RU2385291C1 - Method of production of high purity crystal silicon (versions) - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: process is carried out in two stages in reactor of plasma furnace at temperature above 1500°C. Quartz grit as a silicon containing compound is introduced into the reactor at the first stage of reduction, while mixture of carbon monoxide with hydrogen taken at volume ratio 1:1 is introduced as a reducer. According to one version of the invention an anode is connected to a current conducting case of the reactor and to melted silicon, further quartz grit is added into the reactor; and mixture of methane, propane or acetylene with oxygen taken at volume ratio 2:1 is introduced into the reactor as a reducer. The process can be performed with alternate current of frequency above 5 kHZ.

EFFECT: production of high purity silicon by ecologically safe method with low losses and low prime cost.

3 cl, 3 ex

Description

The invention relates to processes and apparatus for producing crystalline silicon of high purity.

The chloride method is used in the world practice for the production of crystalline silicon of semiconductor quality, while technical silicon powder is hydrochlorinated to form gaseous chlorine-containing silicon compounds (chlorosilanes). After purification and separation of the gases leaving the reactor where silicon hydrochlorination takes place, especially pure trichlorosilane is released, which undergoes hydrogen reduction, and as a result, PACs are obtained with a total impurity content of less than 10 ^-4 wt.%. The obtained polycrystalline silicon (PCC) is used for the manufacture of mono- and multicrystalline ingots for use in semiconductor manufacturing. The method is described in the book "Technology of semiconductor silicon" edited by E.S. Falkevich. M .: Metallurgy, 1992. The implementation of the chloride method is associated with complex and expensive equipment, the technology is complex and multi-link. The use of chlorine requires the observance of special conditions to ensure the safety of the environment.

The closest in essence to this invention is the technical solution set forth in the article by A.A. Bakhtin, L.V. Chernyakhovsky, L.P. Kishchenko, P.S. Menshikov "The influence of the quality of raw materials on the production of high-purity silicon", g. "Non-ferrous metals", N 1, 1992, pp. 29-32.

To obtain silicon suitable in the technology of manufacturing solar cells, the carbothermal reduction method of high-purity quartz with a boron and titanium content of less than 1 · 10 ^-4 wt.% And a total impurity content of 1 · 10 ^-3 % was used. Graphite with a total impurity content at the above levels was used as a reducing agent.

The mixture of quartz and graphite was briquetted, the size of the briquettes was 2-6 mm. The briquettes were loaded into an electric arc furnace with a capacity of 100 kVA, the temperature in the reaction zone exceeded 1900 ° C, the ratio of carbon to quartz in the briquettes was 0.6 kg of carbon per 1 kg of SiO ₂ . The duration of the process was 2 hours. The maximum extraction of silicon reached 67-71%, and the purity of the obtained silicon was not lower than 99.98%. Such silicon, according to the authors, is suitable for subsequent purification and manufacture of solar cells.

However, this method does not provide silicon of the desired degree of purity for solar cells, which is associated with difficulties in maintaining a high degree of purity of briquettes, low chemical activity of graphite and high consumption of carbon-graphite electrodes. Environmental problems remain unresolved due to the need to clean large flows of high-temperature gases at the outlet of the electric arc furnace from dust of silicon dioxide, which leads to the high cost of the obtained silicon.

An object of the present invention is to provide high-purity silicon in an environmentally friendly manner with low silicon losses and low cost.

The technical result is achieved in that in the proposed method for producing crystalline silicon of high purity, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, the process is carried out in two stages in a plasma furnace reactor at a temperature above 1500 ° C, in the first stage of recovery as silicon-containing compounds are introduced into the reactor quartz grains, as a reducing agent is introduced a mixture of carbon monoxide with hydrogen, taken in volume ratio enii 1: 1, in a second stage purification of silicon fed into the reactor chemically active towards impurity gases and then evacuated reactor volume.

In a variant of the method for producing high-purity crystalline silicon, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, the reduction process is carried out in a DC plasma furnace reactor at a temperature above 1500 ° C, the anode is connected to the conductive reactor body and molten silicon, introduced into the reactor is a quartz pellet, and as a reducing agent, a mixture of methane, propane or acetylene with oxygen, taken in a volume ratio, is introduced into the reactor SRI 2: 1.

In a variant of the method for producing high-purity crystalline silicon, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, the reduction process is carried out in a plasma furnace reactor at a temperature above 1500 ° C under alternating current with a frequency above 5 kHz, quartz grains are introduced into the reactor, and as a reducing agent, a mixture of methane, propane or acetylene with oxygen taken in a volume ratio of 2: 1 is introduced into the reactor.

Both process steps are carried out in the same furnace. At the first stage, in a closed working space of a plasma furnace (reactor), high-purity quartz in the form of quartz grains with a total impurity content of about 10 ^-4 % is reduced to elemental silicon by reaction (1)

As a reducing agent, a gaseous mixture of carbon monoxide and hydrogen is used. The content of impurities in the reducing agent is not more than 10 ^-4 %.

Example 1

Quartz grains are loaded into a highly pure siliconized graphite reactor installed in a closed furnace under a plasma torch. The reactor is heated and at a temperature above 1800 ° C, a mixture of carbon monoxide and hydrogen in a volume ratio of 1: 1 is fed into the reactor with a plasmatron.

At a temperature of 1900-2000 ° C, silicon dioxide is almost completely reduced to silicon. The total balance reaction is represented by equality (1). The source of thermal energy necessary for the course of the reaction is plasma. Gaseous carbon dioxide, water vapor and unreacted hydrogen formed by reaction (1) are continuously removed from the reactor through a gas outlet pipe under an exhaust hood where the final afterburning of hydrogen takes place.

Silicon is removed from the reaction zone and is collected at the bottom of the reactor.

At the second stage, silicon is cleaned at a temperature above 1500 ° C due to the fact that the gas introduced into the plasmatron, which is chemically active with respect to impurities, for example, moist oxygen or chlorine, forms compounds with impurity atoms that pass into slag or sediment upon settling or are removed by evaporation.

When interacting with oxygen, part of the impurities is in the form of oxides and their compounds, mainly low-volatile silicates of Ba, Ca, Mg, Al, Zr and volatile B ₂ O ₃ , P ₂ O ₅ .

When interacting with chlorine, part of the impurities forms volatile compounds.

The final stage of purification of molten silicon consists in the subsequent evacuation of the furnace reactor volume for a more complete degassing of the silicon melt. The result is silicon with a purity of 99.9995%.

Example 2. In contrast to example 1, in the first stage of reduction, a mixture of gaseous hydrocarbon, for example methane (by reaction 2), propane or acetylene, with pure oxygen or atmospheric oxygen, taken in a volume ratio, is used to obtain a mixture of carbon monoxide and hydrogen in a plasma torch stream 2: 1.

The plasma torch operates on direct current, and the anode is connected to the electrically conductive reactor vessel and to the molten silicon. The purity of the resulting silicon is at a level suitable for the manufacture of solar cells.

Example 3. Unlike example 2, the plasma torch operates on alternating current with a frequency of at least 5 kHz. Both electrodes can be made of pure silicon. As a result of the experiment, silicon was obtained with a purity of 99.9995% with a silicon recovery of 90%. The presence of several furnace plants allows you to organize a continuous process for producing pure silicon, alternating recovery and post-treatment.

A comparison of the technology that is the subject of this invention with the prototype shows the following advantages:

- the developed method allows to obtain the purity of silicon in the processed ingots at the level of 99.9995%, higher compared to the prototype, and to increase the yield of silicon from 70% to 90%;

- obtaining high purity silicon is achieved mainly through the use of a free of impurities reducing agent in gaseous form, sealing the reactor volume from the external environment;

- sharply reduced losses of silicon due to the elimination of entrainment in the form of dust;

- in a single technological cycle, silicon is refined to solar quality;

- there are no emissions of toxic gases, which guarantees the environmental safety of the method.

Claims (3)

1. A method of producing crystalline silicon of high purity, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, characterized in that the process is carried out in two stages in a plasma furnace reactor at a temperature above 1500 ° C, in the first stage of recovery as a silicon-containing compound quartz grains are introduced into the reactor, and a mixture of carbon monoxide and hydrogen taken in a volume ratio of 1: 1 is introduced as a reducing agent in the second stage of purification to emniya introduced into the reactor chemically active towards impurity gases are evacuated and the reactor volume. 2. A method for producing crystalline silicon of high purity, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, characterized in that the reduction process is carried out in a DC plasma furnace reactor at a temperature above 1500 ° C, the anode is connected to a conductive reactor vessel and to the molten silicon, quartz grains are introduced into the reactor, and a mixture of methane, propane or acetylene with oxygen taken in volume is introduced into the reactor as a reducing agent th 2: 1 ratio. 3. A method for producing crystalline silicon of high purity, including thermal reduction of a silicon-containing compound to elemental silicon using a carbon-containing substance, characterized in that the reduction process is carried out in a plasma furnace reactor at a temperature above 1500 ° C on alternating current with a frequency above 5 kHz, introduced the reactor is a quartz pellet, and as a reducing agent, a mixture of methane, propane or acetylene with oxygen taken in a volume ratio of 2: 1 is introduced into the reactor.