Recycling method for broken silicon wafers of photovoltaic cells

Abstract

The invention discloses a method for recycling broken silicon wafers of photovoltaic cells. The recovery method comprises the following steps: filling silicon materials: adding broken silicon wafers into the bottom of a crucible, then alternately filling according to a manner of blocky silicon materials and the broken silicon wafers, and filling the last layer with the broken silicon wafers; the crucible filled with the silicon material is put into an ingot furnace, heating is performed after vacuumizing, the silicon material is molten, crystal growth is performed, and the crystal growth comprises an initial crystal growth stage, a middle crystal growth stage and a later crystal growth stage. And the heating temperature and the heat-insulating velvet lifting / descending rate of each stage of crystal growth are controlled. According to the method, the conductivity between the silicon materials can be improved, melting of the silicon materials in the crucible can be accelerated, and the purification time of the polycrystalline cast ingot is shortened. In addition, compared with the whole loading of the broken silicon wafers, the method provided by the invention can improve the loading weight of the crucible. Moreover, according to the method disclosed by the invention, more impurities can be segregated to the tail part of the polycrystalline silicon cast ingot, and the polycrystalline silicon cast ingot meeting the solar grade silicon purity can be obtained by cutting off the tail part, so that the qualification rate of the cast ingot is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic cells, and in particular to a method for recycling broken silicon wafers of photovoltaic cells. Background Art

[0002] The rapid development of solar photovoltaic cells has generated broken silicon wafers at both the slicing and cell production stages of the production line, leading to a continuous increase in the amount of discarded broken silicon wafers. Recycling broken silicon wafers not only reduces environmental pollution but also effectively lowers production costs in the photovoltaic industry.

[0003] Remelting and recycling broken silicon wafers involves chemically treating them to remove surface impurities, then remelting them into polycrystalline or monocrystalline silicon for use in the manufacture of new silicon wafers. Broken silicon wafers contain various metallic impurities and elements diffused during battery manufacturing, such as element B in TopCon cells. Even after acid and alkaline washing, their purity still falls short of the 99.9999% required for solar-grade silicon, requiring further purification. Existing processes for producing ingots from discarded broken silicon wafers struggle to guarantee this purity.

[0004] Therefore, providing a method for recycling broken silicon wafers of photovoltaic cells to obtain silicon ingots that can meet the purity of solar-grade silicon is a technical problem that needs to be solved urgently. Summary of the Invention

[0005] In view of the above technical problems existing in the prior art, the purpose of the present invention is to provide a method for recycling broken silicon wafers of photovoltaic cells.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for recycling broken silicon wafers from photovoltaic cells. The recycling method uses an ingot casting furnace, which includes a crucible and a heater. The heater is used for heating to radiate heat to the crucible wall. The outer side of the crucible wall is provided with insulation fleece, which is used for heat preservation. The recycling method comprises the following steps:

[0008] Filling silicon material:

[0009] The silicon material includes broken silicon wafers and block silicon material, and the filling method includes: adding broken silicon wafers to the bottom of the crucible, then filling the crucible with block silicon material and broken silicon wafers alternately, and finally filling the last layer with broken silicon wafers;

[0010] Multicrystalline ingot growth:

[0011] The crucible containing the silicon material is placed in an ingot casting furnace, vacuumed, and then heated to melt the silicon material and perform crystal growth, wherein the crystal growth includes an early stage of crystal growth, a middle stage of crystal growth, and a late stage of crystal growth;

[0012] In the initial stage of crystal growth, the heating temperature is controlled to be T1, T1 is 1427° C. to 1433° C. (for example, it can be 1427° C., 1428° C., 1429° C., 1430° C., 1431° C., 1432° C. or 1433° C.), the lifting rate of the thermal insulation fleece is first controlled within the range of 2.5 cm / h to 4 cm / h (for example, it can be 2.5 cm / h, 2.6 cm / h, 2.8 cm / h, 2.9 cm / h, 3 cm / h, 3.3 cm / h, 3.6 cm / h or 4 cm / h) for a period of time t1, and then controlled within the range of 0.47 cm / h to 0.55 cm / h (for example, it can be 0.47 cm / h, 0.48 cm / h, 0.49 cm / h, 0.5 cm / h, 0.52 cm / h or 0.55 cm / h) for a period of time t2;

[0013] In the middle stage of the crystal growth, the heating temperature is controlled to be T2, T2 is 1427° C. to 1433° C. (for example, it can be 1427° C., 1428° C., 1429° C., 1430° C., 1431° C., 1432° C. or 1433° C.), and the lifting rate of the thermal insulation fleece is in the range of 0.25 cm / h to 0.45 cm / h (for example, it can be 0.25 cm / h, 0.27 cm / h, 0.3 cm / h, 0.33 cm / h, 0.36 cm / h, 0.38 cm / h, 0.4 cm / h, 0.42 cm / h or 0.45 cm / h, etc.);

[0014] The late stage of crystal growth includes a first stage and a second stage which are carried out in sequence. In the first stage, the heating temperature is controlled at 1427°C to 1433°C (for example, it can be 1427°C, 1428°C, 1429°C, 1430°C, 1431°C, 1432°C or 1433°C, etc.), and the thermal insulation fleece neither rises nor falls; in the second stage, the heating temperature is controlled at 1423°C to 1426°C (for example, it can be 1423°C, 1424°C, 1425°C or 1426°C, etc.), and the descending rate of the thermal insulation fleece is 0.4cm / h to 0.5cm / h (for example, it can be 0.4cm / h, 0.42cm / h, 0.43cm / h, 0.45cm / h, 0.46cm / h, 0.48cm / h or 0.5cm / h, etc.).

[0015] In the method of the present invention, two types of heat insulation fleece lifting rates are set at the initial stage of crystal growth, which not only ensures that the silicon liquid is quickly condensed and crystallized, but also obtains a suitable crystallization rate.

[0016] The method of the present invention utilizes a defined filling pattern, adding broken silicon wafers to the bottom of the crucible. Subsequently, the filling pattern is alternating between bulk silicon and broken silicon wafers, with the final layer being filled with broken silicon wafers. In this method, the broken silicon wafers at the bottom serve as seed crystals. The alternating filling pattern and the final layer of broken silicon wafers improves conductivity between the silicon materials, accelerates melting of the silicon material in the crucible, and reduces the purification time of polycrystalline ingots. Furthermore, compared to a method that only fills the crucible with broken silicon wafers, the method of the present invention can reduce the weight of the crucible.

[0017] Furthermore, during the crystal growth process, the segregation coefficient of impurities such as B and Ga is less than 1. During growth, the impurities in the crystals that grow first are less, and the impurities are gradually enriched in the tail. At the same time, the impurity distribution is related to the crystal growth rate. For example, if the growth rate is uniform, the impurities in the crystal can be more evenly segregated into the liquid phase; if the growth rate is fast, the crystal crystallization time is short, the impurities in the crystal are less segregated into the liquid phase, and the impurity content of the crystal is high. The present invention can make the fluctuation of the growth rate smaller by regulating the heating temperature and the lifting / lowering rate of the insulation fleece at each stage of crystal growth, and the temperature drop during crystal growth is slower, the impurities have sufficient time to diffuse, and the impurities are more segregated into the silicon liquid, so that the impurities are more segregated to the tail of the polysilicon ingot, and there are fewer impurities in the middle area of ​​the silicon ingot. By cutting off the tail, a polysilicon ingot that meets the solar grade silicon purity can be obtained, thereby improving the qualification rate of the ingot.

[0018] It should be noted that it is very critical to maintain time t1 under a certain insulation fleece rate condition. If the insulation fleece lifting rate at this stage is too fast, the cooling rate of the silicon liquid in the crucible will be too fast, and the crystal growth rate will quickly reach a relatively high value. The growth rate at the initial stage of growth is too fast, and the diffusion of impurities in the crystal into the silicon liquid is reduced, and the crystal has many impurities, which will lead to a reduction in the volume share of the solar-grade silicon ingot after casting. If the insulation fleece lifting rate at this stage is too slow, the cooling rate of the silicon liquid will be too slow, and the crystal growth rate will be too slow, which will affect the initial growth rate of the crystal. In the middle stage, the heater temperature needs to be controlled to accelerate crystal growth. However, the way to adjust the heater temperature makes it difficult to control the crucible temperature, and the pulling rate fluctuates greatly in the middle stage of crystal growth, which also leads to a reduction in the diffusion of impurities in the crystal, affecting the volume share of the solar-grade silicon ingot.

[0019] Compared with single crystal Czochralski purification, the method of the present invention can obtain polycrystalline ingots with higher purification efficiency, which can be increased by more than 30%.

[0020] The present invention does not specifically limit the type of bulk silicon material, and it can be, for example, pot bottom material or floor material.

[0021] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0022] Preferably, the total mass of the silicon chips in the crucible is m1, the total mass of the silicon chunks in the crucible is m2, and the ratio of m1 to m2 is 0.4 to 0.6, for example, 0.4, 0.45, 0.5, 0.55, or 0.6. Within this range, both the crucible loading weight and the consumption of the silicon chips are guaranteed, while also maintaining a certain melting efficiency. Because the gaps between the silicon chips conduct heat less well than chunks, when chunks are used within this ratio, when the silicon material begins to melt, the chunks melt first, allowing the molten silicon to enter between the silicon chips, thereby accelerating their melting.

[0023] Preferably, the time in the initial stage of crystal growth accounts for 23% to 34% of the total crystal growth time, for example, it may be 23%, 25%, 27%, 28%, 30%, 32% or 34%.

[0024] Preferably, the initial crystal growth time is 7 hours to 10 hours, for example, it can be 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours or 10 hours.

[0025] Preferably, t1 is 1.5 h to 3 h, for example, it can be 1.5 h, 1.7 h, 1.8 h, 2 h, 2.2 h, 2.5 h, 2.8 h or 3 h.

[0026] Preferably, t2 is 5.5h to 7h, for example, it can be 5.5h, 5.6h, 5.8h, 6h, 6.3h, 6.5h, 6.7h or 7h.

[0027] Preferably, the time in the middle crystal growth phase accounts for 50% to 65% of the total crystal growth time, for example, it may be 50%, 52%, 54%, 55%, 57%, 58%, 60%, 62% or 65%.

[0028] Preferably, the time in the middle stage of crystal growth is 18 hours to 30 hours, for example, it can be 18 hours, 20 hours, 22 hours, 23 hours, 25 hours, 26 hours, 28 hours or 30 hours.

[0029] Preferably, the time in the later stage of crystal growth accounts for 12% to 16% of the total crystal growth time, for example, it may be 12%, 13%, 14%, 15% or 16%.

[0030] Preferably, the time for the later stage of crystal growth is 4 hours to 8 hours, for example, it can be 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours.

[0031] Preferably, the middle stage of crystal growth includes three stages carried out in sequence, the heating temperatures of the three stages are the same, and as time goes on, the insulation fleece lifting rates of the three stages are respectively 0.33cm / h to 0.4cm / h (for example, 0.33cm / h, 0.34cm / h, 0.35cm / h, 0.36cm / h, 0.37cm / h, 0.38cm / h, 0.39cm / h or 0.4cm / h, etc.), 0.23cm / h to 0.28cm / h (for example, 0.23cm / h, 0.24cm / h, 0.25cm / h, 0.26cm / h, 0.27cm / h or 0.28cm / h, etc.) and 0.43cm / h to 0.47cm / h (for example, 0.43cm / h, 0.44cm / h, 0.45cm / h, 0.46cm / h or 0.47cm / h, etc.).

[0032] As a preferred technical solution of the recovery method of the present invention, after the vacuum is evacuated, the furnace pressure is maintained at 2.5 torr to 3.5 torr, for example, 2.5 torr, 2.6 torr, 2.7 torr, 2.8 torr, 2.9 torr, 3.0 torr, 3.1 torr, 3.2 torr, 3.3 torr, 3.4 torr, or 3.5 torr. Silicon melting and crystal growth are carried out under this low-pressure condition, which can accelerate the volatilization of impurities in the silicon liquid (for example, mainly removing B and Ga impurities), thereby increasing the qualified area of ​​the polysilicon ingot. In addition, the low pressure can accelerate the melting rate.

[0033] In one embodiment, the qualified area of ​​the polysilicon ingot is increased by more than 3%, and the melting time is saved by 1 hour.

[0034] Preferably, the average particle size of the silicon chips filled in the crucible decreases gradually as the crucible moves away from the bottom of the crucible.

[0035] Preferably, the larger surface of the silicon block is close to the crucible wall. Because heat is radiated from the heater to the crucible wall, which then transfers it to the silicon, the contact between the larger surface of the silicon block and the crucible wall increases the initial melting rate. This allows for faster heat transfer to the silicon block during heating, melting it first.

[0036] Preferably, the silicon chips are ground using a mill before use to a particle size of ≤1 mm, after which impurities are removed to obtain pre-treated silicon chips. This arrangement has the advantages of improving the quality of the crucible and, by reducing the silicon chips to the above particle size, more thoroughly removing impurities.

[0037] Preferably, the method for removing impurities is at least one of acid washing and alkali treatment.

[0038] Preferably, the ground silicon chips are sieved to separate the chips with a particle size of less than 0.3 mm. These chips are used to fill the final layer of silicon chips, and the remaining chips are used to fill the bottom of the crucible or in the alternating filling step. This method can better improve the crucible filling quality.

[0039] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The method of the present invention uses the broken silicon wafers at the bottom as seed crystals. Alternating the filling and filling the last layer with broken silicon wafers can improve the conductivity between the silicon materials, accelerate the melting of the silicon material in the crucible, and reduce the purification time of the polycrystalline ingot. Moreover, compared with filling the crucible entirely with broken silicon wafers, the method of the present invention can increase the weight of the crucible.

[0042] Furthermore, the present invention can make the fluctuation of growth rate smaller by regulating the heating temperature and the lifting / lowering rate of the insulation fleece in each stage of crystal growth, and the temperature drop during crystal growth is slower, the impurities have sufficient time to diffuse, and the impurities are more condensed into the silicon liquid, so that the impurities are more condensed to the tail of the polysilicon ingot, and there are fewer impurities in the middle area of ​​the silicon ingot. By cutting off the tail, a polysilicon ingot that meets the purity of solar-grade silicon can be obtained, thereby improving the qualification rate of the ingot.

[0043] (2) The method of the present invention can improve the quality of the crucible obtained, thereby increasing the total length of the polycrystalline ingot. Moreover, the volume percentage of the usable ingot and the volume percentage of the solar-grade silicon ingot can also be increased. In the polycrystalline silicon ingot prepared by the method of the present invention, the length percentage of the usable ingot is greater than 93%, and the volume percentage of the solar-grade silicon ingot is greater than 70%, preferably greater than 73%. DETAILED DESCRIPTION

[0044] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0045] The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0046] The broken silicon wafers used in the present invention come from the slicing end of the photovoltaic cell production line and / or the production end of the cell. The broken silicon wafers are crushed using a grinder to a particle size of ≤1 mm, acid-washed and alkaline-treated to remove impurities, and sieved to obtain broken silicon wafers with a particle size of less than 0.3 mm and broken silicon wafers with a particle size between 0.3 mm and 1 mm, which are used in various embodiments.

[0047] In the embodiment of the present invention, the requirement for solar-grade silicon ingots is (the purity of solar-grade silicon must reach 99.9999%).

[0048] Example 1

[0049] A method for recycling broken silicon wafers from photovoltaic cells is provided. The recycling method uses an ingot casting furnace, which includes a crucible and a heater. The heater is used for heating and radiating heat to the crucible wall. The outer side of the crucible wall is provided with insulation fleece, which is used for heat preservation. The recycling method includes the following steps:

[0050] Filling silicon material:

[0051] The silicon material includes broken silicon wafers and bulk silicon material (the mass ratio of broken silicon wafers to bulk silicon material is 0.5), and the filling method includes: adding broken silicon wafers with a particle size ranging from 0.3 mm to 1 mm to the bottom of the crucible, then adding bulk silicon material (pot bottom material), with the larger surface of the bulk silicon material close to the crucible wall, and finally adding broken silicon wafers with a particle size less than 0.3 mm;

[0052] Multicrystalline ingot growth:

[0053] The crucible containing the silicon material is placed in an ingot casting furnace, vacuumed and maintained at a furnace pressure of 3 Torr, heated, the silicon material is melted and crystal growth is carried out, wherein the crystal growth includes an early crystal growth stage, a middle crystal growth stage and a late crystal growth stage;

[0054] At the initial stage of the crystal growth, the heating temperature is controlled to be T1, T1 is 1430°C, and the lifting rate of the insulation fleece is first controlled to be 3 cm / h for a period of time t1, t1=2h, and then controlled to be 0.5 cm / h for a period of time t2, t2=7h;

[0055] In the middle of the crystal growth, the heating temperature is controlled to T2, T2 is 1430°C, and the lifting rate of the insulation fleece is first controlled to 0.35 cm / h for 9 hours, then controlled to 0.25 cm / h for 7 hours, and finally controlled to 0.45 cm / h for 5 hours;

[0056] The latter stage of crystal growth includes a first stage and a second stage, which are performed sequentially. In the first stage, the heating temperature is controlled at 1430°C, and the insulation fleece is neither raised nor lowered, and maintained for 4 hours. In the second stage, the heating temperature is controlled at 1426°C, and the insulation fleece is lowered at a rate of 0.5 cm / h for 1 hour. An ingot is grown, and the tail portion enriched with impurities is removed to obtain a solar-grade silicon ingot. The tail portion includes ingots that can be reused through purification and waste ingots.

[0057] The total length of the ingot, the length of the solar-grade silicon ingot and the length of the silicon ingot that can be purified and reused are shown in Table 1.

[0058] Example 2

[0059] A method for recycling broken silicon wafers from photovoltaic cells is provided. The recycling method uses an ingot casting furnace, which includes a crucible and a heater. The heater is used for heating and radiating heat to the crucible wall. The outer side of the crucible wall is provided with insulation fleece, which is used for heat preservation. The recycling method includes the following steps:

[0060] Filling silicon material:

[0061] The silicon material includes broken silicon wafers and bulk silicon material (the mass ratio of broken silicon wafers to bulk silicon material is 0.6), and the filling method includes: adding broken silicon wafers with a particle size ranging from 0.3 mm to 1 mm to the bottom of the crucible, then adding bulk silicon material (pot bottom material), with the larger area of ​​the bulk silicon material close to the crucible wall, and finally adding broken silicon wafers with a particle size less than 0.3 mm;

[0062] Multicrystalline ingot growth:

[0063] The crucible containing the silicon material is placed in an ingot casting furnace, vacuumed and maintained at a furnace pressure of 3.5 torr, heated, and the silicon material is melted and crystal growth is carried out, wherein the crystal growth includes an early stage of crystal growth, a middle stage of crystal growth, and a late stage of crystal growth;

[0064] At the initial stage of crystal growth, the heating temperature is controlled to be T1, T1 is 1432°C, and the lifting rate of the insulation fleece is first controlled to be 3.5 cm / h for a period of time t1, t1=3h, and then controlled to be 0.55 cm / h for a period of time t2, t2=6.5h;

[0065] In the middle of the crystal growth, the heating temperature is controlled to be T2, T2 is 1433°C, and the lifting rate of the insulation fleece is first controlled to be 0.34 cm / h for 8 hours, then controlled to be 0.26 cm / h for 7 hours, and finally controlled to be 0.43 cm / h for 6 hours;

[0066] The latter stage of crystal growth includes a first stage and a second stage, which are performed sequentially. In the first stage, the heating temperature is controlled at 1432°C, and the insulation fleece is neither raised nor lowered, and maintained for 3 hours. In the second stage, the heating temperature is controlled at 1425°C, and the insulation fleece is lowered at a rate of 0.4 cm / h for 2 hours. An ingot is grown, and the tail portion enriched with impurities is removed to obtain a solar-grade silicon ingot. The tail portion includes ingots that can be reused through purification and waste ingots.

[0067] The total length of the ingot, the length of the solar-grade silicon ingot and the length of the silicon ingot that can be purified and reused are shown in Table 1.

[0068] Example 3

[0069] A method for recycling broken silicon wafers from photovoltaic cells is provided. The recycling method uses an ingot casting furnace, which includes a crucible and a heater. The heater is used for heating and radiating heat to the crucible wall. The outer side of the crucible wall is provided with insulation fleece, which is used for heat preservation. The recycling method includes the following steps:

[0070] Filling silicon material:

[0071] The silicon material includes broken silicon wafers and bulk silicon material (the mass ratio of broken silicon wafers to bulk silicon material is 0.4), and the filling method includes: adding broken silicon wafers with a particle size ranging from 0.3 mm to 1 mm to the bottom of the crucible, then adding bulk silicon material (pot bottom material), with the larger area of ​​the bulk silicon material close to the crucible wall, and finally adding broken silicon wafers with a particle size less than 0.3 mm;

[0072] Multicrystalline ingot growth:

[0073] The crucible containing the silicon material is placed in an ingot casting furnace, vacuumed and maintained at a furnace pressure of 2.5 torr, heated, and the silicon material is melted and crystal growth is carried out, wherein the crystal growth includes an early stage of crystal growth, a middle stage of crystal growth, and a late stage of crystal growth;

[0074] At the initial stage of crystal growth, the heating temperature is controlled to be T1, T1 is 1429°C, and the lifting rate of the insulation fleece is first controlled to be 3.2 cm / h for a period of time t1, t1=2.5h, and then controlled to be 0.48 cm / h for a period of time t2, t2=7h;

[0075] In the middle of the crystal growth, the heating temperature is controlled to T2, T2 is 1428°C, and the lifting rate of the insulation fleece is first controlled to 0.37 cm / h for 8.5 hours, then controlled to 0.25 cm / h for 6.5 hours, and finally controlled to 0.45 cm / h for 6 hours;

[0076] The latter stage of crystal growth includes a first stage and a second stage, which are performed sequentially. In the first stage, the heating temperature is controlled at 1430°C, and the insulation fleece is neither raised nor lowered, and maintained for 3.5 hours. In the second stage, the heating temperature is controlled at 1425°C, and the insulation fleece is lowered at a rate of 0.45 cm / h for 1.5 hours. An ingot is grown, and the tail portion enriched with impurities is removed to obtain a solar-grade silicon ingot. The tail portion includes ingots that can be reused through purification and waste ingots.

[0077] The total length of the ingot, the length of the solar-grade silicon ingot and the length of the silicon ingot that can be purified and reused are shown in Table 1.

[0078] Example 4

[0079] The difference between this embodiment and embodiment 1 is that the mass ratio of broken silicon wafers to bulk silicon material is 0.3.

[0080] Example 5

[0081] The difference between this embodiment and embodiment 1 is that the mass ratio of broken silicon wafers to bulk silicon material is 0.7.

[0082] Comparative Example 1

[0083] The difference between this comparative example and Example 1 is that the bulk silicon material is replaced by broken silicon wafers with a particle size ranging from 0.3 mm to 1 mm. Therefore, in this comparative example, the silicon material does not contain bulk silicon material, and all of it is broken silicon wafers.

[0084] Comparative Example 2

[0085] The difference between this comparative example and Example 1 is that the process parameters in the later stage of crystal growth are: controlling the heating temperature at 1430° C., and keeping the insulation fleece neither rising nor falling for 5 hours.

[0086] Comparative Example 3

[0087] The difference between this comparative example and Example 1 is that the process parameters in the later stage of crystal growth are: controlling the heating temperature at 1426° C., and the descending rate of the insulation fleece at 0.5 cm / h, which is maintained for 5 hours.

[0088] Comparative Example 4

[0089] The difference between this comparative example and Example 1 is that the process parameters in the initial stage of crystal growth are: the heating temperature is controlled to be T1, T1 is 1430°C, the lifting rate of the insulation fleece is first controlled to be 4.2 cm / h and maintained for a period of time t1, t1=2h, and then controlled to be 0.5 cm / h and maintained for a period of time t2, t2=7h.

[0090] Comparative Example 5

[0091] The difference between this comparative example and Example 1 is that the process parameters in the initial stage of crystal growth are: the heating temperature is controlled to be T1, T1 is 1430°C, the lifting rate of the insulation fleece is first controlled to be 2.3 cm / h and maintained for a period of time t1, t1=2h, and then controlled to be 0.5 cm / h and maintained for a period of time t2, t2=7h.

[0092] In this comparative example, the rate of lifting the insulation fleece during the maintenance time t1 is too slow, the cooling rate of the silicon liquid is too slow, and the crystal growth rate is too low, which affects the initial growth rate of the crystal. In the middle stage, it is necessary to control the heater temperature to accelerate crystal growth. However, the method of adjusting the heater temperature makes it difficult to control the crucible temperature, and the pulling speed fluctuates greatly in the middle stage of crystal growth, which also leads to reduced diffusion of impurities in the crystal, affecting the volume proportion of solar-grade silicon ingots.

[0093] test:

[0094] (1) After the silicon material is filled, the mass m1 of the crucible filled with silicon material is measured, and the mass m2 of the crucible itself is subtracted to obtain the mass of the crucible.

[0095] (2) Record the distance of the cut tail and test the purity of the resulting polysilicon ingot product.

[0096] See Table 1 for the results.

[0097] Table 1

[0098]

[0099] Note: The length percentage of usable ingots and the volume percentage of solar-grade silicon ingots are both based on the total length of the ingot.

[0100] As can be seen from Table 1, the present invention has the following beneficial effects: (1) The method of the present invention uses the broken silicon wafers at the bottom as seed crystals, alternately fills the material and fills the last layer with broken silicon wafers, which can improve the conductivity between the silicon materials, accelerate the melting of the silicon material in the crucible, and reduce the purification time of the polycrystalline ingot. Moreover, compared with filling all the broken silicon wafers, the method of the present invention can increase the weight of the crucible. Furthermore, by regulating the heating temperature and the lifting / lowering rate of the insulation fleece at each stage of crystal growth, the present invention can make the fluctuation of the growth rate smaller, and the temperature drop during crystal growth is slower, the impurity diffusion time is sufficient, and the impurities are more condensed into the silicon liquid, allowing more impurities to be condensed to the tail of the polycrystalline silicon ingot, and less impurities in the middle area of ​​the silicon ingot. By cutting off the tail, a polycrystalline silicon ingot that meets the purity of solar-grade silicon can be obtained, thereby improving the qualified rate of the ingot. (2) The method of the present invention can improve the quality of the obtained crucible, thereby increasing the total length of the polycrystalline ingot. Moreover, it can also increase the volume proportion of the available ingot and the volume proportion of the solar-grade silicon ingot. In the polycrystalline silicon ingot prepared by the method of the present invention, the length of the usable ingot accounts for more than 93%, and the volume of the solar-grade silicon ingot accounts for more than 70%, preferably more than 73%.

[0101] By comparing Example 1 with Examples 4-5 and Comparative Example 1, it can be seen that, compared with simple broken silicon wafers, the method of using broken silicon wafers and bulk silicon materials for loading can improve the charging quality, thereby increasing the total length of the polysilicon ingot, the volume proportion of the solar-grade silicon ingot and the volume proportion of the usable ingot. Furthermore, the mass ratio of broken silicon wafers to bulk silicon materials is preferably in the range of 0.4 to 0.6.

[0102] By comparing Example 1 with Comparative Examples 2-3, it can be seen that in the later stage of crystal growth, if there is a lack of a step in which the insulation fleece does not rise or fall, or if there is a lack of a step in which the insulation fleece falls, it will lead to a decrease in the volume share of solar-grade silicon ingots and a decrease in the volume share of usable ingots.

[0103] By comparing Example 1 with Comparative Examples 4-5, it can be seen that in the early stage of crystal growth, if the insulation fleece lifting rate during the time t1 is too fast or too slow, the volume proportion of the solar-grade silicon ingot will decrease.

[0104] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A method for recycling broken silicon wafers of photovoltaic cells, characterized in that: The equipment used in the recovery method is an ingot casting furnace, which includes a crucible and a heater. The heater is used for heating to radiate heat to the crucible wall. The outer side of the crucible wall is provided with heat insulation fleece, and the heat insulation fleece is used for heat preservation. The recovery method comprises the following steps: Filling silicon material: The silicon material includes broken silicon wafers and block silicon material, and the filling method includes: adding broken silicon wafers to the bottom of the crucible, then filling the crucible with block silicon material and broken silicon wafers alternately, and finally filling the last layer with broken silicon wafers; Multicrystalline ingot growth: The crucible containing the silicon material is placed in an ingot casting furnace, vacuumed, and then heated to melt the silicon material and perform crystal growth, wherein the crystal growth includes an early stage of crystal growth, a middle stage of crystal growth, and a late stage of crystal growth; In the initial stage of crystal growth, the heating temperature is controlled to be T1, T1 is 1427℃~1433℃, and the lifting rate of the insulation fleece is first controlled within the range of 2.5cm / h~4cm / h for a period of time t1, and then controlled within the range of 0.47cm / h~0.55cm / h for a period of time t2; In the middle stage of the crystal growth, the heating temperature is controlled to be T2, T2 is 1427°C to 1433°C, and the lifting rate of the insulation fleece is in the range of 0.25cm / h to 0.45cm / h; The late stage of crystal growth includes a first stage and a second stage carried out in sequence. In the first stage, the heating temperature is controlled at 1427℃~1433℃, and the insulation fleece does not rise or fall; in the second stage, the heating temperature is controlled at 1423℃~1426℃, and the descending rate of the insulation fleece is 0.4cm / h~0.5cm / h.

2. The recycling method according to claim 1, wherein: The total mass of the broken silicon wafers in the crucible is m1, the total mass of the silicon chunks in the crucible is m2, and m1 / m2 is 0.4-0.

6.

3. The recycling method according to claim 1 or 2, characterized in that The time of the initial crystal growth period accounts for 23% to 34% of the total crystal growth time; Preferably, the initial crystal growth time is 7h to 10h; Preferably, t1 is 1.5h to 3h; Preferably, t2 is 5.5h to 7h; Preferably, the time in the middle stage of crystal growth accounts for 50% to 65% of the total crystal growth time; Preferably, the time in the middle stage of crystal growth is 18 hours to 30 hours; Preferably, the time of the late crystal growth period accounts for 12% to 16% of the total crystal growth time; Preferably, the time of the later stage of crystal growth is 4 hours to 8 hours.

4. The recycling method according to any one of claims 1 to 3, characterized in that The middle stage of crystal growth includes three stages carried out in sequence. The heating temperatures of the three stages are the same. As time goes by, the insulation fleece lifting rates of the three stages are 0.33cm / h to 0.4cm / h, 0.23cm / h to 0.28cm / h and 0.43cm / h to 0.47cm / h respectively.

5. The recycling method according to claim 1, characterized in that: After the vacuum is evacuated, the furnace pressure is maintained at 2.5 torr to 3.5 torr.

6. The recycling method according to any one of claims 1 to 5, characterized in that: The average particle size of the crushed silicon wafers filled decreases in the direction away from the bottom of the crucible.

7. The recycling method according to any one of claims 1 to 6, characterized in that: The surface of the bulk silicon material with a larger area is close to the crucible wall.

8. The recycling method according to any one of claims 1 to 7, characterized in that: Before use, the broken silicon wafers are ground using a grinder to reduce the particle size of the broken silicon wafers to ≤1 mm, and then impurities are removed to obtain pre-treated broken silicon wafers.

9. The recycling method according to claim 8, characterized in that: The method for removing impurities is at least one of pickling and alkali treatment.

10. The recycling method according to claim 8 or 9, characterized in that: The ground silicon chips are sieved to separate the chips with a particle size of less than 0.3 mm and used to fill the last layer of silicon chips. The remaining silicon chips are used to fill the bottom of the crucible or for the alternating filling step.

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