WO1998021162A1

WO1998021162A1 - An insulating refractory type material and a method of making such a material

Info

Publication number: WO1998021162A1
Application number: PCT/GB1997/003077
Authority: WO
Inventors: Ian James Mann
Original assignee: Thermal Ceramics Limited
Priority date: 1996-11-09
Filing date: 1997-11-10
Publication date: 1998-05-22
Also published as: AU4877997A; GB9623459D0; EP0944559A1; GB2319247A

Abstract

Disclosed are a refractory type material and a method of making same. The insulating refractory type material comprising a crystalline relict of a structure which comprised one or more non-ceramic fibres selected from glass fibres and mineral fibres, one or more reactive additives, and a binder/flocculation system. The material is manufactured by a method comprising: (1) mixing one or more non-ceramic fibres selected from glass fibres and mineral fibres in a suspension fluid with one or more reactive additives to produce a slurry; (2) flocculating the mixture with a binder/flocculation system comprising one or more components; (3) forming a shaped product by drawing the suspension fluid off through a shaping tool; (4) drying the shaped product; and (5) firing the shaped product to convert it into a crystalline relict.

Description

DESCRIPTION

AN INSULATING REFRACTORY TYPE MATERIAL AND

A METHOD OF MAKING SUCH A MATERIAL

The present invention relates to an insulating refractory type material and a method of making such a material. More particularly, it relates to an insulating refractory type material which can be produced as a board or in more complex shaped forms from fibres which would not ordinarily withstand being used at temperatures over 1000°C, such as mineral fibres and glass fibres.

True refractory materials are generally considered to be materials which can withstand being used to temperatures over 1000°C. They include, for example :

Acid refractory materials such as fireclays, china clays (kaolin) , silica, flint, chalcedony, ganister and titanium dioxide;

Neutral refractory materials such as, graphite, charcoal, coke, chromite and various carbides ; and

Basic refractory materials such as, lime, magnesia, various materials composed of alumina, dolomite and many of the rarer refractory oxides, particularly zirconia.

Ceramics are materials made from inorganic 2 chemicals (excluding metals and alloys) by high temperature processing. These are resistant to chemical attack and include refractories, glasses, cement and cement products, vitreous enamels, abrasives, china, porcelain, clay wares and alumina.

Over the years, the classification of refractory and refractory type materials including mineral fibres and ceramic fibres has become haphazard. Originally mineral fibres were classified as those fibres produced directly from naturally occurring ores, however now many mineral fibres are produced from synthetic compositions. The first ceramic fibres were produced using kaolin, a naturally occurring clay, but now almost all ceramic fibres are produced from synthetic raw materials. For the purposes of this document, Refractory Ceramic Fibres (RCF) can be considered to be those fibres with classification temperatures greater than 1260°C; Mineral Fibres as those having classification temperatures in the range of 500°C to 1100°C and Glass Fibres which although not generally classified by temperature would not generally be considered to have classification temperatures higher than 1000°C.

Refractory Ceramic Fibres have, for many years, been converted into boards and more complex shaped forms through a vacuum forming process. Papers and thin boards are produced with rota forming or Foudrinier paper making machines. In all cases, the process is basically the same. Refractory ceramic fibres, binders and fillers are mixed in a slurry and the slurry is then filtered through a perforated tool, plate or belt. The filter process is driven by a differential pressure which is provided in some cases by a vacuum pump, in others by a positive pressure, and in the remainder by gravity.

Whilst having desirable properties, it is thought that RCF's may be carcinogenic and those with fibres having a diameter below 3μm may cause respiratory disease.

It is an aim of the present invention to produce an insulating refractory type material, for example, one which can withstand arduous conditions of heat or erosion, with properties similar to those materials made with RCF's, but which is less likely to cause health problems .

The materials of the present invention should preferably have the following properties : They can be produced in board or more complex shaped form;

They have similar thermal conductivity compared to boards and more complex shaped products made from RCF ' s ;

They have dimensional stability at temperatures of up to 1260°C; and they are substantially resistant to thermal shock.

It is another aim of the present invention to produce an insulating refractory material which has a lower thermal conductivity than normal RCF based products .

By low thermal conductivity is meant below 0.3 Wm-1 K at 500 °C, more preferably below 0.2 Wm-IK at 500°C and most preferably below 0.15 Wm-1 K at 500°C.

It has surprisingly been found that a product with the desired properties can be produced using non- ceramic fibres, more particularly glass fibres or mineral fibres which fibres would be expected to soften at temperatures below 1000°C.

The mineral fibres include diabase fibres, rockwools, and slagwools. Such fibres had not previously been used in products for use as high temperature insulating refractory - type materials. It was assumed that such fibres, particularly glass fibres, were .unsuitable because:

They have a low melting point;

They include fluxes which increase levels of vitrification in mixed compositions; and in the case of glass fibres were known to prove difficult to disperse in slurries.

According to one aspect of the present invention there is provided an insulating refractory type material comprising a crystalline relict of a structure which comprised one or more non-ceramic fibres selected from glass fibres and mineral fibres, one or more reactive additives and a binder/ flocculation system.

According to another aspect of the present invention there is provided a crystalline relict obtainable from a material comprising: i) one or more non-ceramic fibres selected from glass fibres and mineral fibres, ii) one or more reactive additives, and iii) a binder/flocculation system.

The structure may additionally have comprised an opacifier to cut radiant transmission through the material .

The opacifier may be Titania, Rutile, Ilmenite, Zirconium Silicate, Zirconia, or any other material or mixture of materials with good radiation scattering properties. These may be present in the structure in an amount of from 5% to 50% by weight of the total weight.

Where an opacifier is present the preferred material comprises Ilmenite or Rutile in amounts of between 20% and 30% by weight of the material total weight .

In the material of the present invention the non-ceramic fibres have been reacted with the reactive additives and binder/flocculation system, by heating to 6 above the structures devitrification point, to produce a crystalline relict.

The crystalline forms produced in the material on firing will be determined by the starting components of the material . By changing the non- ceramic fibres, reactive additives, and binder/flocculation system different crystalline forms within the material can be obtained. These crystalline forms include anorthite, a refractory material suitable for use up to 1250°C, which has the formulation CaAl₂Si₂0₂, and mullite, a refractory material suitable for use up to 1700°C, which has the formulation 3Al₂0₃.2Si0₃.

A refractory type material comprising these crystalline forms can be produced from, for example, the following mixture.

E Glass Fibre 26%

Tabular Alumina 60%

Silica 8.5%

Cationic Starch 5.5%

On firing to 1200°C this produces a rigid material which retains the relict microstructure of the original, but which is now crystalline. The presence of these crystalline forms can be determined by X-ray diffraction analysis. Alternatively, a refractory material including Anorthite can be produced from, for example, the following mixture.

Slagwool 25%

Tabular Alumina 59% Silica 9%

Cationic Starch 7%

On firing to 1200°C this produces a rigid material which retains the relict microstructure of the original, but which is now crystalline. The presence of anorthite can be determined by X-ray diffraction analysis .

Cordierite is a refractory material which is suitable for use at temperatures up to 1350 °C and which has excellent thermal shock resistance due to its very low coefficient of thermal expansion.

Typically for Alumino Silicate materials the coefficient of thermal expansion is about 0.0075deg^"\ while for Cordierite the coefficient of thermal expansion is about 0.0025 deg^"1.

A refractory type material with Cordierite as its major crystalline relict can be produced from, for example, the following mixture.

E Glass Fibre 24% Tabular Alumina 29% Talc 28% Silica 11 . 5 %

Cationic Starch 7 . 5 %

On firing to 1200°C this produces a rigid material which retains the microstructure of the original, but which is crystallised. The presence of Cordierite as a major phase is suggested by its thermal expansion curve and can be confirmed by X-Ray diffraction analysis.

It is the formation of a material in which the original non-ceramic fibres are converted to crystalline relicts within the body of the material, ie they are devitrified whilst retained within or embedded in the reactive additive and binder/flocculation system which gives rise to their unique properties. The presence of a crystalline structure can be confirmed by X-Ray diffraction analysis.

To achieve the desired properties it is essential that the conversion from a vitreous phase to a predominantly crystalline microstructure takes place whilst the morphology of the material remains substantially unchanged.

Thus, for example, if the material comprises a glass fibre and appropriate reactive additives the crystalline forms may, .for example, include Mullite, Anorthite, Cordierite, and Celsian.

Of course it will be appreciated that the material of the invention may be produced for firing by the end user in service.

According to another aspect of the present invention there is provided a material comprising: i) one or more non-ceramic fibres selected from glass fibres and mineral fibres, ii) one or more reactive additives, and iii) a binder/flocculation system.

Preferably, the starting mixture will comprise the components in the following weight percent :

Non-ceramic Fibres: 5% - 50%

Reactive additive : 25% - 85%

Binder/flocculation system: 4% - 40% Other eg. Opacifier 0% - 50% based on the materials total weight .

The preferred non-ceramic fibres include, for example :

Glass fibres, such as, for example, E or R type glass fibres; and Manmade mineral fibres, such as, for example, Inorphil produced by Lapinus Oy; Diabase fibres including labradorite and pyroxenes; Rockwool, such as for example, that produced by Rockwool Pic; and Slagwool , such as for example, that produced by Fibrox Inc .

The preferred reactive additives include, for example :

Powders based on refractory compounds, including Alumina, Silica, Alumino Silicates, Magnesia, Magnesia-Alumino-Silicates, Calcium Silicates, Titania, Zirconia and Zirconium Silicate;

Ground Refractory Materials;

Cenospheres processed from coal ash; or mixtures of any of the above.

The preferred binder/flocculation systems include, for example;

Silica Sol such as, for example, those available from Nalco of Chicago Illinois or Akzo PQ Silicas of Amsterdam Netherlands. These may be mixed with starch such as, that available from Tunnel Avebe;

Alumina Sol such as, for example that obtainable from Condea Chemie;

Clay such as, for example, Hywhite Atlas from English China Clays; and

Aluminium Sulphate/Polymer such as, for example, polyvinyl alcohol, stabilised vinyl acetate, and acrylic copolymers .

Where the composition includes magnesium containing material the crystalline forms may, for example, include Cordierite in the fired product.

The most preferred non-ceramic fibres are glass fibres or slagwool; The most preferred reactive additive is an alumina, and the most preferred binder/flocculation system is a silica/starch system.

The preferred material comprises one including the most preferred components and would typically comprise non-ceramic fibres, reactive additives and a binder/flocculation system in the following weight percents :

Non-ceramic Fibres, e.g. Glass Fibres 5% - 50%

Reactive additive, e.g. Alumina 25%- 85%

Binder/Flocculation system e.g. Silica Sol (Dry) , and 3% - 25%

Starch 1% - 15% based on the materials total weight.

More preferably the preferred composition ranges are :

Non-ceramic fibres, e.g. Glass Fibres 15% - 35%

Reactive additive, e.g. Alumina 50% - 70% Binder/Flocculation system e.g. Silica Sol (Dry), and 5% - 10%

Starch 2% - 8% based on the materials total weight.

Of course the ranges of the components can include subranges taken between the general and preferred ranges .

The most preferred composition comprises; Glass Fibre 26% Alumina 60% Silica Sol 8.5% Starch 5.5%

Such a composition exhibits shrinkage of less than 1% after 24 hours soak at 1300°C and has a thermal conductivity similar to a Ceramic board.

According to a further aspect of the present invention there is provided a method of producing a material of the invention comprising:

(1) mixing one or more non-ceramic fibres selected from glass fibres and mineral fibres in a suspension fluid with one or more reactive additives to produce a slurry;

(2) flocculating the mixture with a binder/flocculation system comprising one or more components ;

(3) forming a shaped product by drawing the suspension fluid off through a shapihg tool,

(4) drying the shaped product; and

(5) firing the shaped product to convert it into a crystalline relict.

Where the non-ceramic fibres contain organic material it is preferred to fire the non-ceramic fibres before mixing so as to remove the organic material present . The shaping tool may be planar, for the production of boards, or a more complex design for the formation of more complex forms.

The suspension fluid, usually water, (though other fluids are envisaged) may be removed by filtration and is more preferably drawn off under pressure, the preferred method being under vacuum.

Alternatively, the intermediate product, namely the product produced at the end of stage 4 may be fired in situ by the end user of the product.

According to a further aspect of the present invention there is provided a material comprising:

(3) forming a shaped product by drawing the suspension fluid off through a shaping tool, and

(4) drying the shaped product.

The fibres are preferably chopped fibres. Preferably the fibres have a diameter of between 3μm and 12μm, although larger diameters can be used, and a length of 3mm to 12mm. However, at diameters greater than 12μm the fibre count is substantially reduced. Higher concentrations of fibre, by weight, compensate for this but diminish the refractoriness of the finished material.

The preferred fibres are E-glass type roving or slagwool, but other types or blends of fibre can be used.

The most preferred glass fibre is normally chopped strand fibre, but other product forms could be used.

Preferably the reactive additive is an alumina or clay. Alumina has been found to give better results in terms of refractoriness and sag resistance. It can be tabular, calcined or fused. Preferably, the reactive additive has a particle size of 40μm or less. The optimum particle size has been found to be in the order of 5μm. Very small particle sizes have been found to increase forming time and larger particles sizes do not disperse well, and tend to fall out of suspension.

Preferably the binder/floceulation system is a Silica Sol/Starch system. Such a binder/flocculation system is economical and gives a hard product. Silica Sol is available in a range of concentrations, and preferably the amount of dry silica added to the finished material should be between 2% and 25% by weight of the total weight. With a higher binder content the product has been found to be stronger and has improved high temperature creep properties. A higher binder content has also been found to lead to higher thermal conductivity and lower resistance to thermal shock. The addition of starch at approximately 2/3 the weight of the dry silica initiates flocculation.

Flocculation is a key part of the process. In normal mixing components are distributed randomly throughout the mixture . However when the mixture has been flocculated, the mixture clumps together, and in this case the reactive additives and the binder are attracted to the surface of the fibres. Their proximity to the fibres is critical when devitrification commences as it makes their chemical composition available to the crystals forming in the fibres.

Other binder/flocculation systems can be used. For example, alumina sol produces softer products, but the finely divided alumina helps to extend refractoriness. Clay based binders also produce softer products .

In the method of the invention the components can be mixed as follows :

Mixing can be done with a single speed mixer although more preferably a dual speed mixer or two separate mixers are used. Ideally the first mixer is a high speed conventional or high shear mixer and is used to separate and disperse the non-ceramic fibres. A I D lower speed is preferably used to mix in the binder/flocculation components and to hold the flocculated mix in suspension while the product is formed.

The slurry is then applied to the appropriate shaping tool and the water drawn off. It is then dried, preferably in a hot air batch or a continuous dryer at about 90°C to 100°C. If discolouration is to be prevented, which is not essential, the upper limit is critical. Lower temperatures can be used, but produce longer drying times . The product can also be dried with forced air, microwave infra-red or radio frequency drying techniques . The product may be machined by methods known to the man skilled in the art, such as, for example laser, water or mechanical methods. The end product may be fired, to about 1200°C to produce the crystalline microstructure, or the firing can be carried out in situ when the product has been put to use, such as, for example, as a refractory lining.

The invention will be further described by way of example only with reference to the following compositions and method of manufacture :

EXAMPLE 1

Refractory type material comprising anorthite, and mullite Starting Material: E Glass Fibre 26% Tabular Alumina 60% Silica 8.5%

Cationic Starch 5.5%

EXAMPLE 2

Refractory type material comprising Cordierite

Starting material: E Glass Fibre 24% Tabular Alumina 29% Talc 28%

Silica 11.5%

Cationic Starch 7.5%

Compositions according to example 1 and 2 above are made by a mixing procedure substantially as follows :

Typical mixing procedure :

1. Fill a tank with approximately 2000 litres of water .

2. Make up materials to a total of 30kg (1.5% Solids)

3. Add materials except starch and silica and agitate for about 1-2 minutes or until totally wetted and dispersed.

4. Add Silica and agitate for about 2 minutes.

5. Add starch and agitate for about 2 minutes.

The above sequence is not critical i.e. the Starch can be added before the Silica. The silica and the starch should not, however, be added together.

The mixture is then ready for vacuum forming.

Following vacuum forming the products are dried to convert them to crystalline relicts.

The presence of these crystalline relicts can be confirmed by x-ray diffraction.

Examples of the type of results obtained are shown with reference to examples 1 and 2

Analysis by x-ray diffraction of the product obtained after firing example 1 identified the following phases :

A1₂0₃ - corundum

CaAl₂Si₂0₃ - anorthite Al₂Si0₅ - sillimanite

More specific analysis identified the presence of :

Na₂0 0.29%

MgO 0.16%

A1₂0₃ 60.98% SiO- 30.67%

P₂0₅ < 0.05%

K₂0 0.35%

CaO 6.83%

TiO, < 0.05%

Mn₃0₄ < 0.05% v₂o₅ < 0.05%

Cr₂0₃ < 0.05%

Fe₂0₃ 0.24%

BaO < 0.05% Zr0₂ < 0.05% ZnO < 0.05% SrO < 0.05%

The thermal expansion curve (Fig. 1) of the product obtained after firing example 2 suggested cordierite was present.

Analysis by x-ray diffraction (Fig. 2) confirmed the presence of cordierite (Mg₂Al₄Si₅0_ιa) and minor amounts of spinel (MgAl₂0₄) and corun,dum (A1₂0₃) . The phases identified are noted below:

CORDIERITE-STD Major phases :

Al₆Si₂0₁₃ - mullite

Mg₂Al₄Si₅0₁₈ - magnesium aluminium silicate (cordierite) Minor phase : A1₂0₃ corundum

CORDIERITE 3962 Major phases : (Ca,Na) (Si,Al)₄0_a sodium calcium aluminium silicate ( anorthite ) Mg₂Al₄Si₅0₁₈ - magnesium aluminium silicate ( cordierite ) Minor phases : MgAl₂0₄ spinel

A1₄0₃ corundum

Claims

1. An insulating refractory type material comprising a crystalline relict of a structure which comprised one or more non-ceramic fibres selected from glass fibres and mineral fibres, one or more reactive additives, and a binder/flocculation system.

2. An insulating refractory type material as claimed in claim 1, comprising the components in the following weight percents :

Non-ceramic Fibres 5% - 50%

Reactive additive 25% - 85%

Binder-Flocculation system 4% - 40% based on the materials total weight.

3. An insulating refractory type material as claimed in claim 2 wherein the components are present in the following weight percents:

Non-ceramic Fibres 15% - 35%

Reactive additive 50% - 70%

Binder/Flocculation system^'-. 12% - 18% based on the materials total weight.

4. An insulating refractory type material as claimed in any of the preceding claims, wherein the non-ceramic fibres are selected from glass fibres, diabase fibres, rockwools and slagwools .

5. An insulating refractory type material as claimed in any of the preceding claims wherein the reactive additives are selected from: powders based on refractory compounds, including Alumina, Silica, Alumino Silicates, Magnesia, Magnesia-Alumino-Silicates, Calcium Silicates, Titania, Zirconia and Zirconium Silicate;

Ground Refractory Materials;

Cenospheres processed from coal ash; or mixtures of any of the above .

6. An insulating refractory type material as claimed in claim 5 wherein the reactive additive comprises alumina.

7. An insulating refractory type material as claimed in any of the preceding claims, wherein the binder/flocculation system is selected from silica sol, alumina sol, clay, aluminium sulphate/polymer and silica sol/starch.

8. An insulating refractory type material as claimed in any of the preceding claims, wherein the binder/flocculation system is a silica sol/starch system.

9. An insulating refractory type material as claimed in any of the preceding claims which further comprises an opacifier.

10. An insulating refractory type material as claimed in claim 9, wherein the opacifier is selected from Titania, Rutile; Ilmenite; Zirconium silicate and Zirconia .

11. An insulating refractory type material as claimed in claim 9 or 10, wherein the opacifier is present in an amount of from 5% to 50% by weight based on the materials total weight.

12. An insulating refractory type material as claimed in claim 11 in which the opacifier is present in an amount of from 20% to 30% by weight based on the materials total weight.

13. An insulating refractory type material as claimed in any of the preceding claims, wherein the fibres have a diameter of between 3 and 12 μm and a length of 3 - 12 mm.

14. An insulating refractory type material ^'as claimed in any of claims 1 to 8, wherein the components are present in the following weight percents

E Glass Fibre 26% Tabular Alumina 60% Silica 8.5%

Cationic Starch 5.5%.

15. An insulating refractory type material as claimed in any of claims 1 to 8 , wherein the components are present in the following weight percents

Slagwool 25% Tabular Alumina 59% Silica 9%

Cationic Starch 7%.

16. An insulating refractory type material as claimed in any of claims 1 to 8 , wherein the components are present in the following weight percents

E Glass Fibre 24%

Tabular Alumina 29%

Talc 28%

Silica 11.5%

Cationic Starch 7.5%.

17. A method of producing a material of the invention comprising:

(3) forming a shaped product by drawing the suspension fluid off through a shaping tool,

(4) drying the shaped product; and

(5) firing the shaped product to convert it into a crystalline relict.

18. A crystalline relict obtainable from a material comprising: i) one or more non-ceramic fibres selected from glass fibres and mineral fibres, ii) one or more reactive additives, and iii) a binder/flocculation system.

19 . A material comprising : i) one or more non-ceramic fibres selected from glass fibres and mineral fibres, ii) one or more reactive additives, and iii) a binder/flocculation system.

20. A material as claimed in claim 19, in which the one or more reactive additives and binders are distributed in close proximity, in clumps, to the surface of the fibres.

21. A method of producing a material as claimed in claim 19 or 20, comprising:

(4) drying the shaped product.