Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2001-01-05
2004-06-08
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S340000
Reexamination Certificate
active
06746655
ABSTRACT:
This invention concerns a method for cleaning the SiO
2
particles, by heating a fill of the particles in a reactor with a vertically oriented center axis and thereby exposing it to a treatment gas which is conducted at a given flow velocity from the bottom to the top through the reactor and the fill.
Furthermore, the invention concerns a device for the implementation of the method according to the invention, comprising a reactor with a vertically oriented center axis for accepting a fill of SiO
2
particles, with a gas inlet for feeding a treatment gas into an area of the reactor essentially below the fill, and with a gas outlet for discharging the treatment gas from an area of the reactor above the fill.
Moreover, the invention concerns cleaned SiO
2
grain from naturally occurring raw material.
From SiO
2
particles, quartz glass products are molten for the chemical and optical industry and for the manufacture of optical fibers and semiconductors. There are high requirements on the purity of the quartz glass products. Especially alkaline metals, alkaline earth metals, heavy metals, iron, carbon and free or combined water may have a detrimental effect on the desired properties of the quartz glass products. Correspondingly high are thus the purity requirements for the raw materials of quartz glass products. Quartz glass raw materials within the meaning of this invention are amorphous or crystalline particles, for example SiO
2
particles of naturally occurring quartz, or contaminated synthetically produced grains, granulates, or recycling material.
A method for the continued cleaning of quartz powder by means of thermochlorination has been described in EP-A1 737 653. It has been suggested therein to continuously feed the quartz powder to be cleaned, having a mean grain size ranging between 106 &mgr;m and 250 &mgr;m, into an electrically heated quartz glass rotary furnace in which it will pass in succession 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 is subsequently treated in the reaction chamber at a temperature of about 1,300° C. with a gas mixture of chlorine and hydrochloric gas. The alkaline and alkaline earth contaminations of the quartz powder here react with the chloric gas mixture, forming gaseous metal chlorides. The treatment gas and the gaseous reaction products will subsequently be exhausted.
The known cleaning method leads to a significant reduction of alkaline and alkaline earth contaminations in the quartz powder. The purity of the quartz powder can be yet improved even further by repeated passage of the cleaning process. However, in many quartz powder applications—such as for example as a starting material for quartz glass components in the use of semiconductor manufacture or for optics—the purity of the starting materials will be subject to extremely high requirements which cannot be achieved with the known method or only under great expenditure of time, material and cost.
With the known method, the cleaning effect depends on the reaction period of the quartz powder 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 would have to be expected with increasing temperatures. However, at high temperatures, agglomerates will form due to the softening of the grain which will interfere with further access of the treatment gas to the surface of the individual grains. The cleaning effect by the treatment gas which primarily acts on the grain surface will thus be reduced. Furthermore, the cleaning effect depends on the dwell period of the quartz powder in the reaction chamber. Coarse grain powder usually passes the reaction chamber faster than fine grain powder. Thus, different purities may result which may even be different within one charge, depending on the temperature, the grain fraction or throughput. This complicates the reproducibility of the known cleaning method.
In the cleaning method according to DD-PS 144 868, fluid quartz sand Is continuously charged from above into a vertically oriented reactor. The quartz sand fill passes the reactor continuously from top to bottom. The quartz sand fill therein passes a heating up zone, a thermochlorination zone and a cooling down zone. To avoid oxygen from penetrating into the chlorination zone and thus to prevent the reformation of the chlorides formed during chlorination into metal oxides, an inert gas or nitrogen gas curtain is produced at the entrance area and at the exit area of the chlorination zone.
Wet chemical treatments are also customary for removal of contaminations on the surface of naturally occurring quartz sand. In such a method as is described for example in U.S. Pat. No. 4,983,370, the quartz sand is pretreated—prior to the cleaning process by thermochlorination—first by means of a two-stage flotation process, a magnetic separator and a subsequent caustic treatment in hydrofluoric acid.
A cleaning method and a device for the implementation of the method in accordance with the aforementioned species are known from EP-A 1 709 340. Therein is suggested to continuously feed SiO
2
powder, manufactured by means of flame hydrolysis, to a vertically oriented reactor to remove chlorine, and to treat the powder fill in a countercurrent with a gas mixture of water vapor and air which is conducted through the reactor from the bottom to the top. In the area of the fill, the gas mixture has a linear gas velocity ranging between 1 and 10 cm/s and a temperature ranging between 250° C. and 600° C. The gas flow will flow through the fill forming a so-called “fluidized bed”, and the fill will be slightly raised.
It has been shown that the degree of purity of the SiO
2
particles such as it is required for use in semiconductor and optical fiber manufacture cannot be achieved by means of the known methods. Especially the contaminations with the chemical elements Li, Na, K, Mg, Cu, Fe, Ni, Cr, Mn, V, Ba, Pb, C, B and Zr cannot be sufficiently removed by means of the known methods.
This invention is accordingly based on the task of providing an improved method for cleaning the SiO
2
particles and of also providing a suitable simple device therefor; as well as of specifying SiO
2
grain from naturally occurring raw material and cleaned by using the method according to the invention, such grain being particularly suitable especially for the manufacture of semi-finished or finished quartz glass products for semiconductor and optical fibers production.
In view of the method, this task is solved in accordance with the invention such that a chlorous treatment gas is being used that—in the area of the fill—is adjusted to a treatment temperature of at least 1,000° C. and to a flow velocity of at least 10 cm/s.
A chlorous treatment gas is used. Many of the contaminations contained in SiO
2
particles react with the treatment gas at temperatures of above 1,000° C. to form gaseous metal chlorides or other volatile compounds which can be exhausted from the reactor via the exhaust gas. Aside from a chlorous component, the treatment gas may contain additionally other components—such as fluorine, iodine or bromine, inert gases or hydrogen for example—which are specially suitable for the removal of specific contaminations or for adjustment of specific properties of the SiO
2
or for the heat transfer between treatment gas and particles. For economic reasons, free chlorine is undesirable in the treatment gas so that the chlorous component will contain chlorine in a combined but reactive form.
The treatment gas will be passed through the fill at a flow velocity of at least 10 cm/s. This will ensure that gaseous compounds of the contaminations will be removed as fast as possible from the particles and discharged from the reactor. Moreover, due to the fast gas exchange, the SiO
2
particles are continuously and fast provided with unspent treatment gas so that the rate of chemical reaction between treatment gas
Becker Jörg
Nowak Joachim
Heraeus Quarzglas GmbH & Co. KG
Johnson Edward M.
Silverman Stanley S.
Tiajoloff Andrew L.
Tiajoloff & Kelly
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