GB2124202A

GB2124202A - Aluminosilicate fibres

Info

Publication number: GB2124202A
Application number: GB08229846A
Authority: GB
Inventors: Masafumi Yamamoto
Original assignee: Isolite Babcock Refractories Co Ltd
Current assignee: Isolite Babcock Refractories Co Ltd
Priority date: 1982-07-23
Filing date: 1982-10-19
Publication date: 1984-02-15
Also published as: JPS5921567A; JPH0114182B2

Abstract

A refractory host insulating material having improved thermal shrinkage properties is produced by raising the temperature of amorphous aluminosilicate fibres formed on 35 to 65% by weight of Al2O3, 30 to 60% by weight of SiO2, 1.5 to 6% by weight of Cr2O3, the balance being Fe2O3 and other impurities, from room temperature to 950 DEG C to 1150 DEG C over a period, of not more than 10 minutes, retaining the heated amorphous fibres at the elevated temperature for 5 to 15 minutes, and subsequently rapidly cooling the hot amorphous fibres to room temperature.

Description

SPECIFICATION Process for production of refractory heat insulating material This invention relates to a process for the production of a refractory heat insulating material, and more particularly to a process for producing a refractory heat insulating material by subjecting amorphous fibres of a specific composition to a thermal treatment under prescribed conditions.

A refractory heat insulating material of amorphous alumino-silicate fibres which is produced by melting a raw material composed of 40 to 70% by weight of Al203, 30 to 60% by weight of SiO2, and a small amount of Fe203 and other impurities and converting the moltern material into fibres by blowing and a refractory heat insulating material which is produced by thermally treating the aforementioned refractory heat insulating material at a temperature exceeding the devitrifying temperature thereof thereby including mullite (3at203 2Si02) crystals in the vitreous texture thereof and lowering the thermal shrinkage thereof are known in the art.Such amorphous refractory heat insulating materials, however, have the disadvantage that when exposed to temperatures exceeding about 1 000 C for extended periods, mullite crystals grow in the vitreous texture with the result that the fibres become embrittled, so that the materials betray their deficiency in lasting resistance to heat and freedom from thermal shrinkage and give rise to problems during prolonged use at elevated temperatures. The refractory heat insulating material in which mullite crystals occur in the vitreous texture, enjoys improvement in thermal shrinkage only for a brief period. During prolonged standing the mullite crystals grow progressively. Thus, the problems arising in use remain and the material suffers from degradation of strength over a long period.

A refractory heat insulating material which contains about 1 to 6% by weight of chromium oxide is added to the aforementioned composition in also known in the art. Owing to the incorporation of chromium oxide, this material exhibits improved thermal shrinkage properties as compared with the basic material, but has the disadvantage that, it shrinks during the initial stages of use to a not insignificant extent and suffers a degradation in quality.

According to this invention we propose a process for the production of a refractory heat insulating material, comprising raising the temperature of amorphous fibres formed of 35 to 65% by weight of Awl203, 30 to 60% by weight of SiO2, 1.5 to 6% by weight of Cr2,03, and the balance being Fe203 and other impurities, from room temperature to 950 to 1150 over a period of not more than 10 minutes, retaining the heated amorphous fibres at the elevated temperature for a period of from 5 to 1 5 minutes, and subsequently rapidly cooling the hot amorphous fibres to room temperature.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is a diagram of X-ray diffraction curves showing the relationship between the change of chromium oxide content and the growth of mullite crystals; Figure 2 is a diagram of X-ray diffraction curves showing the relationship between the temperature of thermal treatment and the growth of mullite crystals; and Figure 3 is an electron photomicrograph showing, at 10,000 magnifications, an etched portion of a fibre obtained by the process of the present invention.

If the amount of Cr203 added is less than 1.5% by weight, crystallisation of mullite crystals occurs conspicuously as evidenced by the X-ray diffraction curves of Fig. 1, if the amount exceeds 6% by weight, the excess yields no further improvement in lowering the shrinkage due to prolonged heating. If the temperature of the thermal treatment is less than 950"C, no improvement in thermal shrinkage is obtained as will be shown below in the examples and, if the temperature exceeds 11 50'C, crystallisation of the mullite crystals begins to occur.

This is illustrated in the examples and the X-ray diffraction curves of Fig. 2 represents the test results obtained with a material containing 3.8% by weight of Cr203. Also the heat insulating material produced tends to be brittle and does not permit easy handling.

If the duration of the thermal treatment exceeds 1 5 minutes, the material exhibits a linear shrinkage ratio of at least, and never less than, 2% when it is heated for an extended period at a temperature exceeding 1 300 C and, if the duration is less than 5 minutes, the thermal treatment fails to produce an effect.

Heating of the amorphous fibres to the elevated temperature should be carried out in less than 10 minutes, otherwise when the material produced is heated for an extended period at a temperature exceeding 1300"C, it fails to retain a linear shrinkage ratio below 2% and, further its component fibres become so rigid as to render the handling thereof difficult. The rate of temperature increase, thereof, is preferably in the range of 100" to 1 000 C/minute.

EXAMPLE A raw material composed of 40% by weight of Awl203, 56% by weight of SiO2, 3.3% by weight of Cr203, and 0.7% by weight of impurities was melted. The molten material was converted by blowing into amorphous fibres having diameters of from 2.5 um and maximum length of 1 50 mm. Blankets 25 mm in thickness and 0.14 in specific gravity were produced from the amorphous fibres and thermally treated under the conditions shown in the following table. The products of the thermal treatment were heated at varying temperatures from 1100" to 1400 for 24 hours and at the end of the heating, were measured to determine linear shrinkage ratio.The results are shown in the table: Linear shrinkage ratio (%) by heating

5 EE O)OOV) Sample E ON w 0 D E F w O of d this o CDmg) O p,r of 100'C 5 minute 800'C 5 QR 1200'C 5 minute 1000'C 5 minute 1000'C 20 minute thermal treatment m --'C F.~ X o oc u o ~ increasing increasing increasing increasing increasing No thermal speed speed speed speed speed. treatment Heating P) 1OOO'Cjminute 1000'Cjminute 1000'C/minute 5O'C/minute e .= cr 0.2 1.7 0.3 0.4 0.1 1.7 o 0.7 2.9 0.1 0.9 1.5 2.9 a 1.0 3.8 0.5 2.1 2.1 3.8 O CLIB 1.4 4.5 0.8 2.3 4.8 4.5 The product showed The product The ) b signs of comminution O O O = E and Attribute of and degradation and showed signs of product was brittle and communication at E o a o 0 3.Eo to handle. elevated tempera C.C C 2 m E F gD oi X q c O c oO n c E : c E 0:L.c o c 0 ' m / N N q w en c 4- / c X X X X Fo ~ / a 0O O 0O 0O ~ U Q - / I o ~ N X q i ro It is noted from the table that the blanket which had undergone thermal treatment according to this inyenton exhibited a linear shrinkage ratio of not more than 2% and still retained flexibility after 24 hours heating at 1 300'C.

The outstanding results of the thermal treatment may be explained by a postulate that the incorporation of Cr203 and the rapid heating shortens the distance of diffusion of components in the vitreous texture, producing numerous minute separate phases of locally varying component ratios around Cr203 cores, and impeding the growth of crystals. When the fibres in which such separate phases have been produced are exposed to elevated temperatures while in actual service, the occurrence and growth of crystals are curbed since the separate phases destined to form cores of crystals are very small and numerous. Even the crystals which occur later are similarly small and numerous. Consequently, the fibres are stabilised by the numerous fine crystals, they are well able to withstand degradation and comminution and enjoy long service life at elevated temperatures. Fig. 3 is an electron photo-micrograph showning at 10,000 magnification, an etched portion of a fibre obtained by the treatment of the present invention. It is noted from the photograph that the fibre is entirely made up of minute separate phases. A fibre lacking separate phases and crystals would be thoroughly dissolved when subject to etching.

Claims

CLAIMS:

1. A process for the production of a refractory heat insulating material, comprising raising the temperature of amorphous fibres formed of 35 to 65% by weight of Awl203, 30 to 60% by weight of SiO2, 1.5 to 6% by weight of Cr203, and the balance being Fe203 and other impurities, from room temperature to 950e to 11 50'C over a period of not morethan 10 minutes, retaining the heated amorphous fibres at the elevated temperature for a period of from 5 to 1 5 minutes, and subsequently rapodly cooling the hot amorphous fibres to room temperature.

2. A process according to claim 1 wherein the rate of increase in temperature is in the range 100" to 1000"C/minute.

3. A process for the production of a refractory heat insulating material, substantially as hereinbefore described.