WO2000001639A1

WO2000001639A1 - Refractory compositions

Info

Publication number: WO2000001639A1
Application number: PCT/GB1999/001953
Authority: WO
Inventors: Jay Tu
Original assignee: Foseco International Ltd
Current assignee: Foseco International Ltd
Priority date: 1998-07-01
Filing date: 1999-07-01
Publication date: 2000-01-13
Anticipated expiration: 2001-01-01
Also published as: GB9814205D0; ID23568A; AR018957A1; AU4917499A

Abstract

A dry, heat-settable refractory composition comprises a particulate refractory material, a particulate thermosetting organic binder, and a particulate element, compound or alloy which will react with oxygen to produce a refractory bond. The particulate refractory material may comprise a material selected from the following materials: alumina, magnesia, zircon, silica, silicon carbide, calcined dolomite, chrome magnesite, olivine, forsterite, an aluminosilicate, chromite, carbon, and mixtures comprising one or more of these materials. The particulate thermosetting organic binder may comprise a phenol-formaldehyde resin, a urea-formaldehyde resin, a melamine-formaldehyde resin, an epoxy resin, a silicone, or a mixture comprising one or more of these binders. The particulate element, compound or alloy which will react with oxygen to produce a refractory bond may comprise aluminium, magnesium, silicon, calcium boride (CaB6), boron carbide (BC4), calcium silicide (CaSi2), ferrosilicon (FeSi2), a magnesium-aluminium alloy having the formula Mg17Al12, or a mixture comprising one or more of these materials.

Description

REFRACTORY COMPOSITIONS

This invention relates to refractory compositions which are used in their dry state to produce refractory articles, for example linings for furnaces or metallurgical vessels such as ladles, tundishes or launders.

Refractory products, whether they be pre-formed articles for a particular application, or linings for furnaces or metallurgical vessels, have usually been produced by methods, such as ramming, trowelling, spraying or casting, using wet refractory compositions in the form of an aqueous slurry or a mouldable or castable mass. Such methods have disadvantages in that they are time consuming, since apart from the initial forming process they require a drying step, and often additional calcining or sintering steps. It is also necessary to maintain the composition in a suitable condition for application, and this can be a problem because the solid materials tend to segregate. Some of the methods, for example, spraying, require relatively complex equipment which in use can become blocked, and all the wet methods require an on-site supply of water, and sometimes compressed air, for producing the compositions in a form suitable for application.

Alternative methods have been proposed which involve the use of substantially dry refractory compositions, which contain a chemical compound which has chemically or physically bound water associated with it, and which can be set or hardened by heat after forming or application.

US - A - 2499729 describes a refractory composition for lining surfaces such as moulds and furnace walls consisting of a mixture of a comminuted dry refractory substance and sodium metasiiicate nonahydrate which gives up water of crystallisation when heated above 43° C and produces self-tempering and setting of the mixtures on application to the surface to be lined. EP - A - 0064863 describes a method of applying a monolithic refractory layer within a metallurgical vessel using a substantially dry particulate mixture comprising at least 70% by weight of refractory aggregate, at least 0.5% by weight of thermosetting resin, from about 0.5% to 10% by weight of an inorganic binder and optionally from about 0.5% to 10% of an inorganic hydrate.

WO - A - 92/09542 describes a thermally activated, dry refractory composition which is used for the production of a new refractory lining or for the repair of an existing refractory lining in a furnace or a high temperature vessel. The composition may consist of from about 35% to about 85% by weight of a refractory aggregate and from about 15% to about 50% by weight of a hydrated material containing from about 5 to about 9 moles of water in crystalline form, the water constituting from about 7% to about 35% by weight of the total composition. The hydrated material is present in a sufficient amount with respect to the amount of chemically bound water contained therein, to provide moisture to the composition to cause self-flowability of the composition when the composition is applied to a hot surface to be lined or repaired.

WO- A - 94/14727 describes a substantially dry, self-hardening, thermally activated refractory composition comprising a particulate refractory material, an inorganic binder having associated therewith chemically or physically bound water, and an element or compound which will react exothermically with the inorganic binder. Suitable inorganic binders having chemically bound water associated therewith are crystalline hydrated salts such as silicates, carbonates, sulphates, nitrates, borates and phosphates. The element or compound which will react exothermically with the inorganic binder may be for example ferrosilicon, calcium oxide, magnesium oxide, aluminium or cement.

In all the above documents the hydrated compound used in the refractory compositions is principally a sodium metasiiicate hydrate. Refractory compositions containing silica suffer from disadvantages when used to form linings or other articles which are to contact molten iron, and in particular iron for producing ductile iron castings, because silica reacts with carbon in the iron resulting in silicon pick-up, and also with the iron itself forming an iron silicate and hence increasing the amount of slag formed during casting. Silica will also react with or be reduced by aluminium in aluminium-killed steel and this can cause silicon pick-up, and also nozzle clogging in continuous casting of the steel. Furthermore, the presence of sodium can significantly decrease the refractoriness of the composition.

It has now been found that a dry, heat-settable refractory composition which does not suffer from these disadvantages can be produced using a combination of an thermosetting organic binder and an element, compound or alloy which will react with oxygen to produce a refractory bond

According to the invention there is provided a dry, heat-settable refractory composition comprising a particulate refractory material, a particulate thermosetting organic binder, and a particulate element, compound or alloy which will react with oxygen to produce a refractory bond.

The particulate refractory material used in the composition will be chosen according to the application for which the composition is intended, taking account of such factors as the temperature of the molten metal with which the composition is to come into contact, and whether the particulate refractory material will react with the molten metal. Examples of refractory materials which may be used include alumina (for example bauxite or corundum), magnesia (for example calcined or dead burnt magnesite), zircon, silica, silicon carbide, calcined dolomite, chrome magnesite, olivine, forsterite, an aluminosilicate (for example mullite, kyanite, andalusite or chamotte), chromite and carbon, and mixtures of one or more of these materials. The particulate refractory material may also be wholly or partly a low density material such as aluminosilicate or glass hollow microspheres such as EXTENDOSPHERES, fly ash floaters or cenospheres such as FILLITE, or light weight high-alumina aggregates or grogs (one such material is available commercially under the trade name PL.ASMAL), in order to improve the heat-insulating properties of the refractory composition.

Silica free linings based on alumina and/or magnesia are preferred for refractory compositions intended for use for lining applications for vessels which are to contain molten iron, and in particular molten ductile iron. The presence of silica is disadvantageous for the same reasons that sodium silicate is disadvantageous as a binder in similar lining applications. Silica will react with carbon to form silicon and carbon monoxide giving rise to silicon pick-up, and, because silicon and carbon are not in equilibrium in liquid iron, prevention of carbon loss. Silica will react with iron oxide present from rust or from overheated scrap producing fayalite (Fe₂Si0 ) and giving rise to increased slag. Silica will also cause magnesium loss as it will react with magnesium to produce magnesium silicate (MgSi0₃) in the form of stringy-type dross inclusions.

Boron-containing additives are often used as sintering aids in refractory linings. However, it is preferred that the refractory compositions of the invention are boron free because boron is a powerful carbide forming element and as little as 0.002% boron can result in intercellular carbides and a deterioration in the mechanical properties of cast iron.

The thermosetting organic binder may be for example a phenol-formaldehyde resin, a urea-formaldehyde resin, a melamine-formaldehyde resin, an epoxy resin or a silicone.

The preferred thermosetting organic binder is preferably a novolak phenol-formaldehyde resin. Thermosetting novolak phenol-formaldehyde resins usually contains hexmethylenetetramine as cross-linking and curing agent, and on heat curing such resins produce an unpleasant smell. Therefore in order to produce a refractory composition which in use is as environmentally friendly as possible, it is preferred that the phenol-formaldehyde resin contains only the minimum amount of hexamethylenetetramine needed to ensure efficient curing of the resin in order to eliminate the problem of the unpleasant smell as far as is possible.

The element, compound or alloy which will react with oxygen to produce a refractory bond may be for example aluminium, magnesium, silicon, calcium boride (CaB₆), boron carbide (BC₄), calcium suicide (CaSi₂), ferrosilicon (FeSi₂) or a magnesium-aluminium alloy having the formula Mg₁₇AI₁ . If desired, more than one element, compound or alloy may be used.

The Mg₁₇Ali2 alloy is preferred. The alloy has a low eutectic temperature in the binary Mg-AI system, and on heating to a temperature above 450° C, it will react exothermically with oxygen to form Mg-AI-spinel (MgAI₂0₄) plus magnesium oxide producing a highly refractory ceramic bond, and affording protection for the carbon bond formed by decomposition of the thermosetting organic binder. Such a bond is free of alkaline elements which are present when using an alkali metal silicate binder, and is thermochemically compatible with a wide variety of particulate refractory materials which may be used. A further advantage results from the fact that when the reaction to form Mg-AI-spinel and MgO takes place there is a small but permanent volume expansion, because when the refractory composition is used to produce a lining the expansion tends to compensate for the shrinkage which can occur due to high temperature sintering, and improves the thermal stress resistance.

The refractory composition of the invention will usually contain 85 -95% by weight of particulate refractory material, preferably alumina and/or magnesia, 1 - 10% by weight of thermosetting organic binder, preferably a novolak phenol-formaldehyde resin containing a low amount of hexamethylenetetramine as curing agent, 1 - 10% by weight of an element, compound or alloy which will react with oxygen to produce a refractory bond, preferably the alloy Mg₁₇AI₁₂. In order to improve the performance of the composition when used as a lining material in specific applications the refractory composition may, if desired, also contain other constituents, such as a sintering agent, which may itself be a compound or alloy which will react with oxygen to produce a refractory bond, such as calcium boride (CaB₆), boron carbide (BC₄), calcium suicide (CaSi₂) or ferrosilicon (FeSi₂), or a material which will make the composition more resistant to wetting by molten metal, for example zirconia or boron nitride. For example a sintering agent may be desirable when the composition is to be used as a lining which is to be in contact with molten grey, malleable or ductile iron, and both a sintering agent and a material which will improve the non-wetting properties of the composition may be desirable when the composition is to be used as a lining which is to be on contact with molten aluminium or aluminium alloys.

When present these additional constituents will usually amount to up to 10% by weight of the refractory composition.

The dry heat-settable refractory compositions of the invention may be prepared simply by thoroughly mixing together the individual components. The particle size of the individual components will usually be in the range of a maximum of 5000 microns down to less than 44 microns.

The compositions may be used to produce refractory articles by various means, for example by filling a suitable mould, or in the case of the production of a lining in a metallurgical vessel by filling the space between the surface to be lined and a suitable former or mandrel with the composition, and heating the composition, and then removing the former or mandrel if used. The heating procedure used will be dependent on such factors as the actual composition, its density, its mass, the mould or lining thickness, the complexity of the lining geometry, and the heat energy input. The compositions will usually be heated to a temperature of 150° to 350°C. The refractory compositions of the invention are dry flowable mixtures of particulate materials, which may be readily used for a variety of applications, and in particular for producing facing or backing linings for furnaces, for metallurgical vessels such as ladles, tundishes or launders, or for producing refractory shapes, for use in the treatment and/or casting of a variety of molten metals such as iron, steel or aluminium.

Phosphoric acid or metal phosphates are commonly used as binders or sintering additives in lining refractories. The compositions of the invention can be produced without the need to include phosphorus-containing compounds. This is advantageous when the compositions are used as linings in the casting of iron because phosphorus reacts with iron to form needle-shaped iron phosphate which can cause machining difficulties and embrittlement when casting grey or ductile iron.

The fact that the compositions are dry is also advantageous in cold-start practice (i.e. when molten metal is poured into a lined vessel without preheating of the lining) compared with known linings which contain water. The water in the known linings on contact with molten metal would be transformed into H₂, 0₂, [H] and [O] (the latter two being soluble elements), giving rise to reoxidation and hydrogen defects, and resulting in oxide defects in metals in general and promoting carbide formation (for example in iron) or embrittlement (for example in steel) respectively.

If desired, when the compositions are used to produce a lining, the set lining may be coated with a thin layer of a refractory material, for example a zirconia coating when the lining is used for steel, a zirconia or silicon carbide coating when the lining is used for grey, malleable or ductile iron, or a boron nitride coating when the lining is used for aluminium.

The following Examples will serve to illustrate the invention :- EXAMPLE 1

A dry, heat-settable refractory composition was prepared by mixing together the following components in the proportions indicated :-

% By Weight Bauxite (- 20 mesh ASTM) 87.0

Bauxite (- 48 mesh ASTM) 5.0

Phenol-formaldehyde resin (- 200 mesh ASTM ) 3.5

Mg₁₇AI₁₂ alloy (- 200 mesh ASTM) 4.5

The phenol-formaldehyde resin was a proprietary resin having a very low hexamethylenetetramine content available under the trade name DURITE RD-201C from Borden Inc.

The composition was used to a produce cylinder 50mm in diameter and 50mm in height by filling the composition into a steel tube and heating the filled tube in an electric furnace at 200° C for about 2 hours. After cooling the hardened cylinder was removed from the tube.

The gas permeability of the cylinder was determined using a GF permeability tester of the type used for determining the permeability of foundry sand test cores.

The composition was also used to produce rectangular bars (25mm x 25mm x 150mm) by filling the composition into multiple cavities of a steel tool. The filled tool was heated in an electric furnace at 200° C for about 2 hours. After cooling the hardened bars were removed from the tool and their density was determined. The transverse strength or modulus of rupture (MOR) of the bars was determined using a Soiltest Versa-Loader at a constant loading rate of 2mm/minute.

The density, MOR and permanent linear change (PLC) were also determined after the bars had been fired at 1600° C for 2 hours.

The following results were obtained :-

Permeability at 22° C 68 AFS

Density after 2 hours at 200° C 1.83 g/cm³

Density after 2 hours at 1600° C 1.76 g/cm³

PLC after 2 hours at 1600° C -2.8 %

MOR after 2 hours at 200° C 1900 kPa

MOR after 2 hours at 1600° C 8700 kPa

EXAMPLE 2

% By Weight

Bauxite (- 20 mesh ASTM) 40.0

Bauxite (- 48 mesh ASTM) 20.0 90% alumina grog (PLASMAL nominally

-10 + 40 mesh ASTM) 30.0

Phenol-formaldehyde resin (200 mesh ASTM) 4.5 Mg₁₇AI₁₂ alloy (200 mesh ASTM ) 5.5 Th e phenol-formaldehyde resin was the same as that used in Example

The composition was used and tested as described in Example 1 except that the MOR was also determined on bars which had been heated to 1100° C for 2 hours and then cooled.

The following results were obtained :-

Permeability at 22° C 37 AFS

Density after 2 hours at 200° C 1.65 g/cm³

Density after 2 hours at 1600° C 1.59 g/cm³

PLC after 2 hours at 1600° C -2. 0 %

MOR after 2 hours at 200° C 1300 kPa MOR after 2 hours at 1100° C 490 kPa

MOR after 2 hours at 1600° C 4400 kPa

EXAMPLE 3

% By Weight Dead burnt magnesite (-20 mesh ASTM) 82.0

Hard burnt magnesite (MAG CHEM 10 PR-30 -16 mesh ASTM) 10.0

Phenol-formaldehyde resin (-200 mesh ASTM) 3.5

Mg₁₇AI₁₂ alloy (-200 mesh ASTM) 4.5 The phenol-formaldehyde resin was the same as that used in Example

After mixing the composition was poured into the bottom of a lip-pour ladle. A mandrel coated with a fluorocarbon aerosol release agent was placed inside the ladle cavity, and secured to the ladle shell, and the space between the mandrel and the inner surface of the ladle was filled with the composition to produce a lining. The mandrel was heated for 12-16 minutes per 50 kg of the lining composition using a gas torch until the lined turned dark brown in colour, and the mandrel was then allowed to cool and removed. The lining was then preheated to 1000 to 1200° C. This required a burner of 5270 kJ/hr gas input for 100 kg of lining material. After preheating of the lining, and before being filled with molten metal, the ladle was covered with a refractory fibre blanket to minimised heat loss.

The composition was also used and tested as described in Example 1 , and the following results were obtained :-

Permeability at 22° C 8.9 AFS

Density after 2 hours at 200° C 1.72 g/cm³

Density after 2 hours at 1600° C 1.66 g/cm 3

PLC after 2 hours at 1600° C -3.3 %

MOR after 2 hours at 200° C 2000 kPa

MOR after 2 hours at 1600° C 2300 kPa

EXAMPLE 4

% By Weight Dead burnt magnesite (-20 mesh ASTM) 30.0 Hard burnt magnesite (MAG CHEM 10 PR-30 -16 mesh ASTM) 45.0

Dead burnt magnesite (-100 mesh ASTM) 15.0

Phenol-formaldehyde resin (-200 mesh ASTM) 4.5

Mg₁₇AI₁₂ alloy (-200 mesh ASTM) 5.5

The phenol-formaldehyde resin was the same as that used in Example

1.

The composition was used and tested as described in Example 1.

A hardened plate was also produced from the composition and the rate of heat loss through the composition and the total heat loss after 1 hour were determined using the plate on AMITEC equipment of the type described in British Patent No. 1018703.

The following results were obtained :-

Permeability at 22° C 28 AFS

Density after 2 hours at 200° C 1.56 g/cm³

Density after 2 hours at 1600° C 1.50 g/cm³

PLC after 2 hours at 1600° C -3.7 %

MOR after 2 hours at 200° C 1400 kPa

MOR after 2 hours at 1600° C 1900 kPa

Rate of heat loss 258 J/cm².min. Total heat loss after 1 hour 17.6 kJ/cm²

The rate of heat loss and total heat loss after 1 hour were also determined using the same equipment on a typical cast refractory ladle lining having a density of 2.6 g/cm³. The rate of heat loss was 488 J/cm².min and total heat loss was 36.2 kJ/cm². EXAMPLE 5

% By Weight Bauxite (- 20 mesh ASTM) 93.5

Phenol-formaldehyde resin (- 200 mesh ASTM ) 3.5

CaB₆ alloy (-325 mesh ASTM) 0.5

Mg₁₇AI₁₂ alloy (- 200 mesh ASTM) 2.5

The composition was used and tested as described in Example 1 except that the PLC was not determined and the MOR was determined only on bars which had been heated for 2 hours at 1200° C.

The following results were obtained :-

Permeability at 22° C 74 AFS

Density after 2 hours at 200° C 1.81 g/cm³

MOR after 2 hours at 1200° C 2400 kPa

Claims

1. A dry, heat-settable refractory composition comprising a particulate refractory material, a particulate thermosetting organic binder, and a particulate element, compound or alloy which will react with oxygen to produce a refractory bond.

2. A composition according to Claim 1 , in which the particulate refractory material comprises a material selected from the following materials: alumina, magnesia, zircon, silica, silicon carbide, calcined dolomite, chrome magnesite, olivine, forsterite, an aluminosilicate, chromite, carbon, and mixtures comprising one or more of these materials.

3. A composition according to Claim 1 or Claim 2, in which the particulate refractory material comprises hollow microspheres.

4. A composition according to any preceding claim, in which the particulate thermosetting organic binder comprises a phenol-formaldehyde resin, a urea-formaldehyde resin, a melamine-formaldehyde resin, an epoxy resin, a silicone, or a mixture comprising one or more of these binders.

5. A composition according to Claim 4, in which the particulate thermosetting organic binder comprises a novolak phenol-formaldehyde resin.

6. A composition according to any preceding claim, in which the particulate element, compound or alloy which will react with oxygen to produce a refractory bond comprises aluminium, magnesium, silicon, calcium boride (CaB₆), boron carbide (BC₄), calcium suicide (CaSi₂), ferrosilicon (FeSi₂), a magnesium-aluminium alloy having the formula Mg₁₇AI-ι₂, or a mixture comprising one or more of these materials.

7. A composition according to any preceding claim, comprising 85 - 95% by weight of particulate refractory material, preferably alumina and/or magnesia.

8. A composition according to any preceding claim, comprising 1 - 10% by weight of thermosetting organic binder, preferably a novolak phenol-formaldehyde resin.

9. A composition according to any preceding claim, comprising 1 - 10% by weight of an element, compound or alloy which will react with oxygen to produce a refractory bond, preferably Mg₁₇AI₁₂

10. A composition according to any preceding claim, further comprising a sintering agent comprising up to 10% by weight of the refractory composition.

11. A composition according to any preceding claim, in which the particle size of the individual components is no greater than 5000 microns, and preferably no smaller than 40 microns.

12. A method of forming a refractory article, comprising filling a mould or other space to define the shape of the article, with a refractory composition according to any preceding claim, and heating the composition to a temperature of at least 150°C to cause the refractory composition to set.

13. A method according to Claim 12, in which the refractory article comprises a refractory lining for a furnace or a metallurgical vessel, and the space which is filled with the refractory composition is defined by the surface to be lined and a former and/or mandrel, the method further comprising removing the former and/or mandrel after the refractory composition has set.

14. A method according to Claim 13, further comprising the step of coating the set liner with a layer of refractory material.

15. A refractory article, preferably a liner, or a refractory shape for use in the treatment and/or casting of molten metal, formed by means of a method according to any one of claims 12 to 14.