JP2006193346A - Refining method of silicon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently removing boron in molten silicon in the production of silicon requiring high purity for e.g. solar cell. <P>SOLUTION: This refining method of silicon is characterized in that Na<SB>2</SB>CO<SB>3</SB>(5) or a mixture 6 of Na<SB>2</SB>CO<SB>3</SB>and SiO<SB>2</SB>is blown into molten silicon together with a carrier gas through a bottom hole 2 or an immersed pipe 3 of a container housing the molten silicon 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a scouring method for increasing the purity of silicon used in solar cells and the like.

  High purity silicon is required for silicon used in solar cells and the like. In particular, various removal methods have been proposed for impurities that are difficult to remove, such as boron. One method is to blow some solid or some gas into the molten silicon together with the carrier gas.

Patent Document 1 proposes a method of blowing Ca (OH) ₂ or CaCO ₃ using Ar, H ₂ O, or CO ₂ as a carrier gas. In Patent Document 2, although carrier gas is not described, slag having a density lower than that of silicon, specifically, slag based on Na ₂ O—SiO ₂ is continuously supplied from the bottom hole. A method has been proposed.
JP-A-9-202611 JP-A-8-073209

However, in the method of blowing Ca (OH) ₂ or CaCO ₃ using Ar, H ₂ O, or CO ₂ as a carrier gas, the B concentration in silicon is set at a solar cell level of 0. It is impossible to reduce it to 3 ppm or less.

The first reason is that the B concentration contained as an impurity in Ca (OH) ₂ or CaCO ₃ is high. Since it can be estimated that a commercially available product contains several ppm of B, the B concentration in silicon cannot be significantly reduced below 1 ppm even if it is added to silicon by any method.

The second reason is that the density of CaO—SiO ₂ slag formed by Ca (OH) ₂ or CaCO ₃ is larger than that of silicon. If the time for which the slag is in contact with silicon is too long, a reverse B phenomenon occurs in which B moves to silicon. Therefore, in order to further reduce the B concentration in silicon, it is necessary to remove the added slag after an appropriate time. However, since the CaO—SiO ₂ slag formed by Ca (OH) ₂ or CaCO ₃ has a higher density than silicon, the slag sinks in the molten silicon. Since the precipitated slag cannot be removed while hot, the B concentration in silicon cannot be significantly reduced below 1 ppm.

On the other hand, in the method of continuously adding slag based on Na ₂ O—SiO ₂ from the bottom hole, the B concentration contained as an impurity in the slag is low. Furthermore, the density of the slag is smaller than that of silicon and is easy to remove. However, the oxidizing power of slag is weak and it cannot be said that boron removal ability is high.

  An object of the method of the present invention is to provide a silicon refining method that efficiently removes boron from silicon and lowers the B concentration in silicon to a solar cell level.

The present invention has been made to solve the above problems. (1) Na ₂ CO ₃ or a mixture of Na ₂ CO ₃ and SiO ₂ together with a carrier gas from a bottom hole or a dip tube of a container containing molten silicon. (2) A silicon scouring method according to (1), wherein an acidic oxide having a lower density than the molten silicon is added onto the molten silicon. 3) The generated slag is removed at least once during or after scouring, and the silicon scouring method according to (1) or (2), (4) an oxidizing gas or an inert gas as the carrier gas Or a mixed gas of both, wherein the silicon scouring method according to (1), and (5) Ar gas is used as the inert gas. Scouring method of silicon, (6) a method of scouring the acidic oxide as characterized by using the SiO ₂ (2) silicon as claimed, (7) the _Na 2 CO _3, or a _Na 2 CO ₃ and SiO ₂ The silicon scouring method according to (1), wherein the boron concentration in the molten silicon before blowing the mixture is 20 mass ppm or less.

  By using the method of the present invention, the boron concentration in silicon can be efficiently reduced.

As shown in FIG. 1, the present invention includes Na ₂ CO ₃ (5) or a mixture 6 of Na ₂ CO ₃ and SiO ₂ together with a carrier gas from a bottom hole 2 or a dip tube 3 in molten silicon 1. It is a method of blowing.

In the experiments by the inventors, the boron removal ability was clearly higher when Na ₂ CO ₃ was blown and when Na ₂ CO ₃ and SiO ₂ were blown than when Na ₂ O—SiO ₂ was blown. . Furthermore, when Na ₂ CO ₃ and SiO ₂ are blown, the boron removal ability can be improved by adjusting the amount of Na ₂ CO ₃ and SiO ₂ so that the molecular weight ratio is Na ₂ CO ₃ / SiO ₂ > 1. Is preferable. Na ₂ CO ₃ is to generate is decomposed to be added CO ₂ into the molten silicon is believed that oxidation of the CO ₂ is effective in B removal. Further, the SiO _2, since it is considered that an effect of promoting the decomposition of the Na ₂ O and CO ₂ of _Na 2 CO _3, who when blowing and the SiO _{2 Na 2} CO ₃ is more preferable. The boron removal capability referred to here relates to, for example, the flow rate of solid to be blown, the time, etc. required to reduce the boron concentration in a certain amount of molten silicon from 10 ppm to 1 ppm. The smaller the flow rate and time, the higher the removal ability. Compared to the conventional method of continuously adding Na ₂ O—SiO ₂ , the method of the present invention in which Na ₂ CO ₃ of the present invention or Na ₂ CO ₃ and SiO ₂ is blown is more effective in the flow rate and time of the solid to be blown. Therefore, it can be said that it is excellent as a boron removal method.

On the other hand, the boron removing ability is almost the same when Na ₂ CO ₃ or Na ₂ CO ₃ and SiO ₂ are blown, and when conventional Ca (OH) ₂ and CaCO ₃ are blown, and from about 10 ppm. Boron removal up to about 1 ppm can be similarly performed. However, as described above, it is considered that Ca (OH) ₂ and CaCO ₃ have a B concentration contained as an impurity of about several ppm. Thus, when Ca (OH) ₂ or CaCO ₃ is blown, it is difficult to remove boron from about 1 ppm to about 0.3 ppm. This is because the boron concentration in the molten silicon and the boron concentration of Ca (OH) ₂ or CaCO ₃ are the same level. On the other hand, the B concentration of impurities of Na ₂ CO ₃ and SiO ₂ is on the order of 0.1 ppm, and if the slag floating on the silicon is appropriately removed so that recovery B does not occur, the B concentration in the silicon can be reduced. It can be lowered to 0.3 ppm or less which is a solar cell level.

Further, _Na 2 CO _3, or slag that is formed when bubbling and _Na 2 CO ₃ and SiO ₂ is lower density than silicon is easily removed to float on the silicon.

In the method of adding slag together with the carrier gas, there is a possibility that slag and silicon are scattered and there is a problem that the yield of silicon is lowered. In the method of the present invention, as shown in FIG. 1, an acidic oxide 4 having a lower density than the molten silicon 1 is added onto the molten silicon 1, thereby forming a layer on the molten silicon to prevent scattering of the silicon 1. Can be suppressed. As an acidic oxide having a lighter density than molten silicon, SiO ₂ is preferable. This is because SiO ₂ is inexpensive and has a low boron concentration.

Na ₂ CO _3, or carrier gas used for blowing and the SiO _{2 Na 2} CO _3, the oxidizing gas, an inert gas or both mixed gas is desired. As the oxidizing gas, O ₂ or CO ₂ is preferable. Oxidizing gas enhances the boron removal effect due to the oxidizing action of the gas itself, but also oxidizes silicon, which may reduce the yield of silicon. Therefore, a mixed gas of oxidizing gas and inert gas is used. It is preferable. In contrast to the oxidizing gas, the inert gas does not contribute to the boron removal effect and does not oxidize silicon. As the inert gas, Ar is particularly inexpensive and effective. In order to prevent a decrease in the yield of silicon while maintaining an appropriate boron removal effect, the ratio of the oxidizing gas in the mixed gas is desirably 2% or less of the total.

The initial boron concentration of the molten silicon is desirably 20 mass ppm or less. The yield of silicon may be reduced by oxidation of silicon due to the action of Na ₂ CO ₃ or oxidizing gas. If it is attempted to reduce the initial boron concentration from a level exceeding 20 ppm by mass to 0.3 ppm by mass or less, which is a silicon level for solar cells, a long scouring may be required. In addition, since the scouring time becomes longer, the yield of silicon due to oxidation may be reduced, and a sufficient amount of silicon may not be obtained.

  As described above, by using the silicon scouring method of the present invention, boron in silicon can be efficiently removed and the B concentration can be lowered to the solar cell level.

A trial experiment was conducted in which CaCO ₃ or Na ₂ CO ₃ was blown into molten silicon with an oxidizing gas or an inert gas and refined. The silicon mass was 10 kg. Moreover, carbon was used for the crucible, and an alumina tube was used for the dip tube, and the total treatment time was 120 minutes. As shown in Table 1, conditions insufflation, comparative examples blowing Comparative Example 2, CaCO ₃ blowing Comparative Example 1, CaCO ₃ blowing CaCO ₃ in Ar gas and _{O 2} gas Ar gas and CO ₂ gas in the Ar gas 3, Na ₂ O and the SiO ₂ 1: example 1, _Na 2 CO ₃ to blow Comparative example 4, _Na 2 CO ₃ blown in Ar gas at Ar gas and _{O 2} gas at 1 molecular weight ratio and Ar gas Example 2 for blowing with CO ₂ gas, Example 3 for blowing Na ₂ CO ₃ with Ar gas, and Example 4 for blowing Na ₂ CO ₃ and SiO ₂ with Ar gas at a molecular weight ratio of 2: 1. Further, in each comparative example and each example, a method in which 5 kg of silica sand was added on molten silicon was not used. Furthermore, in the comparative example 4 and each Example in which slag floats on silicon, slag was removed every 30 minutes. In Comparative Examples 1 to 3, since the slag sinks and the slag cannot be removed in the hot state, the slag was blown for 30 minutes, then cooled, and the slag and silicon were separated four times. The flow rate of gas to be blown was 15 L / min. Table 2 shows the boron concentration and silicon yield after the treatment with respect to the boron concentration of 13 ppm in the initial silicon.

Comparing changes in B concentration between each comparative example and each example, in Comparative Examples 1 to ₃ using CaCO ₃ , all reached steady state at about 1 ppm, and Na ₂ O—SiO ₂ was used as slag. Comparative Example 4 was found to decrease to 1 ppm or less after 90 minutes and to about 0.4 ppm after 120 minutes. On the other hand, it was found that each example using Na ₂ CO ₃ as slag can reduce the B concentration to 0.03 ppm or less. However, in any method, it was found that when silica sand was added onto molten silicon, the yield was higher than when it was not added.

It is a schematic diagram of the present invention.

Explanation of symbols

1 silicon,
2 bottom hole,
3 Dip tube
4 acidic oxides,
5 Na ₂ CO ₃ ,
6 A mixture of Na ₂ CO ₃ and SiO ₂ .

Claims (7)

Bottom hole or dip tube _Na 2 CO ₃ with carrier gas from, or _Na 2 CO ₃ and scouring method of the silicon, characterized in that blowing a mixture of SiO ₂ container with molten silicon.   2. The silicon scouring method according to claim 1, wherein an acidic oxide having a lower density than the molten silicon is added onto the molten silicon.   The silicon scouring method according to claim 1 or 2, wherein the generated slag is removed at least once during or after scouring.   2. The silicon refining method according to claim 1, wherein an oxidizing gas, an inert gas, or a mixed gas of both is used as the carrier gas.   The silicon scouring method according to claim 4, wherein Ar gas is used as the inert gas. 3. The silicon scouring method according to claim ₂ , wherein SiO2 is used as the acidic oxide. _2. The silicon refining method according to claim 1, wherein a boron concentration in the molten silicon before blowing the Na ₂ CO ₃ or a mixture of Na ₂ CO ₃ and SiO ₂ is 20 ppm by mass or less.

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