GB2277885A

GB2277885A - Ceramic filter and membrane

Info

Publication number: GB2277885A
Application number: GB9222857A
Authority: GB
Inventors: Richard John Wakeman; John Leslie Henshall; Swee Gim Ng
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-10-31
Filing date: 1992-10-31
Publication date: 1994-11-16
Also published as: GB9222857D0

Abstract

A filter element or membrane comprises basically silicon nitride but also contains some other elements which affect its properties, such as Al, O, Y, Ca, Mg, Li, Nd, La, Ce, Sm, Dy and Yb. The material may form a porous support for a finer membrane of the same material or a monolithic filter element, and can be in the form of a flat section, a tube, a cylindrical block with parallel passages or a layer form on a mesh support. Its porosity exceeds 25% and its pore size falls in the range <0.005 - 1000 microns. Filtration may be assisted by sonic or ultrasonic vibrations and/or a constant, varying or alternating electric field. Applications include separation of fluids from each other and from solids by filtration, dialysis, reverse osmosis etc.

Description

Description POROUS SELF-SUPPORTING CERAMIC FILTER AND MEMBRANE This invention relates to porous self-supporting ceramic materials, more particularly to porous low density silicon nitride and its alloys and methods for their use. The term alloy is defined to mean any combination of elements where they are chemically combined and where said elements are defined in the following embodiment. The porous materials comprise holes in their external surfaces interconnected bv pores so forming a permeable structure.

Solid-fluid and droplet-fluid filtration and separation technologies play an important role in industry, especially in the removal of fine solid and fluid particulates and colloids-from liquid and gas dispersions. Filtration of solids, or organic species such as cells, from a fluid stream is undertaken to either collect the filter deposit for use or disposal, or to enhance the quality of the fluid.

Ceramic membrane micro- and ultra- filters have been available for a number of years. A common arrangement for these is to form the ceramic membrane units into tubes or flat sheets or log modules having multi-channels and incorporate these into a liquid circulation system such that the liquid to be filtered passes from a pump via a pipe to the membrane where it is flowed tangentially to the surface of the membrane. The known ceramic materials used in these systems are silica, alumina, silicon carbide, an alumino-silicate and a partially stabilised zirconia. These are usable only over limited ranges of pH and can be produced with only a limited pore size range and are brittle and can have low mechanical strength. For example, a zirconia membrane disintegrates after immersion for a period in a caustic soda solution at 600 C.

The present invention aims to provide an improved porous ceramic filter material that, for example, has a greater chemical resistance and has greater physical strength and can be made available in a wide range of pore sizes. A series of monolithic and composite filters and membranes based on silicon nitride and its alloys and compounds has been developed. These filters are made with porosities in excess of 25%, high permeabilities and controlled pore sizes. For example, the composite structure of a sialon membrane laid down on a silicon nitride filter can be controlled during its manufacture to have a mean flow pore size of 0.4 micron, a maximum flow pore size of 0.6 micron, a porosity in excess of 50%, and a permeability of 6x10-14 m2. The same composite structure has favourable corrosion resistance. For example, there is no deterioration of the structure or of its filtering characteristics after immersion for long periods in either three molar hydrochloric acid or three molar caustic soda solutions at 70"C, and there is no deterioration of its structure or filtering capabilities after being heated to 7000C in an oxidising atmosphere.

These filters may be used to replace existing ones and to enable filtration to take place in aggresive environments beyond the capabilities of existing filter and membrane materials.

A device, or devices, may be used to simultaneously apply a field or fields to influence the filtration process. The fields may be electrical, magnetic, sonic or ultrasonic.

Accordingly, the present invention provides a self-supported porous ceramic filter or membrane having a porosity greater than 25% and having interconnecting pores said pores having a mean pore diameter of less than 1000 microns, said ceramic being in the physi- cal form of a flat sheet, corrugated sheet, tube or log, which contains one or more layers of silicon nitride based material. The silicon nitride derived filter may be infiltrated, or otherwise bonded, to a support of ceramic, metal or polymer.

In a preferred form of this invention, the porous ceramic filter or membrane material comprises of the elements silicon, aluminium, oxygen and nitrogen combined chemically or combined chemically together with other chemical elements which affect the processing and properties including yttrium, calcium, magnesium, lithium, neodynium, lanthanum, cerium, samarium, dysprosium and ytterbium.

In a preferred embodiment of the invention the filters and membranes can be used in their flat or corrugated sheet forms or their tubular or log forms as a crossflow filter in which a particle carrying fluid is flowed tangential to the surface of said filters or membranes whilst a cleaned fluid is collected having passed through the said filters or membranes. An alternative use of the said filters or membranes is in a dead-end filter unit where the feed flow is predominantly normal to the filtering surface. Either the crossflow or the dead-end filtration unit may comprise a plurality of the self-supporting porous filters or membranes which may be arranged in series or in parallel or in a series-parallel configuration with each filter or membrane in the configuration having the same or different pore size from its adjacent filter or membrane. For microfiltration the required pore size would be in a range from 0.1 to 10 microns, for ultrafiltration an appropriate pore size would be in a range from 0.005 to 0.1 micron, and even smaller pore sizes would be used in reverse osmosis.

For filtration of a gas, pore sizes up to 1000 microns may be used.

In addition to micro- and ultra-filtration and reverse osmosis, the filter and membrane materials of the present invention may also have applicability in techniques such as electrofiltration, electroacoustic filtration, dialysis, di afiltrat ion, electro- dialysis, membrane distil]ation, pervaporation, immobilised cell culture and hot or cold gas separations. Thus it will be appreciated that the terms filtration or separation are meant to include separation techniques such as gas-liquid, gas-gas and liquid-liquid separations.

A further embodiment of the invention is that the pore size may increase from one face of the filter or membrane through the thickness of its porous structure to the opposing face. For example, the porous structure may comprise two or more layers of the silicon nitride based ceramic, each layer may have a differing chemical composition according to the previous description, with the smallest pores in the layers forming the faces in contact with the fluid to be filtered.

The present invention will now be described by way of example with reference to the accompanying drawings. Figures 1 to 4 show cross-sectional schematic diagrams of specific embodiments of these filters and membranes.

In Figures l(a)-(c), which represent flat plate filters or membranes, the fluid can flow from one side, H, through the filter to the other, L, or vice versa. In addition, there may be fluid flowing parallel to either or both surfaces H or L. Item 1 is a porous silicon nitride based ceramic alloy. Items 2 and 6 may also be porous silicon nitride based ceramic alloy or alternative porous ceramic alloys. As shown in Figure 1(b), one surface H of item 2 may be covered in whole or in part by a layer or plurality of layers. For example, three layers of porous material(s), items 3, 4 and 5, at least one layer of which is a silicon nitride ceramic alloy. Alternatively, as depicted in Figure 1(c), both surfaces H and L may be covered in whole or in part by a layer or plurality of layers of porous material(s), for example items 7 to 11, at least one of which is a silicon nitride based ceramic alloy.

In addition, items 1, 2 and 6 may be corrugated, rather than flat, plates.

Figures 2(a)-(c) are cross-sectional schematic diagrams of tubular forms of the filter. The carrier fluid flows either along the hollow internal channel of the tube, I, and/or the external surface, O. In addition, fluid is transferred from the inside of the tube to the outside, or vice versa, through the interconnected pores in the filter. Item 12 is a porous silicon nitride based ceramic alloy. Items 13 and 16 may also be porous silicon nitride based ceramic alloy or alternative porous ceramic alloys. As shown in Figure 2(b), the internal or external surface of the tube may be covered in whole or in part by a layer or plurality of layers of porous materials. This is exemplified in Figure 2(b) with two layers attached to the internal surface of the tube, items 14 and 15. At least one of these layers is a silicon nitride based ceramic alloy. Alternatively, as depicted in Figure 2(c), both surfaces I and O may be covered in whole or in part by a layer or plurality of layers, for example items 17 to 20, of porous material(s) at least one of which is a porous silicon nitride based ceramic alloy.

In addition, the cross-sections of items 12, 13 and 16 depicted schematically in Figures 2(a)-(c) may be non-circular, for example oval or rectangular.

Figure 3 is a schematic cross-sectional diagram of the log form of of the filter, item 21.

This consists of a plurality of hollow internal channels C in a porous ceramic body. The carrier fluid flows either along these internal channels, C, and/or the external surface, S.

In addition fluid is transferred either between the internal channels or between the internal channels and the external surface. In addition the surfaces of the channels and/or external surface of the log, item 21, may be covered in whole or in part by a layer or plurality of layers of porous materials. Either item 21 or one of the layers is a porous silicon nitride based ceramic alloy.

In addition, the cross-sections of item 21 and the hollow internal channels C in Figure 3 may be non-circular, for example oval or rectangular.

Figures 4(a) and (b) are schematic diagrams of a wire mesh, item 22, into which a porous ceramic is introduced, item 23. This structure may be coated in whole or in part by a layer or plurality of layers, item 24, of porous ceramic or ceramics of which one would be a silicon nitride based alloy. A fibrous mat of material may be used as an alternative for item 22.

Claims

1. A ceramic porous self-supporting filter based upon silicon nitride, but also containing other chemical elements which affect the processing and properties, including aluminium, oxygen, yttrium, calcium, magnesium, lithium, neodynium, lanthanum, cerium, samarium, dysprosium and ytterbium.

2. A ceramic porous self-supporting membrane based upon silicon nitride, but also containing other chemical elements which affect the processing and properties, including aluminium, oxygen, yttrium, calcium. magnesium, lithium, neodynium, lanthanum, cerium, samarium, dysprosium and ytterbium.

3. A ceramic alloy filter according to Claim 1 bonded to a ceramic membrane according to Claim 2 to form a composite filter-membrane structure where the number of filters in the composite structure may be equal to or greater than one and where the number of membranes in the composite structure may be equal to or greater than one.

4. A ceramic alloy filter or membrane according to Claims 1,2 and 3 which is bonded into or onto a supporting mat.

5. A filter or membrane according to Claims 1,2,3 or 4 which comprises a porous ceramic having a porosity in excess of 25% internal phase volume and having interconnected pores, said pore sizes increasing from one major face through the thickness of the filter, membrane or filter-membrane composite to the other major face, the smaller pores being of the desired size for the required filtration effect.

6. A filter or membrane according to Claims 1,2,3 or 4, in which the pore size is from 10 to 1000 microns.

7. A filter or membrane according to Claims 1,2,3 or 4, in which the pore size is from 0.1 to 10 microns.

8. A filter or membrane according to Claims 1,2,3 or 4, in which the pore size is from 0.005 to 0.1 microns.

9. A filter or membrane according to Claims 1,2,3 or 4, in which the pore size is less than 0.005 micron.

10. A filter or membrane according to Claims 6,7,8 or 9, used in the presence of an constant, varying or alternating electric field applied to the filtration process.

11. A filter or membrane according to Claims 6,7,8 or 9, used in the presence of a sonic or ultrasonic field applied to the filtration process.

12. A filter or membrane according to Claims 6,7,8 or 9, used in the presence of electric, sonic or ultrasonic fields which are being applied in any combination or simultaneously within the filtration process.