KR101340601B1 - Recovery method of elemental silicon by electrolysis in non-aqueous electrolyte from silicon sludge - Google Patents

Abstract

The present invention relates to a recovery method of elemental silicon by the electrolysis in non-aqueous electrolyte from silicon sludge. The recovery method of the elemental silicon is capable of directly recovering the elemental silicon using the electrolysis at low temperatures (less than 200°C) during a production process of silicon, is capable of controlling the structure of the silicon by a simple process and the change of electrolysis condition, and is capable of being continuously processed by adding silicon salt.

Description

Recovery method of elemental silicon by electrolysis in non-aqueous electrolyte from silicon sludge}

The present invention relates to a method for recovering elemental silicon from silicon sludge using an electrolytic method in a non-aqueous electrolyte.

In the process of cutting a wafer from a silicon ingot, a cutting slurry containing carbon silicon and the like having a size of 20 μm is used. In this process, sludge containing silicon, silicon carbide, metal powder and cutting oil is discharged, and waste sludge can be effectively separated and recovered and reused or recycled as a useful resource. It can be said.

In general, smelting of silicon can be obtained by reacting concentrated silicon dioxide (SiO ₂ ) at high temperature using a reducing agent such as carbon, which has a purity of about 98%, and thus its use is limited. In addition, there is a problem that consumes a lot of energy in the smelting process, emits a large amount of carbon dioxide and contains impurities. The purification process is essential to be used in high value added industries such as semiconductor, photovoltaic and secondary battery industries. Purification of silicon can achieve more than 99.9% purity by conversion-distillation-reduction into halogen compounds, but the process is very complicated, energy-consuming and handling of toxic substances, but alternative processes have yet to be developed. It is not.

In addition, in order to be used as a semiconductor or a solar cell, a thinning process is required, which requires wafer-making by melt-solidification followed by ingot cutting, or a deposition method such as chemical vapor deposition or sputtering. The process must be operated at high vacuum and is very expensive because the continuous process is very difficult.

In the past decades, a lot of research has been done on the process of making silicon thin films easily, but there is no alternative. The electrolytic method is a simple process and a simple operation of voltage, current, etc. to obtain a material of the desired characteristics, due to its characteristics is a process capable of continuous mass production. However, due to the low redox equilibrium potential and high reduction overvoltage, it is very difficult to reduce electrolytic reduction in ordinary aqueous solutions, and even a small amount of reduced silicon is reoxidized with oxygen in aqueous solution, making it impossible to reduce elemental silicon electrolysis.

According to the literature investigation, a method for electrolytic reduction of silicon by electrolytic method in non-aqueous electrolyte has been studied. Cohen and Huggins prepared SiF ₆ by electrolysis in LiH-KF process (U. Cohen and RA Huggins, 1976: Silicon Epitaxial Growth by Electrodeposition from Molten Fluorides, J. Electrochem. Soc., 123, pp. 381- 383.) Research began on monosilicon and polycrystalline silicon. Elwell et al. Investigated the possibility of continuous deposition of thin-film silicon in molten LiF-KF-NaF-K ₂ SiF ₆ (D. Elwell and GM Rao, 1988: Electrolytic production of silicon, J. Appl. Electrochem., 18, pp. 15-22.). They studied electrodeposition using molten salt at high temperature of 500 ~ 1400 ℃, which requires complicated process and high temperature process, which consumes a lot of cost and energy. Aprotic solvents (AK Agrawal and AE Austin, 1981: Electrodeposition of silicon from solutions of silicon halides in aprotic solvents, J. Electrochem. Soc., 128, pp. 2292-296.) Molten salts (KL Carleton , JM Olson, and A. Kibbler, 1983:.... Electrochemical nucleation and growth of silicon in molten fluorides, J. Electrochem Soc, 130, pp silicon in electrolytic reduction at 782-786) as a source of silicon K ₂ SiF ₆ , Na ₂ SiF ₆ , SiO _2, etc. were used to reduce silicon in hot molten salt, and it was reported that a uniform thin film of up to 0.25 μm thickness could be formed (J. Gobet and H. Tannenberger, 1986: Electrodeposition of silicon). from a nonaqueous solvent, J. Electrochem. Soc., 133, pp. C322.). In addition, Nicholson et al. Recently reported that silicon can be electrodeposited even at low temperature molten salt using a non-aqueous solvent, but they reported that the electrodeposited silicon exhibited strong oxidative properties to form silica (JP Nicholson, 2005: Electrodeposition of silicon). from nonaqueous solvents, J. Electrochem. Soc., 152, pp. C795-802.).

It has been reported that electrolytic reduction of highly active metals such as aluminum, titanium, and magnesium, which is impossible in aqueous electrolytes, is possible by using an ionic liquid having a wide electrochemical window and excellent electrical conductivity as an electrolyte (F. Endres, 2002). : Ionic Liquids: Solvents for the electrodeposition of metals and Semiconductors, Phys. Chem. Chem. Phys., 3, pp. 144-154.). Highly conductive electrolytes that minimize hydrogen evolution using organic solvents have been studied by Austin (AE Austin, 1976: US Pat. 3,990,953 and Nov. 9 (1976) CA 86: 10098c.). Generally, electrolytes are prepared using aprotic solvents such as silicon halide, propylene carbonate (PC), or tetrahydrofuran (THF), and they need a supporting electrolyte because they are not conductive. Austin's work used several organic electrolytes by adding tetrabutylammonium perchlorate (Bu ₄ NClO ₄ ) as the silicon halide and supporting electrolyte, and was described by Bucker and Amick (ER Bucker and JA Amick, 1980: US Pat. Oct. 16 (1980) CA 92: 10098.). A recent study of electrochemical reduction of silicon using PC has been reported by the Fukunaka Group (Y. Nishimura and Y. Fukunaka, 2007: Electrochemical reduction of silicon chloride in a non-aqueous solvent, Electrochimca Acta, 53, pp. 111-116. In the XPS analysis, the electrodeposited silicon was exposed to air and oxidized, and Munisamy et al. Studied the initial growth of silicon by electrolysis in acetonitrile (T. Munisamy and AJ Bard, 2010: Electrodeposition of Si from organic solvents and studies related to initial stages of Si growth, Electrochimca Acta, 55 (11, 15), pp. 3797-3803.). Abedin et al. Reported the electrodeposition of silicon using a room temperature ionic liquid (SZ El Abedin, N. Borissenko, and F. Endres, 2004: Electrodeposition of nanoscale silicon in a room temperature ionic liquid, Electrochem. Comm., 6, Mallet et al. reported the first synthesis of silicon nanowires in the same way (J. Mallet, M. Molinari, F. Martineau, F. Delavoie, P. Fricoteaux, and M. Troyon, 2008). : Growth of silicon nanowires of controlled diameters by electrodeposition in ionic liquid at room temperature, Nano Lett. 8 (10), pp. 3468-3474.).

They all reported a study on the electrolytic reduction of silicon at room temperature, and did not solve the problem of reduced silicon oxidation. In addition, it is not known whether the electrodeposited silicon oxide is reduced together with the dissolved oxygen of the electrolyte, or the electrodeposited porous silicon is oxidized when exposed to air, and through annealing before the electrodeposited silicon is exposed to air. Some attempts to stabilize have been attempted but have not yet been resolved.

Therefore, there is an urgent need for the development of electrolytic conditions in the recovery of elemental silicon from the silicon sludge by electrolysis in a simpler process compared to the existing complicated process.

In this study, silicon tetrachloride was prepared by mechanically removing cutting oil and metal impurities through the separation process from silicon sludge, and then roasting the separated silicon and silicon carbide mixtures with chloride. Silicon tetrachloride was dissolved in a non-aqueous solvent having conductivity, and silicon in elemental form was directly reduced by electrolysis. The crystallization theory, composition and presence of impurities were investigated from the analysis of the final electrodeposits.

While the present inventors are studying a method for recovering silicon from the silicon sludge through electrolysis, silicon tetrachloride may be prepared to directly reduce elemental silicon by electrolysis using a non-aqueous electrolyte having conductivity, and then heat treatment. By greatly improving the electrolytic process efficiency of silicon and excellent in the stabilization efficiency of silicon, it was completed the present invention.

Accordingly, the present invention is to provide a method for recovering elemental silicon from the silicon sludge using an electrolytic method in a non-aqueous electrolyte.

According to the present invention, after the production of silicon tetrachloride from the silicon sludge, the non-aqueous electrolyte can be used to control the silicon structure as well as to simplify the manufacturing process and to easily control the electrolytic parameters. Provided is a method for recovering elemental silicon using electrolysis.

In the method of recovering silicon according to the present invention, it is possible to directly reduce elemental silicon by electrolytic method at low temperature (below 200 ° C.) when manufacturing silicon, and to control silicon structure by simple process and change of electrolytic conditions. The addition of silicone salts allows for continuous operation.

1 is a schematic view of a process for producing silicon tetrachloride from silicon sludge.
FIG. 2 is a diagram showing the results of analysis using an X-ray diffraction and an electron microscope of a solid component from which cutting oil and iron powder are separated from silicon sludge.
FIG. 3 is a diagram showing electrochemical behavior measured by cyclic dislocation method using [EMIM] TFSI electrolyte solution containing 0.1M silicon tetrachloride and an electrode as gold.
Figure 4 is a view showing the SEM-EDS results of the silicon reduced in the gold electrode.
5 is a view showing the XPS measurement results of the silicon recovered by the present invention.
6 is a view showing the XRD measurement results of the silicon recovered by the present invention.

The present invention

(a) mixing the silicon sludge and the organic solvent to separate the cutting oil in the waste sludge;

(b) separating the iron (Fe) from the silicon sludge from which the cutting oil is separated using a magnetic separator;

(c) adding chlorine to the silicon sludge from which the iron has been removed, followed by heating to prepare silicon tetrachloride (SiCl ₄ );

(d) placing an electrolytic cell having a conductive electrode under a high purity inert gas atmosphere into a conductive non-aqueous solvent in which silicon tetrachloride is dissolved; And

(e) applying power to the electrolytic cell to reduce silicon at the cathode and to form a silicon thin film on the electrode surface; It provides a method of recovering elemental silicon, including.

Hereinafter, the present invention will be described in detail.

In the silicon recovery method of the present invention, step (a) is a step of separating the cutting oil in the silicon sludge, by mixing the organic solvent and waste sludge which can be mixed with the cutting oil selectively to dissolve the cutting oil to separate the cutting oil from the silicon sludge do.

The organic solvent is preferably chloroform, ethyl acetate, tetrahydrofuran (THF), dichloromethane (CH ₂ Cl ₂ ), but is not limited thereto.

Step (b) is to remove iron (Fe) powder contained in the silicon sludge from which the cutting oil is separated. Iron powder is produced by friction of a wire saw and silicon, and is separated using a magnetic manufacturing machine. The magnetic separator is preferably a magnetic flux density of 500 gauss.

Step (c) is a step for preparing silicon tetrachloride (SiCl ₄ ) from the silicon sludge after iron is separated. The silicon sludge from which the iron has been removed is a mixture of silicon and silicon carbide, and is heated after the addition of chlorine to recover silicon tetrachloride. The heating is preferably performed for 30 to 90 minutes at 800 ~ 1200 ℃.

In the step (d), the basic electrolytic cell structure for the electrolytic reduction of silicon includes a three-way electrode system that additionally uses a reference electrode for the precise monitoring of the voltage applied to the cathode of an electrolytic cell consisting of a working electrode and a counter electrode. use.

The electrode may be any metal having conductivity, and preferably two or more selected from the group consisting of gold, platinum, and copper, but are not limited thereto.

In the step (d), as the high purity inert gas, at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon may be used, and argon may be preferably used. By recovering the silicon in a high purity inert gas atmosphere, there is an effect that can prevent the oxidation of the silicon.

In step (d), a conductive non-aqueous solvent capable of dissolving silicon tetrachloride may be used as an electrolyte. The conductive non-aqueous solvent is a conductive non-aqueous solvent containing TFSI (bis (trifluoromethylsulfonyl) imide) anion, [BMPy] TFSI (1-Butyl-3-methyl-pyridinium bis (trifluoromethylsulfonyl) imide), PP13TFSI (1-methyl -propylpiperidinium bis (trifluoromethylsulfonyl) imide) and [EMIM] TFSI (1-Ethyl-3-methyl-imidazolium bis (trifluoromethylsulfonyl) imide), including but not limited to one or more selected from the group consisting of.

In addition, the conductive non-aqueous solvent may further include PC (Propylene carbonate), DCM (Dichloromethane), THF (Tetrahydrofuran), DCA (dicyanamide) or NMP (N-methylpyrrolidone). The material has an effect of improving the electrical conductivity of the electrolyte solution without affecting the dissociation of silicon while having a very low viscosity.

Before step (d), washing the electrode and the cell with a mixture of sulfuric acid (H ₂ SO ₄ ) and hydrogen peroxide (H ₂ O ₂ ); And (d '') drying the non-aqueous solvent at 80-120 ° C. for 20-30 hours; As shown in FIG. The sulfuric acid and hydrogen peroxide of step (d ') are preferably used in a mixture of 50: 50% by volume, and impurities of the electrode and the cell can be removed by this step. By passing through the step (a ''), the water contained in the trace amount in the non-aqueous solvent can be removed.

In the step (e), power is supplied to the electrolytic cell to reduce silicon at the cathode and a silicon thin film is formed on the electrode surface. The two metal electrodes are placed in an electrolyte in which silicon is dissolved, as in general electroplating. When the cathode is added, reduction of silicon occurs at the cathode, and silicon electrodeposits are formed on the electrode surface.

After the step (e) may further comprise the step of heat treatment for 30 to 90 minutes at 800 ~ 900 ℃ under an inert gas atmosphere. The inert gas includes but is not limited to nitrogen, helium, argon, neon, xenon and the like. By including the heat treatment step there is an effect of removing impurities and stabilization of the electrodeposited silicon.

As described above, when recovering the silicon by the recovery method according to the present invention, it is possible to directly improve the element-type silicon by the electrolytic method, and the process is easy compared to the existing complex process, and because it is a low temperature process, mass production is possible. Cost savings are excellent. In addition, the method for recovering silicon according to the present invention has the effect that the manufacturing process is simple and the silicon can be continuously recovered while repeatedly dissolving silicon tetrachloride.

Hereinafter, preferred embodiments of the present invention will be described in order to facilitate understanding of the present invention. However, the following examples are provided only for the purpose of easier understanding of the present invention, and the present invention is not limited by the examples.

< Example 1 > Silicone From sludge Of silicon tetrachloride  Produce

An overview of the process for producing silicon tetrachloride as a pretreatment for the recovery of silicon from silicon sludge is shown in FIG. 1.

Dichloromethane (CH ₂ Cl ₂ ) was mixed with silicon sludge as an organic solvent that can be mixed well with cutting oil in the silicon sludge, and the cutting oil was selectively dissolved from the silicon sludge by stirring at 300 rpm or more for 3 hours. After dissolving the cutting oil, the solid phase (Si + SiC) and the liquid phase (organic oil) separated the solid silicon, the silicon carbide and the liquid organic oil by filtration and centrifugation. Iron powder contained in the solid-phase mixture from which the oil-soluble oil was separated was separated by magnetic manufacturing with a magnetic flux density of 500 gauss, and the mixture was separated into 1 mol / L hydrochloric acid, high liquid ratio 1: 2, After stirring for 2 hours at room temperature, precipitated, washed with distilled water and dried. The solid phase component separated to iron powder was analyzed by X-ray diffraction and electron microscopy.

As shown in Figure 2, the iron content in the separated solid powder of iron powder is removed to less than 0.1% by weight relative to the total solid weight, it was confirmed that the mixture of silicon and silicon carbide.

Thereafter, 5 wt% of carbon powder was added to the mixture of concentrated silicon and silicon carbide, charged in a furnace, infused with chlorine gas, chlorinated and roasted at 1000 ° C. for 1 hour, and the distilled gas was condensed at 25 ° C. It was prepared in the form of silicon tetrachloride (SiCl ₄ ).

< Example  2> From silicon tetrachloride  Electrolytic Recovery Of Silicon

1. Experimental conditions for electrolytic recovery of silicon

The basic electrolytic cell structure for the electrolytic reduction of silicon was used a ternary electrode system, and the electrolyte solution [EMIM] TFSI dissolved in silicon tetrachloride. In order to prevent oxidation of silicon, experiments were carried out in a glove box loaded with high purity argon gas (5N, oxygen content within 1 ppm, water vapor content within 3 ppm). The effect of the voltage window and the electrolytic conditions (electrode, ionic liquid, silicon concentration, etc.) on the electrochemical properties of ionic liquids was investigated by cyclic voltammetry using Potentiostat / Galvanostat (Solartron 1287). Measured. The voltage ranged from -4 to 2 V (vs. OCV) and the scan rate was measured at 10 mV / s.

The working electrode was based on Au and the electrochemical oxidation / reduction behavior of silicon was investigated. Platinum wire (Pt wire) was used as a counter electrode (counter electrode, 4 cm ² ) and a reference electrode (QRE, quasi reference electrode), respectively. In order to remove impurities, all electrodes and cells were washed with a 50:50 volume% mixture of sulfuric acid (H ₂ SO ₄ ) and hydrogen peroxide (H ₂ O ₂ ) before the experiment, and then used in the experiment. In addition, all ionic liquids were dried for 24 hours in a 100 ^° C vacuum oven before the experiment to remove the traces of water contained in the electrochemical experiment. In order to analyze the shape, composition and crystallinity of elemental silicon prepared by electrolytic method, elemental silicon was prepared by electrostatic potential method (-1.9 V vs. Pt-QRE) from [EMIM] TFSI dissolved in 0.5 M of silicon tetrachloride. The experiment was conducted to reduce directly.

2. Electrolytic Recovery of Silicon

Electrochemical behavior of the [EMIM] TFSI electrolyte and the electrode using gold (Au) is shown in FIG. 3.

As shown in FIG. 3, the electrochemically stable electrochemical window was about 3 V and the negative electrode potential was -2.5 V (vs. Pt-QRE). In the electrolyte of 0.5 M SiCl ₄ , it was confirmed that a strong reduction current was expected to be expected in the reduction of stable silicon considering the negative limit potential of [EMIM] TFSI. Looking at the cyclic potential behavior in the electrolyte containing silicon tetrachloride (SiCl ₄ ), the first reduction current starts to increase from 0 V and there is a second reduction region where the reduction current density increases rapidly at about -2.2 V. In the first reduction zone, tetravalent silicon ions are expected to be changed into other compounds. In the second reduction zone starting from -2.2 V, the ionic silicon and the intermediate silicon compound are expected to be reduced to silicon. . In particular, it was confirmed that impurities are not likely to be included in the reduction reaction at a potential that is expected to be a reduction current of silicon at a potential higher than the reduction limit potential.

< Experimental Example 1 > Silicone Electrodeposition layer  analysis

1. Analysis method

The shape of the silicon was observed using a field emission scanning electron microscope (FE-SEM, JEOL, JSM-6500F), and the composition of the electrodeposition layer was X-ray spectrometer attached to the FE-SEM (energy). dispersive spectrometer (EDS). Impurity identification of the electrodeposited silicon was analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Quantera SXM), and the surface analysis was performed with depth analysis at a depth of about 50 nm from the surface. . In addition, the crystallinity of silicon was confirmed using an X-ray diffractometer (X-ray diffractometer, XRD, Rigaku D / MAX-2000, Cu-Ka).

2. Electrodeposition layer  Analysis

SEM-EDS results of the silicon reduced in the gold electrode are shown in FIG. 4.

As shown in Figure 4, the shape of the silicon is gathered in the form of particles of about 100 nm, EDS analysis confirmed that the reduced silicon with the working electrode gold.

Since the silicon was obtained in a stable section identified in the cyclic potential curve, other impurities except for the working electrode did not appear, but a large amount of oxygen was detected. Experiments to measure the reduction of silicon and the properties of ionic liquids are carried out in a glove box in an argon atmosphere, and it is unlikely that silicon has been reduced to oxides. Since the moisture inside was removed, the detected oxygen was exposed to the outside while preparing the specimen and moving to the analyzer. 5 shows the results of measuring XPS for surface and impurity analysis.

As shown in FIG. 5, the amount of reduced silicon was very low at 31.3%, and no impurities other than oxygen were found. Silicon was detected in the form of SiO ₂ and confirmed through depth analysis to determine whether the reduction of the surface or oxide proceeded. Looking at the results of the inside 20 nm from the surface, it was confirmed that the intensity of the SiO ₂ peak is lowered from the surface to the inside, the Si peak appears. After 20 nm, a peak of complete Si appears, and as a result, SiO ₂ was not generated during electrolytic reduction, and it was confirmed that only the surface was oxidized while exposed to the outside for analysis. In addition, it was confirmed that pure silicon was reduced because the component and chloride ion of the electrolyte were not detected.

XRD was measured to confirm the crystallinity of the electroreduced silicon is shown in FIG. Based on the peak of gold used as the working electrode, the silicon peak was analyzed, and it was confirmed that silicon was present in polycrystalline or amorphous form because several silicon crystal planes were mixed.

Claims (11)

(a) mixing the silicon sludge and the organic solvent to separate the cutting oil in the waste sludge;
(b) separating the iron (Fe) from the silicon sludge from which the cutting oil is separated using a magnetic separator;
(c) adding chlorine to the silicon sludge from which the iron has been removed, followed by heating to prepare silicon tetrachloride (SiCl ₄ );
(d) placing an electrolytic cell having a conductive electrode under a high purity inert gas atmosphere into a conductive non-aqueous solvent in which silicon tetrachloride is dissolved; And
(e) applying power to the electrolytic cell of step (d) to reduce silicon at the cathode and to form a silicon thin film on the electrode surface;
A method for recovering elemental silicon comprising a. The method of claim 1, wherein the organic solvent in step (a) is chloroform (chloroform), ethyl acetate (ethyl acetate), tetrahydrofuran (THF), dichloromethane (CH ₂ Cl ₂ ), characterized in that the element form Recovery method of silicon. The method of claim 1, wherein the magnetic separator in the step (b) has a magnetic flux density of 500 gauss. The method for recovering elemental silicon according to claim 1, wherein the heating in step (c) is performed at 800 to 1200 ° C for 30 to 90 minutes. The method of claim 1, wherein the electrode in step (d) is at least two selected from the group consisting of gold, platinum and copper. The method for recovering elemental silicon according to claim 1, wherein the inert gas in step (d) is at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon. The method of claim 1, wherein the conductive non-aqueous solvent of step (d) is a conductive non-aqueous solvent containing a trifluoromethylsulfonyl (bis) anion (TFSI), [BMPy] TFSI (1-Butyl-3-methyl-pyridinium bis) (trifluoromethylsulfonyl) imide), PP13TFSI (1-methyl-propylpiperidinium bis (trifluoromethylsulfonyl) imide) and [EMIM] TFSI (1-Ethyl-3-methyl-imidazolium bis (trifluoromethylsulfonyl) imide) A recovery method of elemental silicon. The method of claim 7, wherein the conductive non-aqueous solvent is propylene carbonate, dichloromethane, tetrahydrofuran, dicyanamide, or N-methylpyrrolidone. The method for recovering elemental silicon, characterized in that it further comprises a). The method of claim 1, wherein before step (d)
(d ') washing the electrode and the cell with a mixture of sulfuric acid and hydrogen peroxide; And
(d '') drying the non-aqueous solvent at 80-120 ° C. for 20-30 hours; Method for recovering elemental silicon, characterized in that it further comprises. According to claim 1, After the step (e) further comprising the step of heat treatment for 30 to 90 minutes at 800 ~ 900 ℃ under inert gas atmosphere, element recovery method silicon. The method of claim 10, wherein the inert gas is at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon.

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