WO2001023315A1

WO2001023315A1 - Methods of making inorganic fibres

Info

Publication number: WO2001023315A1
Application number: PCT/GB2000/003590
Authority: WO
Inventors: Gary Anthony Jubb
Original assignee: Morgan Crucible Co PLC
Current assignee: Morgan Advanced Materials PLC
Priority date: 1999-09-27
Filing date: 2000-09-19
Publication date: 2001-04-05
Anticipated expiration: 2002-03-27
Also published as: GB9922834D0; GB2353996B; AU7301700A; GB2353996A

Abstract

A method of forming fibres comprising SrO, Al2O3, and a fibre forming additive, the method comprising the steps of either: a) mixing: i) a compound of strontium which on heating loses a volatile component, ii) Al2O3, and iii) a fibre forming additive; b) calcining the mixture to remove the volatile component; c) melting the calcined mixture or: a) mixing: i) a compound of strontium which on heating loses a predominantly non-toxic volatile component, ii) Al2O3, and iii) a fibre forming additive; b) forming granules from the mixture of a sufficient size to resist ejection from a melt by outgassing; c) melting the granules and extracting the predominantly non-toxic volatile component either followed by: d) disrupting the melt to form fibres.

Description

METHODS OF MAKING INORGANIC FIBRES

This invention relates to methods of making man-made inorganic oxide fibres containing strontium oxide. The invention also relates to products formed by such methods.

Inorganic fibrous materials are well known and widely used for many purposes (e.g. as thermal or acoustic insulation in bulk, mat, or blanket form, as vacuum formed shapes, as vacuum formed boards and papers, and as ropes, yarns or textiles; as a reinforcing fibre for building materials; as a constituent of brake blocks for vehicles). In most of these applications the properties for which inorganic fibrous materials are used require resistance to heat, and often resistance to aggressive chemical environments.

Inorganic fibrous materials can be either glassy or crystalline. Asbestos is an inorganic fibrous material one form of which has been strongly implicated in respiratory disease.

It is still not clear what the causative mechanism is that relates some asbestos with disease but some researchers believe that the mechanism is mechanical and size related. Asbestos of a critical size can pierce cells in the body and so, through long and repeated cell injury, have a bad effect on health. Whether this mechanism is true or not regulatory agencies have indicated a desire to categorise any inorganic fibre product that has a respiratory fraction as hazardous, regardless of whether there is any evidence to support such categorisation. Unfortunately for many of the applications for which inorganic fibres are used, there are no realistic substitutes.

Accordingly there is a demand for inorganic fibres that will pose as little risk as possible (if any) and for which there are objective grounds to believe them safe. One line of study has proposed that if inorganic fibres were made that were sufficiently soluble in physiological fluids that their residence time in the human body was short; then damage would not occur or at least be minimised. As the risk of asbestos linked disease appears to depend very much on the length of exposure this idea appears reasonable. Asbestos is extremely insoluble.

As intercellular fluid is saline in nature the importance of fibre solubility in saline solution has long been recognised. If fibres are soluble in physiological saline solution then, provided the dissolved components are not toxic, the fibres should be safer than fibres that are not so soluble. The shorter the time a fibre is resident in the body the less damage it can do.

Such fibres are exemplified by the applicant's earlier International Patent Applications WO93/15028 and W094/15883, which disclose saline soluble fibres usable at temperatures of 1000°C and 1260°C respectively. Fibres of these types are on sale under the trade mark SUPERWOOL™.

Refractory fibres generally are formed by the steps of: -

a) melting the constituents in a crucible to form a melt; b) allowing a stream of melt to leave the crucible, and; c) disrupting the melt stream to form fibres.

The disruption can be by many methods but typically a jet of air is used (blowing) or the stream impinges on spinning discs (spinning) although other methods such as centrifugal formation of fibres have been proposed.

In WO96/04214 the applicants revealed that whereas strontium aluminate compositions do not appear to form fibres when formed from a melt, such compositions including additives such as silica do form fibres when blown from a melt. Such fibres appear to hydrate in the manner of calcium aluminate fibres and additionally show the potential for high temperature use. The content of this application in its entirety is incorporated herein by reference. However such fibres, although readily formable, have processing problems when made in large quantities. Sfrontium oxide is a strongly alkaline and irritant material. Handling strontium oxide in large quantities can therefore be hazardous to workers. Accordingly the applicants looked to other materials as a source of strontium that might pose fewer risks in handling.

Strontium is found chiefly as the minerals celestite (SrS0₄) and strontianite (SrC0₃). Use of a sulphate such as SrS0₄ would be hazardous since on melting copious amounts of sulphur dioxide would be produced which would be hazardous and corrosive and would require complex and expensive extraction apparatus.

Much strontium carbonate is made by converting the sulphate to the carbonate but even this contains sufficient residual sulphate to pose a pollution problem when foπning the melt. Additionally use of either a sulphate such as SrS0₄ or a carbonate such as SrC0₃ poses processing problems in that evolution of gases on melting can be so vigorous as to eject raw material from the melting crucible. In the glass-making industries problems with excessive outgassing are generally solved by pre-melting cullet and dissolving the gas-generating materials in the melt so formed. However this is not practicable for strontium aluminate refractory fibres because of the need to provide a composition having low amounts of impurities; because of the high temperatures required in melting the ingredients; and because the amount of gas-generating material is high in comparison with glass making compositions. Accordingly the present invention provides a method of foπning fibres comprising SrO, A1₂0₃, and a fibre-forming additive, the method comprising the steps of a) mixing:- i) a compound of strontium which on heating loses a volatile component ii) A1₂0_3> and iii) a fibre foπning additive b) calcining the mixture to remove the volatile component c) melting the calcined mixture d) disrupting the melt to form fibres.

Alternatively, the present invention provides a method of forming fibres comprising SrO, A1₂0₃, and a fibre-forming additive, the method comprising the steps of:- a) mixing: - i) a compound of strontium which on heating loses a predominantly non-toxic volatile component ii) A1₂0_3> and iii) a fibre forming additive b) forming granules from the mixture of a sufficient size to resist ejection from the melt by outgassing c) melting the granules and extracting the predominantly non-toxic volatile component d) disrupting the melt to form fibres The first alternative has the advantages that:- a) no additional gas extraction is required at the melting step, evolution of gas taking place primarily or entirely on calcining b) a stable non-irritant intermediate (in the form of the calcined mixture) is produced which is easy to handle and/or transport c) it does not involve significant outgassing at the melting stage d) extraction of gas takes place at the calcining stage where it is easier to provide than at the melting stage but has the disadvantage that an additional processing step is required. The second alternative has the advantages that:- a) a stable non-irritant intermediate (in the form of the granulated mixture) is produced which is easy to handle and/or transport b) although there is significant outgassing at the melting stage the granulated product is sufficiently heavy to resist ejection from the melt c) the granules provide a continuous path between granules by which the gas can escape without causing explosive eruptions in the batch above the melt. but has the disadvantage that an additional processing step is required and gas extraction is required at the melting step. If no localised extraction is present then the amount of toxic volatile components in the predominantly non-toxic volatile component must be kept low to minimise worker exposure. For example, if no extraction is provided and SrC0₃ is used as the strontium compound it must have a low S0₃ content, e.g. of < 0.1%, preferably < 0.05%.

For both alternatives, the A1₂0₃ and/or the fibre forming additive or part thereof may be in the form of compounds with a volatile component that is lost on melting.

Advantageously the strontium is in the form of a carbonate.

The invention extends to the intermediate granulated products and to the products of the methods. The scope of the invention is made clear in the appended claims with reference to the following description and the drawing Fig.l, which is a graph of compositions comprising SrO, A1₂0₃ and Si0₂.

The compositions plotted in Fig.l include those described in WO96/04214. Such compositions may contain impurities and these should amount to less than 10%, preferably to less than 5%, of the composition so that the compositions comprise more than 90%, preferably more than 95% SrO-Al₂0₃-Si0₂. Advantageously the compositions contain less than 3wt%, more advantageously less than 2wt% and still more advantageously less than lwt% incidental impurities. The compositions were tested for shrinkage and the results, normalised to 100% SrO-Al₂0₃-Si0₂, are plotted on the three co-ordinate graph of Fig.1. Contours are drawn around the compositions tested as having shrinkage of less than 3.5% at 1260°C, 1300°C, 1400°C, 1450°C, 1500°C, and 1550°C respectively.

When made on a laboratory scale manufacture was easy. However when made in pre-production test quantities of 25kg or more evolution of S0₃ originating from the traces present in the strontium carbonate supplied would have led to rapid departure of workers from the laboratory were it not for good extraction facilities in the laboratory. Additionally evolution of carbon dioxide from the strontium carbonate led to rapid and explosive departure of powder from the melting chamber.

Provision of high capacity gas extraction in a commercial fibre forming plant would add considerably to the cost and complexity of the plant. Loss of powder from the melting chamber would not only create a mess in a commercial fibre forming plant it would also make it difficult to form fibres of a consistent quality.

When strontium carbonate, alumina, and silica were granulated or calcined as mentioned above the problem of explosive evolution of gas was solved. Calcining also solved the problem of S0₃ evolution. Once a pool of molten material is formed it is possible to add un-calcined or un- granulated material without the problem of explosive evolution of gas. Accordingly one method of producing fibre includes the step of providing an initial charge of calcined or granulated material, melting these materials to form a melt pool, and adding un-calcined or un-granulated material to the melt pool at a sufficiently low rate not to produce explosive evolution of gas.

A prefeπed range of compositions specified in WO96/04214 was that the fibres comprise at least 90%, preferably at least 95%, by weight SrO, A1₂0₃, and a fibre forming additive, and had a composition comprising: - SrO 41.2wt% - 63.8wt%

A1₂0₃ 29.9wt% - 53.1 wt%.

The applicant's cuπently prefeπed range is that formed by the >1400°C contour of Fig.l and in particular the composition:-

SrO 58 ± 0.5 wt%

A1₂0₃ 30 ± 0.5 wt%

Si0₂ 12 ± 0.5 wt%

incidental impurities < 3wt%, preferably less than 2wt%, more preferably less than lwt%.

which shows a good compromise between formability (the Si0₂ giving ease of manufacture) and high temperature performance.

Claims

1. A method of forming fibres comprising SrO, A1₂0₃, and a fibre foπning additive, the method comprising the steps of:- a) mixing: - i) a compound of strontium which on heating loses a volatile component ii) A1₂0_3> and iii) a fibre forming additive b) calcining the mixture to remove the volatile component c) melting the calcined mixture d) disrupting the melt to form fibres.

2. A method of forming fibres comprising SrO, A1₂0₃, and a fibre forming additive, the method comprising the steps of:- a) mixing:- i) a compound of strontium which on heating loses a predominantly non-toxic volatile component ii) Al₂θ_3> and iii) a fibre forming additive b) forming granules from the mixture of a sufficient size to resist ejection from a melt by outgassing c) melting the granules and extracting the predominantly non-toxic volatile component d) disrupting the melt to form fibres

3. A method according to 1 or claim 2 in which the A1₂0₃ and/or the fibre forming additive or part thereof is in the form of compounds with a volatile component that is lost on melting.

4. A method according to any preceding claim in which the compound of strontium is a carbonate.

5. A method according to any preceding claim in which subsequent to melting step c) additional materials are added comprising a mixture of:- i) a compound of strontium which on heating loses a volatile component ii) A1₂0_3> and iii) a fibre forrning additive

6. A method of producing granules for use in the method of claim 2, comprising the steps of:- a) mixing:- i) a compound of strontium which on heating loses a predominantly non-toxic volatile component ii) A1₂0_3> and iii) a fibre forming additive b) forming granules from the mixture of a sufficient size to resist ejection from a melt by outgassing

7. The method of claim 6 in which the A1₂0₃ and/or the fibre forming additive or part thereof is in the form of compounds with a volatile component that is lost on melting

8. The method of any of claims 6 to 7 in which the compound of strontium is a carbonate.

9. The method of any preceding claim in which the fibre is needled to form blanket.

10. The method of any preceding claim in which the fibres comprise at least 90% by weight SrO, A1₂0₃, and a fibre forming additive, and have a composition comprising:- SrO 41.2wt% - 63.8wt%

A1₂0₃ 29.9wt% - 53.1wt%

11. The method of any preceding claim in which the fibres comprise at least 90% by weight SrO, A1₂0₃, and Si0₂ and have a composition falling within the 1400°C contour of Fig. 1.

12. The method of claim 1 1 in which the fibres fall within the 1450°C contour of Fig.l.

13. The method of claim 12 in which the fibres fall within the 1500°C contour of Fig.l.

14. The method of claim 13 in which the fibres fall within the 1550°C contour of Fig.l.

15. The method of claim 11 in which the fibre has the composition: SrO 58 ± 0.5 wt%

A1₂0₃ 30 ± 0.5 wt%

Si0₂ 12 ± 0.5 wt%

with incidental impurities amounting to less than 3wt%.

16. The method of claim 15 in which incidental impurities amount to less than 2wt%.

17. The method of claim 16 in which incidental impurities amount to less than lwt%.