A method for preparing high-purity silicon by recycling diamond wire-cut silicon powder using electric field coupled directional solidification technology

Abstract

The present invention discloses a method for preparing high-purity silicon by recycling diamond wire cut silicon powder using electric field coupled directional solidification technology, wherein the waste silicon powder from diamond wire cut is ground, and a solution is prepared with HF:H2O:C2H5OH=1:3:1 for pickling, and then centrifugally dried and pressed into blocks; the silicon powder block is vacuum induction melted, and an electric field is introduced during the solidification process after the insulation is completed to control the temperature gradient at the solid-liquid cross section, so that some protruding parts at the solid-liquid interface are remelted, thereby maintaining the stability of the solid-liquid interface, inhibiting the growth of dendrites, and significantly improving the yield during the recovery process. The introduction of the electric field can promote convection of silicon liquid, ensure the uniformity of impurities and temperature in the silicon liquid, make the impurity segregation effect during the solidification process more obvious, further improve the purity of silicon ingots, and increase the purity of recycled silicon to 5N. This technology can not only prevent environmental pollution of silicon powder waste, but also reuse silicon resources, thereby saving industrial costs in the crystalline silicon solar cell industry.

Description

Technical Field

[0001] The invention relates to a method for preparing high-purity silicon by recycling silicon powder cut by diamond wire using electric field coupling directional solidification technology, and belongs to the technical field of photovoltaic solid waste resource recycling and silicon material preparation. Background Art

[0002] As a clean and renewable energy, solar energy has a broad development prospect. Among them, crystalline silicon solar cells have maintained a market share of more than 90%, and crystalline silicon has become a strategic resource in the new century. However, as the current mainstream silicon ingot processing method, diamond wire cutting silicon ingot technology, in its production process, about 30-35% of ultrafine silicon particles are lost to silicon powder waste. Among these wastes, high-purity silicon accounts for about 90%, which has important recycling and reuse value. Due to the imperfect technology for recycling silicon powder waste, a large amount of high-purity silicon powder is wasted, causing economic losses and creating ecological and environmental risks.

[0003] Since silicon powder produced by diamond wire cutting contains silicon oxide and other metal impurities, there are problems such as insufficient purity and low yield when recovering high-purity silicon. At present, the main technologies for the recycling and reuse of waste materials from diamond wire cutting include pickling, induction melting, slag refining, high-temperature non-transferred arc assisted vacuum melting, etc. However, these methods are hindered from recycling silicon waste due to different defects, resulting in low purity of recycled silicon, less than 99.0%; or insufficient yield of high-purity silicon, which can only reach 40-60%; for example, in the study of the journal article "Recovery of High-Purity Silicon from Single Crystal and Polycrystalline Silicon Cutting Waste" (Material Science and Technology, 2011), Guo Jing et al. used the pickling and ingot casting process to obtain a silicon ingot with a purity of 98.71% and a silicon recovery rate of 43.33%. Therefore, there is an urgent need for a recycling method that can both improve the purity of recycled silicon and improve the yield. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the present invention recovers the silicon powder waste generated in the production of crystalline silicon solar cells and prepares high-purity silicon through electric field coupled directional solidification technology, which can not only prevent the environmental pollution caused by the silicon powder waste, but also enable the silicon resources to be reused, saving the industrial cost of the crystalline silicon solar cell industry, and solving the problems of insufficient purity and low yield when recycling high-purity silicon in the prior art.

[0005] To achieve the above-mentioned purpose, the present invention provides a method for preparing high-purity silicon by recycling diamond wire-cut silicon powder using electric field coupled directional solidification technology, comprising the following steps:

[0006] S1. Take the solid waste of diamond wire cutting silicon powder and grind it into waste powder with a particle size of 10-20um; prepare a pickling solution with hydrofluoric acid, water and ethanol in the ratio of HF:H2O:C2H5OH=1:3:1, and pickle the waste powder with the pickling solution under the action of a magnetic stirrer. The mass ratio of the waste powder to the pickling solution is 4-5:1, the pickling reaction temperature is 30-50°C, and the pickling time is 140-160min; the stirring rate of the magnetic stirrer is 1100-1400r / min.

[0007] Under storage process conditions, silicon powder particles undergo surface oxidation to form an amorphous SiO2 oxide layer that gradually grows. Some metal impurity particles migrate to the inside of the oxide layer, accumulate and remain inside the oxide layer, making them difficult to remove in conventional acid leaching. The corrosion of the oxide layer by hydrofluoric acid exposes the retained metal impurities, thereby promoting the removal of metal impurities. The combined use of hydrofluoric acid and ethanol can enhance the pickling and impurity removal effect and eliminate the effect of organic solvent adhesion on acid treatment; magnetic stirring allows a large amount of metal impurities attached to the surface of silicon micropowder to fully react with the acid and be removed.

[0008] S2. The waste powder mixed liquid obtained after the acid washing treatment in step S1 is subjected to solid-liquid separation by centrifugation, and is continuously rinsed with deionized water and centrifuged 4-6 times until the pH value of the washing liquid is neutral, thereby obtaining preliminarily purified silicon powder.

[0009] S3, placing the preliminarily purified silicon powder obtained in step S2 in a vacuum drying oven for drying, the temperature of the drying oven is set at 140°C, and the drying time is 6-8 hours; the moisture content of the silicon powder after drying reaches below 0.9%.

[0010] S4, the dried silicon powder obtained in step S3 is pressed and molded by a briquetting machine, and the pressure applied by the briquetting machine is 10-30Mpa. The dried silicon powder is not easy to add powder, and the "powdering" phenomenon is easy to occur during the smelting process. The dried silicon powder is briquetized to solve the "powdering" phenomenon.

[0011] S5. Place the pressed silicon powder block obtained in step S4 into a graphite crucible and perform electromagnetic induction heating melting. The entire melting process is carried out under the protection of an argon atmosphere with a pressure of 50 Pa. The temperature is raised to 1520-1600°C at a heating rate of 20-30°C / min. After the temperature stabilizes, the temperature is maintained for 0.8-1.2h to obtain a silicon powder melt.

[0012] S6. Place the silicon powder melt obtained after the insulation of step S5 together with the graphite crucible in an electric field coupled directional solidification device, and perform directional solidification with the assistance of electric field coupled thermal effect; the electric field coupled directional solidification device is relatively equipped with two conductive molybdenum wires, and the conductive molybdenum wires are connected to the external electric field to form a coupling effect. The electric field setting condition is 15A direct current, and the stretching rate of the crucible is 20mm / h. After completing the solidification and impurity removal process under electric field assisted conditions, high-purity silicon is obtained.

[0013] By coupling the thermal effect of the electric field applied in the directional solidification equipment, the supercooling of the solid-liquid interface during the solidification of the silicon ingot can be adjusted. On the one hand, the temperature gradient at the interface can be reduced and the stability of the solid-liquid interface can be increased. On the other hand, the thermal effect can remelt the protruding part of the interface, inhibit the growth of dendrites, expand the columnar crystal area, and improve the yield. Due to the introduction of the electric field, the convection of the silicon melt can also be enhanced, and the segregation effect of impurities in the melt can be improved.

[0014] The electric field coupled directional solidification equipment comprises a furnace shell, a flange is arranged on the top of the furnace shell, two conductive molybdenum wires are arranged opposite to each other at the center of the flange and the bottom of the furnace shell, and the conductive molybdenum wires are connected to an external electric field; a crucible advancing support is arranged at the bottom of the furnace shell, a crucible base is fixedly installed on the upper part of the crucible advancing support, a crucible sleeve is arranged on the crucible base, a graphite crucible is installed inside the crucible sleeve, an insulation layer is arranged between the graphite crucible and the crucible sleeve, and an insulating sleeve is installed inside the graphite crucible; exhaust holes and air inlet holes are respectively arranged at the upper and lower parts of the furnace shell, and the air inlet is used to communicate with the air supply port of an inert gas supply device; the graphite crucible is used to hold silicon powder melt; the height of the conductive molybdenum wire on the flange can be adjusted, and it is adjusted to the conductive molybdenum wire to contact the upper surface of the silicon melt according to the amount of silicon melt in the graphite crucible.

[0015] The electric field coupled directional solidification technology utilizes the characteristics of metal impurities segregation to enrich the impurities with small segregation coefficient in silicon to the top of the ingot to improve the purity of silicon. When an electric field is applied during the solidification process, for the coexistence of solid and liquid, the conductivity of the liquid phase is lower than that of the solid phase. Therefore, the current passes through the solid phase first, and the thermal effect generated by the solid phase is greater than that of the liquid phase, causing some protrusions in the solid phase to remelt, ensuring the stability of the solid-liquid interface, promoting the growth of crystals into spherical grains, and expanding the columnar crystal area in the ingot, thereby improving the yield of the ingot.

[0016] In the above technical solution, preferably, the mass ratio of the waste powder to the pickling solution is 5:1, the pickling reaction temperature is 40°C, and the pickling time is 150 minutes.

[0017] In the above technical solution, preferably, the pressure for pressing the dried silicon powder to form is 20 MPa; the electric field coupled directional solidification equipment is heated to 1560°C at a heating rate of 25°C / min, and the temperature is kept constant for 1 hour after the temperature stabilizes to obtain a silicon powder melt.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The method of preparing high-purity silicon by recycling diamond wire-cut silicon powder using electric field-coupled directional solidification technology introduces an electric field during the solidification process to control the temperature gradient at the solid-liquid cross section, causing some protruding parts at the solid-liquid interface to remelt, maintaining the stability of the solid-liquid interface, inhibiting the growth of dendrites, and promoting the growth of grains into spherical grains, significantly improving the yield of the recycling process. In addition, the introduction of the electric field can promote convection of silicon liquid, ensure the uniformity of impurities and temperature in the silicon liquid, make the impurity segregation effect during the solidification process more obvious, further improve the purity of the silicon ingot, and increase the purity of the recovered high-purity silicon to 5N. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the electric field coupled directional solidification equipment;

[0021] Figure 2 It is a schematic diagram of the solidification and impurity removal process;

[0022] In the figure: 1-conductive molybdenum wire; 2-flange; 3-furnace shell; 4-insulating sleeve; 5-graphite crucible; 6-exhaust hole; 7-insulating layer; 8-crucible sleeve; 9-crucible base; 10-crucible push support; 11-silicon powder melt; 12-air inlet; 13-external electric field; 21-silicon melt; 22-melt convection; 23-impurity diffusion direction during segregation; 24-metal impurities; 25-polysilicon; 26-solid-liquid interface protrusion remelting. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. The composition mass fractions of the raw material (silicon powder waste cut by diamond wire) and the recycled high-purity silicon sample are semi-quantitatively analyzed by X-ray fluorescence spectroscopy (XRF), and the instrument used for particle size analysis is Malvern laser particle size analyzer Master Size 2000. In the test results of each component of the silicon powder waste cut by diamond wire in the embodiment, the content of Si is 91.70wt%, the content of Al is 0.75wt%, the content of Na is 0.36wt%, and the content of Ca is 0.33wt%.

[0024] Example 1

[0025] Take 2005.5g of solid waste of diamond wire cutting silicon powder and grind it into waste powder with a particle size of 20um; prepare hydrofluoric acid, water and ethanol into a pickling solution in the ratio of HF:H2O:C2H5OH=1:3:1, and pickle the waste powder with the pickling solution under the action of a magnetic stirrer. The mass ratio of the waste powder to the pickling solution is 5:1, the pickling reaction temperature is 40°C, and the pickling time is 150min; the stirring rate of the magnetic stirrer is 1400r / min. The waste powder mixed liquid obtained after the pickling treatment is centrifuged for solid-liquid separation, and is continuously rinsed with deionized water and centrifuged 6 times until the pH value of the washing liquid is neutral to obtain preliminarily purified silicon powder; the purified silicon powder is placed in a vacuum drying oven for drying, the drying oven is set at a temperature of 140°C, the drying time is 8 hours, and the moisture content of the dried silicon powder reaches 0.5%; the dried silicon powder is pressed and formed by a briquetting machine, and the pressure applied by the briquetting machine is 20Mpa; the pressed silicon powder block is placed in a graphite crucible and then subjected to electromagnetic induction heating smelting, and the smelting is carried out under the protection of an argon atmosphere throughout the process. The pressure of the gas atmosphere is 50Pa; the temperature is raised to 1560℃ at a heating rate of 25℃ / min, and the temperature is kept for 1.0h after the temperature stabilizes to obtain a silicon powder melt; the silicon powder melt obtained after the insulation is completed is placed in an electric field coupled directional solidification device together with a graphite crucible, and directional solidification is performed under the assistance of electric field coupled thermal effect; the electric field coupled directional solidification device is relatively provided with two conductive molybdenum wires, and the conductive molybdenum wires are connected to an external electric field to form a coupling effect, the electric field setting condition is 15A direct current, the crucible stretching rate is 20mm / h, and high-purity silicon is obtained after the solidification and impurity removal process is completed under electric field assisted conditions. The purity of the high-purity silicon product of Example 1 is 5N according to XRF analysis; 1656.74g of product is obtained, and the yield is 82.61%.

[0026] Example 2

[0027] Take 2601.8g of solid waste of diamond wire cutting silicon powder and grind it into waste powder with a particle size of 10um; prepare a pickling solution with hydrofluoric acid, water and ethanol in the ratio of HF:H2O:C2H5OH=1:3:1, and pickle the waste powder with the pickling solution under the action of a magnetic stirrer. The mass ratio of the waste powder to the pickling solution is 4:1, the pickling reaction temperature is 50°C, and the pickling time is 140min; the stirring rate of the magnetic stirrer is 1100r / min. The waste powder mixed liquid obtained after the pickling treatment is centrifuged for solid-liquid separation, and is continuously rinsed with deionized water and centrifuged 4 times until the pH value of the washing liquid is neutral to obtain preliminarily purified silicon powder; the purified silicon powder is placed in a vacuum drying oven for drying, the drying oven is set at 140°C, the drying time is 6 hours, and the moisture content of the dried silicon powder reaches 0.8%; the dried silicon powder is pressed and formed by a briquetting machine, and the pressure applied by the briquetting machine is 30Mpa; the pressed silicon powder block is placed in a graphite crucible and subjected to electromagnetic induction heating smelting, and the smelting is carried out under the protection of argon atmosphere. The pressure of the argon atmosphere is 50Pa; the temperature is raised to 1600℃ at a heating rate of 20℃ / min, and the temperature is kept for 1.2h after the temperature stabilizes to obtain a silicon powder melt; the silicon powder melt obtained after the insulation is placed in an electric field coupled directional solidification device together with a graphite crucible, and directional solidification is performed under the assistance of electric field coupled thermal effect; the electric field coupled directional solidification device is relatively equipped with two conductive molybdenum wires, which are connected to an external electric field to form a coupling effect, the electric field setting condition is 15A direct current, the crucible stretching rate is 20mm / h, and high-purity silicon is obtained after the solidification and impurity removal process is completed under electric field assisted conditions. The purity of the product high-purity silicon is 5N according to XRF analysis; 2219.34g of product is obtained, and the yield is 85.30%.

[0028] Example 3

[0029] The electric field coupled directional solidification device described in the present invention is as shown in the attached Figure 1 As shown, it includes a furnace shell 3, a flange 2 is arranged on the top of the furnace shell 3, two conductive molybdenum wires 1 are arranged opposite to each other at the center of the bottom of the flange 2 and the furnace shell 3, and the conductive molybdenum wires 1 are connected to the external electric field 13; a crucible advancing support 10 is arranged at the bottom of the furnace shell 3, a crucible base 9 is fixedly installed on the upper part of the crucible advancing support 10, a crucible sleeve 8 is arranged on the crucible base 9, a graphite crucible 5 is installed inside the crucible sleeve 8, a thermal insulation layer 7 is arranged between the graphite crucible 5 and the crucible sleeve 8, and an insulating sleeve 4 is installed inside the graphite crucible 5; exhaust holes 6 and air inlet holes 12 are respectively arranged on the upper and lower parts of the furnace shell 3, and the air inlet 12 is used to communicate with the air supply port of the inert gas supply device; silicon powder melt 11 is contained in the graphite crucible 5.

[0030] The solidification and impurity removal process of the electric field coupled directional solidification equipment is as follows Figure 2 As shown, the silicon powder block in the graphite crucible is heated and melted to form silicon melt 21, and melt convection 22 occurs under the action of the coupled electric field; during coagulation, metal impurities 24 diffuse in the impurity diffusion direction 23, and some protruding parts at the solid-liquid interface are remelted 26, and high-purity polycrystalline silicon 25 is formed after solidification is completed under electric field assisted conditions.

[0031] Comparative Example 1

[0032] 3500.5g of solid waste from diamond wire cutting silicon powder was taken, and it was cleaned by pickling with hydrochloric acid and hydrofluoric acid. The cleaned material was washed with water until neutral, and dried to 1.8wt% of water after solid-liquid separation. It was molded at 500MPa pressure and melted in a vacuum induction furnace at a vacuum degree of 10Pa, a melting temperature of 1600℃, and a heat preservation of 0.5h to make a silicon sample. The purity of the product silicon was 98.73% according to XRF analysis; 1603.23g of product was obtained, and the yield was 45.80%.

[0033] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing high-purity silicon by recycling diamond wire-cut silicon powder using electric field coupled directional solidification technology, characterized in that: The method comprises the following steps: S1. Take the solid waste of diamond wire cutting silicon powder and grind it into waste powder with a particle size of 10-20 um; prepare a pickling solution with hydrofluoric acid, water and ethanol in a ratio of HF:H2O:C2H5OH=1:3:1, and pickle the waste powder with the pickling solution under the action of a magnetic stirrer. The mass ratio of the waste powder to the pickling solution is 4-5:1, the pickling reaction temperature is 30℃-50℃, and the pickling time is 140-160 min; the stirring rate of the magnetic stirrer is 1100-1400 r / min; S2, separating the waste powder mixture obtained after the acid washing treatment in step S1 by centrifugation, washing with deionized water and centrifuging 4-6 times continuously until the pH value of the washing liquid is neutral, and obtaining preliminarily purified silicon powder; S3, placing the preliminarily purified silicon powder obtained in step S2 in a vacuum drying oven for drying, with the drying oven set at a temperature of 140° C. for a drying time of 6-8 h; S4, the dried silicon powder obtained in step S3 is pressed and molded by a briquetting machine, wherein the pressure applied by the briquetting machine is 10-30 Mpa; S5, placing the pressed silicon powder block obtained in step S4 into a graphite crucible and performing electromagnetic induction heating melting, the melting process is carried out under the protection of an argon atmosphere, and the pressure of the argon atmosphere is 50 Pa; heating the temperature to 1520°C-1600°C at a heating rate of 20-30°C / min, and keeping the temperature for 0.8-1.2 h after the temperature stabilizes, to obtain a silicon powder melt; S6, placing the silicon powder melt obtained after the insulation in step S5 together with the graphite crucible in an electric field coupled directional solidification device, and performing directional solidification under the assistance of electric field coupled thermal effect; The electric field coupled directional solidification equipment is relatively equipped with two conductive molybdenum wires, which are connected to an external electric field to form a coupling effect. The electric field setting condition is 15A direct current, and the stretching rate of the crucible is 20 mm / h. High-purity silicon is obtained after the solidification and impurity removal process is completed under electric field assisted conditions.

2. The method according to claim 1, characterized in that: The moisture content of the silicon powder after drying in step S3 is less than 0.9%.

3. The method according to claim 1, characterized in that: The electric field coupled directional solidification equipment described in step S6 comprises a furnace shell (3), the top of the furnace shell (3) is provided with a flange (2), two conductive molybdenum wires (1) are arranged at the center of the bottom of the flange (2) and the furnace shell (3), and the conductive molybdenum wires (1) are connected to the external electric field (13); the bottom of the furnace shell (3) is provided with a crucible advancing support (10), the upper part of the crucible advancing support (10) is fixedly mounted with a crucible base (9), and the crucible base (9) is provided with A crucible sleeve (8), wherein a graphite crucible (5) is installed inside the crucible sleeve (8), a heat-insulating layer (7) is provided between the graphite crucible (5) and the crucible sleeve (8), and an insulating sleeve (4) is installed inside the graphite crucible (5); exhaust holes (6) and air inlet holes (12) are provided at the upper and lower parts of the furnace shell (3), respectively, and the air inlet holes (12) are used to communicate with the air supply port of an inert gas supply device; the graphite crucible (5) is used to hold silicon powder melt (11).

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