Note: Descriptions are shown in the official language in which they were submitted.
~697~
This invention relates to a process For treatlng sol(ltions
which contain metallic materials and more particularly it relates to
an improved electrochemical process for decreasing the metallic content
of a solution.
In ~arious industries, solutions are utilized which contain
metallic materials, and the disposal of these poses a significant
pollution problem. For example, in the metal plating industries, the
plating baths contain copper, zinc and similar metals and various hexa~
valent chromium compounds are frequently added to much of the cooling
water used in various industrial processes, to inhibit corrosion and
retard the growth of algae. Additionally, the effluent from numerous
processes, such as chlor-alkali processes, frequently contains mercury
or lead. Although heretofore, various chemical techniques have been
proposed for the treatment of such metallic containing effluents, these
ha~e generally been either inefficient or to expensive or have resulted
in the formation of products whose dispos,ll presents as many pollution
problems as the metallic materials themselves. Accordingly, there has
recently been a great deal of effort expended in the development of new
and different processes for the treatment of these metallic containing
effluent solutions.
In Belgium patent 739,684, for example, there is described
an electrochemical technique wherein a semi-conductive bed of solid
particles is used to oxidize various substances to non-toxic forms.
Another process, utilizing an electrochemical technique, is described
in New Scientist, June 26, 1969, page 706. In these and similar pro-
cesses which have recently been proposed, the electrochemlcal systems
utillzed ha~e been found to be both inefficient, and/or uneconomical
and require frequent changing of the bed of particles which is utilized.
Accordingly, these systems have not met with any appreciable commercial
utilization.
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It is, therefore, an object of the present invention to
provide an improved process for treating solution~ containiny
metallic materials so as to reduce the metallic content of such
solutions.
A further object of the present invention is to provide
an improved process for reducing the metallic content of a solu-
tion by means of an efficient and econornical electrochemical treat-
ment.
Additionally it is an object of the present invention
to provide an improved process for reducing the hexavalent
chromium content of a solution by means of an efficient and
economical electrochemical treatment.
These and other objects will become apparent to those
skilled in the art from the description o~ the invention which
follows.
Pursuant to the above objects, the present invention
includes a process for treating a solution containing metallic
materials to decrease the metallic content thereof which com-
prises passing an electric current through the solution which
contains the metallic materials, which solution is contained as
the electrolyte in a cell, said cell having at least one positive
and one negative electrode, between which the current is passed,
the electrodes being separated by a diaphragm, and wherein the
electrolyte also contains a bed of particles, distributed therein
such that the porosity of the bed is from about 40 to 8~/o, poro-
sity defined as
1 ¦volume of particles ~ X 100
volume of cell wherein -
~articles are distributed
By carrying out the electrochemical tre,atment of the solutions
containing metallic materials in this manner, it has ~een found ; -
to be possible to reduce the concentrations of these metals in
the solutions from the parts per million level to the parts per
~illion level.
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~4~97~
More specifically, in the practice of the method of the
present inven-tion, the solutions which are electrolyzed to
effect the reduction in the metallic content thereof, i.e., the
electrolyte
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~69~75~
solutions in the cell, may be various solutions which contain metallic
materials although, preferably, these are aqueous solutions. These
solutions may contain varying amounts of the metallic materials,
solutions containing as much as 10% by weight and as little as one
part per million of the metallic material being suitable for treatment
in accordance with the process of the present invention to effect a
reduction of the metal content. In referring to the metallic material
in the solutions, it is intended to include not only the metals them-
selves, and particularly the heavy metals such as lead, mercury, copper,
zinc, cadmium and the like, but also these metals in ionic form, such
as Pb+2, Hg~2, Hg~Cr~6and the like. These may be present as various
compounds or complexes, both organic and inorganic. Additionally,
since it is believed that the removal of the metallic materials from
the solutions treated by the present process involves reduction, the
materials going through various electrochemical reductions and result-
~ ing ultimately in the metal itself which is deposited out at the
; cathode, the solutions treated may also contain various reduced states
of the metallic materials as well.
~he solutions containing metallic materials which are to be
treated in accordance with the present method may come ~rom various ~ -~
sources. Thus, for example, they may be effluent stréams from industrial
plants which have relatively high concentrations of the metallic materials,
as have been indicated heretofore. Additionally, however, the solutions
; treated may have a relatively low concentration of metallic materials,
e.g. one part per million or less, which solutions may come from municipal
or other water treating plants. Thus, the method of the present invention
may be used not only to reduce the relatively high content of metallic
materials in industrial and similar waste streams, but, additionally,
may also be used to effect substantially complete removal of relatively
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6979
small amounts of me-tallic materials, as a final puri~ication
step in the treatment of water intended for human consumption.
The solutions treated rnay also contain various other components,
in addition to the metallic materials and may include mixed
effluent streams from several different industrial processes.
Thus, for example, the solutions may contain, in addition to
the metallic materials of mercury, lead, cadmium and zinc,
various chloride materials, such as chlorinated organics,
chlorine, HCl, hypochlorites, hypochlorous acid, as well as
sulfates, fluorides, phosphates, and the like, as are typically
present in plating bath and chlor-alkali process effluents. Such -
solutions are, however, merely exemplary of the effluent
solutions which may be treated.
The pH of the solution to be treated may vary over a
wide range, being either acidic, neutral or basic, pH values of
from about 1 to 14 having been found to be suitable. Desirably,
where lead is the metal being removed, the pH is from about 4 to
7, and a pH of from about 6 to 13 being preferred when the
metal is mercury and a pH of from about 5 to 10 with a pH range
~f from about 6 to g being preferred when the metal is chromium,
Depending upon the makeup o~ the metal-containing solution which
is to be treated, adjustment of the pH may be done by the
addition of various "support" electrolytes to the metallic
solution. Suitable "support" electrolytes which may be used are -
aqueous solutions of borates, ammonia, sodium chloride, sulfuric
acid, calcium chloride, sodium cyanide, chloroacetates, sodium
hydroxide, sodium bicarbonate, hydrochloric acid, and the like. ~
The temperature of the electrolyte, i.e., the solution ~;
being treated, may also vary over a wide range, the only criteria
~30 being ~hat at the temperature used, the electrolyte remain a ~ ;
liquid. Thus, temperatures within the range of about 0 to 100C
have been found, generally, to be suitable. For economy in
operation, however, it has frequently been found to be preferred
to utilize these
~5-
~14~7g
solutions at ambient temperatures. Similarly, the present process
is desirably carried out at atmospheric pressure although either sub-
or super atmospheric pressures may be employed, if desired. It has
been found in some instances, however, that the use of elevated
temperatures, e.g.60-75C. may be desirable in effecting a more rapid
reduction in the ~etallic content, depending upon the particular
"support" electrolyte, pH range, type and concentration of metal which
are used.
As has been noted hereinabove, the electrolyte, i.e., the
solution being treated, is contained, during treatment, in a suitable
electrolytic cell and contains a bed of particles which are distributed
in the electrolyte in the cell, such that the porosity of the bed
ranges from about 40 to 80%, porosity being defined as:
l- volume of particles ~ X 100
volume of cell wherein
the particles are
; ~ distributed ~ ~
By determining the density of the particles used and weighing them,
the term "volume of the particles" in the above porosity formula
may be replaced by the value for the weight of the particles divided
`
by the true density of the particles. The particles density can be
measured by filling a one liter container with particles, the weight of
which is known. Then electrolyte is added to the container to fill
~ the voids between the particles, the amount of electrolyte ~eeded being: .:
measured as it is added. The true density of the particles, in grams
per cm3, is the weight of the particles in grams divided by the true
volume of the particles in cm3. The true volume of the particles is
the bulk volume minus the volume of the voids in the particle bed, the
latter being the volume of the electrolyte which is added to the one
- liter container. Thus, the true volume of the particles in this instance
would be 1000 cubic centimeters minus the volume of the voids, i.e. the
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volume of electrolyte added to the container.
It will, of course, be apparent that the porosity of the bed
of particles maintained in the electrolyte which is being treated in
the cell may be varied and that with different types oF particles,
under the same operating conditions or with similar particles under
different operating conditions, chan~es in the bed porosity will take
place. Thus~ the true density of the particles will vary depending
upon the porosity of the par~icles themselves, e.g., graphite as com-
pared to glass beads, with similar variations in density being effected
by the electrolyte itself because of the difference in the surface
tension of various electrolyte solutions. Additionally, since the
particles of the bed are generally dispersed or distributed by the flow
of the electrolyte through the cell, variations in the flow character-
istics w;ll also result in changes in the bed porosity.
To illustrate this latter situation, if a one liter container
were ~illed with particles of a particular size and shape, using the
formula given above, the porosity of this bed of particles would be:
volume of particles in cc ~ X 100 ~ -
1000 cc
If the same quant;ty of partlcles were then distributed by the flow
of the electrolyte, such that the volume of the bed now reached two
liters, using its same formula, the porosity of the bed is now
volume of particles in cc ~ X 100
\ 2000 cc J ~ ;
C1early, in the second instance, the porosity of the bed has increased
As has been noted above, the porosity of the bed of particles dispersed
in the electrolyte may range from about 40 to ~0%. In many instances,
- a preferred range for the bed porosity is from about 55 to 75% with a :
.: . .
speci~ically preferred range being from about 60% to 70%. ~
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i97~
The particles ernployed to from the porous bed in the present
process typically are solid, particulate materials that may be con-
ductive, non-conductive or semi-conductive. By "conductive" it is
meant that the material of which the particles are made will normally
be an electron-conducting material. Where the particles are conductive,
they may have a metallic surface, either by virtue of the particles
themselves being metallic or by being made of non-conductive material
on which a metallic surface has been deposited. Typical of the con-
ductive materials which may be employed are the metals of Group VIII
of the Periodic Table, such as ruthenium and platinum, as well as
graphite, copper, silver, zinc. Additionally, the conductive particles
may be electrically conductive metal compounds, such as ferrophosphorus,
; the carbides, borides or nitrides of various metal such as tantalum,
titanium, and zirconium, or they may be various electrically conductive
metal oxides, such as lead dioxide, ruthenium dioxide, and the like.
Where the particles are non-conductive, they may be made of various
materials such as glass, Teflon~ coated glass, and they may also be
sand, spheres and chips of various polymers such as polystyrene.
Exemplary of various semi-conductive materials of which the particles
may be made are fly ash, oxidized ferrophos(ferrophosphorus~, zirconia,
alumina, conductive glasses.
The particles used desirably range in size from about 5 to
5000 microns, with particle sizes of from about S0 to 2000 microns
being preferred. In many instances, a particularly preferred range of
particle size has been found to be from about 100 to 800 microns.
Although it is not essential to the successful operation of the process
o~ the present invention that all of the particles in the porous bed
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distributed in the electrolyte have the same size, for the most
preferred operation of the process, it has been found to be desirab1e
if the range of particle size is maintained as small as is practical.
It has further been found that the density of the particles
used should be such, that in conjunction with the si~e and shape of
the particles, it will provide the proper balance between the drag
force created by the electrolyte motion and the buoyancy and gravita-
tional forces required to achieve particle dispersion or distribution
at the desired bed porosity. Thus, where the particle dispersion is
established against or in opposition to the buoyancy force, the
particle densities typically may range from about 0.1 (less than the
density oF the electrolyte) to about 1.0 grams per cc. Where the par- -
ticle dispersion is achieved against or in opposition to the gravita-
tional force, the particle densities typically may range from about
l.l to lO grams per cc. and preferably from about 1.5 to 3.5 grams
per cc. The most preferred operating conditions have been found to
be when the particles are dispersed throughout the electrolyte, within
the cell, during the movernent of the electrolyte and when the particles
are more dense than the electrolyte.
The electrolytic cell may be oF any suitable material and
configuration which will permit electrolysis of the metallic containing
solution to effect a reduction in its metal content and which will ~ 1
permit retention of the porous bed of particles in the electrolyte,
within the cell. Exemplary of suitable materials of construction which
may be used for the cell are various plastics, such as the polyacrylates,
polymethacrylates, polytetrahaloethylenes, polypropylenes, and the like,
rubber, as well as materials conventionally used in the construction of
chlor-alkali cells such as concretes. Additionally, the cell may be
made of metal, such as iron or steel. In such instances, electrically
_ 9 _ ;:
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insulating coatings should be provided on the metal surfaces in the cell
interior or electrical insulation provided between the metal of the
cell and the electrode.
The size of the electrolytic cell may also vary widely,
depending upon the nature and quantity of the metallic containing
solution which is to be treated. Thus, where appreciable quantities are
involved, as in the treatment of industrial wastes or as a part of a
water purification system , the cell may be relatively large and include
a multiplicity of treating zones, whereas for the treatment of water
for~ndividual home use, appreciably smaller units may be utilized,
similar in size to conventional "soft-water" treating units. Additionally,
the cell may be of a suitable size so as to be portable, for use at
camp sites, and the like. Typically, the cell will have a suitable
inlet and outlet means for introducing and removing the solution to be
treated, means for retaining the porous bed of particles dispersed in
the electrolyte within the cell, means for supporting at least one positive
and one negative electrode in contact with the electrolyte in which the
porous bed of particles is distributed and, if desired, a diaphragm
dlsposed between the p~sitive and negative electrodes.
The electrolytic-cell has within it at least one positive
and one negative electrode. These are disposed within the cell so
- as to be in contact with the electrolyte in which is distributed -
the porous bed of particulate material. These electrodes may be formed
of various materials, as are known to those in the art. Typical of
suitable electrode materials which may be used are graphite, ruthenium
dioxide and, noble metals and their alloys, such as platinum, iridium,
and the like, both as such and as deposits on a base metal such as
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6'97~
titanium, tantalum, and the like; conductive cornpounds such as lead
dioxide, ~langanese dioxide, and th~Like; ~letals, such as cobalt,
nickel, copper, tungsten bronzes, and the like; and refractory metal
compounds, such as the nitrides and borides of tantalum~titanium,
zirconium, and the like.
The positive and negative electrodes will be positioned within
the electrolytic cell so as to be separated sufficiently to permit the
flow of the electrolyte through the cell and the movement of the particles
within the electrolyte. It will be appreciated, of course, that as the
separation between the electrodes is incr~s~d the voltage necessary to
effect the desired reduction in the metallic impurity content of the
electrolyte will also increase. Accordingly, in many instances it has
been found to be desirable that the separation between the positive and
negative electrode in the cell is from about 0.1 to 5.0 centimeters,
with a separation of from about 0.3 to about 3.0 centimeters being pre-
ferred and a separation of from about 0.5 to 2.0 centimeters being
particularly preferred. Although particular reference has been made
to an electrolytic cell having one positive and one negative electrode,
-
it wlll be appreciated that the cell may be provided with a plurality --~
; of electrode pairs, in much the same manner that such a plurality of
electrodes are normally utilized in various comMercial, large scale
electrolytic continuous processes.
It will, of course, be appreciated that in addition to the
amount of electrode separation, the flow of the electrolyte through
the electrode area will also be dependent upon the size and density
of the partic:las which are distributed in the electrolyte to form the
porous bed. Typically, this flow, which is described in terms of
,
the`linear flow velocity of the electrolyte, will be within the range
of from about 0.1 to 1000 centimeters per second. A preferred electolyte
flow velocity has been found to be from about 0.5 to 100 centimeters
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per second with a flow velocity of from about l to lO cent~meters per
second being specifically preferred. Under these operating conditions,
current densities within the range of about l.0 to 500 milliamps per
square centimeter have been found to be typical of those which are
utilized.
To further illustrate the present invention, reference is
made to the accompanying drawing which is a schematic diagram of a
system incorporating the electrolytic cell of the invention. As shown
in the drawing, this system includes an electrolytic cell (1) having a
fluid inlet (6) and a fluid outlet (9). Within the cell (l) are dis-
posed a positive electrode (2) and a negative electrode (3). Although
these electrodes are shown as being separated by a diaphragm (4), in
many instances, the use of such a diaphragm has not been found to be
necessary. Where such a diaphragm is used, e.g., to control the par-
ticles in the anolyte or catholyte compartments, the diaphragm may beformed of various materials, such as a Teflo ~, coated screen, Fiber-
glass~, asbestos, porous ceramics and the like. The important criteria
for the materials of which these diaphragms are made are that they
permit the passage of the hexavalent chromium ions and are not adversely
affected by the solutions being treated. In regard to the former, it
is believed that the reduction in hexavalent chromium content is effected
in the present process by the reduction of the Cr 6 to Cr 3 at the cathode
and the subsesquent precipitation of the Cr 3, probably as a trivalent
chromium hydroxide. Thus, the diaphragm serves not only to control the
particles in the anode and/or cathode compartments of the cell but,
additionally it helps minimi~e the back migration of the Cr 3 to the
anode where it would be oxidized to Cr 6. It is for this reason, that ~ -
in many instances it has been found that increased reduction in the hexa-
valent chromium content may be obtained when a diaphragm is used.
.
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Depending upon the particular makeup of the solution being treated, its
pH and temperature, however, satisfactory reduction of the hexavalent
chromium content can also be obtained without a diaphragm in some
instances. An electrolyte (8) is provided within the cell, which elect-
rolyte is a solution containing metallic material. A source (5) of thiselectrolyte is provided, from which the electrolyte may be introduced
into the cell through the inlet (6). Distributed within the electrolyte
(8) are particles (7), which particles are distributed randomly through
the electrolyte, the nature of the distribution depending upon the
electrolyte flow, size and density of the particles, density of the ..
electrolyte, and the like. The electrolyte (8) is pumped into the cell
(1) through the inlet (6) from the electrolyte source (5) and exits from .
the cell through the outlet (9) for recirculation through line (12) or
for subsequent processing through line (13), as is desired. If desired, ~-
dual electrolyte sources, cell inlets and outlets may be provided so
: that the introduction of electrolyte into the anode and cathode com-
partments of the cell may be separately controlled. The cell is further ..
provided with screens (10) and (11), screen (11) serving to support the ..... .
particles in the cell and screen (10) serving to maintain the particles
2n within the cell and prevent their discharge through the outlet (9). As
the distance between the screens (10) and (11) is changed, the volume
`: of that portion of the cell in which the particles are distributed will :~
likewise vary, thus, varying the porosity of the bed of particles which
is maintained within the cell.
- 25 While it is nct intended to restrict the operability of the .
'-
present invention by an theory of operation, the use of particles in
an electrolytic cell in the manner which has been described, has
been found to have the following advantages. In a conventional elect- :.
rolytic cell, such as a chlor-alkali cell, the amount of electrode
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surface at which the electrolytic reaction is conducted is de-
pendent upon the surface area of the el~ctrodes. Typical, this
surface will be about 1O3 times 105 cm . With a typical cell
volume of about 3.5 times 106 cm3, the resulting ratio of the
electrode area per cell volume is about 0.037 cm /cm3, By the
use of conductive particles in an electrolytic reaction, as in the
process of the present invention, there is a significant increase
in the surface area at which the electrolytic reaction may occur,
In Chemical and Process Enqineerinq, Fehruary 1968, page 93,
there is described a cell containing an electrolyte having
particles therein. It was calculated that the electrolyte con-
taining the particle~ has an electrode area of about 11,500 cm
and that the volume of the cell is about 153 cm3. This gives a
ratio of electrode area to cell volume of about 75 cm2/cm3 which,
clearly, is significantly higher than that of an electrolytic
cell having conventional electrodesO
Additionally, it is believed that by the use of the -
particles in the electrochemical reaction, a mass transport
; phenomenon may be taking place. In this the contact of metallic
materials with the particles and electrodes is dependent upon a
number of variables, including the electrolyte flow rate, the
particle size, density and type, and the concentration of the
metallic material. From a consideration of all of the above
variables, it has been found that the one condition which has an
effect upon all of them i~ the porosity of the bed of particles
; and that this porosity, as defined hereinabove, is the detarrnining
factor that makes po~sible a commercially feasible operation.
Moreover, in the process of the present invention, the
removal of the hexavalent chromium contaminates from the solutions
treated is believed to be effected by cathodic reduction of the
Cr~6 to Cr~3. The Cr~3 materials form a precipitate, probably
a trivalent chromium hydroxide, which is removed from the solution
in any convenient manner,
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such as Filtration, settling, centrifuging or the like. Thus, in
this process, it has been found that little, if any, of the hexavalent
chromium is removed from the solution by being plated out on the
electrodes and/or particles of the porous bed as chromium ~etal.
In order that those skilled in the art may better understand
the present invention and the manner in which it may be practiced, the
following specific examples are given. In these examples, unless
otherwise indicated, temperatures are in degrees centigrade and parts
and percents by weight. It is to be appreciated, however, that these
examples are merely exemplary of the present invention and the manner
in which it may be practiced and are not to be taken as a limitation i -
thereof.
In the following Examples 1.5 liters of aqueous O.lNCaC12,
O.lN NaCl or O.lN HCl solutions, containing about 500 parts per million
lead were used. The solution was circulated through apparatus similar
to that shown in the drawing, having an electrode cross-sectional area
of 460 cm2, for 15 minutes to allow for equilibration. A 50 milliliter
- sample was then withdrawn and analyzed for pH and lead content. The
analyses showed substantially no reduction from the original lead
content of about 500 parts per million, indicating little if any
absorption on the particles or electrodes in the cell. The solution
was then electrolyzed under the conditions indicated in the following
table. The electrolyte was then drained from the apparatus and again
analyzed for pH and lead content. All lead analyses were done by
atomic absorption technique. In these E~amples, there was no diaphragm
used in the cell, the particles were glass beads, having a particle
size of 500 microns, the anode was graphite, the cathode was stainless
steel and ~he separation between the anode and cathode was 0.7 centi-
meters. The electrolyte Flow rate was adjusted during the electrolysis
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so as to have a porosity of the bed of glass bead particles of 67%.
The current density used in all cases was 15 milliamps/cm2. Using
this procedure, the following results were obtained:
Time of Initial Final
Electrolyte Initial Final Electrolysis Pb Content Pb Content
Ex. Solution p~l pH Minutes (ppm) (ppm)
1 O.lN CaC12 4.40 2.00 120 435 4.8
; 2 O.lN CaC12 4.40 3.80 60 487 0.5
3 0.1 NaCl 5.05 5.80 60 570 ~ 0.13
4 O.lN Hcl 1.20 1.30 60 415 33
The procedure of the preceding Examples was repeated using
similar apparatus having an electrode cross-sectional area of 100 cm2.
From 700-800 milliliters of the electrolyte solution was circulated
; through the cell. The cathode used was nickel, the anode graphite and
the separation between the electrodes was 0.4 cm. The electrolyte flow
was adjusted so that the porosity of the bed of the glass bead particles ~ -
was 65%. Using this procedure, the following results were obtained:
Time of Initial Final
Electrolyte Initial Final Electrolysis Pb Content Pb Content
Ex. Solution pH pH Minutes (ppm) (ppm)
5 O.lN CaC12 5.10 7.11 60 470 ~ 0.2
6 O.lN CaC12 5.05 1.68 180 570 9.6
7 O.lN HCl 1.20 0.93 180 500 3.8
The procedure of Examples 5-7 was repeated with the exception
~ that the electrolyte used contained mercury, rather than lead. In
Examples 8, 9 and 10, the el~ctrolyte solution was the filtrate obtained
by filtering an industrial mercury containing waste effluent slurry
through a coarse porosity sintered glass crucible. In the remaining
Examplesg the electrolyte was obtained by mixing 50 grams of the slurry
with 1 liter of a 1.3N NaOCl solution and filtering the resulting
solution through #42 Whatman filter paper, the resulting filtrate being
used as the electrolyte. The solid resulting from the filtration of
- 15a -
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the original effluent slurry was Found by X-ray analysis to contain Fe,
Ca, K, S, and C1, minor amounts of Ba and Hg; and traces of Ni and Si.
The filtrate which was obtained was found to contain, in addition to Hg,
Cl and K and traces of Zn and S. Analysis of the filtrate for Hg was
done by a modified Dow procedure using a Beckman Mercury Vapor Meter.
The condition under which these solutions were electrolyzed
were as follows:
Example 8 - Nickel anode; graphite cathode; 1.0 cm electrode
separation, glass bead bed porosity 67%; current
density 20 milliamps/cm2
Example 9 - Same as Example 8 except the anode was platinum
coated titanium and the current density was 50
milliamps/cm2
Example 10 - Graphite anode and cathode, 0.4 cm electrode
separation; glass bead bed porosity 65%; current
density 20 milliamps/cm for first 240 minutes
and 50 milliamps/cm2 for last 60 minutes
Example 11 - Same as Example 10 except current density was
50 milliamps/cm2 for first 120 minutes and 100
milliamps/cm for last 120 minutes
Example 12 - Same as Example 10 except current density was 50
- milliamps/cm for first 60 minutes and 100 milli-
; amps/cm For last 180 minutes and HCl was added
to electrolyte to obtain indicated initial pH.
Using this procedure the following results were obtained:
Final Time of Initial Hg Final Hg
Initiai Final Electrolysis Content Content
Example pH pH (minutes) (ppm) (ppm)
8 13.5 12.5 120 1.6 0.53
9 13.4 13.3 120 1.4 0.1
13.15 13.40 300 0.8 0.06
11 13.15 12.50 240 15~ 12
12 6.20 7.25 2~0 255 60
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_AMPLE 13
A solution containing Z00 ppm cyanide and 165 pprn Cu+ and
having a pH of 13.05, was treated in appparatus similar to that shown in
the drawing. 700 cc of this solution, which was a waste effluent from a
5 cyanide copper electroplating bath, were circulated through the apparatus
at a flow velocity of 2.6 cm/sec., to provide a porosity of 70% in the
bed of graphite particles, which particles has a particle size of 840-2000
microns. The anode used was graphite, the cathode nickel, the area of
each electrode was 100 cm2 and the electrode separation was 1.35 cm.
After electrolysis for 52 minutes, at a current density of 15 milliamps/cm2
and a voltage within the range of 2-3 volts, the Cu content was 5 ppm,
the solution pH was 13.0 and the cyanide content was~0.5 ppm. Addi-
tionallyS the cathode was found to have a characteristic copper coating.
EXAMPLE 14
.
The procedure of Example 13 was repeated with the exception
that the solution treated was the effluent from a cyanide zinc electro-
plating bath having a pH of 12.54, a cyanide content of 200 ppm and a
zinc ion content of 141 ppm. The graphite particles were of a size of
595-840 microns, the flow velocity was 2.0 cm/sec. to produce a bed
20 porosity of 70~ and the electrode separation was 0.4 cm. After elect-
rolysis for 110 minutes at 15 milliamps/cm2 and a voltage of 2-2.8
volts, the pH was 12.8, the zinc ion content was 33 ppm and the cyanide
content ~0.5 ppm. Additionally, there was characteristic zlnc coating
on the cathode.
25 EXAMPLE 15
The procedure of Examples 1 4 was repeated with the exception
that 3.0 liters of copper cyanide solution containing 1353 ppm Cu and
2,000 ppm CN- was used. Periodically a 50 ml sample of the solution was
withdrawn and analyzed for copper using atomic absorption technique.
Using this procedure, the following results were obtained.
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Electrolysis Time Cu Concentration
(Minutes) (PPm? _ _ pH
Start 1353 12.~
559 12.57
93 12.50
120 21 12.~0
150 11 12.35
180 1.8 12.25
210 1.5 12.15
240 1.0 12.20
The procedure of Examples 1-~ was repeated using a zinc cyanide
plating bath which had been diluted to 16,000 ppm CN , 11,260 ppm zinc
and 0.44 NNaOH and a copper cyanide solution which had 16,000 ppm CN ,
12,000 ppm copper and 0.5 NKOH. These solutions were electrolyzed using
a current density of 30 milliamps/cm2 and the following results were
obtained:
Electrolysis Support
Time Initial Metal Final Metal Electrolyte
Example(minutes~ Content Content Added
16 325 11,260 ppm zinc 21.0 ppm zinc none
17 330 11,260 ppm zinc 139 " " l.ON NaCl
18 450 12,000 ppm copper 236 ppm copper none
Additionally3 in the following ExaMples, an aqueous chrome
plating bath solution was used. This solution, which initially contained
4~.4 ounces/gallon CrO3, 0.2 ounces/gallon Cr~3 and 0.3 ounces/gallon SO~=
and had a pH of 0.6, was diluted with water to form a solution containing
200 parts/million Cr+6 and 200 parts/million Cr+3 and having a pH of about
2.5. In each Example, 700 cubic centimeters of this solution were cir-
culated through apparatus sim;lar to that shown in the drawing with theexception that the electrolytic cell did not contain a diaphragm. The
solution was circulated for 15 minutes to allow for equilibration and
a 50 co. sample was withdrawn and analyzed for hexavalent chromium
content and pH. The solution was then electrolyzed under the conditions
indicated in the following table. Thereafter, the electrolyte was again
analyzed for Cr content and pH. The Cr content of the solution was
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measured polarographically and the total chromium content of the
solution was determined by atomic absorption. The anode used was
graphite, the cathode nickel and the separation between the anode and
cathode was 0.4 centimeters. Except where otherwise indicated, the
particles used were graphite, having a particle size of 590 to 840
microns, the flow velocity was 0.7 centimeters/second and the bed
porosity was 70%. In those Examples having an initial pH above
2.4-3.0, NaOH was added to the solution to adjust the pH to the
values shown. In all Examples, the initial Cr+6 content of the
solution was 200 parts/million. Using this procedure, the following
results were obtained:
Current Time of Final Cr 6
Density 2 Electrolysis Initial Final Content
Example Milliamps/cm (minutes) pH pH(parts/million)
l9(a) 15 50 2.9 3.6 297
420 10.0 5.8 3
21 23 240 10.1 6.5 18
22(b) 30 190 3 0 6.1 127
23 30 180 2.4 6.5 7
24 ~ 30 250 10.2 6.6 5
240 7.0 6.6 3
26 30 120 12.7 12.7 201
(a) No particles used. Flow velocity = 3.2 cm/sec.
(b) Particles used were glass beads having a size of 500
~ microns.
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The procedure of the preceding Examples was repeated with the
exception that the electrolytic cell contained a Fiberglass~D diaphragm,
as shown in the drawing, and 700 cc. of the solution were circulated
through both the anolyte and catholyte compartments. Using this
procedure the following results were obtained~
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Current Time of Final Cr 6
Density 2 Electrolysis Initial Final Content
Example (milliamps/cm ) (minutes) p~l pH(parts/million)
27(a) 15 210 6.9 6.6 4.8
28(a) 30 180 6.9 7.5 1.1
2g 30 180 Anolyte- 6.7 6.0 2.2 -
Catholyte-6.7 9.9 0.4
(a) A common electrolyte source or reservoir was used in
these Examples.
From all of the above results, it can be seen that although,
appreciable reductions in the hexavalent chromium content are obtained
in many instances where a diaphragm is not used, the reduction is
consistently lower with a diaphragm, particularly where the electrolyte
pH is relatively high. It is for this reason that in the most preferred
embodiment of the present process a diaphragm is used. It is to be
noted that the increase in the final Cr 6 content in Examples 19 (a)
and 26, over the 200 parts/million initially present, is believed to
have been caused by the anodic oxidation of some of the Cr 3 present to
Cr+6, which the absence of a diaphragm permitted.
While there have been described various embodiments of the
invention, the compositions and methods described are not intended to
be understood as limiting the scope of the invention, as it is realized
that changes therewithin are possible and it is further intended that
; each element recited in any of the following claims is intended to be
;~ understood as referring to all equivalent elements for accomplishing
substantially the same result in substantially the same or equivalent
manner, it being intended to cover the invention broadly in whatever
form its principle may be utilized.
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