RU2519460C1 - Production of silicon with the use of aluminium subchloride - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed method comprises reduction of silicon from vapours of silicon compounds with chlorine or silicon with chlorine and hydrogen at mixing of said vapours with vapours of lower aluminium chlorides at 1000-1250°C in transfer gas flow. Said gas represents the mix of hydrogen with argon or the mix of hydrogen with helium containing 2-20 molar parts of hydrogen per one part of silicon compounds vapours. Note here that aluminium lower chlorides of required purity are produced from aluminium metal of 99.0-99.8% purity and gaseous aluminium trichloride or hydrogen chloride or chlorine by multiple reiteration of sublimation and decomposition of formed lower aluminium chlorides. Note also that precipitated silicon crystals are subjected to heat treatment at above 577°C and less than 1400°C, processed by hydrochloric acid and subjected to refining remelting.

EFFECT: higher purity of silicon, lower production costs.

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Description

The invention relates to the metallurgy of silicon, in particular to a technology for producing polycrystalline silicon for photovoltaics.

Currently, the bulk of polycrystalline silicon in the world is produced by hydrogen reduction of trichlorosilane during the endothermic reaction on the surface of silicon current conductors heated by electric current to a temperature of ~ 1000 ° C (Siemens process), or in fluidized bed reactors [E. Falkevich, Pulner E.O. Semiconductor silicon technology. M .: Metallurgy, 1992. - 408 p .; W.O. Filtvedt, M. Javidi. Development of fluidized bed reactors for silicon production. Solar Energy Materials & Solar Cells 94 (2010) 1980-1995].

The disadvantage of these methods is the relatively low productivity, due primarily to the low yield of the reduction reaction, during which less than 30% of the silicon that is part of trichlorosilane is reduced. The rest of the silicon in trichlorosilane, when reacted with hydrogen, forms by-products (SiCl ₄ , SiCl ₂ H ₂ , etc.), which require subsequent complex conversion to trichlorosilane. In addition, the disadvantages of the Siemens process are the inability to use other types of feedstock, in addition to trichlorosilane, the high cost of electric energy and the fire hazard of production using hydrogen heated to high temperature at elevated pressure.

Known metallothermal methods for producing silicon from its halides using as metal reducing agents active in halogen (usually chlorine or fluorine). Unlike the Siemens process, the metallothermal reduction reaction proceeds with the release of heat and does not require electric or thermal energy, but, on the other hand, expensive reagents are used - high-purity reducing metals.

A known method of reducing silicon dioxide with metallic magnesium (RU 2452687, publ. 27.12.2011). Preliminary purification of silicon dioxide is carried out by obtaining an intermediate compound - ammonium hexafluorosilicate, from which purified SiO ₂ is then obtained. Then it is mixed with pre-purified vacuum distillation with powdered magnesium and the resulting mixture is ignited. During the exothermic reaction, elemental silicon is formed in the form of a powder and magnesium oxide, which is then washed with hydrochloric acid.

The disadvantage of this method is the production together with silicon of mechanically mixed magnesium oxide with it in a comparable amount and the high consumption of hydrochloric acid for the removal of this oxide, as well as the need for preliminary production of pure magnesium.

A known method of reducing sodium tetrafluoride silicon (SiF ₄ ), previously obtained by thermal decomposition of Na ₂ SiF ₆ (RU 2415809, publ. 04/20/2010). The reduction of SiF ₄ vapor to silicon occurs at 500 ° C mainly on the surface of molten sodium.

The disadvantage of this method is the necessity of washing the obtained silicon from sodium fluoride, as well as the need for preliminary obtaining pure sodium.

A known method of reducing silicon tetrachloride by vapors of zinc at 950 ° C (US 2773745, publ. 11.12.1956). Recovery is carried out in a quartz flow reactor. A mixture of the resulting zinc chloride and unreacted zinc is removed from the reactor and condensed in a separate vessel. In this way, silicon is obtained with a purity of 99.9%.

The disadvantage of this method is the relatively low yield of silicon, due to the unsatisfactory thermodynamics of the process, and the need for preliminary obtaining pure zinc.

A known method of reducing trichlorosilane zinc (WO 2008/034577, publ. 03/27/2008). SiHCl ₃ vapors are passed through zinc heated to 850 ° C and placed in a graphite crucible. The resulting zinc chloride is condensed in a separate vessel, and silicon is heated to 1500 ° C to remove residual zinc.

The disadvantage of this method is the relatively low yield of silicon, due to the unsatisfactory thermodynamics of the process, and the need for preliminary obtaining pure zinc.

Closest to the claimed is a method of subchloride reduction of silicon tetrachloride [Parfenov OG, Pashkov GL New Approach to Silicon Metallurgy // Doklady Chemistry. - 2008 .-- Vol.422. - Part 1. - p.225-226; Zakirov R.A., Parfenov O.G., Pashkov G.L. Subchloride metallurgy of silicon. Abstracts of the VI International Conference “Silicon - 2009”, Novosibirsk, July 7–10, 2009, Novosibirsk: Institute of Chemical Technology SB RAS, P.25; K. Yasuda, K. Saegusa, T.N. Okabe. New Method for Production of Solar-Grade Silicon by Subhalide Reduction. Materials Transactions, Vol.50, No. 12, 2009, p. 2873-2878]. This method uses the ability of aluminum subchloride vapors (AlCl _x , x <3) to reduce silicon from a compound with chlorine:

SiCl ₄ + 2AlCl = Si + 2AlCl ₃

or from compounds with chlorine and hydrogen, for example trichlorosilane:

SiHCl ₃ + 3 / 2AlCl = Si + 3 / 2AlCl ₃ + 1 / 2Н ₂ .

Aluminum subchloride is prepared by reacting 99.99% pure aluminum and gaseous aluminum trichloride at a temperature of 1000-1250 ° C in an argon atmosphere and then it is fed to a reduction reactor in which polycrystalline silicon is obtained by reacting silicon tetrachloride with aluminum subchloride.

The disadvantage of this method is the low quality of polycrystalline silicon, because as a result of the disproportionation of excess aluminum subchloride on the surface of the silicon crystals appear aluminum particles (figure 2). The aluminum content in polycrystalline silicon is at the level of tenths of a percent. In addition, expensive high-purity aluminum is used to produce aluminum subchloride.

The objective of the invention is to improve the quality of the obtained polycrystalline silicon, namely obtaining silicon of the required purity for photovoltaics, increasing the efficiency of the process through the use of a less expensive reducing agent.

The problem is solved in that in a method for producing polycrystalline silicon, including its reduction from vapors of compounds of silicon with chlorine or silicon with chlorine and hydrogen by mixing these vapors with vapors of lower aluminum chlorides in a stream of transporting gas, characterized in that as the transporting gas is used a mixture of hydrogen with argon or helium, containing from 2 to 20 molar parts of hydrogen per molar part of the vapor of silicon compounds, while obtaining lower aluminum chlorides of the required purity lead from metal aluminum with a purity of 99.0-99.8% and gaseous aluminum trichloride, or hydrogen chloride, or chlorine by repeatedly repeating the process of sublimation and decomposition of the resulting lower aluminum chlorides. Precipitated silicon crystals are subjected to heat treatment at temperatures above 577 ° C and below 1400 ° C, and then treated with hydrochloric acid and subjected to refining remelting.

The proposed method is illustrated in figure 1, which shows a block diagram of the production of silicon using aluminum subchloride.

The block diagram shows a reactor for subchloride purification of aluminum and synthesis of aluminum subchloride 1, an evaporator of silicon-containing liquid 2, a reactor for subchloride reduction of silicon 3, an acid washing apparatus 4, a heat treatment unit for silicon crystals 5, and a refining melting unit 6.

The method is as follows.

Technical grade aluminum (99.0-99.8%) in reactor 1 is subjected to subchloride purification based on the ability of aluminum at a temperature above 1000 ° C to restore its trichloride to volatile subchloride by the reaction:

$$\text{2Al}+{\text{<figure-callout id="3" label="ÀlÑl" filenames="00000004.png" state="{{state}}">ÀlÑl</figure-callout>}}_{\text{3}}=\text{3AlCl (1)}$$

and the ability of the resulting subchloride to disproportionate to aluminum and its chloride at a lower temperature inverse to (1) the reaction. Spacing apart the zones of synthesis of subchloride and its disproportionation, it is possible to obtain aluminum, and accordingly, its subchloride of purity 99.99999 or higher.

For the synthesis of aluminum subchloride, chlorine or hydrogen chloride can be used as starting reagents, which are fed instead of aluminum trichloride to a reactor for subchloride purification of aluminum and synthesis of aluminum subchloride 1.

Then, aluminum subchloride of the required purity enters the subchloride reduction reactor 3, into which a starting silicon-containing (liquid under normal conditions) substance, for example, SiCl ₄ or SiHCl _{3 of the} required purity, is supplied from the evaporator 2. A gas is supplied to the subchloride reduction reactor as a transporting gas, which does not form stable compounds with the obtained products under normal conditions (for example, helium, argon, or mixed with hydrogen). It was found that the kinetics of the process and the size of the obtained silicon crystals differ for argon and hydrogen. This is due to a number of factors, including the increased thermal conductivity of hydrogen relative to argon, which is important for bulk silicon nucleation during exothermic reactions. In this case, the composition of the initial gas mixture may include hydrogen in an amount of 0-20 molar parts per molar part of the vapor of volatile silicon compounds. Recovery is carried out in stages and is described by the following generalized reactions (n = 1 ÷ 20):

$${\text{SiCl}}_{\text{four}}+\text{2AlCl}+{\text{<figure-callout id="2" label="nH" filenames="00000004.png" state="{{state}}">nH</figure-callout>}}_{\text{2}}=\text{Si}+{\text{2AlCl}}_{\text{3}}+{\text{nH}}_{\text{2}}\text{(2)}$$

$$SiHC{\mathrm{<figure-callout\; id="3"\; label="l"\; filenames="00000004.png"\; state="\{\{state\}\}">l</figure-callout>}}_3+3/2\text{AlCl}+\mathrm{<figure-callout\; id="2"\; label="n\; H"\; filenames="00000004.png"\; state="\{\{state\}\}">n\; </figure-callout>}{H}_{}{}_2=Si+3/2{\text{<figure-callout id="3" label="AlCl" filenames="00000004.png" state="{{state}}">AlCl</figure-callout>}}_{\text{3}}+(n+\mathrm{one}/2)H_2(3)$$

On the surface of the silicon crystals formed droplets of aluminum (figure 2). To remove them, the cooled silicon crystals are etched with acid, for example, hydrochloric acid in a washing apparatus 4, and then in an inert medium for silicon and aluminum crystals (argon, helium, hydrogen) they are heated in a heat treatment unit 5 to a temperature of 577-600 ° C or higher, but up to the temperature of their fusion. At this temperature, an abnormally fast diffusion of dissolved aluminum in silicon crystals to the surface of these crystals is observed [Shengunov, D.V., Pis'ma Zh. Tekh. Fiz., 1997, vol. 23, no. 11, p. 83-87]. Then, these crystals are again washed in hydrochloric acid from the aluminum released on their surface. In this way, the main impurity introduced during the reduction process is removed from the polycrystals of silicon. If necessary, more thorough cleaning of the silicon is subjected to refining remelting in the Czochralski furnace or in the form of a rod - crucible free zone melting 6.

The method is confirmed by specific examples.

Example 1. Aluminum grade A7 (99.7%) in an amount of 1 g is loaded into a crucible and placed at the inlet of the reactor, where a temperature of 1200 ° C is maintained with continuous supply of argon. Then 2.5 g of technical grade aluminum chloride from a separately heated vessel (sublimator) is fed to the inlet of the reactor, where it selectively interacts with the aluminum therein to form 2 g of AlCl. The resulting subchloride enters the disproportionation section, where at a temperature of 800 ° C it disproportionates to aluminum trichloride and aluminum metal. Both products enter the reactor inlet for the next purification step. At the same time, the supply of aluminum of technical purity to the reactor inlet is terminated. Depending on the composition of the initial impurities in technical grade aluminum, such a cleaning cycle is repeated 3-6 times. Next, the aluminum subchloride of the required purity obtained at the last stage of purification is sent to the reduction part of the reactor, where it reacts at a temperature of 1100-1200 ° С with vapors of SiCl ₄ (6 ml ^{3 of the} initial liquid) of the required purity in the composition of the gas mixture with hydrogen and argon (in the ratio 1: 1: 1 mol.) To obtain 0.5 g of silicon.

 Example 2. Conditions as in example 1, but the composition of the source gas mixture includes hydrogen in an amount of 5 molar parts per part of the supplied silicon tetrachloride. In this case, the silicon yield increased to 0.7 g.

Example 3. 1.5 g of aluminum grade A7 (99.7%) is loaded into a crucible, which is placed in an oven. The furnace is heated to 1200 ° C, after which the supply of hydrogen chloride in an amount of 1.3 l (pressure 0.1 MPa), which when interacting with aluminum forms AlCl, is started. After passing through cyclic purification, aluminum subchloride reduces SiCl ₄ (6 ml ^{3 of the} initial liquid). The result is obtained as in example 1.

Example 4. Conditions as in example 3, but instead of hydrogen chloride, chlorine in an amount of 0.65 l is supplied to the reactor (pressure 0.1 MPa). The result as in example 1.

Thus, the proposed method for producing silicon using aluminum subchloride can improve the quality of the resulting silicon and improve the efficiency of the process through the use of a less expensive reducing agent.

Claims (1)

A method of producing polycrystalline silicon, including its reduction from vapors of silicon compounds with chlorine or silicon with chlorine and hydrogen by mixing these vapors with vapors of lower aluminum chlorides at a temperature of 1000-1250 ° C in a stream of a transporting gas, characterized in that use as a transporting gas a mixture of hydrogen with argon or a mixture of hydrogen with helium containing from 2 to 20 molar parts of hydrogen per molar part of the vapor of silicon compounds, while the production of lower aluminum chlorides of the required purity made of metal aluminum with a purity of 99.0-99.8% and gaseous aluminum trichloride or hydrogen chloride or chlorine by repeating the process of sublimation and decomposition of the resulting lower aluminum chlorides, the precipitated silicon crystals are subjected to heat treatment at temperatures above 577 ° C and below 1400 ° C, and then treated with hydrochloric acid and subjected to refining remelting.

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