VITREOUS SILICA

Abstract: A vitreous silica feedstock material as a substitute for quartz crystal powder is made by creating an amorphous silica powder of a BET-surface area (J. Amer. Chem. Soc. 1938, 60, 309) of at least 10 m<2>g<-1> and subsequently reducing the BET-surface area by a factor of at least 10. The amorphous starting material can, for example, be fume silica or produced by hydrolysis of silicon tetrachloride with water and the reduction in BET-surface area can be carried out by heating to high temperature (e.g. above 1100 DEG C) in air or an atmosphere containing oxygen, halogen, helium or hydrogen.

Description: Vitreous silica This invention relates to an improved method of fabricating a vitreous silica article by fusing a vitreous silica powder and to a method of preparing a feedstock material to substitute for quartz crystal powder in a fusion process to manufacture a vitreous silica article. Articles of vitreous silica, rods, tubes, vessels and other fabricated items, are widely used in modern industry. The majority 5' of these are made by fusion of natural quartz crystal powders. However, with increasing emphasis being laid on product quality, there is a frequent requirement for a purity of vitreous silica which cannot readily be obtained using natural quartz crystal as feedstock, even though such crystal feedstock may have been sub¬ jected to a complex refinement process. The conventional answer to this problem is to use so-called synthetic vitreous silica, which is made on an industrial scale by a vapour deposition process, for example the deposition of a vitreous silica soot formed by vapour phase hydrolysis of silicon tetrachloride to form a porous "preform" , which is then subjected to a dehydration process, followed by a high temperature vitrification step, whereby the pores are removed, and a fully densified glass is manufactured. An alter¬ native large scale route to synthetic vitreous silica involves the vapour phase hydrolysis or oxidation of silicon tetrachloride in a flame or plasma, and direct deposition of synthetic silica glass from the flame or plasma onto a substrate, typically silica. In this way substantial ingots of synthetic vitreous silica can be built up in a single stage process. These methods have several disadvantages. The range of geometrical shapes efficiently produced in the deposition stage is restricted, and the deposition rate is itself limited by the need to ensure that an acceptable proportion of the silica formed in the flame or plasma is deposited on the substrate. Methods of making synthetic silica articles by alter¬ native routes have been proposed. Many workers have endeavoured to make such articles by casting a gel, pre¬ pared by hydrolysis of an organo-silicate ester, and careful dehydration and firing of the cast gel. A serious disadvantage of such routes is the lengthy time required for dehydration of the gel if cracking of the cast gel is to be avoided. It is furthermore not yet possible to make large articles by this method. Alternatively, ceramic technology has been used to make sintered silica shapes by pressing a silica powder made by hydrolysis of silicon tetrachloride with water into the required shape and sintering and subsequently glazing the shaped article. This route to silica articles appears more economical, but suffers from several defic¬ iencies. Easy handling of the silica powder is critical when such powders are required to be of maximum purity, but the prior art hydrolysis reaction leads to a silica gel product which is difficult to handle, and on dehydra- tion gives a powder of high surface area and low density, impossible to fabricate into high quality synthetic vitr¬ eous silica articles using conventional equipment and techniques. The use of synthetic silica powders, manu¬ factured by hydrolysis of silicon chemicals, has thus not yet proved to be a viable route for the production of synthetic vitreous silica articles. The manufacture of fused quartz or vitreous silica articles from natural crystalline feedstocks is, in con¬ trast, well known. Thus quartz crystal powders, either natural or synthetic, may be fused in a flame, for example an oxy-hydrogen flame and thus a vitreous silica form may be built-up. Fusion may also be effected in an elec¬ trically heated gas flow, or plasma, provided by an elec- trie arc, by one or more arc plasma torches, or by an induction plasma torch, operating for example at radio frequency. Alternatively, quartz crystal powders may be fused within an electrically heated container, at atmospheric or reduced pressure, and advantageously in the presence of hydrogen, helium or a mixture thereof. The present invention seeks to combine the advantages of using an established prior art fusion technique, but replacing the quartz crystal powder with a synthetic vitreous silica powder of high purity and uniform consist- ency. Synthetic silica powders may be made in many differ¬ ent forms, with a wide range of chemical purities. In general, however, it is required that each metallic impur¬ ity be present in an amount less than 10 ppm by weight, and preferably less than 1 ppm by weight. The preferred feedstocks for the manufacture of such powders are there¬ fore chemicals which may be readily purified, and silicon halides and halosilanes, and organo-silicate esters are thus of particular interest. Silicon tetrachloride, and trichlorosilane are preferred raw materials, being readily available • at high purity as by-products in the manufacture of electronic grades of silicon. These raw materials may be economically converted to fine powder amorphous silica by combustion in an oxy-hydrogen or similar flame, or by heating in a plasma- heated oxygen flow. Alternatively, silicon tetrachloride may be hydrolysed with water to form on drying and removal of hydrochloric acid, an amorphous silica gel powder. These amosphous silica powders, however, are not suitable for use in a conventional fusion process, because they have very high surface area and consist of agglomer¬ ates of many ultrafine silica particles. As a result, if subjected to a conventional fusion process, they evolve a large amount of gas, and result in a product contamin¬ ated with many small bubbles. By the method of the present invention, a high surface area amorphous intermediate powder may serve as a precursor powder and be converted to a low surface area amorphous powder, which may be used in place of quartz crystal powder in a conventional fusion process. . According to one aspect of the invention, a method of fabricating a vitreous silica article by fusing an amorphous silica powder is characterised in that prior to fusing to form the article, the BET-surface area of the powder is not above 5 m 2g-1 and has been reduced by a factor of at least 10 relative to that of an amorphous silica precursor powder. Suitably the reduction in the BET-surface area is at least 50 times and can be more than 500 times. Desirably the BET-surface area of the powder after 2 -1 surface area reduction is not above 3 m g and preferably is not above 1 m g The particle size may be tailored to suit the specific fusion process envisaged but conveniently at least 90% w/w of the amorphous silica powder subjected to fusing consists of particles of size (i.e. agglomerate size) in excess of 50 micrometres, preferably in excess of 100 micrometres and typically between 125 and 600 micrometres, i.e. such particles are retained on a sieve of mean aper¬ ture 125 micrometres, but pass through a sieve of mean aperture 600 micrometres. Throughout this specification the term "BET-surface area" should be taken to mean the surface area as measured by gas absorption, based on the technique of Brunauer, Emmett, and Teller described in the J. Amer. Chem. Soc. 1938, βifj, 309. According to a further aspect of the invention, a method of preparing a feedstock material to substitute for quartz crystal powder in a fusion process for the manufacture of a vitreous silica article comprises creating an amorphous silica precursor powder of BET-surface area of at least 10 m 2g-1 by precipitation from a silicon-con- taining vapour or solution, and subsequently reducing the BET-surface area of the precursor powder by at least a factor of ten. Desirably the surface area reduction is effected in an aqueous phase treatment followed by drying and, if necessary, grinding to a suitable grain size, followed by heating to sinter the individual particles in the granu¬ lar agglomerate. Alternatively in certain circumstances, a dry agglomeration process, such as heating the dry high surface area silica powder, may be used to increase grain size and decrease the surface area of the product. The present invention thus comprises taking a high purity amorphous silica powder of high surface area, de- rived for example from liquid or vapour phase hydrolysis of a silicon halide, by oxidation in the vapour phase, or by condensation of a high temperature silica-containing vapour, and converting such a powder to a more easily handled low surface area vitreous silica grain suitable for fusion in a conventional process. The process of surface area reduction typically in¬ cludes heating the intermediate powder in an appropriate furnace, preferably in a gas atmosphere controlled with respect to composition and/or pressure. The composition of the gas may be chosen to ' enhance the chemical purity of the powder being treated by removal of volatile contamin¬ ants and/or to control the nature of the process of surface area reduction or sintering. Thus an atmosphere containing chlorine or chlorinating species may advantageously be used to effect a purificiation of the powder in the course of heat treatment, by combination with hydroxyl (OH) groups on the surface or within the silica lattice, also by remov¬ al of contaminating metal ions by formation of volatile chlorides. Addition of other halogen species may also help in this process. Again an atmosphere of helium, hydrogen, or a mixture thereof may be employed, and under these circumstances, both gases being soluble in vitreous silica, the risk of entrapment of insoluble gas during the sintering process is greatly reduced, and in conse¬ quence the tendency to form a bubbly product on fusion is also minimised. A mixture of helium and halogen-con¬ taining gas may be used simultaneously to effect controlled sintering, and purification of the powder. It may further be advantageous to operate at reduced pressure where it is required to sinter the powder with minimum take-up of gas. At least 90% w/w of the substitute feedstock material, i.e. synthetic vitreous silica grain, should consist of particles of a size not less than 50 micrometres, prefer¬ ably greater than 100 micrometres and ideally between 125-600 micrometres. The substitute feedstock material may arise from one of the above processes with an accept¬ able particle size distribution, but if not, the size can be adjusted either by comminution, if oversize, or by an agglomeration process if undersize. If a comminution process is used, it is desirably selected so as to minimise contamination in the crushing and seiving operations. Thus grinding media made from high purity silica may be advantageously used. Several forms of high temperature furnace can be used, including a static bed furnace, a shaft kiln and a rotary kiln. A gas-sealed rotary kiln appears particu¬ larly advantageous in that it permits, inter alia, short residence times, uniform treatment temperature and continu- ous operation. While in some situations it is envisaged that the powder will be converted to a free-flowing grain before delivery to the high temperature process for surface area reduction, in another situation the high surface area precursor, e.g. silica fume, may be fed in a raw state (i.e. as amorphous fume, not free flowing grain) into a rotary kiln when, under suitable conditions, a process of dry agglomeration may be effected and the pro¬ duct emerges as roughly spherical sintered agglomerates of size and density which may be controlled by varying, for example, the treatment time, the temperature and the atmosphere. Other methods of reducing the BET-surface area of the precursor powder may also be employed. One which has proved promising is the process of spray drying and calcination, whereby a gel or suspension of the precursor powder may be atomised, and dehydrated, and even partially or fully calcined in flight. The invention will now be further described with the following Examples: Example 1 An amorphous silica powder was obtained by hydrolysis of high purity silicon tetrachloride with high purity water, followed by dehydration and removal of hydrochloric acid by heating at temperatures up to 300°C. The dried gel was crushed in a ball mill made from synthetic vitreous silica with synthetic vitreous silica balls and sieved to separate the fraction between 100 and 1000 micrometres. The separated fraction had a BET-surface area of 750 m 2g-1 The separated fractiφff thus prepared was placed in a gas purged vessel made from fused quartz, which was heated within an electrically heated muffle furnace. The temperature of the furnace was raised to 800°C over 90 minutes, while oxygen was passed over the gel powder. After 3 hours the temperature was further increased to 1100°C at a heating rate 2°C/min, and held at 1100°C for between 1 - 19 hours. The resultant vitreous silica powder had a BET-surface area < 1 m 2g-1, and was used to replace quartz crystal powder in a variety of known fusion processes, and readily fused to produce bubble- free glass of high purity. Suitable processes include oxy-hydrogen flame fusion, electric arc or plasma fusion, fusion within an electrically heated vessel under reduced pressure, or under a controlled pressure of hydrogen, helium or a mixture thereof. Example 2 A granular hydrated gel powder prepared as in Example 1, had a BET-surface area of 800 m 2g—1, and particle size range 100 - 1000 micrometres. The powder was heated to 800 C C in a flow of oxygen (180 cm 3min—1) over a period of 1.5 hours. The oxygen flow was replaced with helium (180 cm 3min-1) mixed with chlorine (180 cm3min-1 ), and the temperature was raised to 1000 C C over a period of 1.5 hours. The flow of chlorine was stopped, the powder was heated for 1 hour at 1100°C under helium (180 cm 3min-1). Finally, to remove dissolved gases, the powder was heated in air for 16 hours at 1220°C. Densification as a result of pore collapse was achieved, and the water content of the powder was substantially reduced. The product powder had a BET-surface area of < 1 m 2g-1 and could be used to replace quartz crystal powder in a var¬ iety of quartz fusion processes. When fused in an elec¬ tric arc furnace, the product glass was low in bubble content, and had very low hydroxyl (OH). Example 3 Fume silica, particle size 0.01 micrometres and BET-surface area of approximately 175 m 2g—1 was mixed 3 with high purity water in proportions of 500 cm water to 200 g fume. The suspension gelled on standing and was dried at 100 β C. The solid, friable mass thus produced was comminuted and sieved to size fraction range 100 - 500 micrometres - using high purity silica grinding media. The low density granular powder produced had a BET-surface area of approximately 160 m 2g-1 and could be sintered using the process described in Example 1 or sintered and substantially dehydrated using the process described in Example 2, to give a product powder of BET- ι -1 surface area < 1 m g Example 4 A granular powder produced substantially as in Example 3, had a BET-surface area of 207 m 2g-1 and a grain size in the range 100 to 1000 micrometres. The powder was heated in oxygen flow (180 cc/min) to 800°C at a heating rate of 8.9°C/min. The oxygen flow was 3 -1 replaced by a mixture of He (180 cm min ) and chlorine (180 cm 3 min -1 ) and the temperature was held at 800°C for a period of 1 hour. The supply of chlorine was then stopped and with pure helium as the flowing gas the tem- perature was raised to 1100°C and held at that temperature for 1 hour. Finally, the supply of helium was stopped and the powder was heated in a flow of air to 1150°C and held at that temperature for 17 hours to yield a 2 -1 granular powder of BET-surface area < 1.0 m g . Example 5 A granular powder produced as in Example 3, was heated in helium flow at a heating rate of 8.9°C/min to a temperature of 900 G C. Chlorine gas at a flow rate of 180 cm 3 min-1 was added to the helium flow and the temperature was held at 900°C for a period of 2 hours. The mixed He/Cl- flow was replaced by a flow of pure oxygen and further heating to 1100°C was carried out 3 -1 in the O- flow (at a rate of 180 cm min ). The temper¬ ature of 1100°C was held for 1 hour. Final sintering in air was carried out at 1200°C for 1 hour and produced a powder with a BET-surface area < 1.0 m 2g-1. The above process yielded a powder having contamination by individ¬ ual metallic elements of ' generally less than 1 ppm and contamination by individual alkali metals of less than 0.1 ppm. Example 6 55 gm of fume silica of primary particle size 0.01 3 micrometres, a bulk density of 0.05 to 0.1 g/cm and a BET-surface area of 200 m 2g-1 was heated in air to 1200"C at a heating rate of 5°C/min in a rotary furnace, and held for 1 hour at 1200°C. Free flowing powder in the form of roughly spherical granules of sizes in the 5 mm was produced having a BET-surface Example 7 The process described in Example 6 was carried out with the air replaced by a dehydrating and purifying atmosphere (of mixed He and Cl^ ) at a temperature in the range of 900 to 1200°C. A high purity powder with a 2 -1 BET-surface area of less than 1 m g was produced with metallic contamination < 1 ppm and low hydroxyl (OH) content. Example 8 Fume silica particles of size 0.01 micrometres and BET-surface area of 175 m 2g—1 was blended with water in proportions 750 cm water to 250 g fume, to yield a moist crumb-like material. This material was fed di¬ rectly into a rotary furnace at 600°C to yield a granu- lated powder with grains sized in the range 0.1 to 3 mm. Direct granulation eliminates necessity for crushing and grinding of the dried granular gel, and such powder can be further processed as in Examples 4, 5 and 6, to yield a suitable high purity feedstock. The product powder from each of Examples 4 to 8 may be used to replace quartz crystal powder in any of a variety of known fusion processes.

Claims: CLAIMS 1. A method of fabricating a vitreous silica article by fusing an amorphous silica powder characterised in that prior to fusing to form the article, the BET-surface area of the powder is not above 5 m 2g-1 and has been reduced by a factor of at least 10 relative to that of an amorphous silica precursor powder. 2. A method according to claim 1, characterised in that the BET-surface area has been reduced at least 50 times. 3. A method according to claim 1, characterised in that the BET-surface area of the powder after surface area reduction is not above 3 m 2g-1 and preferably is not above 1 m g 4. A method according to claim 3, characterised in that at least 90% w/w of the amorphous silica powder subjected to fusing has particles of size in excess of 50 micrometres. 5. A method of preparing a feedstock material to substitute for quartz crystal powder in a fusion process for the manufacture of a vitreous silica article which comprises creating an amorphous silica precursor powder of BET-surface area of at least 10 m 2g- 1 by precipitation from a silicon-containing material, and subsequently reducing the BET-surface area of the precursor powder by at least a factor of ten. 6. A method as claimed in claim 5, in which at least 90% w/w of the substitute feedstock material con¬ sists of particles of a size between 50 and 600 micro¬ metres. 7. A method as claimed in claim 5, in which the amorphous silica powder is processed at elevated tem¬ perature in an atmosphere containing a halogen. 8. A method as claimed in claim 5, in which the reduction in BET-surface area occurs in an atmosphere consisting of. a gas selected from at least one of chlorine, a chlorine-containing gas, hydrogen and helium. 9. A method as claimed in claim 5, characterised in that the amorphous silica powder is obtained by hydrolysis of silicon te 3 tra©hloride and the reduction in BET-surface area is effected by heating to a tempera¬ ture of at least 1100°C. 10. A method as claimed in claim 5, characterised in that the amorphous silica powder is fume silica and the reduction in BET-surface area includes a step of mixing the powder with water. 11. A method as claimed in claim 5, characterised in that the amorphous silica powder is fume silica and the reduction in BET-surface area includes heating to a temperature of at least 1100°C.

Inventors: HUSSAIN TASADUQ, SAYCE IAN GEORGE, HUSSAIN, TASADUQ, SAYCE, IAN, GEORGE
Applicants: TSL GROUP PLC
Application year: 1987
Main classification: C03C1
IPC codes: C03B19, C03C1