Leaching process

An increasing problem with the pulping chemical recovery system, is the presence of chloride and potassium in the recovery boiler. Chloride and potassium increase inter alia the stickiness of carryover deposits and dust particles to the recovery boiler tubes, which accelerate fouling, corrosion and plugging of the recovery boiler. As the environmental legislation becomes more stringent, the degree of system closure increases. The present invention relates to a process by which the collected precipitator dust is leached, at a temperature exceeding 50.degree. C., for a residence time sufficient to get a chloride and potassium enriched leach solution and to remove at least a part of the content of metal ions in a solid phase. Said leach solution is electrochemically treated, preferably in an electrodialysis cell, in order to remove at least a part of the chloride and potassium therein. 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 and bleaching chemicals.

The present invention relates to an environmentally friendly process for 
reducing the content of chloride and metal ions in a liquid inventory of a 
chemical pulp mill. 
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.degree. C. to reduce the residence time for the pulp produced. 
Therefore, the cooking is carried out in a pressure vessel known as a 
digester. 
BACKGROUND OF THE INVENTION 
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. 
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. 
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. 
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 about 
5 to about 15% by weight of the sodium entering the recovery boiler, which 
corresponds to about 50 to about 150 kg dust per ton pulp, if the dust is 
calculated as sodium sulphate. 
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. 
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. The 
content is moderate in mills using caustic make-up contaminated with 
sodium chloride or in mills that at least partially recover spent bleach 
liquids from stages using chlorine-containing bleaching agents. 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 becomes a severe problem, unless 
the content can be controlled by purging the system in some 
environmentally acceptable way. 
A further problem in the chemical recovery system, in the treatment of 
spent liquors and recirculation of the purified process liquids, is the 
content of metal ions. In the treatment of the spent liquors, especially 
when using electrochemical methods, the metals are harmful. Metal ions 
such as calcium (Ca) and magnesium (Mg) may precipitate on the membranes 
and cause damage on the membranes. Ca and Mg may also form sparingly 
soluble salts which are clogging the compartments of the cell, thus 
leading to an interruption in the production due to restoration of the 
cells. 
Several methods have been proposed to overcome the problem with chloride 
and potassium build-up in pulping chemical recovery systems. One example 
is evaporation of cooking liquid to recrystallize sodium chloride and 
potassium chloride. Also known is leaching of precipitator dust and 
discarding the leach liquid rich in chloride. 
According to Tran et al., Pulp Paper Canada 91(5): T185-T190 (1990), the 
easiest and most effective way to control chloride, as well as potassium, 
in the chemical recovery cycle today, is by directly discarding the 
precipitator dust. Therefore, still the most commonly used method is 
removal of part of the precipitator dust from the system, and subsequent 
deposition on land or discharge to water. However, this will not only be 
environmentally unacceptable, but also result in a loss of valuable 
cooking chemicals. 
U.S. Pat. No. 5,352,332 discloses a process for recycling bleach plant 
filtrate. Precipitator dust is collected and treated by leaching with 
water or by evaporation crystallization from a water solution. The thus 
formed salt solution is discharged to sewer or recovered for its chlorine 
value. 
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. 
JP-A-55022051 discloses a process for reduction of chloride where 
precipitator dust is washed with a Glauber's salt solution, whereafter a 
part of the washing solution is treated by electrodialysis to remove 
chloride. 
CA 1059271 discloses a process for reduction of chloride in a pulp mill 
recovery system. Precipitator dust is leached with hot water at a 
temperature of 60-100.degree. C. Chloride is precipitated from the leached 
solution by cooling crystallization. Solid sulphate is recycled to the 
black liquor. Acid (sulphuric acid) is added in the leaching to lower the 
pH in order to precipitate sulphate.

SUMMARY OF THE INVENTION 
The present invention relates to a process by which the content of 
chloride, potassium and other metal 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 optionally together with 
make-up chemicals, collecting precipitator dust formed, leaching the 
precipitator dust with a leaching liquid at a temperature exceeding 
50.degree. C., for a residence time sufficient to form a chloride and 
potassium enriched leach solution and to remove at least a part of the 
content of metal ions in a solid phase. The thus formed solid phase, 
comprising inter alia metal compounds and organic material, is separated 
from the chlorine and potassium enriched leach solution, whereupon said 
leach solution is electrochemically treated, preferably in an 
electrodialysis cell in order to remove at least a part of the chloride 
and potassium therein. 
BACKGROUND OF THE INVENTION 
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. 
Another 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. 
A further advantage is the reduction of metal ions in the recirculation 
liquid, which is important when using electrolysis in the treatment of 
waste liquids. 
The process is energy efficient, has low investment costs and offers a 
possibility to remove chloride, potassium and metal ions, with a minimum 
loss of valuable substances like sodium and sulphate. In the 
electrochemical embodiment, the cells can be operated at very high current 
densities, which result in low investment cost for cells and membranes. 
By the present process, chloride may be removed from the precipitator dust 
by leaching with a saturated, or near saturated aqueous sulphate solution. 
Similar result might be possible to reach by leaching with water, but with 
higher loss of precipitator dust. 
Potassium and sodium are alkali metals present in the spent liquors. 
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 containing an 
alkali metal is a sulphate recovery system. Preferably, the recovery 
system for pulping chemicals containing an alkali metal, is a kraft 
recovery system. 
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. 
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. 
With the present process, all or a portion of the precipitator dust 
collected and withdrawn from the recovery system is leached with a 
leaching liquid and treated electrochemically. The proportion between the 
amount of dust electrochemically treated and recycled directly to the flow 
of spent liquor, can be chosen with respect to the initial content of 
chloride and potassium ions in the dust. 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.sub.2 
SO.sub.4 80-85% by weight, Na.sub.2 CO.sub.3 2-8% by weight, NaCl 2-8% by 
weight, NaHSO.sub.4 +Na.sub.2 S.sub.2 O.sub.7 &lt;2% by weight, K.sub.2 
SO.sub.4 5-10% by weight, K.sub.2 CO.sub.3 0.5-1% by weight, KCl&lt;1% by 
weight, metal ions such as Ca, Fe, Mg, P, Si, Mn, Zn, Mo&lt;1% by weight and 
organic material&lt;1% by weight. 
The leaching should be performed at a temperature exceeding 50.degree. C., 
in order to reach a maximum amount of potassium chloride in the leach 
solution and a minimum amount in the separated solid phase. Below 
50.degree. C. the content of potassium chloride in the leach solution will 
be poor, and most of the potassium will remain in the solid phase, which 
is unfavourable. The upper temperature is limited by practical reasons. 
There is generally no advantages of performing the leaching above 
100.degree. C. The leaching is preferably performed in the range from 
above 50.degree. C. up to about 90.degree. C., suitable from about 
60.degree. C. up to about 80.degree. C., and most preferably from about 
65.degree. C. up to about 75.degree. C. 
The residence time of the leaching is preferably at least about 1 minute. 
The upper residence time is not critical, but have to be set by 
process-technical reasons. However, any improved leaching results have not 
been observed exceeding about 1080 minutes. The residence time is 
preferably in the range from about 5 minutes up to about 1080 minutes, 
suitably from about 5 minutes up to about 180 minutes. 
The chloride and potassium enriched leach solution is separated from the 
solid phase of the leached precipitator dust, by e.g. filtration, 
centrifugation, sedimentation etc. The leach solution can be filtered 
before the electrochemical treatment to remove undissolved, precipitated 
or flocculated compounds. By this preferred filtering, especially the 
content of calcium is reduced, but also the content of phosphate, 
aluminium and silicon are reduced to a considerable extent. In filtering 
the solution, mainly flocculated organic compounds and precipitated 
inorganic compounds are removed. The filter can be of any conventional 
type, e.g. a drum, belt or table filter with or without vacuum being 
applied. 
According to a preferred embodiment, the separated solid phase can be 
further treated, e.g. by filtering and addition of water, in order to get 
a second solid phase comprising mainly of metal compounds, metal ions, 
organics, sodium sulphate and carbonate. The filtrate, mainly water, 
separated from the second solid phase may be recirculated to the leaching 
step. The thus formed second solid phase can be further treated in order 
to produce acid and alkali, and to separate compounds of silicon, 
phosphate, metal ions and other harmful compounds for the process. The 
thus separated compounds can be deposited, recycled or reused e.g. for the 
production of metals. The remaining solid phase is preferably added to the 
black liquor, and subsequently incinerated in the recovery boiler. 
Inorganic or flocculated organic impurities, are suitably precipitated and 
separated as solid phase in the leaching step. Organic material comprises 
residues of e.g. lignin, resin and fibres. Calcium, magnesium, silicon, 
phosphate, aluminium, iron and manganese are the most important examples 
of sparingly soluble inorganic impurities present as cations in the 
solution. The content in the leach solution of these cations can be 
reduced down to an acceptable level by raising the pH sufficiently, at 
which inorganic compounds remain in the solid phase, mainly metal 
hydroxides such as MgOH.sub.2 and also carbonate, e.g. CaCO.sub.3. 
The pH in the leaching step can be in the range from about 6 up to about 
14, suitably from about 7 up to about 12 and preferably from about 10 up 
to about 12. The pH can be adjusted by adding sodium hydroxide. Below a pH 
about 6, CO.sub.2 will be formed, inter alia from the carbonate. 
The added leaching 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 from a recirculated and depleted 
chloride solution from the electrochemical treatment. If water is added, 
it can be either fresh water or purified process water. 
Calcium is detrimental to the cells, in the preferred electrochemical cell 
treatment. Carbonate may also be added to the leaching, especially if the 
carbonate content in the precipitator dust is low or zero, in order to 
precipitate metal ions, preferably calcium. Carbonate may also be added to 
the dust, prior to the leaching, or to the recycled chloride depleted 
solution. The added amount of carbonate depends on the precipitator dust 
composition and additional carbonate is added to reach a total amount of 
carbonate. There is generally no advantages exceeding a total amount of 
10% by weight of carbonate. The amount of carbonate added is preferably in 
the range from about 0 up to about 10% by weight leaching solution 
content, suitably from about 2 up to about 10% by weight, most preferably 
from about 4% up to about 10% by weight. Carbonate is preferably added in 
solid form as sodium carbonate. 
When a water solution of sulphate is added as a leaching liquid, sodium 
sulphate may be at least partially precipitated and separated in the solid 
phase, along with the separation of metals and organics. 
After the leaching step, the chloride enriched leach solution is 
dechlorinated by a electrochemical treatment. 
According to an embodiment, a nanofiltration treatment can be carried out 
by filtering the leach solution before the electrochemical treatment, at 
high pressure, through a filter, which is more selective for monovalent 
ions such as Cl.sup.- and K.sup.+, than larger ions e.g. sulphate 
(divalent). The filters are preferably negatively charged in order to 
repel e.g. sulphate ions. Thus, a chloride and potassium enriched 
concentrate is separated from a sulphate concentrate, depleted of 
chloride, and further brought to the electrochemical treatment. The 
concentrated sulphate solution may be recycled to the leaching step. A 
nanofiltration treatment is also possible, for further purification, on 
the diluate or on the concentrate from the electrochemical treatment. 
The electrochemical treatment is preferably carried out by electrodialysis 
by transferring the chloride ions over an anion selective membrane by 
applying an electrical current perpendicular to the membrane surface. 
Dissolved cations are transferred in the opposite direction over a cation 
selective membrane. A large number of alternating anion and cation 
selective membrane can be arranged in a stack between an anode and a 
cathode to give diluate and concentrate chambers. The treatment in the 
cell gives a salt solution with chloride as the dominant anion and a 
precipitator dust solution which is depleted with respect to chloride. The 
electrochemical treatment is preferably performed in a stack with anion 
selective membranes which are more selective for monovalent anions, e.g. 
chloride, compared to divalent anions, e.g. sulphate. 
The pH is preferably adjusted before the leach solution reaches the 
electrochemical treatment, preferably with sodium hydroxide, hydrochloric 
acid or sulphuric acid. The pH in the electrochemical treatment should 
preferably not exceed about and not be below about 2, in order not to 
damage the membranes. 
Preferably the desalination is performed by electrodialysis of the 
resulting salt solution, normally essentially or entirely consisting of 
inorganic materials, to form a diluate with reduced salt concentration and 
a first electrodialysis concentrate of the salts in solution. The diluate, 
mainly comprising sodium sulphate, can be at least partly recycled to the 
leaching step. The diluate may also be recycled to other places in the 
pulp mill. The first electrodialysis concentrate of feed leach solution 
mainly comprising harmless inorganic salts like sodium chloride and 
potassium chloride, can be sewered to the sea. It is, however, possible to 
recover the inorganic salts, especially if there are mainly 
chloride-containing salts, and purify these further, e.g. to produce acid 
and alkali, or for use in a plant for the production of sodium chlorate 
aimed for bleaching. In this case the pulp mill may be closed in a very 
broad sense. 
It is possible to obtain a 3 M chloride solution with only about 0.1 to 
about 0.3 M sulphate, by an electrodialysis treatment with a current 
efficiency for chloride removal between 80-90%. The concentrate may 
comprise from about 5 up to about 200 g/l sodium chloride and from about 
0.5 g/l sulphate up to saturation. 
Part or all of the chloride depleted solution can also be electrochemically 
treated in a membrane cell to give acid and caustic which can be used as 
internal supply for adjustment of pH in the mill. 
The electrodes used in the electrochemical treatment, can be of the 
conventional type. The anode and the cathode may be made of the same 
material. The material of the cathode may be steel or nickel, suitably 
nickel, graphite, titanium, coated titanium or activated nickel. Suitable 
anodes are made of lead, graphite, titanium, coated titanium, lead oxides, 
tin oxide, tantalum or titanium, or combinations thereof. 
The temperature in the cells should preferably not exceed 50.degree. C. 
since the membranes can be damaged at temperatures beyond 50.degree. C. 
But the membranes in the future may withstand temperatures exceeding 
50.degree. C. Thus, the limit is not critical but set of technical 
reasons. 
The current density may be in the range from about 0.2 up to about 10 
kA/m.sup.2, suitably in the range from 0.5 up to 5 kA/m.sup.2 and 
preferably in the range from 1 up to 3 kA/m.sup.2. 
The current efficiency for removal of chloride should be maintained above 
about 50%. The current efficiency is suitably maintained in the range from 
about 55 up to about 100% and preferably in the range from about 65 up to 
about 100%. 
An embodiment of the process of the present invention will now be described 
in more detail with reference to figures. FIG. 1 shows a schematic 
description of an electrochemical plant where chloride and potassium are 
removed from precipitator dust. FIG. 2 shows an example of a flow-chart of 
an electrodialysis cell. 
FIG. 1 shows roughly a process where dust (1), formed in a recovery boiler 
and collected in a dry-bottom electrostatic precipitator, is brought to a 
leaching step (2). A solid phase (3) is separated from a chloride, 
potassium and sulphate enriched leach solution (4). The leach solution is 
preferably further brought to an electrodialysis cell (5). The 
electrodialysis treatment result in a chloride and potassium enriched 
solution (6) which is separated and preferably brought to further 
treatment. The chloride and potassium depleted solution (7), enriched on 
inter alia sodium sulphate, may be recirculated to the leaching step (2). 
The separated solid phase (3) in the leaching, may be subjected to a 
treatment (8), e.g. by filtration, in order to form a second solid phase 
(9) comprising metals, carbonate, sulphate and organics. Additional water 
(10) may also be added in the treatment step (8). The liquid (11), mainly 
water, can be recirculated to the leaching step (2). In the leaching step 
additional carbonate may be added (12). Carbonate may also be added to the 
dust (1) or to the recycled solution (7). 
FIG. 2 shows, in a preferred embodiment, an electrodialysis cell comprising 
at least one anion selective (MA) and one cation selective (C) membrane 
between an anode and a cathode. Normally the cell comprises multiple pairs 
of alternating anion selective and cation selective membranes between one 
anode and one cathode. The electrodialysis treatment is preferably 
performed in a stack with anion selective membranes which are more 
selective for monovalent anions (MA), e.g. chloride, compared to divalent 
anions, e.g. sulphate. Pairs of membranes form between said compartments 
with inlets and outlets for feeding liquids to and withdrawing liquids 
from said compartments. At the anode, an anode-solution (30) is added and 
at the cathode, a cathode-solution (31) is added. When the leach solution 
(32) is fed into the cell, the monovalent anions, e.g. chloride, will 
migrate through the monoanion selective membrane (MA) towards the anode 
and the cations, e.g. potassium and sodium ions, will migrate through the 
cation selective membrane (C) towards the cathode. The water solution will 
be depleted in salt, i.e. diluate (33). The chloride enriched concentrate 
(34) may be prepared in every other compartment. The diluate can be 
recycled at least partially to the leaching step or to other places in the 
pulp mill. The diluate can also be subjected to one or more desalination 
treatments, preferably to one or more electrodialysis treatments (35) for 
further reduction of the salt content therein. It is preferred to operate 
the electrodialysis stacks at a high current density to minimize the size 
and the investment cost. Electrodialysis can be performed in 
electrodialysis stacks operating in parallel and/or in series, and with 
liquid stream flow in parallel and/or in series. 
The obtained diluate can be further desalinated in additional 
electrodialysis stacks operating at lower current densities to obtain a 
higher degree of desalination before a preferred recycle to the leaching 
step, or other liquors in the pulp mill. 
The part of the diluate that is not recycled to the leaching can be 
desalinated in a separate electrodialysis stack to obtain an almost salt 
free diluate which can be recycled to a pulping process with no risk of 
getting problems with chlorides in the recovery system. 
The concentrate (34) is suitably formed in every second chamber of the 
electrodialysis cell and to the chambers are added concentrated solution 
(32). The compartments may contain only chloride and harmless inorganic 
salts in concentrations from about 5 up to about 200 grams per liter and 
may be sewered, e.g. to the sea. It is, however, possible to recover the 
inorganic salts, which may be mainly chloride-containing salts, and purify 
these further for use e.g. in a plant for production of sodium chlorate 
for bleaching. In this case the pulp mill may be closed in a very broad 
sense. In case heavy metals or other metals harmful to the pulping process 
are present in the bleach effluent, these may be separated in the 
electrochemical stage and collected in the concentrate stream, where they 
may be removed by conventional brine purification processes, many of which 
are well-know e.g. from patents belonging to this applicant and others. 
The conversion in the cells should preferably exceed about 50%. 
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 
80 g of a precipitator dust having a carbonate content of 6% by weight, was 
dissolved in 120 ml saturated sodium sulphate solution, with a content of 
17 g/l sodium chloride solution. The temperature was 65.degree. C. during 
the leaching. The slurry was stirred for 5 minutes and thereafter the 
solution was filtered. Tests have been made at pH 6, 10 and 12. At each 
pH-value tests have been made without any addition of extra carbonate, and 
with an addition of 4% by weight solid phase carbonate. 
TABLE I 
______________________________________ 
Addition 
of car- Concent- 
bonate ration Reduction 
Element %! pH mg/l! %! 
______________________________________ 
Calcium 0 6 13,8 76 
4 6 6,2 89 
0 10 13,6 76 
4 10 7,0 88 
0 12 13,0 77 
4 12 10,2 83 
Barium 0 6 0,16 92 
4 6 &lt;0,1 100 
0 10 0,8 58 
4 10 0,18 91 
0 12 0,56 70 
4 12 0,78 61 
Manganese 0 6 1,8 92 
4 6 0,06 100 
0 10 1,8 91 
4 10 0,06 100 
0 12 0,7 97 
4 12 0,98 96 
Silicon 0 6 50 13 
4 6 34 40 
0 10 52 8 
4 10 36 36 
0 12 56 0 
4 12 62 0 
______________________________________ 
As evident from the Table, the content of Ca and other metals are reduced 
dramatically when carrying out the leaching step in accordance with the 
process of the present invention. 
A test was also made with the above mentioned precipitator dust, at varying 
temperatures and residence time. The result is presented in Table II 
below. 
TABLE II 
______________________________________ 
Residence time 
Temperature 
Potassium concen- 
minutes! C.degree.! 
tration g/l! 
______________________________________ 
5 30 20 
67 42 
80 49 
82 48 
180 33 27 
65 35 
65 44 
82 54 
83 56 
1080 25 22 
25 25 
70 48 
______________________________________ 
As evident from the Table above, the concentration of potassium increases 
with increasing temperature. 
EXAMPLE 2 
80 g of a precipitator dust having a carbonate content of 0% by weight, was 
dissolved in 120 ml saturated sodium sulphate solution, with a content of 
17 g/l sodium chloride solution (pH 10). The temperature was 65.degree. C. 
during the leaching. The slurry was stirred for 5 minutes and thereafter 
the solution was filtered. At each pH-value tests have been made without 
any addition of extra carbonate, and with an addition of 2, 6 and 10% by 
weight, solid sodium carbonate. When carbonate and dust were added, the pH 
increased as evident from Table III. In test 5 no carbonate was added, but 
instead the pH was raised to 12 by addition of alkali. 
TABLE III 
______________________________________ 
Addition Calcium 
of car- filtrate 
Calcium 
bonate conc. reduction 
Test % wt! pH mg/l! 
% wt! 
______________________________________ 
1 0 10,4 7 94 
2 2 11,2 4,8 96 
3 6 11,6 5,6 95 
4 10 11,6 3,6 97 
5 0 + NaOH 12 7,2 94 
______________________________________ 
As evident from the Table, a substantial reduction of calcium can be made 
by adding carbonate. The reduction of calcium is dependent on the 
carbonate addition, not on the pH. 
EXAMPLE 3 
A test with electrodialysis of precipitator dust have been made in a lab 
cell equipped with monoanion and cation selective membranes. The initial 
concentrations of chloride, potassium and the current density in the 
diluate solution have been varied according to Table IV. The cell, with an 
electrode area of 1.72 dm.sup.2, was equipped with ten membrane pairs. The 
anion selective membranes were monoanion selective membranes of type 
Neosepta ASV.RTM. and the cation selective membranes were of the type 
Neosepta CMV.RTM.. Platinum wires, one on each side of the ten membrane 
pairs, were used to measure the membrane voltage. Samples of the brine and 
diluate were taken every half hour and an analyses of chloride, sulphate 
sodium and potassium ion concentrations were done. The initial 
concentration of sodium chloride in the brine solution was about 0.5 M. 
The electrode rinse solution was 50 g/l sodium sulphate. The results are 
evident from Table IV. 
TABLE IV 
______________________________________ 
Current Current 
Current 
density eff. % Cl.sup.- 
eff. % K.sup.+ 
Test kA/m.sup.2 ! 
Cl.sup.- %! 
K.sup.+ +/-10% +/-5% 
______________________________________ 
1 1,5 8,7 4,8 90 20 
2 0,3 2,9 2,8 93 20 
3 2,5 9,7 2,2 100 10 
______________________________________ 
As evident from Table IV chloride and potassium can be sufficiently removed 
over a wide range and with relatively high current efficiency.