CN103043665A - Preparation method for silicon powder - Google Patents

Abstract

A method for preparing silicon powder relates to an industrial silicon boron removal process and a method for preparing high-purity ultrafine silicon powder. A preparation method of silicon powder capable of significantly reducing boron content is provided. The obtained silicon powder can be used as an ideal low-boron raw material in the subsequent process of purifying solar-grade polysilicon by metallurgical methods, that is, the pickling and impurity removal process. 1) Using metallurgical grade silicon material as the raw material, put the raw silicon material in the crucible, and heat it electromagnetically through the induction coil to melt the silicon material; 2) When the silicon material is melted, add a slagging agent to control the heating power, so that the silicon liquid in the reaction process The temperature of the silicon liquid is kept at 1550-1850 °C. After the slagging and refining is completed, let it stand still to remove the slagging agent floating on the upper layer of the silicon liquid; 3) Control the temperature of the silicon liquid at 1500-1700 °C, then open the valve and start the atomization The silicon liquid enters the atomizer through the guide tube; the air pressure at the air outlet of the atomizer is adjusted to make the silicon liquid fly out of the atomizer into the atomization chamber in the form of a spray, and the silicon powder is collected by the receiving turntable.

Description

A kind of preparation method of silicon powder

technical field

The invention relates to a process for removing boron from industrial silicon and a method for preparing high-purity ultrafine silicon powder, in particular to a method for preparing silicon powder suitable for preparing silicon materials for solar cells.

Background technique

Facing the increasingly severe energy crisis and environmental degradation, governments of various countries are scrambling to focus on the development of new energy sources. my country is one of the countries with rich reserves of solar energy resources. The area with annual sunshine hours greater than 2,000 hours accounts for more than two-thirds of the total area of the country. Solar power generation has great potential for development in my country. At present, solar cells mainly include crystalline silicon solar cells, thin-film solar cells, dye-sensitized solar cells, and quantum dot matrix solar cells. Among them, crystalline silicon cells account for more than 90%, which is the most important type of solar cells at present and in the future.

The preparation of crystalline silicon solar cells requires silicon materials with a purity not lower than 6N, and the content of impurity element boron is less than 0.3ppmw. Boron is a main acceptor element in semiconductor silicon, and the boron-oxygen complex formed in silicon seriously restricts the minority carrier lifetime of solar cells and has a bad influence on the power generation efficiency of solar cells. Therefore, boron removal is a key task in various silicon purification methods. In the chemical method (improved Siemens, silane method, etc.), low-boron high-purity polysilicon is obtained through silicide rectification, reduction and other steps. In the metallurgical process, due to the high segregation coefficient (0.8) and low high-temperature saturated vapor pressure of boron in silicon, it is difficult to achieve the purpose of removal by traditional directional solidification or vacuum melting, and the method of slagging and refining is required. Purify.

Slagging refining is a method of obtaining high-purity silicon by adding a slagging agent to form a slag phase in the molten state to remove boron impurities in silicon. The key factors depend on the smelting temperature, the alkalinity of the slag liquid, the oxygen potential and the distribution coefficient of boron. Through theoretical calculation and experimental verification, the segregation coefficient of boron oxide in slag phase and silicon is much smaller than that of boron in silicon. Suzuki and Sano from Japan, Viana Teixeira and Kazuki Morita from the University of Tokyo, Japan (Leandro Augusto Viana Teixeira and Kazuki Morital, Removal of Boron from Molten Silicon Using CaO–SiO ₂ Based Slags, ISIJ International, Vol.49(2009), No.6, pp.783–787.) have carried out exploratory experimental studies, and the boron removal effect with a distribution ratio of 2.4 can be obtained under the condition of 1823K. Metallurgical Laboratory of Xiamen University (Cai Jing, Luo Xuetao, etc., Research progress on boron removal from high-purity metallurgical silicon, Materials Herald, 2009, 23 (12): 81-84) using CaO-SiO ₂ -CaF ₂ -BaO slagging system The pilot test showed that the B content was successfully reduced to 0.15-0.35ppmw at a slag-silicon ratio of 2:1-1:2 and a temperature of 1650-1750°C. In 2011, Luo Xuetao et al. (Chinese patent ZL201110040875.0; Chinese patent ZL201110000994.3) used the method of slagging combined with pickling to reduce the boron in metal silicon from 8ppmw to below 0.3ppmw. In 2012, Ma Wenhui of Kunming University of Science and Technology and others used CaO-SiO ₂ -Li ₂ O and CaO-SiO ₂ -LiF slag systems to reduce the boron content from 22ppmw to 1.3ppmw at 1823K.

The research results of F.A.Trumbore, G.L.Vick et al. show that below 1500K, the solid solubility of boron in silicon decreases significantly as the temperature decreases, and impurities tend to segregate and remain in the grain boundaries. Usually, the silicon liquid after slagging and refining is poured into the ingot mold to cool and solidify into large silicon ingots, and then pickling and impurity removal are carried out after crushing and grinding, which often causes secondary metal pollution and Increase processing costs. Therefore, the traditional slagging refining method has certain limitations.

Contents of the invention

The purpose of the present invention is mainly to provide a method for preparing silicon powder that can significantly reduce the boron content in view of the deficiencies in the prior art of slagging and boron removal technology. The obtained silicon powder can be used as an ideal low-boron raw material in the subsequent process of purifying solar-grade polysilicon by metallurgical methods, that is, the pickling and impurity removal process.

The technical proposal of the present invention is to purify the polysilicon by high-temperature slagging, and remove the slag liquid after the slagging reaction is completed; then, the silicon liquid is atomized by an atomizer to form low-boron silicon powder. The boron content can be reduced to 0.05-0.2ppmw at one time, which can meet the requirements of the subsequent process of purifying solar-grade polysilicon by metallurgical methods, that is, the pickling and impurity removal process.

The present invention comprises the following steps:

1) Using metallurgical grade silicon material as the raw material, placing the raw material silicon in a crucible, and electromagnetically heating the silicon material through an induction coil to melt the silicon material;

2) When the silicon material is melted, add a slagging agent and control the heating power to keep the temperature of the silicon liquid at 1550-1850°C during the reaction process. After the slagging and refining is completed, let it stand still to remove the slagging agent floating on the upper layer of the silicon liquid ;

3) Control the temperature of the silicon liquid at 1500-1700°C, then start the atomizer, the silicon liquid enters the atomizer through the guide tube, adjust the air pressure at the air outlet of the atomizer, so that the silicon liquid flies out of the atomizer in a spray shape Enter the atomization chamber, and collect the silicon powder by the receiving turntable.

In step 1), the metallurgical-grade silicon can be silicon powder or silicon block, the purity of which is preferably above 99%, and the content of boron can be 5-30 ppmw; the crucible is preferably a graphite crucible.

In step 2), the mass ratio of the slagging agent to the silicon material can be 3:1 to 1:2; when the silicon material is no more than 20kg, the slagging and refining time is 15 to 30min; When the silicon material is 50kg, the slagging and refining time is 30-60 minutes; when the silicon material is 50-100kg, the slagging and refining time is 60-120 minutes; the standing time can be 10-20 minutes. The slag-forming agent can be a mixture of silicon dioxide, sodium fluoride and sodium salt. According to mass percentage, silicon dioxide can be 25% to 40%, sodium fluoride can be 8% to 20%, and sodium salt can be 40% to 60%; the sodium salt can be sodium carbonate or sodium bicarbonate.

In step 3), the diameter of the guide tube can be 2-8mm; the atomizer gas used can be nitrogen, oxygen, argon or a mixture of the above three gases, when the gas from the atomizer When the gas outlet flows out, the air pressure can be 0.5-3MPa; the vertical distance from the liquid outlet of the atomizer to the receiving turntable is 0.3-1.5m, and the temperature of the receiving turntable can be 25°C-100°C.

Compared with prior art, mechanism of the present invention and beneficial effect are as follows:

According to the theory of metallurgical physics and chemistry, at high temperature, boron in silicon interacts with free oxygen ions and oxygen in the molten state, and enters the slag phase in the form of borate. By adjusting the formula of the slagging agent and changing its alkalinity, the concentration of free oxygen ions and oxygen potential in the melt can be adjusted. Considering the loss of silicon and the degree of boron removal reaction, the optimal temperature and melting time are selected for smelting.

After standing still, the high-temperature silicon liquid enters the atomizer through the guide tube, and the silicon liquid is atomized into fine droplets under the action of the atomizing gas by adjusting the air pressure at the gas outlet of the atomizer. During the atomization process, under the action of the gas, the silicon liquid overcomes the surface tension and viscous force, and deforms and breaks into droplets. Therefore, the ratio between the mass flow rate of the atomized gas and the mass flow rate of the metal liquid (GMR ) is an important parameter affecting the quality of the final silicon powder. This parameter determines the mass distribution, particle size distribution, cooling rate, etc. of the atomization cone, which directly affects the quality of silicon powder. When the diameter of the draft tube is constant, the mass flow rate of silicon liquid can be regarded as a certain value. To adjust the ratio (GMR) between the mass flow rate of the atomizing gas and the mass flow rate of the metal liquid, it is only necessary to control the air pressure at the gas outlet of the atomizer.

The high-temperature silicon liquid turns into liquid particles under the action of gas, and is quenched on the receiving turntable. Through the rapid cooling effect, the reverse diffusion of boron from the remaining small amount of slag phase into the silicon phase is suppressed during the cooling process of the silicon liquid. On the other hand, the boron (B) element in the high-temperature silicon liquid reacts chemically with the oxygen and moisture in the air to form B _x O _y H _z compounds, which mainly contain B ₃ O ₆ H ₃ and BO ₂ H , and a small amount of BO ₃ H ₃ and BOH, etc., the generated B _x O _y H _z compounds will quickly volatilize from the silicon phase in the form of gas, so as to achieve the purpose of further boron removal. In addition, by controlling the temperature of the receiving turntable, an appropriate subcooling degree can be provided for the solidification process of the liquid droplets, and the nucleation probability and crystallization rate of the silicon liquid can be improved.

The present invention confirms key parameters by verifying the process experiment of removing B impurities in industrial silicon: such as slagging reaction temperature, reaction time, air pressure at the gas outlet of the atomizer, vertical distance from the liquid outlet of the atomizer to the receiving turntable, and the receiving turntable The temperature and other setting ranges, the results show that the B content in industrial silicon is significantly reduced, and it has broad application prospects.

The invention proposes a preparation process of silicon powder. The silicon powder with low boron content is obtained by atomizing the silicon liquid after slagging and refining, which can be directly used as the raw material of the pickling process.

Description of drawings

Fig. 1 is a schematic diagram of the device and process for preparing silicon powder in various embodiments of the present invention.

Fig. 2 is a schematic structural diagram of the atomizer in Fig. 1 .

Detailed ways

Referring to Figures 1 and 2, the device for preparing silicon powder in each embodiment of the present invention includes an induction coil 1, a graphite crucible 2, a heat preservation plate 3, a valve 4, a draft tube 5, an atomizer 6, an atomization chamber 7 and a receiving turntable 8 . The symbol P1 in Fig. 1 is silicon liquid, and the symbol P2 is silicon powder.

The heat preservation plate 3 is wrapped on the outer wall of the graphite crucible 2, the induction coil 1 surrounds the outer periphery of the heat preservation plate 3, the valve 4 is located at the upper end of the guide tube 5, the guide tube 5 is located at the center of the bottom of the graphite crucible 2, and the atomizer 6 is located under the graphite crucible 2, The atomization chamber 7 is located under the atomizer 6 , and the receiving turntable 8 is located in the atomization chamber 7 and below the atomizer 6 .

The atomizer 6 includes an outer wall 61 , an air bag 62 , an air outlet 63 and a liquid outlet 64 . The airbag 62 is wrapped by the outer wall 61, and the gas in the airbag 62 flows out from the gas outlet 63, reacts with the silicon liquid flowing out from the liquid outlet 64, and atomizes the silicon liquid.

Adopt above-mentioned device to prepare the preparation method embodiment of silicon powder of the present invention as follows:

Example 1

A metallurgical grade silicon material with a B content of 5.1 ppmw was taken and melted in a graphite crucible. Add slagging agent (sodium bicarbonate 50%, silicon dioxide 30%, sodium fluoride 20%) when the silicon material is melted, slagging and refining boron removal time is 0.5h, control the temperature in the graphite crucible at 1600 ° C ~ 1650 ° C, static Leave it on for 10-20 minutes. Remove the slagging agent floating on the upper layer of the silicon liquid; open the valve, the silicon liquid flows into the atomizer from the diversion tube, the diameter of the diversion tube is 5mm, control the air pressure of the atomizer outlet to 0.5MPa, the liquid outlet of the atomizer is connected to the acceptor The distance of the turntable is 0.3m. The boron content in the obtained silicon powder was 0.24ppmw. The average particle size of silicon powder is 55 μm.

Through comparative tests, the B content in silicon prepared by ordinary technology (same slagging refining temperature, time and formula) is 0.55ppmw. It can be seen that the boron content in the silicon powder obtained in this embodiment is significantly reduced.

Example 2

The process is the same as in Example 1, the difference is that the slagging agent formula is 40% sodium bicarbonate, 40% silicon dioxide, and 20% sodium fluoride. When slagging and refining boron removal, the temperature in the graphite crucible is controlled at 1650°C to 1700°C. ℃, slagging and refining time 1h, the diameter of the guide tube is 4mm, the air pressure at the gas outlet of the atomizer is controlled at 1MPa, and the distance between the liquid outlet of the atomizer and the receiving turntable is 0.5m. The boron content in the obtained silicon powder was measured to be 0.21 ppmw. The average particle size of silicon powder is 48 μm.

According to comparative tests, the boron content in silicon prepared by ordinary technology (same slagging refining temperature, time and formula) is 0.51ppmw. It can be seen that the boron content in the silicon powder obtained in this embodiment is significantly reduced.

Example 3

The process is the same as in Example 1, the difference is that the slagging agent formula is 60% sodium carbonate, 32% silicon dioxide, and 8% sodium fluoride. During slagging, refining and boron removal, the temperature in the graphite crucible is controlled at 1650°C to 1700°C , slagging and refining time is 0.5h, the diameter of the guide tube is 6mm, the air pressure at the gas outlet of the atomizer is controlled at 1.5MPa, and the distance between the liquid outlet of the atomizer and the receiving turntable is 0.75m. The boron content in the obtained silicon powder was measured to be 0.19 ppmw. The average particle size of silicon powder is 41 μm.

According to comparative tests, the boron content in silicon prepared by ordinary technology (same slagging refining temperature, time and formula) is 0.49ppmw. It can be seen that the boron content in the silicon powder obtained in this embodiment is significantly reduced.

Example 4

The process is the same as in Example 1, the difference is that the slagging agent formula is 60% sodium bicarbonate, 25% silicon dioxide, and 15% sodium fluoride. When slagging and refining boron removal, the temperature in the graphite crucible is controlled at 1700°C to 1750°C. ℃, slagging and refining time 1h, control the gas outlet air pressure of the atomizer to 2MPa, and the distance between the liquid outlet of the atomizer and the receiving turntable to be 1m. The measured boron content in the obtained silicon powder was 0.15 ppmw. The average particle size of silicon powder is 38 μm.

According to comparative tests, the boron content in silicon prepared by ordinary technology (same slagging refining temperature, time and formula) is 0.38ppmw. It can be seen that the boron content in the silicon powder obtained in this embodiment is significantly reduced.

Example 5

The process is the same as in Example 1, the difference is that the slagging agent formula is 40% sodium carbonate, 40% silicon dioxide, and 20% sodium fluoride. When slagging and refining boron removal, the temperature in the graphite crucible is controlled at 1700°C to 1750°C , slagging and refining time 1h, control the gas outlet air pressure of the atomizer to 3MPa, and the distance between the liquid outlet of the atomizer and the receiving turntable to be 1.5m. The boron content in the obtained silicon powder was measured to be 0.12 ppmw. The average particle size of silicon powder is 33 μm.

According to comparative tests, the boron content in silicon prepared by ordinary technology (same slagging refining temperature, time and formula) is 0.35ppmw. It can be seen that the boron content in the silicon powder obtained in this embodiment is significantly reduced.

Claims (10)

1. a preparation method of silicon powder, is characterized in that it comprises the following steps: 1) Silicon material melting: metallurgical grade silicon material is used as raw material, the raw material silicon is placed in a crucible, and the silicon material is melted by electromagnetic heating through an induction coil; 2) Boron removal by slagging and refining: When the silicon material is melted, add slagging agent and control the heating power to keep the temperature of the silicon liquid at 1550°C to 1850°C during the reaction. Slagging agent on the upper layer of silicon liquid; 3) Control the temperature of the silicon liquid at 1500-1700°C, then open the valve and start the atomizer; the silicon liquid enters the atomizer through the guide tube; adjust the air pressure at the outlet of the atomizer to make the silicon liquid fly out in the form of a spray The atomizer enters the atomization chamber, and the silicon powder described in the present invention is collected by a receiving turntable. 2. The preparation method of a kind of silicon powder as claimed in claim 1, it is characterized in that in step 1), described silicon material is silicon powder or silicon block, and purity is more than 99%, and boron content is 5～30ppmw. 3. the preparation method of a kind of silicon powder as claimed in claim 1 is characterized in that in step 1), described crucible is graphite crucible. 4. the preparation method of a kind of silicon powder as claimed in claim 1 is characterized in that in step 2), when the silicon material is no more than 20kg, the slagging and refining time is 15～30min; when the silicon material is 20～50kg , the slagging and refining time is 30-60 minutes; when the silicon material is 50-100 kg, the slagging and refining time is 60-120 minutes; the standing time is 10-20 minutes. 5. The preparation method of a kind of silicon powder as claimed in claim 1, is characterized in that in step 2), described slagging agent adopts the mixture of silicon dioxide, sodium fluoride and sodium salt. 6. the preparation method of a kind of silicon powder as claimed in claim 5 is characterized in that described slagging agent is by mass percentage, and silicon dioxide is 25%～40%, sodium fluoride is 8%～20%, sodium Salt is 40% to 60%. 7. the preparation method of a kind of silicon powder as claimed in claim 5 or 6 is characterized in that described sodium salt adopts sodium carbonate or sodium bicarbonate. 8. A method for preparing silicon powder as claimed in claim 1, characterized in that in step 3), the diameter of the draft tube is 2-8mm. 9. the preparation method of a kind of silicon powder as claimed in claim 1 is characterized in that in step 3), the gas that described atomizer adopts is the mixed gas that nitrogen, oxygen, argon or above-mentioned three kinds of gases form ; When the gas flows out from the gas outlet of the atomizer, the pressure is 0.5~3MPa. 10. the preparation method of a kind of silicon powder as claimed in claim 1, it is characterized in that in step 3), the vertical distance of described accepting turntable apart from atomizer liquid outlet is 0.3～1.5m, and the temperature of accepting turntable is 25℃～100℃.

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