FR2670397A1 - Improvement to the process for the manufacture of semipermeable membrane on a flexible porous support - Google Patents

Abstract

Improvement to the process of manufacture of flexible semipermeable membranes by continuous electrodeposition of one or more layers of sinterable inorganic particles on a porous or potentially porous conductive flexible strip. Application to microfiltration, ultrafiltration and reverse osmosis.

Description

IMPROVEMENT IN THE SEMI-MEMBRANE MANUFACTURING PROCESS
PERMEABLE ON SOFT POROUS SUPPORT
FIELD OF THE INVENTION
The present invention relates to an improvement to the process for manufacturing a semi-permeable membrane by electrodeposition of finely divided mineral solid on a flexible porous conductive support in the form of a sheet or lattice, process described in the main French application N0 89 04197 filed March 14, 1989.

OBJECT OF THE INVENTION
The Applicant has sought to improve the process described in the main application, in particular to economically obtain semi-permeable membranes in the form of flexible strips, of flexible sheets of small thickness.

DESCRIPTION OF THE INVENTION.

The solution found consists in an improvement to the process for manufacturing a flexible element for micro filtration, ultrafiltration, reverse osmosis consisting of a semi-permeable mineral membrane adhering to the surface of a flexible, electrically conductive support, comprising the following stages a) one or more inert mineral solid products are finely dispersed in a liquid medium such as water, water mixed with organic solvents at least partially water-soluble, in the presence of a water-soluble coating resin electrodepositable, b) an electrophoresis bath is prepared by the optional addition of water, organic solvents at least partially water-soluble, and said coating resin, to said mineral solid product finely dispersed in a liquid medium, c) after having put said electrophoresis bath in contact with said flexible porous electrically conductive support of any geometry taken as an electrode and after having placed a metal counter-electrode in said bath, a layer of mineral solid product (s) coated with resin which is electrocoagulated is deposited by electrophoresis, d) a new layer of product may possibly be electrodeposited mineral solid, or several layers successively, of increasingly fine particle size, and / or of different chemical nature, by modifying the electrical deposition conditions, e) heat treating said flexible porous conductive support coated with one or more layers solid mineral product coated with electrocoagulated resin so as to remove the solvent and organic matter and to sinter the layer of mineral powder until the desired microporous ceramic layer is obtained, an improvement characterized in that one chooses as said flexible conductive support, a flexible strip of great length (1), porous, possibly by means of cells (2), or susceptible to become porous by a subsequent heat treatment, in that said flexible strip (1) is positioned in and out of the bath (3) by guide means, is provided with means allowing the continuous unwinding of said flexible strip (1) in the bath (3) so as to ensure that any fraction of the strip has a predetermined residence time in the bath, in that one electrodes, over all or part of the length of said flexible strip (1) immersed in the bath, a layer of mineral solid product coated with coagulated resin on said flexible strip (1) serving as a cathode and taking place continuously in the bath then in that, after having optionally washed the flexible strip coated with said layer at the outlet of the bath (3), it is subjected to a continuous heat treatment so as to remove at least the water and the solvents from said layer.

According to the invention, the porous flexible strip (1) comprises the meshes or lattices formed by weaving (see FIG. 5b), knitting (see FIG. 5a), or any other known means of interweaving or assembling continuous conductive wires. or discontinuous, making it possible to obtain porous flexible strips of great length, typically more than 10 m.

As a conducting wire, a metallic wire, a graphite wire or fiber can be chosen, any wire, in particular of plastic or ceramic material, made conductive for example by depositing metal on the surface or by incorporating, into the mass of the wire, a conductive powder, metallic in particular.

The flexible strip (1) also comprises perforated conductive strips obtained from solid conductive strips, which can be made of metal but also of graphite, of plastic film metallized on the surface. It also includes flexible bands which, initially non-porous at the electrodeposition stage, will become so at the stage of the final heat sintering treatment. Thus, metallized plastic films, known for their tightness and their gas barrier properties, can be used according to the invention, as are films made conductive by incorporating conductive powder, in particular metallic, into the mass of the film.

When high mechanical characteristics are sought, it is preferably chosen as a flexible strip (1) serving as a support for the materials which will mechanically reinforce the filter membrane obtained according to the process.

It may be advantageous to locally remove a small portion of flexible strip (1) from electroplating in order to easily make electrical contact with the uncoated and therefore conductive part of the filter membrane (16) obtained according to the invention, or else in order to be able to subsequently perform mechanical assembly of the filter membrane, for example by gluing, welding, brazing.

For this, one can use any known means constituting a temporary mask (18,19,20) on a surface. This temporary mask is made of insulating material or covered with insulating material so as not to be itself covered, when passing through the bath (3), with an electrodeposited layer of mineral solid product coated with resin. As an example of a temporary mask, a narrow strip of adhesive tape (18) can be fixed before the electrodeposition on one edge of the flexible strip (see FIG. 7a), or evenly spaced transverse strips (19) (see FIG. 7b). , or even small magnetic pads (20) on either side of the flexible strip (1) (see Figure 7c).

These flexible supports (1) have a mass per m2 of between 5 and 1500 g. Their thickness is small, generally between 5 pm and 2 mm, but preferably between 20 and 300 pm. For example, in the case of a flexible strip made up of a mesh obtained by simple interlacing of warp and weft threads, the thickness is substantially equal to twice the thickness of the thread generally between 20 and 200 pm. Furthermore, commercially available thin films ranging from 5 μm to 100 μm are commercially available.

 In the case of a flexible woven strip, the cells (2) are formed by the space delimited by the conducting son of the warp and weft and are generally of polygonal section, in particular of square section. In the case of perforated conductive strips, each perforation of the strip constitutes a cell (see FIG. 5c). In general, it is important according to the invention to have, on the one hand, cells of size between 5 and 1000 μm and preferably between 20 and 100 μlm, the size of a cell being its largest dimension. , and on the other hand to have a large alveolar surface but compatible with sufficient mechanical strength of the conductive mesh or of the perforated conductive strip. To measure the alveolar surface, the orthogonal projection of the alveoli in the plane of the flexible strip is considered. (1) and we consider the fraction of the plane occupied by the projected alveolar surface. The projected cellular surface represents from 10 to 80% of the total surface and preferably from 20 to 50%. As a first approximation, we can consider that the value of this alveolar surface is close to the value of the pore volume of the flexible strip expressed in% of the total volume of the flexible strip.

In the case of a flexible band (1) either knitted, or in the form of felt or mat, the band is characterized by its porous volume, the concept of "alveolus" used in the case of a woven band not suitable not. According to the invention, this pore volume must be between 10 and 80% of the total volume.

According to the invention, when the electrical conduction results from the use of metal or metallic alloy, any metal or metallic alloy can be used, either capable of forming a wire making it possible to fabricate a metallic mesh, or capable of forming a perforable metallic strip. , either can be deposited on a support to form a conductive coating, or can form a conductive powder incorporable into a matrix.

Preferably, the metals or metal alloys which retain mechanical characteristics during the heat treatment at high temperature, typically at 1000 ° C., are chosen.

As other considerations governing the choice of flexible strip (1), there is also availability on the market, cost per m2, mechanical characteristics, corrosion resistance. Mention may be made, as supports satisfying all these criteria, of the metal fabrics used for the screening of powders, for example the stainless steel fabrics.

In general, the flexible strips (1) according to the invention are in the form of a strip of great length and width which may exceed 1 m, wound on an axis so as to form a roll (4) which can be easily handled and removed from the mold. at the head of the electrophoresis bath (3).

Figure 1 shows in vertical section a manufacturing line for implementing the method according to the invention, which, although not limiting, will allow a better understanding of the invention.

This production line comprises - means for positioning the roll (4) of flexible strip (1) (not shown in FIG. 1), - a tank (5) containing the electrophoresis bath (3) provided with a roll conductor (6) located above the bath, two rollers (7) for positioning the strip in the bath and drive rollers (8) located at the outlet of the bath intended to ensure continuous and regular advancement of the strip . The rollers (8) also provide mechanical tension to the metal support so that it has a negligible deflection in the bath. The rollers (7) and (8) can be made of rubber.

The tank (5) is provided with a counter-electrode (9) consisting of one or two conductive planes (15), possibly perforated, immersed in the bath. When two planes are used, they are located on either side of the flexible strip (14) and are not necessarily parallel if a non-homogeneous electric field is desired. They can be made integral by lateral metal uprights so that the whole forms a parallelepiped "cage".

The tank (5) is provided with devices known in themselves making it possible to ensure a constant level and composition of the bath, for example using a bath inlet (10) and an overflow ( 11).

- downstream of the electrophoresis bath, optional means for washing the strip (14) obtained after electrodeposition, shown diagrammatically by a spraying device (12).

- Strip drying means (14), shown diagrammatically by a drying tunnel (13).

- Possibly means for cutting the dried strip into formats which will be subjected to a subsequent treatment, or means for winding the dried strip on a large diameter mandrel.

- an automatic control device based on setpoints and control of the entire installation making it possible to choose and regulate the main process parameters such as the speed of advance of the support / tape as well as the electrical parameters (voltage, plating current density).

One can imagine other configurations, for example that described in FIG. 2 which avoids any contact of the wet strip (14) with guide or drive rollers thanks to its vertical arrangement.

This diagram shows a counter-electrode (9) with conductive planes not parallel to the flexible strip (1,14) and allowing a gradual increase in the electric field as the strip advances, which is favorable for get a dense deposit. There is also shown diagrammatically in FIG. 2 (in dotted lines) the winding of the strip (16) on the mandrel.

In order to avoid any contact between the wet strip (14) and guide or drive rollers, it is also possible to use a perforated support, for example in the manner of a photographic film, as indicated in FIGS. 3a and 3b.

The implementation of the method according to the invention uses the specifications contained in the main application, in particular with regard to the nature of the electrophoresis bath (3) and the electrical parameters, however, preferably, the voltage applied between the electrode (cathode) and the counter electrode (anode) is between 80 and 250V, the current density is between 2 and 20 mA / cm2, the dry extract of the bath (M + O) is between 10 and 25 % and the ratio "mineral filler / resin" (M / O) is between 0.2 and 2.

The speed of movement of the support (1) / of the strip (14) is chosen as a function of the particular geometry of the installation, in particular it is important to ensure a sufficient time of electrodeposition of the strip, time which is generally between 30 and 180 s.

To ensure a deposition time of 90 s with a 3 m long counterelectrode, the speed must therefore be 2 m / min.

 While the initial strip (1) can be strongly discontinuous, whether it is for example a woven or perforated strip, there is obtained, after electrodeposition of the resin loaded with mineral particles, a continuous strip (14), without pores or orifices, having a slight surface undulation recalling the presence of a metal support, as shown diagrammatically in FIG. 4a.

According to the invention, it is possible to carry out the final continuous sintering heat treatment by installing strip or format scrolling ovens. In practice, it is simpler to proceed in two stages and first to obtain a flexible strip, free of water and of solvents by passing the strip (14) through a drying tunnel, in the form of formats or reel which can be handled without special precautions, then obtaining the semi-permeable membranes (16) according to the invention by subjecting these formats or reels to a heat treatment at high temperature under conditions defined in the main application, such as the speeds temperature rise, the duration of the temperature stages, the nature of the atmosphere, the cooling rate.

Given the small potential thickness of the support and that of the strip obtained after electrodeposition, it remains flexible even after the high temperature heat treatment during which the sintering of the mineral charge electrodeposited takes place.

FIG. 4b typically represents a membrane (16) obtained after heat treatment: the thickness of the membrane is typically close to 10 μm and the corrugations are more marked after than before the heat treatment.

By modifying the dry extract of the bath and the mineral filler content and its particle size, it is possible to obtain membranes with a thickness of between 1 and 30 μm and an average porous diameter of between 0.01 and 3 rm.

According to the invention, it is also possible, preferably before the step of drying the strip (14), to circulate it in several baths in series, in particular two baths, loaded with mineral powders which are different in chemical nature and / or granulometry. Preferably, the second bath will make it possible to obtain a membrane layer of porous diameter smaller than the underlying layer formed by the passage in the first bath. In this case, the counter electrode is preferably chosen so that the second deposition takes place only on one side of the strip in order to obtain, after heat treatment, an asymmetric membrane with a surface layer of small thickness (typically 0.5 to 5 μm) and of small porous diameter (typically of 0 , 01 to 1
deposited on a layer of greater thickness (typically from 5 to 30 μm) and of greater porous diameter (typically from 0.5 to 3 μm).

Surprisingly, the invention has made it possible to obtain a continuous filter membrane (16) even starting from a support with cells of relatively large dimensions or with large pore volume. The tests have shown that the upper limit size was, in the case of square cells obtained with a wire of 50 rm in diameter, around 200 rm (edge). With a constant cell surface, this size limit increases with the diameter of the wire.

The invention makes it possible to manufacture, in a very economical and reproducible manner, thin membrane filters (16) possibly mechanically reinforced. These filter membranes above all make it possible to obtain compact filtration modules with a large filtering surface per m3, typically 150 to 200 m2 per m3.

It should be noted that the undulations of the filter membrane obtained according to the invention are advantageous for the flow of liquids in the filtration modules which, when the membranes are strictly flat, have perforated grids (17) to keep the membrane at a distance. internal partitions and facilitate the circulation of liquids on either side of the membrane (see Figure 6a). The filter membranes (16) of the invention allow, thanks to their relief, the removal of the means used previously for this purpose (see Figure 6b).

In addition, the corrugations of the filter membrane facilitate the flow of liquids because they ensure turbulent flow at lower circulation speeds of the liquids to be treated (of the order of 1 to 2 m / s) than those necessary (of around 3 to 5 m / s) in the case of a filter membrane with a flat surface.

The filter membranes of the invention find application in micro filtration, ultrafiltration and reverse osmosis. In addition, the filter membranes of the invention have an internal conductive reinforcement (the flexible starting strip (1)) and are therefore usable in applications where this particular feature can be highlighted. By way of example, mention may be made of the use of electrical conductivity to heat the filter membrane by direct Joule effect, or by induced currents, and accelerate filtration.

- use of electrical conductivity to create an electric field, polarize the membrane and facilitate / prevent the passage through the membrane of electrically charged species.

According to the invention, it is also possible to obtain filter membranes of various shapes by shaping either the strip obtained after drying of the wet strip (14), or the filter membrane (16) obtained after the final heat treatment.

Thus, for example, it is easy to obtain a tubular filtering membrane according to the invention starting from a metal support with masked edges (for example as shown diagrammatically in FIG. 7a), by assembling by any known means (solder, glue, welding ) the bare edges so as to obtain a filter tube of any length and diameter, as shown diagrammatically in FIGS. 8a to 8c.

EXAMPLE
The example which follows illustrates the invention.

A stainless steel filter cloth having the following characteristics was chosen as flexible band (1) - wire diameter: 40 rm - fabric weight: 450 g / m2 - mesh: 50 pm - fabric width: 30 cm - projected alveolar surface / total flat surface: 20%
An electrophoresis bath was prepared based on zirconia powder and on electrodepositable resin Residrol SWE 5186 as in Example 1 of the main application.

The M / O ratio (Zirconia / Residrol) was set at 0.7 and the dry extract (E.S.) at 16%.

A laboratory electrodeposition device similar to that shown diagrammatically in FIG. 1 was used and comprising a tank (5) with a capacity of 20 1 provided with a counter electrode 50 cm long.

The electroplating was carried out under the following conditions - speed of advancement of the strip: 0.3 m / min - electroplating voltage: 150 V - current density: 5 mA / cm2 (actual surface of electroplating)
At the outlet of the tank (5), the strip coated with an electrodeposited layer was dried continuously in an oven at 250 ° C., then was cut into formats of 30 cm × 30 cm which were heat treated at 100 ° C. to sinter the zirconia according to the methods described in example 1 of the main application.

 A filter membrane (16) -represented in section in FIG. 4b was obtained, the active part of which has an average thickness of the order of 10 rm.

An identical test, but carried out with a fabric having a mesh of 400 μm, showed that in this case there was no formation of a filtering membrane between the metal wires of the canvas, but only overlapping of these metal wires.

DESCRIPTION OF THE FIGURES
Figure 1: diagram in vertical section of an installation for carrying out the invention.

Figure 2: diagram in vertical section of a variant of the installation of Figure 1.

Figure 3 a and b: diagram of a support with lateral perforations (fig. 3a), lateral and central (fig. 3b) in the case of very wide support.

Figure 4a: diagram in section perpendicular to the support (1) of the wet strip (14) obtained after electrodeposition.

The hatched part represents the electrodeposited layer.

Figure 4b: diagram of a photograph (magnification of 400), in section perpendicular to the support (1) of the final membrane obtained after heat treatment of the wet strip (14). The hatched part represents the active part of the filtration membrane (16).

Figure Sa: top view of a flexible knitted band
Figure 5b: top view of a flexible woven band
Figure 5c: top view of a flexible strip obtained by forming cells in a solid strip.

Figure 6a: diagram of a filtration module according to the prior art, in section perpendicular to the plane of this module
Figure 6b: diagram of a filtration module according to the invention, in section perpendicular to the plane of this module.

Figure 7a: section of a flexible strip (1), perpendicular to the direction of travel of the strip, provided with longitudinal strips of adhesive tape (18) used to mask the edges of the strip.

Figure 7b: top view of a flexible strip (1) provided with transverse strips of adhesive tape (19) serving as a mask.

 Figure 7c: top view and in longitudinal section of a flexible strip (1) provided with magnetic pads (20) serving as a mask.

FIG. 8a: diagram of a very long filter tube (22) obtained by winding the filter band (16) provided with masked edges (23) and assembly edge to edge so as to form a tube while sealing the area of junction (21) of the edges.

Figure 8b: sectional diagram of the filter tube of Figure 8a.

Figure 8c: diagram of a filter tube (22) obtained by bending a portion of filter tape (16) provided with masked edges (23) and assembling the opposite edges so as to form a tube while sealing the area of junction (21) of the edges.

Figure 9: photograph, top view, of a portion of filter tape obtained in the example according to the invention.

Claims (24)

1 - Improvement in the manufacturing process of a flexible microfiltration, ultrafiltration, reverse osmosis element consisting of a semi-permeable mineral membrane adhering to the surface of a flexible porous electrically conductive support, comprising the following steps a ) one or more inert mineral solid products are finely dispersed in a liquid medium such as water, water mixed with organic solvents at least partially water-soluble, in the presence of a water-soluble electrodeposable coating resin, b) prepares an electrophoresis bath by optional addition of water, organic solvents at least partially water-soluble, and said coating resin, to said mineral solid product finely dispersed in a liquid medium, c) after having put said electrophoresis bath in contact with said flexible porous electrically conductive support of any geometry taken as an electrode and after having placed a counter-el metal electrode in said bath, a layer of mineral solid product (s) coated with resin is deposited by electrophoresis, which is electrocoagulated, d) a new layer of mineral solid product, or more, may be electrodeposited successively layers, of increasingly fine particle size, and / or of different chemical nature, by modifying the electrical deposition conditions, e) heat treating said flexible porous conductive support coated with one or more layers of coated solid mineral product of electrocoagulated resin so as to remove the solvent and the organic materials and to sinter the layer of mineral powder until the desired microporous ceramic layer is obtained, improvement characterized in that a flexible conductive support is chosen, as said flexible of great length (1), porous, possibly by means of cells (2), or liable to become porous by a process subsequent thermal treatment, in that said flexible strip (1) is positioned in and out of the bath (3) by guide means, is provided with means allowing the continuous unwinding of said flexible strip (1) in the bath so as to ensure that any fraction of the strip has a predetermined residence time in the bath, by electrodeposition, over all or part of the length of said flexible strip (1) immersed in the bath, a layer of mineral solid product coated with resin coagulated on said flexible strip (1) serving as cathode and taking place continuously in the bath then in that, after having possibly washed the flexible strip coated with said layer (14) at the outlet of bath (3), it is subjected to a continuous heat treatment so as to remove at least water and solvents from said layer. 2 - The method of claim 1 wherein said flexible strip (1) porous comprises the mesh, lattice formed by weaving, knitting or any other known means of interweaving son or portions of son electric conductors. 3 - A method according to claim 2 wherein the son or portions of electrically conductive son are chosen from metallic son, graphite son or fiber, son of non-conductive material, made electrically conductive by depositing metal on the surface or incorporating electrically conductive powder, in particular metallic powder, in the mass. 4 - Process according to claim 1 wherein said flexible porous strip (1) is obtained by perforation of a solid conductive strip, in particular a metal strip. 5 - Method according to any one of claims 2 to 4 wherein the pore volume of said flexible porous strip (1) is between 30 and 95 * of the total volume. 6 - The method of claim 1 wherein said flexible strip (1) liable to become porous by heat treatment is a plastic film made electrically conductive by surface metallization or incorporation of conductive powder, in particular metallic, in the mass. 7 - Method according to any one of claims 1 to 6 wherein said flexible strip (1) has a mass per m2 of between 5 and 1500 g. 8 - Method according to any one of claims 1 to 7 wherein said flexible strip has a thickness between 5 rm and 2 mm. 9 - Process according to any one of claims 3, 5, 7 and 8 in which said flexible strip (1) is chosen from among the stainless steel wire cloths of type 304 or 316 and of mesh between 50 and 100 rm. 10 - Method according to any one of claims 1 to 9 wherein the counter-electrode (9) comprises one or two planar surfaces, possibly perforated in whole or in part. 11 - The method of claim 10 wherein the (the) surface (s) plane (s) is (are) not parallel to the flexible strip (1, 14) so as to have a gradual increase in the electric field at level of the band as it progresses in the bath. 12 - Method according to any one of claims 1 to 11 which comprises a device for ensuring a constant level and composition of bath and an automatic piloting and control device for choosing and regulating the main parameters such as the speed d advancement of the flexible strip (1.14) and the electrical parameters (voltage, electrophoresis current density). 13 - Method according to any one of claims 1 to 12 wherein the wet strip portion (14) covered with said layer is dried without there having been contact of said layer with a positioning means and / or setting in motion of said wet strip (14) so as not to equalize the surface of said layer. 14 - A method according to any one of claims 1 to 13 wherein there is circulated, preferably before the drying step, the flexible strip (1,14) in two baths successively. 15 - Method according to any one of claims 1 to 14 wherein the parameters defining the method are as follows - applied voltage: from 80 to 250 V - current density: from 5 to 20 mA / cm2 - dry extract of the bath (MsO) : from 10 to 25% - "mineral / resin filler" ratio (M / O): from 0.2 to 2 16 - Method according to any one of claims 1 to 15 in which a fraction of the flexible strip is temporarily masked (1 ) before electrodeposition, on its two opposite faces, so as to obtain a semipermeable membrane (16) allowing easy connection to its flexible conductive strip. 17 - Semi-permeable membrane in the form of a flexible strip obtained according to any one of claims 1 to 16. 18 - Semi-permeable membrane according to claim 17 having an effective thickness between 1 and 30 µm and an average porous diameter between 0.01 µm and 3 µm. 19 - Semi-permeable membrane according to claim 17 comprising a layer of thickness between 0.5 and 5 µm and an average porous diameter between 0.01 and 1 µm and a layer of thickness between 5 and 30 µm and with an average porous diameter of between 0.5 and 3 μm, so as to form an asymmetrical membrane. 20 - Membrane according to any one of claims 17 to 19 comprising a corrugated surface whose amplitude of the corrugation is between 10 and 100 μm, so as to facilitate the tangential flow of the fluid to be filtered and / or of the filtrate in particular in filtration modules. 21 - Membrane according to any one of claims 17 to 19 which locally comprises an area without semipermeable membrane so as to allow easy electrical connection of the flexible conductive strip (1) and / or to allow the shaping of said membrane.   22 - Application of the membranes according to any one of claims 17 to 21 in the manufacture of filtration modules having a "filtering surface" / module volume "high ratio. 23 - Application of membranes according to any one of claims 17 to 21 in the manufacture of filtration modules comprising an alternating stack of watertight partitions and membranes with corrugated surface so as to avoid the use of perforated grids intended to facilitate tangential flow of the fluid to be filtered. 24 - Application of the membranes according to claim 21 to electrofiltration and / or filtration with temperature control at the level of the filtering membrane. 25 - Application of the membranes according to claim 21 in the manufacture of filter tubes of any length and diameter.