Patented May 11, 1954 2,678,282 UNITED STATEs PATENT OFFICE 2,678,282 PROCESS FOR MANUFACTURING SYN THETIC NORGANIC SILICATES OR THE LIKE Clifford Jones, Eccleston, St. Helens, England, as signor to Pilkington Brothers Limited, Lanca shire, England, a British company No Drawing. Application June 1, 1950, Seria No. 165,595 5 Claims. (CI. 06-65) 2 This invention relates to the manufacture of natural mullite and Sillimanite. They discounted a synthetic mullite. the suggestion of Bradley and Roussin that their Pure mullite is a very desirable refractory "Porzite' was a new aluminium silicate, and they material because it consists of closely interlock identified three forms of mullite, viz: ing crystals which give it good mechanical 5 1. or mullite.--This contains 72% of Al2O3 and strength, and because it contains no glassy ina 28% of SiO2 by weight: its formula is trix liable to drain out of the material in Serv ice as a result of its fusibility being lowered by external chemical attack. This form was not identified in any of the By the calcination at high temperature, usually O natural samples, but it was prepared syntheti above 1400° C., of Kaolin (Al2O3.2SiO2.2H2O), or cally by grinding together pure silica (SiO2) and of Kyanite or sillimanite (which are two dif pure alumina (Al2O3) in the correct proportions, ferent crystalline forms of Al2O3SiO2), or of and calcining the mixture at 1750. There is other aluminium silicates, it is relatively easy to no discloSure of the material being moulided into promote the growth of crystals of mullite, the 5 Shaped articles. formula of which is approximately 3Ai2O3.2SiO2: 2. 3 mullite-This contains 72% to 78% of but in the material So produced there is a residue Al2O3 and 28% to 22% of SiO2 by weight: its of excess silica (SiO2), usually in the form of a formula is approximately 3Al2O3.2SiO2. glassy matrix. This glassy matrix of Silica is The X-ray spectrum for this material was detrimental in refractories for tank-furnaces 20 found to correspond to that found for mullite used for the melting of glass from the raw in crystals in electrically fused (electro-cast) tank gredients because, in the conditions existing in furnace blocks, and to that for natural mullite such furnaces, it is liable to be attacked by al fi'On North America. A Substance very simi kaline dustS and furnace gaseS as a result of lar crystailographically was prepared artificially which its fusibility is lowered and it drains out 2 5 by mixing pure Silica (SiO2) with pure alumina, f the refractory material leaving an open por (Al2O3) in excess of the 72 to 28 alumina-to ous structure which may disintegrate in Service. Silica, ratio and calcining the mixture at i800. Thus, if a mullite refractory can be made in 3. y mullite.- This was found in Samples of which there is no glassy matrix that can cause natural material from the Isle of Mull (hence weakness by draining out of the structure in the 30 the name "mullite'). A material very similar to presence of alkaline dusts, etc., full advantage it was prepared by repeating the process de can be taken of the characteristically good me Scribed in ... above for the Synthesis of a mullite, chanical and chemical properties of pure mullite but replacing 1% of the alumina (Al2O3) by 1% as a refractory for high temperature service. of titania (TiO2) or by less than 1% of ferric Attempts to make an all-mullite refractory 35 oxide (Fe2O3). by introducing the correct quantity of alumina, Accordingly, the present invention provides a (Al2O3) to combine with this excess silica, usually process for the manufacture of Synthetic Inull result in a mixture of mullite, corundum (a ite wherein an intimate mixture of alumina, and particular crystalline form of Al2O3); and silica. aluminium silicate bonded by means of an ethyl For example, We have found that the calcina 40 Silicate is calcined at a temperature of at least tion of a mixture of crushed sillimanite (natural 1500° C., the proportions of the ingredients be rock) and alumina in the proportions present in ing such that the ratio of alumina to silica in mullite results in a material containing mullite the calcined mixture is between 72% and 78% and Corundum. by weight of Al2O3 and between 28% and 22% A. J. Bradley and A. L. Roussin (cf. Transac 45 by weight of SiO2. - tions of the Ceramic Society, (1932), 31, 422), The calcination temperature should be at have described mixtures of porcelains which on least 1500, but a higher temperature, e. g. of X-ray examination gave spectra similar to but i650 to 1700° C. may be attained when it is re not identical with that of mullite, and they Sug quired to produce a very dense material. gested that the material should be called “Por 50 The aluminium silicate (such as Kyanite or zite.' sillimanite) should be of high purity and both H. P. Rooksby and J. H. Partridge, (cf. Jour the aluminium silicate and the alumina must nal of the Society of Glass Technology, (1939), be finely ground (i. e. must pass through a 200 23, 338), have described results of the X-ray mesh Sieve or finer). examination of the structures of artificial and 55 Preferably, the binding agent is an ethyl silicate 2,678,282 3 4. or mixture of ethyl silicates, such as the material substitute for natural mullite Which comprises sold as silicon ester and usually containing 40 to calcining an intimate mixture of alumina, alu 47% by weight of silica (SiO2). The silicon ester minium silicate and ethyl silicate as a binding may conveniently be used in Solution in Water Or agent, at a tenperature of between 1500° C. and as a dispersion in an alcohol-water mixture, and 800° C., the initial proportions of the ingre the quantity of silicon ester thus introduced into dients being such that in the calcined mixture the the mixture can be adjusted according to the ratio of alumina to silica is between 72% and consistency required for consolidating the final 78% by Weight of Al2O3 and between 28% and Inix, as little as 1% by Weight of the dry mixture 22% by weight of SiO2. inay be appropriate in Some cases, and experi 0 2. A process for the manufacture of a synthetic nets have shown that a quantity of 5% by Substitute for natural mullite which comprises Weight of the dry mixture has produced very effi calcining an intimate mixture of alumina, alu cient results. iaiiium silicate and an ethyl silicate contain he ingredients should be very thoroughly ing between 40% and 47% by Weight of SiO2 as a mixed. When the Silicon ester breaks down, the 5 binding agent, at a temperature of between 1500 pure silica produced is in a very finely divided C. and 1800° C., the initial proportions of the State and is more reactive than ordinary fine ingredients being such that in the calcined mix ground silica. Since this silica is introduced as a ture the ratio of alumina, to Silica, is between 72% solution or dispersion it may be aSSunned to be and 78% by weight of Al2O3 and between 28% and very uniformly distributed. 20 22% by weight of SiO2. The following example illustrates how the 3. A process for the manufacture of a synthetic process of the invention may be carried into effect: Sui:stitute for natural nullite Which comprises 500 gms. of a natural rock sillimanite contain calcining an intimate mixture of alumina, alu ing 58% (=290 gms.) of Al2O3 and 39%. (=195 (niiniuin Silicate and an ethyl Silicate as a bind guns.) of SiO2 crushed to 200-mesh were inti 25 ing agent, at a temperature of between 1650 and imately mixed with 500 gms. of calcined alumina, 1700° C., the initial proportions of the ingredients Al2C3, crushed to 200-mesh and 247 c. c. of silicon icing Such that in the calcined mixture the ratio ester (containing 46.5% by weight of silica equiva of alumina, to silica is between 72% and 78% by lent to 112 gms, of SiO2) in an alcohol-Water So weight of Al2O3 and between 28% and 22% by lution consisting of 60 c. C. of alcohol and 27 c. c. Weight of SiO2. of water. Thus the ratio of total alumina, to total silica in the mixture was 72 to 28. The mixture, 4. A process for the manufacture of a Synthetic after shaping with a required article, was calcined Substitute for natural mullite which comprises at between 1650° C. and 1700° C. by heating in a intimately mixing together alumina, aluminium gas-air fired furnace. The product Was a Syn 35 silicate and an aqueous alcoholic Solution of an thetic annullite. ethyl silicate, calcining the mixture thus pro By slightly increasing the ratio of alumina to duced at a temperature of between 1500° C. silica, Bmullite may be prepared in a similar and 1800° C., the initial proportions of the in alle. gredients being such that in the calcined mix In the manufacture of refractories intended 40 ture the ratio of alumina to Silica is between 72% for use in the making of glass, experience has and 78% by weight of Al2O3 and between 28% and shown that the aluminium compound herein de 22% by weight al SiO2. scribed should be calcined at a temperature of 5. A process for the manufacture of a synthetic at least 1500° C. substitute for natural mullite Which comprises The mixture is eminently suitable for pressing, intimately mixing together alumina, Sillimanite haramering, jolting or slip-casting to the re and an aqueous alcoholic Solution of an ethyl quired shape before calcining. The moisture con silicate, calcining the mixture thus produced at a tent, appropriate to the shaping process used is temperature of between 1650 and 1700 C., the oitained by adjustment of the quantity of Water initial proportions of the ingredients being Such in the mixture and by allowing the mixture to that, in the calcined mixture the ratio of alumina, dily out if necessary-it hardens fairly quickly in to silica is between 72% and 78% by Weight of Warin air. Al2O3 and between 28% and 22% by Weight of The porosity of the material produced can be SiO2. reduced by crushing after calcining, then re-bond References Cited in the file of this patent ing with an organic binder Such as polyvinyl 55 alcohol, and shaping and recalcining. UNITED STATES PATENTS A synthetic mullite manufactured according to Number Nane Date the present invention may be used as a bonding 2,450,327 Cogan et al. ------Sept. 28, 1948 material for certain other refractories, for ex ample corundum, but not with materials such as 60 FOREIGN PATENTS zirconium with which the mullite would react to Number Country Date lower the melting point of the mixture. 957,817 France ------1949 claim: 242,455 Germany ------1912 1. A process for the manufacture of a synthetic