This invention relates to binding of refractory grain, specifically to a method of forming refractory articles using mixed metal complexes based on alkoxides or derivatives thereof, as binders.

Alkoxides, particularly alkoxides of aluminium, zirconium and titanium have been previously proposed as binders for refractory grain, as they 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 form a refractory residue which links the refractory grains together.

Chromium as chromia is well known as a component of refractory compositions as it enhances resistance to thermal shock and reduces erosion by slags. Hence, it has been suggested that liquid gellable binders for refractory compositions may be based on chromium compounds. Binders of this type are disclosed inter alia in GB 2063848. In GB 2143809 a binding system comprising a gel derived from a chromium-aluminium double alkoxide has been used to bind alumina grain and basic refractory grains. The gel obtained from the double alkoxide is thought to produce a matrix containing a chromia-alumina solid solution on firing.

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 instance, alumina and zirconia.

EP 0063034 describes a rigid coherent gel for binding refractories prepared by hydrolysing an aluminium alkoxide or aroxide and causing or allowing the hydrolysate to set. Conveniently, the hydrolysis is carried out in the presence of a catalytic amount of an amine hydrolysis catalyst such as an aminoalcohol and in the presence of a mutual solvent such as isopropanol. The use of a mixed alkoxide of aluminium and another metal such as magnesium is also contemplated as a precursor of a rigid coherent gel.

Mixed metal complexes based on alkoxides have also been previously described in GB 1588521 where the preparation of complexes containing two or more metals selected from aluminium, titanium and zirconium is effected by reacting less than one mole of an alkanolamine with one mole of a mixture of two or more metal alkoxides with the remaining active sites on the metal complex being occupied by groups derived from a polyhydric alcohol. These complexes have been used as additives to increase the structure in aqueous polymer systems containing protective organic colloids, such as emulsion paints, solvent based foundry paints, paint strippers and drilling muds.

According to the present invention, there is provided a method - for producing a material capable of forming a rigid coherent

gel which comprises contacting water with a mixed metal complex comprising an alkoxide or derivative thereof containing at least two metals which give refractory oxides, complexed by reaction with at least two compounds selected from the group comprising aminoalcohols, polyhydric alcohols and 3-keto-esters, with the liberated alcohols being removed by distillation.

In a preferred aspect of the invention, there is provided a method for producing a material capable of forming a rigid coherent gel which comprises contacting water with a mixed metal complex comprising an alkoxide or derivative thereof containing at least two metals which give refractory oxides, complexed by reaction with a compound selected from at least one amino alcohol, together with a polyhydric alcohol or β-keto-ester, with the liberated alcohols being removed by distillation.

On firing the gel gives a refractory oxide residue comprising a homogeneous mixture of the oxides of the metals used. When oxides are capable of reacting at high temperature to form new phases, such as in the cases of alumina and silica to form mullite and magnesia and alumina forming spinel, then the reation will proceed much more readily from oxides derived from gels than from separately mixed oxide powders.

The material is particularly useful as a binder in binding refractory grain which may be incorporated with the mixed metal complex prior to the addition of water.

When using a process to provide green shapes from refractory grain suitable for firing employing precursors of rigid coherent gels as described in earlier specifications, there have been difficulties in obtaining reproducible results owing to inconsistency in gel times. Using the process of the present invention it has been found that gel times show consistency, enabling an easily controlled and reliable process to be employed to obtain refractory articles.

It is also a particular advantage that the only material required in addition to the mixed metal complex used according to the invention to produce a rigid coherent gel is water. This has not been achieved before with metal organic compounds as it is usual to have to have a plurality of compounds separately mixed with the grain to be bound to promote gelation. This may contribute to the observed inconsistencies in gel times. If water is added to simple mixtures of the components of the complexes of the invention rigid coherent gels are not formed. In some cases, however, it may be beneficial to add a small amount of a lower alcohol such as isopropanol to increase the fluidity of the mix to enable more efficient dispersion of the grain employed or to improve flow properties of the mix, but this is not essential.

In a preferred aspect of the invention, bodies in which both the bonding phase and the grain have substantially the same oxide composition are produced on firing. The binders now act as sintering aids.

It is important to use mixed metal complexes instead of simple mixtures of different metal-organic compounds. It has previously been recognised that a major problem in forming homogeneous multicomponent gels is the unequal hydrolysis and condensation rates of the metal alkoxides. The use of simple mixtures gives gels which, even though they may have the same oxide composition as the grain, are often non-homogeneous and form more than one crystalline phase on firing. For example in the case of mullite formation, it has been shown that when the gel is non-homogeneous, the product obtained on firing contains cristobalite and < ^χ -alumina or y-alumina with little mullite.

The mixed metal complexes suitable for use as binders according to the invention comprise the products obtained by treating an alkoxide mixture or a mixed alkoxide of at least two metals which give refractory oxides, with a complexing agent which comprises one or more aminoalcohols such as mono-, di- or triethanolamines together with a β- keto-ester such as ethyl acetoacetate or a polyhydric alcohol which is preferably a glycol such as ethylene glycol or a monopropylene glycol, and removing liberated alcohols by distillation. The alcohols may be removed at atmospheric pressure or under vacuum. If desired, water vapour may be introduced during the distillation to polymerise the complex. If necessary the product may be diluted with a polyhydric alcohol such as ethylene glycol or propylene glycol to increase its fluidity. The metal contents of the complex are preferably controlled to give an oxide analysis on firing susbtantially the same as

that of the refractory grain which is bonded.

According to the invention, mixed metal complexes suitable for use as binders for refractory grain can be prepared in this way from the alkoxides of calcium, magnesium, aluminium, chromium, silicon, titanium and zirconium. The alkoxides used may be represented as __(0R) _n where M is Ca, Mg, Al, Cr, Si, Ti or Zr; R is a lower alkyl group containing up to 6 carbon atoms, preferably 2-4 carbon atoms, and n is the valency of the particular metal. The alkoxides used may also be polyalkoxides, for instance polyalkoxysilanes (alkyl polysilicates) , or they may be a mixture of simple alkoxides M(0R) _n and polyalkoxides, or they may be double alkoxides.

In practice any refractory grain can be used in the process of the invention. However as aforementioned, the invention is particularly useful for grains having a mixed oxide analysis wherein the metal contents of the complex are controlled to give an oxide analysis on firing substantially the same as that of the refractory grain which is bonded. Examples of grains having a mixed oxide analysis include the aluminosilicates such as mullite or sillimanite or calcined clays; cordierite; zircon and the products obtained by firing or sintering mixtures of alumina and zircon; the -aluminium titanate' grains; magnesia-chromes and chrome-magnesias; spinel grains; products obtained by fusing or sintering mixtures .of alumina and chromia. It is possible to use mixtures of grains such as alumina and silica, provided they are in a finely divided state (eg-150μm) to ensure homogeneity

of the grain mix. In another instance fine magnesia mixed with fine alumina giving the oxide stoichiometry of spinel may be used.

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 an 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.

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 2O3 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 to monoclinic transformation of

zirconia, which by absorbing energy from the cracks increases the toughness of the article. The use of the binder derived from a complex containing zirconium as a component of the mixed metal complex according to the invention on firing 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.

Using the binders of the invention, any of the processes currently used for shaping the articles prior to firing are suitable. 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 the more complex shapes and where requirements for the finished refractory shapes 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 thickness is obtained.

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 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.

The invention is further illustrated by reference to the following examples.

Example 1

The preparation of a mixed metal complex suitable for binding

- grain obtained by fusing or sintering mixtures of alumina and zircon is described. The reaction occurring during fusing or sintering mixtures of alumina and zircon may be represented as:

2ZrSi0 + 3A1 ₂ 03 3Al ₂ 0 ₃ .2Si0 + 2Zrθ2

Ideally, the mixed metal complex should have the oxide stoichiometry of 3AI2O3.2Siθ2« 2Zrθ2- In practice, however, an oxide stoichiometry within the range 3Al2θ3.1-2Siθ2-2Zrθ2 is suitable.

Aluminium isopropoxide (6 moles-1224g) was mixed with technical ethyl silicate (150g) containing silicon equivalent

to 40% w/w Siθ2 (1 mole), and a solution of zirconium n-propoxide in n-propanol (934g) which comprised 70% zirconium n-propoxide and 30% n-propanol, equivalent to 2 moles Zrθ2- The mixture was heated and ethyl acetoacetate (1560g) was added under reflux over a period of 45 minutes. Monopropylene. glycol (1185g) was then added and the n-propanol solvent and liberated alcohols were removed by distillation, giving a mixed metal complex (3775g) having an oxide stoichiometry of 3 l2θ3.Siθ2-2Zrθ2- The product was used to bind a grain mix prepared from the material obtained by fusing a mixture of alumina and zircon.

To obtain a mixed metal complex of. oxide stoichiometry 3AI2O3.2Siθ2- Zrθ2 the quantity of technical ethyl silicate

_* is increased to 300g.

Example 2

Aluminium isopropoxide (2448g) and tetraethoxysilane (ethyl orthosilicate) (832g) were mixed and to the mixture was added monoethanolamine (780g), then diethanolamine (780g), in the course of 45 minutes, under reflux. Ethylene glycol (1178g) was now added, under reflux, during 35 minutes. The liberated ethanol and isopropanol were removed by distillation at atmospheric pressure over the range 90-125°C, any remaining volatiles being removed by vacuum distillation. 2740g of volatiles were removed. Propylene glycol (456g) was added to the residue to give a product of useable fluidity.

The product was a complex containing aluminium and silicon, formed from the aminoalcohols, monoethanolamine and diethanolamine, and the polyhydric alcohol ethylene glycol. The product was suitable for use in binding aluminosilicate grains, for instance mullite grain. The complex has the oxide stoichiometry I2O3..S1O2 of 3:2.

When mixed with water the complex sets to a rigid coherent gel having the following characteristics:

Wt Of complex Wt of water Gel ti

⁽ g ⁾ (g) (min)

30 10 10 clear coherent gel

30 20 21 clear coherent gel

30 30 30 clear coherent gel

20 30 40 coherent gel

10 30 >60 gel time too long for practical use 30 5 >60 weak gel

A refractory brick was prepared from the above complex and a refractory grain mix comprising fine alumina grain and coarse sintered mullite grain, as described in GB 1451548 in the following stages:

Stage 1 - 50% of water (w/w) was added to the complex and thoroughly mixed.

Stage 2 - 5-8% by weight of the mixture from stage 1 was added to the refractory grain mix and thoroughly mixed.

Stage 3 - The mixture from stage 2 was poured into a mould, vibrated to remove air, then allowed to set.

Stage 4 - The shape was removed from the mould, air-dried for

24 hours, then dried at 100°C for 48 hours.

Stage 5 - The green shape was fired from ambient temperature to 1740°C in 3_ hours, held at 1740°C for one hour, then cooled to ambient temperature (16 hours).

A good refractory brick was obtained.

Example 3

Aluminium isopropoxide (1224 g) , tetraethoxysilane (ethyl orthosilicate) (416g) and zirconium prop late (654 g) were mixed and ethyl acetoacetate(1560 g) was added to the mixture followed by ethylene glycol (1178 g). The liberated alcohols were removed by distillation, 1879g of volatiles being removed. Propylene glycol (456g) was added to the residue to give a product of workable fluidity.

The product was a complex containing aluminium, silicon and zirconium, formed from ethyl acetoacetate and the polyhydric alcohol ethylene glycol. It may be used for binding aluminosilicate grains, for instance mullite grain, or for binding grain prepared from the material obtained by sintering or fusing mixtures of alumina and zircon. The complex had the oxide stoichiometry Al ₂ θ3:Siθ2 Zrθ2 of 3:2:2.

The complex was found to gel rapidly when mixed with water. Addition of methanol or ethanol was found to prolong the gelation time. Mixing 5% w/w water with the complex gave a paste-like gel which required about 24 hours to harden completely to a clear gel. Mixing 10% w/w water with the complex gave a clear gel in about 18 minutes.

A refractory brick was prepared from the above complex and a refractory grain mix comprising fine alumina grain and coarse sintered mullite grain, as described in GB 1451548, according to the following stages:

Stage 1 - 5-8% of water (w/w) was added to the complex and thoroughly mixed.

Stage 2 - ^' 5-8% by weight of the mixture from stage 1 was added to the refractory grain mix and thoroughly mixed.

Stage 3 - The mixture from stage 2 was poured into mould, vibrated to remove air, then allowed to set in the mould for about six hours.

Stage 4 - The shape was removed from the mould, air-dried for

48 hours, then dried at 70-80°C for 48 hours.

Stage 5 - The green shape was fired from ambient temperature to 1740°C in 3. hours, held at 1740°C for one hour, then cooled to ambient temperature (16 hours).

Example 4

The magnesium double alkoxide Mg[Al ⁽ 0ip _r ⁾ _x ⁽ 0 _seC B _U ⁾ y-2 where x + y is 4, iPr represents isopropyl and secBu is sec-butyl, was prepared by treating a mixture of magnesium and aluminium having the atomic ratio of 1:2 with an equimolar mixture of isopropanol and sec-butanol.

A mixture of magnesium powder and aluminium powder

(300g -atomic ratio 1:2) was added in four lots of 75g each to an equimolar mixture of isopropanol and sec-butanol (4288g) which was maintained under reflux, Mercuryl acetate (l_5g) was added to initiate the reaction. The yield of double alkoxide was 94%, based on the volume of hydrogen evolved.

The above double alkoxide (75g), triethanolamine (19.75g) and ethylene glycol (15.6g)were mixed. The liberated isopropanol and sec-butanol were removed by distillation at atmospheric pressure, any remaining volatiles being removed by vacuum distillation. Ethylene glycol (20g) was added to the residue to give a product of useable fluidity. The product is a complex containing magnesium and aluminium formed from the aminoalcohol triethanolamine and the diol ethylene glycol, having the following gelation characteristics. All gels were clear coherent gels.

The gel formed when the weight of water added per lOOg product is in the range 160-220g is a clear rigid coherent gel suitable for binding refractory grain. The small variation in gel time produced by large changes in the amount of water added is important. This shows that in this range the gel-forming system is very tolerant to errors in measuring the amount of water added to cause gelation.

A refractory grain mix comprising coarse fused spinel grain and fine alumina grain which may be bonded with the clear rigid coherent gel containing magnesium and aluminium and formed as described is as follows :

Fused spinel - 3/16 + 1/8 9 parts by weight

Fused spinel - 1/8 + 1/16 36 parts by weight

Fused spinel - 1/16 + 22 45 parts by weight

Tabular alumina -48 12 parts by weight

Alcoa calcined alumina 25 parts by weight A2 - 325

The screen and mesh sizes for the fused spinel grain are those given in B.S.410:1976.

The mesh sizes given for the alumina grains are Tyler standard mesh sizes.

Example 5

Ten replicates of the complex prepared according to the method given in Example 4 (5g) were mixed with water (8g) and the times required for the mixtures to set to clear coherent gels were noted.

The mean setting time was 44.30 minutes with a standard deviation of 0.622.