JP4073864B2 - Silicon purification method and silicon - Google Patents

Description

  The present invention relates to a silicon purification method and silicon, and more particularly to a silicon purification method and silicon used in the manufacture of solar cells.

  Iron, aluminum, copper, silicon, and the like rarely exist alone in nature, and most exist as compounds such as oxides. Therefore, when these are used as a structural material, a conductive material, a semiconductor material, or the like, it is necessary to remove impurities by reducing these oxides. However, impurities cannot be sufficiently removed only by reducing these oxides and the like. Therefore, it is necessary to further reduce the amount of impurities contained therein. This process of reducing the amount of impurities is called purification.

  For example, in the refining of iron used as a structural material, by bringing pig iron taken from a blast furnace into contact with a molten oxide called slag, impurities such as phosphorus and sulfur that significantly impair toughness are incorporated into the slag. Impurity content in pig iron has been reduced. For carbon, which is an impurity that can determine the mechanical strength of steel, oxygen gas is blown into pig iron to oxidize the carbon in pig iron and remove it as carbon dioxide gas, thereby adjusting the amount of carbon in pig iron. ing.

  Also, in the purification of copper used as a conductive material, the fact that the segregation coefficient of impurities, which is the ratio between the impurity concentration in solid copper and the impurity concentration in molten copper in an equilibrium state, is small, Impurity concentration in the solid copper is reduced by solidifying the copper in a state at a slow speed that is close to the equilibrium state.

In refining silicon used as a semiconductor material, silicon with a purity of 98% or more obtained by reducing silica is converted into a gas such as silane (SiH ₄ ) or trichlorosilane (SiHCl ₃ ), and these gases are further converted into a bell jar furnace. Polycrystalline silicon having a purity of about 11 N is obtained by decomposition or reduction with hydrogen. By growing the polycrystalline silicon into a single crystal, silicon used for manufacturing an electronic device such as an LSI can be obtained. In order to obtain silicon used for manufacturing an electronic device, a very complicated manufacturing process and strict manufacturing process management are required, and thus the manufacturing cost is inevitably high.

  On the other hand, demand for solar cells has been growing rapidly in recent years due to increased awareness of energy issues such as depletion of fossil fuel resources and environmental issues such as global warming. The purity of silicon required for silicon used in the manufacture of solar cells is about 6N. Therefore, the non-standard product of silicon for electronic devices that has been used in the manufacture of solar cells so far has an excessive quality for solar cells.

  Until now, there was no problem because the amount of non-standard products for electronic devices exceeded the demand for solar cells. However, in the near future, it is expected that the demand for solar cells will exceed the amount of non-standard products for electronic devices, and there is a strong demand for the establishment of inexpensive manufacturing technology for silicon for solar cells. As such a technique, a technique of refining by a metallurgical method using the above-described oxidation-reduction reaction or solidification segregation has attracted attention in recent years.

  Of the impurities contained in silicon for solar cells, the segregation coefficients of phosphorus and boron are both large. Therefore, it is known that a purification method using coagulation segregation has little effect on removing phosphorus and boron.

  Thus, regarding the removal of phosphorus, Patent Document 1 discloses a method of releasing phosphorus into the gas phase by holding molten silicon under a reduced pressure atmosphere.

Regarding the removal of boron, Patent Document 2 discloses a method of irradiating the molten silicon surface with plasma of a mixed gas containing an inert gas and water vapor. Further, Patent Document 3 discloses a method in which a torch obtained by burning hydrogen and oxygen is immersed in molten silicon. Patent Document 4 discloses a method of blowing a processing gas while stirring molten silicon. Patent Document 5 discloses a method of continuously adding slag into molten silicon. These boron removal methods all remove boron oxide from molten silicon.
Japanese Patent No. 2905353 Japanese Patent No. 3305352 US Pat. No. 5,972,107 JP 2001-58811 A Japanese Patent No. 2851257

  However, in these disclosed methods for purifying silicon, it cannot be said that the removal rate of boron is sufficient.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a silicon purification method capable of efficiently purifying silicon by improving the boron removal rate and silicon obtained by the method.

The present invention blows a carbon-containing gas containing at least one selected from the group of carbon monoxide, carbon dioxide, and hydrocarbons into silicon after melting, and carbon and carbonization before and / or after melting of silicon. This is a silicon purification method in which at least one of silicon is mixed .

  The present invention is also a method for purifying silicon in which at least one of carbon and silicon carbide is mixed before and / or after melting of silicon.

  In the silicon purification method of the present invention, it is preferable to mix slag before and / or after melting of silicon.

  In the silicon purification method of the present invention, the slag preferably contains silicon oxide.

  In the silicon purification method of the present invention, the slag preferably contains an alkali metal oxide.

  In the silicon purification method of the present invention, the slag preferably contains at least one selected from the group consisting of alkali metal carbonates, alkali metal hydrogencarbonates and alkali metal silicates.

  In the method for purifying silicon according to the present invention, a gas blowing tube is immersed after silicon is melted, and a carbon-containing gas containing at least one selected from the group consisting of carbon monoxide, carbon dioxide and hydrocarbons is blown into the gas. It is preferable to blow carbon-containing gas into the silicon by introducing it into the tube.

  In the silicon purification method of the present invention, it is preferable that at least a part of the gas flow path of the gas blowing tube is made of an oxidation resistant material.

  In the silicon purification method of the present invention, it is preferable to rotate the gas blowing tube in the silicon while blowing the carbon-containing gas into the silicon.

  ADVANTAGE OF THE INVENTION According to this invention, the removal rate of boron can be improved, and the silicon purification method which can refine | purify silicon efficiently, and the silicon obtained by the method can be provided. Therefore, according to the present invention, silicon for solar cells can be produced efficiently and inexpensively.

(apparatus)
FIG. 1 shows a schematic sectional view of a preferred example of a part of the apparatus used in the present invention. In the drawings of the present application, the same reference numerals denote the same or corresponding parts.

  1, the apparatus used in the present invention includes a melting furnace 1 having a stainless steel wall surface, a graphite crucible 2, an electromagnetic induction heating device 3, and a graphite gas blowing tube 4. And the molten silicon 8 is inject | poured into this crucible 2, and the molten slag 9 is further added as needed.

  Here, the gas blowing tube 4 is provided with a stirring unit 5 in the lower part thereof. Further, the gas blowing tube 4 has a rotation drive mechanism (not shown) for rotating the stirring unit 5 in the molten silicon 8 at the upper part thereof, and a molten silicon for immersing the stirring unit 5 in the molten silicon 8. And an elevating mechanism (not shown) for disengaging from 8. In addition, a hollow gas flow path 7 serving as a passage for a gas such as a carbon-containing gas is formed inside the gas blowing tube 4 including the stirring unit 5. Further, a sealing mechanism 12 is provided at a portion where the gas blowing pipe 4 penetrates the wall of the melting furnace 1 to ensure the sealing inside the melting furnace 1 and to enable the gas blowing pipe 4 to rotate.

  FIG. 2 shows a schematic side view and a schematic bottom view of a part of the gas blowing tube 4. As shown in FIG. 2, the stirring unit 5 has a structure in which a plurality of blades 10 project radially outward from the gas blowing tube 4, and a gas blowing port 6 is formed at each tip of the blade 10. . However, the shape of the stirring unit 5 is not limited to the above shape as long as the bubbles 11 or the molten slag 9 of the carbon-containing gas shown in FIG. 1 can be uniformly dispersed in the molten silicon 8.

(Purification method)
Hereinafter, a preferable example of the method for purifying silicon according to the present invention will be described.

  First, solid raw material silicon and, if necessary, slag are placed in the crucible 2 of the above apparatus, and the crucible 2 is heated by the electromagnetic induction heating apparatus 3 with the space in the melting furnace 1 as an inert gas atmosphere such as argon. And the temperature of raw material silicon | silicone and slag is raised by the heat transfer from the crucible 2, and these are fuse | melted. The melt thus obtained is maintained at a predetermined processing temperature, typically 1450 to 1600 ° C. When slag is added, the molten silicon and the molten slag are completely separated into two layers before the melt is stirred.

  Next, the gas blowing pipe 4 is lowered by the lifting mechanism, and the gas blowing pipe 4 and the stirring unit 5 are immersed in the molten silicon 8 in the crucible 2 as shown in FIG. Then, while the carbon-containing gas introduced into the hollow gas flow path 7 of the gas blowing tube 4 is blown into the molten silicon 8 from the gas blowing port 6, the gas blowing tube 4 is rotated by the rotation drive mechanism in the direction indicated by the arrow. The molten silicon 8 is stirred.

  By doing in this way, the bubble 11 of the carbon-containing gas blown into the molten silicon 8 and the molten slag 9 added if necessary are refined, and the bubble 11 of the carbon-containing gas and the molten slag 9 added if necessary. It can be uniformly dispersed in the molten silicon 8. Then, the reaction between the molten silicon 8, the molten slag 9 added if necessary, and the carbon-containing gas is promoted throughout the molten silicon 8, and an oxide of boron contained in the molten silicon 8 is generated. The oxide is removed from the molten silicon 8 by vaporization or the like. Therefore, in the present invention, the carbon-containing gas is uniformly dispersed in the molten silicon 8, and boron can be removed from the entire molten silicon 8 almost simultaneously, so that the boron removal rate is improved and silicon is purified efficiently. It can be done.

(Carbon-containing gas)
In the present specification, the carbon-containing gas refers to a gas containing at least one selected from the group consisting of carbon monoxide, carbon dioxide, and hydrocarbons. When the carbon-containing gas is blown into the molten silicon 8, the inventors have found that the boron removal rate is improved and silicon can be purified efficiently, and the present invention is completed. It was. Here, as the hydrocarbon, for example, methane (CH ₄ ), ethane (C ₂ H ₆ ) or the like is used.

Examples of components other than carbon monoxide, carbon dioxide, and hydrocarbons in the carbon-containing gas include carrier gases such as argon (Ar) and nitrogen, and oxidizing gases such as water vapor (H ₂ O) and oxygen. In particular, when water vapor is contained in the carbon-containing gas, since water vapor is more oxidizing than carbon monoxide and carbon dioxide, the removal rate of impurities such as boron can be further improved. The amount of water vapor in the carbon-containing gas is about 2 to 70 vol% (volume) of the entire carbon-containing gas by using a normal humidifier and setting the dew point in the carbon-containing gas to a range of typically 20 to 90 ° C. %) Can be easily controlled. Further, an inert gas such as argon (Ar) is preferably used as the carrier gas.

  The total volume ratio of carbon monoxide, carbon dioxide and hydrocarbon in the carbon-containing gas is preferably 1% by volume or more and 50% by volume or less of the total volume of the carbon-containing gas. When this ratio is less than 1% by volume, it tends to be difficult to improve the silicon purification efficiency by the carbon-containing gas, and when it exceeds 50% by volume, carbon mixed in the molten silicon can be removed. This is because it tends to be difficult. In this case, a gas such as argon or water vapor may be blown from the gas blowing pipe 4, and carbon monoxide, carbon dioxide, or hydrocarbon may be blown from a place other than the gas blowing pipe 4.

  The introduction pressure of the carbon-containing gas is preferably greater than 1 atmosphere, and more preferably in the range of 0.15 MPa to 0.3 MPa. In these cases, even when the molten slag 9 having a high viscosity is mixed in the molten silicon 9, the carbon-containing gas tends to be blown out stably.

(Gas blowing pipe)
Graphite is preferably used as the material for the gas blowing tube 4 and the stirring unit 5, and graphite is more preferably used as the material for the crucible 2. This is because graphite does not melt even when it comes into contact with the molten silicon 8 having a temperature exceeding 1400 ° C., and it is easy to process. When graphite is used as the material of the crucible 2, the gas blowing tube 4 or the stirring unit 5, a carbon-containing gas, particularly a carbon-containing gas containing at least one of carbon monoxide and carbon dioxide is introduced into the molten silicon 8. By blowing, the graphite crucible 2, the gas blowing tube 4, and the stirring unit 5 can be used for a longer time.

  That is, conventionally, water vapor has been mainly used as a gas for oxidizing impurities such as boron. However, in the course of further research by the present inventors, when water vapor is used, the inner surface of the graphite crucible 2, the outer surface of the gas blowing tube 4, and further, as time for purifying silicon elapses. It has been found that the gas flow path 7 of the gas blowing tube 4 and the gas flow path surface of the stirring unit 5 are consumed by the reaction with water vapor, which is an oxidizing gas.

  Due to the consumption of the graphite member, carbon is mixed in the molten silicon 8 and, as a result, the purification ability of silicon is to be improved. However, the thickness of the crucible 2 and the gas blowing tube 4 is increased. The problem that surface strength became worse and the usable period of these members was shortened emerged. Further, the diameter of the gas outlet 6 is enlarged, the bubbles 11 of the carbon-containing gas are not refined, and the problem that the time for refining silicon becomes longer is also revealed.

  The temperature of the molten silicon 8 is maintained at about 1450 to 1600 ° C. as described above. Therefore, the crucible 2 in contact with the molten silicon 8, a part of the gas blowing tube 4 and the stirring unit 5 are heated to the same level as the temperature of the molten silicon 8. Further, due to heat transfer from the molten silicon 8, the portion of the gas blowing tube 4 near the surface of the molten silicon 8 is heated to about 500 ° C. or more. In such an environment where graphite is easily oxidized, it is considered that if an oxidizing gas such as water vapor contacts these graphite members, these graphite members are easily oxidized.

  Therefore, in the present invention, a carbon-containing gas, particularly a carbon-containing gas containing at least one of carbon monoxide and carbon dioxide, is blown into the molten silicon 8, so that the gas blowing pipe 4 and the gas flow path surface of the stirring unit 5 can be used. It is possible to suppress the erosion of graphite due to oxidation of oxygen, and to increase the carbon concentration in the molten silicon 8 to reduce the amount of carbon eluted into the molten silicon 8 from the crucible 2, the gas blowing tube 4 and the stirring unit 5. it can. Therefore, in the present invention, graphite members such as the crucible 2 and the gas blowing tube 4 can be used for a longer period than before. This leads to the fact that the device can be operated for a long period of time and silicon for solar cells can be manufactured efficiently and inexpensively.

  Further, as a material for the crucible 2, the gas blowing tube 4 and the stirring unit 5, it is conceivable to use a material that is not easily oxidized (oxidation resistant material) instead of graphite. For example, a material such as silicon carbide or silicon nitride can be used as an alternative to graphite. However, since it is very difficult to manufacture large members such as the crucible 2, the gas blowing tube 4 and the stirring unit 5 using these oxidation resistant materials, the manufacturing cost of these members becomes very high. .

  Moreover, as another example used for the material of the crucible 2, the gas blowing tube 4, and the stirring part 5, it is also conceivable to use oxide ceramics. In particular, for aluminum oxide (alumina), it is possible to produce a large member as described above, and the production cost of the member is also low. However, oxide ceramics may be severely eroded by the molten slag 9 or the like.

  Therefore, it can be said that it is most desirable to use graphite as the material of the members constituting the apparatus used in the present invention, in particular, the crucible 2, the gas blowing tube 4, and the stirring unit 5, but the gas flow is caused by the oxidizing gas. In order to prevent the passage 7 from being oxidized and the gas blowing pipe 4 from being consumed more reliably, it is preferable that a part of the gas flow path 7 of the gas blowing pipe 4 is made of an oxidation resistant material.

  Note that in this application, an oxidation resistant material means that the appearance or mechanical strength is remarkably high even when it comes into contact with a gas containing 2 vol% or more of an oxidizing gas such as water vapor or oxygen at a temperature of 1412 ° C. or higher, which is the melting point of silicon. A known material such as alumina, silicon nitride, or silicon carbide can be used, but alumina is particularly preferable because it is excellent in strength at high temperatures and resistance to oxidizing gas, and is inexpensive.

  The method for producing the gas flow path 7 with an oxidation resistant material is not particularly limited, and a gas flow path 7 may be formed by inserting a tube made of an oxidation resistant material and covering the inner surface of the gas blowing tube 4. A paste-like oxidation resistant material may be applied to the gas flow path 7, or a thin film of the oxidation resistant material may be formed by vapor deposition or vapor phase growth.

(Stirring)
When slag having higher specific gravity than raw material silicon is added, the gas blowing pipe 4 is rotated after the stirring unit 5 is lowered near the interface between the molten silicon layer as the upper layer and the molten slag layer as the lower layer. Is preferred. In this case, the bubbles 11 and the molten slag 9 of the carbon-containing gas blown out from the gas blowing port 6 are more easily dispersed in the molten silicon 8. Then, the carbon-containing gas, the molten silicon 8, and further the molten slag 9 added if necessary are mixed very efficiently in the crucible 2, and the contact area between the phases is remarkably increased. In such a state, the oxidation reaction of impurities such as boron in the molten silicon 8 by the oxidizing gas that can be contained in the carbon-containing gas or the like or oxygen supplied from the molten slag 9 that is added as necessary is remarkably accelerated. .

  Further, by stirring so that the molten slag 9 is more uniformly dispersed in the molten silicon 8, the function of the molten slag 9 as an oxidizing agent can be efficiently extracted. However, it is not necessary that the entire amount of slag is melted, and even if a part of it is in a solid state, almost the same effect can be obtained. It is desirable to keep both silicon and slag in a molten state.

  In addition, when slag is added to silicon, silicon may be in a solid state or a molten state, and the slag to be added may be in a solid state or a liquid state. In the present invention, it is not necessary to add slag to silicon.

  When slag is not added to the raw silicon or when slag having a specific gravity smaller than that of the raw silicon is added, the gas blowing pipe 4 is rotated after the stirring unit 5 is lowered below the molten silicon 8. preferable.

(Slag)
As the slag that can be used in the present invention, for example, a mixture of silicon oxide and calcium oxide is used. For example, as can be seen from the SiO ₂ —CaO binary phase diagram described on page 109 of Advanced Physical Chemistry for Process Metallurgy (issued in 1997), silicon oxide and silicon oxide at about 1460 ° C. or higher, which is higher than the melting point of silicon, about 1412 ° C. Slag which is a mixture with calcium oxide can be made into a molten state.

  The fact that silicon oxide powder is useful as an oxidizing agent is disclosed in, for example, Patent Document 2 and Patent Document 3 described above, but silicon oxide powder has poor wettability with molten silicon 8 and a large amount of silicon oxide. Since the powder cannot be added into the molten silicon 8, the silicon purification rate may be limited. Therefore, since the wettability with the molten silicon 8 can be improved by using a mixture of silicon oxide and calcium oxide as the slag, a large amount of oxidizing agent necessary for the purification process of silicon is introduced as the molten slag. It becomes possible.

  In addition, when using the slag which consists of a mixture of a silicon oxide and a calcium oxide, it is preferable to use what has a silicon oxide as a main component. In the case where a material mainly composed of calcium oxide as described in Patent Document 5 is used, the function as an oxidizing agent for impurities such as boron tends to be weakened.

  However, when a slag containing silicon oxide as a main component is used, the molten slag 9 may adhere to the gas outlet 6 and the gas outlet 6 may be blocked by the molten slag 9. Since slag containing silicon oxide as a main component generally has a high viscosity, it is considered that it is difficult to peel it once it has adhered.

  Therefore, the present inventors have found that the clogging of the gas outlet 6 can be suppressed by mixing one or more kinds of alkali metal oxides such as lithium oxide and sodium oxide with slag mainly composed of silicon oxide. . It is considered that the viscosity of the molten slag 9 was decreased by mixing an alkali metal oxide with the slag, and adhesion to the gas outlet 6 was suppressed.

  When mixing an alkali metal oxide with the slag, the alkali metal oxide may be mixed directly. However, since the alkali metal oxide reacts with water and changes to a hydroxide, it exhibits strong alkalinity. , Handling may require attention.

  Therefore, at least one selected from the group consisting of alkali metal carbonates, bicarbonates or silicates can be mixed with the slag. For example, by mixing lithium carbonate, lithium hydrogen carbonate, or lithium silicate with slag and heating, the same effect as when lithium oxide is mixed with slag containing silicon oxide as a main component can be obtained.

  By mixing sodium carbonate, sodium hydrogen carbonate or sodium silicate with slag containing silicon oxide as the main component and heating, the same effect as mixing sodium oxide with slag containing silicon oxide as the main component can be obtained. It is done.

  Moreover, it cannot be overemphasized that the slag material used in this invention is not limited to the above-mentioned thing. For example, additives such as aluminum oxide, magnesium oxide, barium oxide, or calcium fluoride generally used in the refining field such as steel may be mixed as appropriate.

(Carbon, silicon carbide)
The present inventors have found that the removal rate of boron is improved even when carbon-containing gas is blown or when at least one of solid carbon and silicon carbide is mixed instead of blowing carbon-containing gas. It was. Here, carbon and / or silicon carbide may be mixed in the crucible 2 before melting of silicon, or may be mixed after melting of silicon. Moreover, carbon and / or silicon carbide may be mixed in the necessary amount all at once, or may be mixed intermittently or continuously little by little.

  Here, the total amount of carbon and / or silicon carbide to be mixed is preferably 10% by mass or more of the molten silicon. In this case, the boron removal rate tends to be further improved. On the other hand, when carbon is mixed excessively, stirring of molten silicon may not be good, and it becomes difficult to remove carbon mixed in molten silicon. The total amount of silicon carbide is preferably 50% by mass or less of the molten silicon.

Example 1
By mixing scrap silicon containing 65 ppm of boron and semiconductor grade silicon of purity 11N at a mass ratio of approximately 1: 8, a raw material silicon having a boron concentration adjusted to about 7 ppm is produced. did.

  1 kg of raw material silicon whose boron concentration is adjusted to about 7 ppm is put in a crucible 2 shown in FIG. 1, and the inside of the melting furnace 1 is set to an argon gas atmosphere of 1 atm and the crucible 2 is heated using an electromagnetic induction heating device 3. Then, the raw material silicon was melted and held at 1550 ° C. At this time, in order to measure the boron content before treatment, about 20 g of molten silicon 8 was extracted, and 5 g was used for the measurement.

A carbon-containing gas consisting of a mixed gas of argon (Ar) and carbon dioxide (CO ₂ ) (Ar volume ratio 70%, CO ₂ volume ratio 30%) was introduced into the gas blowing tube 4 at a pressure of 0.2 MPa. The gas blowing pipe 4 was lowered by the lifting mechanism so that the stirring unit 5 was positioned below the molten silicon 8. The flow rate of the carbon-containing gas was 3.0 L / min.

  After confirming that the carbon-containing gas was blown into the molten silicon 8, the gas blowing pipe 4 was rotated at 400 rpm by a rotating mechanism, and a purification process was performed for 2 hours. When the boron content before and after the purification treatment was measured, it was 7.0 ppm before the purification treatment and 3.4 ppm after the purification treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the purification treatment were not significantly different from those before the treatment.

(Example 2)
A purification treatment was carried out in the same manner as in Example 1 except that a mixed gas of argon (Ar) and carbon monoxide (CO) (Ar volume ratio 70%, CO volume ratio 30%) was used as the carbon-containing gas. Was done. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 3.6 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

(Example 3)
As a carbon-containing gas, a mixed gas of argon (Ar), carbon monoxide (CO), and carbon dioxide (CO ₂ ) (volume ratio of Ar 70%, volume ratio of CO 15%, volume ratio of CO ₂ 15%) The purification treatment was performed in the same manner as in Example 1 except that it was used. When the boron content before and after the treatment was measured, it was 7.0 ppm before the treatment and 3.5 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

Example 4
As a carbon-containing gas, a mixed gas of argon (Ar), carbon monoxide (CO), carbon dioxide (CO ₂ ) and water vapor (H ₂ O) (Ar volume ratio 70%, CO volume ratio 10%, CO ₂ The purification treatment was performed in the same manner as in Example 1 except that the volume ratio of 10% and the volume ratio of H ₂ O were 10%. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 3.2 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

(Example 5)
In this example, a mixture of silicon oxide (SiO ₂ ) powder and calcium oxide (CaCO ₃ ) powder in a mass ratio of 65:35 was used as the slag material. 1 kg of raw material silicon whose boron concentration is adjusted to about 7 ppm and the above slag material mixed at a mass ratio of 4: 1 is put in the crucible 2, and the space in the melting furnace 1 is made an argon gas atmosphere of 1 atm. The raw silicon and slag material were melted by heating the crucible 2 using the electromagnetic induction heating device 3 and kept at 1550 ° C. Since the molten slag has a larger specific gravity than the molten silicon, most of the molten slag was precipitated at the bottom of the crucible 2. At this time, in order to measure the boron content before treatment, about 20 g of molten silicon was extracted so that molten slag was not mixed, and 5 g was used for the measurement.

As a carbon-containing gas, a mixed gas of argon (Ar), carbon monoxide (CO), carbon dioxide (CO ₂ ) and water vapor (H ₂ O) (Ar volume ratio 70%, CO volume ratio 10%, CO ₂ the volume ratio of 10%, the vicinity of the interface between of H ₂ O volume ratio of 10%) was introduced at a pressure of 0.2MPa to the gas blowing pipe 4, the gas blow-out port 6 of the stirring portion 5 and the molten silicon layer molten slag layer Thus, the gas blowing pipe 4 was lowered by the lifting mechanism. The flow rate of the carbon-containing gas was 3.0 L / min.

  After confirming that the carbon-containing gas was blown into the molten silicon 8, the gas blowing pipe 4 was rotated at 400 rpm by a rotating mechanism, and a purification process was performed for 2 hours. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 0.6 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr.

  On the other hand, a large amount of slag adhered to the lower end of the gas blowing tube 4 and the outer surface of the stirring unit 5, and a part of the gas blowing port 6 was blocked.

(Example 6)
In this example, a mixture of silicon oxide (SiO ₂ ) powder and lithium carbonate (Li ₂ CO ₃ ) powder in a mass ratio of 62:38 was used as the slag material. Lithium carbonate (Li ₂ CO ₃ ) is decomposed into lithium oxide (Li ₂ O) and carbon dioxide (CO ₂ ) during heating. Since the ratio of the molecular weight of lithium oxide (Li ₂ O) to carbon dioxide (CO ₂ ) is approximately 30:44, silicon oxide (SiO ₂ ) and lithium oxide (Li ₂ O) are in a molten state of the slag material. It is considered that the mass ratio is about 80:20.

  Except for the slag material, the purification treatment was performed in the same manner as in Example 5, and the boron content before and after the treatment was measured.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr.

  On the other hand, adhesion of slag to the lower end of the gas blowing tube 4 and the outer surface of the stirring unit 5 was very small, and the gas blowing port 6 was completely open. The diameter of the gas outlet 6 that was 1 mm was 1.2 mm, and hardly changed.

(Example 7)
Purified in the same manner as in Example 1 except that a mixed gas of argon (Ar) and carbon dioxide (CO ₂ ) (Ar volume ratio 95%, CO ₂ volume ratio 5%) was used as the carbon-containing gas. Processing was performed. When the boron content before and after the treatment was measured, it was 7.0 ppm before the treatment and 4.0 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

(Example 8)
Purified in the same manner as in Example 1 except that a mixed gas of argon (Ar) and carbon dioxide (CO ₂ ) (Ar volume ratio 90%, CO ₂ volume ratio 10%) was used as the carbon-containing gas. Processing was performed. When the boron content before and after the treatment was measured, it was 7.0 ppm before the treatment and 3.6 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

Example 9
Purification was performed in the same manner as in Example 1 except that a mixed gas of argon (Ar) and carbon dioxide (CO ₂ ) (Ar volume ratio 50%, CO ₂ volume ratio 50%) was used as the carbon-containing gas. Processing was performed. When the boron content before and after the treatment was measured, it was 7.0 ppm before the treatment and 3.3 ppm after the treatment.

  Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment.

(Example 10)
Purification was performed in the same manner as in Example 1 except that a mixed gas of argon (Ar) and carbon dioxide (CO ₂ ) (Ar volume ratio 45%, CO ₂ volume ratio 55%) was used as the carbon-containing gas. Processing was performed. When the boron content before and after the treatment was measured, it was 7.0 ppm before the treatment and 3.3 ppm after the treatment.

Slight graphite consumption was observed in the gas blowing tube 4 in contact with the carbon-containing gas and the gas flow path 7 in the stirring unit 5. The thickness of the bottom portion of the stirring unit 5 where the graphite was most consumed was 1 mm smaller. That is, the thickness of graphite was reduced at a rate of 0.5 mm / hr. However, the appearances of the gas blowing tube 4 and the stirring unit 5 after the treatment were not significantly different from those before the treatment. However, in this example, since the ratio of carbon dioxide (CO ₂ ) in the carbon-containing gas was too high, it was difficult to remove the carbon remaining in the molten silicon 8 by the decompression process.

(Example 11)
The purification treatment was performed in the same manner as in Example 1 except that an alumina tube was inserted so as to cover the surface of the gas flow path 7 of the gas blowing tube 4. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 3.5 ppm after the treatment, and the silicon purification ability was the same as in Example 1.

In this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all. This is because the carbon supply into the molten silicon 8 due to the exhaustion of the gas blowing tube 4 is suppressed by inserting the alumina tube, but the carbon dioxide (CO ₂ ) in the carbon-containing gas contains the molten silicon 8 in the molten silicon 8. This is probably due to the supply of carbon.

(Example 12)
Instead of the carbon-containing gas, a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) is used, and carbon powder having a diameter of about 10 μm. Was purified in the same manner as in Example 11 except that 1 g per minute was mixed in molten silicon 8. When the boron content before and after the treatment was measured, it was 7.1 ppm before the treatment and 2.5 ppm after the treatment, and the silicon purifying ability was larger than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 13)
A purification treatment was performed in the same manner as in Example 12 except that silicon carbide powder having a diameter of about 10 μm was mixed in molten silicon 8 at a rate of 4 g / min instead of carbon powder. When the boron content before and after the treatment was measured, it was 7.3 ppm before the treatment and 2.0 ppm after the treatment, and the silicon purifying ability was larger than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 14)
Mixed gas of argon (Ar), methane (CH ₄ ) and water vapor (H ₂ O) as a carbon-containing gas (Ar volume ratio 80%, CH ₄ volume ratio 10%, H ₂ O volume ratio 10%) The purification treatment was performed in the same manner as in Example 11 except that was used. When the boron content before and after the treatment was measured, it was 7.6 ppm before the treatment and 2.6 ppm after the treatment.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 15)
The purification treatment was performed in the same manner as in Example 6 except that an alumina tube was inserted so as to cover the surface of the gas flow path 7 of the gas blowing tube 4. When the boron content before and after the treatment was measured, it was 7.3 ppm before the treatment and 0.5 ppm after the treatment.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 16)
As a carbon-containing gas, a mixed gas of argon (Ar), carbon monoxide (CO), carbon dioxide (CO ₂ ) and water vapor (H ₂ O) (Ar volume ratio 40%, CO volume ratio 20%, CO ₂ The purification treatment was performed in the same manner as in Example 15 except that the volume ratio of 20% and the volume ratio of H ₂ O was 20%. When the boron content before and after the treatment was measured, it was 7.5 ppm before the treatment and 0.2 ppm after the treatment, and the silicon purifying ability was improved as compared with Example 15.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 17)
Instead of the carbon-containing gas, a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) is used, and carbon powder having a diameter of about 10 μm. Was purified in the same manner as in Example 11 except that 95 g of molten silicon was mixed in 95 g of molten silicon 8. When the boron content before and after the treatment was measured, it was 7.1 ppm before the treatment and 3.2 ppm after the treatment, and the silicon purifying ability was slightly higher than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 18)
Instead of the carbon-containing gas, a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) is used, and carbon powder having a diameter of about 10 μm. Was purified in the same manner as in Example 11 except that 100 g of the mixture was mixed in 100 g of molten silicon 8. When the boron content before and after the treatment was measured, it was 7.1 ppm before the treatment and 2.6 ppm after the treatment, and the silicon purifying ability was considerably higher than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 19)
Instead of the carbon-containing gas, a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) is used, and carbon powder having a diameter of about 10 μm. Was purified in the same manner as in Example 11 except that 500 g was mixed in 500 g of molten silicon 8. When the boron content before and after the treatment was measured, it was 7.3 ppm before the treatment and 2.4 ppm after the treatment, and the silicon purifying ability was considerably larger than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all.

(Example 20)
Instead of the carbon-containing gas, a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) is used, and carbon powder having a diameter of about 10 μm. Was purified in the same manner as in Example 11 except that 600 g was mixed in 600 g of molten silicon 8. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 2.5 ppm after the treatment, and the silicon purifying ability was considerably larger than that of Example 11.

  Also in this embodiment, the surface of the gas flow path 7 of the gas blowing tube 4 and the alumina tube were not consumed at all. However, in the present example, since the amount of carbon mixed was too large, it was difficult to remove the carbon remaining in the molten silicon 8 by filtering with a filter.

(Comparative Example 1)
Example 1 except that a mixed gas of argon (Ar) and water vapor (H ₂ O) (Ar volume ratio 70%, H ₂ O volume ratio 30%) was used instead of the carbon-containing gas. The purification treatment was performed in the same manner. When the boron content before and after the treatment was measured, it was 7.4 ppm before the treatment and 4.4 ppm after the treatment.

  Clear consumption of graphite was observed in the gas blowing pipe 4 and the gas flow path 7 in the stirring section 5 in contact with the mixed gas. The thickness of the bottom part of the stirring part 5 where the consumption of graphite was the most intense was 4 mm smaller. That is, the thickness of graphite was reduced at a rate of 2 mm / hr. The diameter of the gas outlet 6 was increased from 1 mm to 3 mm.

Despite the higher oxidizing power of water vapor (H ₂ O) than carbon monoxide (CO) and carbon dioxide (CO ₂ ), the ability to purify silicon was lower. This is presumably because the bubbles of the mixed gas became larger due to the increase in the diameter of the gas outlet 6 and the rate of oxidation reaction of impurities such as boron decreased.

(Comparative Example 2)
The purification treatment was performed in the same manner as in Comparative Example 1 except that an alumina tube was inserted so as to cover the surface of the gas flow path 7 of the gas blowing tube 4. When the boron content before and after the treatment was measured, it was 7.2 ppm before the treatment and 5.0 ppm after the treatment, and the silicon purification ability was further reduced as compared with Comparative Example 1. In Comparative Example 2, since the alumina tube was inserted, the surface of the gas flow path 7 of the gas blowing tube 4 was not consumed, but the amount of carbon supplied into the molten silicon by that amount. This is thought to be due to a decrease in

  In the above examples and comparative examples, after performing the purification process for the predetermined time, the gas blowing pipe 4 is raised by the elevating mechanism until the stirring unit 5 is located sufficiently above the surface of the molten silicon 8, Boron content was measured by taking out several g of molten silicon 8 for measuring the boron content after treatment. Further, when the slag material is added as in Examples 5, 6, 15 and 16, the molten silicon 8 and the molten slag 9 are allowed to stand for a few minutes in order to sufficiently separate them, and the molten slag 9 does not mix. I did it.

  In the above, the boron content was measured by ICP (inductively coupled plasma) emission spectrometry.

  In the above, a mixture of semiconductor grade silicon and boron-containing scrap silicon was used as the raw material silicon. However, the raw material contains impurities other than boron, for example, a purity of about 98%, which is widely used industrially. Needless to say, the effect of the present invention is exhibited even with silicon.

  In Examples 5, 6, 15, and 16, the solid state slag was added to the solid state silicon and melted. However, the solid state slag was added to the molten silicon and melted. However, the same effect can be obtained, and even if molten slag is added to solid silicon and then melted, the same effect is obtained, and molten slag is added to molten silicon. Needless to say, the same effect is exhibited even if these are kept in a molten state.

  Furthermore, the place where the present invention is applied is not limited to the present embodiment. For example, the mixing amount of slag, the flow rate of the carbon-containing gas, the rotational speed of the gas blowing tube 4 and the like are the raw material silicon to be processed. The amount should be appropriately selected so as to be in an optimum state depending on the amount of the crucible or the shape of the crucible 2.

  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.

  According to the present invention, silicon can be efficiently purified by improving the boron removal rate. Therefore, according to the present invention, since silicon for solar cells can be produced at low cost, the present invention is suitably used for the production of solar cells.

It is typical sectional drawing of a part of preferable example of the apparatus used for this invention. It is the typical side view and typical bottom view of some preferable examples of the gas blowing pipe used for the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Melting furnace, 2 crucible, 3 Electromagnetic induction heating apparatus, 4 Gas blowing pipe, 5 Stirring part, 6 Gas blowing port, 7 Gas flow path, 8 Molten silicon, 9 Molten slag, 10 Wings, 11 Bubbles, 12 Seal mechanism.

Claims (9)

The molten silicon is blown with a carbon-containing gas containing at least one selected from the group consisting of carbon monoxide, carbon dioxide and hydrocarbons , and at least of carbon and silicon carbide before and / or after melting of the silicon. A method for purifying silicon, comprising mixing one of the two .   A method for purifying silicon, comprising mixing at least one of carbon and silicon carbide before and / or after melting of silicon. The method for purifying silicon according to claim 1 or 2 , wherein slag is mixed before and / or after melting of the silicon. The method for purifying silicon according to claim 3 , wherein the slag contains silicon oxide. The method for purifying silicon according to claim 4 , wherein the slag contains an oxide of an alkali metal. The slag, characterized in that it comprises at least one selected from the group of alkali metal carbonates, alkali metal hydrogen carbonates and alkali metal silicates, the purification of silicon according to claim 4 or 5 Method. By immersing the gas blowing tube after the melting of the silicon, and introducing the carbon-containing gas containing at least one selected from the group of carbon monoxide, carbon dioxide and hydrocarbons into the gas blowing tube, the carbon containing wherein the blowing gas to the silicon, the purification method of silicon according to any one of claims 1 to 6. The method for purifying silicon according to claim 7 , wherein at least a part of a gas flow path of the gas blowing pipe is made of an oxidation resistant material. While blowing the carbon-containing gas into the silicon, characterized in that rotating the gas blowing tube with said silicon, the purification process of silicon according to claim 7 or 8.

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