Chemistry of inorganic compounds – Silicon or compound thereof – Elemental silicon
Reexamination Certificate
2000-06-23
2002-05-28
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Elemental silicon
C423S348000, C423S350000
Reexamination Certificate
active
06395249
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a production process and apparatus for economical and efficient production of high purity silicon, for example, Si with a purity of 99.999% or higher. Si of such purity can be utilized as Si for solar batteries.
BACKGROUND ART
Solidifying purification may be mentioned as a general process for metal purification. In the case of Si, heat-melted Si is cooled to solidification during which time the impurity elements other than Si are concentrated and condensed in the last solidified portion, and this portion is removed to obtain the purified Si. This process is based on the fact that most impurity elements have a small segregation coefficient for Si, but since the segregation coefficient of boron or phosphorus is close to 1 they are therefore difficult to remove by this process; in practice, therefore, it is difficult to achieve a purification of 99.999% or higher from Si with a purity of about 99% that is easily obtained by solidifying purification alone.
A common production process for high purity Si is a process widely employed in industry known as the Siemens process, whereby high purity Si is obtained by utilizing chlorides of Si; however, it is mainly suited for semiconductor related purposes, and while the purity achieved is above 99.999999% which is much better than the 99,999% required for solar battery Si, the production cost is high rendering it unsuitable for solar battery production.
One example of an attempt at easier production of Si at a purity that can be utilized in solar batteries is the process disclosed in Japanese Unexamined Patent Publication SHO No. 63-79717, whereby SiO gas is emitted from silica stone and metallic silicon and the gas is reduced by carbon kept at 1600 to 2400° C. Another process found in U.S. Pat. No. 875,285 also reduces SiO with carbon, but neither of these processes deal with purification and no process is described for obtaining high purity Si. Reduction by carbon generally leaves carbon residue in the resulting Si since carbon is a solid, and it is therefore difficult to obtain high purity Si.
As a method of overcoming this problem, it has been considered that reducing SiO with a gas such as high purity hydrogen could minimize inclusion of impurities from the reducing agent. Below the melting point of SiO at about 1730° C., SiO can only take the form of a solid or a gas with a pressure below the saturation vapor pressure, and in general, reaction with a reducing agent for reduction of solid or vapor SiO to obtain Si does not proceed very readily. The following process has been proposed as a solution.
Japanese Unexamined Patent Publication SHO No. 62-123009 describes a process in which silicon tetrachloride, trichlorosilane, silane and a silicon alcoholate are heat decomposed or flame heat decomposed to produce a fine particle aggregate of silicon monoxide and/or silicon dioxide, and the fine particle aggregate is reduced in a reducing atmosphere at 200° C. or above to produce silicon. The size of the fine particles forming the fine particle aggregate is given as 10-100 nm, but since such fine particles are very highly reactive, they were thought to be susceptible to reduction. High purity silicon tetrachloride, trichlorosilane, silane and silicon alcoholate products are industrially produced and the resulting SiO fine particle aggregates are expected to be of high purity, so that reduction thereof gives high purity Si. However, silicon tetrachloride, trichlorosilane, silane and silicon alcoholate are costly, and therefore the final resulting Si is also costly.
One problem common to reduction processes occurs when the impurities in SiO become concentrated in the Si produced. The Si obtained by reduction of SiO is under 64 wt % of the SiO, and since the impurities in the SiO are not removed by high temperature reducing atmospheres, the impurities become concentrated in the Si which is under 64 wt % of the SiO, so that the resulting Si has a lower purity than the original SiO.
In U.S. Pat. No. 3,010,797 there is described a process in which silicon and silica are reacted to obtain SiO vapor which is reduced by hydrogen, and particularly a process in which it is reduced by hydrogen that has permeated through palladium or the like, or a process in which it is reduced by hydrogen in the copresence of it platinum. This process uses SiO obtained by reaction of silicon and silica, and both the silicon and silica starting materials are inexpensive and readily obtainable so that there is no problem with starting material cost. However, the following problems must be dealt with.
The first problem in U.S. Pat. No. 3,010,797 is that a large amount of hydrogen is necessary for hydrogen reduction of the SiO vapor obtained from the silicon and silica. While 90.5% of the total amount of Si contained in the SiO was obtained according to Example 1 of U.S. Pat. No. 3,010,797, the hydrogen required for this was 6 times the stoichiometric amount. When palladium is used in Example 3 of U.S. Pat. No. 3,010,797, hydrogen is required at 20 times the stoichiometric amount in order to obtain 86.5% of the total Si in the SiO. Since one mole of Si is about 28 g and one mole of hydrogen is about 22.4 L at room temperatures atmosphere, even if 100% of the Si in the SiO were obtained it would require 134-448 L of hydrogen to obtain about 28 g of Si by the reaction in this example. This is also clear from claim 1 of U.S. Pat. No. 3,010,797, where it is stated that an excess of hydrogen over the stoichiometric amount is necessary for reduction. This can be expected since most SiO is highly stable against reduction reaction, whether in gaseous or solid form. Except for specially synthesized SiO fine particle aggregates such as described in Japanese Unexamined Patent Publication SHO No. 62-123009 cited above, most SiO is stable and requires an excess of hydrogen for reduction by hydrogen to produce Si. From an industrial standpoint, a process that requires a few hundred liters of hydrogen to obtain 28 g of Si makes it difficult to achieve inexpensive production of Si, and hence improvement is desired in this aspect.
The second problem in U.S. Pat. No. 3,010,797 is that palladium or platinum is used for hydrogen reduction of SiO, as stated in claim 1 and throughout the specification. Use of these precious metals necessitates a more expensive reaction apparatus, while it is impossible to negate the risk of contamination, by these precious metals, of the resulting Si.
As described above, with processes for obtaining Si by reduction of SiO it is difficult to obtain high purity Si due to the inclusion of solid carbon when carbon is used as the reducing agent. Also, when a reducing gas is used as the reducing agent the reduction proceeds slowly and an excess of reducing gas is necessary. Another problem has been the required use of special costly SiO such as fine particle aggregates. Furthermore, the problem common to reduction processes is that the impurities contained in the SiO concentrate in the resulting Si, so that the resulting Si has a lower purity than the original SiO.
As another prior art finding that should be mentioned, U.S. Pat. No. 3,660,298 teaches that SiO vapor causes the disproportionation reaction: 2SiO→Si+SiO
2
at about 1800° C. According to the present invention there is provided, as will be explained below, a process whereby SiO
2
solid is produced with Si, and impurities are concentrated in the SiO
2
solid to increase the purity of the Si produced. However, the SiO
2
by-product at 1800° C. is liquid, and impurities will not concentrate in liquid SiO
2
so that the purity of the Si produced cannot be increased. According to the process of the present invention it is possible to reduce the boron, etc. that cannot be removed by the aforementioned solidifying purification, and this is highly useful.
Moreover, while the present invention allows higher purification even in the process of obtaining Si from SiO solid, as just explained, production of high purity Si is favored when the SiO of the pre
Kiyose Akihito
Kondo Jiro
Nogami Atsushi
Shimada Haruo
Tokumaru Shinji
Kenyon & Kenyon
Nguyen Ngoc-Yen
Nippon Steel Corporation
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