Solar cell silicon waste purification method

Abstract

The invention provides a method for purifying silicon waste of a solar cell. The method comprises the following steps: cleaning the silicon waste with an organic solvent, and removing impurities with a mixed acid solution; the organic solvent is a polar organic solvent of which the dielectric constant is greater than or equal to 18.3 and the dipole moment is greater than or equal to 1.66 D; the molar ratio of hydrofluoric acid in the mixed acid solution to total acid is 1-6%, and the molar concentration of the total acid in the mixed acid solution is 1.5-4 mol / L. According to the method, the crystalline silicon cutting waste is cleaned through the polar organic solvent, metal impurities in the crystalline silicon cutting waste can be better removed with the assistance of the mixed acid solution with the specific concentration and molar ratio, and the recovery rate of silicon in the crystalline silicon cutting waste is greatly increased; and moreover, the use amount of hydrofluoric acid in the mixed acid solution is greatly reduced, and production of high-purity silicon with high recovery rate is possible.

Description

Technical Field

[0001] The invention relates to the technical field of silicon waste treatment, and in particular to a method for purifying solar cell silicon waste. Background Art

[0002] In recent years, the photovoltaic industry of solar cells based on crystalline silicon has developed rapidly. In industry, in the process of using crystalline silicon to prepare battery materials, crystalline silicon needs to be cut into silicon slices. During the cutting process, nearly 40% of the silicon will enter the cutting fluid in the form of ultrafine powder to form cutting waste, resulting in a waste of silicon resources, increased costs and environmental pollution. Therefore, the effective recovery of silicon from cutting waste is an urgent concern of the global photovoltaic industry.

[0003] Impurities in crystalline silicon cutting waste mainly include organic impurities and metal impurities. Organic impurities come from organic additives used in the slicing process. In the process of purifying and recycling cutting waste, most of them use heat treatment to remove organic additives. For solar cell applications, the element impurity level cannot exceed 1×10 -6 The effect of metal impurity removal determines the effect of crystalline silicon purification and recovery. The existing metal impurity removal technology for crystalline silicon waste is generally carried out through wet acid leaching process. The research group of Professor Xing Pengfei of Northeastern University and the research group of Professor Ma Wenhui of Kunming University of Science and Technology have conducted detailed research on the wet acid leaching process. Comprehensive research found that in the process of wet acid leaching, there are limitations when a single acid solution is used to remove metal impurities in cutting waste. Because each acid has different reaction kinetics with different metals, when a single acid is used for impurity removal, the acid and different metal reactions will interfere with each other, resulting in low metal removal rate. Therefore, the focus of wet acid leaching is to select a mixed acid with a suitable ratio for acid leaching and impurity removal.

[0004] At the same time, due to the long-term exposure of crystalline silicon cutting waste to an oxygen environment, an uneven oxide layer with a certain thickness is formed on the surface. The oxide layer is mainly composed of amorphous SiO 2 In order to remove the metal oxides on the surface of silicon waste and improve the removal rate of metal impurities in silicon waste, domestic and foreign scholars mostly add a large amount of hydrofluoric acid to the mixed acid solution to remove impurities and recycle silicon waste. The mass proportion of hydrofluoric acid in the mixed acid solution is usually ≥50%. The molecular weight of hydrofluoric acid is small, and the molar proportion is higher than that after conversion. Although this method of adding a large amount of hydrofluoric acid can fully remove metal impurities, the recovery rate of silicon waste is low because hydrofluoric acid reacts with silicon. Usually, the silicon recovery rate does not exceed 70%. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the defect in the prior art that a mixed acid solution containing high concentration of hydrofluoric acid is used in the removal of impurities from silicon waste, resulting in a low silicon recovery rate.

[0006] To this end, the present invention provides a method for purifying solar cell silicon waste, wherein the silicon waste is cleaned with an organic solvent and then impurities are removed with a mixed acid solution; the organic solvent is a polar organic solvent with a dielectric constant ≥18.3 (20°C) and a dipole moment ≥1.66D; the molar ratio of hydrofluoric acid to total acid in the mixed acid solution is 1-6%, and the molar concentration of total acid in the mixed acid solution is 1.5-4 mol / L.

[0007] The dielectric constant is a physical quantity that describes the material's ability to store electrical energy in an electric field. A higher dielectric constant usually means that the solvent has a stronger polarity. The dielectric constant mainly reflects the static polarization ability of the solvent.

[0008] The dipole moment is the distance between the centers of positive and negative charges in a molecule multiplied by the amount of charge. Its unit is Debye (D). A larger dipole moment indicates that the molecule has a stronger polarity.

[0009] Total acid refers to all acids contained in the mixed acid solution.

[0010] For example, the molar ratio of hydrofluoric acid to total acid in the mixed acid solution is 1%, 2%, 3%, 4%, 5%, 6%, etc., and the molar concentration of the mixed acid solution is 1.5mol / L, 1.8mol / L, 2mol / L, 2.5mol / L, 3mol / L, 3.5mol / L, 4mol / L, etc.

[0011] For example: polar organic solvents are acetonitrile with a dielectric constant of 37.5 (20°C) and a dipole moment of 3.92D, ethanol with a dielectric constant of 24.3 (20°C) and a dipole moment of 1.69D, methanol with a dielectric constant of 32.6 (20°C) and a dipole moment of 1.71D, isopropanol with a dielectric constant of 18.3 (20°C) and a dipole moment of 1.66D, n-propanol with a dielectric constant of 20.0 (20°C) and a dipole moment of 1.68D, and acetone with a dielectric constant of 20.7 (20°C) and a dipole moment of 2.91D.

[0012] Furthermore, the purity of the organic solvent is above 99.5%;

[0013] And / or, the polar organic solvent is preferably an organic solvent having a dielectric constant ≥ 20.7 and / or a dipole moment ≥ 2.91D; more preferably, the polar organic solvent is ethanol or acetone.

[0014] In the step of cleaning with the organic solvent, the volume mass ratio of the organic solvent to the silicon waste to be cleaned is 5-15 mL / g; for example, the organic solvent used for every 1 g of silicon waste to be cleaned is 5 mL, 6 mL, 7 mL, 8 mL, 9 mL, 10 mL, 11 mL, 12 mL, 13 mL, 14 mL, 15 mL, etc.

[0015] And / or, in the step of removing impurities with the mixed acid solution, the volume mass ratio of the mixed acid solution to the silicon waste to be removed is 5-15 mL / g; for example, the amount of the mixed acid solution used per 1 g of silicon waste to be removed is 5 mL, 6 mL, 7 mL, 8 mL, 9 mL, 10 mL, 11 mL, 12 mL, 13 mL, 14 mL, 15 mL, etc.

[0016] Furthermore, the step of washing with an organic solvent comprises: adding the silicon waste to be washed into an organic solvent, heating and refluxing, and then filtering to obtain filter residue.

[0017] Furthermore, the heating reflux temperature is 70-90°C.

[0018] Furthermore, the step of washing with an organic solvent also includes: vacuum drying the filter residue and then thermally decomposing it to obtain silicon powder, and the silicon powder is the silicon waste to be removed in the subsequent step of removing impurities with a mixed acid solution.

[0019] Furthermore, the temperature of the thermal decomposition is 500-700°C.

[0020] Further, the mixed acid solution comprises sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in mass percentages of 42-46wt%, 42-46wt% and 16-8wt% respectively;

[0021] The concentration of sulfuric acid in the sulfuric acid solution is 3-4 mol / L; and / or the concentration of hydrochloric acid in the hydrochloric acid solution is 3-4 mol / L; and / or the concentration of hydrofluoric acid in the hydrofluoric acid solution is 0.5-1 mol / L.

[0022] Furthermore, the step of removing impurities from the mixed acid solution comprises: adding the silicon waste to be removed into the mixed acid solution, heating and refluxing, filtering to obtain filter residue, and vacuum drying.

[0023] Furthermore, the silicon waste purification method further comprises the step of pre-treating the silicon waste, wherein the pre-treating step comprises:

[0024] The silicon waste is washed with water and then filtered;

[0025] The filter residue is vacuum dried and ground, and the silicon waste obtained after the grinding is the silicon waste to be cleaned used in the subsequent step of cleaning with an organic solvent.

[0026] The technical solution of the present invention has the following advantages:

[0027] 1. The present invention provides a method for purifying solar cell silicon waste, wherein crystalline silicon cutting waste is cleaned by using a polar organic solvent such as ethanol, acetone, etc., with a dielectric constant of ≥18.3 and a dipole moment of ≥1.66D. The polar organic solvent can not only effectively reduce the organic matter content in the silicon waste, but also effectively corrode the silicon surface oxide layer, thereby facilitating the removal of metal elements therein during subsequent acid treatment, overcoming the obstacle of metal impurity removal caused by the oxide layer, and improving the removal rate of metal impurities; therefore, the present invention uses a polar organic solvent to treat the silicon waste before using a mixed acid solution to remove metal impurities, which can effectively achieve the effect of assisting the mixed acid solution to better remove metal impurities in the crystalline silicon cutting waste; and, because the polar organic solvent destroys the oxide layer in the crystalline silicon cutting waste, it can achieve a substantially equivalent or even better metal impurity removal effect at a lower hydrofluoric acid content, thereby effectively and significantly reducing the amount of hydrofluoric acid in the mixed acid solution on the basis of satisfying the metal impurity removal effect. Specifically, the polar organic solvent in the present invention has a large dielectric constant and dipole moment, and can effectively stabilize charged species (such as ions or polar molecules); when the metal ions on the surface of the metal oxide come into contact with the solvent molecules, the solvent molecules can surround these ions through electrostatic interactions, thereby reducing the Coulomb force between them and making the metal oxide easier to dissociate; at the same time, the functional groups (such as hydroxyl, amino, carbonyl, etc.) in the highly polar organic solvent can act as ligands to form coordination compounds with the metal ions, and this coordination effect can further destroy the crystal structure of the metal oxide and promote its dissolution. Therefore, after the organic matter on the surface of the silicon waste and the metal oxide in the silicon surface oxide layer are dissolved by the polar organic solvent of the present invention, the mixed acid solution can fully contact the surface of the silicon waste (without the barrier of organic matter and oxide layer), and then react the metal impurities in the silicon into metal salts dissolved in water, so as to achieve the removal of metal impurities in the silicon; in the present invention, hydrofluoric acid plays a more important role in reacting with the silicon dioxide oxide layer and then destroying the oxide layer in the acid solution impurity removal, therefore, when a strong polar organic solvent is not used for pre-treatment, the hydrofluoric acid needs to react with the non-silicon dioxide oxide layer and the silicon dioxide oxide layer at the same time, thereby resulting in the need for a large amount of hydrofluoric acid to effectively solve the problem that the oxide layer blocks the mixed acid solution from contacting the metal in the silicon, therefore, more hydrofluoric acid is needed in the mixed acid solution to better achieve the purpose of removing metal impurities in the silicon, and after the introduction of a polar organic solvent with a dielectric constant ≥20 or / and a dipole moment ≥1.6D, the polar organic solvent can take on the role of dissolving the non-silicon dioxide oxide layer, thereby achieving the purpose of reducing the hydrofluoric acid content in the subsequent mixed acid solution.Experimental verification has shown that even when the molar proportion of hydrofluoric acid in the mixed acid solution is only 1-6% of the total acid, metal impurities can still be effectively removed. Moreover, since the proportion of hydrofluoric acid in the mixed acid solution is significantly reduced, the problem of increased silicon loss due to reaction with silicon caused by excessive hydrofluoric acid content is avoided, and the recovery rate of silicon in crystalline silicon cutting waste is greatly improved, making it possible to produce high-purity silicon (purity ≥99.7%) with a high recovery rate (recovery rate ≥80%).

[0028] 2. The present invention provides a method for purifying solar cell silicon waste, which uses acetone or ethanol to clean the crystalline silicon cutting waste, and can also fully remove organic impurities in the crystalline silicon cutting waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 This is the infrared spectrum test chart of Test Example 1 of the present invention;

[0031] Figure 2 Thermogravimetric analysis diagrams of silicon powder and dry silicon powder obtained after the organic solvent cleaning step in Example 1 in Experimental Example 2;

[0032] Figure 3 This is the thermogravimetric analysis diagram of the silicon powder and the water-washed silicon powder obtained after the organic solvent cleaning step in Example 2 in Experimental Example 2. DETAILED DESCRIPTION

[0033] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0034] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0035] The resistivity of the deionized water used in the present application is ≥18MΩ·cm;

[0036] In the embodiment of the present application, the vacuum degree during vacuum drying is ≤-0.095MPa;

[0037] The organic solvents in the examples of the present application are all EL grade (HPLC grade).

[0038] Example 1

[0039] This embodiment provides a method for purifying solar cell silicon waste, comprising the following steps:

[0040] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0041] Organic solvent cleaning: 8 g of pretreated silicon powder was added to acetone (10 mL of acetone was used for every 1 g of silicon powder), then heated to 80°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered out to obtain the residue. The residue was vacuum dried at 80°C for 24 h, placed in a tube furnace, and thermally decomposed at 500°C to obtain thermally decomposed silicon powder.

[0042] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 4 mol / L sulfuric acid solution, 4 mol / L hydrochloric acid solution and 0.8 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 46 wt%, 46 wt% and 8 wt%, respectively.

[0043] Example 2

[0044] This embodiment provides a method for purifying solar cell silicon waste, comprising the following steps:

[0045] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0046] Organic solvent cleaning: 8 g of pretreated silicon powder was added to isopropanol (10 mL of isopropanol was used for every 1 g of silicon powder), and then the temperature was raised to 70°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered to obtain the residue. The residue was vacuum dried at 80°C for 24 h, and then placed in a tube furnace and thermally decomposed at 500°C to obtain thermally decomposed silicon powder.

[0047] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 3 mol / L sulfuric acid solution, 3 mol / L hydrochloric acid solution and 0.5 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 42 wt%, 42 wt% and 16 wt%, respectively.

[0048] Example 3

[0049] This embodiment provides a method for purifying solar cell silicon waste, comprising the following steps:

[0050] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0051] Organic solvent cleaning: 8 g of pretreated silicon powder was added to 80 wt% ethanol (10 mL of ethanol was used for every 1 g of silicon powder), then heated to 90°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered out to obtain the residue. The residue was vacuum dried at 80°C for 24 h, placed in a tube furnace, and thermally decomposed at 700°C to obtain thermally decomposed silicon powder.

[0052] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 2 mol / L sulfuric acid solution, 2 mol / L hydrochloric acid solution and 1 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 46 wt%, 46 wt% and 8 wt%, respectively.

[0053] Example 4

[0054] This embodiment provides a method for purifying solar cell silicon waste, comprising the following steps:

[0055] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0056] Organic solvent cleaning: 8 g of pretreated silicon powder was added to acetone (5 mL of acetone was used for every 1 g of silicon powder), and then the temperature was raised to 80°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered to obtain the residue. The residue was vacuum dried at 80°C for 24 h, and then placed in a tube furnace and thermally decomposed at 500°C to obtain thermally decomposed silicon powder.

[0057] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 4 mol / L sulfuric acid solution, 4 mol / L hydrochloric acid solution and 0.8 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 46 wt%, 46 wt% and 8 wt%, respectively.

[0058] Example 5

[0059] This embodiment provides a method for purifying solar cell silicon waste, comprising the following steps:

[0060] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0061] Organic solvent cleaning: 8 g of pretreated silicon powder was added to acetone (15 mL of acetone was used for every 1 g of silicon powder), and then the temperature was raised to 80°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered to obtain the residue. The residue was vacuum dried at 80°C for 24 h, and then placed in a tube furnace and thermally decomposed at 500°C to obtain thermally decomposed silicon powder.

[0062] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 4 mol / L sulfuric acid solution, 4 mol / L hydrochloric acid solution and 0.8 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 46 wt%, 46 wt% and 8 wt%, respectively.

[0063] Comparative Example 1

[0064] This comparative example provides a method for purifying solar cell silicon waste, which is different from Example 1 only in that an equal amount of deionized water is used instead of acetone in the organic solvent washing step.

[0065] Comparative Example 2

[0066] This comparative example provides a method for purifying solar cell silicon waste, comprising the following steps:

[0067] Pretreatment: The crystalline silicon cutting waste was stirred and washed three times with deionized water, the mass ratio of the washing waste to deionized water was 1:40, and after washing, the filter residue was obtained by suction filtration, and the filter residue was vacuum dried at 80°C for 24 hours to obtain silicon blocks, and then the silicon blocks were ground into 16 mesh to obtain silicon powder;

[0068] Solvent cleaning: 8 g of pretreated silicon powder was added to deionized water (10 mL of deionized water was used for every 1 g of silicon powder), then heated to 80°C and refluxed for 2 h. After cooling to room temperature, the residue was filtered to obtain the residue. The residue was vacuum dried at 80°C for 24 h, placed in a tube furnace, and thermally decomposed at 500°C to obtain thermally decomposed silicon powder.

[0069] Mixed acid solution impurity removal: take thermal decomposition silicon powder and add it to the mixed acid solution (10 mL of mixed acid solution is used for every 1 g of thermal decomposition silicon powder), then heat it to 70°C and reflux with stirring for 3 hours, cool it to room temperature and filter it to obtain the filter residue, and vacuum dry the filter residue at 80°C for 24 hours to obtain purified silicon powder; the mixed acid solution is a mixed solution of 4 mol / L sulfuric acid solution, 4 mol / L hydrochloric acid solution and 0.8 mol / L hydrofluoric acid solution, and the mass percentages of sulfuric acid solution, hydrochloric acid solution and hydrofluoric acid solution in the mixed acid solution are 25wt%, 25wt% and 50wt%, respectively.

[0070] Comparative Example 3

[0071] This comparative example provides a method for purifying solar cell silicon waste, which is different from Example 1 only in that an equal amount of tetrahydrofuran is used instead of acetone in the organic solvent washing step, and in order to achieve a better impurity removal effect, the silicon content (%) in the recovered silicon powder is increased to about 99.9%, and the mass percentage of the hydrofluoric acid solution in the mixed acid solution is increased to 30wt%, that is, the mass percentages of the sulfuric acid solution, the hydrochloric acid solution, and the hydrofluoric acid solution in the mixed acid solution are 35wt%, 35wt%, and 30wt%, respectively. The rest is the same as Example 1.

[0072] Comparative Example 4

[0073] This comparative example provides a method for purifying solar cell silicon waste, which is different from Example 1 only in that an equal amount of toluene is used instead of acetone in the organic solvent washing step, and in order to achieve a better impurity removal effect, the silicon content (%) in the recovered silicon powder is increased to about 99.9%, and the mass percentage of the hydrofluoric acid solution in the mixed acid solution is increased to 30wt%, that is, the mass percentages of the sulfuric acid solution, the hydrochloric acid solution, and the hydrofluoric acid solution in the mixed acid solution are 35wt%, 35wt%, and 30wt%, respectively. The rest is the same as Example 1.

[0074] Test Example 1

[0075] The purified silicon powders of Example 1, Example 2 and Comparative Example 1, and the silicon powder (washed silicon powder) after pretreatment of Example 1 were tested by infrared spectroscopy, and the infrared spectroscopy of the silicon powder was characterized by a NICOLET 6700 Fourier transform infrared spectrometer. Specifically, the infrared analysis was performed by potassium bromide tabletting method. The sample was ground into powder, dried at 80°C in vacuum for 12 hours, and added into the powdered potassium bromide according to the mass ratio of sample: KBr = 1mg: 100mg, and tabletted for determination. Parameter settings: room temperature dry environment, air background subtraction, 32 scans, resolution 4cm -1 , wavelength range: 4000-380cm -1 The test results are shown in Figure 1 .

[0076] Reference Figure 1 It can be seen that compared with Comparative Example 1, the silicon powder treated and purified in Example 1 has a -1 The -OH vibration peak around 2700cm -1 、28000cm -1 -CH 2 、-CH 3 The stretching vibration peak basically disappeared, and the silicon powder treated and purified in Example 1 was at 1700cm -1 The -C=O vibration peak around 1350cm is significantly weakened, indicating that the organic matter content in the silicon waste is reduced after washing with organic solvent. The silicon powder treated and purified in Example 1 and Example 2 has a peak at 1350cm -1 The vibration peak at 1350cm -1 The vibration peak at may be Si-O-Si or Si-Si. It can be seen that cleaning silicon waste with acetone solution or ethanol can effectively corrode the oxide layer on the silicon surface. This may be the reason why ethanol or acetone solution can assist in removing metal impurities in crystalline silicon cutting waste.

[0077] Test Example 2

[0078] Thermogravimetric analysis was performed on the silicon powder obtained after the organic solvent washing step in Example 1 and Example 2, the silicon powder pretreated in Example 1 (water-washed silicon powder), and the silicon powder raw material (dry silicon powder) without any treatment in Example 1. The analysis results are shown in Figure 2 and Figure 3 ,in Figure 2 Thermogravimetric analysis of silicon powder and dry silicon powder obtained after the organic solvent cleaning step in Example 1, Figure 3 Thermogravimetric analysis diagrams of silicon powder obtained after the organic solvent cleaning step in Example 2 and water-washed silicon powder.

[0079] Reference Figure 2 and Figure 3 It can be seen that silicon powder has multiple inflection points in the range of 200-500℃, proving that organic matter with different boiling points remains in the silicon powder in this range. After ethanol cleaning, some of the organic matter is removed, but the best effect is acetone solution cleaning. After acetone solution cleaning, the weight loss rate of silicon powder does not exceed 0.1%, which is close to a straight line, proving that acetone treatment is the best.

[0080] Test Example 3

[0081] The metal impurity concentrations in the silicon powder purified and recovered in Examples 1-5 and Comparative Examples 1-4 and the pretreated silicon powder in Example 1 were tested respectively. After the metal impurities were treated according to the national standard (GB / T14849.4-2014), ICP-AES (OPTIMA, 8000) or ICP-MS (DRC, Two) was used for quantitative detection; the test results are shown in Table 1.

[0082] Table 1

[0083]

[0084]

[0085] By comparing the above Examples 1-4 with Comparative Example 1, it can be seen that the use of polar organic solvents with a dielectric constant ≥18.3 and a dipole moment ≥1.66D instead of water for cleaning can achieve the purpose of significantly reducing metal impurities under the same mixed acid solution, and Examples 2-4 can achieve an effect substantially equivalent to that of Example 1; and by comparing Example 1 with Comparative Examples 2-4, it can be seen that the use of the polar organic solvent in the present invention can significantly reduce the amount of hydrofluoric acid in the mixed acid solution when compared with other solvents to achieve a substantially equivalent metal impurity removal effect; by comparing Example 1 and Examples 4 and 5, it can be seen that, relative to silicon waste, the higher the amount of polar organic solvent used, the better the impurity removal effect, but after the amount of polar organic solvent is increased to 10 mL / g, especially after 15 mL / g, the purity is not significantly improved, therefore, the volume-to-mass ratio of the amount of polar organic solvent used in the present invention relative to silicon waste is preferably 1-15 mL / g.

[0086] Test Example 4

[0087] The silicon content of the silicon powder purified and recovered in the embodiment and the comparative example was tested respectively using an X-ray fluorescence spectrometer (XRF, AxioSmAX). The test results are shown in Table 2.

[0088] The recovery rates of the purified and recovered silicon powders in the embodiment and the comparative example were calculated respectively. The calculation results are shown in Table 2. The calculation formula is as follows:

[0089]

[0090] V is the recovery rate (%); c 1 is the silicon content in the recovered silicon powder (%); m 1 is the mass of silicon powder after recovery (g); c 0 is the silicon content in the initial silicon powder (%); m 0 is the initial silicon powder mass (g).

[0091] Table 2

[0092]

[0093]

[0094] The silicon content / % in the above Table 2 is the silicon content (%) in the silicon powder after recovery. By comparing Examples 1 to 5 in the above Table 2 with Comparative Example 1, it can be seen that by using a polar organic solvent with a dielectric constant ≥ 18.3 and a dipole moment ≥ 1.66D instead of water for cleaning, the purpose of significantly improving the purity of the silicon product can still be achieved under the condition of using a mixed acid solution with a lower hydrofluoric acid content to ensure the recovery rate (recovery rate ≥ 82%), so that the purity of the silicon product reaches more than 99.7%; and by comparing with Comparative Examples 2-4, it can be seen that after treatment with other solvents, in order to obtain a substantially equivalent purity, it is necessary to increase the amount of hydrofluoric acid in the mixed acid solution, which leads to a significant reduction in the recovery rate of the final silicon product, and the recovery rate of the silicon product is not higher than 78%.

[0095] In summary, the present invention uses a polar organic solvent with a dielectric constant ≥18.3 and a dipole moment ≥1.66D to treat silicon waste before using a mixed acid solution for impurity removal, thereby significantly improving the purity of the silicon product while reducing the hydrofluoric acid content in the mixed acid solution to ensure the recovery rate, and the effect is very significant.

[0096] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for purifying solar cell silicon waste, characterized in that: The silicon waste is washed with an organic solvent and then impurities are removed using a mixed acid solution; The organic solvent is a polar organic solvent having a dielectric constant ≥18.3 and a dipole moment ≥1.66D; The molar ratio of hydrofluoric acid to total acid in the mixed acid solution is 1-6%, and the molar concentration of total acid in the mixed acid solution is 1.5-4 mol / L.

2. A method for purifying solar cell silicon waste according to claim 1, characterized in that: The purity of the organic solvent is above 99.5%; And / or, the polar organic solvent is a polar organic solvent having a dielectric constant ≥ 20.7 and / or a dipole moment ≥ 2.91D.

3. A method for purifying solar cell silicon waste according to claim 1 or 2, characterized in that: In the step of washing with an organic solvent, the volume mass ratio of the organic solvent to the silicon waste to be washed is 5-15 mL / g; And / or, in the step of removing impurities with the mixed acid solution, the volume mass ratio of the mixed acid solution to the silicon waste to be removed is 5-15 mL / g; And / or, the polar organic solvent is ethanol or acetone.

4. A method for purifying solar cell silicon waste according to claim 1 or 2, characterized in that: The step of washing with an organic solvent comprises: adding the silicon waste to be washed into the organic solvent, heating and refluxing, and then filtering to obtain filter residue.

5. A method for purifying solar cell silicon waste according to claim 4, characterized in that: The heating reflux temperature is 70-90°C.

6. A method for purifying solar cell silicon waste according to claim 4, characterized in that: The step of washing with an organic solvent also includes: vacuum drying the filter residue and then thermally decomposing it to obtain silicon powder.

7. A method for purifying solar cell silicon waste according to claim 6, characterized in that: The temperature of the thermal decomposition is 500-700°C.

8. A method for purifying solar cell silicon waste according to any one of claims 1 to 7, characterized in that: The mixed acid solution comprises a sulfuric acid solution, a hydrochloric acid solution and a hydrofluoric acid solution in amounts of 42-46wt%, 42-46wt% and 8-16wt% respectively; The concentration of the sulfuric acid solution is 3-4 mol / L, and / or the concentration of the hydrochloric acid solution is 3-4 mol / L, and / or the concentration of the hydrofluoric acid solution is 0.5-1 mol / L.

9. A method for purifying solar cell silicon waste according to any one of claims 1 to 8, characterized in that: The step of removing impurities from the mixed acid solution comprises: adding the silicon waste to be removed into the mixed acid solution, heating and refluxing, and then filtering to obtain the filter residue and vacuum drying.

10. A method for purifying solar cell silicon waste according to any one of claims 1 to 9, characterized in that: The step of pre-treating the silicon waste is also included, and the pre-treating step comprises: The silicon waste is washed with water and then filtered to obtain a filter residue; The filter residue was dried in vacuo and ground.

Images