Cleaning and liquid contact with solids – Processes – Including melting
Reexamination Certificate
1999-12-02
2002-06-18
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Including melting
C134S019000, C134S025100, C134S025400
Reexamination Certificate
active
06406552
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns a method for cleaning the SiO
2
grain, by means of heating the SiO
2
grain comprising contaminations to a temperature at which the contaminations soften or form melting agglomerates with the SiO
2
grain, and the separation of contaminations and SiO
2
grain. Furthermore, the invention concerns a device for the implementation of the method.
DISCUSSION OF PRIOR ART
DE-C1 33 21 589 describes a generic method for the cleaning of quartz sand and a device for the implementation of the method. For the separation of mineral contaminations in quartz sand, present for example as intergrowth of feldspar or garnet with quartz grains, it is proposed to heat a screen fraction of the quartz sand, sized 180 &mgr;m to 250 &mgr;m, to a temperature of 1,370° C. in an electrically heated rotary furnace with an SiC rotary tube. Due to this heat treatment performed over a period of 30 minutes, the contaminations soften so as to form melting agglomerates with each other or with quartz grains. In contrast, the quartz grains themselves do not soften so that they will essentially retain their original size and morphology. After cooling down, the melting agglomerates are screened out or separated from the purified quartz sand by means of air sifting.
This method enables a batch-wise cleaning of quartz sand from contaminations, under the prerequisites that the contaminations will bind in melting agglomerates and that the melting agglomerates are larger than the employed screen fraction of the quartz grain. It has been shown, however, that these prerequisites are not always met and that, moreover, the melting agglomerates are mechanically unstable and easily disintegrate again during the separation process and thus cannot be easily removed from quartz sand by means of screening or air sifting.
For many applications of SiO
2
powder—for example as the starting material for quartz glass components to be used in semiconductor manufacture or for optics, the starting materials' purity is subject to extremely high requirements which can be met by the known method only with great expenditures of time, materials and costs. To avoid any contaminations due to abrasion during the cleaning process, high-purity, partly high-temperature resistant, expensive device components are required—such as rotary tubes made of SiC, for example.
Some of these disadvantages are avoided by a cleaning method suitable for the continuous cleaning of quartz powder, as described in EP-A1 737 653. The quartz powder to be cleaned, with a mean grain size between 106 &mgr;m and 250 &mgr;m, is continuously fed to an electrically heated rotary furnace of quartz glass in which it runs successively through a preheating chamber, a reaction chamber and a gas desorption chamber. In the preheating chamber, the quartz powder is heated to approx. 800° C., and subsequently treated in the reaction chamber at a temperature of about 1,300° C. with a gas mixture of chlorine and hydrogen chloride. The quartz powder's alkali and alkaline earth contaminations will react with the chloric gas mixture, forming gaseous metal chlorides. The treatment gas and the gaseous reaction products are subsequently exhausted.
In this manner, especially such contaminations can be removed which can pass over into the gas phase through hot chlorination. The known method thus achieves a significant reduction of alkali and alkaline earth contaminations in the quartz powder. However, the method is unsuitable for contaminations which cannot be removed by chlorination. Moreover, the degree of purification depends on the quartz powder's reaction period with the chloric gas mixture and on the reaction temperature. At higher temperatures, chlorine reacts faster with the metallic contaminations so that a better cleaning effect could be expected with increasing temperatures. However, the softened grain tends to form agglomerates which impedes free access of the treatment gas to the individual grains' surface and thus reduces the cleaning effect of the treatment gas.
SUMMARY OF THE INVENTION
This invention is accordingly based on the task of specifying a method for the cleaning of SiO
2
grain which achieves high grain purity at comparatively little expenditure of time, material and costs, and of providing a simple device suitable for the implementation of the method.
In view of the method, this task is solved on the basis of the initially described cleaning method according to the invention such that SiO
2
grain is fed to and heated in a gas stream which is directed towards an impingement plate, the SiO
2
grain being accelerated in the direction of the impingement plate such that softened contaminations or melting agglomerates adhere to the impingement plate and cleaned SiO
2
grain is removed from the impingement plate.
The SiO
2
grain can consist of natural crystalline quartz or of quartz glass grain which in turn may be made of natural quartz or of synthetic starting materials. The contaminations of the SiO
2
grain may be either mineral substances such as usually found in natural quartz or substances which were imported into the grain during preparation of the raw materials or in the course of further processing, for example due to abrasion.
SiO
2
grain of crystalline quartz melts at approx. 1700° C. whereas no defined melting point can be assigned to amorphous SiO
2
grain, but much rather a gradual viscosity decrease is observed with increasing temperature. The melting points of purely mineral contaminations or of metallic abrasions—such as steel for example—are usually at temperatures of under 1500° C. Due to mixing or alloying of the contaminations with substances from the SiO
2
grain, the melting or softening temperatures may be even somewhat lower.
The SiO
2
grain comprising contaminations is fed to the gas stream and heated therein to a temperature at which the contaminations will soften which is understood to also include melting, or where the contaminations with SiO
2
grain form softened melting agglomerates. Hereinafter, the molten or softened contaminations and the melting agglomerates comprising contaminations will be called “softened contamination particles”. By means of the gas stream, the grain including the softened contamination particles are thrown onto the impingement plate. Since, in the method according to the invention, the grain is heated and fed in the gas stream without contact to the furnace walls, no sticking of the grain to the furnace walls need be expected; and other marginal conditions for the cleaning processes are inapplicable, such as the abrasion resistance or temperature stability of a furnace material. The grain can thus be heated to very high temperatures at which even those contaminations soften, melt up or form melting agglomerates which cannot be removed by the generic method due to their high melting or softening temperatures.
The separation of contaminations from the remaining grain is due to the softened contamination particles or at least part thereof adhering to the impingement plate; however, no pure SiO
2
grain will do so, or very little thereof. Decisive for the degree of separation shall be difference between the adhesive capacities of softened contamination particles on the one hand and pure SiO
2
grain on the other hand at the impingement plate. The respective adhesive capacities in turn essentially depend on the viscosity immediately before the impingement plate. Best adhesive capacity can generally be expected in a viscosity range of doughy consistency. Ideally, all softened contamination particles will adhere to the impingement plate, but no pure SiO
2
grain. Thus, the SiO
2
grain is not or only slightly softened in the area of the impingement plate.
After impacting on the impingement plate, the non-adhesive SiO
2
grain will be removed from there. The simplest manner is by the force of gravity whereby the SiO
2
grain drops to the bottom at a right angle to the impingement plate. However, the impingement plate may also be inclined with regard to th
El-Arini Zeinab
Heraeus Quarzglas GmbH & Co. KG
Tiajoloff Andrew L.
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