TECHNICAL FIELD

The present invention relates to a ceramic filter, such as a ceramic monolithic filter suitable for membrane filtration in liquid media and a method for production of a ceramic filter.

BACKGROUND

Membrane filtration processes present several advantages over traditional processes, such as high retention of contaminants, continuous operation, high throughput, easiness to scale up and to operate, small footprint and energy efficiency (Essalhi and Khayet, 2015; Gohil and Ray, 2017; Kim et al., 2017).

Ceramic membranes have several advantages, including higher thermal, mechanical and chemical stability; well-defined pore size distribution; higher hydrophilicity; longer membrane lifetimes; high fluxes at low pressures; higher porosity; and lower fouling (Hofs et al., 2011; Kayvani Fard et al., 2018; Padaki et al., 2015).

To an extent depending on the size of the materials to be filtered out, a distinction is made between microfiltration (MF, ultrafiltration (UF) and nanofiltration (NF). The smaller the grains to be filtered out, the higher the demands imposed on the filter technology and the filter membranes used.

Asymmetric filter membranes usually consist of a thin top-layer responsible for the separating, and a porous ceramic support with single or multiple intermediate layers imparting the required mechanical strength to the composite membrane.

FR 2948295B1 discloses a preparation of a membrane for the tangential filtration of a fluid to be treated containing abrasive particles, which compromise a inorganic porous support composed of carbon, silicon aluminate, silicon carbide or oxide metal pure or in admixture impregnated at the zone for receiving the active inorganic phase separation, with a suspension of inorganic particles, followed by a consolidation step by sintering the active inorganic phase separation.

WO 2006/049940 A2 describes a ceramic multi-channel monolith, which may be composed of one of several oxide or non-oxide ceramic materials with a coat of a substantially infiltrating slurry. The channels are distributed in parallel, between one face and the opposing face of the monolith with a density of 50-400 channels/in ² .

US 10,413,870 B2 discloses a filter membrane of a multi-layer configuration for filtration of a medium. The filter membrane has at least one first layer that has as the main component an oxide ceramic material and is provided with at least one second layer that has a main component a non-oxide ceramic material. The first layer is a carrier layer and the second layer is a separation layer that filters the medium and generates a retentate and a permeate.

US 9,512,041 B2 discloses a process for fabricating a ceramic membrane include providing a porous substrate, at least one intermediate layer disposed on the porous support and an outermost filtration layer comprised of alumina particles having a multi-modal size distribution.

DISCLOSURE OF INVENTION

An objective of the invention is to provide a method of producing a ceramic filter, such as a monolithic ceramic filter suitable for membrane filtration in liquid media, which method results in a ceramic filter of high mechanical and chemical stability and at the same time may be economically attractive.

In an embodiment, it is an objective of the invention to provide a method of producing a ceramic filter with a membrane layer having a desired mean pore size and preferably with a relatively narrow pore size distribution.

In an embodiment, it is an objective of the invention to provide a method of producing a ceramic filter with a membrane layer where the membrane has a relatively narrow mean pore size and at the same time provide that the ceramic filter has a desirable high flux.

In an embodiment, it is an objective of the invention to provide a method of producing a ceramic filter with a membrane layer where the membrane may be produced with a relatively high accuracy to have preselected mean pore size.

In an embodiment, it is an objective of the invention to provide a method of producing a high-quality ceramic filter at a relatively low cost. In an embodiment, it is an objective of the invention to provide a ceramic filter suitable for membrane filtration in liquid media, which filter has high mechanical and chemical stability and at the same time a desirable high flux.

In an embodiment, it is an objective of the invention to provide a ceramic filter with a membrane layer, which membrane layer has a small pore size and at the same time allow a high flow rate through the ceramic filter.

These and other objects have been solved by the invention as defined in the claims and/or as described herein.

It should be emphasized that the term "comprises/comprising" when used herein is to be interpreted as an open term, i.e. it should be taken to specify the presence of specifically stated feature(s), such as element(s), unit(s), integer(s), step(s) component(s) and combination(s) thereof, but does not preclude the presence or addition of one or more other stated features.

Reference made to "some embodiments" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with such embodiment(s) is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in some embodiments" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the skilled person will understand that particular features, structures, or characteristics may be combined in any suitable manner within the scope of the invention as defined by the claims.

The term "substantially" should herein be taken to mean that ordinary product variances and tolerances are comprised.

Throughout the description or claims, the singular encompasses the plural unless otherwise specified or required by the context.

The term particle size and grain size are used interchangeable.

In an embodiment, the particle size(s) (and accordingly grain size(s)) means the primary particle size(s). This may preferably exclude sizes of aggregates and agglomerates. The primary particle size may e.g. be determined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) laser scattering or laser diffraction or a combination comprising at least one of these. In an embodiment, the particle size(s) (and accordingly grain size(s)) means particle size(s) comprises secondary particle size(s), preferably including sizes of aggregates and agglomerates. The particle size(s) comprises secondary particle sizes may e.g. be determined by sieving optionally combined with other particle size determination method(s).

The term disc is applied to mean a raised annulus shape. Annulus is here used as the mathematical term describing a ring-shaped object between two concentric circles. The term 'raised annulus' is here used to render height to the annulus, giving a hollow cylinder of greater diameter than height and greater average wall thickness than height where the height is the average height determined parallel to the center axis of the annulus.

The terms "calcination" and "sintering" are herein used to designate a heat treatment at a temperature resulting in fusing particles together. For easy distinguishing the term "sintering" has been applied for designating the heat treatment applied for forming the ceramic structure front layer and the term "calcination" has been applied designating the heat treatment for forming the membrane layer.

All features of the invention and embodiments of the invention as described herein, including ranges and preferred ranges, may be combined in various ways within the scope of the invention, unless there are specific reasons not to combine such features.

The method of the invention of producing a ceramic filter the method comprises

• providing a porous ceramic structure comprising a porous ceramic structure front layer and a slurry comprising oxide and/or hydroxide ceramic particles,

• coating the slurry of ceramic particles onto at least a first surface portion of the ceramic structure front layer without the slurry fully penetrates the porous ceramic structure,

• drying the coated ceramic structure and calcination the ceramic particles to form a membrane layer.

To ensure a high mechanical and chemical stability of the ceramic filter, the ceramic structure front layer comprises sintered non-oxide ceramic material. The ceramic filter is advantageously a monolithic ceramic filter meaning that it is a rigid, single unit structure.

After a carefully analysis and experimentation, the inventors of the invention have found that the very attractive properties of the porous ceramic structure, such as high mechanical and chemical stability may be combined with relatively low cost of production of the membrane layer. Also, it has been found that the fixation and/or adhesion of the membrane layer to the ceramic structure is surprisingly stable and durably even for filtration at high pressure difference over the filter.

It has been found that he combination of a ceramic structure front layer comprising sintered non-oxide ceramic material with a membrane layer of oxide ceramic material generates a ceramic filter with highly beneficial properties. Since the oxide and/or hydroxide ceramic particles may be calcinated at a substantially lower temperature than the sintering temperature of the non-oxide ceramic material of the ceramic structure, the production of the membrane layer may be produced much faster and at a lower cost than where both the ceramic structure and the membrane layer comprises non-oxide ceramic material.

In addition, it has been found that the membrane layer produced from oxide and/or hydroxide ceramic particles may be produced with a narrower mean pore size than where the membrane layer comprises non-oxide ceramic material. Further, it has been found that due to the relatively low calcination temperature the resulting mean pore size and pore size distribution may be controlled to a high degree, e.g. to obtain a desired mean pore size with a desired narrow pore size distribution, to thereby ensuring a relatively high flux even where the mean pore size is selected to be narrow.

In a desired embodiment the method comprises producing the ceramic structure front layer, comprising providing a green ceramic structure comprising non-oxide ceramic particles and one or more carbon containing additives and sintering the green ceramic structure in inert environment, such as in a vacuum atmosphere or in an inert gas atmosphere, such as in argon, nitrogen or a mixture thereof.

The one or more carbon containing additives may be as described below, for example comprising an organic binder.

It has been found that when the sintering of the green ceramic structure is performed in an inert environment, the sintering comprises a pyrolysis of the additive resulting in formation of carbonaceous residues, which - due to the inert environment - will remain in pores of the ceramic structure front layer.

In an embodiment, the method comprises producing the ceramic structure front layer, comprising providing a green ceramic structure comprising non-oxide ceramic particles and one or more carbon containing additives and sintering the green ceramic structure comprising decomposing the additive to obtain carbonaceous residues located in pores of the ceramic structure front layer.

These carbonaceous residues, e.g. in the form of free carbon, may thereafter serve as a blocking agent preventing particles of the slurry to penetrate fully into the pores of the ceramic structure front layer even where the particles of the slurry are relatively small compared to the ceramic structure front layer mean pore size. Thereby a membrane with much smaller mean pore size may be applied onto a ceramic structure front layer with relatively large mean pore size. Thereby a filter with a filter membrane with very small pores and a support structure comprising the ceramic structure front layer with the larger mean pore size may be applied, wherein due to the much larger pore size of the ceramic structure front layer the flow resistance through the filter will be very low. It has been found that the filter may have a surprisingly high flux while simultaneously having a very narrow membrane layer mean pore size.

Advantageously, the method comprises coating the slurry of ceramic particles onto at least the first surface portion of the ceramic structure front layer without intermediate oxidative purification of the porous ceramic structure front layer.

Advantageously, the method comprises coating the slurry of ceramic particles onto at least the first surface portion of the ceramic structure front layer at a stage where carbonaceous residues are located in pores of the ceramic structure front layer.

The inventors of the present invention has thereby provided a method to obtain a ceramic filter high mechanical and chemical stability and with a membrane layer having relatively and selected small mean pore size and at the same time allow a high flow rate or flux through the filter. The ceramic filter may thus be used in efficient membrane filtration even under elevated temperatures and/or harsh chemical conditions or abrasive conditions.

Advantageously the calcination of the ceramic particles is performed at a temperature below the melting point of the oxide and/or hydroxide ceramic particles. To provide a high control of the resulting pore size of the membrane layer it may in an embodiment be desired to keep the calcination temperature relatively low and optionally increasing the calcination time. In an embodiment, the calcination is performed at a temperature of about 1300 °C or less, of about 1200 °C or less, such as of about 1100 °C or less, such as of a temperature of about 1000 °C or less such as at a temperature of from about 600 °C to about 1300 °C.

In an embodiment, the maximal calcination temperature is up to 2/3, such as up to ¹ /2 of the highest melting point in Kelvin of the oxide and/or hydroxide ceramic particles.

Preferred calcination temperatures are within 600 - 1300 °C, with an optimal temperature or temperature range depending on the oxide or hydroxide. In an embodiment, it is desired to perform the calcination at a temperature within 0.45- 0.55 times the melting temperature given in Kelvin (or average melting temperature) of the particles, preferably at the Tammann temperature of the particles or less than the Tammann temperature.

In an embodiment, the Tammann temperature of a component may be determined as 0.5 Tm, where Tm is its melting point in Kelvin of the particles.

The calcination of the ceramic particles may be performed partly or fully in inert environment, such as in a vacuum atmosphere or in an inert gas atmosphere, such as in argon, nitrogen, or a mixture thereof. For example, a first part of the ceramic particles may be performed in inert environment, followed by submitting an oxidative gas to the treatment environment.

The oxidative may partly or fully remove the carbonaceous residues in the ceramic structure front layer.

In an embodiment, the calcination of the ceramic particles is performed in an oxidative gas atmosphere.

The oxidative gas atmosphere may for example be air and/or oxygen enriched air.

Advantageously, the ceramic structure front layer has a narrow pore size distribution wherein at least 50 % by volume of the ceramic structure front layer pores has a pore size diameter within 0.5 to 2 times the ceramic structure front layer mean pore size, such as wherein at least 90 % by volume of the ceramic structure front layer pores has a pore size diameter within 0.5 to 2 times the ceramic structure front layer mean pore size.

In an embodiment, the ceramic structure front layer has a monomodal pore structure comprising pores having a mean pore size with a narrow pore size distribution. Preferably, at least 50 % by volume of the macropores has a pore size diameter within ± 5 % from the mean pore size, preferably at least about 75 % by volume of the macropores has a pore size diameter within ± 5 % from the mean pore size, preferably at least about 95 % by volume of the macropores has a pore size diameter within ± 5 % from the mean pore size.

By providing the mean pore size with a narrow pore size distribution a high flux through the ceramic structure front layer may be ensured, while simultaneously the mechanical strength of the ceramic structure front layer and support for the membrane layer may be optimized.

The pore size distribution and the mean pore size may be determined using capillary-flow porosimetry, which is a standard method for pore size determination in porous solids.

Advantageously, the mean pore size of the ceramic structure front layer is between 0.05 and 100 pm, such as between 2 and 100 pm, such as between 4 and 50 pm, such as between 6 and 25 pm.

The mean pore size of the membrane layer may advantageously be smaller than the mean pore size of the ceramic structure front layer, such that the actual filtering is provided by the membrane layer, such that any solids passing through the membrane layer also passes through the ceramic structure front layer. Thereby the risk of filter being blocked is very small and the filter may be applied for filtering large volume of liquid suspensions.

In an embodiment, the membrane layer mean pore size is at least about 50% smaller than the ceramic structure front layer pore size. As mentioned above one beneficial effect of the membrane layer provided from the oxide and/or hydroxide ceramic particles is that the mean pore size may be provided with a high accuracy, since the relatively low calcination temperature ensures a relatively low risk of inhomogeneous shrinking or collapse of pores of the membrane layer. Thereby the membrane layer may be produced to have a selected mean pore size and a narrow pore size distribution even where the mean pore size is selected to be relatively small.

In practice, it is desired that the pores of the membrane layer have a narrow pore size distribution wherein at least 50 % by volume of the membrane layer pores has a pore size diameter within 0.5 to 2 times the membrane layer mean pore size, such as wherein at least 90 % by volume of the membrane layer pores has a pore size diameter within 0.5 to 2 times the membrane layer mean pore size.

In an embodiment, the membrane layer mean pore size is up to about 3 % of ceramic structure front layer pore size, such as up to about 1 % of ceramic structure front layer pore size, such as up to about 0.1 % of the ceramic structure front layer pore size.

It has been found that membrane layer mean pore may be as low as about 1 nm or less and still be provided with a narrow pore size distribution, to thereby ensuring a relatively high flux through the membrane layer.

Advantageously, the membrane layer has a mean pore size of from about 1 nm to about 3 pm, such as from about 50 nm to about 2 pm, such as from about 100 nm to about 1 pm.

In an embodiment, the pores of the membrane layer has a narrow pore size distribution wherein at least 50 % by volume of the membrane layer pores has a pore size diameter within ± 5 % from the membrane layer mean pore size, preferably at least about 75 % by volume of the membrane layer has a pore size diameter within ± 5 % from the membrane layer mean pore size, preferably at least about 95 % by volume of the membrane layer pores has a pore size diameter within ± 5 % from the membrane layer mean pore size.

The ceramic structure may advantageously consist of the ceramic structure front layer. The ceramic structure consisting of the ceramic structure front layer is simpler to produce than where the ceramic structure comprises additional elements.

In an embodiment, the ceramic structure one or more support structure supporting the ceramic structure front layer, such as one or more ceramic and/or metallic support structures having larger mean pore size than the ceramic structure front layer pore size. The one or more support structures may e.g. comprise an additional porous ceramic layer where the additional ceramic layer has a mean pore size which is equal to or preferably larger than the mean pore size of the ceramic structure front layer.

The additional ceramic layer may be made of the ceramic materials as described herein for the ceramic structure front layer. Advantageously the additional ceramic layer and the ceramic structure front layer are made from the same ceramic material(s).

In an embodiment, the support structure is a metallic structure.

The support structure should advantageously provide less flow resistance than the ceramic structure front layer.

The ceramic structure front layer comprises at least one non-oxide ceramic component.

The ceramic structure front layer may for example comprises one or more of the ceramic components selected from alumina, zirconia, boride, nitride, silicon carbide or any combinations comprising one or more of these. Preferably, the ceramic structure front layer comprises at least one ceramic component selected from boride, nitride or carbide.

The amount of ceramic structure front layer oxide and/or hydroxide ceramic components should advantageously be kept low to ensure a homogeneous open pore structure. It has been found that oxide and/or hydroxide ceramic components in the ceramic structure front layer comprising non-oxide components may result in an increase in pore size distribution.

It may be desired that the ceramic structure front layer comprises less than 10 mol- %, such as less than 5 mol-%, such as less than 1 mol-% of oxide and/or hydroxide ceramic components. Preferably, the ceramic structure front layer is substantially free of oxide and/or hydroxide ceramic components. In practice, the grains of non oxide and/or hydroxide ceramic materials may comprise traces of oxide and/or hydroxide ceramic materials.

In an embodiment, porous ceramic structure front layer may in principle comprise any non-oxide ceramic components. Advantageously the ceramic structure front layer comprises one or more ceramic components selected from non-oxide ceramics or any combinations thereof, such as preferably but not limited to borides, nitrides or carbides, preferably silicon carbide or any combinations comprising one or more thereof.

In an embodiment, the ceramic structure front layer is a non-oxide ceramic structure front layer, preferably consisting of one or more of the ceramic components boride, nitride or carbide.

In an embodiment, the support of the ceramic structure front layer comprises one single ceramic component.

In an embodiment, the support of the ceramic structure front layer comprises or consist of silicon carbide.

In an embodiment, the ceramic structure front layer is of composite material, preferably comprising two or more different ceramic components.

In an embodiment, the method comprises producing the ceramic structure front layer, the method comprising

• selecting a first ceramic powder with a first mean grain size,

• selecting a second ceramic powder with a second mean grain size that is substantially smaller than the first mean grain size,

• mixing of the first and second ceramic powders with one or more additive to form a paste,

• shaping the paste to a green ceramic structure, and

• sintering the green ceramic structure at a temperature sufficiently high to at least partly sintering the ceramic grains.

The support may for example be produced using the method described in US 7,699,903, preferably with the modification that the sintering is performed in an inert environment.

Advantageously, the size ratio between the mean grain size of the first ceramic powder and the mean grain size of the second ceramic powder lies in the range of approximately 10: 1 to 2:1, such as 6: 1 to 3:1. The mean grain size of the first ceramic powder and/or the mean grain size of the second ceramic powder advantageously has/have a narrow grain size distribution, preferably at least 90 % by weight of the first ceramic powder and/or the second ceramic powder are within about 0.5 and about 2 times the medium grain size of the respective ceramic powder.

The grain size is advantageously determined in accordance with ISO 8486-121996- Bonded abrasives (determination and designation of grain size distribution Part I: Macrogrits F4 to F220), ISO 8486-2zl9967Bonded abrasives— (Determination and designation of grain size distribution— Part 2: Microgrits F230 to F1200) and/or, JAPANESE INDUSTRIAL STANDARD JIS R6001 (1998) (Abrasive Grain Size Distribution).

In an embodiment, the mean grain size of the first ceramic powder is from about 5 pm to about 50 pm and the mean grain size of the second ceramic powder is from about 0.5 pm to about 10 pm, such as wherein the mean grain size of the first ceramic powder is from about 10 pm to about 30 pm and the mean grain size of the second ceramic powder is from about 1 pm to about 5 pm.

Advantageously, the first and/or the second ceramic powder comprises grains of ceramic components selected from alumina, zirconia, boride, nitride, silicon carbide or any combinations comprising one or more of these. Preferably the first and/or the second ceramic powder comprises grains of at least one ceramic component selected from boride, nitride or carbide, more preferably the first and/or the second ceramic powder comprises or consists of silicon carbide.

Ad described above it is desired that the amount of oxide and/or hydroxide ceramic components is kept low or is fully absent.

Advantageously, the first and/or the second ceramic powder comprises grains of ceramic components comprises less than about 10 % by weight of oxide and/or hydroxide ceramic particles, such as less than about 5 % by weight of oxide and/or hydroxide ceramic particles, such as less than about 1% of oxide and/or hydroxide ceramic particles. Preferably, the first and the second ceramic powder is substantially free of oxide and/or hydroxide ceramic particles.

Preferably, the first and the second ceramic powder is free of oxide and/or hydroxide ceramic particles beyond minor unavoidable traces. The additives mainly have the purpose of providing a good adhesion between the grains of the ceramic powders and to ensure a good processability. Advantageously the additives comprise one or more of a binder, a plasticizer, a dispersant, a surfactant a lubricant or any combinations thereof. Usually, the additives will comprise organic components, such as an organic binder.

The paste may be shaped to practically any desired shape. In principle, any method of shaping may be applied. Examples of suitable methods include casting, isostatic pressing, 3D printing, injection molding, extruding, cutting or any combinations thereof.

When the paste is shaped, the shaped structure is referred to as a "green ceramic structure".

The shaping advantageously comprises providing the green ceramic structure to have an elongate shape or a flat annular shape. Such as a cylinder shape or an angular prism shape, with one or more elongate partly or entirely through going channels, such as channels of polygonal, circular or elliptical shape or any combinations thereof.

In an embodiment, the ceramic support has an elongate shape comprising a first and a second end faces and wherein the at least one channel is a through going channel, preferably passing through the first and the second end faces.

The sintering preferably comprises pyrolyzing the additive(s). The additives will pyrolyze at a much lower temperature than the final sintering temperature. It is desired to allow the additive to fully pyrolysis before heating to the final sintering temperature.

In an embodiment, the sintering comprises treating the green ceramic structure at a pyrolyzing temperature followed by treating at a sintering temperature or with a sintering energy for a sufficient time to bind the ceramic grains to form the porous ceramic structure.

Methods of sintering are well known in the art. Preferably, the sintering comprises treating the green ceramic structure at a final sintering temperature for a sufficient time and/or providing sufficient energy to bind the ceramic grains to form the ceramic support. Methods such as solid-state sintering, such as liquid phase sintering, such as pressure less sintering, such as flash sintering, such as spark plasma sintering, microwave sintering.

In an embodiment, the final sintering temperature is at least about 1300 °C, such as at least about 1600 °C, such as up to about 2200 °C.

The oxide and/or hydroxide particles of the slurry may comprises or consist of particles selected from metal oxides and/or metal hydroxides.

The slurry for forming the membrane layer should advantageously be free of non oxide ceramics beyond unavoidable traces. In an embodiment, the slurry for forming the membrane layer is fully free of borides, nitrides and/or carbides.

The parent metal of both oxides and hydroxides may advantageously be chosen from transition metals and semimetals, advantageously group 4 and group 13 metals, advantageously Zirconium hydroxide, zirconium oxide, aluminum hydroxide or aluminum oxide.

Metal hydroxide will generally be converted to the oxide form during the calcination process. However, where the particles comprises metal hydroxide, it is desired that the calcination is performed in an oxidative atmosphere.

In an embodiment, the oxide and/or hydroxide particles of the slurry may comprises or consist of particles selected from oxides and/or hydroxides of scandium, zirconium, aluminum, gallium, indium, germanium, antimony, lanthanum, samarium, hafnium or any combinations comprising one or more thereof.

In an embodiment, the oxide and/or hydroxide particles of the slurry consist of Zirconium (IV) hydroxide and/or zirconia.

Preferably, the slurry is substantially free of non-oxide ceramic components capable of forming part of the membrane.

Advantageously, the oxide and/or hydroxide particles have a mean grain size of less than about 25 % of the mean grain size of the second ceramic powder, preferably less than about 10 % of the mean grain size of the second ceramic powder.

To ensure a membrane layer with a desired small mean pore size the oxide and/or hydroxide particles may advantageously have a mean grain size of less than about 6 mih, such as from about 0.1 nm to about 1 miti, such as from about 1 nm to about 400 nm.

In an embodiment, the particles are selected to have a relatively narrow grain size distribution.

To ensure a desired narrow pore size distribution, the oxide and/or hydroxide particles are advantageously selected to have grain size distribution wherein at least 50 % by number of the particles have a particle size diameter within ± 5 % from the medium particle size, preferably at least about 75 % by number of the particles have a particle size diameter within ± 5 % from the medium particle size, preferably at least about 95 % by number of the particles have particle size diameter within ± 5 % from the medium particle size.

The medium grain size (D50) is the grain size where about 50 % by volume of the grains have a grain size smaller or equal to the medium grain size.

The selection of the grain size of the particles may influence the final pore distribution in the support.

In an embodiment, the particles have a grain size distribution comprising that at least 90 % by weight of the grains is within 0.5 times to 2 times the D50 grain size.

In an alternative embodiment, the oxide and/or hydroxide ceramic particles have a bimodal grain size distribution comprising a fraction of nanoparticles with a larger D50 grain size and a fraction of nanoparticles having another smaller D50 grain size distribution.

The invention also comprises variations where the oxide and/or hydroxide ceramic particles has a multimodal grain size distribution, such as a trimodal or a higher modal distribution.

The slurry is advantageously a colloidal suspension, such as an aqueous or organic suspension. The suspension may advantageously comprise water, poly vinyl alcohol, acetic acid, ethanol, organic binder or any combinations comprising one or more of these. Advantageously, the slurry comprises one or more additive, such as more of a binder, a plasticizer, a dispersant, a surfactant a lubricant or any combinations thereof.

In an embodiment, the membrane forming slurry further comprises a binder, such as an organic binder or an inorganic binder.

In an embodiment the membrane forming slurry further comprises a dispersant, such as an organic dispersant or an inorganic dispersant.

In an embodiment the membrane forming slurry further comprises a solvent such as an aqueous solvent, such as water or an organic solvent or a combination hereof.

The amount of particles in the slurry may be selected in dependence of the desired pore size distribution. The slurry of particles may for example comprise of from about 1 % by weight to about 60 % by weight of the particles. In an embodiment, the slurry of particles comprise from about 10 to about 40 % by weight of the particles.

The membrane layer is advantageously applied onto the support. The thickness of the membrane layer is advantageous very low, such as up to about 80 pm, such as from about 1 to about 25 pm.

The pore size of the membrane layer is advantageously appropriately sized for the membrane application. The main purpose of the membrane layer is to provide separation of particles, micelles and molecules from a liquid suspension or solution.

In principle the membrane layer pores may have any pore size distribution, ensuring that the larger pores are not too large i.e. at least not larger than the mean pore size of the ceramic structure front layer. Hence, to control the pore size and ensure a high flux, it may be beneficial to ensure that the pore size distribution is relatively narrow.

The coating the slurry of ceramic particles onto at least a first surface portion of the ceramic structure front layer may comprise applying the slurry to the first surface portion using any method for wetting and covering at least a first surface portion of the ceramic structure front layer provided that it is performed without the slurry fully penetrates the porous ceramic structure.

In an embodiment, the coating comprises applying the slurry onto the first surface portion of the ceramic structure front layer by a method comprising spreading, spraying, dipping, flowing or any combinations comprising one or more of these.

In an embodiment, the method comprises applying the slurry onto the first surface portion of the ceramic structure front layer by a method comprising providing a turbulent flow over the first surface portion of the ceramic structure front layer. The turbulent flow may e.g. be provided by a flow having a velocity of from about 1 to about 20 m/s.

The application of the slurry onto the first surface portion may e.g. be performed as described in patent WO 2015/018420 A1 using a turbulent flow application procedure but preferably without applying a pressure over the layer.

Advantageously, the slurry for the coating is formulated such that particle concentration and viscosity of the slurry are selected such that the turbulent flow can be provided at an adequate velocity.

In an embodiment, the dynamic viscosity of the slurry is less than 100 cP, such as 80 cP, such as 60 cP.

In an embodiment, it may be sufficient to apply one single layer of the slurry, in particularly where the particle content in the slurry is relatively high.

Advantageously, the method comprises repeating the step of applying slurry, optionally with in between steps of drying and optionally with in between steps of calcination.

In an embodiment, the method comprises repeating the steps of coating, drying and calcinating two or more times, preferably until the formed membrane layer has a preselected thickness, such as a thickness up to about 80 pm, such as from about 1 to about 25 pm.

In an embodiment, the method comprises repeating the steps of coating, drying and calcinating two or more times using identical slurry. The repeated steps of coating may comprise two or more steps of coating and drying followed by calcination of the two or more layers.

In an embodiment, the method comprises repeating the steps of coating, drying and calcinating two or more times using at least one slurry different from a first slurry, wherein the at least one different slurry preferably differs from the first slurry in that the oxide and/or hydroxide particles of the different slurry have a smaller mean particle size than the oxide and/or hydroxide particles of the first slurry. Thereby a property gradient, such as a pore size gradient may be provided in the membrane layer.

In an embodiment, the at least one different slurry preferably differs from the first slurry in that it comprises particles comprising one or more different ceramic oxide and/or hydroxide.

In an embodiment, the method comprises applying the slurry to the portion of the ceramic structure front layer to allow a fraction of the oxide and/or hydroxide ceramic particles to penetrate into the ceramic structure front layer to a depth of up to about 10 times the mean pore size of the ceramic structure front layer. A minor penetration of the oxide and/or hydroxide ceramic particles into the ceramic structure front layer may ensure a very high fixation strength between the ceramic structure front layer and the membrane layer. However, for ensuring a high flux it may be desired that the penetration of the oxide and/or hydroxide ceramic particles into the ceramic structure front layer is only a relatively small penetration depth.

In an embodiment, the method comprises applying the slurry to the portion of the ceramic structure front layer to allow a fraction of the oxide and/or hydroxide ceramic particles to penetrate into the ceramic structure front layer to a depth of up to about 5 times the mean pore size of the ceramic structure front layer, such as up to a depth of the mean pore size of the ceramic structure front layer.

In an embodiment, the method comprises applying the slurry to the portion of the ceramic structure front layer, wherein the carbonaceous residues in the pores of the ceramic structure front layer prevent the particles of the slurry to penetrate into the ceramic structure front layer to a depth exceeding 5 times the mean pore size of the ceramic structure front layer, such as to a depth exceeding 2 times the mean pore size of the ceramic structure front layer such as to a depth exceeding the mean pore size of the ceramic structure front layer.

The amount of carbonaceous residues in the ceramic structure front layer depends on the amount of organic constituents of the additive in the green paste forming the green ceramic structure. Thus, by increasing the amount of organic constituents of the additive in the green paste forming the green ceramic structure, the amount of carbonaceous residues will also increase and thereby provide an increased barrier for the slurry and the particle thereof to penetrate into the pores of the ceramic structure front layer.

In an embodiment, the method comprises applying the slurry to the portion of the ceramic structure front layer to allow a fraction of the oxide and/or hydroxide ceramic particles to penetrate into the ceramic structure front layer to a depth of up to about 200 pm, such as at most at a depth of about 100 pm, such as at most a depth of about 50 pm.

In an embodiment, the method comprises applying the slurry to the portion of the ceramic structure front layer to allow a fraction of the oxide and/or hydroxide ceramic particles to penetrate into the ceramic structure front layer to a depth of up to about 10 pm, such as at most a depth of 5 pm, such as at most a depth of 2 pm.

To ensure a desired penetration depth, the amount of organic constituents of the additive in the green paste forming the green ceramic structure, the oxide and/or hydroxide particles, the viscosity of the slurry and/or the method of coating the slurry onto the ceramic structure front layer is selected to provide that the hydroxide particles penetrates into the ceramic structure front layer to a penetrating depth of at most 200 pm, such as at most at a depth of 100 pm, such as at most a depth of 50 pm or preferably even less, such as at most 5 pm, such as at most, 2 pm, such as at most 1 pm.

Advantageously, the method comprises keeping a second surface portion of the ceramic structure free of the slurry, wherein the method preferably comprises masking the second surface portion of the ceramic structure during the application of the slurry.

Advantageously, the method comprises applying the slurry of the oxide and/or hydroxide ceramic particles in a sufficient amount to provide the membrane layer to have a thickness of at least about two times the mean pore size of the ceramic structure front layer. In principle, it is desired the keep the membrane layer as thin as possible, because the thicker the membrane layer the higher pressure-difference may be required for a desired flow velocity through the membrane. On the other hand, a too thin membrane layer may result in an inadequate filtration and/or durability of the membrane layer. The desired thickness of the membrane layer, therefore, depend largely on the expected use of the ceramic filter. In an embodiment, the membrane layer has a thickness of at least about five times the mean pore size of the ceramic structure front layer, such as up to about 500 times the mean pore size of the ceramic structure front layer, such as up to about 100 times the mean pore size of the ceramic structure front layer, such as up to about 50 times the mean pore size of the ceramic structure front layer.

In an embodiment, the method comprises producing the membrane layer to have a thickness of up to about 80 pm, such as from about 1 to about 25 pm, preferably the method comprises applying the slurry of the oxide and/or hydroxide ceramic particles in a sufficient amount to provide the membrane layer to have a thickness up to about 80 pm, such as from about 1 to about 25 pm.

After drying of the slurry, the porous ceramic structure is subjected to a calcination process, comprising heating treating of the porous ceramic structure in a furnace or reactor at a temperature and for a time period sufficient for calcining the oxide and/or hydroxide ceramic particles.

As mentioned above the calcination temperature of the particles forming the membrane layer may be kept relatively low. Advantageously, the calcination temperature is lower than the sintering temperature, such as at least about 100 °C lower, such as at least about 200 °C lower, such as up to 1200 °C lower, such as up to about 1000 °C lower than the sintering temperature.

Advantageously, the method comprises subjecting the ceramic structure front layer and membrane layer to an oxidative purification for removing optional carbonaceous residues.

The purification step of removing carbonaceous residues may be performed after and/or simultaneously with the step of calcination. Where the calcination is performed in inert atmosphere, it is especially desired to perform a purification after the calcination.

Advantageously, the step of purification comprises removing carbonaceous residues from the pores of the ceramic structure front layer and optionally from the pores of the membrane layer.

The step of purification may conveniently comprise subjecting the ceramic structure front layer and membrane layer to a heat treatment as at least about 400 °C, such as at least about 500 °C, such as at least about 600 °C in an oxidizing atmosphere.

The porous ceramic structure is advantageously a monolithic structure to thereby provide that the filter is a monolithic filter.

The porous ceramic structure advantageously, comprises an outer peripheral surface and at least one through going channel defined by a channel surface, wherein the first surface portion of the ceramic structure front layer is at least a portion on the channel surface or a portion of the outer peripheral surface. Thereby the ceramic filter may be arranged in a filter housing allowing a flow into the channel and out via the outer peripheral surface where the first surface portion is provided by the channel surface or the opposite flow direction where the first surface is provided by the outer peripheral surface.

In an embodiment, the porous ceramic structure has an elongate shape, such as a cylinder shape or an angular prism shape, with one or more elongate through going channels, preferably arranged in a honeycomb structure or a circular structure.

In an embodiment, the ceramic filter comprises an arrangement of channels comprising parallel located channels with a cross-sectional density of up to 40 channels per 1 in ² (0.000645 m ² ).

In an embodiment, the porous ceramic structure comprises at least one through going channel, preferably a plurality of parallel through going channels, wherein the channels are preferably arranged with parallel center axis, preferably the one or more channels comprises a cross-sectional shape selected from polygonal, circular or elliptical or any combinations thereof.

In an embodiment, the porous ceramic structure is disc shaped e.g. by having a hollow short length cylinder shape comprising an annular wall, having an inner wall channel surface defining an inner periphery and an outer wall peripheral surface defining an inner and a front and a rear end surfaces. The height (the short length cylinder) is lower than the width defined by the annular body between the inner and outer periphery.

To provide that the flux over the filter is desirably high, it is desired that the channel(s) is/are not too narrow.

Advantageously, the at least one channel has a cross-sectional minimum dimension, such as a minimum diameter of at least about 1 mm, such as at least about 5 mm, preferably the at least one channel it is surrounded by 1 mm or more of ceramic structure material. The minimum dimension e.g. the diameter may vary along the length of the channel or it may advantageously be substantially constant along the length of the channel.

This structure ensures a very high flux and at the same time ensures that the retention of particles or micelles may relatively high for a selected cut of particle size of particles suspended in the liquid to be filtered, such as even for a selected cut of particle size below 50 nm particle, micelle or molecule size.

According to current theory, it is believed that the relatively low flow resistance is caused by the macropore structure, which forms a network of passage through the ceramic structure front layer while the pore size of the membrane layer ensure the very high retention of small particles or micelles without resulting in any excessive increase in flow resistance.

The invention also comprises a ceramic filter, such as a monolithic filter.

The ceramic filter comprises a porous ceramic structure comprising a ceramic structure front layer and a membrane layer covering at least a first surface portion of the ceramic structure front layer, wherein the ceramic structure front layer comprises sintered non-oxide ceramic material and wherein the membrane layer comprises an oxide and/or hydroxide ceramic membrane layer.

The ceramic filter may be obtained by the method described herein and may advantageously comprises the ceramic components as described herein.

Advantageously, the ceramic structure front layer is of non-oxide ceramic material having a mean pore size of between 0.05 and 100 pm, such as between 2 and 100 mih, and the membrane layer is of oxide ceramic material having a mean pore size of from about 1 nm to about 3 pm.

BRIEF DESCRIPTION OF THE EXAMPLES AND DRAWING

The invention is being illustrated further below in connection with selected examples and embodiments and with reference to the figures. The figures are schematic and may not be drawn to scale.

Figure 1 shows the illustration of an embodiment of a multichannel monolith ceramic filter of SiC with an oxide membrane in the inner channel.

Figure 2 shows the illustration of an embodiment of a monotube monolith ceramic filter of SiC with an oxide membrane on the outer surface.

Figure 3 shows an embodiment of a disc shaped ceramic filter of SiC with an oxide membrane onto the outer surface of the disc.

Figure 4 shows pore size distribution of the SiC support of example 1.

Figure 5 shows the pore size distribution of the membrane made by ZrC>2 in an embodiment of a multichannel ceramic filter of the invention according to example 2.

Figure 6a shows cross section of the membrane and ceramic structure front layer of ZrCh/SIC structure of the invention according to example 2.

Figure 6b shows a surface section of the membrane and ceramic structure front layer of respective ZrC>2 and SIC of an embodiment of the invention according to example 2.

Figure 7 shows the pore size distribution of the membrane made by AI2O3 of a multichannel filter of an embodiment of the invention according to example 3.

Figure 8a shows cross section of the membrane and ceramic structure front layer of respective AI2O3 and SIC a filter of an embodiment of the invention according to example 3.

Figure 8b shows surface section of the membrane and ceramic structure front layer of respectively AI2O3 and SIC of an embodiment of a filter of the invention according to example 3. Figures 9 shows a picture of a prototype disc shaped filter of ZrC>2 membrane on top of SiC ceramic structure front layer.

Figure 10 shows the pore size distribution of the membrane made by ZrC>2 of a disc shaped support forming the ceramic structure front layer of an embodiment of the invention according to example 4.

Figure 11a shows cross section of the membrane and ceramic structure front layer of respective ZrC>2 and SIC of an embodiment of a filter of the invention according to example 4.

Figure lib shows a surface section of the membrane and support of respective ZrC>2 and SIC of an embodiment of a filter of the invention according to example 4.

The ceramic filter shown in figure 1 is seen in a perspective view la and a front view lb. The filter has an elongate shape with a length L and a plurality of parallel through going channels lc. Preferably, either the inner wall defining the channel or the outer wall peripheral surface comprises the oxide membrane as described above.

The ceramic filter shown in figure 2 is seen in a perspective view 2a and a front view 2b. The filter has an elongate shape and is shaped as a hollow cylinder with a length L and with one single through going channels 2c.

The ceramic filter shown in figure 3 is seen in a perspective view 3a and a front view 3b. The filter is shaped as a disc with an annular body 3d surrounding a through going channel 3c and with a wall thickness W larger than its height H. It has a front surface 3e and an opposite not shown rear surface.

Advantageously the membrane will be on the front and rear surfaces 3e and on the inner periphery or the outer periphery of the annular body 3d, depending on the application.

In the shown embodiment, the disc is symmetrical around the center axis of the channel 3c. In variations there of the disc has an asymmetrical shape in the width W and/or the height varies along the annulus. For example, in an embodiment the width at a first annular location is X and in a second opposite location is X + a and the height FI at the first location is Y and in the second opposite location is Y - 2a. Example 1

A number of monolithic porous ceramic structures were produced using the method described in US 7,699,903. For each porous ceramic structure, a paste of a-SiC powder with well-defined particle size distribution was produced and shaped into a multi-channel monolith ceramic structure, dried, and sintered at an appropriate temperature. The monolith structures were shaped to have a cylindrical form with a length L of 305 and a diameter of 25.4 mm. Each monolithic structure contained 30 elongate through going channels extending in the length of the monolithic structure. Each channel was 3 mm in diameter.

To obtain monoliths with a finer or coarser core, namely SiCf and SiCc, two different first ceramic powders with respective first mean particle sizes were used: 17.3 pm (fine) and 36.5 pm (coarse). Each of these first ceramic powders were mixed with a second ceramic powder with a second mean particle size about 0.1 times the respective first mean particle size.

The monolithic porous ceramic structures had a monomodal macropore structure, with a mean pore size of 7.38 pm. The pore size distribution of the SiC support is shown in figure 4.

Example 2

A membrane was applied to a monolithic porous ceramic structure of example 1. which therefor formed the ceramic structure front layer as described herein. A suspension of sub-p sized ZrC>2 particles was prepared and applied onto the inner surface of the channels. The suspension was dried and calcined, to form a ZrC>2 membrane. The membrane had a mean pore size of 117 nm and a thickness of 40- 50 pm. Figure 5 shows pore size distribution of the membrane.

The membrane resulted in that the external surface roughness was reduced without altering the composition of the body portion of the support. Figure 6 (left) shows the cross section of the membrane of ZrCk and support (ceramic structure front layer) of SiC, with a good adhesion of the membrane and support. Figure 6 (right) shows the defect-free membrane, with special focus in the square of the good joining of the grain during the calcination step. The obtained filter with SiC support and oxide membrane was analyzed to calculate the clean water permeability. This was performed by filtration of the filter with clean water feed and a permeability value of around 800 L-hrm ^bar ¹ was obtained.

Example 3

A monolithic porous ceramic structure of example 1 was coated a suspension of sub- m sized AI ₂ O ₃ particles, which was prepared and applied onto the inner surface of the channels. The suspension was dried and calcined, to form an AI ₂ O ₃ membrane. The membrane had a mean pore size of 210 nm and a thickness of 40-50 pm.

Figure 7 shows pore size distribution of the membrane.

The membrane resulted in that the external surface roughness was reduced without altering the composition of the body portion of the support. Figure 8 (left) shows the cross section of the membrane of AI ₂ O ₃ and support of SiC, with a good adhesion of the membrane and support (ceramic structure front layer). Figure 8 (right) shows the defect-free membrane, with special focus in the square of the good joining of the grain of AI ₂ O ₃ during the sintering step.

The obtained filter with membrane was analyzed to calculate the clean water permeability. This was performed by filtration of the membrane with clean water feed and a permeability value of around 1800 L-hrm ^bar ¹ was obtained.

Example 4

A number of monolithic porous ceramic structures were produced using the method described in US 7,699,903 and in the same way of example 1. Each ceramic structure (ceramic structure front layer) was shaped to have a disc shape as illustrated in figure 3 with an outer diameter of 152 mm and an inner diameter of 26 mm.

A membrane was applied to the monolithic porous ceramic structures. A suspension of sub-p sized ZrC>2 particles was prepared and applied onto the front and rear surfaces (Fig. 3, 3e) and on the outer periphery of the annular body of the disc. The suspension was dried and calcined, to form a ZrC>2 membrane. The membrane had a mean pore size of 880 nm and a thickness of 50-80 pm. Figure 9 is a picture showing the prototype hybrid membrane developed and figure 10 shows pore size distribution of the membrane.

The membrane resulted in that the external surface roughness was reduced without altering the composition of the body portion of the support. Figure 11 (left) shows the cross section of the membrane of ZrCb and support of SiC, with a good adhesion of the membrane and support. Figure 11 (right) shows the defect-free membrane, with special focus in the square of the good joining of the grain during the sintering step.

The obtained filters with membranes were analyzed to calculate the strength by four bending breaking tests. The hybrid membranes have a strength of 46±6 MPa.

The ceramic supports obtained in examples 2-4 were further analyzed.

It was found that the coating and calcination with oxides or hydroxides resulted in homogeneous and defect-free membranes on top of supports (ceramic structure front layers) without infiltration into the support. In all the examples, there were a good deposition of the membrane on top of the support with a good adhesion between the oxide membrane and non-oxide support.

With the appropriate calcination temperature of the membrane i.e. temperatures between 600 and 1300 °C as described above, there is a perfect sintering of the particles in the membrane with a good joining between them and reduction of the pore size.