FILTER COMPRISING A SILICONE CARBIDE SEPARATOR LAYER

Abstract

A filter for the filtration of a fluid, such as a liquid, includes or is constituted by a support element made from a porous ceramic material, at least a portion of the surface of the support element being covered with a porous membrane separating layer, the membrane separating layer being constituted essentially of silicon carbide (SiC), its porosity being between 10% and 70% by volume, the median diameter of its pores being between 50 nanometers and 500 nanometers, its mean thickness being between 1 micrometer and 30 micrometers, and its tortuosity being less than 1.7.

Claims

1 A filter for the filtration of a fluid, comprising or constituted by a support element made from a porous ceramic material, at least a portion of a surface of said support element being covered with a membrane separating layer, wherein: said membrane separating layer is constituted essentially of silicon carbide (SiC), a porosity of the membrane separating layer is between 10% and 70% by volume, a median pore diameter in the membrane separating layer is between 50 nanometers and 500 nanometers, a mean thickness of the membrane separating layer is between 1 micrometer and 30 micrometers, a tortuosity of the membrane separating layer is less than 1.7.

2. The filter as claimed in claim 1, wherein a median size of the grains constituting the membrane separating layer is less than 400 nm and greater than 80 nm.

3. The filter as claimed in claim 2, wherein the median size of the grains constituting the membrane separating layer is less than 300 nm.

4. The filter as claimed in claim 1, wherein the mean thickness of the separating layer is between 4 and 15 micrometers.

5. The filter as claimed in claim 1, wherein the median pore diameter in the separating layer is between 100 and 300 nm.

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

7. The filter as claimed in claim 1, additionally comprising one or more intermediate layers positioned between the material constituting the support element and the material constituting the membrane separating layer.

8. The filter as claimed in claim 1, wherein the intermediate layer or layers are essentially constituted of SiC.

9. The filter as claimed in claim 1, wherein at least one intermediate layer exhibits a median pore diameter at least two times greater than that of the membrane separating layer.

10. The filter as claimed in claim 1, wherein the intermediate layer or layers exhibit(s) a mean thickness of between 5 and 100 micrometers.

11. The filter as claimed in claim 1, wherein said support element is provided in the form of a plate, of a disk, of a tube or of a parallelepiped.

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

13. The filter as claimed in claim 1, wherein an open porosity of the material constituting the support element is between 20% and 70%.

14. The filter as claimed in claim 1, wherein the tortuosity of the support element is greater than 1 and/or less than 2.

15. A process for the manufacture of a filter as claimed in claim 1, comprising: preparing a slip starting from a powder of silicon carbide particles with a mean size of between 0.05 and 0.4 micrometers, applying said slip to a support element, under conditions making possible the formation of a layer of the slip on at least a part of an external surface of said support, drying then firing under a nonoxidizing, atmosphere at a temperature of between 1400° C. and 1600° C., and in said temperature range for a time sufficient to obtain the membrane separating layer, said layer being constituted essentially of silicon carbide.

16. The process for the manufacture of a filter as claimed in claim 15 , additionally comprising, before applying said slip, depositing at least one intermediate layer starting from a slip comprising a powder of silicon carbide particles, at least a fraction of the grains of which exhibits a median diameter of less than 1 micrometer and greater than 0.1 micrometer.

17. A method comprising filtrating a liquid with a filter as claimed in claim 1.

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

19. The filter as claimed in claim 1, wherein the tortuosity of the membrane separating layer is less than 1.5.

20. The filter as claimed in claim 3, wherein the median size of the grains constituting the membrane separating layer is less than 250 nm.

Description

[0114] 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.

[0115] In the appended figures:

[0116] FIG. 1 illustrates a conventional configuration of a tubular filter according to the current art, along a cross section plane P.

[0117] FIG. 2 is a microscopy photograph of a filter showing the membrane separation layer within the meaning of the present invention.

[0118] FIG. 1 illustrates a crosswise 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 cross section plane P. The filter comprises or most often is constituted by a support element 1 made from a porous, preferably non-oxide, inorganic material. The element conventionally exhibits a tubular form with a longitudinal central axis A, 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 formed from a porous inorganic material allowing 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 membrane separating layer 5 deposited on a tie intermediate, as illustrated by the electron microscopy photograph shown in FIG. 2. This membrane separating layer 5 (or membrane) comes into contact with said fluid circulating in said channels and makes possible the filtration thereof.

[0119] In FIG. 2, an electron microscopy photograph taken on a channel 4 of FIG. 1 has been shown. The porous support 100 of high porosity, a first intermediate layer 101, a second intermediate layer 102 making possible the attachment of the membrane separating layer 103 of finer porosity, are observed in this figure.

[0120] According to another configuration, not represented, of another filter according to the invention, the latter is configured in order for the fluid to be treated to initially pass through the external wall, the permeate being collected by a reception means. According to such a configuration, the membrane filtering layer is advantageously deposited on the external surface of the filter and covers at least a part of it.

[0121] Such a configuration is often referred to as FSM (Flat Sheet Membrane). Reference can be made to the publication available on the website: http://www.liqtech.com/img/user/file/FSM_Sheet_F_4_260214V2 .pdf.

[0122] The examples which follow are provided solely by way of illustration. They are not limiting and make it possible to better understand the technical advantages associated with the implementation of the present invention.

[0123] 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: [0124] 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 is found.) [0125] 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 and as represented in the appended FIG. 1.

[0126] More specifically, the fired monoliths exhibit round channels with a hydraulic diameter of 2 mm, the peripheral half-moon 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.

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

[0127] The raw monoliths thus obtained are dried by microwave for a time sufficient to bring the content of non-chemically-bonded water to less than 1% by weight.

[0128] 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 mean pore distribution diameter of the order of 25 micrometers, as measured by mercury porosimetry.

[0129] The grains exhibit a median size of approximately 20 micrometers.

EXAMPLE 1 (COMPARATIVE ACCORDING TO WO2016097661A1)

[0130] According to this example, a membrane separating 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: An intermediate layer for attachment of the separating layer is formed, as a first step, from a slip, the inorganic formulation of which comprises 30% by weight of a powder of SiC grains (Sika DPF-C), the median diameter D.sub.50 of which is approximately 11 micrometers, 20% by weight of a powder of SiC grains (Sika FCP-07), the median diameter D.sub.50 of which is approximately 2.5 micrometers, and 50% of deionized water.

[0131] A slip of the material constituting the membrane filtration layer is also prepared, the formulation of which comprises 40% by weight of SiC grains (d.sub.50 around 0.6 micrometers) and 60% of deionized 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 DINC33-53019.

[0132] These two layers are deposited successively according to the same process described below: the slip is introduced into a tank with stirring (20 revolutions/min). After a phase of deaeration under a gentle vacuum (typically 25 millibar) while maintaining agitation, the tank is put under an excess pressure of approximately 0.7 bar in order to be able to coat the inside 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 with the slip on the internal wall of the channels of the support, the excess is discharged by gravity.

[0133] The supports are subsequently dried at ambient temperature for 10 minutes, then at 60° C. for 12 h. The supports, thus dried, are subsequently fired under nitrogen at a temperature of 1600° C. for 2 h at ambient pressure.

[0134] A cross section is produced on the filters thus obtained. The structure of the membrane is observed and studied with a scanning electron microscope according to the methods described below.

EXAMPLE 2 (COMPARATIVE)

[0135] According to this example, the procedure is identical to that of example 1 but the firing of the coated supports is carried out at 1600° C. under argon, the time interval being 30 minutes between 1400 and 1600° C.

EXAMPLE 3 (COMPARATIVE)

[0136] According to this example, the procedure is identical to that of example 1 but the firing of the coated supports is carried out at 1550° C. under argon, the time interval being 30 minutes between 1400 and 1550° C.

EXAMPLE 4 (ACCORDING TO THE INVENTION)

[0137] According to this example, the procedure is identical to that of the preceding example 3 but the coated supports are fired under argon at 1650° C. instead of 1550° C., the time interval being 30 minutes between 1400 and 1650° C. A third layer which becomes the separating layer is subsequently deposited. This layer is obtained by deposition of a slip, the formulation of which corresponds to 7% by weight of a powder of green SiC grains, the median diameter D.sub.50 of which is approximately 0.18 micrometer, corresponding to a specific surface of 50 m.sup.2/g, and the remainder being deionized. In particular for the slip of this separating layer, the pH is adjusted to between 9 and 10 by the addition of sodium hydroxide solution. In order to thicken the slip, to control the sedimentation and to obtain a film-forming effect for a good surface appearance after drying, 0.01 to 0.03 of Tylose MH4000P2 is added with respect to the amount of water. A mean thickness of 7 micrometers is deposited by monitoring the weight of the slip fixed to the substrate. The supports, thus coated, are fired again up to 1550° C. under argon at ambient pressure, the time interval being 30 minutes between 1400 and 1550° C.

EXAMPLE 5 (ACCORDING TO THE INVENTION)

[0138] According to this example, the procedure is identical to that of example 3 but: [0139] the separating layer of grains, the median grain diameter D.sub.50 of which is approximately 0.18 micrometer, is deposited directly on a single intermediate tie layer formed from a slip, the inorganic formulation of which comprises 18% by weight of a powder of SiC grains (Sika DPF-C), the median diameter D.sub.50 of which is approximately 11 micrometers, 9% by weight of a powder of SiC grains (Sika 1500F), the median diameter D.sub.50 of which is approximately 2 micrometers, 3% by weight of a powder of SiC grains, the median diameter D.sub.50 of which is approximately 0.18 micrometer, the remainder being 70% of deionized water, and [0140] the separating layer is obtained by deposition of a slip, the formulation of which exhibits 7% by weight of a powder of green SiC grains, the median diameter D.sub.50 of which is approximately 0.18 micrometer, prepared as in example 4. A mean thickness of 7 micrometers is deposited by monitoring the weight of the slip fixed to the substrate.

[0141] The supports, thus coated, are fired again at ambient pressure at 1550° C. under argon, the time interval being 30 minutes between 1400 and 1550° C.

EXAMPLE 6 (COMPARATIVE)

[0142] Unlike the previous example, the intermediate layer is formed from a slip, the inorganic formulation of which comprises 30% by weight of a powder of SiC grains (Sika DPF-C), the median diameter D.sub.50 of which is approximately 11 micrometers, 20% by weight of a powder of SiC grains (Sika FCP-07), the median diameter D.sub.50 of which is approximately 2.5 micrometers, and 50% of distilled water. A separating layer identical to that of the preceding example is deposited on the intermediate layer by monitoring the weight of the slip fixed on the substrate.

EXAMPLE 7 (COMPARATIVE)

[0143] Unlike example 3, the final firing of the supports, thus coated, is carried out this time at a temperature of 1100° C. for 2 hours under pure nitrogen.

[0144] This example thus appears in accordance with the teaching of the application published under FR 2 587 026 A1 for the production of a membrane filter made of SiC.

EXAMPLE 8 (COMPARATIVE)

[0145] Unlike the preceding example, the final firing of the coated supports is carried out this time at a temperature of 1700° C. for 2 hours under argon according to the teaching of EP 2 559 470 A1 for the production of a membrane made of SiC.

EXAMPLE 9 (COMPARATIVE)

[0146] Unlike example 6, the separating layer is obtained from a slip, the inorganic composition of which is as follows: 20% by weight of grains of the powder of α-SiC, the median diameter d.sub.50 of which is of the order of 0.6 micrometer, 53% by weight of the powder of metallic silicon grains, the median diameter d.sub.50 of which is approximately 4 micrometers, 27% of amorphous carbon powder, the median diameter D.sub.50 of which is approximately 1 micrometer.

[0147] The supports, dried as described above, are finally fired under argon at a temperature of 1470° C. for 4 h at ambient pressure.

[0148] The characteristics and the properties of the filters thus obtained are measured as follows:

[0149] For each example, a sample of membrane on its support is placed in a scanning electron microscope equipped with a focused ion probe (FIB or focused ion beam). Slices are made in the thickness of the membrane layer along parallel planes making possible the observation of different sectional planes in order to represent a cross section of the membrane layer. In order to obtain a very good resolution between the ceramic walls and the porosity of the membrane throughout its thickness, a photograph of at least one million pixels is taken using the electron microscope according to a mode using secondary electrons (SE mode). The magnification used is such that the width of the image is between ten and one hundred times the mean size of the particles of the membrane separating layer, approximately 50 times according to the examples. On the basis of these photographs, a segmentation is carried out using ImageJ software in order to differentiate the porosity of the grains, attention being paid to the depth of field. For each photograph, the surface proportion of porosity of the membrane layer is determined. The mean of the proportions of the photographs taken themselves along in different planes within the membrane is a value representative of the volume of porosity of this membrane.

[0150] The tortuosity of examples 3 to 6 was determined using the iMorph© software.

[0151] The results are given in table 1, as are the following properties for which the experimental protocols followed are described below: [0152] a) A measurement of specific flow or flow rate (relative water flow rate) is carried out on the filters according to the following method: At a temperature of 25° C., a fluid consisting of demineralized water charged with 5.10.sup.−3 mol/l of KCl feeds the filters to be evaluated under a transmembrane pressure of 0.5 bar and a circulation speed in the channels of 2 m/s. The permeate (water) is recovered at the periphery of the filter. The measurement of the characteristic flow rate of the filter is expressed in 1/min per filtration surface area in m.sup.2 after 20 hours of filtration. In the table, the flow rate results have been expressed by reference to the data recorded for comparative example 3. More specifically, a value of greater than 100% indicates an increased flow rate with respect to the reference (example 3) and thus an improvement in the filtration capacity. [0153] b) The abrasion resistance is measured by subjecting a filter produced according to the preceding examples a slip comprising 2 g/liter of a SiC powder with a median diameter of 250 micrometers passing through the channels of the filter according to a speed of 2 m/second. The loss of permeability (standard to be supplied) is measured after 20 hours of testing. Example 3 is regarded as reference (100). A lower value than 100, for example of 80, means a relative loss of permeability, lower by 20% with respect to the reference, and thus a better abrasion resistance. [0154] c) The test of resistance to intensive backwashing consists in subjecting the filter to 1000 pulses of water under a pressure of 3 bar/second every minute so that the liquid passes through the porous walls countercurrentwise. The increase in permeability which can result from degradation of the membrane is measured. Example 3 is taken as reference (100). A lower value than 100, for example of 80, means a relative loss of permeability, lower by 20% with respect to the reference (and thus a better resistance to the mechanical stresses brought about by backwashing). [0155] d) Turbidity test:

[0156] The procedure is carried out according to the following method: Synthetic dirty water comprising clay, salt, oil and surfactants, according to contents equal to 100 ppm, 4000 ppm, 300 ppm and 2 ppm respectively, is used. The dirty water feeds, at a constant temperature of 25° C., the filters to be evaluated under a transmembrane pressure of 0.5 bar and a circulation speed in the channels of 3 m/s. The filtrate (purified water) is recovered at the periphery of the filter. In order to estimate the filtration performance of the filter, the turbidity of the filtrate is measured continuously by means of a turbidimeter of Beam-Turbidy Meter Series LAT N1 type supplied by Kobold Instrumentation, after filtration cycles. Example 3 is regarded as reference (100). A higher value after the test on the turbidity thus corresponds to a poorer quality of filtration of the liquid.

[0157] The characteristics and the properties of the filters and of the membrane separating layer (designated in the table below by membrane) obtained according to examples 1 to 9 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.) (comp.) (invention) (invention) (comp.) Intermediate DPF-C (11 μm) DPF-C (11 μm) DPF-C (11 μm) DPF-C (11 μm) DPF-C (11 μm) DPF-C (11 μm) layer No. 1 (d.sub.50 FCP07 (2.5 μm) FCP07 (2.5 μm) FCP07 (2.5 μm) FCP07 (2.5 μm) 1500F (2 μm) FCP07 (2.5 μm) powders) SIC (0.18 μm) Intermediate no no no SiC no no layer No. 2 (d.sub.50 (0.6 μm) powder) Intermediate no no no yes no no firing (1650° C./Ar) Separating SiC SiC SiC SiC SiC SiC layer (0.6 μm) (0.6 μm) (0.6 μm) (0.18 μm) (0.18 μm) (0.18 μm) (d.sub.50 powder) T.sub.max firing of 1600° C./Ar 1600° C./Ar 1550° C./Ar 1550° C./Ar 1550° C./Ar 1550° C./Ar the separating layer Characteristics of the final membrane filter after firings SiC content by 99.3 >99.0 >99.0 >99.0 >99.0 >99.0 weight of the membrane (%) Cumulative 86 83 85 95 52 51 mean thickness of the separating and intermediate layers (micrometers) Intermediate layer after firing Median pore 1500 1000 1000 2000 900 2000 diameter (nm) Pore volume 42 42 40 45 42 45 (%) Median grain 10 10 10 10 5 10 size (micrometers) Membrane separating layer after firing Mean 45 35 35 7 7 2 thickness (micrometers) Median pore 250 413 249 236 242 550 diameter (nm) Pore volume 40 40 31 45 44 52 (%) Tortuosity no no 1.8 1.4 1.4 1.2 index measured measured Median grain 605 620 600 185 188 223 size (nm or nanometers) Properties of the membrane filter Specific flow 95 150 Ref. = 100 150 140 148 rate index Abrasion 98 50 Ref. = 100 36 35 34 resistance index Resistance 87 45 Ref. = 100 25 26 88 index after 1000 backwashings (pulse 3 bar/sec) Turbidity index Similar to A little Reference Similar to Similar to Too high the higher than the the with reference the reference reference respect to reference the reference Example 7 Example 8 Example 9 (comp.) (comp.) (comp.) Intermediate layer No. DPF-C (11 μm) DPF-C (11 μm) DPF-C (11 μm) 1 (d.sub.50 powders) FCP07 (2.5 μm) FCP07 (2.5 μm) FCP07 (2.5 μm) Intermediate layer No. no no no 2 (d.sub.50 powder) Intermediate firing no no (1650° C./Ar) Separating layer SIC SIC SIC (0.6 p.m) (d.sub.50 powder) (0.6 μm) (0.6 μm) Si (4 μm) C (1 μm) T.sub.max firing of the 1100° C./2 h/N.sub.2 1700° C./2 h/Ar 1470° C./4 h/Ar separating layer Characteristics of the final membrane filter after firings SiC content by weight >98.5 >99.0 >99.0 of the membrane (%) Cumulative mean 85 81 80 thickness of the separating and intermediate layers (micrometers) Intermediate layer after firing Median pore diameter 900 1650 1000 (nm) Pore volume (nm) 39 44 42 Median grain size 10 11 10 (micrometers) Membrane separating layer after firing Mean thickness 45 25 30 (micrometers) Median pore diameter 200 800 220 (%) Pore volume (%) 31 45 40 Tortuosity index 1.9 1.2 >1.9 Median grain size 600 620 350 (nm or nanometers) Properties of the membrane filter Specific flow rate index 85 250 105 Abrasion resistance 125 29 98 index Resistance index 112 10 120 after 1000 backwashings (pulse 3 bar/sec) Turbidity index Similar to the Too high with Similar to the reference respect to the reference reference

[0158] The results collated in table 1 which precede indicate that examples 4 and 5 according to the invention exhibit the best combined performance qualities in the various tests and measurements carried out. The membrane filters of these examples have in particular a much better resistance to backwashing while exhibiting a very high abrasion resistance and a very high specific flow rate without deterioration in their selectivity.

[0159] In the end, the results collated in the table indicate that the material used according to the invention to manufacture the membrane separating layer can only be obtained following certain processing conditions, not yet described in the prior art.