GB1576312A

GB1576312A - Kilns prepared for firing of ceramic articles

Info

Publication number: GB1576312A
Application number: GB267876A
Authority: GB
Original assignee: MORGANITE CERAMIC FIBRES Ltd
Current assignee: MORGANITE CERAMIC FIBRES Ltd
Priority date: 1977-04-18
Filing date: 1977-04-18
Publication date: 1980-10-08

Description

(54) KILNS PREPARED FOR FIRING OF CERAMIC ARTICLES (71) We, MORGANITE CERAMIC FIBRES LIMITED, of Neston, Wirral, Merseyside, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be paricularly described in and by the following statement: The invention relates to kilns prepared for or in use in the firing of ceramic articles.

An essential requirement in kiln furniture is that it shall support articles being fired so that there is no unnacceptable distortion in the transition from the green to the fully fired state. Conventional ceramics, which retain both shape and physical strength at firing temperatures, are therefore used and their disadvantages of fragility and waste of heat in repeated firing - often the mass of the kiln furniture is far greater than that of the articels being produces - are accepted.

We have found that ceramic fibre, preferalby bonded ceramic fibre, normally used for its heat insulating properties and therefore apparently wholly unsuitable for any application where ready access of heat is essential, makes highly successful kiln furniture. Ceramic fibre can readily be formed, by filter casting for example, into shapes which can be used both to support articles being fired and to allow access of heat, particularly between individual pieces of the kiln furniture or through openings provided in the pieces.

Thus the invention lies in a kiln prepared for or in use in the firing of ceramic articles, wherein the articles are supported by tiles, posts, girders saggers, ringers, setters, batts or other kiln furniture of porosity (void volume) at least 60% and made from or comprising ceramic fibre as herein defined.

The great advantage of the new kiln furniture is its low thermal capacity, but it has other advantages of resistance to thermal shock (in start up or during firing for example from air leaks) and ability to be handled without the constant breakages found with conventional furniture. The ceramic fibre structure can of course be crushed by rough handling, but it is not fragile in the sense 6f being liable to ready breakage when knocked, or when subjected to the vibration and shock of transport through travelling kilns. Also, fibres and bonding agents of high purity may readily be used, giving furniture particularly suitable for firing products that must be kept from taking up contaminants.

Suitable materials for the furniture depend on the firing temperature it has to withstand.

Many fibrous refractory compositions soften and shrink increasingly when heated, but for the purposes of the present invention a 'ceramic fibre' must, when tested as a uniformly heated, unrestrained body of the unbonded fibre, withstand continuously a temperature of 1000"C without showing a linear shrinkuge above 5%. Any fibre or fibre mix that will not give at least this performance is not a 'ceramic fibre' for the purposes of the invention. The 'service temperature' of a ceramic fibre, as referred to herein is the maximum temperature it will withstand continuously, when so tested, without showing a linear shrinkage above 5%.For some applications the shrinkage is desirably better than that qualifying the fibre as a 'ceramic fibre', preferably no more than 5% at the service temperature of the kiln, for example 1200 , 1300 or 1400"C or higher.

It is much more difficult to quantify softening particularly under load, and for this reason empirical parameters best applicable to individual operating conditions are adopted, the fibre being initially selected on shrinkage.

Among suitable fibres are the two general types of alumino-silicate fibre that exist: the first, intended for use up to 12600C, normally contains 43-55% A1203 and 42-54% SiO2, though other oxides may be present; the second, intended for use up to 14000C, normally contains 60-65% A1203 and 35-40% Si02, with sometimes the addition of other oxides. Such alumino-silicate fibres sold for example as grades of 'TRITON KAOWOOL' (TRITON ' and 'KAOWOOL' are Trade Marks), can be produced from naturally occurring minerals or from blends of A1203 and Si02 with various oxide additives, for example by melting the raw materials in a furnace and then allowing a stream of the molten alumino-silicate to be blasted into fibres by high pressure air or steam or by allowing the stream to fall onto a rotating disc or drum.

Alumino-silicate fibres cannot be generally used above 1400"C due to the unacceptably high shrinkage and softening characteristics which result. Where such conditions exist fibres made from more refractory materials are used. These include pure alumina, zirconia, magnesia, mullite and other high refractories, for example those which can be obtained in fibre form under the trade names 'SAFFIL' alumina and 'SAFFIL' zirconia, manufactured by I.C.I., and 'ZIRCAR' zirconia, manufactured by Union Carbide, or as mullite fibres e.g. those manufactured by Babcock and Wilcox. ('SAFFIL' and ZIR CAR' are Trade Marks). These fibres have service temperatures up to 2000"C.

Alternatively, high service temperatures (above 1400"C) may be attained by use of the ceramic fibre mixes of our co-pending U.K. patent application No. 14536/74 (1 506 152) in which comparatively inexpensive alumino-silicate ceramic fibres are upgraded by a proportion of alumina or other high-refractory fibres such as are mentioned above.

The definition of 'ceramic fibre' above further includes the higher grades of the mixed materials of our U.K. patent specification No. 1, 328,493. In that specification 'ceramic' fibres, i.e. fibres there defined as containing 40% or more alumina are mixed with mineral wool, which contains 3% to 20% alumina. For example the use of 30% 'TRITON KAOWOOL' (Trade Mark) with 70% of a typical mineral wool, by weight, gives a material of service temperature 1150"C. These materials can therefore be used for the kiln furniture of the invention in less demanding applications.

The kiln furniture for example tiles, posts, girders, saggers, ringers, setters and batts, may be made from refractory fibre compositions, bonded or not, using techniques which are conventional in themselves for making ceramic fibre articles. The fibre may in particular be bonded where the fibres touch but otherwise of open structure. For example the compositions can be used to prepare fibrous articles such as blankets, felts, blocks, boards, special shapes and paper products by air or water deposition. The porosity (void volume) of the furniture is at least 6%and generally 65%, 70% or more, depending on the balance required between strength and low thermal capacity.Also usable are castables, concretes, and cements ramming, tamping and mouldable materials made by dry or wet blending; and pressed and injection moulded articles made by the use of clay or plastic forming equipment; all containing fibre and of low density, at least 60% void volume. Ropes, threads, cloths and other woven or unwoven textiles made by the use of conventional textile machinery can be used, and the fibres may be used in the bulk form in which they are produced, and processed into desired articles and shapes with no additional ingredients.

The fibres may be associated with a form of reinforcement, particularly where they are to be used in load bearing applications. Reinforcement may be by one of two methods.

One method is by internal means whereby rods, tubes, mesh, rings, tiles or other appropriate rigid articles or shapes made from ceramic not containing fibre are embedded within the body of the article during manufacture. The other method is by external means whereby the fibrous article is supported within a rigid refractory framework or other support of such ceramic.

All fibrous articles may be surface treated, after manufacture, with a binder, refractory wash or similar auxiliary agent. Instances of the desirability of such treatment are included amongst the following examples, which illustrate the invention.

Example 1 The following composition was prepared 90% by weight 12600C 'TRITON KAOWOOL' alumino silicate fibre 5% by weight 'SYTON' (Trade Mark) x 30 colloidal silica bonding agent 5% by weight cationic starch The composition was formed into a slurry with water, and batts (Fig. 1) were made from the slurry by vacuum deposition.

These batts were found to be useful up to 12600C showing less than 5% shrinkage at that temperature Example 2 The following composition was prepared 80% by weight 12600C 'TRITON KAOWOOL' alumino silicate fibre 10% by weight 'SAFFIL' alumina fibre 5% by weight 'SYTON' x 30 colloidal silica bonding agent 5% by weight cationic starch The composition was formed into a slurry with water and batts made from the slurry by vacuum deposition.

These batts were found to be useful up to 1400"C, showing less than 5% shrinkage at that temperature, and were superior to the batts in Example 1, their shrinkage at 12600C being about 2%, because of the presence of the alumina fibre.

Example 3 The following composition was prepared 65% by weight 'TRITON KAWOWOOL' alumino silicate fibre 25% by weight 'SAFFIL' alumina fibre 5% by weight 'SYTON' x 30 colloidal silica bonding agent 5% by weight cationic starch The composition was formed into a slurry with water and batts made from the slurry by vacuum deposition.

These batts were fitted to a kiln car top and exposed to normal production conditions in a kiln. The batts were exposed to 7 x 36 hour production cycles incorporating a heat soak at 1200"C - 1400"C. Performance was satisfactory with shrinkage values of less than 3 % being recorded.

Example 4 Identical batts to those in Example 3 were tested in a different kiln operating at 12800C. Results after approximately 10 production cycles were considered satisfactory.

Example 5 Identical batts to those in Examples 3 and 4 were coated with a 'ZIRCOSIL' (Trade Mark) refractory wash it is a colloidal preparation of zirconia and silica to give an external hard coat to the batt. These were exposed to conditions as in Example 4 and resulted in better handleability and longer service life.

Example 6 Saggars (Fig. 2) manufactured from the composition and using the manufacturing technique referred to in Example 3 were fitted into a normal production line and subjected to one cycle during which the maximum temperature was 12800C, followed by 12 cycles with a maximum temperature of 13200C. The saggars performed satisfactorily though they showed some evidence of deformation due to softening in service as the service life was extended.

Example 7 Following the experience in Example 6 saggars were manufactured from the composition as detailed in Example 3. The composition was formed into a slurry with water and the saggars made from the slurry by vacuum forming. Reinforcing refractory rods were embedded into the base of the saggar (Fig. 3) during the vacuum forming process.

The reinforced saggars were exposed to 10 production cycles during which the temperature reached 13600C. They were found to perform satisfactorily with no evidence of sagging due to softening.

Example 8 Saggars were prepared as in Example 7 and all finished surfaces coated with 'ZIR COSIL' refractory wash. These saggars gave improved handleability when exposed to conditions similar to those in Example 7.

WHAT WE CLAIM IS: 1. A kiln prepared for or in use in the firing of ceramic articles, wherein the articles are supported by tiles, posts, girders, saggers, ringers, setters, batts or other kiln furniture of porosity (void volume) at least 60% and made from or comprising ceramic fibre as herein defined.

2. A kiln according to claim 1, wherein the furniture is formed of ceramic fibre which is unbonded or is bonded where adjacent fibres touch but otherwise of open structure.

3. A kiln according to claim 2, wherein the furniture has been made by deposition of the fibre, optionally with a bonding agent, from a fluid medium.

4. A kiln according to claim 3, wherein the furniture has been made by filter casting under suction from an aqueous or other liquid medium.

5. A kiln according to any one of claims 2 to 4, wherein the furniture has a porosity of 65%, 70%, or more.

6. A kiln according to any preceding claim wherein the kiln furniture is of or comprises an alumino silicate fibre of service temperature up to 1400"C; a pure alumina, zirconia, magnesia, mullite or other high refractory fibre of service temperature above 1400"C; a mixture of such alumino silicate and high refractory fibre; or a mixture of mineral wool containg 3 to 20% alumina and ceramic fibre containing 40% or more alumina, by weight.

7. A kiln according to claim 1, wherein the furniture has been made from a plastic or dry mix of the ceramic fibre and a particulate ceramic, said particulate ceramic bonding and being reinforced by the ceramic fibre.

8. A kiln according to claim 7, wherein the ceramic fibre is a fibre or fibre mix as set out in claim 6.

9. A kiln according to any preceding claim wherein the furniture is reinforced by rods, tubes, mesh, rings, tiles or other bodies of a rigid ceramic, not containing fibre, embedded within the furniture or by an external framework or other support of such ceramic.

10. A kiln according to any preceding claim, wherein the furniture has been treated after manufacture with a binder or refractory wash.

11. A kiln according to any preceding claim, wherein the furniture is arranged to provide spaces between indivdual pieces of furniture for access of heat or the kiln furniture itself is provided with openings for the purpose.

12. A kiln according to any one of claims 1 to 6, wherein ths furniture is made from board or paper or blanket, felt, rope, thread, cloth or other unwoven or woven textile, manufactured in the shape required for use

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Claims

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12600C being about 2%, because of the presence of the alumina fibre.

Example 3 The following composition was prepared 65% by weight 'TRITON KAWOWOOL' alumino silicate fibre 25% by weight 'SAFFIL' alumina fibre 5% by weight 'SYTON' x 30 colloidal silica bonding agent 5% by weight cationic starch The composition was formed into a slurry with water and batts made from the slurry by vacuum deposition.

These batts were fitted to a kiln car top and exposed to normal production conditions in a kiln. The batts were exposed to 7 x 36 hour production cycles incorporating a heat soak at 1200"C - 1400"C. Performance was satisfactory with shrinkage values of less than 3 % being recorded.

Example 4 Identical batts to those in Example 3 were tested in a different kiln operating at 12800C. Results after approximately 10 production cycles were considered satisfactory.

Example 5 Identical batts to those in Examples 3 and 4 were coated with a 'ZIRCOSIL' (Trade Mark) refractory wash it is a colloidal preparation of zirconia and silica to give an external hard coat to the batt. These were exposed to conditions as in Example 4 and resulted in better handleability and longer service life.

Example 6 Saggars (Fig. 2) manufactured from the composition and using the manufacturing technique referred to in Example 3 were fitted into a normal production line and subjected to one cycle during which the maximum temperature was 12800C, followed by

12 cycles with a maximum temperature of 13200C. The saggars performed satisfactorily though they showed some evidence of deformation due to softening in service as the service life was extended.

Example 7 Following the experience in Example 6 saggars were manufactured from the composition as detailed in Example 3. The composition was formed into a slurry with water and the saggars made from the slurry by vacuum forming. Reinforcing refractory rods were embedded into the base of the saggar (Fig. 3) during the vacuum forming process.

The reinforced saggars were exposed to 10 production cycles during which the temperature reached 13600C. They were found to perform satisfactorily with no evidence of sagging due to softening.

Example 8 Saggars were prepared as in Example 7 and all finished surfaces coated with 'ZIR COSIL' refractory wash. These saggars gave improved handleability when exposed to conditions similar to those in Example 7.

WHAT WE CLAIM IS: 1. A kiln prepared for or in use in the firing of ceramic articles, wherein the articles are supported by tiles, posts, girders, saggers, ringers, setters, batts or other kiln furniture of porosity (void volume) at least 60% and made from or comprising ceramic fibre as herein defined.
2. A kiln according to claim 1, wherein the furniture is formed of ceramic fibre which is unbonded or is bonded where adjacent fibres touch but otherwise of open structure.
3. A kiln according to claim 2, wherein the furniture has been made by deposition of the fibre, optionally with a bonding agent, from a fluid medium.
4. A kiln according to claim 3, wherein the furniture has been made by filter casting under suction from an aqueous or other liquid medium.
5. A kiln according to any one of claims 2 to 4, wherein the furniture has a porosity of 65%, 70%, or more.
6. A kiln according to any preceding claim wherein the kiln furniture is of or comprises an alumino silicate fibre of service temperature up to 1400"C; a pure alumina, zirconia, magnesia, mullite or other high refractory fibre of service temperature above 1400"C; a mixture of such alumino silicate and high refractory fibre; or a mixture of mineral wool containg 3 to 20% alumina and ceramic fibre containing 40% or more alumina, by weight.
7. A kiln according to claim 1, wherein the furniture has been made from a plastic or dry mix of the ceramic fibre and a particulate ceramic, said particulate ceramic bonding and being reinforced by the ceramic fibre.
8. A kiln according to claim 7, wherein the ceramic fibre is a fibre or fibre mix as set out in claim 6.
9. A kiln according to any preceding claim wherein the furniture is reinforced by rods, tubes, mesh, rings, tiles or other bodies of a rigid ceramic, not containing fibre, embedded within the furniture or by an external framework or other support of such ceramic.
10. A kiln according to any preceding claim, wherein the furniture has been treated after manufacture with a binder or refractory wash.
11. A kiln according to any preceding claim, wherein the furniture is arranged to provide spaces between indivdual pieces of furniture for access of heat or the kiln furniture itself is provided with openings for the purpose.
12. A kiln according to any one of claims 1 to 6, wherein ths furniture is made from board or paper or blanket, felt, rope, thread, cloth or other unwoven or woven textile, manufactured in the shape required for use

or cut from bulk stock.
13. A kiln prepared for or in use in the firing of ceramic articles, wherein the articles are supported by kiln furniture substantially as herein described in any one of the Examplebs.