Note: Descriptions are shown in the official language in which they were submitted.
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E457
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,
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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
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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
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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.
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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
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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
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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,
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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
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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.
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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.
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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.
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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:
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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.
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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.
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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.
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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:
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U)
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U)
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tn LL
0 > Q v c~ ~n Q ~ o b~ ~n M ~ ~
N O~ 16 1-
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w
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~
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F- - 1n 4n ~ tn ~ M tn l/'I tn
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'~ a i a~ oo Q o~n ~
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a ~o r- co
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CA 02219550 1997-10-29
18
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.
