JPH09142823A - Purification of metal silicon and device for purifying the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying metal silicon, capable of solving the difficulty of the removal of boron from the silicon melted with plasma and more effectively purifying the metal silicon than conventional methods, and to provide a device for purifying the same. SOLUTION: This method for purifying the metal silicon comprises blowing a plasma jet flow comprising an inert gas on the surface 2 of the melted silicon in a melted metal silicon bath to remove impurities contained therein. Therein, steam is mixed with the inert gas or blown on the surface of the melted silicon in the melted metal silicon bath, and a direct magnetic field is further applied so that the direction of the flux is parallel to the surface of the melted silicon in the melted metal silicon bath.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of metallic silicon for solar cells, and more particularly to a technique for purifying low-purity commercial metallic silicon to a purity that can be used for solar cells.

[0002]

2. Description of the Related Art The purity of metallic silicon (hereinafter simply referred to as silicon) used as a raw material of metallic silicon for solar cells is required to be 99.9999%, that is, so-called six nines or more. However, conventionally, silicon for semiconductors (purity is about eleven nine) has been used as the silicon for solar cells, which is very expensive. Therefore, in order to obtain more inexpensive silicon for solar cells, it has been studied to purify commercially available silicon having a purity of about 99.5% to increase its purity and use it as a raw material for silicon for solar cells.

On the other hand, the above-mentioned commercially available silicon contains various impurity elements. Among them, metal elements such as Fe, Al and Ti are disclosed in, for example, Japanese Unexamined Patent Publication No. 5-124809.
As can be seen in Japanese Laid-Open Patent Publication No. 2003-242242, a technique for purifying and removing by a unidirectional solidification method has been developed by utilizing the fact that these elements have a small solid-liquid partition coefficient with respect to silicon. However, boron, which is a nonmetallic element, has a solid-liquid partition coefficient of about 0.
8 is much larger than that of metallic elements, and it is difficult to reduce the target concentration by the unidirectional solidification method. Therefore, another refining technology to replace the unidirectional solidification method is required, and many research and development activities are required. It has been done.

Among the various boron removal techniques proposed so far, the most influential one uses a thermal plasma. For example, Japanese Patent Application No. 4-228414 discloses a refining method of removing boron by injecting a plasma jet using an inert gas containing water vapor as an operating gas into molten silicon. However, this purification method has a problem that the boron removal reaction proceeds only in the plasma-irradiated portion of the surface of the molten silicon and cannot be removed in other portions, so that the treatment time for removing boron is long. Further, JP-A-5-139713 discloses
In addition to the plasma irradiation method that uses an inert gas containing water vapor as the working gas, we propose a method of injecting an inert gas from the bottom of the container holding the molten silicon to enhance stirring, and thereby the boron removal treatment. Time savings have been achieved. However, with this method, it is difficult to provide a bottom blowing tuyere without contaminating the molten silicon bath, and the molten silicon is subjected to strong agitation, which significantly damages the tuyere, resulting in an overall life of the container. Another problem of shortening occurred.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems of the technique for removing boron from molten silicon by plasma melting and to provide a more efficient method and apparatus for purifying metallic silicon. There is.

[0006]

Means for Solving the Problems The inventor has conducted extensive studies in order to achieve the above-mentioned object, and focused on utilizing electromagnetic force to improve the stirring force, and completed the present invention. That is, the present invention is a method for purifying metal silicon in which a plasma jet stream composed of an inert gas is jetted onto the molten metal silicon bath to remove impurities contained therein, and water vapor is mixed with the inert gas or separately described above. The method for purifying metal silicon is characterized in that a direct current magnetic field is applied such that the magnetic flux is sprayed onto the surface of the molten metal silicon bath and the direction of the magnetic flux is parallel to the surface of the molten metal silicon bath.

The present invention is also a method for purifying metallic silicon, characterized in that a magnetic flux density distribution is applied to the molten metallic silicon bath. Furthermore, the present invention provides a molten metal silicon holding container, a plasma torch which is above the holding container and which generates a plasma jet stream of an inert gas between the furnace bottom electrode, and steam to the inert gas. In a refining apparatus for metallic silicon provided with a steam supply means for mixing or separately spraying on the bath surface of the molten metal bath, a magnet is provided on the outside of the side wall of the holding container so as to generate a DC magnetic field whose magnetic flux direction is parallel to the molten metal surface. The apparatus is also a refining apparatus for metallic silicon, which is characterized in that the magnet is an electromagnet. Furthermore, the present invention provides a holding container for molten metal silicon, a plasma torch above the holding container for generating a plasma jet stream of an inert gas between the furnace bottom electrode, and steam for the inert gas. In a refining apparatus for metallic silicon provided with a water vapor supply means that is mixed with gas or is sprayed onto the molten metal silicon bath surface, a means for generating an uneven magnetic flux density distribution is provided in the molten metal silicon bath. And a magnetic pole of a magnet disposed on the outside of the side wall of the holding container in which the magnetic flux density generating means is set to approximately 80% or less of the cross-sectional area of the molten metal silicon bath, or , The magnetic flux density generating means,
With a plurality of magnets arranged on the outside of the side wall of the holding container,
This is a refining apparatus for metallic silicon, characterized in that adjacent polarities are different from each other.

By using such an invention, it is possible to solve the above-mentioned problems of the technique for removing boron from molten silicon by plasma melting, and more efficient purification of metallic silicon becomes possible.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention provides a transfer type direct current plasma in which a counter electrode for a plasma torch is provided at the bottom of a container for holding molten metal silicon, and a molten direct current is directly applied to the melt.
The basic form is to dissolve the metallic silicon to be purified. The plasma by the transfer type operation has an advantage that it is superior in thermal efficiency as compared with the non-transfer type operation. At that time, a current flows in the molten metal silicon bath perpendicularly to the molten metal surface, and the DC flux density B is applied so that the magnetic flux becomes parallel to the molten metal surface with this current density as J. But,
This is an important point of the present invention.

The application of this magnetic flux density induces in the bath an electromagnetic force F given by the vector product of vector F = vector J * vector B (1).
Therefore, looking at the effect of this electromagnetic force on the surface of the metal silicon bath, the plasma irradiation part where the boron removal reaction occurs has the highest current density at the same time, and the electromagnetic force given by equation (1) is stronger than the surroundings. . Therefore, as shown in FIG. 1, not only a part of the molten metal silicon stays in the plasma irradiation unit 4 for a long time, but it moves toward the container peripheral portion by the electromagnetic force 6 and, at the same time, other than the plasma irradiation unit 4. A flow occurs in which the retained molten metal is newly moved to the irradiation unit 4. The flow 3 of the molten metal acts as a force for stirring the molten metal silicon, and effectively acts on the boron removal reaction.

Further, in plasma melting, as shown in FIG. 2, it is general that the area of the counter electrode installed at the bottom of the melt is large with respect to the cross-sectional area of the arc electrode at the tip of the plasma torch 7. In this case, the current distribution in the melt spreads concentrically from the center axis of the arc as the molten metal approaches the molten metal bottom, and a current component parallel to the molten metal surface is generated. This current component and the direct-current magnetic field applied parallel to the molten metal surface induce six electromagnetic force components in the depth direction. In the present invention, not only the direction parallel to the molten metal surface but also the stirring in the depth direction is promoted. In addition,
The DC magnetic field was applied using an electromagnet and / or a permanent magnet.

Further, in the present invention, in order to promote the stirring in the depth direction, a spatial magnetic flux density gradient is provided in the molten metal silicon bath. As a result, a distribution is generated in the electromagnetic force 6 given by the equation (1), and as illustrated in FIGS. It is pushed up in a direction parallel to the molten metal surface by pressing against the inner wall or another portion receiving the electromagnetic force 6 in the opposite direction. On the other hand, the melt subjected to the relatively weak electromagnetic force 6 is pushed down to the bottom of the molten metal by the weight of the pushed-up portion, and as a result, a depthwise flow is generated in the molten metal silicon bath. Various means can be considered for imparting the magnetic flux density gradient. For example, as shown in FIG. 3A, by making the area of the magnetic poles sufficiently smaller than the cross-sectional area of the bath, it is possible to give a magnetic flux density distribution having an extreme value to the magnetic pole portions with a single magnet. FIG.
As shown in (a), a plurality of magnets may be used and the polarities of the magnetic poles may be combined in various ways to provide a desired magnetic flux density distribution.

Incidentally, Japanese Patent Application Laid-Open No. 5-254817 proposes a method of stirring the silicon bath by applying a moving magnetic field. In this method, a complicated mechanism and structure for moving the magnetic field are proposed. is required. On the other hand, the present invention has an advantage that an effective stirring force can be easily obtained by the permanent magnet and / or the DC electromagnet, and the stirring pattern can be easily changed depending on the size and arrangement of the magnetic poles.

By the way, an inert gas is suitable as the plasma working gas used in the present invention, and in particular, A
r, He and mixed gas thereof are preferably used. Further, the steam may be mixed with the plasma working gas in a predetermined amount, but in a system separate from the working gas,
Alternatively, it can be added directly to the silicon bath surface. In addition,
As the steam supply means necessary for mixing and addition, known means may be used. The amount of water vapor added is preferably 1 mol / min or less per 1 kg of metallic silicon. If the amount exceeds this amount, a silica film is likely to be formed on the molten metal surface of the metal silicon, and the boron removal reaction is hindered. The molten metal silicon holding container is not particularly limited as long as it can hold the molten metal silicon in the temperature range of 1600 to 1700 ° C., but it is not contaminated due to the material forming the container 1 and facing the plasma torch 7. Considering the conductivity of the electrode, it is desirable to use the water-cooled copper container 1. In particular, since the water-cooled copper container 1 itself functions as the counter electrode of the plasma torch 7, there is no need to provide any means for ensuring the continuity of the plasma current, which is truly convenient. However, the present invention does not prevent the use of a container made of other material such as a quartz crucible or a silica stamp material by providing an appropriate conducting means.

Hereinafter, the contents of the present invention will be described in detail with reference to Examples, but the present invention is not limited to the specific conditions shown in the following Examples.

[0016]

【Example】

(Example 1) In a refining apparatus according to the present invention, a water-cooled copper container 1 was charged with 10 kg of commercially available metallic silicon having a boron content of about 6 ppm, which was irradiated with Ar plasma arc with a current of 900 A to obtain a molten state. did. When a uniform magnetic field of 10 mT was applied to the molten metal silicon surface 2 by a magnet arranged outside the wall of the container 1 so that the magnetic flux was parallel,
The molten metal flow 3 as shown in FIGS. 1 and 2 was visually observed. After collecting a sample for analysis of boron concentration from this molten metal, 0.5 mol /
Addition of water vapor was started at a rate of min (water vapor supply means is not shown). Two hours after the start of the addition of the steam, the steam was stopped, and a sample for analysis of the boron concentration was taken again.
These two analytical samples were subjected to ICP (Inductively Coupled Plasma Emission) analysis to compare the boron concentration in the molten metallic silicon before and after the addition of water vapor.

Example 2 As shown in FIG. 3, a plate-shaped electromagnet was horizontally arranged around the outside of the same water-cooled copper container 1 as in Example 1. At that time, the size of the magnet was set to 20% of the cross-sectional area of the melt. Metallic commercially available metallic silicon was irradiated with a plasma arc 8 under the same conditions as in Metal Example 1, and the metallic silicon was used as a molten metal, and the electric current of the electromagnet was adjusted so as to be 10 mT in the vicinity of the magnetic pole. By applying this magnetic field, the flow of the molten metal as shown in FIG. 3 was visually observed. Then, steam addition was performed in the same procedure as in Example 1, and the boron concentrations in the molten metallic silicon before and after the steam addition were compared.

(Embodiment 3) As shown in FIG. 4, two sets of horizontal plate-shaped electromagnets are arranged in two stages around the outside of the same water-cooled copper container as in Embodiment 1 so that the magnetic poles are opposite to each other. .
Commercially available metallic silicon was irradiated with a plasma arc under the same conditions as in Example 1, a molten metal of molten metallic silicon was held, and the electric current of the electromagnet was adjusted so as to be 10 mT in the vicinity of the magnetic pole. By applying this magnetic field, a flow of molten metal as shown in FIG. 4 was obtained by visual observation. Then, steam addition was performed in the same procedure as in Example 1, and the boron concentrations in the molten metallic silicon before and after the steam addition were compared.

(Embodiment 4) As shown in FIG. 5, two sets of plate-shaped electromagnets are arranged vertically in two rows around the outside of the same water-cooled copper container as in Embodiment 1, and the polarities thereof are opposite to each other. I did it. Commercially available metallic silicon was irradiated with a plasma arc under the same conditions as in Example 1 to obtain a molten metal of molten metallic silicon,
The current of the electromagnet was adjusted so that it was 10 mT in the vicinity of the magnetic pole. By applying this magnetic field, the flow of the molten metal as shown in FIG. 5 was visually observed. Then, steam addition was performed in the same procedure as in Example 1, and the boron concentrations in the molten metallic silicon before and after the steam addition were compared.

(Comparative Example) Using the same water-cooled copper container and commercially available metallic silicon as in Example 1, a molten metal of molten metallic silicon was obtained under the same conditions of plasma irradiation and the like, and steam was added. The boron concentration in molten metallic silicon was compared. However, in this case, no magnetic field was applied.

Table 1 shows the analysis results of the analytical samples collected in the above-mentioned Examples and Comparative Examples only with respect to the boron concentration. The values before addition of steam are shown in the column [B ₀ ] and the values after addition of steam for 2 hours are shown in the column [B]. From Table 1,
It is clear that boron, which is usually difficult to remove, has dropped to 1 ppm. Also, the boron removal rate is dlog
When calculated by ([B ₀ ] / [B]) ÷ 2 hr, it is also found that the example using the purification method according to the present invention shows a boron removal rate that is more than twice that of the comparative example. Furthermore, FIG.
Shows the relationship between the magnetic field and the average boron removal rate in the magnet arrangement of Example 4. According to the refining method of the present invention in which a magnetic field is applied, the average boron removal rate is clearly improved as compared with the comparative example having no magnetic field.

[0022]

[Table 1]

[0023]

As described above, according to the present invention, high-purity silicon having a low boron content was obtained by using inexpensive commercially available metallic silicon as a starting material, and at that time, the manufacturing time was significantly longer than that of the conventional one. Could be shortened to

[Brief description of the drawings]

FIG. 1 is a plan view showing a situation on a molten metal surface when a DC magnetic field is applied to a molten metal silicon by a refining method according to the present invention.

FIG. 2 is a side sectional view of FIG.

3A and 3B are diagrams showing a case where a magnetic flux density distribution is formed in a molten metal silicon bath, FIG. 3A is a magnet layout diagram in Example 2, and FIG. 3B is a side view seen from point A of FIG. It is a figure which shows the exercise condition of a silicon bath.

4A and 4B are diagrams showing a case where a magnetic flux density distribution is formed in a molten metal silicon bath, FIG. 4A is a magnet layout diagram in Example 3, and FIG. 4B is a side view seen from a point B of FIG. 4A. It is the schematic which shows the exercise condition of a silicon bath.

5A and 5B are diagrams showing a case where a magnetic flux density distribution is formed in a molten metal silicon bath, FIG. 5A is a plan view showing a magnet arrangement in Example 4, and FIG. 5B is a view from point C of FIG. 5A. FIG. 3C is a side view, and FIG. 6C is a schematic view showing the movement state of the silicon bath on the side surface viewed from the point D in FIG.

FIG. 6 is a diagram showing the relationship between the average boron removal rate from commercially available metallic silicon and the applied magnetic field when the purification method according to the present invention is applied.

[Explanation of symbols]

 1 Water-cooled copper container (holding container) 2 Surface of molten metal silicon bath 3 Flow of molten metal 4 Plasma irradiation part 5 External magnetic field 6 Electromagnetic force 7 Plasma torch 8 Plasma arc 9 Cross section of molten metal silicon 10 Current distribution in molten metal 11 Electromagnet Iron core 12 Electromagnetic coil 13 Electromagnetic power supply 14 Magnetic pole position

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Baba No. 1 Kawasaki-cho, Chuo-ku, Chiba City Research Institute of Kawasaki Steel Co., Ltd. (72) Yasuhiko Sakaguchi No. 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co. In-house (72) Inventor Hisae Terashima 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Technical Research Institute

Claims (6)

[Claims] 1. A method for refining metallic silicon in which a plasma jet stream composed of an inert gas is sprayed onto a molten metal silicon bath surface to remove impurities contained therein, and water vapor is mixed with the inert gas or separately melted. A method for purifying metallic silicon, which comprises spraying onto a molten metal surface of a metal silicon bath and applying a direct-current magnetic field so that the direction of magnetic flux is parallel to the molten metal silicon bath surface. 2. The method for refining metallic silicon according to claim 1, wherein a magnetic flux density distribution is applied to the molten metallic silicon bath. 3. A molten metal silicon holding container, a plasma torch for generating a plasma jet stream of an inert gas between the holding container and a furnace bottom electrode, and a water vapor mixed with the inert gas. In a metallic silicon refining apparatus separately equipped with a water vapor supply means for spraying on the molten metal silicon bath surface, a magnet is arranged on the outside of the side wall of the holding container so as to generate a DC magnetic field whose magnetic flux direction is parallel to the molten metal surface. A refining device for metallic silicon, which is characterized by being installed. 4. The apparatus for purifying metallic silicon according to claim 3, wherein the magnet is an electromagnet. 5. A molten metal silicon holding container, a plasma torch for generating a plasma jet stream of an inert gas between the holding container and the furnace bottom electrode, and a water vapor mixed with the inert gas. A metal silicon refining apparatus separately provided with a steam supply means for spraying the molten metal silicon bath on the surface of the molten metal, wherein the means for generating an uneven magnetic flux density distribution is provided in the molten metal silicon bath. Silicon refining equipment 6. The magnetic flux density generating means comprises a plurality of magnets arranged outside a side wall of the holding container, and the polarities of adjacent magnets are different from each other. Refining equipment for metallic silicon.