Method for separating dissolved species by electrodialysis

Electrodialysis is used to separate at least one species of a first group of elements present in ionized form from at least one species of a second group of elements present in nonionized form in water-containing solutions, provided that the pH of the solutions and the oxidation state of species of the second group have the appropriate values. If necessary, either the pH, the oxidation state or both are adjusted in the solution fed to the electrodialysis unit such that species of the first group are present in ionized form and species of the second group are present in a nonionized form. The method is applicable to a large number of solutions from metallurgical and chemical processes. Species of the first group include at least one element of groups IA, except Fr, IB, IIA, IIB, IIIA except B, IIIB, IVB, VIIB and VIII except Os of the periodic table; the elements V, Cr, Sn, Pb and Bi; and acids of N, S, F, Cl, Br and I. Species of the second group comprise acids containing at least one element including B, C, Si, Ge, P, As, Sb, Se, Te and F. Electrodialysis is carried out in one of more stages at 10-500 A/m.sup.2 and 0.degree.-60.degree. C. while minimizing water splitting, and using alternating cationic and anionic permselective membranes forming alternating concentrate and diluate cells between an anode and a cathode. The ionized species migrate through the membranes from the diluate to the concentrate cells and the nonionized species remain in the diluate cells. Solution concentrated in ionic species is withdrawn from the concentrate cells, and solution containing the nonionized species is withdrawn from the diluate cells. The electrode compartments are rinsed with appropriate rinse solutions.

This invention relates to a method for the separation of dissolved species 
by electrodialysis and, more particularly, to a method for the separation 
by electrodialysis of ionized from nonionized species in aqueous systems. 
BACKGROUND 
In the treatment of solutions that contain one or more elements in 
cationic, anionic or radical form dissolved therein, a separation of one 
or more ions or radicals can be accomplished by using electrodialysis. In 
electrodialysis, a feed solution is circulated through an electrodialyzer 
cell that comprises an anode and a cathode positioned in an anode and a 
cathode compartment, separated from each other by a multiplicity of 
alternating anionic and cationic selective exchange membranes. The 
alternating membranes and associated gaskets in appropriate arrangement 
form alternating concentrate and diluate cells. A direct current is 
applied between the electrodes and solution is fed to the diluate cells. A 
concentrate enriched in the ions that have passed through the membranes is 
withdrawn from the concentrate cells. 
Electrodialysis has been used in a multitude of applications which include 
the separation of cations, anions, monovalent from multivalent ions, ionic 
species of the same or different electrical charge, acids from elements in 
ionic form, amphoteric elements, compounds of different conductances and 
degree of dissociation in solution, and ionic species from nondissociated 
organic compounds. 
The removal of ionized species by electrodialysis is well documented but 
few references deal with the separation of ionized species from nonionized 
species. According to U.S. Pat. No. 2,854,393, electrodialysis is used for 
the fractionation of cations, anions, organic and inorganic ions, the 
separation of nonionic and nondissociated compounds in solution, as well 
as the separation of different compounds of different conductances and 
different degrees of dissociation in solution. Examples of separations of 
nonionic compounds all relate to organic compounds. Separation of 
compounds of different degrees of dissociation are only mentioned in 
passing, and relate to aqueous solutions of sodium chloride, boric acid, 
sodium acetate and acetic acid. All these fractionations and separations 
are carried out, however, with the application of a mechanical or 
gravitational accelerating force resulting in the formation of separate 
fluid strata in the cell from which solutions are withdrawn that contain 
an increased concentration of one of the species to be separated. 
The separation of chloride ions and impurities from boric acid by 
electrodialysis has been discussed by Russian authors (Chem. Abs., vol. 
60, 11404b and vol. 78, 140772p). The separation of calcium ions from 
phosphoric acid by electrodialysis is disclosed in Belgian Patent 643 464. 
Japanese unexamined application 79 132 496 discloses electrodialysis of an 
aqueous sodium silicate solution at pH 1-3 and containing 3-25% SiO.sub.2 
together with 0.5N sodium chloride solution. Sodium ions and sulfuric acid 
were removed from a silica sol. 
It appears from these references that separations of ionized from 
nonionized species by electrodialysis has only been carried out with a 
very limited number of elements. 
SUMMARY OF THE INVENTION 
We have now found that a very large number of elements in ionized form can 
be separated by electrodialysis from a number of elements that can be 
present in aqueous systems in nonionized form. More particularly, we have 
found that a separation by electrodialysis can be effected between at 
least one element of a first group of elements that is present in ionized 
form in solution and at least one element of a second group of elements 
that is present in nonionized form, provided that the pH of the solution 
and the oxidation state of elements of the second group have the 
appropriate values. If necessary, either the pH or the oxidation state or 
both are adjusted such that the separation can be effected. The 
electrodialysis for the separation of ionized from nonionized species is 
carried out along conventional lines in an electrodialyzer cell wherein a 
cathode compartment containing a suitable cathode and an anode compartment 
containing a suitable anode are separated from each other by a plurality 
of alternating suitable cation permselective and suitable anion 
permselective exchange membranes which form alternating concentrate and 
diluate cells. Generally, an anionic membrane is arranged next to the 
anode compartment and a cationic membrane next to the cathode. In certain 
embodiments a cationic membrane is next to the anode compartment and an 
anionic membrane is next to the cathode compartment, or anionic membranes 
are next to both compartments in case solutions contain metal ions that 
can be cathodically deposited. 
A feed solution containing dissolved species is fed into the diluate cells 
of an electrodialysis unit. If necessary, the feed solution is adjusted to 
a predetermined value of the pH or the oxidation state of the element 
desired to be present in nonionized form or predetermined values of both 
the pH and the oxidation state, such that at least one species of a first 
group of elements of the periodic table of elements is present in the 
solution in ionized form and at least one species of a second group of 
species containing elements of the periodic table of elements is present 
in the solution in nonionized form. The feed solution is a 
water-containing solution that may also contain organic components. A 
suitable direct current is applied between the electrodes in the electrode 
compartments of the unit, and the value of the current is sufficient to 
effect the separation while substantially preventing water splitting. The 
ionized species migrate through the membranes from the solution in the 
diluate cells into solution in the concentrate cells. The nonionized 
species substantially remain in the diluate cells. During electrodialysis, 
the circulating solution in the concentrate cells becomes concentrated in 
the ionized species, and the circulating solution in the diluate cells 
becomes depleted in the ionized species. A solution is withdrawn from the 
concentrate cells that is concentrated in ionized species and a solution 
is withdrawn from the diluate cells that is depleted in ionized species. 
Electrodialysis is carried out under turbulent conditions in the cells, at 
up to 60.degree. C., a differential membrane pressure of less than 150 kPa 
and a current density in the range of 10 to 500 A/m.sup.2. The electrode 
compartments are preferably rinsed with appropriate, circulating rinse 
solutions, or a common rinse solution. The electrodialysis may be carried 
out in single or multi-stage to effect the desired degree of separation 
and concentration. The method of the invention is particularly useful in 
the treatment or purification of metallurgical and chemical process 
solutions, waste solutions, waste water and the like. 
It is a principal object of the present invention to provide a method for 
the separation of dissolved species by electrodialysis. It is another 
object to provide a method for the separation by electrodialysis of 
ionized species from nonionized species in water-containing systems. It is 
yet another object to provide a method for the electrodialytic separation 
of species in a solution adjusted in the value of the pH or the value of 
the oxidation state or both. These and other objects of the invention will 
become clear from the following detailed description of the embodiments of 
the method according to the invention. 
According to the main embodiment of the invention, there is provided a 
method for the separation by electrodialysis of dissolved species in a 
water-containing solution derived from metallurgical and chemical 
processing comprising the steps of forming a feed solution, said feed 
solution comprising a) a concentration of at least one species selected 
from a first group consisting of the elements of groups IA except 
francium, IB, IIA, IIB, IIIA except boron, IIIB, IVB, VIIB, and VIII 
except osmium of the periodic table of the elements; the elements 
vanadium, chromium, tin, lead, and bismuth; and acids of nitrogen, sulfur, 
fluorine, chlorine, bromine and iodine; and b) a concentration of at least 
one species selected from a second group consisting of acids containing an 
element chosen from the group consisting of boron, carbon, silicon, 
germanium, phosphorus, arsenic, antimony, selenium, tellurium and 
fluorine; said feed solution having a value of the pH, and species of said 
second group in said solution having a value of the oxidation state such 
that said at least one species of said first group is present in said 
solution substantially in ionized form and said at least one species of 
said second group is present in said solution substantially in nonionized 
form; feeding said feed solution to the diluate cells of an 
electrodialysis unit comprising a multiplicity of alternating suitable 
cation permselective exchange membranes and suitable anion permselective 
exchange membranes, said membranes defining alternating diluate and 
concentrate cells, 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 sufficient to effect said separation while 
substantially preventing water splitting; passing flows of solutions 
through the diluate and the concentrate cells at a linear velocity 
sufficient to maintain turbulent flow in said diluate and concentrate 
cells; withdrawing a diluate from said diluate cells, said diluate 
containing substantially said concentration of said at least one species 
of said second group; and withdrawing a concentrate from said concentrate 
cells, said concentrate containing an increased concentration of said at 
least one species of said first group. 
Preferably, the elements of said first group are present in the ionized 
form and are selected from the group consisting of H.sup.+, Li.sup.+, 
Na.sup.+, K.sup.+, Cs.sup.+, Be.sup.2+, Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, 
Ba.sup.2+, Ti.sup.4+, Zr.sup.4+, V.sup.5+, Cr.sup.3+, Mn.sup.2+, 
Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Rh.sup.3+, Ir.sup.4+, Ni.sup.2+, 
Pd.sup.2+, Pt.sup.4+, Cu.sup.2+, Ag.sup.+, Au.sup.+, Zn.sup.2+, Cd.sup.2+, 
Hg.sup.2+, Al.sup.3+, Ga.sup.3+, In.sup.3+, Tl.sup.+, Sn.sup.4+, 
Pb.sup.2+, Bi.sup.3+, NO.sub.3-, SO.sub.3.sup.2-, SO.sub.4.sup.2-, 
Cl.sup.-, Br.sup.-, I.sup.- and F.sup.-. 
Preferably, the species of said second group are present in the nonionized 
form and are selected from the group consisting of H.sub.3 BO.sub.3, 
H.sub.2 CO.sub.3, HCO.sub.2 H, H.sub.2 SiO.sub.3, H.sub.2 GeO.sub.3, 
H.sub.3 PO.sub.4, H.sub.3 AsO.sub.4, HAsO.sub.2, HSbO.sub.2, H.sub.2 
SeO.sub.3, H.sub.2 TeO.sub.4 and HF. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Solutions that can be treated in the process of the invention are solutions 
that may be present in or are obtained from or during metallurgical and 
chemical processing of materials, and include metallurgical and chemical 
intermediate, final and waste solutions. Solutions that can be treated may 
be prepared from solid materials that have been solubilized into a 
water-containing solution. The solutions are generally water-containing 
solutions containing dissolved species of cations, anions or radicals of 
elements of the periodic table of elements. The solutions may also contain 
organic liquids as part of a solution, but a sufficient amount of water 
must be present to provide electrical conductivity and for the species to 
be present and remain in dissolved form. The nature of the organic 
compounds must be such that any fouling of the membranes is limited to a 
practical and economical level. Thus, to achieve separations of dissolved 
species in an organic solution some water must be present or added. 
Suitable organic liquids include water-immiscible liquids of low viscosity 
and water-miscible liquids. Viscous oils or greases, for example, are 
unsuitable because of potential membrane fouling. 
The species that can be separated by the method according to the invention 
are divided in two groups. The first group comprises elements of the 
periodic table of elements that may be present in ionized form in a 
water-containing solution, and the second group comprises species that 
contain elements that may be present in nonionized form. More 
specifically, the first group includes the ionized form of the elements of 
groups IA except francium, IB, IIA, IIB, IIIA except boron, IIIB that 
includes the lanthanides and actinides, IVB, VIIB and VIII except osmium; 
the elements V, Cr, Sn, Pb and Bi; and the ionized form of acids of the 
elements N, S, F, Cl, Br and I. The acids included in the first groups may 
include, for example, HNO.sub.3, H.sub.2 SO.sub.4, H.sub.2 SO.sub.3, HF, 
HCl, HBr, HI, HOCl and HClO.sub.3. 
The second group includes species that contain the elements B, C, Si, Ge, 
P, As, Sb, Se, Te and F. The species that contain the elements recited for 
the second group may be present in nonionized form, and the species 
usually include hydrogen and oxygen. Thus the species of the second group 
consist of acids containing an element chosen from B, C, Si, Ge, P, As, 
Sb, Se, Te, and F. The acids included in the second group may include, for 
example, H.sub.3 BO.sub.3, H.sub.2 CO.sub.3, HCO.sub.2 H, H.sub.2 
SiO.sub.3, H.sub.2 GeO.sub.3, H.sub.3 PO.sub.4, HAsO.sub.2, H.sub.3 
AsO.sub.4, HSbO.sub.2, H.sub.2 SeO.sub.3, H.sub.2 TeO.sub.4 and HF. These 
species occur in water-containing solutions in nonionized form when either 
the pH of the solution or the oxidation state of the element of the second 
group, or both, has the appropriate value. Hydrogen fluoride appears in 
the recitations for both groups, as HF can be present in solution in 
either ionized or nonionized form. 
Generally, at least one member of the first group is separated from at 
least one member of the species of the second group. Although the 
separation according to the invention can be carried out for all of the 
elements, acids and species recited for the first and second groups, in 
practise, solutions usually contain a limited number of the elements and 
acids of the first group that are to be separated from a limited number of 
nonionized species of the second group. 
The solution to be treated may be fed directly to an electrodialysis cell 
if the species to be separated are present in the desired ionized and 
nonionized form. In many cases, it is necessary to adjust the pH of the 
solution or the oxidation state of the element of the second group in the 
solution, and in some cases both, to ensure that the species of the first 
group are present in the ionized form and the species of the second group 
are fixed in the nonionized form. 
The specific value of the pH and/or the oxidation state is dependent on the 
solution and the species of the first and the second groups that are 
present. An adjustment of the pH may be carried out by adding suitable 
acidic or alkaline material in an amount sufficient to attain the desired 
value of the pH. An adjustment of the value of the oxidation state of an 
element of the second group may be carried out by oxidizing or reducing 
the solution as required by any one of a number of suitable methods. The 
adjustment of the oxidation state is usually carried out by adding 
suitable oxidizing or reducing agent such as, for example, oxygen, ozone, 
hydrogen peroxide, chlorine, hydrazine, or sulfur dioxide, in an amount 
necessary to attain the desired value of the oxidation state. 
Alternatively, oxidation or reduction of solution may be accomplished 
electrolytically. The specifying of a solution redox potential represents 
a convenient guide for determining the equilibrium condition of the 
electrochemical reactions involved in the oxidation or reduction. 
Adjustment of pH and/or oxidation state are carried out such that the 
species of the first group remain present in solution in ionized form and 
remain separable from species of the second group. 
For elements of the second group, the practical values of the pH and the 
values of the oxidation state that cause species of the second group to be 
present in water-containing solutions in the preferred nonionized form are 
given in Table I. Also given, as a guide for achieving the preferred 
oxidation states of the elements of the second group, are values of the 
redox potential based on standard conditions, as derived from 
thermodynamic data. Predictable deviations from these values based on, for 
example, temperature and chemical activity of a given system can be 
expected. 
TABLE I 
______________________________________ 
Preferred Pure Solution 
Oxidation Redox Potential 
Preferred 
Element pH State in mV Species 
______________________________________ 
B &lt;5 +3 200 to 1200 
H.sub.3 BO.sub.3 
C &lt;6 +4 400 to 1000 
H.sub.2 CO.sub.3 
C &lt;4 +2 &lt;-400 HCO.sub.2 H 
Si &lt;9 +4 &gt;-200 H.sub.2 SiO.sub.3 
Ge &lt;7 +4 &gt;-100 H.sub.2 GeO.sub.3 
P &lt;1.8 +5 &gt;-150 H.sub.3 PO.sub.4 
As 0.5 to 5.5 
+3 &gt;200 HASO.sub.2 
As 0.5 to 3.5 
+5 &gt;200 H.sub.3 AsO.sub.4 
Sb 1 to 10 +3 -100 to 400 
HSbO.sub.2 
Se 0.5 to 2 +4 800 to 900 
H.sub.2 SeO.sub.3 
Te &lt;6 +6 &gt;1200 H.sub.2 TeO.sub.4 
F &lt;2 -1 100 to 1100 
HF 
______________________________________ 
It is further understood that the pH and redox potential values in Table I 
are the maximum values or broadest ranges for preferred species of the 
elements of the second group, and that in many cases lower values or 
narrower ranges must be used to ensure that species of the first group 
remain present in ionized form. Again, the redox potentials are given as a 
guide for solutions in which the species of the second group are 
predominant. Deviations from the given values can be expected for more 
complex solutions. Nevertheless, the conversion to the preferred oxidation 
state can usually be achieved by standard methods. 
To recapitulate, with or without adjusting either pH or oxidation state or 
both, the species of the first group are present in the ionized form and 
are selected from at least one ion of the group consisting of H.sup.+, 
Li.sup.+, Na.sup.+, K.sup.+, Cs.sup.+, Be.sup.2+, Mg.sup.2+, Ca.sup.2+, 
Sr.sup.2+, Ba.sup.2+, Ti.sup.4+, Zr.sup.4+, V.sup.5+, Cr.sup.3+, 
Mn.sup.2+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Rh.sup.3+, Ir.sup.4+, 
Ni.sup.2+, Pd.sup.2+, Pt.sup.4+, Cu.sup.2+, Ag.sup.+, Au.sup.+, Zn.sup.2+, 
Cd.sup.2+, Hg.sup.2+, Al.sup.3+, Ga.sup.3+, In.sup.3+, Tl.sup.+, 
Sn.sup.4+, Pb.sup.2+, Bi.sup.3+, NO.sub.3-, SO.sub.3.sup.2-, 
SO.sub.4.sup.2-, Cl.sup.- , Br.sup.-, I.sup.- and F.sup.-, and the species 
of the second group are present in the nonionized form and are at least 
one species selected from the group consisting of H.sub.3 BO.sub.3, 
H.sub.2 CO.sub.3, HCO.sub.2 H, H.sub.2 SiO.sub.3, H.sub.2 GeO.sub.3, 
H.sub.3 PO.sub.4, H.sub.3 AsO.sub.4, HAsO.sub.2, HSbO.sub.2, H.sub.2 
SeO.sub.3, H.sub.2 TeO.sub.4 and HF. 
Any solids content in the feed solution is reduced prior to electrodialysis 
to less than 5 ppm, preferably less than 1 ppm, by any one of a number of 
suitable means. The feed solution is fed to an electrodialysis unit. The 
electrodialysis unit comprises a multiplicity of vertically arranged, 
alternating cation permselective exchange membranes and anion 
permselective exchange membranes, a cathode compartment and an anode 
compartment. The cation and anion permselective exchange membranes are 
membranes that have a high permselectivity for the ionized species of the 
first group and have a low permselectivity for the nonionized species. 
Suitable cation permselective membranes are, for example, strongly acidic 
membranes which have a membrane matrix of a styrene di-vinyl benzene 
copolymer on a polyvinyl chloride base and possess sulphonic acid radicals 
(R--SO.sub.3 H) as active groups. The active groups comprise 3-5 
milli-equivalents per gram of dry resin which is satisfactory to provide 
the desired selectivity for cations. In particular, we have found that 
suitable cationic permselective membranes are Selemion.TM.-type membranes 
such as Selemion.TM. CMR, Selemion.TM. CMV, Selemion.TM. CSV and 
Selemion.TM. CSR. 
Suitable anion permselective membranes are, for example, basic membranes 
with tertiary amine or quaternary ammonium active groups, such as, for 
example, derived from trimethylamine (for example, 
R--N(CH.sub.2).sub.3.Cl), at 3-5 milli-equivalents per gram of dry resin, 
and having a matrix of a styrene di-vinyl benzene copolymer on a polyvinyl 
chloride base. Other suitable anion permselective membranes are certain 
aliphatic anion membranes which have a lower electrical resistance. 
Selemion.TM. ASV, Selemion.TM. ASR, Selemion.TM. AMV and Selemion AAV are 
particularly suitable. The Selemion.TM. membranes, which are manufactured 
by the Asahi Glass Company of Tokyo, Japan, have the desired properties. 
It is understood that membranes with similar properties produced by other 
manufacturers, such as Neosepta.TM. membranes produced by the Tokuyama 
Soda Co. Ltd. of Japan, and Ionac.TM. membranes produced by the Ionac 
Chemical Company, are similarly suitable, and that the use of combinations 
of other membranes may yield the desired results. 
The alternating cationic and anionic membranes form a number of alternating 
diluate cells and concentrate cells that are situated between the anode 
compartment and the cathode compartment. The anode and the cathode in the 
respective compartments are made of suitable materials, i.e., those that 
are compatible with the generation of oxygen at the anode and the 
evolution of hydrogen at the cathode. For example, the anode can be made 
of platinum or platinum-coated titanium and the cathode of stainless 
steel. The cathode can also be advantageously made of a material with a 
low hydrogen overvoltage such as platinum-coated titanium, to favour 
hydrogen evolution over the deposition of material on the cathode. A 
source of direct current is connected to the electrodes. 
The feed solution is fed to the diluate cells, and diluate is withdrawn 
from the diluate cells. A concentrate, i.e. a solution concentrated in 
ionized species of the first group, is withdrawn from the concentrate 
cells, 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 
cells. This can be achieved by passing solution through the cells at a 
sufficient rate, such as achieved by recycling a portion of the diluate 
and a portion of the concentrate to the diluate and concentrate cells, 
respectively. 
During electrodialysis, 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 cells. 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 cells is less 
than the desired withdrawal rate of concentrate from the concentrate 
cells, it will be necessary to feed a receiving solution to the 
concentrate cells. For example, the receiving solution may be chosen from 
water, dilute acid and a dilute salt solution compatible with the solution 
being treated and the general operation of the electrodialysis. 
In the cathode and anode compartments the predominant reactions are 
hydrogen and oxygen evolution, respectively. However, small amounts of 
material may deposit on the anode and cathode, respectively. Deposition on 
the electrodes is undesirable and should be kept at a minimum. Deposition 
can be controlled and kept at a minimum by a number of means such as by 
arranging the membranes in the electrodialysis unit such that an anionic 
membrane forms an end membrane, i.e., is the membrane next to the cathode 
compartment (a cationic membrane is then next to the anode compartment), 
by selecting a large enough electrode rinse flow to minimize the 
concentration of any contaminants, i.e. species transferred from the feed 
solution, by maintaining a pH of less than about four in the cathode 
compartment, or by using a cathode material with a low hydrogen 
overpotential to promote the evolution of hydrogen over metal deposition. 
For example, a cathode material of platinum-coated titanium may be 
advantageously employed. Any one of the above described means may be used 
alone or in combination with one or more of the other means to control 
deposition at the electrodes. 
The cathode and the anode compartments are rinsed with circulating rinse 
solutions. If less control of conditions is permissible, a common rinse 
solution may be circulated to both the electrode compartments. The rinse 
solutions may be chosen from water, dilute acid and sodium sulfate 
solution maintained at a pH in the range of about 0 to 4. The solutions 
are preferably solutions of sodium sulfate. Suitable rinse solutions 
contain sodium sulfate in a concentration in the range of about 0.1 to 
1.0M, and are maintained at a pH at values in the range of about 0.5 to 
4.0. Values in the higher end of the pH range are preferred for rinsing 
the anode. The rinse solutions are circulated through the electrode 
compartments at rates sufficient to give a differential pressure across 
the membranes of less than about 150 kPa, preferably less than about 50 
kPa. The rates are generally in the range of about 25 to 90 L/h.m.sup.2, 
and preferably in the range of about 40 to 80 L/h.m.sup.2. Rates in these 
ranges ensure low concentrations of contaminants that may deposit or yield 
undesirable compounds such as, for example, arsine when arsenic is present 
in the solution. A portion of the rinse solutions may be removed from 
circulation and be replaced with a substantially equal portion of fresh 
solution. The concentration of contaminants in the rinse solutions is 
preferably maintained at about 50 mg/L or less. 
During electrodialysis, the ionized species in the feed solution pass from 
the diluate cells to the concentrate cells through the cationic and 
anionic permselective membranes respectively, leaving substantially all 
nonionized species in the diluate cells. The gases evolved at the 
electrodes are carried from the cathode and anode compartments in the 
rinse solutions. 
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 60.degree. C., i.e. from about 0.degree. to 60.degree. C. At the 
higher temperatures, the process is more efficient but the life of the 
membranes is reduced. The process is preferably operated with solution 
temperatures in the range of about 20.degree. to 50.degree. C. 
Feed rates to the electrodialysis unit may be selected in the range of 
about 2 to 40 L/h.m.sup.2 per membrane pair, the selected value being 
dependent on the concentrations of species in the feed solution and the 
value of the current density. The flow rate of solutions through the 
concentrate and diluate cells should provide turbulent flow. The flow 
rates through the concentrate and diluate cells and the anode and cathode 
compartments should be substantially balanced in order to maintain a 
differential pressure across the membranes not exceeding about 150 kPa, 
preferably, less than 50 kPa. 
The current applied to the electrodes is controlled such that the membrane 
current density (applied current per membrane surface area) is sufficient 
to effect the desired separation of species but minimizes water splitting. 
The current is equivalent to a current density in the range of about 10 to 
500 A/m.sup.2, the particular value selected being a function of the 
concentrations of species in the feed solution. Below about 10 A/m.sup.2, 
the ionic transfer rate is too low and above about 500 A/m.sup.2 there may 
not be enough ionized species to replenish the ions transferred from the 
diffusion layer at the membrane and, as a result, water splitting and/or 
loss of permselectivity would occur to an undersirable extent. Water 
splitting and permselectivity loss can be substantially obviated when 
operating with current densities in the preferred range of about 50 to 350 
A/m.sup.2 under conditions of turbulence in the concentrate and diluate 
cells. 
The electrodialysis may be carried out in one or in more than one stage. 
Although one-stage electrodialysis may be effective to reduce 
concentrations of ionized species in the diluate to the desired low 
concentrations, it may be desirable to have more than one stage. In more 
than one stage, the stages are preferably connected in series, diluate 
withdrawn from one stage being fed to the diluate cells of a subsequent 
stage whereby concentrations of ionized species may be further reduced. If 
desired, the concentrate may be further concentrated by electrodialysis, 
by feeding the concentrate withdrawn from concentrate cells from a first 
stage electrodialysis to the diluate cells of a subsequent stage. Such a 
step may be advantageous to effect further separation. Diluate from such a 
subsequent stage of electrodialysis of concentrate may be returned as feed 
to the first stage electrodialysis. The concentrate and diluate recovered 
from the process may be subjected to further treatment. Further treatment 
may be effected by electrodialysis or other known treatment methods for 
the recovery and separation of values, especially when the concentrate and 
diluate contain multiple species. 
If needed, the membranes may be cleaned periodically and mechanically to 
remove any solid deposits. The membranes may also be cleaned chemically 
with a suitable acid solution followed by adequate rinsing with water. The 
electrodes may be cleaned with dilute sulfuric acid. 
The invention will now be illustrated by means of the following 
non-limitative examples.

EXAMPLE 1 
Various simulated metallurgical process solutions were treated using the 
method of the invention in a seven compartment electrodialyzer having 
alternating Selemion.TM. CMV cationic and Selemion.TM. AMV anionic 
membranes. The end compartments were the anode and cathode compartments 
separated from their respective neighbouring compartment by an anionic and 
a cationic membrane, respectively. A platinum anode and a stainless steel 
cathode were used. 
The diluate cells were initially filled with the feed solution and the 
concentrate cells with dilute sulfuric acid (pH 2). A rinse solution 
containing 15 g/L Na.sub.2 SO.sub.4 adjusted to pH 2 with sulfuric acid 
was circulated through both electrode compartments. Direct current was 
applied between the electrodes and the electrode current density was 
allowed to decrease towards the end of each test as the ionized species 
were being depleted in the diluate cells. Oxygen and hydrogen evolved at 
the anode and cathode, respectively, and some metal deposition occurred on 
the cathode. 
In the first test, a synthetic smelt-gas scrubber solution containing 
arsenic and cadmium was treated after the pH was adjusted to 1.7 with the 
addition of 5.5 g/L H.sub.2 SO.sub.4. In the second test, a zinc and 
arsenic-containing solution with a pH of 4.7 was treated; no pH adjustment 
was necessary. In the third test, a copper, nickel and arsenic-containing 
feed solution was treated after pH adjustment to pH 0.8 by addition of 20 
g/L H.sub.2 SO.sub.4. In all three tests, arsenic was present as AS(III), 
which is suitable for separation by the process. The redox potential in 
the three cases ranged from 270 mV to 350 mV as measured with a 
Pt(Ag/AgCl) probe. The test results are given in Table II. 
TABLE II 
______________________________________ 
Concentration of Species 
Test Time Current Density 
in Diluate in g/L 
No. in h in A/m.sup.2 As Cd Zn Cu Ni 
______________________________________ 
1 0 50 11.5 5.0 -- -- -- 
3 -- 10.5 0.88 -- -- -- 
4 -- 10.7 0.22 -- -- -- 
5 10 10.5 0.05 -- -- -- 
2 0 25 4.5 -- 5.1 -- -- 
4 -- 4.5 -- 1.7 -- -- 
5 10 4.45 -- 1.0 -- -- 
3 0 150 11.0 -- -- 5.10 0.94 
5 60 10.0 -- -- 0.74 0.14 
______________________________________ 
These tests show that ionized species are effectively separated from a 
nonionized species by electrodialysis at an appropriate pH of the feed 
solution. 
The following examples illustrate tests wherein metallurgical process 
solutions were treated in an electrodialyzer having 11 diluate and 10 
concentrate cells separated by anionic membranes from the electrode 
compartments, and using electrodes made of platinum-coated titanium. 
Cationic Selemion.TM. CMV and anionic Selemion.TM. AMV membranes were 
used. After completion of the tests the separation factors S.sub.A/B were 
calculated, wherein A represents an ionized species and B a nonionized 
species. Separation factors are defined as 
##EQU1## 
wherein A(conc.), A(dil.), B(dil.) and B(conc.) represent the 
concentrations of A and B in the concentrate and the diluate. 
EXAMPLE 2 
A synthetic scrubber purge solution from a lead smelter, containing Cd, Cl 
and F ions, H.sub.2 SO.sub.4 and As (in the trivalent state), was fed into 
a diluate stream recirculating through the diluate cells. A concentrate 
stream, initially dilute H.sub.2 SO.sub.4 (pH 2), was circulated through 
the concentrate cells. The electrode compartments were rinsed with a 
circulating rinse solution containing 15 g/L Na.sub.2 SO.sub.4 maintained 
at pH 2 with addition of H.sub.2 SO.sub.4 as required. To ensure turbulent 
conditions in the diluate and concentrate cells, the linear velocity of 
solutions through the cells was maintained at 5 cm/sec. The rinse solution 
was circulated at a rate such that the differential pressure between 
diluate and rinse streams was less than 20 kPa. 
The feed solution had a pH of 1.8, requiring no adjustment, and was fed at 
7 L/h.m.sup.2 (membrane pair area). Rinse solution was fed at 44 
L/h.m.sup.2 (electrode area). Electrodialysis was carried out for 7 hours 
at 32.degree. C. and a constant current density of 135 A/m.sup.2. The 
concentrations of species in feed, diluate and concentrate at the end of 
the test are given in Table III. 
TABLE III 
______________________________________ 
Concentration in g/L 
Species Feed Diluate Concentrate 
______________________________________ 
As 11.5 11.3 1.1 
Cd 4.0 0.1 21.0 
Cl 1.2 0.07 6.1 
F 1.4 0.9 1.8 
H.sub.2 SO.sub.4 
5.0 0.5 28.5 
______________________________________ 
The separation factors were calculated: S.sub.Cd/As =2157, S.sub.Cl/As =895 
and .sup.S H.sub.2 SO.sub.4 /As=586. 
The results show that Cd, Cl and H.sub.2 SO.sub.4 are efficiently removed 
from arsenic as the nonionized species but the removal of F was only 35%. 
Fluoride removal can be increased by neutralizing diluate to pH 3.5 to 5.0 
to dissociate HF more completely, and feeding neutralized solution to a 
second stage of electrodialysis. As a result of the membrane arrangement 
and the high rinse solution flow rate, metal deposition on the cathode was 
negligible. 
EXAMPLE 3 
Using the same method, rinse solution, rates and velocity as in Example 2, 
a feed solution, such as may be encountered in metallurgical processing to 
produce copper, and containing Ni and Cu ions, H.sub.2 SO.sub.4 and As as 
As(III) was electrodialyzed at 37.degree. C. for 7 hours at a constant 
current density of 190 A/m..sup.2. The feed solution had a pH of 1.6 and a 
redox potential of 330 mV (Pt-Ag/AgCl). The results are given in Table IV. 
TABLE IV 
______________________________________ 
Concentration in g/L 
Species Feed Diluate Concentrate 
______________________________________ 
As 3.8 3.3 2.1 
Cu 4.4 0.12 33.0 
Ni 1.0 0.04 6.2 
H.sub.2 SO.sub.4 
4.0 0.3 34.0 
pH 1.6 2.3 0.6 
______________________________________ 
The results show that Cu, Ni and H.sub.2 SO.sub.4 were efficiently 
separated from arsenic. Metal deposition on the cathode was negligible. 
The separation factors were calculated: 
EQU S.sub.Cu/As =432, S.sub.Ni/As =244 and .sup.S H.sub.2 SO.sub.4 /As=178. 
EXAMPLE 4 
In the production of zinc, the leaching of antimony containing feed stocks 
results in the solubilization of Sb in the Zn electrolyte (small amounts 
of antimony may also be added to Zn electrolyte, along with other 
additives, for efficient Zn electrowinning). 
Using the same method, rinse solution, rates and velocity as in Example 2, 
a feed solution such as may be encountered in the production of zinc, and 
containing Zn, trivalent Sb and sulfuric acid was electrodialyzed for 7 
hours at two different values of the pH and current densities. To maintain 
Zn in ionized form a pH value of less than 5.4 had to be maintained while 
Sb was present as the nonionized form HSbO.sub.2. The results are given in 
Table V. 
TABLE V 
______________________________________ 
Separa- 
Current tion 
Density Species Concentration in g/L 
factor 
Test in A/m.sub.2 
and pH Feed Diluate 
Concentrate 
S.sub.Zn/Sb 
______________________________________ 
1 100 Zn 3.8 0.7 26.0 13.5 
Sb 4.2 2.9 8.0 
pH 1.9 1.6 1.0 
2 85 Zn 4.8 0.6 34.0 151 
Sb 4.2 4.0 1.5 
pH 3.0 2.8 1.8 
______________________________________ 
The results show that Zn can be effectively separated from Sb and that the 
value of the pH has a marked effect on the separation, the separation 
factor increasing about elevenfold when the pH was increased from 1.9 to 3 
(possibly reducing the presence of any SbO.sup.+ ions). 
EXAMPLE 5 
Using the same method, rinse solution and velocity as in Example 2, a 
synthetic scrubber purge solution containing Cd and As mostly as As (III) 
was subjected to multistage electrodialysis after adjustment of the pH to 
a value of about 2.0. If As is present as As (III) the range of pH values 
is 0.5 to 5.5, 2 to 4 being preferred, and when present as As (V) the 
range is 0.5 to 3.5, preferably a value of 1.5. 
The concentrate and diluate streams generated in the first stage 
electrodialysis were treated separately in a second electrodialysis. The 
results are given in Table VI. 
TABLE VI 
__________________________________________________________________________ 
Stage Feed Current Species 
Concentrations 
Separation 
and rate density 
Temp. 
and in g/L Factor 
feed L/h .multidot. m2 
A/m.sup.2 
.degree.C. 
pH Feed 
Diluate 
Conc. 
S.sub.Cd/As 
__________________________________________________________________________ 
first, 
7 130 32 As 11.5 
11.3 
1.0 
original Cd 4.0 
0.12 
22.0 
feed pH 2.0 
1.9 1.4 2072 
second, 
14 100 30 As 11.3 
11.2 
0.8 
1st stage Cd 0.12 
0.02 
0.9 
diluate pH 1.9 
1.8 1.3 630 
second, 
3.5 150 31 As 1.0 
0.9 0.3 
1st stage Cd 22.0 
0.3 47.0 
concentrate pH 1.4 
1.4 1.2 470 
__________________________________________________________________________ 
The results show that an arsenic and cadmium-containing solution can be 
effectively treated in two stages for the separation of these species 
yielding a final As-containing stream substantially free of Cd and a final 
Cd-containing stream substantially free of As. 
EXAMPLE 6 
A solution from a zinc plant containing Zn and Ge was treated for the 
recovery of a diluate suitable for the recovery of germanium. Using the 
same method, rinse solution, flow rates and velocity as in Example 2, feed 
solution was adjusted to pH 2 with H.sub.2 SO.sub.4. A suitable pH for the 
separation may be chosen in the range of 1 to 5. 1.2 L of the feed 
solution was recirculated through the diluate cells for 100 minutes, and a 
concentrate stream (initially dilute H.sub.2 SO.sub.4 and pH 2) was 
recirculated through the concentrate cells. The current density was 
allowed to vary from a maximum of 320 A/m.sup.2 to 30 A/m.sup.2 at the end 
of the 100 minute period. The temperature was 28.degree. C. The results 
are given in Table VII. 
TABLE VII 
______________________________________ 
Concentration in g/L 
Species Feed Diluate Concentrate 
______________________________________ 
Ge 1.7 1.5 0.7 
Zn 2.4 0.03 9.2 
______________________________________ 
A portion of the original solution was treated with NaOH to increase the pH 
to a value of 9 to precipitate Ge. The precipitate contained 28% Ge and 
39% Zn. When the diluate from the electrodialytic separation was similarly 
treated, the precipitate contained 66% Ge and 1.4% Zn. The results show 
that Ge was effectively separated from Zn, and a Ge concentrate was 
upgraded from 28 to 66% while reducing Zn contamination to 1.4%. Further 
upgrading can be obtained in two stages of electrodialysis with recycle of 
the second stage diluate to the first stage and generation of a more 
concentrated zinc solution. 
EXAMPLE 7 
This example illustrates the separation of Hg, Na and Cl from Se by 
treating crude calomel (Hg.sub.2 Cl.sub.2) recovered from a treatment of 
roaster gases. The crude calomel, containing on a dry weight basis 74.3% 
Hg, 0.65% Se, 3.65% Pb and sodium was made into a slurry. The slurry was 
treated with chlorine gas until the redox potential had increased from 
350mV to 1160mV (Pt vs. Ag/AgCl) and substantially all of the mercury had 
dissolved and leaving lead in the residue. The resulting solution 
contained 75 g/L Hg, 1.46 g/L Se and 10.2 g/L Na. The solution was diluted 
with water to give a feed solution for electrodialysis containing 37.5 g/L 
Hg, 0.73 g/L Se and 5.1 g/L Na. The pH of the feed solution was 1.1, and 
required no adjustment. Using the same method as in Example 2, 
electrodialysis is carried out at a current density of 320 A/m.sup.2 and a 
feed rate of feed solution to the diluate cells of 5 L/h.m.sup.2. Major 
portions of the Hg and Na are transferred to the concentrate streams while 
Se substantially remains in the diluate stream. The results of analysis of 
concentrate and diluate streams are shown in Table VIII. 
TABLE VIII 
______________________________________ 
Concentration in g/L 
Species Diluate Concentrate 
______________________________________ 
Hg 7.2 55.6 
Na 1.6 8.3 
Se 1.3 0.03 
______________________________________ 
The separation factors are calculated to be: 
EQU S.sub.Hg/Se =335 and S.sub.Na/Se =225 
EXAMPLE 8 
This example illustrates the separation of As and B from a brine containing 
mostly the chlorides and some sulfates of Na, K, Li, Ca and Mg. The pH of 
the brine was 7.3, and, to effect a separation, the pH was adjusted to 1.3 
by the addition of hydrochloric acid. During electrodialysis the pH of the 
diluate was maintained at 2.0. 
Using the same method, rinse solution, flow velocity and differential 
pressure as in Example 2, electrodialysis was carried out at ambient 
temperature, a current density of 250 A/m.sup.2 and a feed rate of 
pH-adjusted feed solution to the diluate cells of 7 L/h.m.sup.2. The 
electrodialysis unit was an Asahi Glass Model CS-0 unit provided with 
Selemion.TM. CMV and ASR membranes. Electrodialysis was continued for 
eight hours. The compositions of the feed solution final diluate and final 
concentrate, and the calculated separation factors are given in Table IX. 
TABLE IX 
__________________________________________________________________________ 
Concentration in mg/L 
Stream Na G Li Ca Mg HCl 
SO.sub.4 
As B 
__________________________________________________________________________ 
Feed 100 
97 600 
10 11000 
1400 
2650 
2800 
365 
Diluate 
10 6 60 0.5 
800 230 
700 3200 
435 
Concentrate 
610 
590 
3500 
75 65000 
8200 
12300 
770 
140 
S.sub.M/As 
254 
409 
242 
623 
338 148 
73 
S.sub.M/B 
190 
305 
181 
466 
252 110 
55 
__________________________________________________________________________ 
The results show that As and B can be separated from Na, K, Li, Ca, Mg, HCl 
and H.sub.2 SO.sub.4 by the method of the invention. 
EXAMPLE 9 
This example illustrates that Ge, As and F can be separated from Na, K, Mg, 
Zn, Fe, Ni, Co, HCl and H.sub.2 SO.sub.4. The germanium and arsenic were 
in the oxidation states of four and three, respectively. Using the same 
method rinse solution, velocity and differential pressure as in Example 2, 
electrodialysis was carried out for eight hours at ambient temperature, a 
current density of 250 A/m.sup.2 and a feed rate of feed solution to the 
diluate cells of 7 L/h.m.sup.2. The pH of the diluate was maintained at 
1.6 by the addition of sulfuric acid to the incoming feed. The flow rate 
of the diluate was 5.95 L/h.m.sup.2 and that of the concentrate was 1.05 
L/h.m.sup.2. The compositions of the feed, diluate and concentrate streams 
are shown in Table X. 
TABLE X 
______________________________________ 
Concentrations in mg/L 
Species Feed Diluate Concentrate 
______________________________________ 
Na 1000 200 5300 
K 20 4 102 
Mg 800 200 3900 
Zn 15000 4000 73000 
Fe 1100 240 5700 
Ni 595 160 2850 
Co 200 70 900 
Cl 6770 660 39200 
H.sub.2 SO.sub.4 
6800 2500 27600 
F 110 63 295 
Ge 460 520 120 
As 1200 1200 1200 
______________________________________ 
Some separation factors were calculated to be: 
EQU S.sub.Cl/F =12.7, S.sub.Cl/Ge =25.7 and S.sub.Cl/As =59.4 
It is noted that the value of the pH of 1.6 maintained in the diluate was 
favourable for achieving a nearly complete separation from germanium, but 
represented a compromise for the simultaneous separation from arsenic and 
fluorine. For efficient separation from arsenic alone, a pH value of 
between two and four is preferred, while for efficient separation from 
fluorine (HF) a pH value of 0.5 is preferred. 
EXAMPLE 10 
This example illustrates the separation of Zn, Fe, Na, Mg and H.sub.2 
SO.sub.4 from Si, F, As and P by treating impure wet-process phosphoric 
acid. Using the method described in Example 2, the phosphoric acid was fed 
at a rate of 7 L/h.m.sup.2 to the electrodialysis cell. The apparent pH of 
the acid was 0.08. Electrodialysis was carried out at 500 A/m.sup.2 and 
37.degree. C. The diluate and concentrate stream were analyzed. The 
oxidation state of the species and their concentrations in the process 
streams are given in Table XI. 
TABLE XI 
______________________________________ 
Oxidation 
Concentrations in g/L 
Species State Feed Diluate 
Concentrate 
______________________________________ 
Zn +2 25.2 9.9 78.5 
Fe +2 21 9.9 55.5 
Na +1 0.14 0.08 0.5 
Mg +2 22 8 70 
H.sub.2 SO.sub.4 
-- 32 5 135 
Si +4 3.4 3.2 1.0 
F -1 21.3 22.4 0.8 
As +3 0.25 0.27 0.07 
P +5 21 20 5 
______________________________________ 
The separations achieved are reflected in the calculated separation factors 
shown in Table XII. 
TABLE XII 
______________________________________ 
Species (M) 
Separation factors 
Zn Fe Na Mg H.sub.2 SO.sub.4 
______________________________________ 
S.sub.M/Si 25 18 11 28 86 
S.sub.M/F 220 160 100 245 760 
S.sub.M/As 31 22 14 34 100 
S.sub.M/P 32 22 14 35 110 
______________________________________ 
Wet process phosphoric acid contains, in addition to the above-noted 
impurities, certain elements such as Mn, Y, La, U, Al, Cr, Ca that are 
present in parts per million. These elements can also be separated but 
their separation is masked by the high concentrations of the elements 
specified in Table XI. The appropriate separations may be effected by 
diluting the acid prior to electrodialysis, with subsequent concentration 
of the product streams or, alternatively, by multistage electrodialysis.