FILTERS COMPRISING OXYGEN-DEPLETED SiC MEMBRANES

Abstract

A filter for the filtration of a fluid, such as a liquid, includes or composed of a support element made of a porous ceramic material, the element having 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, wherein 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 layer is made of a material essentially composed of sintered grains of silicon carbide (SiC), and the weight content of elemental oxygen of the layer is less than 0.5%.

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 having 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, wherein: 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; said porous separating membrane layer is made of a material essentially composed of sintered grains of silicon carbide (SiC), a weight content of elemental oxygen of the porous separating membrane layer is less than 0.5%.

2. The filter as claimed in claim 1, wherein the SiC represents more than 97% of the weight of the material constituting the porous separating membrane layer.

3. The filter as claimed in claim 1, wherein an atomic concentration of oxygen measured by XPS at a surface of the SiC grains is less than 10%, on the basis of the total amount of the elements Si, C and O.

4. The filter as claimed in claim 1, wherein a carbon/oxygen ratio measured by XPS at a surface of the SiC grains is greater than 4.

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

6. The filter as claimed in claim 5, wherein a median size of the SiC grains in said material is between 20 nanometers and 10 micrometers.

7. The filter as claimed in claim 1, wherein the weight content of elemental oxygen of the material constituting the porous separating membrane layer is less than or equal to 0.4%.

8. 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.

9. 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.

10. 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.

11. A separating membrane layer as described in claim 1, made of a material essentially composed of sintered grains of silicon carbide (SiC), the weight content of elemental oxygen of the layer being less than 0.5%.

12. The separating membrane layer as claimed in claim 11, wherein an atomic concentration of oxygen measured by XPS at a surface of the SiC grains is less than 10%, on the basis of the total amount of the elements Si, C and O.

13. A process for manufacturing a separating membrane layer as claimed in claim 11, in a tangential or frontal filter, comprising: preparing a slip from a powder of silicon carbide particles having a mean size of between 20 nanometers and 10 micrometers, applying said slip to the support element, under conditions that enable 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 inert gas atmosphere at a temperature between 1400° C. and 2000° C. and for a time sufficient to obtain a separating membrane layer on their internal surface of said channels essentially composed of sintered silicon carbide grains, performing a treatment for eliminating some of the residual elemental oxygen present at a surface of said grains.

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

15. The filter as claimed in claim 7, wherein the weight content of elemental oxygen of the material constituting the porous separating membrane layer is less than or equal to 0.3%.

16. The filter as claimed in claim 8, 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.

17. The process as claimed in claim 13, wherein the treatment for eliminating some of the residual elemental oxygen present at the surface of said grains is performed by the action of hydrofluoric acid or by a heat treatment under a hydrogen-containing reducing atmosphere.

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

Description

[0101] In the appended figures:

[0102] FIG. 1 illustrates a conventional configuration of a tubular filter according to the current art, along a transverse cross-sectional plane P.

[0103] FIG. 2 is a microscopy photograph of a filter showing the separation membrane layer within the meaning of the present invention.

[0104] 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 schematic view of the transverse cross-sectional plane P. The filter comprises or generally is composed of a support element 1 made of a porous inorganic material, preferably a non-oxide material. The element conventionally has 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 an adhesion 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 enables the filtration thereof.

[0105] An electron microscopy photograph taken on a channel 4 of FIG. 1 has been given in FIG. 2. Seen in this figure are the porous support 100 of high porosity and the primer layer 102 enabling the attachment of the separating membrane layer 103 of finer porosity.

[0106] According to another configuration, not represented, of another filter according to the invention, this other filter is configured so that the fluid to be treated initially passes 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 thereof.

[0107] The following examples are provided solely by way of illustration. They are not limiting and make it possible to better understand the technical advantages relating to the use of the present invention:

[0108] The supports according to all the examples are identical and are obtained according to the same experimental protocol which follows:

[0109] Mixed in a mixer are: [0110] 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 having a median diameter of the order of 60 micrometers and 25% by weight of a second powder of particles having 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). [0111] 300 g of an organic binder of the cellulose derivative type.
Water is added in an amount of around 20% by weight relative to the total weight of SiC and of organic additive and mixing is carried out until a homogeneous paste is obtained, the plasticity of which allows the extrusion of a structure of tubular shape, the die being configured for obtaining monolith blocks, the channels and the external walls of which have a structure according to the desired configuration as represented in the appended FIGS. 1 to 2. Thus, 5 to 10 green supports having a diameter of 25 mm and a length of 30 cm are synthesized for each configuration.

[0112] More specifically, the fired monoliths have round channels with a hydraulic diameter of 2 mm, the peripheral semicircular channels represented in the figures having 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 or open front area is obtained by calculating the ratio as a percentage of the area covered by the sum of the transverse cross sections of the channels to the total area of the corresponding transverse cross section of the porous support.

[0113] The green monoliths thus obtained are dried by microwave for a time sufficient to bring the content of water that is not chemically bound to less than 1% by weight.

[0114] The monoliths are then fired up to a temperature of at least 2100° C. which is maintained for 5 hours. The material obtained has an open porosity of 43% and a mean pore distribution diameter of the order of 25 micrometers, as measured by mercury porosimetry.

Example 1 (Comparative)

[0115] According to this example, a separating membrane layer made of silicon carbide is then deposited on the internal wall of the channels of a support structure as obtained above, according to the process described below:

A primer for adhesion of the separating layer is formed, in a first step, from a slip, the mineral 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.
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.
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, measure at 22° C. according to the standard DINC33-53019.

[0116] These two layers are successively deposited according to the same process described below: the slip is introduced into a tank with stirring (20 rpm). 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.

[0117] Next, the supports are dried at ambient temperature for 10 minutes and then at 60° C. for 12 h. The supports thus dried are then fired under argon at a temperature of 1400° C. for 2 h at ambient pressure.

[0118] A transverse cut is made through the filters thus obtained. The structure of the membrane is observed and studied with a scanning electron microscope.

Example 2 (According to the Invention)

[0119] According to this example, the procedure is identical to that of example 1, but the filter obtained is subjected to an additional treatment by immersing in a concentrated solution of hydrofluoric acid (20 vol %), followed by successive rinsings to bring its pH back to 6.

Example 3 (Comparative)

[0120] According to this example, the procedure is identical to that of example 1, but the filter obtained is additionally subjected to a steam oxidation treatment at 350° C. for 8 hours.

[0121] The properties and the characteristics of the filters thus obtained are measured as follows:

[0122] The mean thickness of the successive layers obtained for each example is measured by image analysis on the basis of the electron microscopy photographs.

[0123] 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 is around 250 nm for all the examples.

[0124] The other results as measured as indicated above are given in the following table 1.

[0125] The details of the measurement of flow (relative flow rate of water) carried out are given below:

[0126] At a temperature of 25° C. a fluid consisting of demineralized water feeds the filters to be evaluated under a transmembrane pressure of 0.5 bar and a circulation rate in the channels of 2 m/s. The permeate (water) is recovered at the periphery of the filter. The characteristic flow rate measurement of the filter is expressed in L/min per 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. Specifically, a value greater than 100% indicates an increased flow rate relative to the reference (example 1) and therefore an improvement in the filtration capacity.

[0127] The characteristics and the properties of the filters obtained according to examples 1 to 3 are given in table 1 below.

TABLE-US-00001 TABLE 1 Example 2 according Reference to the Comparative example 1 invention example 3 Additional treatment none HF Steam immersion oxidation Weight content of SiC of the 99.0 99.3 98.8 membrane (%)* Weight content of elemental 0.50 0.24 0.81 oxygen of the membrane (%)** Atomic percentage at the membrane surface*** Si 40.5 42.2 42.2 C 44.2 49.0 37.6 O 15.3 8.8 20.2 C/O ratio*** 2.9 5.6 1.9 Mean thickness of the membrane 45 45 45 (micrometers) Median pore diameter of the 250 250 250 membrane (nanometers) Relative flow measurement 100 138 61 *Measured according to standard ANSI B74.15-1992-(R2007) **Measured by LECO ***Measured by XPS

[0128] The results combined in the preceding table 1 indicate that example 2 according to the invention has a much higher filtration capacity than that of the reference filter (example 1).

[0129] The analysis of the data listed in the preceding table makes it possible to directly correlate this higher capacity with the amount of residual oxygen in the membrane layer.

[0130] Finally, the results listed in the table also indicate that the material used according to the invention for manufacturing the separating membrane layer (membrane in table 1) can only be obtained following certain process conditions not yet described in the prior art, that aim to very greatly limit the amount of oxygen present therein and more particularly at the surface of the SiC grains constituting the membrane.