JP2538044B2 - Metal silicon decarburizing lance and decarburizing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a decarburizing lance for blowing a gas to remove carbon contained in metallic silicon and a method for decarburizing metallic silicon. The present invention relates to a technique for removing carbon contained in materials, in particular, for producing polycrystalline silicon, which is a raw material for solar cells, efficiently and inexpensively.

[Conventional technology]

Conventionally, as the silicon for electronic materials, for example, the polycrystalline silicon used as a raw material for solar cells, metallurgical silicon (MG-Si) obtained from silica stone (SiO ₂ ) and carbon is used, for example, in the following (1), (2), It is refined through a gasification process as shown in (3).

MG-Si + 3HCl → SiHCl ₃ + H ₂ (1) SiHCl ₃ + H ₂ → Si + 3HCl (2) 4SiHCl ₃ → Si + 3SiCl ₄ + 2H ₂ (3) The silicon obtained in this way is called a gasification process. However, the increase in energy consumption and the need for process control and quality control have been one of the factors that make the high-purity silicon obtained in this process expensive.

On the other hand, according to the method for producing metallic silicon by reducing SiO _{2 in} an arc furnace using a carbonaceous material, as disclosed in JP-A-61-117110, a gasification process is performed. High-purity silicon can be obtained efficiently and inexpensively.

[Problems to be Solved by the Invention]

However, the silicon obtained by the method disclosed in Japanese Patent Laid-Open No. 61-117110 cannot prevent carbon from being mixed in from raw materials or furnace materials, and it is necessary to remove it.

It is considered that carbon in metallic silicon exists in a solid solution state or in a state of silicon carbide (SiC), but in the case of metallic silicon used as a raw material for electronic materials, especially for solar cells, precipitates such as SiC are present. It is necessary to decarburize in advance because it causes deterioration of product characteristics.

An object of the present invention is to provide a method for efficiently removing carbon in metallic silicon, so that high-purity silicon can be obtained without a gasification process.

[Means for solving the problem]

The present invention has developed a lance in which a refractory core tube is coated with silica as a lance for decarburizing high-purity silicon.

Further, according to the method of the present invention, an inert gas is blown onto the surface of the molten silicon, and an oxidant is blown together with the inert gas by using a decarburizing lance in which the refractory core tube is covered with silica in the molten silicon. A method for decarburizing metallic silicon is provided.

In this case, the oxidizing agent is preferably oxygen, water, or silica, and the supply amount of the oxidizing agent may satisfy one of the following expressions.

2 ≦ O ₂ equivalent oxidizing gas% (4) 0.05 ≦ silica (g) / silicon (kg) (5) [Function] Carbon in metallic silicon is in solid solution state or silicon carbide (SiC) state It is thought to exist in.

When used as a raw material for electronic materials, for example, a raw material for solar cells, a deposit such as SiC is considered to be a cause of impairing the characteristics of the product, and therefore needs to be decarburized in advance.

It is considered that decarburization of molten silicon proceeds according to the following equation (6).

[C] + [O] = CO (g) (6) According to Nozaki et al. (J.Rad. Chem, 32 (1976) p43-50), the partial pressure of CO in equation (6) is silicon. Immediately above the melting point of P _CO = 2 × 10 ^-7 [C] · [O] (7)

However, P _CO : partial pressure of CO in gas phase (atm) [C]: concentration of C in molten silicon (ppm) [O]: concentration of O in molten silicon (ppm).

Therefore, the carbon concentration [C] (ppm) in the molten silicon is [C] = P _CO / 2 × 10 ⁻⁷ [O] (8). From the equation (8), it is clear that the carbon concentration in the molten silicon is reduced by reducing the CO partial pressure in the gas phase or increasing the oxygen concentration in the molten silicon. The CO partial pressure in the gas phase can be reduced by, for example, spraying an inert gas on molten silicon to cover the surface with the inert gas, or reducing the pressure while spraying the inert gas. it can.

To increase the oxygen concentration in the molten silicon, for example, an oxidizing agent may be added to the molten silicon. Any oxidant may be used as long as it contains oxygen as a constituent element. In this case, as the method of adding the oxidizing agent, for example, an oxidizing gas such as H ₂ O, O ₂ or the like may be sprayed, or may be sprayed into silicon, or both may be used in combination. Alternatively, silica may be placed on the surface of the molten silicon and / or blown with gas into the silicon. In the case of blowing into silicon, silica having a large amount of water adsorbed thereon can also be used. Alternatively, the molten silicon may be held in a silica crucible or a silica stamped crucible.

Next, the lance for blowing the oxidizer into the molten silicon will be described. Silicon for electronic materials, for example, polycrystalline silicon used as a raw material for solar cells, needs to have a purity of 99.999% by weight or higher, and therefore the concentration of impurities other than carbon must also be kept low. Therefore, when performing the above-mentioned decarburization by gas injection, it is necessary to select the material of the injection tube so as not to contaminate the silicon.

Therefore, first, an attempt was made to blow gas by using a quartz (SiO ₂ ) tube that was free from the risk of contamination, but quartz was softened and the quartz tube floated on the silicon bath surface, and continuous blowing was not possible.

Then, injection was tried using a mullite tube (main component: 3Al ₂ O ₃ .2SiO ₂ ) and a magnesia tube (main component: MgO).
As a result, it succeeded in blowing for 3 hours or more. However, as shown by curves 21 and 22 in FIG. 3, Al and Mg, which are the main components of the mullite tube and the magnesia tube, enter the silicon, and it has been found that the silicon is contaminated.

Here, the present inventors have developed a decarburizing lance that does not contaminate silicon and enables stable gas injection for a long time. That is, the lance of the present invention is a blow lance in which the outer wall of a refractory core tube such as a mullite tube or magnesia is coated with quartz. This lance does not soften above the melting point of silicon and allows stable gas injection. For example, in a simple case, a triple tube (the second tube in which the inside and the outside of the mullite tube 11 are covered with the quartz tube 12) The drawings (a) and (b)) may be used.
In this case, stable gas injection for 1 hour or more was possible with 1 / min gas injection, the Al concentration in silicon did not change as shown by the curve 23 in FIG. 3, and no contamination was observed. The quartz tube 12 is thin in the portion immersed in the silicon bath, which is due to softening and movement of the silicon bath. In the structure shown in FIGS. 2 (a) and 2 (b), an alumina tube, an aluminum nitride tube, a boron nitride tube, a magnesia tube, a graphite tube or the like may be used as the refractory core tube other than the mullite tube.

In addition, as shown in FIGS. 2 (c) and 2 (d), silica (Si) is formed around the refractory core tube 11 made of mullite, alumina, magnesia, aluminum nitride, boron nitride graphite or the like.
A structure covered with an O ₂ ) stamp may be used.

〔Example〕

 Next, examples of the present invention will be described below.

FIG. 1 is a vertical sectional view of an apparatus used for decarburizing silicon. A molten silicon 3 is held in a quartz crucible 1 supported by a graphite crucible 2. Top blowing lance 4 to inert gas 5
Is sprayed on the upper surface of the molten metal, and an oxidizing agent is blown into the molten silicon 3 together with an inert gas from the decarburizing lance 6 according to the present invention.
8 is a heater.

The outer diameter is 178 mm, the height is 178 mm, and the wall thickness is 5 mm.
5 kg of molten silicon was charged into a container as shown in FIG. 1 charged in a graphite crucible 2 having a height of 20 mm and a height of 200 mm, and the temperature was raised to 1500 ° C. The metal silicon decarburizing lance shown in FIGS. 2 (a) and 2 (b) is immersed in molten silicon to supply an inert gas containing an oxidizer, and Ar is sprayed onto the bath surface at 20 / min for decarburization. Scouring was carried out for 80 to 120 minutes. The decarburizing lance was a mullite tube having a refractory core tube 11 having an inner diameter and an outer diameter of 6 mm and 10 mm, respectively, and a quartz tube was used as a tube for covering the mullite tube. The results are shown in the following table.

The purity in the table indicates the value when a metal element other than silicon is treated as an impurity.

First, the case where an oxidizing gas is used as the blowing gas will be described.

50 Nl / min of Ar from the top blowing lance 4 of silicon 3 to the surface
The carbon concentration in silicon when sprayed at was as shown by curve 31 in FIG. Under the same conditions, in silicon
The change in carbon concentration in each silicon when Ar is blown at 5 Nl / min and Ar-10% O ₂ is blown at 5 Nl / min is the first.
It became like curves 32 and 33 in the figure. By blowing gas into the molten silicon 3, decarburization proceeds efficiently. In particular, by blowing oxygen gas together with an inert gas, decarburization proceeds more efficiently. FIG. 5 shows the relationship between the O ₂ equivalent gas concentration (%) of the gas mixed in the blown gas and the time (τ) when the carbon concentration during decarburization becomes half the initial concentration.

When the O ₂ -converted gas concentration was 2% or more, there was almost the same effect. In addition, when Ar-20% O ₂ gas was blown, silica was generated at the blow-in port, and stable gas supply could not be performed.

Next, a case where silica is mixed as an oxidant in the blown gas will be described. FIG. 6 shows the relationship between the blown silica (g) / silicon (kg) at this time and the time when the carbon concentration during decarburization becomes half.

If the ratio of silica (g) to silicon (kg) is 0.05 or more, there is almost the same decarburizing effect. Silica (g) / Silicon (kg)
When exceeds 0.5, the amount of silica powder floating on the silicon surface increases rapidly, and the ratio of silica powder contributing to the decarburization reaction decreases.

〔The invention's effect〕

The metal silicon decarburizing lance of the present invention is used when carbon in silicon is removed by gas blowing, has durability in a Si molten metal, and does not contaminate the Si molten metal. Therefore, the decarburization method in which the inert gas is blown onto the surface of the molten silicon and the inert gas containing the oxidizing agent is blown into the molten silicon by using this decarburizing lance, is used to efficiently remove the carbon in the metallic silicon. Can be removed.

Therefore, as raw material for electronic materials, for example, silicon used as raw material for solar cells, it is possible to decarburize and use the one obtained by reducing SiO _{2 in} an arc furnace using a carbonaceous material. became. This makes it possible to produce high-purity silicon at a low cost regardless of the gasification process, which is a great contribution to the industry.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a crucible for explaining the method of the present invention,
2 is an explanatory view showing the structure of the silicon decarburizing lance of the embodiment of the present invention, FIG. 3 is a graph showing the relationship between the material of the blowing lance and the impurity concentration change in silicon, and FIG. 4 is the silicon. Fig. 5 is a graph showing the carbon concentration in the inside, and Fig. 5 is a graph showing the relationship between the O ₂ equivalent oxidizing gas concentration in the blown gas and the time when the carbon concentration becomes half after starting the decarburization of silicon. Figure 6 shows blown silica (g) / silicon (kg)
2 is a graph showing a relationship with time when the carbon concentration becomes half after starting decarburization of silicon with respect to. 1 ... Quartz crucible, 2 ... Graphite crucible, 3 ... Molten silicon, 4 ... Top blowing lance, 5 ... Inert gas, 6 ... Decarburizing lance, 7 ... Inert gas containing oxidant, 8 ... Heater , 11 …… Fireproof core tube 12 …… Quartz tube, 13 …… Silica stamp

Front page continuation (72) Inventor Fumio Araya 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Ken Nagahashi 1 Kawasaki-cho, Chiba-shi Kawasaki Steel Co., Ltd. Research Headquarters (56) References Japanese Patent Publication Sho 33-9860 (JP, B1)

Claims (4)

(57) [Claims] 1. A metal silicon decarburizing lance having a dipping portion in which the inner and outer surfaces of a refractory core tube are coated with silica. 2. A method for decarburizing molten silicon, wherein an inert gas is blown onto the surface of the molten silicon, and the lance according to claim 1 is immersed in the molten silicon, and the inert gas containing an oxidant is introduced from the lance. A method for decarburizing molten silicon, which comprises blowing. 3. The method according to claim 2, wherein the oxidant is oxygen, water or silica. 4. The method according to claim 2, wherein the supply amount of the oxidant satisfies any of the following expressions. 2 ≦ O ₂ equivalent oxidizing gas% 0.05 ≦ silica (g) / silicon (kg)