[0001] The present invention relates to an environmental-friendly process for reducing the
content of potassium ions in a liquid inventory of a chemical pulp mill.
[0002] In the production of a chemical pulp, chips of lignocellulose-containing material
are cooked in an alkaline or acid aqueous solution. This cooking liquid contains inorganic
pulping chemicals to improve the dissolution of lignin. The cooking is normally carried
out at a temperature above 100°C to reduce the residence time for the pulp produced.
Therefore, the cooking is carried out in a pressure vessel known as a digester.
[0003] In the production of sulphate pulp, soda pulp and sulphite pulp with an alkali metal
as a base, normally sodium, it is possible to recover the inorganic pulping chemicals
in the spent liquor leaving the digester. It is vital both to economy and environment
to recover these pulping chemicals to the largest possible extent. This is achieved
in the pulping chemical recovery system, which essentially transfers the used inorganic
pulping chemicals into a chemical state, where they can be re-used for cooking.
[0004] An essential part of the recovery system is the recovery boiler, where the spent
liquor is burned. Normally, make-up chemicals are added to the spent liquor before
the recovery boiler to make up for the chemicals lost during cooking and recovery.
The spent liquor is sprayed into the lower part of the boiler, previously at a relatively
low temperature to remove free water. Modern recovery boilers operate at a high temperature
to reduce the content of sulphur in the flow gases leaving the boiler. Higher up in
the boiler, gases and vapours of light hydrocarbons and decomposition products are
volatilized. This is known as pyrolysis. Then, the pyrolysis products are burned after
mixing with air or oxygen. The solid carbon-based residue which remains after complete
pyrolysis of the organics is then heterogeneously burned. The solid particles formed
are collected as a dust in precipitators at the top of the recovery boiler, to reduce
the release of solid material to the surrounding atmosphere.
[0005] A substantial and increasing problem with the pulping chemical recovery system, is
the presence of chloride and potassium in the spent liquor entering the recovery boiler.
These elements tend to reduce the capacity of the recovery boiler to produce useful
chemicals. Thus, chloride and potassium increase the stickiness of carryover deposits
and dust particles to the recovery boiler tubes, which accelerate fouling and plugging
in the upper part of the recovery boiler. Chloride also tend to increase the corrosion
rate of superheater tubes.
[0006] Chloride and potassium are concentrated in the dust formed during the combustion
of spent liquor in the recovery boiler. The dust is collected in dry-bottom or wet-bottom
electrostatic precipitators. The dust mainly consists of sodium and potassium salts,
where sulphate, carbonate and chloride are the dominant anions. The amount of dust
corresponds to from about 5 up to about 15% by weight of the sodium entering the recovery
boiler, which corresponds to from about 50 up to about 150 kg dust per tonne pulp,
if the dust is calculated as sodium sulphate.
[0007] Today, normally all of the precipitator dust collected and withdrawn from the recovery
boiler is recycled to the flow of spent liquor to be burned in the boiler. When the
concentration of chloride or potassium is too high, a portion of the precipitator
dust is withdrawn from the system and discharged or deposited.
[0008] The largest source of potassium is the wood, and the intake will depend on the wood
source generally varying from about 0,5 to 5 kg per tonne pulp. The hardwood species
usually contains larger amounts of potassium than softwood species. Besides, the content
of chloride in the spent liquor can be very high for coastal mills, if the raw material
consists of logs floated in seawater. As the environmental legislation becomes more
stringent regarding pulp mill discharges to air and water, the degree of system closure
increases. This means that even a small input of chloride and potassium becomes a
severe problem, unless the content can be controlled by purging the system in some
environmentally acceptable way.
[0009] Several methods have been proposed to overcome the problem with chloride and potassium
build-up in pulping chemical recovery systems. The use of organic ion exchangers have
been proposed as a unit operation for treatment of precipitator dust, but this has
mainly been for softening purposes, e.g. to reduce the content of multivalent metals
that would harm membranes in subsequent electrolysis of the precipitator dust. Chloride
and potassium removal are preferably carried out in a common waste water treatment.
Chloride can be removed efficiently by e.g. electrodialysis, while potassium still
is difficult to remove efficiently electrochemically.
[0010] For instance, WO-A1-9404747 discloses a process, in which the content of chloride
in a recovery system for pulping chemicals can be reduced. The process comprises collecting
precipitator dust, dissolving the dust in water to produce an aqueous solution of
precipitator dust, whereupon said aqueous solution is electrolysed in a cell for production
of chlorine or hydrochloric acid in the anolyte. Use of ion exchange is suggested
as a pretreatment before the electrolyses, chiefly to remove divalent ions such as
Ca
2+ and Mg
2+.
[0011] Caron J. R. et al, "Metals management in a closed kraft mill bleach plant", Pulping
Conference, TAPPI (1995), p. 1155-1160, have investigated metals removal from recycled
chlorine dioxide bleach plant filtrate with ion exchange resins.
[0012] The present invention relates to a process by which the content of potassium ions
in a recovery system for pulping chemicals can be reduced. The process comprises bringing
spent liquor to a recovery boiler, burning said spent liquor, collecting precipitator
dust formed, forming a solution by dissolving the precipitator dust in a liquid, where
the solution of precipitator dust is subjected to a treatment with an inorganic ion
exchange material in order to remove at least a part of the potassium therein.
[0013] An advantage of the present process is the possibility to reduce the content of potassium
in the liquid inventory and more particularly in the spent liquor entering the recovery
boiler. By the present process, the problem of sticky deposits in the recovery boiler
can be substantially reduced. This means an improved energy efficiency as well as
a higher degree of recovery of the pulping chemicals.
[0014] The present invention can be used in the production of a chemical pulp and especially
for production of a sulphate pulp, soda pulp or sulphite pulp with an alkali metal
as base. A kraft pulp is a special type of sulphate pulp, where the pulp is under-cooked
to produce a dark-coloured pulp of exceptional strength. The present invention can
also be used in the production of sulphate, soda or sulphite pulps with an alkali
metal as base, where the cooking processes have been modified, combined or extended
compared to the normal pulping techniques. Suitably, the present process is applied
where the recovery system for pulping chemicals is a kraft recovery system.
[0015] A liquid inventory is the total quantity of various liquids in a mill, with varying
contents of active or activatable cooking liquid components. The liquid inventory
of a sulphate mill, mainly consists of white liquor, black liquor, green liquor and
spent liquor entering the recovery boiler. The spent liquor to be burned in the present
process, is a used cooking liquid withdrawn from a digester, optionally with added
make-up chemicals. Potassium and sodium are alkali metals present in the spent liquors.
[0016] The amount of precipitator dust formed depends mainly on the temperature in the boiler,
the ratio between sodium and sulphur in the spent liquor and the raw material and
sulphidity of the cooking process. A high temperature in the lower part of the boiler
to reduce the sulphur content in the flow gases, increases the amount of dust formed.
[0017] With the present process, all or a portion of the precipitator dust collected and
withdrawn from the recovery system is treated with an inorganic ion exchange material.
[0018] The composition of precipitator dust formed in recovery boilers vary considerably
depending on type and origin of wood, cooking process, purity of make-up chemicals,
temperature in the boiler, type of precipitator etc. However, irrespective of these
factors the dust mainly consists of sodium and potassium salts, where sulphate, carbonate
and chloride are the dominant anions. A typical composition of precipitator dust from
a kraft recovery system is Na
2SO
4 80-85% by weight, Na
2CO
3 2-8% by weight, NaCl 2-8% by weight, NaHSO
4 + Na
2S
2O
7 < 2% by weight, K
2SO
4 5-10% by weight, K
2CO
3 0.5-1% by weight, KCl < 1% by weight, metal ions such as Ca, Fe, Mg, P, Si, Mn, Zn,
Mo < 1% by weight and organic material < 1% by weight.
[0019] Natural as well as synthetic inorganic ion exchange material can be used. Suitable
inorganic ion exchange materials are aluminosilicates, hydrous oxides, acid salts
of polyvalent metals or salts of heteropolyacids. Preferably use is made of aluminosilicates
such as zeolites, and most preferably zeolites are used as ion exchange material.
[0020] Zeolites are inorganic crystalline compounds mainly consisting of SiO
2 and Al
2O
3 in tetrahedral co-ordination. In the present invention, zeolites also relate to other
crystalline compounds of zeolite structure, such as aluminium phosphates. Such crystalline
compounds of zeolite structure which can be used in the present invention are defined
in W.M. Meier et al, Atlas of zeolite structure types, sec. ed., Butterworths, London,
1987, which is hereby incorporated by reference in the present application. Many zeolites
occur naturally, but most commercially used zeolites are synthetically produced. These
zeolites function as adsorbents or molecular sieves and may, depending on the size
of the cavities and the nature of the zeolite surface, be used to increase or decrease
the taking-up of specific chemical compounds. In the present invention, a very essential
property of the zeolites is their selectivity towards potassium relative to sodium.
[0021] A suitable zeolite can be selected from the group consisting of mordenite, chabazite,
clinoptilolite, zeolite A and zeolite Y. Preferably use is made of mordenite. A wide
variety of zeolites are available on the market. For instance, Wessalith P is a A-zeolite
manufactured by Degussa. Zeolite Y EY250, Zeolite Y N3S, Zeolite BMH and Zeolite Sodium
Mordenite EM120 is manufactured by Eka Chemicals AB.
[0022] Example of hydrous oxides which can be used are hydrous titanium oxide, iron hydroxide,
hydrous stannic oxide, hydrous zirconium oxide, silica gel etc. Crystalline antimonic
acid is known to exhibit the following selectivity series Li>K>Rb>Na.
[0023] Suitable acid salts of polyvalent metals used as ion exchange material may be zirconium
phosphate crystal or titanium phosphate crystals.
[0024] The amount of inorganic ion exchange material used may vary within wide limits. Thus,
the amount of inorganic ion exchange material used may be up to 1 tonne/tonne of dry
precipitator dust and e.g. lie in the range of from about 1 kg/tonne up to about 1000
kg/tonne of dry precipitator dust, suitably in the range of from about 10 kg/tonne
up to about 1000 kg/tonne of dry precipitator dust and preferably in the range of
from about 100 kg/tonne up to about 500 kg/tonne of dry precipitator dust. Suitably,
the amount of inorganic ion exchange material used is based upon the ion exchange
capacity of the actual inorganic material and the amount of potassium in the precipitator
dust. The inorganic ion exchange material is preferably used in excess of the stoicheometric
amount of potassium.
[0025] The potassium and chloride containing solution is treated with an inorganic ion exchange
material. The ion exchanged material, enriched on potassium, is then preferably filtered
and washed with water, whereafter the spent ion exchanger material can be deposited
in a land fill. The solution depleted on potassium and chloride, can be recycled in
the pulp mill or forwarded to another step in the treatment of precipitator dust for
mixing or dilution. The separation and washing can preferably take place in a centrifuge,
a filter press or a vacuum filter.
[0026] Alternatively, after separating the ion exchanged slurry from the solution, it can
be regenerated with a sodium rich aqueous solution. This could for example be a solution
of the Na
2SO
4 salt cake obtained from a chlorine dioxide generator (or from a crystallisator, or
a chloride concentrate from an electrochemical cell). Separation, washing, regeneration
and dewatering then may take place in the same piece of equipment operating in a continuos
mode. Even the ion exchange may take place in the separation equipment. This can be
done batchwise in a filter press or continuos eg. in a centrifuge or a rotating filter
of vacuum or pressure type.
[0027] In an embodiment of the present invention, the inorganic ion exchange material is
made in the form of granules or pellets and are placed as a fixed bed in a column,
thus forming an ion exchanger. The precipitator dust solution, rich on potassium,
is forced to flow through the bed of ion exchange material to which the potassium
is adsorbed. The reaction zone proceeds down the column as the upper layers of ion
exchange material reaches equilibrium with the solution. At the end of the work cycle
when the ion exchanger becomes exhausted, the ion exchange material is backwashed
(regenerated) with a sodium electrolyte. The flow of potassium rich solution is simultaneously
switched to another column. A number of columns can be arranged in parallel so that
the continuos operation is ensured. The advantage with this embodiment, columns arranged
in parallel, over the previously described is that the operation is continuous and
that a more efficient ion exchange can be achieved as multiple equilibrium stages
are obtained (cf. McKabe-Thiele diagram).
[0028] The amount potassium removed in a precipitator dust solution subjected to a treatment
with an inorganic ion exchange material, can be above about 40 %, suitably above about
50 %, preferably above about 60 % and most preferred above about 70 %.
[0029] A potassium free or potassium depleted stream which has undergone the inorganic ion
exchange, is preferably recycled to the weak black liquor or may, in the cases where
the water balance allows, be mixed with the strong black liquor and fed directly into
the recovery boiler in the recovery system.
[0030] A solution of precipitator dust will also commonly have a pH between about 7 and
about 11, within which range most ion exchange material are stable and thus preferred.
However, the pH is not critical since many ion exchange material work satisfactory
outside the pH range 7-11.
[0031] The ion exchange is suitably performed in the range from above 0°C up to about 100°C
and preferably from about 20°C up to about 60°C.
[0032] The residence time for the suitable batchwise ion exchange is preferably at least
about 1 minute. Suitably the residence time is at least about 1 hour, preferably at
least about 2 hours and most preferred at least about 5 hours. The upper residence
time is not critical, but have to be set by process-technical reasons. The flow rate
for the suitable continuous operation of the ion exchange is suitably at least from
about 0.1 up to 20 BV/h (Bed Volume per hour), preferably from about 1 up to 10 BV/h
and most preferred from about 2 up to 6 BV/h. The ion exchange is preferably carried
out by a continuous operation.
[0033] In one embodiment of the invention the potassium concentration of the solution is
increased by leaching the precipitator dust with a liquor. The added liquid may comprise
of water, or water solutions of sulphate or carbonate. Added sulphate may be alkali
metal, preferably sodium sulphate, suitably at least a part derives from a recirculated
solution, depleted of chloride and potassium, for instance from a suitable electrochemical
treatment or recrystallisation. If water is added, it can be either fresh water or
purified process water. The potassium enriched leach solution is separated from the
solid phase of the leached precipitator dust, by e.g. filtration, centrifugation,
sedimentation etc..
[0034] The concentrate in the slurry obtained from the leaching step may comprise from about
1 g/l up to about 60 g/l potassium, and will be saturated with sulphate.
[0035] The solution depleted of potassium obtained from the ion exchange may comprise from
0 g/l up to about 60 g/l potassium (counted as e.g. K
+, K
2SO
4, KCl).
[0036] The inorganic ion exchange according to the present invention is preferably combined
with another process. This could for example be a process for leaching and electrodialysis
of precipitator dust. The ion exchange can also be combined with processes where precipitator
dust is split in electrolysis cell or a electrodialysis cell with bipolar membranes.
The advantage is, especially when removing the potassium prior to electrolysis, that
NaOH with little or no potassium impurities can be prepared.
[0037] The inorganic ion exchange according to the present invention can also be combined
with a process where sodium sulphate is re-crystallised from the precipitator dust.
In another embodiment, the inorganic ion exchange according to the present invention
can be advantageously combined with a process where chloride ions are ion exchanged
from the precipitator dust (for instance the Precipitator Dust Purification System,
PDP™).
[0038] Another possible application is to use the inorganic ion exchange according to the
present invention on bleach filtrates recovered back to the chemical recovery.
[0039] Embodiments of the process of the present invention will now be described in more
detail with reference to figures. Figure 1 shows a schematic description of the use
of an inorganic ion exchanger in a process for treatment of precipitator dust applying
a combination of leaching and electrodialysis treatments. Figure 2 shows the application
of an inorganic ion exchanger in yet another embodiment in combination with a recrystallisation
of sodium sulphate from precipitator dust.
[0040] In the embodiment of fig. 1, precipitator dust (1) can be mixed with a near saturated
solution of sodium sulphate (2) in a leach tank (3) where the mixed solution can be
subjected to a leaching treatment. The chloride and potassium compounds in the precipitator
dust are leached out and the potassium and chloride enriched solution (31) is brought
to an ion exchange column (8). A slurry (4) is separated on a filter (5), a centrifuge
(not shown) or the similar. The filter cake (6), virtually free of chloride and potassium,
is recovered to the strong black liquor. The filtrate (7) is recycled (71) to the
leach tank (3) or forwarded (72) to the ion exchange column (8) where potassium ions
are removed (9), and the potassium depleted filtrate, still containing a high concentration
of chloride ions (10), can be further brought to the diluate chamber(s) (13) of an
electrodialysis stack (11) holding mono-anion selective membranes (12). When an electric
field is applied to the electrodialysis stack the chloride ions are removed by migration
through the anion selective membranes to the concentrate (brine) chamber(s) (14).
The remaining chloride and potassium depleted sodium sulphate solution (2) can be
recycled for use as leachant in the first step (3). The electrodialysis stack (11)
may also comprise cation selective membranes through which cations such as sodium
and potassium are removed by migration. The concentrated sodium chloride solution
prepared in the electrodialysis step (15) can be recirculated back to the concentrate
chamber(s) of the cell, but a part may be purged and used for regenerating (16) the
ion exchange material. In addition, water can be added to dilute the concentrated
chloride solution (17).
[0041] The advantage with implementing potassium ion exchange in the process described in
fig. 1 is obvious. Chloride and sulphate ions can efficiently be separated in the
electrodialyser as the divalent sulphate ions will be repelled by the mono-anion selective
membranes. Unfortunately, cation exchange membranes cannot exhibit a similar selectivity
for potassium versus sodium. Consequently, only a minor part of the potassium will
be removed in the electrodialysis step and without any selective potassium removal
the concentration of potassium will increase in the solution until the solubility
limit for glacerite (NaK
3(SO
4)
2) is reached. As a consequence, the main part of the potassium will follow the cake
resulting in a low removal efficiency for potassium. With the ion exchange the potassium
concentration can be kept below solubility and potassium removal efficiency is improved.
[0042] In fig.2, an embodiment is shown where an ion exchanger is combined with a process
where sodium sulphate is re-crystallised from the precipitator dust. Precipitator
dust (21) can be mixed with water (22) (preferably condensate from the crystallisator
(28)) in a dissolving tank (23). The dissolved solution (24) is forwarded to an ion
exchange column (25), where potassium, are removed (26) and the potassium depleted
filtrate (27), may further be brought to a crystallisator (28) where sodium sulphate
is re-crystallised from the precipitator dust. The solution (29) containing mother
liquor and sodium sulphate crystals is brought to a separation step with conventional
technique, for instance a vacuum drum filter (30) and an addition of water (31), where
the sodium sulphate crystals (32) (Na
2SO
4) is separated from the mother liquor (33). Condensed water (22) from the re-crystallisation
(28) may be recycled to the dissolving step (23). The sodium sulphate crystals can
be recycled to the strong black liquor, alternatively can a small portion of the crystals
be dissolved in water and used for regeneration of the ion exchanger. The mother liquor
is preferably discharged.
[0043] The process shown in fig.2, purges a stream of the mother liquor to control the potassium
and chloride levels. As described in connection to fig 1. above, the limiting factor
is the solubility of glacerite (NaK
3(SO
4)
2). When the potassium level is kept down, either by ion exchanging the feed solution
or a small side stream of mother liquor, the chloride levels in the mother liquor
can be increased and the purge flow can be reduced. In other words less sodium and
sulphate are lost with the purge. The concentration of potassium in the precipitator
dust solution may vary between 0 and about 35 g/l. After ion exchange, the solution
fed to the crystallizator may have a potassium concentration down to 0 g/l.
[0044] The invention and its advantages are illustrated in more detail by the following
examples which, however, are only intended to illustrate the invention and not to
limit the same. The percentages and parts used in the description, claims and examples,
refer to percentages by weight and parts by weight, unless otherwise specified.
Example 1
[0045] A 20 wt% ash solution was prepared from a kraft mill precipitator dust, with a content
in the ash of 32 wt% Na, 42 wt% SO
4, 3.0 wt% K, 3.4 wt% Cl, 16 wt% CO
3 and impurities < 4 wt%. The pH of the solution was adjusted by 50% H
2SO
4 to pH 7 or 9 and filtered through an OOH filter paper. After that the solution contained
about 7.5 g/l potassium. The ash solution was mixed with the zeolite and left for
stirring for at least 10 minutes. The samples that were run at 50°C the solution was
heated and stirred at the same time. When the samples reached 50°C they were stirred
for 5 minutes more. For filtration of the solution, a Millipore equipment and a nylon
filter paper with a pore size of 0.45 µm were used. The zeolites used in the examples
were of two different types. The Wessalith P is a zeolite manufactured by Degussa.
Sodium Mordenite EM 120 manufactured by Eka Chemicals in Bohus, Sweden. The tested
zeolites were powder form. Analyses of potassium content were done on the ash, the
start solution and on the filtrate. The experiments were run at pH 7, and at room
temperature with different amounts of zeolites. In tables I and II below the results
of the removal of potassium in % for each are shown. The amount of zeolite/100 ml
ash solution has been recalculated as dry zeolite. The removal of potassium is calculated
on the concentration in the start solution and the filtrate as follows:
![](https://data.epo.org/publication-server/image?imagePath=1998/28/DOC/EPNWA1/EP97203861NWA1/imgb0001)
[0046] Table I shows the results from the experiments with Wessalith P.
Table I
Test: |
1 |
2 |
3 |
4 |
5 |
Amount zeolite [g]/100 ml solution |
3,9 |
5,8 |
7,8 |
31 |
38,8 |
K in start solution, [g/l] |
7,2 |
7,2 |
7,2 |
7,4 |
7,4 |
K after treatment, [g/l] |
6,5 |
6,1 |
5,9 |
3,9 |
3,7 |
Removal of K+ [%] |
10% |
15% |
18% |
47% |
50% |
[0047] As evident from table I, the removal of potassium with Wessalith P was efficient
and increases with increasing addition of the zeolite.
[0048] Table II shows the results from the experiments with Zeolite Sodium Mordenite EM120.
Table II
test: |
1 |
2 |
3 |
4 |
5 |
6 |
Amount zeolite [g]/100 ml solution |
3,9 |
7,8 |
15,5 |
23,3 |
31 |
38,8 |
K in start solution, [g/l] |
7,7 |
7,7 |
7,7 |
7,2 |
7,2 |
7,2 |
K after treatment, [g/l] |
6,2 |
6,0 |
4,3 |
3,3 |
2,8 |
2,4 |
Removal of K+ [%] |
19% |
22% |
44% |
54% |
61% |
67% |
[0049] As shown in table II, the removal of potassium with Zeolite Sodium Mordenite EM120
was further improved, resulting in about 15 % higher removal rate than with the Wessalith
P zeolite.
Example 2
[0050] A 20 wt% ash solution was prepared from the precipitator dust used in example 1.
The pH of the solution was adjusted by 50% H
2SO
4 to pH 8. After that the solution contained about 7.7 g/l potassium. The ash solution
was mixed with the zeolite and left for stirring for 5 hours at a temperature of 80°C.
The zeolite used was Sodium Mordenite EM120 manufactured by Eka Chemicals in Bohus,
Sweden. The zeolite was in powder form. 100ml solution/20g zeolite was used in this
test. The start concentration of potassium is 7,7g/l. Analyses of potassium content
and the calculation of amount potassium removed were done in accordance with example
1. Samples were taken once every hour from the solution with the zeolite. The stirrer
was turned off and sample was taken from the clear phase. In table III below the results
of the removal of potassium in % for each hour is shown.
Table III
Time [h]: |
1 |
2 |
3 |
4 |
5 |
Removal of K+ [%]: |
32.5 |
33.8 |
35.1 |
36.4 |
40.3 |
[0051] As evident from table III, the removal of potassium with Zeolite Sodium Mordenite
EM120 increased with time.
Example 3
[0052] For comparative reasons, Amberlite IRC-718 (a cationic exchange resin) manufactured
by Rohm and Haas was used in an experiment. Before start the resin was regenerated
as follows:
1. 300ml Amberlite was added to 1200ml 4% HCl and stirred for 30min.
2. Wash with water (Filter paper and funnel).
3. The Amberlite was added to 3000ml 4% NaOH and stirred for 30min.
4. Wash with water (water and Amberlite were mixed and stirred for some minutes and
wash of the Amberlite as in 2, this was repeated several times). pH 10,9 after 3,5
hour.
[0053] A 20 wt% ash solution was prepared from the precipitator dust used in example 1.
The pH of the solution was adjusted by 50% H
2SO
4 to pH 10. The solution contained after that about 7,5 g/l potassium. The ash solution
was mixed with the wet ion exchanger and stirred for 30min at 40 °C. After that the
solution was filtered. Analyses of potassium content and the calculation of amount
potassium removed were done in accordance with example 1. In table IV below the results
of the removal of potassium in % for each are shown. The amount of zeolite/100 ml
ash solution has been recalculated as dry zeolite.
Table IV
Test |
1 |
2 |
3 |
Amount zeolite g / 100 ml solution |
25 |
37,5 |
50 |
K in start solution [g/l] |
7,5 |
7,5 |
7,5 |
K after treatment [g/l] |
6,0 |
5,5 |
5,1 |
Removal of K [%] |
20 |
26 |
32 |
[0054] As evident from table IV, the removal of potassium with Amberlite IRC-718 is poor,
compared to the inorganic ion exchange material used in examples 1 and 2.
Example 4
[0055] In this experiment a continuous ion exchange was carried out. The equipment used
were a glass column, a conduit for continuously separating a bleed from the top of
the column to a balance, a balance with the feed solution in connection to a pump
for continuously feeding the solution from the bottom of the column and a pump control
unit. In each trial the column was packed with fresh zeolite granules. Potassium were
analysed by AAS (Atomic Absorption Spectra). In trials no 1-3 a real precipitator
dust solution were used. A 5 wt % solution was prepared from Weyerhaeuser precipitator
dust (see example 1). The pH was adjusted by 50 % H
2SO
4 to pH of 7 and the solution was filtered through a 00H filter paper. A new solution
was prepared in each trial. Table V shows the precipitator dust solution content.
Table V
K g/l |
Na g/l |
Cl g/l |
SO4 g/l |
pH |
1,5 |
∼16 |
∼1,7 |
∼21 |
7 |
[0056] The used regeneration solutions has been 3 M NaAc and 3M NH
4 Cl. The variables can be seen in table VI. Re
x = regeneration. IE
x = ion exchange. BV=Bed Volume. Trial no.1 is Wessalith MS 330, no.2 is Ammonium Mordenite
and no.3 is Sodium mordenite.
Table VI
Trial No. |
Amount Zeolite [g] |
Flow [BV/ h] |
IE time [min] |
Temp [° C] |
Performed series |
Regeneration (Re), Solution, time |
1 |
30.1 |
18 |
100 |
25 |
Re0 IE1 Re1 IE2 |
NH4Cl 90 minutes |
2 |
27 |
18 |
100 |
25 |
Re0 IE1 Re1 IE2 |
NaAc 90 minutes |
3 |
31 |
18 |
100 |
25 |
Re0 IE1 Re1 IE2 |
NH4Cl 90 minutes |
[0057] In table VII below, the continuous ion exchange on precipitator dust solution is
shown.
Table VII
Zeolite type / trial No |
IE-stage |
Rem % K 20 minutes |
Rem % K 50 minutes |
Rem % K 70 minutes |
Rem % K 100 minutes |
Wessalith MS 330 / trial no.1 |
IE1 |
41,1 |
26,9 |
22,5 |
18,2 |
|
IE2 |
39,9 |
26,7 |
21,9 |
17,7 |
Ammonium Mordenite EM 032 / trial no. 2 |
IE1 |
50,4 |
35,8 |
30,9 |
25,7 |
|
IE2 |
45,1 |
33,2 |
28,4 |
23,0 |
Sodium mordenite EM 120 / trial no.3 |
IE1 |
65,2 |
52,8 |
44,5 |
36,2 |
|
IE2 |
76,5 |
44,3 |
37,2 |
29,6 |
[0058] As evident from table VII above, it is possible to achieve a high removal of potassium
from a precipitator dust solution when using a continuous operation. The results from
these trials also show that the zeolites can be regenerated and used again.
1. A process for reducing the content of potassium ions in a recovery system for pulping
chemicals by bringing spent liquor to a recovery boiler, burning said spent liquor,
collecting precipitator dust formed, forming a solution by dissolving the precipitator
dust in a liquid, characterised in that the solution of precipitator dust is subjected to a treatment with an inorganic
ion exchange material (8;25) in order to remove at least a part of the potassium therein.
2. A process according to claim 1, characterised in that the inorganic ion exchange material contain an aluminosilicate.
3. A process according to claim 1 or 2, characterised in that the inorganic ion exchange material contain a zeolite.
4. A process according to claim 3, characterised in that said zeolite is selected from the group consisting of mordenite, chabazite,
clinoptilolite, zeolite A and zeolite Y.
5. A process according to any of the preceding claims, characterised in that the treatment is carried out batchwise, with a residence time of at least
about 2 hours.
6. A process according to any of claim 1-4, characterised in that the treatment is carried out in a continuous operation, with a flow rate
from about 1 up to 10 BV/h.
7. A process according to claim 1, characterised in that the inorganic ion exchange material is used in a process for leaching (3)
and electrodialysis (11).
8. A process according to claim 1, characterised in that the inorganic ion exchange material is used in a process for re-crystallisation
(28) of sodium sulphate from a chloride and potassium enriched mother liquor.
9. A process according to claim 1, characterised in that the inorganic ion exchange material is regenerated with an internal chloride
concentrated solution (16) received in the process for removal of potassium and chloride
ions from precipitator dust.
10. A process according to claim 1, characterised in that the amount potassium removed from said precipitator dust solution, subjected
to the treatment with said inorganic ion exchange material, is above about 50 %.