JPH02267110A - Lance for decarburizing metal silicon and decarburization - Google Patents

Abstract

PURPOSE:To decarburize metal silicon efficiently and inexpensively without contaminating silicon by immersing a lance having an inner and an outer faces of refractory core tube coated with silica in metal silicon melt while blowing an inert gas against the surface molten silicon and introducing a gas containing an oxidizing agent into the metal silicon melt. CONSTITUTION:Metal silicon melt 3 heated and molten by a heater 8 is retained in a quartz crucible 1 supported by a graphite crucible 2. An inert gas 5 is blown from an upper spray lance 4 against the surface of the melt 3. With the introduction of the gas, an inert gas 7 containing an oxidizing agent is sent from a decarburizing lance 6 immersed in the melt 3. Oxygen, water or silica is used as the oxidizing agent. In the decarburizing lance 6, the inner and the outer faces of a refractory core tube 11 of mullite, etc., are covered with silica such as a quartz tube 12, etc. The decarburizing lance 6 has durability in the silicon melt 3 and will not contaminate the silicon melt. Consequently, the silicon melt 3 is efficiently decarburized so that polycrystalline silicon useful for electronic materials, especially raw material for solar battery can be inexpensively obtained.

Description

Detailed Description of the Invention [Industrial Application Field 1] The present invention relates to a decarburization lance for blowing gas to remove carbon contained in metal silicon, and a method for decarburizing metal silicon. The present invention relates to a technology for removing carbon contained in polycrystalline silicon, which is a raw material for materials, particularly solar cells, in order to efficiently and inexpensively manufacture it.

[Prior Art] Conventionally, silicon for electronic materials, such as polycrystalline silicon used as a raw material for solar cells, has been produced using metallurgical silicon (MG-3i) obtained from silica stone (SiOz) and carbon, for example.
It is purified through the gasification process shown in (1), (2), and (3) below.

M G -S i + 3 [-1CI2→S i t
(CI23 + ) (2S i l-I C, +23
+ H2-S i + 3 It C+24SiHCR
3=S i +3 S i CQ 4 + 2 H2...
... (3) Silicon obtained in this way has problems such as increased energy consumption and the need for process control and quality control due to the gasification process, and the high quality obtained by this process This was one of the reasons why pure silicon became expensive.

On the other hand, according to the method of producing metallic silicon by reducing 5i02 in an arc furnace using carbonaceous materials, as disclosed in Japanese Patent Application Laid-open No. 61-117110, there is no need to go through the gasification process. High purity silicon can be obtained efficiently (and inexpensively).

[Problem to be Solved by the Invention 1] However, the silicon obtained by the method disclosed in JP-A-61-117110 cannot avoid being contaminated with carbon from raw materials or furnace materials, and it is necessary to remove this. There is.

Carbon in metallic silicon is in a solid solution state or in silicon carbide (S
i C) However, in metallic silicon used as a raw material for electronic materials, especially solar cells, the presence of precipitates such as SiC can cause deterioration of the product properties, so it must be prepared in advance. It is necessary to decarburize.

An object of the present invention is to provide a method for efficiently removing carbon from metal silicon, thereby making it possible to obtain high purity silicon without going through a gasification process.

[Means for Solving the Problems 1] The present invention has developed a lance for decarburizing high-purity silicon in which a refractory core tube is coated with silica.

In addition, according to the method of the present invention, an inert gas is sprayed onto the surface of the molten silicon, and an oxidizing agent is blown into the molten silicon together with the inert gas using a decarburization lance whose refractory core tube is coated with silica. A method for decarburizing metallic silicon is provided.

In this case, the oxidizing agent is preferably oxygen, water, or silica, and the amount of the oxidizing agent supplied is preferably such that it satisfies one of the following formulas.

2≦O2 equivalent oxidizing gas%・・・・・・−(4)0
．． 05≦Silica (g)/Silicon (kg)・・・・・・
(5) [Function] Carbon in metallic silicon is in a solid solution state or in silicon carbide (S
It is thought to exist in the state iC).

When used as a raw material for electronic materials, for example, a raw material for solar cells, it is necessary to decarburize the material in advance, since precipitates such as SiC are considered to be a cause of impairing the properties of the product.

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

[C] + [0] =CO(g')...
・(6) Nozaki et al. (J, Rad, Chem, 3
2 (According to 19761 p43-501, C in equation (6)
The partial pressure of O is just above the melting point of silicon.

PC8=2X10-7 [C] [O]・-=(7)
is given by

However, P co : Partial pressure of CO in the gas phase [atml[C] :
Concentration of C in molten silicon tppml[0]: concentration of C in molten silicon fppm).

Therefore, the carbon concentration [C]lppm) in molten silicon is [C] =Pco/2xlO-7[0] -=-(8)
becomes. From equation (8), it is clear that the carbon concentration in molten silicon can be reduced by decreasing the 00 partial pressure in the gas phase or by increasing the oxygen concentration in molten silicon. The 00 partial pressure in the gas phase can be reduced, for example, by spraying an inert gas onto the molten silicon to cover the surface with the inert gas, or by creating a reduced pressure state while spraying the inert gas. can.

In order to increase the oxygen concentration in the molten silicon, for example, an oxidizing agent may be added to the molten silicon.

The oxidizing agent may be one containing oxygen as a constituent element. In this case, the method of adding the oxidizing agent is, for example, H20,
In some cases, an oxidizing gas such as 02 or the like may be blown into the silicon, or both may be used in combination. or placing silica on the molten silicon surface and/or
Alternatively, it may be blown into silicon together with a gas. When blowing into silicon, silica that has absorbed a large amount of water can also be used. Alternatively, molten silicon may be held in a silica crucible or a crucible stamped with silica.

Next, a lance for injecting an oxidizing agent into molten silicon will be explained. 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 more, so it is also necessary to keep the concentration of impurities other than carbon low.

Therefore, when decarburizing by blowing gas as described above, it is necessary to select the material of the blowing pipe so as not to contaminate the silicon.

First, an attempt was made to blow gas into the bath using a quartz (Si02) tube that is free from contamination, but the quartz softened and floated to the surface of the silicon bath, making continuous blowing impossible.

Next, a mullite tube (main component = 3AI2203/2Si
Blowing was attempted using a magnesia tube (main component: Mg0). As a result, we succeeded in blowing for over 3 hours. However, as shown by curves 21 and 22 in FIG. 3, it was found that Ag and Mg, which are the main components of the Murai tube and the magnesia tube, entered the silicon and the silicon was contaminated.

Here, the present inventors have developed a decarburization lance that does not contaminate silicon and allows stable gas injection for a long period of time. That is, the lance of the present invention is a mullite tube,
This is a blowing lance with a refractory core tube made of magnesia, etc., whose outer wall is coated with quartz. This lance does not soften at temperatures above the melting point of silicon, allowing stable gas injection.
For example, a simple one may be a triple tube (FIGS. 2(a) and 2(b)) in which the inside and outside of a mullite tube 11 are covered with quartz tubes 12.

In this case, stable gas injection for more than 1 hour was possible with gas injection at a rate of 1°C/min, and the Af2 concentration in the silicon did not change as shown by curve 23 in Figure 3, and no contamination was observed. . The quartz tube 12 is thin in the portion immersed in the silicone bath, and this is due to softening and movement of the silicone bath. In the structures shown in FIGS. 2(a) and 2(b), the refractory core tube may be an alumina tube, an aluminum nitride tube, a boron nitride tube, a magnesia tube, a graphite tube, etc. in addition to a mullite tube.

In addition, as shown in Figure 2 (C) and (d),
Mullite, alumina, magnesia, aluminum nitride,
Silica (Si
O2) A structure coated with a stamp may be used.

〔Example〕

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

FIG. 1 is a longitudinal sectional view of an apparatus used for decarburizing silicon. Molten silicon 3 is held in a quartz crucible 1 supported by a graphite crucible 2. An inert gas 5 is blown onto the upper surface of the molten metal from the top blowing lance 4, and an oxidizing agent is blown into the silicon depletion 3 together with the inert gas from the decarburization lance 6 according to the present invention. 8
is a heater.

5 kg of molten silicon was charged into a container as shown in FIG. 1, in which a quartz crucible 1 was placed in a graphite crucible 2 having an outer diameter of 220 mm and a height of 200 mm, and the temperature was raised to 1500°C. The metal silicon decarburization lance shown in Figures 2 (a) and (b) is immersed in molten silicon, an inert gas containing an oxidizing agent is supplied, and Ar is sprayed onto the bath surface for 20 minutes to decarburize the silicon. 80-120m1n was carried out. The decarburization lance was a gas injection lance in which the refractory core tube 11 was a mullite tube with an inner diameter of 6 mm and an outer diameter of 10 mm, respectively, and a quartz tube was used as the tube covering it. The results are shown in the table below.

The purity in the table indicates the value when metal elements other than silicon are treated as impurities.

Outer diameter: 178 mm, height: 178 mm, wall thickness: 5 mm First, a case where an oxidizing gas is used as the blown gas will be explained.

Apply 50 N of Ar to the surface of silicon 3 from top-blowing lance 4.
The carbon concentration in silicon when sprayed at a rate of 2/min was as shown by curve 31 in FIG. Under the same conditions, when Ar is injected into silicon at a rate of 5NI2/min,
The change in carbon concentration in each silicon when Ar-10%02 is injected at 5N2/min is shown in curve 3 in Figure 1.
It became like 2.33. By blowing gas into the molten silicon 3, decarburization progresses efficiently. Above all,
By blowing oxygen gas together with inert gas, decarburization progresses more efficiently. FIG. 5 shows the relationship between the 02 equivalent gas concentration (%) of the gas mixed into the blown gas and the time (T) when the carbon concentration during decarburization becomes half of the initial concentration.

Almost the same effect was obtained when the 02 equivalent gas concentration was 2% or more. In addition, when blowing Ar-20%02 gas, silica was generated at the blowing port, making it impossible to stably supply the gas.

Next, a case will be described in which silica is mixed into the blown gas as an oxidizing agent. FIG. 6 shows the relationship between the blown silica (g)/silicon (kg) at this time and the time required for the carbon concentration to be halved during decarburization.

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

[Effects of the Invention] The lance for decarburizing metal silicon of the present invention is used when removing carbon from silicon by blowing gas,
It has durability in the S1 molten metal and does not contaminate the Si molten metal. Therefore, an efficient decarburization method that uses a combination of blowing an inert gas onto the surface of the molten silicon and using this decarburization lance to inject an inert gas containing an oxidizing agent into the molten silicon is more efficient. It became possible to remove carbon.

Therefore, 5i02
It has become possible to use the product obtained by reducing the carbon by decarburizing it by the method of the present invention.

This makes it possible to produce high-purity silicon at low cost without using a gasification process, which will greatly contribute to industry.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a crucible for explaining the method of the present invention;
Fig. 2 is an explanatory diagram showing the structure of a lance for silicon decarburization according to an embodiment of the present invention, Fig. 3 is a graph showing the relationship between the material of the blowing lance and the change in impurity concentration in silicon, and Fig. 4 is a graph showing the relationship between the material of the injection lance and the change in impurity concentration in silicon. Fig. 5 is a graph showing the relationship between the 02-equivalent oxidizing gas concentration in the blown gas and the time taken for the carbon concentration to become half after starting decarburization of silicon. The figure shows the blown silica (g)/silicon (k)
It is a graph showing the relationship between the time when the carbon concentration becomes half after starting the decarburization of silicon for g). ■...Quartz crucible 2...Graphite crucible 3...Silicon molten metal 4...Top-blowing lance 5...Inert gas 6...Decarburization lance 7--Inert gas containing oxidizing agent

Claims (1)

[Scope of Claims] 1. A lance for metal silicon decarburization having an immersion part in which the inner and outer surfaces of a refractory core tube are coated with silica. 2. In the method for decarburizing molten silicon, in addition to blowing an inert gas onto the surface of the molten silicon, the lance according to claim 1 is immersed in the molten silicon, and an inert gas containing an oxidizing agent is blown from the lance. A method for decarburizing molten silicon characterized by: 3. The method according to claim 2, wherein the oxidizing agent is oxygen, water or silica. 4. The method according to claim 2 or 3, wherein the supply amount of the oxidizing agent satisfies the following formula. 2≦O_2 equivalent oxidizing gas% 0.05≦Silica (g)/Silicon (kg)