RU2327639C2 - Method of producing high purity silicon - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to obtaining a high pure silicon which can be used at production of solar elements. Pure silicon dioxide is melted at a temperature of 1900°C and a mixture of powders of pure silicon and silicon dioxide taken at a stoichiometric ratio is introduced into the melt. The resulted gaseous silicon monoxide is then reduced in a gaseous phase with pure methane at a temperature of 2300-2500°C and elementary silicon is produced.

EFFECT: allows to upgrade purity of processed silicon and to reduce its production cost.

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Description

The invention relates to the field of non-ferrous metallurgy, in particular to the production of high-purity silicon, which can be used in the manufacture of solar cells.

A known method of producing high purity silicon from trichlorosilane by its reduction with hydrogen. This produces silicon semiconductor purity, which is used in the electronics industry. The method is described in the book "Technology of semiconductor silicon" edited by E.S. Falkevich. M .: Metallurgy, 1992, p. 107. However, the low yield of the product (less than 10%) leads to its high cost, which makes the method unprofitable for obtaining "solar" silicon. In addition, the process of obtaining trichlorosilane and its purification is complex, expensive and environmentally hazardous.

A known method of producing high-purity silicon carbothermal reduction of high purity quartz in an electric arc furnace using a carbon reducing agent. The method is described in the article by A. A. Bakhtin, L. V. Chernyakhovsky, L. P. Kishchenko, P. S. Menshikov “Influence of the quality of raw materials on the production of high-purity silicon”, journal “Non-ferrous metals”, No. 1, 1992 ., p.29-32. The disadvantages of this method are the large losses of pure starting materials in the recovery process and the high content of impurities for solar silicon (up to 2 · 10 ^-2 wt.%).

Closest to this invention is the technical solution proposed in the patent of the Russian Federation No. 2173738 dated 12/23/1999. To obtain silicon suitable in the technology of manufacturing solar cells, the process of reducing silicon dioxide (natural quartz) in two stages is used - first to silicon monoxide, and then to elemental silicon:

Such a process allows combining the production of silicon with its purification from impurities, since the volatility of most impurities is much lower than the volatility of silicon monoxide SiO under the conditions of its formation.

At the first stage, high-purity silicon is melted in a crucible and crushed high-purity quartz is introduced into the crucible at a temperature of 1900 ° C. The silicon monoxide formed in this process enters the second stage, where it is reduced to elemental silicon. To do this, finely dispersed carbon — soot — is introduced into the SiO stream, and the reduction process proceeds in suspension at a temperature of 1900–2000 ° С. The resulting liquid silicon is then subjected to directional crystallization.

The method allows to obtain silicon with a purity of 99.998 wt.%, Which corresponds to a total content of impurities in it of 0.002 wt.% Or 20 ppm (parts per million). The degree of extraction of silicon is 94.7-95%. However, this method has the following disadvantages:

- low productivity of the process, since the initial liquid silicon in the crucible is quickly consumed to obtain silicon monoxide and the crucible must often be replaced;

- more than 20% of high-purity silicon source material, which is contaminated by impurities during the formation of silicon monoxide, is wasted;

- the purity of the obtained silicon is insufficient for its use in modern solar energy. In particular, it contains a lot of carbon, which requires significant costs for its removal.

The aim of the invention is to improve the quality of silicon and reduce the cost of its manufacture.

This goal is achieved in that in a method for producing high purity silicon, comprising introducing solid silicon dioxide into a silicon-containing melt with the formation of silicon monoxide and then reducing silicon monoxide to elemental silicon with a carbon reducing agent, silicon dioxide is used as a silicon-containing melt, into which a mixture of dioxide powder is introduced silicon with elemental silicon, taken in a stoichiometric ratio, and methane is used as a carbon reducing agent.

The essence of the invention lies in the fact that in the reaction of the formation of SiO a mixture of powders of silicon dioxide and elemental silicon is consumed, and the molten silicon dioxide in the crucible serves as a reaction medium and solvent, but is not consumed. With the accumulation of high concentrations of impurities in the melt, it is replaced, but the number of replacements is an order of magnitude lower compared to the prototype. Material losses from pollution of quartz are small, since its cost is 10-15 times lower than the cost of silicon of the same purity. As a result, the productivity of the process rises and material costs are reduced.

The reduction reaction of silicon dioxide is carried out in an electric arc installation, which consists of an argon plasma torch, a powder dispenser and a reaction chamber with a crucible heating system. The reduction temperature of SiO ₂ to SiO is 1900 ° C. The reaction products are transported by argon to the SiO reduction unit.

The reduction of the monoxide is carried out in the gas phase with pure methane CH ₄ at a temperature of 2300-2500 ° C. Methane is heated to the indicated temperature using an arc plasmatron. The reduction reaction is carried out in a vortex reactor with a tangential feed of reducing gas. In this case, the silicon formed is deposited on the walls of the reactor and flows down into the silicon receiver in the form of a liquid film. The temperature of liquid silicon in the receiver crucible is maintained at a level of 1500 ° C.

The use of methane reduces silicon carbon pollution due to the lack of direct contact of liquid silicon with carbon (soot). In addition, it is more technologically advanced compared to the use of fine soot, and the reaction in the gas phase is more controllable and reproducible. As a result, the quality of the resulting silicon is improved compared to the prototype.

An example of the method

As starting materials for obtaining high-purity silicon, quartz grains with a purity of 99.99 wt.% SiO ₂ and crystalline silicon in granules of the same purity are used. To prepare a mixture of quartz and silicon powders, these materials are ground to a particle size of 70-100 microns and dosed in a ratio of SiO ₂ : Si = 2.14: 1 by weight. Then the powders are stirred for an hour in a drum mixer and the finished mixture is loaded into the reactor feeder. The loading mass in the feeder is about 30 kg. For loading into the crucible, quartz grains are used without preliminary grinding (with a particle size of 6-8 mm). This grain is loaded into a graphite crucible coated with silicon nitride in an amount of 30 kg.

The entire system for the process is sealed, pumped out in vacuum to a residual pressure of ~ 10 ^-4 mm Hg and washed with argon for 30 minutes. Then, all electric heaters are brought to operating temperatures and plasmatrons are put into operation to heat and supply gas flows of argon and methane to the system. After obtaining the molten quartz in the crucible, a mixture of SiO ₂ and silicon powders is fed through a dispenser to the plasma torch nozzle. The reaction products (silicon monoxide) are transferred by a stream of argon to a SiO reduction reactor, where they are mixed with a stream of methane heated to 2400 ° C. The resulting liquid silicon is collected in the receiver crucible. In a continuous process, 15 kg of silicon are obtained in 3 hours with the parameters given below.

The results of the example and other experiments are shown in the table.

Table Indicators units rev. Prototype The inventive method The content of the amount of impurities in silicon ppm twenty 8-10 Silicon carbon content ppm 10-15 4-5 The degree of extraction of silicon % 95 95 High Purity Silicon Performance kg / h four 5 (ppm - 10 ^-4 wt.%)

The comparison of the method, which is the subject of this invention, with the prototype shows the following advantages:

- the developed method allows to obtain silicon containing less impurities, especially carbon, in comparison with the prototype, i.e. significantly improves the quality of the resulting product;

- the productivity of the process for producing high purity silicon is increased by 20%, which reduces production costs.

Claims (1)

A method for producing high purity silicon, comprising introducing solid silicon dioxide into a silicon-containing melt to form silicon monoxide and then reducing silicon monoxide to elemental silicon with a carbon reducing agent, characterized in that silicon dioxide is used as a silicon-containing melt, into which a mixture of silicon dioxide and elemental powders is introduced silicon taken in a stoichiometric ratio, and methane is used as a carbon reducing agent.