JP2011178586A - Method for refining polycrystalline silicon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing polycrystalline silicon of solar cell quality while stably regenerating zinc chloride when polycrystalline silicon is produced from silicon tetrachloride by a zinc reduction method. <P>SOLUTION: Regarding the method for refining polycrystalline silicon, crude metallic silicon is chlorinated into silicon tetrachloride, the silicon tetrachloride is distilled to separate impurities, and the refined silicon tetrachloride is brought into contact with metallic zinc to cause reduction, thus zinc chloride and high purity metallic silicon are generated. The method includes: (1) a wet reaction step of adding a sodium hydroxide aqueous solution to the generated zinc chloride to obtain a zinc hydroxide-sodium chloride aqueous solution; (2) an electrolysis step of electrolyzing the sodium chloride aqueous solution obtained in the step (1) so as to be separated into a chlorine gas generated from the surface of an anode, a hydrogen gas generated from the surface of a cathode and the remaining sodium hydroxide aqueous solution; and (3) a reduction step of reducing the zinc hydroxide obtained in the step (1) using the hydrogen gas generated from the cathode in the step (2)to generate metallic zinc. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for purifying polycrystalline silicon used as a material for solar cells by a zinc reduction method.

  In recent years, with an increase in demand for solar cells, there has been a demand for an increase in the supply of metal polycrystalline silicon as a raw material. Generally, metal polycrystalline silicon used as a raw material for solar cells is manufactured using non-standard products and scrap generated when manufacturing ultra-high-purity silicon necessary for semiconductor manufacturing, which improves production technology. This was largely affected by the decrease in the amount of non-standard products generated and the trend of the semiconductor market, and the supply was not stable.

In addition, polycrystalline silicon for solar cell applications does not need to be of ultra-high purity like silicon for semiconductor applications. It is inefficient to produce silicon for solar cell applications only.
For this reason, there has been a growing trend to search for a process for producing high-purity polycrystalline silicon for silicon for solar cells at low cost and in large quantities directly from low-cost, readily available crude metal silicon.

As a method for producing polycrystalline silicon having a quality equivalent to that for solar cells, many production methods are conventionally known, but a method called a zinc reduction method as described in Patent Document 1 is usually used. It is used.
In this zinc reduction method, crude metal silicon containing a large amount of impurities is reacted with chlorine to produce silicon tetrachloride, which is then rectified by distillation to separate impurities, and then zinc is added as a reducing agent. In this method, silicon tetrachloride is reduced to obtain high-purity metallic silicon.

  The zinc reduction method is considered to be the most suitable method for producing silicon for solar cells because of the quality and production cost of the high-purity silicon obtained. In addition, the zinc reduction method produces zinc chloride as a by-product when silicon tetrachloride is reduced and produced, so this zinc chloride is separated into zinc and chlorine gas by molten salt electrolysis. It can be operated in a closed state, such as being sent to the upstream process for reuse, and is also an efficient and environmentally-friendly process.

Japanese Patent Laid-Open No. 11-92130

However, in the gas discharged from the apparatus that performs the zinc reduction reaction, in addition to the generated zinc chloride gas that is the main component, there is a tendency that silicon generated in a fine powder form is also mixed, and the generated zinc chloride gas It was difficult to carry out molten salt electrolysis of zinc chloride collected by cooling, condensing and recovering. This is because if silicon is mixed into the zinc chloride molten salt, the current efficiency of the electrode reaction is remarkably reduced. If the concentration of the mixed silicon is too high, molten salt electrolysis itself may be impossible.
If the reaction to separate metal zinc and chlorine gas from zinc chloride does not proceed in molten salt electrolysis, the chlorination reaction of crude metal silicon and the reduction reaction that separates purified silicon tetrachloride into metal silicon and zinc chloride are smoothly performed. Can't do so, and the process may stop.

  For this reason, before the generated zinc chloride is subjected to molten salt electrolysis, a treatment for removing silicon or the like by performing distillation in advance or using melt filtration may be performed. However, there are problems such as complicated processes and increased generation of waste, and it has been difficult to actually operate the silicon purification process by the zinc reduction method.

  Under such circumstances, the present invention provides a method for producing polycrystalline silicon of solar cell quality while stably regenerating zinc chloride when producing polycrystalline silicon from silicon tetrachloride by the zinc reduction method. It is to provide.

In order to solve the above-mentioned problems, the first invention of the present invention is to reduce silicon tetrachloride obtained by chlorination of crude metal silicon by bringing zinc into contact with purified silicon tetrachloride from which impurities have been separated by distillation. And a method for purifying polycrystalline silicon, which comprises the following steps (1) to (3):
(1) A wet reaction step in which a sodium hydroxide aqueous solution is added to the generated zinc chloride to obtain zinc hydroxide and a sodium chloride aqueous solution.
(2) Electrolysis step of separating the aqueous sodium chloride solution obtained in the step (1) into chlorine gas generated from the anode surface, hydrogen gas generated from the cathode surface, and the remaining sodium hydroxide aqueous solution. .
(3) A reduction step of reducing the zinc hydroxide obtained in the step (1) using the hydrogen gas generated from the cathode in the step (2) to produce metallic zinc.

  The second invention of the present invention is a method for purifying polycrystalline silicon in which the sodium hydroxide aqueous solution obtained in the step (2) of the first invention is used as the sodium hydroxide aqueous solution in the step (1). .

  According to a third aspect of the present invention, the chlorine gas obtained in the step (3) of the first aspect separates oxygen mixed by the decompression process into liquefied chlorine, and the liquefied chlorine is the crude metal silicon. A method for purifying polycrystalline silicon, characterized in that it is used for chlorination from chlorosilane to silicon tetrachloride.

The present invention has the following industrially significant effects.
1. Stabilization of zinc and chlorine from zinc chloride stabilizes the operation of the zinc reduction process.
2. Since the product of the refinement process such as the alkali used as well as the metal zinc and chlorine gas used are circulated and used, the cost of producing polycrystalline silicon is reduced.
3. Since distillation and melt filtration for separating impurities from zinc chloride are not required, the equipment can be simplified and the amount of waste generated can be reduced.

It is a schematic flowchart which shows the closed cycle in this invention. It is a manufacturing process flowchart of this invention. It is the schematic of the zinc reduction test apparatus used in the Example.

  In the present invention, when zinc chloride produced as a by-product is electrolyzed in an aqueous solution in place of molten salt electrolysis, each of separated and refined metal zinc, sodium hydroxide, and chlorine gas is polycrystallized in a silicon purification process by a zinc reduction method As a result of extensive and detailed investigations on the silicon purification method, zinc hydroxide and sodium chloride aqueous solution are generated as intermediate compounds without direct electrolysis, so that the gas and liquid generated in each process can be reused. The present inventors have found that a closed cycle of the manufacturing process can be achieved, and have reached the present invention.

The present invention is constituted by the following outline.
After the crude metal silicon is chlorinated and converted to silicon tetrachloride, the silicon tetrachloride is reduced by bringing the silicon tetrachloride into contact with metal zinc and purified silicon tetrachloride, which is separated from impurities by distillation. In the purification method to be produced,
1. Zinc chloride, which is by-produced when silicon tetrachloride is reduced, is subjected to a wet reaction treatment using sodium hydroxide, and separated by filtration to produce zinc hydroxide and an aqueous sodium chloride solution.

  The aqueous sodium chloride solution produced in 2.1 is separated into chlorine gas, hydrogen gas, and aqueous sodium hydroxide solution by electrowinning using an ion exchange membrane. Chlorine gas is used for chlorination of crude metal silicon, The aqueous sodium oxide solution is used for wet reaction treatment of zinc chloride.

  3. Zinc hydroxide is reduced and roasted using hydrogen gas generated when electrolytically collecting a sodium chloride aqueous solution to form zinc and water vapor, and this zinc is used in the reduction reaction of silicon tetrachloride to produce silicon.

  That is, in view of problems such as poor electrodeposition that occurs when zinc chloride produced as a by-product is collected by molten salt electrolysis or wet electrolysis, without direct electrolysis, through a wet reaction treatment with sodium hydroxide, The sodium chloride aqueous solution produced is separated and recovered from chlorine, hydrogen, and sodium hydroxide aqueous solutions by applying a common salt electrolysis method using an ion exchange membrane, while zinc chloride is removed from the sodium hydroxide aqueous solution. This method includes an efficient method of treating by-product zinc chloride, in which zinc hydroxide recovered as a solid when subjected to wet reaction treatment is reduced using hydrogen generated by electrolysis of a sodium chloride aqueous solution.

  Usually, in the production of polycrystalline silicon for solar cell applications, it is premised that it has the performance of exhibiting a predetermined photoelectric conversion efficiency. Therefore, polycrystalline silicon used for solar cells is equivalent to 6 to 7N. It is required to ensure purity. In order to achieve such high purity, the present invention is based on a method for producing polycrystalline silicon by a zinc reduction method of silicon tetrachloride.

FIG. 1 shows a schematic flow diagram of a method for producing high-purity polycrystalline silicon according to the present invention, and FIG. 2 shows a flow diagram of the production process. The production method is roughly composed of the following five steps.
(A) Zinc reduction step of silicon tetrachloride (1) Wet reaction step of zinc chloride with sodium hydroxide aqueous solution (2) Electrolysis step of sodium chloride aqueous solution (3) Reduction reaction step of zinc hydroxide (b) Crude metal silicon Chlorination purification process

In the entire process, a closed cycle is configured as shown in FIG. The step (a): SiCl ₄ zinc reduction step and the step (b): chlorination purification step of crude metal silicon have been performed conventionally, and are newly provided in the present invention (1 ) To (3) will be described in detail.

Step (1): wet reaction step of zinc chloride with sodium hydroxide Add water to the recovered zinc chloride and perform repulp washing to separate solids of silicon fine particles. In addition, zinc is precipitated in the form of zinc hydroxide, filtered off and the liquid is recovered as an aqueous sodium chloride solution. The reaction formula in this step is shown in the following chemical formula 1.

Step (2): Electrolysis step of aqueous sodium chloride solution Sodium chloride is electrolyzed using an ion exchange membrane electrolysis method to generate chlorine from the anode and hydrogen from the cathode as shown in the following chemical formula 2. The sodium hydroxide aqueous solution is recovered.

In the present invention, wet electrolysis (salt electrolysis) by ion exchange membrane electrolysis is a technique that is currently widely used, and is often used in various industrial fields as a method for producing a simple structure that does not require auxiliary materials. Although it is a technology, the quality conditions of the sodium chloride aqueous solution to which this ion exchange membrane electrolysis method is applied are generally that the acceptable concentration of the SiO ₂ concentration in the solution in the salt electrolysis apparatus is 5 ppm or less and the concentration of the accepted sodium chloride is 300 g. / L is desirable.

Step (3): Zinc hydroxide reduction reaction step Zinc hydroxide produced by wet reaction of zinc chloride with sodium hydroxide is decomposed by low-temperature heating to become zinc oxide. This zinc oxide is reduced by heating to a high temperature in a hydrogen stream to recover zinc (see Chemical Formula 3 below). The recovered zinc is used for zinc reduction of silicon tetrachloride in the step (a).

Hereinafter, the flow of the present invention will be described with reference to the polycrystalline silicon manufacturing process flow chart of FIG.
[Chlorination and purification of crude metal silicon: step (b)]
First, in order to purify polycrystalline silicon for solar cells, crude metal silicon is chlorinated, and by utilizing the difference in boiling point temperature between silicon chloride and impurity chlorides present in trace amounts by rectification operation. Impurities are removed and silicon tetrachloride purified to a high purity is recovered as a liquid.

[Reduction of silicon tetrachloride with zinc: step (a)]
Next, the silicon tetrachloride is vaporized and supplied as a gas, and zinc zinc is vaporized and brought into contact with the silicon tetrachloride gas at a temperature of about 1000 ° C. to perform a zinc reduction reaction, thereby producing a high Crystalline silicon is produced and recovered, and zinc chloride is produced as a by-product during the reaction.

[Wet reaction of zinc chloride with aqueous sodium hydroxide: step (1)]
The zinc chloride produced as a by-product in the zinc reduction reaction is dissolved in water and washed with repulp to extract the solid silicon residue contained in it, and then sodium hydroxide is added to the aqueous solution from which the residue has been removed. The reaction was carried out by adding, and the mixture was sufficiently stirred and filtered. The residue is zinc hydroxide, and the filtered solution is an aqueous sodium chloride solution.

[Ion-exchange membrane electrolysis of aqueous sodium chloride solution: step (2)]
This aqueous solution of sodium chloride is subjected to wet electrolysis by ion exchange membrane electrolysis without being discarded, and separated into chlorine gas, sodium hydroxide aqueous solution, and hydrogen gas.
The obtained chlorine gas is returned to the upstream chlorination treatment step (step (b)) for reuse, and the aqueous sodium hydroxide solution is also used for the upstream wet treatment step (step (1)). Increase process recyclability by returning and reusing.
The quality of the sodium chloride aqueous solution used in the step (2) in the present invention can be easily and stably obtained when the SiO ₂ concentration is 2 ppm or less. If the zinc is dissolved beforehand in the water, it can be adjusted to a wide range of concentrations including the above-mentioned accepted sodium chloride concentration.

[Reduction reaction of zinc hydroxide: step (3)]
Zinc hydroxide recovered as a precipitate in the wet reaction step (1) is decomposed when heated at about 200 ° C. to remove water vapor and recovered as zinc oxide.
This zinc oxide was placed in a furnace into which hydrogen gas separated and recovered by electrolysis of sodium chloride in the step (2) was introduced, and heated and reduced at 1300 ° C. in a hydrogen gas atmosphere. The zinc vapor generated by the reduction sprays molten lead droplets onto the zinc vapor and melts the zinc in the molten lead by quenching condensation from 1300 ° C to 700 ° C to prevent reoxidation and lower the temperature by further cooling. The metal zinc and the metal lead are separated by a method using the difference in zinc concentration dissolved in the lead.

  The separated metallic lead is repeatedly used again for zinc vapor recovery, while metallic zinc is recovered as 4N level high-purity metallic zinc by performing purification by distillation to remove lead in zinc. The recovered metallic zinc is repeatedly reused as a reducing agent for silicon tetrachloride in the upstream step (a).

Hereinafter, the present invention will be described in more detail with reference to examples and drawings.
First, in order to purify silicon for solar cells, silica as a raw material was mixed and charged into a heat treatment furnace together with carbon and heated according to the manufacturing process flow chart shown in FIG. 2, and the silicon purity was roughly refined to 98-99%. Crude metal silicon was produced. The produced crude metal silicon was reacted with chlorine gas to produce silicon tetrachloride. Next, the silicon tetrachloride was purified by a rectification operation using the difference in boiling point temperature between the impurities and silicon tetrachloride to obtain purified silicon tetrachloride.

  Next, by using the zinc reduction method test apparatus of FIG. 3 as Example 1, the quartz of the ceramic electric tubular furnace 1 in which the purified silicon tetrachloride vapor and zinc vapor were kept at a temperature of about 1000 ° C. Contact was made in the furnace core tube 2 to cause a zinc reduction reaction to produce polycrystalline silicon having the desired purity. The produced polycrystalline silicon was individually recovered from the deposits on each part of the furnace inner wall, including those deposited in a dendritic shape inside the furnace core tube 2. Further, by-product zinc chloride was trapped in the recovery tank 10. The recycling process of the recovered zinc chloride is shown below.

  In FIG. 3, 1 is a ceramic electric tubular furnace, 2 is a quartz furnace core tube (transparent quartz tube), 3 is a K thermocouple, 4 is a honeycomb, 5 is a thermostat, 6 is a thermometer, and 7 is silicon tetrachloride vapor. Hood surrounding the generator, 7a is a steam exhaust draft, 8 is a gas flow meter, 9 is an Ar gas pipe, 10 is a zinc chloride recovery tank (trap bottle), 11 is a pipe, 12 is an exhaust line from the recovery tank, 13 is a dehydration and deoxygenation part (absorption bottle) of chlorine component, 14 is a hood surrounding a zinc chloride recovery tank, 20 is a heated unitube, and 21 is a three-way valve.

In Example 1, 5.2 g of metallic zinc (solid zinc) was charged into a transparent quartz tube 2 installed in the furnace core tube of the tubular furnace 1 of FIG. Silicon tetrachloride gas obtained by vaporizing 7.5 mL of liquid silicon tetrachloride in a constant temperature bath 5 at 58 ° C. is supplied into the transparent quartz tube 2 filled with zinc vapor and kept at a temperature of 1000 ° C. A zinc reduction reaction was carried out in contact with zinc to produce and recover high-purity polycrystalline silicon, and at the same time, zinc chloride was recovered as a by-product.
The amount of silicon (including SiO ₂ ) in the by-produced zinc chloride was 4 ppm. Next, 10.2 g of zinc chloride, from which zinc chloride has been repulped with pure water to remove silicon and impurities, is processed according to the schematic flow chart of the manufacturing process of FIG. Metallic zinc was obtained.

  The details are as follows. From 10.2 g of zinc chloride, 7.1 g of zinc hydroxide and a sodium chloride aqueous solution are produced in the step (1), and a cation exchange membrane electrolysis treatment of the sodium chloride aqueous solution produces chlorine gas, hydrogen gas. An aqueous sodium hydroxide solution was obtained. This hydrogen gas was used in step (3) to reduce zinc hydroxide to produce 4.7 g of metallic zinc and water. The sodium hydroxide aqueous solution was recycled as sodium hydroxide to be added when the zinc chloride was wet-treated in the step (1). Chlorine gas can be used for chlorination / purification of crude metal silicon in the step (b).

The amount of metal zinc produced and the amount of metal zinc added were compared to determine the recovery rate of metal zinc, and its resource saving properties were evaluated. The amount of input metal zinc in the table is described by subtracting the unreacted zinc content.
In the evaluation of resource saving, it was assumed that the more reusable items collected, the better the resource saving.

  As in Example 1, in order to purify polycrystalline silicon for solar cells, first, silica as a raw material is mixed and charged into a heat treatment furnace together with carbon and heated to roughly refine the silicon purity to 98-99%. Crude metal silicon was prepared. The produced crude metal silicon was reacted with chlorine gas to produce silicon tetrachloride.

  Next, after obtaining purified silicon tetrachloride, the purified silicon tetrachloride vapor and the zinc vapor were put into the quartz furnace core tube 2 in the same manner as in Example 1 using the zinc reduction method test apparatus shown in FIG. A zinc reduction reaction is caused to react to produce polycrystalline silicon for a solar cell with a predetermined purity, and the substances deposited on each part of the inner wall of the furnace, including those deposited in a dendritic shape inside the furnace core tube 2, are individually It was collected. Further, zinc chloride produced as a by-product was recovered from the trapped in the recovery tank 10, the inside of the pipe leading to the recovery tank through the exhaust pipe from the inside of the furnace to the outside, and the inner wall of the exhaust line after the recovery tank. The recycling process of the recovered zinc chloride is shown below.

In Example 2, 19.4 g of metallic zinc (solid zinc) was charged into the transparent quartz tube 2 installed in the furnace core tube of the tubular furnace 1 of FIG. Silicon tetrachloride gas obtained by vaporizing 28.0 mL of liquid silicon tetrachloride in a constant temperature bath 5 at 58 ° C. is supplied into the transparent quartz tube 2 filled with zinc vapor, and kept at a temperature of 1000 ° C. A zinc reduction reaction was carried out in contact with zinc to produce and recover high-purity polycrystalline silicon, and at the same time, zinc chloride was recovered as a by-product.
The amount of silicon (including SiO ₂ ) in the by-produced zinc chloride was 80 ppm. Next, 37.2 g of zinc chloride, which has been subjected to repulp washing to remove silicon and impurities until the mixed amount of silicon becomes 5 ppm or less, is treated according to the schematic flow chart of the manufacturing process of FIG. 17.0 g of metallic zinc was obtained.

The details are as follows. From 37.2 g of zinc chloride, 25.8 g of zinc hydroxide and a sodium chloride aqueous solution are produced in the step (1), and cation exchange membrane electrolysis of the sodium chloride aqueous solution produces chlorine gas, hydrogen Gas and aqueous sodium hydroxide were obtained. Hydrogen gas was used in step (3) to reduce zinc hydroxide to produce 17.0 g of metallic zinc and water.
The amount of metal zinc produced and the amount of metal zinc added were compared to determine the recovery rate of metal zinc, and its resource saving properties were evaluated.

(Conventional example)
In the same manner as in Example 1, after obtaining purified silicon tetrachloride from metal silicon, a zinc reduction reaction was performed to purify silicon for solar cells having the desired purity.

The zinc chloride produced as a by-product was purified from the zinc chloride recovery tank and all the zinc chloride recovered from locations other than the recovery tank. Evaluation was conducted in the same manner as in Example 1, and the results are shown in Table 1.
At this time, silicon (including SiO ₂ ) contained in zinc chloride was 90 ppm.

DESCRIPTION OF SYMBOLS 1 Ceramic electric tubular furnace 2 Quartz furnace core tube 3 K thermocouple 4 Honeycomb 5 Thermostatic bath 6 Thermometer 7 Hood surrounding the silicon tetrachloride vapor generation part 8 Gas flow meter 9 Ar gas piping 10 Zinc chloride recovery tank (trap bottle )
11 Pipe 12 Exhaust line from recovery tank 13 Dehydration and deoxygenation part of chlorine component (absorption bottle)
14 Hood that surrounds the zinc chloride recovery tank 20 Unitube to be heated 21 3-way valve

Claims (3)

In a method for purifying polycrystalline silicon in which zinc tetrachloride is reduced by contacting zinc with purified silicon tetrachloride obtained by distilling silicon tetrachloride obtained by chlorination of crude metal silicon and separating impurities, and zinc chloride and high-purity metal silicon are produced. A method for purifying polycrystalline silicon comprising the following steps (1) to (3):
(1) A wet reaction step in which a sodium hydroxide aqueous solution is added to the generated zinc chloride to obtain zinc hydroxide and a sodium chloride aqueous solution.
(2) Electrolysis step of separating the aqueous sodium chloride solution obtained in the step (1) into chlorine gas generated from the anode surface, hydrogen gas generated from the cathode surface, and the remaining sodium hydroxide aqueous solution. .
(3) A reduction step of reducing the zinc hydroxide obtained in the step (1) using the hydrogen gas generated from the cathode in the step (2) to produce metallic zinc.   2. The method for purifying polycrystalline silicon according to claim 1, wherein the aqueous sodium hydroxide solution obtained in the step (2) is used as the aqueous sodium hydroxide solution in the step (1).   The chlorine gas obtained in the step (3) separates oxygen mixed by decompression to become liquefied chlorine, and the liquefied chlorine is used for chlorination from the crude metal silicon to silicon tetrachloride. The method for purifying polycrystalline silicon according to claim 1.

Images