WO1988001990A1

WO1988001990A1 - Improved binders for refractories

Info

Publication number: WO1988001990A1
Application number: PCT/GB1987/000629
Authority: WO
Inventors: Ayub Mahomed Adam; Harold Garton Emblem; Ian Malcolm Mackinnon; Peter Womersley
Original assignee: Manchem Limited
Priority date: 1986-09-09
Filing date: 1987-09-09
Publication date: 1988-03-24
Also published as: GB8621713D0; EP0282521A1

Abstract

A binder for refractory grain capable of forming a rigid coherent gel which leaves at least one refractory residue on firing, comprises an hydroxy group-containing colloid complexed with at least one aqueous miscible metal organic compound.

Description

IMPROVED BINDERS FOR REFRACTORIES

This invention relates to the binding of refractory grain, specifically to an improved method of forming refractory articles using a complexed organic hydroxyl-group containing colloid as a binder.

When refractory shapes are formed by pressing, a refractory powder mix is moistened with an organic binder to give 'green' strength after pressing, then the pressed shape is fired to remove the binder and form a ceramic bond.

Polyhydroxy compounds such as celluloses are well known as fugitive binders in the production of refractory articles by conventional pressing techniques, but their use has previously been limited to articles generally of a simple shape as the articles pass through a fragile phase when the binder is incinerated prior to sintering of the refractory grain.

Alternative proposals have included the use of petroleum waxes and wax emulsions as fugitive binders but these do not eliminate the problem.

Refractory metal alkoxides and acylates, particularly alkoxides of aluminium, zirconium and titanium, have been previously proposed as binders for refractory grain. These have the advantage of not only being capable of being hydrolysed to a gellable hydrolysate to give 'green' strength to refractory shapes, but on firing forming a refractory residue which binds the refractory grains together.

Binders derived from organic silicates, which hydrolyse and gel under appropriate conditions to give a rigid coherent gel have also been widely used. On firing silica is obtained and a ceramic bond is formed. Silica is recognised as a refractory oxide but its refractory properties do not compare favourably with the refractory properties of, for example, alumina or zirconia.

Water soluble chelates of metals such as titanium and zirconium have previously been proposed for use in aqueous polymer systems in the gelation of protective organic colloids. GB 922456 describes the use of water soluble titanium chelates as additives to impart thixotropy to emulsion compositions containing a water soluble, hydroxy group-containing organic colloid. In GB 1102427 water soluble zirconium and aluminium chelates are proposed for use in similar systems. GB 1588521 describes mixed metal complexes having improved gelling characteristics again for use in aqueous polymer systems, particularly emulsion paints. GB 1093465 describes zirconium-organic complexes suitable for use in the gelation of protective colloids such as used in the production of emulsion paints, water based paints, inks, cosmetic preparations, carpet backings, adhesives, paint strippers, polishes and textiles sizes. However, none of these systems has been suggested for use as binders in the production of refractory articles. It is according to the present invention that there is provided a binder for use in binding refractory grain comprising an hydroxy group-containing organic colloid complexed by the addition of at least one aqueous miscible metal organic compound which on firing leaves at least one refractory residue.

It is intended to include within the scope of this specification metal chelates such as described in GB 1093465 within the definition of aqueous miscible metal organic compound.

Using such binding systems it is possible to extend the use of cellulose type binders or other hydroxy group-containing organic colloid binders to include the manufacture of articles of any desired shape as on firing articles made with binders of the invention, the binder acts as a sintering aid which gives the advantage of improved hot strength and therefore improved dimensional stability leading to final products having lower porosities than heretofore where cellulosic binders alone have been employed.

In practice the metal organic compounds are selected from alkoxides, acylates and organic complexes of calcium, magnesium, aluminium, chromium, titanium, zirconium and silicon, or polymers thereof, and depending on the composition of the refractory grain selected, mixtures of two or more compounds may be preferred. Thus, for mullite grain, a mixture of aluminium and silicon compounds which would give an oxide residue of approximately the same stoichiometry as mullite may be preferred.

It is in a preferred aspect of the invention that the metal organic compounds selected for use are those which on firing leave an oxide residue substantially identical to the oxide composition of the grain used. In this way the desirable properties of the fired refractory are not adversely affected. For example, the presence of different refractory oxides in the final product may lead to reduced thermal shock resistance or reduced resistance to attack by slags.

However, matching of the oxide contents is not an essential feature of the invention and in some aspects is not as beneficial as using other binders according to the invention. For example, castable compositions prepared from alumina, mullite or aluminosilicate, usually include a clay or bentonite to give plasticity. The elimination of clay or bentonite would improve refractory properties which would be a significant advantage. Using the binders according to the invention eliminates the need for clay or bentonite. If desired, only part of the clay or bentonite need be replaced by binders according to the invention.

For instance the martensitic phase transformation of zirconia can be used to increase the strength and fracture toughness of refractory articles. On cooling through the transformational range, large particles of tetragonal zirconia transform more readily into the monoclinic form than do smaller particles. When the particle size of the zirconia is sufficiently small a metastable tetragonal form of zirconia can be retained to ambient temperature. The addition of stabilising oxides such as CaO, MgO and Y2O3 to zirconia is known to increase the maximum size of the metastable tetragonal form of zirconia which can be retained at ambient temperature. Under the influence of an applied force, the stress field around a crack tip can induce the tetragonal - monoclinic transformation of zirconia, which by absorbing energy from the cracks increases the toughness of the article. The use of the binder derived from water-soluble zirconium compounds according to the invention gives zirconia in a form which will increase the toughness of a refractory article. The toughness of a refractory article prepared according to the invention, may also be improved when fine unstabilised zirconia forms part of the refractory grain mix.

Other metal compounds may be selected to give an oxide residue which on firing converts to particular compounds having special properties. For instance, magnesium plus aluminium alkoxide give an oxide residue which converts readily to spinel. In another instance, aluminium plus titanium alkoxide gives an oxide residue converting to 'aluminium titanate', which has a very low coefficient of expansion which would lead to improvement of resistance to thermal shock.

When the metal organic compound is an aluminium or zirconium compound a silica aquasol may also be included. Preferably the silica aquasol is present in an amount to give a binder having the oxide stoichiometry of mullite or zircon.

The aqueous miscible hydroxyl-group containing colloids may be cellulose derivatives such as cellulose-oxirane reaction products, for example hydroxyethyl cellulose or hydroxypropyl cellulose; carboxymethyl celluloses; partially hydrolysed polyvinyl acetates or acetate/butyrals; homopolymers and copolymers of acrylic acid, acrylamide and methacrylic acid, particularly their sodium or ammonium salts; polyvinyl alcohols; gums, for example, xanthan gum and guar gum; dextrins; starches; gelatin; alginates such as sodium and ammonium alginate; lignosulphonates.

There is merit in adding more than the minimum quantity of metal organic compound necessary to form a complex with the hydroxy-group containing organic colloid, because this will help the binder act as a sintering aid. Using more than the minimum quantity of metal organic compound than is necessary to form a complex with the colloid will give improved strength at the stage in the firing where all the organic material has burned away.

If the colloid, as defined herein, is non-ionic, the cross-linking bonds formed by the complex are not as strong as with other colloids and are usually formed under alkaline conditions. However, this complex has the advantage that it is not easily precipitated by an excess of metal compound. Anionic colloids, e.g. the carboxymethyl celluloses, are economically attractive to use but are usually supplied as the sodium or potassium salts which may form low melting point glasses on firing. The ammonium salts are therefore preferred. It is also possible to obtain the free acid by ion-exchange procedures. These colloids give strong cross-linked bonds resulting in very strong gels at low solution concentrations.

In a modification of the invention when using the polyacrylate group of colloids, it is possible that the desired metal may be incorporated into the polymer system by forming the appropriate salt.

Examples of suitable refractory powders include the silicas; alumina and the aluminosilicates such as calcined clays, sillimanite and mullite; zircon; zirconia; magnesia and magnesia-chromes, chrome-magnesias and spinel; carbides such as silicon carbide and tungsten carbide; nitrides such as boron nitride, silicon nitride and the sialons; products obtained by fusing or sintering mixtures of alumina and zircon.

The refractory powder mix may be a mixture of coarse and fine grains of particle size distribution to give close packing. Coarse and fine grains of more than one composition may be used. This is for manufacturing a refractory shape. When the article required is an engineering ceramic component, the powder may consist of micron-sized spheres of more or less uniform size. In this case the powder is typically zirconia or alumina.

Using the binders of the invention the process for shaping the articles prior to firing can be any of the conventional processes currently used. For instance a slurry of a refractory grain mix and a binder according to the invention may be poured into a suitable mould and allowed to set. Articles may also be shaped by processes such as hand ramming, machine moulding, pressing or extruding. Pressing may be by toggle press, vibro-press or isostatic press, which is preferred for more complex shapes and where requirements for the finished refractory article are stringent as in well-blocks and nozzles used in molten metal discharge arrangements in casting ladles.

The binders of the invention may also be used in slurries which are used in the preparation of metal-casting moulds by the investment process. In this process a slurry of a fine refractory powder suspended in a bonding liquid is used to coat an expendable pattern, then a coarse refractory is dusted on to the coated pattern. The sequence of dipping and dusting is repeated until the desired mould thickness is obtained. Alumina, zircon and zirconia are examples of fine refractory materials suitable for preparing the slurry.

The proportion of binding liquid to refractory grain depends on the process used to shape the article. The liquid requirement will depend on the particle size distribution of the refractory grain mix. Processes requiring a slurry will of course, need more liquid than processes such as hand-ramming, pressing or extruding. As a rough guide, hand-ramming or pressing processes will require at least 50ml binding liquid per 1kg of refractory grain mix. More than this is often necessary.

Particular embodiments of the invention for preparing refractory articles are described in the following examples.

Example 1

An aluminium complex was prepared by adding 80% w/w lactic acid solution (168.75ml) with stirring and cooling, to an aluminium oxychloride solution (215ml) containing 12.5% by weight Al. The mixture was stirred for 30 minutes then neutralised with aqueous ammonia solution 0.88 sp.gr (85ml). This complex has 1.5 mole lactic acid per aluminium atom.

5 parts by weight of a 2% w/w aqueous solution of the ammonium salt of a high viscosity grade carboxymethyl cellulose were added to a mixture of tabular alumina and calcined alumina (100 parts). After mixing thoroughly, the aluminium complex (5 parts) was added and mixed until the mixture was homogeneous. The resulting mixture was cast into a mould and allowed to set then stand for one hour. The 'green' shape was removed from the mould, allowed to dry overnight and then fired at 1600°C.

A hard refractory body was obtained. The alumina mix contained coarse and fine particles of size distribution to give close packing. The size distribution was as follows:

Tabular alumina grain Parts by weight

B.S. 410 1976 screen or sieve mesh number (and aperture)

-1/4+8 (-6.35mm+2.0mm) 6

-8+14 (-2.9mm+1.18mm) 24

-14+25 (-1.18mm+600μm) 31

-25+48 (-600μm+355u ) 7

-48 (-355μm) 8

-100 (-150μm) 7

BACO MA95 grade calcined alumina 17

For the production and properties of tabular alumina, see B.L. Bryson, Jnr. Refractories Jnl. Nov 1971 pp 6-9 BACO is a registered trade mark.

Example 2

5 parts by weight of a 2% w/w solution of polymethacrylic acid neutralised by the addition of aqueous ammonia solution, were added to an alumina grain mix (100 parts) as described in Example 1. The aluminium complex prepared as described in Example 1 (5 parts) was then added and mixed until the mixture was homogeneous. The resulting mixture was cast into a mould and allowed to set then stand for one hour. The 'green' shape was removed from the mould, allowed to dry overnight and then fired at 1600°C.

A hard refractory body was obtained. Example 3

10 parts by weight of a 2% w/w solution of polyacrylic acid neutralised by the addition of aqueous ammonia solution, were added to an alumina grain mix (100 parts) as described in Example 1. 10 parts of an aluminium complex prepared in a manner similar to that described in Example 1 but containing 2 moles lactic acid per aluminium atom were added and mixed until the mixture was homogeneous.

A shape was cast and fired as described in Example 1 and a hard refractory body was obtained.

Example 4

A zirconium complex was prepared by adding glacial acetic acid (120g) to 291.3g of zirconium carbonate paste (31.3% Zr), allowing to liquefy and then adding with stirring 80% lactic acid solution (281g) followed by aqueous ammonia solution 0.88 s.g. (243g) with stirring and cooling. Water (322g) was added finally to give, after filtration, a clear, light brown product containing 7.5% Zr and 2.5 moles lactic acid per zirconium atom.

This zirconium complex was used to replace the aluminium complex in Example 1 to obtain a hard refractory body on firing at 1600°C.

Example 5

80% lactic acid solution (250 ml) was added slowly with stirring and cooling to aluminium chlorhydrate solution containing 12.5% by weight of aluminium (431 ml). The mixture was stirred for 90 minutes, kept cool and neutralised to pH7 with 0.88 s.g. aqueous ammonia solution (181 ml). On addition of the ammonia solution a gelatinous precipitate formed which re-dissolved during stirring. The complex was shown to have 1.125 moles of lactate per aluminium atom.

25g of the above solution was mixed with an aqueous solution of an ammonium salt of polymethacrylic acid (25g). A soft gel was formed immediately on mixing. After 24 hours ageing, a hard coherent gel was formed.

Example 6

The gel prepared according to Example 5 was used to bind the alumina grain mix containing coarse and fine alumina particles as described in Example 1. 5% w/w of an ammonium salt of polymethacrylic acid was added to the alumina grain mix and thoroughly mixed, then 5% w/w of the solution of the aluminium complex was added and mixed. The mixture was poured into a suitable mould and allowed to stand for 24 hours. The shape was then removed from the mould and dried at 90°C for 43 hours before firing. The shape was fired from ambient temperature to 1740°C in 3i hours, held at 1740°C for one hour, then cooled in the furnace to ambient temperature (16 hours). A good refractory block was obtained.

Example 7

The ammonium salt of polymethacrylic acid and the aluminium complex whose preparation is described in Example 5 was used with the alumina grain mix of Example 1 to prepare a ramming mix according to the following stages.

Stage 1 5% w/w of the solution of the ammonium salt of polymethacrylic acid was added to the refractory grain and well mixed.

Stage 2 2.5% w/w of the aluminium complex whose preparation is given in Example 5 was added and well mixed.

Stage 3 2.5% w/w of aluminium chlorhydrate solution (containing 12.5% aluminium by weight) was added and well mixed.

The resulting mix was plastic in consistency and was suitable for hand-ramming . A brick was made by hand-ramming the mix into a suitable mould. The brick was allowed to air-dry for 24 hours then dried at 100°C for 48 hours before being fired as described in Example 6. A good refractory brick was obtained.

Example 8

The ammonium salt of polymethacrylic acid and the aluminium complex whose preparation is described in Example 5 was used with the refractory grain mix comprising coarse sintered mullite grain and fine alumina grain, as described in

GB 1451548 to prepare a ramming mix according to the following stages. Stage 1 5% w/w of a solution of the ammonium salt of polymethacrylic acid was added to the refractory grain mix and well mixed.

Stage 3 2.5% w/w of aluminium chlorhydrate solution (containing 12.5% w/w aluminium) was added and well mixed.

The resulting mix had a plastic consistency and was suitable for hand-ramming. A brick was made by hand-ramming the mix into a suitable mould. The brick was allowed to air-dry for 24 hours, then dried at 100°C for 48 hours before being fired as described in Example 5. A good refractory brick was obtained, with a modulus of rupture approximately equal to that of an ethyl silicate bonded brick made following the procedure given in GB 1451548.

Claims

1. A binder for use in binding refractory grain comprising an hydroxy group-containing organic colloid complexed with at least one aqueous miscible metal organic compound which, upon firing, leaves at least one refractory residue.

2. A binder according to claim 1 wherein on firing the binder has substantially the same oxide analysis as the refractory grain being bound.

3. A binder according to claim 1 or 2 wherein the hydroxy group-containing compound is a cellulose derivative.

4. A binder according to claim 1, 2 or 3 wherein the metal organic compound is selected from the alkoxides, acylates and organic complexes of a metal selected from calcium, magnesium, aluminium, chromium, titanium, zirconium and silicon and polymers or combinations thereof.

5. A binder according to claim 4 which contains a silica aquasol and the metal is selected from aluminium and zircon.

6. A binder according to any preceding claim wherein the hydroxy group-containing colloid is selected from cellulose-oxirane reaction products, carboxymethyl celluloses, partially-hydrolysed polyvinyl acetates or acetate/butyrals, homopolymers and co-polymers of acrylic acid, acrylamide and methacrylic acid and their salts, polyvinyl alcohols, gums such as xanthan gum and guar gum, dextrins, starches, gelatin, alginates and their salts and lignosulphonates.

7. A binder according to any preceding claim wherein an excess of metal organic compound is used above that necessary to form a complex with the hydroxy group-containing organic colloid.

8. A binder according to claim 6 wherein an anionic colloid is used in the form of its ammonium salt.

9. A binder according to claim 8 wherein the anionic colloid is carboxymethyl cellulose.

10. A binder according to claim 6 wherein the colloid is a polyacrylate used in the form of its metal or ammonium salt.

11. Use of a binder as claimed in any one of claims 1 to 10 in binding refractory grain to form a refractory shape.

12. A refractory article produced by firing a refractory shape prepared according to claim 11.