FILTRATION MEMBRANE WITH HIGH CHEMICAL RESISTANCE

Abstract

The present invention relates to a filtration membrane with high chemical resistance comprising a porous support with a primary titanium oxide powder and a secondary titanium oxide powder, wherein the primary powder has a grain size between 10 and 50 microns, and the secondary powder has a grain size at least 2 times smaller than the grain size of the primary powder, the grain size of the secondary powder being larger than 5 microns, and wherein the primary powder represents at least 50% by weight with respect to the total weight of the porous support, such that the porous support has a pore size between 1 and 7 microns and a percentage of porosity greater than 30%.

Claims

1. A filtration membrane with high chemical resistance comprising a porous support with a primary titanium oxide powder and a secondary titanium oxide powder, wherein the primary powder has a grain size between 10 and 50 microns, and the secondary powder has a grain size at least 2 times smaller than the grain size of the primary powder, the grain size of the secondary powder being larger than 5 microns, and wherein the primary powder represents at least 50% by weight with respect to the total weight of the porous support, such that the porous support has a pore size between 1 and 7 microns and a percentage of porosity greater than 30%.

2. The filtration membrane with high chemical resistance according to claim 1, wherein the primary titanium oxide powder represents at least 70% by weight with respect to the total weight of the porous support.

3. The filtration membrane with high chemical resistance according to claim 1, wherein the titanium oxide support is sintered at a temperature comprised between 1300 and 1500? C.

4. The filtration membrane with high chemical resistance according to claim 1, additionally comprising one or more porous layers of ceramic material deposited on the porous support.

5. The filtration membrane with high chemical resistance according to claim 4, wherein the porous layers of ceramic material has a pore size between 1 and 1000 nm.

6. The filtration membrane with high chemical resistance according to claim 4, wherein the ceramic material of the porous layers is selected from the group consisting of Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, or SiO.sub.2.

Description

DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows a perspective view of a filtration membrane with a tubular morphology.

[0024] FIG. 2 shows a longitudinal section view of the membrane of the preceding drawing.

[0025] FIG. 3 is a graph showing the pore size distribution of the porous support of the filtration membrane of the invention.

[0026] FIG. 4 is a graph showing the deionized water permeability of the porous support of the filtration membrane of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] FIG. 1 shows the morphology of an example of filtration membrane according to the invention. The filtration membrane comprises a porous support (1) made of a single ceramic material, and inner channels (2) through which the liquid to be filtered is circulated.

[0028] FIG. 2 shows a cross-section of the filtration membrane in which the inner channels (2) are shown lengthwise. There are deposited on the inner walls of said channels (2) one or more porous layers (3) of ceramic material, acting as a semi-permeable physical barrier capable of separating the substances contained in the liquid to be filtered by means of applying pressure and according to the pore size thereof. So most of the liquid continues through the inner channels (2) of the membrane, and substances with sizes smaller than the pore size of the layers (3) of ceramic material are tangentially filtered through the porous layers (3) and the porous support (1) itself, where this filtered liquid is referred to as permeate.

[0029] The filtration membrane has a tubular geometry with external diameters between 8 and 80 mm and a length up to 2000 mm. The membrane has a single-channel structure, or a multi-channel structure of up to 85 channels, with internal channel diameters between 1 and 10 mm.

[0030] The porous support is made entirely of titanium oxide (TiO.sub.2) with purity greater than 95%, where some traces of impurities may be present due to the raw material used.

[0031] The porous support is made using titanium oxide of at least two different grain sizes. The porous support therefore comprises a primary titanium oxide powder having a grain size between 10 and 50 microns and a secondary powder having a grain size at least 2 times smaller than the grain size of the primary powder, the grain size of the secondary powder being larger than 5 microns.

[0032] To make the porous support, a ceramic paste with titanium oxide powders of a different grain size, water, and other organic compounds such as plasticizers, binders, or lubricants, is prepared. After mixing and kneading the components, the ceramic paste is extruded to obtain a single-channel or multi-channel porous support, depending on the nozzle used in the extrusion. The green extruded porous support is subjected to thermal treatment with a sintering temperature comprised between 1200 and 1500? C.

[0033] The porous layers (3) of ceramic material deposited on the inner channels (2) of the porous support (1) have a pore size between 1 and 1000 nm. The ceramic material from which the porous layers (3) of ceramic material are made is selected from the group consisting of aluminum oxide (Al.sub.2O.sub.3), titanium oxide (TiO.sub.2), zirconium oxide (ZrO.sub.2), or silicon oxide (SiO.sub.2).

[0034] According to the present invention, since the gradient of sizes of the different powder granules used is not too large, the pore size distribution is also narrow, which favors a homogenous adhesion of the layers deposited on the porous support. Furthermore, the use of primary powder with a grain size between 10 and 50 microns allows obtaining a support with pore sizes that are suitable for depositing layers that give rise to microfiltration, ultrafiltration, and nanofiltration membranes. Specifically, the mean pore size obtained in the support of the invention is comprised between 1 and 7 microns.

[0035] By adapting the sintering temperature to the grain size distribution used in each case, high porosities above 30% are also obtained, maintaining high mechanical strength greater than 40 MPa in three-point bending strength tests. The combination of mean pore size and high porosity allows obtaining a support with a high filtration capacity exceeding 5000 l/hm2 bar in tests with deionized water.

[0036] The use of a single material in the composition of porous supports also favors resistance to acidic and alkaline products used both in actual filtration processes and in chemical washes. Titanium oxide has a high resistance against etchings, and given the high purity of the porous supports (>95%) and the absence of other compounds having a different composition acting as weak points for these etchings, such as inorganic binders, for example, the mechanical strength of the supports is not compromised, where it remains above 35 MPa after etching with HNO.sub.3 with a concentration of 4 wt % at 70? C. or with NaOH with a concentration of 1.5 wt % at 90? C.

[0037] According to a first embodiment of the invention, the porous support is made using a primary powder having a grain size of 30 microns in a percentage of 70-80% and a secondary powder having a grain size of 15 microns in a percentage of 20-30%.

[0038] Table 1 shows the properties of the porous support obtained according to the first embodiment of the invention. The percentages of the powders used are expressed in percentage by weight (weight of the component in relation to the total weight of the ceramic composition of the porous support). The same table also shows the porosity (mean pore size and percentage of porosity measured with the mercury intrusion porosimetry technique), the mechanical strength before and after etchings, and the deionized water permeability of the obtained supports.

TABLE-US-00001 TABLE 1 TiO.sub.2 with a mean granule size of 30 microns [%] 70-80 TiO.sub.2 with a mean granule size of 15 microns [%] 20-30 Mean pore size [?m] 4-6 Porosity [%] 30-40 Three-point bending strength (MOR) [MPa] ?45 MOR after HNO.sub.3 (4 wt %) for 500 hours at 70? C. [MPa] ?35 MOR after NaOH (1.5 wt %) for 500 hours at 90? C. [MPa] ?35 Deionized water permeability [l/hm2bar] >7000

[0039] FIG. 3 shows the pore size distribution of a porous support developed following the composition of the first embodiment of the invention. In this case, the obtained mean pore size is comprised between 4-6 microns, showing a narrow distribution.

[0040] The permeability curve of the support developed following the composition of the first embodiment of the invention can be seen in FIG. 4, and it can be seen that after testing for 300 seconds, permeability remains above 7500 l/hm2 bar.

[0041] According to a second embodiment of the invention, the porous support is made using a primary powder having a grain size of 30 microns in a percentage of 85-90% and a secondary powder having a grain size of 15 microns in a percentage of 10-15%

[0042] Table 2 shows the properties of the porous support obtained according to the second embodiment of the invention. As can be seen, the properties of the porous support remain the same, where the percentage of porosity, mechanical strength, and permeability remain as high as in the porous support of the first embodiment of the invention; however, when the percentage of granules of the secondary powder having a smaller size decreases, the mean pore size of the support increases.

TABLE-US-00002 TABLE 2 TiO.sub.2 with a mean granule size of 30 microns [%] 85-90 TiO.sub.2 with a mean granule size of 15 microns [%] 10-15 Mean pore size [?m] 6.5-7 Porosity [%] 30-40 Three-point bending strength (MOR) [MPa] ?45 MOR after HNO.sub.3 (4 wt %) for 500 hours at 70? C. [MPa] ?35 MOR after NaOH (1.5 wt %) for 500 hours at 90? C. [MPa] ?35 Clean water permeability [l/hm2bar] >7000