SiC-NITRIDE OR SiC-OXYNITRIDE COMPOSITE MEMBRANE FILTERS

Abstract

A filter for the filtration of a fluid includes or is composed of a support element made of a porous ceramic material, the element exhibiting a tubular or parallelepipedal shape including, in its internal portion, a set of adjacent channels separated from one another by walls of the porous inorganic material, in which at least a portion of the channels and/or the external surface are covered with a porous separating membrane layer for contacting the fluid to be filtered circulating in the channels and making possible the tangential or frontal filtration of the fluid. The layer is made of a material including a mixture of silicon carbide and of at least one compound chosen from silicon nitride or silicon oxynitride, the content by weight of elemental nitrogen, with respect to the content by weight of SiC in the material constituting the porous separating membrane layer, is between 0.02 and 0.15.

Claims

1. A filter for the filtration of a fluid, comprising or composed of a support element made of a porous ceramic material, said support element exhibiting a tubular or parallelepipedal shape delimited by an external surface and comprising, in its internal portion, a set of adjacent channels with axes parallel to one another and separated from one another by walls of said porous inorganic material, in which: at least a portion of said channels are covered on their internal surface with a porous separating membrane layer and/or at least a portion of said external surface is covered with a porous separating membrane layer; wherein: said porous separating membrane layer is made of a material comprising a mixture of silicon carbide (SiC) and of at least one compound chosen from silicon nitride and silicon oxynitride, a content by weight of elemental nitrogen, with respect to a content by weight of SiC in said material constituting the porous separating membrane layer, is between 0.02 and 0.15.

2. The filter as claimed in claim 1, wherein the content by weight of elemental nitrogen in said material constituting the porous separating membrane layer is between 2 and 10%.

3. The filter as claimed in claim 1, wherein the SiC represents between 50 and 95% of the weight of the material constituting the porous separating membrane layer.

4. The filter as claimed in claim 1, wherein the material constituting the porous separating membrane layer comprises less than 2% by weight of metallic silicon.

5. The filter as claimed in claim 1, wherein the silicon carbide, the silicon nitride and the silicon oxynitride together represent at least 95% of a total weight of the material constituting the porous separating membrane layer.

6. The filter as claimed in claim 1, the wherein a porosity of the porous separating membrane layer is between 30 and 70% and a median pore diameter is between 10 nanometers and 5 micrometers.

7. The filter as claimed in claim 1, wherein the material of the porous separating membrane layer is essentially composed of SiC grains bonded together by a phase essentially composed of silicon nitride and/or of silicon oxynitride.

8. The filter as claimed in claim 7, wherein a median size of the SiC grains in said material of the porous separating membrane layer is between 20 nanometers and 10 micrometers.

9. The filter as claimed in claim 1, wherein said porous separating membrane layer is made of a material essentially composed of a mixture of silicon carbide and of silicon nitride and optionally of residual metallic silicon.

10. The filter as claimed in claim 1, wherein a content by weight of oxygen of the material constituting the porous separating membrane layer is less than or equal to 1%.

11. The filter as claimed in claim 1, wherein said porous separating membrane layer is made of a material essentially composed of a mixture of silicon carbide and of silicon oxynitride and optionally of residual metallic silicon.

12. The filter as claimed in claim 1, wherein the porous support element comprises or is composed of a material chosen from silicon carbide, SiC, recrystallized SiC, silicon nitride, silicon oxynitride, silicon aluminum oxynitride or a combination of these.

13. The filter as claimed in claim 1, wherein an open porosity of the material constituting the porous support element is between 20 and 60%, a median pore diameter of the material constituting the porous support element being between 5 and 50 micrometers.

14. The filter as claimed in claim 1, additionally comprising one or more primer layers arranged between the porous ceramic material constituting the porous support element and the material constituting the porous separating membrane layer.

15. A separating membrane layer as described in claim 1, made of a material comprising a mixture of silicon carbide (SiC) and of at least one compound chosen from silicon nitride or silicon oxynitride, the content by weight of nitrogen, with respect to the content by weight of SiC in said material constituting the porous separating membrane layer, being between 2 and 15%.

16. A process for the manufacture of a separating membrane layer as claimed in claim 15, in a tangential or frontal filter, comprising: preparing a slip from a powder of silicon carbide particles and from a metallic silicon powder, in a ratio by weight between the two powders (w.sub.SiC/w.sub.Si) of between 0.03 and 0.30, and from water, applying said slip to the support element under conditions which make possible the formation of a thin layer of the slip on the internal part of the channels of said filter, drying and then firing under nitrogen at a temperature of greater than 1200 C. and for a time sufficient to obtain said separating membrane layer on their internal surface of said channels.

17. A method comprising utilizing a filter as claimed in claim 1 for the filtration of liquids.

18. The filter as claimed in claim 1, wherein the fluid is a liquid.

19. The filter as claimed in claim 12, wherein silicon carbide is liquid-phase or solid-phase sintered SiC, the silicon nitride is Si.sub.3N.sub.4, and silicon oxynitride is Si.sub.2ON.sub.2.

20. The process as claimed in claim 16, wherein the filter is a tangential filter.

21. The method as claimed in claim 17, wherein the filter is utilized for filtering an aqueous liquid.

Description

[0104] FIG. 1 illustrates a conventional configuration of a tubular filter according to the current art, along a plane of cross section P.

[0105] FIG. 2 is a microscopy photograph of a filter showing the separating membrane layer within the meaning of the present invention.

[0106] FIG. 1 illustrates a tangential filter 1 according to the current art and in accordance with the present invention, as used for the filtration of a fluid, such as a liquid. FIG. 1 represents a diagrammatic view of the plane of cross section P. The filter comprises or generally is composed of a support element 1 made of a porous inorganic material, preferably a non oxide. The element conventionally exhibits a tubular shape with a longitudinal central axis A, its shape being delimited by an external surface 2. It comprises, in its internal portion 3, a set of adjacent channels 4, with axes parallel to one another and separated from one another by walls 8. The walls are made from a porous inorganic material which allows the filtrate to pass from the internal part 3 to the external surface 2. The channels 4 are covered on their internal surface with a separating membrane layer 5 deposited on a tie primer, as illustrated by the electron microscopy photograph given in FIG. 2. This separating membrane layer 5 (or membrane) comes into contact with said fluid circulating in said channels and makes possible the filtration thereof.

[0107] An electron microscopy photograph taken on a channel 4 of FIG. 1 has been given in FIG. 2. The porous support 100 of high porosity, the primer layer 102 making possible the tying of the separating membrane layer 103 of finer porosity, are observed in this figure.

[0108] The examples which follow are provided solely by way of illustration. They are not limiting and make possible a better understanding of the technical advantages relating to the use of the present invention.

[0109] The supports according to all the examples are identical and are obtained according to the same experimental protocol which follows.

[0110] The following are mixed in a kneader: [0111] 3000 g of a mixture of the two powders of silicon carbide particles with a purity of greater than 98% in the following proportions: 75% by weight of a first powder of particles exhibiting a median diameter of the order of 60 micrometers and 25% by weight of a second powder of particles exhibiting a median diameter of the order of 2 micrometers. (Within the meaning of the present description, the median diameter d.sub.50 denotes the diameter of the particles below which 50% by weight of the population of said particles occurs). [0112] 300 g of an organic binder of the cellulose derivative type.
Water, approximately 20% by weight with respect to the total weight of SiC and of organic additive, is added and kneading is carried out until a homogeneous paste is obtained, the plasticity of which makes possible the extrusion of a structure of tubular shape, the die being configured in order to obtain monolithic blocks, the channels and the external walls of which exhibit a structure according to the desired configuration which is represented in the appended FIGS. 1 and 2. More specifically, the fired monoliths exhibit round channels with a hydraulic diameter of 2 mm, the peripheral semicircular channels represented in the figures exhibiting a hydraulic diameter of 1.25 mm. The mean thickness of the external wall is 1.1 mm and the OFA (Open Front Area) of the inlet face of the filter is 37%. The OFA is obtained by calculating the ratio as percentage of the area covered by the sum of the cross sections of the channels to the total area of the corresponding cross section of the porous support.

[0113] For each configuration, 5 to 10 crude supports with a diameter of 25 mm and with a length of 30 cm are thus synthesized.

[0114] The crude monoliths thus obtained are dried by microwave radiation for a time sufficient to bring the content of not chemically bound water to less than 1% by weight.

[0115] The monoliths are subsequently fired up to a temperature of at least 2100 C., which is maintained for 5 hours. The material obtained exhibits an open porosity of 43% and a distribution mean pore diameter of the order of 25 micrometers, as measured by mercury porosimetry.

EXAMPLE 1 (COMPARATIVE)

[0116] According to this example, a separating membrane layer made of silicon carbide is subsequently deposited on the internal wall of the channels of a support structure as obtained above, according to the process described below.

[0117] A tie primer for the separating layer is formed, in a first step, from a slip, the inorganic formulation of which comprises 30% by weight of a powder of grains of black SiC (Sika DPF-C), the median diameter d.sub.50 of which is approximately 11 micrometers, 20% by weight of a powder of grains of black SiC (Sika FCP-07), the median diameter d.sub.50 of which is approximately 2.5 micrometers, and 50% of deionized water.

[0118] A slip of the material constituting the filtration membrane layer is also prepared, the formulation of which comprises 50% by weight of SiC grains (d.sub.50 of approximately 0.6 micrometer) and 50% of demineralized water.

[0119] The rheology of the slips was adjusted, by addition of the organic additives, to 0.5-0.7 Pa.Math.s under a shear gradient of 1 s.sup.1, measured at 22 C. according to the standard DIN C 33-53019.

[0120] These two layers are successively deposited according to the same process described below: the slip is introduced into a tank with stirring (20 revolutions/min). After a phase of deaerating under slight vacuum (typically 25 millibar) while continuing to stir, the tank is overpressurized to approximately 0.7 bar in order to be able to coat the interior of the support from its bottom part up to its upper end. This operation only takes a few seconds for a support with a length of 30 cm. Immediately after coating the slip over the internal wall of the channels of the support, the excess is discharged by gravity.

[0121] The supports are subsequently dried at ambient temperature for 10 minutes and then at 60 C. for 12 h. The supports thus dried are subsequently fired under argon at a temperature of 1430 C. for 4 h.

[0122] A cross section is taken over the filters thus obtained. The structure of the membrane is observed and studied with a scanning electron microscope.

EXAMPLE 2 (ACCORDING TO THE INVENTION)

[0123] According to this example, a separating membrane layer made of a composite silicon carbide/silicon nitride material is deposited on the internal wall of the channels of a support structure as described above and identical to that of example 1, according to the process described below.

[0124] A layer of tie primer for the separating layer is formed in a first step from a slip, the inorganic formulation of which comprises 30% by weight of a powder of grains of black SiC (Sika DPF-C), the median diameter d.sub.50 of which is approximately 11 micrometers, 15% by weight of a powder of grains of black SiC (Sika FCP-07), the median diameter d.sub.50 of which is approximately 5 micrometers, 5% of a silicon (Silgrain Micro 10), the median diameter d.sub.50 of which is approximately 3 m, and 50% of deionized water.

[0125] A slip for the material constituting the separating membrane layer is also prepared but the formulation of which this time comprises 36% by weight of SiC grains with a median particle diameter d.sub.50 of the order of 0.6 micrometer, 4% of metallic silicon with a median particle diameter d.sub.50 of approximately 3 microns and 60% of deionized water.

[0126] The rheology of the slips is adjusted to 0.5-0.7 Pa.Math.s at 1 s.sup.1. In order to control the rheology of these slips and to observe a viscosity typically approximately Pa.Math.s under a shear gradient of 1 s.sup.1, measured at 22 C. according to the standard DIN C 33-53019. These layers are deposited according to the same process as for example 1. The coated supports are subsequently fired under nitrogen according to a temperature rise of the order of 10 C./h up to 1430 C. under stationary conditions for 4h.

EXAMPLE 3 (ACCORDING TO THE INVENTION)

[0127] According to this example, the procedure is identical to that of example 2 but 0.04% of iron oxide Fe.sub.2O.sub.3 provided by Bayferrox with a median diameter of approximately 0.7 micrometer, i.e. 0.5% with respect to the weight of silicon, is added to the slip for the material constituting the separating membrane layer.

EXAMPLE 4 (COMPARATIVE)

[0128] According to this example, the procedure is identical to that of example 2 but amounts by weight of 8% of metallic silicon and 32% of SiC grains per 60% of demineralized water are introduced into the slip in order to form the material of the separating membrane layer.

[0129] Likewise, the primer layer was adapted with the same silicon content, such that its inorganic formulation comprises 30% by weight of a powder of grains of black SiC (Sika DPF-C), the median diameter d.sub.50 of which is approximately 11 micrometers, 12% by weight of a powder of grains of black SiC (Sika FCP-07), the median diameter d.sub.50 of which is approximately 5 micrometers, 8% of silicon (Silgrain Micro 10), the median diameter d.sub.50 of which is approximately 3 m, and 50% of deionized water.

EXAMPLE 5 (COMPARATIVE)

[0130] According to this example, the procedure is identical to that of example 2 but the sintering temperature is brought to 1800 C. for 2 hours under nitrogen.

EXAMPLE 6 (COMPARATIVE)

[0131] According to this example, the procedure is identical to that of the preceding example 2 but the final firing of the coated supports is carried out this time at a temperature of 1100 C. for 2 hours and under pure nitrogen. This example thus appears in accordance with the teaching of the applications EP 0 219 383 and EP 2 484 433 for the preparation of an SiC membrane filter.

[0132] The properties and the characteristics of the filters thus obtained are measured as follows.

[0133] The mean thickness of the successive layers obtained for each example is measured by image analysis on the basis of the electron microscopy photographs.

[0134] The mean thickness of the separating layer is of the order of 40 micrometers for all the examples. The median pore diameter of the separating membrane layer varies between 200 and 250 nm for all the examples.

[0135] The other results as measured as indicated above are given in the following table 1.

[0136] The details of other experimental protocols followed are given additionally below: [0137] a) A measurement of flow (relative flow rate of water) is carried out on the filters according to the following method: [0138] At a temperature of 25 C., a fluid composed of demineralized water feeds the filters to be evaluated under a transmembrane pressure of 0.5 bar and a rate of circulation in the channels of 2 m/s. The permeate (the water) is recovered at the periphery of the filter. The measurement of the flow rate characteristic of the filter is expressed in 1/min per square meter of filtration surface area after filtering for 20 h. In the table, the flow rate results have been expressed with reference to the data recorded for comparative example 1. More specifically, a value of greater than 100% indicates an increased flow rate with respect to the reference (example 1) and thus an increase in the filtration capacity. [0139] In the case of the measurement of flow rate under demineralized water and salts, the demineralized feed water contained a load of 5.10.sup.3 mol/l of KCl. [0140] b) The measurement of the depth of scratching of the separating membrane layer, an essential longevity factor of the filter, also known as scratch test, is carried out using a Rockwell C diamond spheroconical point forming a conical angle of 120, the radius of curvature of the point being 200 microns. The point is driven at an unchanging rate of 12 mm/min according to an incremental load of 1N per step of 1 mm over a measurement length of 6 mm. Several passes can be carried out. The deterioration in the coating is a combination of the elastic and/or plastic indentation stresses, of the frictional stresses and of the residual internal stresses within the layer of material of the coating. The depth of penetration of the indenter is measured after a sixth pass at the 4N step. The degree of depth of scratching was measured as percentage with respect to the reference (example 1) set at 100. The degree of resistance of examples 2 to 5 is calculated by determining the ratio of depth of the indenter of the example divided by the depth of the indenter measured with regard to example 1, a degree of less than 100% representing a greater scratch resistance than the reference. [0141] c) The resistance to chemical attack was determined by immersing a sample of the separating membrane layer in a beaker filled with a 0.1M HCl solution at 80 C. for 24h with gentle stirring. The nitrogen content of the solution is measured by ion-exchange chromatography. The degree of deterioration of the membrane is measured by the loss of nitrogen with reference to the initial nitrogen content of the membrane, before the chemical attack of HCl. [0142] A degree of resistance of 100% is set for the reference example (example 1). A degree of less than 100% corresponds to the degree of deterioration of the membrane with respect to the reference.

[0143] The characteristics and the properties of the filters obtained according to examples 1 to 6 are given in table 1 below.

Other tests carried out by the applicant company have shown that the composition of the primer had no or virtually no influence on the properties described above of filtration and of durability of the separating membrane.

TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 (comp.) (inv.) (inv.) (comp.) (comp.) (comp.) Content by weight >99.0 84.5 83.1 67.6 >99.0 >98.5 of SiC in the membrane (%)* Content by weight <0.05 5.1 5.7 11.4 0.1 <0.05 of elemental nitrogen in the membrane (%)** Content of residual nd 1.2 0.5 2.0 nd nd silicon in the membrane (%)*** Type/Content of Fe.sub.2O.sub.3/ catalyst [wt %/wt [0.5%] initial Si] N/SiC ratio by <0.005 0.06 0.07 0.17 <0.005 <0.02 weight of the membrane Content by weight 0.5 0.8 1.0 nm 0.2 >0.5 of elemental oxygen in the membrane (%)** Firing of the 1430 C./4 h/ 1430 C./4 h/ 1430 C./4 h/ 1430 C./4 h/ 1800 C./2 h/ 1100 C./2 h/ membrane Ar N.sub.2 N.sub.2 N.sub.2 N.sub.2 N.sub.2 Mean thickness of 45 45 45 45 45 45 the separating membrane (micrometers) Median pore 190 190 nm nm 650 200 diameter of the separating membrane (nm) Degree of 100 67 59 91 100 >>150 scratching of the membrane Measurement of 100 155 145 150 120 nm flow rate relative to demineralized water Measurement of 100 nm 275 nm nm nm flow rate relative to demineralized water + salts Resistance to 100 92 98 79 nm nm chemical attack, 80 C., pH1 (HCl) nd = not determined; nm = not measured *Measured according to standard ANSI B74.15-1992-(R2007) **Measured by Leco ***Measured according to standard ANSI B74-151992 (R2000)

[0144] The results combined in the preceding table 1 indicate that examples 2 and 3 according to the invention exhibit the best combined performances in the different tests and measurements carried out. In particular, the filters having a filtering membrane according to the invention exhibit a high mechanical strength (scratch test) and also a greater filtration capacity. In addition, they appear to be more resistant to acid attacks.

[0145] According to example 5 according to the invention, it is observed that an excessively high firing temperature prevents the formation of the nitride and results finally in nitrogen contents which are too low to obtain the desired improvement. In the end, the results combined in the table indicate that the material used according to the invention to manufacture the separating membrane layer can only be obtained following certain processing conditions not yet described in the prior art.

[0146] The comparative example 6 (for which the temperature of calcination under nitrogen is only 1100 C.) exhibits a very high degree of scratching, that is to say a low mechanical strength. The data given in table 2 thus show that such a temperature, which is too low, does not make possible the insertion of elemental nitrogen into the material constituting the membrane.