by the addition of solid sodium chloride thereto.
6. The process of claim 4 wherein the concentration of sodium chloride in said resulting solution is increased by the evaporation of said resulting solution.
7. The process of claim 3 wherein said mother liquor is recycled to the liquid subjected to said evaporation.
8. The process of claim 1 wherein the weight ratio of sodium sulphate to sodium chloride in said recovered mixture exceeds about 0.17 and said recovered mixture of sodium chloride and sodium sulphate is separated by forming an aqueous solution containing said sodium sulphate and said sodium chloride, concentrating said aqueous solution to deposit sodium sulphate therefrom to the substantial saturat-ion of said aqueous solution with sodium chloride, separating said deposited sodium sulphate from the concentrated solution, undersaturating said concentrated solution with respect to sodium sulphate while maintaining said concentrated solution saturated with respect to sodium chloride, increasing the concentration of the resulting solution with respect to sodium sulphate thereby depositing sodium chloride from the solution, and removing the deposited sodium chloride from the resulting mother liquor.
9. The process of claim 8 wherein said aqueous solution of sodium sulphate and sodium chloride is concentrated by evaporating water therefrom.
10. The process of claim 9 wherein said undersaturat-ing of said concentrated solution is carried out by cooling said concentrated solution to a lower temperature than that of the concentrated solution at the end of the evaporation.
11, The process of claim 10 wherein the concentration of sodium sulphate in the resulting solution is increased by the addition of solid sodium sulphate thereto.
12. The process of claim 9 wherein said mother liquor is recycled to the liquid subjected to said evaporation.
13. The process of claim 1 wherein said recovered mixture of sodium chloride and sodium sulphate is separated by leaching the solid mixture to dissolve selectively one of the components together with a minor amount of the other component and to leave a solid mass of substantially pure other component.
14. The process of claim 1 wherein said recovered mixture of sodium chloride and sodium sulphate is separated by physical fractionation of crystal sizes.
15. The process of claim 1 wherein said recovered mixture of sodium chloride and sodium sulphate is separated by froth flotation of one of the components.
16. The process of claim 1 wherein said chlorine dioxide is formed in admixture with chlorine by reducing said sodium chlorate in an acid reaction medium in accordance with the equation:
ClO3- + Cl- + 2H+ ? ClO2 + 1/2Cl2 + H2O
said hydrochloric acid provides all the stoichiometric requirement of chloride ions and more than half the total stoichiometric acid requirement, and the remainder of the stoichiometric acid requirement is provided by said sulphuric acid.
ClO3- + Cl- + 2H+ ? ClO2 + 1/2Cl2 + H2O
said hydrochloric acid provides all the stoichiometric requirement of chloride ions and more than half the total stoichiometric acid requirement, and the remainder of the stoichiometric acid requirement is provided by said sulphuric acid.
17. The process of claim 16 wherein said acid reaction medium is maintained substantially at its boiling point to evaporate water from said reaction medium, the chlorine dioxide and chlorine being mixed with the evaporated water and said mixture of sodium chloride and sodium sulphate is precipitated from the reaction medium by said evaporation.
18. The process of claim 1 wherein said recovery and regeneration steps include concentrating said spent pulping liquor, furnacing said concentrated spent pulping liquor to provide a solid smelt, dissolving said smelt in aqueous material to form a green liquor and causticizing said green liquor to obtain said white liquor.
19. The process of claim 1 wherein said series of bleaching and purification steps involves a first stage bleach-ing with chlorine, chlorine dioxide or a mixture thereof, a first stage caustic extraction using sodium hydroxide solution, a second stage bleaching with chlorine dioxide, a second stage caustic extraction with sodium hydroxide solution and a third stage bleaching with chlorine dioxide.
20. The process of claim 19 including separating said pulp and spent pulping liquor while washing said pulp after formation thereof and before passage to said series of bleach-ing and purification operations, and washing said pulp after each said bleaching and caustic extraction, the wash water utilized in said latter washing passing countercurrently to the pulp through said series of bleaching and purification steps.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB4396373 | 1973-09-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1057909A true CA1057909A (en) | 1979-07-10 |
Family
ID=10431150
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA209,010A Expired CA1057909A (en) | 1973-09-19 | 1974-09-10 | Production of chlorine dioxide |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1057909A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112479232A (en) * | 2020-12-16 | 2021-03-12 | 深圳清华大学研究院 | Floatation separation method for industrial by-product sodium chloride and sodium sulfate |
-
1974
- 1974-09-10 CA CA209,010A patent/CA1057909A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112479232A (en) * | 2020-12-16 | 2021-03-12 | 深圳清华大学研究院 | Floatation separation method for industrial by-product sodium chloride and sodium sulfate |
| CN112479232B (en) * | 2020-12-16 | 2023-04-11 | 深圳清华大学研究院 | Floatation separation method for industrial by-product sodium chloride and sodium sulfate |
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