GB2093013A - Process for the manufacture of ceramic fibre containing granular heat-resistant or refractory materials - Google Patents

Abstract

The manufacture of a granular, heat-resistant or refractory material includes mixing 100 parts by weight of loosened ceramic fibres with 2 to 15 parts by weight of clay and/or other common refractory additives, 1 to 10 parts by weight organic bonding agent and 5 to 100 parts by weight water. The resultant mixture is compressed by a volume factor of at least three, dried and then comminuted.

Description

SPECIFICATION Process for the manufacture of ceramic fibre containing, granular, heat-resistant or refractory materials The invention relates to a process for the manufacture of ceramic fibre containing, granular, heat-resistant or refractory materials, the material manufactured in accordance with the process and to the use of such material.

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 1 2 74 490 describes a combustion chamber for ovens which is made by forming a fibre mass mixed with bonding agent and in which the concentration of bonding agent decreases across 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 suitable for high loads to the fact that one of its wall surfaces is compact and hard whilst the opposing surface is soft and flexible.

In the process described 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, fibre spraying compositions are disclosed in DE-AS 26 18 813 which, in addition to a large proportion of inorganic fibres, contain low proportions of a bonding agent or other inorganic additives and a chemical additive bonder and also 5 to 20% by weight of an oil to avoid dust formation. When using these fibre spraying compositions it is specifically stated that the inorganic fibres, such as rock wool, are used in a loosened state.

It is an object of the present invention to provide a process for the production of ceramic fibre containing, granular, heat-resistant or refractory materials which in the processed state have a higher strength, in particular with respect to mechanical stressing, and which can be used with particular advantage in so called fibre spraying compositions.

According to the present invention there is provided a process for the manufacture of a granular, heat-resistant or refractory material including the following steps: a) 100 parts by weight ceramic fibres, 2 to 1 5 parts by weight clay and/or Awl203 and/or SiO2 and/or aluminium hydroxides and/or magnesia and/or titanium dioxide and/or chromium oxide and 1 to 10 parts by weight organic bonding agent, calculated in solid form, are thoroughly mixed with 5 to 100 parts by weight water, and b) the mixture obtained in step a) is compressed by a volume factor of at least 3, dried and then comminuted.

The invention embraces also the ceramic fibre containing, granular, heat-resistant or refractory material produced by the method which is found to have particularly advantageous properties.

The ceramic fibres used in the inventive process can be all usual fibres of this type, e.g. rock wool or fibres based on aluminium silicate, preferably with a particularly high Awl203 content in the region of 45 to 95% by weight. Naturally mixtures of different ceramic fibres can also be used.The fibres are, however, preferably based on Awl203 and SiO2 with at least 40% by weight Awl203 and are preferably capable of being used at temperatures in excess of 11 00'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 1 1 ooec, but such fibres may be and preferably are used as a subsidiary component in addition to those whose use temperature is above 1 100'C.

The clay used in the inventive process can be a conventional clay or a special bonding clay, e.g. bentonite. This clay is preferably used in an amount of 2 to 1 5 parts by weight to 100 parts by weight of the ceramic fibres. Additionally, up to 10 parts by weight of other refractory additives can be used in the process, examples of which are porcelain dust and fire clay. The other refractory components which may be present instead of or as well as the clay in the mixture are Al2O3 and/or SiO2 and/or magnesia and/or titanium dioxide and/or chromium oxide, all of which are preferably in very finely divided form, and/or aluminium hydroxides.

These are components whose use is known in the refractory field. The term "very finely divided" is used here to mean that these components are present in a very finely divided or collodial state. The very finely divided refractory materials preferably have a grain size of less than 50 ,um, more preferably less than 10 ym. Particularly when using such materials in the collodial state, such as colloidal SiO2 or colloidal aluminium oxide, it is possible to use only low amounts of bonding agent, namely near to the lower threshold value of 1 part by weight of such a bonding agent.

Advantageously the total proportion of clay and/or other very finely divided components plus other refractory additives, if present, is 20 parts by weight to 100 parts by weight of the ceramic fibres.

The organic bonding agents used in the process in accordance with the invention may be conventional bonding agents of this type and molasses, sulphite lye or waste and most particularly, methyl cellulose are preferred.

The organic bonding agent is commonly used in the form of a concentrated solution, however, a part of it, advantageously up to 50% by weight, can also be used in solid finely divided form. If methyl cellulose is used as the bonding agent, this is commonly used in the form of a 5% by weight solution in water. Sulphite waste can be used with a conventional solid content of about 50% by weight, however it is also possible to use dried finely pulverized sulphite waste.

The mixing of the components in step a) of the process in accordance with the invention can occur in a conventional mixer, for example in a Drais mixer or mixer FM 600 D manufactured by the Loedige company of Paderborn, West Germany.

In an advantageous embodiment of the process in accordance with the invention the ceramic fibres are used in loosened or separated form. For this, commercially available fibres are loaded in the state in which they are delivered into an impact mixer (such as a Turbulent Rapid Mixer made by Drais) in which the fibres, which are commonly delivered as a fibre bundle, are converted into loosened fibres. Such a mixer is in principle similar to a plougher mixer with additional rapidly rotating blades and comprises a mixing unit with rapidly rotating knife heads so that any conglomerations which may be present in the fibres, which are in part present in a strongly compressed form, are loosened up without the fibres being unacceptably strongly crushed or comminuted.

Naturally it is also possible to mix the components which are to be added in step a), namely the clay and/or the other finely divided components, the other refractory additives if present and the organic bonding agent, if this is added in solid form, with the fibres in such an impact mixer whereby loosening up of the fibres and a particularly good and homogeneous mixing with the various components occur simultaneously. Subsequently, the dissolved bonding agent, if any, and water are added and mixed in.

The mixture produced in step a) of the process in accordance with the invention must be compressed by a volume factor of at least 3. This can advantageously occur in an extruder.

Alternatively this compression can be effected in a rotary table press or a conventional briquetting device or any other conventional type of press. The compression must be effected by a volume factor of at least 3 and advantageously the compression is effected by a volume factor of 3 to 6. The maximum volume factor of the compression in in practice about 12 to 14.

Subsequently the compressed product is dried, preferably at temperatures between 110 and 1 80 C, until the water contained in it is substantially or completely removed. The pieces thus obtained are comminuted to the desired granulate, the maximum grain size being preferably 8 mm and advantageously 6 mm. The communution can however also be set to a predetermined range, for example a product can without difficulty be obtained with a grain size between 2 and 3 mm or a maximum grain size of up to 2 mm or 3 mm by comminution in a conventional crushing device and, if necessary, sieving out of the desired grain sizes.

The granular material obtained by the process in accordance with the invention is found to have a density of 0.7 to 1.75 g/cm3 and a pore volume of the order of 35 to 75%.

The granular mixture obtained by the inventive process is particularly suitable for use in fibre spraying compositions. For this purpose the material is either fed in dry form to a spraying nozzle at the head of which it is mixed with water and an inorganic bonding agent or additive or a slurry is produced of the granular material with water and at least one inorganic bonding agent and then sprayed. Either phosphate bonding agents or alternatively hydraulic setting bonding agents, such as Portland cement or highly refractory cements such as high alumina cement, can be added as such additives, or the addition of other common additives is also possible when using the material as a fibre spraying composition. Spraying is also possible with the addition of water, bonding agent and ceramic fibres.The use of granular materials in accordance with the invention in fibre spraying compositions results in the advantage that these compositions require less water for spraying, whereby the water saving can be up to 50%.

Furthermore the breaking up of the fibres is substantially reduced. Such fibre spraying compositions are particularly useful for the insulating lining of heat treatment furnaces or as an insulating layer over refractory materials in furnace outlets. The compositions can also be used for subsequent insulation on the fire side of existing outlets, e.g. during their repair. In this case an additional high temperature resistant bonding agent, for instance a phosphate bonding agent or highly aluminous cement, must of course be present in the fibre spraying composition when spraying it.

A particularly advantageous usage of the fibre spraying compositions relates to the spraying of the ceilings of boiler rooms. Such ceiling linings for boiler rooms are described, for example, in German Auslegeschrift 28 32079 in which mats of a temperature resistant fibre material are secured to the ceiling by means of mountings and the last mat layer is completely covered with a protective layer of a high temperature resistant light building material. When using spraying compositions including the granular material in accordance with the invention it is possible to spray such roof linings with the inventive fibre spraying compositions, which, after drying, result in a compact insulating layer which requires no cover plate.

A further advantage of the granular material in accordance with the invention resides in that the organic bonding agent in the fibre granulates, be they used as such or in the form of additives to other refractory materials or refractory moulded bodies, partially or completely burns out at higher temperatures, e.g. at temperatures above 500"C. This results in the individual grains of the material no longer containing bonding agent so that the elastic properties of the ceramic fibres reappear.When using such a ceramic fibre granulate, for instance as an additive in a refractory composition, the advantage is therefore given that after the burning out of the organic bonding agent the individual grains of the fibre granulate are elastic and this advantageous property is imparted to heat-resistant or refractory components manufactured from such materials with the result that stresses to which they are subjected can be accommodated and compensated for. This results in the fact that such heat-resistant or refractory components which contain the fibre granulate in accordance with the invention have a lower tendency to crack formation since the stresses are relieved by the individual elastic grains of the fibre granulate.

In the process in accordance with the invention different quantities of water may be used in the production of the mixture in step a). The water quantity used depends primarily on the device in which the mixture obtained in step a) is subsequently compressed in step b). If this compression is effected in a briquetting device or a rotary table press, water quantities of 5 to 25 parts by weight are sufficient, however if the compression is effected in an extruder the quantity of water to be added in this step a) should be higher and may be as high as 100 parts by weight. The amount of water to be used can, however, be determined without difficulty by means of simple prior experiments. The water quantity used depends also on how large is the quantity added in step a) of clay, finely divided Awl203 or the other finely divided components referred to.Particularly when using colloidal SiO2 and colloidal Al2O3 larger water quantities can be advantageous.

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

In these examples ceramic fibres A with 47% Awl203 and 53% SiO2 or fibres B with 95% Al2O3 and 5% SiO2 were used.

Examples 1 to 5 Th following compositions were used: Example 1 2 3 4 5 Ceramic fibres A 100 - - - 50 Ceramic fibres B - 100 100 100 50 Bonding clay (with 35% Awl203) 1 5 6 - 4 Chromium oxide, < 63 itm - 4 4 - Colloidal SiO2 - - 6 2 4 Colloidal Al2O3 - - - - 6 Solid methyl cellulose 6 - - 4 1 Solid sulphite waste - 7 2 - Fire clay powder 2 - - - - Water 25 10 15 12 25 The ceramic fibres were mixed with the bonding clay and/or the other refractory components for 5 minutes in an Eirich mixer. The organic bonding agent or mixture of bonding agents was then put in and finally the water added. The resulting mixture was mixed for a total of 20 minutes.

The mixture was then compressed in a briquetting device (for instance that manufactured by Kloeckner-Humboldt-Deutz) by the volume factors given below, then dried for 1 2 hours at 1 20 C and finally comminuted to a maximum grain size of about 6 mm. The following properties were determined on the fibre granulates obtained: Example 1 2 3 4 5 Weight per unit volume, R (g/cm3) 1.25 1.09 1.15 1.20 1.23 Compression factor 5.4 7.2 6.8 6.0 6.5 Pore volume, Pg, (Vol. %) 49.5 69.7 68.0 53.8 62.6 Examples 6 to 10 The compositions of Examples 1 to 5 were used again but loosened fibres were used. The loosening of the fibres was effected in an impact mixer (such as that manufactured by Drais) for 5 minutes. Subsequently the remaining additives were added and mixed in for 2 minutes.

The resultant mixture was compressed at the differing compression factors shown below in a hydraulic press into blocks of 250 x 1 25 x 30 mm which were dried for 1 2 hours at 1 20eC and subsequently comminuted to a maximum grain size of 6 mm. The following properties were determined on the fibre granulates obtained: Example 6 7 8 9 10 R (g/cm3) 1.10 0.95 1.01 1.04 1.09 Compression factor 6.0 7.9 7.2 6.9 7.3 Pg (Vol. %) 56.0 73.5 71.7 59.9 66.6 Example ii The composition of Example 6 was used again with the difference that 80 parts by weight water were mixed in. The compression was effected by a volume factor of 3.2 in an extruder whose nozzle had a cross-section of 250 x 190 mm. The crude clups discharged from the extruder were cut off in suitable lengths and dried for 24 hours at 1 204C. Subsequently the dried clumps were comminuted to a maximum grain size of 3 mm. The following properties were measured on the fibre granulate obtained: R (g/cm3) 0.95 Compression factor 3.2 Pg (Vol. %) 62.7

Claims (16)

1. A process for the manufacture of a granular, heat-resistant or refractory material including the following steps: a) 100 parts by weight ceramic fibres, 2 to 1 5 parts by weight clay and/or Al203 and/or SiO2 and/or aluminium hydroxides ad/or magnesia and/or titanium dioxide and/or chormium oxide and 1 to 10 parts by weight organic bonding agent, calculated in solid form, are thoroughly mixed with 5 to 100 parts by weight water, and b) the mixture obtained in step a) is compressed by a volume factor of at least 3, dried and then comminuted.

2. A process as claimed in Claim 1 in which bentonite is used as the clay.

3. A process as claimed in Claim 1 or Claim 2 in which the ceramic fibres are in loosened form.

4. A process as claimed in any one of Claims 1 to 3 in which the bonding agent is molasses.

5. A process as claimed in any one of Claims 1 to 3 in which the bonding agent is sulphite lye in solid form and/or in solution.

6. A process as claimed in any one of Claims 1 to 3 in which the bonding agent is methyl cellulose in solid form and/or in solution in water.

7. A process as claimed in any one of the preceding claims in which up to 10 parts by weight of one or more conventional refractory additives are aded in step a).

8. A process as claimed in Claim 7 in which the refractory additive is porcelain powder or fire clay.

9. A process as claimed in one of the preceding claims in which the compression in step b) is carried out by a volume factor of 5 to 8.

10. A process as claimed in any one of the preceding claims in which 5 to 25 parts by weight water are added.

11. A process as claimed in any one of Claims 1 to 9 in which the compression in step b) is carried out by extrusion and in step a) 25 to 100 parts by weight water are added.

1 2. A process as claimed in any one of Claims 1 to 10 in which the compression is carried out in a briquetting device.

1 3. A process substantially as specifically herein described with reference to any one of Examples 1 to 11.

14. A ceramic fibre containing, granular, heat-resistant or refractory material manufactured by a process as claimed in one of Claims 1 to 1 3.

15. The use of a material as claimed in Claim 14 in a fibre spraying composition.

16. The use of a material as claimed in Claim 14 as an additive in heat-resistant or refractory components which, in use, are subjected to stresses.