FLOW SPACERS FOR ELECTRODIALYSIS APPARATUS AND METHOD FOR MAKING THEM

Abstract: An improved electrodialysis stack (10) and method of making the same wherein the border surface areas (46, 48, 50) of the flow spacer frames (26, 28, 30) are conditioned by chemical etching, the peripheral inner edges of the frame border are machined smooth, the etched border surface areas are coated with a thin layer of inert grease such as sil­ icone, and placed under pressure with other electrodialysis components to form a closed system closely packed and es­ sentially solid stack which is free from cross leakage between compartments (12, 16, 20) and seepage therefrom.

Description: FLOW SPACERS FOR ELECTRODIALYSIS APPARATUS AND METHOD FOR MAKING THEM This invention relates to electrodialysis apparatus including alternated layers of metal electrodes, flow spacers, and permselective membranes, al pressed tightly together in a stack. In such apparatus different liquids under pressure flow through separate chambers. As a consequence, leak-free operation is an essential requirement. Electrodialysis apparatus has been used for the concentration or the demineralization of electrolytes from diverse solutions¯¯con-taining electrolytes such as saline water, electroplating waste, radioactive waste, organic acid solutions and the like. In such electrodialysis apparatus, it is quite important to prevent leakage of the different liquids from one cell compartment to another since such leakage may cause contamination of the dialysed liquid and render the electrodialysis operation inoperable or diminish its efficiency. It is important, also, to prevent external leakage, as from the areas between the cell compartments, particularly where the liquids may have acid, alkali or other toxic properties contaminating to the environment. Various proposals have been made in the prior art for preventing leakage of the liquids between and from cell compartments in electrodialysis apparatus. Thus, the use of multi-ply membrane spacers is disclosed in U. S. Patent No. 3878,O86 issued to David B. Haswell et al. on April 15, 1975 and in U. S. Patent No. 4,319,978 issued to William G. Millman on March 16, 1982. In patent 3,878,086 the outer plies are selected to be more rigid than those in the interior and are held together by a water-resistant pressure-sensitive adhesive. The resultant spacer is adhesively bonded to membranes on each of the sides thereof. The plies in patent 4,319,978 are held together by a water-resistant pressure sen sitive adhesive with the outer plies being made of compressible material and those in the interior of more rigid material. The spacer so formed is assembled in a stack without being adhesively bonded to the membranes thus facilitating disassembly of the stack for servicing without tearing of the membranes that tends to result when the membranes are adhesively bonded to the spacer as in patent 3,878,978. In U. S. Patent No. 3,933,617 issued to Tosinori Yamamoto et al., low leakage of liquid between the cells of a filter-press type electrodialysis apparatus is attributed to the feeding and discharge means of the cell being formed separately from the body of the cell frame and made of a rigid material having a thickness preferably less than that of the cell frame. The cell frame is formed of elastic material whereby, upon assembly, the cell frames are compressed while the feeding and discharge means are substantially noncompressed. U. S. Patent No. 4,217,200 issued to Ora Kedem et al. on August 12, 1980 discloses a membrane unit for use in electrodialysis apparatus comprising a heatsealable frame framing and forming an integral entity with an electrodialysis membrane. U. S. Patent No. 4,303,493 issued to Klemens Kneifel et al. on December 1, 1981, discloses sealing layers or coatings of -foamed -epoxy resin applied by a-screen printing method adjacent or adjoining sealing surfaces of the sealing frame such as the chamber bores and supply and discharge channels. In U. S. application for patent bearing Serial No. 691,095, filed on January 14, 1985 by Emmanuel Korngold for Electrodialysis Apparatus for the Chemical Maintenance of Electroless Copper Plating Baths and assigned to the assignee of the present invention, the disclosure of which application, by reference, is incorporated herein, there is disclosed an electrodialysis apparatus comprising alternated layers of metal electrodes, Teflon flow spacers and thin Teflon gaskets, all pressed tightly together. (The generic name for Teflon, which is a trademark of E. I. DuPont deNemours and Company, is tetrafluoroethylene.) High pressure liquids of three types flow through the separate compartments of the apparatus, the Teflon caskets being provided to preclude leakage Such prior art arrangements for achieving leak-free operation are characterized by a number of disadvan tages. They are overly complicated requiring additional components such as separate gaskets or other sealing layers or complicated structures including, in some cases, water-resistant pressure sensitive adhesives that add undesirably to the cost of manufacture and assembly and disassembly when necessary for servicing, and moreover, tend to increase the likelihood of operation failure. Thus, there exists a need and a demand in the art for simplification and improvement in electrodialysis apparatus to the end of achieving leak-free operation which is necessary for the successful commercial operation of the apparatus. The present invention was devised to fill the technological gap that exists in this respect. An object of the invention is to provide an improved leak-free electrodialysis apparatus flow spacer frame structure and a method for the making thereof. Another object of the invention is to provide an improvement in a flow spacer frame structure used in electrodialysis apparatus, which structure imparts leak free operation of the apparatus in a simple and efficient, cost-effective, way. Still another object of the invention is to provide such an improvement in flow spacer frame structures for electrodialysis apparatus that enables a significant reduction in the cost of manufacture and assembly of the apparatus, and that facilitates disassembly of the apparatus for servicing, when and if necessary, while minimizing any tendency to damage of the membranes or other apparatus components. A further object of the invention is to provide such an improvement to the flow spacer frame structures for electrodialysis apparatus that enables the elimination of the Teflon gaskets that are provided in the aforementioned application for patent bearing Serial No. 691,095 of Emmanuel Korngold thereby significantly reducing the number of components that are required. This becomes of particularly significant importance in the commercial scale up of the apparatus. In accomplishing these and other objectives of the present invention, the flow spacer frays of electrodialysis apparatus, which apparatus may be similar to that disclosed in the aforementioned Korngold application Serial No 691,095, are made of Teflon with the border portion surfaces of the frames being etched and coated with a thin layer of an inert grease such as silicone grease. The etching may be effected by known means, for example, chemical means such as those that are employed in the printed circuit industry to microroughen nonconductive articles, particularly plastic articles employed as substrates for printed circuit boards, to provide bonding sites for electroless copper plating. Typically, such etching is an inorganic etching with a strong acid or base. In a preferred embodiment, the improved electrodialysis flow spacer frame structures of the present invention are made by a method involving the steps of: (a) etching both sides of a Teflon workpiece or blank having the desired thickness and outer dimensions desired for the flow spacer frame; (b) cutting out a window in the etched workpiece with the dimensions of the window being those desired for the electrodialysis apparatus flow space, leaving a substantial border portion surface surrounding the window; (c) machining the inner peripheral edges of the border portion of the etched workpiece to provide smooth surfaces thereat; and (d) coating the etched border portion surfaces of the workpiece with a thin layer of an inert grease such as silicone grease. When the flow spacer frame structure resulting from the practice of this method according to the present invention is placed under pressure in face-to-face relation with a substantially identical flow frame structure coated with a thin layer of inert grease on the adjacent etched border portion surfaces of the frame structures and with an electrodialysis permselective membrane therebetween, a leak-free seal is obtained between the permselective membrane and the adjacent etched border portion surfaces of the flow spacer frame structures even though only moderate pressure is used to compress the frame structures together. Such a leak-free seal is also obtained when the resulting flow spacer structure is placed under similar pressure in face-to-face relation with a dimensionally stable and planar metallic cathode or anode or with a Teflon or other plastic planar end plate used in an electrodialysis assembly for holding the various components together under pressure in stacked relation. Being smooth as a result of the machining thereof, the inner peripheral edge surfaces of each such flow spacer frame are substantially free of any tendency for the electrodeposition thereon of metal involved in the operation of the electrodialysis apparatus, for example, the plating out of copper thereon in an electrodialysis apparatus used for the chemical maintenance of electro less copper plating baths. In another contemplated embodiment of the invention, a Teflon flow spacer frame with a window machined therein may have an etch applied to the border portion surfaces on both sides thereof, following which etching the inner peripheral edges of the border portion may be machined to provide smooth surfaces thereat, and the etched border surfaces coated with a thin layer of silicone grease. The flow spacer frame structure produced according to this method is substantially identical to that produced according to the method first described above and is operable to provide a leak-free seal in electrodialysis apparatus, as above described. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. With this summary of the invention, a detailed description follows with reference being made to the accompanying drawings which form part of the specification, of which: Fig. 1 is a schematic exploded view illustrating a single electrodialytic cell unit utilizing the leak-free etched Teflon flow spacer structure according to the present invention; and Fig. 2 is a plan view of one of the leak-free etched Teflon flow spacer structures of Fig. 1. Referring now to the drawings and more particularly to Fig. 1 thereof, an apparatus comprising a threecompartment electrodialytic cell 10 for chemically maintaining an electroless copper plating bath is shown as including a cathode compartment 12 which contains a dimensionally stable planar cathode electrode 14, an anode compartment 16 which contains a dimensionally stable planar anode electrode 18, and an intermediate electroless copper or regenerating compartment 20. Compartment 20 is defined by anion permselective mem branes 22 and 24 which separate the electroless copper compartment 20 from the cathode compartment 12 and the anode compartment 16, respectively. A first flow spacer frame 26, a plan view of which is shown in Fig. 2, separates membrane 22 from cathode 14. A second similar flow spacer frame 28 separates membrane 22 from membrane 24. A third similar flow spacer frame 30 separates membrane 24 from the anode 18. The three flow spacer frames may be substantially identical except for the arrangement of flow passageways or ports through the border portions thereof as required for the passage of different solutions in each of the three compartments 12, 16 and 20, in a manner known in the art and fully explained in the aforementioned Korngold application Serial No. 691,095. While, for convenience of illustration, the several components of the electrodialytic cell have been shown in spaced apart relation in Fig. 1, it will be understood that in a practical embodiment thereof these components would be arranged in a closed system in a closely packed and essentially solid relation, with per forated spacers 32, 34 and 36 positioned in the window opening of frames 26, 28 and 30, respectively, providing physical support for the membranes 22 and 24. For main taining the electrodialytic cell 10 in such closely packed relation there is provided at the cathode end of the stack a plastic press block 38 and a steel backup plate 40. A similar plastic press block 42 and steel backup plate 44 are provided at the anode end of the stack. Suitable steel rods (not shown) extending between the steel plates 40 and 44 may be provided to hold various components of the electrodialytic cell together under pressure. In accordance with the invention, the borders surfaces of each of the flow spacer frames 26, 28 and 30 are conditioned by etching in known manner, as described hereinbefore, to provide thereon, on both sides of the border surfaces, microroughened surfaces, for example, that are visibly roughened when viewed under a 10X lens. The microroughened border surfaces of frames 26, 28 and 30 that are visible in Fig. 1 of the drawing have been designated by reference number 46, 48 and 50, respectively. It will be understood that the opposite sides of frames 26, 28 and 30 that are not visible in the drawing are also microroughened by etching. Before assembly of the electrodialytic cell 10, the etched or roughened border surfaces on both sides of the frames 26, 28 and 30 are coated with a thin layer of an inert grease, silicone grease being preferred since it has been found to coat the etched border surfaces uni formly and reliably. When the resulting flow spacer frame structures 26, 28 and 30 are assembled to form an electrodialytic cell such as the electrodialytic cell 10, and placed under pressure, as shown, in face-to-face relation with the anion permselective membranes 22 and 24, the cathode electrode 14 and the anode electrode 18, a leak-free seal is obtained between the permselective membranes 22 and 24 and the respectively associated frame structures 26, 28 and 28, 30, between frame structure 26 and cathode electrode 14, and between frame structure 30 and anode electrode 18. In an operative embodiment of the invention no leaks of any kind were found, even though only moderate pressure was used to compress the electrodialysis stack. It will be understood that, if desired, the border surfaces of the plastic blocks 38 and 42 adjacent the respectively associated cathode electrode 14 and anode electrode 18 may also be etched and coated with silicone grease, particularly in the region of the passageways or ports that are provided therein for the flow of the various solutions, thereby to preclude any leakage or seepage in these regions. Thus, in accordance with the invention, there has been provided an improved leak-free electrodialysis apparatus flow spacer frame structure and a method for the making thereof. The improved flow spacer frame structure is characterized not only by the relative ease of manufacture thereof in a simple, efficient and costeffective way, but also by the ability thereof to impart leak-free operation to the cell while enabling the use of Teflon sealing gaskets as used in the prior art to be dispensed with. While the invention has been illustrated in the drawing in connection with use thereof in a single electrodialytic cell, it will be understood it may be employed efficaciously in electrodialysis apparatus made up of a number of electrodialytic cells that may be connected in series, parallel, or series-parallel arrangements in a manner known in the art. With this description of the invention in detail, those skilled in the art will appreciate that modifications may be made in the invention without departing from the spirit thereof. Therefore, it is not intended that the scope of the invention be limited to the specific embodiment illustrated and described. Rather, it is intended that the scope of the invention be determined by the appended claims and the equivalents thereof.

Claims: CLAIMS 1. A method of making a flow spacer frame (26,28,30) for an electrodialysis apparatus (10) comprising the steps of: (a) etching both sides of the border portions of a flow spacer frame surrounding a window having dimensions of the size desired for the electrodialysis apparatus, (b) machining the inner peripheral edges of the border portions (46,48,50) of said flow spacer frame to provide smooth surfaces thereat, and (c) coating the etched border portions of said inert flow spacer frame with a thin layer of inert grease, whereby, when the resulting flow spacer frame is placed under pressure in face-to-face relation with a substantially identical flow spacer frame with a permselective membrane (22,24) therebetween, a leak-free seal is obtained between the permselective membrane and the adjacent etched border portions of said flow spacer frames with the smooth inner peripheral edge surfaces of said flow spacer frames being substantially free from any tendency for the electrodeposition thereon of metal involved in the operation of the electrodialysis apparatus. 2. A method as set forth in claim 1 wherein the etching of said border portions (46,48,50) of said flow spacer frame is effected chemically. 3. A method as set forth in claim 2 wherein said flow spacer frame is made of a plastics material. 4. A method as set forth in claim 3 wherein said flow spacer is made of tetrafluoroethylene and the inert grease is a silicone grease. 5. The method of making a flow spacer frame for an electrodialysis apparatus comprising the steps of: (a) chemically etching both sides of a plastics workpiece having the desired thickness and outer dimensions desired for the flow spacer frame (26,28,30), (b) cutting out a window in the etched workpiece with the dimensions of the window being those desired for a flow space in the electrodialysis apparatus and leaving a substantial border portion surface (46,48,50) surrounding the window, (c) machining the inner peripheral edges of the border portion of the etched workpiece to provide smooth surfaces thereat, and (d) coating the etched border portion surfaces of said workpiece with a thin layer of inert grease, whereby, when the resulting flow spacer frame is placed uner pressure in face-to-face relation with a substantially identical flow spacer frame with a permselective membrane (22,24 therebetween, a leak-free seal is obtained between the permselective membrane and the adjacent etched border portion surfaces of said flow spacer frames with the smooth inner peripheral edge surfaces of said flow spacer frames being substantially free from any tendency for the electrodepostion thereon of metal involved in the operation of the electrodialysis apparatus. 6. A method as set forth in claim 5 wherein said workpiece is made of tetrafluoroethylene and the inert grease is silicone grease. 7. A method as set forth in claim 5, or claim 6 wherein a plurality of such flow spacer frames are assembled in a closely packed closed system stack (10) to form the various electrodialysis compartments (12,16,20) with the thickness of each of said flow spacer frames being in accordance with the depth desired for the respective electrodialysis cell compartmentments. 8. A flow spacer frame for electrodialysis apparatus comprising: a plastics frame (26,28,30) having etched border portion surfaces (46,48,50) on both sides thereof surrounding a window, said window having dimensions of a size desired for the electrodialysis apparatus, and with the inner peripheral edge surfaces of the border portion surfaces which define the window being smooth, whereby, when said plastics frame is placed under pressure in face-to-face relation with a substantially identical flow spacer frame with an inert grease on the adjacent etched border portions of said frames, and with a permselective membrane (22,24) therebetween, a leak-free seal is obtained between the adjacent etched border portion surface with the smooth inner peripheral edge surface of the border portion of the flow spacer frames being substantially free of any tendency for the electrodeposition thereon of metal that may be involved in the operation of the electrodialysis cell. 9. A flow spacer frame as set forth in claim 8 wherein the border portion surfaces of said plastics frame are chemically etched. 10. A flow spacer frame as set forth in claim 9 wherein the material of which said plastics frame is made is tetrafluoroethylene. 11. A plurality of flow spacer frames as defined by claim 10 assembled in a closely packed closed system stack (10) with a silicone grease on the etched border portion surfaces thereof to form the various cell compartments (12,16,20) of the electrodialysis apparatus with the thickness of each of the flow spacer frames being in accordance with the depth desired for the respective cell compartments. 12. Electrodialysis apparatus comprising fluid containing cells formed of alternate membranes (22,24) and frame members (26,28,30) having border surfaces (46,48,50) and stacked and pressed together in face-to-face relation to form a consolidated apparatus, characterised in that the border surfaces of adjacent frame members are etched and coated with an inert grease for precluding leakage or seepage of fluid between and from said cells (12,16,20) of said apparatus. 13. Electrodialysis apparatus as set forth in claim 12 whrein said frame members are mede of plastics. 14. Electrodialysis apparatus as set forth in claim 12 wherein said frme-members are made of tetrafluoroethylene mand said inert grease is silicone grease.

Inventors: KRULIK, GERALD A.
Applicants: MORTON INTERNATIONAL, INC.
Application year: 1987
Main classification: B01D61
IPC codes: B01D61, B01D63