Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1999-03-15
2001-01-30
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S335000, C423S241000, C423S24000R, C423S499100
Reexamination Certificate
active
06180077
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the purification of SiO
2
granulate by supplying the SiO
2
granulate into a reaction zone where the granulate is heated and exposed to a treatment gas which contains a halogen-containing component, whereby the treatment gas reacts with contaminants in the SiO
2
granulate under formation of volatile reaction products which are removed from the reaction zone. The invention furthermore relates to an apparatus for the implementation of the said process.
FIELD OF THE INVENTION
A process of this kind, for a continuous purification of quartz powder, and an apparatus for the implementation of the process, are described in EP A1 737,653. It is proposed there to continuously supply the quartz powder to be cleaned, which has an average grain size of 106 &mgr;m to 250 &mgr;m, into an electrically heated revolving cylindrical kiln of quartz glass, where the powder successively passes through a preheat chamber, a reaction chamber and a gas desorption chamber. In the preheat chamber the quartz powder is heated to a temperature of about 800° C. and is then treated in a reaction chamber at a temperature of about 1,300° C. by a mixture of chlorine and hydrogen chloride gas. Alkali and alkaline earth contaminants in the quartz glass powder react with the chlorine-containing gas mixture to form gaseous metal chlorides. The treatment gas and the gaseous reaction products are then evacuated.
The known purification process results in a marked reduction of alkali and alkaline earth in the quartz powder. Purity of the quartz powder can be improved even more by multiple passes through the purification process. However, in many of the quartz powder uses, for example as starting material for quartz glass components in semiconductor manufacture or in optics, there are particularly high demands regarding purity of the starting material which in the known process can only be achieved with great expenditure of time, material and money.
In the known process, the purification effect is dependent on the duration of the reaction between the quartz powder and the chlorine-containing gas mixture, and on the temperature of the reaction. Chlorine reacts faster with the metallic contaminants at higher temperatures so that a better purification would be expected as the temperature increases. However, due to the softening of the granulate at high temperatures, agglomerates are formed which restrict further access of the treatment gas to the surface of the individual granules. This therefore reduces the purification effect of the treatment gas which primarily acts on a granule's surface. In addition, the quartz glass rotary kiln is not designed for treatment temperatures above about 1,400° C. Furthermore, the purification effect depends on how long the quartz powder remains in the reaction chamber. Coarse powder usually passes through the reaction chamber faster than fine powder. This can result in varying degrees of purity which can differ even within a single charge, depending on temperature, grain size, or throughput rate. This makes the known purification process difficult to reproduce.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a process for the purification of SiO
2
granulate in which a high purity of the granulate can be achieved in a reproducible manner while incurring relatively low expenditures in time, material and money, and to provide a simple apparatus suitable for implementation of the said process.
As far as concerns the process, the object is achieved on the basis of the purification process described above in that the purification gas comprises a fuel gas with a hydrogen-containing component which is burned in the reaction zone, forming a fuel gas flame to which the SiO
2
granulate is exposed.
In the process according to the invention, the SiO
2
granulate is exposed to a fuel gas flame, is heated therein, and is at the same time purified.
The purification is effected by the halogen-containing component of the treatment gas. Halogen-containing components may be fluoride, chlorine, bromium, iodine, gaseous chemical compounds of these elements and mixtures of these elements and compounds. The halogen-containing component is supplied into the reaction zone together with the fuel gas, or separately.
The granulate is heated by the reaction heat during the combustion of the hydrogen-containing component of the fuel gas. What is meant here by combustion is any form of rapid exothermic oxidation of the hydrogen-containing component which makes heating of the granulate possible. The hydrogen-containing components may be for example hydrogen or hydrocarbon compounds such as propane or acetylene. Oxygen, oxygen compounds, halogens and halogen compounds may be used for example as combustion co-reactants for the combustion of the hydrogen-containing component.
As regards the adjustment of temperature, the fuel gas flame is more flexible than the rotary kiln used in the known purification process. In particular, higher temperatures may be achieved with the fuel gas flame and the fuel gas flame permits rapid temperature changes. High temperatures are favorable for the reaction of the halogen-containing component with contaminants, especially with alkaline contaminants such as potassium or lithium. The SiO
2
granulate can be exposed to rapid temperature changes in the fuel gas flame and thus be heated instantly. Bursting of the SiO
2
granules is possible in such event, primarily bursting of granules with internal tension caused by prior damage and structural defects. Foreign atoms are frequently the cause of such defects. Bursting causes the foreign atoms to reach the free surface where they can be easily removed.
In addition to an increase of treatment gas reactivity due to high temperatures, short-lived or reactive atoms can be created in the fuel gas flame, such as for example halogen radicals which are distinguished by a particularly effective purification action.
The granulate can be supplied to the reactive zone continuously, so that the process according to the invention is also suitable for continuous purification. The SiO
2
granulate may be present as crystalline quartz granulate or as amorphous quartz glass granulate. The SiO
2
granulate is exposed to the fuel gas flame and heated by it. The heating usually takes place in the hottest, visible area of the fuel gas flame; however, the location of the heating is not decisive for the process according to the invention, of substance is merely that the fuel gas flame act upon the SiO
2
granulate which is heated and at the same time purified.
An application of the process has been shown to be particularly favorable where the fuel gas comprises the halogen-containing component, whereby the fuel gas flame is formed by an exothermic reaction between the hydrogen-containing component and the halogen-containing component.
The SiO
2
granulate is heated and at the same time purified by the fuel gas flame in this method as well. Oxidation of the hydrogen-containing component results from an exothermic reaction with the halogen-containing component. The halogen-containing component is supplied directly into the fuel gas flame where the component can develop its greatest purifying effect due to the relatively high flame temperature. Because of the high temperatures the fuel gas flame promotes creation of halogen radicals which, due to their high reactivity, can have a positive effect on the purification of the SiO
2
granulate.
A fuel gas has been shown to be particularly advantageous in which gas the halogen-containing component is present in a stoichiometric surplus in relation to the hydrogen-containing component. The stoichiometric surplus relates to the reaction products of halogen and hydrogen. An excess of halogen favorably affects formation of metal halides versus metal oxides. Insofar as metal oxides are already present as contaminants in the granulate, the halogen can replace the oxygen or it can prevent further oxidation. An advantage of this met
Griffin Steven P.
Heraeus Quarzglas GmbH & Co. KG
Ildebrando Christina
Tiajoloff Andrew L.
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