GB2347143A

GB2347143A - Refractory composition

Info

Publication number: GB2347143A
Application number: GB0002133A
Authority: GB
Inventors: Philip Hodgson
Original assignee: BRIGHTCROSS MANUFACTURING LIMI
Current assignee: BRIGHTCROSS MANUFACTURING LIMI
Priority date: 1999-02-24
Filing date: 2000-02-01
Publication date: 2000-08-30
Also published as: GB9904083D0; GB0002133D0

Abstract

A composition for use in the manufacture of thermally insulating refractory articles comprises a biogenic silica material. The composition also includes a fibrous material and a binding agent. The biogenic silica material is substantially devoid of cristobalite. In one embodiment of the invention the fibrous material is a mineral fibre. For preference, this silica is formed from calcined rice husks and the binder is either starch or a phenolic resin.

Description

omposition for Use in the Manufacture of Thesmally InsulaUng Refractory ArtçicleE This invention relates to the manufacture of thermally insulating refractory articles, to the articles themselves, and to compositions and formulations used in manufacturing the articles.

Since its introduction in the 1950's, a large range of thermally insulating refractory articles have been manufactured using refractory ceramic fibre.

However, under European Directive 97/69/EC, refractory ceramic fibre has been classifie as a category 2 carcinogen"substances regarde as probably carcinogenic to humans". Under Duty of Care, users of ceramic fibre products have an obligation to seek suitable"safe"alternatives.

Various"safer"alternatives have been used to replace ceramic fibres, for example, mineral fibres, and newly developed high biosolubility ceramic fibres.

Also formulations containing insulating fly ash or cenospheres have been used.

These alternatives work admirably for some applications. However they are not suitable for all applications. There are limitations in terms of cost, insulating properties or temperature capability, depending on the application and on the material used.

For example, formulations containing mineral wools or biosoluble ceramic fibres work well in applications involving contact with molten aluminium (e. g. in foundries making aluminium castings), but because of their relatively low melting point they will not work in contact with molten iron or steel at temperatures in excess of 1300 C (e. g. in foundries making ferrous castings). Difficulties have also been experienced with formulations containing these fibres in some domestic appliance applications involving direct flame impingement, for example fibre firebacks in coal/log effect gas fires, where unacceptable cracking due to shrinkage can occur.

Formulations containing substantially fly ash or cenospheres as the insulating component generally tend to be of higher density than those made from refractory ceramic fibres, and they generally have inferior insulating properties to ceramic fibre products.

Formulations containing polycrystalline high alumina fibres such as Saffil (ICI) and Fibermax (Carborundum, Unifrax) will work well in all the above applications, but their very high cost (20-30 x ceramic fibre) precludes them from most of these applications.

A rapid and economical method of producing fibre shapes, whether containing refractory ceramic fibres, mineral wools, biosoluble ceramic fibres, polycrystalline fibres, glass fibres, wood or other organic fibres is by"vacuum forming". This method involves making a dilute aqueous slurry containing the fibres, a binder or binders, for example colloidal silica and/or starch, and (optionally) a filler ; a porous hollow forming tool of the desired shape is immersed into the slurry and suction is applied to the inside of the tool, the carrier liquid is drawn through the tool by the suction, and the fibre and binder (s) are filtered onto the surface of the tool ; the tool is removed from the slurry and excess water is removed by the continuing suction (dewatering) ; the fibre shape is carefully removed from the tool, maintaining its formed shape and it is then dried, resulting in a rigid insulating product.

The thermal insulating properties of shapes formed by the above method result from the myriads of air pockets between the fibres-the finer the fibres, the smaller are the air pockets, and the better are the insulating properties.

The range of applications for such vacuum formed products depends upon the properties of the fibres used and the type of binder and (optionally) filler. In applications where the temperature of use of the product is higher than the melting point of the fibre, it is understood that the fibre will start to melt at the exposed surface, and will lose its fibrous structure, resulting in progressive melting and collapse of the product. Such melting can be arrested by the use of refractory fillers which fill the voids between the fibres. However, the use of refractory fillers will normally compromise the insulating properties of the product.

According to one aspect of this invention there is provided a biogenic silica material for use in the manufacture of a thermally insulating refractory product, wherein the biogenic silica material is substantially devoid of cristobalite.

According to another aspect of this invention there is provided the use of a biogenic silica material in the manufacture of a refractory thermally insulating product, wherein the biogenic silica material is substantially devoid of cristobalite.

According to another aspect of this invention there is provided a composition for use in the manufacture of a thermally insulating refractory product comprising a biogenic silica material, a fibrous material and a binding agent, the biogenic silica material being substantially devoid of cristobalite.

The composition may include a filler which may be ignitable when heat is applied thereto to provide an exothermic rection.

While not wishing the invention to be restricted to any particular theory relating thereto, it is believed that the preferred embodiment of the present invention overcomes the limitations of the prior art by filling the voids between the fibre with biogenic silica, which is a highly refractory, highly insulating material. Previous investigations have revealed that biogenic silica has a micro porous structure and it is believed that it arrests the melting of the fibres of the surface of the product and prevents the collapse of the fibre structure, maintaining the structural and dimensional integrity of the product.

The biogenic silica material may have been previously formed from processing biological matter derived from plants. Suitable such matter has been found to be rice hulls. The biogenic silica material can be obtained by processing the biological matter to remove the carbon containing compound and to substantially prevent the formation of cristobalite. In one example, the process involves heating rice hulls to a suitable temperature and for a suitable period of time in an oxidising atmosphere to remove by combustion the carbon containing compound and yet substantially prevent the formation of cristobalite. The processing of rice hulls in the above way is known and results in a product referred to as rice hull ash.

The biogenic silica material may comprise at least 60% silica, preferably at least 90% silica, more preferably at least 93% silica.

The fibrous material may be a mineral fibre, preferably a synthetic mineral fibre, for example a fibre sold under the trade mark ROCKWOOL. The composition may include 0-40% w/w mineral fibre, more preferably, substantially 20% w/w mineral fibre.

The composition may further inclue an organic fibre, for example wood pulp. The composition may include 0-20% w/w organic fibre, more preferably, substantially 6% w/w organic fibre. The avantage of the provision of an organic fibre in the preferred embodiment is that it improves the toughness of the product.

In one embodiment, the composition comprises substantially 10% w/w to substantially 90% w/w of said biogenic silica material. Desirably, the composition comprises substantially 10% w/w to substantially 80% w/w of said biogenic silica material, preferably substantially 20% w/w to substantially 50% w/w of said biogenic material, and more preferably substantially 40% w/w of said biogenic silica material.

The binding agent may comprise a colloidal silica sol and may also include a starch. Alternatively, the binding agent may be a resin, suitably a phenolic resin, for example phenol-formaldehyde resin.

The amount of binding agent present in the composition is preferably in the range of substantially 2% w/w to substantially 50% w/w, preferably substantially 2% w/w to substantially 20% w/w.

The filler may be a refractory filler which may be in a particulate or powder form. The refractory filler may comprise china clay and/or alumina. The alumina may be in the form of a powder having a size of substantially-200 mesh.

Preferably the amount of filler is substantially 25% w/w. In an embodiment, in which alumina and china clay are provided as the filler material, the amount of alumina may be substantially 17% w/w, the amount of china clay may be substantially 8% w/w.

The biogenic silica material is preferably in an amorphous state and advantageously has a porous skeletal structure.

According to another aspect of this invention there is provided a formulation comprising a composition as described above and a carrier material.

The carrier material may be a liquid, suitably water. The amount of carrier material is preferably dependent upon the characteristics of the slurry required. For example, in some situations more concentrated slurry may be required than in others.

In one embodiment, a composition having a mass of substantially 10kg is mixed with substantially 400 litres of water.

According to another aspect of this invention there is provided a method of manufacturing refractory and/or thermally insulating article comprising the steps of providing a formulation as described above, shaping some of the solid material in the formulation into a desired shape and removing the carrier material from the shaped solid material.

The shaping of the solid material into the desired shape may be carried out by known techniques, for example vacuum forming the material onto formers.

The removal of the carrier material from the shaped solid material may comprise drying the shaped solid material to remove substantially all water therefrom.

According to another aspect of this invention there is provided a refractory and/or thermally insulating article comprising a composition as described above.

An embodiment of the invention will now be described by way of example only.

Refractory heat insulating articles, in the present case, sleeves, are manufactured using a composition containing a biogenic silica material in the form of rice hull ash.

The composition comprises the following ingredients : 2000g mineral fibre 800g china clay 4000g rice hull ash 1600g alumina (-200 mesh) 600g cationic starch 1200 mls of silica sol (40% silica) The above composition was disperse in 400 litres of water to form a slurry.

A suitable mineral fibre which could be used in the above composition is sold under the trade mark ROCKWOOL and has been exonerated from classification as a carcinogen in EU Commission Directive 97/69/EC.

The rice hull ash used in the above composition can be prepared by heating rice hulls in a furnace to temperatures in the range of about 800 to 1400 C.'The rice hulls pass through the furnace during a period of about 3 minutes, and the atmosphere in the furnace is oxidising. An example of the preparation of such rice hull ash is described in U. S. patent specification No.

4, 555, 448.

The use of this process results in all the carbon containing matter in the rice hulls being burned off such that substantially only silica remains. The silica so produced is in an amorphous state, having a porous skeletal structure.

This process substantially prevents the formation of the crystalline form of silica known as cristobalite, which is carcinogenic. In effect, the rice hull ash contains substantially undetectable amounts of cristobalite. In one application of this process, it is possible to produce a rice hull ash that contains of the order of 1% carbon.

The skilled person will realise that rice hull ash used in the above described embodiment and produced by the above described process is different to rice hull ash produced as a by product from incineration of rice hulls in such processes as gasification or electricity production. Rice hull ash derived from these processes usually contains significant proportions of cristobalite and of unburned carbon.

Porous formers to define the shape of the insulating sleeves are provided and the insulating sleeves are manufactured by immersing porous formers into the slurry. A vacuum is applied to the inside of the formers, and the solid material in the slurry is sucked onto the formers. The formers and the solid material thereon are removed from the slurry. The vacuum is continued to be applied to the formers to remove water from the solid material thereon. The sleeves are then removed from the formers and dried at substantially 125 C.

The above description relating to the formation of sleeves is only one example of refractory articles that can be made from the above slurry or similar slurries. Other such articles are for example, pouring cups, ladles, tap cones all of which are for use at smelters, mills and foundries. Domestic articles can also be manufactured, for example fire backs, pads, diffusers and artificial coals and logs. Other known techniques can be used to manufacture these and other articles.

Various modifications can be made without departing from the scope of the invention. For example, other materials can be used in addition to, or as an alternative to, rice hulls, for example rice stalks, equisetum, certain bamboo palm leaves e. g. Palmyra palms.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS 1. A composition for use in the manufacture of a thermally insulating refractory product comprising a biogenic silica material, a fibrous material and a binding agent, the biogenic silica material being substantially devoid of cristobalite.
2. A composition according to Claim 1 wherein the biogenic silica material may comprise at least substantially 60% silica.
3. A composition according to Claim 2 wherein the biogenic silica material comprises at lease substantially 90% silica.
4. A composition according to Claim 2 or 3 wherein the biogenic silica material comprises at least substantially 93% silica.
5. A composition according to any preceding claim wherein the fibrous material is mineral fibre.
6. A composition according to Claim 5 wherein the composition inclues 0 to substantially 40rob w/w mineral fibre.
7. A composition according to Claim 5 or 6 wherein the composition inclues substantially 20% w/w mineral fibre.
8. A composition according to any preceding claim further including an organic fibre.
9. A composition according to Claim 8 wherein the composition inclues 0 to substantially 20% w/w organic fibre.
10. A composition according to Claim 8 or 9 wherein the composition inclues substantially 6% w/w organic fibre.
11. A composition according to any proceeding claim wherein the composition inclues substantially 10% w/w to substantially 90% w/w biogenic silica material.
12. A composition according to any preceding claim wherein the composition inclues substantially 20'r6 w/w to substantially 50% w/w biogenic silica material.
13. A composition according to any preceding claim wherein the composition inclues substantially 40% w/w of said biogenic silica material.
14. A composition according to any preceding claim wherein the binding agent comprises a colloidal silica sol and a starch.
15. A composition according to any of Claims 1 to 13 wherein the binding agent is a resin.
16. A composition according to any Claim 15 wherein the resin is a phenolic resin.
17. A composition according to any preceding claim wherein the composition inclues substantially 2% w/w to substantially 50% w/w binding agent.
18. A composition according to any preceding claim wherein the composition inclues substantially 2% w/w to substantially 20% w/w binding agent.
19. A composition according to any preceding claim including a filler which may be ignitable when heat is applied thereto to provide an exothermic rection.
20. A composition according to Claim 19 wherein the filler is a refractory filler in particulate or powder form.
21. A composition according to Claim 20 wherein the filler comprises china clay and/or alumina.
22. A composition according to Claim 21 wherein the alumina is in the form of a powder having a size of-200 mesh.
23. A composition according to any of Claims 19 to 22 wherein the composition inclues substantially 2 5% w/w filler.
24. A composition according to Claim 21, 22 or Claim 23 when dependent upon Claim 21 or 22 wherein the composition inclues substantially 17% w/w alumina and substantially 8% w/w china clay.
2Z. A composition according to any preceding claim wherein the biogenic silica material is in an amorphous state and has a porous skeletal structure.
26. A formulation comprising a composition as claimed in any preceding claim and a carrier material.
27. A formulation according to Claim 26 wherein the carrier material is a liquid.
28 A formulation according to Claim 26 or 27 wherein the carrier material is water.
29. A method of manufacturing refractory and/or thermally insulating article comprising the steps of providing a formulation as claimed in Claims 26, 27 or 28, shaping some of the solid material in the formulation into a desired shape and removing the carrier material from the shaped solid material.
30. A method according to Claim 29 wherein the shaping of the solid material into the desired shape is carried out by vacuum forming the material onto formers.
31. A method according to Claim 29 or 30 when dependent on Claim 28 wherein the removal of the carrier material from the shaped solid material comprises drying the shaped solid material to remove substantially all water therefrom.
32. A biogenic silica material for use in the manufacture of a thermally insulating refractory produce, wherein the biogenic silica material is substantially devoid of cristobalite.
33. The use of a biogenic silica material in the manufacture of a refractory thermally insulating product, wherein the biogenic silica material is substantially devoid of cristobalite.
34. A composition according to Claim 1 substantially as herein described.
35. A formulation according to Claim 26 substantially as herein described.
36. A method according to Claim 29 substantially as herein described.
37. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.