MANUFACTURE OF SHAPED ARTICLES FROM REFRACTORY POWDER

Abstract: A method of making a shaped article comprises mixing a refractory powder with an aqueous acidic phosphate solution, subjecting the mixture thus formed to high shear to form a plastic mass, shaping the mass, and hardening the shaped mass at a temperature below the sintering temperature of the refractory powder. By this method articles of useful hot-strength can be obtained without having to employ the high temperatures required for sintering.

Description: This invention relates to the manufacture of sheet-form material from refractory powder. Such material can be made, without using the high temperatures required for sintering, by using as binder prior to the shaping operation an organic polymeric material, for example a partially hydrolysed polyvinylacetate at a level of about 10% by weight. Such a binder is, however, combustible above about 250° C, and when it burns the sheet is seriously weakened by its loss. The present invention utilises an incombustible, inorganic binder, namely an aqueous acidic phosphate solution. Two well-known examples of such a solution are aqueous phosphoric acid and aqueous aluminium hydrogen phosphate solution, both of which have for many years been used as binders in the manufacture of unfired chemically bonded bricks from high-alumina aggregrates, that is to say, high-alumina material whose particles are predominantly fairly coarse, typically 3 mm in largest dimension. The use of aqueous acidic phosphate solutions as non-combustible binders for refractories is described in GB-A-2 004 266, US-A-3 379 544 and EP-A-0 178 154. In GB-A-2 004 266 a refractory mix composition is prepared from refractory powder comprising mainly alumina and, as binder, aqueous 85% phosphoric acid. The mixture of powder and aqueous phosphoric acid is subjected to high intensity mixing. To test their suitability for use as plastic ramming masses, the products of high intensity mixing are allowed to set in air for 24 hours and then hardened at 104° C or 816° C. In US-A-3 379 544 there is described an air-setting dry particulate mortar mixture prepared by mixing 2-12% by weight of liquid phosphoric acid of high phosphorus pentoxide content with a refractory high alumina material, a clay or zircon sand. In EP-A-0 178 154 mortars, coatings, and casting, gunning or ramming mixtures are prepared by mixing a magnesium aluminium spinel refractory with an acidic phosphate and water, and optionally with a non-reactive filler such as tabular alumina. The present invention provides phosphate-bonded refractory material which is in sheet form and is of relatively high flexural strength. According to the invention, there is provided a method of making sheet-form material from refractory powder, by (a) mixing the powder with an aqueous acidic phosphate solution and a plasticity-improving clay, to form a mixture free from combustible binder; (b) subjecting the mixture to high shear; and (c) hardening the product of (b) in shaped form, at a temperature below the sintering temperature of the refractory powder; characterised in that i at least 50% by weight of the refractory powder is of particle size below 200 micrometers, ii the clay is included in the mixture in an amount not greater than 10% by weight, and iii the mixture is subjected to high shear by passing it through a twin-roll mill, and the cohesive plastic mass thus formed is hardened as a coherent sheet below the sintering temperature of the refractory powder. The proportion of aqueous acidic phosphate solution required to form a suitable mixture is, perhaps owing to the viscous nature of these solutions, relatively small; and the mixture itself, which takes the form of a damp crumbly mass, undergoes a remarkable change in consistency on being subjected to high shear by means of a twin-roll mill, forming a cohesive plastic mass. The plastic mass formed by high shear will ordinarily end up adhering to one only of the rolls, and can easily be parted therefrom as a unitary mass, and then laid flat in a tray for hardening. Depending on the precise nature of the ingredients present in the refractory powder, hardening may be possible at ambient temperature, for example 20° C, as in Example 1a later in this specification. Often, however, it will be desired to expedite hardening and/or improve strength, and this can be done by heating above 50° C, preferably above 200° C, as in Example 1b. It will sometimes be convenient to carry out at least the initial stage of hardening under pressure, in order to consolidate and prevent distortion of the mass. The invention can be applied to the treatment of a wide range of refractory powders, for example alumina, titania, zirconia, zircon, and calcined clays. As already indicated, an ordinary clay, such as one of the kaolinitic clays ball clay and china clay, is included in the refractory powder to improve its plasticity under high shear, in an amount up to 10% by weight of the whole, ie of the shaped mass to be hardened. To speed up hardening at ambient temperature, there may be included in the refractory powder magnesium oxide (up to about 15% by weight of the whole) or aluminium hydroxide (up to about 20%). To improve the flexural strength and impact strength of the hardened product, inorganic fibres such as glass fibres, fibrous wollastonite or ceramic fibres may be included in the mixture; and impact strength can also be improved by inclusion of non-fibrous particles of a layer silicate such as mica or talc. Such additives for improving strength may form up to 25% by weight of the whole. Organic fibres, such as fibres of polyacrylonitrile, polypropylene, or the kind of polyamide available under the registered trade mark KEVLAR, may be included to give impact strength at temperatures up to about 200°C, at a level no greater than about 2% by weight, so that the loss of hot strength if they are burnt out is not serious. The following Examples further illustrate the invention and in particular show its application to the production of strong, refractory sheet material having good machinability and useful flexural strength at elevated temperatures (eg 850°C, at the level encountered during the operation of electrical switchgear. Example 1 Calcined alumina powder (particle size 45 µm; surface area, 1 m²/gram) was mixed in a Z-blade mixer with dead-burnt magnesia (91% MgO; particle size<0.2 mm) and ball clay (particle size: 90%<5 µm; 60% <0.5 µm) and aluminium phosphate solution (40% aqueous Al(H₂PO₄)₃; pH 0.8) was added. Mixing was continued until a crumbly mass had formed. Its composition was: % by weight Alumina 75 Magnesia 1 Ball clay 7 Al phosphate solution 17 100% The crumbly mass was removed from the Z-blade mixer and charged to a twin-roll mill (roll diameter, 0.3 m; nip, 4mm; speed, 10 rpm) and was repeatedly passed through the nip of the mill until it had become a cohesive plastic mass adhering to one of the two roll. The mass was removed from the roll as a coherent sheet, of thickness about 4mm, and the sheet was set aside to harden at ambient temperature. After 3 days the sheet was cut into strips for testing. a) One batch was subjected to flexural strength tests 7 days after initial formation. Average flexurual strength was 38 MPa at room temperature. The strips showed negligible creep at 500°C, good retention of strength at that temperature and useful flexural strength at 850°C. b) Two further batches were put into furnaces, at 400°C and 600°C respectively. After 4 hours they were removed, allowed to cool and tested for flexural strength at room temperature. Results were: (i) '400°C' sample: 40 MPa (ii) '600°C' sample: 51 MPa c) Samples b(i) were immersed in water at 20°C for 24 hours and then tested for flexural strength. On average, this was 28 MPa. Example 2 Following generally the procedure of Example 1, sheet material was prepared from the following ingredients in the following proportions by weight: % by weight Calcined china clay 38 Alumina 38 Magnesia 2 Ball clay 2 Al phosphate solution 20 100% The calcined china clay was that sold under the trade mark MOLOCHITE, calcined at 1350°C and with particle size 98%<75 µm. After hardening at room temperature for 7 days, flexural strength was 30 MPa. The product had excellent resistance to thermal stock, as assessed by a standard arc-resistance test. Examples 3 to 5 Following generally the procedure of Example 1, products were prepared from the following ingredients in the following weight proportions , with hardening at room temperature, and were found to have the strengths shown.

Claims: A method of making sheet-form material from refractory powder, by (a) mixing the powder with an aqueous acidic phosphate solution and a plasticity-improving clay, to form a mixture free from combustible binder; (b) subjecting the mixture to high shear; and (c) hardening the product of (b) in shaped form, at a temperature below the sintering temperature of the refractory powder; characterised in that i at least 50% by weight of the refractory powder is of particle size below 200 micrometers ii the clay is included in the mixture in an amount not greater than 10% by weight, and iii the mixture is subjected to high shear by passing it through a twin-roll mill, and the cohesive plastic mass thus formed is hardened as a coherent sheet below the sintering temperature of the refractory powder. A method according to claim 1 in which hardening is carried out at a temperature above 50° C. A method according to claim 1 in which hardening is carried out at a temperature above 200° C. A method according to any of claims 1 to 3 in which at least the initial stage of hardening is carried out under pressure. A method according to any of claims 1 to 4 in which the refractory powder comprises alumina. A method according to any of claims 1 to 4 in which the refractory powder comprises a calcined clay. A method according to any of claims 1 to 6 in which the plasticity-improving clay is ball clay or china clay. A method according to any of claims 1 to 7 in which the refractory powder also contains magnesium oxide or aluminium hydroxide. A method according to any of claims 1 to 8 in which the aqueous acidic phosphate solution is a solution of aluminium phosphate. A method according to any of claims 1 to 8 in which the phosphate solution is phosphoric acid.

Inventors: ATKINSON, ALAN WILLIAM, SMITH, DAVID NIGEL
Applicants: T&N MATERIALS RESEARCH LIMITED
Application year: 1987
Main classification: C04B28
IPC codes: C04B40, C04B35