JP3844849B2 - Method for producing polycrystalline silicon and zinc chloride - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a method for producing polycrystalline silicon and zinc chloride by reducing silicon tetrachloride with zinc. Regarding the production of polycrystalline silicon, it can be carried out continuously from the step of producing polycrystalline silicon by a reduction reaction to the step of separating by-products, and from the step of producing silicon tetrachloride as an intermediate compound as necessary. It is a manufacturing method of polycrystalline silicon which can constitute a closed cycle until the process of electrolyzing a by-product. In addition, the production of zinc chloride is a method for producing zinc chloride that can be carried out continuously from the step of producing zinc chloride by a reduction reaction to the step of separating and recovering zinc chloride.
[0002]
[Prior art]
With the recent spread of solar power generation, solar cell manufacturing technology is applied in various fields such as semiconductor silicon, amorphous silicon, polycrystalline silicon, etc., and technological development is remarkable in any field. The characteristics required for solar cells at the beginning of development were to improve the performance with the main purpose of improving the photoelectric conversion efficiency. However, with the widespread use of solar cells, the trend toward lower prices has come to be directed. For this reason, for example, in the manufacture of a polycrystalline silicon substrate, low-priced materials such as an extraordinary product of polycrystalline silicon manufactured as semiconductor silicon as a raw material and a residual material of single crystal silicon have come to be used. It is insufficient in quantity and there is a limit to reducing the price. For this reason, the development of a low-cost polycrystalline silicon manufacturing technique is required for the development of a solar cell having sufficient price competitiveness.
[0003]
As a method for producing high-purity polycrystalline silicon, many production methods have been conventionally proposed (for example, semiconductor silicon crystal engineering, pages 15 to 22, Maruzen, issued on September 30, 1993). Among them, the most representative production method is the Siemens Method in which trichlorosilane (SiHCl ₃ ), which is an intermediate compound, is reduced with hydrogen (H ₂ ). In this production method, vaporized high-purity trichlorosilane is introduced into a reaction furnace together with high-purity hydrogen, and the trichlorosilane is decomposed according to the reaction formula (A) below, and both ends are supported by graphite electrodes at about 1100. Polycrystalline silicon is vapor-phase grown on the surface of a polycrystalline silicon mandrel heated to ° C.
[0004]
SiHCl ₃ + H ₂ → Si + 3HCl (A)
As another production method, an ethyl method for producing granular polycrystalline silicon is known. In this manufacturing method, a fluidized bed reactor is used, a silicon fine powder as a seed is fluidized in the reactor, and a mixed gas of monosilane (SiH ₄ ) and hydrogen is introduced into the reactor, 600 to 700 Monosilane decomposes in a flowing atmosphere heated to ° C. At this time, granular polycrystalline silicon is produced through the reaction formula (B) below. In this ethyl method, energy saving in fluid reaction, high reactivity and high yield can be expected.
[0005]
SiH ₄ → Si + 2H ₂ (B)
However, there is a problem that must be solved when these high-purity polycrystalline silicon manufacturing methods are used for solar cells. First, in the Siemens method, a large amount of electric power is consumed for heating the polycrystalline silicon mandrel, so that the electric power consumption rate becomes worse. Furthermore, since only a small proportion of the trichlorosilane introduced into the reactor contributes to the production of polycrystalline silicon, the production efficiency is extremely low. For this reason, it is not suitable as a manufacturing method of the silicon raw material for solar cells which aims at price reduction. On the other hand, as described above, the ethyl method is advantageous in terms of production cost compared to the Siemens method, but it is still insufficient for solar cells, and further cost reduction is necessary.
[0006]
Instead of the above-described method for producing polycrystalline silicon, JP-A-54-84824 discloses a method for producing polycrystalline silicon for solar cells (for photoelectric conversion) by reducing a silicon compound. This manufacturing method is a method in which a silicon compound in a gaseous state is reduced by a reducing agent. By using zinc as a reducing agent and at least one metal such as tin and lead together in a liquid state, a thin film is obtained. It is characterized in that a polycrystalline silicon is produced. Certainly, according to this manufacturing method, since the reaction conditions are applied at a relatively low temperature of around 750 ° C., the working efficiency is excellent, and since polycrystalline silicon is produced in a thin film shape, It is advantageous. However, in terms of quality, there is a problem that the inclusion of heavy metal impurities is inevitable and the purity of silicon cannot be ensured. In addition, the disclosed polycrystalline silicon manufacturing method is difficult to continue the reaction process, and is not intended to be a closed cycle process, so the production efficiency cannot be improved and the cost reduction is insufficient. is there.
[0007]
[Problems to be solved by the invention]
The first object of the present invention is to use the reduction reaction in molten zinc, which has a high reaction efficiency with silicon tetrachloride, in view of the problems of conventional polycrystalline silicon production technology. It can be implemented continuously up to the step of separating the product, and if necessary, it can be configured in a closed cycle from the step of producing silicon tetrachloride as an intermediate compound to the step of electrolyzing the by-product, and the production efficiency is An object of the present invention is to provide a method for manufacturing polycrystalline silicon that is high, stable in quality, and compatible with low prices.
[0008]
The second object of the present invention is to carry out continuously to the process of separating and recovering zinc chloride simultaneously produced in the reduction reaction in the above-mentioned polycrystalline silicon production process, and is inexpensive and quality stable zinc chloride. It is in providing the manufacturing method of.
[0009]
[Means for Solving the Problems]
In the manufacture of silicon for solar cells, it is premised that the solar cell silicon has a performance that exhibits a predetermined photoelectric conversion efficiency. For this reason, polycrystalline silicon provided for solar cells and silicon raw materials for producing the same are required to have a high purity required for this, specifically 6N (99.9999%) and 7N (99.99999%), respectively. It is required to ensure purity. In order to achieve such high purity, the manufacturing method employed is to form a high-purity silicon by halogenating metal silicon to produce an intermediate compound, which is then reduced and decomposed without secondary contamination. Is effective.
[0010]
As a result of various studies, the present inventors have used silicon tetrachloride (SiCl ₄ ) as an intermediate compound and reduced it with molten zinc (Zn) as shown in the reaction formula (C) below. We have found that manufacturing silicon (Si) is excellent in productivity and effective in terms of price.
[0011]
SiCl ₄ + 2Zn → Si + 2ZnCl ₂ (C)
In the above formula (C), a reaction system in which almost all of silicon tetrachloride (SiCl ₄ ) progresses to silicon (Si) from the viewpoint of the equilibrium of the reaction, and also as a process, silicon tetrachloride (SiCl ₄ ) A mixture of silicon (Si) produced in the reduction process and zinc chloride (ZnCl ₂ ), which is a by-product, and possibly mixed zinc (Zn) can be extracted from the reaction vessel immediately after the reduction reaction. The extracted mixture is stored in a separation container, and zinc chloride (ZnCl ₂ ) and zinc (Zn) in the mixture can be evaporated and separated. If such a production method with high production efficiency is adopted, low-cost polycrystalline silicon can be easily produced.
[0012]
In the above process, zinc chloride (ZnCl ₂ ) produced together with silicon (Si) in the reduction step of silicon tetrachloride (SiCl ₄ ) can be extracted from the reaction vessel immediately after the reduction reaction as a mixture. Zinc chloride (ZnCl ₂ ) in the mixture can be separated and recovered by storing in a separation container. The recovered zinc chloride can be produced at low cost, but is stable in quality, and can be used in a wide range of fields as a pretreatment agent for galvanization, an electrolytic component of a dry battery, and the like.
[0013]
In order to establish a low-cost production method of polycrystalline silicon, it is necessary to configure a closed cycle. That is, in the above manufacturing method, it is necessary to rely on external purchase of silicon tetrachloride (SiCl ₄ ) or zinc (Zn), and zinc chloride (ZnCl ₂ ) as a by-product is not effectively used. Therefore, if zinc chloride (ZnCl ₂ ) taken out in the separation process is electrolyzed, metal zinc (Zn) and chlorine (Cl ₂ ) can be recovered separately and reused, making the manufacturing process a closed cycle. Achieved and further cost reduction will be realized.
[0014]
The present invention has been completed based on the above findings, and is a method for producing polycrystalline silicon having the following (1) to (2) as a gist (see FIG. 1).
[0015]
(1) Silicon tetrachloride in liquid or gaseous state is reduced with molten zinc in the reaction vessel 5, and the resulting mixture containing polycrystalline silicon and zinc chloride is taken out of the reaction vessel 5, and the mixture is put into the separation vessel 10. This is a method for producing polycrystalline silicon and zinc chloride characterized in that after containing zinc chloride and polycrystalline silicon in a mixture and separating them from the separation container, both are collected (hereinafter referred to as “first production”). Method ").
[0016]
(2) Among the above (1), a method for producing polycrystalline silicon, wherein zinc chloride produced as a by-product in the reduction of silicon tetrachloride is separated from the mixture, and then electrolyzed to recover metallic zinc and chlorine, A method for producing polycrystalline silicon, characterized in that the recovered metallic zinc is used again as a reducing agent for the silicon tetrachloride, and / or the recovered chlorine is used for chlorination of metallic silicon provided as a previous step. (Hereinafter referred to as “second manufacturing method”).
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a method for producing polycrystalline silicon having a purity of 7N (99.99999%), which is mainly used as a raw material for producing polycrystalline silicon for solar cells. A description will be given by dividing into the second manufacturing method. The first manufacturing method includes a method of manufacturing zinc chloride that is generated simultaneously with the manufacture of polycrystalline silicon.
[0018]
1. FIG. 1 is a diagram for explaining an apparatus configuration of a first production method, that is, a production method ranging from melting of metallic zinc to separation of a reaction byproduct after a reduction reaction. The target processes in the first production method are as described above: (1) the step of supplying raw materials, (2) the step of reducing silicon tetrachloride with molten zinc, (3) the step of separating the mixture and (4) This is a step of taking out polycrystalline silicon, and these are sequentially processed. Hereinafter, the content of each process is demonstrated.
[0019]
(1) Process for supplying raw materials In order to facilitate supply to the reaction furnace, metallic zinc as a reducing agent is melted. The dissolution tank 3 charged with metallic zinc is installed and heated in a dissolution tank 2 equipped with a heating device 2a. From the viewpoint of energy saving, the heating temperature of metallic zinc should be kept just above the melting point (420 ° C.) of metallic zinc, and is preferably in the range of 450 to 550 ° C. Since the purity of the reducing agent greatly affects the purity of the silicon produced, it is desirable that the purity of the metal zinc be 4N (99.99%) or higher.
[0020]
Silicon tetrachloride as an intermediate compound is accommodated in the SiCl ₄ tank 1 at room temperature and kept in a liquid state (melting point: -70 ° C.). Since the purity of silicon tetrachloride also greatly affects the purity of the polycrystalline silicon produced, it is desirable that the contained heavy metals be 50 ppb or less.
[0021]
The molten zinc and silicon tetrachloride tanks 1 and 2 are maintained in an inert gas (for example, argon gas) atmosphere at atmospheric pressure, and the supply to the reaction vessel 5 is pressurized with the inert gas. The supply method is adopted. By this system, molten zinc and silicon tetrachloride can be supplied into the reaction vessel 5 separately or simultaneously.
[0022]
(2) Step of reducing silicon tetrachloride with molten zinc The reduction reaction is carried out in a closed reaction vessel 5 installed in the reduction furnace 4, and by reducing silicon tetrachloride as an intermediate compound with molten zinc. A mixture of polycrystalline silicon and zinc chloride as a by-product halide is obtained. The reduction furnace 4 has a built-in heating device 4a, and zinc inside and by-product zinc chloride are held in a molten state. The heating temperature is preferably as low as possible from the viewpoint of energy saving and prevention of contamination of the produced polycrystalline silicon, but is preferably about 800 ° C. in order to secure a certain reaction rate. Silicon tetrachloride is dripped from the top through a nozzle or blown directly into the molten zinc. When blowing directly into the molten zinc, the reaction area can be secured not only in the horizontal direction but also in the vertical direction in the reaction vessel 5, so that the production amount per unit area is increased and the apparatus can be made compact. .
[0023]
As shown by the formula (C), with the supply of silicon tetrachloride, the silicon tetrachloride is reduced by molten zinc to produce polycrystalline silicon powder and zinc chloride. Since the boiling point of zinc chloride is 732 ° C., the generated zinc chloride is discharged as vapor out of the reaction vessel 5. Moreover, the polycrystalline silicon powder produced | generated simultaneously is very fine, and is discharged | emitted outside the reaction container 5 as the mixture 7 with the vapor | steam of the above-mentioned zinc chloride. The zinc reduced by the reaction is supplied from the dissolution tank 3 continuously or intermittently in an amount corresponding to the zinc.
[0024]
(3) Step of separating the mixture The reaction by-product is separated in a separation vessel 10 disposed inside the separation furnace 9. When using the evaporative separation method, a perforated plate for preventing scattering is installed in the separation vessel 10 in order to prevent scattering of the granular polycrystalline silicon accompanying evaporation. A mixture 7 composed of polycrystalline silicon, zinc chloride and, in some cases, zinc is introduced from the upper part of the separation vessel 10, and then the whole separation vessel 10 is heated by a heating device 9 a built in the separation furnace 9, and chlorinated. Polycrystalline silicon is recovered by evaporating and separating zinc and zinc. The heating temperature at this time is desirably as low as possible from the viewpoint of preventing contamination of polycrystalline silicon, and a range of 450 to 1000 ° C. is employed. The pressure in the separation vessel 10 may be atmospheric pressure, but is preferably about 10 to ^{3 -3} Torr in order to promote evaporation and separation. On the other hand, the evaporated and separated reaction by-product, such as zinc chloride, is discharged out of the separation vessel 10, condensed through the heat exchanger 11, and recovered.
[0025]
When the liquid filtration method is used, the separation container 10 is provided with a porous fine ceramic filter having high temperature characteristics and little contamination, and the mixture 7 introduced into the separation container 10 is pressure-filtered.
[0026]
(4) Step of taking out polycrystalline silicon By repeating the above separation, impurities are completely removed and purified polycrystalline silicon is manufactured. Such polycrystalline silicon is taken out from the lower part of the separation container 10. The purity of the extracted polycrystalline silicon is 7N (99.99999%) grade, and can be applied for solar cells.
[0027]
Similarly, zinc chloride separated from the polycrystalline silicon is taken out from the separation container 10 and used as a pretreatment agent for galvanization, an electrolytic component of a dry battery, or the like.
[0028]
2. FIG. 2 illustrates an example of the second manufacturing method, that is, a process in which metal zinc recovered by electrolysis of zinc chloride taken out in the separation step is used again as a reducing agent for silicon tetrachloride. It is a figure to do. As described above, in the first manufacturing method, metallic zinc and silicon tetrachloride as reducing agents are premised on purchase from the outside, and zinc chloride taken out in the separation step is also discharged out of the system. In the production of polycrystalline silicon for solar cells, it is effective to configure a closed cycle in order to achieve a lower cost production method. Therefore, as an example of the second manufacturing method, in FIG. 2, zinc chloride discharged from the separation vessel, condensed by a heat exchanger and recovered, and zinc metal recovered by electrolysis is used as a reducing agent. It is.
[0029]
FIG. 3 shows another example of the second production method, that is, metal zinc recovered by electrolysis of zinc chloride taken out in the separation step is used again as a reducing agent for silicon tetrachloride, and further recovered chlorine is used. It is a figure explaining the process used for the chlorination treatment of the metallic silicon provided as a previous process. In the manufacturing method shown in FIG. 3, a closed cycle is achieved as compared with the manufacturing method shown in FIG. Specifically, a process for distilling silicon tetrachloride generated by chlorinating metal silicon as a pre-process in the manufacturing process of polycrystalline silicon is provided, and the molten zinc chloride discharged from the separation vessel is condensed. The solution is electrolyzed to recover metallic zinc and chlorine gas, using the recovered metallic zinc as a reducing agent, and simultaneously using the recovered chlorine gas for the chlorination treatment in the previous step.
[0030]
The electrolysis used in the second production method may employ a conventional method for generating chlorine gas while precipitating and recovering metallic zinc on the electrode, based on the reaction of the following formula (D). .
[0031]
ZnCl ₂ → Zn + Cl ₂ (D)
The production of silicon tetrachloride in the previous step is based on the reaction of the following formula (E), but may be a conventional method comprising two steps of production of crude silicon tetrachloride and purification thereof.
[0032]
Si + 2Cl ₂ → SiCl ₄ ... (E)
[0033]
【Example】
The effects of the method for producing silicon for solar cells of the present invention will be described based on Examples 1 and 2 divided into first and second production methods.
[0034]
Example 1
Polycrystalline silicon was manufactured by the first manufacturing method shown in FIG. The operating conditions in each process from the supply of the raw material to the removal of the polycrystalline silicon are as follows.
[0035]
(1) Process of supplying raw material The purity of metallic zinc used was 4N (99.99%), and it was heated to 500 ° C. after being placed in a melting tank to be in a molten state. As an intermediate compound, silicon tetrachloride having a purity of 4N (99.99%) metal impurity of 20 ppb or less was used. The supply to these reaction vessels was performed while pressurizing silicon tetrachloride to 1 kg / cm ² with argon gas and adjusting the flow rate using a control valve and a flow meter. About molten zinc, argon gas was supplied in the melt | dissolution tank and the supply amount was adjusted by changing the flow volume of argon gas.
[0036]
(2) Reduction process First, molten Zn is supplied as a reducing agent into the reaction vessel, heated to 800 ° C. and stabilized, and a silicon tetrachloride supply nozzle is immersed in the molten zinc from the top of the reaction vessel. Then, silicon tetrachloride was supplied into the molten zinc. With the supply of silicon tetrachloride, a mixture of polycrystalline silicon and zinc chloride is formed. The supply rate of silicon tetrachloride was 1.6 kg-SiCl ₄ / m ² min at maximum when the height from the nozzle tip to the molten zinc layer was 0.5 m. The produced zinc chloride evaporates and becomes a gaseous mixture with the finely divided polycrystalline silicon produced at the same time, and is transferred to the separation vessel.
[0037]
(3) Step of separating the mixture The mixture was separated by an evaporation separation method and a liquid filtration method. In the case of the evaporative separation method, the inside of the separation vessel was 500 ° C. and 1 Torr. The end point of the separation was determined by changing the control output of the heating furnace.
[0038]
On the other hand, in the case of the liquid filtration method, a mixture of zinc chloride and polycrystalline silicon is heated to 500 ° C., and after zinc chloride and zinc contained therein are completely dissolved, 0.2 kg of porous silica is used. The solution was filtered under a pressure of / cm ² .
[0039]
(4) After the separation of the polycrystalline silicon is completed, the high-purity granular silicon is taken out. When the extracted silicon was analyzed, the purity was 7N (99.99999%), and it was confirmed that it could be applied for solar cells. Furthermore, it can be seen that the manufacturing cost of the silicon raw material manufactured in this way is 40, which is lower than that of the case where the polycrystalline silicon manufactured by the conventional Siemens method is taken as 100.
[0040]
(Example 2)
In the second production method, polycrystalline silicon was produced by recovering metallic zinc from the zinc chloride separated and discharged in the step shown in FIG. Processes (1), (2) and (3) were made the same conditions as in Example 1. The electrolysis process was operated under the following conditions.
[0041]
That is, the electrolysis is performed at a bath temperature of 500 ° C., a current density of 760 A / m ² , a bath potential of 3 V, a bath current of 2.26 A, and a zinc chloride (ZnCl ₂ ) melt concentration of 56 Zn-g / liter. The composition was adjusted. The purity of Zn recovered by electrolysis was 99.998% and was used again as molten Zn.
[0042]
When the polycrystalline silicon produced by this method was analyzed, the purity was 7N (99.99999%), which was a quality without any problem for solar cells. Assuming that the manufacturing cost of polycrystalline silicon manufactured by the conventional Siemens method is 100, the cost of silicon manufactured by this method is calculated as 30, which is a low price.
[0043]
【The invention's effect】
According to the method for producing polycrystalline silicon of the present invention, after the reduction reaction, it can be carried out continuously up to the step of separating reaction by-products, and from the step of producing silicon tetrachloride as an intermediate compound as necessary. Since it can be configured in a closed cycle up to the step of separating the by-product, it is possible to manufacture polycrystalline silicon that is stable in terms of quality and that is low in price with high production efficiency.
[0044]
Further, according to the method for producing zinc chloride of the present invention, since it can be continuously produced from zinc chloride produced simultaneously in the production process of polycrystalline silicon, it is possible to provide inexpensive and quality stable zinc chloride. it can.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram illustrating an apparatus configuration in a first production method, that is, a production method ranging from melting of metallic zinc to separation of reaction byproducts after a reduction reaction.
FIG. 2 is a diagram for explaining an example of a second production method, that is, a step of using metal zinc recovered by electrolysis of zinc chloride taken out in the separation step as a silicon tetrachloride reducing agent again.
FIG. 3 shows another example of the second production method, that is, metal zinc recovered by electrolysis of zinc chloride taken out in the separation step is used again as a reducing agent for silicon tetrachloride, and further recovered chlorine. It is a figure explaining the process used for the chlorination treatment of the metal silicon provided as a previous process.
[Explanation of symbols]
1: SiCl ₄ tank, 2: dissolution tank 3: dissolution tank, 4: reduction furnace 5: reaction vessel, 7: mixture 8: molten zinc layer, 9: separation furnace
10: Separation container
2a, 4a, 9a: Heating device

Claims (7)

Silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in the reaction vessel, and a mixture containing the generated polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and the mixture is placed in a separation vessel. A method for producing polycrystalline silicon, comprising separating the zinc chloride and then collecting polycrystalline silicon from the separation container. The method for producing polycrystalline silicon according to claim 1, wherein the mixture is mainly composed of vapor of zinc chloride and polycrystalline silicon powder. The method for producing polycrystalline silicon according to claim 1, wherein the separation of zinc chloride is evaporation separation or liquid filtration. The zinc chloride produced as a by-product in the reduction of silicon tetrachloride is separated from the mixture and then electrolyzed to recover metal zinc and chlorine, and the recovered metal zinc is used again as the silicon tetrachloride reducing agent, or The method for producing polycrystalline silicon according to any one of claims 1 to 3, wherein the recovered chlorine is used for a chlorination treatment of metallic silicon provided as a previous step. Silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in the reaction vessel, and a mixture containing the produced polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and the mixture is placed in a separation vessel. A method for producing zinc chloride, comprising separating zinc chloride and then collecting the zinc chloride from the separation vessel. 6. The method for producing zinc chloride according to claim 5, wherein the mixture is mainly composed of vapor of zinc chloride and polycrystalline silicon powder. The method for producing zinc chloride according to claim 5, wherein the separation of zinc chloride is evaporation separation or liquid filtration.

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