JP2856839B2 - Silicon purification method - Google Patents

Description

The present invention relates to a method for purifying silicon, and more particularly, to a method for producing high-purity silicon used as a raw material for solar cells and the like.

[Conventional technology]

The high-purity silicon used in solar cells has a specific resistance of, for example, 0.1 Ωcm or more. However, such silicon (Si) requires the impurity content to be reduced to the order of ppm. On the other hand, although various techniques have been conventionally studied, boron (B) and carbon (C) are the most difficult elements to remove.

In this regard, for example, Japanese Patent Application Laid-Open No. 63-218506 discloses that B is removed by melting Si under thermal plasma obtained by high-frequency excitation. In this method, in the first step, Si is dissolved by plasma excited by high frequency, and in the second step, 0.005 to 0.05% oxygen (O ₂ ) and 1 to 9
A method of treating with a plasma using a mixed gas with Ar containing 9.995% hydrogen (H ₂ ) as a plasma generation gas is described.

However, in such a method, since all of melting and refining of Si are performed by using plasma having low heat utilization efficiency, a great burden is caused economically. Since the area is limited to a relatively small area, it is a technique that is not suitable for mass production for use in solar cells due to poor productivity, and it is being refined because the temperature of Si rises excessively locally. There is a disadvantage that the loss (scattering and evaporation) is large and the oxygen concentration in the plasma gas cannot be increased.

As for C, for example, as shown in Japanese Patent Application No. 61-132874, it has been shown that decarburization can be achieved by stirring Si melted in a silica crucible by blowing Ar gas under reduced pressure. This method has the disadvantage that the decarburization rate is low and the productivity is poor.

[Problems to be solved by the invention]

The present invention solves the above-mentioned problems of the prior art,
An object of the present invention is to provide a method for economically and easily removing B and C in Si, which has conventionally required a great deal of energy and time.

[Means for solving the problem]

In order to solve the above-mentioned problems, the present invention uses a DC arc furnace having an upper electrode having a hollow hole, and converts powdery silicon into oxidizing gas and / or powdery silica together with a carrier gas through the hollow hole of the electrode. Another object of the present invention is to provide a method for purifying silicon, which comprises charging into a furnace together with a flux to dissolve the silicon.

[Action]

The present invention, in order to solve the above-described problems, is to simultaneously dissolve Si and remove B and C using a DC arc furnace that has high energy efficiency and is economical and can be used easily as compared with high-frequency excitation plasma. .

The present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view of an example of an apparatus in which the present invention is implemented.

In the figure, 1 is a furnace body, 2 is a furnace lining refractory, 3 is a furnace lid, 4 is molten Si, 5 is a lower electrode, 6 is an upper electrode having a hollow hole, 7 is a granular Si hopper, 8 is a powdery Si hopper. Granular Si, 9 is a carrier gas inlet, 10 is an arc-fired portion, 11 is a water-cooling unit, 12 is a valve for adjusting the amount of charged granular silicon, and 13 is a valve for adjusting the amount of carrier gas introduced. The silica powder or the flux can be mixed with the particulate silicon as required and charged into the furnace.

The method of the present invention uses a DC arc furnace having a structure in which one of a pair of electrodes is attached to a furnace bottom and the other is inserted from the upper part of the furnace as shown in FIG. By charging the powder or the powdered Si together with the oxidizing gas or silica together with the carrier gas into the ultra-high-temperature arc firing point formed between the electrodes,
This is to simultaneously promote the dissolution of Si and the removal of B and C by oxidation.

The atmosphere for this treatment is usually carried out under atmospheric pressure, but when carried out under reduced pressure, the effect of removing B and C is further enhanced.

In order to melt solid Si with an arc or the like, the melting itself requires time and energy, and furthermore, B in Si,
In order to carry out a scouring reaction for removing C and the like, it is usually carried out after Si is completely dissolved.

However, if the melting and refining can be performed simultaneously while continuously supplying the powdered solid Si as in the present invention, the operation becomes very simple and economically advantageous.

For such a purpose, a DC arc furnace in which one side of a pair of electrodes is mounted on the furnace bottom and the reflection side is inserted from the furnace top as in the present invention has a very advantageous effect. In other words, the electrode inserted into the furnace from the upper part of the furnace is a hollow electrode, and Si is continuously melted and melted by continuously charging powdery and granular Si into the arc firing point from the hollow hole. The deposited Si accumulates at the furnace bottom and acts as an electrode. In this case, the direct current is used as the electric power, so that the temperature of the arc-fired portion is several thousand degrees high, and as soon as the Si is melted, the deboron and decarbonization reactions proceed. In order to carry out such an operation advantageously, the particle size of the granular Si to be added is desirably 10 mm or less in diameter. Further, it is considered that B and C are removed as oxide gases, and it is necessary to supply some oxidizing agent simultaneously with Si. On the other hand, experiments have shown that Ar is used as a carrier gas for Si, and a small amount of oxidizing gas such as water vapor (H ₂ O) and oxygen (O ₂ ) is added to this gas, or powdered silica is used. The addition was found to be effective. Silica (SiO ₂ ) has O ₂ that can oxidize B and C, but the others do not bring impurities with only Si. The same effect can be obtained by adding this silica even if the supplied silicon powder is partially oxidized and supplied.

Graphite can be used for the lining of the furnace when C is not removed, but when C is removed, silica or a refractory containing silica as a main component is used. Graphite is used as the material of the hollow electrode, but by taking this hollow electrode as the cathode, the effect of electrode consumption can be ignored.

When an oxidizing gas is used, a ceramic tube is inserted into the hollow hole of the hollow electrode, and Si and the oxidizing gas are passed through the tube to avoid a reaction between the oxidizing gas and the graphite electrode. it can. O ₂ , H ₂ O and the like are preferably used as the oxidizing gas.

When promoting the removal of B, CaO, C
This can be effectively achieved by supplying a flux such as aF ₂ or CaCl ₂ together with the silica powder or alone.

Oxidizing gas and powdered silica or flux are
Each can be used alone or in combination.

〔Example〕

Using a small experimental furnace equipped with a DC power supply with an output of 40 kW and having the structure shown in Fig. 1, Silkg with an average particle size of 3 mm was supplied in 60 minutes under the following four conditions using argon 5 / min as a carrier gas. Was dissolved. The supplied Si has a B concentration of 15 ppm and a C concentration of 80 ppm.

(1) The atmosphere is set to the atmospheric pressure, and the water vapor of the carrier gas is 100 ml /
The B and C concentrations after the treatment when min was added are shown in Table 1 as Experiment No. 1.

(2) In the same manner as (1), the carrier gas is O ₂ gas at 100 ml /
The results after the treatment when min was added are shown in Table 1 as Experiment No. 2.

(3) In the same manner as (1), instead of water vapor, the average particle size
The results obtained when 150 μm silica powder was added at 2 g / min are shown in Table 1 as Experiment No. 3.

(4) The result of supplying a mixed flux of CaO / CaF ₂ = 1/1 at 1 g / min instead of silica powder in the same direction as (3) is shown as Experiment No. 4 in Table 1.

(5) The results obtained when the experiment was performed under the reduced pressure of 10 ⁻³ atm by the same method as (1) are shown in Table 1 as Experiment No. 5.

In each of the experiments, the B and C concentrations after the treatment were significantly reduced.

〔The invention's effect〕

According to the present invention, Si can be purified by an economical and simple method, and it is particularly effective as a preliminary purification technique when a step such as directional solidification is provided after the purification treatment according to the present invention.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an example of an apparatus used for carrying out the present invention. DESCRIPTION OF SYMBOLS 1 ... Furnace body, 2 ... Furnace lining refractory 3 ... Furnace lid, 4 ... Melted Si 5 ... Lower electrode, 6 ... Upper hollow electrode 7 ... Granular Si hopper, 8 ... Granular Si 9: Carrier gas inlet, 10: Arc flash point 11: Water-cooled section, 12, 13, Valve

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C01B 33/037

Claims (1)

(57) [Claims] 1. A direct current arc furnace having an upper electrode having a hollow hole is used. In the furnace, powdery and granular silicon is mixed with an oxidizing gas and / or powdered silica or flux through a hollow hole of the electrode together with a carrier gas. A method for purifying silicon, wherein the method is carried out by dissolving in silicon.