Method for preparing regenerated silicon by refining photovoltaic cutting silicon sludge in a medium frequency furnace

Abstract

The present application relates to the technical field of silicon preparation, and discloses a method for preparing regenerated silicon by refining photovoltaic cutting silicon mud in a medium-frequency furnace. The method comprises the following steps: screening the photovoltaic cutting silicon mud, removing iron by magnetic separation, removing dust under negative pressure, and drying the silicon mud at low temperature to obtain dry-base silicon mud; obtaining the contents of silicon oxide, metal impurities and non-metal impurities by sampling the granular levels, and determining the adding amounts of residual silicon oxide regulator, slagging desulfurizer, first refining agent and second refining agent, respectively; feeding the mixed furnace charge into a medium-frequency induction melting furnace, and sequentially performing the following steps: preheating, softening and coalescing silicon particles to form a silicon-rich phase, slagging separation by the first refining agent, high-temperature refining, adding the second refining agent in batches, power cycle adjustment and second slag separation, to obtain silicon liquid; sampling and detecting before casting, adding corresponding refining agent and retesting when exceeding the limit, and casting, cooling, crushing and screening after meeting the standard, to obtain regenerated silicon. The method is used for preparing regenerated silicon.

Description

Technical Field

[0001] This invention relates to the field of silicon preparation technology, and in particular to a method for preparing recycled silicon by refining photovoltaic cutting silicon mud in an intermediate frequency furnace. Background Technology

[0002] Photovoltaic cutting silicon sludge originates from the silicon wafer cutting process. Its silicon element primarily exists in the form of elemental silicon particles. Besides elemental silicon, it also contains varying proportions of silicon oxide and impurities such as iron, aluminum, calcium, boron, and phosphorus, making it a silicon-containing raw material for preparing recycled silicon. This type of raw material differs from natural silica, which primarily uses silica as its silicon source. Its processing aims to remelt, refine, and control the composition of existing elemental silicon, rather than achieving complete carbothermic reduction of silica in a submerged arc furnace. Current technologies for processing silicon materials from silicon cutting waste or diamond wire cutting silicon sludge typically include solid-liquid separation, drying, mixing with slagging agents or fluxing components, smelting and refining in a medium-frequency furnace or medium-frequency induction furnace, cooling and shaping, and silicon slag separation. The core of these processes is to transform silicon-containing materials into usable silicon products through pyrometallurgical smelting and slag-forming.

[0003] For example, CN110078077B discloses a method for preparing metallic silicon from diamond wire-cut silicon sludge using medium-frequency melting. This method involves mixing diamond wire-cut silicon sludge with a desiccant, binder, and water to form granules, drying them, and then melting them in a medium-frequency furnace. During the melting process, magnesium flux and slag agents such as CaO, CaF, and MgO are added. The molten silicon and slag are then poured into a mold for cooling to obtain metallic silicon. The document also discloses a method for controlling the moisture content of the dried silicon sludge particles to be below 10%. This type of technical solution can already recover and prepare silicon from diamond wire-cut silicon sludge using medium-frequency melting.

[0004] Other existing technologies employ medium-frequency induction furnaces for slag refining of silicon cutting waste. For example, Chinese patent document CN109052407A discloses a method where silicon cutting waste is separated into solid and liquid phases and dried, then mixed with a slag-forming agent composed of CaO, SiO2, and flux, and melted at a constant temperature of 1450–1700°C in a medium-frequency induction furnace. After cooling and slag separation, high-purity silicon is obtained. This type of approach demonstrates that medium-frequency induction furnace slag refining has been used for the recycling and purification of silicon cutting waste.

[0005] However, in the actual use of photovoltaic cutting silica sludge to prepare regenerated silicon, the photovoltaic cutting silica sludge is not a silicon source with uniform composition. The content of components such as silicon oxide, iron, aluminum, calcium, boron, and phosphorus may vary between different batches, and even between different particle sizes within the same batch of photovoltaic cutting silica sludge. Conventional overall sampling cannot accurately reflect the compositional baseline of the raw material. If residual silica regulators, slagging agents, or refining agents are added in a fixed proportion and directly smelted in an induction furnace, the slagging and refining processes cannot match the fluctuations in the content of silicon oxide, metallic impurities, and non-metallic impurities in the incoming material. This easily leads to problems such as a mismatch between the amount of residual silica regulator added and the silicon oxide content during the smelting stage, unstable impurity bonding during the slagging stage, and fluctuations in impurity residue during the refining stage.

[0006] Meanwhile, existing medium-frequency furnace smelting processes for silicon production often treat silicon sludge drying, batching, and medium-frequency smelting as continuous steps, typically focusing on overall slag formation or overall smelting. This approach lacks sufficient differentiated control over metallic impurities such as iron, aluminum, and calcium, as well as non-metallic impurities such as boron and phosphorus, making it difficult to adjust the process according to the changing states of different types of impurities during smelting and refining. Especially when the target product is 2202 grade recycled silicon, the limits for iron, aluminum, and calcium are relatively low. If there is a lack of effective coordination between raw material testing before smelting, the in-furnace refining process, and the verification of the silicon melt before casting, it can easily lead to batch-to-batch fluctuations in the composition of the finished product, increasing the need for subsequent remelting or reprocessing.

[0007] Furthermore, fine-grained silicon-containing materials and pulverized components in photovoltaic cutting silicon sludge are easily incorporated into the slag during the smelting process. If the silicon melt and slag cannot be differentiated in stages during the smelting and refining process, the stability of the silicon melt composition before casting can be easily affected. Although existing technologies have disclosed the basic route for smelting silicon cutting waste using medium-frequency furnaces or medium-frequency induction furnaces, there is still a lack of targeted technical solutions on how to establish corresponding refining control processes based on the fluctuations in the content of different types of impurities in photovoltaic cutting silicon sludge.

[0008] Therefore, in the process of refining and preparing regenerated silicon from photovoltaic cutting silicon mud in an intermediate frequency furnace, how to coordinate and control the correction of residual silicon oxide content, remelting of elemental silicon, slag formation, refining, slag removal, and composition verification before casting, based on the differences in the content of silicon oxide, metallic impurities, and non-metallic impurities in photovoltaic cutting silicon mud, so as to stably prepare regenerated silicon of the target grade, is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0009] To overcome the aforementioned technical deficiencies, the present invention aims to provide a method for preparing regenerated silicon by refining photovoltaic cutting silicon sludge in an intermediate frequency furnace. This invention solves the problem in the prior art where fluctuations in the composition of photovoltaic cutting silicon sludge lead to difficulties in coordinating batching, slag formation, refining, slag removal, and composition control before casting. This is achieved through a technical solution that employs particle size distribution analysis—determining the amount of addition based on silicon oxide, metallic impurities, and non-metallic impurities—stage-by-stage melting and refining in an intermediate frequency furnace—stage-by-stage separation of the first and second slags—and pre-casting sampling verification and separate addition of refining agents.

[0010] This invention discloses a method for preparing recycled silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace, comprising the following steps: The photovoltaic cutting silica mud is screened, magnetically separated to remove iron, subjected to negative pressure dust removal and drying to obtain dry silica mud with a moisture content of less than or equal to 1%. Dry silica mud was sampled and tested by particle size fraction to obtain the mass content of silica, metal impurities, and non-metallic impurities. Among them, metal impurities include iron, aluminum, and calcium, and non-metallic impurities include boron and phosphorus. The amount of residual silica regulator added is determined based on the silica mass content; the amount of slag-forming desulfurizer and the first refining agent added is determined based on the metallic impurity mass content; and the amount of the second refining agent added is determined based on the non-metallic impurity mass content. The residual silica regulator is a carbonaceous regulator used to correct fluctuations in silica content in photovoltaic cutting silica sludge. The first refining agent comprises, by mass, 45–70 parts calcium oxide, 20–40 parts silica, 2–8 parts magnesium oxide, and 3–10 parts calcium fluoride; the second refining agent comprises, by mass, 20–45 parts sodium carbonate, 25–50 parts calcium oxide, 10–25 parts silica, and 3–12 parts calcium fluoride. The dry silica sludge, residual silica regulator, and slag-forming desulfurizer are mixed to obtain a mixed furnace charge. The mixed furnace charge is fed into a medium-frequency induction melting furnace, preheated at 800-900℃, and melted at 1300-1450℃ to soften and aggregate the elemental silicon particles in the photovoltaic cutting silicon mud to form a silicon-enriched phase that can be separated from the floating slag phase. At the end of the melting stage, the first refining agent is added, and after settling, the first molten slag floats to the surface and is separated to obtain a silicon-enriched phase that has undergone preliminary impurity removal. The silicon-enriched phase, after preliminary impurity removal, is heated to 1550-1650℃ to form a silicon liquid and refined. The second refining agent is added in two parts, and the power cycle is adjusted after each addition of the second refining agent. After standing, the second slag is separated to obtain the silicon liquid. Before casting, the molten silicon is sampled and tested to determine whether the test results meet the component limit requirements. The component limit requirements include iron content ≤0.20%, aluminum content ≤0.20%, calcium content ≤0.02%, boron content ≤0.01%, and phosphorus content ≤0.01%. If any of the test results exceeds the corresponding limit, the first or second refining agent is added according to the excess item, and the mixture is allowed to stand and separate again. After the test results or the retest results after standing and separating again meet the component limit requirements, the molten silicon is cast, cooled, crushed, and screened to obtain recycled silicon.

[0011] Preferably, the particle size fraction sampling includes dividing the dry silica mud into coarse-grained dry silica mud and fine-grained dry silica mud; detecting the mass content of silicon oxide, iron, aluminum, calcium, boron, and phosphorus in the coarse-grained and fine-grained dry silica mud respectively; and then determining the mass content of silicon oxide, metal impurities, and non-metallic impurities in the dry silica mud according to the mass ratio of coarse-grained and fine-grained dry silica mud.

[0012] Preferably, coarse-grained dry silica mud and fine-grained dry silica mud are sampled and tested three or more times; when the difference between the maximum and minimum test values ​​of any test item in the same particle size dry silica mud is greater than 15% of the arithmetic mean of the multiple test values ​​of that test item, the particle size dry silica mud is remixed and resampled and tested.

[0013] Preferably, determining the amount of residual silica conditioner added includes: When the silica content is 8% to 15% by mass, the amount of residual silica regulator added is 8% to 10% of the mass of the mixed furnace charge; when the silica content is greater than 15% and less than or equal to 25% by mass, the amount of residual silica regulator added is 10% to 12% of the mass of the mixed furnace charge; when the silica content is greater than 25% by mass, the amount of residual silica regulator added is 12% to 14% of the mass of the mixed furnace charge.

[0014] Preferably, the amount of slag-forming desulfurizing agent added is determined as follows: when the mass content of metal impurities is less than or equal to 1.0%, the amount of slag-forming desulfurizing agent added is 4% to 5% of the mass of the mixed furnace charge; when the mass content of metal impurities is greater than 1.0% and less than or equal to 1.8%, the amount of slag-forming desulfurizing agent added is 5% to 6.5% of the mass of the mixed furnace charge; when the mass content of metal impurities is greater than 1.8%, the amount of slag-forming desulfurizing agent added is 6.5% to 8% of the mass of the mixed furnace charge.

[0015] Preferably, determining the amount of the first refining agent includes: when the mass content of metal impurities is less than or equal to 1.0%, the amount of the first refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of metal impurities is greater than 1.0% and less than or equal to 1.8%, the amount of the first refining agent added is 1.0% to 1.6% of the mass of the mixed furnace charge; when the mass content of metal impurities is greater than 1.8%, the amount of the first refining agent added is 1.6% to 2.2% of the mass of the mixed furnace charge.

[0016] Preferably, after adding the first refining agent, the mixture is kept at 1300–1450°C for 5–15 minutes, then allowed to stand for 5–12 minutes, and then the first slag is separated.

[0017] Preferably, after separating the first slag and before heating to 1550~1650℃, intermediate sampling and testing are performed on the silicon-enriched phase after preliminary impurity removal to obtain the mass content of molten iron, molten aluminum, and molten calcium. When at least one of the mass content of molten iron, molten aluminum, and molten calcium exceeds the corresponding limit in the composition limit requirements, the temperature is maintained at 1300~1450℃ and a first refining agent is added. The mass of the added first refining agent is 0.2%~0.6% of the mass of the mixed furnace charge.

[0018] Preferably, determining the amount of the second refining agent includes: when the mass content of non-metallic impurities is less than or equal to 0.01%, the amount of the second refining agent added is 0.3% to 0.6% of the mass of the mixed furnace charge; when the mass content of non-metallic impurities is greater than 0.01% and less than or equal to 0.03%, the amount of the second refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of non-metallic impurities is greater than 0.03%, the amount of the second refining agent added is 1.0% to 1.5% of the mass of the mixed furnace charge.

[0019] Preferably, the first addition of the second refining agent is 40% to 70% of the total mass of the second refining agent, the second addition is the remaining amount of the second refining agent, and the second addition is 5 to 15 minutes after the first addition.

[0020] Compared with existing technologies, the above technical solution has the following advantages: I. This invention obtains the mass content of silicon oxide, metal impurities, and non-metallic impurities by sampling dry-based silica mud in different particle sizes. Based on this, the amount of residual silica regulator, slag-forming desulfurizer, first refining agent, and second refining agent to be added is determined. This allows the batching process to match the actual composition fluctuations of photovoltaic cutting silica mud, avoiding the problems of mismatched addition of residual silica regulator, insufficient slag formation, or unsuitable addition of refining agent caused by raw material fluctuations when using a fixed ratio. This is beneficial for the stable preparation of target grade recycled silicon.

[0021] Second, this invention distinguishes between metallic impurities such as iron, aluminum, and calcium and non-metallic impurities such as boron and phosphorus. At the end of the melting stage, a first refining agent is added and the first slag is separated. In the high-temperature refining stage, a second refining agent is added in stages and the second slag is separated. This allows different types of impurities to enter the corresponding slag system at different temperature stages, which is beneficial to improving the control accuracy of impurities such as iron, aluminum, calcium, boron, and phosphorus in regenerated silicon.

[0022] Third, based on the addition of the second refining agent in two stages, the present invention performs power cycle adjustment with alternating high-power and low-power stages after each addition. This causes induced disturbance in the silicon melt during the high-power stage and reduces melt entrainment during the low-power stage. This is beneficial for the migration of non-metallic impurities and their reaction products to the slag-liquid interface and for maintaining a stratified state, thereby improving the removal stability of non-metallic impurities such as boron and phosphorus.

[0023] Fourth, this invention samples and tests the molten silicon before casting. If any of the components of iron, aluminum, calcium, boron, or phosphorus exceeds the target grade control index, a first or second refining agent is added according to the excess component, and the mixture is allowed to stand and separate again. After retesting and confirming that the target grade control index is met, casting, cooling, crushing, and screening are carried out. This allows the component verification to be performed before casting and forming, reducing the probability of the finished product being returned to the furnace or reprocessed.

[0024] V. This invention establishes a furnace number for each production furnace and assigns the raw material test results, batching parameters, refining agent addition amount, pre-casting test results, and finished product test results to the same furnace number. When the current production furnace has metal impurities or non-metal impurities exceeding the limit, the refining agent addition amount and holding time of the next production furnace are adjusted accordingly. At the same time, blocky recycled silicon, fine-grained recycled silicon, and silicon-containing dust of different excess types are recycled separately, which is beneficial to improve the utilization rate of silicon materials and improve the batch consistency of recycled silicon composition in continuous production. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall process flow of a method for preparing recycled silicon by refining photovoltaic cutting silicon mud in an intermediate frequency furnace according to the present invention.

[0026] Figure 2 This is a timing diagram of the power cycle adjustment during the refining stage of the intermediate frequency furnace in this invention.

[0027] Figure 3 This is a block diagram of dynamic proportioning and cross-production furnace feedback control based on detection results in this invention.

[0028] Figure 4 This is a comparison of X-ray diffraction patterns of photovoltaic cutting silicon mud raw materials and recycled silicon finished products of this invention.

[0029] Figure 5 This is a schematic diagram of the microstructure of the recycled silicon product of this invention.

[0030] Figure 6 This is a cross-sectional schematic diagram of the slag-liquid separation state inside the medium-frequency induction melting furnace of the present invention.

[0031] Figure 7 This is a comparative bar chart showing the refining and impurity removal effects of the present invention. Detailed Implementation

[0032] The following is in conjunction with the accompanying drawings. Figures 1 to 7 This method will be explained in detail.

[0033] This method is used to prepare recycled silicon from photovoltaic cutting silicon sludge as silicon-containing raw material. The core processing targets are elemental silicon, silicon oxide, and components such as iron, aluminum, calcium, boron, and phosphorus in the photovoltaic cutting silicon sludge. Silicon in the photovoltaic cutting silicon sludge mainly exists in the form of elemental silicon particles, while silicon oxide participates as an accompanying component in batching correction and slag system adjustment. This method revolves around the process of remelting elemental silicon, refining and impurity removal, slag separation, and casting in a medium-frequency induction melting furnace to obtain recycled silicon of the target grade.

[0034] In one implementation, such as Figure 1 As shown, the photovoltaic cutting silica mud is first screened, magnetically separated to remove iron, subjected to negative pressure dust removal, and dried at low temperature to obtain dry silica mud with a moisture content of less than or equal to 1%. Screening can be performed using a combination of coarse and fine screens. The coarse screen is used to remove large agglomerates, lining material fragments, or other mechanical inclusions, while the fine screen is used to obtain silica mud particle sizes suitable for subsequent drying, sampling, and batching. Magnetic separation to remove iron can be performed using a high-intensity magnetic separator to remove iron filings, iron grinding debris, and magnetic metal particles. Negative pressure dust removal is performed before and after screening, transfer, and drying to collect fine silica-containing dust and reduce the loss of pulverized material. Drying is carried out using a hot air circulation method, and the drying temperature can be controlled between 75 and 115℃. During the drying process, the moisture content of the photovoltaic cutting silica mud is monitored. If the moisture content is greater than 1%, drying continues until dry silica mud with a moisture content of less than or equal to 1% is obtained.

[0035] After obtaining the dry silica mud, it is sampled and graded by particle size. Specifically, the dry silica mud is divided into coarse-grained dry silica mud and fine-grained dry silica mud. Both coarse-grained and fine-grained dry silica mud are sampled and tested at least three times. The tests include the mass content of silicon dioxide, iron, aluminum, calcium, boron, and phosphorus. The coarse-grained and fine-grained dry silica mud can be further divided according to the sieve particle size range. For example, particles larger than a preset sieve aperture size are classified as coarse-grained dry silica mud, and particles smaller than or equal to the preset sieve aperture size are classified as fine-grained dry silica mud.

[0036] To improve sampling representativeness, if the difference between the maximum and minimum test values ​​for any test item in the same particle size dry silica mud exceeds 15% of the arithmetic mean of multiple test values ​​for that item, the dry silica mud of that particle size should be remixed and resampled for testing. This judgment process can be expressed by the following calculation formula:

[0037]

[0038]

[0039]

[0040] in, This represents the difference between the maximum and minimum detected values ​​of a certain test item in dry silica mud of the same particle size, expressed in % (%). This represents the maximum value among three or more tests for this test item, expressed in % (%). This represents the minimum value among three or more tests for this test item, expressed in % (%). This represents the arithmetic mean of three test results for this item, expressed in % (%). , and These represent the first, second, and third test values ​​for the same test item, respectively, in percentage (%). The above calculations are used to determine whether samples of the same particle size need to be remixed and resampled for testing.

[0041] When determining the overall composition of dry-based silica mud, a weighted calculation is performed based on the mass ratio of coarse-grained and fine-grained dry-based silica mud. Taking the mass content of silica as an example, the following formula can be used for calculation:

[0042]

[0043] in, This indicates the silica content by mass of the dry-based silica mud, expressed in % (%). This indicates the silica content (by mass) in coarse-grained dry silica mud, expressed in % (%). This indicates the silica content (by mass) in fine-grained dry silica mud, expressed in % (%). This indicates the mass of coarse-grained dry silica mud, in kg. This indicates the mass of fine-grained dry silica mud, expressed in kg. The mass content of iron, aluminum, calcium, boron, and phosphorus can also be determined using the same weighting method. The mass content of metallic impurities is the sum of the mass contents of iron, aluminum, and calcium, while the mass content of non-metallic impurities is the sum of the mass contents of boron and phosphorus.

[0044] For example, if a batch of dry silica mud contains 600 kg of coarse-grained dry silica mud and 400 kg of fine-grained dry silica mud, with a silica content of 9.2% by mass for the coarse-grained dry silica mud and 12.5% ​​by mass for the fine-grained dry silica mud, then the silica content of this batch of dry silica mud is:

[0045]

[0046] Therefore, the amount of residual silica regulator to be added for this batch of dry silica mud can be determined based on a silica mass content of 10.52%.

[0047] In one embodiment, the amount of residual silica regulator added is determined based on the silica mass content. When the silica mass content is 8%–15%, the amount of residual silica regulator added is 8%–10% of the mass of the mixed furnace charge; when the silica mass content is greater than 15% and less than or equal to 25%, the amount of residual silica regulator added is 10%–12% of the mass of the mixed furnace charge; when the silica mass content is greater than 25%, the amount of residual silica regulator added is 12%–14% of the mass of the mixed furnace charge. The residual silica regulator can be one or more of petroleum coke powder, charcoal powder, and graphite powder, with a fixed carbon mass content greater than 85% and a particle size of 100–300 mesh. The purpose of adding the residual silica regulator is to correct the fluctuations in silica content in photovoltaic cutting silicon sludge and to adjust the local carbon potential and slag state during the melting and refining process; it is not used to completely reduce silica from natural silica as the main silicon source through carbothermic reduction.

[0048] In one embodiment, the amount of slag-forming desulfurizing agent and the amount of the first refining agent added are determined based on the mass content of metallic impurities. When the mass content of metallic impurities is less than or equal to 1.0%, the amount of slag-forming desulfurizing agent added is 4% to 5% of the mass of the mixed furnace charge, and the amount of the first refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of metallic impurities is greater than 1.0% and less than or equal to 1.8%, the amount of slag-forming desulfurizing agent added is 5% to 6.5% of the mass of the mixed furnace charge, and the amount of the first refining agent added is 1.0% to 1.6% of the mass of the mixed furnace charge; when the mass content of metallic impurities is greater than 1.8%, the amount of slag-forming desulfurizing agent added is 6.5% to 8% of the mass of the mixed furnace charge, and the amount of the first refining agent added is 1.6% to 2.2% of the mass of the mixed furnace charge. The slag-forming desulfurizing agent may include one or more of calcium oxide, calcium carbonate, calcium fluoride, and sodium carbonate, and its particle size may be 100 to 300 mesh. The first refining agent comprises, by weight, 45-70 parts calcium oxide, 20-40 parts silicon dioxide, 2-8 parts magnesium oxide, and 3-10 parts calcium fluoride. Each component can be made of industrial-grade powder with a purity of 95% or higher and a particle size of 100-300 mesh. It should be dried before use.

[0049] In this embodiment, the first refining agent is used to promote the incorporation of metallic impurities such as iron, aluminum, and calcium, as well as oxide inclusions, into the first molten slag. Its mechanism of action is as follows: at the end of the melting stage (1300–1450°C), the elemental silicon particles have softened, aggregated, and formed a silicon-enriched phase. The calcium oxide, silicon dioxide, and magnesium oxide in the first refining agent form a multi-component slag system, which absorbs and enriches iron-, aluminum-, and calcium-containing inclusions, as well as oxide inclusions. (The main reaction equations include:) or ; The generated metal oxides are then absorbed by the multi-component slag system composed of calcium oxide, silicon dioxide, and magnesium oxide, reacting to form a composite silicate phase that enters the first molten slag. The calcium fluoride in the first refining agent is used to lower the melting point of the composite slag system and significantly improve the slag fluidity, thereby promoting the full flotation and separation of the first molten slag system during the settling process.

[0050] In one embodiment, the amount of the second refining agent added is determined based on the mass content of non-metallic impurities. When the mass content of non-metallic impurities is less than or equal to 0.01%, the amount of the second refining agent added is 0.3% to 0.6% of the mass of the mixed furnace charge; when the mass content of non-metallic impurities is greater than 0.01% and less than or equal to 0.03%, the amount of the second refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of non-metallic impurities is greater than 0.03%, the amount of the second refining agent added is 1.0% to 1.5% of the mass of the mixed furnace charge. The second refining agent comprises, by mass, 20 to 45 parts of sodium carbonate, 25 to 50 parts of calcium oxide, 10 to 25 parts of silicon dioxide, and 3 to 12 parts of calcium fluoride. Each component can be an industrial-grade powder with a purity of 95% or higher and a particle size of 100 to 300 mesh. After mixing, it is sealed and stored, and dried before use.

[0051] In this embodiment, the second refining agent is mainly used for the deep removal of non-metallic impurities such as boron and phosphorus, which have low separation coefficients and are difficult to remove conventionally. Its impurity removal chemical principle lies in the high-temperature refining stage at 1550–1650°C, where sodium carbonate in the second refining agent undergoes high-temperature decomposition (the main reaction equation is: This provides an extremely high local alkalinity and weak oxidizing environment at the slag-liquid interface. The boron and phosphorus elements dissociated in the silica melt are oxidized to their corresponding oxides (reaction equation: ...). ; ), and quickly react with strong alkalinity A slag-forming reaction occurs, producing a thermodynamically stable complex of sodium borate and sodium phosphate (the main reaction equation is: ; The aforementioned stable salts can be firmly locked and enriched in the second slag. Combined with the alternating adjustment of high and low power cycles, secondary entrainment is effectively avoided, thereby improving the removal stability of non-metallic impurities such as boron and phosphorus.

[0052] After determining the amounts of each additive, the dry silica mud, residual silica modifier, and slag-forming desulfurizer are mixed to obtain a mixed furnace charge. During mixing, the residual silica modifier and slag-forming desulfurizer can be pre-mixed before being mixed with the dry silica mud. The mixing time can be 6–12 minutes. The mixed furnace charge can be fed into a medium-frequency induction melting furnace through a closed feeding device.

[0053] When starting up a medium-frequency induction melting furnace, the initial output power should be set to 50% of the rated power, and the furnace should maintain a stable power increase operation until the material temperature reaches 800℃. The initial output power can be determined using the following formula:

[0054] in, This indicates the initial output power of the medium-frequency induction melting furnace, in kW. This indicates the rated power of the medium-frequency induction melting furnace, in kW.

[0055] like Figure 1 and Figure 6 As shown, after the mixed charge is fed into the medium-frequency induction melting furnace, it is first preheated at 800–900℃ to remove residual moisture and low-boiling-point volatile components. Then, it is melted at 1300–1450℃ to bring the mixed charge into a molten state. At the end of the melting stage, the first refining agent is added. After adding the first refining agent, the furnace is held at 1300–1450℃ for 5–15 minutes, and then allowed to stand for 5–12 minutes to allow the first slag to form and float to the surface. The first slag is then separated to obtain a silicon-containing elemental melt. After the first slag is separated, the furnace enters a refining stage at 1550–1650℃.

[0056] It should be noted that the silicon element in the photovoltaic cutting silicon sludge processed by this method mainly exists in the form of elemental silicon particles. The final stage of the 1300–1450℃ melting phase is not the reaction stage of complete carbothermic reduction of silicon dioxide in traditional silica to generate elemental silicon, but rather a stage in which the elemental silicon particles in the photovoltaic cutting silicon sludge soften, aggregate, and form a silicon-enriched phase. The purpose of adding the first refining agent in this stage is to preferentially introduce metallic impurities such as iron, aluminum, and calcium, as well as oxide inclusions, into the first slag, and to allow the first slag to float and separate during the settling process. After separating the first slag, the silicon-enriched phase, after preliminary impurity removal, continues to be heated to 1550–1650℃ to form a silicon melt and undergoes high-temperature refining. The second refining agent is used in this stage to further control non-metallic impurities such as boron and phosphorus, as well as residual inclusions.

[0057] During the refining stage at 1550–1650℃, the second refining agent is added in two stages. The first addition is 40%–70% of the total mass of the second refining agent, and the second addition is the remaining amount. The second addition is done 5–15 minutes after the first addition. Power cycle adjustments are performed after each addition of the second refining agent. Figure 2 As shown, the power cycle adjustment includes a high-power stage and a low-power stage. The output power of the high-power stage is 85%–100% of the rated power of the medium-frequency induction melting furnace, lasting 3–8 minutes; the output power of the low-power stage is 60%–80% of the rated power, lasting 2–6 minutes. When the mass content of non-metallic impurities is less than or equal to 0.01%, two power cycle adjustments are performed; when the mass content of non-metallic impurities is greater than 0.01% but less than or equal to 0.03%, three power cycle adjustments are performed; and when the mass content of non-metallic impurities is greater than 0.03%, four power cycle adjustments are performed. The high-power stage generates strong induced disturbance in the molten silicon, while the low-power stage reduces the melt entrainment intensity, keeping inclusions and reaction products that have entered the slag-molten interface floating. After the second refining agent is added in stages and the power cycle is adjusted, the molten silicon is obtained by settling and separating the second slag.

[0058] Before casting, the molten silicon is sampled and tested for iron, aluminum, calcium, boron, and phosphorus content. Target grade control indicators include: iron content ≤ 0.20%, aluminum content ≤ 0.20%, calcium content ≤ 0.02%, boron content ≤ 0.01%, and phosphorus content ≤ 0.01%. If at least one of iron, aluminum, or calcium exceeds its corresponding limit, a first refining agent is added; if boron or phosphorus exceeds its corresponding limit, a second refining agent is added. After adding the refining agent, the mixture is held at 1550–1650℃ for 5–15 minutes, then allowed to stand for another 8–15 minutes, and the third slag is separated. Once the test results or the retest results after the second standing and stratification meet the target grade control indicators, the molten silicon is cast, cooled, crushed, and screened to obtain recycled silicon.

[0059] The overall silicon recovery rate can be determined using the following formula:

[0060]

[0061] in, The overall silicon recovery rate is expressed as % (%). This indicates the mass of recycled silicon obtained in this production batch, in kg. This indicates the silicon content by mass in recycled silicon, expressed as % (%). This indicates the mass of dry silica mud added in this production batch, in kg. This indicates the silicon content by mass in dry silica mud, expressed in % (%). This indicates the mass of silicon-containing recycled material added in this production batch, in kg. This indicates the silicon content (%) in the silicon-containing recycled material. When no silicon-containing recycled material is added in a particular production batch... The value is 0.

[0062] In one implementation, the process of performing the above steps is defined as a production furnace run. A furnace run number is established for each production furnace run, and the mass content of silica in the dry-based silica mud, the mass content of metallic impurities, the mass content of non-metallic impurities, the amount of residual silica modifier added, the amount of slag-forming desulfurizing agent added, the amount of the first refining agent added, the amount of the second refining agent added, the sampling and testing results before casting, and the test results of the regenerated silica composition are all attributed to the same furnace run number. For example... Figure 3 As shown, when at least one of iron, aluminum, and calcium in the recycled silicon of the current production batch exceeds the corresponding limit, the amount of the first refining agent added in the next production batch will be increased by 0.2 to 0.6 percentage points; when boron or phosphorus in the recycled silicon of the current production batch exceeds the corresponding limit, the amount of the second refining agent added in the next production batch will be increased by 0.1 to 0.4 percentage points, and the holding time at 1550 to 1650°C in the next production batch will be extended by 5 to 10 minutes.

[0063] In one embodiment, after crushing and screening, lumpy recycled silicon, fine-grained recycled silicon, and silicon-containing dust are obtained. When at least one of iron, aluminum, and calcium in the lumpy or fine-grained recycled silicon exceeds the corresponding limit, the corresponding lumpy or fine-grained recycled silicon is used as the first silicon-containing return material and incorporated into the mixed charge of the next production furnace. When boron or phosphorus in the lumpy or fine-grained recycled silicon exceeds the corresponding limit, the corresponding lumpy or fine-grained recycled silicon is used as the second silicon-containing return material and incorporated into the refining stage at 1550–1650°C of the next production furnace. The silicon-containing dust is incorporated into dry-based silica sludge after magnetic separation to remove iron and moisture content testing, and the moisture content of the silicon-containing dust before incorporation is less than or equal to 1%.

[0064] Example 1

[0065] This embodiment uses photovoltaic cutting silica mud with low silica and low impurity content as raw material. After screening, magnetic separation for iron removal, negative pressure dust removal, and low-temperature drying, the photovoltaic cutting silica mud yields dry-based silica mud with a moisture content of 0.8%. After particle size fractionation sampling, coarse-grained dry-based silica mud accounts for 62% of the mass, and fine-grained dry-based silica mud accounts for 38%. After three sampling tests, the difference between the maximum and minimum test values ​​for each test item in both coarse-grained and fine-grained dry-based silica mud is less than or equal to 15% of the corresponding arithmetic mean, eliminating the need for resampling and testing.

[0066] In this embodiment, the silica content of the dry-based silica sludge, calculated by mass percentage, is 10.5%, the metallic impurity content is 0.80%, and the non-metallic impurity content is 0.008%. Accordingly, the amount of residual silica regulator added is determined to be 9.0% of the mixed furnace charge mass, the amount of slag-forming desulfurizing agent added is determined to be 4.5% of the mixed furnace charge mass, the amount of the first refining agent added is determined to be 0.8% of the mixed furnace charge mass, and the amount of the second refining agent added is determined to be 0.4% of the mixed furnace charge mass. The first refining agent, by mass, comprises 55 parts calcium oxide, 30 parts silica, 5 parts magnesium oxide, and 6 parts calcium fluoride. The second refining agent, by mass, comprises 28 parts sodium carbonate, 42 parts calcium oxide, 20 parts silica, and 6 parts calcium fluoride.

[0067] This embodiment uses 1000kg of mixed furnace charge as the calculation basis. The amount of residual silica regulator added is 90kg, the amount of slag-forming desulfurizing agent added is 45kg, the amount of the first refining agent added is 8kg, and the amount of the second refining agent added is 4kg. After the mixed furnace charge is fed into the medium-frequency induction melting furnace, it is preheated at 850℃ for 30min and melted at 1380℃ for 20min. At the end of the melting stage, the first refining agent is added, and the temperature is held at 1380℃ for 10min, then allowed to stand for 8min to separate the first slag, obtaining a silicon-containing elemental melt. Subsequently, the temperature is raised to 1600℃ for refining. The second refining agent is added in two parts. The first addition is 60% of the total mass of the second refining agent, and the second addition is the remaining amount of the second refining agent. The second addition is 10min after the first addition. Power cycle adjustment is performed after each addition of the second refining agent. Since the mass content of non-metallic impurities is less than or equal to 0.01%, two power cycle adjustments are performed. In each power cycle, the high-power phase is 90% of the rated power and lasts for 5 minutes, while the low-power phase is 70% of the rated power and lasts for 4 minutes. After refining, the mixture is allowed to stand for 12 minutes, and the second slag is separated to obtain molten silicon.

[0068] Pre-cast sampling and testing showed that the silicon molten metal contained 0.150% iron, 0.120% aluminum, 0.011% calcium, 0.005% boron, and 0.004% phosphorus, all meeting the target grade control indicators. The silicon molten metal was then cast, naturally cooled, crushed, and screened to obtain recycled silicon. The overall recovery rate of the recycled silicon obtained in this embodiment was 88.5%. Silicon-containing dust generated during crushing and screening was collected and its moisture content was tested to be 0.5%. After magnetic separation to remove iron, it was used as raw material in the next batch of dry-based silicon sludge for recycling.

[0069] Example 2

[0070] This embodiment uses photovoltaic cutting silica mud with medium silica and medium impurity content as raw material. After screening, magnetic separation for iron removal, negative pressure dust removal, and low-temperature drying, the photovoltaic cutting silica mud is obtained as a dry-basis silica mud with a moisture content of 0.6%. After particle size fractionation sampling, the coarse-grained dry-basis silica mud accounts for 55% of the mass, and the fine-grained dry-basis silica mud accounts for 45%. After three sampling tests, the difference between the maximum and minimum detected values ​​of boron mass content in the fine-grained dry-basis silica mud was greater than 15% of the arithmetic mean of this test item. Therefore, the fine-grained dry-basis silica mud was remixed and resampled for testing. After retesting, the fluctuations in each test item met the sampling requirements.

[0071] In this embodiment, the silica content of the dry-based silica sludge, calculated by mass percentage, is 20.2%, the metallic impurity content is 1.50%, and the non-metallic impurity content is 0.025%. Accordingly, the amount of residual silica regulator added is determined to be 11.0% of the mixed furnace charge mass, the amount of slag-forming desulfurizing agent added is determined to be 6.0% of the mixed furnace charge mass, the amount of the first refining agent added is determined to be 1.3% of the mixed furnace charge mass, and the amount of the second refining agent added is determined to be 0.8% of the mixed furnace charge mass. The first refining agent, by mass, comprises 60 parts calcium oxide, 28 parts silica, 6 parts magnesium oxide, and 6 parts calcium fluoride. The second refining agent, by mass, comprises 35 parts sodium carbonate, 35 parts calcium oxide, 20 parts silica, and 8 parts calcium fluoride.

[0072] This embodiment uses 1000 kg of mixed furnace charge as the calculation basis. The amount of residual silica regulator added is 110 kg, the amount of slag-forming desulfurizing agent added is 60 kg, the amount of the first refining agent added is 13 kg, and the amount of the second refining agent added is 8 kg. After the mixed furnace charge is fed into the medium-frequency induction melting furnace, it is preheated at 880℃ for 35 min and melted at 1400℃ for 25 min. At the end of the melting stage, the first refining agent is added, and the temperature is held at 1400℃ for 12 min, then allowed to stand for 10 min. The first slag is separated to obtain silicon-containing elemental melt. Before heating to the high-temperature refining stage, intermediate sampling and testing of the silicon-containing elemental melt are performed. The test results show that iron, aluminum, and calcium do not exceed the corresponding limits, so no additional first refining agent is added.

[0073] The silicon-containing melt was then heated to 1620℃ for refining. The second refining agent was added in two stages: 55% of its total mass was added in the first stage, and the remaining amount was added in the second stage, 12 minutes after the first addition. Because the non-metallic impurity content was greater than 0.01% and less than or equal to 0.03%, three power cycles were performed. In each power cycle, the high-power phase was 92% of the rated power for 6 minutes, and the low-power phase was 68% of the rated power for 4 minutes. After refining, the mixture was allowed to stand for 15 minutes, and the second slag was separated to obtain molten silicon.

[0074] Pre-cast sampling and testing showed that the silicon melt contained 0.180% iron, 0.160% aluminum, 0.015% calcium, 0.008% boron, and 0.007% phosphorus, all meeting the target grade control indicators. The silicon melt was then cast, naturally cooled, crushed, and screened to obtain recycled silicon. The overall recovery rate of the recycled silicon obtained in this example was 87.2%.

[0075] Example 3

[0076] This embodiment uses photovoltaic cutting silica mud with high silica and high impurity content as raw material. After screening, magnetic separation for iron removal, negative pressure dust removal, and low-temperature drying, the photovoltaic cutting silica mud yields dry-basis silica mud with a moisture content of 0.9%. After particle size fractionation sampling, coarse-grained dry-basis silica mud accounts for 48% of the mass, and fine-grained dry-basis silica mud accounts for 52%. After three sampling tests and calculations based on mass percentages, the dry-basis silica mud contains 28.5% silica, 2.10% metallic impurities, and 0.045% non-metallic impurities.

[0077] Accordingly, the amount of residual silica regulator added is determined to be 13.5% of the mass of the mixed furnace charge, the amount of slag-forming desulfurizing agent added is determined to be 7.5% of the mass of the mixed furnace charge, the amount of the first refining agent added is determined to be 1.8% of the mass of the mixed furnace charge, and the amount of the second refining agent added is determined to be 1.2% of the mass of the mixed furnace charge. The first refining agent, by mass, includes 65 parts of calcium oxide, 25 parts of silica, 6 parts of magnesium oxide, and 8 parts of calcium fluoride. The second refining agent, by mass, includes 42 parts of sodium carbonate, 32 parts of calcium oxide, 18 parts of silica, and 8 parts of calcium fluoride.

[0078] This embodiment uses 1000 kg of mixed furnace charge as the calculation basis. The amount of residual silica regulator added is 135 kg, the amount of slag-forming desulfurizing agent added is 75 kg, the amount of the first refining agent added is 18 kg, and the amount of the second refining agent added is 12 kg. After the mixed furnace charge is fed into the medium-frequency induction melting furnace, it is preheated at 900℃ for 40 min and melted at 1450℃ for 30 min. At the end of the melting stage, the first refining agent is added, and the temperature is held at 1450℃ for 15 min, then allowed to stand for 12 min. The first slag is then separated to obtain a silicon-containing elemental melt.

[0079] After separating the first slag and before heating to 1550–1650°C, intermediate sampling and testing of the silicon-containing melt were conducted. The test showed that the calcium content was 0.023%, exceeding the limit of less than or equal to 0.02%. Therefore, the temperature was maintained at 1450°C, and a first refining agent was added. The mass of the added first refining agent was 0.4% of the mass of the mixed furnace charge. After adding the first refining agent, the temperature was held for 8 minutes and then allowed to stand for 8 minutes. After separating the slag again, the temperature was raised to 1650°C for high-temperature refining.

[0080] During the refining stage at 1650℃, the second refining agent was added in two stages. The first stage added 70% of the total mass of the second refining agent, and the second stage added the remaining amount, 15 minutes after the first addition. Because the non-metallic impurity content was greater than 0.03%, four power cycles were performed. In each power cycle, the high-power phase was 96% of the rated power for 8 minutes, and the low-power phase was 65% of the rated power for 6 minutes. After refining, the mixture was allowed to stand for 15 minutes, and the second slag was separated to obtain molten silicon.

[0081] Pre-cast sampling and testing showed that the boron content in the molten silicon was 0.012%, exceeding the limit of less than or equal to 0.01%. Therefore, a second refining agent was added, at a mass of 0.3% of the mixed furnace charge. After addition, the mixture was held at 1620℃ for 10 minutes, then allowed to stand for another 12 minutes, and the third slag was separated. Retesting showed that the molten silicon contained 0.190% iron, 0.170% aluminum, 0.018% calcium, 0.009% boron, and 0.008% phosphorus, all meeting the target grade control indicators. The molten silicon was then cast, naturally cooled, crushed, and screened to obtain recycled silicon. The overall recovery rate of recycled silicon obtained in this example was 86.8%.

[0082] Cross-production furnace feedback implementation

[0083] This embodiment illustrates the feedback control between different production furnaces in a continuous production process. The process of continuously executing the above steps is denoted as production furnace A1, production furnace A2, and production furnace A3, respectively. Pre-cast sampling and testing of production furnace A1 showed an iron content of 0.220%, exceeding the limit of less than or equal to 0.20%. Therefore, the corresponding blocky or fine-grained recycled silicon from production furnace A1 was used as the first silicon-containing return material and incorporated into the mixed charge of production furnace A2. Simultaneously, the amount of the first refining agent added in production furnace A2 was increased by 0.3 percentage points from the original set value. Pre-cast sampling and testing of production furnace A2 showed a boron content of 0.012%, exceeding the limit of less than or equal to 0.01%. Therefore, the corresponding blocky or fine-grained recycled silicon from production furnace A2 was used as the second silicon-containing return material and incorporated into the 1550–1650℃ refining stage of production furnace A3. Simultaneously, the amount of the second refining agent added to production furnace A3 was increased by 0.2 percentage points from the original set value, and the holding time at 1550–1650℃ was extended by 8 minutes. Testing of production furnace A3 showed that all indicators met the target grade control indicators.

[0084] Comparative Example

[0085] Comparative Example 1 used photovoltaic cutting silica sludge from the same source as in Example 2, but without particle size fractionation sampling, and instead of determining the addition amounts based on silica mass content, metallic impurity mass content, and non-metallic impurity mass content, a fixed proportion of ingredients was used. A common slagging agent was added all at once in a medium-frequency induction melting furnace, and the product was directly cast after melting. The finished product obtained in Comparative Example 1 contained 0.350% iron, 0.250% aluminum, 0.045% calcium, 0.018% boron, and 0.022% phosphorus, failing to meet the target grade control indicators.

[0086] Comparative Example 2 used photovoltaic-cut silicon sludge from the same source as in Example 2, and employed the same particle size fractionation sampling and batching methods. However, the second refining agent was added all at once, and no power cycle adjustment was performed. The finished product obtained in Comparative Example 2 contained 0.160% iron, 0.140% aluminum, 0.018% calcium, 0.025% boron, and 0.028% phosphorus by mass. This result shows that although the metal impurity indicators met the corresponding limits, boron and phosphorus did not.

[0087] Data table

[0088] Table 1 shows the fluctuations in raw material composition and dynamic batching across multiple batches. This table illustrates the different residual silica regulators, first refining agent, second refining agent, and power cycle adjustment times corresponding to different silica mass contents, metallic impurity mass contents, and non-metallic impurity mass contents.

[0089] Table 1 Raw Material Characteristics and Refining Parameters

[0090]

[0091] Table 2 is the feedback control tracking table across production furnaces. This table explains how the test results of the current production furnace affect the amount of refining agent added, the holding time, and the destination of returned materials in the next production furnace.

[0092] Table 2 Production Furnace Information Table

[0093]

[0094] Table 3 is a comparison table of the test results of the core indicators of the finished products. This table is used to illustrate the differences in the indicators of the finished products under different embodiments and comparative examples.

[0095] Table 3. Element Content and Recovery Rate

[0096]

[0097] Table 4 shows the staged test results of the first, second, and third slags in Example 3. This table illustrates the differences in impurity composition of the slags produced at different refining stages, thus supporting the staged use of the first refining agent, the second refining agent, and the refining agent added before casting.

[0098] Table 4. Composition of Slag Samples

[0099]

[0100] As shown in Tables 1 to 4, under different conditions of silicon dioxide content, metallic impurity content, and non-metallic impurity content, the implementation method of fractional sampling, dynamically determining the addition amount, separating the first slag, adding the second refining agent in stages, adjusting the power cycle, and sampling and verifying before casting can ensure that the mass content of iron, aluminum, calcium, boron, and phosphorus in the recycled silicon meets the target grade control indicators. Compared with the comparative example of fixed-ratio batching and direct induction furnace melting, the finished product composition of the examples shows less fluctuation and a higher overall silicon recovery rate; compared with the comparative example without power cycle adjustment, the examples show more stable control over boron and phosphorus.

[0101] Table 5. Effect of the timing of adding the first refining agent on the separation state of the first slag.

[0102] As shown in Table 5, when the first refining agent was added at 1200℃, the silicon particles were not fully softened and aggregated, resulting in a mixture of slag and semi-molten charge and a low separation rate of the first slag. When the first refining agent was added at the initial melting stage at 1300℃, although a silicon-enriched phase was formed locally, significant entrainment still existed. In Examples 1 to 3, the first refining agent was added at the end of the melting stage. At this point, the elemental silicon particles in the photovoltaic cutting silicon sludge had softened and aggregated to form a silicon-enriched phase. After settling, the first slag could continuously float to the surface, and the separation rate of the first slag was 88.7%–93.2%. Therefore, adding the first refining agent at the end of the melting stage at 1300–1450℃ and separating the first slag is not based on the premise of complete carbothermic reduction of silicon dioxide, but rather on the formation of a silicon-enriched phase from the elemental silicon raw material and the subsequent floating and separation of the slag phase.

[0103] like Figure 4 As shown in the X-ray diffraction comparison of photovoltaic-cut silicon sludge raw materials and recycled silicon products, the photovoltaic-cut silicon sludge raw materials exhibit impurity peaks corresponding to silicon oxide and other impurities in addition to silicon phase diffraction peaks; the spectra of the recycled silicon products mainly show characteristic silicon phase diffraction peaks, with a decrease in the number and intensity of impurity peaks. This spectrum can be used to illustrate that after pyrometallurgical smelting, staged refining, and multiple slag separations, the impurity phase components in silicon-containing raw materials are reduced.

[0104] like Figure 5 As shown, Figure 5 The left side shows the surface morphology of blocky regenerated silicon. Figure 5 The right side shows the cross-sectional morphology of fine-grained regenerated silicon. Both images use a scale of 10 μm. The left image shows the interparticle gaps, local pits, and surface texture on the surface of the bulk regenerated silicon. The right image shows the cross-sectional texture formed after the fine-grained regenerated silicon was crushed and sieved, as well as localized particle agglomeration areas. Because the left image corresponds to the surface area of ​​the bulk regenerated silicon, and the right image corresponds to the cross-sectional area of ​​the fine-grained regenerated silicon, the two images exhibit different particle sizes and morphological characteristics under the same 10 μm scale due to the different objects of observation and fracture states.

[0105] right Figure 5 When performing SEM-EDS spot or area scanning on the areas shown, the detection areas can be the surface texture area and interparticle gap area of ​​the blocky regenerated silicon on the left, and the cross-sectional texture area and particle agglomeration area of ​​the fine-grained regenerated silicon on the right. The detection results can be used to demonstrate that these areas are mainly composed of silicon, with low levels of inclusions such as oxygen, iron, aluminum, and calcium, thus proving that visible inclusions in the finished regenerated silicon product are reduced after staged refining and multiple slag separations. The control of low boron and phosphorus content should not rely solely on EDS detection; ICP-OES or ICP-MS detection results should be used as the basis for composition control.

[0106] like Figure 6 As shown, in the slag-liquid separation state after settling in the medium-frequency induction melting furnace, the slag layer is located above the silicon liquid layer, which illustrates the feasibility of separating the first, second, and third slags. The first, second, and third slags can be removed from the upper layer of silicon liquid through the slag discharge port or by skimming, and then proceed to subsequent refining or casting steps.

[0107] like Figure 7 As shown in the bar chart comparing the refining and impurity removal effects, the residual contents of iron, aluminum, calcium, boron, and phosphorus in the embodiment are all lower than those in the comparative example, and the residual contents of boron and phosphorus are lower than those in the comparative example without power cycle adjustment. This chart corresponds to the data in Table 3 and is used to illustrate the synergistic effect of the phased addition of the second refining agent and the power cycle adjustment on the control of non-metallic impurities.

[0108] It should be noted that the embodiments of the present invention have better implementability and are not intended to limit the present invention in any way. Any person skilled in the art may use the above-disclosed technical content to change or modify it into equivalent effective embodiments. However, any modifications or equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing recycled silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace, characterized in that, Includes the following steps: The photovoltaic cutting silica mud is screened, magnetically separated to remove iron, subjected to negative pressure dust removal and drying to obtain dry silica mud with a moisture content of less than or equal to 1%. The dry silica mud was sampled and tested by particle size fraction to obtain the mass content of silicon oxide, the mass content of metal impurities, and the mass content of non-metallic impurities. The metal impurities included iron, aluminum, and calcium, and the non-metallic impurities included boron and phosphorus. The amount of residual silica regulator added is determined based on the silica mass content, the amount of slag-forming desulfurizer added and the amount of the first refining agent added are determined based on the metal impurity mass content, and the amount of the second refining agent added is determined based on the non-metallic impurity mass content. The residual silica regulator is a carbonaceous regulator used to correct fluctuations in the silica content in the photovoltaic cutting silica sludge. The first refining agent comprises, by mass, 45-70 parts calcium oxide, 20-40 parts silica, 2-8 parts magnesium oxide, and 3-10 parts calcium fluoride; the second refining agent comprises, by mass, 20-45 parts sodium carbonate, 25-50 parts calcium oxide, 10-25 parts silica, and 3-12 parts calcium fluoride. The dry silica sludge, the residual silica regulator, and the slag-forming desulfurizer are mixed to obtain a mixed furnace charge. The mixed furnace charge is fed into a medium-frequency induction melting furnace, preheated at 800-900°C, and melted at 1300-1450°C to soften and aggregate the elemental silicon particles in the photovoltaic cutting silicon mud to form a silicon-enriched phase that can be separated from the floating slag phase. The first refining agent is added at the end of the melting stage. After settling, the first slag floats to the surface and is separated to obtain a silicon-enriched phase that has undergone preliminary impurity removal. The silicon-enriched phase, which has undergone preliminary impurity removal, is heated to 1550-1650°C to form a silicon liquid and is then refined. The second refining agent is added in two batches, and the power cycle is adjusted after each addition of the second refining agent. After standing, the second slag is separated to obtain the silicon liquid. Before casting, the molten silicon is sampled and tested to determine whether the test results meet the component limit requirements. The component limit requirements include iron content ≤0.20%, aluminum content ≤0.20%, calcium content ≤0.02%, boron content ≤0.01%, and phosphorus content ≤0.01%. If any test item exceeds the corresponding limit in the component limit requirements, the first refining agent or the second refining agent is added according to the item exceeding the limit, and the mixture is allowed to stand and separate again. After the test results or the retest results after standing and separating again meet the component limit requirements, the molten silicon is cast, cooled, crushed, and screened to obtain recycled silicon.

2. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 1, characterized in that, The particle size fraction sampling includes dividing the dry silica mud into coarse-grained dry silica mud and fine-grained dry silica mud; detecting the mass content of silicon oxide, iron, aluminum, calcium, boron, and phosphorus in the coarse-grained dry silica mud and the fine-grained dry silica mud respectively; and then determining the mass content of silicon oxide, metal impurities, and non-metallic impurities in the dry silica mud according to the mass ratio of the coarse-grained dry silica mud and the fine-grained dry silica mud.

3. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 2, characterized in that, The coarse-grained dry silica mud and the fine-grained dry silica mud are sampled and tested three or more times respectively; when the difference between the maximum and minimum test values ​​of any test item in the same particle size dry silica mud is greater than 15% of the arithmetic mean of the multiple test values ​​of that test item, the particle size dry silica mud is remixed and resampled and tested.

4. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 1, characterized in that, Determining the amount of residual silica modifier added includes: When the silica content is 8% to 15% by mass, the amount of residual silica regulator added is 8% to 10% of the mass of the mixed furnace charge; when the silica content is greater than 15% and less than or equal to 25% by mass, the amount of residual silica regulator added is 10% to 12% of the mass of the mixed furnace charge; when the silica content is greater than 25% by mass, the amount of residual silica regulator added is 12% to 14% of the mass of the mixed furnace charge.

5. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 4, characterized in that, The amount of slag-forming desulfurizing agent added is determined as follows: when the mass content of the metal impurities is less than or equal to 1.0%, the amount of slag-forming desulfurizing agent added is 4% to 5% of the mass of the mixed furnace charge; when the mass content of the metal impurities is greater than 1.0% and less than or equal to 1.8%, the amount of slag-forming desulfurizing agent added is 5% to 6.5% of the mass of the mixed furnace charge; when the mass content of the metal impurities is greater than 1.8%, the amount of slag-forming desulfurizing agent added is 6.5% to 8% of the mass of the mixed furnace charge.

6. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 5, characterized in that, The determination of the amount of the first refining agent is as follows: when the mass content of the metal impurities is less than or equal to 1.0%, the amount of the first refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of the metal impurities is greater than 1.0% and less than or equal to 1.8%, the amount of the first refining agent added is 1.0% to 1.6% of the mass of the mixed furnace charge; when the mass content of the metal impurities is greater than 1.8%, the amount of the first refining agent added is 1.6% to 2.2% of the mass of the mixed furnace charge.

7. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 6, characterized in that, After adding the first refining agent, keep it at 1300-1450℃ for 5-15 minutes, then let it stand for 5-12 minutes, and then separate the first slag.

8. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 7, characterized in that, After separating the first slag and before heating to 1550~1650℃, intermediate sampling and testing are performed on the silicon-enriched phase that has undergone preliminary impurity removal to obtain the mass content of molten iron, molten aluminum, and molten calcium. When at least one of the mass contents of molten iron, molten aluminum, and molten calcium exceeds the corresponding limit in the composition limit requirements, the temperature is maintained at 1300~1450℃ and the first refining agent is added. The mass of the added first refining agent is 0.2%~0.6% of the mass of the mixed furnace charge.

9. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 1, characterized in that, Determining the amount of the second refining agent includes: when the mass content of the non-metallic impurities is less than or equal to 0.01%, the amount of the second refining agent added is 0.3% to 0.6% of the mass of the mixed furnace charge; when the mass content of the non-metallic impurities is greater than 0.01% and less than or equal to 0.03%, the amount of the second refining agent added is 0.6% to 1.0% of the mass of the mixed furnace charge; when the mass content of the non-metallic impurities is greater than 0.03%, the amount of the second refining agent added is 1.0% to 1.5% of the mass of the mixed furnace charge.

10. The method for preparing regenerated silicon by refining photovoltaic-cut silicon sludge in an intermediate frequency furnace according to claim 9, characterized in that, The first addition of the second refining agent is 40% to 70% of the total mass of the second refining agent, and the second addition is the remaining amount of the second refining agent. The second addition is 5 to 15 minutes after the first addition.

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