JPH11209119A - Method and apparatus for removing boron from metallic silicon - Google Patents

Abstract

(57) [Summary] The present invention relates to the production of silicon for solar cells,
It is an object of the present invention to provide a method and an apparatus for removing boron from metal silicon, which can remove boron more quickly than ever before. SOLUTION: On the surface of molten metal silicon in a molten state,
When removing the boron contained in the metallic silicon by spraying a mixed gas obtained by adding steam to a plasma gas composed of an inert gas, the predetermined relationship between the molten metal temperature at which a silicon oxide film is not formed and the amount of added steam is met. The temperature of the molten metal and / or the amount of added steam are adjusted so that the temperature of the molten metal and the amount of added steam are adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for removing boron from metallic silicon. This is an efficient removal technique.

[0002]

2. Description of the Related Art In order to exhibit required semiconductor characteristics, a silicon substrate used for a solar cell must contain boron in an amount of 0.1%.
It is necessary to reduce it to the range of 1 to 0.3 ppm. However, since it has been very difficult to reduce boron to the above level from metallic silicon as a raw material containing about 10 ppm of boron, more boron removal techniques have been studied than before.

For example, Japanese Patent Application Laid-Open No. Sho 63-218506 discloses a method of "irradiating metallic silicon held in a silica container with a high-temperature plasma gas to melt the metallic silicon and oxidize and remove boron." Is disclosed. that time,
As a first step, a mixed gas of hydrogen and argon is used. As a second step, 0.005 to 0.05 vol% oxygen, 1 to 9
A mixed gas composed of 9.995% by volume of hydrogen and the balance of argon is used as the plasma generating gas.

However, Japanese Patent Application Laid-Open No. Sho 63-218506 discloses this technique.
The method described in the above publication includes (1) melting with a plasma gas having poor heat utilization efficiency, which is not economical, (2) the melting region of metal silicon is narrow, there is no mass production, (3) scattering of silicon, There are disadvantages such as a large evaporation loss, a low oxygen concentration in the plasma gas, and a low removal rate.

Accordingly, the applicant of the present invention has disclosed "Economic boron removal technology in which a large amount of metallic silicon can be dissolved,"
No. 228414. That is, "Metal silicon as a raw material is melted and held by induction heating, resistance heating, etc. in a vessel lined with silica or a refractory containing silica as a main component, and high-temperature, high-speed plasma・
Gas is blown to vaporize and remove boron as an oxide. " At this time, 0.1 to 10 vol% of water vapor was added to the argon gas used as the plasma gas. Accordingly, Japanese Patent Application Laid-Open No. 63-21 / 1988
No. 8506, the above (1) to (3)
The disadvantages of (1) have been remarkably improved, and silicon for solar cells can be mass-produced economically and inexpensively.

However, in the technique described in Japanese Patent Application Laid-Open No. 4-228414, a large amount of a silica coating is formed on the surface of the molten metal, so that not only does the temperature of the molten metal rise, but also the removal speed of boron is slow and the processing time is lengthened. There was a disadvantage. That is, in this case, since the productivity does not increase as much as expected, the manufacturing cost of silicon for solar cells does not become low.

Further, Japanese Patent Application Laid-Open No. Hei 5-139713 discloses that
In addition to the above technology, it is proposed that a tuyere for gas injection be provided at the bottom of the container, and a mixed gas of an inert gas and an oxidizing gas be injected from the tuyere to enhance stirring and increase the boron removal rate. ing. However, this technique also has a restriction on the flow rate of the gas to be blown in, probably because the tuyere is of a straight pipe type, and the molten metal flows back into the tuyere when gas blowing is stopped. There was a problem in workability, for example, it was not possible to stop the embedding.

[0008]

SUMMARY OF THE INVENTION In view of the foregoing, the present invention provides a method and an apparatus for removing boron from metallic silicon, which can remove boron more quickly than before in the production of silicon for solar cells. The purpose is.

[0009]

Means for Solving the Problems To achieve the above object, the inventor of the present invention disclosed in Japanese Patent Laid-Open Nos.
The technology described in Japanese Patent Publication No. 139713 was reviewed, and intensive studies were made on increasing the deboronation rate. As a result, it was found that the formation of an oxide film on the surface of the molten metal depends on the amount of steam added and the temperature of the molten metal at the time of blowing, and this finding was used to suppress the formation of the film.

That is, according to the present invention, when a mixed gas obtained by adding water vapor to a plasma gas comprising an inert gas is sprayed on a molten metal surface of a molten metal silicon to remove boron contained in the metal silicon, A metal characterized by adjusting the temperature of the molten metal and / or the amount of added steam so that the molten metal temperature and the amount of added steam match the relationship between the temperature of the molten metal where the silicon oxide film is not formed and the amount of added steam. This is a method for removing boron from silicon.

Further, the present invention provides a container for holding metallic silicon in a molten state, and a plasma generator for blowing a plasma gas comprising an inert gas onto a molten metal surface of the metallic silicon.
In a device for removing boron from metallic silicon, comprising a torch and a pipe for supplying water vapor to be mixed with the plasma gas, a thermometer for actually measuring the temperature of the molten metal, a flow meter for actually measuring the amount of added steam, and An arithmetic unit for storing a relationship between the determined temperature at which the silicon oxide film is not formed and the amount of added steam, and performing a correction operation so that the input measured values of the temperature and the added amount of steam match the relationship; The apparatus for removing boron from metallic silicon is provided with a temperature adjusting means for adjusting the temperature and / or a flow rate adjusting means for adjusting the amount of added steam on the basis of the output of.

Further, according to the present invention, the thermometer may be a radiation thermometer that is not in contact with the surface of the molten metal, or the temperature adjustment means may include:
An apparatus for removing boron from metallic silicon, characterized in that it is a regulator of the current flowing to the electrode of the plasma torch, or the flow regulating means is a flow regulating valve provided in the steam supply pipe. According to the present invention,
Since the formation of a silicon oxide film on the molten metal silicon surface is suppressed, boron removal is more efficient than before,
It can be done more quickly. As a result, the production of solar cell silicon has been improved, and the manufacturing cost has been reduced.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below, taking into account the circumstances that led to the present invention. The inventor
In order to achieve the above object, a conventional boron removal method was performed, and the formation state of silicon oxide was observed in detail. as a result,
Although the reason is not clear, it was found that the formation of the film was affected by the temperature of the molten metal and the amount of water vapor added to the plasma gas, and had a relationship as shown in FIG. In other words, there is a region where the coating film is not formed and the temperature of the molten metal and the amount of added steam. Therefore,
The inventor has focused on this relationship, and has devised a deboronating method and apparatus capable of suppressing the formation of a coating film.

First, FIG. 3 shows an example of a conventional apparatus for performing a method for removing boron from metallic silicon. that is,
A plasma torch 3 for generating a plasma gas 2 composed of an inert gas for heating and dissolving the metal silicon 1, and a non-transition type for applying a voltage and current between an anode 4 and a cathode 5 attached to the torch 3 A plasma power source 6, a supply means 7 (chute) for the metal silicon 1, a container 8 for holding the molten metal silicon in a molten state, and an induction heating coil 9 for protecting the container 8 and melting the object to be heated are built-in. It is made of graphite or water-cooled copper container. Here, the non-transfer type plasma power source 6 includes the anode 4 and the cathode 5 attached to the torch 3.
And an arc 11 is generated only in the torch 3 and the plasma generating gas 19 is heated there, thereby injecting the high-temperature plasma gas 2 from the tip of the torch. . Since the plasma gas 2 generated by this method does not heat the object to be heated by the arc, the thermal efficiency is inferior to that of the transfer-type plasma (heated by the arc), but has the advantage that the stirring power of the melt is strong. In addition, the plasma torch 3 is also provided with a nozzle 13 for blowing steam 14 and a pipe 10 for oxidizing boron or carbon.

Next, the deboroning apparatus according to the present invention is devised from such a conventional apparatus (see FIG. 3), and is provided with the following instruments and means. First, FIG.
As shown in FIG. 1, a thermometer 12 for actually measuring the temperature of the molten metal 1 and a flow meter 15 for actually measuring the amount of added steam. Then, a predetermined relationship between the temperature at which the silicon oxide film is not formed and the amount of added steam (see FIG. 1) is stored,
An arithmetic unit (specifically, an arithmetic unit that performs a correction operation so that the input measurement values of the temperature and the amount of added steam match the above-described relationship.
Computer) 17 and the output of the arithmetic unit 17
There is provided a temperature adjusting means for adjusting the temperature and / or a flow rate adjusting means for adjusting the amount of added steam.

Here, the thermometer 12 may be a thermocouple type, but in the present invention, a non-contact radiation thermometer is used in order to avoid frequent failures in contact with the molten metal 1. preferable. In the present invention, it is preferable that the above-mentioned adjustment of the molten metal temperature is performed by raising or lowering the plasma gas temperature. For this purpose, a temperature controller (not shown) for flowing a current to the electrode of the plasma torch 3 is used as a temperature controller.
But used exclusively. The temperature can be adjusted by separately changing the flow rate of the plasma gas 2, but in this case, the amount of added steam also needs to be changed at the same time, which complicates the adjustment. Further, in the present invention, the flow rate adjusting means is preferably a flow rate adjusting valve 16 provided in the steam supply pipe 10. This is the easiest adjustment means. Note that the steam is generated by supplying water to the steam generator 18.

On the other hand, the deboroning method according to the present invention can be easily performed by using such an apparatus. It is shown in Figure 1
As shown by arrows and alphabetical symbols in FIG.
That is, the symbol A changes the temperature while keeping the amount of added steam constant, the symbol B changes the amount of added steam while keeping the temperature constant, and the symbol C changes both the temperature and the amount of added steam. It is. These three methods depend on the position of the actually measured temperature and the amount of added steam in FIG.
What is necessary is just to judge and select suitably. Of course, the selection is automatically performed by the arithmetic unit. Further, in the present invention, the temperature and the amount of added steam are always measured, but these adjustments may be performed at fixed time intervals. This is to prevent the frequency of adjustment from becoming too high and disrupting operations.

[0018]

EXAMPLES (Example 1) 30 kg of metallic silicon was charged into the container shown in FIG. 3 and melted. Immediately thereafter, the actual measurement of the molten metal temperature and the flow rate of steam was started, and the temperature and / or the amount of added steam were adjusted according to the boron removal method according to the present invention. For this reason, the coating surface of the silicon oxide on the surface of the molten metal has been extremely reduced as compared with the related art. Then output 1
A non-transferred plasma gas generated by a 00 kW plasma torch and a mixed gas obtained by adding 10 vol% of steam to the molten metal surface were sprayed at a rate of 300 liter / min to start deboron removal.

After a lapse of 120 minutes in such a state, the resistivity of the sample collected from the molten metal became 1.5 ohm.cm. Therefore, it was determined that deboron was completed, and the molten metal was cast into a mold (not shown). And solidified. 8 from bottom of ingot
When a sample was taken within 0% and analyzed for boron, the concentration of the boron was 0.1 ppm. These values were all acceptable for silicon for solar cells, and the processing time was shortened. (Comparative Example) With the apparatus shown in FIG. 3, the operation of deboron was started when the temperature of the molten metal reached 1620 ° C. according to the conventional method with the amount of steam added being 10 vol%. Other conditions are the same as those of the embodiment. The formation of the silicon oxide film progressed on the surface of the molten metal with the lapse of time, and after 50 minutes, almost the entire surface was covered. Because of that 3
After the elapse of 60 minutes, the specific resistance value of the collected sample finally became 1.5 ohm · cm, so it was determined that deboron was completed,
The blowing gas was switched to argon gas and deoxidized.

As a result, the sample from the ingot after solidification had a boron concentration of 0.1 ppm, which was satisfactory as a component of silicon for solar cells. However, in this case, the time required for tapping the mold from the start of the blowing of the plasma gas 2 (hereinafter referred to as processing time) was as long as 6 hours.

[0021]

As described above, according to the present invention, in the production of silicon for solar cells, the removal of boron can be performed more quickly than in the past. As a result, the time required for manufacturing the silicon is reduced, the productivity of the silicon for solar cells is improved, and the manufacturing cost is reduced.

[Brief description of the drawings]

FIG. 1 is a relational diagram showing the influence of the temperature of a molten metal and the amount of added steam on the formation of a film on the surface of the molten metal.

FIG. 2 is a longitudinal sectional view showing a device for removing boron from metallic silicon according to the present invention.

FIG. 3 is a longitudinal sectional view showing an apparatus for removing boron from metallic silicon according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metallic silicon (molten metal) 2 Plasma gas 3 Plasma torch 4 Anode 5 Cathode 6 Non-transition type plasma power supply 7 Supply means 8 Holding vessel (vessel) 9 Induction heating coil 10 Pipe 11 Arc 12 Thermometer 13 Steam nozzle 14 Steam Reference Signs List 15 flow meter 16 flow control valve 17 arithmetic unit 18 steam generator 19 gas for plasma gas generation (argon)

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroyuki Baba 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref. In Kawasaki Steel Research Institute (72) Inventor Masamichi Abe 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki (72) Inventor Yoshihide Kato 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture, Japan Kawasaki Steel Engineering Laboratory

Claims (5)

[Claims] 1. A method in which a mixed gas obtained by adding water vapor to a plasma gas comprising an inert gas is sprayed on a molten metal surface of a molten metal silicon to remove boron contained in the metal silicon. Adjusting the temperature of the molten metal and / or the amount of added steam so that the molten metal temperature and the amount of added steam match the relationship between the temperature of the molten metal where the silicon film is not formed and the amount of added steam. Removal method. 2. A container for holding metallic silicon in a molten state, a plasma torch for blowing a plasma gas comprising an inert gas to a molten metal surface of the metallic silicon, and a water vapor mixed with the plasma gas. In the apparatus for removing boron from metallic silicon provided with a pipe, a thermometer for actually measuring the temperature of the molten metal, a flowmeter for actually measuring the amount of added steam, and a predetermined temperature at which a silicon oxide film is not formed and an amount of added steam. A computing unit that performs a correction operation so that the input measured values of the temperature and the amount of added steam match the relationship, based on an output of the computing unit,
An apparatus for removing boron from metallic silicon, comprising: a temperature adjusting means for adjusting the temperature and / or a flow rate adjusting means for adjusting the amount of added steam. 3. The apparatus for removing boron from metallic silicon according to claim 2, wherein the thermometer is a radiation thermometer that is not in contact with the surface of the molten metal. 4. The apparatus for removing boron from metallic silicon according to claim 2, wherein said temperature adjusting means is an adjuster for a current flowing to an electrode of said plasma torch. 5. The apparatus for removing boron from metallic silicon according to claim 2, wherein said flow rate adjusting means is a flow rate adjusting valve provided in said steam supply pipe.