CA2219550C - A method of recovery of chemical compounds from a pulp mill - Google Patents
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- CA2219550C CA2219550C CA 2219550 CA2219550A CA2219550C CA 2219550 C CA2219550 C CA 2219550C CA 2219550 CA2219550 CA 2219550 CA 2219550 A CA2219550 A CA 2219550A CA 2219550 C CA2219550 C CA 2219550C
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Abstract
The build up of unwanted components, particularly potassium and chloride values, in pulp mill recovery cycles is avoided by processing pulp mill recovery cycle precipitator ash. The ash first is dissolved in water and the aqueous solution then is evaporated to crystallize sodium sulfate, which then can be returned to the pulp mill recovery cycle. The mother liquor from such crystallization is subjected to extractive crystallization to recover further quantities of crystalline sodium sulfate for return to the pulp mill recovery cycle. The purge stream containing chloride values is removed from the cycle.
Description
TITLE OF INVENTION
A METHOD OF RECOVERY OF CHEMICAL COMPOUNDS
FROM A PULP MILL
FIELD OF INVENTION
The present invention relates to the removal of components from a pulp mill, particularly potassium and chloride values, to avoid buildup of such components in the spent liquor recovery cycle. Furthermore, the invention provides for the purification and recovery of the chloride value for re-use.
BACKGROUND TO THE INVENTION
In the kraft pulping process, chloride and potassium ions enter with the wood and chemicals, but have no natural purge points in the pulping and chemical recovery loop. In the recovery cycle, chloride and potassium ions become enriched in the flue gas dust in the recovery boiler, and decrease the melting point of the dust. This accumulation can lead to plugging of the recovery boiler tubes, which decreases boiler capacity and causes a loss of pulp mill production, as well as corrosion of boiler tubes where these materials are deposited. As kraft pulp mills increase their degree of water re-use and closure and tighten their liquor recovery loop, the build-up of chloride and potassium in the recovery cycle can become a serious problem. This problem is especially true for coastal mills using wood containing high amounts of chloride, and for mills pulping wood containing high amounts of potassium.
Furthermore, as bleached kraft pulp mills begin to practice the recovery of spent bleaching filtrates in addition to the pulping liquors, for example as described in U.S. Patent No. 5,352,332, the chloride ions from the bleaching chemicals, for example, chlorine, chlorine dioxide and sodium hypochlorite, also accumulate in the recovery cycle, as described above.
In order to control the concentration of chloride and potassium in the recovery cycle, the flue gas dust collected at the electrostatic precipitator can simply be sewered, but such activity wastes a significant amount of sodium sulfate which is also present in the dust in the electrostatic precipitator, which then has to be made up with purchased chemicals, increasing the overall operating cost of the pulp mill operation.
Alternatively, the precipitator ash can be "leached" in an attempt to dissolve the more soluble sodium chloride from the less soluble sodium sulfate, for example, as described in U.S. Patent No. 3,833,462. However, this method requires very high concentrations of chloride, for example, as may be found in a coastal pulp mill and potassium in the ash to drive the separation of the chemicals, and so leaching does not effectively remove these ions from the ash generated in most mills. In addition, in practice, the selectivity of the leaching process is poor, so that the steady-state concentration of chloride and potassium in the ash remains high.
This effect means that sizeable amounts of sodium sulfate may be lost, requiring the purchase of make-up chemicals, and that other measures may still have to be taken to minimize plugging and corrosion in the boiler, for example, operation at lower temperatures or the use of exotic metallurgy. Furthermore, because of the fine particle size of precipitator ash, the filtration requirements are excessive for the large amounts of ash produced in modern mills.
Alternatively, it has been proposed to use bipolar membrane electrodialysis to separate sodium sulfate from chloride and potassium ions in kraft mill precipitator ash, for example, as described in WO 96/19282. However, the nature and sensitivity of the membranes and electrodes in these processes requires that any organic compounds, as well as certain other inorganic impurities, such as calcium and magnesium, be removed from the ash before the electrodialysis step, making the overall process extremely uneconomic with regard to both capital and operating and maintenance costs.
Alternatively, it has also been proposed to use polymers, such as polyethylene glycol, to remove chloride from kraft mill precipitator ash, for example, as described by Prakash and Moudgil (Proceedings of the 1994 International Non-Chlorine Bleaching Conference, Miller Freeman Inc., California 1994). However, this process requires the use of excessively large quantities of the treatment chemical, on the order of 100 to 250%
of the quantity of ash, to remove only modest amounts of chloride (less than 70%). Potassium removal was not disclosed in this paper but would be expected to be low, based on the chemistry of the system. In addition, the use of large quantities of treatment chemical also necessitates an impractical degree of material recovery to preserve economic feasibility and minimize adverse environmental impact of the polymer.
Finally, the removal of chloride and potassium ions from kraft mill precipitator ash, by either disposal of ash or by methods described in the prior art, means that valuable process chemicals may be lost, most notably the sodium chloride, which is required as a feed stock for the production of sodium chlorate, the precursor chemical for chlorine dioxide, used as a bleaching chemical in the pulp mill.
SUbIlKARY OF INVENTION
The present invention uses a two-step process to first separate and purify a majority of the sodium sulfate from kraft mill precipitator ash without the need for any additional chemicals, followed by a second-stage treatment to purify the resulting concentrated sodium chloride solution, thereby minimizing treatment volume and capital costs for the second operation.
In the present invention, chloride and potassium ions are removed from kraft mill or other pulp mill precipitator ash by dissolving the ash in water, and then selectively crystallizing sodium sulfate from the resulting aqueous solution, conveniently via evaporation of water from the aqueous solution. The sodium sulfate crystals are then separated from the mother liquor, for example, via filtration or centrifuge. The purified sodium sulfate generally is returned to the kraft mill to make up sodium and sulfur values, while chloride and potassium ions remain in solution and are purged from the system by removal of the resulting aqueous stream.
The purge stream may be further treated, such as by extractive crystallization with an appropriate solvent, to remove additional sodium sulfate from the purge stream to leave a purge stream containing sodium chloride and containing potassium ions, which can be used in the production of sodium chlorate. The remaining mother liquor may discharged, such as by sewering.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 of drawings is a flow sheet illustrating one embodiment of the present invention.
GENERAL DESCRIPTION OF INVENTION
In the present invention, precipitator ash from a kraft mill recovery boiler, typically containing sulfates, chlorides and carbonates of sodium and potassium, first is dissolved in water to form an aqueous solution thereof and then evaporated. As water is evaporated from the aqueous solution, the solubility of sodium sulfate in the aqueous medium will be exceeded and sodium sulfate then crystallizes from the aqueous solution. Sodium chloride and potassium chloride, with substantially higher solubilities, remain in solution.
However, the concentration of chloride ions in the remaining solution increases, since the sodium chloride concentration never exceeds the solubility limit in this system. Furthermore, the solubility of sulfate ions in 5 the solution is further decreased by the high concentration of sodium chloride in the solution, a phenomenon known as the "common ion effect". The process, therefore, preferentially crystallizes purified sodium sulfate from the solution, while the potassium and chloride ions remain in solution, thereby resulting in a more selective separation. The process also yields sodium sulfate crystals of appropriate and controllable size for better washing and filtrate extraction.
Sodium sulfate exhibits an inverse solubility relationship with temperature. Above a critical temperature, the amount which can be dissolved decreases as the temperature increases. Below this temperature, sodium sulfate crystallizes as the decahydrate (Na2SO9=10H20), which is undesirable for this application, because of the additional water load it would bring back to the liquor cycle. Accordingly, it is preferred to effect the crystallization of sodium sulfate at a temperature above about 30 C.
An initial high concentration of potassium in the precipitator ash may lead to the formation of glaserite (K3Na(S04)2) crystals. Glaserite co-crystallizes with sodium sulfate and the two cannot be simply separated from each other. A pulp mill in this situation must first purge precipitator catch and make up the sodium sulfate losses with potassium-free chemicals, in order to decrease the potassium concentration to an acceptable operating level which avoids the formation of glaserite.
When this operating level has been achieved, the Chloride Removal Process provided herein can then be used to remove incoming potassium on a continuous basis, preventing the concentration from building back up to problem levels. Alternatively, sufficient make-up sodium sulfate may be added to the precipitator ash to depress the concentration of potassium below the point of glaserite formation and the process of the invention effected thereon.
Organic material may also be present in the precipitator ash, especially if the mill is using a direct-contact evaporator on the recovery boiler.
Laboratory trials have shown that this organic material may also shift the solubility curves for the inorganic components of the precipitator ash. For example, when the ash contains high levels of organic material, glaserite and/or burkeite (2NaZSO4=NaZCO3) have been found to form at concentrations below those predicted by the four-component inorganic system. If sufficient carbonate is present in the ash, burkeite is formed regardless of the content of organic material. A
combined crystallization of sulfate and carbonate permits a more complete recovery of sodium values. While burkeite formation can be, at times, desirable as it recovers additional sodium, this is not the case for glaserite formation, as mentioned above.
Depending upon the individual needs of the mill and specific sodium/sulfur balance, the feedstock ash may also be pre-acidified with sulfuric acid, in effect, converting the Na2CO3 into Na2SO4, so that more of the sodium values can be recovered (along with additional sulfur) as sodium sulfate. Alternatively, sodium sesquisulfate or spent generator acid originating from the chlorine dioxide generation process may be used in the acidification of ash.
The purified sodium sulfate crystallized from the solution of precipitator ash optionally may be returned to the kraft liquor cycle, providing make-up sodium and sulfur values. Alternatively, the purified sodium sulfate may be treated electrochemically to produce sodium hydroxide and sulfuric acid which may be used in the pulp mill, for example, by bipolar membrane electrodialysis, or by using a multi-compartment electrochemical cell divided by one or more cation exchange membranes, or by using a three-compartment cell employing both an anion and a cation exchange membrane.
The purge stream resulting from crystallization of the sodium sulfate and containing the chloride and potassium ions also is saturated in sodium sulfate. This stream may then be further treated with an appropriate organic solvent, for example, methanol, which may be -either purchased or recovered from the pulping cycle, or ethanol, to further separate sodium sulfate from sodium chloride. Sodium sulfate is not only relatively insoluble in these pure solvents, but also in solvent-water mixtures, while sodium chloride remains relatively soluble. Other applicable solvents include alcohols, ethers, amines and ketones. As shown in the Examples below, the addition of a suitable solvent to the purge stream crystallizes a substantial portion of the remaining sodium sulfate. This second treatment, therefore, effectively recovers sodium chloride, which remains in the solution phase, of suitable quality for feed to, for example, a chlor-alkali or sodium chlorate plant. The purge stream may optionally be concentrated, or partially concentrated, before the addition of the solvent.
After separation of the sodium sulfate crystals resulting from the addition of organic solvent, the organic solvent may be recovered from the mother liquor for re-use, for example, by fractionation/distillation or via membrane pervaporation.
The purge stream may optionally be acidified, for example, with hydrochloric acid, to remove carbonate ions therefrom as carbon dioxide. Alternatively, the carbonate may remain with the sodium chloride stream, displacing a portion of the carbonate used in the brine demineralization process in a chlorate plant.
Any potassium which remains with the purified sodium chloride stream may be used to displace the potassium chloride used for perchiorate control in a sodium chlorate plant, as described in U.S. Patent No.
5,681,446, assigned to the assignee herein.
DESCRIPTION OF PREFERRED EMBODIMENT
One embodiment of the invention is illustrated in Figure 1. Precipitator ash from a kraft pulp mill recovery boiler is fed through line [1] and dissolved in water or condensate fed through line [2]) in a mix tank [3]. The dissolved ash is fed through line [4] to one or more evaporator/crystallizers [5], where water is evaporated as steam in line [6]. This steam may be condensed for ash dissolution (line [2]) or may alternatively be used elsewhere in the plant. A slurry of mother liquor and crystallized sodium sulfate is withdrawn from the evaporator/crystallizer through line [7] and fed to a separation device [8], for example, a vacuum drum washer or centrifuge. Washed sodium sulfate crystals are removed through line [9] for return to the plant or alternate disposal.
The filtrate from the separation device [8] is removed through line [10] and mixed with an appropriate solvent fed through line [11]) in a mix tank [12]. This combined stream is fed through line [13] to a crystallizer [14], where a further amount of purified sodium sulfate is removed through line [15], for optional washing and disposal as in line [9]. The fiitrate from the crystallizer [14] is fed through line [16] to a solvent regenerator [17], for example, fractionation, distillation, or membrane separation. The purified aqueous sodium chloride solution containing potassium ions is removed through line [18] for further use, for example, in the production of sodium chlorate or chlorine and alkali. The regenerated solvent may be recycled back to the mix tank [12] through line [19] and combined with fresh solvent, as required, from line [11] .
EXAMPLES
Example 1 This Example illustrates the prior art teaching process of separation of sodium chloride and sodium sulfate in precipitator ash.
Leaching tests were carried out to separate sodium chloride from sodium sulfate, following a method similar to that proposed in U.S. Patent No. 3,833,462. A 50%
v/v mixture of pulp mill precipitator ash was prepared using a solution of 290 g/L sodium chloride (NaCl) and 100 g/L sodium sulfate (Na2SO4), to simulate the use of recycled liquor for the leaching. The pH was adjusted to 3.2 with sulfuric acid in order to recover sodium carbonate (Na2CO3) as Na2SO4, and to improve filtration.
However, because of the small size of the undissolved ash particles, the cake was slow to filter and prone to cracking and pinholing, which impaired filtration efficiency. After washing, the resulting cake (and its entrained liquor) was found to contain a higher concentration of chloride than the original ash (3.2 wt%
NaCl, vs. 2.5 wt% in the original ash) . Additional washing may have removed more of the chloride-containing liquor, but would have also dissolved more of the sodium sulfate.
Example 2 This Example illustrates one embodiment of the process of the invention.
A continuous pilot scale evaporator-crystallizer was constructed to evaluate the process of the invention.
Precipitator ash from a pulp mill (Mill "B") was dissolved in water and the aqueous solution was evaporated to crystallize sodium sulfate therefrom.
Table 1 presents the results and mass balance from 5 a sample experimental run. It can be seen from the results presented in Table 1 that the content of sodium and sulfate in the dry crystals increases, while the chloride and potassium content decrease dramatically as compared to the original dry feed ash.
Table 1: Purification of Fly Ash via Evaporative Crystallization: Results of Pilot-Scale Trials (Mill "B") Compound Dry Feed Crystallizer Unwashed "Dry"
(wt%) Ash Filtrate Wet Crystals Crystals (calculated) Sodium (Na+) 29.9 9.6 24.3 32.3 Sulfate (SO42-) 60.3 13.6 48.4 67.2 Chloride (Cl-) 4.3 5.3 2.0 0.2 Potassium (K+) 4.4 3.2 1.2 0.1 Carbonate (C032-) 1.1 2.1 0.8 0.2 water/other --- 66.3 23.3 ---* "Dry" crystal composition is calculated assuming that the filtrate in the unwashed crystals has the same composition as the crystallizer liquor, and adjusting accordingly.
Examples 3 to 5 These Examples illustrate further processing of the mother liquor from the sodium sulfate crystallization.
Samples of the purge stream from the primary evaporation-crystallization process carried out as described in Example 2 on precipitator ash from two mills (Mill "A" and Mill "B") were combined with an appropriate amount of methanol (Tables 2 and 3) or ethanol (Table 4) to yield a total of 250 mL with the volume ratios as shown in Tables 2, 3 and 4 below. The mixtures were stirred and allowed to stand and then filtered to remove crystalline sodium sulfate. The results obtained are shown in the following Tables 2, 3 and 4:
Table 2: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "A") Volume - % Sulfate Removal Chloride Purity Methanol (molar basis) [Cl-]/[Cl-+SO-4]
(molar basis) 0% --- 34.8 %' 30% 84.4% 74.5%
40% 96.6% 92.1%
50% 99.1% 97.4%
60% 99.4% 98.9%
70% 99.7% 99.6%
* average brine solution concentration.
Table 3: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "B") Volume - % Sulfate Removal Chloride Purity Methanol (molar basis) [Cl-]/[Cl-+SO-4]
(molar basis) 0% --- 57.1%*
30% 91.5% 91.5%
40% 97. 8 % 97 .2 %
50% 99.5% 99.2%
60% 99.7% 99.7%
70% 100.0% 99.9%
80% 100.0% 100.0%
90% 100.0% 100.0%
* average brine solution concentration.
Table 4: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "B") Volume - % Sulfate Removal Chloride Purity Ethanol (molar basis) [Cl-]/[Cl-+SO"4]
(molar basis) 0% --- 54.0 %' 40% 96.4% 96.4%
50% 99.0% 98.9%
60% 99.4% 99.7%
70% 99.8% 99.9%
80% 100.0% 100.0%
90% 100.0% 100.0%
* average brine solution concentration.
As may be seen from the results presented in Tables 2 to 4, a very high removal efficiency of sodium sulfate may be achieved by employing the extractive crystallization and the residual aqueous medium is highly purified with respect to chloride ion content.
Examples 6 to 13 These Examples illustrate the use of additional solvents for extractive crystallization of sodium sulfate.
Samples of the purge stream from the primary evaporation-crystallization process of Example 2 were combined with various solvents as shown in Table 5 below. The mixtures were stirred and allowed to react for one hour, and then filtered.
The results obtained are shown in the following Table 5:
w a w W V
+ -R
Ll 1 Q ~+ M Q [~ O~ t~ ~D
. U cL [~ '-' 00 C ~O O v ~O M 00 V1 l~
O O O~ 01 O~ ON O\ ON 01 ON
U) D
w ~ U
U) O
U O N N
tn LL
0 > Q v c~ ~n Q ~ o b~ ~n M ~ ~
N O~ 16 1-N
w W
~
a 0 o 0 0 0 0 0 0 0 0 0 0 0 F- - 1n 4n ~ tn ~ M tn l/'I tn z aQrJ O O O O O O O O O O O O
W tn ~O ~O v) ~D v') ~D l- tn v) h ~D
J
U) w " ~ ~ = ~ a _ = a ~
> a~ ~ >' ~ =~ ~ ~
'~ a i a~ oo Q o~n ~
U) Ca W ci. Q Q 0 O
z W
a ~o r- co w As may be seen from the results presented in Table and in above Tables 2 to 4, alcohols, ethers, ketones, and amines all effectively precipitate the sulfate ions out of solution, leaving the residual aqueous medium 5 highly purified with respect to chloride ion content.
Solvents which from a bi-phasic mixture are less effective and less desirable.
SUNMARY OF DISCLOSURE
In summary of this disclosure, the present invention provides a novel procedure for providing a purge of potassium and chloride ions from a pulp mill recovery cycle without loss of valuable chemicals.
Modifications are possible within the scope of this invention.
A METHOD OF RECOVERY OF CHEMICAL COMPOUNDS
FROM A PULP MILL
FIELD OF INVENTION
The present invention relates to the removal of components from a pulp mill, particularly potassium and chloride values, to avoid buildup of such components in the spent liquor recovery cycle. Furthermore, the invention provides for the purification and recovery of the chloride value for re-use.
BACKGROUND TO THE INVENTION
In the kraft pulping process, chloride and potassium ions enter with the wood and chemicals, but have no natural purge points in the pulping and chemical recovery loop. In the recovery cycle, chloride and potassium ions become enriched in the flue gas dust in the recovery boiler, and decrease the melting point of the dust. This accumulation can lead to plugging of the recovery boiler tubes, which decreases boiler capacity and causes a loss of pulp mill production, as well as corrosion of boiler tubes where these materials are deposited. As kraft pulp mills increase their degree of water re-use and closure and tighten their liquor recovery loop, the build-up of chloride and potassium in the recovery cycle can become a serious problem. This problem is especially true for coastal mills using wood containing high amounts of chloride, and for mills pulping wood containing high amounts of potassium.
Furthermore, as bleached kraft pulp mills begin to practice the recovery of spent bleaching filtrates in addition to the pulping liquors, for example as described in U.S. Patent No. 5,352,332, the chloride ions from the bleaching chemicals, for example, chlorine, chlorine dioxide and sodium hypochlorite, also accumulate in the recovery cycle, as described above.
In order to control the concentration of chloride and potassium in the recovery cycle, the flue gas dust collected at the electrostatic precipitator can simply be sewered, but such activity wastes a significant amount of sodium sulfate which is also present in the dust in the electrostatic precipitator, which then has to be made up with purchased chemicals, increasing the overall operating cost of the pulp mill operation.
Alternatively, the precipitator ash can be "leached" in an attempt to dissolve the more soluble sodium chloride from the less soluble sodium sulfate, for example, as described in U.S. Patent No. 3,833,462. However, this method requires very high concentrations of chloride, for example, as may be found in a coastal pulp mill and potassium in the ash to drive the separation of the chemicals, and so leaching does not effectively remove these ions from the ash generated in most mills. In addition, in practice, the selectivity of the leaching process is poor, so that the steady-state concentration of chloride and potassium in the ash remains high.
This effect means that sizeable amounts of sodium sulfate may be lost, requiring the purchase of make-up chemicals, and that other measures may still have to be taken to minimize plugging and corrosion in the boiler, for example, operation at lower temperatures or the use of exotic metallurgy. Furthermore, because of the fine particle size of precipitator ash, the filtration requirements are excessive for the large amounts of ash produced in modern mills.
Alternatively, it has been proposed to use bipolar membrane electrodialysis to separate sodium sulfate from chloride and potassium ions in kraft mill precipitator ash, for example, as described in WO 96/19282. However, the nature and sensitivity of the membranes and electrodes in these processes requires that any organic compounds, as well as certain other inorganic impurities, such as calcium and magnesium, be removed from the ash before the electrodialysis step, making the overall process extremely uneconomic with regard to both capital and operating and maintenance costs.
Alternatively, it has also been proposed to use polymers, such as polyethylene glycol, to remove chloride from kraft mill precipitator ash, for example, as described by Prakash and Moudgil (Proceedings of the 1994 International Non-Chlorine Bleaching Conference, Miller Freeman Inc., California 1994). However, this process requires the use of excessively large quantities of the treatment chemical, on the order of 100 to 250%
of the quantity of ash, to remove only modest amounts of chloride (less than 70%). Potassium removal was not disclosed in this paper but would be expected to be low, based on the chemistry of the system. In addition, the use of large quantities of treatment chemical also necessitates an impractical degree of material recovery to preserve economic feasibility and minimize adverse environmental impact of the polymer.
Finally, the removal of chloride and potassium ions from kraft mill precipitator ash, by either disposal of ash or by methods described in the prior art, means that valuable process chemicals may be lost, most notably the sodium chloride, which is required as a feed stock for the production of sodium chlorate, the precursor chemical for chlorine dioxide, used as a bleaching chemical in the pulp mill.
SUbIlKARY OF INVENTION
The present invention uses a two-step process to first separate and purify a majority of the sodium sulfate from kraft mill precipitator ash without the need for any additional chemicals, followed by a second-stage treatment to purify the resulting concentrated sodium chloride solution, thereby minimizing treatment volume and capital costs for the second operation.
In the present invention, chloride and potassium ions are removed from kraft mill or other pulp mill precipitator ash by dissolving the ash in water, and then selectively crystallizing sodium sulfate from the resulting aqueous solution, conveniently via evaporation of water from the aqueous solution. The sodium sulfate crystals are then separated from the mother liquor, for example, via filtration or centrifuge. The purified sodium sulfate generally is returned to the kraft mill to make up sodium and sulfur values, while chloride and potassium ions remain in solution and are purged from the system by removal of the resulting aqueous stream.
The purge stream may be further treated, such as by extractive crystallization with an appropriate solvent, to remove additional sodium sulfate from the purge stream to leave a purge stream containing sodium chloride and containing potassium ions, which can be used in the production of sodium chlorate. The remaining mother liquor may discharged, such as by sewering.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 of drawings is a flow sheet illustrating one embodiment of the present invention.
GENERAL DESCRIPTION OF INVENTION
In the present invention, precipitator ash from a kraft mill recovery boiler, typically containing sulfates, chlorides and carbonates of sodium and potassium, first is dissolved in water to form an aqueous solution thereof and then evaporated. As water is evaporated from the aqueous solution, the solubility of sodium sulfate in the aqueous medium will be exceeded and sodium sulfate then crystallizes from the aqueous solution. Sodium chloride and potassium chloride, with substantially higher solubilities, remain in solution.
However, the concentration of chloride ions in the remaining solution increases, since the sodium chloride concentration never exceeds the solubility limit in this system. Furthermore, the solubility of sulfate ions in 5 the solution is further decreased by the high concentration of sodium chloride in the solution, a phenomenon known as the "common ion effect". The process, therefore, preferentially crystallizes purified sodium sulfate from the solution, while the potassium and chloride ions remain in solution, thereby resulting in a more selective separation. The process also yields sodium sulfate crystals of appropriate and controllable size for better washing and filtrate extraction.
Sodium sulfate exhibits an inverse solubility relationship with temperature. Above a critical temperature, the amount which can be dissolved decreases as the temperature increases. Below this temperature, sodium sulfate crystallizes as the decahydrate (Na2SO9=10H20), which is undesirable for this application, because of the additional water load it would bring back to the liquor cycle. Accordingly, it is preferred to effect the crystallization of sodium sulfate at a temperature above about 30 C.
An initial high concentration of potassium in the precipitator ash may lead to the formation of glaserite (K3Na(S04)2) crystals. Glaserite co-crystallizes with sodium sulfate and the two cannot be simply separated from each other. A pulp mill in this situation must first purge precipitator catch and make up the sodium sulfate losses with potassium-free chemicals, in order to decrease the potassium concentration to an acceptable operating level which avoids the formation of glaserite.
When this operating level has been achieved, the Chloride Removal Process provided herein can then be used to remove incoming potassium on a continuous basis, preventing the concentration from building back up to problem levels. Alternatively, sufficient make-up sodium sulfate may be added to the precipitator ash to depress the concentration of potassium below the point of glaserite formation and the process of the invention effected thereon.
Organic material may also be present in the precipitator ash, especially if the mill is using a direct-contact evaporator on the recovery boiler.
Laboratory trials have shown that this organic material may also shift the solubility curves for the inorganic components of the precipitator ash. For example, when the ash contains high levels of organic material, glaserite and/or burkeite (2NaZSO4=NaZCO3) have been found to form at concentrations below those predicted by the four-component inorganic system. If sufficient carbonate is present in the ash, burkeite is formed regardless of the content of organic material. A
combined crystallization of sulfate and carbonate permits a more complete recovery of sodium values. While burkeite formation can be, at times, desirable as it recovers additional sodium, this is not the case for glaserite formation, as mentioned above.
Depending upon the individual needs of the mill and specific sodium/sulfur balance, the feedstock ash may also be pre-acidified with sulfuric acid, in effect, converting the Na2CO3 into Na2SO4, so that more of the sodium values can be recovered (along with additional sulfur) as sodium sulfate. Alternatively, sodium sesquisulfate or spent generator acid originating from the chlorine dioxide generation process may be used in the acidification of ash.
The purified sodium sulfate crystallized from the solution of precipitator ash optionally may be returned to the kraft liquor cycle, providing make-up sodium and sulfur values. Alternatively, the purified sodium sulfate may be treated electrochemically to produce sodium hydroxide and sulfuric acid which may be used in the pulp mill, for example, by bipolar membrane electrodialysis, or by using a multi-compartment electrochemical cell divided by one or more cation exchange membranes, or by using a three-compartment cell employing both an anion and a cation exchange membrane.
The purge stream resulting from crystallization of the sodium sulfate and containing the chloride and potassium ions also is saturated in sodium sulfate. This stream may then be further treated with an appropriate organic solvent, for example, methanol, which may be -either purchased or recovered from the pulping cycle, or ethanol, to further separate sodium sulfate from sodium chloride. Sodium sulfate is not only relatively insoluble in these pure solvents, but also in solvent-water mixtures, while sodium chloride remains relatively soluble. Other applicable solvents include alcohols, ethers, amines and ketones. As shown in the Examples below, the addition of a suitable solvent to the purge stream crystallizes a substantial portion of the remaining sodium sulfate. This second treatment, therefore, effectively recovers sodium chloride, which remains in the solution phase, of suitable quality for feed to, for example, a chlor-alkali or sodium chlorate plant. The purge stream may optionally be concentrated, or partially concentrated, before the addition of the solvent.
After separation of the sodium sulfate crystals resulting from the addition of organic solvent, the organic solvent may be recovered from the mother liquor for re-use, for example, by fractionation/distillation or via membrane pervaporation.
The purge stream may optionally be acidified, for example, with hydrochloric acid, to remove carbonate ions therefrom as carbon dioxide. Alternatively, the carbonate may remain with the sodium chloride stream, displacing a portion of the carbonate used in the brine demineralization process in a chlorate plant.
Any potassium which remains with the purified sodium chloride stream may be used to displace the potassium chloride used for perchiorate control in a sodium chlorate plant, as described in U.S. Patent No.
5,681,446, assigned to the assignee herein.
DESCRIPTION OF PREFERRED EMBODIMENT
One embodiment of the invention is illustrated in Figure 1. Precipitator ash from a kraft pulp mill recovery boiler is fed through line [1] and dissolved in water or condensate fed through line [2]) in a mix tank [3]. The dissolved ash is fed through line [4] to one or more evaporator/crystallizers [5], where water is evaporated as steam in line [6]. This steam may be condensed for ash dissolution (line [2]) or may alternatively be used elsewhere in the plant. A slurry of mother liquor and crystallized sodium sulfate is withdrawn from the evaporator/crystallizer through line [7] and fed to a separation device [8], for example, a vacuum drum washer or centrifuge. Washed sodium sulfate crystals are removed through line [9] for return to the plant or alternate disposal.
The filtrate from the separation device [8] is removed through line [10] and mixed with an appropriate solvent fed through line [11]) in a mix tank [12]. This combined stream is fed through line [13] to a crystallizer [14], where a further amount of purified sodium sulfate is removed through line [15], for optional washing and disposal as in line [9]. The fiitrate from the crystallizer [14] is fed through line [16] to a solvent regenerator [17], for example, fractionation, distillation, or membrane separation. The purified aqueous sodium chloride solution containing potassium ions is removed through line [18] for further use, for example, in the production of sodium chlorate or chlorine and alkali. The regenerated solvent may be recycled back to the mix tank [12] through line [19] and combined with fresh solvent, as required, from line [11] .
EXAMPLES
Example 1 This Example illustrates the prior art teaching process of separation of sodium chloride and sodium sulfate in precipitator ash.
Leaching tests were carried out to separate sodium chloride from sodium sulfate, following a method similar to that proposed in U.S. Patent No. 3,833,462. A 50%
v/v mixture of pulp mill precipitator ash was prepared using a solution of 290 g/L sodium chloride (NaCl) and 100 g/L sodium sulfate (Na2SO4), to simulate the use of recycled liquor for the leaching. The pH was adjusted to 3.2 with sulfuric acid in order to recover sodium carbonate (Na2CO3) as Na2SO4, and to improve filtration.
However, because of the small size of the undissolved ash particles, the cake was slow to filter and prone to cracking and pinholing, which impaired filtration efficiency. After washing, the resulting cake (and its entrained liquor) was found to contain a higher concentration of chloride than the original ash (3.2 wt%
NaCl, vs. 2.5 wt% in the original ash) . Additional washing may have removed more of the chloride-containing liquor, but would have also dissolved more of the sodium sulfate.
Example 2 This Example illustrates one embodiment of the process of the invention.
A continuous pilot scale evaporator-crystallizer was constructed to evaluate the process of the invention.
Precipitator ash from a pulp mill (Mill "B") was dissolved in water and the aqueous solution was evaporated to crystallize sodium sulfate therefrom.
Table 1 presents the results and mass balance from 5 a sample experimental run. It can be seen from the results presented in Table 1 that the content of sodium and sulfate in the dry crystals increases, while the chloride and potassium content decrease dramatically as compared to the original dry feed ash.
Table 1: Purification of Fly Ash via Evaporative Crystallization: Results of Pilot-Scale Trials (Mill "B") Compound Dry Feed Crystallizer Unwashed "Dry"
(wt%) Ash Filtrate Wet Crystals Crystals (calculated) Sodium (Na+) 29.9 9.6 24.3 32.3 Sulfate (SO42-) 60.3 13.6 48.4 67.2 Chloride (Cl-) 4.3 5.3 2.0 0.2 Potassium (K+) 4.4 3.2 1.2 0.1 Carbonate (C032-) 1.1 2.1 0.8 0.2 water/other --- 66.3 23.3 ---* "Dry" crystal composition is calculated assuming that the filtrate in the unwashed crystals has the same composition as the crystallizer liquor, and adjusting accordingly.
Examples 3 to 5 These Examples illustrate further processing of the mother liquor from the sodium sulfate crystallization.
Samples of the purge stream from the primary evaporation-crystallization process carried out as described in Example 2 on precipitator ash from two mills (Mill "A" and Mill "B") were combined with an appropriate amount of methanol (Tables 2 and 3) or ethanol (Table 4) to yield a total of 250 mL with the volume ratios as shown in Tables 2, 3 and 4 below. The mixtures were stirred and allowed to stand and then filtered to remove crystalline sodium sulfate. The results obtained are shown in the following Tables 2, 3 and 4:
Table 2: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "A") Volume - % Sulfate Removal Chloride Purity Methanol (molar basis) [Cl-]/[Cl-+SO-4]
(molar basis) 0% --- 34.8 %' 30% 84.4% 74.5%
40% 96.6% 92.1%
50% 99.1% 97.4%
60% 99.4% 98.9%
70% 99.7% 99.6%
* average brine solution concentration.
Table 3: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "B") Volume - % Sulfate Removal Chloride Purity Methanol (molar basis) [Cl-]/[Cl-+SO-4]
(molar basis) 0% --- 57.1%*
30% 91.5% 91.5%
40% 97. 8 % 97 .2 %
50% 99.5% 99.2%
60% 99.7% 99.7%
70% 100.0% 99.9%
80% 100.0% 100.0%
90% 100.0% 100.0%
* average brine solution concentration.
Table 4: Purification of Crystallizer Mother Liquor via Extractive Crystallization (Mill "B") Volume - % Sulfate Removal Chloride Purity Ethanol (molar basis) [Cl-]/[Cl-+SO"4]
(molar basis) 0% --- 54.0 %' 40% 96.4% 96.4%
50% 99.0% 98.9%
60% 99.4% 99.7%
70% 99.8% 99.9%
80% 100.0% 100.0%
90% 100.0% 100.0%
* average brine solution concentration.
As may be seen from the results presented in Tables 2 to 4, a very high removal efficiency of sodium sulfate may be achieved by employing the extractive crystallization and the residual aqueous medium is highly purified with respect to chloride ion content.
Examples 6 to 13 These Examples illustrate the use of additional solvents for extractive crystallization of sodium sulfate.
Samples of the purge stream from the primary evaporation-crystallization process of Example 2 were combined with various solvents as shown in Table 5 below. The mixtures were stirred and allowed to react for one hour, and then filtered.
The results obtained are shown in the following Table 5:
w a w W V
+ -R
Ll 1 Q ~+ M Q [~ O~ t~ ~D
. U cL [~ '-' 00 C ~O O v ~O M 00 V1 l~
O O O~ 01 O~ ON O\ ON 01 ON
U) D
w ~ U
U) O
U O N N
tn LL
0 > Q v c~ ~n Q ~ o b~ ~n M ~ ~
N O~ 16 1-N
w W
~
a 0 o 0 0 0 0 0 0 0 0 0 0 0 F- - 1n 4n ~ tn ~ M tn l/'I tn z aQrJ O O O O O O O O O O O O
W tn ~O ~O v) ~D v') ~D l- tn v) h ~D
J
U) w " ~ ~ = ~ a _ = a ~
> a~ ~ >' ~ =~ ~ ~
'~ a i a~ oo Q o~n ~
U) Ca W ci. Q Q 0 O
z W
a ~o r- co w As may be seen from the results presented in Table and in above Tables 2 to 4, alcohols, ethers, ketones, and amines all effectively precipitate the sulfate ions out of solution, leaving the residual aqueous medium 5 highly purified with respect to chloride ion content.
Solvents which from a bi-phasic mixture are less effective and less desirable.
SUNMARY OF DISCLOSURE
In summary of this disclosure, the present invention provides a novel procedure for providing a purge of potassium and chloride ions from a pulp mill recovery cycle without loss of valuable chemicals.
Modifications are possible within the scope of this invention.
Claims (7)
1. A method for the removal of chloride ions and potassium ions from precipitator ash from the recovery boiler of a spent pulping liquor recovery cycle of a pulp mill, which comprises:
dissolving the precipitator ash in water to form an aqueous solution, selectively crystallizing sodium sulfate from the aqueous solution to form crystalline sodium sulfate and a mother liquor, separating the crystalline sodium sulfate from the mother liquor for optional return to the recovery cycle, subjecting the mother liquor to extractive crystallization using methanol to recover further quantities of sodium sulfate, and purging the mother liquor containing potassium ions ana chloride ions.
dissolving the precipitator ash in water to form an aqueous solution, selectively crystallizing sodium sulfate from the aqueous solution to form crystalline sodium sulfate and a mother liquor, separating the crystalline sodium sulfate from the mother liquor for optional return to the recovery cycle, subjecting the mother liquor to extractive crystallization using methanol to recover further quantities of sodium sulfate, and purging the mother liquor containing potassium ions ana chloride ions.
2. The method of claim 1 wherein, following separation of the additional sodium sulfate for return to the recovery cycle, methanol is removed from the purged mother liquor for re-use to leave a purge stream containing sodium chloride.
3. The method of claim 2 wherein the purge stream containing sodium chloride is acidified to remove carbonate ions.
4. The method of claim 2 wherein the purge stream containing sodium chloride is forwarded to sodium chlorate production.
5. The method of claim 1 wherein said selective crystallization of sodium sulfate is effected by evaporating water from the aqueous solution of precipitator ash at a temperature of above 30°C to form anhydrous crystalline sodium sulfate.
6. The method of claim 1 wherein co-crystallization of glaserite during crystallization of sodium sulfate is avoided.
7. The method of claim 1 wherein said precipitator ash contains sodium carbonate and said precipitator ash is acidified with sulfuric acid, sodium sesquisulfate or spent generator acid from a chlorine dioxide generation process to convert sodium carbonate to sodium sulfate prior to said selective crystallization step.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US3027096P | 1996-11-01 | 1996-11-01 | |
| US60/030,270 | 1996-11-01 |
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| CN102432038B (en) * | 2011-10-09 | 2013-07-17 | 中国海诚工程科技股份有限公司 | Method and device for removing chlorine and potassium ions in fly ash produced by alkali recovery furnace and recovering sulfate radical |
| CN103159235B (en) * | 2011-10-09 | 2014-09-24 | 中国海诚工程科技股份有限公司 | Method for removing chlorine and potassium ions and recycling sulfate radicals in flying ash in soda recovery furnace |
| SE1451569A1 (en) * | 2014-12-17 | 2015-12-01 | Aprotech Engineering Ab | Process for production of a fertilizer comprising potassiumsulfate |
| FI129150B (en) * | 2017-09-25 | 2021-08-13 | Andritz Oy | Method of controlling the chemical balance of a pulp mill |
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