US3862893A

US3862893A - Electrolytic cell method for transfer of dispersed solids from one liquid electrolyte to another with suppression of transfer of dispersing liquid

Info

Publication number: US3862893A
Application number: US491387A
Authority: US
Inventors: Allan B E Gilchrist
Original assignee: SCM Corp
Current assignee: Akzo Nobel Paints LLC
Priority date: 1974-07-24
Filing date: 1974-07-24
Publication date: 1975-01-28
Anticipated expiration: 1992-01-28

Abstract

Dispersed electrodepositable coating binder and other codispersed electrocoating components co-depositable therewith can be recovered from one liquid dispersion into another liquid dispersion, which may be more concentrated than the dispersion from which recovery is made, without transfer of liquid dispersing medium therebetween by supplying the two dispersions respectively to two compartments of an electrolytic cell, each compartment containing an electrode, where the compartments communicate through a freely and non-selectively permeable nonconductive separator effective for arresting turbulent transmission of liquid dispersions therethrough in the absence of hydraulic pressure difference across the separator and charging the electrodes from an external source of direct current emf to attract dispersed electrodepositable coating binder through the separator toward the electrode in the compartment containing the dispersion into which recovery of binder is desired. Simultaneous transfer of liquid dispersing medium from one dispersion into the other is prevented by controlling the hydraulic pressure difference across the porous separator to be effectively negligible. Electrodeposition of binder onto the electrode in the cell compartment containing the dispersion into which dispersed electrodepositable binder is recovered and toward which that binder is attracted can be prevented, if desired, by sufficiently vigorous agitation of the dispersion in immediate contact with that electrode.

Description

United States Patent 1191 Gilchrist 1 1 .Fan. 28, 1975 41 ELECTROLYTIC CELL METHOD FOR TRANSFER OF DISPERSED souos FROM NE LIQUID ELECTROLYTE To ANOTHER WITH SUPPRESSION OF TRANSFER OF DISPERSING LIQUID [75] Inventor: Allan B. E. Gilchrist, Westlake.

v Ohio 1 7'3 Assignee: SCM Corporation/Cleveland, Ohio 221 Filed: July 24, 1,974 21 Appl. NOQ: 491,387

521 U.S. Cl 204/180 R 51 1m.C1 B0ld 13/02 581 Field of Search 204/180 R,180 1?, 301,

[56] References Cited UNITED STATES PATENTS 892,188 6/1908 Schwerin 204/180 R 1,022,523 4/1912 Whitney 204/180 R 1,986,920 1/1935 Cross 204/180 RX 2,944.952 7/1960 McMinn, Jr. 204/180R 3,682,806

8/1972 Kinsella ct al. 204/181 Primary Examiner- -John H. Mack Assistant ExaminerA. C. Prescott Attorney, Agent, or Firm-James B. Wilkens co-dispersed electrocoating components codepositable therewith can be recovered from one liquid dispersion into another liquid dispersion. which may be more concentrated than the dispersion from which recovery is made. without transfer of liquid dispersing medium therebetwcen by supplying the two dispersions respectively to two compartments of an electrolytic cell, each compartment containing an electrode, where the compartments communicate through a.freely'and non-selectively permeable nonconductive separator effective for arresting turbulent transmission of liquid dispersions therethrough in the absence of hydraulic pressure difference across the separator and charging the electrodes from an external source of direct current emf to attract dispersed electrodepositable coating binder through the separator toward the electrode in the compartment containing the dispersion into which recovery of binder is desired. Simultaneous transfer of Iliquiddispersing medium from one dispersion into the other is prevented by controlling the hydraulic pressure difference across the porous'separator to be effectively negligible. Electrodeposition of binder onto the electrode in the cell compartment containing the dispersion into which dispersed electrodepositable binder is recovered and toward which that binder is attracted can be prevented, if desired, by sufficiently vigorous agitation of the dispersion in immediate contact with that electrode.

10 Claims, 4 Drawing Figures,

PATENTEU JAN 2 81975 SHEET 2 OF 2 FIG.3

ELECTROLYTIC CELL METHOD FOR TRANSFER OF DISPERSED SOLIDS FROM ONE LIQUID ELECTROLYTE TO ANOTHER WITI-I SUPPRESSION OF TRANSFER OF DISPERSING LIQUID BACKGROUND OF THE INVENTION The present invention relates to recovery of electrodepositable coating binder from a dilute liquid dispersion thereof (the term dispersion as used herein includes a solution) by treating such a dispersion in an electrolytic cell apparatus in which an electrical potential difference is established by an external source between the electrodes to electrically drive dispersed electrodepositable coating binder toward one of those electrodes. More particularly, this invention is related to my earlier invention of Process and Apparatus for Electric Cell Treatment of Liquor, described in US. Pat. No. 3,748,244 the teachings of which are incorporated herein by reference, which deals with apparatus and method for complete or partial electrolytic separation of electrodepositable components, such as resinous or polymeric electrocoating binders, from other components of a liquid electrolyte while preventing an insulating accumulation of electrodeposited material on the electrode toward which the electrodepositable components are attracted by employing a porous electrode and pumping the electrolyte rapidly therethrough, where thecell apparatus also incorporates a grossly porous non-conductive turbulence arrester to suppress turbulent inter-mixing of electrolyte from the vicinity of the electrode toward which electrodepositable components are attracted with electrolyte from the vicinityof the other,i.e, counter, electrode.

The present invention is also related to my earlier invention of Process for Electrolytic Treatment of Liquors Using Pressure Cell With Porous Electrode Means, described in US. Pat. No. 3,679,565 the teachings of which are incorporated herein by reference, which deals with complete or partial separation of electrodepositable components, such as resinous or polymeric electrocoating binders, from other components of a liquid electrolyte by treatment in an electrolytic cell apparatus having at least one porous electrode, where the electrodepositable material tends to form a fluent electrodeposit upon one of the electrodes and that fluent electrodeposit isprevented from accumulating on that electrode to the extent of significantly increasing the internal electrical resistance of the cell by inducing sufficiently intense relative motion between the electrode upon which electrodeposition tends to occur and the electrolyte in the immediate vicinity thereof by such means as rapidly spinning that electrode or employing as that electrode a porous conductor and pumping electrolyte therethrough or by heating 7 that electrode.

The present invention is more remotely related to my earlier invention of Apparatus for the Treatment of Liquors Using Porous Deposition Electrodes, described in US. Pat. No. 3,798,150, which deals with pressure cell apparatus for the electrolytic treatment of conductive liquors containing electrodepositable material, where the electrodes to be polarized for attracting electrodepositable material are tubular porous electrodes, it being contemplated therein that the liquor containing electrodepositable material would be fed to the interior of the tubular electrodes and the electrodepositable material electrophoretically attracted to and electrodeposited upon such tubular electrodes would be flushed through the walls of the porous electrodes by the flow of a small portion of the liquor therethrough into a surrounding collection zone by maintaining a small hydrostatic pressure head within the tubes.

In this branch of the prior art, :recovered electrodepositable material is collected in dispersed or solubilized form, but at a greater concentration, in a portion of the same liquid medium from which such recovery takes place. Thus, either a portion of that liquid medium is used to wash through a porous electrode material electrodeposited thereon and to resolubilize that material into a collection zone otherwise separated from the feed stream of liquor to be treated or else a substantial portion of the initial liquid medium in which the electrodepositable material to be recovered is initially'dispersed is removed from the cell in the vicinity of the counter electrode charged to opposite polarity from the. electrode toward which electrodepositable material is electrophoretically attracted and upon which it tends to electrodeposit and, since the liquor near the counter electrode will have been depleted of electrodepositable material by electrically induced migration thereof toward the deposition electrode, the removal of such depleted liquor necessarily leaves behind a liquor enriched in electrodepositable material (so long as removal thereof by net electrodeposition or otherwise is substantially prevented) and this enriched liquor can be collected from any convenient location in the cell such as by withdrawal through a porous deposition electrode. The process of the present invention differs from this branch of the prior art in that electrodepositable material is transferred under the influence of an electrical potential established between two electrodes from one body of liquid in which it is dispersed to a second body of liquid, in which it also appears in dispersed form, without the necessity of any transfer of liquid medium between those two bodies of liquid dispersion and, in fact, with provision being made to largely prevent such transfer of more than an insignificant amount of liquid medium, as will be hereinafter more particularly described.

A principal utility of the present invention is in connection with the electrocoating art, in which a paint or other surface coating is applied to an electrically conductive object temporarily'connected as an electrode in an electrolytic cell apparatus and immersed in a body of liqud electrocoating dispersion comprising a solvent or dispersing medium and electrodepositable coating binder dispersed therein, by electrically charging the object to be coated to a potential, relative to a counter electrode also in contact with the dispersion, such that dispersed binder will be electrically attracted toward and electrodeposited upon the object to form a coating less conductive than the dispersion from which .it was deposited. The electrocoated, object is typically removed from the liquid dispersion, disconnected from the source of electrical potential, allowed to drain a portion of the liquid dispersion mechanically entrained by not electrodeposited thereon back into the electrocoating cell and rinsed to remove the remainder of the liquid dispersion mechnically entrained but not electrodeposited thereon. The electrocoated object is then usually subjected to additional coating and/or coating curing operations.

Liquid electrocoating dispersions often contain, in addition to electrodepositable coating binder, ionizing solubilizer for the binder, pigment, surfactant, curing accelerator, anti-foamer, fungicide and other components commonly found in organic coatings and paints. Many of such additional components will electrodeposit together with electrodepositing binder to form the insulating coating, but ordinarily not in proportion to their concentration in the dispersion. The solvent or dispersing medium is ordinarily water, but may be some other suitable medium. The dispersed coating binder is ordinarily an organicpolymer which ioniz'es sufficiently in the liquid solvent or dispersion medium, usually with the assistance of an ionizing solubilizer, to be reasonably stable against flocculation or precipitation and to be electrodepositable upon a suitably charged electrode, but may be an ionizable low molecular weight material which will polymerize during or subsequent to electrodeposition to form an adherent binder polymer for the protective coating in its final form. The coating binder may be chosen from a broad range of polymer types on the basis of availability, cost, processability and the properties desired in the coating to be formed by the electrocoating process. A typical formulation for anodic deposition would have a maleinized linseed oil binder with a morpholine solubilizer in water. A typical formulation for cathodic deposition would have an organic polymer with tertiary amino groups as binder and lactic acid as solubilizer and be dispersed in water.'

It is frequently desirable to recover electrodepositable material, and particularly electrodepositable binder, from the rinse effluent of an electrocoating operation so that the electrodepositable material can be recycled to the electrocoating cell and also so that the rinse water can be either reused or discharged without contaminating the environment. A number of methods have been proposed and/or used for recovering electrodepositable material from electrocoating rinse water. The most promising of these previously known methods appear to be ultra-filtration, reverse osmosis and electrolytic methods such as those described in my own patents referred to above. A drawback of the ultrafiltration and reverse osmosis techniques is that a great quantity of water must be forced out of the dilute rinse effluent by pressuring it through a filter membrane impermeable, or at least preferentially less permeable, to dispersed electrodepositable material and it is difficult to make such filters or membranes both strongly selective in permeability and highly permeable to water or other dispersing medium. In contrast, the process of the present invention involves the transfer of only a rela-' tively small amount of electrodepositable material con-- tained in the dilute electrocoating rinse effluent into a more concentrated dispersion where it can be reused without inordinate (or even any) dilution of the electrocoating bath dispersion.

Another problem which arises in the electrocoating art is that as material is electrodeposited onto the object being coated, and thereby removed from the-system, solubilizer remains behind in the electrocoating bath dispersion. If it is not to build up to large excess concentrations therein, it must be either separately removed, for example by absorption onto an ion exchange resin, or the electrodepositable material added to replenish the bath must be formulated with only so much solubilizer as will be incidentally lost from the electrocoating operation by evaporation, carry-out in the rinse effluent, etc. Removing excess solubilizer from portions of electrocoating bath dispersion involves an extra process step and may produce local instability of dispersion, while employing solubilizerstarved replenishment formulations may lead to difficulty and non-uniformity in redispersion upon addition to the electrocoating bath. The process of the present invention can be used to transfer excess solubilizer out of the electrocoating bath dispersion into the rinse effluent at the same time and in the same apparatus that the electrode-positionable material is being transferred from the rinse effluent into the electrocating bath dispersion and without concurrently transferring more than insignificant amounts of solvent or dispersing medium.

SUMMARY OF THE INVENTION An object of the present invention is to provide apparatus and method for transferring electrodepositable coating binder from one liquid dispersion thereof to a second liquid in which the transferred binder will re- 'main in dispersed and electrodepositable form until electrodeposited.

Another object is to provide apparatus and method for such transfer of electrodepositable coating binder from a liquid dispersion in which it is relatively dilute into a liquid dispersion in which it is relatively concentrated.

Another object is to provide apparatus and method for such transfer where substantial transfer of solvent or dispersing medium from the dilute liquid dispersion to the concentrated liquid dispersion is prevented.

A particular object is to provide apparatus and method for such transfer of electrodepositable coating binder from electrocoating rinse effluent to a portion of electrocoating bath liquid electrolyte.

A further particular object is to provide apparatus and method for such transfer of electrodepositable coating binder from electrocating rinse effluent into electrocoating bath liquid electrolyte while simultaneously transferring ionizing solubilizer for said binder from the electrocoating bath liquid electrolyte into the electrocoatingrinse effluent.

In all of these objects it is contemplated that electrodepositable material in addition to the electrodepositable coating binder, such as pigment, etc., may be present and recovered simultaneously with the electrodepositable coating binder.

These object are achieved according to the present invention by employing an electrolytic cell divided in two half-cell compartments, each containing a half-cell electrode, by a separator which is electrically nonconductive, freely and non-selectively permable through at least a portion of its area to liquid dispersions containing electrodepositable coating binder, and effective for arresting turbulent transmission of liquid contents of either of the half-cell compartments to the other of the half-cell compartments in the'substantial absence of hydraulic pressure difference across the permeable portion of the separator while permitting hydraulic and electrical communication of respective liquid contents of the two half-cell compartments through the separatonA liquid dispersion of electrodepositable coating binder from which binder is to be recovered is supplied to one of the half-cell compartments so as to make contact with the half-cell electrode thereof and with the permeable portion of the separator. A liquid into which binder is to be recovered is supplied to the other of the half-cell compartments so as to make contact with the half-cell electrode thereof and with the permable portion of the separator. An electrical potential difference of polarity and magnitude effective to drive electrodepositable coating binder from the liquid dispersion from which it is to be recovered through the permeable portion of the separator into the liquid into which it is to be recovered is impressed upon the half-cell electrodes, thereby depleting the liquid dispersion from which binder is to be recovered and enriching the liquid into which the said binder is to be recovered with respect to electrodepositable coating binder concentration. Where it is desirable to prevent the transfer of any substantial amount of solvent or dispersing medium from one liquid to the other, approximate hydraulic equilibrium is established between the two liquids where they communicate through the permeable portion of the separator to substantially prevent hydraulic flow ofliquid therethrough.

In a particularly useful embodiment of the method of the present invention, the liquid dispersion from which binder is to be recovered is relatively dilute in binder concentration and the liquid dispersion into which binder is to be recovered is relatively concentrated in binder of substantially the same composition as the binder to be recovered. In another particularly useful method embodiment, the dilute dispersion from which binder is to be recovered is rinse effluent from an electrocoating process and the liquid into which binder is to be recovered is a portion of the electrocoating bath liquid electrolyte.

In any of its embodiments, it is contemplated that the method of this invention may involve the simultaneous recovery of other electrodepositable coating components simultaneously with recovery of the binder and also that other non-electrodepositable components may be present in the liquids supplied to the half-cell compartments. Where continuous operation is desired, liquid inlet and outlet means are incorporated in each half-cell compartment. Where it is desired to substantially prevent transfer of solvent or dispersingmedium through the separator, means for establishing and maintaining approximate hydraulic equilibrium across the permeable portion of the separator are incorporated in the apparatus and used in the process to prevent such transfer.

An additional particular embodiment involves apparatus and method generally as described above, but wherein the half-cell compartment into which binder is to be recovered is itself an electrocoating cell in which the electrodepositable coating binder may be electrodeposited upon a removable object comprising the half-cell electrode thereof. Ordinarily in this embodiment a counter-electrode would also be present in this half-cell compartment in addition to the counterelectrode in the other half-cell compartment from which binder is to be recovered. In this embodiment, no measures would be taken to prevent an insulating accumulation of electrodeposited coating binder upon the half-cell electrode of the half-cell compartment into which binder is to be recovered.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a simple, open-top electrolytic cell suitable for practicing the method of the present invention wherein hydraulic pressure difference across thepermeable separator is controlled by overflow of liquid through weir outlets and wherein agitation, if desired, is induced by a motor-driven stirrer.

FIG. 2 shows a closed-top electrolytic cell adapted for continuous operation wherein agitation of the liquid contents can be induced by rapid recirculation of those liquid contents though a venetian-blind electrode and wherein hydraulic pressure difference across the porous separator can be controlled by adjusting the relative rates atwhich liquid is supplied, recirculated, and withdrawn.

FIG. 3 shows an elevation cross-section of a cell in which recovery of electrodepositable binder is directly into an electrocoating tank wherein electrodeposition of binder onto electrode workpieces is simultaneously occurring and where the hydraulic pressure difference across the porous separator is controlled by means of an overflow weir outlet on one side of the separator floating on liquid on the other side of the separator.

FIG. 4 shows a plan cross-section of the cell of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1, the shell or body I of the cell holds the two bodies of liquid which are to be treated in the cell. The cell is divided by a non-selectively permeable separator 2 into two half-cell compartments 3 and 4, each of which contains a half-cell electrode, respectively 5 and 6. The liquid dispersion from which electrodepositable coating binder is to be recovered. is supplied to compartment 3 and the liquid into which binder is to be recovered is supplied to compartment 4. Hydraulic pressure difference across the separator can be substantially eliminated by supplying additional amounts of the appropriate liquid to these respective half-cell compartments 3 and d so as to maintain the liquid levels therein at the height of the respective overflow weirs 7 and 8, which are fixed at equal height. The weirs discharge overflowed liquids through outlets 9 and 10, respectively. When it is desired to prevent insulating accumulation of electrodeposited material on electrode 6, this may be accomplished by agitation of the liquid in half-cell compartment 4 by means of the stirrer driven by a motor 101, the intensity of agitation required for this purpose being greater, the greater is the electrical potential gradient in the immediate vicinity of electrode 6. Electrode 6 is indicated as positively charged with respect to electrode 5, which would adapt this cell for recovery of anodic electrocoating material from dispersion in the liquid introduced into halfcell compartment 3 through separator 2 into the liquid introduced into half-cell compartment 4. It should be understood that reversing the polarity of the electrodes so as to make electrode 6 negative with respect to electrode 5 would adapt the cell for corresponding recovery of cathodic electrocoating material. Whichever polarity is required may be produced by connecting the electrodes to a conventional external source of direct current. I

FIG. 2 shows a vertical cross-section of a closed cell particularly adapted for continuous operation. The body 11 is divided by non-selectively permeable separator 12 into two half-cell compartments l3 and I4 and electrodes 15 and I6 are'located respectively within those compartments. Electrode is a porous electrode constructed from metal turnings confined between two spaced metal screens or perforated metal compositions can move freely therethrough without mechanical entrapment. Porous electrode is sealed around its periphery to the body 11 of the cell, or alternatively to an inward extension of liquid outlet 20, so that a liquid dispersion of electrodepositable coating binder supplied to half-cell compartment 13 through liquid inlet 19 must flow through porous electrode 15 in order to reach liquid outlet 20. Electrode 16 in halfcell compartment 14 is a venetian-blind style electrode comprising a spaced series of electrically interconnected metal slats such that dispersed electrocoating material will pass freely through the spaces between the slats thereof. Liquid into which electrodepositable coating binder is to be recovered is introduced into half-cell compartment 14 between electrode 16 and separator 12 by introducing it through liquid inlet 21, valve 22, and connecting inlet 23. Pump 25 causes liquid to be withdrawn from compartment 14 through electrode 16 and into liquid outlet 24 where it passes through pump 25 and a portion is recirculated to compartment 14 by way of inlet 23 and a portion is removed through valve 26 and liquid outlet 27 therefrom. The rate of liquid flow through electrode 16 is made sufficiently high to prevent an insulating accumulation of electrodeposited material thereon by adjusting the rate of recirculation of liquid through outlet 24, pump 25 and inlet 23. Ordinarily this recirculating flow will be much greater than the input and output flows through inlet 21 and outlet 27, respectively. Open tubes 17 and 18 set into the top of the cell in

compartment

13 and 14, respectively, serve to vent any gases generated during operation of the cell and to detect any substantial hydraulic pressure difference across the porous separator '12. If these tubes are transparent, the pressure difference across the separator can be visually observed as a difference in the height of rise of liquid in the two tubes and corrective action to equalize those pressures by adjusting the setting of outlet valves 26 to affect the back pressure in half-cell compartment 14 or by adjusting the setting of a similaroutlet valve (not shown) in the outlet lines 20 to affect the back pressure in half-cell compartment 13. Automatic control could be provided by instrumented optical detection of the liquid levels in tubes 17 and 18 or by detection of those heights by providing suitable conventional electrical circuits to be closed or opened by virtue of the electrical conductivity of the liquid dispersions as they respectively rise to touch or fall to break contact with suitable electrical contacts exposed within those tubes. The signal from any such automatic pressure difference detector can be utilized to automatically actuate adjustment of outlet valves 26 and/or a similar outlet valve in-outlet lines -20 from compartment 13, if such valves are provided-with conventional solenoid or other actuators. Again, the electrode 16 is shown as an anode and electrode 15 as a cathode, renderring the cell suitable for recovery of anodic electrocoating binder from dispersion in the liquid introduced into half-cell compartment 13 by electrical migration through porous separator 12 into the liquid introduced into half-cell compartment 14; reversal of this polarity would render this cell suitable for correspondingly recovering cathodic electrocoating binder. Direct current of the appropriate polarity is supplied by connecting the electrodes to a conventional external emf source.

FIG. 3 is a cross-section elevation and FIG. 4 a crosssection plan view ofa cell adapted for recovering electrodepositable binder from a liquid dispersion thereof directly into an electrocoating bath liquid electrolyte dispersion in a cell where electrocoating of an object is proceeding simultaneously with the recovery of binder. The view of FIG. 3 corresponds to section BB of FIG. 4 and the view of FIG. 4 corresponds to section AA FIG. 3. A liquid containing dispersed electrodepositable coating binder is introduced into half-cell com partment 33 of cell 31 through liquid inlet 40. Half-cell compartment 33 is divided from the reminder of this cell, identified as half-cell compartment 34, by a vertical cylindrical separator 32, a lower portion of which is non-selectively permeable to passage of dispersed electrocoating material. Compartment 33 also contains a solid electrode 35 approximately concentric with the separator 32. Half-cell compartment 34 is filled to a suitable level with a liquid electrolyte dispersion of an electrodepositable coating binder and workpiece electrode 36 supported by a conventional supporting member 41 and connected to an external source of direct current emf either through the supporting member 41 or by separate attachment of an electrical lead is immersed therein. The half-cell electrode 35 and optional additional counterelectrodes 42 are charged to opposite polarity from electrode 36 by connection to the external emf source. The polarities indicated, namely, the workpiece electrode 36 as anode relative to the halfcell electrode 35, and optional additional counterelectrodes 42, as cathode is suitable for use with anodic electrocoating compositions. It should be understood, however, that by reversing the polarity of the electrodes, cathodic electrocoating compositions can be treated in the present invention. In continuous operation, liquid in compartment 33 from which electrodepositable coating binder has been recovered by electrical migration from dispersion therein through the permeable portion of the separator 32 into the electrocoating bath liquid electrolyte dispersion in half-cell compartment 34 by overflow into the floating weir located in the upper region of compartment 33. From the weir, liquid drains through vertically extensible liquid outlet 39. The overflow height of weir 37 is maintained equal to the liquid level of the contents of half-cell compartment 34 by attachment of the weir to flotation devices 38, such as hollow metal spheres, which will float on the surface of the liquid in compartment 34, thereby insuring that substantial hydraulic pressure difference across the porous portion of the separator 32 will not occur. Means conventional in the electrocoating art for replenishing, filtering, cooling, agitating to maintain uniformity and for moving workpieces into and out of the liquid dispersion in cell compartment 34 would normally be provided, but are not shown.

The freely andnon-selectively permeable separator which divides the cell into two half-cell compartments must be constructed of non-conductive material in order to avoid electrodeposition thereon. In one particularly simple and readily available form, the separator may comprise a woven screen or cloth of glass or plastic thread of about 1 millimeter in diameter having apertures in the weave of about 0.2 to 0.4 millimeter. Several such screens can be placed in series, either in contact with each other or spaced apart, to form the porous separator. In another form the separator may comprise a rigid plastic sheet perforated by a plurality of holes of about 0.2 to 0.4 millimeters in diameter or a parallel series of such perforated rigid plastic sheets spaced slightly apart from each other. It is important that the perforations or apertures in the separator be sufficiently large so that the largest components of the electrocoating composition to be treated can pass freely therethrough. For typical electrocoating compositions, apertures of 0.2 millimeters will be adequately large, but the treatment of dispersions of many electrocoating compositions, andv especially those containing no pigments, considerably smaller apertures will suffice. Permeable separators having perforations or apertures considerably larger than 0.2 millimeters, and even considerably larger than 0.4 millimeters, may be employed in the practice of the present invention so long as two conditions are satisfied.

The first condition is that the separator must substantially arrest turbulent transmission of the liquidcontents of either half-cell compartment through the separator into the other half-cell compartment in the absence of substantial hydraulic pressure difference across the separator. For any given intensity of agitation of liquid cell'contents, more layers and/or greater thickness must be provided in the separator structure if large apertures therein are adopted. Similarly, if liquid cell contents are to be more intensely agitated, then either the maximum satisfactory aperture size will be reduced or the minimum satisfactory number of layers or thickness of the permeable separator will be increased. The second condition to be met in considering the maximum size of apertures in the permeable separator is that although the apertures must be large enough to freely pass the largest dispersed component of the electrocoating composition to be treated, nonetheless, there must be sufficient resistance to flow therethrough to permit detection of hydrauylic pressure difference across the separator in order that corrective action may be taken to substantially eliminate that pressure difference and thereby to substantially prevent the hydraulic flow of liquid through the separator which would necessarily take place if such hydraulic pressure difference were not otherwise relieved. The more sensitive is the pressure difference detecting means adopted, the smaller will be the required resistance to hydraulic flow through the porous separator that is required to activate it. Where a batchwise operation or a closed loop recirculating operation is involved, no such resistance to flow at all is required, since in such cases liquid flow through the barrier can be readily detected by directly observing the volumes of the respective liquids present in the system on oppo- I continuous or intermittent additions and withdrawals of materials from the system, some means for detecting the occurrence of hydraulic flow through the separator by observing the hydraulic pressure difference which produces that flow will be required in order to relieve that pressure difference and eliminate the corresponding liquid flow through the permeable separator.

Suitable means for detecting hydraulic pressure difference across the porous separator include, in appropriate contexts, provision for overflow of liquids from the two half-cell compartments at equal fixed heights or for the overflow of liquid from one half-cell compartment at a height automatically adjusted to equal the height of liquid in the other half-cell compartment by flotation thereon or otherwise, instrumented detection of the absolute or relative heights of liquid in the two half-cell compartments by means of optical devices, mechanical flotation devices or by detection of the presence or absence of electrical conductivity as the electrically conductive dispersions rise or fall respectively through the level of the detection zone, but any other means for detecting hydraulic pressure difference across the separator or for detecting the liquid flow through the separator resulting from such hydraulic pressure difference may be employed. Suitable means for preventing such liquid flow through the porous separator as a result of hydraulic pressure difference across the porous separator include, in appropriate contexts, manually controlled addition or withdrawal of appropriate liquid dispersion in one or both half-cell compartments so as to reequalize the liquid levels therein, manual or automatic adjustment of inlet and/or outlet valves controlling the inflow and/or outflow ofliquid dispersion from either or both ofthe halfcell compartments so as to equalize the back pressure in the two half-cell compartments; or direct overflow removal of excess liquid dispersion from either or both of the half-cell compartments, where the overflow levels are either fixed or automatically adjusted so as to provide equal liquid heights in both half-cell compartments as the appropriate liquid dispersion or dispersions are supplied to keep the corresponding liquid level or levels at the overflow height. Other means for accomplishing the same function may of course may be employed.

The intensity of liquid agitation required to prevent an insulating accumulation of electrodeposited material on the electrode toward which electrodepositable material is electrically driven will. increase as the rate at which such electrodeposition would take place in the absence of agitation is increased. Thus, for any given compositions of the two liquid dispersions to be treated in a given cell, the intensity of agitation required to prevent insulating accumulation of electrodeposited material on the attracting electrode will increase as the electrical potential difference between the two half-cell electrodes is increased, but will decrease as the temperature in the immediate vicinity of the attracting electrode is raised. A certain-amount of electrodeposition on the attracting electrode typically occurs as the operation of a cell for the practice of the method of the present invention is commenced, but unless the net accumulation of such electrodepositable material is terminated before such insulating accumulation has proceeded to the extent that it significantly increases the electrical resistance of that electrode, the process will become increasingly inefficient. If the potential difference across the half-cell electrode is maintained indefinitely, further net accumulation of electrodeposited binder upon the attracting electrode will ultimately terminate even in the absence of liquid agitation, but only when the electrode resistance has substantially increased to the point where the rate of deposition has been reduced to equal the rate of resolubilization and the current flow in the cell has been correspondingly reduced. Where the cell is to be operated for recovery of electrodepositable coating binder directly into an electrocoating bath liquid electrolyte dispersion wherein electrocoating of workpiece electrodes is simultaneously proceeding, the accumulation of electrodeposited material on the workpiece electrode ordinarily does continue to a condition'of substantial, but nottotal electrical insulation thereof, but efficient continued operation of the cell is permitted by removal of such insulated workpiece electrodes and replacement thereof by fresh uninsulated workpiece electrodes. By an insulating accumulation of electrodeposited material is meant a deposit which appreciably increase the effective resistance of the electrode upon which it is deposited.

The magnitude of the direct current electrical potential difference to be impressed across the half-cell electrodes can vary over a considerable range depending somewhat upon the composition-of the dispersions to be treated, the shape and distance of separation of the half-cell electrodes, the intensity of fluid agitation that is available,-the rate of gas production at the half-cell electrodesthat can be tolerated, the temperature, and other factors. Generally, potential differences across the half-cell electrodes which will produce current flow in the cell in the range from about 0.025 to aboutiLO amp/(decimeter) atthe surface of the electrode toward which dispersed electrodepositable coating binder is attracted will be found most satisfactory. At significantly lower current densitiesthe rate of recovcry of binder would ordinarily be regarded as undesirably slow. At significantly higher'current densities the rate of heating of the liquid dispersion in the cell and the rate of gas evolution from the electrodes would ordinarily be regarded as presenting serious problems, but where efficient cooling means for the liquid cell contents are available and where a high rate of gas evolution can be tolerated, then an electrical potential which will, produce current densities substantially greater than 1.0 amp./(decimeter) may be used in the practice of the present invention. Where the process is being carried out by recovering electrodepositable binder directly into an electrocoating bath liquid electrolyte dispersion by electrical attraction toward workpiece electrodes being simultaneously electrocoated therefrom, the maximumtolerable current density will ordinarily be limited by the adverse effects'of'too high current density upon the quality of coating being electrodeposited upon those workpiece electrodes. Furthermore, it is usually found that there isa threshold potential difference across the half-cell electrodes of a few volts magnitude, below which no recovery of electrodepositable binders occurs and above which the rate of recovery of such binder increases continuously as the potential differences is increased. The polarity of increases potential difference is, of course, to be selected according to'the type of electrodepositable coating binder to be recovered, the attracting electrode being positively charged for recovery of anodic electrocoating binders and negatively charged for recovery of cathodic electrocoating binders.

The shell or body of the cell should be constructed of non-conductive material, such as rigid plastic, vulcanized rubber, glass or ceramic. It may also be constructed of metal if a non-conductive lining of plastic, paint or other electrically insulating material is provided. Use of a non-conductive material of construction for the body of the cell is important for otherwise binder may electrodeposit in substantial quantities upon the inner surface of the cell body and the electrical migration of dispersed binder will be correspondingly shortcircuited.

The'process of this invention may be used with liquid dispersion containing electrodepositable coating binder of any composition so long as it will migrate in the liquid dispersion in response to an electrical potential gradient impressed therein. A great many such electrodepositable coating binders are known in the electrocoating art. Typical of binders suitable for deposition upon an anode is a maleinized linseed oil resin containing ionizable carboxylic acid groups. Typical of binders suitable for electrodeposition upon a cathode is an organic polymer containing tertiary amine groups, ionizable to form quaternary ammonium' groups. Usually such electrodepositable coating binders are dispersed in water, but other suitable liquid dispersing media can be used, and usually also an ionizing solubilizer is included to obtain and stabilize dispersion, including solution, of a greaterconcentration of binder than would otherwise be possible. Many such solubilizers are well-known in the electrocoating art. Water soluble hydroxy amines are typical of ionizing solubilizers used with anodic depositing electrocoa'tingresins and soluble acids such lactic acid are typical of ionizing solubilizers used with cathodic depositing electrocoating resins. Such soluble ionizing solubilizers, if present in the dispersions used in the process of the present invention, will migrate in the opposite direction from the dispersed electrodepositable binder in response to the electric field produced by charging the electrodes. Pigments, extenders, fillers, curing agents,fungicides, leveling agents, etc., which comprise part of an electrocoating composition intended to be deposited with the binder during electrocoating will also tend to be recovered together with the electrodepositable coating binder in the process of the present invention. While it would be possible to operate this process so as to recover electrodepositable material from a more concentrated liquid dispersion thereof into a less concentrated dispersion liquid dispersion, this process will find its chief utility in recovering such material from dilute dispersion into concentrated dispersion, and most particularly in recovering electrodepositable coating binders from dilute rinse effluent from an electrocoating operation in to a portion of the electrocoating bath liquid elecyrolyte dispersion.

EXAMPLE A closed electrolytic cell of the general type shown in FIG. 2 was constructed of polymethylmethacrylate sheets using neoprene gaskets to seal the joints. The anode consisted of seven stainless steel plates 4 inches X 1 inch spaced about 5% inch part in a tilted venetian blind arrangement and located in a 3 inches X 4 inches anode compartment approximately l k inches thick, and having a total volume of 300 cc. The cathode consisted of 10 grams of stainless steel wool mat confined between two stainless steel screens 3 inches X2 Va inches spaced if; inch apart and located in a cathode compartment which tapered from 3 inches X 4 inches in cross-section to 2 A inches X 3 a in cross-section from the face of the porous separator to the face of the cathode. The opposite face of the cathode opened into a liquid collection region about 2 54; inches X 3 Vi inches in cross-section by inch thick. The cell was divided into anode compartment and cathode compartment by a porous separator consisting of 8 layers of 20 mesh per inch polypropylene screen and three layers mesh per inch cotton gauze. The combined total volume of cathode compartment and cathode collection region was approximately 100 cc. The cathodeanode separation was approximately 1 16 inches. Inlet, outlet, and vent tube connections were provided for each half-cell as shown in FIG. 2.

A 20 percent resin solids solution was prepared from a phenolic modified maleinized linseed oil electrodeposi-table resin of the sort described in Examplel of my U.S. Pat. No. 3,230,162, incorporated herein by reference, having the following composition:

Resin 600. grams Triethylaminc 7.5 grams Diisopropanolamine 128. grams Butyl glycol 60. grams Deionized water 2204.5 grams This solution had a p I-I of 8.0 and a resistivity of outlet back to this reservoir. Similarly, 1290 cc. of the concentrated (8.9 percent) dispersion was placed in a reservoir connected by plastic tubing through a pump to the inlet of the anode compartment of the cell and plastic tubing was connected from the outlet of the anode compartment back to this reservoir. A coil of copper tubing through which cooling water was circulated was immersed in the liquid in this anode reservoir. The pumps were adjusted to circulate 100 cc. per minute of the dilute dispersion through the cathode compartment and 1100 cc. per minute of the concentrated dispersion through the anode compartment. The electrodes were connected to a conventional direct current source of 400 volts and the cell was run for 47 minutes to stabilize its operation. The current flow was initially about 0.4 amps, but after about 7 minutes stabilized at I approximately 1.0 amps, the decrease in the resistance indicating that some intermixing of the two dispersions had occurred while the cell was initially being filled. A light deposit was observed to form on the anode plates, but this deposit did not continue to build up with time. After 47 minutes operation the dispersion circulating through the cathode compartment was analyzed to contain 1.8 percent resin solids and the dispersion circulating through the anode compartment was analyzed to contain 7.0 percent resin solids. At this time a new cathode reservoir containing 560 cc. of the dilute (1.06 percent) dispersion was connected to the cell, replacing the orginal cathode reservoir, without interrupting the circulation of dilute dispersion to the cathode compartment or the current flow in the cell, which continued at approximately 1 amp. After another 20 minutes of operation, the dilute dispersion circulating through the cathode compartment was found to contain only 0.6 percent resin solids and the cathode resservoir and its fluid contents were again replaced-with a new reservoir containing 500 cc. of, the dilute (1.06 percent) resin dispersion. Operation continued for another 20 minutes with approximately 1 amp current flowing in the cell. At this time both the cathode and anode reservoirs containing the dilute and concentrated dispersions of resin solids respectively were disconnected from the cell and the flow of current was terminated. The liquid in the cathode reservoir was analyzed to contain 0.3 percent resin solids and that in the anode reservoir 6.8 percent solids. Assuming that the contents of the small half-cell compartments in this experiment were of the same concentration as the dispersions in the corresponding reservoirs being circulated through those compartments at any given time, a cumulative total of 7.5 grams of resin solids was found to have been transferred out of the two portions of the dilute (1.06 percent) dispersion circulated through the cathode chamber during the final two twenty minutes periods of operation. During the same time, a cummulative total of 15 cc. of total liquid volume was transferred from the dilute dispersions circulated through the cathode compartment into the concentrated dispersion circulated through the anode compartment. The final analysis of the dispersion which had been circulated through the anode compartment showed 6.8 percent resin solids, with only a 15 cc. increase in total volume. While the direct analyses do not appear to positively show that the 7.5 grams of resin solids transferred out of the dilute dispersions circulated through the cathode compartment were transferred into dispersion in the concentrated solution which was circulated through the anode compartment, nonetheless this must have occured since no additional deposit on the anode was observed nor was any separation of solids from any of the solutions noted. More importantly, no significant decrease in the current flow in the cell had occurred, thereby negating any possibility that more than a trivial portion of the 7.5 gram of resin solids transferred out of the solution circulated through the cathode compartment had electrodeposited upon the anode. The lack of direct detection of the transferred resin solids in the anode compartment liquid dispersion is within the expected error limit of those determinations, which were made by evaporation to constant weight of 30 cc. aliquots in a 350 F. oven. Back pressure in the two half-cell compartments was equalized by equalizing the height of liquid rise in the two vent tubes by adjusting the discharge height of the flexible plastic outlet tubes from the respective half-cell compartment.

What is claimed is:

1. In a process for recovering electrodepositable coating binder from a dilute liquid dispersion thereof utilzing an electrolytic cell apparatus divided into two half-cell compartments, each containing a half-cell electrode, by a separator which is electrically nonconductive, freely and non-selectively permeable through at least a portion of its area to liquid dispersions containing electrodepositable coating binder, and effective for arresting turbulent transmission of liquid contents of either of the half-cell compartments to the other of the half-cell compartments in the substantial absence of hydraulic pressure difference across the permeable portion of the separator while permitting hydraulic and electrical communication of respective liquid contents of the two half-cell compartment through the separator, the improvement which comprises:-

a. supplying to one of the half-cell compartments, to make contact with the half-cell electrode thereof and with the permeable portion of the separator, the dilute liquid dispersion of electrodepositable coating binder from which binder is to be recovered;

b. supplying to the other of the half-cell compartments; to make contact with the half-cell electrode thereof and with the permeable portion of the separator, a concentrated liquid dispersion containing dispersed electrodepositable coating binder of substantially the same composition as the binder to be recovered and at a concentration greater than the binder concentration in the dilute liquid dispersion;

c. establishing substantially hydraulic pressure equilbrium between the dilute and concentrated dispersions in the respective half-cell compartments where those dispersions communicate through the separator to substantially prevent hydraulic flow through the separator; and

d. impressing upon the half-cell electrodes in the respective half-cell compartments, from an external emf source, an electrical potential difference of polarity and magnitude effective to drive electrodepositable coating binder from the dilute liquid dispersion through the separator into the concentrated liquid dispersion, thereby depleting the diluteliquid dispersion in electrodepositable coating binder.

2. The process of claim 1 wherein the dilute and concentrated liquid dispersions of electrodepositable coating binder to be treated are continuously supplied to and corresponding depleted dilute dispersion and enriched concentrated dispersion produced thereby are continuously withdrawn from the respective half-cell compartments.

3. The process of claim 2 wherein the dilute liquid dispersion of electrodepositable coating binder is obtained from rinsing freshly electrocoated objects.

4. The process of claim 3 wherein the concentrated liquid dispersion comprises a portion of the liquid electrolyte contents of an electrodeposition cell for electrocoating articles. I

5. The process of claim 4 wherein the half-cell compartment to which the concentrated liquid dispersion is the electrodepositable coating binder on a removable object comprising the half-cell electrode thereof.

6. The process of claim 5 wherein the half-cell compartment to which the concentrated liquid dispersion is supplied additionally contains a counter electrode in contact with the concentrated liquid dispersion supplied thereto and charged from an external emf source to an electrical potential, relative to that impressed upon the half-cell electrodes, such that electrodepositable coating binder is repelled from the counter electrode toward the half-cell electrode of the half-cell compartment to which the concentrated liquid dispersion is supplied.

7. The process of claim 2 wherein the dilute and concentrated liquid dispersions of electrodepositable coating binder also each contain ionizing solubilizer for that binder.

8. The process of claim 7 wherein the ionizing solubilizer in the concentrated liquid dispersion is driven by the electrical potential difference impressed upon the half-cell electrodes from the concentrated liquid dispersion through the separator into the dilute liquid dispersion, thereby depleting the concentrated liquid dispersion and enriching the dilute liquid dispersion in the ionizing solubilizer.

9. The process of claim 1 wherein an insulating accumulation of electrodeposited product of the electrodepositable coating binder upon the half-cell electrode of the half-cell compartment to which the concentrated liquid dispersion is supplied is substantially prevented and the coating binder -is substantially retained in dispersed and electrodepositable condition in the enriched concentrated dispersion thereby produced.

10. The process of claim 9 wherein an insulating accumulation of electrodeposited product of the electrodepositable coating binder upon the half-cell electrode of the half-cell compartment to which the concentrated liquid dispersion is supplied is substantially prevented by inducing vigorous relative motion between the concentrated liquid dispersion in that half-cell compartment and the half-cell electrode thereof.

Claims

1. IN A PROCESS FOR RECOVERING ELECTRODEPOSITABLE COATING BINDER FROM A DILUTE LIQUID DISPERSION THEREOF UTILZING AN ELECTROLYTIC CELL APPARATUS DIVIDED INTO TWO HALF-CELL COMPARTMENTS, EACH CONTAINING A HALF-CELL ELECTRODE, BY A SEPARATOR WHICH IS ELECTRICALLY NON-CONDUCTIVE, FREELY AND NONSELECTIVELY PERMEABLE THROUGH AT LEAST A PORTION OF ITS AREA TO LIQUID DISPERSIONS CONTAINING ELECTRODEPOSITABLE COATING BINDER, AND EFFECTIVE FOR ARRESTING TURBULENT TRANSMISSION OF LIQUID CONTENTS OF EITHER OF THE HALF-CELL COMPARTMENTS TO THE OTHER OF THE HALF-CELL COMPARTMENTS IN THE SUBSTANTIAL ABSENCE OF HYDRAULIC PRESSURE DIFFERENCE ACROSS THE PERMEABLE PORTION OF THE SEPARATOR WHILE PERMITTING HYDRAULIC AND ELECTRICAL COMMUNICATION OF RESPECTIVE LIQUID CONTENTS OF THE TWO HALFCELL COMPARTMENT THROUGH THE SEPARATOR, THE IMPROVEMENT WHICH COMPRISES: A. SUPPLYING TO ONE OF THE HALF-CELL COMPARTMENTS, TO MAKE CONTACT WITH THE HALF-CELL ELECTRODE THEREOF AND WITH THE PERMEABLE PORTION OF THE SEPARATOR, THE DILUTE LIQUID DISPERSION OF ELECTRODEPOSITABLE COATING BINDER FROM WHICH BINDER IS TO BE RECOVERED; B. SUPPLYING TO THE OTHER OF THE HALF-CELL COMPARTMENTS, TO MAKE CONTACT WITH THE HALF-CELL ELECTRODE THEREOF AND WITH THE PERMEABLE PORTION OF THE SEPARATOR, A CONCENTRATED LIQUID DISPERSION CONTAINING DISPERSED ELECTRODEPOSITABLE COATING BINDER OR SUBSTANTIALLY THE SAME COMPOSITION AS THE BINDER TO BE RECOVERED AND AT A CONCENTRATION GREATER THAN THE BINDER CONCENTRATION IN THE DILUTE LIQUID DISPERSION; C. ESTABLISHING SUBSTANTIALLY HYDRAULIC PRESSURE EQUILBRIUM BETWEEN THE DILUTE AND CONCENTRATED DISPERSIONS IN THE RESPECTIVE HALF-CELL COMPARTMENTS WHERE THOSE DISPERSIONS COMMUNICATE THROUGH THE SEPARATOR; AND PREVENT HYDRAULIC FLOW THROUGH THE SEPARATOR; AND D. IMPRESSING UPON THE HALF-CELL ELECTRODES IN THE RESPECTIVE HALF-CELL COMPARTMENTS, FROM AN EXTERNAL EMF SOURCE, AN ELECTRICAL POTENTIAL DIFFERENCE OF POLARITY AND MAGNITUDE EFFECTIVE TO DRIVE ELECTRODEPOSITABLE COATING BINDER FROM THE DILUTE LIQUID DISPERSION THROUGH THE SEPARATOR INTO THE CONCENTRATED LIQUID DISPERSION, THEREBY DEPLETING THE DILUTE LIQUID DISPERSION IN ELECTRODEPOSITABLE COATING BINDER.

4. The process of claim 3 wherein the concentrated liquid dispersion comprises a portion of the liquid electrolyte contents of an electrodeposition cell for electrocoating articles.

5. The process of claim 4 wherein the half-cell compartment to which the concentrated liquid dispersion is supplied is operated as an electrocoating cell to produce a coating comprising electrodeposited product of the electrodepositable coating binder on a removable object comprising the half-cell electrode thereof.

9. The process of claim 1 wherein an insulating accumulation of electrodeposited product of the electrodepositable coating binder upon the half-cell electrode of the half-cell compartment to which the concentrated liquid dispersion is supplied is substantially prevented and the coating binder is substantially retained in dispersed and electrodepositable condition in the enriched concentrated dispersion thereby produced.