GB2143516A

GB2143516A - Lightweight refractory bricks

Info

Publication number: GB2143516A
Application number: GB08418551A
Authority: GB
Inventors: Michael Neuenburg; Heinz Schermer
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1983-07-21
Filing date: 1984-07-20
Publication date: 1985-02-13
Also published as: IT1174173B; IT8421355A0; IT8421355A1; GB2143516B; DE3326271C2; GB8418551D0; FR2549468A1; DE3326271A1; BE900094A

Abstract

A method for manufacturing refractory lightweight bricks comprises shaping, drying and firing a slurry with fine grained refractory material and foam mixed into the slurry. The slurry is produced from 75 to 88 parts by weight quartz, aluminium oxide or mullite and 25 to 12 parts by weight water with at least 25% by weight of the refractory grain having a grain size less than 3 mu m and all of it having a grain size less than 0.2 mm. 0.2 to 1.5 parts by weight magnesium montmorillonite, 0.1 to 1 parts by weight caustic magnesium oxide and foam are added to this mixture and mixed in. The resulting foam slurry is then moulded, dried and fired. Silica bricks thus manufactured are particularly suitable for use in the dome of glass bath furnaces.

Description

SPECIFICATION Method of manufacturing refractory bricks The invention relates to a method of manufacturing refractory bricks of the type which includes making a slurry of at least partially fine grained, refractory material and optionally various additives and mixing it with a separately produced foam and then shaping the mixture and then drying and firing it.

DE-OS 1 471 314 discloses a method of manufacturing porous refractory materials in which a separately manufactured foam is added to a slurry and the mixture is then treated with a precipitating agent, thereby increasing the viscosity of the mixture so that it no longer flows.

Ground fire clay is particularly referred to as the starting material for the slurry but the use of quartz is also mentioned in this Offenlegungschrift. DE-OS 2 023 089 discloses a method of manufacturing expanded, fired granular compositions or shaped bodies from pulverulent, mineral materials, in particular from rock dust, in which a foam is added to a pasty mass of this mineral material. A thinning agent can be used to reduce the viscosity of the pasty mass.

DE-OS 1 965 008 discloses a method of manufacturing ceramic lightweight products in which pulverised ceramic starting material is used with an aqueous colloidal dispersion of silicon oxide or aluminium oxide and a bubble or foaming agent which forms gas at high temperatures.

It is an object of the present invention to provide a method of manufacturing foamed refractory lightweight bricks based on SiO2 and/or Al203 and/or mullite by which refractory lightweight bricks with high purity, low thermal conductivity and high strength can be manufactured without using additional bonding agents.

According to the present invention there is provided a method of manufacturing refractory bricks which includes making a slurry of 75 to 88 parts by weight refractory granular material and 1 2 to 25 parts by weight water, the refractory material comprising quartz and/or aluminium oxide and/or mullite, 100% of the refractory material having a grain size of 0.2 mm or less and 25% by weight of the refractory material having a grain size of 3 um or less, mixing 0.2 to 1.5 parts by weight magnesium montmorillonite, calculated as solid material, into the slurry in the form of a 2 to 6% by weight paste in water, and mixing 0.1 to 1 parts by weight caustic magnesium oxide, calculated as solid material, dispersed in water into the slurry, mixing the slurry obtained thereby with a foam and then shaping, drying and firing the resultant mixture.

When the granular refractory material is mixed with the water it may be present initially in the desired particle size or the particle size may be greater than that required in which event it may be wet ground to the desired size with the water. The quartz and aluminium oxide may be used alone or may be mixed in the composition of mullite.

Preferably the magnesium montmorillonite is added in an amount, calculated as solid material, of 0.2 to 0.6 parts by weight, with respect to 75 to 88 parts by weight of the refractory material. When the latter is quartz it may comprise quartz glass or other quartz material.

Preferably the amount of added caustic magnesium oxide is 0.1 to 0.3 parts by weight, calculated as solid material, which is dispersed in a small quantity of water.

In a preferred embodiment of the method 1 to 10 mm is removed from the surface of the refractory bricks after the firing. In this manner refractory lightweight bricks are obtained which are homogeneous in their properties over their entire cross-section and dimensionally exact.

When shaping and drying the bricks a small amount of the foamer migrates to the surface and apparently takes soluble materials with it. Particularly when the bricks are of quartz material, their surface becomes, in particular by reason of the firing, partially crystalline which could impair the homogeneity of the properties of the brick and the surface layer is therefore conveniently removed.

In a further preferred embodiment pouring clay is used as the finely divided aluminium oxide which is a commercially available product and comprises very finely ground Al203. When using such pouring clay it is not necessary in the first step of the method further to reduce the grain size by a wet grinding process. An intimate mixing of water and pouring clay can be performed satisfactorily in a mixing vessel.

When using aluminium oxide in the production of the slurry, it is preferred that the firing of the bricks is performed at a temperature between 1 500 to 1 700 C. When using a quartz material or quartz glass in the production of the slurry a firing temperature of either 1000 to 1 100'C or of 1100 to 1500'C, in particular 1350 to 1450"C may be used. When firing up to a maximum of 1 1 OO4C a refractory lightweight brick is obtained which can be rapidly heated up and retains its properties at temperatures up to 11 00'C.

In the case of firing at 1100 to 1 500 C, a silica brick is obtained with a crystalline phase comprising substantially Cristobalite which, as is known, must be heated up slowly to 1 000 C and by reason of its internal crystalline structure can be used up to temperatures of about 1 650"C. Such bricks are particularly suitable for use in the dome of glass bath furnaces.

A particular advantage of the method of the present invention is that the green, i.e. undried, shaped bricks can be removed from the mould after a relatively short time and must only then be dried and fired.

It is of course possible to manufacture relatively large refractory lightweight bricks and then to produce bricks of the desired size by cutting the large brick. In this event the removal of the surface layer occurs as a matter of course, at least at the cut sides of such bricks.

The starting materials used for the manufacture of the refractory lightweight bricks are materials which are known per se. The term "quartz material or quartz glass" is to be understood as comprising an amorphous granular silicon dioxide.

In the method of the present invention, if the refractory material requires grinding such grinding is continued until its grain size is sufficiently small and, in particular until at least 25% by weight has a grain size of 3 ,um or less. At least 25% of the refractory starting material must be present in such a very finely divided form if reasonable bonding times are to be achieved after pouring the mixture into moulds.

When quartz material or quartz glass is used as the starting material, at least 40% by weight is advantageously present with a grain size below 3 ,um. When aluminium oxide is used as the starting material, the proportion below 3 ,um is conveniently limited to 25 to 40% by weight and a grain size of up to 0.2 mm is present in the slurry. These features have a favourable effect on the brick as regards low shrinkage on firing.

When manufacturing the refractory bricks in accordance with the invention coarse grained refractory additives with a grain size of up to about 1 mm can be added to the slurry, as is known per se. When manufacturing silica based bricks using silica as the starting material, fragments of fire silica bricks with a grain size of O to 1 mm can be used as a refractory additive, whereby a reduction of the cost of the refractory material is obtained by comparison with the relatively expensive quartz material. The silica material provided as an additive already contains the crystalline SiO2 modifications converted from the starting material by the firing.

Zirconium silicate or zirconium oxide can also be used as further refractory additives.

The refractory additive can be added to the slurry up to an amount equal to that of the fine grained refractory material already contained in the slurry. If larger amounts are added, the strength of the refractory bricks naturally decreases.

To reduce the water requirement when carrying out the method in accordance with the invention it can be advantageous to add a thinner which is known per se. The addition of this thinner occurs advantageously after the wet grinding process.

In the method in accordance with the invention the magnesium montmorillonite added in the second step acts as a so-called standardising agent, and the lower threshold of 0.2 parts by weight of added magnesium montmorillonite is dictated by the fact that below this value the magnesium montmorillonite no longer has an effect as a suspending agent whilst an increase of the added amount above 1.5 parts by weight brings no further improvement as regards its effect.

The caustic magnesium oxide added as a dispersion in water in the method in accordance with the invention serves as a reaction initiator. Normally, one part by weight caustic magnesium oxide is dispersed in 3 to 10 parts by weight water. This caustic magnesium oxide causes a bonding of the finely divided refractory materials so that a solidification of the mixture occurs after pouring it into moulds.

The method in accordance with the invention may be carried out in a conventional manner, i.e. the refractory materials and the water in the given proportions are firstly wet ground (or wet mixed if the starting materials are already in a sufficiently fine form i.e. at least 25% by weight thereof have a grain size of 3 ,um or less). Then the magnesium montmorillonite is added in the given quantity in the form of a paste to the wet material and mixed in for a time of e.g. 1 to 2 minutes. Then the dispersion of the caustic magnesium oxide is added and also homogeneously mixed in for 1 to 5 minutes, advantageously 1 to 2 minutes.

At the same time as the production of the slurry mixture, which optionally includes large grained refractory additives, the foam necessary for the manufacture of the bricks in accordance with the invention is produced in a manner known per se in a second mixing vessel. For this purpose the foaming agent and water, commonly in a volume ratio of 1 :10, are converted into a stable foam by means of a high speed mixer.

Commercially available products such as e.g. ammonium alkyl ether sulphate with an aromatic sodium sulphonate or sodium alkyl polyglycol ether sulphate are suitable as the foaming agent.

When using quartz material as the starting material surface active polypeptide polyoxyalkylenes (e.g. Airocel PK sold by Mearl Corp., New Jersey, U.S.A.) have proved to be particularly suitable.

This foam is then mixed into the slurry and the density of the finished product can be controlled by choosing the proportions of slurry and foam. If, for instance, a density of 0.6 g/cm3 is desired when using quartz material as the starting material, about 20 1 foam must be used to 10 I slurry (containing 14.4 kg fine grained quartz material), though this quantity may, if necessary, be further increased if a slight collapsing of the foam occurs during the mixing process. This is however possible without difficulty with the aid of the usual expert knowledge and simple prior experiments.

After mixing the slurry with the foam the mixture thus obtained is poured into the desired mould, advantageously a folding mould, and after about 1 2 to 30 hours the moulded brick can be removed from the mould. Advantageously, tbe brick is then dried in air for 24 hours, or with larger bricks for 48 hours, and after this it may be further dried in a suitable drying oven at a temperature which is steadily increased from 40"C to 11 0'C.

After the drying, i.e. after removal of the physically bound water, the firing is performed at temperatures which differ in dependence on the refractory starting material used. When quartz material is used a temperature increase of 1 50tC/h up to a firing temperature of 11 00'C is suitable and when using a mixture of quartz material and silica fragments a temperature increase of 40"C/h up to a firing temperature of 1400"C is advantageously used. When using aluminium oxide as the refractory starting material advantageously a temperature increase of 80"C/h to a firing temperature of 1 500 to 1 700 C is advantageously used.When using mullite as the starting material the temperature increase is advantageously 40"C/h up to a firing temperature of 1500 to 1 650'C.

Depending on the dimensions of the brick the firing may be carried out for 1 to 5 hours after reaching the firing temperature.

The invention will be described in more detail with reference to the following examples: Example 1 To manufacture a quartz material refractory lightweight brick, quartz material with over 99% by weight SiO2 was used as the refractory starting material. 80 Parts by weight precomminuted quartz material with a grain size of up to 1 mm were mixed with 20 parts by weight water and finely ground in a grinding vessel for 1 2 hours until 40% by weight of the quartz material had a grain size of less than 3 ,um. In the grinding vessel (which is termed an Attritor) the material to be ground and grinding balls of corundum were agitated by the blades of a shaft extending through the vessel.

In a separate process, a 4% by weight paste was produced from 1 part by weight magnesium montmorillonite and 24 parts by weight water with the aid of a high speed mixer.

This magnesium montmorillonite paste was added to the wet ground refractory material in an amount of 1 part by weight magnesium montmorillonite with respect to 80 parts by weight quartz material and mixed in for 2 minutes.

x .15 parts by weight caustic magnesium oxide dispersed in 0.5 parts by weight water were then added to this mixture.

Separately from the production of the slurry, a foam had been previously produced using polypeptide polyoxyalkylenes as the foamer, for which purpose 10 parts by volume water were used with 1 part by volume foamer and converted into a stable foam by means of a high speed mixer.

1 8 I foam were stirred into 91 of the slurry.

After pouring the mixture into a mould and then removing the bricks from the mould after 1 8 hours the bricks were dried for 24 hours in air. Subsequently the brick was further dried in a dryer whose temperature was increased from 40"C to 11 0 C for a further 48 hours.

After the drying, the brick was fired for 4 hours at a temperature which was increased by 1 50 C/h up to a firing temperature of 11 00'C.

The quartz material refractory lightweight brick obtained had the following properties: SiO content % 98.5 3 Gross density gysm 0.6 Compression strength Xmn2 7.5 Total porosity % 71 0 Thermal conductivity w/mK at 400 C 0.21 0 at 600 C 0.24 at 900 0C 0.32 Maximum use temperature OC 1100 Example 2 In this example, an aluminium oxide was used as the refractory starting material.

A calcined clay was used which was already sufficiently finely grained, i.e. more than 70% by weight had a grain size less than 3 ,um. 33 parts by weight of the calcined clay, 33 parts by weight aluminium oxide with a grain size from 0 to 0.3 mm and 1 7 parts by weight aluminium oxide with a grain size from 0 to 0.2 mm together with 17 parts by weight water and 0.1 parts by weight sodium polyphosphate as a thinner were mixed in a stirring vessel.

In addition, 0.5 parts by weight magnesium montmorillonite in the form of a 4% by weight paste in water were added and mixed in for 2 minutes. Finally a reaction initiator in the form of 0.12 parts by weight caustic magnesium oxide in the form of an aqueous 20% by weight dispersion were added and also mixed in for 2 minutes.

One part by volume of this slurry was mixed with 3 parts by volume of a foam and formed into bricks and dried in the manner described in Example 1. Ammonium alkyl ether sulphate with an aromatic sodium sulphonate were used as the foamer in the ratio of 1 part by volume foamer to 10 parts by volume water.

The bricks were fired at a temperature which was increased by 80'C/h up to a firing temperature of 1 700'C and maintained for 3 hours at this temperature. The properties of the aluminium oxide lightweight bricks obtained were as follows: Al 203 content 3 ca.99 3 Gross density g/cm 0.92 Compression strength N/mm2 17 Total porosity $ 77 0 Thermal conductivity w/mK at 400 C 1.0 at 600 C 0.9 0 at 900 C 0.8 0 Maximum use temperature C 1850 Example 3 This example describes the manufacture of a so-called silica refractory lightweight brick.

The quartz material slurry manufactured in Example 1 was used. After the removal of the wet ground material from the mixer, fragments of fired silica bricks were firstly mixed with the quartz material in a weight ratio of 1:1. The fragments of the silica bricks had a grain size of O to 1 mm.

The addition and mixing in of magnesium montmorillonite and caustic magnesium oxide and the further processing into bricks were the same as in the method of Example 1.

After the drying the bricks were fired with a temperature increase of 40"C/h for five hours at 1450"C and slowly cooled in accordance with the crystalline structure formed. The silica refractory lightweight bricks obtained had the following properties: SiO content % ca.97 Gross density g/cm3 0.57 Compression strength N/mm2 4.4 Total porosity 8 76 Thermal conductivity w/mK at 400 0C 0.29 at 600 C 0.34 at 600 C 0.34 at 900 0C 0.44 Maximum use temperature OC 1650 Example 4 In this example a refractory lightweight brick was manufactured from a mixture of quartz material and aluminium oxide in the composition of mullite. Quartz material slurry as wet ground material from Example 1 in an amount of 28.8 parts by weight containing 5.8 parts by weight water and 23 parts by weight quartz material of which 40% by weight had a grain size of less than 3 ,um was used. 1 7 parts by weight calcined clay (with more than 70% by weight having a grain size less than 3 pm, as in Example 2), 40 parts by weight aluminium oxide with a grain size 0 to 0.3 mm and 14.2 parts by weight water were added to the quartz material slurry and mixed in. (With respect to the sum of SiO2 and Al203 it can be calculated that a composition of about 72.5% by weight Al203 and 27.5% in weight SiO2 was obtained.) As in Example 1, magnesium montmorillonite paste in the amount of 1 part by weight magnesium montmorillonite and 0.15 parts by weight caustic magnesium oxide were added and mixed in. 2.5 parts by volume of foam of Example 1 were added to 1 part by volume slurry to produce a foam slurry from which refractory lightweight bricks in the composition of mullite were obtained by shaping, drying and firing.

Claims

1. A method of manufacturing refractory bricks which includes making a slurry of 75 to 88 parts by weight refractory granular material and 1 2 to 25 parts by weight water, the refractory material comprising quartz and/or aluminium oxide and/or mullite, 100% of the refractory material having a grain size of 0.2 mm or less and 25% by weight of the refractory material having a grain size of 3 ym or less, mixing 0.2 to 1.5 parts by weight magnesium montmorillonite, calculated as solid material, into the slurry in the form of a 2 to 6% by weight paste in water, and mixing 0.1 to 1 parts by weight caustic magnesium oxide, calculated as solid material, dispersed in water into the slurry, mixing the slurry obtained thereby with a foam and then shaping, drying and firing the resultant mixture.

2. A method as claimed in Claim 1 in which the refractory material is mixed with the water by wet grinding the refractory material in the water.

3. A method as claimed in Claim 1 or Claim 2 in which 0.2 to 0.6 parts by weight magnesium montmorillonite, calculated as solid material, are added.

4. A method as claimed in any one of Claims 1 to 3, in which 0.1 to 0.3 parts by weight caustic magnesium oxide, calculated as solid material, dispersed in water are added.

5. A method as claimed in any one of the preceding Claims in which a layer 1 to 10 mm is removed from the surface of the bricks after the firing.

6. A method as claimed in any one of the preceding Claims in which aluminium oxide is used which comprises pouring clay.

7. A method as claimed in any one of the preceding Claims in which aluminium oxide is used and the bricks are fired at a temperature of between 1 500 and 1700"C.

8. A method as claimed in any one of the preceding Claims 1 to 6 in which quartz is used and the bricks are fired at a temperature of between 1000 and 11 00'C or 1100 and 1 500 C.

9. A method of manufacturing ceramic bricks substantially as specifically herein described in any one of the accompanying examples.

10. Refractory bricks manufactured in accordance with a method as claimed in any one of the preceding Claims.