Note: Descriptions are shown in the official language in which they were submitted.
2033S45
METHOD FOR REMOVAL OF MO~OVALENT IONS FROM
METAL SULFATE SOLUTIONS BY ELECTRODIALYSIS
This invention relates to the removal of monovalent ions from
metal sulfate solutions by electrodialysis.
BACKGROUND OF THE INVENTION
Most hydrometallurgical processes for the recovery of metals
in a sulfate system involve the formation and treatment of
metal sulfate solutions and may include e]ectrowinning,
solvent extraction or electrorefining. These processes may
encounter problems with the presence of monovalent ions,
especially halides such as chloride, fluoride and bromide, or
cations such as sodium, potassium and thallium. Such
monovalent ions may be naturally present in the ore or
concentrate or may be added during the processing of the ore
or concentrate. For example, small amounts of chloride,
usually in the form of sodium chloride, may be added to obtain
a good quality copper or to control the presence of silver.
In many cases it will be important to control the amount of
monovalent ions, especially halides, in recirculating
solutions, particularly in processes for producing high-purity
metals. In many of these processes, a removal step may be
required or is actually a step in the process. For example,
according to US 4 338 168, copper concentrates are leached
with a chloride and bromide-containing solutionl and solution
is subsequently purified by adding copper to remove the
halides. This process has the disadvantage of requiring the
2 X033545
addition of copper for halide removal. According to US 4 698
139, base metals that include Cu, Co, Ni, Mn, Mg, Zn and Fe
are electrolytically recovered from materials containing
chloride and fluoride. The materials are leached, metal
sulfates are crystallized, the crystals are washed with return
acid, then leached in water, and metal is recovered by
electrolysis. The halides are removed from the
crystallization mother liquor by evaporation. The
disadvantages of this process include the inclusion of halides
in the crystallized sulfates and the formation of halides by
evaporation. According to US 4 874 436, a high-purity copper
is electrolytically deposited in a diaphragm cell from a
nitric acid electrolyte also containing silver. Chloride is
added to precipitate silver, and after the removal of silver,
the solution is recirculated to the cell. This process will
require the careful control of chloride addition and
concentration to prevent the deleterious effects of chloride
in the cell. It must also be noted that it is important to
control the halide content of electrolytes used in
electrolytic processes to avoid anode scaling, erosion and
undesirable halogen generation. Other disadvantages of the
various halide removal processes or steps include the often
incomplete removal of the halides requiring additional removal
steps, and high costs.
In other processes it may also be necessary to control the
content of halides or to remove halides to avoid their harmful
effects on processes and equipment. Such processes may
3 X033545
include the treatment or recovery of sodium or potassium
sulfate, especially when the pure salts are desired.
The removal of ions from solutions could be carried out by
methods that may include electrodialysis, which is well
documented. The removal of monovalent ions from zinc sulfate
electrolyte by electrodialysis has been patented by the same
assignee as of the present invention (US Patent 4 715 939).
Neither this reference nor the other references, however,
disclose the use of electrodialysis in purification of metal
sulfate-containing solutions for the recovery of metal other
than zinc, or the removal by electrodialysis of monovalent
ions such as ions of chlorine, bromine, fluorine, sodium,
potassium and thallium from metal sulfate solutions containing
metals other than zinc.
SUMMARY OF THE INVENTION
We have now found that the halides as well as monovalent
cations can be efficiently removed from metal sulfate
solutions containing dominant metal other than zinc by
electrodialysis.
According to the invention, metal sulfate-containing solution
containing at least one metal other than zinc, and containing
monovalent ions, is treated by passing solutions through an
electrodialysis unit to remove both monovalent anions and
monovalent cations or monovalent anions alone. The method of
the invention may also be successfully used for the removal
2033545
of monovalent anions from solutions that contain monovalent
cations such as sodium and potassium. Preferably, the metal
other than zinc is chosen from the group consisting of Cu, Co,
Ni, Mn, Fe, Mg, Cd Na and K. A metal of this group may be
present in a dominant amount with other metals of this group
being present in lesser amounts. The electrodialysis unit
includes a number of alternating concentrate and diluate
compartments separated by alternating cationic and anionic
membranes, and an anode and a cathode compartment containing
an anode and a cathode, respectively. The anionic and
cationic membranes are selected from suitable monovalent ion
permselective membranes for the removal of both monovalent
anions and cations. When selective removal of monovalent
anions is desired and removal of monovalent cations is not
required, monovalent anion permselective membranes are used,
and the cationic membranes are selected from the generally
available cationic membranes. Metal sulfate-containing
solution is passed through the diluate cells and the current
applied to the electrodes causes monovalent ions to pass from
the diluate compartments through the membranes into the
concentrate in the concentrate compartments. Any metal
deposition on the electrodes is controlled by one or more of
a number of means that include the controlling of the
compositions and flow rate of an electrode rinse solution that
is circulated through the anode and the cathode compartmentsi
the arranging of the alternating membranes such that the anode
compartment and cathode compartment are separated from the
adjacent diluate or concentrate compartments by a monovalent
5 ,~033~4~
anion permselective membrane; and the use of a cathode
material that favours hydrogen evolution over metal
deposition. Because halide, especially fluoride, removal is
pH dependent, the pH of the electrolyte is carefully
controlled within predetermined ranges. The range is
dependent on the solution being treated, usually on what metal
is dominant. Generally, values of the pH are in the range of
about one to nine. A less strict control of pH is required
when the anion removal is restricted to anions of chlorine or
bromine or both. The electrodialysis may be carried out in
one or more stages depending on the concentration of
monovalent ions in the metal sulfate solutions to be treated
and/or the desired level of these ions and the metal ions in
the treated solutions. ~y choosing appropriate conditions,
the method of the invention can result in the effective
removal in one or more stages of 90~ or better of the
monovalent ions, especially the ions of chlorine, bromine,
fluorine, sodium, potassium and thallium from the rnetal
sulfate solutions. In many cases, anions of chlorine,
fluorine and bromine may be substantially completely removed.
The content of the monovalent ions in treated solutions may
be readily controlled in the process or in the treated
solution.
According to the invention, there is provided a method for the
treatment by electrodialysis of metal sulfate-containing
solution containing at least one metal other than zinc and
containing concentrations of monovalent ions including at
6 2033545
least one ion chosen from the group consisting of cations of
thallium, sodium and potassium, and of anions of chlorine,
bromine and fluorine, said method comprising the steps of
feeding metal sulfate-containing solution to diluate
compartments of an electrodialysis unit comprising a
multiplicity of alternating monovalent cation permselective
exchange membranes and monovalent anion permselective exchange
membranes, said membranes defining alternating diluate and
concentrate compartments, an anode compartment and a cathode
compartment, an anode positioned in the anode compartment and
a cathode positioned in the cathode compartment; applying an
electrical current between the anode and the cathode at a
value such that the value of the corresponding current density
is in the range of about 10 A/m2 to 500 A/m2; feeding metal
sulfate-containing solution at a controlled pH having a value
in ranges between values from about 1 to about 9 to said
diluate compartments; circulating flows of diluate and
concentrate through the diluate and concentrate compartments,
respectively, at a linear velocity sufficient to maintain
turbulent flow in said compartments; withdrawing at least a
portion of the circulating diluate as the treated metal
sulfate-containing solution with reduced concentrations of
said monov~lent ions; and withdrawing at least a portion of
the circulating concentrate.
According to a second embodiment, there is provided a method
for the treatment by electrodialysis of metal sulfate-
containing solution containing at least one metal other than
7 2~3354~
zinc and containing concentrations of monovalent anions
including at least one anion chosen from the group consisting
of anions of chlorine, bromine and fluorine, said method
comprising the steps of feeding metal sulfate-containing
solution to diluate compartments of an electrodialysis unit
comprising a multiplicity of alternating cation exchange
membranes and monovalent anion permselective exchange
membranes, said membranes defining alternating diluate and
concentrate compartments, an anode compartment and a cathode
compartment, an anode positioned in the anode compartment and
a cathode positioned in the cathode compartment; applying an
electrical current between the anode and the cathode at a
value such that the value of the corresponding current density
is in the range of about 10 A/m2 to 500 A/m2; feeding metal
sulfate-containing solution at a controlled pH having value
in ranges between values from about 1 to about 9 to said
dilute compartments; circulating flows of diluate and
concentrate through the diluate and concentrate compartments,
respectively, at a linear velocity sufficient to maintain
turbulent ~low in said compartments; withdrawing at least a
portion of the circulating diluate as the treated metal
sulfate-containing solution with reduced concentrations of
monovalent anions; and withdrawing at least a portiGn of the
circulating concentrate.
Preferably, the at least one metal other than zinc is chosen
from the group consisting of copper, cobalt, nickel,
manganese, iron, magnesium, cadmium, sodium and potassium.
8 Z~33~45
Preferably, one Gf the metals of this group is present in a
dominant amount in the metal sulfate-containing solution.
It is an aspect of the present invention to provide a method
for the removal of monovalent ions from metal sulfate-
containing solutions containing metal other than zinc.
It is another aspect to provide a method for the removal of
both monovalent cations and monovalent anions from metal
sulfate-containing solutions containing metal other than zinc
by electrodialysis.
It is a further aspect to provide a method for the selective
removal of monovalent anions from metal sulfate-containing
solutions containing metal other than æinc by electrodialysis.
It is yet another aspect to provide a method for treating
metal sulfate-containing solution by electrodialysis for the
removal of monovalent ions and controlling the concentration
of the monovalent ions in treated solution.
These and other aspects of the present invention will become
apparent from the following detailed description of the method
of the present invention.
DETAILED DES~RIPTION
Metal sulfate solutions that may be treated according to the
method of the present invention include solutions obtained
from or present in the treating of metal-containing materials
such as ores, concentrates and metallurgical process
intermediate products with sulfuric acidl or solutions used
9 ~ 354S
for extracting metal by electrolytic or solvent extraction
processes. Metal sulfate solutions treated also include
solutions encountered in the processing of magnesium, sodium
and potassium salts. The metal sulfate solutions contain at
least one metal other than zinc. The at least one metal other
than zinc may be present in a dominant amount and is,
preferably, chosen from the group consisting of Cu, Co, Ni,
Mn, Fe, Mg, Cd, Na and K. The metal sulfate solutions may
also contain other metals that may accompany the preferred
metal in the sulfate solutions such as, for example, arsenic,
antimony, zinc, lead, bismuth, silver, tin and calcium, as
well as the metals of above-recited group present in amounts
smaller than the amount of the metal that i5 dominant in the
solutions. The metal sulfate solutions include monovalent
ions comprising at least one monovalent cation or at least
one monovalent anion. Thus, for example, the metal sulfate
solution containing monovalent ions may be a copper sulfate-
containing solution wherein copper is the dominant metal, and
other metals such as zinc, arsenic, iron, silver, nickel and
other metals may be present in ionic form. The metal sulfate
solutions are subjected to a treatment to remove monovalent
anions such as the anions of chlorine, bromine, fluorine, and
monovalent cations such as the cations of sodium, potassium
and thallium. Any nitrate, if present in solution, will be
removed with the other monovalent ions. As will be explained
hereinafter, only monovalent anions may be selectively removed
or both monovalent anions and cations may be removed. As will
be explained, the method may also be used to remove monovalent
10 ;~033~4~
anions from ~etal solutions wherein the metal is a monovalent
ion such as sodium or potassium.
Metal sulfate-containing solution, present in a process
wherein such solution occurs or is treated, is fed to an
electrodialysis unit. The electrodialysis unit comprises a
multiplicity of vertically arranged, alternating monovalent
anion permselective exchange membranes and cation exchange
membranes or monovalent cation permselective exchange
membranes, a cathode compartment and an anode compartment.
The choice of membranes is important. When only monovalent
anions are to be removed, the use of a combination of
monovalent anion permselective membranes and general cation
exchange membranes (limited permselectivity for mono over
multivalent cations) makes it possible to remove monovalent
anions selectively from the solution. This combination of
membranes can be advantageously used when monovalent cations
are present in small amounts. When both monovalent anions and
monovalent cations are to be removed, a combination of
monovalent anion and monovalent cation permselective membranes
is used. Such combination will, therefore, make it possible
to separate monovalent ions from multivalent ions, and to
concentrate the monovalent cations and the monovalent anions.
The metal sulfate-containing solution thereby becomes depleted
in these ions.
We have found that suitable monovalent cation permselective
membranes are, for example, strongly acidic membranes which
11 ~(3;~35~5
have a membrane matrix of a styrene di-vinyl benzene
co-polymer on a polyvinyl chloride base and possess sulfonic
acid radicals (R-SO3H3 as active groups. The active groups
comprise 3-4 milli-equivalents per gram of dry resin which is
satisfactory to provide the desired selectivity for monovalent
ions. Suitable monovalent cation permselective membranes are
specially treated SelemionTM CMV, SelemionTM Experimental A
(specially treated on one face), and Selemion1-M Experimental
B or SelemionTM CSR (both surfaces specially treated) and
specially treated SelemionTM CMR. If the object is to remove
only monovalent anions such as anions of chlorine, bromine and
fluorine, and not monovalent cations, the choice of the cation
membrane can be extended to include others available on the
market such as, for example, those with sulfonic acid
radicals (R -SO3 H) as the active groups at 3-5
milli-equivalents per gram of dry resin, e.g., SelemionTM CMV.
Suitable monovalent anion permselective membranes are, for
example, strongly basic membranes with quaternary ammonium
active groups, such as, for example, derived from
trimethylamine (for example, R-N(CH2)3.Cl)l at 3-4
milli-equivalents per gram of dry resin, and having a matrix
of a styrene di-vinyl benzene co-polymer on a polyvinyl
chloride base. SelemionTM ASV, ASS or ASR, which is
permselective for monovalent anions, particularly anions of
chlorine, bromine and fluorine, is particularly suitable.
12 X033545
It is understood that membranes with similar properties such
as NeoseptaTM CM-l, NeoseptaTM CMS, NeoseptaTM ACS, ~eoseptaTM
CLE~E and IonacTM MC 3470, are suitable, and that the use of
combinations of other membranes may yield the desired results~
The alternating cation and anion membranes form a number of
alternating diluate compartments and concentrate compartments
which are situated between the anode compartment and the
cathode compartment. The anode and cathode are made of
suitable materials. For example, the anode can be made of
platinum-coated titanium and the cathode of stainless steel.
The cathode can also be advantageously made of a material for
which the hydrogen overvoltage is lowered, such as platinum-
coated titanium, in order to favour hydrogen evolution over
the deposition of metal. A source of direct current is
connected to the electrodes.
The metal sulfate-containing solution, preferably free of
suspended solids, is fed as feed solution to the diluate
compartments. A depleted solution or diluate, i.e. a treated
metal sulfate solution, is withdrawn from the diluate
compartments. A concentrate, i.e. a solution concentrated in
monovalent ions, is withdrawn from the concentrate
compartments, preferably at a rate equal to the rate of the
net water transfer from the diluate to the concentrate during
the electrodialysis. It is important to maintain turbulent
conditions in the concentrate and diluate compartments. This
can be achieved by passing solution through the compartments
13 2033S45
at a sufficient rate. Preferably, at least a portion of the
diluate and at least a portion of the concentrate are
circulated to the diluate and concentrate compartments
respectively, mainly to ensure turbulent conditions, but also
to achieve the desired removal and concentration of monovalent
ions. The feed solution is conveniently added to the portion
of circulating diluate. A portion of the circulating diluate
is withdrawn as treated metal sulfate-containing solution
having a reduced content of monovalent ions. A portion of the
circulating concentrate is also withdrawn. The withdrawn
treated solution and the withdrawn concentrate may be treated
further, if desired and as will be described.
During electrodiilysis, water transport occurs by osmosis and
electro-osmosis usually in opposing directions and at
different rates. The net water transport generally occurs in
the direction from the diluate to the concentrate
compartments. This water transport is sufficient, in most
cases, to form concentrate stream flows adequate for
withdrawal. In those cases wherein the net water transfer
rate to the concentrate compartments is less than the desired
withdrawal rate of concentrate from the concentrate
compartments, it will be necessary to feed a receiving
solution to the concentrate compartments. For example, the
receiving solution, compatible with the general operation of
the electrodialysis unit, may be chosen from water, dilute
sulfuric acid and a dilute salt solution such as, for example,
a dilute sodium sulfate solution.
14 20335A5
In the cathode and anode compartments the predominant
reactions are hydrogen and oxygen evolution, respectively.
However, small amounts of metal may deposit on the cathode.
The deposition may be controlled by arranging the membranes
in the electrodialysis unit such that anion permselective
membranes form the end membranes, i.e., are the membranes next
to the electrode compartments; selecting a large enough
electrode rinse flow at a controlled pH to minimize the
concentration of metal ions; or by using a cathode made of a
suitable material to promote the evolution of hydrogen over
metal deposition, such as, for example, a cathode material of
platinum-coated titanium.
The cathode and the anode compartments are rinsed separately
or with a common rinse solution circulated to both the
electrode compartments. The rinse solution may be chosen from
dilute sulfuric acid and, preferably, sulfuric acid-sodium
sulfate solution maintained at a pH in the range of about 0
to 4, values in the higher end of the range being preferred
for more efficient fluoride removal. A portion of the rinse
solution may be removed from circulation and be replaced with
a substantially equal portion of fresh solution so that the
metal concentration in the rinse solution is maintained at
about 150 mg/L or less.
During electrodialysis, the monovalent cations and anions in
the metal sulfate-containing feed solution pass from the
diluate compartments to the concentrate compartments through
15 20;~354~;
the monovalent permselective cation and anion membranes
respectively, leaving substantially all multivalent cations
and anions in the diluate compartments. In the embodiment for
the selective removal of monovalent anions, the monovalent
anions pass from the diluate to the concentrate compartments
through the monovalent anion permselective membranes, leaving
monovalent cations and substantially leaving the multivalent
ions in the diluate compartments. The gases evolved at the
electrodes are carried from the cathode and anode compartments
in the rinse solution. Both embodiment may be used for the
removal of monovalent anions from metal sulfate solutions.
In those solutions wherein the metal is sodium or potassium,
the monovalent anions are effectively removed. As the sodium
or potassium is usually present in such solutions in a high
concentration, a portion of the monovalent cations is also
removed with the concentrate that contains the removed anions.
The concentration of the metals in the concentrate may be
reduced, as will be described.
The electrodialysis unit may be operated with solution
temperatures in the range of from just above the freezing
temperature of the solution to as high as about 60C, i.e.
from about 0C to 60C, preferably from about 20C to 50C.
The method is conducted with a metal sulfate-containing feed
solution that has a pH controlled at values that do not cause
undesirable reactions such as, for example, precipitation of
metal as hydroxide or basic metal sulfate. The pH values
16 X0335~5
depend on the metals present in the feed solution, especially
the dominant metal. &enerally, the pH values are in ranges
between values from about one to about nine. Specifically,
for effective removal of monovalent cations and, especially
the monovalent anions, the pH of a copper sulfate-containing
solution should be maintained in the range of about 1 to 5.5,
preferably 3 to 5; for a cadmium sulfate-containing solution
in the range of about 1 to 5.5, preferably 2.5 to 5; for a
nickel or cobalt sulfate-containing solution in the range of
about 1 to 6, preferably 4 to 5; for a ferrous iron sulfate-
containing solution in the range of about 1 to 6, preferably
4 to 6; for a manganese sulfate-containing solution in the
range of about 1 to 7, preferably 4 to 7; for a magnesium
sulfate-containing solution in the range of about 1 to 7,
preferably 4 to 6.5; and for a sodium or potassium sulfate-
containing solution in the range of about 1 to 9, preferably
4 to 8. We have also found that the removal of anions of
fluorine is especially sensitive to the pH due to the
formation of hydrogen fluoride ions at a pH below about 3.5.
For effective fluoride removal, the pH of the diluate and
concentrate streams containing fluoride is, therefore,
preferably at a value of not less than about 2 and, to enhance
fluoride removal, most preferably at a value in the range of
about 3.5 to 9, depending on the metals in the feed.
The flow rate of solutions through the concentrate and diluate
compartments should be such that the linear velocity is
sufficient to obtain turbulent flow. The flows of solutions
17 X033~45
through the concentrate and diluate compartments and the anode
and cathode compartments should be substantially balanced in
order to maintain a differential pressure across the membranes
that does not exceed about 150 kPa and is preferably in the
range of from 0 kPa to about 50 kPa.
Feed rates to the electrodialysis unit may be selected in the
range of about 2 L/h.m2 to 60 L/h.m2 per membrane pair, the
selected value being dependent on the monovalent ion
concentrations in the metal sulfate-containing solution and
the value of the current density.
The process can be operated with a current applied to the
electrodes such that the equivalent membrane current density
(applied current per effective membrane surface area) is in
the range of about 10 A/m2 to 500 A/m2. Below about 10 A/m2,
the ionic transfer rate is too low, while above about 500 A/m2
the rate of replenishing monovalent ions at the membrane
diffusion layer is too low, with resulting water splitting
and/or loss of permselectivity. Water splitting and
permselectivity loss are substantially obviated when operating
with current densities in the preferred range of about 50 A/m2
to 300 A/m2.
Although electrodialysis may be effective in one stage to
reduce concentrations of monovalent ions to the desired low
concentrations or to substantially zero, it may be desirable
to have more than one stage of electrodialysis. In more than
2(1335~5
18
one stage, the stages are preferably connected in series,
diluate withdrawn from one stage being fed to the diluate
compartments of a subsequent stage whereby concentrations of
monovalent ions may be further reduced to the desired level.
Using one or more stages, the anions of chlorine, bromine and
fluorine may be substantially completely removed so that the
treated solution is substantially free of chloride, fluoride
and bromide.
If desired, the concentra~e may be further concentrated by
electrodialysis. Concentrate withdrawn from concentrate
compartments from the first stage electrodialysis is fed to
the diluate compartments of a second stage. Such a step may
be advantageous to reduce loss of metal with the concentrate,
as concentrate is usually discarded after treatment as an
effluent. Diluate from such a second electrodialysis of
concentrate may be returned as feed to the first stage
electrodialysis. Reduction of loss of metal may be
particularly desirable when solutions of, for example, sodium
or potassium sulfate are treated for the removal of halides.
By a judicious selection of the size of the electrodialysis
unit or the use of a staged process, the method may be used
for the effective control of concentrations of monovalent ions
in the treated solutions. Thus, the concentration of the
monovalent ions is controlled in one or more stages and
treated solution having the desired concentration is
recovered. If the concentration of monovalent ions is below
ZC~33S~S
19
the desired value, the desired monovalent ions may be added
to attain the desired value.
If needed, the membranes may be cleaned periodically to remove
any deposits such as of calcium sulfate or fluoride,
ma~nesium fluoride, or basic metal sulfates. The membranes
may be cleaned physically or chemically with a suitable acid
solution such as, for example, a 15% solution of acetic acid,
a 2 M hydrochloric acid or dilute sulfuric acid followed by
adequate rinsing with water. ~he electrodes may be cleaned
with dilute sulfuric acid.
The invention will now be illustrated by means of the
following non-limitative examples. The apparatus used in the
tests described in the examples consisted of an
electrodialysis unit having ten membrane pairs with a total
effective membrane area of 1720 cm2. The unit was of the so-
called sheet flow design (liquid flows in a sheet-like fashion
in a relatively straight line from the inlet to the exit
ports), and anionic membranes were employed adjacent the
electrode compartments.
The cation permselective membranes were SelemionTM CMR
membranes, and the anion permselective membranes were
SelemionTM ASR membranes. The cathode was made of stainless
steel, and the anode was made of platinum coated titanium.
In all tests, diluate was circulated through the diluate
compartments of the unit at a velocity of from 5 to 7 cm/s,
20 Z03354~
and concentrate was circulated through the concentrate
compartments of the unit at a velocity of from 3 to 6 cm/s.
Feed solution was added to the circulating diluate.
Example 1
A predominantly copper sulfate-containing solution, containing
43 g/L Cu, 9 g/L Ni, 1.2 g/L Zn, 1.2 g/L Fe, 0.245 g/L Cl,
0.131 g/L F, 0.300 g/L Br, 0.380 g/L Na and 0.100 g/L K, was
subjected to electrodialysis in the electrodialysis unit. The
solution was fed at a rate of 14 L/h.m2. A current was
applied between the electrodes to give a current density of
100 A/m2. The temperature was maintained at 40C. The
electrode compartments were rinsed with a rinse solution
containing 5 g/L Na2SO4, maintained at a pH of 2.0 + 0.1, and
circulated at a rate of 50 L/h.m2. The unit was operated for
24 hours. The circulating diluate was controlled at a pH in
the preferred range of 3 to 5.
The final diluate solution was analyzed and was found to
contain 43 g/L Cu, 9.2 g/L Ni, 1.2 g/L Zn, 1.2 g/L Fe, 0.038
g/L F, 0.210 g/L Na, 0.040 g/L K, no Cl, Br was below the
detection limit. The final concentrate solution was analyzed,
and was found to contain 2.4 g/L Cl, 2.2 g/L Br, 0.930 g/L F,
1.60 g/L Na and 0.38 g/L K (bromide analysis was carried out
with an ion chromatography interference apparatus). As can
be seen from the results, the chloride and bromide were
completely removed from the copper sulfate solution, while the
removal of fluoride was 72%. The fluoride removal is more
2033S4S
21
efficient than usually obtained when a predominantly zinc
ulfate-containing solution containing a similar amount of
fluoride is treated.
Example_2
A predominantly magnesium sulfate-containing solution,
containing 105 g/L MgSO4.7 H2O, 1.155 g/L Cl and 0.300 g/L Br,
was subjected to electrodialysis in the electrodialysis unit.
The solution was fed at a rate of 8.75 L/h.m2. ~he current
applied between the electrodes gave a current density of 100
A/m2. The temperature was maintained at 39C. The electrode
compartments were rinsed with a rinse solution containing 5
g/L Na2SO4~ maintained at a pH of 2.0 + 0.1, and circulated at
a rate of 50 L/h.m2. The unit was operated for 8 hours. The
circulating diluate was maintained during operation at a pH
value in the preferred range of 4 to 6.5.
After the operation was completed the diluate and concentrate
were analy~ed. The diluate was found to contain 95 g/L
MgSO4.7H2O, 0.087 g/L Cl and Br below the detection limit.
The concentrate was found to contain 135 g/L MgSO4.7H2O, 4.1
g/L Cl and 2.1 g/L Br.
The high magnesium content of the concentrate suggests the
possible transfer of monovalent complexed anionic magnesium
species.
203~S45
22
The results show that the anions of chlorine and bromine were
effectively removed from the feed solution recovered as
diluate.
ExamPle 3
A predominantly cadmium sulfate-containing solution,
containing 37 g/L Cd, 7.4 g/L Zn, 5.3 g/L Ni, 3.6 g/L Fe, 5.2
g/L Cu, 0.346 g/L Mg, 0.091 g/L Mn, 0.2 g/L Cl, 0.2 g/L Br,
0.063 g/L F, 0.90 g/L Na, 0.22 g/L K and 0.030 g/L Tl, was fed
at a rate of 8.75 L/h.m2. The current density was 100 A/m2.
The temperature was maintained at 38C. The electrode
compartments were rinsed with a rinse solution containing 5
g/L Na2SO4 maintained at a pH of 2.0 + 0.1, and circulated at
a rate of 50 L/h.m2. The solution was fed to the unit for 8
hours. During operation, the pH of the circulating diluate
was maintained in the preferred range of 2.5 to 5.
After operation was completed, the diluate and the concentrate
were analyzed. The diluate was found to contain 35 g/L Cd,
7.0 g/L Zn, 5.0 g/L Ni, 3.4 g/L Fe, 4.9 g/L Cu, 0.326 g/L Mg,
0.086 g/L Mn, 0 g/L Cl, 0.0195 g/L F, Br was below the
detection limit, 0.40 g/L Na, 0.15 g/L K and 0.017 g/L Tl.
The concentrate was found to contain 42 g/L Cd, 8.4 g/L Zn,
5.9 g/L Ni, 4.1 g/L Fe, 5.9 g/L Cu, 0.397 g/L Mg, 0.102 g/L
Mn, 5.9 g/L Cl, 0.560 g/L Br, 0.175 g/L F, 1.5 g/L Na, 0.2 g/L
K and 0.063 g/L Tl. The results show that the halides and
thallium were effectively removed from the feed solution. The
~033SA5
23
metal losses to the concentrate were relatively high. The
metal losses may be minimized by using a two-stage process.
Example 4
A predominantly sodium sulfate-containing solution, containing
16.3 g/L Na, 0.040 g/L K, 2.120 g/L Cl, 0.110 g/L F and 0.100
g/L Br, was treated for the removal of halides. The feed rate
of the solution was 28 L/h.m2. The current density was 200
A/mZ, and the solution was fed to the unit for a period of 8
hours. The electrode compartments were rinsed with feed
solution maintained at a pH of 2.0 + 0.1 at a rate of 10
L/h.m2. The pH of the circulating diluate was maintained in
the preferred range of from 4 to 8.
The recovered diluate was analyzed and found to contain 13 g/L
Na, 0.028 g/L K, 0.450 g/L Cl, 0.040 g/L F and Br below the
detection limit.
The recovered concentrate was analyzed and found to contain
35 g/L Na, 0.120 g/L K, 13.8 g/L Cl, 0.600 g/L F and 0.590 g/L
Br.
The test results show that halides may be effectively removed
from a sodium and potassium sulfate solution. The metal
content of the concentrate may be reduced by subjecting the
concentrate to a second stage electrodialysis.
The results of the above examples show that monovalent anions
24 Z033S4S
and cations may be effectively removed from metal sulfate-
containing solutions containing a metal chosen from copper,
nickel, manganese, iron, magnesium, cadmium, sodium and
potassium, one of these metals being present in a dominant
amount. ~he results also show that anions of chlorine, and
bromine may be substantially removed from metal sulfate-
containing solution, and that halides may be effectively
removed from monovalent cation-containing solutions.
It is understood that changes and modifications may be made
in the embodiments of the invention without departing from the
scope and purview of the appended claims.
