JP2011042523A - Refining method for silicon for solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a zinc reduction method for refining crude metal silicon to produce silicon for solar cells, wherein the silicon for solar cells is produced in high productivity and a low cost by efficiently treating zinc chloride obtained as a by-product. <P>SOLUTION: In the refining method for silicon for solar cells, silicon tetrachloride purified by removing impurities from crude silicon tetrachloride prepared by chlorinating metal silicon is reduced by being contacted with metal zinc to produce silicon. The zinc chloride obtained as a by-product when the silicon tetrachloride is reduced by the metal zinc is sorted by a level of oxygen contained in the zinc chloride and subjected to processing for obtaining the metal zinc and chlorine by electrolysis or processing for obtaining crude zinc oxide. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

As the demand for solar cells increases, the demand for metallic silicon as a raw material is also increasing.
Silicon used as a raw material for solar cells is refined from low-grade crude metal silicon having a purity of around 99%, which is obtained by reducing silica mined in a mine, but it has an ultra-high purity reaching 99.99999999% used in semiconductors. Compared with silicon for semiconductors, even a low-grade product that has been purified to a purity of around 99.9999% can be used sufficiently.

  Conventionally, however, there is no method for easily obtaining silicon of such purity, and expensive non-standard silicon for semiconductors and scrap are often used. When this silicon for semiconductors is used, there is a problem that supply is unstable due to high cost and competition with the semiconductor market. Therefore, when used for solar cells, quality as high as that of semiconductors is not necessary, so a method for obtaining silicon for solar cells having the purity required for solar cells directly from crude metal silicon produced in large quantities at low cost has been studied. .

Among them, a method called a zinc reduction method has been reviewed.
In this zinc reduction method, the above-mentioned crude metal silicon is used as a raw material, and this is reacted with chlorine gas to produce silicon tetrachloride. The silicon tetrachloride is distilled to separate impurities, and then contacted with zinc to form a tetrachlorosilane. In this method, silicon chloride is reduced to obtain zinc chloride and silicon.
When this zinc reduction method is used, it is difficult to obtain ultra-high purity silicon for semiconductors, but it is possible to purify silicon of quality that can be used for solar cells at low cost.

  Further, in the zinc reduction method, zinc chloride is produced as a by-product when silicon tetrachloride is reduced and metal silicon is produced. If this zinc chloride is electrolyzed, it can be separated into zinc and chlorine gas. The separated zinc can be used again for the reduction of silicon tetrachloride, and the chlorine gas on one side can be sent to the upstream chlorination step of the crude metal silicon for use, and a closed and easy-to-handle process can be constructed.

  In addition, when silicon tetrachloride is reduced in contact with zinc, the reaction is performed at a temperature of about 700 ° C. to 1200 ° C., but the by-product zinc chloride is the first because of the relationship between boiling point (732 ° C.) and melting point (365 ° C.). It is produced as a gas or liquid and is transported away from the reaction site to be cooled and deposited as a liquid or solid. This liquid or solid zinc chloride is recovered and electrolyzed.

  The method of electrolyzing the recovered zinc chloride is mainly used by dissolving zinc chloride and performing wet electrolysis as an aqueous solution, and by melting zinc chloride at a high temperature and performing molten salt electrolysis. Various impurities may be mixed in the zinc and chlorine gas separated by electrolysis of zinc or zinc chloride, and depending on the degree of mixing, it may not be electrolyzed or may not be transferred to the next silicon purification process. Most of these impurities volatilize when zinc and silicon tetrachloride react even if they are mixed, or volatilize due to differences in temperature during reaction and vapor pressure of zinc. There are many things that are different.

  However, it is known that oxygen, among impurities, is difficult to separate from zinc, chlorine gas, and the like and has a great influence when mixed. For example, oxygen contained in raw materials at a level of several tens of ppm will spread to the composition and properties of the electrolyte within a short period of time, resulting in problems in electrolytic treatment, reducing the efficiency of zinc recovery, and reducing the lifetime of the electrolyte. May be damaged. Therefore, it is necessary to suppress the oxygen concentration of by-product zinc chloride to less than 100 ppm.

Specifically, when melt electrolysis is used, if oxygen is present in the molten zinc chloride electrolytic bath, oxygen gas may be generated together with chlorine gas from the negative electrode. When this oxygen gas is mixed in the reduction process of silicon tetrachloride, oxygen interferes with the reduction of silicon, and silica is generated, resulting in a loss. In addition, silicon with high oxygen quality can cause deterioration in the quality of solar cells.
Therefore, management of the oxygen quality contained in zinc chloride by-produced in this way has become more important.

As a method for producing metal silicon by such a zinc chloride method, for example, there is a method disclosed in Patent Document 1.
In this method, silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in a reaction vessel, a mixture containing the produced polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and the mixture is stored in a separation vessel. Then, after the zinc chloride in the mixture is separated, the polycrystalline silicon is recovered from the separation container.

  In Patent Document 2, silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in a reaction vessel, and a mixture containing the produced polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and then the mixture is contained in the mixture. In a method for producing high purity silicon in which zinc chloride is separated and polycrystalline silicon is recovered, the separated zinc chloride is electrolyzed to recover metallic zinc and chlorine, and the recovered metallic zinc is again recovered into silicon tetrachloride. In addition, a method for producing high-purity silicon used for chlorination of metallic silicon to produce silicon tetrachloride by synthesizing recovered chlorine with hydrogen to produce hydrogen chloride is disclosed.

  Patent Document 3 uses metal silicon as a raw material, generates chlorine tetrachloride by acting chlorine gas, and further purifies it by rectification, and the high purity silicon tetrachloride and metal zinc vaporized at a high temperature are 900. In gas phase zinc reduction process in which high purity silicon is produced by contacting gas and gas in a reactor at ˜1100 ° C., zinc chloride by-produced in the reactor is electrolyzed with chlorine gas and high purity metal electrolytic zinc The chlorine gas is reacted with the main silicon metal after purification to form silicon tetrachloride, and the electrolytic zinc is vaporized again and reacted with silicon tetrachloride to circulate and produce high-purity silicon with few by-products. The method is shown.

  Furthermore, in the method disclosed in Patent Document 4, silicon tetrachloride and zinc are subjected to a gas phase reaction in a reaction furnace at a temperature of 910 to 1200 ° C. to obtain high-purity polycrystalline silicon. This is a method for producing polycrystalline silicon in which a silicon seed crystal is charged, silicon is deposited on the seed crystal, and a part of by-product zinc chloride is mixed with a raw material gas to form a circulating gas.

  On the other hand, in Patent Document 5, zinc oxide is produced by reacting zinc-containing ore with oxygen, this zinc oxide is reduced with carbon to generate zinc gas, and the generated zinc gas is distilled and condensed to melt. In a method for producing high-purity silicon in which a zinc smelting process for collecting zinc and a process for reducing silicon tetrachloride, which is a silicon raw material, are reduced to zinc, silicon tetrachloride is reduced with molten zinc in a reaction vessel during zinc reduction. A method for producing high-purity silicon for recovering polycrystalline silicon is shown.

JP 11-11925 A Japanese Patent Laid-Open No. 11-92130 JP 2003-34519 A JP 2003-342016 A Japanese Patent Laid-Open No. 11-60228

  However, in each of these conventional methods, regarding the handling of by-produced zinc chloride, after collecting the partially produced zinc chloride, a method is disclosed in which a part is mixed with the raw material gas and used for reaction rate control. However, there was no suggestion regarding the handling of by-produced zinc chloride to maintain the quality of zinc chloride and to operate the process smoothly.

  Therefore, in view of such a situation, the present invention is a zinc reduction method for refining solar cell silicon from crude metal silicon, by efficiently treating by-product zinc chloride, solar cell silicon with high productivity, It is intended to provide a zinc reduction method that can be produced at low cost.

  The first invention of the present invention for solving the above-mentioned problems is to contact silicon tetrachloride by contacting silicon tetrachloride purified by separating impurities from silicon tetrachloride obtained by chlorination of metal silicon and metal zinc. In the method of obtaining silicon by reduction, zinc chloride produced as a by-product when reducing silicon tetrachloride by contacting with zinc metal is classified according to the oxygen grade contained in the zinc chloride and is separated from metallic zinc by electrolytic treatment. This is a method for purifying silicon for solar cells, which is divided into a step of obtaining chlorine and a step of obtaining crude zinc oxide.

  According to a second aspect of the present invention, in the first aspect, the classification according to the oxygen grade contained in the zinc chloride uses the oxygen grade of 100 ppm as the category value, and the zinc chloride having an oxygen grade of less than 100 ppm is subjected to electrolytic treatment by the electrolytic treatment. And a process for obtaining chlorine, and zinc chloride having an oxygen grade of 100 ppm or more is treated by a process for obtaining crude zinc oxide.

  According to a third aspect of the present invention, in the first aspect or the second aspect, the step of obtaining metallic zinc and chlorine by electrolytic treatment is performed by a molten salt electrolysis method using molten zinc chloride as an electrolytic solution, or by using zinc chloride. This is a method for purifying silicon for solar cells, in which a dissolved aqueous solution is separated into metallic zinc and chlorine using any one of the electrowinning methods using an electrolytic solution, and the separated chlorine is recovered by dehydration and deoxygenation.

  According to a fourth aspect of the present invention, in the first aspect or the second aspect, the step of obtaining the crude zinc oxide is performed by adding water to zinc chloride and containing sodium hydroxide, potassium hydroxide, or sodium carbonate. And then the slurry obtained by the alkali treatment is separated into a solid and a liquid component by solid-liquid separation treatment, and the obtained solid is dried and in the presence of air. It is roasted and separated into crude zinc oxide and chlorine components that volatilize from the solid content, and the separated liquid components are separated into chlorine gas, hydrogen gas, and sodium hydroxide or potassium hydroxide by electrowinning using an ion exchange membrane. This is a method for purifying solar cell silicon.

  The fifth aspect of the present invention is the liquid crystal according to the fourth aspect, wherein chlorine gas obtained by electrolytically collecting the separated liquid component using an ion exchange membrane is used for chlorination of metal silicon. This is a method for purifying silicon for solar cells used for alkali treatment in the step of obtaining crude zinc oxide from sodium hydroxide or potassium hydroxide obtained by electrowinning components using an ion exchange membrane.

  According to the present invention, when refining solar cell silicon from low-purity metal silicon, chlorine gas used when chlorinating metal zinc or metal silicon used as a reducing agent during the process is more efficient. This makes it possible to purify silicon for solar cells, which is inexpensive and resource-saving because it can be recycled.

It is a schematic flowchart explaining the manufacturing process of this invention. It is a schematic flowchart of the manufacturing process used in Example 1 of this invention, and is a figure at the time of using soda ash for the process of obtaining crude zinc oxide. It is a schematic flowchart of the manufacturing process used in Example 2 of this invention, and is a figure at the time of using sodium hydroxide for the process of obtaining crude zinc oxide. It is a schematic explanatory drawing of the manufacturing apparatus of the silicon for solar cells used in the Example.

  As a result of diligent research, the inventors of the present invention have obtained an observation result that zinc chloride is often recovered not in one place but in a plurality of places in the production process of metallic silicon. Thus, it has been studied in detail and devised, and the silicon purification method of the present invention has been invented.

Depending on the temperature of the zinc reduction reactor, this zinc chloride is produced in the form of gas or melt, but depending on the cooling state of the piping during the process of taking it out from the reduction reactor, the zinc chloride on the inner wall of the piping etc. There is a certain amount of adhesion.
Also, the amount of zinc chloride deposited in the front and rear areas of the regular collection site may reach a ratio that cannot be ignored as a whole, and in that case, these zinc chlorides are separated from the inner wall of the pipe. The quality of zinc chloride deteriorated due to the influence of oxygen mixed in from the gas phase in the surrounding environment during the operation of oxygen mixing and peeling from the tube wall.

  Therefore, if the oxygen grade of zinc chloride is ignored and the zinc chloride is used together, the influence from places with a large impurity level will affect the whole, and these are distinguished by the level of the impurity level. The inventors of the present invention have found that the problem can be solved if a method of optimizing the resource recovery method from recovered zinc chloride is used in the method of handling separately.

FIG. 1 is a schematic flow diagram showing an example of a solar cell silicon manufacturing process including a resource recovery method from the recovered zinc chloride.
In the manufacturing process shown in FIG. 1, the recovered zinc chloride having an oxygen grade of less than 100 ppm (accepted) is separated into metallic zinc and chlorine gas by electrolytic treatment, whereas the zinc chloride having an oxygen grade of 100 ppm or more (failed). Is subjected to an alkali treatment and then separated into a solid and a liquid component by solid-liquid separation.

This solid content is separated into crude zinc oxide and a chlorine component by the crude zinc oxide treatment, and smelted from the crude zinc oxide to metallic zinc.
On the other hand, chlorine gas is separated from the liquid component and can be used for chlorination of metallic silicon. Hydrogen gas and sodium hydroxide or potassium hydroxide are separated as other by-products.

  In the present invention, zinc chloride with high oxygen quality is an alkaline agent for an aqueous solution from which residues are removed after repulp washing to remove solid silicon residues and the like partially contained by dissolving in water. A certain amount of sodium carbonate (soda ash) was added and reacted, and the resulting insoluble zinc carbonate was further washed with water and repulp as necessary, and the remaining slurry was filtered and dehydrated. The solids are then dried and roasted, and depending on the temperature achieved there, some remaining chlorine component is reduced by volatilization if necessary, and the zinc content or crude zinc oxide with a chlorine content of several percent or less Solidify as

The obtained crude zinc oxide is smelted as a raw material used for existing dry zinc smelting to recover metallic zinc and can be used again for refining metallic silicon. The ratio of zinc chloride to be electrolyzed and zinc chloride treated in the zinc smelting process via crude zinc oxide is preferably adjusted within a range of 70:30 to 100: 0 by weight.
In the electrolytic treatment, even if the oxygen component, which is an impurity element, may be mixed into the crude zinc oxide, the crude zinc oxide itself is primarily composed of oxygen compounds. There will be no obstacles in each process.

  Furthermore, after repulp washing solid silicon residues and the like that are partially contained by dissolving zinc chloride in water, when adding an alkaline agent to the aqueous solution from which the residues have been removed, the chlorine component is a sodium chloride aqueous solution, or It becomes a dissolved component of an aqueous solution of potassium chloride as an aqueous solution of chloride.

  Since the chlorine component is reused as raw material chlorine in the chlorination treatment of metallic silicon, which is the first step in the zinc reduction method, the aqueous solution of chloride is effectively used without being discharged. Specifically, an aqueous solution of chloride was subjected to wet electrolysis by an ion exchange membrane electrolysis method and separated into chlorine gas, sodium hydroxide aqueous solution or potassium hydroxide aqueous solution, and hydrogen gas.

The chlorine gas obtained in this step is returned to the chlorination treatment step, which is an upstream step, and reused to improve the process recyclability.
Moreover, the sodium hydroxide aqueous solution or potassium hydroxide aqueous solution obtained here can also be recycled as an alkaline agent added when processing zinc chloride with high oxygen quality.

  Regarding the classification of the oxygen grade of zinc chloride in the present invention, the evaluation of the oxygen grade of zinc chloride may be a time-consuming method of performing an actual analysis for each operation, but the fluctuation of the oxygen grade over time accompanying the operation is observed in advance. , A method of sorting zinc chloride at a place corresponding to the change over time, such as immediately after the timing when the produced silicon is taken out of the reduction furnace and other operation periods, or immediately after the start of operation and other operation periods, etc. A method of classifying the processing method according to the above may be used.

  In addition, zinc chloride collected at a place designated as a place where zinc chloride produced as a by-product is collected in advance is treated in an electrolytic treatment process, and zinc chloride produced as a by-product in other places is collected into a crude zinc oxide process. It is also possible to classify them so that they are processed by

  In this classification of zinc chloride, the reason for dividing the value of oxygen content at 100 ppm as a boundary is that zinc chloride having an oxygen content of less than 100 ppm and having low oxygen grade is suitable for metal zinc without causing any trouble by electrolytic treatment. It can be separated into chlorine and high-quality zinc chloride having an oxygen content of 100 ppm or more, and is classified because it is not suitable for electrolytic treatment because it generates excessive oxygen.

  If the amount of zinc chloride by-product with high oxygen quality increases, the proportion of zinc chloride that cannot be applied to electrolytic treatment increases, and the process may not be able to move smoothly. By using a method that separates and treats zinc chloride in accordance with conditions that can be treated, the process for purifying silicon for solar cells is made smooth.

  Furthermore, according to the present invention, since chlorine gas as a raw material can be recovered from zinc chloride containing a large amount of impurities such as oxygen, not only recovery of zinc components but also zinc chloride recovered from a zinc reduction furnace and recovered Regardless of the quality, the recovery amount of chlorine gas is kept constant, so that the chlorine balance of the process can be maintained.

  The details will be further described using the following examples.

  In order to purify silicon for solar cells, first, silica as a raw material is mixed and charged into a heat treatment furnace together with carbon in accordance with a schematic flow chart of the manufacturing process shown in FIG. 2, and the silicon purity is coarsened to 98 to 99%. Purified 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 impurity and silicon tetrachloride to obtain purified silicon tetrachloride.

  Next, by using the zinc reduction method test apparatus shown in FIG. 4, the quartz core of the ceramic electric tubular furnace 1 in which the purified silicon tetrachloride vapor and the zinc vapor are kept at a temperature of about 1000 ° C. Contact was made in the tube 2 to cause a zinc reduction reaction to produce silicon having the desired purity. The generated silicon was individually recovered from the deposits on each part of the furnace inner wall, including those precipitated in a dendritic shape inside the furnace core tube 2.

  In FIG. 4, 1 is a ceramic electric tubular furnace, 2 is a quartz 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 generation. Hood enclosing the part, 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 a chlorine component, 14 is a hood surrounding a zinc chloride recovery tank, 20 is a heated unitube, and 21 is a three-way valve.

Therefore, as shown in the schematic flow diagram of FIG. 2, the impurity content state grasping (oxygen content) for each collection place in the apparatus of FIG. 4 was performed.
The zinc chloride by-produced in the apparatus of FIG. 4 was also deposited on the zinc chloride recovery tank 10, the inside of the pipe 11 from the furnace to the recovery tank 10, the inner wall of the exhaust line 12 after the recovery tank 10, and the like.
When zinc chloride taken out from the recovery tank 10 and zinc chloride taken out from each part in the pipe were analyzed for oxygen quality, the oxygen quality of the zinc chloride recovered in the recovery tank 10 was as low as around 50 ppm, The oxygen grade of zinc chloride produced as a by-product other than that was 150 ppm, which was high enough to affect the electrolytic treatment. In addition, the cause of the difference in the oxygen grade contained depending on the location can be attributed to a difference in temperature or contamination level of each adhering member, or a difference in the degree of contamination from the atmosphere at the time of recovery.

Zinc chloride having an oxygen grade of 50 ppm taken out from the recovery tank was electrolyzed to recover metallic zinc and chlorine gas.
On the other hand, zinc chloride with a high oxygen grade of 150 ppm extracted from other than the recovery tank is solidified as crude zinc oxide by the process shown in FIG. 1 and smelted as a raw material used in the existing dry zinc smelting, and the metallic zinc is It was collected. In addition, even if the oxygen component that is an impurity element in the electrolytic treatment may be mixed into the crude zinc oxide, the crude zinc oxide itself is primarily an oxygen compound, so in any subsequent dry zinc smelting It is considered that no trouble occurs in each process.

The amount of metal zinc input was compared with the amount of metal zinc recovered from zinc chloride having an oxygen grade of 50 ppm and the amount of metal zinc recovered from zinc chloride having an oxygen grade of 150 ppm, and the recovery rate of metal zinc was determined. The resource saving properties were evaluated and listed in Table 1.
In addition, the recovery rate of metallic zinc assumes that the whole amount of zinc can be obtained from zinc chloride, obtains the amount of metallic zinc from the amount of recovered zinc chloride, and compares it with the amount of metallic zinc input. The case where the recovery rate exceeded 75% was indicated as “◯”, the range between 75% and 50% was indicated as “Δ”, and the case where the recovery rate was less than that was indicated as “X”.
In the evaluation of resource saving, the more items that can be reused, the better the resource saving.

  Further, the chlorine component became a dissolved component as a sodium chloride aqueous solution, and this sodium chloride aqueous solution was subjected to wet electrolysis by an ion exchange membrane electrolysis method to separate into chlorine gas, sodium hydroxide aqueous solution, and hydrogen gas. The separated chlorine gas is recycled to the crude metal silicon chlorination process, which is an upstream process, to improve the recyclability of the process.

  Similarly to Example 1, in order to purify solar cell silicon, first, silica as a raw material was mixed and charged into a heat treatment furnace together with carbon in accordance with the schematic flow chart of the manufacturing process shown in FIG. Produced crude metal silicon roughly refined to 98-99%. The produced crude metal silicon was reacted with chlorine gas to produce silicon tetrachloride.

  Next, after obtaining purified silicon tetrachloride, the vapor of purified silicon tetrachloride and zinc vapor were reacted in a quartz furnace core tube in the same manner as in Example 1 using the zinc reduction method test apparatus shown in FIG. Then, a zinc reduction reaction was caused to produce silicon for solar cells having a predetermined purity, and the substances deposited on each part of the furnace inner wall including those that were precipitated in a dendritic shape inside the furnace core tube were individually collected.

Therefore, as shown in the schematic flow chart of FIG. 3, the impurity content state grasping (oxygen content) for each collection place in the apparatus of FIG. 4 was performed.
The by-produced zinc chloride was divided into zinc chloride having a low oxygen grade of 50 ppm and high zinc chloride having a high oxygen grade of 150 ppm, which was taken out from the zinc chloride recovery tank. In addition, it accumulates on the inside of the pipe leading from the inside of the furnace through the exhaust pipe to the recovery tank, and on the inner wall of the exhaust line after the recovery tank, and is taken out from the recovery tank and each part in the pipe In each impurity composition of zinc chloride, a certain concentration or more of impurities not suitable for the zinc recovery step by wet electrolytic treatment, which is the subsequent step in Example 2, is recovered in the zinc chloride recovered from the inside of the piping at some locations. It was included in.

  Therefore, the low oxygen grade zinc chloride obtained from the recovery tank was electrolyzed and separated into metallic zinc and chlorine gas and recovered.

  On the other hand, zinc chloride having a higher impurity content level was stored until a certain amount was reached. Thereafter, zinc chloride with a high impurity content level is dissolved in water by the method shown in FIG. 3 to repulp the solid silicon residue partially contained therein, and then sodium hydroxide is added to the aqueous solution from which the residue has been removed. Added. Although the chlorine component became an aqueous solution of sodium chloride, this aqueous solution of sodium chloride is effectively used without being discharged because it is reused as raw material chlorine in the chlorination treatment of metallic silicon, which is the first step in the zinc reduction method. Specifically, the sodium chloride aqueous solution was subjected to wet electrolysis by an ion exchange membrane electrolysis method, and separated into three gases, chlorine gas, sodium hydroxide aqueous solution, and hydrogen gas, and recovered. In addition, the sodium hydroxide aqueous solution obtained here is recycled as sodium hydroxide added when processing zinc chloride with high oxygen quality.

  Further, the zinc hydroxide slurry precipitated during the addition of sodium hydroxide is filtered and dehydrated, and then the solid content obtained is dried and roasted, and then the crude zinc oxide having a chlorine content of several percent or less. As a raw material crude zinc oxide in dry zinc smelting, metal zinc was recovered.

  The amount of metal zinc charged in the same manner as in Example 1 was compared with the amount of metal zinc recovered from zinc chloride having an oxygen grade of 50 ppm and the amount of metal zinc recovered from zinc chloride having an oxygen grade of 150 ppm. The recovery rate was determined, and its resource saving properties were evaluated.

  In the method shown in Example 2, since the alkali metal to be used can be reused as well as the charged metal zinc and chlorine gas, the running cost in the process operation can be further reduced.

(Comparative Example 1)
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. At the time of the zinc reduction reaction, Ar gas used as a carrier had a low purity and contained oxygen.

  As for by-product zinc chloride, the oxygen grade of zinc chloride recovered from the zinc chloride recovery tank was 120 ppm, and the oxygen grade of the other recovered zinc chloride was 200 ppm. Using these zinc chlorides, in the same manner as in Example 1, zinc metal was produced by the two systems of zinc chloride treatment shown in FIG. The recovery rate with respect to the amount of input metal zinc was evaluated in the same manner as in Example 1, and is also shown in Table 1.

(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 by electrolytic treatment using all the zinc chloride collected from the zinc chloride collection tank and other places than the collection tank. The same evaluation was made and the results are shown in Table 1.
At this time, the oxygen grade contained in the zinc chloride was mixed from 50 ppm to 150 ppm.

1 Ceramic electric tube furnace 2 Quartz core tube (transparent quartz tube)
3 K thermocouple 4 Honeycomb 5 Thermostatic chamber 6 Thermometer 7 Hood surrounding the silicon tetrachloride vapor generating section 7a Steam exhaust draft 8 Gas flow meter 9 Ar gas pipe 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 (5)

In a method of obtaining silicon by reducing silicon tetrachloride by contacting silicon tetrachloride purified by separating impurities from silicon tetrachloride obtained by chlorination of metal silicon and metal zinc,
Zinc chloride produced as a by-product when silicon tetrachloride is brought into contact with metallic zinc and reduced is classified according to the oxygen grade contained in the zinc chloride, and a process of obtaining metallic zinc and chlorine gas by electrolytic treatment and crude zinc oxide A process for purifying silicon for solar cells, wherein the process is performed separately from the step of obtaining The classification based on the oxygen grade contained in the zinc chloride uses an oxygen grade of 100 ppm as the category value,
Zinc chloride having an oxygen grade of less than 100 ppm is treated by a step of obtaining zinc metal and chlorine gas by electrolytic treatment,
2. The method for purifying silicon for solar cells according to claim 1, wherein the zinc chloride having an oxygen grade of 100 ppm or more is treated in a step of obtaining crude zinc oxide.   The step of obtaining metallic zinc and chlorine gas by the electrolytic treatment is performed using either a molten salt electrolysis method using molten zinc chloride as an electrolytic solution or an electrowinning method using an aqueous solution in which zinc chloride is dissolved as an electrolytic solution. 3. The method for purifying silicon for solar cells according to claim 1, wherein the silicon gas is separated into zinc and chlorine gas, and the separated chlorine gas is recovered by dehydration and deoxygenation treatment. 4. In the step of obtaining the crude zinc oxide, water is added to zinc chloride, and then alkali treatment is performed by adding an alkali agent containing sodium hydroxide, potassium hydroxide, or sodium carbonate, and then the slurry obtained by the alkali treatment is treated. It is separated into solid and liquid components by solid-liquid separation process to filter,
The separated solid content is dried and roasted in the presence of air, and separated into crude zinc oxide and a chlorine component that volatilizes from the solid content,
The separated liquid component is separated into chlorine gas, hydrogen gas and sodium hydroxide or potassium hydroxide by electrowinning using an ion exchange membrane.
The method for purifying silicon for solar cells according to claim 1 or 2. Chlorine gas obtained by electrolytic collection of the separated liquid component using an ion exchange membrane is used for chlorination of metallic silicon,
5. The sodium hydroxide or potassium hydroxide obtained by electrolytically collecting the separated liquid component using an ion exchange membrane is used for alkali treatment in the step of obtaining the crude zinc oxide. Of refining silicon for solar cells.