GB2227241A

GB2227241A - Ceramic fibre binders

Info

Publication number: GB2227241A
Application number: GB8900632A
Authority: GB
Inventors: Harold Garton Emblem; John Hargreaves; Stanley John Shelley
Original assignee: Clinotherm Ltd
Current assignee: Clinotherm Ltd
Priority date: 1989-01-12
Filing date: 1989-01-12
Publication date: 1990-07-25
Also published as: GB8900632D0

Abstract

A composition for binding ceramic fibres or articles made from ceramic fibres comprises either an aqueous solution of lithium silicate and a chromium salt or a silica aquasol which is thickened with a natural clay mineral together with a finely divided refractory material which is an aluminosilicate or vitreous fused silica. The composition comprising lithium silicate and a chromium salt may also contain a hydrous aluminum oxide, optionally together with chopped ceramic fibres and/or alpha-alumina.

Description

" CERAMIC FIBRE BINDERS DESCRIPTION This invention relates to new compositions for binding ceramic fibres.

British Patent Specification No. 2,199,823A and European published patent application No.

0,276,899 describe a refractory cement containing chopped ceramic fibres and a hydrous aluminium oxide, which contains aqueous lithium silicate as the setting component. British Patent Specification No. 1,423,167 describes a method of binding together ceramic fibres or articles made from ceramic fibres using as binders a silica aquasol thickened with a natural or synthetic clay. The binder may also contain finely divided refractory material.

According to the present invention a composition for binding ceramic fibres or articles made from ceramic fibres comprises either an aqueous solution of lithium silicate and a chromium salt or a silica aquasol which is thickened with a natural clay mineral together with a finely divided refractory material which is an aluminosilicate or vitreous fused silica.

According to another aspect of the invention a composition for binding ceramic fibres or articles made from ceramic fibres comprises an aqueous solution of lithium silicate and a chromium salt together with a hydrous aluminium oxide optionally containing either chopped ceramic fibres or alpha-alumina or both.

The lithium silicate and the chromium salt should not form a precipitate when mixed. The preferred chromium salt is chromium acetate, the preferred chromium acetate being a chromic acetate.

The addition of hydrous aluminium oxide enhances the setting of the composition.

One suitable aqueous solution of lithium silicate is sold under the trade name of CRYSTAL L29 lithium silicate and has the following properties: Sp.Gr. 200C/20oC - 1.15 Wt ratio Si02 : Li20 - 11.3 Molar ratio SiQ2 : Li20 - 5.6 %Li20 (weight/weight) - 1.55 tSi02 (weight/weight) - 17.50 % Total solids (weight/weight) - 19.05 Chromic acetates and their preparation are described in M.J.Udy, "Chromium, Vol.I, Chemistry of Chromium and its Compounds", pp. 229-230 (Rheinhold Publishing Corporation, New York, N.Y. 1956). The preparation, manufacture and properties of chromic acetates are also described in Gmelin's Handbuch der Anorganische Chemie, 8th Edn., System No. 52, Chrom.

Teil B. pp 385-404 (Verlag Chemie GmbH, Weinheim, 1962). One method of preparing a chromium acetate is to treat acetic acid with an excess of hydrous chromic oxide. The solutions of chromium acetate sold by Lancashire Chemical Works Ltd. under the trade name CAC are examples of suitable chromium acetate solutions. Solution CAC-10 has a specific gravity of 1.21 and a Cr203 equivalent of 15% by weight. One example of a suitable composition is: CAC-lO chromium acetate solution - 10 volumes Crystal L29 lithium silicate solution - 1 volume These are mixed immediately before use.

The preferred hydrous aluminium oxide is boehmite. A suitable boehmite material is the GC Powder of Keith Ceramic Materials. This material has the following particle size distribution: Above 63 microns - 7% Passing 45 microns - 80% Passing 25 microns - 60% The chopped ceramic fibre should preferably have a mean average diameter of 3 - 4 microns. The fibre is preferably between 2 cm and 10 cm. In a composition which is to be used for joining refractory fibres, the length of the ceramic fibres in the composition is preferably between lcm and 5 cm.

The fibres in the composition may be alumina fibres or aluminosilicate fibres or they may be the filaments and refractory fibres whose preparation is described in British Patent Specification No. 2,173,179A, also in European published patent application No.

0,197,686 also in British Patent Specification No.

2,184,430A.

Suitable alpha-alumina materials include the BACO MS 95D grades. These are described in BACO Data Sheet 913 of January 1985. (BACO is a registered trade mark). The maximum percentage of MA 95D1; MA 95D2 and MA 95D3 materials retained on a 350 mesh B.S. 410, 1976 sieve (45 microns) is 0.58 w/w. The median mono crystal diameter of these materials is 5 microns. Another suitable alpha alumina material is the 0 115-25 material of VAW (Vereinigte Aluminium-Werke A.G.).This material has the following properties: Sieve analysis > 63 microns max 0.5% > 32 microns 5% < 20 microns 85% crystal diameter 2

1.3 microns In all the compositions of the invention the fine refractory material should desirably all pass a 72 mesh B.S. 410, 1976 sieve (212 microns) and preferably be much finer. Small amounts of coarser refractory material can be added, if necessary.

In compositions using a silica aquasol it is essential that the clay material should not cause coagulation or gelation of the silica in the silica aquasol. Examples of suitable clay materials are the clay minerals bentonite and hectorite, of these clay minerals bentonite is preferred. The preferred silica aguasols are silica aquasols in which the particle size is small, i.e. not exceeding 50 millimicrons and the silica content at least 30% by weight. A preferred silica aquasol is one in which the silica particle size is 11 - 16 millimicrons average, with a silica content of about 30% by weight and a pH of about 10. One such silica aquasol is sold under the registered trade mark NALFlOC as Nalfloc N 1030.

One method of preparing compositions which use a silica aquasol is to treat the clay material with water, mix the treated clay material with the silica aquasol, then add the finely divided refractory material. The volume of water used may be up to 1.3 times the volume of the silica aguasol, but no more than equal volumes of water and silica aquasol are preferred. Distilled or deionised water is preferred.

The ratio silica aquasol : clay is preferably between 5 : 1 and 20 : 1 Volume (cm ) : weight (g). The amount of refractory material added will depend on the particle size distribution of the material and the consistency required for the composition. The ratio silica aquasol : refractory powder will usually be between 1:50 and 1:22 volume (cm ) : weight (g). The compositions may be set by heating for a few hours at (approximately) 800C. On firing, they give refractory residues which bind the ceramic fibres.

Another method of preparing compositions which use a silica aguasol is to dilute the silica aquasol with water. The volume of water used may be up to 1.3 times the volume og the silica aquasol. If the aquasol is to be diluted, no more than equal volumes of water and silica aquasol are preferred. If the composition has to be stored in cold weather, a small amount of glycerol or ethylene glycol may be added at this stage. The required amount of clay mineral (for example, bentonite) may now be added, to form a fluid slip, to which the fine refractory material may be mixed in as required. The refrctory material may be added in two separate quantities, to enable the refractory material to wet properly. The ratio silica aquasol : refractory material will usually be between 1:5.0 and 1:2.2 volume (cm ) : weight (g).

The ratio silica aquasol : clay mineral is preferably between 5:1 and 20:1 volume (cm ):weight (g).

The compositions may be set by heating for a few hours at (approximately) 800C. On firing, they give refractory residues which bind the ceramic fibres.

Examples of aluminosilicate materials suitable for use as the fine aluminosilicate material in compositions using a silica aquasol include mullite (fused mullite or sintered mullite), sillimanite and kyanite, also aluminosilicate refractory materials obtained by calcining china clays or ball clays. One example of a suitable aluminosilicate refractory material obtained by calcining china clay is the material sold under the registered trade mark MOLOCHITE.

Compositions prepared using a silica aquasol and a vitreous fused silica as the refractory material are particularly suitable for bonding ceramic fibres to cordierite bricks, or to cordierite/mullite bricks, because the expansion characteristics of these compositions match the expansion characteristics of the bricks.

The compositions of the invention may be used to bind fibres such as mineral wool fibres, also refractory oxide ceramic fibres such as alumina and aluminosilicate ceramic fibres. They may also be used to bind the filaments and fibres whose preparation is described in British Patent Specification No.

2,173,179A, also in European published Patent Application No. 0,197,686 and in British Patent Specification No. 2,184,430A.

The compositions of the invention particularly the compositions of the invention prepared using a silica aquasol, may be used in the construction of the thermal insulating units and in the procedures described in British Patent Specification Nos. 1,585,866; 1,585,877 and 2,109,903A, also in United States Patent Specification Nos. 4,194,036 and 4,324,602.

The compositions of the invention are also suitable for use as the cement required in the construction of the furnace elements described in British Patent Specification No. 1,555,459 and in United States Patent Specification No. 4,344,753, also in Australian Patent Specification No. 520,229.

Compositions based on silica aquasols are preferred.

The compositions of the invention can also be used to join rigid shapes made from ceramic fibres by the procedures described in British Patent Specification No. 1,302,462. Compositions based on silica aquasols are preferred. When compositions based on silica aquasols are used in placing ceramic fibres on the hot face of a furnace, usually about 2.75 litres of composition per sq.metre of surface will be required and about half as much again when joining fibre to fibre.

The compositions of the invention may be used in the binding of ceramic fibres by spraying.

Compositions based on an aqueous solution of lithium silicate and a chromium salt (for example chromium acetate) may be used to bind ceramic fibres following the procedures given in British Patent Specification No. 2,199,823A and in European Published Patent Application No. 0,276,899.

The invention is illustrated by the following examples: EXAMPLE 1 A composition for binding ceramic fibres was prepared by mixing just before use 10 volumes of CAC-lO chromium acetate solution and 1 volume of Crystal L29 lithium silicate solution. This composition was used to bind alumina ceramic fibres, which formed the surface (hot face) of a furnace, by spraying the composition on to the surface., at a loading of approximately 0.6 cm3 per cm2. A hard rigid surface was formed on the ceramic fibre when the furnace was fired, firing temperature 15500C. The bonding composition was found to retard chemical attack on the fibre during operation of the furnace.

EXAMPLE 2 200 cm3 distilled or deionised water was added to 25g bentonite, all passing a 350 mesh B.S.410, 1976 sieve (45 microns) to form a cream.

300 cm3 of Nalfloc N1030 silica aquasol was now added, together with 1200g of Nolochite, all passing 72 mesh B.S.410, 1976 sieve (212 microns), then stirred, giving a thin slip, which thickened on standing. The composition was used to join rigid shapes made from an aluminosilicate ceramic fibre.

EXAMPLE 3 To 30 litres of Nalfloc N1030 silica sol were added 19 litres of deionised water and 1 litre ethylene glycol. 4 kg bentonite, all passing a 200 mesh B.S. 410 1976 sieve (75 microns) were now added with stirring as thickener. This mixture should now be allowed to age for at least 30 minutes. It can be stored for up to two days before adding the fine refractory material.

To 20 litres of the above mixture 32 kg Molochite, all passing 200 mesh B.S. 410 1976 sieve (75 microns) were added slowly with stirring. Five minutes later a further 12 kg Molochite, all passing 200 mesh B.S. 410 1976 sieve (75 microns) were added slowly with stirring. The composition was used to join rigid shapes made from ceramic fibre.

The composition may be used immediately after preparation or may be stored in sealed containers for use when required. The storage life in sealed containers is several days at ambient temperature.

EXAMPLE 4 (a) The composition prepared as described in Example 3 was applied by trowelling onto the brick lining of a furnace, at a loading of about 2.75 litres of composition per sq.metre. Chopped alumina or aluminosilicate ceramic fibre was placed on the treated lining, which was allowed to air-dry. The composition set hard when the furnace was fired, joining the ceramic fibre to the brick lining.

(b) The composition prepared as described in Example 3 was used in the construction of the thermal insulating units and in the procedures described in British Patent Specification Nos.

1,585,866 and 1,585,867.

EXAMPLE 5 A composition for binding ceramic fibres was obtained by adding vitreous fused silica (all passing a 200 mesh B.S. 410 1976 sieve (75 microns) to the mixture of Nalfloc 1030 silica aquasol, water, ethylene glycol and bentonite, whose preparation is described in Example 3.

The ratio silica aquasol volume (cm ) : vitreous fused silica (g) must be betweemn 1:5.0 and 1:2.2.

Claims

1. A composition for binding ceramic fibres comprising an aqueous solution of lithium silicate and a chromium salt.

2. A composition according to Claim 1 further comprising a hydrous aluminium oxide.

3. A composition according to-Claim 2 further comprising chopped ceramic fibres and/or alpha alumina.

4. A composition for binding ceramic fibres comprising a silica aquasol thickened by a natural clay mineral and a finely divided refractory being an aluminosilicate or a vitreous fused silica.

5. Ceramic fibres bonded by a composition according to any previous claim.