JP2012111672A - Method for purifying silicon and purified silicon - Google Patents

Abstract

A method for purifying silicon capable of purifying silicon with high purity is provided.
The present invention relates to a silicon refining method for obtaining purified silicon from silicon scrap, a step of melting silicon scrap to form a molten silicon, and solidifying molten silicon obtained from the molten silicon to form a silicon lump. Forming a silicon melt by melting silicon scraps under a first pressure to form a silicon melt, and a second pressure lower than the first pressure. Degassing below.
[Selection] Figure 1

Description

  The present invention relates to a silicon purification method and purified silicon, and more particularly, to a silicon purification method and purified silicon from silicon scrap generated when silicon is machined.

High-purity silicon used for semiconductor integrated circuits and the like is made from metal silicon having a purity of 98% or more obtained by reducing carbon of silica with carbon, and synthesizes trichlorosilane (SiHCl ₃ ) by a chemical method. This is purified by a distillation method and then reduced to obtain high-purity silicon having a so-called 11N (Eleven-Nine) level (Siemens method). However, this high-purity silicon inevitably becomes an expensive material because of the amount of energy used for complicated manufacturing plants and reduction.

  On the other hand, the purity required for silicon used in the production of solar cells is about 6N. Therefore, non-standard products of high-purity silicon such as those for semiconductor integrated circuits become excessively high quality for solar cells. Direct metallurgical refining from metal silicon having a purity of about 2N to 3N in parallel with the recycling of high-purity silicon obtained from each process of manufacturing semiconductor integrated circuits in order to reduce the cost of solar cells Has been tried.

For example, Patent Document 1 discloses a silicon purification method of the flowchart shown in FIG. The conventional method for purifying silicon from metallic silicon described in Patent Document 1 includes the following steps.
A. Metallic silicon is melted under vacuum, the phosphorus contained therein is vaporized and dephosphorized, and then unidirectional solidification is performed to remove impurity components from the molten metal to obtain an ingot (see step S1b and step S2b). .
B. The impurity-enriched portion of the ingot is cut and removed.
C. The remaining part after cutting and removing is redissolved, and boron and carbon are oxidized and removed from the molten metal in an oxidizing atmosphere. Subsequently, argon gas or a mixed gas of argon and hydrogen is blown into the molten metal to deoxygenate (see step S3b).
D. The molten metal after the deoxidation is cast into a mold and solidified in one direction to obtain an ingot (see step S4b).
E. The impurity concentrated portion of the ingot obtained by unidirectional solidification is cut and removed, and this is further sliced to obtain a silicon substrate.

  In recent years, the use of natural energy as an alternative to oil and the like has attracted attention due to environmental problems, and in particular, the production of solar cells that can easily convert solar energy into electrical energy has been increasing. I'm following. For this reason, the demand for raw material silicon has been growing rapidly, and the shortage of silicon for solar cells has become apparent.

  In the manufacturing process of single crystal or polycrystalline silicon wafers widely used for IC chips, solar cells, etc., about 60% of the raw material silicon is discarded and disposed of in waste liquid in processes such as cutting, chamfering or polishing. For this reason, the load on the production cost of the silicon wafer that is the final product and the environmental load associated with disposal are serious problems.

  Therefore, conventionally, a method has been proposed in which silicon waste discarded in the waste liquid is recovered in the steps such as cutting, chamfering or polishing, and purified silicon is purified by purifying the recovered silicon waste ( For example, see Patent Document 2). Hereinafter, with reference to FIG. 7, a conventional silicon purification method from silicon scrap described in Patent Document 2 will be described.

  First, in step S1c, a solid slurry (cake) is separated by filtering waste slurry generated when slicing single crystal or polycrystalline silicon with a wire saw (solid content separation step).

  Next, in step S2c, the solid content (cake) separated as described above is put into a washing tank together with an organic solvent such as acetone and stirred to perform organic solvent washing (organic solvent washing step).

  Next, in step S3c, the solid content (cake) after organic cleaning is washed with water to remove an organic solvent such as acetone, and then acid cleaning is performed using a mixed solution of an aqueous hydrogen fluoride solution and sulfuric acid (acid cleaning). Process).

  Finally, in step S4c, the solid content (cake) after the acid cleaning is dried and then pulverized, and by using the difference in density between the silicon powder and the abrasive grains made of silicon carbide, classification using an airflow classifier Silicon powder is separated and high-purity silicon powder is recovered (airflow classification process).

Japanese Patent No. 3325900 JP 2001-278612 A

  However, in the silicon refining method described in Patent Document 2, since there is no significant difference in density and particle size between silicon and silicon carbide, the silicon powder and the abrasive grains made of silicon carbide are sufficiently obtained by the airflow classification process. There was a problem that it could not be separated. Moreover, since the particle diameter of the recovered silicon powder is as small as about 1 μm, there is a problem that the influence of the surface oxide film cannot be ignored and the silicon powder contains several percent of oxygen.

  Furthermore, in order to obtain a silicon substrate using this silicon powder, it is necessary to melt the silicon powder, but since the particle diameter of the silicon powder is small, the thermal conductivity tends to be low when melting. For this reason, there is a problem that a part of the gas is not melted and gas remains in the molten silicon, and as a result, good purified silicon cannot be obtained.

  The above problem also applies to the case where purified silicon is prepared by melting silicon waste. That is, when obtaining purified silicon through a silicon scrap melting step and a silicon lump forming step, it is possible to obtain good purified silicon even when silicon dust having a small particle diameter is contained in the silicon scrap. A similar problem arises that it is impossible.

  In view of the above circumstances, an object of the present invention is to provide a silicon purification method capable of purifying silicon with high purity and purified silicon obtained by the method.

  The present invention is a silicon purification method for obtaining purified silicon from silicon scrap, a step of melting silicon scrap to form a silicon melt, and a step of solidifying molten silicon obtained from the silicon melt to form a silicon lump. The step of forming the molten silicon includes a step of melting silicon scraps under a first pressure to form a molten silicon, and deaeration of the molten silicon under a second pressure lower than the first pressure. And a silicon purification method comprising the steps of:

  In the present invention, the first pressure is preferably 152 Torr or more and 760 Torr or less, and the second pressure is preferably 76 Torr or more and less than 152 Torr.

  In the present invention, the first pressure is preferably 380 Torr or more and 760 Torr or less.

  In the present invention, the step of forming the molten silicon preferably includes a step of placing the degassed molten silicon under a first pressure.

  In the present invention, it is preferable to reduce the pressure to a second pressure at a speed of 38 Torr / min or less from the first pressure.

  Moreover, in this invention, it is preferable to further include the process of collect | recovering silicon | silicone waste from the waste material generated when machining a silicon | silicone.

  Moreover, in this invention, it is preferable that a silicon | silicone waste contains silicon powder with a particle diameter of 10 micrometers or less.

  Moreover, in this invention, it is preferable to further include the process of pre-cleaning silicon waste, before melting silicon waste.

  Further, the present invention provides the above-described silicon purification method, wherein (1) a step of washing the silicon mass with at least one of the first acid solution and the first basic solution, (2) phosphorus from the silicon mass. It is preferable to further include at least one of a step of removing, (3) a step of removing metal from the silicon lump by segregation, and (4) a step of removing metal from the silicon lump by leaching.

  The present invention is a purified silicon purified by the above-described silicon purification method, wherein the boron content in the purified silicon is 0.3 ppm or less, the phosphorus content is 0.3 ppm or less, and the iron content Is a purified silicon having a carbon content of 100 ppm or less and an oxygen content of 100 ppm or less.

  ADVANTAGE OF THE INVENTION According to this invention, the refinement | purification silicon obtained by the refinement | purification method of silicon which can refine | purify silicon with high purity, and that method can be provided.

It is a flowchart of an example of the silicon purification method of this embodiment. It is a schematic sectional drawing which shows an example of the silicon purification apparatus used suitably for the silicon purification method of this embodiment. It is a figure for demonstrating the state of a silicon molten metal. It is a figure for demonstrating a hot water discharge process. It is a figure for demonstrating a hot water discharge process. 10 is a flowchart of a silicon purification method described in Patent Document 1 of the related art. 10 is a flowchart of a conventional silicon purification method described in Patent Document 2.

  Embodiments of the present invention will be described below. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts.

  FIG. 1 shows a flowchart of an example of the silicon purification method of the present embodiment. Hereinafter, an example of the silicon purification method of the present invention will be described with reference to FIG.

<Recovery process of silicon scrap>
First, in step S1a, a step of collecting silicon scraps generated when silicon machining is performed (a step of recovering silicon scraps).

  Here, the machining of silicon is not particularly limited, and examples thereof include machining such as cylindrical grinding, cutting, polishing, slicing, cutting, and peripheral cutting of silicon. The silicon machining is performed using, for example, at least one selected from the group consisting of a coolant (water or lubricating oil or a mixture thereof), a cutting blade, a wire, loose abrasive grains, and fixed abrasive grains. May be.

  As the free abrasive grains, for example, powdery abrasive grains can be used, and they are generally used in a free state in water and / or lubricating oil. In addition, as the fixed abrasive, for example, powdery abrasive can be used, and is generally used in a state of being fixed to the wire with a binder or the like.

The material of the loose abrasive and the material of the fixed abrasive are not particularly limited. For example, at least one selected from the group consisting of silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), or diamond (C) is used. Those containing seeds can be used.

  Silicon scrap generated when silicon is machined can be recovered from wastes (waste liquid and dust collected) by silicon machining. Silicon scrap is a powdery substance containing silicon powder generated by silicon machining.

The waste liquid produced by machining silicon is selected from the group consisting of (2) to (5) below, for example, in addition to the silicon powder produced when silicon is shaved, depending on the machining of silicon. At least one species is included.
(2) Moisture (3) Abrasive grains generated from fixed abrasive grains or free abrasive grains used in silicon machining (4) Metal components generated from cutting blades or wires (5) Oil components of lubricating oil Recovery of silicon waste from waste liquid by machining is, for example, (i) a method of acquiring silicon waste by capturing and drying solids using a filter or a centrifuge, and (ii) recovering waste liquid. It can be performed by a method including at least one of a method for obtaining silicon waste by heating or distillation, and (iii) a method for obtaining silicon waste by using a flocculant with respect to the waste liquid.

<Pre-melting cleaning process>
Next, in step S2a, it is preferable that a step of pre-cleaning silicon scrap is performed before melting the silicon scrap recovered in the step of recovering silicon scrap (pre-melting cleaning step).

  For the cleaning, a cleaning liquid containing at least one selected from the group consisting of an acid solution (second acid solution), an organic solvent, and water can be used. Thereby, at least one impurity selected from the group consisting of boron, phosphorus, a metal component, and an oil component can be removed from silicon scrap.

  The acid solution (second acid solution) used for the acid cleaning of the silicon scrap is not particularly limited. For example, at least one selected from the group consisting of hydrochloric acid, sulfuric acid, hydrogen fluoride aqueous solution, and the like is used. More than once. The organic solvent is not particularly limited. For example, the organic solvent can be used one or more times using an organic solvent containing at least one selected from the group consisting of isopropyl alcohol and ethanol.

  Moreover, although the cleaning method of a silicon | silicone waste is not specifically limited, For example, the method etc. which immerse a silicon | silicone waste in said acid solution (2nd acid solution) and stir etc. are mentioned. In addition, the pre-melting cleaning process may be a process for cleaning silicon waste.

<Melting process>
Next, in step S3a, a process is performed in which the silicon scrap cleaned in the pre-melting cleaning process is melted to form a silicon melt (melting process).

  In this step, silicon scrap is melted under a first pressure to form a molten silicon (silicon melt forming step), and the molten silicon is deaerated under a second pressure lower than the first pressure. Step (deaeration step) is performed.

  The silicon scrap melting method is not particularly limited. For example, after the silicon scrap is contained in a heat-resistant container capable of controlling the atmospheric pressure, the heat-resistant container is heated under the first pressure, and then the atmospheric pressure is set to the second pressure. The method of adjusting to a pressure etc. is mentioned. In the present embodiment, a case will be described in which this step is performed using the silicon purification apparatus 100 shown in FIG.

(Configuration of silicon purification equipment)
First, the configuration of the silicon purification apparatus 100 will be described.

  FIG. 2 is a schematic cross-sectional view showing an example of a silicon purification apparatus suitably used in the silicon purification method of the present embodiment. In FIG. 2, the silicon purification apparatus 100 includes a decompression container 8, and the internal pressure of the decompression container 8 can be controlled by a pressure control unit (not shown). The pressure controller is preferably capable of introducing an inert gas such as argon, helium and nitrogen into the decompression vessel 8. In addition, the decompression vessel 8 is provided with an insertion port 9 for introducing silicon scrap inside.

  Inside the decompression vessel 8, a melting unit 13 is installed that includes a heat-resistant container 10 for storing silicon waste, a heating unit 11 for heating the heat-resistant container 10, and a drive unit 12 for moving the heat-resistant container 10. . When the heating unit 11 disposed on the outer periphery of the heat-resistant container 10 heats the heat-resistant container 10, silicon scraps accommodated in the heat-resistant container 10 can be melted.

  The material of the heat-resistant container 10 is not particularly limited as long as it has heat resistance against the temperature at which silicon scrap is melted. For example, at least one selected from the group consisting of carbon, mullite, alumina, magnesia, and a water-cooled copper mold A container made of can be used. The heating unit 11 is not particularly limited as long as it can heat the heat-resistant container 10 to a temperature at which silicon scraps can be dissolved. For example, a resistance heating device or an induction heating device may be used. it can. The drive unit 12 can tilt the heat-resistant container 10 to discharge the molten silicon in the heat-resistant container 10.

  The decompression vessel 8 further includes a first housing portion 14 for housing the molten silicon discharged from the heat-resistant container 10 and a second housing portion 15 for housing residues adhering to the heat-resistant container 10. And are arranged. It is preferable that the 1st accommodating part 14 and the 2nd accommodating part 15 are arrange | positioned on the conveyance part 16. FIG. With this configuration, the positions of the first housing portion 14 and the second housing portion 15 can be freely moved. Further, it is preferable that a residue collection unit 17 for scraping the residue adhering in the heat-resistant container 10 and moving it into the second storage unit 15 is installed inside the decompression vessel 8.

  The 1st accommodating part 14 will not be specifically limited if a molten silicon can be accommodated, A carbon crucible, a water-cooled copper crucible, a silica crucible, the crucible which sintered sand etc. can be used. The 2nd accommodating part 15 will not be specifically limited if a residue can be accommodated. Here, the residue is an adhesive substance made of silicon oxide, abrasive grains, etc. contained in the molten silicon. Moreover, the residue collection | recovery part 17 is not restrict | limited in particular, For example, it is preferable that it consists of high melting point metals, such as carbon and molybdenum, and the front-end | tip is a spatula-like rod-shaped body.

  The decompression vessel 8 is provided with a discharge pipe 19 connected to the blower 18. A filter 20 is installed in the discharge pipe 19, and the discharge port of the discharge pipe 19 is disposed in the vicinity of the heat-resistant container 10. With this configuration, the vaporized substance rising from the heat-resistant container 10 can be captured by the filter 20 while discharging the vaporized substance out of the decompression container 8. In addition, the structure demonstrated above is only an example of the structure of the refiner | purifier used for this process.

  Hereinafter, a silicon melt forming process and a deaeration process using the above-described silicon purification apparatus 100 will be described.

(Silicon melt forming process)
First, a step of forming a silicon melt under a first pressure is performed (silicon melt formation step).

  Specifically, after the silicon scrap introduced from the inlet 9 is accommodated in the heat-resistant container 10, the pressure in the decompression container 8 is adjusted to 152 Torr or more and 760 Torr or less by a pressure control unit (not shown). Thereby, silicon scraps are placed under a first pressure of 152 Torr or more and 760 Torr or less.

  And when the heating part 11 heats the heat-resistant container 10, a silicon | silicone waste will fuse | melt and a silicon melt will be formed. The heating temperature may be a temperature at which silicon in silicon scraps melts, and may be 1412 ° C. or higher, which is the melting point of silicon. Further, by setting the heating temperature to about 1600 ° C., the separation between silicon and silicon oxide can be further advanced. Furthermore, by setting the heating temperature to 1800 ° C. or lower, the evaporation of silicon from the molten silicon can be suppressed, and the generation of white smoke due to the scattering of oxide in the molten silicon can be suppressed. It should be noted that this step is not limited to the introduction of silicon scrap, pressure adjustment, and heating in this order, and may be a step in which silicon scrap is melted under the first pressure.

  The formed silicon melt contains abrasive grains, silicon oxide, oil components and the like in addition to molten silicon. Under the condition that silicon scrap is melted, silicon oxide formed by, for example, natural oxidation of silicon exists in a state having a higher viscosity than molten silicon. For this reason, by stirring the silicon melt in the heat-resistant container 10 by convection or the like, a silicon oxide having a high viscosity can be attached to the wall of the heat-resistant container 10. In addition, abrasive grains made of silicon carbide or the like can efficiently adhere to the silicon oxide adhering to the wall of the heat-resistant container 10.

Here, the first pressure when forming the silicon melt is preferably 152 Torr or more. When forming the silicon melt, the pressure in the decompression vessel 8 is set to 152 Torr or more, so that it is possible to prevent the silicon oxide (SiO _x ) vapor semi-vaporized from being always generated in the silicon melt. . Thereby, the fall of the collection | recovery rate of silicon | silicone by scattering of the unmelted silicon | silicone waste can be prevented. Further, by setting the upper limit of the pressure in the decompression vessel 8 to 760 Torr or less, the pressure resistance design of the silicon purification apparatus 100 can be simplified.

  As the first pressure, the pressure in the decompression vessel 8 is preferably set to 380 Torr or more and 760 Torr or less. By setting the pressure in the decompression vessel 8 to 380 Torr or more, generation of white smoke due to oxide scattering in the molten silicon can be more efficiently suppressed. Furthermore, silicon scraps may be added to the molten silicon. In addition, this step is preferably performed in an inert gas atmosphere.

(Deaeration process)
Next, a step of degassing the molten silicon under a second pressure is performed (a degassing step).

  Specifically, the pressure control unit (not shown) adjusts the pressure in the decompression vessel 8 to a second pressure of 76 Torr or more and less than 152 Torr, and the molten silicon is placed under this pressure. At least, the molten silicon is preferably placed for at least 1 minute under these conditions, and more preferably for about 5 minutes. The reason for performing this process is as follows.

  Silicon scraps contained in the heat-resistant container 10 are silicon scraps generated when silicon is machined, and include silicon powder having a small particle diameter, for example, a particle diameter of about 1 to 10 μm. The particle diameter can be measured using, for example, a laser diffraction particle size distribution measuring device. In silicon scrap containing silicon powder having such a small particle diameter, the contact area between silicon powders is small and the thermal conductivity thereof is low. Therefore, even if the silicon melt forming step is performed, There will be silicon scrap that has not been done. In this case, bubbles, that is, gases such as air, are present around the undissolved silicon powder in the molten silicon. If there is silicon powder and bubbles that are not dissolved in the molten silicon, the bubbles inhibit the heat conduction in the molten silicon, so that the temperature rise of the entire molten silicon is suppressed. As a result, the melting of the silicon powder is inhibited, so that the separation between the molten silicon and the silicon oxide and the separation between the molten silicon and the abrasive grains are deteriorated.

  After the above-described silicon melt forming step, the pressure applied to the silicon melt is lowered, and the silicon melt is placed under a second pressure of 76 Torr or more and less than 152 Torr, whereby the gas in the silicon melt can be extracted. Moreover, when the gas in the molten silicon escapes, the thermal conductivity of the molten silicon can be improved, and silicon powder having a small particle diameter can be melted. As a result, the molten silicon is uniformly formed, and as a result, the separability between the molten silicon in the molten silicon, the silicon oxide, and the abrasive grains is improved.

  Therefore, by performing the silicon melt forming process and the degassing process in the melting process, as shown in FIG. 3, the silicon melt 21 in the heat-resistant container 10 is made to have a low-viscosity molten silicon 22, silicon oxide 23, and It can be separated from the adhesive substance 25 containing the abrasive grains 24 with high separability.

  For the above reason, the deaeration process is performed after the silicon melt forming process. In particular, by setting the second pressure to a pressure lower than the first pressure, specifically less than 152 Torr, the gas can be effectively extracted from the molten silicon. Moreover, by setting it as 76 Torr or more, generation | occurrence | production of remarkable white smoke and silicon evaporation can be prevented.

In the degassing step, silicon oxide vapor (SiO _x ) semi-vaporized from the molten silicon is easily generated. For this reason, as shown in FIG. 2, it is preferable to suck the vapor generated from the molten silicon by the blower 18 from the discharge pipe 19. Further, it is preferable that the sucked silicon oxide vapor is captured by the filter 20 and further the filter 20 is periodically cleaned. The blower is preferably operated only in this deaeration process.

  Moreover, it is preferable that the rate of pressure decrease is 36 Torr / min or less. Thereby, the jumping phenomenon (splash phenomenon) of the molten silicon due to the sudden drop in pressure from the first pressure to the second pressure can be suppressed. Moreover, it can also suppress that the inside of the pressure-reduced container 8 is contaminated by the generation | occurrence | production speed | rate of silicon oxide vapor | steam being too quick.

  Furthermore, it is preferable to return the pressure in the decompression vessel 8 to 152 Torr or more and 760 Torr or less and put the molten silicon again under the first pressure before performing a silicon lump forming process described later. Thereby, generation | occurrence | production of white smoke can be suppressed.

<Tapping process>
Before performing the silicon lump formation process described later, it is preferable to prevent a silicon oxide or abrasive grains from reattaching to the silicon lump by performing a process of pouring molten silicon from the heat-resistant container 10 (a tapping process). This step will be described with reference to FIG.

  FIG. 4 is a diagram for explaining the hot water process. In FIG. 4, the heat-resistant container 10 that stores the molten silicon is tilted by the drive unit 12 to discharge the molten silicon 22 into the first storage unit 14. By letting out the molten silicon 22 having a low viscosity from the heat-resistant container 10, separation from the adhesive substance 25 is facilitated, and abrasive grains and silicon oxide contained in the molten silicon 22 can be reduced.

  In addition, this step is not limited to the above method, and is performed using a hot water outlet such as a notch provided in the upper part of the heat resistant container 10 or an openable and closable hot water outlet provided in the lower part or the side of the heat resistant container 10. Can do.

  As shown in FIG. 5, after the molten silicon 22 is discharged, the adhesive substance 25 adhering to the wall of the heat-resistant container 10 is scraped out using the residue collection unit 17, and is stored in the second storage unit 15. It may be accommodated. Thereby, the inside of the heat-resistant container 10 can be cleaned, and the heat-resistant container 10 can be easily reused. Moreover, the structure which can convey the 1st accommodating part 14 and the 2nd accommodating part 15 to the exterior of the pressure reduction container 8 by the conveyance part 16 may be sufficient.

<Silicon lump formation process>
Returning to FIG. 1, next, in step S4a, a step of solidifying the molten silicon after the melting step to form a silicon lump is performed (silicon lump forming step).

  Here, the method of solidifying the molten silicon is not particularly limited. For example, as described above, the method of cooling the heat-resistant container in which silicon waste is melted (see FIG. 3) or the method of tilting out the molten silicon (see FIG. 3). 4)), and a method of cooling in the cooling container and the like. 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.

  As the cooling container, 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.

  At least one of silicon oxide and abrasive grains is efficiently removed from the formed silicon mass through the melting step. Therefore, as shown in FIG. 1, the silicon lump after the silicon lump forming step may be used as purified silicon as it is.

<Washing step with first acid solution>
After step S4a, as shown in step S5a, it is preferable to perform a step of washing the silicon mass obtained in the above-described silicon mass formation step with the first acid solution.

In this step, for example, the reattachment impurities reattached when the silicon scrap is melted in the melting step can be removed. Examples of the reattachment impurity include boron, a boron compound (for example, B ₂ O ₃ ), phosphorus, a phosphorus compound (for example, P ₂ O ₅ ), an abrasive grain made of silicon oxide, silicon carbide, and the like.

  Further, in this step, it is possible to remove the metal (iron, aluminum, etc.) and the metal compound (iron silicide, etc.) in the silicon lump simultaneously with the removal of the reattached impurities. In order to improve the removal efficiency, it is preferable to crush the silicon lump appropriately (for example, a diameter of 1 cm to 10 cm).

  The type, temperature, and cleaning conditions of the acid solution used for the cleaning step with the first acid solution are not particularly limited as long as the above-described reattachment impurities can be removed. In order to improve the cleaning effect, an aqueous hydrogen fluoride solution Alternatively, it is preferable to use a mixed solution of an aqueous hydrogen fluoride solution and nitric acid. The hydrogen fluoride concentration of the aqueous hydrogen fluoride solution is preferably 0.5% by mass or more and 10% by mass or less. In the mixed solution of hydrogen fluoride aqueous solution and nitric acid, the hydrogen fluoride concentration is preferably 0.5% by mass or more and 10% by mass or less, and the nitric acid concentration is 0.1% by mass or more and 1% by mass or less. It is preferable.

  Moreover, it is preferable that the temperature of hydrogen fluoride aqueous solution and a mixed solution is room temperature (20 degreeC or more and 30 degrees C or less), and is good also as 30 degreeC or more and 50 degrees C or less in order to raise a cleaning effect. Examples of the cleaning method include a method in which a silicon lump is immersed in a hydrogen fluoride aqueous solution and stirred. In addition, as shown in FIG. 1, it is good also considering the silicon lump after the washing | cleaning process by a 1st acid solution as refined silicon as it is.

<Washing step with first basic solution>
Next, in step S6a, it is preferable to perform a step of washing the silicon block after the washing step with the first acid solution with the first basic solution. The silicon lump after washing with the first acid solution can be used as it is in the washing step with the first basic solution, but before the washing step with the first basic solution, Performing a cleaning step with pure water or the like (removing the first acid solution remaining in the silicon lump) and / or a drying step (heating at room temperature or higher than the melting point of silicon or holding under reduced pressure). preferable.

  In this step, for example, the reattachment impurities reattached to the silicon lump can be removed in the same manner as the cleaning step using the first acid solution. A particularly large cleaning effect can be obtained by combining the cleaning step with the first acid solution and the cleaning step with the first basic solution.

  The kind, temperature, and washing conditions of the basic solution used in the washing step with the first basic solution are not particularly limited as long as the above-mentioned reattachment impurities can be removed. For example, an aqueous sodium hydroxide solution, an aqueous calcium hydroxide solution, or an aqueous sodium carbonate solution can be used. In order to improve the cleaning effect, it is most desirable to use an aqueous sodium hydroxide solution. The solute concentration in the first basic solution is preferably 1% by mass or more and 5% by mass or less, and more preferably 3% by mass or more and 5% by mass or less.

  Moreover, it is preferable that the temperature of the 1st basic solution in the washing | cleaning process by a 1st basic solution is normal temperature (20-30 degreeC), and in order to raise a washing | cleaning effect, you may be about 30-50 degreeC. . Examples of the cleaning method include a method in which a silicon lump is immersed in an aqueous sodium hydroxide solution and stirred.

  By performing the cleaning step with the first acid solution and the cleaning step with the first basic solution in this order, from the silicon lump, from the group consisting of abrasive grains such as silicon oxide and silicon carbide, boron and phosphorus. The at least one selected impurity can be efficiently removed. In addition, as shown in FIG. 1, the silicon lump after the washing step with the first acid solution or the washing step with the first basic solution may be used as purified silicon as it is.

<Washing step with third acid solution>
After the cleaning step with the first basic solution (step S6a), as shown in step S7a, the step of cleaning the silicon mass cleaned with the first basic solution with the acid solution (third acid solution) Is preferably performed.

  As the third acid solution, for example, hydrochloric acid, sulfuric acid, hydrofluoric acid aqueous solution or the like can be used. Although it is desirable that it is the normal temperature (20-30 degreeC) of the 3rd acid solution in the washing | cleaning process by a 3rd acid solution, in order to raise a cleaning effect, you may be about 30-50 degreeC. The cleaning method using the third acid solution is not particularly limited, and examples thereof include a method in which a silicon lump is immersed in an acid solution (third acid solution) and stirred.

  By performing the cleaning step with the third acid solution after the cleaning step with the first basic solution, the portion terminated with -OH on the surface of the silicon lump is replaced with -H to reduce the oxygen concentration. Can do.

<Dephosphorization process>
Further, after any one of steps S4a to S7a, a step of removing phosphorus from the silicon mass (dephosphorization step) can be performed as shown in step S8a.

The method for removing phosphorus from the silicon mass is not particularly limited. For example, the molten silicon melted by heating the silicon mass or irradiating the silicon mass with an electron beam has a pressure of about 10 ⁻² Pa to 10 ⁻¹ Pa. For example, a method of removing phosphorus from molten silicon by installing in a container can be used.

  Note that the molten silicon from which phosphorus has been removed may be solidified again to form a silicon lump, or may be sent to a segregation step (step S10a) to be described later in a molten state. It goes without saying that other components such as oxygen may be removed together with phosphorus by this step.

<Leaching process>
In addition, after any of the steps S4a to S7a, as shown in step S9a, after the silicon lump is crushed, the acid solution (fourth acid solution) and the basic solution (second basic solution) It is also possible to perform a leaching step of cleaning at least one of them. Thereby, impurities, mainly metal components such as iron, can be removed.

  That is, metal components such as iron are precipitated as metal fine particles or metal compounds (especially silicide) at the crystal grain boundaries of the silicon lump, so that these metal components are exposed on the surface of the pulverized powder by crushing the silicon lump. In addition, at least one of an acid solution (fourth acid solution) and a basic solution (second basic solution) can be washed and removed by etching.

  As the acid solution (fourth acid solution), for example, at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, an aqueous hydrogen fluoride solution, and the like can be used. As the basic solution (second basic solution), for example, an aqueous sodium hydroxide solution can be used.

  Further, in this step, the silicon lump is pulverized so that a powder having a particle size of 0.1 to 2 mm, preferably a powder having a particle size of 1 mm or less has a particle size distribution of 98% or more. The particle size distribution of the powder can be measured by a method based on JIS R1629-1997.

  The method for washing at least one of the acid solution (fourth acid solution) and the basic solution (second basic solution) after pulverizing the silicon mass is not particularly limited, but for example, powder after pulverizing the silicon mass A method of immersing and stirring in at least one of an acid solution (fourth acid solution) and a basic solution (second basic solution) can be used.

<Segregation process>
Moreover, it is preferable that the process (segregation process) of removing a metal by segregation is performed after any process of step S4a-step S7a, as shown to step S10a. Thereby, metal components, such as iron, can be removed. In the segregation step, at least one impurity selected from the group consisting of metal components, abrasive grains, and oxygen may be removed. The method for removing the metal component by segregation is not particularly limited. For example, conventionally known unidirectional solidification or rotational segregation can be used.

  As mentioned above, although each process was explained in full detail, in silicon refining, it is not necessary to carry out about each process of Step S5a-Step S7a. Moreover, when performing, it is not necessary to perform all the processes of step S5a-step S7a, For example, you may perform any one process. As described above, it is preferable to perform all the steps in the order shown in FIG.

  Further, the steps S8a to S10a may be performed or not performed in the silicon purification. Moreover, when performing, it is not necessary to perform all the processes of step S8a-step S10a, For example, you may perform any one process. When all the processes are performed, the order of the processes is not limited, but it is particularly preferable to perform the segregation process in step S10a last. Moreover, each process of step S8a-S10a may be after any process of step S4a-S7a.

<Action>
According to the above-described silicon purification method, for example, high-purity purified silicon in which the content of impurities of the types shown in the following (a) to (e) is reduced as described below can be obtained.
(A) Content (weight) of boron (B): 0.3 ppm or less (b) Content (weight) of phosphorus (P): 0.3 ppm or less (c) Content (weight) of iron (Fe): 1 ppm or less (d) Carbon (C) content (weight): 100 ppm or less (e) Oxygen (O) content (weight): 100 ppm or less In order to obtain high purity purified silicon by the silicon purification method of the present invention In this process, a silicon scrap recovery process, a pre-melting cleaning process, a melting process, and a silicon lump forming process are performed. In particular, in the melting process, it is important that a silicon melt forming process and a degassing process are performed. Thereby, even if it is a case where silicon powder with a small particle diameter was contained in silicon scrap, a uniform silicon melt can be formed. Furthermore, since the silicon oxide and silicon carbide are efficiently removed from the molten silicon, as a result, high-purity purified silicon as described above can be obtained.

  Further, after the silicon scrap recovery process, the pre-melting cleaning process, the melting process, and the silicon lump forming process, the cleaning process using the first acid solution and the cleaning process using the first basic solution may be performed in this order. preferable.

  Needless to say, other steps may be included before and after each of the above steps. As an example of another process, a deboron process (by a known method such as plasma refining using an oxidizing gas or a slag method) performed after the melting process and before the silicon lump formation process can be given.

  In addition, since it is not necessary to perform organic cleaning using an organic solvent such as acetone in the silicon purification method of the present invention, high-purity purified silicon as described above can be obtained at low cost.

  As Example 1, purified silicon was prepared by collecting silicon scraps generated when cutting using loose abrasive grains. Hereinafter, it demonstrates in the order which performed each process. Moreover, in the comparative example 1 mentioned later, the process similar to Example 1 was performed except not having performed the deaeration process in a fusion | melting process, and purified silicon was prepared.

<Recovery process of silicon scrap>
As the abrasive grains, green carbon SiC of # 800 (average particle diameter of about 14 μm) to # 1200 (average particle diameter of about 8 μm) was used, and a slurry based on propylene glycol was mixed with the abrasive grains to form a slurry. Then, the silicon wafer was cut using a multi-wire saw and slurry.

The waste liquid generated during the silicon cutting process was collected, and SiC and oil components having a large particle diameter were removed from the waste liquid using a centrifuge. In addition, the centrifugal force at the time of centrifugation was 3000G. The content of impurities (boron, phosphorus, iron, carbon, oxygen, and oil) was measured for the silicon scrap collected by the above method. Moreover, the density (mg / cm ³ ) of silicon scrap was measured.

  The measurement results of the impurity content in the silicon scrap after this step are shown in “After recovery step” in Table 1. In Table 1, B, P, Fe, C, and O represent boron, phosphorus, carbon, and oxygen, respectively.

  Content of boron, phosphorus, and iron in silicon scrap was measured using an ICP mass spectrometer. Carbon and oxygen were measured using a combustion method. Specifically, the carbon content is changed from carbon contained in the sample to carbon dioxide by placing the sample in a magnetic crucible and heating and dissolving the sample in a high-frequency heating furnace in an oxygen stream to react with oxygen. The carbon dioxide concentration was calculated by measuring with infrared rays. The oxygen content is determined by placing the sample in a graphite crucible, heating and melting the sample in a He atmosphere, and reacting with carbon to change oxygen to carbon dioxide, and measuring the carbon dioxide concentration with infrared rays. Calculated. The oil component was measured by a thermobalance method. Specifically, the change in weight after heating the secondary solid content from room temperature to 500 ° C. was measured using a solid thermogravimetric measuring device.

  As shown in Table 1, the content of boron, phosphorus, iron and oil in the silicon scrap was large. In particular, the oil component derived from the coolant was very large. Regarding the content of carbon and oxygen after the recovery process, it is difficult to separate SiC from organic components in the oil and carbon, and oxygen is difficult to separate the natural oxide film of silicon from the oil components and moisture. Therefore, it was not possible to measure.

<Pre-melting cleaning process>
Next, the silicon scrap recovered by the recovery step is washed with the weight of silicon scrap: weight of water = (1: 5) to (1:10), and is solid-liquid separated using a centrifuge to obtain oil. Minute removal was performed. In order to improve the removability of the oil component, washing was performed twice. The centrifugal force at the time of centrifugation was set to 300G. The silicon scraps after centrifugation were washed with the weight of silicon scraps: the weight of dilute sulfuric acid of 5% = (1: 5), and solid-liquid separated using a filter press. Then, after the solid content separated by solid-liquid separation was dried, the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured, and the density and recovery rate were calculated.

  The measurement results, density and recovery rate of the impurity content in the silicon scrap after this step are shown in “After the pre-melting cleaning step” in Table 1. As shown in Table 1, it was found that the boron, phosphorus, iron and oil components can be efficiently removed by performing the pre-melt washing step. In particular, a large amount of heavy metals such as Fe and oil components were removed. In addition, the reduction of the oil component enabled the measurement of carbon and oxygen in silicon scrap. In Table 1, the recovery rate indicates the ratio of the weight of the sample after performing the process to the weight of the sample such as silicon scrap before performing the process.

<Melting process>
Next, after the silicon waste cleaned in the pre-melting cleaning step is melted under a first pressure to form a silicon melt (silicon melt forming step), the silicon melt is placed under a second pressure (degassing). Process), and then, the molten silicon and the sticky substance were separated (bath process). This step was performed using the silicon purification apparatus 100 shown in FIG. This will be specifically described below with reference to FIGS.

(Silicon melt forming process)
Referring to FIG. 2, first, silicon waste after the pre-melting cleaning process was introduced into the decompression vessel 8 from the introduction port 9, and the silicon waste was accommodated in the heat-resistant container 10 made of carbon. Next, argon is introduced as an inert gas into the decompression vessel 8 so that the atmospheric pressure around the heat-resistant vessel 10 is adjusted to be 152 Torr or more and 760 Torr or less (first pressure). Was adjusted to about 1600 ° C. to melt silicon scrap. As a result, molten silicon was formed in the heat-resistant container 10. The molten metal was formed with stirring using a carbon stirring rod.

(Deaeration process)
Subsequently, while the heating temperature was maintained, the pressure in the decompression vessel 8 was reduced so that the atmospheric pressure around the heat-resistant vessel 10 was 76 Torr or more and less than 152 Torr (second pressure). The specific conditions for depressurization were a pressure reduction rate of 38 Torr / min or less, and the pressure was maintained for about 5 minutes when the atmospheric pressure in the heat-resistant container 10 reached about 100 Torr.

And after deaeration was complete | finished, the atmospheric pressure around the heat-resistant container 10 was returned to about 456 Torr. During this step, the silicon oxide vapor was captured by the filter 20 while sucking the silicon oxide vapor by the blower 18 in order to prevent the inside of the vacuum vessel 8 from being contaminated by silicon oxide vapor (SiO _x ) generated from the molten metal.

(Tapping process)
Next, as shown in FIG. 4, the heat-resistant container 10 was tilted by using the drive unit 12, and the molten silicon was discharged from the first storage unit 14.

<Silicon lump formation process>
After melting, the molten silicon accommodated in the first accommodating portion 14 was cooled to solidify the molten silicon, thereby forming a silicon lump. About the silicon lump thus obtained, the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured in the same manner as described above. Further, the density and recovery rate of the recovered silicon mass were calculated.

  The results are shown in “After the melting step” in Table 1. As shown in Table 1, by performing the melting step and the silicon lump forming step, the amounts of carbon and oxygen in the silicon lump are significantly reduced, and the silicon oxide and abrasive grains are efficiently removed. I found out.

<Washing step with first acid solution and first basic solution>
The formed silicon mass was immersed in a 5% by mass hydrogen fluoride aqueous solution (25 ° C.) and washed for 4 hours while stirring. Then, it was immersed in 3 mass% sodium hydroxide aqueous solution (25 degreeC), and was washed for 1 hour, stirring. Thereafter, the aqueous sodium hydroxide solution adhering to the silicon mass was washed away with water, and the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured in the same manner as described above. Further, the density and recovery rate of the recovered silicon mass were calculated.

  The results are shown in “After the post-melting cleaning step” in Table 1. As shown in Table 1, the amount of carbon and oxygen could be significantly reduced. It was also confirmed that boron and iron were slightly removed.

<Dephosphorization process>
Next, the silicon lump after performing the washing process after melting is re-contained in the heat-resistant container 10 and heated to 1650 ° C. or higher under an atmospheric pressure of 1 Torr or less, thereby dissolving the silicon lump and dissolving the molten silicon. Produced. In this state, the molten silicon was held for 10 hours with stirring to remove phosphorus from the molten silicon. Thereafter, the molten silicon was cooled and solidified to produce a silicon lump, and the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured in the same manner as described above. In addition, density and recovery rate were calculated.

  The results are shown in “After dephosphorization step” in Table 1. As shown in Table 1, it was found that phosphorus was efficiently removed by this step. Moreover, the carbon content was also reduced, and it was confirmed that the abrasive grains were efficiently removed.

<Segregation process>
Next, after the silicon lump after the dephosphorization process was melted again to form molten silicon, unidirectional solidification was performed in order to remove metal impurities. More specifically, 250 kg of silicon is melted and unidirectional solidification is performed to obtain a silicon mass of 680 mm square × 235 mm in height, and this silicon mass is cut and removed with a band saw at the top 30 mm, each side 10 mm, and the bottom 10 mm. The inner 660 mm square × 195 mm (200 kg) was used as the silicon raw material for solar cells. Then, in the same manner as described above, the contents of boron, phosphorus, iron, carbon, oxygen and oil components of the silicon raw material for solar cells were measured. In addition, density and recovery rate were calculated.

  The results are shown in “After segregation process” in Table 1. As shown in Table 1, it was found that iron was efficiently removed by this step. When performing unidirectional solidification, if the carbon concentration or oxygen concentration is high, solidification starting from SiC, which has a particularly high melting point, may occur, which may result in insufficient metal impurity removal by segregation. . On the other hand, in the silicon lump in Example 1, since the carbon concentration is already sufficiently low, it can be seen that metal impurities can be easily removed by segregation.

  That is, the content of boron in the purified silicon purified by the silicon purification method of Example 1 is 0.20 ppm, the content of phosphorus is 0.20 ppm, and the content of iron is less than 1 ppm. The carbon content was 100 ppm and the oxygen content was 100 ppm.

  Table 2 shows the results of Comparative Example 1. In Comparative Example 1, as described above, silicon purification was performed by the same process as Example 1 except that the deaeration process was not performed in the melting process. However, the holding time in the dephosphorization process was 11 hours.

<Comparison between Example 1 and Comparative Example 1>
(About melting process)
When Table 1 and Table 2 were compared, it was found that the carbon amount and oxygen amount of the silicon lump after the melting step in Example 1 were sufficiently lower than those in Comparative Example 1. This is because the silicon powder having a small particle diameter remaining in the molten silicon can be melted by the deaeration process, and the molten silicon is uniformly formed. As a result, the molten silicon in the molten silicon and the silicon oxidation This is thought to be because the separability from the object and abrasive grains (SiC) was improved.

  Moreover, it turned out that the density of the silicon lump after the melting step in Example 1 is larger than that in Comparative Example 1. This is presumably because the density of the silicon lump has increased due to the removal of the gas present in the molten silicon in the deaeration process.

(About the cleaning process after melting)
When Table 1 and Table 2 are compared, the carbon mass and oxygen content of the silicon lump after the melting step in Example 1 are sufficiently lower than those of Comparative Example 1, but the post-melt cleaning step The degree of decrease in the carbon content and the oxygen content in the silicon lump due to was equal to or greater than that of Comparative Example 1. This is because, in the silicon lump of Example 1, through the deaeration process, the separability between silicon and silicon oxide and silicon carbide increased, so that silicon oxide and silicon carbide were localized on the surface of the silicon lump. It is thought that That is, it is considered that the removal performance by cleaning is improved because the silicon oxide and silicon carbide are localized on the surface of the silicon lump.

(About dephosphorization process)
Regarding the dephosphorization step, as described above, the holding time in Example 1 was 10 hours, whereas in Comparative Example 1, a holding time of 11 hours was required. This is because it was necessary to extend the holding time due to the occurrence of a jumping phenomenon of the molten metal due to gas escape from the pores during melting and an increase in pressure due to oxygen evaporation. That is, compared with Comparative Example 1, Example 1 can improve the efficiency of phosphorus removal, reduce damage to the device, and increase the life of the device.

  Further, comparing Table 1 and Table 2, the recovery rate after the dephosphorization step was 90% in Comparative Example 1 and 92% in Example 1. This is probably because the jumping phenomenon described above occurred in Comparative Example 1. From this, it was found that according to the method of Example 1, purified silicon can be obtained while maintaining a high silicon recovery rate.

(Segregation process)
Further, when Table 1 and Table 2 were compared, the recovery after the segregation process was 80% in Example 1 and 75% in Comparative Example 1. This is because the carbon concentration portion in the silicon lump of Comparative Example 1 was large, and the removal rate increased. That is, carbon is localized in the upper part of the silicon lump due to segregation, but in Comparative Example 1, since the content of carbon in the molten silicon before segregation is large, the portion where carbon is present at a high concentration becomes large and is removed. The rate increased.

  As Example 2, purified silicon was prepared by collecting silicon scraps generated when cutting using fixed abrasive grains. Hereinafter, it demonstrates in the order which performed each process. In Comparative Example 2, purified silicon was prepared by performing the same process as in Example 2 except that the degassing process was not performed in the melting process.

<Recovery process of silicon scrap>
The diamond was attached to the wire and cut using a fixed abrasive wire plated in order to suppress peeling of the diamond. The silicon wafer was cut using a multi-wire saw while applying a coolant based on propylene glycol.

The slurry waste liquid generated during the silicon cutting process was collected, and the oil component was removed from the slurry using a centrifuge. The centrifugal force at the time of centrifugation was set to 300G. The content of impurities (boron, phosphorus, iron, carbon, oxygen, and oil) was measured for the silicon scrap collected by the above method. Moreover, the density (mg / cm ³ ) of silicon scrap was measured. The method for measuring each component is the same as in Example 1.

  The measurement results of the impurity content in the silicon scrap after this step are shown in “After recovery step” in Table 3. In particular, the oil component derived from the coolant was very large. Moreover, since it was a fixed abrasive wire, it turned out that the content rate of iron which is an abrasive grain component is few compared with the silicon | silicone waste collect | recovered from the loose abrasive (refer Table 1). The content of carbon and oxygen after the recovery process is difficult to separate from carbon and organic components in oil. Oxygen is difficult to separate from natural oxide film of silicon and oil components and moisture. could not.

<Pre-melting cleaning process>
Next, the silicon scrap recovered by the recovery process was cleaned by the same method as the pre-melting cleaning process of Example 1. Then, as in Example 1, after solid-liquid separated solid content was dried, the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured, and the density and recovery rate were further calculated. .

  The measurement results of the impurity content in the silicon scrap after this step are shown in “After the pre-melting cleaning step” in Table 3. As shown in Table 3, it was found that boron, iron and oil components can be efficiently removed by performing the pre-melting cleaning step. In particular, a large amount of heavy metals such as Fe and oil components were removed. In addition, the reduction of the oil component enabled the measurement of carbon and oxygen in silicon scrap. The carbon concentration is about 1000 ppm, which is considered to be caused by peeling and abrasion of diamond from the slice wire.

<Melting process and silicon lump formation process>
A melting step (silicon melt forming step, degassing step and tapping step) was performed by the same method as in Example 1, and then a silicon lump forming step was performed by the same method as in Example 1. About the obtained silicon lump, the content of boron, phosphorus, iron, carbon, oxygen and oil components was measured in the same manner as described above. Further, the density and recovery rate of the recovered silicon mass were calculated.

  The results are shown in “After the melting step” in Table 3. As shown in Table 3, it was found that the amount of carbon and oxygen in the silicon lump was significantly reduced by performing the melting step and the silicon lump forming step. From Table 3, silicon scraps cut with a fixed abrasive wire are recovered, washed before melting, and melted so that the carbon concentration and oxygen concentration are low. I knew it was possible.

  Moreover, the recovery rate after the silicon lump formation step was 70%, which was higher than the recovery rate after the melting step in Example 1 (50%). This is because silicon scrap generated when cutting using fixed abrasive grains has a very small abrasive grain component (carbon) compared to free abrasive grains, so the influence of abrasive grain components is small. This is thought to be due to the good separation between and other components, resulting in a high recovery rate.

<Washing step with first acid solution and first basic solution>
Using the formed silicon lump, a cleaning step with the first acid solution and a cleaning step with the first basic solution were performed in the same manner as in Example 1. Thereafter, the aqueous sodium hydroxide solution adhering to the silicon mass was washed away with water, and the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured in the same manner as described above. Further, the density and recovery rate of the recovered silicon mass were calculated.

  The results are shown in “After the cleaning process after melting” in Table 3. As shown in Table 3, the amount of carbon and oxygen could be significantly reduced. It was also confirmed that boron and iron were slightly removed.

  In addition, silicon scrap cut with a fixed abrasive wire is recovered, washed before melting, and melted so that the carbon concentration and oxygen concentration are low, so that a sufficiently purified silicon lump can be obtained through the steps so far. all right.

<Dephosphorization process>
Next, a dephosphorization step was performed in the same manner as in Example 1. However, the holding time was 8 hours. Thereafter, the silicon mass was produced by cooling and solidifying the molten silicon, and then the contents of boron, phosphorus, iron, carbon, oxygen and oil components were measured in the same manner as described above. In addition, density and recovery rate were calculated.

  The results are shown in “After dephosphorization step” in Table 3. As shown in Table 3, it was found that phosphorus was efficiently removed by this step. Moreover, the carbon content was also reduced, and it was confirmed that the abrasive grains were efficiently removed.

<Segregation process>
Next, after the silicon lump after the dephosphorization step was melted again to form molten silicon, unidirectional solidification was performed in the same manner as in Example 1 to prepare a silicon weight loss for solar cells. Then, in the same manner as described above, the contents of boron, phosphorus, iron, carbon, oxygen and oil components of the silicon raw material for solar cells were measured. In addition, density and recovery rate were calculated.

  The results are shown in “After segregation process” in Table 3. As shown in Table 3, it was found that iron was efficiently removed by this step. It was also found that carbon was also efficiently removed by this step.

  That is, the content of boron in the purified silicon purified by the silicon purification method of the above example is 0.20 ppm, the content of phosphorus is 0.20 ppm, the content of iron is less than 1 ppm, The carbon content was 20 ppm and the oxygen content was 100 ppm.

  Table 4 shows the results of Comparative Example 2. In Comparative Example 2, as described above, silicon purification was performed by the same process as Example 2 except that the deaeration process was not performed in the melting process. However, in the dephosphorization step, the holding time was 8.5 hours.

<Comparison between Example 2 and Comparative Example 2>
(About melting process)
When Table 3 and Table 4 were compared, it was found that the amount of carbon and the amount of oxygen in the silicon lump after the melting step in Example 2 were sufficiently smaller than those in Comparative Example 2. This is because the silicon powder having a small particle diameter remaining in the molten silicon can be melted by the deaeration process, and the molten silicon is uniformly formed. As a result, the molten silicon in the molten silicon and the silicon oxide It is considered that the separability from the abrasive grains (C) was improved. Moreover, the purity of the silicon lump after the melting step of Example 2 is high, and the silicon lump after the melting step can be purified silicon.

  Moreover, it turned out that the density of the silicon lump after the melting step in Example 2 is larger than that in Comparative Example 2. This is presumably because the density of the silicon lump has increased due to the removal of the gas present in the molten silicon in the deaeration process.

(About the cleaning process after melting)
When Table 3 and Table 4 are compared, the carbon mass and oxygen content of the silicon lump after the melting step in Example 2 are sufficiently lower than those of Comparative Example 2, but the post-melt cleaning step The degree of decrease in the carbon content and oxygen content in the silicon lump due to was equal to or greater than that of Comparative Example 2. This is because, in the silicon lump of Example 2, through the deaeration process, the separability between silicon and silicon oxide and silicon carbide was increased, so that silicon oxide and silicon carbide were localized on the surface of the silicon lump. It is thought that That is, it is considered that the removal performance by cleaning is improved because the silicon oxide and silicon carbide are localized on the surface of the silicon lump.

(About dephosphorization process)
Regarding the dephosphorization step, as described above, the holding time in Example 2 was 8 hours, whereas in Comparative Example 2, a holding time of 8.5 hours was required. This is because it was necessary to extend the holding time due to the occurrence of a jumping phenomenon of the molten metal due to gas escape from the pores during melting and an increase in pressure due to oxygen evaporation. That is, compared to Comparative Example 2, Example 2 can improve the efficiency of phosphorus removal, reduce the damage to the device, and increase the life of the device.

  Moreover, when Table 3 and Table 4 were compared, the recovery rate after the dephosphorization process was 92% in Example 2 compared with 92% in Comparative Example 2. This is presumably because the above-described jumping phenomenon occurred in Comparative Example 2. From this, it was found that according to the method of Example 2, purified silicon can be obtained while maintaining a high silicon recovery rate.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The present invention can be widely used when silicon is purified from silicon scrap.

  DESCRIPTION OF SYMBOLS 9 Input port, 10 heat-resistant container, 11 heating part, 12 drive part, 13 fusion | melting part, 14 1st accommodating part, 15 2nd accommodating part, 16 conveyance part, 17 residue collection part, 18 blower, 19 discharge pipe, 20 Filter, 21 Silicon melt, 22 Molten silicon, 23 Silicon oxide, 24 Abrasive grain, 25 Adhesive substance.

Claims (10)

A silicon purification method for obtaining purified silicon from silicon scrap,
Melting the silicon scrap to form a silicon melt;
Including solidifying molten silicon obtained from the molten silicon to form a silicon lump,
The step of forming the silicon melt includes a step of melting the silicon scrap under a first pressure to form a silicon melt, and a deaeration of the silicon melt under a second pressure lower than the first pressure. And a silicon refining method.   2. The silicon purification method according to claim 1, wherein the first pressure is not less than 152 Torr and not more than 760 Torr, and the second pressure is not less than 76 Torr and less than 152 Torr.   The silicon purification method according to claim 1 or 2, wherein the first pressure is 380 Torr or more and 760 Torr or less.   The silicon purification method according to claim 1, wherein the step of forming the silicon melt includes a step of placing the degassed silicon melt under the first pressure.   5. The method for purifying silicon according to claim 1, wherein the pressure is decreased from the first pressure at a speed of 38 Torr / min or less to obtain the second pressure. 6.   The silicon purification method according to any one of claims 1 to 5, further comprising a step of recovering the silicon waste from waste generated when silicon is machined.   The silicon purification method according to claim 1, wherein the silicon scrap contains silicon powder having a particle diameter of 10 μm or less.   The silicon purification method according to claim 1, further comprising a step of pre-cleaning the silicon scrap before melting the silicon scrap. In the silicon purification method,
(1) washing the silicon mass with at least one solution of a first acid solution and a first basic solution;
(2) removing phosphorus from the silicon mass;
(3) removing metal from the silicon mass by segregation;
(4) The silicon purification method according to any one of claims 1 to 8, further comprising at least one step of removing metal from the silicon mass by leaching. A purified silicon purified by the silicon purification method according to claim 1,
The boron content in the refined silicon is 0.3 ppm or less, the phosphorus content is 0.3 ppm or less, the iron content is 1 ppm or less, the carbon content is 100 ppm or less, oxygen Refined silicon whose content of is 100 ppm or less.

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