RU2399583C1 - Silicon tetrafluoride synthesis method - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: invention can be used in making in electronic industry during production of solar cells. Sodium silicofluoride is pre-dried at temperature of up to 300C and residual pressure of up to 2 mm Hg. At the first thermal decomposition step, sodium silicofluoride is heated to 620-650C with constant removal of the released silicon tetrafluoride at residual pressure of up to 30 mm Hg. At the second step the remaining mass is heated to temperature not lower than 750C, while also constantly removing silicon tetrafluoride at pressure not higher than 30 mm Hg. ^ EFFECT: invention enables to avoid agglomeration and melting of the reaction mass and obtain pure silicon tetrafluoride with output of over 99%. ^ 2 tbl, 1 ex

Description

The invention relates to inorganic chemistry, to the production of non-metal fluorides, and in particular to methods for producing silicon tetrafluoride.

Silicon tetrafluoride is used in the electronic industry, in the production of solar cells and fiber optics, and more recently, as a raw material for large-tonnage production of solar and electronic polysilicon.

There are various methods for producing silicon tetrafluoride:

- in the patent of the Russian Federation (Z. No. 2006107767 dated 02.09.2003, IPC C01B 33/08), a method based on fluorination of silicon dioxide in a flare at a temperature of 1500-2000 ° C;

- the authors of RF patent No. 2046095 (IPC C01B 33/10) propose to obtain silicon tetrafluoride by treating an organosoluble salt of hydrofluoric acid with non-volatile mineral acids;

- Volfkovich S.N., Gabrielova M.G. described a method for producing silicon tetrafluoride by thermal decomposition of Na ₂ SiF ₆ by the reaction of Na ₂ SiF ₆ + Q = 2NaF + SiF ₄ , (Q is the amount of heat) at a temperature of 400-800 ° C (Chemical industry, 3, 1951, P. 72 -75).

Various modifications of this method are known, for example, the authors of RF patent 2182558 (IPC C01B 33/107) propose to treat the reaction mass at 300-350 ° C fluorine prior to decomposition, and the authors of RF patent No. 2274603 (IPC C02B 33/107) propose decomposition silicon tetrafluoride at a temperature of 500-800 ° C with the participation of fluorine.

The main purpose of the proposed modifications of the method based on the thermal decomposition of sodium silicofluoride is to obtain relatively small amounts of high-purity silicon tetrafluoride for the electronics industry, fiber optics, various kinds of coatings, etc., i.e. for areas where the main requirement is cleanliness (including siloxane), and the quantities consumed are small.

Due to the rapid growth in the demand for polycrystalline silicon for solar energy and, accordingly, the need to expand its production, interest in cheaper and environmentally safer methods without the use of chlorine in the manufacturing process, in particular, methods based on the reduction of silicon tetrafluoride, has intensified.

So, for example, the technology proposed in RF patent No. 2078034 (IPC C01B 33/03) involves the conversion of silicon tetrafluoride to silicon dioxide, followed by reduction to polysilicon. Here, the presence of small amounts of an impurity such as siloxane is not critical.

In the case of the expansion of the use of such technologies that provide low cost and lower environmental risk, the problem arises of large-scale production of silicon tetrafluoride. So, for example, for the production of 1000-2000 t / g of polysilicon (the productivity of a typical plant for the production of polysilicon), 3500-7000 t / g of silicon tetrafluoride is required, to obtain which it is necessary to process 7000-14000 t / g of sodium silicofluoride. In this case, the main technology requirement is the ability to produce large volumes of polysilicon at an acceptable purity and low cost.

As a prototype of the invention, the method selected in the journal "Chemical industry", 3, 1951, S. 72-75.

According to this method, the decomposition of silicon tetrafluoride takes place at a temperature of 600-620 ° C by the reaction of Na ₂ SiF ₆ → 2NaF + SiF ₄ with continuous removal of the silicon tetrafluoride formed by evacuating the reactor to a residual pressure of 3-5 mm water column. The authors noted that the rate and completeness of the reaction increase with increasing temperature above 620 ° C, but at a temperature above 730 ° C sintering of the reaction mixture is possible. The method was tested on an industrial scale on a tubular furnace with a capacity of more than 1000 t / g for sodium silicofluoride, which is quite close to the required performance. However, silicon tetrafluoride was obtained with an unacceptable impurity content.

The authors of the present invention experimentally established that with a rapid increase in temperature to 650 ° C, a fusion of the reaction mass is observed, which is associated with the formation of a eutectic in the Na ₂ SiF ₆ -NaF system (there is literature on the polymorphic transition of Na ₂ SiF ₆ at a temperature of 646 ° C) .

The technical result of this invention is the development of a method suitable for large-scale production of silicon tetrafluoride by thermal decomposition of sodium silicofluoride, eliminating the possibility of sintering and fusion of the reaction mass and providing a high yield of silicon tetrafluoride (more than 99%) with a purity sufficient to produce solar-grade polysilicon.

The technical result is achieved by the fact that in the known method for producing silicon tetrafluoride, which consists in pre-drying sodium silicofluoride at a temperature of up to 300 ° C and a residual pressure of up to 2 mm Hg, further thermal decomposition is carried out in two stages with continuous removal of the released silicon tetrafluoride with residual pressure not more than 30 mm Hg In the first stage, sodium silicofluoride is heated at a temperature of 620-650 ° C. In the second stage, they are heated to a temperature of at least 750 ° C. The process at each stage is carried out until the rate of release of silicon tetrafluoride to a profitable level.

In the first stage, at a temperature of up to 300 ° C (at this temperature, the vapor elasticity of silicon tetrafluoride over sodium silicofluoride does not exceed 2 mm Hg), the initial salt is dried and the volatile impurities are distilled off. In the second stage, at a temperature of 620-650 ° C, silicon tetrafluoride is distilled (up to 60%) to the point of polymorphic transition, formation of eutectic and sintering. At the stage of heating to 750 ° C, the extraction of silicon terrafluoride to 98-99% ends. The resulting silicon tetrafluoride is collected in a collector cooled to a temperature below 130 ° C. Sodium fluoride after cooling to 300 ° C can be used for deeper purification of silicon tetrafluoride.

The method according to the invention is as follows.

Sodium silicofluoride is distributed in a layer with a thickness of 15-20 mm on a baking sheet, placed in a vacuum oven and dried at a temperature of 280-300 ° C while maintaining a vacuum below 2 mm RT. until the emission of volatile impurities has ceased. Then the temperature rises to 600 - 640 ° C. Heating continues until the intense release of silicon tetrafluoride ceases, after which the temperature rises to 750-770 ° C and is maintained until the degree of extraction reaches a predetermined level. Silicon tetrafluoride released at these stages is pumped to the collector, so that the pressure in the furnace is maintained no higher than 30 mm Hg.

The above example illustrates, but does not limit the scope of the present invention.

Example. Sodium silicofluoride obtained from OJSC South Ural Cryolite Plant was used as a starting material. Its composition is given in table 1

Table 1 №№ Indicator, % Norm
1 grade Sample Number one 2 one Mass fraction of sodium silicofluoride 95 92.3 98.9 2 Mass fraction of free acid 0.15 no no 3 Mass fraction of iron Not normal. 0.055 0.098 four Moisture content one 0.02 0.18 5 Mass fraction of heavy metals 0.05 0.05 0.05 6 Mass fraction of sodium fluoride 12 4.9 0.37 7 Sample weight, kg 48 46

Samples were processed in a vacuum furnace with a chamber volume of 120 l according to the above program. Silicon tetrafluoride was collected in a condenser-evaporator cooled by liquid nitrogen. The results are shown in table 2

table 2 №№ Substance Content The amount of recovered SiF ₄ Recovery% Sample 1 Sample 2 Sample 1 Sample 2 Sample 1 Sample 2 one SiF ₄ ,% 99.7 99.3 24.0 kg 24.2 kg 98 96 2 CO ₂ ppm 38 42 CO, ppm 2 3 Hf ppm Less than 1 Less than 1 CH ₄ ppm Less than 1 Less than 1 H ₂ O, ppm Less than 1 Less than 1 Si ₂ OF ₆ ,% 0.3 0.6

Claims (1)

A method of producing silicon tetrafluoride, which consists in pre-drying sodium silicofluoride at a temperature of up to 300 ° C and a residual pressure of up to 2 mm Hg, characterized in that the further thermal decomposition of sodium silicofluoride is carried out in two stages: in the first stage, sodium silicofluoride is heated to 620- 650 ° C with a constant removal of the released tetrafluoride at a residual pressure of up to 30 mm Hg, and in the second stage, the remaining mass is heated to a temperature of at least 750 ° C with continuous removal of silicon tetrafluoride at a pressure not more than 30 mm Hg