RETICULATE ELECTRODE FOR RECOVERY OF METAL IONS AND METHOD FOR MAKING

Abstract: A cathode for use in an electrolytic cell for recovering metal from solutions containing ions of the metal including a support (12) and a carbon impregnated conductive polymer­ ic foam or mat (14) attached to the support and in substantial electrical contact with the support.

Description: RETICULATE ELECTRODE FOR RECOVERY OF METAL IONS AND METHOD FOR MAKING Technical Field This invention relates to electrolytic cells, and particularly to metal electrowinning. Specifically this invention relates to methods and apparatus for the electrolytic recovery of metals onto reticulate type electrodes from solutions containing ions of the metals. Background of the Invention It is well known that many metals can be recovered from solutions containing ions of the metals using electrolytic techniques. Generally, the solution containing the metal ions is contacted with an anode cathode pair in an electrolytic cell, the metal depositing from ionic solution on the cathode. Traditional concerns of electrolytic cell operations as current efficiency and allowable current density at the cathode apply to such metal recovery cells. In addition, electrolytic recovery of metals brings another concern; recapture of the metal from the cathode, particularly at a relatively low cost and with relative ease. Traditionally, the metal recovery cathode was comprised of a substrate made from an electrically conductive plentiful metal upon which the metal ions in solution were plated. Separation normally entailed melting the electrodeposited metal and the substrate, and subsequently separating the two metals. In some cases it was possible to separate electrodeposit form the substrate by physically cracking the electrodeposited metal from the substrate. Where the substrate metal and the electrodepositing metal were the same, often these problems associated with subsequent separation of cathode substrate and recovered metals was eased.The surface area of these traditional cathodes were often limited in their capability for holding recovered metal. More recently it has been found that openly porous plastics, particularly polymers, have found use as reticulate electrodes in metal ion recovery, providing a greater surface area. In using these polymers for metal ion recovery it has been found essential that these plastics be both conductive and effectively rigid when placed in the electrolytic cell. Without conductivity, electrolytic recovery can become difficult. Without rigidity, maintenance of an effectively low anode cathode spacing within the electrolytic cell can become more complicated. One past proposal for producing an openly porous reticulate form electrode that is both electrically conductive and relatively rigid has been deposition of a metal onto an openly porous polymeric foam or weave or non-woven fabric. Generally a preliminary metal deposition is made onto the polymer by treatment in a chemical bath, and subsequent electroless deposition of a metal such as copper to the polymer. The electroless metal coated polymer, now somewhat conductive, is then subjected to electrodeposition of additional quantities of the metal to produce a relatively rigid, substantially conductive electrode structure for use in recovering metal ions from solution. The combination of both electroless and electrolytic deposition of conductive metals onto the polymer can be economically burdensome in producing a cathode for use in recovering metals from solution. When fully loaded with recovered metals, these cathodes are generally destroyed in removing the metal being recovered. An entirely new cathode is therefore required at each replacement. Particularly where a polymer structure is coated with a metal to make the polymer structure conductive, generally for use as a cathode in an electrolyte cell, it is necessary to attach at least one current feeder. Especially in an acidic solution where electric current is interrupted to cells using polymer cathodes prepared by electroless and electrolytic plating and thus removing cathodic protection generally enjoyed by the cathode during electrolyte operation, this attachment point as well as the conductive metal can be subject to corrosive attack. Electrical communication to the cathode can be weakened by such corrosive action resulting in decreased efficiency of electrolytic metal recovery from solution or an interruption in-recovery. Disclosure of the Invention It is an object of the present invention to provide a reticulate type cathode for use in recovering metal ions from a solution containing the ions. It is a further object of the invention to provide a readily replaceable polymeric cathode not requiring electroless and/or electrolytic application of a conductive metal to the polymeric material to provide electrical conductivity and rigidity. It is a still further object of the invention to provide a cathode configuration or assembly including a separable electrical current distributing and rigidity imparting member and at least one polymeric cathode member. The present invention therefore provides a cathode assembly for use in electrolytic recovery of metal ions from solutions containing the ions. The cathode includes an open supporting structure that is electrically conductive and readily fluid permeable. A portion of this structure is intended for immersion in the solution from which metal ions are being recovered and is attached to a connecting portion by which electrical current is transferred between the supporting structure and a source of electrical current. An openly porous, electrically conductive, carbon impregnated polymeric membrane cathode attachably covers at least a portion of at least one surface of the reticulate structure where immersed. The polymeric cathode is in electrical contact with the supporting structure. The polymeric cathode is attached to the supporting using a conductive adhesive or using fasteners such as staples. These cathode assemblies are utilized to form anode-cathode pairs contained in an electrolytic metal recovery cell. Generally a plurality of the cathode assemblies are arranged along the length of an electrolytic cell each spanning the width of the electrolytic cell; metal ion containing solution being introduced at one end of the cell, and traversing a cell length by passing successively through the openly porous polymeric cathodes arranged within the cell. Use of the openly porous polymeric cathode of the instant invention provides a relatively low cost cathode assembly having a substantial available surface for electrodeposition of metal ions from the solution. The supporting structure provides a relatively rigid cathode assembly assisting in the maintenance of anode-cathode spacing within the cell, and thereby assisting in maintaining desirably low electrical power consumption in operation of the electrolytic cell. The cathode assemblies of the instant invention are relatively readily assembled by covering at least one face of the reticulate structure with the polymer. Once assembled, their unitized construction allows ready changeout as the openly porous structure becomes plugged with accumulating metal deposits. In one preferred embodiment, a portion of the reticulate structure extending above the normal level of solution within the electrolytic cell remains uncovered by the polymeric material. When pluggage of the polymeric cathode occurs, metal laden solution flowing through the cell overflows the plugged polymeric cathode through the uncovered supporting structure above the normal or usual metal laden solution level within the cell providing a visual indication of cathode pluggage. The above and other features and advantages of the invention will become more apparent from the detailed description of the invention which follows considered in conjunction with the accompanying drawings which together form a part of the specification. Description of the Drawings Figure 1 is a perspective view of a cathode assembly made in accordance with the instant invention. Figure 2 is a cross sectional view of a cathode assembly made in accordance with the instant invention and including polymeric cathodes upon both surfaces of the supporting structure. 'Figure 3 is a cross sectional view of a typical electrolytic cell utilizing the cathode assemblies of the instant invention. Best Embodiment of the Invention Referring to the drawings, Figures 1 and 2 show perspective and edge views respectively of a cathode assembly 10. The cathode assembly 10 includes an openly fluid permeable supporting structure 12 and a polymeric cathode 14 attached to the structure 12 and in substantial electrically conducting contact with the structure 12. The supporting structure 12 can be of any electrically conductive, relatively rigid material. Electrically conductive metal mesh, a perforated plate, an interwoven wire grid, a grid formed from an electrically conductive plastic such as Caprez polypropylene available from Alloy Polymers or conductive polyvinyl chloride available from Diamond Shamrock may be utilized. Other suitable or conventional materials may be utilized that readily pass electrical current through 'their structure while permitting ready fluid passage. The supporting structure 12' performs a dual function. Electrical currrent is distributed via the structure to the polymeric cathode 14, and the structure functions to position and support the cathode 14 within an electrolytic cell and to maintain the cathode 14 in a desirable spaced relationship with anodes utilized in the electrolytic cell. The cathode 14 is formed from a conductive polymer. Three polymer forms have been found attractive, foams, fiber weaves and non-woven mats. The polymer utilized should be resistant to corrosive and solvating attack threats posed by whatever metal ion laden solution is to be electrolytically relieved of its metal ion content using the cathode assembly of the instant invention. One preferred form of the conductive polymer is as a foam. Polyurethanes and polyesters have been found to provide effective cathodes although other conductive foams may be satisfactory. The foam should be openly porous, that is, relatively freely passing fluid through .=its thickness--The openlp porous foam should accommodate over the surface of the cathode assembly a liquid flow through the foam at least equal to the volume rate of solution from which metal ion recovery is desired. Another preferred form of the invention is an openly porous woven mat of fibers of a polymer or non-woven mat of the polymer, the mat passing a liquid flow through the mat over the surface of the cathode assembly at least equal to the volume rate of solution from which metal ion recovery is desired. Particularly polyester fiber weaves have been found desirable, though other electrically conductive polymeric fibers may be utilized. The fiber mat or foam polymeric cathode 14 is made conductive by incorporation of carbon into the polymeric cathode 14. Carbon can be included by incorporation into fabric of the foam or mat during formation or by impregnating the foam or mat subsequent to formation. Techniques for carbon inclusion or impregnation are well known. The following materials have been found suitable for use as a polymeric cathode: #202 Urethane foam available from Richards Parents & BR< Murray Inc. ,CC-F-1/8-3SPPI-1Q0, CC-F-1/ & 35PPI-65, and CC-F-7/32-30PPI-65 foams available from Lewcott Chemicals and Plastics. Conductivity of the polymeric foam or mat cathode should generally be greater than about 10,000 ohms per centimeter, and preferably greater than 5,000 ohms per centimeter with preferably materials averaging 3,000 ohms per centimeter or greater. The cathode 14 can vary in thickness over a considerable range. Typically a foam of 0.05 to 1.0 inches is utilized and preferably between 1/16 to 1/2". Thicker foam cathodes 14 tend to spread anode and cathode within a cell to an extent that considerable voltage inefficiency in operating a metal ion recovery cell can be introduced. Thinner cathodes 14 quickly become loaded with recovered metal, requiring undesirably frequent removal and replacement. Porosity of the foam or mat can vary between about 10 and 100 pores per square inch (PPI). Preferred are materials of about 25 to 40 PPI. The polymeric cathode 14 is affixed to the supporting structure 12 in any suitable or conventional manner producing electrical communication between them. Conductive adhesives such as Crest 2014A and B or Crest 173 A and B, two part epoxys, available from Crest Products Corp. or 52-04-4130 conductive latex available from Chomerics Corp., staples 16 or U bolts are used to hold the structure 12 and polymeric cathode 14 in intimate electrical contact. It is important that electrical current be transferred between the reticulate structure 12 and the cathode over a substantial portion of surface portions of the cathode opposing the structure. Where a conductive polymeric reticulate structure is utilized it is sometimes desirable to apply an electrically conductive metal foil (not shown) along an upper edge of the supporting structure whereby electrical current can be transferred along the length of the cathode assembly from a source of electrical current. Otherwise electrical current can be transferred to the cathode assembly 10 at one or more locations along the supporting structure. In the preferred embodiment one or more flange portions 18 of the supporting structure is oriented upwardly from the cathode assembly and can be used both for mounting of the cathode assembly 10 in an electrolytic cell and for conducting electrical current to the cathode assembly. The flange portions 18 can be fabricated in any suitable or conventional manner such as by forming the flange from the supporting structure and bending the portion into position, or by attachment of separately formed flange portions 18. Referring to Figure 3, an electrolyte cell 24 is depicted including a plurality of cathode assemblies 10 and a plurality of anodes 26 arranged in spaced relationship spanning the width of the cell 24. The cathode assemblies 10 generally separate the cell 24 into compartment 28. The cell includes a fluid inlet 30 and outlet 32. Solution containing metal ions to be recovered enters the cell via the fluid inlet and exits via the outlet. The solution passes through each cathode assembly in traversing the cell, and metal ions are thereby brought into intimate contact with the cathode assembly for Where a cathode assembly becomes laden with recovered metal ions, impeding flow of the solution therethrough, the solution level in compartments behind the solution flow will tend to rise as shown at 38 in Figure 3. In the preferred embodiment, the polymeric cathode 14 does not cover the supporting structure to full height, allowing a rising solution level to overflow the cathode 14 through a zone 40 of the supporting structure. Detection of cathode assemblies requiring changeout is thereby facilitated. Anodes, and surfaces of cathodes 14 within the electrolytic cell are desirably separated by between 1/4" and 1/2". Closer spacing is feasible but formation of dendrites upon cathodes 12 closely spaced to anodes can lead to short circuiting. Greater spacing is workable, but can cause unacceptable power inefficiencies. Generally anode cathode spacings in excess of about 5 inches or less than about 1/16" are not desirable. The polymeric cathode 14 may be applied to one or both surfaces of the conductive supporting structure 12, shown at 14 in Figure 2. Selection of one or both surfaces of the structure for coating is in part dependent upon factors such as concentration of metal ions in the solution being treated, flow rate of the solution through the cell, thickness of the cathode 14 being applied, and placement of anodes. Each application is therefore somewhat individualized and should be approached individually. The following examples are offered to further illustrate the invention. EXAMPLE I An open-topped electrolytic cell measuring approximately 6 inches &commat; x 8 inches by 8 inches in height fabricated from Lucite , a duPont product, was provided with a fluid inlet and outlet. Two approximately 6 inch by 6 inch by 3/8 inch thickness of RPM #202 conductive urethane foam squares were placed each in a 6 inch by 6 inch three sided channel frame. The channels were compressed to retain the foam. A strip of aluminum foil was compressed into the foam along the unchanneled edge. The effective open foam area was thereby about 5 x 5 inches. The foam squares were installed in the Lucite cell, the encasing channels adjacent to the walls and bottom of the cell. Three expanded metal mesh anodes measuring approximately 5 inches by 5 inches and coated with Diamond Shamrock TIR-2000, a mixture of tantalum, titanium, and iridium oxides were arranged alternately with the foam squares in the cell. A spacing of approximately 1/2" was established between the surface of each foam square and an opposing electrode. A current feeder was connected to the aluminum foil strip attached to each foam square whereby the square was made cathodic by connection to a current source. 15 liters of a 250 parts per million CuSO4 solution in sulfuric acid was circulated repeatedly through the cell, passing through each of the foam cathodes. pH of the solution was about 1.5. A current of 5 amperes per foot squared of exposed foam was passed between the anodes and the cathodes. The solution circulated at 3 gallons per minute per square foot of foam cathode. Circulation was continued for approximately one hour during which copper concentration in the circulating solution declined to 73 parts per million. Voltage between anodes and cathodes during this period declined from about 3.8 volts to about 2.4 volts. EXAMPLE II A cathode was fabricated by attaching two 6 inch by 6 inch by 1/4 inch RPM +202 conductive foam squares to both faces of a 6 inch by 6 inch by 8 inch aluminum structural mesh having substantial open area. Attachment was made using staples. A 2 inch wide strip of the aluminum remained exposed. The cathode was installed in the cell between and spaced from by 1/2 inch, 2 mesh anodes of the type utilized in Example I. The cathode and exposed aluminum portion of the anodes were connected to an electrical current source. A reservoir of 15 liters of a 210 parts per million CuSO4 solution at a pH of about 1.75 was circulated through the cell at a flow rate of 6 gallons per minute per square foot of foam cathode. After approximately one hour, copper content in the circulating solution had been reduced to 100 parts per million. Voltage between anode and cathode declined from an initial 2.5 volts to a final 2.3 volts. EXAMPLE III A cathode assembly was fabricated by placing an 18 inch by 15 inch wide sheet of expanded aluminum foil mesh between two 18 inch x 15 inch x 1/8 inch thick cathodes formed of carbon impregnated Lewcott polyurethane foam. The three-component cathode assembly was secured by bonding the foam to the aluminum with a conductive epoxy resin adhesive. The cathode was placed in an open electrolytic cell between two Diamond Shamrock TIR-2000 metallic anodes with a spacing between anodes and the foam surfaces of approximately 1/2 inch. A wastewater stream containing approximately 150 parts per million CuSO4 was passed through the electrolytic cell with no recycle for approximately 200 hours at a flow rate of 2 gallons per minute per square foot of exposed cathode area. A direct current of 3 amps per square foot of exposed cathode area was applied to the cell at a constant voltage of 2.8 volts. At the conclusion of the experiment, 300 grams of metallic copper had been removed from the CuSO4 solution. While a preferred embodiment of the invention has been shown and described in detail, it should be apparent that various modifications can be made thereto without departing from the scope of the claims following.

Claims: What is claimed is: 1. A cathode assembly for use in recovering metals from solutions containing ions of the metals comprising: an electrically conductive supporting structure including a portion at least partially immersed in the solution and a connecting portion by which electrical current is transferred between the supporting structure and a source of electrical potential; and an openly porous carbon impregnated poly meric cathode membrane attachably covering at least a portion of at least one surface of the supporting structure and in electrical contact with the supporting structure. 2. The assembly of claim 1 wherein the openly porous carbon impregnated polymer membrane is one of an openly porous foam and an openly porous fiber mat of the polymer. 3. The assembly of claim 1 wherein the polymer is one of a polyurethane and a polyester. 4. A cathode assembly for use in recovering metals from solutions containing ions of the metals comprising: an electrical current conducting supporting structure including a portion at least partially immersed in the solution and a connecting portion by which electrical current is trans ferred between the supporting structure and a source of electrical potential; and an openly porous carbon impregnated elec trically conductive polymeric foam cathode attachably covering at least a portion of at least one surface of immersed portions of the supporting structure. 5. The assembly of claim 4 wherein the openly porous foam is one of carbon impregnated polyurethane and polyester foams. 6. In the assembly of claim 4, the supporting structure having an open area of between 10% and 95%, and the foam having a thickness of between about 0.05 inch and 1.0 inches. 7. A cathode assembly for use in recovering metal from solutions containing ions of the metals comprising: an electrical current conducting supporting structure including a portion at least par tially immersed in the solution and a connecting portion by which electrical current is trans ferred between the supporting structure and a source of electrical potential; and and openly porous woven, carbon impregnated electrically conductive polymeric cathode at tachably covering at least a portion of at least one surface of the supporting structure. 8. The assembly of claim 7 wherein the woven cathode is polyester. 9. In the assembly of claim 7, the supporting structure having an open area of between 10% and 95% and the woven polymer having a thickness of between about 0.05 and 1.0 inches. 10. A method for making an electrode assembly for use in recovering metals from solutions containing ions of the metals comprising: Selecting an electrical current conducting supporting structure having a portion for at partial immersion in the solution and a con necting portion by which electrical current is transferred between the supporting structure and a source of electrical potential; and attaching to the supporting structure an openly porous, electrically conductive carbon cathode membrane covering at least a portion of at least one surface of the supporting structure, the cathode being retained in electrical current transferring relationship with the supporting structure. 11. The method of claim 10 wherein the openly porous, electrically conductive polymeric membrane is one of carbon impregnated polyurethane and polyester foams. 12. The method of claim 10 wherein the openly porous, electrically conductive polymeric membrane is woven polyester. 13. In the method of claim 10, the membrane being fastened using one of an electrically conductive epoxy cement, an electrically conductive latex, staples, and mixtures thereof. 14. In an electrolytic cell for the recovery on a type cathode of ions of metals contained in the cell, the cathodes and corresponding paired anodes being arranged in the cell in electrical current communication with a source of electrical potential, an impoved cathode comprising: an electrically conductive supporting structure including a portion at least partially immersed in the solution and a connecting porticn by which electrical current is trans ferred between the supporting structure and the source of electrical potential; and an openly porous, carbon impregnated, eletrically conductive polymeric cathode selected from a group consisting of poly urethane and polyester foams and matted polyester fibers, attachably covering at least a portion of at least one surface of immersed portion of the supporting structure and in substantial electrical contact with the supporting structure. 15. The improved cathode assembly of claim 14 wherein the cathode is one of a polyurethane and polyester carbon impregnated foam and a carbon impregnated polyester mat. 16. The improved cathode assembly of claim 14 wherein the cathode is between about 0.05 and 1.0 in thickness and the supporting structure is between about 10% and 95% open area.

Inventors: KRUG, GEOFFREY P., PLATEK, GARY F.
Applicants: ELTECH SYSTEMS CORPORATION
Application year: 1982
Main classification: C25C7
IPC codes: C25C7, C02F1