CN109319788B - Method for preparing polycrystalline silicon by refining and directional solidification of silicon-aluminum-calcium alloy - Google Patents

Abstract

The invention belongs to the field of polycrystalline silicon purification, and discloses a method for preparing polycrystalline silicon by refining and directional solidification of silicon-aluminum-calcium alloy. Soak in the liquid for 1 to 2 hours, filter, wash, filter, and dry; 2) put the crude silicon powder obtained in step 1) together with metal aluminum and metal calcium into an induction melting furnace, and pass an inert gas into the furnace body and The temperature in the furnace is raised to 1703-1783° C., the temperature is kept for 1-10 hours, and then cooled to room temperature to obtain a silicon-aluminum-calcium alloy; 3) the silicon-aluminum-calcium alloy obtained in step 2) is ground into powder, and mixed acid I and mixed acid are used in sequence II. Pickling; 4) directional solidification of the pickling product, annealing, to obtain a silicon-aluminum-calcium alloy ingot after secondary refining; 5) cutting off 20 of the head of the silicon-aluminum-calcium alloy ingot after secondary refining; ~40%. The present invention has good removal effect for impurities in polysilicon, especially phosphorus impurities, thereby obtaining high-purity polysilicon.

Description

A method for preparing polycrystalline silicon by refining and directional solidification of silicon-aluminum-calcium alloy

technical field

The invention relates to a method for purifying polycrystalline silicon, in particular to a method for preparing polycrystalline silicon by refining and directional solidification of a silicon-aluminum-calcium alloy.

Background technique

With the development of society, non-renewable energy is continuously consumed, and at the same time, environmental pollution is becoming more and more serious. As a new type of renewable energy, solar energy has attracted widespread attention because of its inexhaustible advantages.

The raw materials of solar cells are mainly polycrystalline silicon materials, and polycrystalline silicon materials have gradually occupied the solar cell market with the advantages of high conversion efficiency and low price. The preparation process of polysilicon mainly includes chemical method and metallurgical method. Among them, the chemical method is to convert industrial silicon into intermediate compounds, and then purify through rectification and other processes, and finally obtain high-purity silicon. Metallurgy mainly removes impurities in silicon through a series of physical processes, and silicon itself does not participate in the reaction. Although the silicon produced by chemical method has high purity and good quality, the chemical method is complicated and expensive. In contrast, metallurgical methods have attracted more and more attention due to their low cost and environmental friendliness.

The metallurgical processes mainly include directional solidification, vacuum refining, blowing slag, hydrometallurgy, plasma refining, etc. However, for multiple impurities in silicon, a single process can only remove one or several impurities, which cannot meet the quality requirements of solar-grade silicon. To effectively remove impurities, it is necessary to combine a variety of different processes to remove impurities according to the different physical and chemical properties of impurities. CN100372762C discloses a method for preparing solar-grade polysilicon, which adopts pickling method to purify polysilicon, but the standard of solar-grade silicon cannot be reached only by the pickling process, and an oxide film will be formed on the silicon surface during the pickling process to prevent the reaction from proceeding.

SUMMARY OF THE INVENTION

The present invention aims to provide a new method for preparing polycrystalline silicon by refining and directional solidification of silicon-aluminum-calcium alloy.

Specifically, the method for preparing polysilicon by refining and directional solidification of a silicon-aluminum-calcium alloy provided by the present invention includes:

1) Put the silicon powder to be purified into the mixed solution of hydrofluoric acid and hydrogen peroxide at 20～35℃, soak for 1～2h, filter, wash with water, filter and dry to obtain the crude silicon powder;

2) Put the crudely extracted silicon powder obtained in step 1) together with metal aluminum and metal calcium into an induction melting furnace, pass an inert gas into the furnace body and make the temperature in the furnace rise to 1703-1783 ℃ and then keep the temperature for 1-10h, Cool to room temperature to obtain a refined silicon-aluminum-calcium alloy;

3) after the silicon-alumina-calcium alloy obtained in step 2) is ground into powder, the mixed acid I of hydrochloric acid and acetic acid and the mixed acid II of hydrochloric acid and hydrofluoric acid are successively used for pickling;

4) directional solidification of the pickling product obtained in step 3), followed by annealing treatment to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) Cut off 20-40% of the head of the silicon-aluminum-calcium alloy ingot after secondary refining obtained in step 4), and the remaining part obtained is polysilicon.

The inert gas can be various existing gases that do not react with the silicon powder to be purified, for example, nitrogen gas, zero-group element gas, and the like. Wherein, the zero-group element gas may be, for example, helium, argon, or the like.

In the present invention, the term "head" refers to the first solidified tip.

Preferably, in step 1), the concentration of hydrofluoric acid in the mixed solution is 1-3 mol/L, and the concentration of hydrogen peroxide is 1.5-2.5 mol/L.

Preferably, the metal aluminum and metal calcium are used in the form of aluminum blocks and calcium blocks, respectively.

Preferably, the atomic percentage of the added amount of the aluminum block and the calcium block is (1.5-2.5):1.

Preferably, relative to 100 parts by weight of the silicon powder to be purified, the total amount of the aluminum block and the calcium block is 80-120 parts by weight.

Preferably, the purities of the aluminum block and the calcium block are each independently 2-3N.

Preferably, the purity of the silicon powder to be purified is 1-3N. In addition, the silicon powder to be purified can be industrial silicon powder.

Preferably, in step 2), the temperature of the heat preservation is 1720-1780° C., and the time is 5-6 h.

Preferably, in step 2), the cooling rate is 5-10°C/min.

Preferably, the concentration of hydrochloric acid in the mixed acid I is 1-3 mol/L, and the concentration of acetic acid is 1-4 mol/L.

Preferably, the concentration of hydrochloric acid in the mixed acid II is 1-3 mol/L, and the concentration of hydrofluoric acid is 1-6 mol/L.

Preferably, the conditions for using the mixed acid I and the mixed acid II for pickling independently include a temperature of 20-60° C., a time of 2-3 hours, and a magnetic stirring of 300-600 rad/min each independently. .

Preferably, the method for preparing polycrystalline silicon by refining and directional solidification of a silicon-aluminum-calcium alloy provided by the present invention further comprises the steps of filtration, washing and drying of the product after two picklings in step 3) in sequence; and/or, In step 5), 20-40% polysilicon cut from the head of the silicon-aluminum-calcium alloy ingot is recovered and reused.

Preferably, the direction of the directional solidification is a vertical direction, and the method used is to fix the heat source, move the crucible from top to bottom, and then perform annealing.

Preferably, the temperature gradient in the vertical direction of directional solidification is set to be 10-30°C/cm, the speed of the crucible moving from top to bottom is 15-25mm/h, and the annealing treatment time is 7-8h.

According to a specific embodiment, as shown in FIG. 1 , the method for preparing polysilicon by refining and directional solidification of a silicon-aluminum-calcium alloy provided by the present invention includes:

1) Soak the silicon powder to be purified in a mixture of hydrofluoric acid and hydrogen peroxide at 20-35°C for 1-2 hours, filter, rinse with deionized water until neutral, filter, and dry to obtain crude silicon pink;

2) Put the crudely extracted silicon powder, aluminum block and calcium block obtained in step 1) into an induction melting furnace, pass argon gas into the furnace body and make the temperature in the furnace rise to 1703-1783 ℃ and then keep for 1-10h, Cool to room temperature to obtain a refined silicon-aluminum-calcium alloy;

3) After grinding the silicon-aluminum-calcium alloy obtained in step 2) into powder, adopt a two-step pickling method: in the first step, the silicon-alumina-calcium alloy powder is subjected to pickling treatment with a mixed acid I of hydrochloric acid and acetic acid, and the pickling temperature is 20 ℃. ～60℃, pickling time is 2～3h, after pickling treatment, filter, wash with deionized water, and dry; in the second step, the dried powder is pickled with mixed acid II of hydrochloric acid and hydrofluoric acid Treatment, pickling temperature is 20～60℃, pickling time is 2～3h, after pickling treatment, filter, wash with deionized water, and dry;

4) The secondary pickling powder obtained in step 3) is subjected to directional solidification, the direction of directional solidification is set as the vertical direction and the temperature gradient in the vertical direction is 10-30 °C/cm, and the adopted method is to fix the heat source, The crucible is moved from top to bottom, and then annealed to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) Cut off 20-40% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) after the secondary refining, and the remaining part obtained is polysilicon; 20-40% polysilicon is recycled and reused.

The invention adopts the method of silicon-aluminum-calcium alloy refining and directional solidification to purify polycrystalline silicon. Before alloy refining, the silicon powder to be purified is subjected to pickling treatment, so that some surface impurities can be removed, and then a CaAl ₂ Si ₂ alloy phase is formed during the alloy refining process. , the CaAl ₂ Si ₂ alloy has a high affinity for impurities, especially phosphorus impurities. In addition, after the alloy is refined, a specific secondary pickling is used for directional solidification, which can further improve the purity of silicon. The annealing treatment after directional solidification can effectively Defects and pores generated during the solidification process of silicon are removed. In summary, the present invention has a good removal effect on impurities in polysilicon, especially phosphorus impurities, through the use of solvent refining, hydrometallurgy, directional solidification and other processes, thereby obtaining high-purity polysilicon. In addition, the method for preparing polysilicon provided by the present invention has low equipment energy consumption, low cost and little environmental pollution, so it has broad application prospects.

Description of drawings

The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of the exemplary embodiments of the present invention in conjunction with the accompanying drawings.

FIG. 1 is a flow chart of a method for preparing polycrystalline silicon by refining and directional solidification of a silicon-aluminum-calcium alloy provided by the present invention.

Detailed ways

The embodiments of the present invention will be described in detail below, and the examples of the embodiments are intended to be used to explain the present invention, but should not be construed as a limitation of the present invention. If no specific technology or condition is indicated in the examples, the technology or condition described in the literature in the field or the product specification is used. The reagents or instruments used without the manufacturer's indication are conventional products that can be obtained from the market.

Example 1

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 25°C for 1 hour. , wherein the concentration of hydrofluoric acid is 1 mol/L, and the concentration of hydrogen peroxide is 1.5 mol/L, then rinsed with deionized water until neutral, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1723 °C, and then kept for 5 h, and then cooled to room temperature at a speed of 5 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First pickle with a mixed acid consisting of 1.5mol/L hydrochloric acid and 1.5mol/L acetic acid, pickle at 30°C for 2h, and the pickling treatment is carried out under magnetic stirring at 350rad/min, and then the acid The washed powder is filtered, washed with deionized water, and dried to obtain primary silicon powder; then the primary silicon powder is acid washed with a mixed acid composed of 1.5 mol/L hydrochloric acid and 1.5 mol/L hydrofluoric acid, at 30 ° C. Pickling was carried out for 2 hours, and the pickling treatment was carried out under magnetic stirring at 350 rad/min, followed by filtration, washing with deionized water, and drying to obtain secondary silica fume;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 20° C./cm, and start the directional solidification lifting device so that the crucible reaches 15 mm. The speed of /h moves from top to bottom, and then annealed for 7 hours to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 20% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon (high-purity silicon). The purity of the polysilicon is 99.99934% as measured by glow discharge mass spectrometer (GDMS). , in which the phosphorus content is 0.24ppmw. The cut-off 20% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

Example 2

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 23°C for 2 hours. , wherein the concentration of hydrofluoric acid is 1.3 mol/L, and the concentration of hydrogen peroxide is 1.8 mol/L, then rinsed with deionized water until neutral, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1757 °C, and then kept for 5.4 hours, and then cooled to room temperature at a speed of 7 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First pickle with a mixed acid consisting of 2mol/L hydrochloric acid and 3mol/L acetic acid, pickle at 40°C for 2.5h, and the pickling treatment is carried out under magnetic stirring at 350rad/min, and then pickle The powder was filtered, washed with deionized water, and dried to obtain primary silicon powder; then the primary silicon powder was pickled with a mixed acid composed of 1.5 mol/L hydrochloric acid and 4 mol/L hydrofluoric acid, and pickled at 40 °C 2.5h, and the pickling treatment was carried out under magnetic stirring at 350 rad/min, and then filtered, washed with deionized water, and dried to obtain secondary silica fume;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 10°C/cm, and start the directional solidification lifting device so that the crucible is 20 mm The speed of /h moves from top to bottom, and then annealed for 7.5h to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 20% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon. The polysilicon has a purity of 99.99924% as measured by a glow discharge mass spectrometer (GDMS), and the phosphorus content is 99.99924%. 0.34ppmw. The cut-off 20% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

Example 3

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 25°C for 1 hour. , wherein the concentration of hydrofluoric acid is 1 mol/L, and the concentration of hydrogen peroxide is 1.5 mol/L, then rinsed with deionized water until neutral, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1758 °C, and then kept for 5 hours, and then cooled to room temperature at a speed of 5 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First, use a mixed acid consisting of 3 mol/L hydrochloric acid and 2 mol/L acetic acid for pickling, pickling at 30 °C for 2 h, and the pickling treatment is carried out under magnetic stirring at 350 rad/min, and then the pickled The powder was filtered, washed with deionized water, and dried to obtain primary silicon powder; then, the primary silicon powder was pickled with a mixed acid consisting of 3 mol/L hydrochloric acid and 4 mol/L hydrofluoric acid, and pickled at 30°C for 2 hours, And the pickling treatment is carried out under magnetic stirring at 350 rad/min, and then the secondary silicon powder is obtained after filtering, washing with deionized water, and drying;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 30°C/cm, and start the directional solidification lifting device so that the crucible is 24 mm The speed of /h moves from top to bottom, and then annealed for 7.4 hours to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 30% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon. The polysilicon has a purity of 99.99968% as measured by a glow discharge mass spectrometer (GDMS), and the phosphorus content is 99.99968%. 0.57ppmw. The cut-off 30% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

Example 4

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 25°C for 1 hour. , wherein the concentration of hydrofluoric acid is 1 mol/L, and the concentration of hydrogen peroxide is 1.5 mol/L, then rinsed with deionized water until neutral, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1778 °C, and then kept for 5 h, and then cooled to room temperature at a speed of 5 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First pickle with a mixed acid consisting of 1.5mol/L hydrochloric acid and 1.5mol/L acetic acid, pickle at 30°C for 2h, and the pickling treatment is carried out under magnetic stirring at 350rad/min, and then the acid The washed powder is filtered, washed with deionized water, and dried to obtain primary silicon powder; then the primary silicon powder is pickled with a mixed acid consisting of 2 mol/L hydrochloric acid and 6 mol/L hydrofluoric acid, and pickled at 30°C 2h, and the pickling treatment was carried out under magnetic stirring at 350 rad/min, and then the secondary silicon powder was obtained after filtering, washing with deionized water, and drying;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 20° C./cm, and start the directional solidification lifting device so that the crucible reaches 15 mm. The speed of /h moves from top to bottom, and then annealed for 7 hours to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 20% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon. The polysilicon has a purity of 99.99955% as measured by a glow discharge mass spectrometer (GDMS), and the phosphorus content is 99.99955%. 0.45ppmw. The cut-off 20% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

Example 5

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 25°C for 1 hour. , wherein the concentration of hydrofluoric acid is 1 mol/L, and the concentration of hydrogen peroxide is 1.5 mol/L, then rinsed with deionized water until neutral, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1723 °C, and then kept for 5 h, and then cooled to room temperature at a speed of 5 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First, use a mixed acid consisting of 3 mol/L hydrochloric acid and 4 mol/L acetic acid for pickling, pickling at 30 °C for 2 h, and the pickling treatment is carried out under magnetic stirring at 350 rad/min, and then the pickled The powder was filtered, washed with deionized water, and dried to obtain primary silicon powder; then, the primary silicon powder was pickled with a mixed acid consisting of 2 mol/L hydrochloric acid and 5 mol/L hydrofluoric acid, and pickled at 30°C for 2 hours. And the pickling treatment is carried out under magnetic stirring at 350 rad/min, and then the secondary silicon powder is obtained after filtering, washing with deionized water, and drying;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 20° C./cm, and start the directional solidification lifting device so that the crucible reaches 15 mm. The speed of /h moves from top to bottom, and then annealed for 7 hours to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 20% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon. The polysilicon has a purity of 99.99924% as measured by a glow discharge mass spectrometer (GDMS), and the phosphorus content is 99.99924%. 0.14ppmw. The cut-off 20% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

Example 6

The flow chart of the method for preparing polysilicon by refining and directional solidification of silicon-aluminum-calcium alloy provided by this embodiment is shown in Figure 1, specifically:

1) Put 15g of industrial silicon powder with a purity of 2N and a particle size of 75μm (phosphorus content is about 30ppmw) into a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ) at 35°C for 2 hours. , wherein the concentration of hydrofluoric acid is 2mol/L, and the concentration of hydrogen peroxide is 2mol/L, then rinsed with deionized water to neutrality, filtered and dried to obtain coarsely extracted silicon powder;

2) The coarsely extracted silicon powder obtained in step 1) is mixed with 13g aluminum block (Al) and 10g calcium block (Ca) and put into the induction melting furnace, and the aluminum block and calcium block purity adopted are both 2N; Argon gas was introduced into the furnace, the induction heating power was turned on, the temperature in the furnace body was raised to 1723 °C, and then kept for 5 h, and then cooled to room temperature at a speed of 5 °C/min to obtain a refined silicon-aluminum-calcium alloy;

3) Grind the once-refined silicon-aluminum-calcium alloy obtained in step 2) into powder, and then carry out two-step pickling. First pickle with a mixed acid consisting of 1.5mol/L hydrochloric acid and 1.5mol/L acetic acid, pickle at 30°C for 2h, and the pickling treatment is carried out under magnetic stirring at 350rad/min, and then the acid The washed powder is filtered, washed with deionized water, and dried to obtain primary silicon powder; then the primary silicon powder is acid-washed with a mixed acid composed of 1.5 mol/L hydrochloric acid and 4 mol/L hydrofluoric acid, and acidified at 30°C. Washing for 2 hours, and the pickling treatment is carried out under magnetic stirring at 350 rad/min, and then filtered, washed with deionized water, and dried to obtain secondary silica fume;

4) Use the secondary silicon powder obtained in step 3) for directional solidification to remove impurities such as B, P, and Ti, set the temperature gradient in the vertical direction of directional solidification to 20° C./cm, and start the directional solidification lifting device so that the crucible reaches 15 mm. The speed of /h moves from top to bottom, and then annealed for 7 hours to obtain a silicon-aluminum-calcium alloy ingot after secondary refining;

5) 20% of the head of the silicon-aluminum-calcium alloy ingot obtained in step 4) is removed, and the remaining part is polysilicon. The polysilicon has a purity of 99.99937% as measured by a glow discharge mass spectrometer (GDMS), and the phosphorus content is 99.99937%. 0.29ppmw. The cut-off 20% polysilicon can be recycled, but the added amount cannot exceed 30% of the silicon raw material.

The preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the specific details of the above-mentioned embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention. These simple modifications All belong to the protection scope of the present invention.

In addition, it should be noted that each specific technical feature described in the above-mentioned specific implementation manner may be combined in any suitable manner under the circumstance that there is no contradiction. In order to avoid unnecessary repetition, the present invention will not describe various possible combinations.

In addition, the various embodiments of the present invention can also be combined arbitrarily, as long as they do not violate the spirit of the present invention, they should also be regarded as the contents disclosed in the present invention.

Claims (10)

1. A method for preparing polycrystalline silicon by refining and directional solidification of silicon-aluminum-calcium alloy is characterized by comprising the following steps:

1) putting the silicon powder to be purified into a mixed solution of hydrofluoric acid and hydrogen peroxide at the temperature of 20-35 ℃, soaking for 1-2 hours, filtering, washing with water, filtering, and drying to obtain crude silicon powder;

2) putting the crude silica powder obtained in the step 1), metal aluminum and metal calcium into an induction smelting furnace, introducing inert gas into the furnace body, raising the temperature in the furnace to 1703-1783 ℃, preserving the temperature for 1-10 h, and cooling to room temperature to obtain primary refined silicon-aluminum-calcium alloy; the cooling speed is 5-10 ℃/min;

3) grinding the silicon-aluminum-calcium alloy obtained in the step 2) into powder, and then sequentially carrying out acid pickling on the powder by using a mixed acid I of hydrochloric acid and acetic acid and a mixed acid II of hydrochloric acid and hydrofluoric acid;

4) directionally solidifying the acid-washed product obtained in the step 3), and then annealing to obtain a secondary refined silicon-aluminum-calcium alloy ingot;

5) cutting off 20-40% of the head of the secondary refined silicon-aluminum-calcium alloy ingot obtained in the step 4), and obtaining the rest part which is polycrystalline silicon.

2. The method for preparing polysilicon by refining and directional solidification of Si-Al-Ca alloy as claimed in claim 1, wherein in step 1), the concentration of hydrofluoric acid and the concentration of hydrogen peroxide in the mixed solution are respectively 1-3 mol/L and 1.5-2.5 mol/L, respectively.

3. The method for preparing polysilicon by refining and directionally solidifying the silico-aluminum-calcium alloy according to claim 1, wherein the metallic aluminum and the metallic calcium are used in the form of aluminum blocks and calcium blocks, respectively; the atomic percentage of the adding amount of the aluminum blocks and the calcium blocks is (1.5-2.5) to 1; the total using amount of the aluminum blocks and the calcium blocks is 80-120 parts by weight relative to 100 parts by weight of silicon powder to be purified; the purity of the aluminum block and the purity of the calcium block are respectively and independently 2-3N; the purity of the silicon powder to be purified is 1-3N.

4. The method for preparing the polycrystalline silicon by refining and directional solidification of the silicon-aluminum-calcium alloy according to any one of claims 1 to 3, wherein in the step 2), the temperature for heat preservation is 1720-1780 ℃ and the time is 5-6 hours.

5. The method for preparing the polycrystalline silicon by refining and directional solidification of the silicon-aluminum-calcium alloy according to any one of claims 1 to 3, wherein the concentration of hydrochloric acid in the mixed acid I is 1 to 3mol/L, and the concentration of acetic acid in the mixed acid I is 1 to 4 mol/L; the concentration of hydrochloric acid in the mixed acid II is 1-3 mol/L, and the concentration of hydrofluoric acid is 1-6 mol/L.

6. The method for preparing polysilicon by refining and directional solidification of Si-Al-Ca alloy according to any one of claims 1 to 3, wherein the acid washing with the mixed acid I and the mixed acid II is carried out at 20 to 60 ℃ for 2 to 3 hours and under magnetic stirring at 300 to 600 rad/min.

7. The method for preparing polysilicon by refining and directionally solidifying the Si-Al-Ca alloy according to any one of claims 1-3, wherein the method further comprises the steps of filtering, washing and drying the product after the two times of pickling in step 3); and/or, in the step 5), recycling and reusing 20-40% of polycrystalline silicon cut from the head of the silicon-aluminum-calcium alloy ingot.

8. The method for refining and directionally solidifying the Si-Al-Ca alloy to prepare the polysilicon as claimed in any one of claims 1 to 3, wherein the directional solidification is in a vertical direction by using a fixed heat source and the crucible is moved from top to bottom.

9. The method for preparing the polycrystalline silicon by refining and directionally solidifying the silicon-aluminum-calcium alloy according to claim 8, wherein the temperature gradient in the vertical direction of directional solidification is set to be 10-30 ℃/cm, and the speed of the crucible moving from top to bottom is set to be 15-25 mm/h; the annealing time is 7-8 h.

10. The method for preparing the polycrystalline silicon by the refining and directional solidification of the silicon-aluminum-calcium alloy according to any one of claims 1 to 3, wherein the method comprises the following steps:

1) putting the silicon powder to be purified into a mixed solution of hydrofluoric acid and hydrogen peroxide at the temperature of 20-35 ℃, soaking for 1-2 hours, filtering, washing with deionized water to be neutral, filtering, and drying to obtain crude silicon powder;

2) putting the crude silica powder obtained in the step 1), an aluminum block and a calcium block into an induction smelting furnace, introducing argon into the furnace body, raising the temperature in the furnace to 1703-1783 ℃, preserving the heat for 1-10 h, and cooling to room temperature to obtain a primary refined silicon-aluminum-calcium alloy;

3) grinding the silicon-aluminum-calcium alloy obtained in the step 2) into powder, and then pickling by adopting a two-step method: firstly, acid pickling the silicon-aluminum-calcium alloy powder with a mixed acid I of hydrochloric acid and acetic acid at the acid pickling temperature of 20-60 ℃ for 2-3 h, filtering after acid pickling, washing with deionized water, and drying; secondly, carrying out acid pickling treatment on the dried powder by using a mixed acid II of hydrochloric acid and hydrofluoric acid at the acid pickling temperature of 20-60 ℃ for 2-3 h, filtering after the acid pickling treatment, washing with deionized water, and drying;

4) performing directional solidification on the secondary pickling powder obtained in the step 3), setting the directional solidification direction as a vertical direction and setting the temperature gradient in the vertical direction as 10-30 ℃/cm, fixing a heat source, moving a crucible from top to bottom, and then performing annealing treatment to obtain a silicon-aluminum-calcium alloy ingot subjected to secondary refining;

5) cutting off 20-40% of the head of the secondary refined silicon-aluminum-calcium alloy ingot obtained in the step 4), and obtaining the rest part which is polycrystalline silicon; and (3) recycling and reusing 20-40% of polycrystalline silicon cut off from the head of the silicon-aluminum-calcium alloy ingot.

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