A method for the manufacture of a ceramic material and its application as kiln furniture

Abstract: The invention relates to a method for the manufacture of a ceramic material starting with a mixture of a MgO source, SiO₂ and Al₂O₃ source as well as chamottes and water. According to the invention a mixture of 10-30% by weight of the MgO source, 10-30% by weight of the SiO₂ and Al₂O₃ source and 40-80% by weight of the chamottes with water are mixed in a ratio of about 1:2, yielding a suspension in which the particle size of the solids ranges from between 1 µ and 1 mm. Then the obtained suspension is spray-dried. Then the granulates are compressed into the desired final shape at a pressure of 150-500 kg/cm², followed by drying at 100°C for about 24 hours. Then the green products are sintered at 1380-1440°C for 20-80 hours, yielding the kiln furniture. As SiO₂ and Al₂O₃ sources chemically pure substances are used. Finally, the invention comprises kiln furniture.

Description: The present invention relates to a method for the manufacture of a ceramic material starting from a mixture of a MgO, SiO 2 and Al 2 O 3 source as well as chamottes and water, and to the application of said ceramic material as kiln furniture. Such a method is known from Fachberichte Sprechsaal , vol. 117 , No. 6, 1984, pp.550-553 . The prior art kiln furniture were manufactured from a mixture of a MgO source, chamotte and as SiO 2 and Al 2 O 3 source, clay. According to this article the kiln furniture thus obtained seem applicable at temperatures up to 1400°C. However, it has been shown that the temperature of 1400°C mentioned in the above article could actually not be reached with the ceramic mixture of that time. The literature shows that cordierite- and mullite-containing materials are not suitable at temperatures above 1380°C. The clay present in the starting mixture in an amount of about 20 % by weight does indeed have favourable properties with regard to the workability of the material in the sense that clay facilitates moulding, but the disadvantage is that clay contains various impurities which in practice are responsible for insufficient resistance to temperatures of 1380°C and higher, leading to disintegration of material. In practice it is therefore not possible to use kiln furniture manufactured from the above-mentioned clay-containing mixture at a temperature above 1380°C, while it is necessary to apply the kiln furniture made from this mixture when firing hard porcelain. The following impurities are found in clay: Fe 2 O 3 , TiO 2 , CaO, Na 2 O and K 2 O. These oxides in particular have an unfavourable effect on the crystallization of cordierite, so that the desired high temperature of above 1380°C cannot be realized. From EP-A-164028 is known a mullite/cordierite composite ceramic with a thermal expansion coefficient close to that of Si at low temperatures which is not suitable for kiln furniture The objective of the invention is to provide a kiln furniture which is resistant to temperatures of 1380°C and higher. To this end the invention provides a method according to claim 1 and a kiln furniture as defined in claim 10 The basic mixture is preferably about 20% by weight of MgO-source, about 20% by weight of SiO 2 and Al 2 O 3 source and 60% by weight of chamotte. Usually good results are obtained when the moisture content of the granulates is about 4% by weight. Sintering should preferably take place at about 1420°C for about 40 hours. The kiln furniture containing about 50% by weight of cordierite and about 50% by weight of mullite was shown to be particularly suitable for application in firing hard porcelain. According to the invention it is essential that as SiO 2 and Al 2 O 3 source chemically pure materials are used. Although clay has the above-mentioned disadvantages, it has the great advantage that it improves the workability of the mixture. Surprisingly it has been shown that a kiln furniture manufactured from the above-mentioned mixture, in which clay is replaced by chemically pure Al 2 O 3 - and SiO 2 -containing substances, is resistant to temperatures of 1380°C and higher. However, it has been shown that the workability of such a mixture leaves something to be desired. To remove this problem one adds to the starting mixture substances improving the process technology, to wit organic substances such as suspension stabilizers, moulding aids, plasticizer, binding agents and/or deflocculants in an amount of 0.1-1.0% by weight, 0.1-1.0& by weight, 0.1-1.0% by weight, 0.1-5.0% by weight and 0.1-1.0% by weight respectively. Good results are obtained when using as suspension stabilizer about 0.2% by weight of polyethylene adduct, as plasticizer 0.2% by weight of a cellulose derivative, as binding agent about 2.0% by weight of methyl cellulose and as the deflocculants 0.2% by weight of a huminate. In general one adds 0.1-8% by weight and preferably about 3% by weight of organic additives, improving the process technology to the starting mixture. Surprisingly it has been shown that such a mixture is especially suitable for the manufacture of kiln furniture, which are resistant to temperature of 1380°C and higher. Moreover, such mixtures are particularly well workable in spite of the absence of clay. Such kiln furniture are especially well suited for the manufacture of hard porcelain, in contrast to the kiln furniture described in Fachberichte of 1984. Examples of binding agents are methyl celluloses, ethyl celluloses, carboxymethyl celluloses, hydroxyethyl celluloses, polyethylene glycols, polyvinyl alcohols, polysaccharides, polyacrylates, alginates and resins. Plasticizer according to the invention that may be used are cellulose derivatives and polyglycol ethers. As suspension stabilizers one might mention polyethylene adducts, wax emulsions, fatty acids, esters and organic phosphoric acid compounds. Finally, suitable deflocculants within the scope of the present invention are huminates, carbonic acid esters and polyelectrolytes + alkali silicates; phosphates. The invention further relates to a kiln furniture suitable for firing hard porcelain, such as table ware, whereby it is of great importance that the kiln furniture has a sustained resistance to a temperature of 1380°C and higher. The invention will now be elucidated by means of the following non-restrictive example. EXAMPLE A kiln furniture was manufactured, starting with a mixture of about 20 parts by weight as MgO source, about 20 parts by weight chemically pure SiO 2 and Al 2 O 3 source and about 60% by weight of chamotte. One then added about 0.2% by weight of huminate as deflocculants, about 0.2% by weight of polyethylene adduct as suspension stabilizer, about 0.2% by weight of cellulose derivative as plasticizer, and about 2,0% by weight of methyl cellulose as binding agent. The mixture thus obtained was subsequently suspended in water in a ratio of 1 part water to 2 parts solids. The grain size of the solid material in the suspension ranged from between 1 µm and 1 mm. The suspension obtained was then granulated by spray-drying, yielding a granulated material having a grain size of from 10 µm to 2 mm and a residual water content of about 4% by weight. The freely flowing granulated material was then moulded into a desired shape of a kiln furniture at ambient temperatures and a pressure of about 19,6 MPa (200 kg/cm 2 ). The green kiln furniture was then dried at 100°C in order to remove the water residue. Finally the green kiln furniture was sintered at about 1420°C for about 40 hours, yielding the desired kiln furniture which, in mineralogical respect, consisted of about 50% by weight of cordierite and about 50% by weight of mullite. This end product possessed the following properties: 1. Ceramic density 2.0 g/cm 3 2. Porosity 30% by volume 3. Mechanical strength 9 µPa 4. Modulus of elasticity 12 GPa The thus obtained kiln furniture was shown to be particularly suitable for the firing of hard porcelain, for example table ware, because of its sustained resistance to temperatures of 1380°C and higher, at which such porcelain has to be fired.

Claims: A method for the manufacture of a ceramic material starting with a mixture of a MgO source, SiO 2 and Al 2 O 3 source as well as chamottes and water, characterized in that the mixture contains 10-30% by weight of the MgO source, 10-30% by weight of the SiO 2 and Al 2 O 3 source, both being chemically pure, 40-80% by weight of the chamottes and water mixed in a ratio water:solids of about 1:2 and after admixture of organic additives needed to improve workability and process technology, yielding a suspension in which the particle size of the solids ranges from between 1 µm and 1 mm, after which said suspension is spray-dried, yielding granulates having a moisture content of 1-10 by weight and a grain size of 10 µm - 2 mm, which granulates are compressed into the desired final shape at a pressure of 19,7-49 MPa (150-500 kg/cm 2 ), followed by drying at 100°C for about 24 hours, after which the green products are sintered at 1380-1440°C for 20-80 hours, yielding a ceramic material applicable as kiln furniture having a cordierite content of 40-60% by weight and a mullite content of 40-60% by weight. A method according to claim 1, characterized in that about 20% by weight of MgO source, about 20% by weight of SiO 2 and Al 2 O 3 source and 60% by weight of chamotte are used. A method according to claim 1 or 2, characterized in that the moisture content of the granulates is about 4% by weight. A method according to claims 1-3, characterized in that sintering is carried out at about 1420°C and for about 40 hours. A method according to claims 1-4, characterized in that the final kiln furniture contains about 50% by weight of cordierite and about 50% by weight of mullite. A method according to claim 1, characterized in that said organic additives are selected from suspension stabilizers, moulding aids, plasticizer, binding agents and/or deflocculants. A method according to claim 6, characterized in that as suspension stabilizer a polyethylene adduct, as plasticizer a cellulose derivative, as binding agent methyl cellulose and as deflocculants a huminate is used. A method according to claim 6 or 7, characterized in that said organic additives are used in an amount of 0.1-8% by weight. A method according to claim 8, characterized in that said organic additives are used in an amount of about 3% by weight. A kiln furniture containing 40-60% wt% cordierite and 40-60 wt% mullite with a sustained resistance to a temperature of ≥ 1380°C without product desintegration.

Inventors: WEBER MONICA JOSEFA, WEBER, MONICA JOSEFA
Applicants: SPHINX TECHNICAL CERAMICS BV, SPHINX TECHNICAL CERAMICS B.V.
Application year: 1995
Main classification: C04B35
IPC codes: C04B35