JP2005247655A - Silicon refining apparatus and its refining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a refining apparatus for removing low-boiling impurities such as P (phosphorus) in Si, and to provide an Si refining method using the apparatus and ensuring high refining efficiency. <P>SOLUTION: The Si refining apparatus for removing impurities in Si from surfaces of sprayed droplets has a vessel whose interior can be evacuated and heated and a nozzle for spraying molten Si in the vessel, wherein a spray nozzle applying the differential pressure between the inlet and outlet sides of the spray nozzle is used. The Si refining method is also provided. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-purity silicon refining apparatus and a silicon refining method, and more particularly to a high-purity silicon refining apparatus for a solar cell substrate and such a silicon refining method.

  As a refining method for improving the Si purity by reducing the concentration of low-boiling impurities such as P (phosphorus) using metal Si or the like as a raw material, for example, the vacuum degassing method of Non-Patent Document 1 is known in the traditional prior art. It has been.

In addition, there is known a method of removing phosphorus, aluminum, and calcium by dropping molten silicon as droplets in a furnace with a vacuum inside the furnace to remove phosphorus, aluminum, and calcium (see, for example, Patent Document 1).
Yushita et al., Journal of the Japan Institute of Metals, Vol. 61, No. 10, 1086-1093 (1997) JP-A-10-182130

  In the conventional prior art, since the surface area relative to the Si melt volume is small, there is a problem that the evaporation rate of impurities is extremely slow and the productivity is low.

  Moreover, in patent document 1, since Si droplet was dripped only once, without giving a special pressure, there existed a problem that a droplet speed was slow and productivity was low.

  Therefore, an object of the present invention is to provide a highly productive refining apparatus and a Si refining method for removing low-boiling impurities such as P in Si.

  The present invention is a Si refining apparatus for removing impurities in Si from a sprayed droplet surface, comprising a container that can be vacuumed and heated, and a nozzle that sprays molten Si into the container. The present invention relates to a refining apparatus characterized by using a spray nozzle that utilizes a differential pressure between an inlet side and an outlet side of the spray nozzle.

  The spray center axis angle of the spray nozzle is preferably in a range exceeding the horizontal direction and not more than vertically above.

  The type of the spray nozzle is preferably a full cone nozzle, a flat spray nozzle, or a Laval nozzle.

  Further, a recovery device having an opening at the bottom in the container, a molten Si recovery device including an opening / closing device for the opening, and a molten Si tank that receives the molten Si from the opening are provided inside the container. It is preferable to include a heating furnace.

  Further, the present invention is a method of atomizing molten Si into a container that can be evacuated and heated, and removing impurities in the Si from the surface of the sprayed liquid droplets. The present invention relates to a method for refining Si, wherein an average moving speed of a melt is 0.2 m / sec or more.

  A method in which the spray central axis angle of the spray nozzle is in a range exceeding the horizontal direction and not more than vertically upward is preferable.

  A method is preferred in which Si droplets dropped in the container are collected and sprayed again into the container.

  A method of keeping the inner wall of the container at 1650 ° C. or higher is preferable.

  The apparatus and method of the present invention can greatly increase the evaporation or removal rate of impurities in the molten Si. Further, since the removal speed is high, the processing amount per unit time can be increased.

  The Si refining apparatus and refining method of the present invention will be described below with reference to the drawings. In the present specification, “a container that can be evacuated and heated” is abbreviated as “vacuum container”.

  FIG. 1 is an explanatory view showing an example of a refining apparatus used for refining Si. In FIG. 1, refining of molten Si is performed in a vacuum vessel 1. The Si melt 3 is stored in a molten Si tank 9 including a differential pressure adjuster 5 and a nozzle 7. The molten Si tank 9 and the nozzle 7 are kept warm by heating means 11 such as a heater so as not to solidify the Si melt 3. Further, the inside of the vacuum vessel 1 can be kept in a vacuum by a vacuum pump 13 and a Si collector 15 for collecting Si droplets is placed. In addition, a well-known apparatus can be used for apparatuses, such as the vacuum vessel 1, the molten Si tank 9, and the heating means 11, used here. The differential pressure adjuster 5 is a device that adjusts the differential pressure in the molten Si tank and the vacuum vessel to a predetermined value. For example, when the bottom plate of the molten Si tank is movable as shown in FIG. 1, the differential pressure adjuster applies a force to the bottom plate to propel the bottom plate or to make it stationary at a fixed position. The pressure is adjusted and the injection of the Si melt is controlled. As a means for imparting propulsive force to the bottom plate of the differential pressure regulator, a hydraulic cylinder or an electric jack can be used as a drive source. Use heat-resistant materials such as graphite. In addition, when the structure is such that the differential pressure between the molten Si tank and the vacuum container is unlikely to be generated, such as when the entire molten Si tank is installed in the vacuum container, the differential pressure regulator may be of a simple mechanism. . For example, a simple pressurizer that pushes up the molten Si tank bottom plate only during the Si melt injection may be used as the differential pressure regulator. Examples of the nozzle 7 include a silicon nitride main body and the outside protected with a carbon composite. The valve 12 is used to cancel the degree of vacuum in the vacuum vessel 1.

Next, a Si refining method using this refining apparatus will be described. The Si melt 3 is stored in a molten Si tank 9 including a differential pressure adjuster 5 such as a pressurizer and a nozzle 7. Here, the type of the nozzle 7 is not particularly limited as long as the Si melt 3 can be sprayed or sprayed, and a conventionally known nozzle can be adopted. However, it is not appropriate that the liquid droplet dispersibility is low as in the case of the direct spray nozzle, For example, a form in which droplets are dispersed in a wider range, such as a full cone nozzle, a flat spray nozzle, or a Laval nozzle, is more desirable. The angle of the injection center shaft port when spraying the Si melt 3 from the nozzle 7 is not particularly limited as long as impurities in the molten Si are removed, but usually exceeds the horizontal direction and is in the range below the upper vertical direction. . By setting such an angle, the sprayed Si droplets are kept in the air for a sufficient time, so that low boiling point substances such as P from the Si droplets can be efficiently evaporated and removed. . Further, the nozzle at that time, the differential pressure between the inlet side and the outlet side, typically, 1 × ¹⁰ 4 Pa or more, and preferably in the range of ^{1 × 10 4 Pa~1 × 10 6} Pa. Alternatively, it is desirable that the average moving speed of the Si melt or droplet when exiting the spray nozzle is 0.2 m / sec or more, preferably in the range of 0.2 m / sec to 50 m / sec. By setting the differential pressure or the average moving speed in this way, the dispersibility of the sprayed droplets is improved, the evaporation rate of impurities such as P from the Si droplets is increased, and the Si refining efficiency is increased. Because. If such a pressure cannot be used, molten Si cannot be injected upward. The spray from the spray nozzle is performed by using, for example, a molten Si differential pressure adjuster 5 attached to the molten Si tank 9. The molten Si tank 9, the conduit 17 connecting the molten Si tank and the nozzle, and the nozzle 7 are kept warm so as not to solidify the molten Si.

  Next, the Si melt 3 is sprayed or sprayed. The molten Si tank 9 and the conduit 17 are arranged outside the vacuum vessel 1, and the nozzle 7 is attached to the bottom wall of the vacuum vessel 1 so that the Si melt 3 can be injected into the vacuum vessel 1. When injecting the Si melt 3 vertically upward, the bottom wall is preferable. Depending on the direction of injection, the nozzle 7 can be attached to the side wall. Injection may be performed continuously or intermittently. In the case of continuous injection, the productivity is high. On the other hand, in the case of intermittent injection, the pressure in the container decreases when there is no injection, so the deimpurity rate per injection increases. Based on these characteristics, it can implement suitably. The Si liquid discharged from the nozzle 7 into the vacuum vessel 1 at high speed is immediately dispersed and atomized. While rising, the Si droplet 19 mainly evaporates low-boiling impurities such as P inside. Then, it falls in the vacuum vessel 1 and accumulates in the Si collector 15. Some impurities evaporate during the fall. At that time, it is desirable to keep the temperature of the inner wall of the vacuum vessel 1, particularly the inner wall of the ceiling and the side wall, usually in the range of 1650 ° C. or higher, preferably 1650 ° C. to 2000 ° C. This is because when the temperature is higher than this temperature, the Si droplet 19 hardly adheres even if it touches the wall. Since it is not necessary to consider the adhesion of the Si droplet 19 to the container 1, the vacuum container 1 can be downsized. Therefore, the equipment cost can be reduced. Of course, since the Si droplet 19 is difficult to adhere to the wall of the container 1, it is not necessary to consider a decrease in the yield of Si.

  According to the present invention, the degree of vacuum in the vacuum vessel can be carried out without being restricted by the condition of high vacuum conditions since the refining performance of Si is high. Here, high vacuum means that the application is indispensable with a special vacuum pump, for example, 0.1 Pa or less.

  FIG. 1 shows an example in which the differential pressure regulator, the molten Si tank, and the heating means are installed outside the vacuum vessel, but these means, in particular, the molten Si tank, the heating means, and the cylinder portion of the differential pressure regulator are vacuum. You may install in an apparatus. By adopting such a configuration, if the inside of the vacuum vessel is evacuated, when the differential pressure regulator is stationary, no differential pressure to the cylinder is generated. Therefore, the configuration of the molten Si tank and the differential pressure regulator is further improved. This is because it can be simplified. Therefore, in addition to the case where the molten Si tank, the heating means, and the cylinder part of the differential pressure regulator are outside the vacuum vessel, the case where the molten Si tank is inside the vacuum vessel is also included in the scope of the present invention.

  Here, the principle of removing impurities such as P from molten Si by evaporation will be described. The evaporation rate of impurities is generally expressed by the following formula 1.

Formula of impurity evaporation rate V (described in general chemical engineering textbooks)
V = (first term: probability that unit concentration impurity on Si surface is released as vapor per unit time) × (second term: impurity concentration on Si surface) ×
(3rd term: evaporation rate coefficient) x
(Section 4: Surface area) (1)
Here, the first term of Equation 1 increases with the temperature rise. The third term of Equation 1 increases due to a decrease in atmospheric pressure or a decrease in impurity concentration in the atmosphere near the evaporation surface.

  In the case of Patent Document 1, in particular, Si is formed into droplets to increase the surface area and to remove impurities. That is, this method focuses on the fourth term of Equation 1. However, it can be said that the third and fourth terms of Equation 1 are the same as those in the conventional prior art.

  In the present invention, as described above, the Si droplets can be moved at a high speed by spraying and spraying the molten Si at a high speed. That is, in addition to the fourth term of Equation 1, the second term and the third term are also noticed. FIG. 2 is a diagram for explaining an example of the principle of removing impurities from Si droplets. First, the effect of the third term will be described. FIG. 2a shows the case where the Si droplet moves, and FIG. 2b shows the case where the Si droplet is stationary. In FIG. 2a, the surface of the droplet contacts the fresh atmosphere 21 in the traveling direction of the Si droplet 19, while the atmosphere 23 is contaminated with impurities behind the traveling direction of the Si droplet. As described above, since the concentration boundary layer of the impurity vapor around the droplet 19 is placed behind, the evaporation of the impurity is effectively performed in a fresh atmosphere. It can be said that the impurities evaporated from the droplet surface are less recondensed to the droplet surface and the net evaporation rate is improved. In FIG. 2b, the periphery of the Si droplet 19 is an atmosphere 23 contaminated with impurities. Next, the effect of the second term will be described. FIG. 3 is a drawing for explaining another example of the principle of removing impurities from Si droplets. In FIG. 3, when the Si droplet 19 travels at a high speed, a frictional force 25 is generated in the direction opposite to the traveling direction with respect to the Si droplet. As a result, a circulating flow 27 in Si is generated in the Si droplet 19. As described above, the friction between the surface of the Si droplet 19 and the surrounding atmosphere generates a strong circulating flow 27 in the droplet, and the impurity concentration difference between the surface of the droplet 19 and the inside disappears, thereby avoiding a decrease in evaporation rate. can do. Therefore, in the present invention, an extremely high temperature or high vacuum is not always necessary.

  On the other hand, even when there is no relative velocity between the droplet and the atmosphere, the impurities in the Si droplet are transported far in the atmosphere by molecular diffusion. If the effect of improving the evaporation rate of impurities by the above-mentioned droplet movement is not larger than the transport effect by molecular diffusion, no significant influence is recognized. Therefore, simply dropping the liquid droplets disclosed in Patent Document 1 is not effective, and it is essential to inject molten Si from the nozzles at a high speed as described above.

  FIG. 4 is an explanatory view showing another example of a refining apparatus used for refining Si. In FIG. 4, the container is divided into two parts in the vertical direction, and both chambers are shut off. The upper part is a vacuum vessel, and the lower part is mainly a heating pressure vessel or a heating furnace.

  First, the case where the vacuum vessel 1 is present will be described. A Si collector 15 is installed in the vacuum container 1. Heating means 11 such as a heater is installed on the wall of the vacuum vessel 1, particularly on the ceiling and side walls. Further, a vacuum pump 13 is attached via a valve 129 in order to evacuate the inside of the vacuum vessel 1. At the bottom of the vacuum vessel 1, a Si recovery device 15a is installed. A first opening 31 is provided at a substantially central portion of the Si recovery device 15a so that the nozzle 7 can be attached. The wall of the opening 31 is the same as the side wall of the Si collector 15a so that the recovered Si 39 does not leak from the first opening 31. Further, a second opening 33 is provided on the bottom wall of the Si recovery device 15a so as to drop the recovered Si into the lower heating pressure vessel 35. The position of the second opening 33 can be determined by the relationship with the position of the lower heating pressure vessel 35. The second opening 33 is a device that controls the fall of the recovered Si, and is provided with a stopper 37 as an opening / closing device that covers the opening 33 and the opening, for example. An opening / closing valve may be used for the opening / closing device. For example, after returning the inside of the vacuum vessel 1 to normal pressure, the stopper 37 is pulled up, the recovered Si 39 is dropped into the Si melt tank 9 provided in the heating pressure vessel 35, and the molten Si can be reused. it can.

  In the heating pressure vessel 35, a conduit 17 connecting the nozzle 7 and the Si melt tank 9 and the Si melt tank 9 are installed. The conduit 17 and the Si melt tank 9 are provided with heating means 11 such as a heater in order to keep the Si from solidifying. Further, the heating pressure vessel 35 is connected to the differential pressure regulator 43 via the valve 241 and further to the vacuum pump 13 via the valve 345. Here, regarding the heating pressure vessel 35, it is not always necessary to pressurize and depressurize the entire inside of the vessel. This is because if only the surroundings of the Si melt tank 9 and the recovered Si 39 dropped from the vacuum vessel 1 are sealed, and only the inside of this is pressurized and depressurized, the gas volume to be manipulated is greatly reduced. Therefore, switching between pressurization and decompression can be performed quickly.

  Next, a Si refining method using this refining apparatus will be described. However, since the members, apparatuses, operating conditions, and the like used in FIG. 2 are the same as those in FIG. 1 unless otherwise described, they have the same functions. The Si refining method will be described focusing on the differences from the above.

  The inside of the vacuum vessel 1 is evacuated by the vacuum pump 13 and, if necessary, the inside of the heating pressure vessel 35 is pressurized by the differential pressure regulator 43 to increase the differential pressure between the inlet side and the outlet side of the nozzle 7, thereby melting Si to the nozzle Let spray from.

  During the spraying operation, the inside of the vacuum container 1 is kept in a vacuum. That is, by operating the vacuum pump 13 and opening the valve 112, the vacuum container 1 is evacuated. The heating pressure vessel 35 is pressurized. That is, the valve 345 is closed, the differential pressure regulator 43 is operated, and the valve 2 41 is opened to apply pressure to the heating pressure vessel 35. At this time, the stopper 37 of the Si recovery device 15a is in a closed state.

  During the dripping operation of the recovered Si 39 to the molten Si tank 9, the inside of the vacuum vessel 1 is kept in a vacuum. On the other hand, the heating pressure vessel 35 is reduced in pressure. That is, the pressurization is stopped by closing the valve 2 41, the valve 345 is opened, and the pressure in the heating pressure vessel 35 is reduced. The stopper 37 of the Si recovery device 15 is opened, and the recovered Si 39 is dropped. The recovered Si 39 can be sprayed from the nozzle 7 again.

  In the case of this layout, the injection operation and the recovery Si dropping operation are performed alternately.

  By adopting such a configuration, it is possible to inject molten Si, collect the dropped molten Si droplet, and reuse it as molten Si, so that long-time de-impurity work is possible in the same apparatus. Become. Moreover, if time is taken, purity can be improved. Therefore, conditions such as temperature, atmosphere, and pressure in the vacuum vessel can be more relaxed.

  Further, since the recovered Si can be returned to the molten Si tank only by changing the pressure in the upper and lower chambers, the structure of the container is simple. Also, pumps that operate at high temperatures are very expensive, but can be operated without using such pumps. Therefore, the equipment cost can be kept low.

  Si refined by the present invention can satisfy the component specifications of Si for solar cell substrates.

  Here, the component specification of Si for solar cell substrates refers to the following composition. In general, it is said that the higher the purity of Si for solar cell substrate, the higher the performance, but the performance improvement rate gradually decreases as the purity increases. On the other hand, the higher the purity of Si, the more rapidly the cost for increasing the purity, so a certain Si purity is the cost-effective optimum. This optimum point differs for each component, and it is desirable to use Si in the following range in order to satisfy the performance having market value as the present solar cell. That is, phosphorus (P) 0.1 ppm or less in mass ratio, arsenic (As) 0.1 ppm, antimony (Sb) 0.1 ppm or less, boron (B) 0.3 ppm or less, gallium (Ga) 0.1 ppm or less, indium (In) 0.1 ppm or less and other metal components 0.1 ppm or less.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the examples.

(Device configuration)
Basically, the equipment shown in FIG. 4 was assumed.

1) Molten Si tank Installed in a heating furnace and kept warm by a heater. Dimensions: Diameter 100mm, Height 500mm, Material: Made of high-purity graphite 2) Nozzle dimensions: Full cone type spray nozzle with outlet diameter of 2mm, Material: Main body Made of high-purity silicon nitride, the periphery of the nozzle is protected with a pressure-resistant cover made of external carbon components , Injection angle: Vertical upward 3) Vacuum container Installed in a vacuum container and kept warm by a heater. Dimensions: diameter 1000 mm, height 5000 mm, material: magnesia-based refractory brick 4) Si recovery device dimensions: diameter 1000 mm, height 500 mm, double cylindrical shape, bottomed, with an opening at the bottom. Material: High-purity graphite, insulated with a heater.

(Operating conditions)
1) Pressure heating furnace, molten Si tank: 200 mPa (held by differential pressure regulator)
Lower heating furnace, vacuum vessel, Si recovery device: 10 Pa (held by vacuum pump)
2) Temperature melting Si tank, Si melt: 1700 ° C
Vacuum vessel wall: 1670 ° C
Si collector: 1450 ° C
3) Procedure / Raw material Si (P concentration: 40 ppm) is melted and poured into a molten Si tank.
-The lower heating furnace is sealed and pressurized to a predetermined pressure (200 mPa).
・ Evacuate the vacuum container to the specified pressure.
-Si is sprayed into the vacuum vessel, and P evaporates while the droplet is falling.
・ Gas evaporated in the vacuum furnace is discharged with a vacuum pump.
-Si droplets fall into the Si collector and accumulate.
-After injecting 50% Si in the tank, the inside of the lower heating furnace is depressurized to 10 Pa.
・ Open the stopper of the Si melt collector.
-The Si melt of the Si melt collector drops into the molten Si tank, and the amount of Si melt in the tank increases.
・ After closing the stopper, increase the furnace pressure and restart the injection of Si melt from the nozzle.
This operation was repeated 20 times.

(Product)
Analytical value of the obtained Si component: P concentration = 0.06 ppm (mass ratio)
The component specification of Si for solar cell substrate was satisfied. The yield was 95%.

  In addition, according to the present invention, about 10 times as much amount of molten Si could be processed as compared with a conventional de-P removal apparatus that does not inject molten Si of the same size.

It is explanatory drawing which shows an example of the refining apparatus used for refining of Si. It is drawing explaining an example of the principle by which an impurity is removed from Si droplet. It is drawing explaining the other example of the principle by which an impurity is removed from Si droplet. It is explanatory drawing which shows the other example of the refining apparatus used for refining of Si.

Explanation of symbols

1 vacuum container,
3 Si melt,
5 Differential pressure regulator,
7 nozzles,
9 Molten Si tank,
11 Heating means,
13 Vacuum pump,
15 Si collector,
17 Conduit,
19 Si droplet,
21 Fresh atmosphere,
23 Atmosphere contaminated with impurities,
25 frictional force,
27 Circulating flow in Si,
29 Valve 1,
31 first opening,
33 second opening,
35 heating pressure vessel,
37 stopper,
39 Recovered Si,
41 Valve 2,
43 Differential pressure regulator,
45 Valve 3.

Claims (8)

  A Si refining apparatus for removing impurities in Si from a sprayed droplet surface, comprising a container that can be evacuated and heated, and a nozzle that sprays molten Si into the container. A refining apparatus comprising a spray nozzle that utilizes a differential pressure between an inlet side and an outlet side.   The refining apparatus according to claim 1, wherein the spray center axis angle of the spray nozzle is in a range exceeding the horizontal direction and not exceeding vertically above.   The refining apparatus according to claim 1 or 2, wherein a type of the spray nozzle is a full cone nozzle, a flat spray nozzle, or a Laval nozzle.   Further, a recovery device having an opening at the bottom in the container, a molten Si recovery device including an opening / closing device for the opening, and a molten Si tank that receives the molten Si from the opening are provided inside the container. The refining apparatus of any one of Claims 1-3 provided with the heating furnace which has.   A method in which molten Si is atomized in a container that can be evacuated and heated, and impurities in Si are removed from the surface of the sprayed droplets, and the average moving speed of the Si melt as it exits the spray nozzle Is a refining method of Si, characterized by being 0.2 m / sec or more.   The method according to claim 5, wherein the spray center axis angle of the spray nozzle is in a range exceeding the horizontal direction and not more than vertically above.   The method according to claim 5 or 6, wherein the Si droplet dropped in the container is collected and sprayed again into the container.   The method according to claim 5, wherein the inner wall of the container is kept at 1650 ° C. or higher.

Images