Process for the production of non-oxide ceramic filtration membranes

Abstract

The present disclosure relates to a method of making oxide and non-oxide ceramic filter elements with high wear resistance, wherein the method of making allows the use of low sintering temperatures in the presence of atmospheric oxygen, wherein the resulting non-oxide filter membranes exhibit typical behavior of non-oxide ceramic filter elements.

Description

Technical Field

[0001] This invention relates to a method for preparing a ceramic filter membrane for nanofiltration purposes in liquid purification methods, and to a ceramic filter element. Background Technology

[0002] Given global population growth, industrialization, and natural disasters, providing clean and drinkable water is one of the emerging challenges that needs to be addressed. Water can be contaminated by bacteria, viruses, protozoa and fungi, bacterial and biological companions, bioactive or toxic substances with high molecular weights, or microplastics that may threaten human health. In addition, industrial wastewater can be contaminated by organic chemicals, dissolved solids, or suspended materials.

[0003] To reduce the amount of pollutants, toxic waste, or the volume of the reaction mixture to be purified in a highly efficient and energy-saving manner, filter membranes are frequently used for filtration or separation purposes, such as in the purification of industrial wastewater or process fluids. These filter membranes can be polymer membranes or ceramic membranes. The latter is generally superior to polymer membranes, especially in filtration or separation methods involving corrosive media such as strongly acidic or alkaline wastewater.

[0004] In the prior art, two types of ceramic filter membranes are known: oxidized and non-oxidized filter membranes.

[0005] Oxidized ceramic filter membranes are composed of particles of metal oxides, including aluminum oxide (Al₂O₃), beryllium oxide (BeO), calcium oxide (CaO), hafnium oxide (HfO₂), iron oxide (FeO / Fe₂O₃), lanthanum oxide (La₂O₃), magnesium oxide (MgO), manganese oxide (MnO₂), silicon dioxide (SiO₂), strontium oxide (SrO), thorium oxide (ThO₂), titanium dioxide (TiO₂), yttrium oxide (Y₂O₃), zirconium dioxide (ZrO₂), or mixtures thereof. The preparation of oxidized ceramic filter membranes is typically achieved via a sol-gel method, in which a carrier surface is coated with a sol containing a precursor metal compound, such as a metal alkoxide. During drying and sintering, the precursor is oxidized to the corresponding metal oxide to form a membrane layer with small pore sizes.

[0006] On the other hand, non-oxide ceramic filter membranes are composed of particles such as silicon carbide (SiC), silicon nitride (Si3N4), tungsten carbide (WC), aluminum nitride (AlN), or boron nitride (BN). Membranes prepared from non-oxide ceramic nanoparticles generally exhibit excellent properties in terms of their resistance to corrosive media and low tendency for fouling due to their low isoelectric point. The sol-gel method is not suitable for the preparation of non-oxide ceramic filter membranes. Therefore, non-oxide ceramic filter membranes are typically prepared by sintering powders with a narrow size distribution. This allows for the tuning of pore size and volume through careful selection of particles. However, the strong covalent bonding within non-oxide ceramic nanoparticles (e.g., between silicon and carbon) makes the sintering process more difficult than that used for preparing oxide ceramic filter membranes due to limited diffusion. To overcome this, and to prevent the formation of metal oxides during sintering, conditions must be carefully selected: sintering can only be successful at extremely high temperatures (e.g., about 2,500 °C) close to the decomposition temperature of the material used and in the absence of oxygen, i.e., by applying a vacuum or sintering in an inert atmosphere. Therefore, non-oxide ceramic filter membranes are difficult to prepare and expensive.

[0007] Alternatively, the required sintering temperature can be lowered by using additives. However, this can have a downside: additives may make precise pore size control difficult, altering the properties of the ceramic filter membrane and reducing its quality. Commonly used sintering aids are variations and derivatives of silica (SiO2), such as glass, borosilicate glass, cristobalite, or mullite. During sintering, these additives form bridges between non-oxide nanoparticles to establish a solid network. Nevertheless, it must be noted that this silica-based composition of ceramic filter membranes limits mechanical strength and resistance to corrosive media. Therefore, the superior properties of non-oxide ceramic materials cannot be fully utilized.

[0008] Therefore, it is highly desirable to provide a ceramic filter membrane that fully utilizes the advantageous properties of ceramic materials, especially non-oxide ceramic materials, but can be prepared at a low sintering temperature, thus eliminating the need for an inert atmosphere during sintering. Summary of the Invention

[0009] This disclosure relates to a ceramic filter element comprising a carrier structure and a filter layer, wherein the filter layer comprises at least a first particle and a second particle, wherein the second particle is selected from oxide ceramic particles, and wherein the first particle and the second particle differ at least in their D... 50 Diameter, characterized in that the particle size Q1(D) of the first particle 50 ) and the particle size Q2(D) of the second particle 50 The ratio (Z) is 2 to 5,000.

[0010] In a second aspect, this disclosure relates to a method for preparing the ceramic filter element, wherein the preparation method includes the steps of...

[0011] a) Provide a carrier structure having a carrier surface, and a coating suspension containing the first particle and the second particle;

[0012] b) Allow the surface of the carrier to contact the coating suspension for a period of time, preferably 10 to 120 seconds, more preferably 60 seconds or 30 seconds;

[0013] c) Removing excess paint suspension without removing residual film from the paint suspension;

[0014] d) Dry the residual film, preferably at a temperature of 60°C to 90°C for 2 to 6 hours; and

[0015] e) Sintering the carrier structure with the residual film.

[0016] f) Optionally repeat steps b) to e), preferably up to 7 times, more preferably up to 5 times. Attached Figure Description

[0017] Figure 1 A cross-sectional photograph of the filter layer containing the first and second particles, obtained by REM after sintering.

[0018] Figure 2 A: A photograph of the surface of the filter layer according to this disclosure. B: Elemental analysis performed by energy-dispersive X-ray diffraction analysis of the membrane.

[0019] Figure 3 The zeta potential of two films according to Example 1 containing 30% by weight ZrO2 as a second particle.

[0020] Detailed Explanation

[0021] This disclosure relates, in a first aspect, to a ceramic filter element comprising a carrier structure and a filter layer, wherein the filter layer comprises at least a first particle and a second particle, wherein the second particle is selected from oxide ceramic particles, and wherein the first particle and the second particle differ at least in their D... 50 Diameter, characterized in that the particle size Q1(D) of the first particle 50 ) and the particle size Q2(D) of the second particle 50 The ratio (Y) is 2 to 5,000.

[0022] One object of the present invention is to provide a ceramic filter element formed on a porous carrier material and exhibiting satisfactory flowability, high permeability to low molecular weight compounds, and high retention of high molecular weight compounds (including particles of a certain size). This filter element should be readily prepared at low cost from a suspension of ceramic compound particles, for example, without requiring high temperatures or an inert atmosphere during sintering. Furthermore, an object of the present invention is to provide a filter element exhibiting high stability to thermal, chemical, and mechanical stresses, resulting in excellent abrasion resistance and favorable cleaning properties. Additionally, those skilled in the art should be able to design the chemical properties of the filter element, such as the zeta potential or hydrophilic or hydrophobic properties of the membrane surface, to easily obtain a filter element optimal for each application. High hydrophilicity and a low contact angle, i.e., low resistance to water transport, can be achieved by using metal carbides contained in the filter layer. Furthermore, the filter element according to this disclosure exhibits a lower tendency to fouling, is easier to clean, and has a very high flux. For example, filter elements with an isoelectric point as high as 3 can be prepared from SiC and 30% by weight of ZrO2 particles, which exhibit unparalleled performance in the separation of oil and water.

[0023] The inventors of this disclosure unexpectedly discovered that by incorporating small oxide ceramic particles into the fabrication process of the disclosed filter element to assist the sintering process, oxide and non-oxide ceramic filter layers can be prepared in a less energy-intensive and less expensive manner. This is highly desirable because the addition of small metal oxide ceramic particles not only improves the strength of the membrane coating but also simultaneously reduces the sintering temperature required for the ceramic particles and eliminates the need for an inert atmosphere in the sintering step, while maintaining product characteristics such as pore size, chemical behavior (e.g., zeta potential and inertness), porosity, and filtration performance. This results in significant savings in fabrication time and energy consumption.

[0024] definition

[0025] In the sense of this disclosure, a ceramic filter element comprises a carrier structure and a filter layer. The carrier structure is a porous material to allow liquids and gases to pass through it. Furthermore, it determines the shape of the filter element and supports the filter layer to provide it with mechanical strength and prevent the filter element from breaking. The filter layer is directly adjacent to the carrier structure and constitutes a porous layer with defined pore sizes to allow liquids and gases to pass through it. Depending on the pore size, compounds of a certain molecular weight and / or aggregates of a certain size cannot pass through the filter layer. During filtration, the compounds or aggregates contained in the feed liquid or gas (also referred to as the "feed") cannot pass through the filter layer and are thus retained in the feed. Therefore, the concentration of the compounds and / or particles in the feed increases, while the liquid passing through the filter layer (also referred to as the "filtrate") contains no said compounds and / or aggregates, or has a reduced concentration of said compounds and / or aggregates compared to the feed. In other words, the pores of the filter element according to this disclosure are free and conductive for liquids and gases; in particular, the filter element according to this application is conductive for liquids.

[0026] In the sense of this disclosure, the filter layer contains at least two different types of particles:

[0027] • Particles of at least one ceramic compound, which determine the average pore size, porosity, chemical properties such as zeta potential, hydrophilic or hydrophobic properties, and stability to corrosive media such as acids and alkalis. For the purposes of this disclosure, the particles of the ceramic compound may be selected from particles of at least one metal oxide, particles of at least one metal carbide, and particles of at least one metal nitride. For the purposes of this disclosure, the particles of the ceramic compound may be considered as "first particles".

[0028] • Particles of at least one metal oxide, which act as an adhesive component to provide strong adhesion between the first particles without requiring high sintering temperatures and / or an inert atmosphere during sintering, and provide high mechanical strength, wear resistance, and high chemical stability (without any associated reduction in the chemical stability of the layer). Therefore, only particles exhibiting high resistance to corrosive chemicals such as acids and alkalis, high sintering activity at low temperatures, particularly below 500°C, and good bonding with the first particles are suitable materials. In the sense of this disclosure, the size of such particles is smaller than the size of the first particles. For the purposes of this disclosure, such particles are referred to as "second particles".

[0029] In the sense of this disclosure, unless otherwise specified, the size of the first and second particles is characterized by their average diameter Q0, i.e., their numerical value D as determined by dynamic light scattering (DLS). 10 D 50 and D 90 Value. D 10A particle diameter is defined as the proportion of particles with a diameter less than or equal to this value, which is 10% of the total number of particles. Therefore, 10% of the particles in a batch have a diameter less than or equal to D. 10 The diameter of the particle is greater than D, while 90% of the particles have a diameter greater than D. 10 The diameter of the value. Therefore, this is the distribution of the number of particles. In a similar way, D 50 This is defined as a particle diameter in which the proportion of particles with a diameter less than or equal to this value is 50% of the total number of particles. Finally, D 90 A particle diameter is defined as the proportion of particles with a diameter less than or equal to this value relative to the total number of particles. In the sense of this disclosure, all particle sizes are determined by DLS. This can be specifically determined using a Nanotrac Flex nanoparticle size analyzer (available from Microtrac MRB).

[0030] The particles disclosed herein can be characterized by their particle size distribution, which is expressed as follows:

[0031]

[0032] In other words, the Z ratio, or Z, is the particle size D of the particles that make up the filter layer. 90 Particle size D of the same particles that make up the filter layer 10 The business.

[0033] The ratio between the first and second particles can be characterized by the ratio of their average particle sizes, expressed as:

[0034]

[0035] In other words, the Y ratio, or Y, is the average particle size D of the first particle that makes up the filter layer. 50 The average particle size D of the second particles constituting the filter layer 50 The business.

[0036] In the sense of this disclosure, the particles of the ceramic compound may be selected from particles of at least one metal oxide, particles of at least one metal carbide, and particles of at least one metal nitride.

[0037] In the sense of this disclosure, at least one metal oxide particle, such as a second particle and / or a first particle, may be selected from oxide ceramic materials, such as aluminum oxide (Al2O3), beryllium oxide (BeO), calcium oxide (CaO), hafnium oxide (HfO2), iron oxide (FeO / Fe2O3), lanthanum oxide (La2O3), magnesium oxide (MgO), manganese oxide (MnO2), silicon dioxide (SiO2), strontium oxide (SrO), thorium oxide (ThO2), titanium dioxide (TiO2), yttrium oxide (Y2O3), zirconium dioxide (ZrO2), and mixtures thereof.

[0038] In a preferred embodiment, the particles of the at least one metal oxide are selected from particles of alumina (Al2O3), silicon dioxide (SiO2), titanium dioxide (TiO2), zirconium dioxide (ZrO2), and mixtures thereof.

[0039] In the sense of this disclosure, at least one metal carbide particle, such as a first particle, may be selected from silicon carbide (SiC), tungsten carbide (WC), boron carbide (B4C), and mixtures thereof.

[0040] In a preferred embodiment, the particles of the at least one metal carbide may be silicon carbide (SiC).

[0041] In the sense of this disclosure, at least one metal nitride particle, such as a first particle, may be selected from silicon nitride (Si3N4), aluminum nitride (AlN), titanium nitride (TiN), boron nitride (BN), and mixtures thereof.

[0042] In a preferred embodiment, the particles of the at least one metal nitride may be selected from silicon nitride (Si3N4), aluminum nitride (AlN), titanium nitride (TiN), and mixtures thereof.

[0043] In the sense of this disclosure, the average aperture is considered to be the D of the aperture. 90 That is, a pore size in which the proportion of pores with a diameter less than or equal to this value is 90% relative to the total number of pores.

[0044] In this disclosure and claims, the use of the term "comprising" does not exclude other elements. For the purposes of this invention, the term "consisting of" is considered a preferred embodiment of the term "comprising." If below a group is defined as comprising at least a certain number of embodiments, this should also be understood as disclosing a group preferably consisting only of those embodiments.

[0045] carrier structure

[0046] According to this disclosure, the carrier is formed of a porous material to allow liquid to pass through. The carrier is formed of a ceramic material. In one embodiment, the carrier is formed of a metal oxide ceramic material. In another embodiment, the carrier is formed of a non-oxide ceramic material, such as one or more metal carbides or one or more metal nitrides.

[0047] In one preferred embodiment, the carrier has an average pore size of ≤30 μm. In another preferred embodiment, the average pore size may optionally be reduced to ≤1.5 μm by coating at least one carrier layer in a stepwise manner. In this case, the optionally coated carrier layer is considered the carrier structure of this disclosure. Reducing the pore size of the carrier by coating a carrier layer may result in a lower defect rate in the filter layer.

[0048] The carrier can have different shapes. There are no particular restrictions on the shape of the carrier. Similarly, there are no particular restrictions on the shape of the filter layer that is supported on / in direct contact with the carrier structure. For example, the carrier can have shapes such as discs, polygonal plates, plates, flat plates, cylinders, box-shaped cylinders, rods, square columns, etc., which can be selected according to the intended use. Except for their thickness, there are no restrictions on the size of the carrier or filter layer, and the size can be selected according to the intended use, as long as the size ensures sufficient strength of the carrier. Those skilled in the art select the thickness and material of the carrier in a manner that provides a filter element with sufficient mechanical strength for the intended use.

[0049] First particle

[0050] In one embodiment of this disclosure, the first particle may be selected from a metal carbide. In another embodiment, the first ceramic nanoparticle is selected from SiC, WC, B4C, and mixtures thereof. In another preferred embodiment, the first particle may be selected from SiC, WC, and mixtures thereof. In yet another preferred embodiment, the first particle may be selected from SiC particles.

[0051] In another embodiment, the first particle may be selected from a metal nitride. In another embodiment, the first particle may be selected from Si3N4, BN, AlN, TiN, and mixtures thereof. In another embodiment, the first particle may be selected from Si3N4, AlN, TiN, and mixtures thereof. In a preferred embodiment, the first particle may be selected from AlN, TiN, and mixtures thereof. In another preferred embodiment, the first particle may be AlN.

[0052] In another embodiment, the first particle may be selected from a metal oxide. In another embodiment, the first particle may be selected from Al₂O₃, BeO, CaO, HfO₂, FeO, Fe₂O₃, La₂O₃, MgO, MnO₂, SiO₂, SrO, ThO₂, TiO₂, Y₂O₃, ZrO₂, and mixtures thereof. In another preferred embodiment, the first particle may be selected from Al₂O₃, SiO₂, TiO₂, ZrO₂, and mixtures thereof. In another preferred embodiment, the first particle may be selected from Al₂O₃, TiO₂, and ZrO₂.

[0053] In one embodiment, the first particle has an average diameter D of 10 nm to 15 μm. 50 In another embodiment, the first particle has a diameter D of 10 nm to 13 μm. 50 In a preferred embodiment, the first particle has a diameter D of 10 nm to 10 μm. 50 In another preferred embodiment, the first particle has a diameter D of 10 nm to 7 μm. 50 In another preferred embodiment, the first particle has a diameter D of 10 nm to 5 μm. 50 In another preferred embodiment, the first particle has a diameter D of 10 nm to 4 μm. 50 In another preferred embodiment, the first particle has a diameter D of 10 nm to 3 μm. 50 In a preferred embodiment, the first particle has an average diameter D of 20 nm to 2 μm. 50 In another preferred embodiment, the first particle has an average diameter D of 50 nm to 1.75 μm. 50 In another preferred embodiment, the first particle has an average diameter D of 100 nm to 1.5 μm. 50 In another preferred embodiment, the first particle has an average diameter D of 100 nm to 1 μm. 50 .

[0054] In another embodiment, the first particle further has a diameter D of 20 nm to 50 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 40 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 30 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 25 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 20 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 15 μm. 90In another embodiment, the first particle has a diameter D of 20 nm to 10 μm. 90 In another embodiment, the first particle has a diameter D of 20 nm to 6 μm. 90 In a preferred embodiment, the first particle has a diameter D of 100 nm to 5,000 nm. 90 In a preferred embodiment, the first particle has a diameter D of 200 nm to 4,000 nm. 90 In a preferred embodiment, the first particle has a diameter D of 250 nm to 3,000 nm. 90 In a preferred embodiment, the first particle has a diameter D of 300 nm to 2,500 nm. 90 In a preferred embodiment, the first particle has a diameter D in the range of 400 nm to 2,500 nm. 90 In a preferred embodiment, the first particle has a diameter D of 500 nm to 2,500 nm. 90 In a preferred embodiment, the first particle has a diameter D of 500 nm to 2,000 nm. 90 .

[0055] In another embodiment, the first particle has a diameter D of 10 nm to 15 μm, preferably 10 nm to 13 μm, more preferably 10 nm to 10 μm, more preferably 10 nm to 7 μm, more preferably 10 nm to 5 μm, more preferably 10 nm to 4 μm, more preferably 10 nm to 3,000 nm, preferably 20 nm to 2,000 nm, more preferably 50 nm to 1,750 nm, more preferably 100 nm to 1,500 nm, and most preferably 100 nm to 1,000 nm. 50 ; and preferably, D 90 The range is 20nm to 6,000nm, more preferably 100nm to 5,000nm, more preferably 200nm to 4,000nm, more preferably 250nm to 3,000nm, more preferably 300nm to 2,500nm, more preferably 400nm to 2,500nm, more preferably 400nm to 2,500nm, and most preferably 450nm to 2,000nm.

[0056] In another embodiment, the first particle has a diameter D of less than 15 μm. 50 In another embodiment, the first particle has a diameter D of less than 14 μm. 50 In another embodiment, the first particle has a diameter D of less than 13 μm. 50 In another embodiment, the first particle has a diameter D of less than 12 μm. 50 In another embodiment, the first particle has a diameter D of less than 11 μm.50 In another embodiment, the first particle has a diameter D of less than 10 μm. 50 In another embodiment, the first particle has a diameter D of less than 9 μm. 50 In another embodiment, the first particle has a diameter D of less than 8 μm. 50 In another embodiment, the first particle has a diameter D of less than 7 μm. 50 In another embodiment, the first particle has a diameter D of less than 6 μm. 50 In another embodiment, the first particle has a diameter D of less than 5 μm. 50 In another embodiment, the first particle has a diameter D of less than 4 μm. 50 In a preferred embodiment, the first particle has a diameter D of less than 3 μm. 50 In another preferred embodiment, the first particle has a diameter D of less than 2,500 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 2,000 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 1,500 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 1,250 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 1,000 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 800 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 700 nm. 50 In another preferred embodiment, the first particle has a diameter D of less than 600 nm. 50 .

[0057] In another embodiment, the first particle has a diameter D greater than 5 nm. 50 In another embodiment, the first particle has a diameter D greater than 10 nm. 50 In a preferred embodiment, the first particle has a diameter D greater than 15 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 20 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 25 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 30 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 35 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 40 nm.50 In another preferred embodiment, the first particle has a diameter D greater than 45 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 50 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 60 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 70 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 80 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 90 nm. 50 In another preferred embodiment, the first particle has a diameter D greater than 100 nm. 50 .

[0058] In another embodiment, the first particle further has a diameter D of less than 60 μm. 90 In another embodiment, the first particle has a diameter D of less than 50 μm. 90 In another embodiment, the first particle has a diameter D of less than 40 μm. 90 In another embodiment, the first particle has a diameter D of less than 30 μm. 90 In another embodiment, the first particle has a diameter D of less than 25 μm. 90 In another embodiment, the first particle has a diameter D of less than 20 μm. 90 In another embodiment, the first particle has a diameter D of less than 15 μm. 90 In another embodiment, the first particle has a diameter D of less than 10 μm. 90 In another embodiment, the first particle has a diameter D of less than 6 μm. 90 In another embodiment, the first particle has a diameter D of less than 5 μm. 90 In another embodiment, the first particle has a diameter D of less than 4 μm. 90 In another embodiment, the first particle has a diameter D of less than 3 μm. 90 In a preferred embodiment, the first particle has a diameter D of less than 2,500 nm. 90 In another preferred embodiment, the first particle has a diameter D of less than 2,000 nm. 90 .

[0059] In another embodiment, the first particle has a diameter D greater than 10 nm. 90 In another embodiment, the first particle has a diameter D greater than 15 nm. 90In a preferred embodiment, the first particle has a diameter D greater than 20 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 30 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 40 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 50 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 70 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 90 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 100 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 150 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 200 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 250 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 300 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 350 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 400 nm. 90 In another preferred embodiment, the first particle has a diameter D greater than 450 nm. 90 .

[0060] What needs to be understood is that, in the selected minimum D 50 Less than the selected maximum D 50 Under the condition that any of the above minimum diameters D 50 It can be used with any maximum diameter D defined in the implementation scheme given above. 50 combination.

[0061] It is also important to understand that, in the selected minimum D 90 Less than the selected maximum D 90 Under the condition that any of the above minimum diameters D 90 It can be used with any maximum diameter D defined in the implementation scheme given above. 90 combination.

[0062] It is also important to understand that in D 50 Diameter less than D 90 Under the condition of diameter, D 50 Any combination of diameters can be used with D 90 Combine any combination of diameters.

[0063] It's also important to understand that the first particle determines the pore size and chemical properties of the filter layer. The first particle exhibits low sintering activity, which is why the presence of the second particle is necessary.

[0064] Second particle

[0065] In one embodiment, the second particle may be selected from a metal oxide. In another embodiment, the second particle may be selected from Al₂O₃, BeO, CaO, HfO₂, FeO, Fe₂O₃, La₂O₃, MgO, MnO₂, SiO₂, SrO, ThO₂, TiO₂, Y₂O₃, ZrO₂, and mixtures thereof. In another preferred embodiment, the second particle may be selected from Al₂O₃, SiO₂, TiO₂, ZrO₂, and mixtures thereof. In another preferred embodiment, the second particle may be selected from Al₂O₃, TiO₂, and ZrO₂. In another preferred embodiment, the second particle may be ZrO₂.

[0066] In one embodiment, the second particle has an average diameter D of 1 nm to 150 nm. 50 In a preferred embodiment, the second particle has an average diameter D of 1 nm to 100 nm. 50 In another preferred embodiment, the second particle has an average diameter D of 1 nm to 50 nm. 50 .

[0067] In another embodiment, the second particle has a diameter D of 3 nm to 400 nm. 90 In a preferred embodiment, the second particle has a diameter D of 3 nm to 300 nm. 90 In a preferred embodiment, the second particle has a diameter D of 5 nm to 200 nm. 90 In a preferred embodiment, the second particle has a diameter D of 5 nm to 100 nm. 90 .

[0068] In another embodiment, the second particle has a diameter D of 1 nm to 150 nm, preferably 1 nm to 100 nm, and more preferably 1 nm to 50 nm. 50 And preferably, D 90 The wavelength range is 3nm to 400nm, more preferably 3nm to 300nm, even more preferably 5nm to 200nm, and most preferably 5nm to 100nm.

[0069] In another embodiment, the second particle has a Z ratio of less than 20. In another embodiment, the second particle has a Z ratio of less than 15. In another embodiment, the second particle has a Z ratio of less than 10. In a preferred embodiment, the second particle has a Z ratio of 1 to 7. In another preferred embodiment, the second particle, as a TiO2 particle, has a Z ratio of 1 to 5. In another preferred embodiment, the second particle, as a ZrO2 particle, has a Z ratio of 1 to 3. As can be understood from the formula for calculating the Z ratio, the Z ratio cannot be less than 1.

[0070] It is important to understand that small particles of metal oxides possess high sintering activity. Therefore, the second particle has the ability to bind together the larger first particle even at lower sintering temperatures, thereby improving the stability of the filter layer without affecting the pore size or chemical properties of the filter membrane, such as zeta potential. In other words, the second particle binds together the first particle, which provides high mechanical strength to the filter element according to this application despite the low sintering temperature.

[0071] It should be understood that the use of second particles, such as metal oxides like Al₂O₃, TiO₂, ZrO₂, and SiO₂, provides high chemical and mechanical stability to the filter element. Second particles of pure metal oxides are therefore superior to particles of clay, mullite, etc. Furthermore, it should be understood that clay, mullite, etc., cannot achieve a Dc of less than 400 nm as required by this disclosure. 90 Granularity provided.

[0072] Filter layer

[0073] According to this disclosure, the filter layer contains at least two different types of ceramic particles, namely, a first particle and a second particle, etc. In one embodiment, the different particles may be determined at least by their numerical value D. 50 The values ​​represent different dimensions. In another embodiment, the different particles may optionally differ in their chemical composition.

[0074] In a further preferred embodiment, the ceramic filter layer comprises two different ceramic nanoparticles, namely a first particle and a second particle, or is composed of two different ceramic nanoparticles, namely a first particle and a second particle.

[0075] • The first particle selected from metal carbides or metal nitrides

[0076] In one embodiment, the filter layer contains first particles selected from metal carbides or metal nitrides.

[0077] In one embodiment, the filter layer comprises at least a first particle selected from metal carbides or metal nitrides, and a second particle, wherein the ratio Y is 2 to 5,000. In a preferred embodiment, the ratio Y is 5 to 4,000. In another preferred embodiment, the Y ratio is 5 to 3,000. In another preferred embodiment, the Y ratio is 5 to 2,000. In another preferred embodiment, the Y ratio is 5 to 1,000. In another preferred embodiment, the Y ratio is 10 to 500. In one preferred embodiment, the ratio Y is 50 to 400. In another preferred embodiment, the ratio Y is 100 to 300. In another preferred embodiment, the ratio Y is 150 to 250.

[0078] In another embodiment, the ratio Y is less than 5,000. In another preferred embodiment, the ratio Y is less than 4,000. In another preferred embodiment, the ratio Y is less than 3,000. In another preferred embodiment, the ratio Y is less than 2,000. In another preferred embodiment, the ratio Y is less than 3,000. In another preferred embodiment, the ratio Y is less than 1,500. In another preferred embodiment, the ratio Y is less than 1,000. In another preferred embodiment, the ratio Y is less than 800. In another preferred embodiment, the ratio Y is less than 600. In another embodiment, the ratio Y is less than 500. In another preferred embodiment, the ratio Y is less than 400. In another preferred embodiment, the ratio Y is less than 300. In another preferred embodiment, the ratio Y is less than 250.

[0079] In another embodiment, the ratio Y is greater than 1. In another preferred embodiment, the ratio Y is greater than 10. In another preferred embodiment, the ratio Y is greater than 50. In another preferred embodiment, the ratio Y is greater than 100. In another preferred embodiment, the ratio Y is greater than 150.

[0080] It should be understood that any minimum Y ratio given above can be combined with any maximum Y ratio given above.

[0081] • Selected from the first particle of a metal oxide

[0082] In another embodiment, the filter layer comprises a first particle selected from metal oxides and a second particle.

[0083] In one embodiment, the filter layer comprises at least a first particle and a second particle selected from metal oxides, wherein the ratio Y is 2 to 5,000. In a preferred embodiment, the ratio Y is 2 to 4,000. In another preferred embodiment, the Y ratio is 2 to 3,000. In another preferred embodiment, the Y ratio is 2 to 2,000. In another preferred embodiment, the Y ratio is 2 to 1,000. In another preferred embodiment, the Y ratio is 2 to 500. In one preferred embodiment, the ratio Y is 2 to 300. In another preferred embodiment, the ratio Y is 3 to 200. In another preferred embodiment, the ratio Y is 5 to 100.

[0084] In another embodiment, the ratio Y is less than 5,000. In another preferred embodiment, the ratio Y is less than 4,000. In another preferred embodiment, the ratio Y is less than 3,000. In another preferred embodiment, the ratio Y is less than 2,000. In another preferred embodiment, the ratio Y is less than 1,500. In another preferred embodiment, the ratio Y is less than 1,000. In another preferred embodiment, the ratio Y is less than 500. In another preferred embodiment, the ratio Y is less than 600. In another embodiment, the ratio Y is less than 500. In another preferred embodiment, the ratio Y is less than 300. In another preferred embodiment, the ratio Y is less than 200. In another preferred embodiment, the ratio Y is less than 150. In another preferred embodiment, the ratio Y is less than 100.

[0085] In another embodiment, the ratio Y is greater than 1. In another preferred embodiment, the ratio Y is greater than 2. In another preferred embodiment, the ratio Y is greater than 3. In another preferred embodiment, the ratio Y is greater than 4. In another preferred embodiment, the ratio Y is greater than 5.

[0086] It should be understood that any minimum Y ratio given above can be combined with any maximum Y ratio given above.

[0087] • Composition of the filter layer

[0088] In one embodiment, the first and second particles exhibit a bimodal numerical distribution of particle size. In other words, the first and second particles each have numerical particle size distributions that overlap only partially, wherein the intersection of these two particle size distributions does not occur at the maximum value of the particle number in either particle size distribution. In a preferred embodiment, the D of the first particle... 10 D greater than the second particle 90 In another preferred embodiment, the particle size distributions do not overlap.

[0089] In one embodiment, the filter layer contains a second particle in an amount of 1 wt% to 50 wt% based on the total weight of the first and second particles. In a preferred embodiment, the filter layer contains a second particle in an amount of 5 wt% to 40 wt% based on the total weight of the first and second particles. In a preferred embodiment, the filter layer contains a second particle in an amount of 5 wt% to 35 wt% based on the total weight of the first and second particles. In a preferred embodiment, the filter layer contains a second particle in an amount of 7 wt% to 35 wt% based on the total weight of the first and second particles. In a preferred embodiment, the filter layer contains a second particle in an amount of 10 wt% to 30 wt% based on the total weight of the first and second particles.

[0090] It is important to understand that technicians can individually select the amount of second particles relative to the amount of first particles for each filter layer, based on usage requirements. A higher amount of second particles improves the mechanical strength and abrasion resistance of the filter layer. On the other hand, it may negatively affect chemical properties, such as zeta potential, and / or pore size, potentially leading to different filtration properties and / or a bimodal distribution of pore size. This could result in an increased risk of membrane clogging and fouling and / or reduced filtrate quality. A lower amount of second particles may reduce the mechanical strength and abrasion resistance of the filter layer due to insufficient bonding between the first particles obtained during sintering. Therefore, durability may be significantly reduced.

[0091] In one embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is ZrO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is TiO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is Al2O3. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is AlN and the second particle is Al2O3. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is AlN and the second particle is ZrO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is AlN and the second particle is TiO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is Si3N4 and the second particle is Al2O3. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is Si3N4 and the second particle is ZrO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is Si3N4 and the second particle is TiO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is BN and the second particle is Al2O3. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is BN and the second particle is TiO2. In another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is BN and the second particle is ZrO2. In a preferred embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is ZrO2. In another preferred embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is TiO2. In another preferred embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is Al2O3. In yet another embodiment, the filter layer comprises a first particle and a second particle, wherein the first particle is SiC and the second particle is ZrO2.

[0092] It is important to understand that the size of the particles contained in the filter layer can be determined after its fabrication, for example, by grating electron microscopy (REM, see below). Figure 1 The chemical composition of the filter layer can be determined after its preparation, for example by elemental analysis using energy-dispersive X-ray diffraction (see [reference]). Figure 2 B).

[0093] Figure 1This shows a magnified REM image of a filter layer containing SiC particles as the first particle and 30% by weight ZrO2 particles as the second particle, which bind to and bind the larger SiC particles together (white circles). Detector = InLens; Signal B = MPSE; Signal = 1.0000; EHT = 5.00 kV; Mag = 250.00 KX; WD = 4.0 mm; image recorded on a ZeissLeo 15340VP.

[0094] Figure 2 Showing REM images (A) of the same filter element used for EDX analysis (B). 1: Peak of CK; 2: Peak of OK; 3: Peak of Si K; 4: Peak of Zr L. A: Detector = InLens; Signal B = MPSE; Signal = 1.0000; EHT = 5.00kV; Mag = 250.00KX; WD = 4.0mm; Image recorded on a Zeiss Leo 15340VP. B: Integrated EDX detector from Zeiss LEO 15340VP.

[0095] Furthermore, it should be understood that the choice of the first particle determines the filtration properties of the filter element. Those skilled in the art can select the first particle based on the intended use of the filter element.

[0096] For example, the pore size and therefore the molecular weight cutoff depend on the first particle. The diameter D of the first particle... 50 The larger the size, the larger the pore size.

[0097] Furthermore, the zeta potential of the filter layer surface also depends on the choice of the first particle. It is known that filter layers composed of metal carbide particles exhibit a typical zeta potential progression across different pH ranges, while the isoelectric point is in the pH range of 2 to 3 (see [link to relevant documentation]). Figure 3 Therefore, the surface of the filter layer carries a negative charge over a wide pH range (i.e., above 3). This is highly desirable for various filtration tasks because negatively charged (organic) compounds are repelled. Thus, not only is the retention of these compounds improved, but the tendency for membrane fouling (e.g., the formation of a fouling layer on the filter layer containing particles and molecules from the feed) is also reduced.

[0098] Figure 3 The slope of the zeta potential relative to pH is shown. Two different elements have been analyzed, both containing SiC as the first particle and ZrO2 as the second particle. Rhomboid: Filter element 1 according to Example 1, containing 30% by weight ZrO2 as the second particle and SiC as the first particle. Sintering was performed at 400°C for 2 hours. Fork-shaped: Filter element 2 according to Example 1, containing 30% by weight ZrO2 as the second particle and SiC as the first particle. Sintering was performed at 500°C for 2 hours.

[0099] Despite the presence of a second particle in the filter layer, these advantageous properties are maintained in the filter layer according to this disclosure.

[0100] Preparation method of ceramic filter element

[0101] In a second aspect, this disclosure relates to a method for preparing the ceramic filter element, wherein the preparation method includes the steps of...

[0102] a) Provide a carrier structure having a carrier surface, and a coating suspension containing a first particle and a second particle;

[0103] b) Allow the surface of the carrier to contact the coating suspension for a period of time, preferably 10 to 120 seconds, more preferably 60 seconds or 30 seconds;

[0104] c) Removing excess paint suspension without removing residual film from the paint suspension;

[0105] d) Dry the residual film, preferably at a temperature of 60°C to 90°C for 2 to 6 hours; and

[0106] e) Sintering the carrier structure with the residual film.

[0107] f) Optionally repeat steps b) to e), preferably up to 7 times, more preferably up to 5 times.

[0108] This disclosure relates to a method for producing the aforementioned ceramic filter element. Therefore, a second aspect of this disclosure relates to a method for preparing a multilayer ceramic filter element according to the first aspect detailed above, wherein a layer is formed by applying a suspension containing first and second particles onto a ceramic carrier structure, which is then sintered and cured at a low temperature without the need for an inert atmosphere.

[0109] In the sense of this disclosure, excess paint suspension is the amount of paint suspension that does not adhere to the solid surface to be coated by means of adhesion. Paint suspension that is carefully removed is also referred to as excess paint suspension. Thus, a film of paint suspension adheres to the surface to be coated by adhesion, while the paint suspension that does not adhere to the surface is gently removed.

[0110] The advantage of the method for preparing ceramic filter elements according to this disclosure is that it saves costs and simplifies the process by reducing the requirements for temperature and atmosphere.

[0111] The method for preparing a ceramic filter element according to this disclosure includes the following steps (a) to (f):

[0112] (a) In order to coat the filter layer, a coating suspension is provided in a first step. The coating suspension may be prepared from crystalline powders of first and second particles and a solvent in the presence of a dispersing additive. Optionally, a binder may be included in the coating suspension.

[0113] It should be understood that the use of crystalline powders of the first and second particles is essential for the method and filter element according to this disclosure. Therefore, methods such as the sol-gel method are excluded. When the sol-gel method is used instead of the crystalline powder of the second particle, the degree to which metal oxide forms on the surface of the first particle during firing cannot be controlled. In other words, the sol of the metal oxide precursor easily covers the entire surface of the first particle. During the conversion of the precursor to the metal oxide, the first particle may be bound together. However, the entire surface of the first particle is covered by the metal oxide. Therefore, the properties of the filter element, such as the zeta potential, are entirely determined by the metal oxide. Therefore, it is impossible to prepare a filter element with the advantageous properties of the non-oxide ceramic filter elements listed above using methods such as the sol-gel method.

[0114] In one embodiment, the coating suspension contains a dispersing additive to promote optimal mixing and prevent accelerated particle aggregation. The dispersing additive may be selected from surfactants, such as carbonic acid, fatty alcohols, inorganic acids, ammonium salts such as tetramethylammonium hydroxide, or polyelectrolytes such as sodium poly(styrene sulfonate). In a preferred embodiment, the dispersing additive is selected from inorganic acids. In another preferred embodiment, the dispersing additive is nitric acid. In another preferred embodiment, the dispersing additive is hydrochloric acid. In another preferred embodiment, the dispersing additive is selected from carbonic acid. In another preferred embodiment, the dispersing additive is acetic acid. In another preferred embodiment, the dispersing additive is tetramethylammonium hydroxide.

[0115] In one embodiment, the coating suspension contains up to 5% by weight of a dispersing additive. In another embodiment, the coating suspension contains up to 3% by weight of a dispersing additive. In a preferred embodiment, the coating suspension contains from 0.1% by weight to 3% by weight of a dispersing additive. In another preferred embodiment, the coating suspension contains from 0.1% by weight to 1% by weight of a dispersing additive.

[0116] In addition, an adhesive can be added to promote sintering and enhance bond strength. This adhesive can be selected from polymers, particularly polyvinyl alcohol, polyvinylpyrrolidone, or cellulose, or mixtures thereof. In one preferred embodiment, the adhesive is polyvinyl alcohol. In another preferred embodiment, the cellulose is selected from methylcellulose and carboxymethylcellulose, or mixtures thereof.

[0117] In another embodiment, the adhesive is present in an amount of ≤15% by weight, preferably ≤10% by weight, based on the total weight of the coating suspension. In another embodiment, the adhesive is present in an amount of at least 0.5% by weight, based on the total weight of the coating suspension. In another preferred embodiment, the adhesive is present in an amount of 0.5% to 15% by weight, based on the total weight of the coating suspension. In another preferred embodiment, the adhesive is present in an amount of 0.5% to 10% by weight, based on the total weight of the coating suspension. In another preferred embodiment, the adhesive is present in an amount of 0.5% to 5% by weight, based on the total weight of the coating suspension.

[0118] In one embodiment, the coating suspension for coating the filter layer contains a second particle in an amount of 1% to 50% by weight based on the total weight of the first and second particles. In a preferred embodiment, the coating suspension contains a second particle in an amount of 5% to 40% by weight based on the total weight of the first and second particles. In a preferred embodiment, the coating suspension contains a second particle in an amount of 5% to 30% by weight based on the total weight of the first and second particles. In a preferred embodiment, the coating suspension contains a second particle in an amount of 7% to 25% by weight based on the total weight of the first and second particles. In a preferred embodiment, the coating suspension contains a second particle in an amount of 10% to 25% by weight based on the total weight of the first and second particles.

[0119] In one embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 1% to 40% by weight based on the total weight of the paint suspension. In a preferred embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 2% to 35% by weight. In another preferred embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 3% to 30% by weight. In another preferred embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 5% to 25% by weight. In another preferred embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 5% to 20% by weight. In another preferred embodiment, the paint suspension comprises a mixture of the first and second particles as described above, in an amount of 7% to 15% by weight.

[0120] In one embodiment, the coating suspension may contain at least 1% by weight of first particles based on the total weight of the suspension, preferably 1% to 70% by weight of first particles, more preferably 1% to 50% by weight of first particles, more preferably 1% to 30% by weight of first particles, more preferably 3% to 20% by weight of first particles, and most preferably 5% to 15% by weight of first particles based on the total weight of the coating suspension.

[0121] In one embodiment, the coating suspension may contain at least 0.1% by weight of second particles based on the total weight of the mixture, preferably 0.1% to 35% by weight of second particles, more preferably 0.1% to 25% by weight of second particles, more preferably 0.1% to 15% by weight, even more preferably 0.1% to 10% by weight of second particles, more preferably 1% to 10% by weight of second particles, and most preferably 1% to 8% by weight of second particles based on the total weight of the coating suspension.

[0122] In one embodiment, the solvent of the paint suspension is water. In another embodiment, the solvent of the paint suspension is selected from aliphatic, straight-chain, or branched alcohols containing 1 to 6 carbon atoms. In another embodiment, the solvent comprises DMSO and / or NMP. In a preferred embodiment, the solvent of the paint suspension is water. In another preferred embodiment, the solvent is ethanol. In another preferred embodiment, the solvent is a mixture of water and ethanol. In another preferred embodiment, the solvent is a mixture of water and DMSO. It should be understood that the total paint suspension must have 100% by weight of total weight. Unless otherwise explicitly stated, solvents constitute unspecified weights (e.g., in the examples). It should also be understood that dispersing additives and binders must be carefully selected to be compatible with the solvent, such as soluble and inert / stable.

[0123] In one embodiment, the coating suspension may be prepared from a base suspension comprising a dispersing additive, a solvent, and a first or second particle.

[0124] The dispersing additives and solvents contained in the base suspension can be the dispersing additives and solvents described above for coating suspensions.

[0125] The base suspension may contain 1% to 80% by weight of first or second particles. In one embodiment, the base suspension contains 1% to 50% by weight of first or second particles based on the total weight of the respective base suspensions. In a preferred embodiment, the base suspension contains 10% to 40% by weight of first or second particles based on the total weight of the respective base suspensions. In another preferred embodiment, the base suspension may contain 10% to 30% by weight of first or second particles based on the total weight of the respective base suspensions. In another preferred embodiment, the base suspension may contain 15% to 30% by weight of first or second particles based on the total weight of the respective base suspensions. In yet another preferred embodiment, the base suspension contains 35% to 45% by weight of first particles based on the total weight of the respective base suspensions.

[0126] In one embodiment, the base suspension containing the first particle may further contain a dispersing additive in an amount of 0.1 wt% to 5 wt% based on the total weight of the base suspension. In another embodiment, the base suspension containing the first particle may further contain a dispersing additive in an amount of 0.1 wt% to 4 wt% based on the total weight of the base suspension. In another embodiment, the base suspension containing the first particle may further contain a dispersing additive in an amount of 0.1 wt% to 2 wt% based on the total weight of the base suspension. In a preferred embodiment, the base suspension containing the first particle may further contain a dispersing additive in an amount of 0.1 wt% to 2 wt% based on the total weight of the base suspension. In another preferred embodiment, the base suspension containing the first particle may further contain a dispersing additive in an amount of 0.1 wt% to 1 wt% based on the total weight of the base suspension.

[0127] In one embodiment, the base suspension containing the second particle may further contain a dispersing additive in an amount of 1% to 20% by weight based on the total weight of the base suspension. In another embodiment, the base suspension containing the second particle may further contain a dispersing additive in an amount of 1% to 15% by weight based on the total weight of the base suspension. In another embodiment, the base suspension containing the second particle may further contain a dispersing additive in an amount of 1% to 10% by weight based on the total weight of the base suspension. In a preferred embodiment, the base suspension containing the second particle may further contain a dispersing additive in an amount of 2% to 10% by weight based on the total weight of the base suspension. In another preferred embodiment, the base suspension containing the second particle may further contain a dispersing additive in an amount of 3% to 7% by weight based on the total weight of the base suspension.

[0128] In one embodiment, the coating suspension can be prepared by mixing the components contained therein, such as first and second particles, dispersing additives, binders, and solvents.

[0129] In another embodiment, the coating suspension can be prepared by mixing a base suspension containing first and second nanoparticles with a binder and a solvent.

[0130] Mixing can be achieved by inputting mechanical energy to obtain a uniform distribution of particles in the coating suspension. Mechanical energy can be applied by stirring, shaking, or grinding, with the energy input ranging from approximately 0.1 kWh / kg suspension to approximately 15 kWh / kg suspension.

[0131] (b) In another step of the preparation method, a filter layer is formed by contacting the surface of the carrier structure with the coating suspension. In a preferred embodiment, the carrier structure is tubular. For coating, the tube is arranged vertically and filled with the coating suspension using a pump. In another preferred embodiment, the tube is completely filled.

[0132] The coating suspension is brought into contact with the carrier surface for a certain residence time. In one embodiment, the residence time is less than 120 seconds. In a preferred embodiment, the residence time is a maximum of 60 seconds. In another preferred embodiment, the residence time is 60 seconds. In another preferred embodiment, the residence time is 30 seconds. In another embodiment, the residence time is at least 10 seconds. In one preferred embodiment, the residence time is from 10 to 120 seconds. In another preferred embodiment, the residence time is from 10 to 60 seconds. In another preferred embodiment, the residence time is 30 or 60 seconds.

[0133] (c) After the residence time, the coating suspension is carefully removed to leave a film on the carrier. In one embodiment, the suspension is discharged from a tube to leave a thin film of the coating suspension on the carrier surface. The amount of suspension discharged from the tube is referred to as excess coating suspension. Excess coating suspension contains first and second particles that have not adhered to the surface to be coated by adhesion. The thickness of the film remaining on the carrier surface can be controlled by the residence time (through concentration polarization effect) and the shear force caused by the velocity of the coating suspension during discharge from the surface. The first and second particles form a layer on the carrier through interlocking between particles and adhesion enhanced by the capillary effect of the solid carrier structure beneath the film. The layer thickness is controlled by the residence time and the particle concentration in the coating suspension.

[0134] (d) Subsequently, the residual film of the coating suspension is dried. In one embodiment, the film is dried in an air atmosphere. In another embodiment, the film is dried in an air atmosphere at room temperature. In a preferred embodiment, the film is dried for at least 12 hours. In another embodiment, the film is dried in an air atmosphere at a temperature of 60°C to 90°C. In a more preferred embodiment, the film is dried in an air atmosphere at a temperature of 60°C to 90°C for 2 to 6 hours.

[0135] The dried film is subjected to a sintering process.

[0136] (e) In the next step of the preparation method, the dried film is solidified by a sintering step.

[0137] Technicians know that the sintering of films containing particles of at least one metal carbide or at least one metal nitride typically requires high temperatures of around 2,000 °C. Under these conditions, especially at temperatures above 900 °C, metal carbides and metal nitrides are particularly prone to oxidation and rapidly form metal oxides. In other words, it is to be understood that non-oxide ceramic particles, such as SiC and Si3N4 as defined above, are readily oxidized at temperatures above 900 °C when the atmosphere contains oxygen, and under certain conditions this is already the case at 700-800 °C. This means that a SiO2 layer forms on the particle surface during sintering. In other words, the resulting ceramic filter element contains particles with a non-oxide ceramic compound in the core but with an oxide material such as SiO2 on the surface. It is to be understood that this oxidation reaction reduces the mechanical and chemical stability of the resulting filter element. Furthermore, the properties of the filter element depend primarily on the surface, such as the zeta potential of the material. Therefore, the sintering of at least one metal carbide or nitride particle typically requires the isolation of oxygen, for example by replacing the atmosphere with an inert atmosphere or by applying a vacuum, to prevent oxidation, also referred to in the sense of this disclosure as "glass formation" (e.g. when silicon carbide or nitride is used).

[0138] However, the inventors of this disclosure unexpectedly discovered that, due to the high sintering activity of the second particles selected from at least one metal oxide due to their small size, the sintering temperature can be significantly reduced to below the critical temperature of 900°C. The second particles can bond larger first particles together, connecting them in a very strong manner to form a highly durable filter layer. Furthermore, the properties of the filter layer are primarily determined by the inherent properties of the first particles. In this way, the surface of the first particles is only partially covered by the metal oxide. In other words, the surface of the first particles is covered by the metal oxide only to the extent that the second particles bond with the first particles. The residual surface of the first particles remains unchanged, thus determining the properties of the filter element, such as the zeta potential.

[0139] In one embodiment, the first particle is selected from at least one metal carbide or at least one metal nitride, and the sintering step can be performed at a temperature of 300°C to 900°C. In another embodiment, the first particle is selected from at least one metal carbide or at least one metal nitride, and the sintering step can be performed at a temperature of 400°C to 900°C. In a preferred embodiment, the first particle is selected from at least one metal carbide or at least one metal nitride, and the sintering step can be performed at a temperature of 400°C to 700°C.

[0140] It should be understood that the metal carbides or metal nitrides do not oxidize at the stated temperature. Therefore, in one embodiment, the sintering step can be performed in an atmosphere containing less than 50% (v / v) of oxygen. In another embodiment, the sintering step can be performed in an atmosphere containing less than 40% (v / v) of oxygen. In yet another embodiment, the sintering step can be performed in an atmosphere containing less than 30% (v / v) of oxygen. In another preferred embodiment, the sintering step can be performed in an air atmosphere.

[0141] In other words, the sintering step according to this disclosure can be carried out at low temperatures and without a controlled atmosphere. Therefore, the method according to this disclosure provides an inexpensive and convenient method for preparing ceramic filter elements, particularly non-oxide ceramic filter elements.

[0142] In another aspect of this disclosure, due to the high sintering activity of the second particles as described above, a film containing a first particle selected from at least one metal oxide can be sintered at a reduced temperature.

[0143] In one embodiment, the first particle is selected from at least one metal oxide, and the sintering step is performed at a temperature of 300°C to 1,400°C. In another embodiment, the first particle is selected from at least one metal oxide, and the sintering step is performed at a temperature of 300°C to 1,200°C. In a preferred embodiment, the first particle is selected from at least one metal oxide, and the sintering step is performed at a temperature of 400°C to 900°C. In another preferred embodiment, the first particle is selected from at least one metal oxide, and the sintering step is performed at a temperature of 400°C to 700°C.

[0144] In one embodiment, the sintering step can be performed at a temperature of 300°C to 1,200°C. In another embodiment, the sintering step is performed at a temperature of 400°C to 1,100°C. In a preferred embodiment, the sintering step is performed at a temperature of 400°C to 900°C. In another preferred embodiment, the sintering step is performed at a temperature of 400°C to 700°C.

[0145] It should be understood that the above-mentioned temperatures are significantly lower than the sintering temperatures used to date. Therefore, the improvement of the preparation method according to this disclosure lies in the fact that it requires less energy and less effort in terms of the sintering atmosphere, while maintaining or even improving the mechanical strength and filtration properties of the filter layer, such as pore size and zeta potential.

[0146] In one embodiment, the filter element to be sintered can be heated to the sintering temperature at a rate of up to 20°C / min. In another embodiment, the filter element to be sintered can be heated to the sintering temperature at a rate of up to 15°C / min. In yet another embodiment, the filter element to be sintered can be heated to the sintering temperature at a rate of up to 10°C / min. In a preferred embodiment, the filter element to be sintered can be heated to the sintering temperature at a rate of 1°C / min to 10°C / min. In yet another preferred embodiment, the filter element to be sintered can be heated to the sintering temperature at a rate of 1°C / min to 5°C / min.

[0147] It is important to understand that excessively rapid temperature increases can lead to thermal stress and cracking in the carrier structure and / or filter layer, which may reduce the durability of the filter element and limit its availability.

[0148] In one embodiment, the filter element can be held at the sintering temperature for 10 to 600 minutes. In another embodiment, the filter element can be held at the sintering temperature for 20 to 500 minutes. In another embodiment, the filter element can be held at the sintering temperature for 20 to 400 minutes. In another embodiment, the filter element can be held at the sintering temperature for 20 to 300 minutes. In a preferred embodiment, the filter element can be held at the sintering temperature for 30 to 300 minutes. In another preferred embodiment, the filter element can be held at the sintering temperature for 30 to 240 minutes.

[0149] It's important to understand that technicians select the sintering time based on temperature and the sintering activity of the second particle. If the sintering time is too short, the second particle cannot tightly bond the first particle together, resulting in low mechanical strength and low stability to chemical stress in the filter layer.

[0150] In one embodiment, the filter element can be cooled to room temperature at a rate of up to 20°C / min after sintering. In another embodiment, the filter element can be cooled to room temperature at a rate of up to 15°C / min after sintering. In another embodiment, the filter element can be cooled to room temperature at a rate of up to 10°C / min after sintering. In a preferred embodiment, the filter element can be cooled to room temperature at a rate of 1°C / min to 10°C / min after sintering. In another preferred embodiment, the filter element can be cooled to room temperature at a rate of 1°C / min to 5°C / min after sintering. In yet another preferred embodiment, the filter element can be cooled to room temperature at a rate depending on the furnace used after sintering without external cooling.

[0151] It is important to understand that faster cooling, such as through thermal shock cooling, can lead to thermal stress and cracking within the carrier structure and / or filter layer.

[0152] The sintering steps according to this disclosure are solid-state sintering. Solid-state sintering occurs when the powder compact is completely densified in a solid state at the sintering temperature, while liquid-state sintering occurs when a liquid phase is present in the powder compact during sintering. In other words, in solid-state sintering, the composition and sintering temperature prevent the formation of a liquid, and all densification is achieved through the reshaping of the powder. This reshaping, most commonly achieved through solid-state diffusion of atoms, is driven by energy reduction achieved by eliminating the solid-gas interface and replacing it with a solid-solid interface. This method is used for high-quality industrial ceramics. It requires the use of fine powder and high temperatures to allow sufficient atomic diffusion to achieve consolidation within a reasonable time.

[0153] In contrast, liquid-phase sintering is a sintering technique that uses a liquid phase to accelerate the particle bonding of the solid phase. Besides the rapid initial particle rearrangement caused by capillary forces, mass transport through the liquid is typically several orders of magnitude faster than through the solid, enhancing the diffusion mechanisms that drive densification. The liquid phase can be obtained by melting a component, forming a eutectic, or by sintering at a temperature between the liquidus and solidus lines of the component. Furthermore, since the softer phase usually melts first, the resulting microstructure typically consists of hard particles within a ductile matrix, improving the toughness of the originally brittle component. However, due to the increased complexity caused by the presence of additional phases and the rapid solidification rate, liquid-phase sintering is inherently more difficult to predict than solid-phase sintering.

[0154] (f) In one embodiment, steps (b) to (d) of the preparation method can be repeated using the same suspension obtained in step (a) until the desired thickness of the filter layer is obtained. In a preferred embodiment, all steps (b) to (d) are performed at least once using the same suspension obtained in step (a). In another preferred embodiment, steps (b) to (d) of the method are performed no more than six times. In yet another preferred embodiment, steps (b) to (d) are performed a maximum of four times using the same suspension obtained in step (a).

[0155] It should be understood that in the second and any further repetitions of steps (b) to (d), the carrier cannot be coated because a layer was coated on it during the first or previous preparation cycle. In this case, an additional layer can be coated on top of the existing layer, which is advantageous for obtaining a thin filter layer.

[0156] In the sense of this disclosure, a single layer composed of materials with the same chemical composition and particle size is considered as a layer, i.e., a filter layer. Example

[0157] Example 1: Preparation of non-oxide ceramic filter membrane by low-temperature sintering

[0158] Preparation of basic solution

[0159] The crystalline ceramic nanoparticles were obtained from commercial suppliers or ground to achieve the desired particle size. The average particle size was measured using dynamic light scattering (DLS) with a NANO-flex machine (from Microtrac Europe GmbH) prior to coating. 10 D 50 and D 90 The value is given.

[0160] The dispersing additive (e.g., acetic acid or tetramethylammonium hydroxide) is mixed with water until a homogeneous solution is obtained. Then, silicon carbide ceramic powder (D...) is added under vigorous stirring. 10 =280nm, D 50 =520nm, D 90 =1060nm; Z=3.8). To obtain a mechanically dispersed mixture with well-distributed particles, mechanical energy is applied using a bead mill. The applied milling energy depends on the ceramic particle size and is selected within the range of 0.1kWh / kg to 15kWh / kg suspension. In this way, a basic suspension containing a 3 / 1 mixture of solvent and crystalline ceramic nanoparticles (by weight, weight (solvent) / weight (nanoparticles)) and dispersing additives can be obtained.

[0161] In a similar manner, a basic suspension of oxide ceramic nanoparticles containing zirconia nanoparticles (D) was prepared. 10 =2nm, D50 =3nm, D 90 =5nm; Z=2.5; a 4 / 1 mixture of solvent and crystalline ceramic nanoparticles, by weight ratio (solvent) / nanoparticles).

[0162] Paint suspension

[0163] Prior to the coating process, both base suspensions were diluted with water and the binder was added while stirring. Unless otherwise stated, the amount of nanoparticles in the suspension is always expressed as weight percent (wt%).

[0164] A suitable coating suspension has the following composition:

[0165] Solvent: H2O (42% by weight)

[0166] • SiC-based suspension (40% by weight)

[0167] • ZrO2-based suspension (15% by weight)

[0168] • 20% by weight polyvinyl alcohol aqueous solution (3% by weight)

[0169] Coated on the carrier

[0170] The above-described coating suspension is filled into the interior of a vertically oriented ceramic carrier tube. The solution is left in the carrier tube for 60 seconds, and then the coating suspension is drained. The remaining film on the inner surface of the tube is dried. The coated tube can then be sintered under the conditions given in Table 1 until sufficient strength is achieved.

[0171] Characterization of ceramic filter membranes

[0172] ·filter

[0173] Prepare oil dispersions in water (pH 6-8). Set the oil concentration above 5200 ppm and mix the dispersions to obtain size D. 3,50 Oil droplets are approximately 1.4 μm in size. In this case, D 3,50 It is volume D 50 Value. In other words, D 3,50 The particle diameter is defined as such that the volume of particles with a diameter less than or equal to this value is 50% of the total particle volume. At a temperature of 40°C, under a cross-flow rate of 2.0 m / s and a transmembrane pressure of 0.5 bar, the dispersion is pumped through ceramic hollow fibers with a SiC / ZrO2 membrane coating on the inner surface.

[0174] During the filtration test, the flow rate remained constant for 7 days without backwashing, indicating the negative surface charge and low fouling tendency of the filter layer.

[0175] Less than 1 ppm of oil was found in the permeate. On the other hand, more than 99.9% of the oil remained in the feed dispersion. This experiment reveals the excellent filtration properties of the filter element.

[0176] ·ζ potential

[0177] Their zeta potentials were measured using a SurPASS 3 machine obtained from Anton Paar (see [link to relevant documentation]). Figure 3 The membrane was further characterized. For analysis, deionized water, potassium chloride (purity ≥99.5%, obtained from Roth), 0.01 N KOH solution (obtained from Roth) as a base, and 0.1 N HCl aqueous solution (obtained from Roth) were used. The measurements were performed at room temperature (23–26 °C) with solutions containing 1 mmol / L KCl (conductivity 11 mS / m, volume 530 mL).

[0178] Measurements of the zeta potential revealed that despite the addition of oxide ceramic nanoparticles as a binder phase, the characteristic shape of the typical curves obtained for silicon carbide was maintained (see [reference needed]). Figure 3 Therefore, due to the membrane's strong hydrophilicity and its repulsive properties against negatively charged components over a wide pH range, the membrane disclosed herein exhibits the same low tendency to fouling as non-oxide ceramic filter membranes.

[0179] • Coating strength with and without chemical impact

[0180] The mechanical properties of the ceramic filter membrane were evaluated by measuring the coating strength of the membrane coated on the ceramic hollow fibers before and after chemical shock. The chemical shock was applied by storing the membrane in a strongly alkaline solution containing NaOH (pH=14) at 95°C for 4 days.

[0181] Mechanical abrasion is tested by a hand abrasion test, which involves vigorously rubbing the membrane with the index finger and thumb.

[0182] Table 1. Ceramic film coating on the exterior of ceramic hollow fibers under chemical shock. forward Results of finger abrasion test

[0183]

[0184] Table 2. Ceramic film coatings on the exterior of ceramic hollow fibers under chemical shock. back Results of finger abrasion test

[0185]

[0186] nd = Not determined; ---: Very low coating strength, the coating can be easily wiped off completely; --: Low coating strength, the coating can be easily partially wiped off; -: Low coating strength, the coating can be partially wiped off with low pressure; +: Medium coating strength, the coating can be partially wiped off with high pressure; ++: High coating strength, the coating can be wiped off in small amounts with high pressure; +++: Very high coating strength, the coating cannot be wiped off even with extremely high pressure. The figures given as % by weight indicate the amount of the second particle based on the total weight of the first and second particles. The SiC powder used has D 10 =280nm, D 50 =520nm, D 90 =1060nm; the ZrO2 powder used has D 10 =2nm, D 50 =3nm, D 90 =5nm; the TiO2 powder used has D 10 =12-17nm, D 50 =17-22nm, D 90 =25-35nm.

[0187] Finger abrasion tests revealed that the membranes of this disclosure are significantly more stable to mechanical stress than membranes prepared solely from silicon carbide. Therefore, the sintering temperature of the membranes of this disclosure is significantly lower compared to comparative membranes. Even after strong chemical shock, the membranes of this disclosure exhibit good mechanical strength, demonstrating their durability and high relevance for filtration purposes, especially in corrosive media.

[0188] Example 2: Preparation of oxide ceramic filter membrane by low-temperature sintering

[0189] Crystalline ceramic oxide particles are obtained from commercial suppliers or ground until the desired particle size and shape are achieved. The average particle size is expressed as a value D measured by DLS prior to coating. 10 D 50 and D 90 The value is given.

[0190] The dispersing additive (i.e., acetic acid) was mixed with water until a homogeneous solution was obtained. Then, Al2O3 powder (D) was added under vigorous stirring. 10 =140nm, D 50 =250nm, D 90=450nm; Z=3.2). To obtain a mechanically dispersed mixture with well-distributed particles, mechanical energy is applied using a bead mill. The applied milling energy depends on the ceramic particle size and is selected within the range of 0.1kWh / kg to 15kWh / kg suspension. In this way, a basic suspension containing a 3 / 2 mixture of solvent and crystalline ceramic nanoparticles (by weight, weight (solvent) / weight (nanoparticles)) and dispersing additives can be obtained.

[0191] In a similar manner, zirconia nanoparticles (D...) were prepared. 10 =2nm, D 50 =3nm, D 90 =5nm; Z=2.5; a 4 / 1 mixture of solvent and crystalline ceramic nanoparticles, by weight ratio (weight (solvent) / weight (nanoparticles)), or titanium oxide nanoparticles (referred to as TiO2-1; D 10 =14nm, D 50 =18nm, D 90 =28nm; Z=2; a 4 / 1 mixture of solvent and crystalline ceramic nanoparticles, by weight ratio (weight (solvent) / weight (nanoparticles)) or titanium oxide nanoparticles (referred to as TiO2-2; D 10 =29nm, D 50 =38nm, D 90 =59nm; Z=2; a 4 / 1 mixture of solvent and crystalline ceramic nanoparticles, by weight ratio (solvent) / weight (nanoparticles) of oxide ceramic nanoparticles as a basic suspension.

[0192] Paint suspension

[0193] Prior to the coating process, both base suspensions were diluted with water, and the binder was added while stirring, as shown in the following example composition:

[0194] Solvent (H2O): 66.0% by weight

[0195] • Basic suspension (Al2O3): 25.0% by weight

[0196] • Basic suspension (ZrO2): 6.0% by weight

[0197] • Adhesive (20% by weight polyvinyl alcohol in H2O): 3.0% by weight

[0198] Coated on the carrier

[0199] The above-described coating suspension is filled into the interior of a vertically oriented ceramic carrier tube. The solution is left in the carrier tube for 60 seconds, and then the coating suspension is drained. The remaining film on the inner surface of the tube is dried. The coated tube can then be sintered until sufficient strength is achieved.

[0200] Characterization of oxide ceramic filter membranes

[0201] • Aperture measurement

[0202] The pore size D was determined by the flow porosity measurement method. 90 The wavelength is 30 to 40 nm. Although this type of filter element does not exhibit a low tendency to fouling, it can be easily manufactured with low energy consumption.

[0203] • Coating strength without chemical impact

[0204] Furthermore, the filter layer exhibits improved mechanical strength and abrasion resistance. Mechanical abrasion was tested using a finger abrasion test, which involved vigorously rubbing the membrane with the index finger and thumb.

[0205] Table 3 Results of finger abrasion test on oxide ceramic film coatings

[0206]

[0207] ---: Extremely low coating strength; the coating can be easily wiped off completely. --: Low coating strength; the coating can be easily partially wiped off. -: Low coating strength; the coating can be partially wiped off with low pressure. +: Medium coating strength; the coating can be partially wiped off with high pressure. ++: High coating strength; the coating can be partially wiped off with high pressure. +++: Extremely high coating strength; the coating cannot be wiped off even with extremely high pressure. The figures given as a percentage by weight indicate the amount of the second particle based on the total weight of the first and second particles. The Al2O3 powder used has D... 10 =140nm, D 50 =250nm, D 90 =450nm; the ZrO2 powder used has D 10 =2nm, D 50 =3nm, D 90 =5nm; the TiO2-1 powder used has D 10 =14nm, D 50 =18nm, D 90 =28nm. The TiO2-2 powder used has D 10 =29nm, D 50 =38nm, D 90 =59nm.

Claims

1. A ceramic filter element comprising a carrier structure and a filter layer, wherein the filter layer contains at least a first particle and a second particle, wherein the second particle is selected from oxide ceramic particles, and wherein the first particle and the second particle differ at least in their numerical value D. 50 Diameter, characterized in that The numerical value D of the first particle 50 The diameter and the numerical value D of the second particle 50 The ratio of diameter (Y) is between 2 and 5,000. The first particle is selected from SiC, Si3N4, AlN, and mixtures thereof; and The second particle has a value D ranging from 1 nm to 50 nm. 50 Diameter; and The second particle is selected from Al2O3, SiO2, TiO2, ZrO2 and mixtures thereof, and Where the values ​​D of the first particle and the second particle are... 50 The diameter was determined by dynamic light scattering. The second particle is present in an amount of 10% to 30% by weight based on the total weight of the first and second particles, and Where the value D 50 The diameter is defined as the particle diameter in which the proportion of particles with a diameter less than or equal to that value is 50% of the total number of particles.

2. The ceramic filter element according to claim 1, wherein... The first particle is selected from metal carbides or metal nitrides, and the ratio Y is from 5 to 3,000.

3. The ceramic filter element according to claim 1, wherein the ratio Y is less than 4,000 or wherein the ratio Y is greater than 10.

4. The ceramic filter element according to any one of claims 1 to 3, wherein the first particle is AlN and the second particle is Al2O3, or wherein the first particle is AlN and the second particle is ZrO2, or wherein the first particle is AlN and the second particle is TiO2.

5. The ceramic filter element according to any one of claims 1 to 3, wherein the first particle is Si3N4 and the second particle is Al2O3, or wherein the first particle is Si3N4 and the second particle is ZrO2, or wherein the first particle is Si3N4 and the second particle is TiO2.

6. The ceramic filter element according to any one of claims 1 to 3, wherein the first particle is SiC and the second particle is ZrO2, or wherein the first particle is SiC and the second particle is TiO2, or wherein the first particle is SiC and the second particle is Al2O3.

7. The ceramic filter element according to any one of claims 1 to 3, wherein the second particle has a numerical value D of 3 nm to 400 nm. 90 Diameter, wherein the numerical value D of the second particle 90 The diameter was determined by dynamic light scattering, and the value D was [not specified]. 90 The diameter is defined as the particle diameter in which the proportion of particles with a diameter less than or equal to that value is 90% of the total number of particles.

8. The ceramic filter element according to any one of claims 1 to 3, wherein the first particle has a numerical value D of 10 nm to 15 µm. 50 Diameter, wherein the numerical value D of the first particle 50 The diameter was determined by dynamic light scattering.

9. The ceramic filter element according to claim 8, wherein the first particle has a numerical value D of 20 nm to 6 µm. 90 Diameter, wherein the numerical value D of the first particle 90 The diameter was determined by dynamic light scattering, and the value D was [not specified]. 90 The diameter is defined as the particle diameter in which the proportion of particles with a diameter less than or equal to that value is 90% of the total number of particles.

10. A method for preparing a ceramic filter element according to any one of claims 1 to 9, wherein the preparation method comprises the steps of... a) Provides a carrier structure having a carrier surface, and a coating suspension containing a first particle and a second particle; b) Allow the surface of the carrier to contact the coating suspension for a period of time; c) Removing excess paint suspension without removing residual film from the paint suspension; d) Dry the residual film; and e) Sintering the carrier structure with the residual film. f) Optional, repeat steps b) to e).

11. The method of claim 10, wherein the first particle is selected from metal carbides or metal nitrides, and wherein the sintering step is performed at a temperature of 300°C to 900°C.

12. The method according to claim 10 or 11, wherein the sintering step is performed in an oxygen-containing atmosphere.

13. The method of claim 10 or 11, wherein the coating suspension comprises at least 1% by weight of the first particles based on the total weight of the mixture.

14. The method of claim 10 or 11, wherein the coating suspension comprises at least 0.1% by weight of the second particles based on the total weight of the mixture.

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