FILTERS COMPRISING SIC MEMBRANES INCORPORATING NITROGEN

Abstract

A filter for the filtration of a fluid, such as a liquid, includes or is composed of a support element made of a porous ceramic material, the element exhibiting a tubular or parallelepipedal shape delimited by an external surface and including, in its internal portion, a set of adjacent channels with axes parallel to one another and separated from one another by walls of the porous inorganic material, in which at least a portion of the channels and/or at least a portion of the external surface are covered with a porous separating membrane layer, wherein the separating membrane layer is made of a material essentially composed of silicon carbide (SiC), and the content by weight of elemental nitrogen of the layer constituting the porous separating membrane layer is between 0.1% and 2%.

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 essentially composed of silicon carbide (SiC), a content by weight of elemental nitrogen of the porous separating membrane layer is between 0.1% and 2%.

2. The filter as claimed in claim 1, wherein the content by weight of elemental nitrogen of the layer constituting the porous separating membrane layer is between 0.1% and 1.5%.

3. The filter as claimed in claim 1, wherein a porosity of the porous separating membrane layer is between 30 and 70%.

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

5. The filter as claimed in claim 1, wherein a median size of SiC grains in said material essentially composed of silicon carbide is between 20 nanometers and 10 micrometers.

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

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

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

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

10. The filter as claimed in claim 1, wherein elemental nitrogen is present at grain boundaries and in the SiC grains constituting the porous separating membrane layer.

11. The filter as claimed in claim 1, wherein the SiC represents more than 95% of the weight of the material constituting the porous separating membrane layer.

12. A separating membrane layer as described in claim 1, made of a material essentially composed of silicon carbide (SiC), said silicon carbide additionally containing nitrogen, a content by weight of elemental nitrogen in said porous separating membrane layer being between 0.1% and 2%.

13. A process for the manufacture of a separating membrane layer as claimed in claim 12, in a tangential or frontal filter, comprising: preparing a slip from a powder of silicon carbide particles having a median size of between 20 nanometers and 10 micrometers, 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 an atmosphere containing nitrogen at a temperature of between 1400° C. and 2000° C. and for a time sufficient to obtain a separating membrane layer on their internal surface of said channels, said layer being essentially composed of silicon carbide containing nitrogen, a content by weight of elemental nitrogen in said layer being between 0.1% and 2%.

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

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

16. The filter as claimed in claim 6, wherein the content by weight of elemental oxygen of the material constituting the porous separating membrane layer is less than or equal to 0.5%.

17. The filter as claimed in claim 7, 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.

18. The method as claimed in claim 14, wherein the filter is utilized for filtering an aqueous liquid.

Description

[0102] The figures associated with the examples which follow are provided in order to illustrate the invention and its advantages, without, of course, the embodiments thus described being able to be regarded as limiting of the present invention.

[0103] In the appended figures:

[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] According to another configuration, not represented, of another filter according to the invention, this other filter is configured in order for the fluid to be treated to initially pass through the external wall, the permeate being collected this time at the outlet of the channels. According to such a configuration, the filtering membrane layer is advantageously deposited on the external surface of the filter and covers at least a portion of it.

[0109] Such a configuration is often known as FSM (Flat Sheet Membrane). Reference may be made to the publication available on the website: http://www.liqtech.com/img/user/file/FSM_Sheet_F_4_260214 V2.pdf.

[0110] 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.

[0111] The supports according to all the examples are identical and are obtained according to the same experimental protocol which follows:

The following are mixed in a kneader: [0112] 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). [0113] 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.

[0114] 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.

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

[0115] 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.

[0116] 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)

[0117] 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: 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 40% by weight of SiC grains (d.sub.50 of approximately 0.6 micrometer) and 60% of demineralized water.

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.

[0119] 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.

[0120] 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 1600° C. for 2 h at ambient pressure.

[0121] 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 (COMPARATIVE)

[0122] According to this example, the procedure is identical to that of example 1 but the filter is finally fired under argon at a temperature of 1800° C., for 2 h and at ambient pressure.

EXAMPLE 3 (ACCORDING TO THE INVENTION)

[0123] According to this example, the procedure is identical to that of example 1 but the filter is finally fired under nitrogen (N.sub.2) at a temperature of 1800° C., for 2 h and at ambient pressure.

EXAMPLE 4 (ACCORDING TO THE INVENTION)

[0124] According to this example, the procedure is identical to that of example 1 but the filter is finally fired under nitrogen (N.sub.2) at a temperature of 1600° C., for 2 h and at ambient pressure.

EXAMPLE 5 (ACCORDING TO THE INVENTION)

[0125] According to this example, the procedure is identical to that of example 1 but the filter is then subjected to a supplementary heat treatment consisting in a firing at 1200° C. for two hours under an atmosphere of 5% H.sub.2/95% N.sub.2 by volume.

EXAMPLE 6 (COMPARATIVE)

[0126] 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.

[0127] The properties and the characteristics of the filters thus obtained are measured as follows:

[0128] The mean thickness of the successive layers obtained for each example is measured by image analysis on the basis of the electron microscopy photographs.

[0129] The mean thickness of the separating layer is of the order of 45 micrometers for all the examples. The median pore diameter of the separating membrane layer varies between 250 and 1100 nm according to the examples.

[0130] The other results as measured as indicated above are given in the following table 1.

[0131] The details of other experimental protocols followed are given additionally below: [0132] a) A measurement of flow (relative flow rate of water) is carried out on the filters according to the following method: [0133] At a temperature of 25° C., a fluid composed of demineralized water containing a load of 5×10.sup.−3 mol/l of KCL 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 filtration surface area in m.sup.2 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 improvement in the filtration capacity. [0134] 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.

[0135] The characteristics and the properties of the filters and of the separating membrane layer (designated by membrane in the table below) obtained according to examples 1 to 6 are given in table 1 below:

TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 (comp.) (comp.) (inv.) (inv.) (inv.) (comp.) Content by weight >99.0 >99.0 >99.5 99.3 99.3 >98.5 of SiC of the membrane (%) Content by weight <0.05 <0.05 0.11 0.36 0.42 <0.05 of elemental nitrogen of the membrane (%) Content by weight 0.5 0.5 0.15 0.25 0.16 >0.5 of elemental oxygen of the membrane (%) Firing of the 1600° C./2 h 1800° C./2 h 1800° C./2 h 1600° C./2 h 1600° C./Ar 1100° C./2 h/ membrane Ar Ar N.sub.2 N.sub.2 1200° C./2 h/ N.sub.2 H.sub.2—N.sub.2 Mean thickness of 45 45 45 45 45 45 the separating membrane (micrometers) Median pore 600 1100 650 250 600 200 diameter of the separating membrane (nm) Degree of 100 63 65 85 90 >>150 scratching of the membrane Measurement of 100 150 140 80 135 not flow rate relative measured to KCl demineralized water

[0136] The results combined in the preceding table 1 indicate that examples 3 and 4 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 very high mechanical strength (scratch test) in comparison with the reference (example 1).

[0137] If reference is made to example 3 according to the invention compared to the reference example 1, the filter according to the invention exhibits a very superior filtration capacity, the sizes of the pores being substantially identical for both samples. Such measurements indicate a substantially increase in the filtration performances, while retaining the same selectivity.

[0138] If reference is made to example 4 according to the invention compared to the reference example 1, it is observed that it became possible, by application of the present invention, to offer highly selective filters, i.e. filters having a very fine pore size, while maintaining an acceptable filtration capacity.

Examples 3 and 4 are also characterized by the high mechanical strength of the filtering membrane layer obtained according to the invention, such an improvement logically leading to a considerably longer expected service life of the filter without significant deterioration of the filtration performances (flow rate, selectivity, etc.)
Example 5 according to the invention shows that the alternative mode of obtaining the membrane layer described above results in the same improvements, especially in terms of flow of permeate at the outlet of the filter.

[0139] 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.

[0140] 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.