CA1046979A - Decreasing the metallic content of liquids by an electrochemical technique - Google Patents

Abstract

DECREASING THE METALLIC CONTENT OF
LIQUIDS BY AN ELECTROCHEMICAL TECHNIQUE

ABSTRACT OF THE DISCLOSURE
A method for decreasing the metallic content of a solution which comprises passing an electric current through a solution containing metallic material, 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, and wherein the electrolyte also contains a bed of particles, distri-buted therein, such that the porosity of the bed is from about 40 to 80%, porosity being defined as

Description

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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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~ ^ -97~1 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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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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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 , 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 :
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speci~ically preferred range being from about 60% to 70%. ~

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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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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, ' . : ' . .' ~ .: ' ~6~7~
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 -7~
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.

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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.

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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~6~37~3 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.
';, , ;~ 15 Flow velocity was 3.2 cm/sec. and bed porosity was 55%.
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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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for decreasing the metallic content of a solution which comprises passing an electric current through a solution containing metallic materials selected from mercury, lead, cadmium and zinc, which solution is contained as the electrolyte in a cell, said cell having at least one positive and at least one negative electrode between which the current is passed, said at least one positive electrode being separated from said at least one negative electrode by a diaphragm, and wherein the electrolyte also contains a bed of dispersed particles, dis-tributed therein such that the porosity of the bed is from about 40 to 80% porosity being defined as:

cathodically reducing the metallic materials to elemental metal until the metallic content is reduced to a desirable level, plating the metal on the cathode, and removing said solution of reduced metallic content from the cell.

2. The method as claimed in claim 1, wherein the electro-lyte solution is an aqueous solution.

3. The method as claimed in claim 2, wherein the initial concentration of the metallic material in the electrolyte solution is from about 1 part per million to 10% by weight.

4. The method as claimed in claim 1, wherein the particles distributed in the electrolyte solution have a density which is greater than that of the electrolyte.

5. The method as claimed in claim 1, wherein the particles distributed in the electrolyte solution are conductive particles.

The method as claimed in Claim 5 wherein the particles are graphite.

The method as claimed in Claim 1 wherein the particles are distributed within the electrolyte by flowing the electrolyte through the electrolytic cell in a direction opposed to the gravitational forces.

The method as claimed in Claim 7 wherein the electrolyte flow velocity through the cell is from about 0.1 to 1000 centimeters per second.

The method as claimed in Claim 1 wherein metal in the electrolyte is lead and the electrolyte solution has a pH of from about 4 to 7.

The method as claimed in Claim 1 wherein the metal in the electrolyte is mercury and the electrolyte solution has a pH of from about 6 to 13.

The method as claimed in Claim 1 wherein the porosity of the bed of particles is from about 55 to 75%.

The method as claimed in Claim 11 wherein the porosity of the bed of particles is from about 60 to 70%.

The method as claimed in Claim 1 wherein the separation between the positive and negative electrode within the cell is from about 0.1 to 5.0 centimeters.