DD291445A7 - METHOD FOR PRODUCING SILKY GLASS FUEL GRANULATE - Google Patents

Abstract

The invention relates to a method of producing a silica glass high granule density granulate, which can be used after melting into silica glass in the semiconductor and light source industry. The aim of the invention is to develop a high-purity product with high Schuettdichte. It is an object of the invention to provide a production method of a melted granulate which, owing to its purity and high rutile density, melts to form a bubble-free or bubble-poor silica glass of high purity. According to the invention, lepidoid silicic acid is used as starting silicic acid, which is finely comminuted to a particle size of less than 60 micrometers and subsequently dewatered at 950 to 1200C. Subsequently, in a known manner, granulated, sintered and granulated or broken again in order to arrive at the desired silica glass melt granules.

Description

Field of application of the invention

The invention relates to a production method of a fused silica granulated high Rütteldichte. These granules can be melted to silica glass, which can be used advantageously in the semiconductor and light source industry.

Characteristic of the known technical solutions

For the production of silica usually find naturally occurring mineral quartz raw materials such. As rock crystal, quartz or quartz used, which are chemically cleaned after a corresponding mechanical comminution and then melted in the form of a melt granules of certain grain fractions according to known technological methods.

Of particular importance is the fact that the production of silica glass, z. B. in rotary vacuum melting equipment, raw materials makes necessary, which from the outset have a high specific grain density. Rock crystal granules having a shaking density of more than 1.2 g.cm ^{-3 are} required for these technologies.

However, the use of natural mineral quartz raw materials is problematic because of the availability, the sufficient purity and the bubble-free fusibility. Among other things, these raw materials contain impurities from compounds of the 3d elements, the elements of the 1st, 2nd and 3rd main group of the periodic table and have gas-liquid inclusions. A thermal and chemical treatment only partially eliminates the impurities contained in the quartz grain and gas-liquid inclusions and therefore rarely leads to a raw material that meets the requirements for the production of silica glass.

There has therefore been no lack of attempts to eliminate this disadvantage by producing high-purity synthetic SiO 2 products; For example, synthetic silica glasses of extremely high purity by a pyro-hydrolytic melting process of silicon halide compounds such. B. SiCI ₄ are produced. Also, methods are known, according to which over a hydrothermal mass crystallization melt granules (low-quartz) are prepared (DE-PS 1190443, DE-PS1052377, SU-PS 734146, DE-OS 3317327). Due to their high technological complexity, however, these methods have become important only for special purposes, since the silica glasses produced from them are energy-intensive and therefore very expensive. Amorphous silicas, as for example by hydrolysis of silicon halides such as SiCI ₄ and SiF ₄ (US Patent 2188121 and DE-PS 2447738) or alkali silicate solution by freezing (DE-PS 2840459, DD-PS 31910 and DE-AS 1269122) are prepared Although they have a high chemical purity, but can not melt directly to a low-bubble or bubble-free silica glass because of the structural water contained in them. Furthermore, they have a low bulk density of 0.05 to 0.5 g / cm ³ , which can be increased by subsequent annealing at 1000 ° C only by a factor of 1.7. (DD-PS 31910) For the production of silica glass from disperse-amorphous hydrolysis silica described sintering process to obtain higher-density products (DD-PS 136608) and (DD-PS 160961) can not apply to amorphous lepidoide silica, since this has a different structure and grain distribution and thus would lead to the required drainage and subsequent sintering only to products of low bulk density.

The aim of the invention is to avoid the disadvantages of the known methods.

Essence of the invention

The invention has for its object to provide a production process of a pure synthetic melt granules high Rütteldichte, which can be melted to a bubble-free or low-bubble fused silica high purity. According to the invention, the preparation of the silica glass is based on the known amorphous lepidoid silica, which has a high purity but whose chemical structure has not yet been clearly clarified. Surprisingly, it was found that when using a lipid silicic acid having a particle size below 60μπι, preferably 30pm, a product is formed, which dehydrate at 950 ⁰ C to 1 200 ⁰ C and by subsequent granulation, sintering at 1 200 ⁰ C to 1600 ⁰ C. and re-granulation or comminution can be converted into a melt granules having a high grain density and still has pores open to the grain boundary. These open microvoids make it possible for complete deaeration to take place during the subsequent fusing to the silica glass, and thus to obtain a very low-bubble or completely bubble-free silica glass. The lepidoide silicic acid used as the starting material is an X-ray amorphous silica which has a flaky structure and which is prepared by known processes, for. B. from technical sodium silicate solution (so-called water glass solution) is obtained by means of ion exchange resins on the sol state in a very pure form by freezing the silica sol. The particle size required according to the invention can be achieved by comminution, in particular grinding, of the lepidoid silica, preferably by wet grinding while avoiding possible impurities by the milling process. But it can also be achieved by freezing the silica sol in this grain size. After a wet grinding, the silica is separated from the mill water and dried at temperatures up to 200 ° C. The resulting powdery material is then thermally treated for complete removal of up to 16Ma .-% contained surface and structural water between 300 and 1150 ⁰ C for a long period of time and subsequently with the addition of a Granulierhilfsmittels, eg polyvinyl alcohol, by known granulation, z. As granulating, granulating or by sieve granulation to granules with a particle size of 0.05 to 2mm diameter mechanically compacted and dried.

The subsequent annealing and sintering treatment is carried out in a known manner at 1300 ⁰ C to 1500 ⁰ C, in which the granulation is removed.

The entry of the annealed granules into the sintering zone results in a sintering densification of the granules, which is essentially dependent on the selected sintering temperature and time.

In fact, in parallel with the sintering process, the conversion into cristobalite takes place, as a result of which the sintering temperature is shifted to considerably higher values and the sintering process then soon comes to a standstill at the set lower temperature. A sufficiently rapid increase in the temperature of the granules, with subsequent holding of the sintering temperature over a short period of time to grain densities of 1.9 to 2.3g · cm " ³ Such highly compressed granules are now mechanically extremely strong and have hollow channels open to the outside, on at Complete degassing takes place in the melting device during subsequent heating, thus permitting bladder-free or bubble-free melting of the granules The thermal process steps, which are carried out continuously in a continuous sintering furnace in a known manner, can also be carried out batchwise until sintering of the granules The product obtained after the sintering process, if it is not already present as granules, can be converted into a particulate material (granules) by known methods The particle size is preferably less than 1 mm, so that a packing density of more than 1.2 g · cm " ^{3 is} reached.

embodiment

The invention will be explained in more detail by way of example, which should not be limited to this example.

The lepidoide silica contaminated by foreign elements of 3ppm Na, 2 ppm Fe, 11 ppm Al, 1 ppm Ti, 1 ppm Ca, 2 ppm K and 0.5 ppm Mg is in a ball mill with grinding media in the mass ratio 1: 3, as well as a mass ratio regrind to dist. Water of 1: 1 ground until the entire ground material has reached a particle size smaller than 30 pm and has a mean grain diameter of 3μηι.

The ground silica is separated from the mill water via a paper filter in a Lakbornutsche by means of water jet pump and dried in a drying oven to constant weight at 175 ° C. Thereafter, the sample in a matte furnace in a muffle furnace for 4 hours at 300 ⁰ C, 8 hours at 850 ⁰ C, 24 hours at 1000 ⁰ C, 1 hour at 1050 ⁰ C and 2 hours at 1100 ⁰ C thermally treated. In order to eliminate the agglomerates which have taken place during the annealing treatment, pretreated lepidoide silica is homogenized in a ball mill for 100 minutes and then separated from the grinding bodies. After sieving through a plastic sieve (1.0 mm mesh size), the finely ground and homogenized lepidoide silica is then transferred in a sieve granulator after addition of a 1% solution of polyvinyl alcohol in a known manner into granules whose largest diameter is 1 mm.

The granules are now dried in a platinum crucible at 120 ° C for three hours, then reacted in a muffle furnace and then heated from room temperature to 145O ⁰ C and sintered for 75 minutes.

At an amorphous silica impurity level of 6 ppm total alkalis, the cristobalite content of the thermally compacted granules is greater than 99% and the grain density is 1.9 g.cm ^-3 . The mean grain diameter of the crushed and sized granules is 0.75 g and the bulk density is 1.25 g. cm " ³ . These granules can now melt into a bubble-poor or bubble-free silica glass.

Claims (8)

1. A process for the preparation of silica glass granules high bulk density by drying a starting silicic acid, granulation, sintering and regranulation, characterized in that is used as starting silicic acid lepidoide silica having a particle size below 60 microns and at about 950 to 1200 ° C. is drained. 2. The method according to item 1, characterized in that the mean grain size of the lepidoid silica is below 8μιη. 3. The method according to item 2, characterized in that the mean grain size of lepidoiden silica is ЗЗt. 4. The method according to item 1, characterized in that the drainage takes place over a period of at least 8 hours. 5. The method according to item 4, characterized in that the dehydration is preferably carried out within 8 to 12 hours. 6. The method according to any one of items 1 to 5, characterized in that the dehydration is carried out in stages, preferably with holding levels at 500.850 and 1100 ° C. 7. The method according to item!, Characterized in that the lepidoide silica is brought by grinding, in particular wet grinding, to the grain size below 60 microns. 8. The method according to item 1 to 7, characterized in that the dehydration is carried out until bubble-free meltability of the melt granules to the silica glass.