GB2093009A

GB2093009A - Moulded articles

Info

Publication number: GB2093009A
Application number: GB8204313A
Authority: GB
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1981-02-16
Filing date: 1982-02-15
Publication date: 1982-08-25
Also published as: DE3105534C2; NL8200598A; SE8200858L; DE3105534A1; FR2499972B1; JPS57145084A; ZA821007B; GB2093009B; NO820440L; BE892072A; DD202135A5; IT8247787A0; FR2499972A1; PL235102A1; CA1190945A; YU31782A

Abstract

A moulded article is made from a composition comprising a mixture of 100 parts by weight ceramic fibres or a mixture of at least 20% by weight ceramic fibres and up to 80% by weight of a fired bonded granular material comprising ceramic fibres, bonding agent and refractory materials with 2 to 20 parts by weight clay and/or other conventional refractory materials, 0 to 8 parts by weight phosphate bonding agent, 0 to 10 parts by weight organic bonding agent, there being at least two parts by weight bonding agent present, with water. The mixture is compressed whilst being moulded into the desired shape and then dried and/or fired. The article has a density of 0.5 to 1.8 g/cm<3> and a hot bending strength at 1000 DEG C of at least 0.8 N/mm<2>

Description

SPECIFICATION Moulded articles The invention relates to moulded articles and is concerned with those articles which have a high mechanical stability at high temperatures and relates also to a process for the manufacture of such articles and their use.

Heat insulating ceramic fibre bodies comprising refractory fibres and organic or inorganic bonding agent having either low strength and high compressibility or high values for their strength, density and constancy of shape are known. Thus DE-AS 1274490 describes a combustion chamber for furnaces which is made by forming out a fibre mass mixed with bonding agent and in which the concentration of bonding agent decreases over the cross-section of the wall. Clays, alkaline silicates, aluminium phosphate, colloidal silica with a proportion by weight of 5 to 35%, optimally 10%, are named as a suitable bonding agent. The fibre body is, however, not capable of adequately resisting high loads due to the fact that one wall surface is hard and compact whilst the opposing wall surface is soft and flexible.

In the process disclosed in DE-AS 27 32 387 a mineral fibre plate prebonded with an organic plastics bonding agent is supposed to be strengthened by soaking with an aqueous slurry of a bonding clay and subsequent tempering. Furthermore, plates are disclosed in European Patent Application 0006362 which contain glass-like inorganic fibres in a matrix of a plastic clay as a reinforcement. The proportion of clay is in the region of 29 to 80% by weight and the proportion of the glass-like inorganic fibres in the region of 15 to 55% by weight of the plate.

It is an object of the invention to provide moulded articles which have improved mechanical and thermal properties and which, in particular, can serve as a replacement for light refractory plates.

According to the present invention there is provided a moulded article manufactured from the following composition: 100 parts by weight of either ceramic fibres or a mixture comprising at least 20% by weight ceramic fibres and up to 80% by weight of a fired, bonded, granular material comprising ceramic fibres, bonding agent and refractory material, 2 to 20 parts by weight clay and/or Al203 and/or SiO2 and/or aluminium hydroxides and/or magnesia and/or titanium dioxide and/or chromium oxide, 0 to 8 parts by weight phosphate bonding agent, calculated as P205, 0 to 10 parts by weight of an organic bonding agent, and 0 to 10 parts by weight other refractory additives, whereby, however, the composition contains at least 2 parts by weight of a bonding agent, the article having a density of 0.5 to 1.8 g/cm3 and a hot bending strength at 1000to of at least 0.8 N/mm2.

The invention relates also to a process for the manufacture of such an article and thus according to a further aspect of the present invention there is provided a process for the manufacture of a moulded article including the following steps: a) 100 parts by weight of either ceramic fibres or a mixture comprising at least 20% by weight ceramic fibres and up to 80% by weight of a fired bonded granular material comprising ceramic fibres, bonding agent and refractory material are thoroughly mixed with 2 to 20 parts by weight clay and/or Al203 and/or SiD2 and/or aluminium hydroxides and/or magnesia and/or titanium dioxide and/or chromium oxide, 0 to 8 parts by weight phosphate bonding agent, calculated as P205,0 to 10 parts by weight of an organic bonding agent, whereby, however, at least 2 parts by weight of a bonding agent are present, water and 0 to 10 parts by weight other refractory additives, b) the mixture obtained in step a) is compressed by a minimum volume factor of 3 when only using ceramic fibres decreasing linearly to 1.5 when using a mixture of 80 parts by weight of the bonded granular material and 20 parts by weight ceramic fibres whilst moulding the mixture to the desired shape, and c) the moulded article manufactured in step b) is dried and/or tempered and/or fired.

The moulded articles in accordance with the invention can be used for many purposes, in particular as a replacement for known light refractory plates. Their advantage is that they have a lower density than such known plates and they have a very narrow pore size distribution. Despite the compression in their manufacture, their thermal conductivity is of the same order as those shaped articles known per se including glass fibres which are not compressed in their manufacture and which are manufactured by a vacuum suction process. By comparison, the articles in accordance with the invention exhibit, however, a substantially higher strength.

By virtue of their high mechanical strength the articles in accordance with the invention are suitable particularly as expansion joint fillers between the bricks of rotary tubular furnaces. For this purpose asbestos has long been used, however the use of asbestos is increasingly resisted due to its adverse effect on the health.

The articles in accordance with the invention can incorporate all conventional ceramic fibres as the organic fibres, such as rock wool or, more preferably, fibres based on aluminium silicate, preferably with an Al203 content of about 40 to 95% by weight. The fibres are, however, preferably based on Al203 and SiO2 with at least 40% by weight Al203 and are preferably capable of being used at temperatures in excess of 1100 C. This will in general exclude inorganic fibres based on, for instance, basalt, slag and glass and natural asbestos fibres whose use temperature is below 1100 C, but such fibres may be, and preferably are, used as a subsidiary component in addition to those whose use temperature is above 1100 C.

The other refractory additives which may be used in the articles in accordance with the invention are those additives conventionally used in shaped fibre articles, such as porcelain powder, fire clay, hollow sphere corundum or vermiculite.

The phosphate bonding agents present in the articles in accordance with the invention may be conventional phosphate-containing bonding agents, e.g. boron phosphate, aluminium phosphate or sodium polyphosphate with a degree of polymerisation n 3 4, and particularly n = 6 to 10.

The organic bonding agents in the articles in accordance with the invention may be those bonding agents commonly used in refractory or heat-resistant shaped articles such as starch, sulphite lye or washings, molasses and, in particular, methyl cellulose. The given amount of bonding agent relates to solid organic bonding agent, i.e. disregarding any water.

Both the phosphate bonding agent and the organic bonding agent can be added both in dissolved form and/or in solid form. When using methyl cellulose, which is commonly used as a 5% by weight aqueous solution, a part of this methyl cellulose is however advantageously used in solid finely divided form, particularly when adding larger quantities of methyl cellulose, since otherwise the quantity of water introduced into the composition by such a bonding agent solution would be too large.

The clay which may be in the articles in accordance with the invention may be a conventional bonding clay or a special clay such as bentonite. This and Al203 and SiO2 and magnesia, and titanium dioxide and chromium oxide, all of which are preferably used in very finely divided form, and the aluminium hydroxides are components whose use is known in the refractory field. The term "very finely divided" is used here to mean that the components are present in a very finely divided or in a colloidal state. The very finely divided refractory materials preferably have a grain size of less than 50 Cim, more preferably less than 10 um.

Particularly when using such materials in the colloidal state, such as colloidal SiO2 or colloidal aluminium oxide, it is possible to use only small quantities of bonding agent, namely close to the lower threshold value of 2 parts by weight. The bonding agent can comprise only a phosphate bonding agent or only an organic bonding agent, advantageously however a mixture of both phosphate and organic bonding agents is used.

The use of about the same parts by weight phosphate bonding agent and methyl cellulose as the organic bonding agent is particularly preferred.

Advantageously the composition of the articles in accordance with the invention contains 5 to 15 parts by weight clay and/or the other said components to 100 parts by weight of the ceramic fibres or ceramic fibre and granular material mixture. Particularly advantageous is the use of a mixture of clay, in particular of bentonite, and 1 or 3 parts by weight of one of the other components referred to above, particularly of colloidal silica.

When manufacturing the articles in accordance with the invention, a mixture is produced of ceramic fibres or the mixture of ceramic fibres with the bonded granular material, and the clay and/or the other components referred to above, the phosphate bonding agent, if present, the other refractory additives, if present, the organic bonding agent, if present, and water. If the phosphate bonding agent and/or the organic bonding agent are added in the form of a solution, commonly an aqueous solution, the addition of further water may not be necessary. During the manufacture in step a) of the process there are preferably 5 to 25 parts by weight of water present to 100 parts by weight of the ceramic fibres.The phosphate bonding agents, such as sodium polyphosphate and monoaluminium phosphate, as well as the organic bonding agents such as sulphite waste or methyl cellulose, can be used in solid ground form but it is also possible to add a portion of these bonding agents in the form of a solution and the remainder in solid form.

The bonding granular material used in the manufacture of the articles in accordance with the invention is preferably of the type described in more detail in British Patent Application No. (Case 3123) of the present applicants which was filed on the same day. Its manufacture includes the following steps:: a) 100 parts by weight ceramic fibres, 2 to 15 parts by weight clay and/or A1203 and/or SiO2 and/or aluminium hydroxides and/or titanium dioxide and/or chromium oxide, optionally up to 10 parts by weight other refractory additives and 1 to 8 parts by weight phosphate bonding agent, optionally with the addition of a plasticising agent, are thoroughly mixed in a mixer with about 2 to 25, or in some cases 2 to 100, parts by weight water, b) the mixture obtained in step a) is compressed by a volume factor of at least 3, and c) the product obtained in step b) is optionally dried and fired at temperatures of 800 to 1550"C and subsequently comminuted.

The materials used in the manufacture of this granular material, i.e. ceramic fibres, clay or other components referred to, the refractory additives and the phosphate bonding agent correspond to the materials as described above. Methyl cellulose is preferably used as the plasticising agent. In the manufacture of the granular material the compression in step b) can be effected in an extruder, a rotary table press or a briquetting device. The mixing of the components in step a) in the manufacture of this fibre granulate can occur in any suitable mixer, for instance in a Drais mixer. Advantageously, loosened ceramic fibres are used in the manufacture of such a granular material, as can also be used in the manufacture of the articles in accordance with the invention. The comminution in step c) in the manufacture of this granular material can be effected in any suitable device and the maximum grain size is preferably 6 mm. This comminution can however be set to a predetermined range, for instance a product can without difficulty be obtained with a grain size between 2 and 3 mm by comminution in conventional crushing devices and, if necessary, sieving out of the desired grain sizes. The granular material obtained thereby is found to have a density of 0.7 to 1.75 g/cm3 and a pore volume of the order of 35 - 75%. The quantity of the plasticising agent which may be added in step a) depends on the compression device used in step b).For example, when using methyl cellulose and compressing in an extruder a quantity of 4 parts by weight methyl cellulose is preferably added, whereby half of this methyl cellulose can be added as a 5% solution in water and the other half as dry methyl cellulose. The quantity of water added can also vary with the compression device so that whilst normally 2 to 25 parts by weight water is adequate, if an extruder is used for the compression the water quantity may be up to 100 parts by weight.

The quantity of water used in the manufacture of the articles in accordance with the invention should be kept as small as possible, advantageously only up to 15 parts by weight water are mixed in with 100 parts by weight of the ceramic fibres and particularly preferably only 10 parts by weight water so that a dough-like mass is obtained.

The advantage of using a mixture of ceramic fibres and a fired granular material comprising ceramic fibres, refractory material and bonding agent resides in that when manufacturing the shaped articles in accordance with the invention a smaller quantity of water is necessary. Thus the quantity of water required depends on the proportion of ceramic fibres and fired granular material in the mixture. The use of a mixture of 50% by weight ceramic fibres and 50% by weight of the granular material has shown itself to be particularly advantageous.

The dough-like mass obtained in step a) when manufacturing the moulded articles is put into a suitable press in step b), for instance a plate press or a table press or even an isostatic press, and pressed for a suitable period of time, this depending on the type of press used. In a plate press the pressing time is commonly 5 to 20 seconds.

It is of importance when manufacturing the articles in step b) of the process in accordance with the invention that the compression is effected by a volume factor of at least 3 when only using ceramic fibres or by a volume factor of at least 1.5 when using a mixture of 80 parts by weight of the granular material and 20 parts by weight ceramic fibres. When using a mixture of ceramic fibres and fired fibre granulate in other proportions this minimum compression factor varies proportionally between 3 and 1.5. Advantageous values forthe compression factor are between 5 and 8 when using only ceramic fibres and 2.5 to 4when using a mixture of 80 parts by weight granular material and 20 parts by weight ceramic fibres. Naturally the advantageous compression factors lie between the values given above with other proportions of ceramic fibres and granular material.

The maximum volume factor of the compression is in practice about 12 to 14 when only using ceramic fibres and about 6 to 8 when using a mixture of 80 parts by weight granular material and 20 parts by weight ceramic fibres.

If the articles in accordance with the invention are in the form of plates these can have a thickness of 1 to 50 mm.

After pressing, the shaped articles are dried in step c) of the process, advantageously at between 110 and 1800C, and/or they can be tempered, e.g. at temperatures between 250"C and 600"C and/or fired, e.g. at temperatures between 800"C and 1650 C. The maximum firing temperature and also the maximum use temperature of the moulded articles depends, however, primarily on the ceramic fibres used in the starting mixture and rather less on the other refractory additives possibly present.

When delivered, ceramic fibres are generally in the form of a loose wool which is, however, partially strongly compressed. In order to enable a better bonding of the fibres by the bonding agent used and a good wetting of the surface by liquids of low concentration, the fibres are advantageously separated or loosened before the moulding.

For this purpose mixing units can be used with rapidly rotating knife heads, so called impact mixers, whereby larger agglomerates present in the fibres as delivered are loosened, without the fibres being thereby unacceptably strongly crushed.

If no granular material is used it is possible to carry out step a) of the process in such an impact mixer. This means that the loosening of the fibres is effected at the same time as the mixing with the components added in this step a), namely clay and/or the other refractory components referred to and optionally an organic bonding agent. In this case, however, only dry solid materials are added in order adequately to achieve the loosening of the agglomerated fibres and also the homogeneous mixing in of the added materials.

Subsequently water and bonding agent, optionally in the form of a solution, are sprayed into the mixing container and mixed in.

Naturally it is, however, also possible to carry out the loosening of the ceramic fibres in an impact mixer and then to add the other materials in another mixer, e.g. a Drais mixer or an Eirich mixer. This mode of operation is particularly appropriate when using vermiculite or hollow sphere corundum as further refractory additives, since otherwise a crushing of these materials would occur, and also when starting with a mixture of ceramic fibres with the fired granular material.

The articles in accordance with the invention have the particular advantage that they have very good thermal insulating properties due to the relatively high content of ceramic fibres but nevertheless a relatively good mechanical stability due to the compression during their manufacture. Furthermore their resistance to sudden changes of temperature, that is to say their thermal shock resistance, is excellent. This thermal shock resistances is preferably in excess of 25 air quenchings measured in accordance with German standard DIN 51068, part 2, on prismatic bodies of, e.g. 124 x 64 x 64 mm. The bodies are repeatedly heated to 9500C and then quenched by blowing them with air at room temperature through an 8 mm nozzle. After cooling the bodies are tested with a bending stress of 0.3 N/mm2.The thermal shock resistance is the number of cycles before failure.

The invention will now be described in more detail with reference to the following Examples.

In these Examples two types of ceramic fibres based on At203 and SiO2 were used, that is to say fibres A with 47% Al203 and 53% SiO2 with a use temperature up to 12600C and fibres B suitable for higher use temperatures with 95% Al203 and 5% SiO2.

The mixtures were partially produced in an impact mixer provided with a rapidly rotating knife head (3000 RPM). In this mixer the fibre material was well loosened and a pourable and fluid fibre material is produced which is uniformly mixed with the mixture components. The mixture comprising the granular material leads on further processing in presses to fibre materials with low to high gross density and a particularly homogeneous composition. If one uses a mixer which has mixing arms rotating with a relatively low velocity, e.g. an Eirich mixer, there is by contrast a less intensive loosening of the fibres and the resultant mixture is not so homogeneous. The 50% monoaluminium phosphate solution was introduced into the mixer in the region of the rapidly rotating knife head as a spray.In this manner a complete wetting of the agglomerate surfaces was achieved with a minimum liquid volume, e.g. 10% achieved with a minimum liquid volume, e.g. 10% by weight MAP = 6.6 litres. Water was subsequently sprayed in in a similar manner.

The water dissolves any dry methyl cellulose which may be present and thus brings about a good green strength of the shaped article.

Manufacture of the fired granular material: a) 100 parts by weight of ceramic fibres 10 parts by weight bonding clay with an Al203 content of 35% by weight and 1.5 parts by weight dry methyl cellulose in powder form were put into an Eirich mixer and mixed together for 10 minutes. Then 10 parts by weight of 50% by weight monoaluminium phosphate solution and 2 parts by weight water were sprayed onto the mass in the mixer whilst continuing to mix and mixed in for a further 30 minutes.

The product was taken out of the mixer and pressed at a pressing pressure of 30 N/mm2 in a plate press into a plate-shaped article with a thickness of 30 mm, whereby a compression by a factor of 5.5 was obtained.

The plate-shaped article was substantially dried at 11 00C for 24 hours in an oven and samples were then fired at different temperatures for 24 hours in each case and subsequently comminuted to a maximum grain size of 3 mm.

The granular materials obtained had the following properties: TABLE I Firing tem peratu re ("C) 800 1350 1510 Weight per unit volume, R, (g/cm3) 1.34 1.52 1.77 Specific weight, S, (g/cm3) 2.60 2.70 2.75 Pore volume, Pg, (Vol.%) 47.7 43.7 35.6 b) The method of manufacture of a) was repeated but an impact mixer was used to loosen the fibres. The pressing pressure in step b) was 10 and 15 N/mm2 on two different samples thereby achieving a compression by a factor of 4 and 5 respectively.

After firing at 1350 C for 24 hours and comminution fibre granulates with the following properties were obtained.

TABLE II Pressing pressure (N/mm2) 10 15 R (g/cm3) 0.7 1.02 Spec. weight (g/cm3) 2.7 2.7 Pg (Vol.%) 74 63 c) The method of manufacture a) was repeated but the proportion of monoaluminium phosphate solution was increased to 15 parts by weight and the proportion of water to 5 parts by weight with the mixing time shortened to 20 minutes. After firing at 13500C for 24 hours and comminution to the desired granulate this had the following properties: TABLE Ill R (g/cm3) 1.29 S (cm3) 2.69 Pg (Vol.%) 53.8 d) The method of manufacture a) was repeated but additionally 8 parts by weight fire clay powder were added in the first step. Furthermore only 8.3 parts by weight of 50% by weight monoaluminium phosphate solution but 4 parts by weight water were added in the mixing step.

The pressing pressure in the compression step b) was 30 N/mm2 which resulted in a compression by a volume factor of 5.2.

The plate-shape article obtained was dried at 180"C and sam ples were fired at the different tem peratu res given in the following Table IV. Subsequently the fired product was comminuted to a maximum grain size of 3mm.

The granular materials obtained had the following properties: TABLE IV Treatment temp. ("C) 180 800 1200 1300 1500 Weight per unit volume R (g/cm3) 1.30 1.26 1.31 1.34 1.48 Spec. weight (g/cm3) 2.60 2.60 2.65 2.68 2.72 Pg (Vol.%) 50.0 51.5 50.5 50.0 45.6 Manufacture of the moulded articles Example 1 The following composition was used: Parts by weight Ceramic fibres A 100 Bonding clay (25% by weight A1203) 10 Dry methyl cellulose 1.5 Monoaluminium phosphate solution, 50% by weight 10 Water 2 The ceramic fibres A were put into an Eirich mixer with the bonding clay and the dry methyl cellulose and mixed for 20 minutes thereby producing a homogeneous mixture.Then the monoaluminium phosphate solution and subsequently the water were sprayed in with the mixer continuing to run and thoroughly mixed in.

Subsequently blocks were pressed out with dimensions 405 x 135 x 50 mm at a pressing pressure of 30 N/mm2. The compression factor was 6.0.

Subsequently the blocks were dried for 4 hours at 11 00C and subsequently fired at differing temperatures for different times.

The properties of the blocks after the firing were as follows: TABLE V Temperature/ firing time 800 C/8h 1350 C/6h 1510 C/6h R(g/cm ) 1.34 1.52 1.77 Pg (Vol.%) 47.7 43.7 35.6 Deformation modulus (N/mm2) 1408 1303 5291 Cold bending strength (N/mm2) 5.0 4.4 8.2 Thermal shock resistance > 25 > 25 > 25 Hot bending strength, 1000 C ( N/m m2) 4.7 6.6 11.6 Hot bending strength, 1200 C (N/mm2) 6.2 5.9 9.6 Linear shrinkage % after 24 hours at 1400"C -3.19 -1.89 1500"C -6.94 -3.35 -0.16 1600 C -10.32 -7.70 -5.45 Chemical analysis:: Al2O3 (%) 44.7 SiO2 (%) 50.7 P205 (%) 2.95 Thermal conductivity (W/m K at 700 C) 0.45 Example 2 The same components were mixed in an impact mixer and moulded at lower pressing pressures. After firing at 13500C blocks with the following properties were obtained (the properties at a pressing pressure of 30 N/mm2 from Table V are set out as well for comparison):: TABLE VI Ex.1 Ex.2 Pressing pressure (N/mm2) 30 20 15 10 Compressing factor 6.0 5.4 4.4 3.5 Firing temperature ("C) 1350 1350 1350 1350 Length of firing (h) 6 24 24 24 R (g/cm3) 1.52 1.34 1.02 0.7 Pg (Vol.%) 43.7 58.0 63.0 74.0 Cold bending strength, (N/mm2) 4.4 4.1 0.9 0.7 Hot bending strength, 1000"C (N/mm2) 6.6 - 2.0 0.8 Hot bending strength, 1200"C (N/mm2) 5.9 5.6 Hot bending strength, 1400 C (N/mm2) - - 2.3 0.9 Thermal conductivity (W/m Kat700 C) 0.45 0.25 0.20 0.16 Example 3 The following composition was used:: Parts by weight Ceramic fibres A 100 Bonding clay (35% by weight Awl203) 10 Dry methyl cellulose 1.5 Monoaluminium phosphate solution, 50% by weight 15 Water 5 Firstly the dry components were put into an Eirich mixer and mixed for 10 minutes. Subsequently the monoaluminium phosphate solution and then the water were sprayed on. After further mixing for 20 minutes the composition was removed from the mixer.

As in Example 1 above, the mixture was pressed into blocks in a press at a pressing pressure of 30 N/mm2.

The compression factor was 5.2.

The pressed blocks were dried at 110 C for 4 hours and subsequently fired for 6 hours at 1 3500C.

The properties of the blocks obtained were as follows: TABLE VII R (g/cm3) 1.20 Pg (Vol.%) 53.8 Hot bending strength at 100 C (N/mm2) 2.1 Cold bending strength (N/mm2) 2.6 Thermal conductivity (W/m K at 700 C) 0.35 A comparison of the blocks produced in Examples 1,2 and 3 shows that when preparing the mixture in an impact mixer and using the same pressing pressure shaped articles can be obtained with a higher cold bending strength. When using an Eirich mixer, i.e. without loosening the ceramic fibres, it is convenient slightly to increase the proportion of phosphate bonding agent and also the proportion of water, the proportion of water being conveniently 8 to 10%.

Examples 4 and 5 The following compositions were used: Parts by weight Ceramic fibres B 100 100 Bonding clay (35% by weight A1203) 10 10 Dry methyl cellulose 1.5 1.5 Monoaluminium phosphate solution, 50% byweight 12 15 Water 3 5 The production of the composition in step a) occurred as in the method of Example 2, i.e. using an impact mixer.

Two portions of the mixture obtained in step a) were pressed in accordance with the method of Example 2 at a pressing pressure of 9 and 20 N/mm2 respectively into blocks, subsequently dried for 4 hours at 1 1 OOC and then fired for 24 hours at 1 350"C. The properties of the blocks were as follows: TABLE VIII Example 4 5 R (g/cm3) 0.52 1.01 Compression factor 3.5 6.7 Pg (Vol.%) 84.2 69.4 Cold bending strength (N/mm2) 0.9 1.9 Hot bending strength, 900"C (N/mm2) 3.4 Hot bending strength, 1000 C (N/mm2) 0.8 Hot bending strength, 1400 C (N/mm2) 0.7 Thermal conductivity (W/m 0K at 7000C) 0.19 0.35 Examples 6 to 9 The following composition was used:: Parts by weight Ceramic fibres A) 25 Ceramic fibres B) 75 Bonding clay (35% by weight A1203) 5 Very finely ground alumina < 44itm 5 Dry methyl cellulose 1.5 Monoaluminium phosphate solution, 50% by weight 10 Water 2 The mixture of Example 6 was prepared in an impact mixer as in the method of Example 2 and pressed at the pressure given in the following Table IX into blocks as in the method of Example 1. An Eirich mixer was used in Examples 7 to 9.

After drying at 110 Cfor 4 hours the blocks were fired for 24 hours at the different temperatures which are also given in the table. The properties of the blocks obtained are given in the table.

A portion of this mixture was also pressed into plates with dimensions 360 x 360 x 18 mm in a hydraulic plate press instead of into blocks with the dimensions given in Example 1. Such plates constitute an excellent firing aid, e.g. as a support for fine-ceramic products or porcelain when being fired.

TABLE IX Example 6 7 8 9 Pressing pressure (N/mm2) 8 30 30 30 Compression factor 3.8 5.3 5.3 5.3 Drying temperature (0C) 110 110 110 110 Firing temperature (OC) 1350 1520 1580 1620 Properties: R g/cm3) 0.57 1.22 1.22 1.31 Pg (Vol. %) 79.9 62.1 62.2 59.3 Cold bending strength {N/mm2) 0.4 3.5 3.S 4.2 Hot bending strength, 1000 C (N/mm2) 0.8 Hot bending strength, 1400QC (N/mm2) 0.8 4.5 4.9 5.6 Thermal conductivity (W/m "K at 700 C) 0.22 0.47 -0.49 0.53 Example 10 The following composition was used:: Parts by weight Fired granular material a) 80 Ceramic fibres B) 20 Bonding clay with 35% by weight Al203 5 Very finely ground alumina, < 44um 5 Very finely ground chromium oxide, < 44 Fm 2 Monoaluminium phosphate solution (50% by weight) 5 Solid sodium polyphosphate 0.5 Water 5 The fired granular material, the clay, the alumina and the chromium oxide were put with the solid sodium polyphosphate into an Eirich mixer, then 5 parts by weight water were sprayed on and mixed for 5 minutes.

Subsequently the 20 parts by weight of ceramic fibres B) were added and mixed in for a further 10 minutes.

Then the monaluminium phosphate solution was added and mixed for a further 10 minutes. The mixture was removed from the mixer, pressed into plates of 405 x 135 x 15 mm at a pressing pressure of-30 N/mm2, subsequently dried for 24 hours at 110 C then fired at 8000C or 1350 Cfor-8 hours.The following properties were determined on the product obtained: TABLE X Firing temperature ("C) 800 1350 R (g/cm3) 1.8 1.8 Compression factor 2.0 2.0 Pg (Vol.%) 35.8 35.8 Cold bending strength (N/mm2) 2.65 6.4 Hot bending strength, 1000 C (N/mm2) 4.5 7.0 Thermal conductivity (W/m "K at 700 C) 0.70 0.65 Example 11 The following composition was used: Parts by weight Ceramic fibres A) 100 Hollow sphere corundum, < 3 mm 10 Clay with 35% Al203 5 Magnesia 2 Solid boron phosphate 4.5 Water 9 The fibres were firstly loosened for 10 minutes in an impact mixer.The hollow sphere corundum, the clay and the water were put into an Eirich mixer, mixed for 5 minutes and then the magnesia and the boron phosphate were added and mixed in for a further 5 minutes. Subsequently the fibres loosened in the impact mixer were put into the Eirich mixer and mixed for a further 20 minutes.

Plates were produced from the mixture in accordance with the method of Example 10. These were fired after drying at 120"C at 800"C or 1350"C. The following properties were determined on the products obtained: TABLE XI Firing temperature ("C) 800 1350 R (g/cm3) 1.15 1.18 Compression factor 4.1 4.1 Pg (Vol.%) 51.9 54.6 Hot bending strength, 1000"C (N/mm2) 2.8 4.1 Thermal conductivity (W/m "K at 700 C) 0.63 0.65 Example 12 The following composition was used:: Parts by weight Ceramic fibres A) 100 Bonding clay 10 Dry sulphite waste 9 Water 9 The ceramic fibres, the clay and the solid sulphite wastewere mixed for 10 minutes in an Eirich mixer, then the water was sprayed on and mixing was finished after a further 10 minutes. Blocks were pressed as in Example 1 at a pressing pressure of 30 N/mm2. After drying at 11 00C for 24 hours these blocks were fired at 800"C or 1350"C. The following properties were determined on the blocks: TABLE XII Firing temperature ("C) 800 1350 R (g/cm3) 1.18 1.28 Compression factor 5.1 5.1 Pg (Vol.%) 54.6 52.6 Hot bending strength, 1000"C (N/mm2) 1.5 2.9 Thermal conductivity (W/m K at 700"C) 0.27 0.28

Claims

1. A moulded article manufactured from the following composition: 100 parts by weight of either ceramic fibres or a mixture comprising at least 20% by weight ceramic fibres and up to 80% by weight of a fired, bonded, granular material comprising ceramic fibres, bonding agent and refractory material 2 to 20 parts by weight clay and/or Al203 and/or SiO2 and/or aluminium hydroxides and/or magnesia and/or titanium dioxide and/or chromium oxide, O to 8 parts by weight phosphate bonding agent, calculated as P2O5, O to 10 parts by weight of an organic bonding agent, and O to 10 parts by weight other refractory additives, whereby, however, the composition contains at least 2 parts by weight of a bonding agent, the article having a density of 0.5 to 1.8 g/cm3 and a hot bending strength at 10000C of at least 0.8 N/mm2.

2. An article as claimed in Claim 1 in which the clay is bentonite.

3. An article as claimed in Claim 1 or Claim 2 which contains porcelain powder, fire clay or hollow sphere corundum as a further refractory additive.

4. An article as claimed in any one of Claims 1 to 3 in which the phosphate bonding agent is sodium polyphosphate or monoaluminium phosphate.

5. An article as claimed in Claim 1 in which the organic bonding agent is methyl cellulose.

6. A shaped article substantially as specifically herein described with reference to any of the accompanying Examples 1 to 12.

7 A process for the manufacture of a moulded article including the following steps: a) 100 parts by weight of either ceramic fibres or a mixture comprising at least 20% by weight ceramic fibres and up to 80% by weight of a fired, bonded granular material comprising ceramic fibres, bonding agent and refractory material are thoroughly mixed with 2to 20 parts byweightclay and/orAl2O3and/or SiO2 and/or aluminium hydroxides and/or magnesia and/or titanium dioxide and/or chromium oxide, 0 to 8 parts by weight phosphate bonding agent, calculated as P205, Oto 10 parts by weight of an organic bonding agent, whereby, however, at least 2 parts byweightofa bonding agentare present, water andOto 10 parts by weight other refractory additives, b) the mixture obtained in step a) is compressed by a minimum volume factor of 3 when only using ceramicfibres decreasing linearly to 1.5 when using a mixture of 80 parts by weight of the bonded granular material and 20 parts by weight ceramic fibres whilst moulding the mixture to the desired shape, and c) the moulded article manufactured in step b) is dried and/or tempered and/or fired.

8. A process as claimed in Claim 7 in which the compression in steb b) is carried out by a factor of 5 to 8 when only using ceramic fibres decreasing linearly to a factor of 2.5 to 4 when using a mixture of 80 parts by weight of the bonded granular material and 20 parts by weight ceramic fibres.

9. A process as claimed in Claim 7 or 8 in which the mixture is moulded into plates whilst compressing it in step b).

10. A process as claimed in any one of Claims 7 to 9 in which the ceramic fibres are loosened ceramic fibres.

11. A process for the manufacture of a moulded article substantially as specifically herein described with reference to any one of the accompanying Examples 1 to 12.

12. The use of the moulded articles as claimed in any one of Claims 1 to 6 as supports for objects to be fired in a furnace.