US3619389A

US3619389A - Electrodeposition system

Info

Publication number: US3619389A
Application number: US863509A
Authority: US
Inventors: Steve Eisner
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1969-10-03
Filing date: 1969-10-03
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09
Also published as: FR2064858A5; DE2048600A1; DE2048600C3; CA927312A; DE2048600B2; SE369315B

Abstract

A method for maintaining anode activity in electrodeposition systems by continuously and repetitively contacting the surface of the anode during the electrodeposition reaction with a plurality of small, dynamically hard particles held in spaced, fixed relationship to one another in or on a supporting matrix.

Description

United States Patent [72] Inventor Steve Eisner Schenectady, N.Y.

[21] Appl. No. 863,509

[22] Filed Oct. 3, 1969 [45] Patented Nov. 9, 1971 [73] Assignee Norton Company Troy, N.Y.

Continuation-impart of application Ser. No. 718,468, Apr. 3, 1968, now abandoned.

[54] ELECTRODEPOSITION SYSTEM 15 Claims, 8 Drawing Figs.

[52] U.S. Cl. 204/143 G, 204/23, 204/35 R, 204/36, 204/DIG. 1O [51] Int. Cl B23p 1/00 [50] Field of Search 204/14, 29,

[56] References Cited UNITED STATES PATENTS 970,755 9/1910 Rosenberg 204/D1G. 10

970,852 9/1910 Rosenberg. 204/D1G. 10 1,214,271 1/1917 Bugbee 204/D1G. 10 1,721,949 7/1929 Edelman 204/DIG. 10 3,156,632 11/1964 Chessin et a1 204/DIG. 10 3,449,176 6/1969 Klass et a1 204/DIG. 1O

313,569 3/1885 Appleton 204/217 701,215 5/1902 Mond 204/36 712,153 10/1902 Reed 204/217X 1,029,965 6/1912 Aylsworth 204/36 1,552,591 9/1925 Batenburg.... 204/224 2,086,226 7/1937 Hoff 204/217 2,997,437 8/1961 Whitaker .4 204/209 3,022,232 2/1962 Bailey et a1. 204/217 X 3,313,715 4/1967 Schwartz, .lr. 204/36 3,334,041 8/1967 Dyer et a1. 204/224 X 3,377,264 4/1968 Duke et a1. 204/290 FOREIGN PATENTS 1,500,269 9/1967 France 204/D1G. 10 OTHER REFERENCES Industrial & Engv Chem, Vol. 61,1 10. 10, Oct. 1969, pp 8- l7, 204/D1G. 10

Primary Examiner-John H. Mack Assistant Examiner-R. J. Fay Ar!0rneysHugh E. Smith and Herbert L. Gatewood ABSTRACT: A method for maintaining anode activity in electrodeposition systems by continuously and repetitively con tacting the surface of the anode during the electrodeposition reaction with a plurality of small, dynamically hard particles held in spaced, fixed relationship to one another in or on a supporting matrix.

PATENTEDNUV 9 l9?! 3. 6 1 9 3 89 INV NTOR S EVE ISNEI? ATTORNEY ELECTRODEPOSITION SYSTEM FIELD OF THE INVENTION While useful in any electrodeposition process wherein reduced anode activity is a problem, the present invention finds particular utility in the fields of electrorefining or electrowinning.

DESCRIPTION OF PRIOR ART The problem of reduced anode activity is generally due to the formation of an oxide layer or layer of some other material on the surface of the anode which increases the electrical resistance of the system. This layer may be depleted in a reactive species which will help dissolve the anode or it may be so concentrated with waste products of the reaction that dissolution of the anode is hindered. It can sometimes be a space charge created by a layer concentrated in ions of the metal comprising the anode. Again, it may be due to intrinsic lack of activity of the anode material. Many times the problem manifests itself by uneven dissolution or activity of the anode.

The prior art has approached the problem by using complexing agents (active anions) to help dissolve the anode. Many of these agents are corrosive; most reduce maximum current efficiency attainable; and many tend to decompose with time, especially at higher temperatures. High temperatures and high solution agitation rates have been employed. These approaches cause problems with evaporation of the solution and do not affect solid layers of the anode. One good approach has been to increase the anode-to-cathode area ratio. This decreases the probability of polarization but, in practice, is limited by space requirements.

SUMMARY The present invention is directed towards a process in which the surface of an anode is continuously and repetitively contacted at extremely short time intervals by what is termed herein as dynamically hard" particles. By this term is meant that the combination of the hardness of the particles, the contact pressure of the particles on the surface of the anode and the speed at which such particles are moving relative to the anode surface is such as to produce an action on such surface sufficient to mechanically activate" the surface. Activating" the surface of the anode within the meaning of the present invention means the removal of any polarization layer and reaction product layer from the anode and the disarrangement of the atoms in the anode surface to a degree suflicient to cause increased activity. The atom disarrangement produced by the present process amounts to introducing activity accelerating defects in a substantially uniform manner in the anode surface such as whole or partial dislocations, vacancies, stacking faults, twins, lattice distortions and the like. In addition to increasing the defect concentration and removing any adherent activity inhibiting anode layer, the process roughens the anode surface and thereby increases the anode area.

The process requires the use of a surface disturbing or activating medium having the characteristics of providing a plurality of small, dynamically hard, relatively inflexible particles held in substantially fixed, spaced relationship to one another and generally normal to the anode surface by a preferably porous matrix or supporting member which also provides a plurality of surfaces extending parallel with and closely adjacent to the anode surface. The process further requires relative motion, during the electrolysis reaction, between the anode surface and the activating medium. In addition, suffcient pressure is applied to said activating medium in a direction normal to the anode surface to cause the aforesaid disarrangement and other effects. The spacing of the particles and the speed of relative movement is such that any given point on the anode surface is contacted or influencedby a particle at extremely short time intervals, e.g., intervals in the range of 6.l l' to 3.8 l0" seconds. Fresh electrolyte is supplied to the anode surface at the same rate through entrapment by those surfaces of the activating medium (which surfaces may be the edges of the particles) parallel with the anode surface. These surfaces sweep fresh electrolyte along with them, the electrolyte reaching such surfaces due to the porosity of the supporting matrix of the activating medium or through proper disposition of the electrolyte supply adjacent the contact area between the activating medium and the anode surface. This serves to carry away excessive concentrations of cations or unwanted products resulting from the electrodeposition reaction.

Accordingly, the principal object of the present invention is the provision of an efficient process for maintaining substantially uniform anode activity which does not possess the defects of the prior art. A further object is to provide for rapid agitation immediately adjacent the surface of the anode.

DRAWINGS FIG. 1 is a schematic representation of the process of this invention as utilized in the electrorefining of copper.

FIG. 2 illustrates diagrammatically a portion of a cross sec tion of one type of porous activating medium useful in the present invention.

FIG. 3 is a chart showing the weight loss (anode activity) of a pure tin anode used with the present process compared with the activity of the same anode in a conventional process.

FIG. 4 is similar to FIG. 3 except that here the anode is a tin alloy (0.] Sb, 0.04 Cu).

FIG. 5 is a chart showing the voltage-amperage relationship for various current densities using a pure tin anode with the present process.

FIG. 6 is a chart similar to FIG. 5 showing the same relationship in a conventional process for the same pure tin anode.

FIG. 7 is a chart similar to FIG. 5 for a 0.1% Sb tin anode used with the present process.

FIG. 8 is a chart similar to FIG. 6 for a 0.1% Sb tin anode used in a conventional process.

DESCRIPTION OF PREFERRED EMBODIMENTS The process of the present invention requires the controlled application under pressure, both normal to and parallel with the anode surface, of a supporting, preferably porous, matrix which supports in closely spaced relationship a plurality of small, relatively inflexible particles. These particles are so positioned in the matrix as to contact any deposit forming on the anode surface. It is necessary to apply sufiicient pressure through this matrix to produce a light scratch pattern in the anode surface. Thus the dynamic hardness of the particles may be substantially greater than the actual hardness, e.g., a resin particle may produce a scratch in a much harder anode surface. This scratch pattern may be visible to the naked eye but, in any case, will be seen under a magnification of 10,000 power or less. While the scratches may be produced by metal removal, preferably the dynamic hardness is so controlled that a displacement of metal atoms on the surface rather than actual removal is the basis for the scratch formation.

By using small, relatively inflexible, nonconductive particles as the activating tool, no spot on the anode surface is covered for any appreciable length of time by the activating particle. These particles are generally randomly distributed over and bonded by a resinous adhesive to at least the anode surfacecontacting side of the matrix and are preferably spaced in fixed relation to one apother over very short spans, e.g., l.25 l0 bhl inches to 5J55 l0 inches. If desired. accurate and nonrandom distribution of the particles on the supporting matrix can be resorted to although this is generally an unnecessary complication. By the term particle" as is used herein is meant not only completely separate and discrete three-dimensional bodies, but also larger bodies with a plurality of points, tips, projections or the like thereon as for instance a relatively hard resinous coating on a fiber wherein the coating contains multiple irregular spaced projections and is generally uneven in nature. The particles, as described herein,

preferably contact or at least influence essentially all of the surface of the anode. The particles themselves may vary widely in size from l l inches to 1.25 l0 inches (average diameter) for example, but should generally be in the size range of from 9X10 inches to 2X I0 inches for best results. The particles can generally be defined as hard, i.e., having a Kn'oop hardness in excess of I00, but the degree of hardness per se is not critical except that control should be exercised not to use a product which is too abrasive for the particular anode being treated. The degree of pressure applied must also be considered with respect to the hardness of the particles and generally with the softer range of particles more pressure normal to the anode surface is required than with the harder range of particles.

As indicated above, the controlling factor is the dynamic hardness of the particles, i.e., the apparent hardness resulting from a combination of the actual Knoop hardness, the pressure applied and the speed with which the particles are moved across the anode surface. A visible indication that the dynamic hardness is sufficiently high is the presence in the anode surface of the scratches visible under I0,000 magnification.

The matrix used to support the activating particles is preferably electrolyte permeable, having a through porosity in the order of at least 6.5 Sheffield units (as measured by a Sheffield porosimeter using a 2 /r-inch ring). Preferably, this matrix is also at least somewhat compressible and deformable so that it can be conformed to irregular surfaced anodes and associated deposits where necessary. As indicated above, the matrix is required to have a plurality of thin-walled surfaces extending between the activating particles to act as electrolyte sweeps. While these surfaces may be the edges of the particles themselves, in the preferred embodiments these thin-walled surfaces define small compartments or pores of either regular or irregular shape which function much like a bucket conveyor in carrying small quantities of electrolyte over the activated anode surface. Many variations of porous supporting matrices have been used, e.g., open mesh screens with activatin g particles adhered to the mesh; nonwoven abrasive articles, both compressed and uncompressed; open cell foam sheets with the activating particles incorporated in or on the foam cell walls; sponge materials containing the required particles and the like. Examples of products which can be used in the present invention as activating media are illustrated in U.S. Pat. No. Re. 221,852 to Anderson which shows an open mesh product having abrasive grains adhered thereto; in U.S. Pat. No. 3,020,139 to Camp et al. which illustrates nonwoven webs having a plurality of hard particles adhered to and along the web fibers; in U.S. Pat. No. 3,256,075 to Kirk et al. which illustrates a sponge containing hard resin impregnated sponge particles; and in U.S. Pat. No. 3,334,041 to Dyer et al. which illustrates a coated abrasive product having perforations through which electrolyte can flow. In this latter instance, the product must be modified for the present process by making it nonconducting, i.e., it essentially becomes a standard coated abrasive product with electrolyte-passing holes therethrough.

Referring now to the drawings, FIG. 1 illustrates the present process as applied to the electrorefining of an impure metal. The system is positioned in tank 10 containing electrolyte 11. A soluble, impure metal anode I2 is moveably mounted within the electrolyte in contact with a porous activating medium in the form ofa continuous belt 14. A cathode 13 of the metal to be deposited is in contact with the other side of belt 14 initially, but at the point in the deposition cycle illustrated in FIG. I, the belt 14 is interposed in contacting relationship between anode I2 and the deposited layer of pure metal 18 on cathode 13. Belt 14 rotates on

idler rollers

15 and 16 and driver roll 17. The belt and associated rollers are capable of adjustment away from the surface of cathode 13 as the deposit 18 builds up. In operation, the activating belt wipes both the electrodeposit surface and also the surface of the anode whereby the anode is assisted in more rapid dissolving.

FIG. 2 shows a highly enlarged and idealized portion of one type of activating media suitable for use in the present invention and illustrates the hard particle-connecting matrix relationship. Reference numeral 25 represents fibers of a nonwoven web (nonconducting fibers such as polyethylene terephthalate or the like) which are anchored one to the other at their points of intersection by an adhesive binder 26. A plurality of small, hard, discrete particles 27 are positioned on the fibers 25 and in the present illustration are held to such fibers by the adhesive 26. At least some of the fibers 25 extend relatively parallel to the anode face 29 as shown at 28 to form the thin-walled cells or electrolyte sweeping members referred to above. (For purposes of illustration, the activating particles 27 are here shown at some distance from the anode face 29 although in operation of the present process they would be in contact therewith.

The following specific examples are furnished for the purpose of illustration of some of the variations possible with the present process and to illustrate the improved results attained therewith.

EXAMPLE 1 An impure copper ingot containing about 96 percent copper was shaped into a rectangular form and used as the anode in the system illustrated in FIG. 1. The activating medium was a nonwoven web of about one-sixteenth inch thickness containing grit 400 aluminum oxide particles bonded thereto by a resin adhesive. The web was laminated on each side of a 20x20 mesh nylon reinforcing fabric to form a continuous belt approximately 6-inch wide. This was run between the anode and a copper cathode. If desired, other conventional cathodes constructed of stainless steel, brass or the like may be substituted for the copper cathode. The entire unit was submerged in a copper sulfate-sulfuric acid solution and a highpurity sound copper deposit was formed on the cathode.

EXAMPLE 2 Illustrative of the results achieved by activating" the anode surface in accordance with the present invention, identical anodes were prepared from pure tin and were used with a conventional copper cathode in a tin plating solution (38 ozJgal. SnSO One of these anodes was treated in accordance with the present invention while the other was not. The results are described below and are graphically shown in FIGS. 3, 5 and 6.

In each instance, the cathode was a 20 in. copper plate while the anode was pure tin having a 1/4 in. anode area. Reaction conditions in the SnSO bath were:

2,880 amps/ft. and 5,760 amps/IL.

5 min.

Room

Electrolysis Time Temperature The electrolyte tank was divided by a fiber glass cloth diaphragm into anode and cathode compartments. In each instance a wheel carrying an activating media as described herein was positioned adjacent the anode and rotated at a speed of 248 surface feet per minute to insure that agitation of the solution was the same. In case 1 (according to the present invention) the surface of the wheel [three-fourths inch wide and 3 inches in diameter and made up of a 15 denier nylon web carrying 400 grit silicon carbide abrasive grain adhered thereto by a resinous adhesive as described in more detail above] was just touching and wiping the anode surface. Wattmeter readings were used to reproduce the load on the drive motor for the wheel in each test. In case 2, the wheel was rotated at a slight distance (one-eighth inch) from the anode surface to insure similar agitation of the solution as in case 1 above but without the actual wiping contact used in case I. The anodes were weighed before and after electrolysis. Voltage readings were taken at the start of electrolysis, then after 1 minute and finally at the end of the run (5 minutes). The current density was held constant for each run. Multiple runs The above illustrates the substantial increase in dissolution rate or conversely the improvement in activity of the anode due to the application of the present process. Also, this effect shows up even more clearly in the graphs of FIGS. 5 and 6 wherein a plot of voltage versus current is shown. FIG. 5 is a plot of the values obtained for case I (present invention) and FIG. 6 illustrates the values obtained for case 2. These curves show that the effect of activation of the anode surface was to reduce the depolarization time so that the voltage required for a given current density was nearly the same at the beginning and the end of the run. When agitation only was employed (case 2), the initial voltage at the higher current density (2,880 aJft?) was over twice the voltage at the end of the run.

Current Final Density Initial Voltage (erupt/ft!) Voltage (5 min.)

Case I 1,140 3 3 Case 2 1,140 3 13 Care I 2,880 8.4 10.6

Case 2 2,880 35 14.6

The above figures and their graphic representation in FIGS. 5 and 6 show that rapid anode depolarization results from the anode activation process of the present invention.

EXAMPLE 3 Current Weight Density Loss (amps/TL) (Grams) Case I 1,140 0.4 Case 2 1,140 0.4 Care 1 2,8110 0.9 Case 2 2.880 0.3

These values are plotted in graphic form in FIG. 4.

Current Density Initial Final (amps/IL) Voltage Voltage Case I [.140 3.4 4.6 Cast! 2 1,160 11.0 11.4 Care I 2,880 15.8 14.9 Case 2 2,380 34.0 14.4

These values are plotted in FIGS. 7 (case I) and 8 (case 2).

EXAMPLE 4 Using a setup similar to that shown in FIG. 1 and described above, anodes were prepared from pure copper. The cathode was also pure copper and the area of both anode and cathode was 54 in). However, instead of an endless belt as in the apparatus of FIG. 1, a rotating porous wheel similar to that of example 2 [3-inch diameter, nylon nonwoven support with silicon carbide grain (400 grit)] was interposed between the anode and cathode and rotated to simultaneously wipe both the anode and the cathode. This wheel was driven at the rate of 315 r.p.m The electrolyte was a solution of copper sulfate (210 grams H,SO (dist. H 0) and 45 grams CuSO /liter) with 2 liters of copper sulfate solution used for each run. Electrolysis time was 5 minutes at room temperature. The current was held constant at S amperes. For comparison, the rotating wheel was spaced one-eighth inch from the surfaces of the anode and cathode and rotated at the same speed. Measurements of initial and final voltages and weight loss of the anodes were taken. "Case 1" identifies the sample run with the activating wiping of the surface while "case 2 shows the same measurements taken with agitation only.

With the identical setup of example 4 except that in this instance the cathode was a 2-inch-by-l l-inch copper plate and the wheel was run against the anode surface only, anodes made from a sample of impure blister copper were used. The average values for multiple runs were:

Initial Final Weight Voltage Voltage Loss Case 1 9.5 3.0 0.60 g. Case 2 25.5 20.3 0.55 g.

The conclusions which can be drawn from examples 4 and 5 are that at this high current density (2,880 a.lft.) there is appreciably less voltage required to anodically dissolve copper (either pure or blister) when the anode is being activated by the present process than when the solution is merely agitated. In the case of example 5, the average voltage required with the present process was around 8 volts whereas with agitation only, the voltage was in excess of 20 volts.

Further, a power saving was also noted since for case 1 the El product was around 40 watts (5 amperes X8 volts) whereas for case 2 the El product was around watts (5 amperes X20 volts). The additional energy for the activation in case 1 was about 1 watt (the wattmeter in the line with the motor driving the wheel used for activation read 57 watts in case 1 and 56 watts in case 2 where the wheel was used for agitation of the solution only).

The type of movement of the activating media over the surface of the anode may be varied widely, i.e., it may be linear as well as rotative; it may be a combination of movements, e.g., a rotating device which is also oscillated as it rotates, etc. The only requirement is that there be relative motion between the two of the order of magnitude herein described and claimed.

Likewise, this relative movement can obviously be achieved with a moving anode and stationary activating media or a combination of movements of both. Simultaneous wiping of the anode and electrodeposit surface on the cathode with the activating medium has proven valuable. Activation of the anode has been found to increase the rate of anode dissolution and to prevent the buildup of anode slimes, particularly in the refining of tin. In some instances, activation of the anode at a differential rate from that employed on the cathode may be desirable. Further, while cathode wiping along with activation of the anode is preferred, the anode surface only may be contacted with the activating medium if desired. The speed of movement can also be varied and generally it can be stated that higher speeds will make depolarization more effective at high current densities.

The activating media described herein may likewise be varied widely, both in shape or configuration and in composition. The requirements of the supporting members and associated dynamically hard particulate materials has been discussed in detail above. Any nonconductive fibrous material capable of resisting erosion by the electrolyte and capable of producing the described supporting matrix may be used for the porous matrix as well as nonfibrous material such as sponge, foam, or the like. The nonconductive particulate activating materials likewise are noncritical in that many materials such as resin particles, abrasive grain, ceramic particles, glass particles, walnut shells or the like can be utilized.

The electrolyte is preferably held at ambient or room temperature, e.g., C., but can be used at temperatures up to the boiling point of the respective electrolyte used in a given setup.

Electrode spacing can vary from as little as one mil up to an electrode gap distance fixed only by the IR drop considered acceptable for the particular operation.

The pressure of the activating medium on the anode, which as indicated above is variable depending on the particular activating particle used and the system in which it is used may either be held relatively constant or varied during the operation of the process within the limits indicated by the requirement of development of a scratch pattern up to the practical limit dictated by the removal of undue amounts of metal.

Anodes which may be used with the present process will vary from insoluble to readily soluble in the electrolyte. For example, lead or zinc anodes in aqueous sulfuric acid, aluminum anodes in molten cryolite, graphite or platinum anodes in brine and, in the use of fuel cells, anodes such as nickel in potassium hydroxide.

The present process does not require the use of highly corrosive or complexing ions, is effective at ordinary temperatures and does not require a high anode-to-cathode area ratio.

1 claim:

I. A process for maintaining and increasing anode activity in electrodeposition processes which comprises discontinuously mechanically activating the surface of the anode only at extremely short repetitive time intervals throughout the period of imposed current flow by contact with and relative motion between such surface and a plurality of small activating particles secured in spaced relationship to one another on a supporting matrix and, coincident with such mechanical activation, supplying quantities of fresh electrolyte at a high flow rate to such surface.

2. A process as in claim 1 wherein the extremely short time intervals are not more than 6.1 10 seconds.

3. A process as in claim 2 wherein the matrix permeable.

4. A process as in claim 2 wherein the activating particles have a dynamic hardness sufiicient to produce a scratch in said anode surface visible under magnification of 10,000 power.v

5. A process as in claim 1 wherein the anode is substantially insoluble in said electrolyte.

6. A process for improving electrodeposition processes comprisin a. establishing a system having an anode, a cathode and an electrolyte therebetween;

b. inter-posing between said anode and said cathode in contact under pressure with a surface of said anode only a matrix having at least on its surface a plurality of spaced activating particles secured in spaced relationship to one another;

c. establishing relative motion between said matrix and said anode surface whereby electrolyte is moved at a high flow rate across said surface;

d. initiating an electrodeposition current flow through said electrolyte and said supporting matrix between said anode and said cathode whereby said anode tends toward decreased activity; and

e. continuing said relative motion during the period of electrodeposition current flow whereby contact of said particles supported by said matrix with said anode surface is repeated at extremely short time intervals to substantially eliminate the tendency of said anode toward decreased activity.

7. A process as in claim 6 wherein said supporting matrix comprises an electrolyte-permeable material.

8. A process as in claim 7 wherein said matrix comprises a porous nonwoven web.

9. A process as in claim 7 wherein said matrix comprises an open-weave fabric.

10. A process as in claim 6 wherein said supporting medium is continuously moved relative to said anode surface.

11. A process as in claim 6 wherein said particles have a dynamic hardness sufficient to produce a scratch in said anode surface visible under magnification of 10,000 power.

12. A process as in claim 6 wherein said anode is soluble in said electrolyte.

13. A process as in claim 6 wherein said anode is insoluble in said electrolyte.

14. A process as in claim 6 wherein said abrasive grains.

15. A process as in claim 6 wherein said supporting medium functions to carry fresh electrolyte into contact with the mechanically activated surface of said anode and to remove spent electrolyte therefrom.

is electrolyte particles comprise

Claims

2. A process as in claim 1 wherein the extremely short time intervals are not more than 6.1 X 10 2 seconds.
3. A process as in claim 2 wherein the matrix is electrolyte permeable.
4. A process as in claim 2 wherein the activating particles have a dynamic hardness sufficient to produce a scratch in said anode surface visible under magnification of 10,000 power.
5. A process as in claim 1 wherein the anode is substantially insoluble in said electrolyte.
6. A process for improving electrodeposition processes comprising: a. establishing a system having an anode, a cathode and an electrolyte therebetween; b. interposing between said anode and said cathode in contact under pressure with a surface of said anode only a matrix having at least on its surface a plurality of spaced activating particles secured in spaced relationship to one another; c. establishing relative motion between said matrix and said anode surface whereby electrolyte is moved at a high flow rate across said surface; d. initiating an electrodeposition current flow through said electrolyte and said supporting matrix between said anode and said cathode whereby said anode tends toward decreased activity; and e. continuing said relative motion during the period of electrodeposition current flow whereby contact of said particles supported by said matrix with said anode surface is repeated at extremely short time intervals to substantially eliminate the tendency of said anode toward decreased activity.
7. A process as in claim 6 wherein said supporting matrix comprises an electrolyte-permeable material.
8. A process as in claim 7 wherein said matrix comprises a porous nonwoven web.
9. A process as in claim 7 wherein said matrix comprises an open-weave fabric.
10. A process as in claim 6 wherein said supporting medium is continuously moved relative to said anode surface.
11. A process as in claim 6 wherein said particles have a dynamic hardness sufficient to produce a scratch in said anode surface visible under magnification of 10,000 power.
12. A process as in claim 6 wherein said anode is soluble in said electrolyte.
13. A process as in claim 6 wherein said anode is insoluble in said electrolyte.
14. A process as in claim 6 wherein said particles comprise abrasive grains.
15. A process as in claim 6 wherein said supporting medium functions to carry fresh electrolyte into contact with the mechanically activated surface of said anode and to remove spent electrolyte therefrom.