KR20130060664A - Manufacturing method of high purity silicon nanofiber by electrolytic refining - Google Patents

Abstract

PURPOSE: A manufacturing method of high purity silicon nanofiber by an electrolytic refining method is provided to manufacture high purity silicon nanofiber from low purity metallurgical grade silicon by controlling contents of silicon halogen compound inside the electrolyte and temperature of the electrolyte during electrolytic refining. CONSTITUTION: A manufacturing method of high purity silicon nanofiber by an electrolytic refining method comprises the following steps: preparing an electrolytic refining device by locating anode and cathode which includes low purity metallurgical grade silicon inside an electrolyzer in which electrolyte is put; and performing electrolytic refining by passing current between the anode and cathode and depositing the nano fiber type high purity silicon to the cathode. The electrolyte comprises molten salt and silicone halogen compound. The content of the silicone halogen compound is 0.1-10 wt% based on the total weight of the electrolyte. In the electrolytic refining step, the temperature of the electrolyte is maintained as 650-750 deg. Celsius. The voltage supplied in the electrolytic refining to the cathode is -0.4 to -1.0 V based on platinum reference electrode. The electric current density in the cathode during electrolytic refining is 1-100mA/cm^2.

Description

Manufacturing method of high purity silicon nanofiber by electrolytic refining

The present invention relates to a method of manufacturing high-purity silicon nanofibers from low-purity silicon, and more particularly to manufacturing high-purity silicon nanofibers from low-purity metallurgical grade silicon using electrolytic refining in the presence of molten salt. It is about how to.

Due to global warming and high oil prices, interest in renewable energy is high, and the government focuses on nurturing renewable energy as a new growth engine to supplement the weakness of domestic energy resources such as depletion of fossil energy and energy fluctuations. In solar power generation is supported by the government's subsidy for solar power generation, solar power facilities are in operation and under construction, and low-cost solar power generation, which has drastically reduced the cost of conventional solar power generation, is preparing for full-scale practical use. A solar cell using the photoelectric effect of a silicon semiconductor as a basic principle has a feature of easily converting solar energy into electricity. The cost down of solar cells, especially the cost down of semiconductor silicon, is very important for the expansion of solar power generation. Semiconductor grade silicon used in the manufacture of semiconductor IC is converted to silicon trichloride (SiHCl ₃ , Trichlorosilane (TCS)) with a purity of 98% or higher purity Subsequently, the silicon chloride is purified by distillation and then reduced by a so-called siemens method to obtain high purity silicon of about 11 N (99.999999999%). This Siemens method is complicated manufacturing plant equipment and very high energy costs in the production of silicon, the high purity silicon of the Siemens method is expensive to manufacture. For this reason, semiconductor-grade silicon is not suitable because of high manufacturing cost due to excessive purity, which is too high for solar cell applications that require an urgent cost down.

In addition, in order to reduce the cost of silicon for solar cells, UMG silicon (upgrading metallurgical grade silicon) made of metal grade silicon with purity of 2N from metallurgical purification in an arc furnace can be used for high purity silicon for solar cells. Purification by directional solidification during the manufacturing process of silicon is excellent for removing many impurities at the same time, except for boron (B) and phosphorus (P). As the segregation coefficient is 0.8 for boron (B), solidification segregation cannot be efficiently carried out in principle, so it is difficult to remove boron (B) in silicon, and the segregation coefficient is 0.35 for phosphorus (P). Difficult to remove

On the other hand, in the US Patent No. US 7,901,561 by electrolytic refining technology, the electrolytic refining process proceeds above the melting point of the silicon, electrolytic refining is about 800 ℃ or more in order to obtain high purity silicon in the general molten salt electrolytic refining method The process is carried out in. In this case, an increase in energy required for electrolytic refining is inevitable, and there is a problem in that the crystals of high purity silicon generated are limited to powder or dendritic phase. In addition, US Patent Publication No. 7,531,472 discloses a method for introducing and growing a crystal growth nucleus under high vacuum in connection with a process for producing silicon nanofibers, in which case the high-vacuum and the substrate in advance to print the nucleus or catalyst productivity The use of this low and high purity raw material has a disadvantage of low economic efficiency.

In general, high-purity silicon nanofibers are used as raw materials for fluorescent materials, lithium secondary battery active materials, etc., so it is required to develop a method for manufacturing them economically and environmentally.

As derived from the above background of the present invention, an object of the present invention is to provide a method for economically manufacturing high-purity silicon used for solar cells and the like using an electrolytic refining method. It is also an object of the present invention to provide a method for producing silicon nanofibers by electrolytic refining.

The inventors of the present invention maintain the temperature of the electrolyte at less than 800 ℃ during the electrolytic refining of low-purity metal-grade silicon, high-purity silicon nanofibers in the case of controlling the content of the silicon halide compound acting as a substantial silicon precursor in the electrolyte to a specific range The present invention was found to be possible in the preparation of the present invention.

In order to solve the above object of the present invention, the present invention comprises the steps of preparing an electrolytic refining apparatus by placing a cathode containing a low-purity metal-grade silicon in the electrolytic cell containing the electrolyte and a cathode at a distance away from the anode; And electrolytic refining by passing a current between the anode and the cathode and electrodepositing high-purity silicon in the form of nanofibers to the cathode, wherein the electrolyte includes a molten salt and a silicon halide compound, and the content of the silicon halide compound. Silver is 0.1 to 10% by weight based on the total weight of the electrolyte, in the electrolytic refining step provides a method for producing high purity silicon nanofibers, characterized in that the temperature is maintained at 650 ~ 750 ℃.

In order to obtain high purity silicon in a general molten salt electrolytic refining method, the electrolytic refining is performed at a process temperature of 800 ° C. or higher, and the high purity silicon electrodeposited product has a granular form or agglomerate form. On the other hand, the method of manufacturing high-purity silicon nanofibers by the electrolytic refining method according to the present invention controls the transfer rate of silicon ions by controlling the flow and temperature of the molten salt to induce unidirectional growth of silicon crystals on the surface of the cathode and the nanometer diameter Silicone fibers can be produced. In addition, when using the method of manufacturing high-purity silicon nanofiber according to the present invention from the low-purity metallurgical grade silicon (99%) of the purity 2N level by using an economical and environmentally friendly electrolytic refining method 4N ( It is possible to obtain high-purity silicon of about 99.99%) or more, and to supply low-cost, high-purity silicon raw materials to the solar cell industry.

1 conceptually illustrates the electrolytic refining apparatus and electrolytic refining step used in the present invention.
2 is a schematic view of a cathode basket containing low purity metallic silicon in the electrolytic refining apparatus used in the present invention and a photograph thereof.
3 is a photograph showing an electrodeposition formed on the surface of the cathode after electrolytic refining as a preferred embodiment of the present invention.
4 is a photograph showing the distribution of silicon form according to the distance from the cathode surface of the electrodeposite formed on the surface of the cathode after electrolytic refining as a preferred embodiment of the present invention.
5 is a photograph showing the shape and size of a silicon electrodeposited material formed on the surface of a cathode after electrolytic refining as a preferred embodiment of the present invention.
FIG. 6 shows cyclic voltammograms of concentrations of K ₂ SiF ₆ in an electrolyte solution containing a KF-LiF eutectic. In FIG. 6, a) is a cyclic voltammogram when the concentration of K ₂ SiF ₆ in the electrolyte is 0% by weight, and b) is 1% by weight of K ₂ SiF ₆ in the electrolyte. Cyclic voltammogram, based on the total weight of the electrolyte containing molten salt and K ₂ SiF ₆ ), and c) is the concentration of K ₂ SiF ₆ in the electrolyte at 5% by weight (molten salt and K). Cyclic voltammogram when the total weight of the electrolyte solution containing ₂ SiF ₆ ) is used, and d) represents 9% by weight of K ₂ SiF ₆ in the electrolyte (molten salt and K ₂ SiF ₆₎ . Cyclic voltammogram when the total weight of the electrolyte is included).
FIG. 7 is a photograph of a high purity silicon electrodeposited body according to the current density and the content of K ₂ SiF ₆ during electrolytic refining according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. Before describing the present invention in detail, terms used in the present invention will be described. In the present invention, "low purity" and "high purity" are relative to each other, for example, when low purity silicon represents silicon having a purity of 3N (99.9%) or less, high purity silicon refers to silicon having a purity of more than 3N. Preferably silicon having a purity greater than 3N and less than or equal to 4N (99.99%). In addition, "molten salt" shows the metal salt which can be melted at the process conditions at the time of electrolytic refining. In addition, "nanofiber" refers to microfibers having a diameter of several nanometers to several hundred nanometers. For example, silicon nanofibers have a structure in which the silicon crystals grow in one direction and have a diameter of about 5 nm to 500 nm and a fiber shape. Indicates.

The present invention relates to a method for producing high purity silicon nanofibers from low purity silicon using an electrolytic refining method, and a method for producing high purity silicon nanofibers according to the present invention includes low purity metal grade silicon in an electrolyte containing an electrolyte. Preparing an electrolytic refining apparatus by placing a cathode at a distance between the anode and the cathode; And electrolytic refining by passing a current between the anode and the cathode and electrodepositing high-purity silicon in the form of nanofibers to the cathode. Hereinafter, the present invention will be described by dividing each step.

Preparing the Electrorefining Device

1 conceptually illustrates the electrolytic refining apparatus and electrolytic refining step used in the present invention. As shown in FIG. 1, the electrolytic refining apparatus is largely an electrolytic cell, an electrolyte contained in an electrolytic cell, an anode contacting an electrolyte solution in an electrolytic cell, a cathode contacting an electrolyte solution in an electrolytic cell and spaced a predetermined distance from the anode, and a potential difference between the anode and the cathode. It consists of a voltage source to which is applied.

The electrolytic cell is also called an electrochemical cell or an electrorefining cell, and the material for forming the electrolytic cell is not particularly limited as long as it can withstand the temperature of the electrolyte maintained during electrorefining.

An electrolytic solution is contained in the electrolytic cell, which contains a molten salt and a silicon halide compound. At this time, the content of the silicon halogen compound is 0.1 to 10% by weight, preferably 1 to 9% by weight based on the total weight of the electrolyte in order to electrodeposit high purity silicon in the form of nanofibers to the anode during electrolytic refining. In addition, the content of the silicon halogen compound is about 0.01 to 2 mol%, preferably 0.02 to 1.9 mol% based on the total moles of the components constituting the electrolyte. If the molten salt constituting the electrolyte is present in the molten state at 650 ~ 750 ℃ process temperature of the electrolytic refining is not limited greatly, considering the characteristics of transferring the oxidized silicon ions to the cathode, It is preferable that it is an alkali metal halogen compound or an alkaline earth metal halogen compound, and it is more preferable that it is a eutectic. Specific examples of the eutectic are KF-LiF (melting point: 492 ° C), LiCl-KCl (melting point: 353 ° C), KCl-KF (melting point: 606 ° C), and KCl-LiF (melting point: 712 ° C) Etc., these may be used independently, respectively and may use 2 or more types together. In the present invention, it is most preferable to use KF-LiF as the molten salt. The composition of the molten salts in each of the eutectes listed above is not very limited, for example a eutectic made of KF-LiF may have a molar ratio of KF to LiF of from 1: 9 to 9: 1, 4: 6 To 6: 4. In addition, the silicon halogen compound constituting the electrolyte substantially acts as a precursor of silicon electrodeposited to the cathode in the electrorefining step of the present invention, and considering the degree of oxidation of the alkali metal to silicon in the molten salt, or It is preferable that it is a silicon halogen compound of alkaline earth metal. Specific examples of the silicon halide compound used in the present invention include K ₂ SiF ₆ , Na ₂ SiF ₆ , Cs ₂ SiF ₆ , and MgSiF ₆ , and these may be used alone or in combination of two or more thereof. . In the present invention, it is most preferable to use K ₂ SiF ₆ as the silicon halogen compound.

In the electrolytic refining apparatus of the present invention, the anode is present in contact with at least a part of the electrolyte contained in the electrolytic cell, and includes low purity metallic grade silicon. For example, the anode may be made of metallic silicon having a purity of 2N (99%) to 3N (99.9%), or may be composed of a low purity metallic silicon and an anode basket accommodating the same, but is not limited thereto. . In addition, the low-purity metallic silicon constituting the anode is preferably in the form of a silicon ingot in consideration of commercial availability and an appropriate level of purity. In addition, the low purity metallic grade silicon may have other forms other than the silicon ingot form, for example, smaller forms such as silicon scrap or silicon granules. Silicon ingots are made of columnar crystals by melting polysilicon and are widely used as materials in the semiconductor field. Meanwhile, in order to manufacture high-purity silicon nanofibers in large quantities using the present invention, scale-up of an electrolytic cell is required, and accordingly, low-purity metallic silicon is preferably present in an anode basket in an electrolytic cell. 2 is a schematic view of a cathode basket containing low purity metallic silicon in the electrolytic refining apparatus used in the present invention and a photograph thereof. Specifically, the anode basket shown in the photograph of FIG. 2 is made of molybdenum metal plate, and excludes the weld in consideration of corrosion of the weld during electrolytic refining. Anode baskets are useful for large amounts of electrorefining, and the shape of the low purity metal grade silicon contained in the anode basket is not very limited. For example, the low purity metal grade silicon may be mixed with silicon scrap or silicon granules. It is preferable to use an anode basket when the shape is small. At this time, it is preferable that the anode basket is not oxidized even when silicon is oxidized at the anode during electrolytic refining, and is formed of a material having a redox potential larger than silicon by at least 0.4V. The material forming the positive electrode basket is not particularly limited as long as the above conditions are satisfied. For example, the material forming the positive electrode basket is one or at least two or more selected from Mo, Ni, W, and Pd.

In the electrolytic refining apparatus of the present invention, the negative electrode is in a state in which at least part thereof is in contact with the electrolyte contained in the electrolytic cell, and is spaced apart from the positive electrode by a predetermined distance, and preferably positioned opposite the positive electrode. The material for forming the cathode is not particularly limited as long as it can withstand the process temperature during electrolytic refining. For example, it is formed of graphite, silver, platinum, stainless steel, etc. In consideration, it is preferable that it is silver. In addition, the shape of the cathode is not particularly limited, such as cylindrical, plate-like, etc., in consideration of the ease of electrodeposition of silicon is preferably plate-like. In consideration of the material and the shape, the cathode is more preferably a plate made of silver.

Electrorefining Step

In the method for producing high purity silicon nanofibers according to the present invention, the electrolytic refining step consists of passing an electric current between the anode and the cathode and electrodepositing the high purity silicon in the form of nanofibers to the cathode. Specifically, as shown in FIG. 1, when a current is passed between the anode and the cathode, silicon ions are generated by an oxidation reaction at an anode made of low-purity metallic silicon, and the silicon ions are reduced at the cathode while passing through an electrolyte. Electrodeposit At this time, the high-purity silicon electrodeposited to the negative electrode after a predetermined time has a nanofiber form of 5 ~ 500nm, preferably 10 ~ 500nm diameter, along with the individual salt constituting the molten salt in the eutectic form together with the high purity silicon Electrodeposited on the cathode.

In order to obtain high-purity silicon electrodeposited on the negative electrode in the method of manufacturing high-purity silicon nanofibers according to the present invention to have a nanofiber form, the temperature of the electrolyte in the electrolytic refining step is preferably maintained at 650 ~ 750 ℃, 660 ~ 740 ℃ It is more preferably maintained at, and most preferably maintained at 680 to 720 ° C. When the temperature of the electrolyte in the electrolytic refining step is less than 650 ° C., high purity silicon is not smoothly electrodeposited on the negative electrode. When the temperature of the electrolyte in the electrolytic refining step exceeds 750 ° C., the shape of the high purity silicon electrodeposited on the negative electrode is dendrite crystal (dendrite). ).

In addition, in the method of manufacturing high-purity silicon nanofibers according to the present invention, the voltage applied to the cathode during electrolytic refining is in the range of the redox potential of K ₂ SiF ₆ in the molten salt in the form of eutectic or It has a lower redox potential than this and is preferably -0.4V to -1.0V relative to the platinum reference electrode. If the voltage applied to the cathode during the electrolytic refining is lower than -0.1V compared to the platinum reference electrode, the amount of impurities electrodeposited on the cathode increases, which may cause a problem in the subsequent separation process of high purity silicon. In the electrolytic refining, the current density is preferably 1 to 100 mA / cm 2, more preferably 20 to 80 mA / cm 2 based on the negative electrode cross-sectional area.

In addition, in the method for producing high purity silicon nanofibers according to the present invention, the entire electrolyte in the electrolytic bath is not physically stirred during electrolytic refining, and at the same time, the temperature difference is preferably 10 ° C. or less, and more preferably 5 ° C. or less. In general, in order to obtain high-purity silicon of coarse crystals such as dendrite by electrolytic refining method, it is advantageous to stir the electrolyte during electrolytic refining to suppress the formation of concentration gradient of molten salt and to smoothly supply silicon ions to the negative electrode. In order to obtain silicon, it is preferable to exclude the physical agitation of the molten salt and to minimize the agitation effect due to convection to maintain a concentration gradient of the molten salt or the like on the surface of the negative electrode. To this end, excluding the physical agitation of the molten salt during electrolytic refining, the local temperature difference of the whole electrolyte can be increased.To prevent this, the temperature difference at the top, bottom, left and right points of the electrolyte is uniformly applied by applying heat uniformly to the entire surface of the electrolyte. It is preferable to manage at 10 degrees C or less.

In addition, the electrolytic refining time in the manufacturing method of the high purity silicon nanofiber according to the present invention is not greatly limited, and depends on the current density, the content of silicon halides in the electrolyte, and the like. However, the electrolytic refining time is preferably 1 to 20 hr, and more preferably 2 to 18 hr at optimum process conditions.

After the electrolytic refining is completed, it is possible to remove the molten salt from the electrodeposition formed on the cathode by a method such as to dissolve in order to obtain a high purity silicon nanofibers.

The method for producing high purity silicon nanofibers according to the present invention can be used to obtain high purity silicon having a purity of 3N (99.9%) from low purity metal grade silicon having a purity of 2N (99%) and having a purity of 3N (99.9%) or less. It can be used to obtain high purity silicon close to 4N (99.99%) from metal grade silicon. For example, the low purity metal grade silicon constituting the anode has a purity of 99.9% or less, and is high purity in the form of nanofibers deposited on the cathode. It is preferred that the purity of the silicon is greater than 99.9%.

Hereinafter, a method for producing high purity silicon nanofibers according to the present invention will be described with a preferred example. In a preferred embodiment of the present invention, the positive electrode is in the form of a silicon ingot, the negative electrode is a silver rolled plate, the molten salt constituting the electrolyte is a LiF-KF eutectic, and the silicon halide compound constituting the electrolyte is K ₂ SiF ₆ . In addition, during electrolytic refining, the temperature of the electrolyte is maintained at 700 ° C.

Unlike conventional powdery silicon electrodeposits under molten salt electrorefining conditions, in order to recover nanofiber silicon, solid KF is formed around the electrodeposite when K ₂ SiF ₆ decomposition and silicon (Si) reduction are performed on the anode surface. It is necessary to keep the molten salt concentration gradient constant on the cathode surface.

K ₂ SiF ₆ added in the molten salt during electrolytic refining is dissolved in the LiF-KF eutectic as shown in Scheme 1 below.

[Reaction Scheme 1]

K ₂ SiF ₆ ⇔ 2K ⁺¹ + Si ⁺⁴ + 6F ^-1

On the other hand, K ₂ SiF ₆ on the surface of the cathode during the electrolytic refining at which the cathode potential is -0.4V to -1V, which is a redox potential range of silicon (Si) and the temperature of the electrolyte is maintained at 700 ° C, is reacted as in Scheme 2 below. . That is, K of K ₂ SiF ₆ is not reduced at the cathode surface.

[Reaction Scheme 2]

^{_{K 2 SiF 6 + 4e - →}} Si + 2KF (S) + 4F -1

At this time, since the oxidation reaction as shown in Scheme 3 is continuously performed on the surface of the anode made of metal-grade silicon (MG-Si), it is possible to supplement the consumption of silicon ions at the cathode.

Scheme 3

Si + 4F ^-1 → SiF ₄ + 4e ^-1

The net reaction of Scheme 2 and Scheme 3 can be represented by the following Scheme 4.

[Reaction Scheme 4]

K ₂ SiF ₆ → SiF ₄ + 2KF _(S) [ΔG _{700 ° C} = -10,457 kJ / mol]

According to Scheme 4, silicon (Si) is continuously reduced on the surface of the cathode during electrorefining, and at the same time, solid KF is formed. In this way, when silicon (Si) crystals grow in solid KF, crystal growth is induced in accordance with the growth direction of solid KF as compared to when grown in a liquid phase to form fibrous silicon (Si) crystals. Therefore, when KF is dissolved by forming a eutectic with LiF again by stirring the surface of the cathode or by vigorous stirring, it may not be difficult to obtain silicon fibers. Therefore, during electrolytic refining, forced agitation should be excluded, and natural convection caused by the up and down temperature gradient of the molten salt should also be suppressed. For this purpose, it is desirable to manage the local temperature difference within the molten salt within 10 ° C.

In addition, the principle of silicon refining by electrolytic refining generally applies a certain amount of overvoltage for process efficiency. At this time, elements having a redox potential similar to that of silicon are oxidized and reduced together. In order to prevent this, when the concentration of K ₂ SiF _{6 in} the electrolyte is increased, a reaction as in Scheme 5 below occurs on the surface of the anode.

Scheme 5

K ₂ SiF ₆ + xM = xMF _{(x + y)} + Si + 2KF

In Scheme 5, M is an impurity such as Ca, Fe, Mn, Mg, Ni, Cu, Cr, Fe, Al, Ti, and B, and M is Ca, Fe, Mn, Mg, Ni, Cu, Cr, Fe X = 2, y = 0, and M = Al, Ti, B, x = 1.33, y = 1.67.

In this case, Ti, Al, Mg, and Ca, which have a negative free energy value, can be purified through a remelting process, and more precious Fe, Mn, Ni, Cr, etc. are separated from the silicon of the negative electrode by physical separation in powder form. Can be. In particular, in the case of B can be removed in the gas phase as shown in Scheme 6 below.

[Reaction Scheme 6]

K ₂ SiF ₆ + 1.33 B = Si + 1.33 BF ₃ (g) ↑ + 2KF [ΔG = -33.735 kJ / mol _{( at 700 ° C)} ]

By inducing the same reaction as in Scheme 5 and Scheme 6, more pure silicon can be obtained. 3 is a photograph showing the electrodeposited material formed on the negative electrode surface after electrolytic refining as a preferred embodiment of the present invention, Figure 4 is a silicon form according to the distance from the negative electrode surface formed on the negative electrode surface after electrolytic refining as a preferred embodiment of the present invention 5 is a photograph showing a distribution, and FIG. 5 is a photograph showing a shape and a size of a silicon electrodeposited body formed on a surface of an anode after electrolytic refining as a preferred example of the present invention. As shown in FIG. 3, when the temperature of the electrolyte is maintained at 700 ° C. during electrolytic refining, the electrodeposited material formed on the surface of the negative electrode is a molten salt used as an electrolyte, depending on the process conditions, and the remainder is pure silicon. to be. In addition, as shown in FIG. 4, the density of silicon in the electrodeposited material formed on the surface of the negative electrode is higher as the distance from the surface of the cathode is higher and lower as it is farther away. In addition, looking at the shape of the silicon in the electrodeposition formed on the surface of the negative electrode, the closer the distance to the negative electrode surface, the coarse silicon crystals of the dendritic or micro-fiber shown in Figure 5 is generated, the farther distance from the negative electrode surface Silicon nanofibers of the order of several nanometers to tens of nanometers as seen at 5 are formed.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are intended to clearly illustrate the present invention and do not limit the scope of protection of the present invention.

1. Measurement of redox potential of silicon

Cyclic voltammetry (CV) experiments were conducted to determine the redox potential of silicon. At this time, a molten salt in the electrolyte was used as a eutectic consisting of 51 mol% LiF and 49 mol% KF. Cyclic voltammetry was used as a control using a sample in which K ₂ SiF ₆ was not added to the eutectic. , CV) experiment was conducted. In addition, K ₂ SiF ₆ was added to the eutectic at different concentrations to prepare redox potential analysis samples, and cyclic voltammetry (CV) experiments were performed on each of the samples. In the cyclic voltammetry (CV) experiment, the temperature of the electrolyte was maintained at 700 ° C, platinum (Pt) was used as the reference electrode, and molybdenum wire (working electrode) was used. Mo wire) was used, and platinum (Pt) was used as a counter electrode.

FIG. 6 shows cyclic voltammograms of concentrations of K ₂ SiF ₆ in an electrolyte solution containing a KF-LiF eutectic. In FIG. 6, a) is a cyclic voltammogram when the concentration of K ₂ SiF ₆ in the electrolyte is 0% by weight, and b) is 1% by weight of K ₂ SiF ₆ in the electrolyte. Cyclic voltammogram, based on the total weight of the electrolyte containing molten salt and K ₂ SiF ₆ ), and c) is the concentration of K ₂ SiF ₆ in the electrolyte at 5% by weight (molten salt and K). Cyclic voltammogram when the total weight of the electrolyte solution containing ₂ SiF ₆ ) is used, and d) represents 9% by weight of K ₂ SiF ₆ in the electrolyte (molten salt and K ₂ SiF ₆₎ . Cyclic voltammogram when the total weight of the electrolyte is included). As shown in FIG. 6, when the concentration of K ₂ SiF ₆ in the electrolyte is 1 wt%, 5 wt%, and 9 wt%, the redox potential of silicon is about -0.58 V and about -0.49 V, respectively, compared to the platinum reference electrode. , And about -0.38V. From the above results, it can be seen that the voltage applied to the cathode during electrorefining should be maintained at -0.4V to -0.6V or lower than that of the platinum reference electrode, and -0.4V to -1.0V is suitable for the platinum reference electrode. Judging.

2. Metal grade  Electrolytic Refining Experiment of Silicon

Example 1.

Fill the electrolyte with an electrolyte consisting of molten salt and K ₂ SiF _6, and place an anode made of metal grade silicon (MG-Si feedstock) and a cathode made of silver plate facing each other at predetermined intervals in the electrolyte. I was. At this time, a molten salt constituting the electrolyte was used as a eutectic consisting of 51 mol% LiF and 49 mol% KF, the content of K ₂ SiF ₆ was 1% by weight based on the total weight of the electrolyte, the silicon ingot as a positive electrode Was processed into a cube having a size of 22 mm x 22 mm x 40 mm. In addition, platinum (Pt) was used as a reference electrode.

Thereafter, the electrolytic cell was maintained in an inert gas atmosphere, and the electrolytic refining was performed by passing a current between the anode and the cathode at a density of 23.8 mA / cm 2 (based on the cathode cross-sectional area) for 16 hours and 40 minutes. At this time, the temperature of the electrolyte was maintained at 700 ℃.

After completion of the electrolytic refining, the electrodeposited material was removed from the cathode, the electrodeposited material was dissolved in distilled water to remove KF primarily, and then LIF was removed using an aqueous 3% hydrochloric acid solution to obtain a high purity silicon electrodeposited product. .

Example 2.

A high purity silicon electrodeposit was obtained in the same manner as in Example 1 except that the content of K ₂ SiF ₆ was 5% by weight based on the total weight of the electrolyte.

Example 3.

A high purity silicon electrodeposit was obtained in the same manner as in Example 1 except that the content of K ₂ SiF ₆ was 9% by weight based on the total weight of the electrolyte.

Example 4.

A high purity silicon electrodeposited material was obtained in the same manner as in Example 1 except that the current density during electrolytic refining was 39.8 mA / cm 2 (cathode cross-sectional area) and the current conduction time was 10 hours.

Example 5.

Except that the content of K ₂ SiF ₆ is 5% by weight based on the total weight of the electrolyte, the current density is 39.8 ㎃ / ㎠ (cathode cross-sectional area) and the current conduction time is 10 hours. In the same manner as in Example 1, a high purity silicon electrodeposit was obtained.

Example 6.

Except that the content of K ₂ SiF ₆ is 9% by weight based on the total weight of the electrolyte, the current density during electrolytic refining is 39.8 ㎃ / ㎠ (cathode cross-sectional area) and the current conduction time is 10 hours. In the same manner as in Example 1, a high purity silicon electrodeposit was obtained.

Example 7.

High purity silicon electrodeposited material was obtained in the same manner as in Example 1 except that the current density during electrolytic refining was 71.6 mA / cm 2 (cathode cross-sectional area) and the current conduction time was 5 hours 35 minutes.

Example 8.

Except that the content of K ₂ SiF ₆ is 5% by weight based on the total weight of the electrolyte, the current density during electrolytic refining is 71.6 ㎃ / ㎠ (cathode cross-sectional area) and the current conduction time is 5 hours 35 minutes. In the same manner as in Example 1, a high purity silicon electrodeposit was obtained.

Example 9.

Except that the content of K ₂ SiF ₆ is 9% by weight based on the total weight of the electrolyte, the current density during electrolytic refining is 71.6 ㎃ / ㎠ (cathode cross-sectional area) and the current conduction time is 5 hours 35 minutes. In the same manner as in Example 1, a high purity silicon electrodeposit was obtained.

 3. Electrodeposited  Form and Purity Analysis of Silicon

The morphology of the high purity silicon deposits obtained in Examples 1-9 were observed by Scanning Electron Microscope (SEM). FIG. 7 is a photograph of a high purity silicon electrodeposited body according to the current density and the content of K ₂ SiF ₆ during electrolytic refining according to an embodiment of the present invention. As shown in FIG. 7, the high purity silicon electrodeposited material obtained in the example of the present invention showed the shape of nanofibers having a diameter of about 500 nm or less. In addition, the purity of the high purity silicon electrodeposits obtained in Examples 1 to 9 were analyzed by Inductively Coupled Plasma (ICP) and the results are shown in Table 1 below.

division MG-Si feedstock Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Element content
(ppm) Al 4729 98.8 47.6 35.5 35.7 39.8 456.0 35.0 59.7 22.7 Ca 1002 487.1 30.1 29.3 36.2 31.9 148.1 31.9 31.3 65.4 Mn 283 1194.9 178.2 89.7 26.6 45.4 24.2 120.2 175.9 793.5 Mg 51 19.9 1.2 1.6 1.6 3.5 6.4 1.3 35.7 0.6 Fe 2444 88.3 39.8 29.6 45.9 62.5 36.1 158.5 44.3 223.6 Ti 367 265.2 47.3 50.5 1.1 4.2 2.1 14.0 11.5 35.9 Ni 95 20.4 19.7 20.9 46.4 32.2 7.4 138.2 5.4 144.7 Cu 44 23.2 9.5 9.0 8.9 10.2 2.9 17.8 7.4 12.3 Cr 20 662.2 308.9 284.4 15.8 17.8 4.8 48.8 18.9 244.1 all 9035 2860.0 682.3 550.5 218.2 247.5 688 565.7 390.1 1542.8

As shown in Table 1, silicon having a maximum purity of 99.98% was obtained from electrolytic refining according to an embodiment of the present invention from metal grade silicon having a purity of 99.09% (MG-Si feedstock).

Although the present invention has been described through the above embodiments as described above, the present invention is not necessarily limited thereto, and various modifications can be made without departing from the scope and spirit of the present invention. In addition, many modifications may be made to adapt a particular situation and material to the teachings of the invention without departing from the essential scope thereof. Therefore, the protection scope of the present invention should be construed as including all embodiments falling within the scope of the claims appended to the present invention.

Claims (16)

Preparing an electrolytic refining apparatus by placing an anode including a low purity metallic silicon in an electrolytic cell containing an electrolyte and a cathode at a distance spaced from the anode; And
And electrolytic refining by passing a current between the anode and the cathode and electrodepositing high-purity silicon in the form of nanofibers to the cathode.
The electrolyte comprises a molten salt and a silicon halogen compound,
The content of the silicon halogen compound is 0.1 to 10% by weight based on the total weight of the electrolyte,
The method of manufacturing a high purity silicon nanofiber, characterized in that the temperature of the electrolyte in the electrolytic refining step is maintained at 650 ~ 750 ℃. The method of claim 1, wherein the molten salt is an alkali metal halogen compound or an alkaline earth metal halogen compound. The method of claim 2, wherein the molten salt is a eutectic. The method of claim 3, wherein the molten salt comprises at least one eutectic selected from the group consisting of KF-LiF, LiCl-KCl, KCl-KF, and KCl-LiF. . The method of claim 1, wherein the silicon halide compound is a silicon halide compound of an alkali metal or a silicon halide compound of an alkaline earth metal. The method of claim 5, wherein the silicon halogen compound of the high purity silicon nanofibers, characterized in that composed of one or more selected from the group consisting of K ₂ SiF ₆ , Na ₂ SiF ₆ , Cs ₂ SiF ₆ , and MgSiF ₆ . Manufacturing method. The method of claim 1, wherein the low purity metallic silicon is in the form of a silicon ingot. The method of claim 1, wherein the low-purity metallic silicon is in the form of silicon scrap or silicon granules. The method of claim 8, wherein the low purity metallic silicon is contained in a positive electrode basket made of a material having a redox potential of at least 0.4 V or more than that of silicon. 10. The method for producing high purity silicon nanofibers according to claim 9, wherein the material forming the anode basket is made of one or more selected from the group consisting of Mo, Ni, W, and Pd. The method of claim 1, wherein the cathode is in the form of a plate made of silver. The method according to any one of claims 1 to 11, wherein the voltage applied to the cathode during the electrolytic refining is -0.4V to -1.0V relative to the platinum reference electrode. The method for producing high purity silicon nanofibers according to any one of claims 1 to 11, wherein the current density at the cathode during electrolytic refining is 1 to 100 mA / cm 2. The method for producing high purity silicon nanofibers according to any one of claims 1 to 11, wherein the whole electrolyte in the electrolytic cell is not physically stirred at the time of the electrolytic refining, and the temperature difference is 10 ° C or less. The method for producing high purity silicon nanofibers according to any one of claims 1 to 11, wherein the electrolytic refining time is 1 to 20 hr. The method of any one of claims 1 to 11, wherein the purity of the low-purity metallic silicon constituting the anode is 99.9% or less, and the purity of the high-purity silicon in the form of nanofibers electrodeposited to the cathode is greater than 99.9%. Method for producing high purity silicon nanofibers, characterized in that.

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