GB2077293A - Electrophoretic separation - Google Patents

Abstract

In electrophoretic dewatering apparatus in which particulate matter from a sludge, slurry or like flowable material is electrophoretically attracted towards a collector electrode, a semi-permeable membrane is provided over the collector electrode with a movable collector portion 30, for example of mesh, adjacent the membrane and between the membrane and the other electrode. The separated material builds up on the collector portion and periodically the collector is raised out of the separation cell and carried to a collection region where the material is scraped off the collector in a stripping tank 39, for example by passing the collector through an apertured plate 32. The collector electrodes are preferably double-sided with two movable collectors, one on each side. <IMAGE>

Description

SPECIFICATION Electrical separation of particulate material from sludges, slurries and like flowable material This invention relates to methods of and apparatus for the electrical separation of particulate material from sludges, slurries and like flowable material.

Electrophoretic dewatering processes are known in which the movement, under the influence of an applied direct electric field, of solid particles to an electrode takes place. The movement is to the anode or to the cathode depending upon the charge residing on the particles. In the case of a negatively-charged particle, such movement is towards the anode. In the case of a sludge, slurry or the like, thickening occurs at this electrode, that is to say the proportion of particulate matter to liquid increases and electro-deposition occurs. If the deposit so formed is coherent, it can be stripped from the electrode. The application of an electric field to such a sludge or slurry also causes electroosmotic transport of water to the cathode and the water so transferred contains very little solid product.Hence electrical dewatering is particularly advantageous for the separation of negativelycharged particles. It nidy be used, for example, for the separation of polyvinylchloride from an emulsion thereof or for the separation of inorganic silicates from a suspension, e.g. a clay slurry.

It is an object of the present invention to provide an improved method of and apparatus for the continuous thickening of a sludge, slurry or like flowable material, permitting removal of the solid.

Many forms of cell have been described in the past for electrophoretic thickening or dewatering of sludges and slurries. One of the major problems however has been the removal of the deposited material after it has collected upon the electrode.

In order to effect continuous operation, there have been a number of proposals for collecting material on a continuously moving screen, e.g. an endless band of fabric, which may be located in front of the collector electrode (usually the anode). Such constructions however require continuously moving the parts within an electro-chemical cell.

See for example British Patent Specifications Nos.

1526560,1525103 and 1525102. Proposals have been made (see for example U.S. Patent Specification No. 4,107,026) for raising the electrode on which the material is deposited, moving strippers into the appropriate position to strip material from the electrode and then lowering the electrode past the stripper whilst at the same time operating a conveyor system to catch the stripped material as it falls away from the electrode and carry it away from the cell. With an electrode on which material is collected on both sides, there must be two separate conveyors spaced apart a sufficient distance that the electrode together with the material deposited thereon can be raised between the conveyors and thus it is necessary to provide guide means to guide the stripped material away from the electrode onto the conveyors.

According to one aspect of the present invention, a method of separating particulate material from a sludge, slurry or like flowable material comprises the steps of feeding the flowable material into a container in the region between two electrodes energised with a direct current so that the particulate material is electrophoretically attracted towards one electrode, referred to hereinafter as the collector electrode, allowing the particulate material to build up as a layer on a semi-permeable membrane forming a planar surface over said collector electrode and to continue to build up over a movable collector, separating the collector with the built-up body of particulate material from said semi-permeable membrane, removing the collector from the container, removing particulate material from the collector and then replacing the collector adjacent said membrane.

With this method, the separation process can continue whilst the collector is being withdrawn and the particulate material removed. The material will continue to build up on the membrane; it is readily possible to replace the collector before any great thickness of build up has occurred and the collector is then left in position until the separated solids have formed a layer over the collector. It is readily possible to remove the solids from the membrane, to which adhesion is only weak. The collector might comprise a mesh or a number of parallel rods; many other types of structure are possible and it is readily possible to ensure that the mass of solids can be lifted up by the collector.

Preferably the collector and the membrane have substantially flat surfaces which are parallel and adjacent one another when the collector is in the container. These surfaces are preferably in vertical planes. The collector can be moved away from the screen, e.g. by bodily movement or by tilting, and then lifted out of the container.

It is convenient to make the collector in the form of an assembly which is relatively thin compared with its height and width (assuming the aforesaid surfaces are vertical) so that collected material can be removed from the collector by drawing the collector, preferably in an upward direction, through an aperture fitting closely around the collector.

It will be noted that this arrangement permits of continuous operation. There is no need to remove any electrodes when a collector is being stripped of material. This facilitates the use of multiple electrode containers.

Individual collectors from various collector electrodes can be removed simultaneously or in sequence for stripping of collected material.

The semi-permeable membrane is preferably such that the solid particles cannot pass through it and it is arranged to cover a region containing the electrode together with the fluid. For separating negatively-charged particles, this electrode is the anode. The fluid in this case is the anolyte: a suitable electrolyte may be fed into the region between the electrode and the semi-permeable membrane to maintain the appropriate liquid level.

According to another aspect of the present invention apparatus for separation of a particulate material from a sludge, slurry or like flowable material using an electric field comprises a container forming a treatment cell for holding the flowable material to be treated, at least one pair of electrodes having facing surfaces with supply means for applying a direct electric current between the electrodes, one of the electrodes of the or each pair having a surface carried by a semi-permeable membrane for the collection thereon of a layer of solids under the influence of an electric field, which electrode is referred to hereinafter as the collector electrode, said electrodes being arranged in substantially vertical planes, a movable collector portion between said semi-permeable membrane and an opposed electrode of said pair, whereby particulate material attracted towards said membrane builds up as a layer on the membrane and over said collector portion, means for moving the collector portion away from the membrane and upwardly out of the container and means for stripping the particulate material and removing it from the collector portion.

The means for moving the collector portion may be arranged to transport it to a stripping position over a conveyor belt, a stripper at said stripping position for stripping collected material from the collector portion so that it falls onto the conveyor belt and means for transporting the stripped collector portion back to the collector electrode in the cell.

Preferably the container has a plurality of electrodes arranged alternately as anode and cathode electrodes so that solid material collects on both sides of each collector electrode. In such an arrangement, electrodes are preferably flat structures arranged in a line normal to their plane.

It is convenient to arrange for the means for moving the collector portion to lift the collector portion, transport it along the length of said line above the treatment tank to a stripping position at one end of the treatment tank and to have a conveyor belt running underneath the treatment tank to collect the stripped material. The various collector electrodes may be stripped in sequence.

Commonly, the collector electrode will be the anode and provision may be made for removing liquid, e.g. water, which is collected electroosmotically at the cathodes.

It will be seen that the above-described arrangement provides a very simple and convenient construction for removal of the collected solid material. Since the material can be allowed to fall away by gravity onto a conveyor extending underneath the electrode, a simple stripping means may be provided, e.g. the electrode may be moved through an apertured structure just large enough to receive the electrode assembly whilst stripping off the accumulated particulate material.

It will be seen that, with this construction the cell can be operated continuously. Only part of the collector electrode structure is removed in order to strip the collected material. This removal can be effected without cessation of cell operation. The rate of collection of solid material would normally be such that the time required for removal of the collector portion, stripping and return of this collector portion, will be quite small compared with the cycle time between successive stripping operations for that electrode. The cell can thus be operated continuously with feed of new material containing solids in suspension into the cell and continuous withdrawal of liquids.

If the material has negatively-charged particles, the liquid can be removed at the cathode by providing a permeable screen over the cathode, through which the liquid can pass and through which it is attracted electro-osmotically.

The removable collector portion of the collector electrode, usually the anode electrode, may comprise a rigid structure, e.g. of rods or a mesh which may be mounted on a frame, which structure is removably secured, e.g. by clamping to the fixed electrode structure. This electrode structure conveniently is a mesh or plate anode secured in a non-conducting frame carrying the semi-permeable membrane on which the solids initially are collected. With a multi-electrode apparatus, separate removable collector portions may be secured on each side of the electrode frame.

In the following description reference will be made to the accompanying drawings in which:~ Figure 1 is a diagrammatic perspective view of a cell for thickening a polyvinylchloride emulsion, the cell being shown as of simplified construction and with the means for removing the solids from the anodes omitted; Figure 2 is a perspective diagram illustrating part of a cell assembly and showing in further detail movable collector portions of the anode assembly; Figures 3, 4 and 5 are respectively a sectional elevation on one side, a partial end elevation and a partial plan view of the lifting means for lifting collector portions away from the anodes of the cell of Figure 2; Figures 6 and 7 are respectively exploded views of an anode and a cathode in the cell of Figure 2; ; Figures 8 and 9 are respectively a perspective view and a plan view of an alternative construction of anode assembly for use in the cell of Figure 2; and Figure 10 is a diagram illustrating part of the mechanism for stripping a collector.

Figure 1 illustrates diagrammatically a cell for thickening of a polyvinylchloride water emulsion by the electrophoretic attraction of the solids towards an anode. The cell is used to dewater the emulsion and to obtain solids which might typically be 80% dry, that is containing only 20% by weight of water.

The apparatus shown in Figure 1 comprises a container 10 containing three cell units 11, 12 and 13. Each cell unit for simplicity is shown with one anode 14 located between two cathodes 1 5, 1 6. Associated with each cell unit is a stripper tank 17.

The P.V.C. emulsion to be thickened is fed into the circulation loop of the cell units 11, 12 and 13.

The circulation loop consists of the inlet ports 19 which are combined to give a common outlet 20.

Electrical connecting means 21 are provided on top of each of the electrodes.

Each anode is of generally hollow construction comprising a metal mesh or plate anode element formed for example of lead dioxide or platinised titanium or ruthenium dioxide having, on each side thereof, a sheet of a semi-permeable membrane.

The anodes are vertical and are spaced between two cathodes. Although Figure 1 shows one anode between two cathodes, conveniently the cell modules are made up with a larger number of anodes and cathodes. Preferably the anodes are double sided with cathodes between the anodes and the end of each cell module.

The means for removing the solids from the cells will be further described later. They are omitted from Figure 1 for the sake of clarity Essentially however collector means are provided between each anode and the adjacent cathode surface, these collector means typically being a mesh or series of rods which are located close to the anode surface. In operation the solid particles are attracted towards the anode electrophoretically but cannot pass through the semi-permeable membranes and hence collect thereon. The layer of solid particles builds up on each of these membranes and continues to build over the adjacent movable collector portion. The particles only have weak adhesion to the membrane and thus, when the collector portion is moved away from the anode, it carries the collected particles with it. It is immaterial if some particles remain adhering to the membrane.The collector elements may be lifted straight out of the cells but it may be preferred to break the weak adhesion between the membrane and the collected mass of particles by moving the collector away from the membrane which is preferably formed as a flat surface. This movement away may be effected by bodily movement of the collector or by tilting movement of the collector.

Once the adhesion between the particles and the rnambrane is broken, the collector portion may be lifted out of the cell, carried to the adjacent stripper tank 1 7 and the particles stripped therefrom. Stripping may be effected in a number of different ways. One convenient manner of doing this is to form the collector portions to be of substantially uniform section in sections transverse to one direction. With vertical flat anodes, as shown in Figure 1, it is convenient to consider horizontal section and the collector portions are arranged to be of uniform dimensions in all horizontal sections. The excess material therefore can readily be removed from a collector portion by raising it through or lowering it through an aperture of just sufficient size that the collector portion can pass through but so that the solid material gathered thereon is removed.One form of apparatus of this nature for stripping the collector portions will be further described later. It will be readily apparent however that other means may be provided for removing collected material. In the stripper tank 1 7, the collector material falls down to the bottom and thence onto a conveyor belt 21 which traverses underneath the tank 10.

With a P.V.C. emulsion in water, the particles are negatively-charged and are electrophoretically attracted to the anodes. The water is electroosmotically attracted to the cathodes. Each cathode conveniently is provided with a filter cloth spaced away from the electrically conductive cathode by a short distance to prevent entry of solid particles into the region between the filter cloth and the cathode. Gas outlets may be provided in the cathode to vent hydrogen. The excess catholyte drawn osmotically to the cathode is removed through cathode outlets 22.

Provision may be made for circulating anolyte into the region between the electrically conductive anode element and the semi-permeable membrane covering it if required. The anolyte fed into this region contains no solid particles. In order to equalise the hydrostatic pressures between the anolyte and the flowable material in the region between the electrodes, the electrolyte level in the anode compartment may be adjusted, for example by an adjustable overflow in the side of the anode frame. This serves as an outlet for anolyte which is circulated if provision is required for circulation of a different anolyte within the anode structure compared with the liquid in the electrolyte.

Provision is made for venting any oxygen evolved at the anode.

Referring now to Figures 2, 3, 4 and 5, there is shown apparatus for removing the collected material from a cell module in which there are two two-sided anodes with three cathodes and in which collector portions on each side of each anode are raised simultaneously. There are thus four collector portions to be simultaneously raised.

These collector portions are shown diagrammatically in Figure 2 as rectangular plates 30. They may be formed for example of mesh or they may be formed, as shown in Figures 3, 4 and 5 as a plurality of rods 31.

The collected solid material on a thin generally plate-like structure such as the structure 30 may be removed by passing the plate through a narrow aperture with movement in the plane of the plate so that the material is scraped off. In the arrangement of the present application, the movement is in a vertical direction through apertures in a plate 32. The collector portions are carried on a plate 29 which can be raised or lowered by means of a pneumatic cylinder 33 with a piston 34 for effecting raising and lowering via a flexible cable 35. The hoisting mechanism is mounted on a trolley 36 which is movable by means of a pneumatic cylinder 37 for traversing the collector means from the cell shown at 38 in Figure 2 to the stripping tank shown at 39.

Automatic control means, not shown, are provided for effecting automatically a sequence in which the collector portions 30 are raised, moved over to the stripping tank 39, lowered, then raised with the material being stripped, in a manner to be described later, as the collector portions are raised. These portions are then traversed back and lowered into the cell 38 for recommencement of a cycle of operation. During this time, the cell electrically continues to operate with the particles being attracted toward the anodes.The time taken to take the collector portions 30 out, strip them and return them, is very short compared with the build up time of the material on the anodes and it is thus readily possible to lower the collector portions back into position so that the collected material can continue to build up until it covers the collector portions again and a further stripping cycle has to be initiated.

The stripping of the material is effected by the plate 32. When the collector portions 30 are lowered into the stripping tank, the plate 32, which normally is above the collector portions 30, is lowered with them into the stripping tank. The plate 32 for this purpose is mounted on sleeves 40. one of which is shown in Figure 10, which sleeves each slide on tube 41 attached to the lifting frame 29. After the assembly has been lowered into the stripping tank 39, at the bottom of the movement, a catch 48 is moved outwardly by a cam 42 and then springs back under the action of a spring 43 so that a shoulder 44 engages the top of the stripper plate 32 so restraining the stripper plate.

The collector portions therefore, when the lifting frame 29 is lifted, are raised upwardly through the appropriate apertures in the stripper plate 32 so stripping the collected material off the collector portions 30. With the continuing upward movement, a cam 45 engages a co-operating cam surface 46 on the lever 48 to release the lever so that the stripper plate is then moved upwardly being carried by the sleeve 40 being engaged by an element 47 mounted on the tube 41. The stripper plate 32 is thus raised and is transported back to the position over the cell 38 but still remaining at the bottom of the collector portions 30. When the collector portions are lowered into the cell however they pass through the stripper plate 32 which rests on an abutment (not shown) on the top of the cell so that the stripper plate remains stationary as the collector portions move downwardly.Thus the collector plate is brought back to the relative position shown in Figure 2.

Figure 6 illustrates in further detail one form of anode assembly. The anode comprises a mesh 50 of a suitable material, e.g. of lead dioxide or platinised titanium or ruthenium dioxide mounted for example in a fibre-glass reinforced plastics casting 51 formed in two parts to facilitate assembly. The electrical connection 52 for the anode is at the top of this assembly and inlet and outlet ports 53 and 54 are provided for the circulation of the anolyte. Semi-permeable membranes in the form of two sheets 56. 57 are provided, one on each side of the anode structure 51 and are clamped in position by glass filled polytetrafluoroethylene clamp plates 58, 59.

The cathode construction is illustrated in Figure 7. This shows one of the end cathodes. it will be readily apparent however that two-sided cathodes may be formed in a generally similar manner. The cathode shown in Figure 7 comprises a stainless steel plate 70 having an electrical connection portion 71 along its top edge. Outlet slots 72 are provided for the catholyte. Over one surface of the plate 70 there is provided a thin polyvinylchloride spacer 73 to hold a sheet 74 of cloth away from the surface of the plate 70 by a distance of 1 to 2 mm. The cloth is clamped in place by a glass-filled polytetrafluoroethylene clamp 75. A two-sided cathode would require a second sheet of cloth, a second spacer and a second clamp on the opposite side of the plate 70.

For the end cathode shown in Figure 7, the catholyte outlet slots 72 leads directly out of the cell module. For a two-sided cathode electrode, suitable outlet means would have to be provided.

The cloth is a small apertured cloth to stop solids passing into the cathode structure. The liquid passes into the structure however electroosmotically. Provision may be made however for circulating liquid through the cathode structure to remove any solids which might enter. This may be done for example by providing holes in the bottom of the cathode plate as shown at 77 through which water can be pumped from the bottom of the cathode up between the cathode plate and the cloth and withdrawn through the apertures 72 which constitute a weir at the top of the assembly.

Figures 8 and 9 illustrate an alternative construction of anode assembly. In this assembly, the anode 80 in the form of a mesh is carried in anode guides 81 in a frame 82. This frame is formed of two L-shaped portions, one forming one vertical side and most of the bottom and the other forming the other vertical side and most of the top, these two portions being joined by adjustable wedges 83 which force the vertical side portions outwardly to tension a membrane in the form of a bag 84 around the assembly so lying on both sides of but spaced away from the mesh 80. At the bottom, a seal portion 85 is provided. The top portion of the frame has a gas vent 86. This arrangement avoids the use of clamp frames for the membrane yet enables the membrane to be tensioned. The absence of clamp frames and their necessary seals reduces the likelihood of leakage.

The membrane can be supported internally if desired by the use of a tensioned open-weave cloth (not shown) which may be carried on its own sub frame within the anode structure.

In operating the above-described apparatus, it is advisable to circulate the electrolyte, that is to say the suspension to be thickened, between the electrodes so that a uniform concentration is maintained. A heat exchanger may be incorporated if necessary to ensure optimum temperature conditions.

It will be seen that a number of cells may readily be assembled on a modular basis.

Conveniently each cell module has its own stripping region as described with reference to Figure 1.

Claims (16)

1. A method of separating particulate material from a sludge, slurry or like flowable material comprising the steps of feeding the flowable material into a container in the region between two electrodes energised with a direct current so that the particulate material is electrophoretically attracted towards one electrode, referred to hereinafter as the collector electrode, allowing the particulate material to build up as a layer on a semi-permeable screen forming a planar surface over said collector electrode and to continue to build up over a movable collector, separating the collector with the built-up body of particulate material from said semi-permeable screen, removing the collector from the container, removing particulate material from the collector and then replacing the collector adjacent said screen.

2. A method as claimed in claim 1 and for treating a suspension in water of negativelycharged particles wherein the collector electrode is the anode and wherein water is removed electro-osmotically at the cathode.

3. A method as claimed in either claim 1 or claim 2 wherein the collector comprises a number of rods.

4. A method as claimed in either claim 1 or claim 2 wherein the collector comprises a mesh.

5. A method as claimed in any of the preceding claims wherein said planar surface is a flat surface.

6. A method as claimed in claim 5 wherein said planar surface is a vertical surface.

7. A method as claimed in any of the preceding claims wherein the collector is of substantially uniform section in sections transverse to one direction and wherein the collected material is removed from the collector by drawing the collector through an apertured element by movement in said one direction.

8. Apparatus for separation of a particulate material from a sludge, slurry or like flowable material using an electric field comprising a container forming a treatment cell for holding the flowable material to be treated, at least one pair of electrodes having facing surfaces with supply means for applying a direct electric current between the electrode, one of the electrodes of the or each pair having a surface covered by a semi-permeable membrane for the collection thereon of a layer of solids under the influence of an electric field, which electrode is referred to hereinafter as the collector electrode, said electrodes being arranged in substantially vertical planes, a movable collector portion between said semi-permeable membrane and an opposed electrode of said pair, whereby particulate material attracted towards said membrane builds up as a layer on the membrane and over said collector portion, means for moving the collector portion away from the membrane and upwardly out of the container and means for stripping the particulate material and removing it from the collector portion.

9. Apparatus as claimed in claim 8 wherein said means for moving the collector portion are arranged to transport it to a stripping position over a conveyor belt and wherein there are provided a stripper at said stripping position for stripping collected material from the collector portion so that it falls onto the conveyor belt and means for transporting the stripped collector portion back to the collector electrode in the cell.

10. Apparatus as claimed in either claim 8 or claim 9 wherein the container has a plurality of electrodes arranged alternatively as anode and cathode electrodes so that solid material collects on both sides of each collector electrode.

11. Apparatus as claimed in claim 10 wherein the electrodes are flat structures arranged in a line normal to their plane.

12. Apparatus as claimed in claim 11 wherein at least some of the electrodes are double-sided electrodes.

13. Apparatus as claimed in either claim 11 or claim 12 wherein said moving means are arranged to lift a collector portion, transport it along the length of said line above the treatment tank to a stripping position at one end of the treatment tank and wherein a conveyor belt for receiving the stripped material runs underneath the treatment tank.

14. Apparatus as claimed in any of claims 8 to 13 and for treating a suspension in water of negatively-charged particles wherein the collector electrode is the anode and wherein means are provided for removing water electro-osmotically attracted to the cathode.

1 5. A method of separating particulate material from a sludge, slurry or like flowable material substantially as hereinbefore described with reference to the accompanying drawings.

16. Apparatus for separating particulate material from a sludge, slurry or like flowable material substantially as hereinbefore described with reference to the accompanying drawings.