JP2007314389A - Silicon refining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon refining method for obtaining solar battery class silicon used for a solar battery or the like where productivity is high and refining cost is reduced. <P>SOLUTION: In the refining method where a rotary cooling body is immersed into molten silicon, and refined silicon is crystallized out on the outer circumferential face of the rotary cooling body, the concentration of impurities in the high purity silicon which is melted at first is made lower than the concentration of impurities in the roughly refined silicon to be additionally charged. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a silicon purification method, and more particularly to a purification method for obtaining high-purity solar cell grade silicon used for solar cells and the like.

  In general, silicon used in a solar cell is better as it contains less impurities such as iron (Fe) or aluminum (Al), and a high purity of 99.9999% by mass or more is required. Conventionally, as a silicon refining method, for example, silicon as a refining raw material is placed in a crucible in advance, the melting furnace is made an inert gas atmosphere, and silicon is melted by a heater to form molten silicon. The material is kept heated to a temperature exceeding the melting point of silicon. Next, a purification method is disclosed in which purified silicon is crystallized on the outer peripheral surface of the rotating cooling body while cooling the rotating cooling body, thereby obtaining high-purity silicon having a purity of 99.9% by mass or more (patent) Reference 1).

JP 63-45112 A

  However, when producing purified silicon having a purity of 99.9999% by mass or more as required for solar cells by the purification method, the impurity concentration of the molten silicon increases while the segregation operation is repeated. The impurity concentration of purified silicon also increases. Therefore, when the segregation operation is repeated a certain number of times, the impurity concentration of the purified silicon exceeds the reference concentration. In such a case, the remaining hot water is discarded, the silicon is newly melted to prepare a molten metal, and then the segregation operation is performed again. If the process of preparing the molten metal, repeating segregation, and discarding the remaining hot water is one cycle, the conventional technique has a problem that the amount of purification in one cycle is small, so that the productivity is low and the purification cost is high.

  An object of the present invention is to provide a silicon purification method that solves the above-described problems in silicon purification and obtains solar cell grade silicon used for solar cells and the like, with high productivity and reduced purification costs. is there.

The present invention relates to a silicon purification method for purifying silicon using a crucible containing molten silicon, a heating means for heating the inside of the crucible, a rotary cooling body, and a silicon replenishing means.
Crystallization of a portion of the molten silicon as purified silicon on the surface of the rotating cooling body while rotating the rotating cooling body in the molten silicon;
Recovering the purified silicon crystallized on the surface of the rotating cooling body;
A step of adding roughly purified silicon in the crucible,
In this silicon purification method, the impurity concentration of the high-purity silicon that is first melted in the crucible is made lower than the impurity concentration of the roughly purified silicon that is additionally installed.

  Further, the present invention is characterized in that the impurity concentration of the high-purity silicon is not more than 0.5 times the impurity concentration of the roughly purified silicon.

  In the present invention, a part of the purified silicon is used as the high-purity silicon.

  According to the present invention, the impurity concentration of the high purity silicon that is first melted in the crucible is lower than the impurity concentration of the roughly purified silicon. In addition to suppressing the increase in the impurity concentration of molten silicon due to repeated segregation, it is possible to obtain purified silicon with a further reduced impurity concentration by performing segregation from high-purity silicon with a low impurity concentration. As a result, the impurity concentration is increased, and the purified silicon that has not been treated as solar cell grade silicon so far can be treated as solar cell grade silicon. Therefore, the amount of purification in one cycle can be greatly increased, the productivity can be improved, and the purification cost can be greatly reduced.

  High-purity silicon with low impurity concentration is first melted and used as molten silicon, but it is expected that the refining cost will increase, but productivity will be improved compared to the cost increase by using silicon with low impurity concentration. Since the effect of cost reduction due to is great, the purification cost can be reduced.

  In addition, according to the present invention, by making the impurity concentration of high-purity silicon 0.5 times or less than the impurity concentration of roughly purified silicon, the amount of purification in one cycle is greatly increased, and the productivity is greatly improved. Can do.

  In addition, according to the present invention, by using a part of purified silicon obtained by purification as high-purity silicon, it is possible to efficiently and inexpensively provide high-purity silicon necessary for each cycle, further reducing the purification cost. can do.

  FIG. 1 is a flowchart showing a silicon purification process according to the present invention. FIG. 2 is a schematic view of a purification apparatus used in the silicon purification method of the present invention.

  The silicon purification apparatus includes a melting furnace 1 that can be sealed, a crucible 2 that is disposed in the melting furnace 1 and accommodates molten silicon 3, a rotating cooling body 4, a heating mechanism 5 that melts silicon, a silicon additional mechanism 7, and the like. ing.

  The melting furnace 1 is formed from a refractory material. The crucible 2 is formed of a material that has low reactivity with silicon and that is less contaminated with molten silicon, such as quartz, graphite, or alumina. The rotating cooling body 4 has a bottomed cylindrical shape, has good thermal conductivity, and does not react with the molten silicon 3 and does not contaminate the molten silicon, such as silicon nitride or graphite. .

  A cooling fluid is sprayed toward the inner wall of the rotating cooling body 4 by a cooling fluid supply mechanism (not shown) to cool the inner wall of the rotating cooling body 4 and crystallize the purified silicon 6 on the outer peripheral surface of the rotating cooling body 4. For the heating mechanism 5, a method used for normal silicon melting, such as a resistance heater or a high-frequency induction heating mechanism, can be applied. The method of the additional mechanism 7 is not limited, and the silicon may be separately melted and then put into the crucible 2, or solid silicon may be put into the crucible 2.

  Next, a silicon purification method according to an embodiment of the present invention will be described with reference to FIGS.

  In step S1, high-purity silicon having an impurity concentration lower than that of roughly purified silicon to be purified (metal grade silicon, purity 98.0 mass% to 99.9 mass%) is placed in the crucible 2 in advance. The impurity concentration of high-purity silicon is preferably 0.5 times or less than the impurity concentration of roughly purified silicon. After the melting furnace 1 is evacuated, an inert gas such as Ar is supplied into the melting furnace 1 to make the inside of the melting furnace 1 an inert gas atmosphere. Although evacuation is not essential, evacuation can provide a more reliable inert gas atmosphere faster than replacement by introducing an inert gas. Then, the silicon in the crucible 2 is heated and melted by the heating mechanism 5 to obtain molten silicon 3. The molten metal temperature of the molten silicon 3 is controlled so as to be maintained at a predetermined temperature equal to or higher than the melting point of silicon.

  In step S2, a part of the rotating cooling body 4 is immersed in the molten silicon 3 by a moving mechanism (not shown). Thereafter, while the cooling fluid is sprayed toward the inner wall of the rotating cooling body 4, the rotating cooling body 4 is rotated, and high purity purified silicon 6 is crystallized on the outer peripheral surface of the rotating cooling body 4 by the principle of segregation solidification. The rotation of the rotating cooling body 4 allows the solidification of the molten silicon to proceed while the impurities discharged from the solidification interface into the liquid phase are dispersed in the entire liquid phase away from the solidification interface. Accordingly, since the solidification of the molten silicon proceeds with a segregation coefficient close to the equilibrium segregation coefficient, the purified silicon 6 having high purity can be crystallized in a short time on the outer peripheral surface of the rotary cooling body 4.

  The method using the rotating cooling body is a preferable method as the purified silicon crystallization method used in step S2 of the present invention, but is not limited to the purified silicon crystallization method. For example, the fixed cooling body and the molten silicon stirring mechanism may be different.

  Further, when silicon is crystallized by the rotating cooling body, the amount of silicon in the crucible decreases. In step S <b> 3, an amount corresponding to the decrease is roughly attached by the additional mechanism 7.

  After crystallizing a predetermined amount of silicon, in step S4, the rotary cooling body 4 and the purified silicon 6 are pulled up from the molten silicon 3 and moved to another location (not shown) or another apparatus in the melting furnace 1 to be solidified. The heated silicon is removed from the periphery of the rotating cooling body 4 by heating and melting, and the purified silicon 6 is recovered in a recovery container.

  In FIG. 1, roughly purified silicon is additionally installed between the purified silicon crystallization step and the purified silicon recovery step, but the timing of adding the roughly purified silicon is not limited to this. It may be after the purified silicon crystallization process, or may be performed in parallel with the purified silicon crystallization process as long as it does not affect the purified silicon crystallization process. Further, it may be supplemented every several purified silicon crystallization steps. In short, it is only necessary to supply an amount of roughly purified silicon necessary for repeating purified silicon crystallization.

  After the purified silicon crystallization process, the purified silicon recovery process, and the roughly purified silicon additional process are performed a predetermined number of times, in step S5, it is determined whether or not the average impurity concentration of the purified silicon exceeds the reference concentration. In that case, the remaining hot water silicon in the crucible 2 is discharged in step S6. If not, the process returns to step S2 to newly crystallize purified silicon 6. The average impurity concentration is a value obtained by averaging the impurity concentrations of all purified silicon recovered so far.

  For discharging the remaining hot water silicon, a method in which the crucible 2 is tilted and discharged while the silicon is melted is preferable because the crucible 2 can be reused, but the crucible 2 and the molten silicon 3 are cooled and solidified in the crucible 2. May be discharged. This completes one cycle of silicon refining, and the crucible 2 is newly charged with high-purity silicon having a lower impurity concentration than that of crudely purified silicon, and the next cycle is reached. As described above, the high-purity silicon having a lower impurity concentration than that of the roughly purified silicon is previously melted in the crucible 2 to perform refined silicon crystallization, refined silicon recovery and roughly refined silicon replenishment, thereby repeating the process. In addition to being able to suppress the increase in impurity concentration of molten silicon due to segregation, purified silicon with reduced impurity concentration can be obtained by performing segregation from high-purity silicon. Purified silicon whose impurity concentration exceeded the reference concentration and could not be treated as solar cell grade silicon can also be treated as solar cell grade silicon. In other words, since the impurity concentration of silicon purified in the first half of one cycle is below the reference concentration, the impurity concentration of silicon purified in the second half of one cycle exceeds the reference concentration, but the average of the entire silicon purified in one cycle The impurity concentration can be lower than the reference concentration. As a result, the amount of silicon that can be purified in one cycle is greater than when purifying the crude silicon itself in a molten state, and the silicon melting time and silicon discharging time can be shortened, improving productivity and reducing purification costs. can do.

Using the method of the present invention, crude silicon can be purified by a two-stage segregation operation.
As shown in FIG. 3, two crucibles 2A and 2c are arranged in order to perform two-stage segregation as the first melting step and the second melting step, and one rotating cooling body 4A is provided for each molten silicon. And the rotating cooling body 4B is installed. Crude purified silicon is charged into the crucible 2A that performs the first-stage segregation, and high-purity silicon is charged into the crucible 2B that performs the second-stage segregation. The molten metal temperature is controlled to be maintained at a predetermined temperature equal to or higher than the melting point of silicon. Next, as the first crystallization step and the second crystallization step, the rotating cooling body 4A and the rotating cooling body 4B are immersed one by one in each molten metal, and silicon is crystallized while blowing a cooling fluid and pulled up.

As a first additional process, the silicon 6A pulled up by the crucible 2A is charged into the second stage crucible 2B and melted. On the other hand, the silicon 6B pulled up by the second-stage crucible 2B is recovered as a recovery step to obtain purified silicon. Meanwhile, as a second additional process, roughly purified silicon is supplied to the molten metal of the crucible 2A from the additional mechanism 7 installed in the melting furnace.
When the impurity concentration in the purified silicon recovered in the recovery step exceeds the reference concentration, the crucible 2A is tilted to discharge the molten silicon. Thereafter, the silicon in the crucible 2B is transferred to the first-stage crucible 2A, and the purified silicon obtained in the recovery process is charged into the second-stage crucible 2B and melted to obtain high-purity silicon in the next cycle. Here, the silicon in the crucible 2B may be actually transferred to the crucible 2A to obtain the initial high-purity silicon. However, a sequence in which the crucible 2B is handled as the crucible 2A without transferring the molten silicon may be used. Even if the silicon 6A crystallized on the surface of the rotary cooling body 4A is melted in the crucible 2B as a recovery process, a large amount of heat is required to re-dissolve the silicon. After being collected at the part, it may be crushed into small pieces of silicon, and then supplied to the crucible 2B by a silicon mounting mechanism as a second mounting process.

  In the second and subsequent cycles, since the impurity concentration of silicon initially melted in the first-stage crucible 2A is lower than the impurity concentration of coarsely purified silicon first melted in the first cycle, a two-stage effect is achieved. As a result, a larger amount of silicon than that obtained in the first cycle can be purified in one cycle.

Example 1
Crude purified silicon having an iron concentration of 20 ppm by mass was purified by the method of the present invention.
As shown in FIG. 4, as a melting step, 260 kg of high-purity silicon having an iron concentration of 10 mass ppm (0.5 times the iron concentration of roughly purified silicon) is charged into a graphite crucible 2 having an inner diameter of 600 mm and a depth of 600 mm. After melting at 3 ° C. for 3 hours, the molten metal temperature was set to 1450 ° C. and held for 2 hours. Next, as a crystallization step, 10 kg of silicon 6 was crystallized in 10 minutes by immersing the rotating cooling body 4 in this molten metal and blowing nitrogen gas as a cooling fluid into the rotating cooling body while rotating. Later, it was lifted from the melt. Next, as a recovery step, the crystallized silicon 6 was heated on the recovery container 9 to melt the silicon, so that it was dropped from the rotating cooling body 4 and recovered as purified silicon in the recovery container 9. In addition, as the additional process, the crude silicon 8 having an iron concentration of 20 mass ppm was supplied to the molten silicon 3 from the additional mechanism 7 installed in the 10 kg melting furnace.

  Steps 2 to 4 were repeated 50 times. When 500 kg of purified silicon was obtained, the average iron concentration of the purified silicon exceeded 0.1 mass ppm. Therefore, the crucible was tilted and the molten silicon was discharged. 130 kg of purified silicon having an iron concentration of 0.1 mass ppm and 130 kg of roughly purified silicon having an iron concentration of 20 mass ppm were put into the crucible 2 and melted to obtain high purity silicon for the next cycle. Thereafter, steps 2 to 4 were repeated in the same manner for purification. Since 130 kg was used as a raw material, the net amount of purified silicon in one cycle was 370 kg.

(Example 2)
When purification was performed using the same method as in Example 1 as high-purity silicon having an iron concentration of 0.1 mass ppm (0.005 times the iron concentration of roughly purified silicon), 800 kg of purified silicon was obtained in one cycle. Obtained. Of these, 260 kg was charged into the crucible 2 and melted and used as high-purity silicon for the next cycle, so the net amount purified in one cycle was 540 kg. By reducing the impurity concentration of high-purity silicon, the net amount of purification obtained in one cycle was greatly increased.

(Comparative Example 1)
As a comparative example, roughly purified silicon having an iron concentration of 20 mass ppm was purified by a conventional method.

  A graphite crucible having an inner diameter of 600 mm and a depth of 600 mm was charged with 260 kg of roughly purified silicon having an iron concentration of 20 mass ppm and melted at 1500 ° C. for 3 hours, and then the molten metal temperature was set to 1450 ° C. and held for 2 hours. A rotary cooling body was immersed in the molten metal, and 10 kg of silicon was crystallized in 10 minutes while blowing nitrogen gas as a cooling fluid and pulled up. The crystallized silicon was recovered as refined silicon, and at the same time, 10 kg of crude refined silicon having an iron concentration of 20 mass ppm was supplied to the molten metal from an additional mechanism installed in the melting furnace. When this operation was repeated 20 times, the iron concentration in the purified silicon exceeded 0.1 mass ppm. The net purified amount obtained in one cycle was 200 kg.

  Therefore, by using the silicon purification method of the present invention, it is possible to remarkably increase the amount of net purification obtained in one cycle, improve productivity, and consequently reduce purification cost.

(Example 3)
When refining roughly refined silicon having an iron concentration of 970 mass ppm by a two-stage segregation operation, it was refined using the method of the present invention.

As shown in FIG. 3, two graphite crucibles 2A and 2B having an inner diameter of 600 mm and a depth of 600 mm are arranged in order to perform two-stage segregation as the first melting process and the second melting process. The rotating cooling body 4A and the rotating cooling body 4B were installed one by one. Crude purified silicon having an iron concentration of 970 mass ppm is applied to the crucible 2A for performing the first stage segregation, and iron concentration of 0.1 mass ppm (about 1.0 of the iron concentration of the roughly purified silicon is applied to the crucible 2B for performing the second stage of segregation. 260 kg of each of (× 10 ⁻⁴ times) silicon was introduced and melted at 1500 ° C. for 3 hours, and then the molten metal temperature was set to 1450 ° C. and held for 2 hours. Next, as the first crystallization step and the second crystallization step, the rotary cooling body 4A and the rotary cooling body 4B are immersed one by one in each molten metal, and 10 kg of silicon is injected for 10 minutes while blowing nitrogen gas as a cooling fluid. Was crystallized and pulled up.

  In the first additional process, the silicon 6A pulled up by the crucible 2A was charged into the second stage crucible 2B and melted. On the other hand, the silicon 6B pulled up by the second-stage crucible 2B is heated in a recovery container as a recovery step to melt the silicon, thereby dropping it from the rotary cooling body 4B and recovering it to obtain purified silicon. Meanwhile, as a second additional step, roughly purified silicon was placed in the melting furnace in the molten metal of the crucible 2A. Meanwhile, as a second additional process, 10 kg of roughly purified silicon having an iron concentration of 970 mass ppm was supplied from the additional mechanism 7 installed in the melting furnace to the molten metal of the crucible 2A.

  Such a purification process was repeated 115 times, and when 1150 kg of purified silicon was obtained, the iron concentration of the purified silicon recovered in the recovery process exceeded 0.1 ppm by mass, so the crucible 2A was tilted to discharge the molten silicon. did. Thereafter, 260 kg of silicon in the crucible 2B is transferred to the first-stage crucible 2A, and 260 kg of purified silicon having an iron concentration of 0.1 mass ppm obtained in step 3B is charged into the second-stage crucible 2B and melted. Cycle high purity silicon. That is, the net purified amount in one cycle was 890 kg.

  Further, after the second cycle of the present embodiment, the impurity concentration of silicon first melted in the first-stage crucible 2A is lower than the impurity concentration of roughly purified silicon melted first in the first cycle. Due to the two-stage effect, it was possible to purify 1500 kg (net 1240 kg) of silicon in one cycle, which was greater than the amount of purification 1150 kg (net purification amount 890 kg) in the first cycle.

(Comparative Example 2)
As a comparative example, when refining roughly refined silicon having an iron concentration of 970 mass ppm by a two-stage segregation operation, it was refined using a conventional method. In order to perform segregation in two stages, two graphite crucibles having an inner diameter of 600 mm and a depth of 600 mm were arranged, molten silicon was charged, and one rotating cooling body was installed for each molten silicon.

  First, roughly refined silicon having an iron concentration of 970 mass ppm was put into a crucible for performing segregation in the first stage, and after melting at 1500 ° C. for 3 hours, the molten metal temperature was set to 1450 ° C. and held for 2 hours. A rotary cooling body was immersed in the molten metal in the crucible, and 10 kg silicon was crystallized in 10 minutes while blowing nitrogen gas as a cooling fluid, pulled up and collected, and then put into a second stage crucible. This operation was repeated until 260 kg of silicon was obtained in the second stage crucible. After that, a rotating cooling body is immersed in each molten metal one by one, and the silicon pulled up by the first stage crucible is collected and put into the second stage crucible, and the silicon pulled up by the second stage crucible is collected. Purified silicon. Meanwhile, 10 kg of roughly purified silicon having an iron concentration of 970 mass ppm was supplied to the molten metal in the first stage crucible from an additional mechanism installed in the melting furnace.

  When segregation was repeated until the average iron concentration of the purified silicon reached 0.1 mass ppm, 750 kg could be purified in one cycle.

  Therefore, as in the case of Example 1, by using the silicon purification method of the present invention, it is possible to remarkably increase the amount of net purification obtained in one cycle, thereby improving the productivity and consequently the purification cost. Can be reduced.

It is a flowchart explaining one Embodiment of this invention. It is the schematic of the silicon | silicone refinement apparatus explaining the silicon | silicone purification method of this invention. It is process drawing explaining Example 2 of this invention. It is process drawing explaining Example 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Melting furnace 2,2A, 2B Crucible 3,3A, 3B Molten silicon 4,4A, 4B Rotating cooling body 5 Heating mechanism 6,6A, 6B Purified silicon 7 Silicon additional mechanism 8 Crude purified silicon 9 Purified silicon recovery container

Claims (3)

In a silicon refining method for refining silicon using a crucible containing molten silicon, a heating means for heating the inside of the crucible, a rotary cooling body, and a silicon additional means,
Crystallization of a portion of the molten silicon as purified silicon on the surface of the rotating cooling body while rotating the rotating cooling body in the molten silicon;
Recovering the purified silicon crystallized on the surface of the rotating cooling body;
A step of adding roughly purified silicon in the crucible,
A silicon purification method, wherein the impurity concentration of high-purity silicon that is first melted in a crucible is made lower than the impurity concentration of the roughly purified silicon to be additionally installed.   2. The silicon purification method according to claim 1, wherein an impurity concentration of the high-purity silicon is 0.5 times or less of an impurity concentration of the roughly purified silicon.   3. The silicon purification method according to claim 1, wherein a part of the purified silicon is used as the high-purity silicon.

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