Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D39/00—Filtering material for liquid or gaseous fluids
  • B01D39/14—Other self-supporting filtering material ; Other filtering material
  • B01D39/20—Other self-supporting filtering material ; Other filtering material of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires
  • B01D39/2068—Other inorganic materials, e.g. ceramics
  • B01D39/2072—Other inorganic materials, e.g. ceramics the material being particulate or granular
  • B01D39/2075—Other inorganic materials, e.g. ceramics the material being particulate or granular sintered or bonded by inorganic agents
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B38/00—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
  • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
  • C04B41/5025—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials with ceramic materials
  • C04B41/5031—Alumina
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
  • C04B41/81—Coating or impregnation
  • C04B41/85—Coating or impregnation with inorganic materials
  • C04B41/87—Ceramics
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/04—Additives and treatments of the filtering material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/04—Additives and treatments of the filtering material
  • B01D2239/0457—Specific fire retardant or heat resistant properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/04—Additives and treatments of the filtering material
  • B01D2239/0471—Surface coating material
  • B01D2239/0478—Surface coating material on a layer of the filter
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/06—Filter cloth, e.g. knitted, woven non-woven; self-supported material
  • B01D2239/065—More than one layer present in the filtering material
  • B01D2239/0654—Support layers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/08—Special characteristics of binders
  • B01D2239/086—Binders between particles or fibres
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/10—Filtering material manufacturing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/12—Special parameters characterising the filtering material
  • B01D2239/125—Size distribution
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
  • B01D2239/12—Special parameters characterising the filtering material
  • B01D2239/1291—Other parameters
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
  • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
  • C04B2111/00793—Uses not provided for elsewhere in C04B2111/00 as filters or diaphragms

Definitions

• the invention relates to a heat-, corrosion- and creep- resistant filter element for separating solid particles from a gaseous or liquid medium, which filter element is made up of a bearing support layer and a coating formed thereon.
• the invention also relates to the said support layer and to the fabrication of the support layer and the filter element.
• honeycomb filters have also been used in power plants; the construction of honeycomb filters is illustrated well by the structures disclosed in US patents 4,632,683 and 5,198,007.
• the problem with these above-mentioned structures is, however, the difficulty of their cleaning owing to their large filtering surface area and the structure which complicates cleaning.
• the structure disclosed in US patent 5,454,947, wherein the coating is inside a tubular filtering element, would be poor for pressurized gases in a power plant application, since the cleaning of the element is difficult owing to the uneven distribution of the cleaning pulse as a consequence of the shape of the body.
• the cleaning pulse may also cause thermal stresses in the coating, and as a result the coating may become detached from the support. Furthermore, the structure of already existing filtering units should be altered radically, since the direction of the gas flow is reverse (from inside out) to that in filter units using "candles.”
• the filtration temperature may be 850...1000 °C during combustion. Since in gasification the gas has to be cooled before combustion in order to remove alkalis and sulfur, the filtration temperature is determined according to the sorbent material. With iron-based sorbents , the gas temperature is at its highest 650 °C and with calcium-based sorbents 980 °C. In connection with the development of new experimental combustion techniques, filtration experiments have also been conducted at temperatures of 870...1100 °C.
• the decrease in the viscosity of the binder additionally reduces the creep resistance of the binder, i.e. its mechanical resistance at high temperatures under load. Reduced creep resistance may cause breakage of the element in the operating conditions.
• Alkali compounds affect silica also through chemical reactions. Typical causative agents of high-temperature corrosion are alkali salts, among them Na 2 C0 3 yielding sodium silicates as reaction products. These corrosion products may, for example in consequence to a change in volume, split off, whereby new surface exposed to reactions is produced.
• the third disadvantage of silica consists of uncontrolled phase changes, particularly in atmospheres containing water vapor. Phase changes cause in the structure internal stress states and cracking, which together or separately weaken the strength of the body significantly.
• silicon carbide serving as a support material serving as a support material
• the problem with silicon carbide serving as a support material is its oxidation to silica at high temperatures. Water vapor in particular promotes this reaction, since the solubility of water vapor in silica is many times that of oxygen, and thus silica formed on the surface of silicon carbide will not protect the silicon carbide under the surface from oxidation. Thus the silica formed as an oxidation product of silicon carbide will further increase the amount of silica in the binder.
• a support structure such as this, containing a large amount of silica is mechanically weak in the operating conditions owing to creep, chemical reactions and uncontrolled phase changes. In the operating environment there thus follows breakage of the filter before the end of the maintenance interval of the filter system.
• Prior known filters also have the disadvantages of the reacting of the silica present in the inorganic binder of the coating layer with a solid or gaseous ingredient being filtered. Thereupon the viscosity of the binder in the coating decreases and the binder is transported in the gas flow, clogging the surface layer. Clogging is also increased by the bonding by sintering of the solid ingredient being filtered to the coating, which may be a consequence of the drop in the viscosity of the binder of the coating. Owing to the clogging, the pressure difference across the filter increases, since the dirt sintered to the filter will no longer be detached by the cleaning pulse. This reduces the efficiency of the filtering apparatus.
• the cause of breakage of a candle filter is often mechanical stress, for example vibrations, a thermal shock, or creep owing to the weight of the candle itself.
• mechanical stress for example vibrations, a thermal shock, or creep owing to the weight of the candle itself.
• changes occurring in the filter structure at high temperatures may reduce its strength to a fraction of its strength at room temperature.
• the load causing the final breakdown may be very small.
• WO 87/01610 disclose a filter with a double-layer structure and the use of ceramic materials as filtering materials in the surface layer.
• the inorganic, most commonly silica-based binder for binding the coating fibers or particles, present in the coating layers described in these patents reacts with the material to be filtered. This may cause ashes to be sintered to the coating, whereupon the coating is clogged.
• the silica-based phase may also be transported in the gas flow and, as a consequence, the porosity and pore size of the coating may increase. If the binder of the coating becomes removed from the surface it is also probable that the coating will easily detach from the support structure.
• the reactions of the silica-based binder of the support also reduces the strength of the body and contributes to the breakage of the element in operation.
• Patent publications FI 92804, US 4,629,483 and WO 87/01610 describe the use of ceramic materials as filter support materials. However, these patents contain no mention that the structural particles of the filter support structure would react chemically with the inorganic binder used in the support, thereby forming, as a reaction product, a new compound which constitutes part of the binder.
• US patent 4,678,758 describes a filter which is intended for the filtering of molten metal, such as iron and ferrous alloys.
• the filter is made up of round, hollow, sintered spheres made up of alumina or some other oxide and having a diameter of 0.2...3 mm. These oxide spheres are reaction- sintered together with the help of a binder component, such as magnesium silicate or calcium silicate, whereby a reaction-sintered product is formed which bonds the spheres.
• a binder component such as magnesium silicate or calcium silicate
• this product is made up of very large spheres and yields a product in which the pore size is very large (which is, of course necessary for operation in the filtering of molten metal), the strength of the filter is very low.
• magnesium silicate or calcium silicate as the binder component, however, a relatively high strength can be obtained.
• the object of the present invention is to provide a novel ceramic support layer for a heat-, corrosion- and creep- resistant filter element, which support layer withstands the filtration of hot fluids, in particular gases within a temperature range of 800...1600 °C, in particular 800...1000 °C, for a long term of use.
• the pore size of the support layer must be such that the pressure loss of the fluid being filtered will remain within acceptable limits.
• Another object of the invention is to provide said filtration element, which consists of a support layer and a coating formed thereon.
• a third object is to provide a novel method for manufacturing said filter element, in which method the support layer and the coating are fired in one step.
• a fourth object of the invention is to provide a coating which does not contain a separate inorganic binder (e.g. silica) which may during operation react with the material being filtered, thereby causing clogging of the filter. Therefore, an option has been developed wherein the coating is made only of a ceramic material which does not contain silica or any other components readily reacting with the material being filtered. No inorganic binder is used for binding the coating particles to each other; instead, the coating particles, made of a chemically resistant material, are in direct contact with each other. Thus the porosity of the coating is produced from the pores remaining between the coating particles attached to each other in a sintering process .
• a separate inorganic binder e.g. silica
• the invention thus relates to a filter element for fluids, the filter element which is resistant to high temperatures, in particular 800...1600 °C, and comprises a support layer and a coating formed thereon, the pore size of the support layer being greater than the pore size of the coating.
• the support layer is made up of inorganic structural particles which are bonded together by a reaction product produced in a reaction-sintering process between the said structural particles and a binder formed by an inorganic starting component. At least 60 % by weight of the structural particle amount consists of structural particles having a diameter of 87...1000 ⁇ m, i.e. -18, +170 mesh, as determined by the screening method.
• the average particle size (d 50 ) of the structural particles also remains within these limits.
• the selection of the material will be most successful when the binder formed by the inorganic starting component is reaction-sintered with the structural particles of the support to a reaction product which will become part of the binder.
• the binder may also contain a reaction product formed as a result of reaction-sintering within a starting component or between the starting components, usually at lower temperatures .
• an “inorganic starting component” is meant in this context not only inorganic compounds but also organo etallic and organoceramic compounds.
• the recommended diameter of the structural particles is above 87 ⁇ m and below 200 ⁇ m, i.e. -74, +170 mesh, in order for the particle size distribution of the support structure to be sufficiently narrow, whereby a pore size distribution optimal for the filter support properties is obtained, for example for strength and permeability.
• the average particle size d 50 is above 87 ⁇ m and below 200 ⁇ m.
• 80 % by weight of the structural particle material consists of particles which pass through a 200 ⁇ m, i.e. 74 mesh, screen of a screen series.
• the structural particles are of alumina (Al 2 0 3 ).
• the alumina particles of the support layer may be of any -Al 2 0 3 , e.g. an -Al 2 0 3 prepared by the Bayer process, i.e. activated (through dehydration of aluminum hydroxide) or calcined (low sodium-content grades, pure grades, tabular alumina, or alumina prepared through molten state).
• the alumina used may also be a natural alumina, i.e. corundum or sapphire. It is especially preferable that the support particles are of synthetic alumina made from molten state. The form of the particles may vary from spherical to angular.
• the starting component of the reaction-product-containing binder according to the invention may be any aluminum silicate mineral, for example kaolinites, halloysite, allophane, chlorite, cyanite, sillimanite, andalusite, ball clays, montmorillonites , illites, or feldspars, but preferably so that the anhydrous composition of the binder is Si0 2 50...56 % by weight and Al 2 0 3 44...40 % by weight, and additionally other oxides 0...10 % by weight, but so that preferably the amount of CaO and MgO present as impurities would be very small, in total at maximum 5 % by weight, in order that the formation of anorthite and cordierite would be avoided, since these compounds are not sufficiently durable in the operating conditions.
• aluminum silicate mineral for example kaolinites, halloysite, allophane, chlorite, cyanite, sillimanite, andalusite, ball clays, montmorillonites ,
• a mineral starting component such as this is easier to sinter than, for example, the mixture of alumina and silica powders mentioned in US patent 4,678,758, since at high temperatures the viscosity of a silica-based phase is, owing to the melt materials and lower alumina amount, lower and thus more fluid than the composition presented in Example 4 of US patent 4,678,758.
• the glass phase properties of the present invention promote the spreading of the melt over the surfaces of the structural particles, improve the reaction-sintering of the silica present in the binder with the alumina serving as structural particles, and thus produce better strength.
• the alumina concentration in the binder can be increased up to 70 % by weight of the chemical composition of the binder by adding as starting components for the binder other alumina- containing minerals, e.g. aluminum hydroxide or bauxite, which take part in the reaction-sintering.
• alumina- containing minerals e.g. aluminum hydroxide or bauxite
• a liquid silicate e.g. tetraethyl orthosilicate
• some aluminum salt e.g. A1(N0 3 ) 3
• the high-temperature binder contains a slight excess of silica in order to make proper reaction- sintering with the support particles possible at high temperatures.
• combinations of all of the three previous ones can be used as starting components for the binder.
• the coating is made up of alumina particles sintered directly to one another, their particle size being substantially smaller than that of the alumina particles of the support layer.
• the invention also relates to a method for the preparation of the novel support layer described above.
• the method is characterized in that - the inorganic structural particles and the inorganic starting component for the binder are mixed with a liquid medium and a suitable organic temporary binder and possibly other auxiliaries to form a slurry or paste,
• the present invention also relates to a method for the fabrication of a novel filter element according to the invention.
• This method in which the firing of the support layer and the coating is carried out in one step, is especially usable if the structural particles of the coating layer and the support layer are chemically of the same material.
• the method does not require the structural particles to be of the same material. If, for example, the structural particles of the support layer are of alumina in which the particle diameter is within a range of 87...1000 ⁇ m, and the structural particles of the coating are of alumina in which the particle diameter is 0.5...50 ⁇ m, the small particles of the coating layer are sintered directly to one another and to the support simultaneously, while the structural particles of the support layer are reaction- sintered to mullite with the aluminum silicate used as the starting component for the binder of the support layer.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Ceramic Engineering (AREA)
• Materials Engineering (AREA)
• Structural Engineering (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Life Sciences & Earth Sciences (AREA)
• Geology (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Filtering Materials (AREA)

Applications Claiming Priority (3)

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• 1996
  • 1996-12-11 FI FI964946A patent/FI103644B1/fi active
• 1997
  • 1997-11-12 AU AU50524/98A patent/AU5052498A/en not_active Abandoned
  • 1997-11-12 WO PCT/FI1997/000687 patent/WO1998025685A1/en not_active Application Discontinuation
  • 1997-11-12 EP EP97913187A patent/EP0948388A1/de not_active Withdrawn

Non-Patent Citations (1)

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