Method for continuously electrolyzing and extracting high-purity silicon from fused silica acid salt

Abstract

The invention provides a method for continuously electrolyzing and extracting high-purity silicon from molten silica, which comprises the following steps of: placing an electrolyte in an electrolytic container, immersing a cathode and an anode in the electrolyte, heating the electrolytic container to 600-900 DEG C in an inert gas atmosphere, switching on the electrode and supplying power to the electrode for more than 1 hour at a constant current with the current density of 25-100mA. Cm <-2 >; and after the electrolysis is finished, cooling the cathode to room temperature in an inert gas atmosphere, taking out, cleaning and drying to finally obtain the high-purity crystalline silicon. Silicon subjected to electro-deposition through the electrolysis method is low in impurity concentration and high in purity, and high-purity silicon with the purity being 99.9998% or above can be obtained. The electrolysis method has the advantages of being low in energy consumption, high in efficiency and the like, and the current efficiency can reach 80% or above based on the method; the electrolyte used for electrodeposition is low in cost, environmentally friendly and free of pollution, and silicon electrodeposited through the electrolysis method is good in compactness, few in hole and basically free of surface cracks.

Description

Technical Field

[0001] The invention relates to a method for sustainably electrolyzing molten silicate to extract high-purity silicon, belonging to the technical field of new energy materials. Background Art

[0002] In industry, silicon raw materials are usually extracted from quartz ores. After a series of purification processes, industrial silicon can be converted into polycrystalline silicon and monocrystalline silicon for use in the photovoltaic industry and the electronics industry. In addition to quartz sand, other ores such as feldspar, mica and zircon can also be used to prepare crystalline silicon, but due to the difficulty of beneficiation of these ores and the high production cost, quartz sand is still the main raw material.

[0003] Amorphous silicon can be prepared by reducing silicon dioxide with magnesium. In the laboratory, magnesium powder can be used to reduce powdered silicon dioxide under high temperature conditions, and then the generated magnesium oxide and unreacted magnesium powder can be removed with dilute acid, and then the residual silicon dioxide can be removed with hydrofluoric acid to finally obtain elemental silicon. However, the amorphous silicon obtained by this method has low purity and is usually in the form of brown-black powder. Crystalline silicon can be prepared by reducing silicon dioxide with carbon in an electric furnace. In the process of industrial production of silicon, electric arc furnaces are usually used to reduce silica (SiO2 content exceeds 99%), and the reducing agents used include petroleum coke and charcoal. When using a DC arc furnace, petroleum coke can completely replace charcoal. Due to the low ash content of petroleum coke (0.3% to 0.8%) and the use of high-quality silica (SiO2 content greater than 99%), high-quality silicon for the manufacture of silicon steel sheets can be directly refined. As for the high-purity silicon required in the electronics industry, it is obtained by reducing trichlorosilane or silicon tetrachloride with hydrogen. High-purity semiconductor silicon can be produced by reducing high-purity trichlorosilane (SiHCl3) or silicon tetrachloride (SiCl4) with hydrogen on a hot silicon rod at about 1200°C. As for ultra-pure single crystal silicon, it can be produced by techniques such as the Czochralski method or the zone melting method.

[0004] Since silicon requires high temperatures during the crystallization process, it involves complex processing and a lot of energy consumption. To solve this problem, since the 1980s, researchers have proposed and explored methods for directly producing silicon at low temperatures in liquid or molten salts. As an industrial technology, the molten salt electrolysis process is widely used in the extraction of key metals such as aluminum and rare earths due to its advantages such as simple process, cost-effectiveness and environmental protection. This method has also been widely used in the extraction and refining of silicon, especially in the manufacture of high-purity silicon films required for solar cells, which are usually prepared by molten salt electrolysis in silicon-containing molten salts. Since the 1970s, research on fluorine-based molten salt electrodeposition of crystalline silicon films has begun, but the current process of molten salt electrolysis to purify silicon still has problems such as high cost and difficult impurity control. Therefore, it is urgent to develop a process for molten salt electrolysis to extract silicon with cheap raw materials. Summary of the invention

[0005] The present invention is used for the sustainable electrolysis extraction of high-purity silicon from molten silicate and provides a process of adding fluoride and chloride molten salt to silicate, wherein the cathode is an inert electrode and the anode is graphite. The fluoride and chloride molten salt added with silicate is subjected to constant current electrolysis at a specific temperature to extract silicon. After the electrodeposition process is completed, the deposited silicon is slowly taken out from the molten electrolyte and cooled in an inert gas atmosphere. Then the sample is taken out, washed with water and ethanol, and then dried at 100°C to finally obtain high-purity crystalline silicon.

[0006] To solve the above technical problems, according to one aspect of the present invention, the present invention provides the following technical solutions:

[0007] A method for sustainable electrolytic extraction of high-purity silicon from molten silicate, which is particularly characterized in that it comprises the following steps:

[0008] 1) Place the electrolyte in an electrolytic container, then immerse the cathode and anode in the electrolyte, heat the electrolytic container to 600-900°C in an inert gas atmosphere, connect the electrodes to a power source and charge at a current density of 25-100 mA cm -2 The constant current is supplied to the electrode for more than 1 hour, the electrolyte includes a silicate and an alkali metal salt, the silicate includes at least one of CaSiO3, K2SiO3, Na2SiO3, Al2SiO5, and MgSiO3, and the alkali metal salt includes at least one of KF, KCl, LiF, LiCl, NaCl, and NaF;

[0009] 2) After the electrolysis is completed, the deposited silicon is slowly removed from the molten electrolyte and cooled in an inert gas atmosphere. The sample is then taken out, washed with water and ethanol, and then dried at 100°C to finally obtain high-purity crystalline silicon.

[0010] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon using molten silicate according to the present invention, in step 1), the silicate accounts for 10%-20% and the alkali metal salt accounts for 80-90% based on the total mass of the electrolyte mixture.

[0011] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the electrolyte is heated to remove moisture in the pretreatment step 1), the heating temperature is 200-300°C, and the insulation time is not less than 12h.

[0012] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate according to the present invention, the power supply in step 1) is less than 24 hours.

[0013] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the cathode in step 1) is an inert electrode.

[0014] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the anode in step 1) is graphite.

[0015] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the electrolysis container in step 1) includes a crucible and a vertical resistance furnace, and placing the electrolyte in the electrolysis container is specifically: loading the electrolyte into the crucible and then placing the crucible in the constant temperature area of ​​the vertical resistance furnace.

[0016] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the heating of the electrolytic container in step 1) is specifically: heating the resistance furnace to a temperature above the melting point of the electrolyte.

[0017] As a preferred embodiment of the method for sustainable electrolytic extraction of high-purity silicon from molten silicate described in the present invention, the crucible in step 1) includes an iron crucible, a quartz crucible, a corundum crucible, a graphite crucible or a nickel crucible.

[0018] The fluorine-alkali salt and chlorine-alkali salt selected in the present invention are used as supporting electrolytes, which have the characteristics of low vapor pressure, low viscosity, wide electrochemical window, high ionic conductivity (and low electronic conductivity) and large decomposition potential range (>3V), and can effectively dissolve reaction products such as oxygen ions, silicates or carbon dioxide during the electrolysis process, and can effectively improve the purity of deposited Si. The selected silicates are used as active silicon sources.

[0019] The present invention mainly achieves the acquisition of high-purity silicon by regulating the current density, electrolysis temperature, electrolysis time and silicon ion concentration in the molten salt. Specifically, based on the scheme of the present invention, it is creatively found that when the electrolysis temperature is controlled at 600-900°C, it can ensure that the molten salt remains in a molten state during the electrolysis process. When the temperature is too low, the molten salt is insufficient to form silicate ions, affecting the deposition rate; when the temperature is too high, the molten salt volatilizes severely, reducing the electrolysis efficiency. The current density has a significant effect on the compactness of silicon deposition, but when the current density is too low, it will lead to the formation of nanowires, accompanied by more inclusions and thus reduce the cleanliness of the deposited silicon; when the current density is too high, silicon powder will be formed, and the density of silicon deposition will be reduced. Based on this, the current density of the present invention should be controlled at 25-100mA·cm -2. In the silicon electrodeposition process, if the electrolysis time is too long or too short, the density of the silicon deposition will be affected. If the time is too short, the deposited silicon will not have time to adhere to the cathode surface. If the time is too long, the grains will grow abnormally, causing holes. Based on this, the electrolysis time of the present invention is controlled to be more than 1 hour, and further controlled to be less than 24 hours, thereby obtaining a dense deposited silicon film with a thickness of 5-60μm. The concentration of silicon ions in the electrolyte will affect the electrolysis efficiency. When the concentration is too low, the silicon in the molten salt cannot provide enough silicon source for electrodeposition and crystallization. Too high a concentration will also cause the molten salt to not melt. Based on this, the proportion of silicate in the electrolyte of the present invention is 10%-20%.

[0020] The present invention deposits silicon in a dual-electrode manner, the cathode is an inert electrode that does not react with silicon, and the deposited silicon adheres to the cathode surface, which is easy to clean and peel off later. After the electrodeposition process is completed, the deposited silicon is slowly taken out from the molten electrolyte and cooled in an inert gas atmosphere. Then the sample is taken out, washed with water and ethanol, and then dried at 100°C to finally obtain crystalline silicon.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1) The silicon electrodeposited by the electrolytic method of the present invention has low impurity concentration and high purity. Based on the method of the present invention, high-purity silicon with a purity of more than 99.9998% can be obtained;

[0023] 2) The electrolysis method of the present invention has the advantages of low energy consumption and high efficiency. Based on the method of the present invention, the current efficiency can reach more than 80%;

[0024] 3) The electrolyte used for electrodeposition of the present invention is low in cost and environmentally friendly and pollution-free;

[0025] 4) The silicon electrodeposited by the electrolytic method of the present invention has good density, few holes, and basically no surface cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0027] Figure 1 Electrolysis device used in the present invention

[0028] Among them, 1-furnace tube, 2-anode, 3-cathode, 4-molten salt, 5-crucible, 6-carbon block, 7-air inlet, 8-air outlet DETAILED DESCRIPTION

[0029] The following will be described clearly and completely in conjunction with the technical solutions in the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The present invention adopts the process of adding silicate to fluoride and chloride molten salt. The cathode is an inert electrode with a thickness of 0.1 to 0.65 mm and a width of 5 to 10 mm. The anode is graphite with a diameter of 5 to 10 mm and a length of 5 cm immersed in the electrolyte. The electrode lead is made of tungsten wire and protected by a quartz tube. Silicon extraction is carried out in the fluoride and chloride molten salt added with silicate through constant current electrolysis at a specific temperature. The temperature is 600°C to 900°C and the constant current density is 25 mA cm -2 ~100mA·cm -2 Under the conditions of , the electrodeposition was carried out for more than 1 hour. After the electrolysis was completed, the cathode was taken out and cooled in an inert gas atmosphere. Then the sample was taken out, washed with water and ethanol, and then dried at 100 ° C to finally obtain high-purity crystalline silicon.

[0031] The preparation mechanism of the present invention is as follows: In the molten salt, the silicon in the silicon source reacts with O2 to generate soluble Si IV -O anions, these anions are reduced to Si atoms on the surface of the inert electrode by gaining electrons. After nucleation and crystallization, a uniform silicon film is gradually formed. IV The O2 separated from the -O anions diffuses toward the graphite anode and leaves the system in the form of CO or CO2. The increase in film thickness and grain size indicates that electrodeposition and crystallization are proceeding continuously.

[0032] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0033] Example 1

[0034] 30 g of LiF with a purity of 99.5%, 60 g of KF with a purity of 99.9%, 10 g of NaF eutectic salt with a purity of 99.9% and 29.5 g of Na2SiO3 were weighed, mixed at 500 r / min for 30 min in a planetary high-energy ball mill, taken out and put into an iron crucible wiped dry with alcohol, and the crucible was placed in a horizontal furnace, high-purity Ar was introduced, and the temperature was raised to 200°C and kept warm for 12 h to remove free water and bound water in the mixed salt.

[0035] A single crystal graphite rod with a diameter of 5 mm and a length of 50 mm was used as the anode, and a graphite sheet with a thickness of 0.65 mm was used as the cathode. The surface of the motor was cleaned with deionized water and anhydrous ethanol in turn and then placed in a 70°C oven to dry for use.

[0036] Use crucible tongs to place the iron crucible containing salt in the constant temperature area of ​​the vertical resistance furnace, keep the crucible horizontal, fix the graphite cathode on one end of the stainless steel rod, and insert it into the middle of the furnace tube through the pipe opening of the furnace cover of the resistance furnace. The same operation is performed on the graphite rod anode. After checking that the airtightness of the resistance furnace is good, introduce high-purity Ar gas and circulating water to raise the temperature of the resistance furnace to 700°C at 5°C / min. After reaching the target temperature, keep it warm for 30 minutes to completely melt the mixed salt.

[0037] Use a multimeter to connect the top of the two electrodes to determine the position of the molten salt and the depth of the electrodes immersed in the molten salt. Both the anode and cathode are immersed 1.5cm. After fixing the electrode position, connect the electrodes to the power supply and set the constant current to 42.75mA (25mA / cmm 2 ), electrolysis time 4h.

[0038] After the electrolysis is finished, the electrode is slowly lifted up and cooled to room temperature at a rate of 5°C / min. The electrode sheet is taken out of the furnace, cleaned and placed in a 70°C oven to dry. Figure 1 .

[0039] It has been determined that the purity of Si obtained by the method of this implementation case is 99.99989%, the thickness of the silicon film is 55μm, the current efficiency of this embodiment is 87.2%, the product has few pores, no obvious gaps are observed, and the density is high.

[0040] Example 2

[0041] In the preparation method of this embodiment, 30g of LiF with a purity of 99.5%, 60g of KF with a purity of 99.9%, 10g of NaF eutectic salt with a purity of 99.9% and 37.2g of K2SiO3 were weighed, and the other preparation conditions were the same as the electrolysis conditions and Example 1. It was determined that the purity of Si obtained by the method of this embodiment reached 99.99985%, the thickness of the silicon film was 53μm, the current efficiency of this embodiment was 94.1%, the product had few pores, no obvious gaps were observed, and the density was high.

[0042] Example 3

[0043] In the preparation method of this embodiment, 30g of LiCl with a purity of 99.5%, 60g of KF with a purity of 99.9%, 10g of NaF eutectic salt with a purity of 99.9% and 56g of CaSiO3 are weighed, and the other preparation conditions are the same as the electrolysis conditions and Example 1. It is determined that the purity of Si obtained by the method of this embodiment is 99.99982%, the thickness of the silicon film is 52μm, the current efficiency of this embodiment is 90.1%, the product has few pores, no obvious gaps are observed, and the density is high.

[0044] Example 4

[0045] The preparation method of this embodiment is the same as that of embodiment 1, except that the current is set to 85.5 mA (50 mA / cm 2 ), and the electrolysis time is 3h. It has been determined that the purity of Si obtained by the method of this embodiment is 99.99987%, the thickness of the silicon film is 56μm, the current efficiency of this embodiment is 89.8%, the product has few holes, no obvious gaps are observed, and the density is high.

[0046] Example 5

[0047] The preparation method of this embodiment is the same as that of embodiment 1, except that the current is set to 171 mA (100 mA / cm 2 ), and the electrolysis time is 1h. It has been determined that the purity of Si obtained by the method of this embodiment is 99.99985%, the thickness of the silicon film is 55μm, the current efficiency of this embodiment is 91.3%, the product has few holes, no obvious gaps are observed, and the density is high.

[0048] Example 6

[0049] The preparation method of this embodiment is the same as that of embodiment 1, except that the electrolysis time is set to 1 hour. According to the measurement, the purity of Si obtained by the method of this embodiment is 99.99981%, the thickness of the silicon film is 10 μm, the current efficiency of this embodiment is 82.1%, the product has few holes, no obvious gaps are observed, and the density is high.

[0050] Comparative Example 1

[0051] The preparation method of this comparative example is the same as that of Example 1, except that the content of added Na2SiO3 is 5.9 g (2 mol%). According to the measurement, the purity of Si obtained by the method of this implementation case is 89.7%, the thickness of the silicon film is 39 μm, the current efficiency of this example is 64.5%, the product has more micropores and poor density.

[0052] Comparative Example 2

[0053] The preparation method of this comparative example is the same as that of Example 2, except that the current is set to 17.1 mA (10 mA / cm 2 ). According to the measurement, the purity of Si obtained by the method of this embodiment is 87.98%, the thickness of the silicon film is 36 μm, the current efficiency of this embodiment is 65.3%, the product has many micropores and poor density.

[0054] Comparative Example 3

[0055] The preparation method of this comparative example is the same as that of Example 1, except that the electrolysis temperature is set to 1000° C. It is determined that the purity of Si obtained by the method of this embodiment is 99.98%, the thickness of the silicon film is 66 μm, the current efficiency of this embodiment is 64.3%, the product has many micropores and poor density.

[0056] Comparative Example 4

[0057] The preparation method of this comparative example is the same as that of Example 1, except that the electrolysis temperature is set to 500° C. It is determined that the purity of Si obtained by the method of this embodiment is 78.7%, the thickness of the silicon film is 28 μm, the current efficiency of this embodiment is 71.9%, the product has many micropores and poor density.

[0058] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for sustainable electrolytic extraction of high-purity silicon from molten silicate, characterized in that: The steps include: 1) Place the electrolyte in an electrolytic container, then immerse the cathode and anode in the electrolyte, heat the electrolytic container to 600-900°C in an inert gas atmosphere, connect the electrodes to a power source and charge at a current density of 25-100 mA cm -2 The constant current is supplied to the electrode for more than 1 hour, the electrolyte includes a silicate and an alkali metal salt, the silicate includes at least one of CaSiO3, K2SiO3, Na2SiO3, Al2SiO5, and MgSiO3, and the alkali metal salt includes at least one of KF, KCl, LiF, LiCl, NaCl, and NaF; 2) After the electrolysis is completed, the cathode is cooled to room temperature in an inert gas atmosphere, taken out, cleaned and dried to finally obtain high-purity crystalline silicon.

2. The method according to claim 1, characterized in that Based on the total mass of the electrolyte mixture, the silicate accounts for 10%-20%, and the alkali metal salt accounts for 80%-90%.

3. The method according to claim 1 or 2, characterized in that The method further comprises: a pretreatment step of heating the electrolyte to remove moisture, wherein the heating temperature is 200-300° C. and the heat preservation time is not less than 12 hours.

4. The method according to claim 1 or 2, characterized in that: In the step 1), the electrolytic container is heated to 600-900° C. and then kept warm for more than 30 minutes.

5. The method according to claim 1 or 2, characterized in that: The cathode in step 1) is an inert electrode.

6. The method according to claim 1 or 2, characterized in that: The anode in step 1) is graphite.

7. The method according to claim 1 or 2, characterized in that: In the step 1), the electrolysis container includes a crucible and a vertical resistance furnace, and placing the electrolyte in the electrolysis container specifically includes: placing the electrolyte in the crucible, and then placing the crucible in the constant temperature area of ​​the vertical resistance furnace.

8. The method according to claim 1 or 2, characterized in that: The crucible includes an iron crucible, a quartz crucible, a corundum crucible, a graphite crucible or a nickel crucible.

9. The method according to claim 1 or 2, characterized in that: The inert gas atmosphere is an atmosphere of argon or nitrogen with a purity of ≥99%.

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