JP5084144B2 - Manufacturing method of high purity silicon - Google Patents

Description

  The present invention efficiently removes boron from metallurgical grade metal silicon (purity of about 98 to 99% by mass) for the purpose of producing high-purity silicon having high utility value as a solar cell raw material. This is a method for improving the durability of the container used for the reaction.

As a conventional technique for removing impurities, particularly boron, from silicon, for example, as disclosed in Non-Patent Document 1, molten silicon and CaO—SiO ₂ , CaO—MgO—SiO ₂ , CaO—BaO—SiO _{2 are used.} And slag such as CaO—CaF ₂ —SiO ₂ is contacted and removed by equilibrium distribution of boron between the slag and silicon.

  Further, in a silicon purification method comprising removing impurities from molten silicon by treating molten silicon with slag as disclosed in Patent Document 1, the slag is continuously or substantially added to the molten silicon. For one or more impurity elements to be continuously added to and removed from the slag, as soon as an equilibrium between the slag and the molten silicon is achieved, the slag is continuously or substantially continuously removed. Methods are known for activation or removal from the molten silicon.

Furthermore, as disclosed in Patent Document 2, a flux containing a basic component is added to silicon having a boron concentration of 100 ppm by mass or less, a flux addition step for melting these, and a nozzle is immersed in the silicon, A silicon purification method comprising a reaction step of blowing an oxidizing gas and a flux removal step of removing flux from silicon, wherein the flux includes a basic flux containing CaO, CaCO ₃ or an alkali metal oxide, preferably a predetermined flux A method using a CaO—CaF ₂ mixed flux having the following composition is known.

Alternatively, as disclosed in Patent Document 3, one or more solids that generate H ₂ O and / or CO ₂ at 1400 ° C. or lower are melted together with a carrier gas such as Ar, H ₂ , and CO. By blowing into the silicon bath, oxidation of silicon at the nozzle tip can be suppressed, and a large amount of H ₂ O and / or CO ₂ that decomposes from the solid can be introduced into the silicon bath. A method for discharging an oxide gas together with a carrier gas is known.

JP-A-8-11208 JP 2003-12317 A JP-A-9-202611 Suzuki et al., Journal of the Japan Institute of Metals, Vol. 54, No. 2, p. 168-172 (1990)

  The method of Non-Patent Document 1 described above attempts to equilibrate and remove boron between slag and silicon, but the boron distribution coefficient (the boron concentration in slag (mass ppm)) / (Defined as boron concentration (mass ppm) in silicon) is as low as 2.0 at most. For this reason, in this method, boron contained in metallurgical grade silicon by several ppm to several tens of ppm by mass, the upper limit concentration of boron generally required as the purity of high-purity silicon for solar cell production is 0.3 mass ppm. Preferably, a large amount of slag is required for removal to 0.1 mass ppm.

  In addition, materials that can be industrially used as a slag-forming material usually contain 1 ppm to several ppm of boron, and boron is removed to the required concentration at the low distribution ratio as described above. It is impossible. Thus, the method of nonpatent literature 1 cannot be utilized industrially.

  Further, in order to improve the above, the method of Patent Document 1 is a method in which molten silicon contained in a container is treated with slag having an ability to remove boron and / or other impurities. Continuously or substantially continuously, and as soon as an equilibrium is achieved between the slag and the molten silicon for one or more impurity elements to be removed, continuously or substantially The slag is continuously inactivated or removed from the molten silicon to reduce the amount of slag used.

  However, even with this method, the distribution coefficient between slag and silicon is essentially the same as that of Non-Patent Document 1, and considering boron contained in materials that can be industrially used as slag forming substances, It is difficult to remove boron to the level required for high purity silicon.

  Further, the method of Patent Document 2 adds a flux containing a basic component to molten silicon, melts the flux, a reaction step of immersing a nozzle in silicon and blowing an oxidizing gas, and a flux from silicon. It is said that it consists of a flux removal process to remove boron, and simultaneously achieves high basicity and high oxygen partial pressure of slag to remove boron efficiently.

  However, blowing an oxidizing gas into molten silicon means that there is no suitable nozzle material that can withstand reactive molten silicon or slag, or the nozzle tip is blocked by oxidized silicon. Have difficulty. Moreover, the removal of boron by this method is that the initial boron concentration of 14 ppm by mass is only reduced to 7.6 ppm by mass after the treatment according to the examples, and the boron tolerance of high-purity silicon for solar cell production is The reality is that it is far away.

In the method of Patent Document 3, one or two or more solids that generate H ₂ O and / or CO ₂ at 1400 ° C. in molten silicon are mixed with a carrier gas such as Ar, H ₂ , CO, etc. By introducing a large amount of H ₂ O and / or CO ₂ oxidizing gas that decomposes from the solid into the silicon bath while preventing nozzle clogging due to silicon oxidation at the nozzle tip by blowing into the bath. In this method, the boron oxide gas is discharged together with the carrier gas.

  However, as described above, molten silicon is very reactive, and the tuyere and nozzle are likely to be corroded and destroyed, making it difficult to realize industrially.

  An object of the present invention is to provide a method for producing high-purity silicon, which can efficiently remove boron from silicon and improve the durability of a reaction vessel for removing boron in silicon. .

As a result of intensive studies to achieve the above object, the present inventors have found that boron can be efficiently removed from silicon and the durability of the reaction vessel can be improved by the following method.
(1) A method of removing boron in silicon by forming a slag by adding one or both of an alkali metal carbonate or a carbonate hydrate and SiO ₂ to a container containing molten silicon. The container used for the boron removal reaction is made of an oxidation-resistant and non-conductive material, and the molten silicon is directly heated by induction heating means while the outline gauge is cooled from the outside. A method for producing high-purity silicon.
(2) The method for producing high-purity silicon according to (1), wherein a main component of the material of the container is a refractory material made of an oxide.
(3) After carrying out the boron removal reaction, the molten silicon is discharged from the container, and then the metal silicon previously melted by another heating and melting means so that the emptied container has a predetermined temperature or higher. method for producing high purity silicon according to the was charged to the vessel, and wherein the preventing temperature drop of the container (1) or (2).
(4) after performing the boron removal reaction, thereby discharging the molten silicon from the reaction vessel and heated at a heating means container emptied, characterized in that to prevent the temperature drop of the container (1 ) Or the method for producing high-purity silicon according to (2).

  According to the present invention, boron can be efficiently removed from silicon, and the durability of the reaction vessel can be improved, so that high-purity silicon having high utility value for solar cells can be produced with high efficiency. As a result, by supplying inexpensive high-purity silicon to the market, it is possible to contribute to lowering the cost of products such as solar cells.

Next, the present invention will be described in more detail. The present inventor has found a method for removing boron in silicon by adding a mixture of SiO ₂ and one or both of an alkali metal carbonate or a hydrate of the carbonate to a container containing molten silicon. Yes.

An important feature of this technology is that an alkali metal carbonate or a mixture of one or both of the carbonate hydrates and SiO ₂ is added to the molten silicon as it is. One or both of the carbonate hydrates and SiO ₂ are not treated to slag.

This is because the alkali metal carbonate or hydrate of the carbonate added to the molten silicon is easily decomposed to generate CO ₂ and alkali metal oxide which are oxidizing substances. This is because boron is oxidized to become boron oxide, which is an acidic compound, and at the same time, migration to a strongly basic alkali metal oxide-SiO ₂ slag is promoted.

On the other hand, by performing such an addition method, an alkali metal carbonate or a hydrate of the carbonate and a strongly basic alkali metal oxide-SiO ₂ slag coexist.

According to the experiments by the present inventors, when an alkali metal carbonate or a hydrate of the carbonate and a strongly basic alkali metal oxide-SiO ₂ slag coexist, It has become clear that there is a problem that durability tends to be lowered.

  The present inventor has intensively studied to improve this problem, and has found a method for efficiently removing boron from silicon while improving the durability of the reaction vessel, leading to the present invention. The contents will be described below.

  One reason for the decrease in the durability of the reaction vessel is that alkali metal carbonates or hydrates of carbonates have a strong oxidizing power, so that a reaction vessel made of a material that is vulnerable to oxidation is severely damaged. . For example, a graphite crucible as described in Patent Document 1 has been conventionally used for melting and refining silicon. However, in our experiments, it is oxidized and damaged, and cannot be used industrially. There was found.

  Further, in the experiments conducted by the present inventors, it was found that the damage was similarly great when a silicon carbide crucible was used.

For this reason, it is desirable to use a reaction vessel made of a refractory material having oxidation resistance, for example, an oxide such as alumina, magnesia, silica, etc. as a main component. For example, when a method of heating a container made of silica, which is a known technique, from the outside by a graphite heater is employed, the alkali metal carbonate or the hydration of the carbonate is not very effective. And strongly basic alkali metal oxide-SiO ₂ slag reacted with silica, which is the material of the container, and it was found that the damage was large.

This is because, when the crucible is heated from the outside, the temperature of the container becomes higher than the melting point (1414 ° C.) of silicon, and an alkali metal carbonate or a hydrate of the carbonate and a strongly basic alkali metal oxide This is because the reaction with —SiO ₂ -based slag is rapid. The same thing happened when a container made of a refractory material made of an oxide such as alumina or magnesia was heated from the outside.

  For this reason, the present inventors focused on suppressing damage to the reaction vessel due to oxidation by adopting the best method in combination with the material of the reaction vessel and the heating method, and the results of earnest experiments In addition, the container used for the boron removal reaction is made of an oxidation-resistant and non-conductive material, and the molten silicon is directly heated by induction heating means while cooling the container from the outside. It was found that the performance can be greatly improved.

  The reason for this is that, since the molten silicon is heated by induction heating means using a non-conductive material for the reaction vessel, only the molten silicon having electrical conductivity generates heat, and the reaction vessel does not generate heat. In addition, since the reaction vessel is cooled from the outside, the reaction vessel is maintained at a lower temperature than silicon. The temperature of the reaction vessel at this time may be set to a temperature appropriately determined by experiments or the like, but according to the knowledge of the present inventor, it is preferably set to 1450 ° C. or lower.

  The temperature of the reaction vessel during the reaction can be confirmed by using a thermometer (thermocouple) attached to the reaction apparatus or intermittently measuring with a consumable thermocouple.

  In addition, since the reaction vessel is made of an oxidation resistant material, damage due to oxidation is less likely to occur.

For this reason, the reaction between the alkali metal carbonate or one or both of the carbonate hydrate, the SiO ₂ slag, and the container material becomes very slow, and it becomes durable enough for industrial use. is there.

Further, by induction heating the molten silicon, the molten silicon is strongly stirred by electromagnetic force. Therefore, the mass transfer rate when boron in the molten silicon moves to the slag is accelerated by stirring, and combined with the effect of adding one or both of the alkali metal carbonate or the carbonate hydrate and SiO _2. Further, more efficient boron removal can be achieved.

  Due to these effects, the partition coefficient of boron between slag and silicon, which is an index for removing boron in silicon, is only about 2.0 in the prior art as described in Non-Patent Document 1 and Patent Document 1. However, it was significantly improved to 5.0 or more, and it was confirmed that boron can be removed with high efficiency.

  In addition, as the above-mentioned oxidation-resistant and non-conductive refractory material, the degree of the characteristic may be set as appropriate, and is not particularly specified, but is a refractory material made of generally used oxide, For example, alumina, magnesia, silica, and a mixture thereof can be used.

  In addition, when the molten silicon in the reaction vessel is induction-heated, Lorentz force acts on the molten silicon, and thereby a pinch force directed from the side surface of the reaction vessel to the inside of the molten silicon acts on the molten silicon. For this reason, since a gap is formed between the molten silicon and the reaction vessel, slag may enter the gap and the inner surface of the reaction vessel may be easily oxidized and damaged. Therefore, an oxidation resistant material is formed on the inner surface of the reaction vessel. Preferably it is formed.

  Examples of the oxidation resistant material formed on the inner surface of the reaction vessel include magnesia, mullite, alumina, silicon nitride, silicon carbide and the like.

  As described above, when the reaction for removing boron from the molten silicon is performed, the reaction efficiency is improved and the durability of the container is dramatically improved.

  However, when boron is removed industrially by the above-described method, it is necessary to discharge molten silicon from the reaction vessel to another vessel when the boron removal reaction is completed. Therefore, after the molten silicon is discharged, the container is emptied, so that the temperature of the reaction container rapidly decreases due to the absence of molten silicon as a heat source.

  Therefore, since the refractory material made of the above oxide generally has a defect that it easily breaks due to thermal stress when the temperature is lowered from a high temperature to a low temperature, There is also a concern that the reaction vessel is cracked.

  For this reason, as a result of intensive studies, the inventors conducted a boron removal reaction, and then discharged the molten silicon from the container, and then the emptied container was at a predetermined temperature or higher, It was found that cracking of the reaction vessel can be prevented by charging metal silicon previously melted by another heating and melting means into the vessel to prevent a temperature drop of the vessel.

  Here, the temperature above the predetermined temperature of the emptied container may be set as appropriate by experiments or the like, but according to the knowledge of the present inventors, it is preferably 1000 ° C. or higher, more preferably 1100 ° C. or higher.

  Here, the temperature of the emptied container can be measured using a thermometer (thermocouple) attached to the reaction apparatus or using a radiation type thermometer.

  Further, the time from when the container is emptied until the metal silicon melted by another heating and melting means is charged into the container is preferably as short as possible. It is preferably within minutes, and more preferably within 2 minutes.

  Similarly, after performing the boron removal reaction, the molten silicon is discharged from the reaction vessel, and the emptied vessel is heated with a heating means to prevent the temperature of the vessel from decreasing. It was found that cracking can be prevented.

  Here, the temperature when the emptied container is heated by the heating means is preferably 1000 ° C. or higher, and more preferably 1100 ° C. or higher, as described above.

  As described above, it is possible to efficiently remove boron from silicon and improve the durability of the reaction vessel in which the removal reaction is performed.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to the following Example.

Example 1
After mixing powdered Na ₂ CO ₃ containing 0.6 mass ppm of boron and powdered SiO ₂ (silica sand) containing 1.5 mass ppm of boron in a ratio of 2: 1 by molecular weight, the diameter It was molded into a spherical shape of about 5 mm (about 0.1 g).

As shown in FIG. 1, an apparatus is used in which an induction heating coil 7 having an output of 5 kHz for directly heating metal silicon and an external cooling means 6 using water as a cooling medium are disposed outside the reaction vessel 5. In addition, 20 kg of metallic silicon containing 15 mass ppm of boron is heated and melted in a reaction vessel 5 made of alumina, and the molded body 1 of the above mixture is added to the silicon by adding means 4 using a non-oxidizing compressed gas (N ₂ ). Added. The molded body was continuously added at a rate of 0.2 kg / min.

  The slag 3 produced by the reaction of the molded body of the added mixture with silicon was continuously discharged from the reaction vessel 5. The addition was stopped when the total amount of the added molded body reached 30 kg, and after 20 minutes, the reaction vessel 5 was tilted to discharge the silicon, and then the silicon melted by another heating and melting means (not shown) within 3 minutes. Moved in.

  Here, the temperature of the molten silicon during the reaction was about 1500 ° C., but the container during the reaction was maintained at about 1400 ° C. at the maximum.

  Further, while the silicon was discharged and the silicon melted by another heating and melting means was transferred within 3 minutes, the temperature decreased, but the minimum temperature could be about 1100 ° C.

  When the silicon component discharged from the reaction vessel was analyzed, the boron concentration in the silicon was 0.25 ppm, which was a high purity that can be used for solar cells. The reaction vessel 5 was still usable even after the above reaction was performed 50 times or more.

(Example 2)
After mixing powdery K ₂ CO ₃ containing 1.0 mass ppm of boron and powdered SiO ₂ (silica sand) containing 1.5 mass ppm of boron in a ratio of 2: 1 by molecular weight, the diameter It was molded into a spherical shape of about 5 mm (about 0.1 g).

Using the apparatus shown in FIG. 1, 20 kg of metallic silicon containing 15 mass ppm of boron is heated and melted in a reaction vessel 5 made of alumina, and the molded body 1 of the above mixture is non-oxidized compressed gas (N ₂ ) in this silicon. It was added by addition means 4 using The molded body was continuously added at a rate of 0.2 kg / min.

  The slag 3 produced by the reaction of the molded product of the added mixture with silicon was continuously discharged from the reaction vessel 5. The addition was stopped when the total amount of the added molded body reached 30 kg, and after 20 minutes, the reaction vessel 5 was tilted to discharge the silicon, and then the silicon melted by another heating and melting means (not shown) within 3 minutes. Moved in.

  Here, the temperature of the molten silicon during the reaction was about 1500 ° C., but the container during the reaction was maintained at about 1400 ° C. at the maximum.

  Further, while the silicon was discharged and the silicon melted by another heating and melting means was transferred within 3 minutes, the temperature decreased, but the minimum temperature could be about 1100 ° C.

  When the silicon component discharged from the reaction vessel was analyzed, the boron concentration in the silicon was 0.27 ppm, which was a high purity that can be used for solar cells. The reaction vessel 5 was still usable even after the above reaction was performed 50 times or more.

(Comparative example)
After mixing powdered Na ₂ CO ₃ containing 0.6 mass ppm of boron and powdered SiO ₂ (silica sand) containing 1.5 mass ppm of boron in a ratio of 2: 1 by molecular weight, the diameter It was molded into a spherical shape of about 5 mm (about 0.1 g).

As shown in FIG. 2, using a device in which a resistance heater is disposed as the heating means 7 on the outside of the reaction vessel 5, 20 kg of metallic silicon containing 15 mass ppm of boron is contained in the reaction vessel 5 made of alumina. The mixture was heated and melted, and the molded body 1 was added to the silicon by an adding means 4 using a non-oxidizing compressed gas (N ₂ ). The molded body was continuously added at a rate of 0.2 kg / min.

  Slag produced by the reaction of the projected molded body with silicon was continuously discharged from the reaction vessel 5. When the total amount of the projected molded body reached 30 kg, the projection was stopped, and after 20 minutes, the reaction vessel 5 was tilted and silicon was discharged.

  Here, the temperature of the molten silicon during the reaction was set to about 1500 ° C., but the temperature during the reaction rose from the outside to about 1550 ° C. because it was heated from the outside.

  Further, while the temperature was lowered while silicon was discharged and then the silicon melted by another heating and melting means was transferred, the temperature was not particularly controlled, so the minimum temperature was lowered to about 500 ° C.

  When the silicon component discharged from the reaction vessel 5 was analyzed, the boron concentration in the silicon was 0.3 ppm, which was a purity that can be used for solar cells. However, the reaction vessel 5 was not damaged when used for the above reaction 10 times or more.

It is the schematic which shows one Example of this invention. It is the schematic which shows a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Molded body 2 Molten silicon 3 Slag 4 Addition means 5 Reaction vessel 6 Cooling means 7 Coil for induction heating 8 Compressed gas piping 9 Heating means

Claims (4)

A method of removing boron in silicon by forming a slag by adding one or both of an alkali metal carbonate or a hydrate of the carbonate and SiO ₂ to a container containing molten silicon, A high-purity silicon characterized in that an oxidation-resistant and non-conductive material is used as a container for boron removal reaction, and the molten silicon is directly heated by induction heating means while cooling the container from the outside Manufacturing method.   2. The method for producing high-purity silicon according to claim 1, wherein a main component of the material of the container is a refractory material made of an oxide. After performing the boron removal reaction, the molten silicon is discharged from the container, and then the metallic silicon previously melted by another heating and melting means is used so that the emptied container has a predetermined temperature or higher. the method of producing high purity silicon according to claim 1 or 2 was charged, and wherein the preventing temperature drop of the container in. After performing the boron removal reaction, thereby discharging the molten silicon from the reaction vessel and heated at a heating means container emptied, claim 1 or 2, characterized in that to prevent the temperature drop of the container A method for producing high-purity silicon as described in 1.

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