Sintered, high-purity granular material containing silicon dioxide

The invention relates to a process for producing a sintered, high-purity, granular material containing silicon dioxide and the granular material itself.

Innumerous methods for producing granular silicon dioxide materials from amorphous silicon dioxide are known. Suitable starting materials can be silicon dioxide prepared by sol-gel processes, precipitated silica or a pyrogenic silicon dioxide. The production process usually encompasses agglomeration of the silicon dioxide. This can be effected by means of wet granulation. In wet granulation, a sol is produced from a colloidal silicon dioxide dispersion by continual mixing or stirring and a gel is produced therefrom by gradual removal of the moisture, and the gel can subsequently be sintered. Production by means of wet granulation is complicated, particularly when a low level of impurities in the granular material is demanded.

It is also possible to obtain the sintered silicon dioxide by compaction of silicon dioxide. Compaction, without binders, of pyrogenic silicon dioxide is difficult because pyrogenic silicon dioxide is very dry and there are no capillary forces which can bring about bonding of the particles. Pyrogenic silicon dioxides have extremely fine particles, a low bulk density, a high specific surface area, very high purity, a very largely spherical primary particle shape and no pores. The pyrogenic silicon dioxide frequently has a high surface charge which makes agglomeration difficult because of electrostatic repulsion.

The compaction of pyrogenic silicon dioxide has therefore not been a practicable route to the production of high- quality sintered products to now.

The use of pyrogenic silicon dioxide is disclosed, for

example, in DE-A-10050434. There, high-purity granular silicon dioxide material is produced by spray drying of AEROSIL ^® 380, from Degussa. This granular material can be classified by means of known methods such as sieving, air sifting or air classification. However, it is also stated in DE-A-10050343 that the classification step leads to impurities, in particular metallic impurities, being able to be introduced into the silicon dioxide and these are often difficult to remove, for example by means of strong acids, and can often not be removed to a sufficient extent.

US4042361 discloses a process for producing fused silica in which pyrogenic silicon dioxide is used. This is incorporated into water to form a castable dispersion, the water is subsequently removed thermally, the solid residue is calcined at from 1150 to 1500 ⁰ C and subsequently ground to produce 1-100 μm granules and vitrified. The purity of the fused silica produced in this way is not satisfactory for today's applications. The production process is complicated and expensive.

WO91/13040 also discloses a process in which pyrogenic silicon dioxide is used for producing fused silica. The process comprises the preparation of an aqueous dispersion of pyrogenic silicon dioxide having a solids content of from about 5 to about 55% by weight; conversion of the aqueous dispersion into porous particles by drying the aqueous solution at a temperature of from about 100 to about 200 °C in an oven and comminuting the dried porous particles; and subsequent sintering of the porous particles at temperatures below about 1200 ⁰ C in an atmosphere having a water partial pressure in the range from 0.2 to

0.8 atmospheres. High-purity fused silica granules having a diameter of from about 3 to 1000 μm are obtained.

EP-A-1717202 discloses a process for producing granular fused silica material by sintering a pyrogenic silicon dioxide which has been compacted to tamped densities of

from 150 to 800 g/1 by a particular process. This process, disclosed in DE-A-19601415 comprises spray drying of silicon dioxide dispersed in water and subsequent heat treatment at from 150 to 1100 ⁰ C. The granular material obtained in this way can be sintered but does not give fully bubble-free fused silica granules.

EP-A-1258456 discloses a process for producing a monolithic glass moulding, in which a silicon alkoxide is hydrolysed and a pyrogenic silicon dioxide powder is subsequently added to form a sol, the sol is subsequently converted into a gel, the gel is dried and subsequently sintered.

EP-A-1283195 likewise discloses sol-gel processes in which silicon alkoxides and pyrogenic silicon dioxide powder are used.

In principle, the processes known from the prior art all follow a scheme in which a silicon alkoxide is firstly hydrolysed, a silicon dioxide powder is produced with formation of a sol which is converted into a gel, the gel is subsequently dried and then sintered. The process encompasses a number of steps, is time-consuming and sensitive to process fluctuations.

It was an object of the present invention to provide a process for producing a granular fused silica material which can be carried out simply and inexpensively compared to the prior art and produces a bubble-free granular fused silica material on sintering.

The invention provides a process for producing a sintered granular material containing silicon dioxide and having a BET surface area of less than 1 m ² /g and a proportion of impurities of less than 50 ppm, characterized in that a mixture which contains silicon dioxide powder and a metal compound is intensively mixed in an atmosphere having a

relative atmospheric humidity of from 10 to 100%, preferably from 40 to 80%, at temperatures of from 0 to 50 ⁰ C by means of a dispersing apparatus, the crumbly mass is divided into pieces, subsequently dried, purified and sintered, where the silicon dioxide powder comprises particles which bear hydroxyl groups bound to silicon atoms, SiOH, on their surface, . has a moisture content of from 0.1 to 10% by weight, based on the silicon dioxide powder, and is present in a proportion, based on the sum of powder and metal compound, of from 40 to 95 mol%, the metal compound . is liquid under the reaction conditions and reacts on contact with water to form the corresponding metal oxide and a volatile compound and the moisture content of the silicon dioxide powder and/or the atmospheric humidity are/is at least sufficient to hydrolyse the metal compound completely, drying is carried out at temperatures of from 20 ⁰ C to

150 ⁰ C at a pressure of from 1000 to 10 mbar for a period of from 1 hour to 5 days, - purification is carried out at from 400 ⁰ C to 1100 ⁰ C in an atmosphere containing one or more reactive compounds and the material is subsequently sintered.

The crumbly mass can advantageously be broken up into pieces having a maximum dimension in one direction of from about 0.2 cm to 2 cm.

The silicon dioxide powder used in the process of the invention comprises particles which bear hydroxyl groups bound to silicon atoms on their surface. The silanol group concentration can preferably be from 0.2 mmol/g to

1 . 2 mmo 1 / g .

Furthermore, the silicon dioxide powder used in the process of the invention has a moisture content of from 0.1 to 10% by weight. The moisture content results from absorption of water from the air and is reversible. The silicon dioxide powder can preferably have a moisture content of from 0.5 to 5% by weight and particularly preferably from 1 to 2.5% by weight.

The process of the invention also allows doped silicon dioxide powders to be used. Here, the doping component is preferably selected from the group consisting of oxides of Ag, Al, B, Ce, Cs, Er, Ga, Ge, Li, K, Na, P, Pb, Ti, Ta, Tl and Zr. Particular preference is given to Al, B, Ti and Zr. The proportion of the doping component can preferably be from 1 ppm to 20% by weight and particularly preferably from 0.1 to 10% by weight. In general, silicon dioxide powders having one or two doping components are used.

Furthermore, silicon-metal mixed oxide powders can also be used. For the purposes of the invention, the silicon-metal mixed oxide powder is defined so that the metal oxide component is present in a proportion of more than 20% by weight. This can particularly preferably be silicon- aluminium mixed oxide powder having a proportion of aluminium oxide of from 50 to 80% by weight. In particular, a pyrogenic silicon dioxide powder can be used in the process of the invention. For the purposes of the present invention, "pyrogenic" means that the silicon dioxide powder is obtained by flame hydrolysis or flame oxidation. Here, a gaseous silicon compound, for example silicon tetrachloride, is reacted in a hydrogen/oxygen flame. Pyrogenic silicon dioxides can, depending on the reaction conditions, be present in the form of very largely aggregated primary particles through to very largely unaggregated particles. Pyrogenic silicon dioxides are very

largely free of pores. The pyrogenic silicon dioxide powder can also be in doped form. The BET surface area of the pyrogenic silicon dioxide powder can preferably be from 10 to 400 m ² /g. Particular preference is given to a pyrogenic silicon dioxide powder having a BET surface area of from 30 to 90 m ² /g.

Pyrogenic silicon dioxide powders have a high purity. Commercially available pyrogenic silicon dioxide powders, for example AEROSIL ^® from Degussa, can be used in the process of the invention. It is also possible to use specifically highly purified silicon dioxide powders such as AEROSIL ^® OX 50 EG in which trace elements such as Al, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Ni, Ti, Zr are present in individual amounts of less than 1 ppm or a total amount of less than 5 ppm.

In the process of the invention, use is additionally made of a metal compound which is liquid under the reaction conditions and reacts on contact with water to form the corresponding metal oxide and a volatile compound. For the present purposes, volatile means that at least 95% of the compound has vaporized within 24 hours at 25°C.

Preference is given to using metal compounds which have Al, B, Ce, Cs, Er, Ga, Ge, Pb, Si, Ti or Zr as metal component. For the purposes of the invention, the term metal compounds also encompasses metalloid compounds. Metal compounds containing Al, B, Si, Ti or Zr as metal component can be particularly preferred.

As metal compounds, preference is given to using metal alkoxides in which the alkoxide group is an aliphatic hydrocarbon radical having from 1 to 6 carbon atoms.

In principle, it can be advantageous to use metal compounds which have a limited reactivity towards water. Such compounds are known to those skilled in the art.

Particular preference is given to using tetraalkoxysilanes Si(OR) ₄ such as tetraethoxysilane (TEOS), tetra-n- propoxysilane, tetraisopropoxysilane, tetra-n-butoxysilane and/or precondensed silanes such as Dynasilan 40, from Degussa. Dynasilan 40 is a mixture of ethyl esters of various polysilicic acids. It is formed from TEOS in the presence of water by partial hydrolysis of the ethoxy groups to hydroxyl groups and subsequent condensation to form siloxane bonds.

In the process of the invention, a silicon dioxide powder and a metal compound are used. The molar proportion of silicon dioxide, based on the sum of silicon dioxide and metal compound, is preferably at least 50 mol% , particularly preferably at least 70 mol% and very particularly preferably at least 85 mol%.

In particular, it has been found to be useful to use a pyrogenic silicon dioxide powder having a BET surface area of from 30 to 90 m ² /g as silicon dioxide powder and TEOS as metal compound.

In the process of the invention, it is also possible for the mixture of silicon dioxide powder and metal compound to additionally contain one or more unreactive, volatile, organic solvents. Here, "volatile" means that at least 95% of the solvent has vaporized within 24 hours at 25°C. "Unreactive" means that the solvent is very largely inert towards silicon dioxide and the liquid metal compound.

As unreactive, volatile, organic solvent, it is possible to use Ci-C ₄ -alcohols or mixtures thereof comprising methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 2-butanol.

In particular, it has been found to be useful to use a pyrogenic silicon dioxide powder having a BET surface area of from 30 to 90 m ² /g as silicon dioxide powder, TEOS as metal compound and one or more Ci-C ₄ -alcohols, preferably

ethanol .

The proportions are preferably, in each case in mol% and based on the sum of silicon dioxide powder, TEOS and C1-C4- alcohols which add up to 100 mol%: - silicon dioxide powder from 20 to 80

- TEOS from 0.5 to 20

- Ci-C ₄ -alcohols from 10 to 65

The purification in the process of the invention is carried out at temperatures of from 400 ⁰ C to 1100 ⁰ C in an atmosphere containing one or more reactive compounds. For the present purposes, "reactive" compounds are compounds which are suitable for reducing the amount of hydroxyl groups and impurities.

Suitable reactive compounds can be chlorine, hydrochloric acid, sulphur halides and/or sulphur oxyhalides. They are usually used in a concentration of from 0.5 to 10% by volume in mixtures with air, oxygen, helium, nitrogen, argon and/or carbon dioxide.

The temperatures in the purification step and the sintering step depend mainly on the composition of the granular material and are known to those skilled in the art.

As dispersing apparatus, preference is given to using a mortar, a 2-roll mill, a 3-roll mill, a ball mill or a planetary kneader. Particular preference is given to 2-roll mills and 3-roll mills.

The invention further provides a sintered granular material containing silicon dioxide which can be obtained by the process of the invention.

The sintered granular material containing silicon dioxide can be, in particular, a granular fused silica material.

The sintered granular material containing silicon dioxide

has only a small proportion of impurities. Preference is given to a granular material having the following proportions of impurities, all in ppb : Al < 600, Ca < 300, Cr < 250, Cu ≤ lO, Fe < 800, K < 80, Li ≤ lO, Mg < 20, Mn ≤ 20, Na < 80, Ni < 800, Ti < 200, V < 5 and Zr < 80.

Particular preference is given to a granular material having the following proportions of impurities, all in ppb: Al < 350, Ca < 90, Cr < 40, Cu < 3, Fe < 200, K < 50, Li < 1, Mg < 10, Mn < 5, Na < 50, Ni < 80, Ti < 150, V < 1, Zr < 3.

The invention further provides for the use of the sintered granular material containing silicon dioxide for producing fused silicas, materials having very low coefficients of expansion (ultra low expansion (ULE) materials) , for photocatalytic applications, as superhydrophilic constituent of self-cleaning mirrors, for optical articles such as lenses, as seal for gases and liquids, as mechanical protective layer and for use in composites.

Examples

Example 1: 0.83 mol of AEROSIL ^® OX50 (moisture content:

1.5% by weight, SiOH concentration: 0.2 mmol/g) is kneaded together with 0.23 mmol of TEOS on a 3-roll mill for a period of 20 min in air having a relative humidity of 60%, the crumbly mass was broken up and subsequently dried at 20 ⁰ C for one day and at 105 ⁰ C for 3 hours. The mass is subsequently treated with thionyl chloride at a temperature of 500 ⁰ C and subsequently sintered in air at a temperature of 1250°C for 30 minutes.

Examples 2 to 15 are carried out analogously, using 2-propanol in place of ethanol in Examples 8 to 12 and using AEROSIL ^® 300 (moisture content: 1.5% by weight) in place of AEROSIL ^® OX50 in Examples 13 to 15. Table 1 shows all starting materials and amounts used.

Glass produced from the granular fused silica material displays extremely few, fine bubbles and differs in this way from glasses produced from commercially available granular fused materials: IOTA 6, from UNIMIN (a few large bubbles), MKC Silica (many bubbles) from Nippon Kasei.

The granular fused silica material of the invention has the following contents (all in ppb) : Al 521, Ca 165, Cr 47, Cu 3, Fe 147, K 44, Li 0.8, Mg 10, Mn 3, Na 68, Ni 113, Ti 132, V 0.5, Zr 3. To determine the metal content, the granular fused silica material is dissolved in a solution containing hydrofluoric acid. The silicon tetrafluoride formed vaporizes and the residue is analysed by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES). The accuracy is about 10%.

Table 1 : Starting materials and amounts

* SiO ₂ = AEROSIL® OX 50, Examples 1-12; AEROSIL® 300, Examples 13-15; **ROH = ethanol, Examples 1-7 and 13-15; 2-propanol Examples 8-12.