EP1301265A1

EP1301265A1 - Method and device for carrying out membrane purification

Info

Publication number: EP1301265A1
Application number: EP01967112A
Authority: EP
Inventors: Christian Hying; Franz-Felix Kuppinger; Gerhard HÖRPEL; Bernd Penth
Original assignee: Creavis Gesellschaft fuer Technologie und Innovation mbH
Current assignee: Creavis Gesellschaft fuer Technologie und Innovation mbH
Priority date: 2000-07-14
Filing date: 2001-06-29
Publication date: 2003-04-16
Also published as: US20040262169A1; AU2001287572A1; JP2004503376A; CA2411935A1; DE10034386A1; WO2002005936A1

Abstract

The invention relates to a method and device for carrying out electrofiltration. Electrofiltration is a commonly known method, which is frequently used in industry for purifying suspensions, such as wastewater resulting from manufacturing processes. Prior art devices used for carrying out electrofiltration have the drawback in that large amounts of expensive metals such as titanium, gold, iridium, platinum and the like have to be used as counter-electrodes to the membrane electrodes. The aim of the invention is to improve the filtration results compared to those of conventional filtration methods and modules without requiring the use of large amounts of expensive metals for the counter-electrodes. To this end, the invention provides a method, according to which the membrane electrodes are displaced, and a device for carrying out said method. The inventive method and device can be used for separating substances.

Description

METHOD AND DEVICE FOR CLEANING THE MEMBRANE

A method and a device for electrofiltration are claimed.

The separation of mixtures is a common problem in the industrial production of substances. Liquid phases containing solids are particularly frequent. These solids, some of which are very small solid particles in the liquid phases, often have to be removed from the liquids before they can be processed further. Such a separation task is e.g. in the beverage industry, where juices are to be separated from the finest solid components, or when cleaning waste water. In the industrial production of plastics, there are often also emulsions or latices in which the plastics are finely distributed in a solution. In this case, the plastic can be separated from the liquid by filtration, in particular by micro- or ultrafiltration. The retentate can be processed further.

Membranes have long been used to separate mixtures of substances. With synthetic membranes, a distinction is made between organic and inorganic membranes.

Membranes are usually made of plastics or inorganic components, e.g. Oxides used. In the known processes in which these membranes are used, e.g. filtration, there is always the problem that the membranes clog after a relatively short period of use. In classic cross-flow filtration, there is a temporal decrease in the transmembrane permeate flow due to deposits on the membrane surface, which means that the amount of substance that flows through the membrane at constant pressure decreases.

This is caused by a secondary flow perpendicular to the wall, since product is drawn off through the filter pores. The solid is convective on the wall or

Membrane surface transported, retained there and also deposited. Although the high overflow velocity in the membrane modules tries to continue the solid to keep in suspension, the solid can no longer be removed near the wall, within the laminar boundary layer. This significantly reduces the passage of material through the membrane. The membranes must be replaced and either laboriously cleaned or disposed of.

In some commercially available membrane filtration systems, the cross-flow effect is not achieved by a high pumping rate, but the necessary flow rate is achieved by a rotating stirrer that sweeps past the membrane surface. Such devices are sold, for example, by Valmet-Flotec. With these membrane filtration systems, too, there is a temporal decrease in the transmembrane permeate flow due to deposits on or in the membrane, that is to say the amount of substance which flows through the membrane at constant pressure is reduced.

With sufficiently stable ceramic membranes, e.g. the backwash principle has prevailed for tubular membranes made of aluminum oxide. The flow direction is suddenly reversed at periodic intervals for a short time by applying a pressure surge from the rear. However, this principle has the disadvantage that it can only be used effectively in liquid filtration. In addition, the membranes are subjected to high mechanical stresses and ultimately only part of the caking can be removed.

Another option for cleaning membranes is the process of electrofiltration. Methods and devices for electrofiltration have long been known in the prior art. For example, in EP 0 165 744, EP 0 380 266 and EP 0 686 420 claims methods which allow gas bubbles to form for cleaning a filter by applying a voltage and carrying out electrolysis on the filter. The gas bubbles clean the filter surface so that longer filter service lives are achieved.

WO 99/15260 also describes a method for separating mixtures of substances by means of a permeable material. In this method it is proposed to use the material as a so-called membrane electrode and through this membrane brief application of an electrical voltage due to the development of gas bubbles in aqueous solutions. This method also requires a counterelectrode the size of the membrane electrode, which, as is generally known, preferably consists of a noble metal.

To achieve a sufficient and uniform gas bubble development on the entire surface of the membrane electrode, it is necessary in all these methods and devices to keep the distance between the counter electrode and the membrane electrode as small as possible. At the same time, the distance between all points of the membrane electrode to the counter electrode must be as equal as possible in order to achieve a uniform strength of the gas bubble development.

The methods described have the disadvantage that the size of the area of the counterelectrode has to correspond almost to the size of the area of the membrane electrode used in order to achieve a uniform gas bubble development. This requires a large amount of material. Since, as the counter electrode material, mostly expensive metals, such as Titanium, iridium, platinum, palladium and gold are used means a high use of materials at the same time high costs. By using expanded metal or grid electrodes, attempts are sometimes made to reduce these costs. Such electrodes often have a basic structure made of titanium, which is coated with mixed metal oxides. Such electrode materials are e.g. available from Heraeus, Degussa-Hüls or Metakem.

The object of the present invention was therefore to provide a method and a device in which the material expenditure for the counterelectrode is less and an improved filtration performance can be achieved.

Surprisingly, it was found that with a method according to claim 1, in which the membrane electrode moves, the material expenditure can be significantly reduced. At the same time, the filtration performance can be improved compared to conventional methods or devices.

The present invention therefore relates to a method according to claim 1 for Electrofiltration, in which a membrane electrode is cleaned by gas bubble development, which is characterized in that the membrane electrode used is moved.

The present invention also relates to a device for electrofiltration, which is characterized in that it comprises at least one rotating membrane electrode and at least one counter electrode which has a smaller shape or contour than the membrane electrode.

The method according to the invention has the advantage that longer filter service lives can be achieved while improving the filtration performance.

The device according to the invention has the advantage that a significantly smaller counterelectrode can be used than in conventional devices, and as a result of this considerable material saving, the costs for the device are significantly lower than in conventional devices according to the prior art.

The method according to the invention for the electrofiltration of substance mixtures is based on the cross-flow principle in combination with an electrofiltration in which a membrane electrode is cleaned by gas bubble development. In conventional cross-flow systems, the filtration performance drops over time due to fouling or other processes on the membrane surface. If the filtration capacity in the device according to the invention drops below a certain limit value, the membrane surfaces are cleaned by applying an electrical current.

In contrast to the known combined methods, in which an attempt is made to keep the solid in suspension by a high overflow rate in the membrane modules, the membrane electrode is moved in the method according to the invention and in this way an attempt is made to keep the majority of the solids in suspension ,

The membrane electrode moves both during the filtration and during the cleaning process. It may be advantageous to reduce the pressure at which the liquid to be filtered is pressed against the retentate side of the membrane electrode during the cleaning process. The pressure ratios during the cleaning process are preferably set such that the pressure on the retentate and permeate sides of the membrane electrode is the same. It may be advantageous to set the pressure on the permeate side of the membrane electrode higher during the cleaning process than on the retentate side in order to generate a current from the permeate side to the retentate side of the membrane electrode, which can support the cleaning process in which solid particles detached by gas bubble development from the membrane electrode be carried away. After cleaning, the pressure conditions are set again to the optimal conditions for the filtration. Usual pressures during the filtration are, for example, a feed pressure of 1.2 to 6 bar, a pressure in the retentate outlet of 1 to 6 bar and a pressure on the permeate side of the membrane from 5.8 to 0.2 bar.

The cleaning process itself is known from the literature described above and is based on the fact that a voltage is applied to a membrane electrode which is sufficiently high to electrolyze one of the liquids present in the substance mixture to be filtered. Water is preferably electrolyzed. Depending on the use of the membrane electrode as an anode or cathode, gas bubbles of hydrogen or oxygen form on the membrane electrode. However, it is also possible to split organic liquids into gaseous components on the membrane electrode.

The application of an electrical voltage charges the membrane electrode, and gas bubbles develop on the membrane surface as a result of the electrical voltage. The resulting gas bubbles cause caking to be blasted off the surface of the membrane. The movement of the membrane electrode makes it easy to ensure that the solid particles detached from the membrane electrode surface are carried away by the membrane electrode. This process can be supported by the above-mentioned pressure reversal or the resulting current reversal.

The movement of the membrane electrode is preferably a rotation. By at the Rotational centrifugal forces generate currents on the membrane electrode surface, which transport the solid particles detached by the gas bubble development to the outside of a rotating, circular or almost circular membrane electrode. The solid particles can be removed from the outside of the membrane electrode, for example with the retentate stream. The cleaning of the membrane electrode surface by means of gas bubble development is significantly improved by the movement of the membrane electrode, in particular by the rotating movement of the membrane electrode.

According to the invention, the membrane electrode particularly preferably rotates more slowly during the cleaning or the cleaning process than during the filtration or the filtration process. During cleaning, the membrane electrode preferably rotates at a rotation speed of 0.1 to 5 min ^"1. During the filtration process, the membrane electrode preferably rotates at a rotation speed of 1 to 500 min ^" 1, very particularly preferably at a rotation speed of 100 to 300 min ^"1 ,

However, it can also be advantageous if the membrane electrode rotates at the same speed during cleaning and during the filtration process. In this case, rotation speeds of 1 to 10 min ^"1 , preferably 1 to 5 min ^{" 1} , are preferred.

An electrical voltage of greater than 1.5 V is preferably applied between the membrane electrode and at least one corresponding counterelectrode for the cleaning process. A current or voltage of a magnitude is preferably applied which ensures that the current strength at the counter electrode is greater than 1 mA / cm ² , preferably greater than 10 mA / cm ² .

The electrical voltage can be pulsed or applied as a permanent voltage. A permanent voltage is preferably used during the cleaning process. By slowly rotating the stack, each point on the membrane surface experiences a pulsating electrical current, which gives the best cleaning effects. After the flow has risen again to the initial value or at least has been improved by the electrical cleaning of the membrane surface, the filtration is continued in normal current-free operation. It can be beneficial during filtration use a higher rotation speed than during the cleaning process.

When a voltage is applied, the membrane is cleaned according to the principle described above, due to the development of gas bubbles on the membrane surface. Depending on the shape of the counter electrode used, the gas bubbles do not form over the entire area of the membrane electrode surface. In order to achieve the formation of the gas bubbles on the entire membrane electrode surface, disk-shaped counter electrodes of the same size as the membrane electrode must generally be present on both sides of the membrane electrode. This connection makes the electrofiltration process considerably more expensive, since the counter electrodes must have noble and expensive metals so that the counter electrode, which is usually the anode, is dimensionally stable. By rotating or moving the membrane according to the invention during the cleaning process, however, it is now also possible to use counter electrodes which have a smaller shape than the membrane electrode. In this case, it only has to be ensured that each area of the membrane electrode is at least once during the cleaning process at a sufficiently small distance from the counter electrode. In such an embodiment of the method according to the invention, gas bubbles develop only in the area of the counter electrodes, since the electric field is strongest here. In order to obtain a complete cleaning of the entire membrane surface, the membrane electrodes are guided past the counter electrodes by slowly rotating the membrane electrode stack.

The method according to the invention can also be used advantageously in dead-end filtration. With this filtration method it is not possible to achieve a sufficiently high overflow rate of the liquid to be filtered through the membrane. The method according to the invention offers the possibility of simulating or achieving an overflow rate by moving the membrane electrode according to the invention. Dead-end filtration is also preferably carried out with a feed pressure of 1.2 to 6 bar and a pressure on the permeate side of the membrane of 5.8 to 0.2 bar.

The method is particularly suitable for carrying out the method according to the invention Device for electrofiltration according to the invention. According to the invention, this device, which is also referred to below as an electrofiltration module, has at least one rotating membrane electrode and at least one counter electrode. The counter electrode preferably has a smaller surface area than the membrane electrode. The rotation of the membrane electrode leads all areas of the membrane electrode surface past the counter electrode.

According to the invention, the membrane electrodes comprise an inorganic membrane that conducts the electrical current. The membrane electrode preferably comprises an inorganic membrane which was produced on the basis of an open -work carrier which conducts the electrical current and which was provided with an inorganic, material-permeable coating comprising titanium dioxide. The membrane according to the invention can preferably be negatively charged by applying an electrical current. Permeable for the purposes of the present invention means that the coating has pores. Depending on the intended use (filtration project), membranes can be used which have suitable maximum pore sizes, so that particles which are larger than the maximum pore size are retained during filtration.

According to the invention, disc-shaped, so-called membrane cushions arranged on an axis are used as membrane electrodes, which preferably have a thickness of 1 mm to 30 mm, particularly preferably of 1 mm to 10 mm. It is also possible to use thinner membrane cushions, with restrictions in the dimensions being imposed by the necessary stability and / or separation performance. The membrane cushions preferably have a round or approximately round shape, the maximum diameter being from 10 to 100 cm, preferably from 10 to 50 cm. The membrane cushions preferably have an opening or bore in their center, the outer diameter of which is from 1 to 9 cm. The opening or bore very particularly preferably has an outer diameter which corresponds to the outer diameter of the shaft or axis.

All membranes that are at least partially electrically conductive are suitable for producing the membrane cushions used as membrane electrodes. Membranes are preferred predominantly used from inorganic constituents, such as ceramic membrane or metal membrane. The production of such ceramic membranes is described, for example, in WO 99/15260, WO 99/15262 or WO 96/00198. Metal membranes can be, for example, metal nets or mesh. Inorganic membranes that are flexible or bendable are very particularly preferably used.

The membrane cushions are e.g. obtainable in that at least one inorganic membrane, which preferably has at least partially electrically conductive properties, is attached to a porous carrier disk or a round or almost round, disk-shaped holder which has a recess, preferably a round recess, in the middle. The attachment can e.g. done by sticking. This happens on both the top and bottom of the carrier disc. The outer edge of the carrier disk is either sealed or made impermeable to the substance using a suitable material or likewise closed with an electrically conductive membrane. The inner edge of the pane is not sealed and not covered with a membrane. In this way, membrane cushions are obtained which are permeable on the flat sides only to substances whose particle size is smaller than the pore size of the membrane used in each case. The outer edge of the membrane cushion is either just as permeable to fabrics as the side surfaces or completely impermeable to fabrics. The inner edge of the membrane cushion is permeable to all substances with a particle size smaller than the pore size of the porous carrier disk.

It can also be advantageous if the membrane cushions are made from membranes into which spacer materials, drainage materials or nonwoven fabric have been incorporated. Such membranes can also be produced in accordance with WO 99/15260 and / or WO 99/15262, in which the required spacer material, the drainage material or the nonwoven fabric is used as the porous carrier material, to which a porous ceramic layer is applied. A porous ceramic layer is preferably applied, which has titanium oxide, which can be made electrically conductive by applying a voltage. The required membrane cushions can be obtained from such membranes, for example by punching out, the outer edges, which would be permeable to substances after punching out, being sealed with appropriate materials, such as, for example, adhesives or glass solder or need to be welded.

It can be advantageous if the porous carrier disk and / or the spacer material, the drainage material or the nonwoven fabric is electrically conductive. However, this is not absolutely necessary as long as the membrane or membrane surface used is electrically conductive.

Stick electrodes are particularly suitable as counter electrodes. However, other shaped electrodes can also be used, e.g. Disc electrodes or cake-shaped electrodes. According to the invention, the counterelectrodes have an identical or smaller shape or contour, preferably a smaller shape or contour than the membrane electrode. Since the membrane electrodes used according to the invention preferably have circular or at least polygonal shapes or contours, electrodes which have a circular section as a contour are particularly preferred as counter electrodes. The circular section preferably has the same outer radius as the contour of the membrane electrode. The circular section can have all sizes smaller than 360 degrees. The counter electrode preferably has a circular section (pie piece) of 60 to 0.1 degrees. The above-mentioned stick electrode can be regarded as a counter electrode with a very small circular section.

The counter electrodes mentioned can be produced regardless of their shape or contour from the materials usually used for electrodes. The counter electrodes of the device according to the invention are preferably made of Ti, Ir, Pt, Au, Pd or alloys which contain these metals. It can also be advantageous to use standard electrodes coated with the aforementioned metals. The choice of standard electrodes is restricted by the fact that the electrodes used or the base body of the electrodes must be dimensionally stable with respect to the solutions or substance mixtures to be treated.

The device for electrofiltration according to the invention can have one or more of the above-mentioned membrane cushions. Likewise, the device according to the invention can have one or more of the counter electrodes mentioned above. Preferably that The electrofiltration module according to the invention has a ratio of counter electrodes to membrane cushions of 0.5 to 1 to 10 to 1. A ratio of 0.5 to 1 is achieved, for example, by arranging exactly one counter electrode between two membrane cushions.

The electrofiltration module according to the invention has at least one membrane cushion, which is arranged on at least one shaft, which has at least partially openings, such that the inner edge of the membrane cushion lies over all openings of the shaft. It can be advantageous if not just one but several membrane cushions are arranged on such a shaft. In this case, at least one opening of the shaft is covered by the inner edge of a membrane cushion. The membrane cushions are firmly attached to the shaft. This can be done in a manner known to those skilled in the art, e.g. done by welding or gluing. One condition for attaching the cushions to the shaft is that it must be ensured that there are no gaps between the inner edge of the membrane cushion and the shaft through which substances can pass. There must be such large gaps between the individual membrane cushions on the shaft that at least one counter electrode can be arranged between two membrane cushions. The distance between the membrane cushions is determined by the arrangement of the openings in the shaft. Against this background, the arrangement of the openings on the shaft is not arbitrary, but must meet the condition mentioned. It can be advantageous to provide spacers between the individual membrane cushions. Such an arrangement of shaft and at least one membrane cushion is referred to below as a membrane electrode stack.

Electrically conductive hollow objects, which preferably have a round or square cross section, such as e.g. Metal pipes can be used. The arrangement of the above-mentioned openings in the sides of the shafts must satisfy the above-mentioned condition that membrane cushions which are arranged above the openings have a sufficiently large distance. Through these openings, the filtrate which has passed through the membrane of the membrane cushion can be transferred into the shaft and can be passed through this shaft to a container.

The electrofiltration module according to the invention preferably has at least one chamber which has at least one inlet and at least one outlet. At least one membrane electrode stack is also installed in this chamber. The membrane electrode stack is preferably installed in the chamber in such a way that the membrane cushions are arranged horizontally or perpendicularly to the standing surface of the chamber when the electrofiltration module is in operation. The membrane electrode stack is preferably installed in the chamber in such a way that the shaft rests in bearings which are integrated in the side walls of the chamber. At least one drive is installed on the shaft, preferably outside the chamber, which enables the shaft to be rotated. A motor is preferably attached to the shaft, which allows the shaft to be rotated at an adjustable speed.

If the filtration module according to the invention is to be used in a filtration according to the dead-end principle, the outlet from the chamber of the filtration module is closed during the filtration process. As with cross-flow filtration, the permeate is passed out of the filtration module through the shaft on which the membrane cushions are arranged. During or after the cleaning process, the drain from the chamber can be opened briefly to flush the cleaned particles out of the chamber.

There is also at least one counter electrode in the chamber. Preferably, there are at least so many counter electrodes in the chamber that the abovementioned ratio of counter electrodes to membrane cushions is maintained. All counter electrodes are preferably connected to one another in an electrically conductive manner. It can be advantageous if there is not only one counter electrode per membrane cushion in the chamber, but at least two or more. When using more than one counterelectrode per membrane cushion, it can be advantageous to arrange the counterelectrodes in such a way that the angle between the counterelectrodes, which are located on a plane between the membrane cushions, is the same.

For reasons of stability, it may be advantageous to attach non-conductive shells to the head of some electrodes and to design the counter electrodes so long that the non-conductive shells lie on the shaft as bearing shells. In this way, the shaft inside the chamber can be supported with an additional bearing. The shaft and thus the membrane electrode stack and the counter electrodes are connected to a current source in such a way that the membrane electrode stack is connected to one pole and the counter electrodes are connected to the other pole. The current source supplies current with a voltage of at least 1.5 V. Direct or alternating current can be used, preferably direct current is used. The direct current is very particularly preferably used in such a way that the membrane electrode stack is switched as the cathode and the counterelectrodes are switched as the anode.

It may be advantageous to install not just one membrane electrode stack in one chamber, but several. All membrane electrode stacks are advantageously connected together as a cathode.

The device according to the invention can be used to carry out the method according to the invention for increasing the filtration performance of membrane filtration systems in the filtration of substance mixtures e.g. according to the cross-flow or dead-end principle.

The device according to the invention is explained in more detail with reference to FIGS. 1 to 4, without the device according to the invention being restricted to these.

In Fig. 1, an electrofiltration module according to the invention is shown schematically. In a chamber Ka, which has an inlet Ei and an outlet Au, there is a shaft W on which a plurality of membrane pads are arranged as membrane electrodes M. The membrane electrodes are electrically connected to the negative pole of the power source (-) via the shaft W, which is hollow and through which the permeate Pe can be removed. The shaft is attached so that it can be rotated. Rod electrodes S are installed between the membrane electrodes M, which are connected to one another in an electrically conductive manner and are connected together to the positive pole of the current source (+).

2, the membrane cushions according to the invention are shown schematically. The views labeled MK la and MK 2a represent a section through a membrane cushion according to the invention. The views labeled MK lb and MK 2b represent the Membrane cushions in supervision.

3 shows four possible arrangements of rod electrodes S in comparison to the membrane electrodes Ml to M4. In addition, two possible arrangements of pie-shaped counterelectrodes T, which have the contour of a circular or ring section, are shown as examples in comparison to the membrane electrodes M5 and M6.

4 shows the basic functioning of an electrofiltration. In the case of electrofiltration, as can also be carried out with the electrofiltration module according to the invention, a stream of material to be filtered is circulated through a filtration module FM. Due to the different pressure on both sides of the filtration membrane, a part of the feed stream Fe, purified as permeate Pe, passes through the filtration membrane Mem into the permeate chamber. The majority of the feed stream, as well as the particles retained by the filtration membrane, return as retentate R to the feed template FV.

Applying a voltage to the membrane (-) and a counter electrode (+), which is also present, causes gas to develop on the membrane through electrolysis. Since the gas evolution Ga takes place directly on the membrane surface, particles that cover the membrane surface are detached therefrom and, if the current is sufficiently high, are flushed back through the filtration module with the retentate into the feed template. In this way, the membrane can be cleaned by applying a voltage to the membrane.

5 shows the basic functioning of an electrofiltration based on the dead-end principle. In the case of electrofiltration, as can also be carried out with the electrofiltration module according to the invention, a material flow Feed) Fe 'to be filtered is moved from the feed template FV into a filtration module FM'. Due to the different pressure on both sides of the filtration membrane Mem ', a part of the feed stream reaches the permeate chamber in a purified form as permeate Pe' through the filtration membrane.

By applying a voltage to the membrane (-) and an existing one Counterelectrode (+) produces gas Ga on the membrane through electrolysis. Since the gas development takes place directly on the membrane surface, particles that cover the membrane surface are detached from it. In this way, the membrane can be cleaned by applying a voltage to the membrane.

The measurement results obtained for the experiments described in the examples are shown graphically in FIGS. 6 and 7.

Example of electrofiltration of a 1% PMMA latex solution

In a filtration device according to the invention, electrofiltration of a 1% polymethyl methacrylate (PMMA) latex was carried out at different rotation speeds. The filtration device had a membrane electrode with an outer diameter of 10 cm. The membrane used to manufacture the membrane electrode had an average pore size of 0.08 μm. In experiments A, B and E, two platinum-coated rod electrodes with a round profile, a length of 10 cm and a diameter of 5 mm made of titanium were used as counter electrodes (anodes). The stick electrodes were arranged parallel to one another above and below the membrane electrode at a distance of 5 mm from the membrane electrode.

For comparison purposes, a test was carried out in test C, in which the test parameters were identical to those from test A, except that no current was applied to the membrane electrode.

Disc electrodes were also used as counter electrodes in experiment D for comparison purposes. These were also platinum-coated discs or rings made of titanium, which in turn were arranged above and below the membrane electrode at a distance of 5 mm parallel to the membrane electrode. In contrast to the membrane electrode, the disk electrodes were not fastened movably.

Experiments E to H were carried out with the same apparatus and the same parameters as experiments A to D, with the difference that a different one Membrane electrode with an average pore size of 0.075 μm was used. In experiment G, as in experiment B, the measurement was without current. The course of the measurement curves is similar accordingly. The course of curves H and D also corresponds. In both experiments, the filtration was carried out by applying a continuous current to the membrane electrode and to a disk electrode (circular section 360 °) as the counter electrode.

In experiment F, pie electrodes with a circular section of 180 ° were used as electrodes, which were congruent, parallel above and below the membrane electrode. The rotation speed was 10 min ^"1 .

In experiment E, the filtering was first carried out at a rotation speed of 1 min ^"1 for a period of 4.5 hours without applying a current. After this period, a current of 2 A was applied.

The assignment of curves A to E to the test parameters can be found in the table below.

6 shows the course of the permeate flow over the test period. As can be seen from the curves, the permeate flow over the test period is 6.5

Hours in experiments A and B almost constant. The curve belonging to experiment C shows a continuous decrease in the permeate flow over the course of the experiment. From the It can be seen from the curve belonging to test D that the decrease in the permeate stream over the test time is still significantly higher for the selected test parameters.

7 shows the course of the permeate flow over the test time for tests E to H. The lower permeate flow due to the smaller maximum pore size of the membrane electrode used can be clearly recognized at the start of the tests. The course of the curve for experiment E initially corresponds to the course of the curve for experiment F, that is to say the permeate flow decreases with the duration of the experiment. After 4.5 hours, i.e. at the point in time at which a current is applied to the membrane electrode, the permeate flow through the membrane increases again, and after approximately half an hour almost reaches the value of the permeate flow through the membrane at the beginning of the Try it. The use of a cake electrode with a circular section of 180 ° (curve F) shows a hardly better filtration performance than the curve with the disk electrode (test E).

Curve G shows a similar course to curve B, which is not surprising since both curves show the course of the permeate flow in the case of an electroless filtration. Curves F and H are almost identical and are similar to the curve of curve D. Filtration with a disk electrode (360 ° circle) or with a cake electrode (180 ° circle) shows little difference at a rotation speed of 10 min ^"1 .

In contrast to the comparative experiment C, in which the filtration is carried out without current and the permeate flow decreases continuously over the duration of the experiment, the permeate stream in the electrofiltration according to experiment A or B remains virtually constant over the entire duration of the experiment. This is due to the cleaning of the membrane by gas bubble development. In experiment A, the gas bubbles develop at every point on the membrane electrode once per minute. In experiment B, gas bubble development occurs twice per minute due to the higher rotational speed, since each area of the membrane electrode comes twice per minute into the area of the electric field at the stick electrode in which the voltage is high enough to electrolytically water the PMMA solution to split into hydrogen and oxygen. In experiments H and D, in which a disc electrode was used and thus a voltage sufficient for the electrolysis of water was permanently applied to each area of the membrane electrode, the continuous development of gas bubbles leads to an even faster decrease in the permeate flow. This phenomenon can probably be explained by the fact that the pores of the membrane electrodes are partially blocked by the strong gas bubble development and therefore no longer contribute to the filtration. The course of curve F further shows that when a cake electrode with a circular section of 180 ° and a rotation speed of the membrane electrode of 10 min ^{"1 is used,} the gas bubble development is still virtually permanent and thus poor filtration results are obtained, as when using a For this reason, the use of electrodes that are not too large should be aimed at, or the rotation speed should be reduced accordingly when using large electrodes (circular section 180 °).

From the course of the curve for experiment E, it can be seen that it is not absolutely necessary to apply a voltage to the membrane electrode or regions of the membrane electrode at regular intervals even at the beginning of the filtration. Rather, it may be sufficient if a voltage is only applied to the membrane electrode or parts thereof when the permeate flow has decreased to a certain limit value.

Claims

claims:

1. A method for electrofiltration, in which a membrane electrode is cleaned by gas bubble development, characterized in that the membrane electrode used is moved.

2. The method according to claim 1, characterized in that the membrane electrode is charged by applying an electrical voltage, and there is a gas bubble development on the membrane surface due to the electrical voltage.

3. The method according to claim 2, characterized in that the electrical voltage is greater than 1.5 V.

4. The method according to claim 2 or 3, characterized in that the electrical voltage is pulsed or applied as a permanent voltage.

5. The method according to at least one of claims 2 to 4, characterized in that the current intensity at the membrane electrode is greater than 1 mA / cm ² .

6. The method according to at least one of claims 1 to 5, characterized in that the membrane electrode rotates.

7. The method according to claim 6, characterized in that the membrane electrode rotates more slowly during cleaning than during the filtration process.

8. The method according to claim 6, characterized in that the membrane electrode rotates at the same speed during cleaning and during the filtration process.

9. The method according to at least one of claims 6 to 8, characterized in that the membrane electrode rotates during the cleaning with a rotation speed of 0.1 to 5 min ^"1 .

10. The method according to at least one of claims 6 or 7, characterized in that the membrane electrode rotates during the filtration process at a rotation speed of 1 to 500 min ^"1 .

11. The method according to claim 10, characterized in that the membrane electrode rotates during the filtration process at a rotation speed of 100 to 300 min ^"1 .

12. Device for electrofiltration, characterized in that it comprises at least one rotating membrane electrode and at least one counter electrode.

13. The apparatus according to claim 12, characterized in that the counter electrode has a smaller shape or contour than the membrane electrode.

14. The device according to claim 12 or 13, characterized in that the membrane electrode comprises a membrane which conducts the electric current.

15. The apparatus according to claim 14, characterized in that the membrane electrode comprises an inorganic membrane, which were produced on the basis of an electrically conductive, open-worked carrier which was provided with a titanium oxide-containing, inorganic permeable coating.

16. The apparatus according to claim 14 or 15, characterized in that the membrane can be charged by applying an electrical current.

17. The device according to at least one of claims 14 to 16, characterized in that the inorganic membranes are attached to porous, round or almost round disc-shaped carriers to form membrane cushions.

18. The apparatus according to claim 17, characterized in that the disc-shaped carrier can be constructed from several layers.

19. The apparatus of claim 17 or 18, characterized in that the membrane cushions have a hole in the middle, the outer diameter of which is from 1 cm to 9 cm.

20. The apparatus according to claim 19, characterized in that the outer diameter of the membrane cushion is from 10 to 100 cm.

21. The device according to at least one of claims 17 to 20, characterized in that the membrane cushions are connected via a rotating shaft as a membrane cathode.

22. The device according to at least one of claims 12 to 21, characterized in that one or more dimensionally stable counter electrodes are present, which are arranged above and / or below a membrane.

23. The device according to claim 22, characterized in that the counter electrodes have at least one of the materials Ti, Ir, Pt, Au, Pd, or mixtures and / or alloys thereof.

24. The device according to at least one of claims 12 to 23, characterized in that an electrolysis of aqueous solutions can be carried out.