GB2063848A - Improvements in the binding of refractory powders - Google Patents

Abstract

A liquid gellable binder for refractory powders which may contain ceramic fibres is based on a solution of a chromium compound or a chromia sol. Examples of suitable compounds are trivalent chromium salts, such as chromic acetate or chromic chloride. The solution of the chromium compound may optionally contain other binders such as an aluminium salt, a magnesium salt, a zirconium salt, a silica aquasol, a silica alcosol, or an acid hydrolysate of a C1-6alkyl silicate. The shaped slurry may be caused to set by the incorporation of a suspension of light magnesia in a mixture of a polyhydric alcohol and water.

Description

SPECIFICATION Improvements in the binding of refractory powders This invention relates to the binding of a solid refractory substance by mixing the solid substance with a liquid gellable binder thereby forming a slurry. The slurry is caused or allowed to set to the desired configuration by gelling the binder and the set slurry is thereafter fired.

The majority of systems of the kind set forth above are based on ethyl silicate as the gellable binder but proposals have been made to use other substances such as soluble zirconium salts.

In accordance with the present invention a soluble chromium compound or a chromia sol is used as a binderfor a solid refractory substance in a system of the kind set out above. Suitable soluble chromium compounds include chromium salts, for example trivalent chromium salts; these are hereafter referred to as chromic salts.

According to the invention a method of preparing a shaped refractory article includes the steps of mixing a chromia sol or a solution of a chromium compound with a refractory substance to form a shapable slurry, forming the slurry to the desired shape and causing or allowing it to set solid by gelation of the chromia sol or chromium compound.

A suitable salt is chromic acetate, which is commercially available as "green chromic acetate" from BDH Chemicals Ltd., or as a spray-dried chromium acetate powder containing 40% w/w Cr203 from Lancashire Chemical Works Ltd. Chromic acetates and their preparation are described in M.J. Udy, "Chromium, Vol. 1, Chemistry of Chromium and its Compounds," pp 229-230 (Reinhold Publishing Corporation, New York, N.Y.

1956). The preparation, manufacture and properties of chromic acetates are also described in Gmeli's Handbuch des Anorganische Chemie, 8th Edn., System No. 52, Chrom, Teil B, pp 385-404 (Verlag Chemi GmbH., Weinheim, 1962). One method of preparing a chromium acetate is to treat acetic acid with an excess of hydrous chromic oxide. Evaporation then gives chromium acetate as a violet crystalline powder.

Other suitable chromium acetate solutions are the chromium acetate solution CAc - 10 and the chromium acetate solution CAc - 136 supplied by Lancashire Chemical Works Ltd. Solution CAc - 10 has a specific gravity of 1.21 and a Cr203 content of 15% w/w. Solution CAc - 136 has a Cr203 content of 20% w/w and an acetate content of 23% w/w. A colloidal chromia sol supplied by Nyacol Inc. may also be used in any of the processes described herein. This can also be used in conjunction with an ethyl silicate hydrolysate, or with silica aquasol or alcosol.The colloidal chromia sol supplied by Nyacol Inc. has a Cr203 content of 22.4% wlw, a particle size of 5 millimicrons (50 Ä), a positive particle charge, a nitrate counter ion concentration of approximately 1.08 mol NO3/mol Cur203, a pH of 2.5 - 3.0, a specific gravity of 1.34 and a viscosity of 5 - 10 ps.

The invention also envisages the use of a binder solution comprising a chromic salt solution and a zirconium salt solution, for instance a solution of chromium acetate and zirconium acetate or a solution of chromic acetate and zirconium oxychloride or a solution of chromic chloride and zirconium oxychloride.

Another binder solution envisaged comprises a silica aquasol or a silica alcosol or an acid hydrolysate of ethyl silicate (or another alkyl silicate in which the alkyl groups contain 1 to 6 carbon atoms) together with a chromic salt solution, for instance chromic acetate. Yet another binder solution envisaged comprises a chromic salt solution and an aluminium salt solution, for instance a solution of chromium acetate or chromic chloride and an aluminium chlorhydrate or aluminium chloride. A further binder solution envisaged comprises a chromic salt solution and magnesium salt solution, for instance a solution of chromic acetate and magnesium acetate in water.

The binder comprising a chromic sol or a solution of a chromium compound, or any of the Binders envisaged above, may be caused to set by allowing them to dry in the air, possibly at an elevated temperature. However it is preferred to cause setting by the action of a catalytic agent such as a suspension of light magnesia in a mixture of water and a polyhydric alcohol. The preparation of this suspension is described in British Patent Specification No. 1,356,247.

To obtain a shaped refractory, the shapes resulting when the slurry of refractory substance and chromia sol or solution of chromium compound has set by gelation of the chromia sol or chromium compound are dried and then are fired. The binders in accordance with the present invention are especially suitable for use with basic refractory aggregates, for example magnesia, magnesia-chrome and chrome-magnesia systems, also with alumina aggregates, for example tabular alumina aggregates. Other examples of suitable refractory aggregates include mixtures of alumina or magnesia grain in which the fines fraction includes chromium sesquioxide (cur203), zircon, zirconia or pigment titanium dioxide. The binders in accordance with the present invention are also suitable for binding ceramic fibres, for example alumina fibres.

In an application of the method in accordance with the present invention a liquid stream of a chromic salt solution is mixed, adjacent a surface to be coated, with another liquid stream containing refractory material in suspension, together with a catalytic agent to cause setting. This "mixed stream" systems forms the subject of British Patent Specification No. 1,527,651. In an embodiment of this method one stream is a solution of chromic acetate and the other stream is a magnesium oxide suspension, as set out above. Coarse refractory grains can be incorporated, as described in British Patent Specification No. 1,527,651. These coarse grains can be magnesia grain, for instance 'Sardamag' sea-water-magnesia grain, or they may be fused magnesia-chrome or fused chrome-magnesia grain.One industrial application envisaged for the chromium binder system in accordance with the present invention using the "mixed stream" procedure, is in the re-lining of furnaces for steel production in which the refractory lining is a basic refractory.

Examples 1 - 5 describe conditions under which a gel suitable for binding a refractory substance is prepared from a water soluble trivalent chromium salt. Examples 6 - 9 describe the use of the gel to bind a refractory substance.

In Examples 1 - 9 the chromium acetate used was "green" chromium acetate. Example 10 describes the use of the gel to bind ceramic fibres for use in vacuum forming of refractory articles.

Example 1 An aqueous solution of chromic acetate was prepared by dissolving 6.23 grams of "green" chromium acetate in 50 ml. warm distilled water. A suspension of light magnesium oxide was prepared by adding 20 ml. distilled water to 4 grams light magnesium oxide, warming the mixture to 70"C then adding 20 ml.

ethylene glycol and warming to 70"C for 15 minutes. The suspension was allowed to cool to ambient temperature before use.

9 ml. of the chromium acetate solution and 6 ml. of the magnesia suspension on mixing gave a soft but coherent gel in 30 - 90 minutes, still weak after 240 minutes.

Example 2 20 ml. of the aqueous chromic acetate solution of Example 1 were concentrated by evaporation to 7 ml.

When cool 6 ml. of magnesia suspension, prepared as described in Example 1 were added. A soft but coherent gel was formed in about 90 minutes, becoming harder after 240 minutes.

In general increasing the amount of magnesia suspension used reduces the gel time, but increasing the proportion of ethylene glycol in the magnesia suspension does not reduce the gel time.

Example 3 An aqueous solution of chromic acetate was prepared by dissolving 7.37 grams of green chromium acetate in 25 ml. distilled water. A suspension of light magnesium oxide was prepared by adding 2 grams of light magnesium oxide to 15 ml. distilled water, then adding 15 ml. ethylene glycol. The mixture was warmed to 70do, then allowed to cool to ambient temperature. 6 ml. of the magnesia suspension were mixed with 9 ml. of the chromium acetate solution. A soft coherent gel was formed in about two hours.

Example 4 A suspension of light magnesium oxide was prepared by adding 25 ml. distilled water to 4 grams light magnesium oxide, then adding 25 ml. ethylene glycol, warming to 70"C then allowing to cool to ambient temperature. The following gelation characteristics were obtained using 5 ml. of the suspension.

With 2 ml. of the chromic acetate solution of Example 3 - Soft coherent gel after 30 minutes.

With 5 ml. of the chromic acetate solution of Example 3 - Soft coherent gel after 30 minutes.

With 6 ml. of the chromic acetate solution of Example 3 - Soft coherent gel after 30 minutes.

With 9 ml of the chomic actetate solution of Example 3. - Soft coherent gel overnight.

This indicates the concentrations and proportions required to obtain a gel in a relatively short time.

Example 5 A larger scale preparation on using the conditions of Example 4 gave the following result.

40 ml. of aqueous chromium acetate solution, prepared as in Example 3 were mixed with 33 ml. of light magnesium oxide prepared as in Example 4. A soft but coherent gel was formed in about 19 minutes. The ge hardened on ageing.

Example 6 Sufficient of the chromium acetate solution - light magnesium oxide suspension of Example 5 just to wet the surface was added to 10 grams graded tabular alumina. This mixture was allowed to set. After two hours, a weak solid was obtained, which on standing overnight became hard. Volatiles were removed by heating to 350"C prior to firing.

Example 7 A suspension of light magnesium oxide was prepared by adding light magnesium oxide to an equal volume of water and ethanediol, in the proportion of 4 grams light magnesium oxide to 25 ml. water plus 25 ml. ethanediol, warming to 70"C for 15 minutes and then cooling to ambient temperature. A solution of chromium acetate was prepared by dissolving "green chromium acetate" in water in the proportion 35.4 grams of "green chromium acetate" to 120 ml. water at ambient temperature.

A refractory shape was made by preparing a slurry from a refractory power comprising: Tabularalumina -1/4 +8 B.S.4l0sieve 141/2 Tabularalumina - 8 + 14 B.S.410sieve 57ȯr Tabularalumina -14 +28 B.S.410sieve 741/2 Tabularalumina -28 +48 B.S.410sieve 17 oz Tabular alumina - 48 B.S.4l0sieve 19 oz Tabular alumina -100 B.S. 410 sieve 101/4 oz Green chromic oxide Cr203 71/4 (chromium sesquioxide) (Durham Chemicals Distributors Ltd.

M100 grade) Calcined alumina MA 95 grade 41 oz To this powder was added 450 ml. of a suspension of light magnesia prepared as above, together with 180 ml. of the chromium acetate solution prepared as above, giving a slurry. The slurry was poured into a mould, which was being vibrated, to give a shape, which was removed from the mould when the slurry had set. A good refractory shape was obtained after drying and firing to 1600"C.

Example 8 A refractory shape was made by preparing a slurry from a refractory powder comprising Sardamag magnesium oxide - 5 + 10 B.S.410 sieve - 48 oz Sardamag magnesium oxide -10 + 25 B.S.410 sieve - 32 oz Sardamag magnesium oxide -25 + 72 B.S.410 sieve - 24 oz Sardamag magnesium oxide - 56 oz ball-milled fines To this powder was added 320 ml. of a suspension of light magnesia prepared as described in Example 7, together with 120 ml. of a chromium acetate solution prepared as described in Example 7, giving a slurry.

The slurry was poured into a mould, under vibration, to give a shape, which was removed from the mould when the slurry had set. Setting occurred about 10 minutes after the slurry was poured into the mould. The cast shape was removed from the mould about an hour after setting. After drying and firing to 1600"C a good, crack-free, refractory body was obtained.

Example 9 A refractory shape was made by preparing a slurry from a refractory power comprising Sardamag magnesium oxide - 5 + 10 B.S.410 sieve - 431/2 oz Sardamag magnesium oxide -10 + 25 B.S.410 sieve - 29 oz Sardamag magnesium oxide -25 + 72 B.S.410 sieve - 211/2 oz.

Sardamag magnesium oxide Ball-milled fines - 501/2 oz Pigment grade titanium dioxide -325 B.S.410 sieve - 5 oz (British Titan Products Ltd.) Green chromic oxide Cr203 M100 grade - 15 oz (chromium sesquioxide) To this powder was added 300 ml. of a suspension of light magnesia prepared as described in Example 7, together with 120 ml. of a chromium acetate solution prepared as described in Example 7 giving a slurry. The slurry was poured into a mould, under vibration, to give a shape, which was removed from the mould when the slurry had set. The cast shape was removed from the mould about an hour after setting. After drying and firing to 1600"C a good, crack-free, refractory body was obtained.

Example 10 (a) A magnesia suspension is prepared by adding 20 ml. of distilled water to 4 grams of light magnesium oxide, warming to 70"C, adding 20 ml. of ethylene glycol and rewarming to 70"C for 15 minutes. The resulting suspension is allowed to cool to ambient room temperature before use.

(b) To the suspension prepared in (a) 60 - 90 ml. of chromium acetate solution CAc - 136 is added and the resulting mixture is stirred.

(c) Water and ceramic fibres are added as required for vacuum forming. Alumina or high alumina fibres must be used.

Claims (10)

1. A method of preparing a shaped refractory article which includes the steps of mixing a chromia sol or a solution of a chromium compound with a refractory substance thereby forming a shapable slurry, forming the slurry to the desired shape and causing or allowing it set solid by gellation of the chromia sol or chromium compound.

2. A method as claimed in Claim 1 wherein the slurry is caused to set by the incorporation of a catalyst.

3. A method as claimed in Claims 1 and 2 wherein the catalyst is a suspension of light magnesia in a mixture of a polyhydric alcohol and water.

4. A method as claimed in any preceding claim wherein the solution of a chromium compound is a solution of a chromic salt optionally containing a component selected from the group comprising an aluminium salt, a magnesium salt, a zirconium salt, a silica aquasol, a silica alcosol and an acid hydrolysate of an alkyl silicate in which the alkyl groups contain from 1 to 6 carbon atoms.

5. A method as claimed in any of the preceding claims wherein the refractory substance is a powder.

6. A method as claimed in any of Claims 1 to 4 wherein the refractory substance comprises ceramic fibres.

7. A method as claimed in any of the preceding claims wherein the chromium compound is chromic acetate.

8. A method as claimed in any of Claims 1 to 6 wherein the chromium compound is chromic chloride.

9. Methods as claimed in Claim 1 and substantially as described in the Examples.

10. A shaped refractory object produced by a method as claimed in any of the preceding claims.