JP2002173311A - Method for purifying molten metal silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying the molten metal silicon industrially in safety. SOLUTION: An inert gas containing steam of x volume % (x>3) is blown into the molten metal silicon 10 in a treating tank as a processing gas at the flow rate of A l/minute and the inert gas is also blown onto the surface of the silicon 10 as a diluting gas at the flow rate of B l/minute at the same time so that parameters x, A and B satisfy xA/(A+B)<3. When the processing gas contains further hydrogen of y volume %, the processing gas and the diluting gas are blown so that the parameters satisfy (x+y)>3 and (x+y)A/(A+ B)<3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying silicon, and more particularly to a method for purifying molten silicon for obtaining high-purity silicon suitable for, for example, solar cells.

[0002]

2. Description of the Related Art In a situation where an energy crisis is expected in the near future due to a decrease in energy resources such as petroleum and an increase in world energy demand, utilization of natural energy is desired. Utilization is expected as an alternative energy that does not destroy the environment at all. Here, cost reduction is indispensable for the spread of solar cells, and one of the major cost factors is the problem of refining silicon raw materials.

[0003] Converting solar energy into electricity utilizes one property of semiconductor silicon. At present, high-purity silicon for semiconductor integrated circuits is used as a raw material for solar cells. Since silicon for integrated circuits needs to be extremely high in purity, the current manufacturing method is to convert silicon having a purity of about 98% obtained by reducing silica in a submerged arc furnace into tricrolsilane to form a gas phase. Gas-phase fractional deposition methods such as the Siemens method of depositing silicon after fractionation and purification by fractionation are employed. The silicon purified in this way has an ultra-high purity of so-called Eleven Nine, but the cost is high due to the use of high energy in the manufacturing process. It hinders the spread of batteries.

It is said that ultra-high purity silicon for solar cells is not required as much as silicon for integrated circuits, and it has long been necessary to develop a purification technology for producing high-purity silicon for solar cells at low cost. , The United States, Europe, and Japan. Among the impurity atoms in silicon, metal impurities such as Fe, Al, and Ti, which adversely affect the performance such as the conversion efficiency of a solar cell, are equilibrium distributed in a liquid with respect to a solid in the solidification of silicon. Since the solid-liquid distribution ratio as the coefficient ratio is large, it can be removed by unidirectional solidification. However, since B and P having a small distribution ratio and close to 1 cannot be reduced to a high purity level required for solar cell production by unidirectional solidification, a purification technique has been separately developed.

[0005] P can be removed by evaporation by holding the molten silicon under a high vacuum. B
About HCTheuerer's experimental results (JOURNAL OF MET
According to ALS, 1956, PP1316-1319), it has been suggested that B in silicon can be removed very efficiently by a fusion treatment of silicon in a hydrogen gas atmosphere containing water vapor. Therefore, based on the results of HC Theuerer's experiment, a method of treating molten silicon with a gas containing water vapor has been developed and studied.

For example, US Pat. No. 4,097,584
According to No. 4, as a method for producing silicon having a B and P concentration of 1 ppm or less, a silicon melt having a B and P concentration of 1 ppm or more is maintained at 1420 to 1480 ° C., and 0.1% or more of steam and 1% A method of blowing the above inert gas containing hydrogen into the molten metal is described. JP-A-4-19
In Japanese Patent No. 3706, silicon containing impurity elements such as B, C, and P is melted, and A is melted from the bottom of the molten metal.
It describes a method of blowing r or H ₂ or a mixed gas thereof, and a method of purifying silicon by a method of adding a high-temperature oxidizing gas such as H ₂ O to these gases and blowing them.

[0007]

SUMMARY OF THE INVENTION A silicon refining technique for decarburization utilizing the experimental results of HC Theuerer has not yet been put to practical use. The reason for this is that there is a risk of explosion due to high concentration hydrogen gas in the space inside the processing tank. That is, it is dangerous that the processing gas blown into the silicon melt is released from the surface of the molten metal and the gas containing high-temperature and high-concentration hydrogen fills the space inside the processing tank, and the airtightness of the processing tank is secured. Must have been. When the processed gas is discharged into the atmosphere from the gas discharge pipe of the processing tank, it is necessary to burn the discharged gas at the outlet to the atmosphere to convert hydrogen into steam and discharge. However, even if such conditions were ensured, the presence of hot explosive gas in the treatment tank caused great anxiety, and HC Theuerer
No industrial process has been realized using the experimental results.

In view of the situation in the prior art,
An object of the present invention is to provide a safe industrial silicon refining method capable of efficiently purifying silicon by blowing a processing gas containing hydrogen or a processing gas containing hydrogen into the molten silicon.

[0009]

In the method for purifying molten silicon according to the present invention, x vol. % Steam is blown at a flow rate of Al / min as a processing gas using an inert gas containing x> 3.
At least when the processing gas is blown,
At the same time, an inert diluent gas
blow at a flow rate of 1 / min, parameters x, A,
And B are set so as to satisfy the relationship of xA / (A + B) <3.

The processing gas is y vol. % Hydrogen (x +
y) may further include a range of> 3, in which case,
Each parameter is (x + y) A / (A + B) <3
Are set so as to satisfy the relationship.

As the inert gas, argon, nitrogen, helium, or a mixed gas thereof can be preferably used.

The treated gas diluted with the inert gas can be discharged into the atmosphere through a discharge pipe provided at the ceiling of the processing tank.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a mixed gas of an inert gas and hydrogen, a hydrogen component is 3 vol. If it is less than 10%, no explosion occurs and it is not a hazardous material. When a processing gas containing an inert gas and water vapor is blown into a molten silicon, the water vapor mainly oxidizes silicon to generate SiO ₂ (solid) or SiO (gas), and at the same time, generates hydrogen gas having the same volume as the reacted water vapor. appear. Assuming that 100% of the water vapor in the bubbles has completed the reaction with the silicon at the time when the bubbles of the processing gas blown into the silicon melt are released from the surface of the melt into the space in the processing tank,
The water vapor component in the processing gas has been changed to the same volume of hydrogen gas. Therefore, it must be considered that the atmosphere gas in the processing tank contains a hydrogen gas component having the same concentration as the water vapor component of the processing gas at the maximum. For example, if the dew point is 50
When the processing gas is argon gas containing water vapor at about 12 vol. % Of water vapor, the atmosphere in the processing tank is 12 vol. % Hydrogen composition can be dangerous.

The inventors of the present invention have developed a method for safely industrializing such a process, in which a process gas is blown into a molten silicon and at the same time, an inert gas is blown onto the surface of the molten silicon to come out of the molten silicon. The hydrogen component in the treated gas was reduced to 3 vol. % Or less. That is, x vol. % Of the steam is blown into the molten silicon at a flow rate of Al / min as a processing gas, the processed gas of Al / min discharged from the surface of the molten metal is at most xvol.
% Hydrogen. Therefore, an inert diluent gas is separately blown onto the molten metal surface at a flow rate of Bl / min to reduce the hydrogen content of the entire atmosphere gas to 3 vol. %, The parameters x, A, and B must be xA / (A + B)
What is necessary is just to set so that <3 may be satisfied. As a result, the concentration of hydrogen in the atmosphere in the processing tank can be kept below the explosion limit, and the processing for purifying the molten silicon can be continued.

When an inert gas containing water vapor and hydrogen is used as the processing gas for the silicon melt, the hydrogen concentration in the gas released from the silicon melt surface is at most the sum of the water vapor concentration and the hydrogen concentration in the processing gas. Never exceed.
Therefore, the inert gas containing water vapor and hydrogen is converted to Al /
min, the Al / min treated gas released onto the surface of the molten silicon at the maximum flow rate of (x + y) vol. % Of hydrogen, a separate inert diluent gas is blown onto the molten metal surface at a flow rate of B1 / min to reduce the hydrogen concentration of the entire atmosphere gas to 3 V.
ol. %, The parameters x, y, A, and B may be set so as to satisfy the relationship of (x + y) A / (A + B) <3. This makes it possible to keep the hydrogen concentration in the atmosphere in the processing tank below the explosion limit and continue the purification process of the molten silicon.

The inert diluent gas which is blown into the silicon melt at the same time as the process gas is blown into the molten silicon is:
Ar, can be used He, and N _2. Since the temperature of the silicon melt is usually maintained at 1430 to 1550 ° C., the temperature of the atmosphere gas on the melt becomes high and the interdiffusion and mixing of the inert gas and hydrogen occur quickly, and the uniformity of the atmosphere gas composition is high. It is estimated that But,
By discharging gas from the ceiling without ignoring the tendency of hydrogen to accumulate on the ceiling in the processing tank due to its lightness, a safer method for purifying molten silicon can be achieved. The processing gas can be blown by dipping a lance tube made of a refractory material such as silica or graphite from the ceiling of the processing tank into the molten silicon, and a bottom-blowing type used in the steel industry can be used. .

FIG. 1 is a schematic cross-sectional view illustrating a purifying apparatus including a treatment tank that can be used in the method for purifying a molten silicon according to the present invention. The silicon refining apparatus includes a melting furnace that can be heated by the electromagnetic induction heating device 4, a lance pipe 6 that blows a processing gas into the silicon melt 10, two pipes 7 that blows an inert gas onto the surface of the silicon melt 10, and a processing furnace. The atmosphere above the silicon melt 10 is kept airtight, including a discharge pipe 8 for discharging a mixed gas of the used gas and the inert dilution gas.

In the melting furnace, a protective crucible 1 made of graphite is provided inside a refractory material 3 such as alumina, and a silica crucible 2 is further provided inside the protective crucible 1. The temperature of the raw material silicon inserted into the silica crucible 2 is increased by the heating of the graphite protection crucible 1 by electromagnetic induction in an inert gas atmosphere, and the raw silicon is melted into the silicon melt 10. Thereafter, since the silicon melt 10 has conductivity, it is directly heated by electromagnetic induction, and is maintained at a predetermined purification processing temperature.

The processing gas used for silicon refining is blown into the molten silicon 10 from the ceiling 9 of the refining device through a lance tube 6 inserted by a separate lifting device (not shown). The lance tube 6 is formed of a material such as silica having corrosion resistance to the molten silicon 10. Ceiling insertion part 1 of lance tube 6 that can move up and down
1 is hermetically supported by suitable sealing means. As a method of injecting the processing gas into the molten silicon 10, a rotary nozzle that is industrially used in the processing of molten aluminum instead of the lance tube 6 (Ryo Otsuka et al .: Light Metal, 40 (1990), p. 290) Can also be used, which could make purification more efficient. Various other methods are also possible as a method of blowing the processing gas into the silicon melt 10, and the present invention is not limited to the specific blowing method.

At the same time as the processing gas used for silicon purification, a diluting gas is blown into the atmosphere on the silicon melt surface to reduce the hydrogen content of the processed gas to 3 vol. %, An inert diluent gas is blown through two tubes 7 made of a heat-resistant material such as graphite inserted from the ceiling 9. However, the method of injecting the inert dilution gas onto the molten metal surface is described in FIG.
However, the number of the blowing pipes 7 is not limited to two. For example, an inert diluent gas may be blown into the atmosphere above the silicon melt from the side of the refiner. The processing gas blown into the refining device becomes a processed gas and is released from the surface of the silicon melt, and is mixed with the inert diluent gas blown onto the surface of the melt to obtain 3 vol. % And is discharged through a discharge pipe 8 provided in a ceiling portion 9.

US Pat. No. 4,097,584 states that it is effective to purify silicon under reduced pressure compared to atmospheric pressure in the atmosphere in the processing tank. 3 vol. %, The purification process can be continued without the danger of explosion in the processing tank and the exhaust gas.

Example 1 5 kg of silicon (B concentration: 5 ppm) was charged into the silica crucible 2 shown in FIG. 1, and Ar was blown from an inert diluent gas blowing pipe 7 so that the inside of the purification apparatus was Ar atmosphere. And then the electromagnetic induction coil 4
Thus, the graphite protection crucible 1 was heated, and the silicon in the silica crucible 2 was melted. The lance tube 6 is lowered from the ceiling 9 into the silicon melt 10 maintained at 1450 ° C., and 10 vol. % Of water containing 2% water vapor
It was immersed while being supplied at a flow rate of min. At the same time as the processing gas flows from the lance pipe 6, Ar diluting gas is blown at a flow rate of 3 l / min from the two inert diluting gas blowing pipes 7 on the molten metal surface, thereby maximizing the hydrogen concentration in the atmosphere gas. 2.5 vol. %, And the purification treatment was performed for 4 hours. The B concentration in the silicon after the treatment was reduced to 2 ppm.

Example 2 5 kg of silicon (B concentration: 5 ppm) was charged into the silica crucible 2 shown in FIG. 1, and Ar was blown from an inert diluent gas blowing pipe 7 so that the inside of the purification apparatus was Ar atmosphere. After that, the graphite protection crucible 1 was heated by the electromagnetic induction coil 4 to melt the silicon in the silica crucible 2. The lance tube 6 is lowered from the ceiling 9 into the silicon melt 10 maintained at 1450 ° C., and 10 vol. % Steam and 10 vol. The gas was immersed while flowing Ar gas containing 2% hydrogen as a processing gas at a flow rate of 2 l / min. At the same time as the processing gas flows from the lance tube 6, Ar diluting gas is blown at a flow rate of 9 l / min from the two inert diluting gas blowing tubes 7 on the melt surface, and the hydrogen concentration in the atmosphere gas is maximized. 2.
0 vol. %, And the purification treatment was performed for 4 hours. The B concentration after the treatment was reduced to 1 ppm.

[0024]

As described above, according to the present invention, in the process of purifying molten silicon, there is no danger of explosion at high temperatures due to hydrogen components contained in the purifier.
A long-term purification treatment can be performed safely, and an industrial purification method can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a refining apparatus that can be used in the method for purifying a molten silicon according to the present invention.

[Explanation of symbols]

1 graphite crucible, 2 silica crucible, 3 alumina refractory material, 4 electromagnetic induction coil, 5 furnace shell, 6 lance tube, 7 inert dilution gas injection tube, 8 treated gas discharge tube, 9 ceiling plate, 10 silicon Molten, 11 Lance tube insertion part.

Claims (4)

[Claims] 1. x vo into silicon melt in a processing tank
l. % Of water vapor in the range of x> 3 is blown at a flow rate of Al / min as a processing gas. At least when the processing gas is blown,
At the same time, an inert dilution gas is blown onto the surface of the silicon melt at a flow rate of B1 / min, and the parameters x, A, and B are xA / (A + B) <
3. A method for purifying molten silicon, wherein the method is set so as to satisfy the relationship of 3. 2. x vo into the silicon melt in the processing tank
l. % Steam and yvol. % Hydrogen (x + y)> 3
Al / min as a processing gas containing an inert gas containing
When the processing gas is blown at least,
At the same time, an inert gas
The parameters x, y, A, and B are (x + y) A /
A method for purifying a molten silicon, wherein the method is set so as to satisfy a relationship of (A + B) <3. 3. The method according to claim 1, wherein the inert gas is argon, nitrogen, helium, or a mixed gas thereof. 4. The processing gas according to claim 1, wherein the processed processing gas diluted with the dilution gas is discharged through a discharge pipe arranged on a ceiling portion of the processing tank. The method for purifying a molten silicon according to the above.