WO2011147866A1 - Method for producing quartz glass granules - Google Patents

Abstract

In a known method for producing quartz glass granules by sintering a bed of pourable SiO₂ granulated material made of porous granulated material particles, the bed is heated and vitrified, forming quartz glass particles. In order to make it possible, proceeding from porous SiO₂ granulated material, to continuously and cost-effectively produce transparent synthetic quartz glass granules having a low hydroxyl group content, according to the invention the vitrification of the porous granulated material particles is carried out in a vacuum or in a helium-containing atmosphere using at least one laser beam, in that the granulated material bed is produced on a carrier and is continuously moved by means of or on said carrier relative to the laser beam during vitrification, wherein the granulated material particles have a mean grain size of between 200 and 1000 μm and the laser beam is directed with glancing incidence at the granulated material bed.

Description

 Process for the preparation of quartz glass grains Description

The present invention relates to a method for the production of quartz glass grain by sintering a bed of free-flowing SiO ₂ granules of porous granules by heating them and vitrifying them to form quartz glass particles. Synthetic SiO ₂ granules consisting of porous granulate particles are obtained by agglomeration and densification of amorphous SiO ₂ powder, as obtained, for example, in the production of synthetic quartz glass in precipitation reactions or in CVD precipitation processes as soot or filter dust. Since the immediate utilization of the soot dust by melting is problematic because of the low bulk density, this is usually precompressed by conventional construction or pressing granulation process. Examples which may be mentioned are roll granulation in a granulating dish, spray granulation, centrifugal atomization, fluidized-bed granulation, granulation processes using a granulating mill, compaction, roll pressing, briquetting, blank production or extrusion.

The resulting discrete, mechanically and optionally also thermally pre-compressed particles are thus composed of a multiplicity of primary particles and are referred to herein as "SiO ₂ granulate particles." In their entirety they form the porous "SiO ₂ granulate". Such porous "SiO ₂ granulate" is easier to handle, but for

Production of transparent, synthetic quartz glass only partially suitable. Because when melting the "SiO ₂ granulate" there is the danger that closed, gas-filled cavities form that are not or only very slowly removed from the high-viscosity quartz glass mass and thus lead to bubbles in the quartz glass, which is why it is for demanding applications in It is usually necessary to produce amorphous transparent quartz glass particles from the porous granules.

State of the art

A generic method for vitrifying synthetic SiO ₂ granules is known from EP 1 076 043. It is proposed to throw porous SiO ₂ granules into a fuel gas flame in order to finely distribute it therein and to vitrify at temperatures of 2000 to 2500 ° C. The granules are preferably obtained by spray or wet granulation of filter dust and has particle sizes in the range of 5 to 300 μιτι. Before vitrification, it can be heated and precompressed by treatment with microwave radiation.

In the fuel gas flame, however, especially large particles are not sufficiently heated, which makes sintering into transparent quartz glass particles difficult. There is also the danger that the quartz glass particles are contaminated by the combustion gases. In particular, a loading with hydroxyl groups when using hydrogen-containing fuel gases is to be mentioned, which is accompanied by a comparatively low viscosity of the quartz glass.

In EP 1 088 789 A2, for the vitrification of porous SiO ₂ granules it is proposed to first purify the synthetically produced granules by heating in an HCl-containing atmosphere, then to calcine them in a fluidized bed and then to glaze under hydrogen or under vacuum or helium.

This is a discontinuous vitrification process with low throughput and a result in the relatively expensive granules.

In the process described in US Pat. No. 5,604,163 A, an SiO ₂ gel is prepared by the polymerization of tetramethoxysilane by the sol-gel method and dried rapidly in vacuo, so that it breaks up to form SiO ₂ granules. The granules are then filled into a sintered container made of quartz glass and sintered in batches in an electrically heated furnace. For this - - The granules are heated at a heating rate of 200 ° C / h to 1 150 ° C and held at this temperature for 35 hours.

Again, it is a discontinuous vitrification process, accompanied by a large thermal inertia of the furnace and consequently long process times with correspondingly high time and cost. In addition, agglomerations of the sintering granulate particles can occur in the furnace, leading to an undefined, porous quartz glass mass.

US 2004/0237588 A1 describes a method for vitrifying a SiO ₂ green body, which is obtained by a slip casting method, by irradiation with a laser under vacuum or helium.

From WO 00/44679 A1 a method for laser Verg lase fine aggregated nanoparticles (aerosils) is known, which are generated for example by flame hydrolysis and are glazed directly after their formation by means of laser.

Technical Problem The invention is therefore based on the object of specifying a method which, starting from porous SiO ₂ granules, enables a continuous and inexpensive production of transparent synthetic quartz glass grains having a low hydroxyl group content.

General Description of the Invention This object is achieved on the basis of a method of the type mentioned above in that the vitrification of the porous granules takes place under vacuum or in a helium-containing atmosphere by means of at least one laser beam by producing the granulate bed on a support and by means of or is moved continuously on this during vitrification relative to the laser beam, wherein the granules have a mean particle size between 200 and 1000 μιτι and the laser beam is directed in grazing incidence on the granulate bed. By vitrification of the bed of porous SiO ₂ granules under vacuum or in a helium-containing atmosphere, a bubble-free or low-bubble melting of the porous granule particles can be achieved. Optionally trapped gases consist of at least 50 vol .-% of helium, with proportions of hydrogen up to 50 vol .-%, which can also easily diffuse out in the further processing of vitrified quartz glass grain, and small amounts of other gases (up to about 5 vol. %) are harmless.

For heating the granulate particles, a laser beam or several laser beams are used. The at least one laser beam is focused on the granulate bed. This results in a locally high temperature on the granules without these being influenced mechanically (by blowing) or chemically (by impurities) by a fuel gas. When heated, the granulate thus does not dodge and it is not loaded with any impurities of a fuel gas. Because of the local heating by means of laser beam, however, only the irradiated bed area is heated and vitrified, so that the granules in the meantime repeatedly cool, which on the one hand reduces sintering together with adjacent particles and caked, and on the other hand contributes to a low thermal inertia of the process. By continuously supplying granules to the laser beam, the method is also continuously carried out.

The relative movement between bed and laser beam can be done by continuous movement of the laser, optical deflection of the laser beam such as an oscillating reciprocation. According to the invention, however, a movement of the bed of granules is provided by the

Produced granulate bed on a support and is continuously moved by means of or on this during vitrification relative to the laser beam.

The carrier is, for example, a pipe, a crucible, a gutter, a plate or a disk. The movement of the bedding is generated by continuously moving the support or granulate bed, such as - - by tumbling, shaking, spinning or stirring. It is essential that the granules are moved relative to the laser beam and the granulation while the net weight of the bed is subjected. The resulting mechanical action contributes to the separation of agglomerated granules. When grazing incidence the laser radiation hits at a more acute angle than 90 degrees on the surface of the granulate bed. The angle of incidence between the laser beam and the bed is preferably less than 50 degrees. As a result, a larger surface area of the bed is heated and glazed compared to the vertical incidence. The fact that relatively small granules are used with an average particle size between 200 and 1000 μιτι, sufficient action of the laser radiation and as uniform as possible glazing is achieved. Granulate particles having a particle size of more than 1000 μιτι can be glazed only slowly by means of the selectively acting laser beam. Particles with a mean particle size of less than 200 μιτι tend to agglomerations.

Has proven particularly useful a procedure in which the movement of the granulate bed is carried out in a rotary tube rotating about a central axis.

The rotary tube is slightly inclined in the longitudinal direction, in order to bring about a transport of the granules from its inlet side to the outlet side. The granulate bed is continuously circulated in the rotary kiln. In the course of this process, grain which is in contact with the rotary tube wall is continuously transported to the rotary tube center on the upper side of the bed in the laser beam and heated and glazed. From there, the wholly or partially glazed grain passes into the interior of the bed or under the bed as it is circulated, and cools down rapidly, while at the same time being exposed to mechanical forces which are exerted by the bed

Weight and the circulation of the bedding are generated. Any agglomerates of the glazed grain are dissolved again.

The granules outside the action of the laser beam and the inner wall of the rotary tube remain relatively cool. The low thermal load on the rotary kiln wall results in a low removal and entry - - of impurities in the granules. The comparatively cooler surface of unirradiated granule particles reduces the risk of sticking and caking.

The rotary tube is preferably arranged in a vacuum chamber, so that there are no difficulties in sealing the inlet and outlet ends of the rotary tube.

For a uniform as possible vitrification of the granules about the same residence times in the area of the incident surface of the laser beam or the laser beams on the bed surface and about the same particle sizes are advantageous. In view of this, it has been found to be useful if the granules have a narrow particle size distribution at which the particle diameter assigned to the D ₉₀ value is at most twice as large as the particle diameter assigned to the D ₀ value.

A narrow particle size distribution shows a comparatively low bulk density, which counteracts agglomeration during desalination. In addition, in the ideally monomodal size distribution of the granules, the weight difference between the particles as a parameter for any separation within the bed, which promotes more uniform vitrification of the bed, is eliminated. It has proven to be advantageous to preheat the granules before glazing by means of a heater to a temperature of 800 ° C to 1200 ° C.

By preheating to a temperature in said range, the granules are not or not appreciably vitrified; However, it results in a faster and energetically favorable vitrification when exposed to the laser beam.

The preheating is preferably carried out in an externally heated chamber into which the laser beam is directed for vitrifying the granules. The preheating of the granules takes place in the same Kannnner as the vitrification, so for example a rotary kiln. The chamber is maintained, for example, by electrical heating at a temperature which produces a temperature in the granular bed in the said range.

Preferably, the chamber has an inner wall made of quartz glass.

Especially at high temperatures, the quartz glass particles can be contaminated by abrasion of the material of the chamber wall. This risk of contamination is counteracted by a chamber inner wall 10 made of quartz glass.

With regard to a fast compaction and a rapid glazing process, a method variant has proven in which the laser beam has an energy density sufficient to heat them when hitting a temperature of at least 1400 ° C.

Furthermore, it has proved to be favorable when the laser beam is generated by means of a CO ₂ laser and at the point of impact on the bed has a focal spot with a diameter of at least 30 mm and a surface power of at least 40 W / cm ² .

By means of CO ₂ laser energy-rich laser radiation with a wavelength of 20 10 m can be generated, which is less strongly absorbed in the transparent, glazed quartz glass than by porous quartz glass. By choosing a working radiation with a wavelength of about 10 μιτι is thus achieved that the laser energy is preferably introduced into the not and not completely vitrified areas of the granular bed.

25 embodiment

The invention will be explained in more detail with reference to an embodiment and a drawing. The sole figure shows a schematic representation - -

1 shows a rotary kiln for carrying out the method according to the invention in a side view.

FIG. 1 shows a vacuum chamber 1 with a connection 2 to a vacuum pump, within which a rotary kiln 3 is mounted on rollers 4. At the outlet end 5 of the rotary kiln 3, a discharge housing 6 is arranged. The inner wall of the rotary kiln 3 is lined with a quartz glass tube. On the outer jacket, a resistance heater 10 is provided, by means of which a bed of 1 1 of SiO ₂ granules can be heated to a temperature of 1 100 ° C. On the inlet side 7 of the rotary kiln 3 several CO ₂ laser 8 (type TLF 3000 Turbo) are arranged with a maximum beam power of 3 kW in the focus. The primary beam is expanded in each case by means of an expanding optics to laser beams 12a, 12b, which are directed in the middle of the rotary tube 3 in grazing incidence on the granular bed 1 1, each with an oval focal spot with a diameter of 20 mm and a maximum area performance of about 50 W / cm ² . The angles of incidence between the surface of the granulate bed 1 1 and the respective laser beam 12a; 12b differ slightly and are between 20 and 30 degrees.

In the following, an exemplary embodiment of the method according to the invention will be described in more detail with reference to the apparatus shown schematically in FIG.

The evacuated (200 mbar) and rotating about its axis of rotation at 8 rev / min rotary kiln 3 is continuously supplied with a feed rate of 15 kg / h amorphous, porous SiO ₂ granules. The granules are obtained by granulation of pyrogenically produced, nanoscale SiO ₂ powder and consists essentially of porous, spherical granules 13 having a particle size distribution with a D 10 value of 300 μm, a D90 value of 500 mm and an average particle Diameter (D50 value) of 400 μιτι. - -

The rotary tube 3 is inclined in the longitudinal direction in the specific angle of repose of the granules 13, so that over its length a uniform thickness of the granulate bed 1 1 sets.

By means of the resistance heater 10, the granulate 13 is heated within the rotary kiln 3 to a temperature around 1 100 ° C, wherein the open porosity remains. The vitrification of the granules 13 takes place when the laser beams 12a, 12b strike.

The preheated granulate bed 1 1 is continuously circulated and thereby to the rotary kiln center in the region of the laser beams 12 a; 12b and glazed. Due to the locally acting laser beams 12a, 12b comparatively few granules are softened. The temperature of the surrounding granules 13 hardly increases. In this way, caking between the granules 13 with each other are largely avoided. If agglomerates nevertheless occur, they are redissolved as a result of the mechanical stress in the moving granulate bed 11. Granulate particles 13 which come into contract with the wall of the rotary tube are also relatively cool (not hotter than 1100 ° C.), so that no adhesion to the quartz glass lining occurs.

The particles which are completely glazed on the quartz glass particles are continuously removed by means of the discharge device 6.

Claims

claims Process for producing quartz glass grain by sintering a bed (1 1) of free-flowing SiO ₂ granules of porous granules (13) by heating them and vitrifying them to form quartz glass particles, characterized in that the vitrification of the porous granules (13) under Vacuum or in an atmosphere containing helium by means of at least one laser beam (12a, 12b) by the granulate bed (1 1) generated on a support and by means of or on this during glazing relative to the laser beam (12a, 12b) is moved continuously, wherein the granules (13) have a mean particle size between 200 and 1000 μιτι and the laser beam (12a, 12b) is directed in grazing incidence on the granulate bed (1 1). A method according to claim 1, characterized in that the movement of the granulate bed (1 1) takes place in a rotating about a central axis rotary tube (3). A method according to claim 2, characterized in that the rotary tube (3) in a vacuum chamber (1) is arranged. Method according to one of the preceding claims, characterized in that the granules (13) have a narrow particle size distribution, wherein the D ₉₀ value associated with the particle diameter is at most twice as large as the D ₀ value associated particle diameter. Method according to one of the preceding claims, characterized in that the granulate particles (13) are preheated by a heater to a temperature of 800 ° C to 1200 ° C before vitrification. 6. The method according to claim 5, characterized in that the preheating takes place in an externally heated chamber into which the laser beam (12a, 12b) is directed for vitrifying the granules (13). 7. The method according to claim 6, characterized in that the chamber has an existing of quartz glass inner wall. 8. The method according to any one of the preceding claims, characterized in that the laser beam (12a, 12b) has an energy density sufficient to heat when hitting the granules (13) to a temperature of at least 1400 ° C. 9. The method according to claim 8, characterized in that the laser beam (12a, 12b) by means of a CO ₂ laser (8) is generated and at the point of impact on the bed (1 1) a focal spot with a diameter of at least 30 mm and a Area performance of at least 40 W / cm ² has.