JP2012020364A - Method for regenerating silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for regenerating silicon, with which the regenerated silicon whose phosphorus concentration is sufficiently lowered can be obtained.SOLUTION: In the method for regenerating silicon, the regenerated silicon having the phosphorus concentration usable as a raw material for solar cells can be obtained from the silicon chips which are produced as by-products when a silicon block is cut, without performing a special dephosphorization treatment. The method for regenerating the silicon comprises the steps of: recovering the silicon chip-containing solid content from waste slurry including the silicon chips, which are produced as by-products when the silicon block is stuck to a dummy material by using an adhesive and a part of the silicon block-stuck dummy material is cut by using a wire with the abrasive grain stuck to the surface and a coolant to obtain a silicon wafer; cleaning the recovered solid content with a first cleaning liquid and a second cleaning liquid different from the first cleaning liquid; melting the cleaned solid content to obtain molten silicon; and solidifying the molten silicon. The dummy material has easy solubility in the first cleaning liquid and the adhesive has easy solubility in the second cleaning liquid.

Description

  The present invention relates to a silicon regeneration method.

  Conventionally, silicon wafers are manufactured by mixing a slurry of loose abrasives such as silicon carbide and kerosene or base oil such as glycol, and winding the wire around a guide roller as a lubricant. In general, a method of cutting the silicon block while feeding it at a high speed (hereinafter referred to as the free abrasive grain method) is generally used. Moreover, in recent years, using a fixed abrasive wire in which abrasive grains are fixed to a wire, the wire is wound around a guide roller and fed at a high speed while being fed to a silicon block for cutting (hereinafter referred to as the following) (Referred to as Patent Document 1). An apparatus for cutting a silicon block to manufacture a silicon wafer is called a multi-wire saw apparatus.

  In addition to the original advantages of the fixed abrasive method, such as faster cutting speed and less uneven thickness of the wafer, compared to the free abrasive method, the abrasive particles can be added to the waste slurry generated by cutting the silicon block. There is an advantage that there is little mixing, and there is an advantage that there is little mixing of heavy metals into the waste slurry due to wire wear. For this reason, the purity of the recycled silicon recovered from the waste slurry generated by cutting the silicon block is high, and it is expected that the silicon can be easily recycled.

JP 2007-180527 A

However, the recycled silicon regenerated from the waste slurry produced by cutting the silicon block by the fixed abrasive method contains about 1 ppm by weight of phosphorus. For this reason, in order to use this regenerated silicon as a silicon raw material for solar cells, a dephosphorization process by a vacuum melting method or an electron beam method is required, which increases the manufacturing cost.
The present invention has been made in view of such circumstances, and provides a silicon regeneration method capable of obtaining reclaimed silicon having a sufficiently reduced phosphorus concentration.

  The present invention cuts a silicon block and regenerates a phosphorus concentration that can be used as a raw material for a solar cell, without performing a special dephosphorization treatment such as a vacuum melting method or an electron beam method from silicon by-products generated as a by-product. A silicon regeneration method for obtaining silicon, wherein the silicon block and the dummy material are bonded to each other using a wire and a coolant in which abrasive grains are fixed to the surface of the dummy block with an adhesive. A part of the silicon wafer is cut to obtain a silicon wafer, and a silicon scrap-containing solid content is recovered from a waste slurry containing by-produced silicon scrap. The recovered solid content is divided into a first cleaning liquid and a different second cleaning liquid. The dummy material includes a step of cleaning, a step of melting the washed solid content to obtain molten silicon, and a step of solidifying the molten silicon. To have a readily soluble, the adhesive provides a silicon reproducing method characterized by having a readily soluble in the second cleaning solution.

  According to the present invention, since the dummy material is readily soluble in the first cleaning liquid, the components of the dummy material contained in the waste slurry are easily removed by cleaning the solid content recovered from the waste slurry with the first cleaning liquid. be able to. As a result, phosphorus contained in the dummy material can be easily removed, and a regenerated silicon having a sufficiently reduced phosphorus concentration can be obtained. For this reason, the step of removing phosphorus when obtaining regenerated silicon, for example, vacuum melting or electron beam melting can be omitted, and solar cell grade silicon can be provided at low cost.

It is explanatory drawing of the cutting process by which the silicon | silicone waste collect | recovered at the collection | recovery process included in the silicon | silicone reproduction | regenerating method of one Embodiment of this invention is discharged | emitted. It is a schematic sectional drawing of the wire used at the cutting process by which the silicon | silicone waste collect | recovered at the collection | recovery process included in the silicon | silicone reproduction | regeneration method of one Embodiment of this invention is discharged | emitted. It is explanatory drawing from the collection | recovery process included in the silicon reproduction | regenerating method of one Embodiment of this invention to a segregation process. It is explanatory drawing of the melting process included in the silicon | silicone reproduction | regenerating method of one Embodiment of this invention.

  The silicon regeneration method of the present invention is a phosphor that can be used as a raw material for solar cells without cutting the silicon block and by performing a special dephosphorization treatment such as a vacuum melting method or an electron beam method from silicon by-products generated as a by-product. A silicon recycling method for obtaining a recycled silicon having a concentration, wherein the silicon block is bonded to a dummy material using an adhesive, and the silicon block is used by using a wire and a coolant in which abrasive grains are fixed to the surface. And a part of the dummy material is cut to obtain a silicon wafer, and a process of recovering the silicon scrap-containing solid content from the waste slurry containing silicon scrap generated as a by-product, and the recovered solid content is different from the first cleaning liquid. 2 a step of cleaning with the cleaning liquid, a step of melting the cleaned solid content to obtain molten silicon, and a step of solidifying the molten silicon. Chromatography material has an easily soluble in the first washing solution, the adhesive is characterized by having a readily soluble in the second cleaning solution.

The phosphorus concentration that can be used as a raw material for a solar cell refers to the phosphorus concentration in silicon that can be used as a raw material for a solar cell without performing a dephosphorization step such as a vacuum melting method or an electron beam method.
In the silicon regeneration method of the present invention, the phosphorus concentration is preferably 0.32 ppm by weight or less.
Thereby, a dephosphorization step such as a vacuum melting method or an electron beam method can be omitted, and a regenerated silicon having a phosphorus concentration that can be used as a raw material for a solar cell can be obtained.
In the silicon recycling method of the present invention, it is preferable that the dummy material is soda glass, and the first cleaning liquid is 0.5% or more and 10% or less hydrofluoric acid.
Since soda glass is easily soluble in hydrofluoric acid, the components of the dummy material contained in the solid content recovered from the waste slurry can be easily removed by washing with the first washing liquid. Thus, phosphorus contained in the dummy material can be easily removed from the solid content.

In the silicon recycling method of the present invention, the adhesive is preferably an epoxy resin adhesive, and the second cleaning liquid is preferably nitric acid of 2% or more and 67.5% or less.
Since the epoxy resin adhesive is readily soluble in nitric acid, the components of the adhesive contained in the solid content recovered from the waste slurry can be easily removed by washing with the second washing liquid. Thereby, phosphorus contained in the adhesive can be easily removed from the solid content.
In the silicon regeneration method of the present invention, it is preferable that the adhesive is easily soluble in the first cleaning liquid.
Thereby, the component of the adhesive contained in the solid content recovered from the waste slurry can be easily removed. Thereby, phosphorus contained in the adhesive can be easily removed from the solid content.

In the silicon regeneration method of the present invention, it is preferable that the wire has a nickel plating layer, and the nickel plating layer has a phosphorus concentration of 0.05% by weight or less.
Thereby, the mixing of phosphorus from the nickel plating layer into the solid content recovered from the waste slurry can be reduced, and the concentration of phosphorus in the regenerated silicon that can be recovered by the present invention can be reduced.
In the silicon regeneration method of the present invention, it is preferable that the step of solidifying the molten silicon includes the step of solidifying the molten silicon discharged from the cooling vessel.
By pouring molten silicon into the cooling vessel, silicon oxide, abrasive grains, etc. can remain in the crucible and only the molten silicon can be solidified. This makes it possible to obtain recycled silicon from which silicon oxide and abrasive grains have been removed.

Preferably, the silicon regeneration method of the present invention further includes a step of heating and melting the silicon lump formed by the step of solidifying the molten silicon to a temperature equal to or higher than the melting point of silicon to solidify in one direction.
Thereby, metal components such as iron can be removed from the regenerated silicon that can be recovered by the present invention.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

Silicon Regeneration Method The silicon regeneration method of this embodiment is a raw material for solar cells that is obtained by cutting the silicon block 1 and using silicon debris produced as a by-product without performing a special dephosphorization treatment such as a vacuum melting method or an electron beam method. Is a silicon regeneration method for obtaining a regenerated silicon having a phosphorus concentration that can be used as a wire, and a wire 8 having abrasive grains fixed on the surface and a coolant in a state where the silicon block 1 is adhered to the dummy material 5 using the adhesive 3 13 is used to obtain a silicon wafer 11 by cutting a part of the silicon block 1 and the dummy material 5 and recovering the silicon waste-containing solid content 26 from the waste slurry 20 containing silicon waste produced as a by-product. A step of cleaning the recovered solid content 26 with a first cleaning liquid and a second cleaning liquid different from the first cleaning liquid; and melting the cleaned solid content 31 to obtain a molten silicon 32 That includes a step, and a step of solidifying the molten silicon 32, the dummy member 5 has a readily soluble in the first cleaning liquid, the adhesive 3, characterized by having a readily soluble in the second cleaning solution.

Moreover, the silicon regeneration method of the present embodiment may include a plurality of cleaning steps for cleaning the solid content with a cleaning liquid, and may further include a segregation step. Moreover, you may perform a leaching process as needed.
Hereinafter, the silicon regeneration method of this embodiment will be described.

1. Cutting Process FIG. 1 is an explanatory diagram of the cutting process, and FIG. 2 is a schematic cross-sectional view of a wire used in the cutting process.
In the cutting step, the silicon block 1 bonded to the dummy material 5 with the adhesive 3 and a part of the dummy material 5 are cut using the wire 8 having the abrasive grains 22 fixed to the surface and the coolant 13. For example, as shown in FIG. 1, the silicon wafer 1 can be manufactured by cutting the silicon block 1 by feeding the silicon block 1 to the wire 8 running at high speed while supplying the coolant.

The silicon block 1 is not particularly limited as long as it is a silicon block used as a raw material for a silicon wafer. For example, the silicon block 1 may be single crystal silicon for semiconductors or polycrystalline silicon for solar cells. The shape of the silicon block 1 may be a cylindrical shape or a quadrangular prism.
The dummy material 5 is capable of fixing the silicon block 1 and is easily soluble in the first cleaning liquid. For example, when the dummy material 5 is made of glass, the first cleaning liquid can be hydrofluoric acid. Further, soda glass can be used for the dummy material 5. This is because soda glass is relatively inexpensive.

The adhesive 3 is not particularly limited as long as it can bond the silicon block 1 and the dummy material 5, but may be, for example, an epoxy resin adhesive. The adhesive 3 can bond the silicon block 1 and the dummy material 5 with a thickness of about 0.1 to 0.2 mm, for example.
The coolant 13 is not particularly limited as long as it acts as a cooling effect and a lubricating oil in the step of cutting the silicon block 1, but for example, a water-soluble oil such as glycol can be used as a base material. Water-insoluble oils for machining can also be used. In addition, when using a reuse coolant for a coolant, the silicon | silicone chips of a pre-cut product may be contained in a coolant.
The wire 8 can be a fixed abrasive wire. In this case, abrasive grains 22 such as diamond are fixed to a bus bar (steel material) 21. Further, the wire 8 can have a nickel plating layer 23 in order to improve the adhesion between the abrasive grains 22 and the wire 8.

By this cutting step, (1) silicon scrap 15 generated by cutting the silicon block 1, (2) wire scrap 16 generated when the wire 8 is worn, and (3) adhesive 3 are cut into the coolant 13. The waste slurry 20 mixed with the waste of the adhesive 17 and (4) the waste of the dummy 18 generated by cutting the dummy 18 is discharged.
The silicon scrap 15 is considered to contain crystalline silicon and silicon oxide. The wire scrap 16 is considered to include scraps and abrasive grains of the bus bar 21. When the wire 8 has the nickel plating layer 23, the wire scrap 15 includes scraps of the nickel plating layer 23. Conceivable.

2. Recovery Process FIG. 3 is an explanatory diagram from the recovery process to the segregation process.
In the recovery step, the waste slurry 20 is subjected to solid-liquid separation, and the solid content 26 including the silicon scrap 15 is recovered. The method for recovering the solid content 26 is not particularly limited. For example, (i) by capturing the solid content of the waste liquid using a filter or a centrifuge and drying it. A method of obtaining the solid content 26, (ii) by collecting and heating or distilling the waste liquid. A method for obtaining the solid content 26 and (iii) using a flocculant for the waste liquid. It is a method including at least one of the methods for obtaining the solid content 26.

3. Washing Step In the washing step, the solid content 26 recovered from the waste slurry 20 is washed with a washing liquid. In the washing step, the solid content 26 may be washed with one washing liquid, and washing with a washing liquid and solid-liquid separation may be performed, and washing with a plurality of washing liquids may be performed. When the solid content 26 is washed with a plurality of washing liquids, after washing with one washing liquid, solid-liquid separation can be performed in the same manner as in the “recovery step”, and washing with other washing liquids can be performed. Moreover, solid content 31 can be obtained by performing solid-liquid separation by the same method as the “recovery step” after the washing step. As the cleaning liquid used in the cleaning process, a cleaning liquid containing at least one selected from the group consisting of an acid, an alkali, an organic solvent, and water can be used.
Further, at least one of the cleaning liquids used in the cleaning process is easily soluble in the dummy material 5. For example, when a soda glass plate is used for the dummy material 5, hydrofluoric acid can be used for the cleaning liquid. Thereby, the dummy material waste 18 contained in the solid content 26 can be easily removed, and the phosphorus contained in the dummy material waste 18 can be easily removed. In addition, the cleaning liquid is preferably one in which the coolant 13 and the adhesive scrap 17 are easily soluble. Moreover, when using hydrofluoric acid for a washing | cleaning liquid, the density | concentration of hydrofluoric acid is not specifically limited, For example, it is 0.5% or more and 10% or less.

In order to remove the coolant 13 contained in the solid content 26, when the coolant 13 is a water-soluble oil, the cleaning liquid is preferably water. In order to remove the adhesive waste 17 contained in the solid content 26, when the adhesive 13 is an epoxy resin adhesive, the cleaning liquid is preferably nitric acid. When nitric acid is used for the cleaning liquid, the concentration of nitric acid is not particularly limited, but is, for example, 2% or more and 67.5% or less.
In order to remove the dummy material waste 18 contained in the solid content 26, when the dummy material 5 is soda glass, the cleaning liquid is preferably hydrofluoric acid. Therefore, in the cutting process, when water-soluble oil is used for the coolant 13, an epoxy resin adhesive is used for the adhesive 3, and a soda glass plate is used for the dummy material 5, the solid content recovered from the waste slurry 20 is water. It is preferable to wash with nitric acid and hydrofluoric acid, respectively. The order of cleaning with these three cleaning liquids is arbitrary, but since the solid content 26 contains more coolant 13 than other impurities, it is preferable to wash with water as the cleaning liquid.

  Of the wire scraps 16 contained in the solid content 26, most of the plating components and wire strands can be dissolved by common inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid. Many of the bonds often remain undissolved in the acid. Therefore, it is desirable that the P concentration during wire plating is low from the viewpoint of regeneration of silicon scraps. For this reason, the nickel plating layer 23 included in the wire 8 can have a phosphorus concentration such that the phosphorus concentration derived from the nickel plating layer 23 in the solid content 26 is 0.1 ppm or less. Thereby, the phosphorus concentration of the solid content 26 derived from the nickel plating layer 23 can be lowered. The phosphorus concentration in the nickel plating layer 23 can be adjusted when the nickel plating layer 23 is formed on the bus bar 21. The phosphorus concentration derived from the nickel plating layer 23 in the solid content 26 can be obtained by the product of the nickel concentration in the solid content 26 and the phosphorus concentration in the nickel plating layer 23.

When the material that is hardly soluble in any of acid, alkali, and organic solvent is used as the dummy material 5, for example, when a carbon plate is used, the dummy material 5 has a phosphorus concentration derived from the dummy material 5 in the solid content 26. The phosphorus concentration derived from the nickel plating layer 23 can have a phosphorus concentration of 0.1 ppm or less. Thereby, the phosphorus concentration of the solid content 26 derived from the dummy material 5 can be lowered. The phosphorus concentration derived from the dummy material 5 in the solid content 26 is L _{1 for} the length of the cut silicon block 1, L _{2 for} the thickness of the adhesive 3, and L _{3 for} the length of the cut dummy material 5. In this case, it can be obtained by the product of L ₃ / (L ₁ + L ₂ + L ₃ ) and the phosphorus concentration in the dummy material 5.

When the adhesive 3 that is hardly soluble in any of acid, alkali, and organic solvent is used, the adhesive 3 is derived from the concentration of phosphorus derived from the adhesive 3 in the solid content 26 and the nickel plating layer 23. It can have a phosphorus concentration such that the sum with the phosphorus concentration is 0.1 ppm or less. Thereby, the phosphorus concentration of the solid content 26 derived from the adhesive 3 can be lowered. The phosphorus concentration derived from the adhesive 3 in the solid content 26 is L _{1 for} the length of the cut silicon block 1, L _{2 for} the thickness of the adhesive 3, and L _{3 for} the length of the cut dummy material 5. In this case, it can be determined by the product of L ₂ / (L ₁ + L ₂ + L ₃ ) and the phosphorus concentration in the adhesive 3.

  When both of the adhesive 3 and the dummy material 5 use materials that are hardly soluble in acids, alkalis, and organic solvents, the adhesive 3 and the dummy material 5 have a concentration of phosphorus derived from the adhesive 3 in the solid content 26 and The phosphorus concentration derived from the dummy material 5 and the phosphorus concentration derived from the nickel plating layer 23 can have a phosphorus concentration that is 0.1 ppm or less. As a result, the phosphorus concentration of the solid content 26 derived from the adhesive 3 and the dummy material 5 can be lowered.

4). Melting process In the melting process, the solid content 31 after the cleaning process is heated to a temperature equal to or higher than the melting point of silicon to form molten silicon 32. The method for melting the solid content 31 is not particularly limited. For example, after the solid content 31 is accommodated in a heat resistant container (crucible), the heat resistant container is heated using a heating device to obtain molten silicon. Can be mentioned. The melting process can be performed using, for example, an apparatus as shown in FIG.
The heat resistant container 30 is not particularly limited as long as the solid content 31 can be melted. For example, a container made of at least one selected from the group consisting of carbon, mullite, alumina, and magnesia is used. Can do.

The heating means 35 is not particularly limited as long as it can be heated to a temperature at which the solid content 31 can be dissolved. For example, a resistance heating device or an induction heating device can be used.
The melting step is preferably performed in an inert gas (nitrogen, argon, helium gas, etc.) atmosphere. This is because an increase in silicon oxide due to the oxidation of silicon can be suppressed. The atmospheric pressure in the melting step can be set to 152 Torr to 760 Torr, preferably 380 Torr to 760 Torr when the solid content 31 is added to the heat-resistant container 30 and melted.

In the melting step, the method of solidifying the molten silicon 32 is not particularly limited. For example, the cooling of the solid silicon 31 together with the heat-resistant container 30 or the method of cooling the molten silicon 32 by other methods such as tilting hot water is used. The method etc. which are accommodated in the container 50 and cooled with the cooling container 50 are mentioned. Examples of the cooling atmosphere include an air atmosphere or an inert gas atmosphere, and the pressure of the cooling atmosphere may be about atmospheric pressure or reduced pressure.
In addition, as the cooling container 50, for example, an inexpensive disposable one such as a silica crucible or a sand crucible or a reusable one such as a carbon mold or a water-cooled copper mold can be used.

The molten silicon 32 is formed by inclining the heat-resistant container 30, making a notch in the upper part of the heat-resistant container 30, or providing a hot water outlet that can be opened and closed on the lower or side surface of the heat-resistant container 30. The hot water can be discharged into the cooling container 50. Since the molten silicon 32 is in the form of hot water having a relatively low viscosity, only hot water having a low viscosity can be discharged from the heat-resistant container 30. Thereby, the molten silicon 32 can be separated from the silicon oxide 32 and discharged. By solidifying the melted molten silicon 32, a silicon lump 43 in which silicon oxide derived from silicon scrap oxide film (SiO _x ) is reduced can be obtained. Further, the abrasive grains 22 mixed from the wire 8 can be reduced together with the silicon oxide 33.

5. Segregation process After the melting process, a process of removing metal by segregation (segregation process) can be performed. By this step, metal components such as iron can be removed.
The method for removing the metal component by segregation is not particularly limited, and for example, conventionally known unidirectional solidification can be used.

6). Silicon Mass According to the above-described silicon regeneration method, for example, a high-purity silicon mass 44 in which the content of impurities of the types shown in the following (a) to (e) is reduced can be obtained.
(A) Boron content (weight): 0.3 ppm by weight or less (b) Phosphorus content (weight): 0.3 ppm by weight or less (c) Iron content (weight): 1 ppm by weight or less ( d) Silicon carbide content (weight): 100 ppm by weight or less (e) Oxygen content (weight): 100 ppm by weight or less

Examination of Phosphorus Contamination Source A study was conducted on the source of phosphorus contamination in the waste slurry 20 in the cutting step.
First, the fixed abrasive grains in which the soda glass plate which is the dummy material 5 is bonded with polycrystalline silicon for solar cells using an epoxy resin adhesive, diamond as the abrasive grains is welded to the busbar (steel material), and Ni plating is applied. The polycrystalline silicon for solar cells was cut using a coolant based on wire and glycol to produce a silicon wafer. The waste slurry discharged during the production of this silicon wafer was subjected to solid-liquid separation to obtain a solid content 26. The ratio of each component contained in the solid content 26 and the phosphorus concentration of the contamination source were examined. The results are shown in Table 1. The “phosphorus concentration derived from the contamination source” in Table 1 is the product of “content ratio of each component in the solid content” and “phosphorus concentration of the contamination source”.

Looking at Table 1, the phosphorus component in the solid content 26 recovered by the fixed abrasive method is as shown in Table 1. The phosphorus component in the silicon scrap, the phosphorus component in the coolant, and the phosphorus component contained in the nickel contained in the wire It was discovered that there are five types of phosphorus component in the adhesive and phosphorus component in the dummy material.
It should be noted that phosphorus is hardly included in the case of silicon scrap when cutting single-crystal silicon for semiconductors, but phosphorus is about 0.3 ppm by weight if it is silicon scrap when cutting polycrystalline silicon for solar cells. May be included.

The coolant may contain about 0.2 ppm by weight or less of phosphorus. Further, the coolant contained in the solid content can be easily removed by washing with water if it is water-soluble, or by washing with kerosene if it is water-insoluble.
For fixed abrasive wires, diamond is welded to the bus bar (steel) and Ni plating is mainly applied to improve the adhesion of diamond, but P is intended to be within 4% or less in order to improve adhesion. May be added. Ni is readily soluble in inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid, but the Ni-P bond is chemically stable and is relatively difficult to remove with a chemical solution.

The adhesive is an adhesive for bonding the polycrystalline silicon and the dummy material, and contains about 100 ppm by weight or less of phosphorus in the adhesive. For example, the phosphorus concentration after curing an epoxy adhesive (W bond, Q bond, F bond) manufactured by Nikka Seiko was 30 to 100 ppm by weight. The adhesive thickness is about 0.1 to 0.2 mm. The presence form of phosphorus in the adhesive is an organophosphorus compound C _a H _b O _c —P (a, b, c: any integer), and is considered to be easily soluble in the acid in which the adhesive dissolves. Epoxy adhesives are readily soluble in nitric acid.

When manufacturing a silicon wafer by cutting polycrystalline silicon, it is necessary to cut the dummy material beyond the polycrystalline silicon to the dummy material. Is also included. Usually, when cutting silicon with a multi-wire saw, the dummy material is also cut by about 1 to 5 mm.
As the dummy material, a plate-shaped material such as silicon edge material, glass, carbon, or wood can be used. Glass is readily soluble in hydrofluoric acid, but carbon and wood are poorly soluble in chemicals, so glass is desirable for the dummy material according to the present invention.

A cheap glass such as soda glass is used. The commercially available soda glass had a phosphorus concentration of 1 to 20 ppm by weight. The presence form of phosphorus in the glass is inorganic phosphoric acid (H ₂ PO ₃ or the like), which is easily soluble in an acid that dissolves the glass.
In FIG. 1, when L _{1 is} the cut length of silicon, L _{2 is the} adhesive thickness, and L _{3 is} the cut length of the dummy material,
Adhesive content = L ₂ ÷ (L ₁ + L ₂ + L ₃ ) × (100% −oil content) ≈0.05 to 0.1%
Dummy material content = L ₃ ÷ (L ₁ + L ₂ + L ₃ ) × (100% −oil content) ≈1 to 4%
It is.

Silicon regeneration experiment Cutting process While applying a coolant based on propylene glycol using a multi-wire saw with a nickel-plated wire (fixed abrasive wire) attached to the wire to prevent diamond peeling Then, the silicon block was cut to produce a silicon wafer.
The dummy material used at the time of cutting was soda glass that is easily soluble in hydrofluoric acid, and the adhesive was an epoxy adhesive that was easily soluble in nitric acid.

2. Recovery process The waste slurry produced in the cutting process was roughly removed by using a centrifugal separator to obtain a solid content. Table 2 shows the concentration of the impurity component contained in the solid content after each step of the “silicon regeneration experiment”.

The solid content recovered by the recovery process contained 50% oil, and B, P, heavy metals, etc. could not be measured.
Moreover, since the cutting process is performed using the fixed abrasive wire, the content rate of the abrasive component is very small compared to the case where the cutting process is performed using the free abrasive grains.

3. Cleaning step In the cleaning step, water cleaning, hydrofluoric acid cleaning, and nitric acid cleaning were performed in this order.
First, the solid content recovered in the recovery process was diluted with solid content: water = (1: 5) to (1:10), and solid-liquid separation was performed with a centrifuge to remove the oil content. In order to improve the removability of the oil component, washing was performed twice.
Table 2 shows the impurity concentration in the solid content after completion of the water washing. In addition, although silicon after completion of the water cleaning contains 30 to 50% of water, in order to perform analysis, the silicon which has been dried in a vacuum at 150 ° C. to remove moisture is analyzed. B, P, Fe, and Ni were measured by an ICP mass spectrometer, C was a combustion method, O was a fluorescent X-ray, and an oil component was measured by a thermobalance. After completion of water washing, the Ni concentration is high, which is considered to be due to the nickel plating of the wire. The C concentration is 2,000 ppm by weight, which is diamond as abrasive grains, carbon C _p H _q O _r (p, q, r: any integer) in residual oil, carbon of adhesive component It is considered that C _s H _t O _u (s, t, u: any integer) is added. O is present at about 10%, but is considered to be the sum of the surface oxide film, oxygen in the residual oil, and oxygen of the adhesive component.

  Since the Ni concentration in the solid content after washing with water is 200 ppm by weight, the phosphorus concentration in the nickel plating of the wire is controlled so that the nickel concentration derived from the nickel plating in the solid content is 0.1 ppm or less. It is necessary to use a wire that is controlled so that the phosphorus concentration in the nickel plating of the wire is 0.05% by weight or less in the cutting step. The nickel concentration derived from the nickel plating in the solid content was determined by the product of the nickel concentration in the solid content and the phosphorus concentration in the nickel plating.

Next, it was stirred for 1 hour with dilute hydrofluoric acid having a solid content of 1% after washing with water (1: 5), followed by solid-liquid separation using a filter press to remove the glass component.
Table 2 shows the impurity concentration of the solid content after completion of the hydrofluoric acid cleaning. The solid content after completion of the hydrofluoric acid cleaning contains 30 to 50% hydrofluoric acid aqueous solution. For analysis, rinse with water until the pH is close to 7, then dry at 150 ° C. in vacuum to remove moisture. Analyzing what was removed. B, P, Fe, and Ni were measured by an ICP mass spectrometer, C was a combustion method, O was a fluorescent X-ray, and oil components were measured by a thermobalance. P does not decrease much after completion of the hydrofluoric acid cleaning. This is because P present in the glass as a dummy material can be reduced with hydrofluoric acid, but P in nickel plating, that is, the Ni-P bond is hardly soluble in hydrofluoric acid. Conceivable.

Next, the hydrofluoric acid washed silicon: 5% nitric acid = (1: 5) was stirred for 1 hour, and solid-liquid separation was performed using a filter press to reduce the adhesive component. Furthermore, in order to remove an acid component, it washed with water until the filtrate became pH 7 vicinity, and dried in 150 degreeC vacuum.
Table 2 shows the impurity concentration of silicon after completion of the nitric acid cleaning and water cleaning. B, P, Fe, and Ni were measured by an ICP mass spectrometer, C was a combustion method, O was a fluorescent X-ray, and oil components were measured by a thermobalance. By washing with nitric acid, the adhesive component can be dissolved, and a slight decrease in C and P is observed. There is almost no difference in the O from the completion of the water washing to the completion of the nitric acid washing. This is probably because most of the O is caused by the surface oxide film of silicon scrap.

4). Melting process After the pre-melting cleaning process, the solid content after the cleaning process was melted. The melting process was performed using a melting furnace as shown in FIG.
In addition, since the solid content generated when cutting using fixed abrasive is very small compared to the case of 0.1% abrasive component and free abrasive, the influence of the abrasive component is small, There are merits in that the separability between Si and other components is good and the recovery rate is high.

First, the furnace body 53 having at least the crucible 30 and the heating means 35 for heating the crucible 30 and the tilting means 49 for tilting the crucible 30 was installed in the chamber 41.
In the crucible 30, the solid content 31 that had been washed before melting was added and melted. The atmosphere was nitrogen, and the atmosphere pressure was 380 Torr to 760 Torr. The temperature in the crucible 30 was set to 1412 ° C., which is the melting point of silicon, or about 1600 ° C. The molten silicon 32 was stirred using a carbon stirring rod 48.

Thereafter, the crucible 30 was tilted using the tilting means 49 to discharge the molten silicon 32 into the first hot water container 50. Since most of the material 33 remaining in the crucible 30 is silicon oxide, it was removed to the second hot water container 51 using physical means 52. The molten silicon in the first hot water container 5 was naturally cooled and solidified to obtain a silicon lump 43.
Table 2 shows the impurity concentration of the silicon lump 43 after the melting step. B, P, Fe, and Ni were measured by an ICP mass spectrometer, and C and O were measured by a combustion method. Since the influence of the surface oxide film is reduced by the agglomeration, the oxygen concentration becomes very small.

5. Segregation process In order to remove metal impurities from the silicon mass 43 after the melting process, unidirectional solidification using segregation was performed.
After the melting step, 250 kg of silicon lump, 250 kg of internal size, was put into a 680 × 680 mm silica crucible, and the silica crucible was put into a unidirectional solidification furnace having a predetermined hot zone (heating zone). After heating to 1450 ° C. to melt the silicon, the silica crucible was lowered at a rate of 10 mm / H and separated from the hot zone to solidify in one direction.
The obtained 250 kg silicon lump (about 680 mm × 680 mm × 230 mm) was cut with a band saw at a lower end of 5 mm, a side end of 5 mm, and an upper end of 10 mm, and then cut into pieces for easy handling.

  Table 2 shows the impurity concentration in the vicinity of the upper end of the silicon lump after the silicon lump is cut with a band saw. The obtained silicon lump can be used as a silicon raw material for solar cells with a phosphorus concentration of 0.28 ppm by weight, without performing vacuum melting or electron beam melting, which is a method for removing phosphorus from silicon. Concentration. Of these, about 0.1 ppm by weight of phosphorus is considered to be caused by the Ni-P plating residue. Moreover, the density | concentration of B and heavy metal was also a density | concentration which can be used as a silicon raw material for solar cells.

    1: Silicon block 3: Adhesive 5: Dummy material 8: Wire 10: Waste slurry container 11: Silicon wafer 13: Coolant 15: Silicon waste 16: Wire waste 17: Adhesive waste 18: Dummy waste 20: Waste slurry 21 : Busbar 22: Abrasive grain 23: Nickel plating layer 25: Cleaning liquid 26, 31: Solid content 27: Cleaning container 28: Stirrer 30, 38: Heat-resistant container (crucible) 32, 37: Molten silicon 33: Silicon oxide 35, 39: heating means 41: chamber 43, 44: silicon block 48: stirring rod 49: tilting means 50: first cooling vessel 51: second cooling vessel 52: scraping rod 53: furnace body

Claims (8)

Silicon obtained by cutting silicon blocks to obtain regenerated silicon having a phosphorus concentration that can be used as a raw material for solar cells, without any special dephosphorization treatment such as vacuum melting method or electron beam method from silicon by-product generated as a by-product A playback method,
In a state where the silicon block is bonded to the dummy material using an adhesive, a silicon wafer is obtained by cutting a part of the silicon block and the dummy material using a wire and a coolant having abrasive grains fixed on the surface. , A step of recovering the silicon scrap-containing solid content from the waste slurry containing silicon scrap generated as a by-product,
Washing the collected solids with a first washing liquid and a different second washing liquid;
Melting the washed solid to obtain molten silicon;
A step of solidifying molten silicon,
The method of reclaiming silicon, wherein the dummy material is easily soluble in the first cleaning liquid, and the adhesive is easily soluble in the second cleaning liquid.   The method according to claim 1, wherein the phosphorus concentration is 0.32 ppm by weight or less. The dummy material is soda glass,
The method according to claim 1 or 2, wherein the first cleaning liquid is 0.5% to 10% hydrofluoric acid. The adhesive is an epoxy resin adhesive,
The method according to any one of claims 1 to 3, wherein the second cleaning liquid is nitric acid of 2% to 67.5%.   The method according to claim 1, wherein the adhesive is easily soluble in the first cleaning liquid. The wire has a nickel plating layer,
The method according to claim 1, wherein the nickel plating layer has a concentration of phosphorus of 0.05% by weight or less.   The method according to any one of claims 1 to 6, wherein the step of solidifying the molten silicon includes a step of solidifying the molten silicon discharged from the cooling vessel.   The method according to any one of claims 1 to 7, further comprising a step of heating and melting the silicon lump formed by the step of solidifying the molten silicon to a temperature equal to or higher than the melting point of silicon and solidifying in one direction.

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