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1. WO2000037694 - MATERIAU GRANULAIRE INORGANIQUE POREUX

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

POROUS INORGANIC GRANULAR MATERIAL

The present invention relates to porous
inorganic granular material.
Porous inorganic granular material, eg
comprising foamed, sintered material, is known from various prior art references, eg GB-B-2067174 , GB-A- 2271987, EP-A-758,633 and EP-A-764617. Such material may be employed as a lightweight granular filler material in a variety of applications. However, the material may not have suitable properties for use in certain types of application or composition in which it could potentially be employed.
According to the present invention in a first aspect there is provided an inorganic foamed extruded material having a -metallic coating.
The metallic coating may be deposited on
inorganic foamed extruded material by use of a suitable known method for depositing metals on inorganic substrates, especially inorganic substrates which are essentially insulators.
For example, the metallic coating may be
deposited by a chemical deposition process, eg involving application of a metal salt to form a coating followed by a process involving a chemical reduction step in which the metal is formed. The chemical deposition process may involve immersion of the inorganic foam material in a vapour or liquid containing a substance, eg metal salt, which may be converted into the metal. For example, the inorganic foamed extruded material may be immersed in an aqueous solution of a metal salt and the salt may be converted to the corresponding metal oxide by the action of heating. Subsequently, the metal oxide may be reduced, eg by contacting at an elevated
temperature with a reducing agent, eg a reducing gas, to provide a coating of the required metal on the surfaces, including the pores, of the inorganic foamed extruded material.
The metal salt may for example be a water soluble, heat decomposible salt of a metallic
element, eg a nitrate, such as a nitrate of one or more of copper, nickel, zinc, silver and gold.
The reduction step may be carried out for example by heating in an atmosphere containing carbon monoxide, eg produced by heating carbon in a
controlled oxygen atmosphere. The carbon may be in the form of granular activated carbon.
The inorganic foamed extruded material employed to provide the metal coated inorganic foamed extruded material of the invention may comprise a foamed extruded ceramic material. Various methods are known in the prior art for the production of foamed ceramic material and the foamed ceramic material to be employed to form the metal coated material according to the present invention may be prepared by one of these various methods. Thus, the foamed ceramic material may be produced by one of the methods described in prior patent specifications GB-A-986,635, GB-B-2,067,174, GB-A-2, 271, 987, EP-A-758,633 and EP-A-764, 617.

The foamed ceramic material may advantageously be prepared by a known method which includes:
(a) preparing a foam from a slurry of a
particulate ceramic forming material, eg a clay;
(b) shaping and optionally breaking or dividing the foam into discrete pellets or prills;
(c) optionally drying the pellets or prills; and
(d) calcining the pellets or prills at an elevated temperature to cause sintering of the particles thereof.
The foamed ceramic material employed to form the coated material according to the present invention itself may be produced directly or indirectly from closed cell ceramic foam granules which comprise bubbles or cells. The bubbles are desirably lOOμm or less in size, especially lOμm to 60μm in size. Such granules are described for example in Applicants ' GB-B-2, 0,271, 987. The bubbles produced by the method described therein are polyhedral bubbles of varying sizes bounded by thin walls, the walls and junctions between walls generally bounding two or more bubbles.

The material which is employed to form the foamed extruded ceramic material (from which the coated material according to the present invention is obtained) may comprise any one or more of the known minerals and/or synthetic materials from which ceramics may be formed, eg as described later.
The foamed ceramic material employed in the material according to the invention may comprise initially after calcination granules, pellets or prills, eg having a length of from 0.5mm to 20mm, especially 1mm to 5mm, and may have a bulk density less than 1000kg. m~3 (lg.cπf3), eg in the range
80kg. m~3 to 700kg. rrf3. The solid material of that material will, following calcination, comprise inorganic particles, eg of aluminosilicate, which have been sintered and fused together. The granules etc may be made finer prior to coating by comminution as described later.
Ceramics comprises a broad class of non-metallic, inorganic materials from which solid articles may be made. Such materials have a high melting or sublimation point. For example, known ceramics include traditional ceramics eg clay products, cements, and the like which have been known and used for many centuries and also ceramics which have found uses in less traditional applications, which are known as "new ceramics" eg various pure or mixed oxides, carbides and nitrides. In general, ceramics are formed from inorganic particulate materials which are either obtained as minerals or are manufactured synthetically or a mixture of both. The ceramic material comprising the ceramic foam material which is coated in the invention is selected from ceramic materials which are suitable for coating with a latex polymer.
Where the inorganic particulate material
employed to produce foamed ceramic material for production of the coated material according to the present invention, the material may comprise one or more naturally occurring silicon-containing compounds, especially one or more silicates or aluminates . Such compounds may comprise one or more silicates of, for example, calcium, magnesium or aluminium. The compound may be a naturally-occurring mineral, such as talc, a clay mineral, mica or wollastonite . Preferably the compound is an
aluminosilicate, for example a clay mineral of the kandite and/or smectite type. Clay minerals of kandite group, for example kaolinite, dickite, nacrite and halloysite, have been found to be
particularly advantageous. "Kaolinite" includes kaolin type clays, ball clays, fire clays and China clays. Such clays occur in nature (and may be used) in the form of kaolinite plus other minerals, eg one or more of illite, mica, quartz and feldspar. The kandite clay mineral may be used in its natural, hydroxylated or hydrous state. Where the
aluminosilicate comprises a smectite clay it may comprise for example one or more of bentonite, hectorite and saponite.
The particulate material employed to produce foamed ceramic material will generally be employed as particulate material incorporated in a suitable liquid medium in which a suitable suspension or dispersion can be formed. Suitable liquid media are known in relation to the formation of ceramic
materials from the various classes of known material. In many cases, especially where the particulate material comprises a mineral, a suitable liquid medium comprises water or an aqueous solution. Foam may be made from the liquid medium by a process involving incorporating a gas in the liquid. The liquid may contain a surface active agent or
surfactant to form a stable froth.
Examples of suitable surface active agents include known cationic, anionic, non-ionic and amphoteric surface active agents.
The gas may for example be air incorporated by agitating the liquid medium to form a froth. The gas may be added to the liquid medium before or after the particulate material (and other optional additives) is added thereto.
Conveniently, as described in GB-B-2, 067, 17 , an aqueous foam containing a surface active agent may be formed prior to addition to the ceramic forming particulate material. The aqueous foam may be added to a paste or slurry containing the particulate material. The addition may conveniently be carried out in an extrusion machine from which foamed ceramic material is to be extruded. The machine may be a screw extruder, eg a co-rotating twin screw extruder. The machine may extrude foamed ceramic material into a plurality of individual elongate portions. The portions may be divided by a divider or by allowing extrudate to fall onto a moving belt which by the action of carrying away the portions causes lengths or portions to break from the extruding material. In any event, the granules, eg pellets or prills so formed may be collected and optionally sized by one or more screen meshes, eg so that only lengths greater than a chosen minimum length, eg a minimum in the range 1mm to 5mm, are selected.

The selected granules may be converted into a sintered ceramic form by calcining as described hereinafter.
Before calcining, the granules are preferably pre-dried to avoid possible damage to the granules in the calciner which might result from the rapid evolution of water if the water content of the foamed granules is excessively high. The granules of foam may therefore be dried to a water content of not more than 1% by weight as a separate step, before being introduced into the calciner. Alternatively, the foam granules may be introduced directly into a calciner which has a preliminary drying zone. If the drying is performed as a separate step, the granules are preferably dried to a water content of not more than 0.5% by weight. Drying may be carried out in a heated atmosphere, eg an oven, at a temperature of from 50°C to 200°C.
The foamed extruded ceramic material produced in the manner described may incorporate one or more additive materials added at one or more of the stages of producing such material or after its production. The foamed ceramic material may, for example,
incorporate one or more of a fluxing material, for example forming from 5 per cent to 50 per cent by weight of the mixture with the particulate material (mineral and/or synthetic material) , the fluxing agent comprising for example mica or feldspar, which subsequently reduces the temperature at which the material may be calcined, a biocide, eg forming up to 1 per cent by weight of the solids portion of the foamed ceramic material, or an organic or inorganic binder or filler or a combustible material, eg forming up to 30 per cent by weight of the solids portion of the foamed ceramic material.
Calcining may be carried out in a known manner. The temperature and time of the calcining will depend on the material being calcined and the amount of fluxing agent present but, for example, material comprising clay may be calcined at a temperature typically in the range 800°C to 1600°C for a period of 5 minutes to 24 hours.
A preferred method of forming the coated ceramic foamed material according to the present invention comprises the following steps:
(a) preparing a foam from an aqueous suspension of a particulate ceramic forming material, eg in a mixture with a fluxing agent;
(b) forming granules by shaping and optionally dividing or breaking the foam;
(c) drying the granules;
(d) calcining the granules at an elevated temperature to form a calcined foamed ceramic
material;
(e) optionally comminuting the calcined
granules of foamed ceramic material; and
(f) metal coating the calcined granules.
The optional comminution which may be employed to reduce the size of the calcined foamed granules may be performed by a device which exerts a gradual pressure or controlled squeezing action on the calcined foamed granules. This action causes the foamed ceramic to fracture at its weakest points, which are generally the thin cell walls nearest the outside surfaces of the granule. The device requires an adjustable discharge gap by which the crushing surfaces are spaced apart during the comminution. Suitable comminuting devices which have such an adjustable discharge gap include roll crushers, cone crushers and gyratory crushers.
The granules or particles produced by
comminution may have an average size in the range 50μm to lOOOμm, eg lOOμm to 500μm. Granules or particles in a particular preferred size range may be separated by a known size fractionation procedure, eg screening.
The metal coated inorganic foamed extruded material according to the first aspect of the
invention may be employed in applications wherein the properties of the inorganic foamed material and of the metallic coating are both functional. For example, the coating provided on granules of the material may be employed to provide a conducting network throughout a porous ceramic material formed from the granules. Porous conducting material which can withstand high temperatures is useful for example to provide interconnect layers in a solid oxide fuel cell .
The metal coated inorganic foamed material may be employed in host or matrix material as a
functional filler to produce lightweight composite materials. For example, the matrix material may comprise a metallic matrix, eg comprising aluminium.

When used in such a composite material, the foamed inorganic material may provide the usual beneficial properties of ceramic additives, eg stiffness, rigidity, low coefficient of thermal expansion, high rupture and impact strength, whilst providing also low density. The metal coating facilitates improved bonding of the foamed inorganic material within the metal matrix by the metal-to-metal bond provided. Such a composite may be
produced in a manner known for the production of metal-ceramic composites, eg by controlled heating to provide diffusion bonding of the components.
The metal coated inorganic material according to the invention may have the following properties:
(i) a bulk density less than 1500kg. m~3, eg 80kg. iτf3 to 1000kg. πf3;
(ii) a bed voidage of 30% to 40%.
Embodiments of the present invention will now be described by way of example with reference to the following illustrative Examples and with reference to the accompanying drawing in which:
Figure 1 is a schematic side view of an
arrangement employed to coat a foamed inorganic material with metal.

EXAMPLE 1
A low density foamed, granular material was produced in the manner described in GB-A-2271987 by producing a foamed aqueous suspension of a kaolin-containing particulate material together with a fluxing agent, extruding the suspension in the form of a wet paste, dividing the extrudate to form
prills, drying the prills in an oven and calcining the prills in a furnace. The calcined prills
resulting had a density of about 0.2g.cm~3 to 0.8g.cm~3 and an average size of a few millimetres. The prills were crushed in a roller crusher and the resulting granules were screened to provide predominantly granules having sizes in the range lOOμm to 250μm.
A sample of the comminuted, screened granules produced in this way was obtained and will now be referred to as the granular sample' . The granular sample was placed in a flask and the flask was
evacuated. A saturated solution of copper nitrate was introduced into the vessel, drawn in by the vacuum. The amount of the solution added was just in excess of that required to flood the pores of the granular sample. The remaining vacuum was then released. The flask was gently heated to drive off water from the granular sample. The resulting dried granular sample was removed from the flask and placed in a ceramic tube having ceramic fibre plugs at each end. The tube was employed in an arrangement as shown in Figure 1 where the tube is indicated by reference numeral 1.
In the arrangement shown in Figure 1, the tube 1 is inserted in a larger coaxial ceramic tube 3 having end seals 5 and 7. An inlet gas pipe 9 and an outlet gas pipe 11 are provided through the end seals 5 and 7 respectively. The larger ceramic tube 3 is packed with granulated activated carbon 6. The assembly comprising the tubes 1 and 3 is inserted in a vertical, cylindrical electrically heated furnace 13 with the inlet and the outlet pipes 9 and 11 external to the furnace. The furnace 13 is heated at a controlled rate with a controlled flow of air applied via the pipes 9 and 11 to raise the temperature of the granular sample to about 850 °C. The contents of the furnace 13 are then allowed to cool naturally in an inert atmosphere of nitrogen applied via the pipes 9 and 11.
In the method described, the following reactions take place. The copper nitrate in contact with the granular sample initially loses water and is
subsequently decomposed to copper oxide. The carbon is oxidised to produce carbon monoxide and carbon dioxide, and the carbon monoxide produced reduces the copper oxide to metallic copper. This resulting layer remains as a coating on the surfaces including the internal pore surfaces of the granular sample. The reaction which occur may be written as follows:

Δh
Cu(N03)2.3H20 > Cu(N03)2 + 3H20
Δh
2Cu (N03 ) 2 > 2CuO + 4N02 + 02
Δh
CuO + CO Cu + C02

where Δh represents applied heat.