MEMBRANE, A METHOD OF IMPROVING A

CERAMIC FILTER MEMBRANE AND THE CERAMIC FILTER MEMBRANE OBTAINED BY THE METHOD TECHNICAL FIELD

The invention relates to ceramic filter membranes and in particular to a method of producing a ceramic filter membrane as well as a method of improving already produced ceramic filter membranes.

BACKGROUND ART Ceramic filter membranes are well known for both gas filtration and liquid filtration, for example for gas exhaustion filtration, water filtration and filtration within the chemical and food industry in general. Ceramic filter membranes have many beneficial properties e.g. high strength, high resistance towards aggressive chemicals and further they can withstand high temperatures, and accordingly such ceramic filter membranes may be regenerated by burning out captured particles.

There are two main flow configurations of membrane processes: cross-flow and dead-end filtrations. In cross-flow filtration the feed flow is tangential to the surface of the membrane, retentate (liquid and solid parts that do not pass through the filter) is removed from the same side further downstream, whereas the permeate flow is tracked on the other side. In dead-end filtration the direction of the fluid flow is normal to the membrane surface. 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. Several methods of producing ceramic filter membranes are known. Generally the methods comprise the step of forming a ceramic paste, forming the paste e.g. by extrusion to the desired shape, drying and burning of and/or sintering the material.

US 7,699,903 describes the production of a multi layered SiC ceramic filter body for cross-flow filtration. The method described comprises providing a main layer (called the carrier) by providing a paste/slurry comprising bimodal grain sizes a-SiC, extruding or casting the paste/slurry to the desired shape and firing the extruded and dried paste to recrystallization of the grains and with a treatment time and temperature such that the smaller grains dissolve and the material thereof deposits to form bonds between the larger grains, to thereby form the SiC carrier. A filter membrane layer is applied onto the carrier by providing a slurry/paste similar to the first paste but with smaller bimodal grain sizes, the second slurry/paste is applied onto the carrier, dried and fired.

EP 1 607 129 describes a method of producing a multi layer ceramic filter membrane from a dispersion comprising an aggregate (e.g. alumina, mulite, cordierite, silicon carbide, pottery scrap), a sintering assistant (alumina, silica, zirconia, titania, glass frit, feldspar, cordierite) and organic binder. The method comprises extruding the dispersion to form a desired body shape, drying and firing. A filtration membrane is applied onto inner wall surfaces of main flow path of the porous body by providing a slurry containing ceramic particles which is smaller than the aggregate of the first dispersion and further comprising organic binder and sintering assistant. The slurry is circulated in the flow path and simultaneously a pressure on an outer peripheral surface side is reduced to ensure deposition of a layer of the slurry. The body is dried and fired and optionally additional layers with smaller ceramic particles are applied. US 2008/0096751 describes a method of producing an UF membrane where a ceramic sol is deposited on the surface of a base member, allowed to drop down owing to the weight of the ceramic sol itself, and discharged from the surface of the base member. The ceramic sol which is not discharged is deposited on the surface of the base member and the ceramic sol is dried and fired. Generally the methods of production of ceramic filter membranes aim at providing a very narrow pore size distribution at the surface in order to have a well-defined particle size cutoff and simultaneously have a high flux (rate of flow through the membrane per surface area of the membrane). The ceramic filter membranes produced today, in particular for liquid filtration, have a relatively narrow surface pore size distribution. However, during drying and firing of the ceramic material formation of cracks takes place depending on the composition of the ceramic material and the method used. DISCLOSURE OF INVENTION

An object of the present invention is to provide a method of producing a ceramic filter membrane comprising a relatively narrow pore size distribution at its surface compared to prior art ceramic filter membranes. In an embodiment it is an object to provide a method of producing a ceramic filter membrane with a high flux and a relatively sharp particle size cutoff.

In an embodiment it is an object to provide a method of improving already produced and optionally used ceramic filter membranes.

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

It has been found that the invention or embodiments thereof have a number of additional advantages which will be clear to the skilled person from the following description.

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.

The term "substantially" should herein be taken to mean that ordinary product variances and tolerances are comprised. According to the invention a very simple method for producing a ceramic filter membrane of a very high quality is provided. The method comprises providing a porous substrate and applying a membrane forming dispersion onto the substrate where the membrane forming dispersion comprises a first fraction of ceramic particles having a larger medium grain size (D-50) and a second fraction of ceramic particles having a smaller medium grain size(D-50) than the first fraction. The dispersion is dried and sintered to form a coherent membrane layer on the porous substrate. The membrane layer has a membrane layer surface facing away from the porous substrate.

The method further comprises subjecting the membrane layer surface to a surface treatment using a surface treatment fluid dispersion comprising a first fraction of ceramic particles having a medium grain size (D-50) which is in the interval from about -10 % to about + 10 % relative to the larger medium grain size of the membrane forming dispersion and a second fraction of ceramic particles having a smaller medium grain size (D-50) than the first fraction of the surface treatment fluid dispersion.

The surface treatment comprises flowing the surface treatment fluid

dispersion in a main flow direction substantially along the membrane layer surface in a turbulent flow while simultaneously providing a pressure difference over the membrane layer to cause that a part of the surface treatment fluid dispersion is passing into the membrane layer. The pressure difference over the membrane layer is provided by applying a pressure difference over a porous structure comprising the porous substrate and the membrane layer. After a selected treatment time the surface treated membrane layer is dried and sintered.

It has been found that the surface treatment results in a high improvement of the produced ceramic filter membrane. The risk that particles larger than the cutoff particle size are passing through the ceramic filter membrane produced by the method is highly reduced or even eliminated. It is believed that the surface treatment results in filling potential cracks or other defects which may be in the membrane layer surface without adding a full layer to the

membrane layer. Due to the selection of the D-50 value of the grain size of the ceramic particles, the flux through the membrane remains substantially unchanged.

The method of the invention can in practice be used for production of any kind of ceramic filter membranes including cross-flow filters and dead-end filters. The method is particularly suitable for use in the production of ceramic filter membranes for liquid filtration such as MF, UF or NF filtration.

The method preferably comprises the following steps i-vi: i. providing a porous substrate;

ii. providing a membrane forming dispersion comprising a first fraction of ceramic particles having a larger medium grain size (D-50) and a second fraction of ceramic particles having a smaller medium grain size(D-50) than the first fraction of the membrane forming dispersion; iii. applying a layer of the membrane forming dispersion onto a surface of the porous substrate, drying and sintering the ceramic particles to form a coherent membrane layer on the porous substrate, the membrane layer has a membrane layer surface facing away from the porous substrate;

iv. providing a surface treatment fluid dispersion comprising a first fraction of ceramic particles having a medium grain size (D-50) which is in the interval from about -10 % to about + 10 % relative to the larger medium grain size of the membrane forming dispersion and a second fraction of ceramic particles having a smaller medium grain size (D-50) than the first fraction of the surface treatment fluid dispersion;

v. surface treating the membrane layer by passing the surface treatment fluid dispersion in a main flow direction substantially along the membrane layer surface in a turbulent flow while simultaneously providing a pressure difference over the membrane layer to cause that a part of the surface treatment fluid dispersion is passing into the membrane layer and

vi. drying and sintering the surface treated membrane. In an embodiment the steps iii-vi are repeated. In most situations it is sufficient to perform steps iii-vi once, however, in situations where there are many cracks or defects it may be advantageous to repeat steps iii-iv.

The porous substrate can in principle be any kind of substrate onto which the membrane forming dispersion can be applied. The porous substrate advantageously has pores with a larger size than the D-50 of the first fraction of ceramic particles of the membrane forming dispersion. The porous substrate is advantageously a ceramic porous substrate. By using a ceramic porous substrate with larger pores than the D-50 of the first fraction of ceramic particles of the membrane forming dispersion, the membrane forming dispersion can be applied in a relatively thin layer while still resulting in a relatively mechanically strong filter membrane. Generally it has been found that thicker layers of membrane forming dispersion results in more cracks or defects during sintering than thinner layers of membrane forming dispersion is thinner.

In an embodiment the porous substrate is a porous ceramic substrate preferably having a substantially larger porosity than the ceramic filter membrane obtained by the method, preferably the porous substrate is obtainable from a method comprising sintering of ceramic particles comprising a first fraction of ceramic particles having a larger medium grain size (D-50) and a second fraction of ceramic particles having a smaller medium grain size(D-50) than the first fraction of the membrane forming dispersion.

In an embodiment the porous substrate is a ceramic porous substrate optionally produced as the ceramic body described in US 6,699,903. The porous substrate can comprise 1, 2, 3 or more layers having different average pore sizes. In an embodiment the porous substrate is an organic porous substrate which burns up during sintering. By this method it is possible to provide very thin filter membranes. The porous substrate can in principle have any shape. In an embodiment the porous substrate comprises a plate shape, a channel shape or a multi-channel shape.

In an embodiment the porous substrate comprises an elongate substrate body, comprising one or more hollow channels arranged in its elongate direction.

In an embodiment the porous substrate comprises a circular substrate body having a periphery and a center axis with a feeding opening and a plurality of channels extending to its periphery.

The shape of the porous substrate is selected in dependence on the desired shape of the final filter membrane.

The membrane forming dispersion is for example applied in a thickness resulting in a membrane layer thickness from about 3 times D-50 of the first fraction of ceramic particles of the membrane forming dispersion to about 1 cm, such as from about 5 times D-50 of the first fraction of ceramic particles of the membrane forming dispersion to about 5mm.

The membrane forming dispersion is advantageously in the form of a paste. The amount of liquid in the paste is selected in dependence on the thickness of the membrane layer. Since the liquid should be dries of prior to sintering, the amount of liquid is advantageously selected to be as low as possible.

After the membrane forming dispersion has been applied the membrane forming dispersion layer is dried and sintered e.g. as described below. After sintering the membrane layer formed from the membrane forming dispersion is cooled down to ambient temperature prior to performing the surface treatment. The cooling down process is preferably a passive cooling down at ambient temperature. A too fast cool down procedure may increase the forming of cracks and defects in the membrane layer.

In practice any ceramic particles can be applied in the present invention alone or in combination with other types.

In an embodiment the ceramic particles comprise particles of ceramic oxides, such as alumina, zirconia, spinel (a combination of magnesium and aluminum oxides), mullite (a combination of aluminum and silicon oxides); ceramic non- oxides, such as carbides, borides, nitrides, silicates and silicides; and combinations and compositions thereof.

In an embodiment the ceramic particles comprise particles of alumina, zirconia, titania, boron nitride, silica, mullite, silicon carbide (SiC), and combinations thereof.

Generally it is preferred that the ceramic particles of non-oxide ceramic material and preferably the ceramic particles are or comprise silicon carbide particles and in particular of the alpha type (a-SiC).

An advantage of using non-oxide ceramic material, such as a-SiC is that no melt phase occurs during recrystallization and so the resulting ceramic body is essentially free of a vitreous structure or a melt phase, which may be formed during sintering using other ceramic particles, in particular where the particles other than a-SiC are used for the second fraction of particles.

SiC recrystallizes without melting simply via surface diffusion or gas transport. Smaller grains generally recrystallize at lower temperature than larger grains. In the recrystallization process the small grains dissolve and the material is deposited again on energetically more favorable points, which is especially where two large grains make contact. Thereby the smaller grains act as an adhesive between larger grains to hold the larger grains in position without change in volume of the ceramic layer from prior to the sintering.

Advantageously the ceramic particles of the second fraction of the membrane forming dispersion and/ or of the second fraction of the surface treatment fluid dispersion are a-SiC particles, since in particular a-SiC particles can be recrystallized in a simple and effective way.

Further information about a-SiC particles and recrystallization thereof can e.g. be found in US 7,699,903.

In an embodiment the ceramic particles of the first fraction of respectively the membrane forming dispersion and the surface treatment fluid dispersion are of a ceramic material having a recrystallization temperature equal to or higher than the recrystallization temperature of the ceramic material of the ceramic particles of respectively the second fraction of the membrane forming dispersion and the second fraction of the surface treatment fluid dispersion. Thereby potential melting, sublimation or recrystallizing of parts of the first fraction of particles can be kept at a low level or even be avoided.

In an embodiment the ceramic particles of the first fraction of respectively the membrane forming dispersion and the surface treatment fluid dispersion are of a ceramic material having a recrystallization temperature which is at least about 5 °C, such as at least about 10 °C or preferably at least about 20 °C higher than the recrystallization temperature of the ceramic material of the ceramic particles of respectively the second fraction of the membrane forming dispersion and the second fraction of the surface treatment fluid dispersion.

In an embodiment the ceramic particles of the first fraction of the membrane forming dispersion are of a ceramic material having a recrystallization temperature which is at least about 50 °C higher than the recrystallization temperature of the ceramic material of the ceramic particles of the second fraction of the membrane forming dispersion. In an embodiment the ceramic particles of the first fraction of the surface treatment fluid dispersion are of a ceramic material having a recrystallization temperature which is at least about 50 °C higher than the recrystallization temperature of the ceramic material of the ceramic particles of the second fraction of the surface treatment fluid dispersion.

In an embodiment the ceramic particles of the first fraction of respectively the membrane forming dispersion and the surface treatment fluid dispersion are of a ceramic material having a melting or sublimation temperature equal to or preferably higher than the recrystallization of the ceramic material of respectively the second fraction of the ceramic particles of the membrane forming dispersion and the second fraction of the surface treatment fluid dispersion.

The difference between the D-50 for the ceramic particles of the first fraction relative to the D-50 for the ceramic particles of the second fraction of the membrane forming dispersion and/or of the surface treatment fluid dispersion can for example be as the difference of D-50 values of the bimodal particle distributions of ceramic particle described in US 7,699,903.

In an embodiment the ceramic particles of the first fraction of the membrane forming dispersion has a D-50 which is at least about two times the D-50 of the second fraction of the membrane forming dispersion.

Advantageously the ceramic particles of the first fraction of the membrane forming dispersion have a D-50 which is at least about 5 times, such as at least about 10 times the D-50 of the second fraction of the membrane forming dispersion. In an embodiment the ceramic particles of the first fraction of the surface treatment fluid dispersion have a D-50 which is at least about two times the D-50 of the second fraction of the surface treatment fluid dispersion. Advantageously the first fraction of the surface treatment fluid dispersion has a D-50 which is at least about 5 times, such as at least about 10 times the D- 50 of the second fraction of the surface treatment fluid dispersion.

It has been found that a very good and homogeneous result can be obtained where the first fraction of the membrane forming dispersion and the first fraction of the surface treatment fluid dispersion have equal D-50.

Thereby any cracks or defects formed in the membrane layer before the surface treatment can be repaired by the surface treatment almost or fully without any visible trace in the final filter membrane. Advantageously the first fraction of the membrane forming dispersion and the first fraction of the surface treatment fluid dispersion are of the same ceramic material.

In an embodiment the second fraction of the membrane forming dispersion and the second fraction of the surface treatment fluid dispersion are of the same ceramic material and preferably have equal D-50.

In an embodiment the first fraction of the ceramic particles of the membrane forming dispersion has a D-50 of from about 0.5 μιη to about 1 μιη and the second fraction of ceramic particles of the membrane forming dispersion has a D-50 of from about 1/10 to about 1/20 of the D-50 of the first fraction. Preferably the ceramic particles of both the first and the second fraction are SiC, preferably at least the particles of the second fraction are a-SiC particles.

In an embodiment the first fraction of the ceramic particles of the surface treatment fluid dispersion has a D-50 of from about 10 % smaller than the D- 50 of the first particles of the membrane forming dispersion to about the D-50 of the first particles of the membrane forming dispersion and the second fraction of ceramic particles of the membrane forming dispersion has a D-50 of from about 1/10 to about 1/20 of the D-50 of the first fraction of the surface treatment fluid dispersion. Preferably the ceramic particles of both the first and the second fraction are SiC, preferably at least the particles of the second fraction are a-SiC particles.

The amount of the first fraction of respectively the membrane forming dispersion and the surface treatment fluid dispersion relative to the amount of respectively the second fraction of the membrane forming dispersion and the second fraction of the surface treatment fluid dispersion is advantageously from about 10:1 to about 1:1 w/w, such as from about 5:1 to about 2:1.

In an embodiment the membrane forming dispersion and/or the surface treatment fluid dispersion further comprises a binder, such as an organic binder or an inorganic binder.

In an embodiment the membrane forming dispersion and/or the surface treatment fluid dispersion further comprises pore forming organic elements, such as organic elements of a desired size or sizes, which pore forming elements will be burned off during sintering, thereby forming pores. Further the membrane forming dispersion and the surface treatment fluid dispersion comprise processing liquid such as water to provide the desired particle concentration and viscosity.

Advantageously the ratio of particles to processing liquid of the membrane forming dispersion is from about 1:0.1 to about 1:10 vol ./vol. Advantageously the ratio of particles to processing liquid of the surface treatment fluid dispersion is from about 1:1 to 1:1000 vol. /vol. On the one hand, the amount of particles should not be too high, since it can then be difficult to obtain a suitable flowability and turbulence. On the other hand, the amount of particles should not be too low since a very low ratio of particles relative to processing liquid results in an increased surface treatment time.

The particle concentration and viscosity of the surface treatment fluid dispersion are preferably selected such that the turbulent flow can be provided at an adequate velocity. In an embodiment the dynamic viscosity of the surface treatment fluid dispersion is less than 0.1 Pa s.

Where the ceramic particles are all SiC, in particular comprising a-SiC particles, the respective dispersions advantageously comprise no binder or any other organic additives.

The surface treating of the membrane layer preferably comprises flowing the surface treatment fluid dispersion in a main flow direction substantially along the membrane layer surface in a turbulent flow at Reynolds number of at least about 2300, preferably larger than about 3000, such as larger than about 5000.

Preferably the method comprising flowing the fluid dispersion along the membrane layer surface with a turbulent flow at Reynolds number of from about 3000 to about 8000.

If the flow is too low, a layer of the surface treatment fluid dispersion may be formed on the top of membrane layer formed by the membrane forming dispersion and simultaneously defects and/or cracks may not be fully repaired. If the flow is too high, the particles of the surface treatment fluid dispersion may not be captured in defects and/or cracks and this may also result in that such defects and/or cracks may not be fully repaired. Advantageously the surface treatment fluid dispersion is caused to flow along the membrane layer surface with a minimum velocity of at least about 2 m/s, preferably at least about 5 m/s, such as at least about 8 m/s, such as at least about 10 m/s.

In an embodiment the surface treatment fluid dispersion is caused to flow along the membrane layer surface with an average velocity of from about 5 to about 20 m/s, preferably from about 8 to about 15 m/s.

In order to ensure that particles pass into the membrane layer and to ensure that particles are captured in defects and/or cracks of the membrane layer formed by the membrane forming dispersion it is desired to apply a pressure difference over membrane. In practice this is done by applying a pressure difference over the substrate and the membrane which together forms a porous structure. This pressure difference is also called a trans-membrane- pressure (TMP).

In an embodiment the TMP during at least a part of the surface treatment of the membrane layer is at least about 10 kPa, such as at least about 50 kPa, such as at least about 100 kPa.

Preferably the TMP during at least a part of the surface treatment of the membrane layer is from about 10 kPa to about 500 kPa.

In an embodiment the TPM during at least about 50 percent of the surface treatment is from about 10 kPa to about 500 kPa.

In an embodiment the pressure difference over the porous structure is a pulsed pressure difference. Thereby a more full repair of cracks and/or defects can be obtained. The pulsed pressure difference can e.g. be provided with a base TMP of at least about 10 kPa and a pulsed TMP of at least about 10 kPa higher than the base TMP.

Advantageously the surface treatment comprises flowing the surface treatment fluid dispersion in a main flow direction substantially along the membrane layer surface in a turbulent flow for a sufficient time to deposit an amount of the ceramic particles of the surface treatment fluid dispersion into openings of the membrane layer, preferably the flowing of the surface treatment fluid dispersion is performed for at least about 5 minutes, such as at least about 30 minutes, such as from 1 hour to 24 hours. The term "substantially along the membrane layer surface" means that the flow is provided in a general direction with a velocity in the direction along the membrane layer surface. When the surface treatment has continued for a sufficient time the surface treatment is terminated and not deposited part of the surface treatment fluid is removed.

In an embodiment the method comprises terminating the surface treatment by stopping the flow of surface treatment fluid dispersion and removing the not deposited part of the surface treatment fluid dispersion prior to removing the pressure difference over the porous structure comprising the membrane layer. Thereby the major part of particles captured in cavities such as cracks and defects of the membrane layer remain there. Preferably the pressure difference over the porous structure with the membrane layer is maintained for at least one minute after termination of the flow of surface treatment fluid dispersion. Optionally an airflow in the direction from the surface of the membrane layer and through the porous substrate is provided to pre-dry the membrane layer. The pre-drying can ensure that almost all of the captured particles remain in the membrane layer.

Preferably the surface treated membrane is subjected to drying for a sufficient time to dry out a substantial amount of liquid. During the drying the surface treated membrane is advantageously held in a stand-still position.

After drying the surface treated membrane is sintered to form the final ceramic filter membrane.

The sintering can e.g. be performed as described in US 7,699,903. Examples of suitable sintering temperatures can also be found herein.

In an embodiment the sintering of the ceramic particles to form a coherent membrane layer on the porous substrate is performed at a temperature and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles, without any substantial melting or sublimation of the first fraction of ceramic particles, preferably the sintering of the ceramic particles to form a coherent membrane layer on the porous substrate is performed at a temperature and for a time sufficient to recrystallize substantially all of the second fraction of ceramic particles, without any substantial melting or sublimation of the first fraction the ceramic particles of the second fraction.

Advantageously the sintering is performed at ambient pressure. In an embodiment the sintering is performed at increased pressure to ensure that the pores do not collapse. In an embodiment the sintering is performed at a slight overpressure of up to about 20 mbar.

Preferably the sintering temperature is at least about 1680°C, for example with a sintering time of from about 1 hour to about 6 hours.

In most situations it is desired that the sintering temperature does not exceed about 2200°C.

The sintering time is usually from about 1 hour to about 6 hours. Where the sintering temperature is relatively low, longer sintering time can be applied.

Advantageously the sintering of the ceramic particles to form a coherent membrane layer on the porous substrate is performed at a temperature and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles, without any substantial recrystallization of the first fraction of ceramic particles, preferably the sintering of the ceramic particles to form a coherent membrane layer on the porous substrate is performed at a temperature and for a time sufficient to recrystallize substantially all of the second fraction of ceramic particles, without any substantial recrystallization of the first fraction of ceramic particles of the second fraction.

The sintering of the surface treated membrane is performed as the sintering of the ceramic particles to form a coherent membrane layer on the porous substrate described above i.e. using same sintering temperature and sintering time. The sintering time can be shorter but preferably the sintering time is at least about 1 hour.

After sintering the final membrane is cooled down to ambient temperature. The invention also comprises a method of improving a ceramic filter

membrane, such as a used membrane. By this method used ceramic filter membranes can be repaired and be reused. The method of improving a ceramic filter membrane is as the method described above excluding the first steps of providing a substrate and forming a membrane layer on the substrate. Instead the surface treatment is performed on a surface of the ceramic filter membrane which thereby will be improved.

The method of improving a ceramic filter membrane comprises a. providing a surface treatment fluid dispersion comprising a first fraction of ceramic particles having a medium grain size (D-50) and a second fraction of ceramic particles having a smaller medium grain size (D-50) than the first fraction of the surface treatment fluid dispersion, where the D-50 of the first fraction of ceramic particles is larger than the average pore size of a membrane surface to be treated and is smaller than the maximum pore size of the membrane surface to be treated.; b. surface treating a surface of the ceramic filter membrane by passing the surface treatment fluid dispersion in a main flow direction

substantially along the membrane surface in a turbulent flow while simultaneously providing a pressure difference over the ceramic filter membrane such that a part of the surface treatment fluid dispersion is passing into the membrane and

c. drying and sintering the surface treated ceramic filter membrane.

The surface treatment fluid dispersion is advantageously as described above, and the method is advantageously performed as in steps iii-vi as described above where the membrane layer is replaced with the ceramic filter

membrane to be improved.

The invention also relates to a ceramic filter membrane obtainable by the method. The ceramic filter membrane advantageously has a cutoff particle size of about 1 pm or less, preferably the filter membrane has a cutoff particle size of about 500 nm or less, such as about 400 nm or less.

In an embodiment the filter membrane has a cutoff particle size of about 100 nm or less, such as about 50 nm or less. Such a ceramic filter membrane has shown to be useful for sterile filtration and has the further beneficial properties that it has a very high flux while simultaneously providing a high certainty against passing of any undesired particles with a size above the cutoff particle size. The rating of cutoff particle size of a filter refers to the diameter of the largest spherical glass particle, which will pass through the filter under laboratory conditions and at a pressure difference not exceeding a damaging point or up tolO bars.

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 features.

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

All features of the inventions including ranges and preferred ranges can be combined in various ways within the scope of the invention, unless there are specific reasons for not to combine such features.

It should be understood that the examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. EXAMPLES

Example 1

Substrate A porous substrate is provided from a bimodal ceramic powder comprising a first particle fraction of a-SiC particles having a D-50 of 30-60 pm and a second fraction of a-SiC particles having a D-50 of about 0.6 pm and where the ratio of the first and the second particle fraction is about 3:1. The bimodal ceramic powder is dispersed in demineralized water to provide a paste and extruded to form a honeycomb shape with an elongate multi-channel structure comprising hollow channels in its elongate direction. The

honeycomb shaped green substrate is dried and sintered at a temperature and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles without any substantial melting or sublimation of the first fraction of ceramic particles. Finally the substrate is cooled.

The porous honeycomb shaped substrate has an outer diameter of about 146 mm, a length of about 865 mm, a porosity of about 43% and a cutoff particle size of about 3 pm.

Membrane forming dispersion A membrane forming dispersion is provided from a bimodal ceramic powder comprising a first particle fraction of a-SiC particles having a D-50 of about 0.6 pm and a second fraction of a-SiC particles having a D-50 of about 0.2 pm and where the ratio of the first and the second particle fraction is about 3:1. The bimodal ceramic powder is dispersed in demineralized water and up to 2 % by volume of the bimodal ceramic powder of pore forming organic particles of up to about 0.6 pm is added to provide a dispersion with a solid content of about 30 % by volume and a viscosity of about 60 cp.

Applying a layer of the membrane forming dispersion The membrane forming dispersion is added to the porous honeycomb shaped substrate using a "Push-Pull" process. The dispersion is pumped into the substrate, and the excessive material is removed by changing the flow direction. Drying and sintering

The coated substrate is dried at 60°C dry air until completely dry.

Thereafter the coated substrate is sintered with a soak-time ranging from 0.5 to 8 hours and at a temperature (2100°C/1700°C) and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles without any substantial melting or sublimation of the first fraction of ceramic particles. Thereby the basic membrane layer is provided.

Providing a surface treatment fluid dispersion

A surface treatment fluid dispersion is provided from a bimodal ceramic powder comprising a first particle fraction of a-SiC particles having a D-50 of about 0.6 pm and a second fraction of a-SiC particles having a D-50 of about 0.2 pm and where the ratio of the first and the second particle fraction is about 3:1. The bimodal ceramic powder is dispersed in demineralized water to a solid content of about 0.1 %.

Surface treating the membrane layer Above mentioned dispersion is in a main flow direction substantially along the membrane layer surface in a turbulent flow to provide a cross flow through the filter membrane walls of the hollow channels. The minimum velocity is about 2 m/s at the exit of the retentate. TMP is kept above 100 kPa.

Treatment time is about 30 minutes. The treatment is terminated by stopping the flow while maintaining a TMP. Drying and sintering

Treated membrane is dried at 60°C dry air until completely dry. The TMP is maintained at a positive value e.g. by the drying air. Thereafter the coated substrate is sintered with a soak-time ranging from 0.5 to 8 hours and at a temperature (1900°C/1600°C) and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles without any substantial melting or sublimation of the first fraction of ceramic particles.

Resulting filter

The final honeycomb shaped cross-flow membrane is substantially free of defects in the membrane layer, and has a tightening effect of the pore size distribution in general. The final honeycomb shaped cross-flow membrane has a cutoff particle size which is less than the cutoff particle size of the corresponding honeycomb shaped cross-flow membrane with basic

membrane layer without the surface treatment. Example 2

Substrate

A porous substrate is provided as described in example 1. The second layer is provided as the basic membrane layer as described in example 1.

Membrane forming dispersion

A membrane forming dispersion is provided from a bimodal ceramic powder comprising a first particle fraction of a-SiC particles having a D-50 of about 0.4 pm and a second fraction of a-SiC particles having a D-50 of about 0.02 pm and where the ratio of the first and the second particle fraction is about 4: 1. The bimodal ceramic powder is dispersed in demineralized water and minor amount of organic binder to provide a dispersion with a solid content of about 30 % by volume and a viscosity of about 60 cp. Applying a layer of the membrane forming dispersion

The membrane forming dispersion is added to the porous honeycomb shaped substrate as described in example 1.

Drying and sintering The coated substrate is dried at 60°C dry air until completely dry.

Thereafter the coated substrate is sintered with a soak-time ranging from 0.5 to 8 hours and at a temperature and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles without any substantial melting or sublimation of the first fraction of ceramic particles. Thereby the basic membrane layer is provided.

Providing a surface treatment fluid dispersion

A surface treatment fluid dispersion is provided from a bimodal ceramic powder comprising a first particle fraction of a-SiC particles having a D-50 of about 0.4 pm and a second fraction of a-SiC particles having a D-50 of about 0.02 pm and where the ratio of the first and the second particle fraction is about 4: 1. The bimodal ceramic powder is dispersed in demineralized water to a solid content of about 0.1 %.

Surface treating the membrane layer

The surface treatment is performed as described in example 1 . Drying and sintering

The surface treated membrane is dried at 60°C dry air until completely dry. The TMP is maintained at a positive value e.g. by the drying air.

Thereafter the coated substrate is sintered with a soak-time ranging from 0.5 to 8 hours and at a temperature and for a time sufficient to recrystallize a major amount of the second fraction of ceramic particles without any substantial melting or sublimation of the first fraction of ceramic particles. Resulting filter

The final honeycomb shaped cross-flow membrane is substantially free of defects in the membrane layer and has a cutoff particle size of 500 nm or less.

Example 3

The filter is produced as in example 1 with the difference that the first particle fraction of the substrate is mullite.

Example 4 The filter is produced as in example 1 with the difference that the first particle fraction of the membrane forming dispersion and of the treatment fluid dispersion is SIC (multi phase).

Example 5 The filter is produced as in example 1 with the difference that the TMP during the surface treatment is about 200 kPa.

Example 6 The filter is produced as in example 1 with the difference that the TMP during the surface treatment is pulsed between about 100 kPa and about 200 kPa.