IE53513B1 - Electrode assembly with ion exchange membrane - Google Patents

Description

This invention is directed to an improved electrode assembly for use in dewatering suspensions of finely divided or colloidal solids in a carrier liquid, for example, a kaolin suspension in water, by application of vacuum, with the vacuum dewatering process augmented by an electric field created by passing an electric current through the suspension by means of a pair of spaced electrodes submerged in the suspension.

Process and apparatus for carrying out dewatering of suspensions is disclosed in U.S. Patent N. 4,168,222, issued 18th September 1979, U.S. Patent No. 4,207,158, issued 10th June 1980, and copending application Serial No. 109,931, filed 4th January 1980. In these earlier disclosures, self-contained hollow electrode assemblies are provided which are normally submerged in the suspension, but bodily removable therefrom for inspection or other purposes. These hollow electrodes comprise two types of wall surfaces: ion-pervious walls for electrode assemblies of one polarity and liquid-pervious walls for electrode assemblies of the opposite polarity. The wall surfaces of the electrode assemblies comprise chemically and electrically neutral filter media or permeable porous membrane backed by a supporting grid, thus presenting a planar electrode surface.

In operation, with electrode assemblies of both types Immersed in the suspension, a source of vacuum is connected to the interior of the electrode assemblies having liquid-pervious walls to provide a controllable pressure differential thereby producing a flow of carrier liquid through the filtration surfaces, while the solids migrate in the opposite direction, under the influence of the electric field, to deposit as cake upon the electrode assemblies having ion-pervious walls. Filtrate liquid, that is, carrier liquid freed of solids, is withdrawn or pumped from the interior of the liquid-filled hollow electrode structure at a controllable rate. 2.

As indicated, cake deposition occurs on the hollow electrode assemblies having ion-pervious surfaces; these electrode assemblies being filled with an electrolyte and having an electrode element therein immersed in the electrolyte, Isolated from direct contact with the suspension. The electrolyte is specially selected for high conductivity and compatibility with the electrode element. By compatibility is meant the relatively non-corrosive character of the electrolyte under the conditions that ordinarily prevail within the hollow electrode assembly. Since decomposition or evolution products and heat are generated at the electrode element within the hollow 1on-perv1ous electrode assembly, provision is made for a flow of electrolyte into and through the electrode chamber so that such foreign products and heat are swept out of the chamber and a relatively constant predetermined electrolyte composition is maintained.

The ion-pervious wall of the electrode assembly in these prior art structures comprises a chemically and electrically neutral filter media or permeable porous membrane which, if film-like in nature or otherwise requiring support, may be backed by a chemically and electrically neutral grid so that a planar electrode filter surface is presented to the slurry being treated. Since the cake will form on this electrode during electrofiltration and must be removed by contact with doctoring blades, a friction cage or spacing means may be provided to protect the filter medium from direct contact with the doctoring blades. The friction cage comprises a thin, open screenwork of relatively hard material covering the filter medium for contact with the doctoring blades while spacing means may comprise strips of plastic materials, such as Delrin acetal resin arranged in a frame-like configuration and of a thickness sufficient to prevent contact between the doctor blade and the filter medium. For the purpose of cake recovery, the electrode assembly may be raised to a position of emergence from the suspension, with the layer of collected solids or cake layer adhering thereto. Since the electrolyte remains within the electrode assembly in the raised position, a vacuum is applied internally of the electrode to reduce the pressure on the filter media and so prevent rupture of the filter media. When the electrode assembly is immersed in operation, the vacuum applied Internally serves to remove gaseous products, such as chlorine or carbon dioxide evolved at the electrode element; 3.

The ion-pervious electrode structures of the prior art applied in the field of clay dewatering exhibit certain operating problems. Since the clay particles in the feed material are of colloidal particle size, a significant amount of such clay particles pass through the filter medium.

This clay accumulates in the chamber of the electrode assembly, contaminates the electrolyte circulating through the clay and restricts and blocks flow of electrolyte through the electrode chamber. Ultimately, the electrode assembly must be taken out of operation, disassembled, cleaned and re-assembled, a time-consuming and expensive procedure.

Further, it has been found that, electrolyte is being pumped from the chamber of the electrode assembly into the bath. This infusion of material into the bath materially and adversely affects the properties of the dewatered product. There is thus a clear need for an electrode assembly which would operate without these disadvantages.

Accordingly, it is an object of this invention to provide an improved electrode assembly for carrying out an electrofiltration process.

The present invention resides in an electrically augmented vacuum filter for dewatering a suspension of solids in a carrier liquid, which comprises an improved electrode assembly Including: a non-conductive frame structure supporting the walls of an electrode chamber, means for circulating an electrolyte through said electrode chamber, at least one electrode element within said electrode chamber, whereby said electrode element is immersed in said electrolyte in 25 operation, said electrode element comprising a terminal which is capable of being electrically connected to a voltage source external to said electrode chamber, at least one of said walls including an anion exchange membrane, and a spacing means being provided to protect said anion exchange membrane from direct contact with a doctoring blade used to remove a cake of said solids deposited on said anion exchange membrane.

Other objects and advantages of the present invention will become apparent from the following description taken on conjunction with the accompanying drawings, in which: 4.

Figure 1 is a schematic view of an electrofiltration apparatus incorporating the electrode assembly of the Invention, Figure 2 is a detailed view of the improved electrode assembly of the invention, and tFigure 3 is a view taken along line 3-3 of Figure 2.

Generally speaking, the electrode assembly of the invention for immersion in a suspension of solids comprises a chamber having at least one wall composed of an Ion-exchange membrane with at least one electrode element within the chamber immersed in an electrolyte.

In the following description, the electrode assembly is cathodic and an anion membrane is used.

Referring to the drawings, there is shown schematically in Figure 1 an electrically augmented vacuum dewatering apparatus of the kind to which this Invention is directed having a tank containing a bath 9 of suspended solids with a pair of anodes 31 and 32 immersed in the bath 9 on opposite sides of a cathodic electrode assembly 15. The tank 10 is provided with a supply connection 11 for feeding the suspension to the tank. This may be a suspension of uniformly dispersed finely divided charged solids of colloidal size. The required depth of the body of the suspension 1n the 7 tank is defined and ensured by an overflow edge 12 associated with an overflow receiving launder 13, providing for full submergence of electrode structures in the tank. Accordingly, feed suspension is applied from the tank, and so that there will be an ever-changing body of the suspension in the tank. Moreover, a circulating pump 14 connected to the tank as at 14a and 14b, keeps the tank contents in motion, thus ensuring suitable dispersion of the solids in suspension, and proper and uniform functioning of the cathodic and anodic electrode surface exposed to the suspension in the tank.

Cathodic and anodic electrode surfaces in the form of self-contained electrode structures of planar configuration, parallel to one another, are thus provided each being constructed and arranged so that it may be be raised vertically in its own plane to a position of emergence from the suspension, and again to be lowered back into the suspension.

. In the present case, the self-contained cathodic electrode structure 15 of hollow construction occupies the centre, as stated above.

Provision Is made for a vertical guide arrangement (not shown) In order to move this electrode structure vertically In Its own plane to a position of emergence from the suspension, and to lower It back Into submergence. Also provided are disposal devices whereby cake material formed on the electrode surfaces from the suspension, is stripped and carried away during the downward return movement of the electrode structure. In one embodiment, these disposal devices are schematically shown to comprise a pair of symmetrically disposed doctor blades 17 and 18 swingable about their horizontal axes as between neutral position and cake stripping position. The cake material thus being stripped may be carried away by conveying means indicated by respective band conveyers 19 and 20.

Of course, the stripping devices may also be constructed and arranged in a manner whereby the cake stripping and removal is effected incident to the upward movement of the electrode structure to emergence. In greater detail shown in Figures 2 and 3, the electrode structure 15 is of a hollow construction comprising a rectangular frame member 21 and a pair of walls 22 comprising a membrane 22a composed of an anionic ion exchange resin connected to the frame member 21. Frame member 21 is of an outwardly open U-shaped cross-section adapted for securement thereto of the ion-permeable walls. Each of the walls 22 is a multi-layered assembly comprising an anion exchange member 22a, a supporting grid 22b and a protective cage 22c, and adapted for deposit thereon of charged solids from the suspension, as a layer or a cake formation.

Fixed to the upper end of frame member 21 are a pair of support brackets 15a for positioning and supporting the electrode structure 15 in the tank 10.

A terminal is provided in the form of vertical rod 26 extending into 30 the interior of this electrode structure 15, and connected therein to the electrode 27 while the exposed top end portion of this rod has a cable connection 26a.

It will be understood that the frame member 21 and the wall members 22 of the electrode structure 15 are electrically neutral and are, consequently, composed of non-conductive materials such as plastics, or are insulated from contact with the electrode 27 and the electric conduits 26 and 26a.

Provision is furthermore made for filling the interior of this electrode structure with a suitable electrolyte (catholyte). In order to maintain a relatively constant electrolyte composition during operation of the electrically augmented vacuum filter, a flow of fresh electrolyte is maintained through the electrode structure. The apparatus for maintaining this electrolyte flow, in simplest form, may be a gravity feed system in which an elevated electrolyte supply tank is connected to the electrode structure by a supply conduit 28 and a waste conduit 29 leads from the electrode structure, perhaps a waste tank. Gas evolved at the cathode is carried out with the depleted electrolyte. Electrolyte circulation may be provided in more sophisticated systems. In some case, it may be desirable to separate the anode gases from the anolyte and inject the gases into the catholyte (for pH control or for other purposes).

Claims (9)

1. .C, I . AJ_M_£

1. An electrically augmented vacuum filter for dewatering a suspension of solids in a carrier liquid, which comprises a cathode electrode assembly including: 5 a non-conductive frame structure supporting the walls of an electrode chamber, means for circulating an electrolyte through said electrode chamber, at least one electrode element within said electrode chamber, whereby said electrode element is immersed in said electrolyte in 10 operation, said electrode element comprising a terminal which is capable of being electrically connected to a voltage source external to said electrode chamber, at least one of said walls including an anion exchange membrane, and a spacing means being provided to protect said anion exchange 15 membrane from direct contact with a doctoring blade used to remove a cake of said solids deposited on said anion exchange membrane.

2. An electrically augmented vacuum filter as claimed in claim 1, which further comprises an anodic electrode assembly including: a non-conductive frame structure supporting the walls of an 20 electrode chamber, means for circulating an electrolyte through said electrode chamber, and at least one electrode element within said electrode chamber, whereby said electrode element Is immersed in said electrolyte in 25 operation, said electrode element comprising a terminal which is capable of being electrically connected to a voltage source external to said electrode chamber, at least one of said walls including a cation exchange membrane.

3. The filter of claim 2, including means for collecting anode gases 30 contained in an anolyte solution and dispelled from said anodic electrode assembly and injecting said anode gases into catholyte of the cathodic electrode assembly of the filter for controlling the pH of said catholyte. 8.

4. The filter of any preceding claim, wherein said spacing means is a friction cage.

5. 6. The filter of claim .4, wherein said, friction cage comprises a thin, open screenwork of relatively hard material covering said anion exchange membrane.

6. The filter of any of claims 1 to 3, wherein said spacing means comprises strips of plastics materials. 10 7 · The filter of claim 6, wherein said strips of plastics materials are arranged in a frame-like configuration and of a thickness sufficient to prevent contact between the doctor blade and said anion exchange membrane.

7. 8. The filter of claim : 6 or 7, wherein said plastics material is an acetal resin. 15

8. 9. The filter of any preceding claim in combination with a gravity feed system, wherein an electrolyte is circulated and continuously replenished by means of the gravity feed system, which comprises an electrolyte supply tank connected to said electrode chamber by a supply conduit for supplying said electrolyte to said electrode and a waste 20 conduit for removing the depleted electrolyte.

9. 10 An apparatus for electrically augmented vacuum filtration of a suspension of solids in a carrier liquid, substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings. Dated this 26th day of February 1982