CALCINED CLAY-CONTAINING COMPOSITIONS AND THEIR PRODUCTION AND USE

Abstract: A method of producing a refractory mullite-containing material which includes adding an aluminous material to a kaolinitic clay-containing material to produce a mixed material and calcining the mixed material, wherein the kaolinitic clay-containing material is in the form of granules containing not more than about 30% by weight of water which have been produced by extrusion of a moist mass of particles of the kaolinitic clay-containing material and the aluminous material comprises powdered alumina whose mean particle size is less than that of the granules and wherein the alumina is applied to coat the granules whereby an alumina-rich coating is formed on the surface of the granules.

Description: CALCINED CLAY-CONTAINING COMPOSITIONS AND THEIR PRODUCTION AND USE This invention is concerned with calcined clay- containing compositions and their production and use. Kaolinitic clays consist predominantly of the mineral kaolinite, and, when kaolinite is calcined to a temperature above about 1200 °C, it undergoes a transformation to a different mineral phase called mullite. Mullite is an aluminosilicate compound having the chemical formula 3Al 2 0 3 .2Si0 2 and has properties which render it very useful in the manufacture of refractory materials, such as construction elements for building the linings of kilns, moulds for casting metals and items of kiln furniture . One difficulty which has been encountered in the calcination of kaolinitic clay-containing compositions to form mullite is that, when the temperature of the kaolinitic clay is raised to about 1350°C, a glassy phase, rich in silica, is formed, and this causes the composition to become very sticky. In conventional processes for making synthetic mullite, this difficulty is generally avoided by calcining a kaolinitic clay-containing composition in the form of a static bed of particles or elements so that, as the temperature is raised through the band in which the glassy phase is formed, the composition is given very little opportunity to coalesce to form larger entities. In a typical conventional process for preparing a mullitic refractory material, a kaolinitic clay raw material is mixed with sufficient water to form a plastic paste which is extruded, and the extruded rods are cut into lengths to form pieces known as bricks. The bricks are thermally dried and stacked in kiln cars which are caused to travel slowly through a tunnel kiln over a period of about 24 hours such that the kaolinitic clay is exposed to a temperature of about 1500°C for this period. A process of the this type carries a high cost in terms of the high initial capital cost of the equipment and the low throughput rate of product material. The process is also relatively inefficient in terms of the utilisation of thermal energy. The rate of calcination and the thermal efficiency would be improved if it were possible to use a moving bed- type kiln, for example a rotary kiln or a fluidised bed, but it has been found that in kilns of this type, as the temperature of the kaolinitic clay material passes through 1350°C, the formation of the glassy phase causes particles of the material to coalesce together to form larger entities which are extremely difficult to reduce in size, and which cause uneven calcination of the material. The object of this invention is to overcome the difficulties described above, and to make it possible to calcine, e.g. in a moving bed type furnace, a kaolinitic clay-containing material in the form pieces of an extruded composition. According to the present invention there is provided a method of producing a refractory mullite- SUBST1TUTE SHEET (RULE 26) containing material which includes adding an aluminous material to a kaolinitic clay-containing material to produce a mixed material and calcining the mixed material, wherein the kaolinitic clay- containing material is in the form of pieces containing not more than about 30% by weight of water which have been produced by extrusion of a moist mass of particles of the kaolinitic clay-containing material, wherein the aluminous material comprises powdered alumina whose mean particle size is less than that of the granules and wherein the alumina is applied to coat the pieces whereby an alumina-rich coating is formed on the surface of the pieces. The said pieces may be granules, pellets, prills, bricks or the like formed by extruding a moist mass of particles of the clay-containing material and dividing or breaking the extrudate so formed. Optionally, the pieces so formed may be at least partially dried prior to calcination. The coated pieces will therefore consist of a multiplicity of smaller particles which are adhered together and the alumina applied will come into contact with only those clay particles near the outer surface of the pieces. The coated pieces may be calcined at a temperature of at least 1200°C for at least 10 minutes . The calcination may be in a moving bed type kiln or furnace, i.e. wherein there is relative movement between the material to be heated and the source of heating (in contrast to the tunnel kilns employed in prior art methods) . The starting material for the method may conveniently be pieces formed by extrusion of a fine kaolin clay or china clay. The pieces upon extrusion may contain less than 30% by weight of water, e.g. from about 20% to about 25%, and more especially from 22% to 24%, by weight of water, and may therefore be in a substantially plastic state when extruded. The plastic kaolinitic clay may be extruded by an extruder or pugmill through an aperture of diameter in the range of from about 1mm to about 10mm, and the extruded strings of clay formed may be chopped to form pieces of length approximately equal to their diameter. Alternatively, the starting material may comprise pieces of a foamed kaolinitic clay- containing material which have been prepared by a process of the kind described in GB-A-2271987. In accordance with the process described in GB-A- 2271987, a substantially dry kaolinitic clay is mixed with water and a surfactant to form a foamed composition, the foamed composition is extruded, the extrudate being divided or broken into pieces in the form of pellets, prills or granules. The pieces may be at least partially dried. In the preparation of pieces which are foamed in the manner specified above, substantially dry kaolinitic clay may be mixed with sufficient water to form a suspension containing from 50% to 68% by weight of the clay. The percentage by weight of the clay in the suspension may be in the range from 58% to 65% by weight. Dispersing agent may be employed to disperse the particles of clay in the suspension. The amount of the dispersing agent used is preferably from 0.05 to 1% by weight, based on the weight of the dry kaolinitic clay. The dispersing agent may be a water soluble inorganic dispersing agent of a known kind, e.g. a condensed phosphate salt or a water soluble salt of a polysilicic acid. Alternatively or in addition, the dispersing agent may be an organic polyelectrolyte of a known kind, e.g. a water soluble salt of a poly (acrylic acid) or a poly (methacrylic acid) or of a co-polymer containing acrylate or methacrylate units. A suitable dispersed suspension may be obtained in a foamed form by introducing into the feed end of an extruder simultaneously a substantially dry kaolinitic clay, and sufficient water containing a dispersing agent for the kaolinitic clay to form a suspension containing from 50% to 68% by weight of the clay, while a separately generated foam is introduced through an inlet which is preferably situated between the inlet and the outlet of the extruder. The extruder may be a compounder-extruder of the type which has two co-rotating parallel screws and a casing along the length of which are provided inlets through which liquid additives may be introduced into the material passing through the extruder. The foam may be generated, for example, by forcing a mixture of air, water and a surfactant under pressure through a fine mesh. The foam may conveniently consist of bubbles of sizes in the range of from 5μm to lOOμm, and most advantageously of bubbles in the size range of from 0 to 60μm. The foam conveniently has a water content of from 20kg to 30kg, and most preferably from 22kg to 26kg, per cubic metre of foam. The foam may be prepared using a suitable surfactant. If an anionic or a non-ionic surfactant is employed, it may be necessary to introduce through an inlet in the casing of the extruder an acid, for example a mineral acid such as sulphuric or hydrochloric acid, or a flocculant for the clay mineral, in order to flocculate the solid material. If, on the other hand, the foam is prepared using a cationic surfactant, for example a higher alkyl trimethyl quaternary ammonium chloride, wherein the higher alkyl group has from 12 to 24 carbon atoms, the suspension of the aluminosiliceous material is flocculated on contact with the foam. The pieces of foamed clay composition formed following extrusion may be dried to a water content of not more than 1% by weight and are preferably dried to a water content of not more than 0.5% by weight. The foamed clay composition may be formed into granules, pellets or prills by extruding the foam in the compounder-extruder through a plurality of small apertures and dividing the resultant extruded strings or allowing the strings to break into discrete pieces of the desired size. The diameter of the pieces formed in this way may generally be not smaller than about 1mm, e.g. in the range from about 1mm to about 10mm, e.g. from 1mm to 5mm. The alumina employed in the method of the invention may comprise a powder which preferably has a particle size distribution such that substantially all of its particles have a diameter smaller than 50μm and its mean particle diameter is in the range of from about Iμm to about lOμm. The pieces of the extruded kaolinitic clay-containing material are preferably mixed with the alumina powder by rolling the pieces in the presence of an excess of the alumina powder in a known commercially available device such as a pan granulator, or other mixing device which subjects its contents to a tumbling or rolling action usually with the assistance of heat. The amount of the alumina powder taken up by the pieces will depend on the specific surface area of the pieces. For example, if the pieces consist of an unfoamed material, the pieces may take up from about 0.5% to about 10%, e.g. from about 1% to about 5% of the alumina powder by weight, based on the dry weight of the pieces. If, on the other hand, the starting material consists of pieces of a dry, foamed kaolinitic clay-containing material, the amount of alumina powder taken up by the pieces will generally be up to 30% by weight, e.g. in the range of from 15% to 25% by weight, based on the dry weight of the pieces. This is because of the much higher specific surface area of the porous, foamed pieces. The pieces of the kaolinitic-clay containing material, with their surface coating of alumina powder, are introduced into a calcining kiln or furnace, e.g. of the moving-bed type. The kiln may conveniently be of the fluid bed type or rotary type. If the kiln is of the rotary type, the pieces may be either directly or indirectly fired. A rotary kiln generally comprises a cylindrical drum of length greater than its diameter which is rotated about its longitudinal axis, which is usually inclined at a small angle with the horizontal. The feed material to be calcined is generally introduced at the higher end of the drum, and travels along the inner wall of the drum towards the lower end. Hot gases which provide the source of heat are passed through the drum at the temperature required to effect the desired calcining process. The hot gases may be passed through the drum either concurrently or counter-currently with the flow of the feed material. In the case of a directly fired kiln, the hot gases come into direct contact with the feed material, calcining operation. It is believed that, when the alumina coated pieces are calcined at a temperature of greater than 1200°C, e.g. at or above 1350°C, a layer, rich in silica, is formed near the surface of the pieces, and the silica reacts with the alumina to form aluminosilicate compounds, such as mullite, which are non-sticky, so that the tendency for adjacent granules to adhere together is removed. The invention allows pieces of the starting material to be calcined without large amounts of costly alumina being consumed since the alumina is required only to act as a surface coating to prevent co- alescence of pieces rather than to form an ingredient of the entire composition to be calcined. In some prior art processes, e.g. as described in GB-A- 886075, GB-A-832278 and US-A-3758318, particles of aluminous material and clay are mixed together to form an entire mixed body of material to be calcined. The mean size of the pieces produced by the method of the invention may be substantially not greater than before calcination. In other words, no size increase takes place due to co-alescence . The alumina coated pieces are preferably calcined in the method of the invention at a temperature of at least 1300 °C for at least 10 minutes. Because a moving bed type kiln may be employed the calcination may be carried out more quickly and more efficiently than in the prior art. For example, in the method of the invention the calcination time may be not greater than one hour. The products of the method of the invention may be used in all of the applications known in the prior art for mullite containing refractory materials, especially the manufacture of kiln furniture articles . Embodiments of the present invention will now be described by way of example with reference to the following Examples. EXAMPLE 1 A kaolinitic clay having a particle size distribution such that 40% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm and a K 2 0 content of not more than 2.0% by weight was mixed with water to form a plastic paste having a water content of 23% by weight. This paste was extruded by means of an extruding device known as a pugmill through a plurality of apertures of diameter 5mm. The resulting extruded clay strings were chopped into short lengths to form pellet-shaped pieces, the length of which was approximately equal to the diameter. The pieces were transferred to a commercially available pan-type pelletiser where they were tumbled with alumina powder having a particle size distribution such that substantially all of the particles had a diameter smaller than 50μm and the average particle diameter was about 5μm. It was found that the clay pieces took up 3.5% by weight of alumina, based on the dry weight of kaolinitic clay to form a coating on the pieces. The alumina coated clay pieces prepared in this way were introduced into a rotary calcining kiln which was directly heated to give a calcining temperature of 1500 °C. The residence time of the pieces in the kiln was 19 minutes. After firing in the kiln, the size distribution of the pieces was found to be substantially the same as that of the pieces before firing. The fired pieces were found to have a density of 2.156, a modulus of rupture of 304.3kg. cm -3 , a mullite content of 57.0% by weight and a coefficient of linear expansion of 2.89 x 10 "6 °C _1 . As a comparison, a further sample of the same raw kaolinitic clay was extruded in the same manner as described above to form pellet-shaped pieces, the length of which was approximately equal to the diameter. However, in this case, the pieces were not surface treated with alumina powder. The pieces were introduced into the same rotary calcining kiln which was heated to a temperature of 1500 °C, the residence time of the granules in the kiln being 19 minutes. It was found that the pieces in this case cohered in the kiln and produced large lumps of calcined material which could not be broken up. EXAMPLE 2 A kaolinitic clay of the same type as used in Example 1 above was introduced in substantially dry form into the feed inlet of a twin screw, co-rotating compounder-extruder . At the same time sufficient water to form a suspension containing 60% by weight of clay together with 0.7% by weight, based on the dry weight of clay, of a sodium tripolyphosphate dispersing agent was injected through inlets provided in the casing of the extruder. An aqueous foam was prepared by forcing a mixture of air, water and a non-ionic surfactant, which was a mixture of higher alkyl dimethylamine oxide compounds, the higher alkyl groups having from 12 to 14 carbon atoms, under pressure through a fine wire mesh. This aqueous foam was introduced into the compounder-extruder through an inlet situated approximately midway between the feed inlet and the extruder plate. The foamed clay composition formed in the compounder-extruder was extruded through a plurality of circular apertures of diameter 3mm. The extruded strings of clay produced were dried at a temperature of about 180 °C for 12 minutes and the resultant dried strings were subjected to gentle crushing to break the strings down into pellet shaped granules of length approximately equal to the diameter. These pellets were then rolled with alumina powder in the manner described in Example 1. It was found that the foamed clay granules took up about 20% by weight of alumina, based on the dry weight of kaolinitic clay. The alumina coated foamed clay pellets prepared in this way were introduced into the rotary calcining kiln and were calcined under the same conditions as were described in Example 1 above. The residence time of the granules in the kiln was 19 minutes.

Claims: CLAIMS 1. A method of producing a refractory mullite- containing material which includes adding an aluminous material to a kaolinitic clay-containing material to produce a mixed material and calcining the mixed material, wherein the kaolinitic clay- containing material is in the form of pieces containing not more than about 30% by weight of water which have been produced by extrusion of a moist mass of particles of the kaolinitic clay-containing material and the aluminous material comprises powdered alumina whose mean particle size is less than that of the pieces and wherein the alumina is applied to coat the pieces whereby an alumina coating is formed on the surface of the granules. 2. A method according to claim 1 and wherein the calcination is carried out at a temperature of at least 1200°C for a time of at least 10 minutes but not greater than one hour. 3. A method according to claim 1 or claim 2 and wherein the calcination is carried out in a kiln or furnace wherein relative movement is applied between a source of heat and the material to be calcined. 4. A method according to claim 1, claim 2 or claim 3 and wherein the amount of alumina taken up by the pieces is less than 30% by weight based on the dry weight of the pieces. 5. A method according to claim 4 and wherein the pieces are of non-foamed material and the alumina coating applied forms from to 0.5% to 10% by weight of the pieces on a dry weight basis. 6. A method according to claim 4 and wherein the pieces are of foamed material. 7. A method according to claim 6 and wherein the alumina coating applied forms from 15% to 25% of the pieces on a dry weight basis. 8. A method according to any one of the preceding claims and wherein the pieces have a mean particle size of at least one millimetre. 9. A method according to any one of the preceding claims and wherein the particles of the powdered alumina have a mean particle size of from lμm to lOμm. 10. A method according to any one of the preceding claims and wherein the powdered alumina powder is applied to the pieces by rolling or tumbling the pieces in the powder. 11. A mullite containing product produced by the method according to any one of the preceding claims. 12. Use of the product according to claim 11 in the manufacture of kiln furniture.

Inventors: HEARLE JONATHAN ANDREW, HEARLE, JONATHAN, ANDREW
Applicants: ECC INT LTD, HEARLE JONATHAN ANDREW, ECC INTERNATIONAL LTD., HEARLE, JONATHAN, ANDREW
Application year: 1999
Main classification: C04B35
IPC codes: C04B33, C04B35