RU2182558C1 - Method of producing silicon tetrafluoride - Google Patents

Abstract

FIELD: inorganic chemistry, particularly, methods of production of metalloid fluorides, more specifically, methods of production of silicon tetrafluoride. SUBSTANCE: method includes holding of initial salt Na₂SiF₆ for 1.5-3 h at temperature of 300-350 C and low pressure decreased down to 5 mm Hg; treatment of reaction mass with fluorine (possibly in mixture with inert gas) for 0.5-1.5 h with supply of fluorine in ratio of Na₂SiF₆:F₂ from 50 to 100; reduction of pressure after discontinuation of fluorine supply; thermal decomposition of Na₂SiF₆ after discontinuation of liberation of gases with simultaneous withdrawal of gaseous silicon tetrafluoride. Realization of proposed method results in obtaining of silicon tetrafluoride up to 99.991 wt.% pure and sodium fluoride up to 99.0 wt.%. Silicon tetrafluoride is used in electronic industry and for imparting water repelling properties to articles, resistance to corrosion and attrition. EFFECT: higher efficiency. 2 cl, 1 dwg, 1 tbl

Description

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

 Silicon tetrafluoride is used in the electronics industry, as well as to give products water-repellent properties, corrosion and abrasion resistance. It is known for its use to improve the hydrophobic properties of crystalline molecular sieves [EP 83-104533], in the synthesis of finely divided silicic acid [US patent 2631083, s. 01/10/1952, op. 03/10/1953], etc.

 Various methods for producing silicon tetrafluoride are known. For example, it is obtained from silicon dioxide by exposure to the latter with fluorine or hydrogen fluoride [UK patent 779804, cl. 1 (2), A13, s. 03/30/1955, op. 07.24.1957, UK patent 1080662, CL S 01 B 33/08, c. 02/05/1964, op. 08/23/1967].

 A known method of obtaining uranium and silicon hexafluoride [patent of Germany 3841222, class. C 01 B 17/45, c. 12/07/1988, op. 06/13/1990, etc.].

Silicon tetrafluoride is obtained by heating hydrofluoric acid [patent EP 254349, cl. C 01 B 17/45, 33/08, c. 07/23/1986]:
H ₂ SiF ₆ -> SiF ₄ + 2HF
Known methods for producing SiF ₄ as a result of thermal decomposition of salts of alkali and alkaline earth metals, for example, sodium, barium, etc.

So, a method for producing sodium fluoride by decomposition of Na ₂ SiF _{6 is} described, according to which silicon tetrafluoride is obtained as an intermediate product [Volfkovich S. N., Gabrielova MG Chemical industry, 3, 1951, p. 72-75]. The process takes place at a temperature of 600-620 ^o With the formation of a solid product of NaF and gaseous SiF ₄ :
Na ₂ SiF ₆ -> 2NaF + SiF ₄
Developing in recent years, the applications of silicon tetrafluoride put forward increased requirements for the purity of this product. As a result, work aimed at solving this problem appeared.

 The known method [US patent 4615872, cl. S 01 B 33/08, op. 10/07/1986], including the hydrolysis of a gas containing silicon fluoride, the interaction of silicofluoric acid with sodium, potassium or barium fluorides, and subsequent thermal decomposition of the formed salt. The result is the target silicon tetrafluoride with a purity of 99% vol.

For the complete extraction of carbon dioxide from silicon tetrafluoride is proposed [Japan patent 4175217, class. S 01 B 33/107, c. 11/08/1990, op. 06/23/1992] pass SiF ₄ through hydrotalcite, which is previously subjected to dehydration. Hydrotalcite is treated at a temperature of 100-900 ^o C, the supply of SiF _{4 is} carried out at a pressure above atmospheric - up to 10 atm.

To obtain pure silicon tetrafluoride, it is passed through a layer of active alumina, previously subjected to dehydration at a temperature of 200-900 ^o [Lay. Japanese application 59-162122 of 09/13/1984, application 58-36611, declared. 03/08/1983]. The purpose of this method is the extraction of siloxane, which is formed at the stage of obtaining silicon tetrafluoride in the presence of an aqueous component contained in the feed. It is extremely difficult to remove the completely aqueous component in the silicon tetrafluoride production step and thus prevent the formation of siloxane.

A known method of producing silicon hexafluoride [application of Japan 10-231114, class. S 01 B 33/107, c. 02/18/1997, op. 2.09.1998, Mitsui Chemical Inc.] The method consists in the fact that salts, for example, Na ₂ SiF ₆ or BaSiF ₆ , are mixed with SiO ₂ and / or Al ₂ O ₃ and heated to 500-800 ^o C. The temperature used depends on on the type of silicate used. This method is closer to the proposed invention, because consists in obtaining SiF ₄ and involves the thermal decomposition of the sodium salt of hexafluorosilicic acid. According to this method, it is preferable to use oxides of silicon and aluminum of 99% purity, with a particle size of from 0.1 to 100 μm, in an amount of from 1 to 20 wt.% Based on hexafluorosilicic acid. The obtained solid component - NaF - contains impurities of the introduced oxides of silicon and / or aluminum, which complicates its use and requires disposal. In addition, the use of these oxides increases the mass of solids involved in the process and requiring additional costs for its heating and movement in the apparatus system.

 The challenge facing the authors of this invention is the development of a method that allows to obtain silicon tetrafluoride of higher purity - not less than 99.99%, by thermal decomposition of silicon hexafluoride, to avoid clumping, without the need for the introduction of silicon and / or aluminum oxides.

This problem is solved by the fact that silicon tetrafluoride is obtained by thermal decomposition of sodium hexafluorosilicate Na ₂ SiF ₆ , while the initial salt is maintained at a temperature of 300-350 ^o With a residual pressure of up to 5 mm Hg for 1.5-3 hours, then under the same conditions, it is treated with fluorine for 0.5-1.5 hours, with a mass ratio of sodium hexafluorosilicate: fluorine from 50 to 100, followed by evacuation and distillation of gaseous impurities, after which the salt subjected to thermal decomposition with the simultaneous removal of silicon tetrafluoride. Fluorine may be fed into a mixture with an inert gas to treat the starting salt.

 The process is carried out on a plant for producing silicon tetrafluoride, the circuit diagram of which is illustrated in figure 1, however, another setup scheme using a reactor is also possible, allowing the process to be carried out at the indicated parameters.

Fluorination can be carried out by fluorine diluted with a pure inert gas, for example, helium, nitrogen, etc. At the preliminary heat treatment stage, impurities are released that can be distilled off under the indicated conditions, in particular water vapor, nitrogen and other adsorbed gases. During fluorine treatment, the remaining impurities interact and transfer to fluorine-containing gases, for example, SiOF ₂ , OF ₂ , COF ₂ , which are removed by evacuation at a residual pressure of about 5 mm Hg.

 The above processing procedures are carried out for a certain time necessary for the transition of impurities to a state in which it is possible to extract them from the feedstock in the solid phase.

 This time at each stage is from 0.5 to 3 hours in the first stage and from 0.5 to 1.5 hours in the second. The longer time of the first stage is due to the fact that it removes difficult to remove impurities that are no longer in the second stage of their distillation. The process can be carried out both continuously and in batch mode.

The method according to the invention is carried out as follows: as a starting material, a commercial product is used - sodium hexafluorosilicate with a basic substance content of Na ₂ SiF _{6 of} about 99.0 wt.% Or more.

 It can be, for example, varieties "h" and "chda" containing about 99.5-99.6 wt. % of the main substance and impurities of sulfates (0.01-0.02%), chlorides (0.1-0.04), iron up to 0.002%, heavy metals up to 0.002%.

Sodium hexafluorosilicate may contain as impurities a certain amount of oxygen-containing compounds, for example, Na ₂ SiOF ₄ , SiO _2, and the like.

The feedstock from the hopper pos. 1 is loaded into the reactor pos. 2, made of nickel, with a capacity of about 1 l, equipped with temperature sensors and pressure gauges. The reactor is checked for leaks by evacuation at a residual pressure of about 5 mm Hg. After that, the temperature in the reactor is increased to 300-350 ^o C and maintained at this temperature and pressure of about 5 mm RT.article. within 1.5-3 hours. Then, fluorine or fluorine, diluted with an inert gas, is fed into the reactor with a mass ratio of Na ₂ SiF ₆ to the fluorine fed, equal to 50-100, the feed being carried out for 0.5-1.5 hours. The ratio of fluorine and inert gas does not matter, only the ratio of Na ₂ SiF ₆ to the missed fluorine is important.

After that, gaseous products are distilled off and / or purged with pure helium or nitrogen, and then the reagents are subjected to thermal decomposition at a temperature of 550-650 ^o C. During the thermal decomposition, the resulting gas phase is continuously withdrawn from the reactor and condensed in the collector pos. liquid nitrogen.

 The spent solid phase, consisting of sodium fluoride, is removed from the reactor and sent to the collection pos. 4.

 Thus, the main differences of this invention from the known one is to carry out preliminary heat treatment, then fluorination and evacuation. The result is a product of high purity - not less than 99.99 wt.%.

 An approximate scheme of the method is given, however, it can be implemented on another installation, allowing to carry out the above procedure.

 The above examples of the process illustrate, but do not limit the scope of this invention.

EXAMPLES OF THE METHOD
EXAMPLE 1

Sodium hexafluoride of grade “h” with a basic substance content of about 99.0 wt.% With a particle size of 200-1000 μm is fed into the container pos. 1. 0.132 kg of Na ₂ SiF ₆ are measured from this container using a batcher and this quantity is transferred to the reactor pos. 2. The reactor is evacuated to a residual pressure of about 5 mm RT. Art. , and raise the temperature to 300 ± 20 ^o C, maintaining these conditions for 3 hours. Then start the flow of gaseous fluorine, introducing it in an amount of 1.7 g for 30 minutes (0.5 hours). After completion of fluorination, the reactor volume is again evacuated to a residual pressure of 3-4 mmHg, as a result of which all gaseous impurities are distilled off, and then the contents of the reactor are subjected to thermal decomposition for 2 hours, heating it to a temperature above 550 ^o С, while removal and condensation of the resulting SiF ₄ , which is collected in the collection pos. 3. Solid metal fluorides are sent to the receiver pos. 4.

 The result is silicon tetrafluoride with a purity of up to 99.991 wt.%, And sodium fluoride with a purity of 99.0 wt.%.

 Other examples of the process were carried out on the same installation, parameters and results are presented in the table.

 For comparison, experiments were carried out without treatment with fluorine (example 9) and without heat treatment of the starting salt (example 10). As a result, products were obtained whose purity does not meet the requirements for them.

Claims (2)

1. The method of producing silicon tertrafluoride, which consists in the thermal decomposition of sodium hexafluorosilicate Na ₂ SiF ₆ , characterized in that the source salt is maintained at a temperature of 300-350 ^o C and a residual pressure of up to 5 mm RT. Art. for 1.5-3 hours, then under the same conditions it is treated with fluorine for 0.5-1.5 hours with a mass ratio of sodium hexafluorosilicate: fluorine from 50 to 100, followed by evacuation and distillation of gaseous impurities, after which the salt is subjected thermal decomposition with simultaneous removal of silicon tetrafluoride.  2. The method according to p. 1, characterized in that the processing of the source salt serves fluorine in a mixture with an inert gas.

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