JPS59164613A - Manufacture of finely powdered silicon of high purity - Google Patents

Abstract

PURPOSE:To manufacture finely powdered Si of high purity at a low cost by mixing K2SiF6 or Na2SiF6 as an Si source with NaN3 or NaH as a reducing agent and by heating the mixture at a specified temp. to cause a reaction. CONSTITUTION:K2SiF6 or Na2SiF6 is mixed with NaN3 or NaH as a reducing agent in 1:6-1:1 molar ratio, and the mixture is heated in an inert gaseous atmosphere under ordinary pressure. The heating temp. is within the range of the decomposition temp. of NaN3 or NaH as a reducing agent to the m.p. of an alkali fluoride such as KF or NaF as a by-product. The K2SiF6 or Na2SiF6 is reduced with Na produced by the decomposition of the NaN3 or NaH to form Si. Since NaF or KF as a by-product and the unreacted starting material stick to the formed Si, and the Si is treated with an aqueous soln. of hydrofluoric acid, washed, and dried to obtain fine Si powder of high purity.

Description

[Detailed description of the invention] The present invention relates to a method for producing high purity fine powder silicon.

In recent years, high-purity fine powder silicon has been increasingly used as a raw material for structural ceramics such as semiconductors, solar cells, and engines.

Conventional methods for producing silicon include reducing silicon dioxide using magnesium, aluminum, carbon, etc., but these methods have drawbacks such as difficulty in obtaining high purity and high cost. Therefore, attention is being focused on a method using alkali silicofluoride as a silicon source. There are two known methods related to the alkali silicofluoride method: One is (a) a method in which alkali silicofluoride is mixed with metallic aluminum and heated, and the other is (b) a method in which alkali silicofluoride is mixed with metallic sodium and heated. .

However, method (a) has the following three drawbacks. That is, (1) unreacted metal aluminum may remain in the product, (2) silicon and metal aluminum may form an alloy, or (2) aluminum fluoride may remain in silicon. Furthermore, metal aluminum is a power-intensive raw material and is expensive.

Furthermore, method (b) also has the following two drawbacks. In other words, ■ Handling of metallic sodium is difficult from a process standpoint, and ■ It is almost impossible to prepare finely powdered metallic sodium. Therefore, the reduction reaction of alkali silicofluoride using metallic sodium proceeds smoothly. That is difficult.

The present inventors have conducted extensive research in order to invent a method for producing high-purity fine powder silicon by reduction of alkali fluorosilicate, which does not have the drawbacks of the above-mentioned known methods. As a result, a mixture of a silicon source material consisting of sodium silifluoride and/or potassium silifluoride and a reducing agent consisting of sodium azide and/or sodium hydride in a molar ratio of 1=6 to 1:1 was obtained. It was discovered that high-purity fine powder silicon can be easily obtained in terms of process by heating and reacting to a temperature above the decomposition temperature of the reducing agent and below the melting point of the alkali fluoride produced, and then treating the reactant with an acid. The present invention was completed. The silicon source raw material and the reducing agent used in the present invention are both easy to purify, allowing the use of highly purified products, and have the advantage of being relatively inexpensive.

The reaction formula for silicon production in the method of the present invention is as follows (1
) and (2).

K2S1F6+4NaN3→ s, + 2 KF+ 4 N, F + 6 N2↑・
...(1) K2S, F6+ 4N H time S, + 2 KF+ 4 N, F + 2 N2↑・Old... (2) Commercially available alkali silicofluorides can be used as the silicon source material. The alkali fluorosilicate used in the present invention preferably has a purity of 98.0% or higher for the purpose of the present invention. Commercially available sodium azide can be used as the reducing agent, but it can be easily purified to a purity of 98.9% or higher by a known method. .0% or more is desirable. Commercially available sodium hydride can be used as the reducing agent, but commercially available hydrogen (for example, purity 99.99% or higher) and metallic sodium (for example, purity 89.8% or higher) can be used. High purity powdered sodium hydride (for example, purity of 99.9% or more) can be obtained by reacting according to the method.

In the method of the present invention, the above-mentioned silicon source material and reducing agent are mixed in advance at a constant molar ratio and then subjected to the subsequent heating reaction.

However, when using a continuous reaction method, the silicon source raw material and the reducing agent are separately supplied into a mixer (note: this is not necessary) and a reaction vessel while maintaining a constant molar ratio for mixing and reduction reaction. may be performed in parallel.

The molar ratio of reducing agent/silicon source material is 1 to 6, preferably
It is 1.2-2.5. If the molar ratio is less than 1, the yield will be very small, and even if the molar ratio exceeds 6, the yield will hardly change, which will not only be uneconomical, but also cause excessive sodium metal to remain in the reaction mixture. This makes subsequent processing complicated.

In the present invention, the silicon source raw material and the reducing agent are used in powder form, but the particle size normally available is sufficient, and since the reactions of formulas (1) and (2) above are solid phase reactions, No pulverization or intimate mixing is required to the extent that fine grinding is involved.

The raw material mixture thus obtained is subjected to the next heating reaction step. The raw material mixture is heated and reacted in a temperature range from above the decomposition temperature of the reducing agent to below the melting point of the alkali fluoride produced by the heating reaction.

In the reaction of equation (1), the alkali azide in the raw material mixture decomposes to produce pure metallic sodium while generating nitrogen at temperatures above 300°C, and at the same time, this metallic sodium reduces the alkali silicofluoride to produce sodium fluoride. becomes.

On the other hand, if the alkali silicofluoride has the necessary and sufficient purity, for example, 99.0% purity, it is possible to finally obtain high-purity fine powder silicon with a purity of 89.9% through the above-mentioned reduction reaction. .

In the reaction of equation (2), the sodium hydride in the raw material mixture decomposes to produce pure metallic sodium while generating hydrogen at temperatures above 425°C, and at the same time, this metallic sodium is produced in the same way as in the reaction of equation (1) above. By reducing the alkali fluorosilicate, high-purity fine powder silicon can be finally obtained.

In the reactions of formulas (1) and (2), the reduction reaction does not proceed smoothly if the heating temperature is lower than the decomposition temperature of the reducing agent.

Further, the practical upper limit heating temperature in this reaction is the temperature immediately before the alkali fluoride produced as a by-product in the reactions of formulas (1) and (2) begins to melt.

(Note: For example, if only sodium fluoride is produced as a by-product as an alkali fluoride, the practical upper limit heating temperature is below the melting point of sodium fluoride, 892°C, and both sodium fluoride and potassium fluoride are produced as an alkali fluoride. In this case, the practical upper limit heating temperature is below the melting point of potassium fluoride, 860°C.) If heated above the melting point of the alkali fluoride, the solid phase decomposition and reduction reaction will not proceed smoothly.

Since the above reaction is a solid-phase reaction and involves the generation of gas, it is preferable to carry out the reaction in an inert gas atmosphere at normal pressure, and furthermore to be able to discharge the generated gas out of the reaction system without any problem.

The reaction time of the above reaction is from 5 minutes to 1000 minutes, preferably from 20 minutes to 200 minutes. Select an appropriate heating time depending on the heating temperature conditions.

In both the reactions of formula (1) and formula (2), the reaction apparatus used is an electric furnace or a so-called reaction furnace in which internal temperature can be controlled by heating from the outside with a burner or the like. The furnace material used must be able to withstand strong basicity due to the nature of the reaction raw materials, but the reaction temperature must be between 300 and 90°C.
Since the temperature is about 00°C, no special furnace material is required. In a laboratory, for example, a quartz tube is used as a furnace, a raw material mixture is placed in a porcelain board, and heated in the furnace to a predetermined temperature for a predetermined period of time.

After the reaction is completed, the reaction mixture is cooled to near room temperature, then taken out from the reactor and subjected to the next step, that is, acid treatment. As can be understood from equations (1) and (2), the purpose of this acid treatment is to remove byproducts (KF, NaF) and unreacted raw materials (NaN) from the reaction mixture after the reaction is completed.
3. NaH and alkali fluorosilicate) are dissolved and removed using an acid aqueous solution, and if necessary, washed with pure water and dried to obtain high purity fine powder silicon.

The reason why acid treatment is used instead of water treatment is that dissolution and removal can be carried out more effectively by using an acid aqueous solution. The acid used for the acid treatment is not limited as long as the above purpose (isolation of fine silicon powder) can be achieved, but considering the ease with which the acid itself is removed after the acid treatment, hydrochloric acid and hydrofluoric acid are is typical.

In the manner described above, high-purity fine powder silicon, which is the object of the present invention, is obtained. The purity of this product can be measured by known methods, such as fluorescent X-ray method (Note: Purity H can be measured up to about 99%), atomic absorption spectrometry (Note: Purity H can be measured up to about 99.998%), etc. The purity of silicon obtained by the method of the invention depends on the purity of the raw materials, but
．． It is about 8% to 99.99%. Further, the particle size of this material can be easily measured using, for example, a transmission electron microscope, and the particle size of silicon obtained by the method of the present invention is extremely fine with an average particle size of 102 to 104[lambda]. The present invention will be explained below with reference to Examples.

Example 1 85g of sodium silifluoride powder with purity θ9.0% (0
, 45 moles) and 87.2 g (1,03 moles) of sodium azide powder with a purity of 89.8% were mixed and placed in a porcelain boat, and the port was placed in a quartz tube furnace and placed in a nitrogen gas atmosphere. The mixture was heated at 400° C. for 60 minutes to react.

After cooling to room temperature, the reaction mixture was acid-treated with 1000 ml of 5% aqueous hydrofluoric acid solution, washed with water and dried to obtain 7.0 g of fine powder silicon. The particle size of the fine powder observed with a transmission electron microscope was 100 to 1,000 particles. Also,
The amount of impurities measured by fluorescence X-ray method was less than 0.1%, and it was confirmed that the silicon purity of the fine powder was 99.8% or more.

Example 2゜208g of potassium silicofluoride powder with a purity of 88.0% (0
, f34 mol) and 104 g (1,80 mol) of sodium azide powder with a purity of 98.8% at 600°C.
11.0 g of finely powdered silicon was obtained in the same manner as in Example 1, except that 2000 + fL of a 10% aqueous hydrochloric acid solution was used as an acid treatment agent for the reaction mixture after the reaction was completed. The purity of this product was 98.9% or more, and the average particle size was 200A.

Example 3 188g of sodium silifluoride powder with a purity of 98.0% (
1 mol) and 99.0% purity sodium hydride powder 4
Using 8g (2 mol), react at 600℃ for 60 minutes,
5% salt as an acid treatment agent for the reaction mixture after the reaction $2000
The same procedure as in Example 1 was carried out except that mM was used, and 13 g of finely powdered silicon was obtained. The purity of this stuff is 98.9%
The average particle size was 150A.

Example 4 138g of potassium silicofluoride powder with a purity of 99.0% (0
, 30 mol) and 14.4 g (o, eo mol) of sodium hydride powder with a purity of 88.0% were used in the same manner as in Example 1, except that the reaction was carried out at 650°C for 50 minutes to form a fine powder. 4.0 g of silicon was obtained. The purity of this product is 99.
9% or more, and the average particle size was 200'A.

that's all 1 82-

Claims (1)

[Claims] A mixture of a silicon source material consisting of sodium silifluoride and/or potassium silifluoride and a reducing agent consisting of sodium azide and/or sodium hydride in a molar ratio of 6 to 1:1 is used as a reducing agent. A method for producing high-purity fine powder silicon, which comprises heating to a temperature above the decomposition temperature of and below the melting point of the alkali fluoride produced to cause a reaction, and treating the reaction product with an acid.