WO2013121504A1 - Silicon purification device - Google Patents

Description

Silicon purification equipment

The present invention relates to a silicon purification apparatus, and more particularly, to a silicon purification apparatus using a solidification segregation phenomenon.

Due to environmental problems, the use of natural energy is attracting attention as an alternative to fossil fuel energy such as oil. Solar cells that generate power using natural energy are actively introduced in Japan and Europe because they do not require large facilities and do not generate noise during operation.

In solar cells, new solar cells made of compound semiconductors such as cadmium tellurium are being developed. However, solar cells using crystalline silicon as a substrate currently occupy a large share in terms of the safety of the substance itself, past use results and price.

It is well known that impurities in silicon used for solar cells affect the quality of solar cells. Therefore, it is preferable to use highly purified silicon for manufacturing a solar cell.

As a method for improving the purity of silicon, metal silicon having a purity of 98% or more obtained by reducing silica is converted into a gas such as silane (SiH ₄ ) or trichlorosilane (SiHCl ₃ ), and the gas is placed in a bell jar furnace. There is a method of hydrogen reduction. A silicon wafer for an electronic device such as LSI (Large Scale Integration) can be manufactured by growing a single crystal of the polysilicon thus obtained.

The silicon used for the electronic device material is required to have a very high purity of 11N (99.99999999999%). As a result, the investment in facilities that convert metallic silicon into gas such as silane and the investment in bell jar furnaces are enormous, and the productivity of high-purity silicon is not so high due to the use of gas phase reactions. However, as a method for purifying silicon for electronic devices, a method of converting metal silicon into a gas such as silane and reducing it with hydrogen in a bell jar furnace must be employed.

However, silicon as a solar cell material is required to have a purity of about 6N (99.9999%). Therefore, silicon obtained by a method in which metal silicon is converted into a gas such as silane and hydrogen reduced in a bell jar furnace satisfies the quality as a solar cell material, but the cost is very high.

From the above, establishment of an inexpensive manufacturing technique for silicon as a solar cell material is strongly demanded, and a technique for refining the above-mentioned metal silicon having a purity of about 98% by a metallurgical technique using solidification segregation or the like. Has attracted attention in recent years.

There are iron, aluminum, titanium and the like that are relatively contained as metal impurity elements present in metal silicon. Typical values of the impurity content in the metal silicon are 100 ppmw to 5000 ppmw for iron, 100 ppmw to 2000 ppmw for aluminum, and 1 ppmw to 10 ppmw for titanium.

Impurities such as iron, aluminum or titanium are known to have a small segregation coefficient (equilibrium segregation coefficient) in silicon. For example, the equilibrium segregation coefficient value of iron in silicon is 6.4 × 10 ⁻⁶ , the segregation coefficient value of aluminum in silicon is 2.8 × 10 ⁻³ , and the segregation coefficient value of titanium in silicon is It is reported to be 7.37 × 10 ⁻⁶ . Therefore, impurities such as iron, aluminum, and titanium can be removed using solidification segregation.

That is, when the silicon melt is solidified, impurities (iron, aluminum or titanium) having a small segregation coefficient are distributed in the silicon melt and hardly taken into the solid. Therefore, the concentration of the impurity in the deposited silicon is lowered, and high purity silicon can be obtained by taking out the solid phase region in which the impurity concentration is lowered.

The impurity concentration C _{S in the} solidified portion changes with the progress of solidification (solid phase ratio) and is expressed by the following equation.
C _S = k × C ₀ × (1−f _S ) ^(k−1) (1)
Here, k is the segregation coefficient, C ₀ is the impurity concentration before purification, and f _S is the solid fraction.
The impurity concentration C _l of the melt portion is expressed by the following equation.
C _l = C ₀ × f _l ^(k−1) (2)
Here, f _l is a liquid phase ratio and f _S + f _l = 1.

When solidification proceeds at a sufficiently low speed and solidifies under the condition that impurities discharged from the solid phase are uniformly diffused into the liquid phase, the equilibrium segregation coefficient k ₀ may be applied as the segregation coefficient. However, when purifying by segregation industrially, a faster solidification rate is preferable from the viewpoint of productivity. If a faster solidification rate, the use of the effective segregation coefficient k _e instead equilibrium segregation coefficient k ₀ as segregation coefficient, the above equation (1), (2) is similarly established.

The effective segregation coefficient k _e is expressed by the following equation.
k _e = k ₀ / {k ₀ + (1−k ₀ ) exp− [Vδ / D]} (3)
Here, V is the moving speed of the solidification interface, δ is the thickness of the impurity concentrated layer, and D is the impurity diffusion coefficient.

From the above equation (3), in order to reduce the effective segregation coefficient k _e while accelerating the solidification speed V it is found to be effective to reduce the thickness δ of the impurity concentration layer. Therefore, the melt at the solid-liquid interface is agitated so that the thickness δ of the impurity concentrated layer is reduced.

As a prior document disclosing a method for purifying silicon, there is JP-A-63-45112 (Patent Document 1). In the method for purifying silicon described in Patent Document 1, the cooling body is rotated in molten silicon while maintaining the molten silicon in a state exceeding the solidification temperature in an inert gas atmosphere, and the outer periphery of the cooling body High purity silicon is crystallized on the surface.

In such a high-purity silicon refining apparatus using a cooling body, a material that does not react with molten silicon and has good thermal conductivity is used as the material of the cooling body, and ceramics or graphite such as silicon nitride can be used. Yes.

Japanese Patent Laid-Open No. 2000-53411 (Patent Document 2) is a prior document disclosing the configuration of a polycrystalline silicon lump manufacturing apparatus. In the polycrystalline silicon lump manufacturing apparatus described in Patent Document 2, nitrogen is used as a cooling fluid, and the used nitrogen is discharged from the cooling fluid discharge port to the outside of the apparatus.

JP-A-63-45112 JP 2000-53411 A

The cooling body causes high-purity silicon to be crystallized by removing heat from the molten silicon on the peripheral surface in contact with the molten silicon by allowing a cooling fluid to flow therethrough. That is, the cooling body is a member that intervenes between the cooling fluid and the molten silicon to exchange heat.

Therefore, the solidification rate of silicon deposited on the peripheral surface of the cooling body is affected by the thermal conductivity of the members constituting the cooling body, and the higher the thermal conductivity, the faster the solidification rate of silicon. Since the solidification rate of silicon is directly related to the productivity of purified silicon, it is desirable to configure the cooling body with a material having a high thermal conductivity in order to improve the productivity.

Also, the member used for the cooling body is required not to react with high-temperature silicon and not to contaminate the silicon. In addition, in view of strength and cost, graphite is most suitable as a material used for the cooling body.

On the other hand, either a gas or a liquid may be used as the cooling fluid to be passed through the cooling body. When a liquid is used as the cooling fluid, it is necessary to cope with the following problems.

First, when the cooling fluid leaks from the cooling body into the furnace, the cooling fluid in contact with the high-temperature molten silicon undergoes rapid volume expansion by being vaporized. When the cooling fluid is vaporized in the sealed furnace, the pressure in the furnace rapidly increases, and there is a possibility that damage to the furnace body and damage to the surroundings due to the ejected high-pressure gas may occur. Therefore, it is necessary to design the cooling body in consideration of the safety factor so that the cooling fluid does not leak in any case.

Also, the same phenomenon can occur when the cooling fluid vaporizes in the flow path in the cooling body, even if the cooling fluid does not leak from the cooling body. In order to prevent the cooling fluid from evaporating in the flow path in the cooling body, it is necessary to appropriately maintain the flow rate of the cooling fluid and the amount of heat received so that the cooling fluid is maintained at a temperature below the boiling point. These also require flow management that takes into account the safety factor.

Thus, when a liquid is used as the cooling fluid, attention must be paid to the device design and flow management. However, if a gas is used as the cooling fluid, a relatively simple device and management can be achieved.

¡Various types of cooling fluid gases can be used, but they are selected in consideration of specific heat and thermal conductivity as properties related to cooling capacity. However, since He which is excellent in thermal conductivity is very expensive, the cooling fluid is selected in consideration of cost.

Also, when graphite is used for the cooling body, it is necessary to consider the reactivity with graphite in a high temperature environment for the following reasons. When the cooling body is immersed in the molten silicon, the cooling body becomes a high temperature due to heat transfer from the molten silicon, and the temperature in contact with the cooling fluid also exceeds 1000 ° C.

When such high-temperature graphite comes into contact with an active gas such as oxygen, the oxidation reaction proceeds and the graphite itself is depleted, so that the strength of the cooling body cannot be maintained finally. For example, since air contains about 20% of oxygen, when the cooling body is used in the air, the cooling body is oxidized and consumed to such an extent that the required strength cannot be maintained after several hours of use.

If an inert gas having low reactivity with graphite is used for the cooling fluid, such a problem can be avoided. Nitrogen gas is mentioned as the cheapest thing among inert gases.

However, when the concentration of oxygen contained as an impurity is high, the same problem as described above arises. Therefore, it is necessary to use nitrogen gas having a low oxygen concentration to such an extent that a problem due to oxidative consumption of the cooling body does not occur.

Considering the cost of replacing the cooling body due to oxidation consumption of graphite, the oxygen concentration of the nitrogen gas is desirably about 100 ppm or less. As a method for producing nitrogen gas that satisfies this condition, an air cryogenic separation method is generally used. When producing nitrogen gas, since air is used as a material, the material cost is low, but the cost for separation and purification of nitrogen is required.

Conventionally, a large amount of nitrogen gas passed through the cooling body has been once used as a cooling fluid and then exhausted into the atmosphere. For this reason, the cost of nitrogen gas has been an obstacle in reducing the cost of silicon purification.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a silicon purifier capable of reducing the purification cost by enabling circulation of an inert gas used as a cooling fluid, particularly nitrogen gas. And

The silicon purification apparatus according to the present invention was supported so as to be immersed in the molten silicon in a state where a crucible for holding the molten silicon, a heating unit for heating the silicon held in the crucible, and a cooling fluid flowing therethrough. A cooling unit, a cooling unit that regenerates the cooling fluid so as to be allowed to flow again through the cooling body by cooling the cooling fluid that has absorbed heat by flowing through the cooling body, and a compressor that compresses the cooling fluid are provided. A cooling body, a cooling unit, and a compressor are connected to each other to form a cooling fluid circulation path.

In one aspect of the present invention, the cooling unit includes a first cooler that takes away a part of the endothermic amount of the cooling fluid that has absorbed heat by flowing through the cooling body, and is cooled by the first cooler to the compressor. A second cooler that cools the cooling fluid before flowing in; and a third cooler that cools the cooling fluid discharged from the compressor. In the circulation path, the cooling body, the first cooler, the second cooler, the compressor, and the third cooler are connected in order.

In one embodiment of the present invention, the silicon purifier further includes a pressure measuring device that measures the pressure in the suction section of the compressor. Based on the measurement result of the pressure measuring device, the operation of the compressor is controlled so that the pressure in the suction portion of the compressor becomes higher than the external air pressure outside the circulation path.

In one embodiment of the present invention, the operation of the compressor is controlled so that the gauge pressure at the suction portion of the compressor is in the range of (0.005 ± 0.005) MPa.

In one embodiment of the present invention, a buffer tank that further suppresses pressure fluctuation of the cooling fluid is further provided adjacent to the compressor in the circulation path.

In one embodiment of the present invention, the silicon purifier further includes a buffer tank that suppresses the pressure fluctuation of the cooling fluid adjacent to the compressor in the circulation path.

In one embodiment of the present invention, the silicon purification apparatus further includes two gate valves so as to sandwich the cooling body in the circulation path. Only the section including the cooling body sandwiched between the two gate valves in the circulation path can be opened to the atmosphere.

In one embodiment of the present invention, the supply path is connected to the upstream side of the cooling body and the discharge path is connected to the downstream side of the cooling body in the above-described section of the circulation path, so that the atmosphere is opened to the atmosphere. Further, the inside of the section can be replaced with a cooling fluid.

In one embodiment of the present invention, one of the two gate valves is located at the connection between the supply path and the circulation path, and the above-mentioned 2 at the connection between the discharge path and the circulation path. The other of the two gate valves is located, and both gate valves are three-way valves.

According to the present invention, it is possible to reduce the refining cost by enabling the circulation and use of nitrogen gas.

It is sectional drawing which shows the structure of the silicon refinement | purification mechanism which concerns on this embodiment. It is sectional drawing which shows the structure of the cooling fluid circulation mechanism which concerns on the same embodiment. It is a graph which shows transition of the temperature of the cooling fluid in each point of the cooling fluid circulation mechanism in an Example, flow volume, and pressure.

Hereinafter, a silicon purification apparatus according to an embodiment of the present invention will be described with reference to the drawings. In the following description of the embodiments, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

The silicon refining apparatus according to this embodiment is roughly composed of a silicon refining mechanism for refining silicon and a cooling fluid circulation mechanism for circulating a cooling fluid. Hereinafter, the configuration and operation of each mechanism will be described.

<Silicon purification mechanism>
FIG. 1 is a cross-sectional view showing the configuration of the silicon purification mechanism according to this embodiment. As shown in FIG. 1, the silicon purification mechanism 100 according to the present embodiment includes a sealed container 110 whose inside can be replaced with an inert gas such as argon or nitrogen in order to prevent oxidation of the silicon and oxidation of the carbon member. I have. The inner surface of the sealed container 110 is covered with a heat insulating material 111.

Further, the silicon purification mechanism 100 can be immersed in the molten silicon 10 with the crucible 114 for holding the molten silicon 10, the heating unit 113 for heating the silicon held in the crucible 114, and the cooling fluid flowing inside. And a cooling body supported by the. The cooling body has an immersion part 120 immersed in the molten silicon 10 and a pipe part 130 whose inside becomes a flow path of the cooling fluid.

The silicon refining mechanism 100 immerses the immersion part 120 of the cooling body in the molten silicon 10 and raises and lowers the cooling body to raise and lower the cooling body in which silicon is deposited on the peripheral surface of the immersion part 120. It has. Furthermore, the silicon purification mechanism 100 includes a rotation drive mechanism (not shown) that rotates the immersion unit 120 of the cooling body in the molten silicon 10. The elevating mechanism and the rotational drive mechanism are attached to the piping section 130 of the cooling body.

The crucible 114 is made of a material that has heat resistance and durability at 1412 ° C. or higher, which is the melting point of silicon, and allows contamination of the raw material silicon within an allowable range. For example, a material mainly composed of one or more of graphite, silicon oxide, silicon nitride, silicon carbide, and the like can be used as the material for the crucible 114. In particular, graphite is suitable as a material for the crucible 114 from the viewpoint of durability, and graphite manufactured by a cold isostatic pressing method (CIP (Cold Isostatic Pressing)) is more preferable.

The heating unit 113 is not particularly limited as long as it can be heated to 1412 ° C. or higher, which is the melting point of silicon. For example, a resistance heating device or an induction heating device using a graphite heater or the like can be used as the heating unit 113.

The internal structure of the cooling body has a double pipe structure. Specifically, the peripheral surface of the immersion part 120 has a cylindrical shape with a closed lower end, but may be chamfered. Moreover, the taper-shaped site | part which gradually became large diameter toward the upper direction from the lower end may be provided in the surrounding surface of the immersion part 120. FIG.

The piping unit 130 includes an introduction pipe 131 that introduces a low-temperature cooling fluid into the immersion part 120, and a delivery pipe 132 that surrounds the outside of the introduction pipe 131 and sends out a high-temperature cooling fluid.

The inside of the introduction pipe 131 communicates with the inside of the inflow pipe 140 into which the cooling fluid flows. The leading end 131 a of the introduction pipe 131 is located inside the immersion part 120. A plurality of openings are formed in the distal end portion 131 a of the introduction tube 131. The plurality of openings are opposed to the inner wall of the immersion part 120. The inside of the introduction pipe 131 and the inside of the immersion part 120 communicate with each other through the plurality of openings.

The inside of the delivery pipe 132 communicates with the inside of the immersion part 120. The inside of the delivery pipe 132 communicates with the inside of the outflow pipe 150 from which the cooling fluid flows out.

As described above, a rotation drive mechanism is attached to the pipe part 130, and the pipe part 130 is held rotatably about the axis. Specifically, the connection part between the introduction pipe 131 and the inflow pipe 140 and the connection part between the delivery pipe 132 and the outflow pipe 150 are constituted by a rotary joint.

The material constituting the immersion part 120 of the cooling body may be selected from the same viewpoint as the crucible 114. However, since the immersion part 120 is a part responsible for heat exchange, a material having high thermal conductivity is preferable as the material constituting the immersion part 120 from the viewpoint of improving the productivity of purified silicon, and graphite is particularly preferable.

The material constituting the cooling pipe section 130 may be the same material as the immersion section 120. Alternatively, in the portion not exposed to the high temperature atmosphere in the sealed container 110, priority is given to strength, and a material different from the material of the immersion part 120, for example, stainless steel is used to connect to the graphite member constituting the immersion part 120. May be.

The immersion part 120 of the cooling body is immersed in the molten silicon 10 while rotating. The immersion part 120 is cooled by spraying the cooling fluid blown out from the plurality of openings onto the inner wall.

As a result, the molten silicon 10 in contact with the peripheral surface of the immersion part 120 is cooled, solidified, and deposited. At this time, a solidification segregation phenomenon occurs, and the impurity concentration of the solidified silicon 11 deposited on the peripheral surface of the immersion part 120 is lower than that of the molten silicon 10. Thereafter, the immersion part 120 is pulled up from the molten silicon 10 by the lifting mechanism, and the solidified silicon 11 is peeled off from the immersion part 120, whereby purified silicon purified to a high purity can be obtained.

<Cooling fluid circulation mechanism>
FIG. 2 is a cross-sectional view showing the configuration of the cooling fluid circulation mechanism according to the present embodiment. The cooling fluid circulation mechanism 200 according to the present embodiment cools the cooling fluid that has absorbed heat by flowing through the cooling body, thereby compressing the cooling fluid, and a cooling unit that regenerates the cooling fluid so that it can flow again through the cooling body. And a compressor. The cooling body, the cooling unit, and the compressor are connected to each other to constitute a cooling fluid circulation path.

As shown in FIG. 2, in the present embodiment, the cooling body, the cooler 250 as the first cooler, the gate valve 233, the cooler 251 as the second cooler, the first buffer tank 260, the compressor 270, A second buffer tank 261, a cooler 252 as a third cooler, a pressure reducing valve 221, a gate valve 234, and a gate valve 231 are connected in order by a pipe to form a circulation path. The cooling fluid circulates in the circulation path in the direction indicated by arrow 1.

Hereinafter, each configuration of the cooling fluid circulation mechanism will be described.
The cooler 250, which is the first cooler, cools the cooling fluid by taking a part of the endothermic amount of the cooling fluid absorbed through the cooling body from the cooling fluid.

If the cooling fluid that has absorbed heat by the cooling body and has reached a high temperature is not cooled and flows through the piping and the piping becomes hot, it is dangerous if, for example, an operator inadvertently touches the piping. Moreover, since high heat resistance is calculated | required by piping, it will be necessary to use piping formed with expensive material.

By cooling the cooling fluid with the cooler 250, it is possible to reduce the above-mentioned danger and reduce the heat resistance required for the piping and use an inexpensive resin piping having mobility. Thus, by providing the cooler 250, the design flexibility of the cooling fluid circulation mechanism 200 can be increased, and the piping cost can be reduced. In order to obtain the above effect, it is desirable to cool the cooling fluid to about 60 ° C. by the cooler 250.

When the cooling fluid is cooled to about 60 ° C. by the cooler 250, a cooler that uses atmospheric heat radiation due to a temperature difference from the outside air can be used as the cooler 250, so that the cost of the cooler 250 is relatively low. Can do.

In the present embodiment, a cooler 251 that is a second cooler for cooling the cooling fluid before being cooled by the cooler 250 and flowing into the compressor 270 is provided. As will be described later, the cooling fluid is pressurized by the compressor 270 in order to circulate and use the cooling fluid. However, since the temperature of the cooling fluid rises when the cooling fluid is pressurized, the cooling fluid 251 before it flows into the compressor 270. Cool the cooling fluid to room temperature.

Some compressors 270 require that the temperature of the compression target gas be reduced to about room temperature in order to suppress oil consumption. In order to efficiently cool a cooling fluid of about 60 ° C. to room temperature, a cold heat source cooled to room temperature or lower is required, and the cost of the cooler is relatively high. Moreover, if the cooling fluid is cooled using a cooler that can be cooled to room temperature or lower in one stage, the cooling efficiency is not good. Therefore, it is preferable to cool the cooling fluid to room temperature or lower in two stages using the cooler 250 and the cooler 251.

Furthermore, in the present embodiment, a cooler 252 that is a third cooler that cools the cooling fluid discharged from the compressor 270 is provided. As described above, since the cooling fluid pressurized by the compressor 270 is adiabatically compressed and the temperature rises, the cooling fluid is cooled again by the cooler 252.

As described above, by cooling the compressed cooling fluid by the cooler 252, a cooling fluid having a high cooling ability cooled to room temperature can be caused to flow into the cooling body. As a result, the productivity of purified silicon can be improved. However, the cooler 252 is not necessarily provided, and whether to install the cooler 252 may be determined in consideration of the installation cost of the cooler 252 and the increased production amount of purified silicon.

The cooling unit is composed of the cooler 250, the cooler 251, and the cooler 252. However, the configuration of the cooling unit is not limited to the above, and any configuration may be used as long as the cooling fluid that has absorbed heat by flowing through the cooling body can be cooled so that the cooling fluid can be recirculated through the cooling body. Here, the regeneration that allows the cooling fluid to flow again means that the cooling fluid is cooled to a temperature at which the immersion part 120 can be cooled so that purified silicon can be deposited in the immersion part 120 of the cooling body.

The compression method is not particularly limited as the compressor 270, but a screw type compressor capable of obtaining a discharge flow rate of about several m ³ / min with a compact device is suitable.

In this embodiment, a buffer tank that suppresses the pressure fluctuation of the cooling fluid is provided adjacent to the compressor 270 in the circulation path. Specifically, a buffer tank 260 is disposed on the intake side of the compressor 270, and a buffer tank 261 is disposed on the discharge side.

In the buffer tanks 260 and 261, since the flow path of the cooling fluid is larger than the pipe, the pressure of the cooling fluid can be stabilized while being reduced.

The pressure in the piping may fluctuate in the circulation path. In that case, by controlling the operation of the compressor 270, a certain amount of pressure fluctuation can be suppressed. However, when a sudden pressure fluctuation occurs, the operation of the compressor 270 cannot follow the fluctuation, and there is a high possibility that the pressure in the circulation path deviates from the control target value.

By providing the buffer tanks 260 and 261 adjacent to the compressor, it is possible to relieve rapid pressure fluctuations in the piping and keep the pressure in the circulation path within the target range. The buffer tanks 260 and 261 are not necessarily provided, and only one of the buffer tank 260 disposed on the intake side and the buffer tank 261 disposed on the exhaust side of the compressor 270 may be provided.

Also, the circulation path is connected to a supply path that supplies cooling fluid from outside the circulation path. Cooling fluid is supplied from the supply path when the cooling path is initially filled with cooling fluid and to compensate for cooling fluid that leaks out of the circulation path and decreases.

The supply path is composed of a cooling fluid supply source 210 and a piping system connecting the supply source 210 and the circulation path. The piping system is provided with a gate valve so that the supply amount of the cooling fluid can be adjusted.

In this embodiment, nitrogen gas is used as the cooling fluid. The concentration of oxygen contained in the nitrogen gas is preferably 100 ppm or less. Such high-purity nitrogen gas can be produced by a cryogenic air separation method or a pressure swing adsorption method (PSA (Prescure Swing Adsorption)). The cooling fluid is not limited to nitrogen gas, but may be other inert gas.

As the supply source 210, a manufacturing apparatus that manufactures nitrogen gas by the above method may be disposed, or a storage apparatus that stores the nitrogen gas manufactured by the manufacturing apparatus may be disposed.

In the present embodiment, three supply paths are provided. The first supply path includes a pressure reducing valve 220 and a gate valve 230, and is connected to a branch point 281 between the gate valve 231 and the gate valve 234 in the circulation path. The second supply path includes a gate valve 235 and a check valve 243 and is connected between the cooler 252 and the pressure reducing valve 221 in the circulation path. The third supply path has a gate valve 236 and a check valve 244, and is connected to the buffer tank 261 in the circulation path.

By connecting the supply path to the upstream side of the cooling body in the circulation path, high purity nitrogen gas can be introduced into the cooling body before being contaminated. However, the connection position and the number of supply paths are not limited to the above, and any supply path that can supply cooling fluid into the circulation path may be used.

Furthermore, a pressure measuring device 271 that measures the pressure in the suction portion of the compressor 270 is provided in the circulation path. As will be described later, based on the measurement result of the pressure measuring device 271, the operation of the compressor 270 is controlled so that the pressure in the suction portion of the compressor 270 is higher than the external air pressure outside the circulation path.

Also, the circulation path is connected to a discharge path that discharges the cooling fluid from the circulation path. The cooling fluid is discharged from the discharge path when the internal pressure of the pipe in the circulation path becomes equal to or higher than a predetermined pressure, or when the inside of the circulation path is opened to the atmosphere by maintenance or the like.

The discharge path includes a discharge port 290 and a piping system that connects the discharge port 290 and the circulation path. The piping system is provided with a pressure release valve having a check function, and is configured to be able to discharge the cooling fluid when the pressure of the cooling fluid exceeds a predetermined value.

In the present embodiment, three discharge paths are provided. The first discharge path has a gate valve 232 and is connected to a branch point 282 between the cooler 250 and the gate valve 233 in the circulation path. The second discharge path has a pressure release valve 240 and is connected to a buffer tank 260 in the circulation path. The third supply path has a pressure release valve 241 and is connected to a buffer tank 261 in the circulation path.

Hereinafter, the operation and control of the cooling fluid circulation mechanism will be described.
At each point in the circulation path, the pressure, flow rate (mass), and temperature of the cooling fluid change. Regarding the flow rate, it is ideal from the viewpoint of reducing the amount of nitrogen gas used that the cooling fluid sealed in the circulation path continues to circulate without increasing or decreasing in the path. However, it is actually difficult to completely suppress the leakage of the cooling fluid from the circulation path.

A small amount of leakage from pipe connections is assumed. Especially when graphite is used as the cooling body, leakage occurs because graphite itself is a porous body, and also in the connection between members made of graphite. Leakage is likely to occur.

In such a leak location, if the pressure in the circulation path is higher than the outside, the cooling fluid will flow out of the circulation path. If the pressure in the circulation path is lower than the outside, the outside air will circulate. It will flow into the route.

When the outside air flows into the circulation path, oxygen in the atmosphere is mixed into the circulation path, so that the oxygen concentration in the nitrogen gas in the circulation path increases, leading to exhaustion of the cooling body. Therefore, it is preferable that the pressure in the circulation path is higher than the external air pressure even at a location where the pressure is lowest. However, if the pressure in the circulation path is too high, the amount of cooling fluid that leaks from the leaked portion increases, which is not preferable because the use efficiency of nitrogen gas decreases.

Therefore, it is preferable to set the pressure target value in the circulation path so that the lowest pressure point does not fall below the external pressure even when the pressure fluctuates. Therefore, it is necessary to design in consideration of the control followability of the compressor 270, disturbance, and the pressure control variation determined by the capacity of the cooling fluid in the circulation path.

Generally, since the variation in pressure control is about ± 0.005 MPa, it is preferable to set the pressure target value at the lowest pressure location to 0.005 MPa as a gauge pressure. In the circulation path shown in FIG. 2, the suction portion of the compressor 270 is the lowest pressure location. Therefore, it is preferable to control the operation of the compressor 270 so that the gauge pressure at the suction portion of the compressor 270 is in the range of (0.005 ± 0.005) MPa.

As described above, the pressure measuring device 271 is provided in the suction portion of the compressor 270. The measurement result of the pressure measuring device 271 is sent to a control unit (not shown). The control unit controls the operation of the compressor 270 based on the measurement result of the pressure measuring device 271.

For example, by controlling the operation of the drive motor of the compressor 270 using an inverter based on a signal from the control unit, fine operation control can be performed and energy loss in the compressor 270 can be suppressed.

Note that the pressure on the discharge side of the compressor 270 is determined by the supply pressure of the cooling fluid to the cooling body. The supply pressure of the cooling fluid to the cooling body is determined in consideration of the following circumstances.

The cooling capacity of the cooling body that determines the amount of silicon solidified in the cooling body is proportional to the flow rate (mass) of the cooling fluid per unit time and the heat exchange rate in the immersion part 120 of the cooling body. From the viewpoint of improving the production volume of purified silicon, it is preferable to increase both the flow rate and the heat exchange rate. However, if the flow rate of the cooling fluid is significantly increased, the supply source 210 and piping are increased in size and the equipment cost is increased. This increases the manufacturing cost of purified silicon.

Meanwhile, in order to increase the heat exchange rate in the immersion part 120, the surface area of the inner wall of the immersion part 120 is increased to increase the contact area with the cooling fluid, or a plurality of openings for spraying the cooling fluid to the inner wall of the immersion part 120 are provided. A method of increasing the spray speed of the cooling fluid by reducing the diameter can be employed.

In any of the above methods, the flow resistance (pipe resistance) of the cooling fluid flowing through the cooling body is increased, and the pressure loss of the cooling fluid occurs. By considering the pressure loss of the cooling fluid and the optimum flow rate of the cooling fluid, the supply pressure of the cooling fluid to the cooling body is determined.

As the compressor 270, a compressor having the ability to pressurize to the supply pressure determined above may be selected, but in order to perform stable operation, the ability to pressurize to a pressure higher than the supply pressure. What it has is preferable.

As described above, when the operation control of the compressor 270 is performed based on the measurement result of the pressure measuring device 271 provided in the suction portion of the compressor 270, the discharge side of the compressor 270, that is, the cooling fluid supply side to the cooling body The pressure may fluctuate.

When the supply pressure of the cooling fluid to the cooling body fluctuates, the inflow amount of the cooling fluid to the cooling body fluctuates, and the cooling capacity of the cooling body changes accordingly. If the cooling capacity of the cooling body changes, it is not preferable because the purified silicon deposition conditions are not stable.

In the present embodiment, since the buffer tank 261 is provided on the discharge side of the compressor 270, the pressurization pressure of the compressor 270 is set higher than the supply pressure of the cooling fluid to the cooling body and discharged from the compressor 270. By storing the cooled cooling fluid in the buffer tank 261, the cooling fluid whose pressure has been reduced to the supply pressure can be supplied to the cooling body.

In order to compensate for the decrease due to the cooling fluid leakage described above, it is necessary to inject additional nitrogen gas into the circulation path. When the additional nitrogen gas is injected, a larger amount of nitrogen gas than the amount actually leaked may be injected.

Nitrogen gas injected more in the circulation path becomes obvious as a pressure increase at any location in the circulation path. In the present embodiment, the pressure release valve 240 connected to the buffer tank 260 and the pressure release valve 241 connected to the buffer tank 261 are set to open at a predetermined pressure.

Therefore, when the pressure in the piping in the circulation path rises and the pressure in the buffer tank 260 or the buffer tank 261 becomes higher than a predetermined pressure, the pressure release valve 240 or the pressure release valve 241 opens and the circulation path By releasing the cooling fluid inside, the pressure in the piping in the circulation path can be maintained at a predetermined pressure.

The following effects can be obtained by injecting a larger amount of nitrogen gas than the amount actually leaked.

Even when the minimum pressure location in the circulation path is maintained at a positive pressure as described above, the pressure in the pipe may fluctuate abruptly and outside air may enter the circulation path. When outside air is mixed, the outside air in the circulation path can be diluted as the amount of nitrogen gas to be injected is increased.

The purity of the cooling fluid in the circulation path can be kept high by injecting the additional cooling fluid while circulating the cooling fluid in the circulation path. As a result, it is possible to suppress the cooling body from being exposed to oxygen and being consumed.

It should be noted that the situation in which outside air is mixed in the circulation path may occur other than in the case of the control malfunction as described above. For example, when the cooling body is taken out of the sealed container 110 for inspection or maintenance, the inside of the cooling body is opened to the outside air.

In this embodiment, two gate valves 233 and 234 are provided so as to sandwich the cooling body in the circulation path, and only the section including the cooling body sandwiched between the two gate valves 233 and 234 in the circulation path. It can be opened to the atmosphere.

With this configuration, it is possible to prevent outside air from entering the section on the compressor 270 side by closing the two gate valves 233 and 234 before the start of inspection or maintenance.

In the present embodiment, in the section including the cooling body in the circulation path, the first supply path is connected upstream of the cooling body, and the first discharge path is downstream of the cooling body. By being connected, the inside of the section opened to the air atmosphere can be replaced with a cooling fluid. Therefore, as shown by a dotted arrow 40 in FIG. 2, the section including the cooling body can be purged with nitrogen gas.

Specifically, after the inspection or maintenance is completed, reconnect the cooling body to the piping. At this time, the two gate valves 233 and 234 remain closed. In this state, by opening the gate valves 230 and 232, the inside of the cooling body can be replaced with nitrogen gas.

After the replacement, the circulation path can be formed again by closing the gate valves 230 and 232 and opening the gate valves 233 and 234.

Note that the distance between the branch point 281 and the gate valve 234 and the distance between the branch point 282 and the gate valve 233 are short in order to prevent outside air from entering the circulation path during this inspection or maintenance. Is preferred. This is because both sections are not easily replaced with nitrogen gas because one end is closed by the gate valve 234 or the gate valve 233 when purging with nitrogen gas.

Therefore, one gate valve 234 of the two gate valves is positioned at the connection portion between the first supply path and the circulation route, and two ports at the connection portion between the first discharge path and the circulation route. Of the gate valves, the other gate valve 233 is preferably located, and both gate valves 233 and 234 are preferably three-way valves.

The time for purging with nitrogen gas may be determined by the volume of the purged section and the flow rate of the nitrogen gas, but is at least longer than the time necessary for nitrogen gas to flow over the volume of the purged section. Furthermore, in consideration of the substitution rate, it is preferable to set the time for the nitrogen gas to flow about several times the volume of the purged section.

Examples of the present invention and comparative examples will be described below.
(Example)
In the examples, the silicon was purified using the silicon purification apparatus provided with the silicon purification mechanism shown in FIG. 1 and the cooling fluid circulation mechanism shown in FIG.

In the silicon refining mechanism, a heat insulating material 111 having a thickness of 100 mm made of a carbon fiber molding material was provided on the inner periphery of a sealed container 110 made of SUS. Furthermore, a resistance heater made of graphite CIP material was disposed inside. A cylindrical crucible 114 made of graphite CIP material and having an outer diameter of 630 mm was installed at the center of the sealed container 110. The crucible 114 can hold 400 kg of molten silicon 10.

Moreover, it was made possible to immerse in the molten silicon 10 while rotating a graphite cooling body from directly above the crucible 114. Nitrogen gas is circulated in the cooling body as a cooling medium. Ar was introduced into the sealed container 110, and the inside of the sealed container 110 was set to a normal pressure atmosphere.

In the above-described silicon purification mechanism, 400 kg of molten silicon 10 was formed by melting in a crucible 114 using silicon containing Fe as an impurity at a concentration of 100 ppmw as a raw material.

A cooling body was immersed in the molten silicon 10 to deposit about 15 kg of purified silicon on the peripheral surface of the immersion part 120. Thereafter, the cooling body was pulled up from the molten silicon 10, and the purified silicon was recovered by crushing the peeled silicon from the immersion part 120.

The process from the immersion of the cooling body to the recovery of the purified silicon is referred to as an immersion recovery process. Then, the immersion collection process was repeated 12 times.

When the molten silicon in the crucible 114 was reduced to about 220 kg, the raw silicon was charged into the about 180 kg crucible 114 and melted, so that the molten silicon 10 in the crucible 114 was again set to 400 kg, and the immersion collection process was repeated. .

FIG. 3 is a graph showing changes in the temperature, flow rate and pressure of the cooling fluid at each point of the cooling fluid circulation mechanism in the example. In FIG. 3, the vertical axis indicates the temperature, flow rate and pressure of the cooling fluid, and the horizontal axis indicates the position in the system shown in FIG.

As shown in FIGS. 2 and 3, in the cooling fluid circulation mechanism, the nitrogen gas flowing out from the cooling pipe delivery pipe 132 to the outflow pipe 150 is about 400 ° C. The cool heat is removed by the cooler 250.

The cooler 250 has a structure in which nitrogen gas flows through the copper pipe and cooling water flows through the outside of the copper pipe. By using water that is shared throughout the silicon purification process as the cooling water to be used, capital investment for newly adding cooling water supply equipment can be suppressed. The cooling water may be one whose water temperature is controlled to about room temperature, and water used when operating the silicon melting apparatus is used.

The nitrogen gas that has passed through the cooler 250 is cooled to about 60 ° C. and then further cooled by the cooler 251. The structure of the cooler 251 is the same structure as the cooler 250, but has a chilling unit (not shown) that cools the cooling water to be used to about 7 ° C.

The nitrogen gas that has passed through the cooler 251 and has been cooled to about 20 ° C. is then pressurized by the compressor 270 after passing through the buffer tank 260 having a capacity of 0.5 m ³ .

The compressor 270 is controlled based on the pressure signal transmitted from the pressure measuring device 271 that measures the pressure in the suction unit of the compressor 270 to the control unit. Specifically, the rotation of the inverter control motor that drives the compressor was PID controlled so that the target value was 0.005 MPa in gauge pressure and the pressure signal was in the range of 0.005 ± 0.005 MPa in gauge pressure. .

In the present embodiment, the pressure of the suction portion of the compressor 270 is controlled, but the buffer tank 260, the coolers 251 and 250, and the outflow pipe 150 located upstream from the suction portion of the compressor 270. The pipe conductance is designed to be sufficiently small. The differential pressure during that time was about 0.001 MPa. The pressure in the vicinity of the connection portion between the delivery pipe 132 and the outflow pipe 150 having the highest possibility of leakage of the cooling fluid was maintained near +0.006 MPa as a gauge pressure.

The pressure release valve 240 connected to the buffer tank 260 was set to open when the internal pressure of the buffer tank 260 exceeded 0.01 MPa.

The capacity of the compressor 270 was set such that the discharge pressure was 0.6 MPa or more and 0.69 MPa or less, and the maximum discharge capacity was 7 m ³ / min.

The nitrogen gas compressed by the compressor 270 was stored in a buffer tank 261 having a capacity of 0.5 m ³ , and the nitrogen gas was set to be released from the pressure release valve 241 when the internal pressure of the buffer tank 261 became 0.69 MPa or more. Further, nitrogen gas was continuously supplied to the buffer tank 261 from the third supply path at a flow rate of 0.6 m ³ / min. As the nitrogen gas, high-purity nitrogen gas having an oxygen concentration of 1 volppm or less produced by a cryogenic air separation device was supplied at a pressure of 0.75 MPa.

A cooler 252 having the same structure as the cooler 251 was provided between the buffer tank 261 and the cooling body. The cooling water introduced into the cooler 252 was supplied from a chilling unit shared with the cooler 251.

The nitrogen gas that has passed through the cooler 252 has a temperature of about 25 ° C., and is then depressurized to 0.45 MPa by the pressure reducing valve 221 and flows into the cooling body.

Inside the piping part 130 and the immersion part 120 of the cooling body, the supply pressure of nitrogen gas was 0.45 MPa, and the flow rate was about 6 m ³ / min (standard state conversion).

The nitrogen gas that has passed through the cooling body flows out from the outflow pipe 150 and returns to the cooler 250 again. In this way, a circulation path is configured.

Further, when the flow rate of the nitrogen gas was measured in the vicinity of the connection portion between the delivery pipe 132 and the outflow pipe 150, it was 5.8 m ³ / min (standard state conversion), and 0.2 m ³ / min (standard state) from the cooling body. About nitrogen gas leaked out of the circulation path.

The flow rate of nitrogen gas in the entire circulation path is adjusted by constantly supplying nitrogen gas to the buffer tank 261 at a flow rate of 0.5 m ³ / min, while leaking from the cooling body at a flow rate of 0.2 m ³ / min, Is released from the pressure release valves 240 and 241 of the buffer tanks 260 and 261 at a flow rate of 0.3 m ³ / min.

Nitrogen introduced into the cooling body is nitrogen gas additionally supplied at 10% of the total flow rate of 6m ³ / min, 0.6m ³ / min, and 90% of 5.2m ³ / min circulates. Nitrogen gas used. Therefore, even if the nitrogen gas is contaminated in any of the circulation paths, the impurities can be diluted with the added high-purity nitrogen gas.

When the oxygen gas concentration in the circulation path was continuously monitored in a sampling path (not shown) branched from the buffer tank 261, an increase in oxygen concentration that was considered to be mixed with outside air was observed once every few days. An oxygen concentration of 500 volppm was confirmed. However, once the oxygen concentration increased, it immediately decreased and reached 100 volppm, which is a level at which oxidation of graphite does not become a problem, in about 20 minutes.

Under the above conditions, the silicon apparatus was operated for one month. The cooling body was inspected and maintained once / day. When the operation was resumed after completion of the maintenance, a purge with nitrogen gas was performed at a flow rate of 2 m ³ / min for 1 minute.

In the operation of the silicon purification apparatus for one month, the amount of nitrogen gas used was about 35000 m ³ . Moreover, although the consumption amount of graphite was measured, the consumption amount was 1 mm or less, and it was not confirmed.

Hereinafter, a comparative example will be described.
(Comparative Example 1)
In Comparative Example 1, silicon was purified by the same silicon purification mechanism as in the example. However, the cooling fluid circulation mechanism according to the present invention was not used, and nitrogen gas was passed as shown by a dotted arrow 40 shown in FIG. Washed away. That is, the cooling fluid was not circulated at all, and nitrogen gas was used for cooling only once. As a result, the amount of nitrogen gas used in the operation of the silicon refining apparatus for one month was about 160000 m ³ .

As can be seen from the above Examples and Comparative Example 1, it is confirmed that by providing the cooling fluid circulation mechanism 200 as in the silicon purification apparatus of the present invention, it is possible to circulate and use the nitrogen gas and to reduce the purification cost. It was.

The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

10 molten silicon, 11 solidified silicon, 100 silicon purification mechanism, 110 sealed container, 111 heat insulating material, 113 heating section, 114 crucible, 120 immersion section, 130 piping section, 131 introduction pipe, 131a tip section, 132 delivery pipe, 140 inflow Pipe, 150 outflow pipe, 200 cooling fluid circulation mechanism, 210 supply source, 220, 221 pressure reducing valve, 230, 231, 232, 233, 234, 235, 236 gate valve, 240, 241 pressure release valve, 243, 244 check Valve, 250, 251, 252 cooler, 260, first buffer tank, 261, second buffer tank, 270 compressor, 281, 282 branch point, 290 outlet.

Claims (9)

A crucible (114) for holding molten silicon (10);
A heating section (113) for heating silicon held in the crucible (114);
A cooling body supported so as to be immersed in the molten silicon (10) in a state in which a cooling fluid flows therein;
A cooling unit that regenerates the cooling fluid so that it can flow again through the cooling body by cooling the cooling fluid that has absorbed heat by flowing through the cooling body;
A compressor (270) for compressing the cooling fluid;
The silicon purifier, wherein the cooling body, the cooling unit, and the compressor (270) are connected to each other to constitute a circulation path of the cooling fluid. The cooling unit is cooled by the first cooler (250) that takes away a part of the endothermic amount of the cooling fluid that has absorbed heat by flowing through the cooling body, and the first cooler (250). A second cooler (251) for cooling the cooling fluid before flowing into the compressor, and a third cooler (252) for cooling the cooling fluid discharged from the compressor,
The cooling body, the first cooler (250), the second cooler (251), the compressor, and the third cooler (252) are connected in order in the circulation path. The silicon purification apparatus as described. A pressure measuring device (271) for measuring the pressure in the suction section of the compressor (270);
Based on the measurement result of the pressure measuring device (271), the operation of the compressor (270) is controlled so that the pressure in the suction portion of the compressor (270) is higher than the outside air pressure outside the circulation path. The silicon purifier according to claim 1. The silicon according to claim 3, wherein operation of the compressor (270) is controlled so that a gauge pressure in the suction portion of the compressor (270) is in a range of (0.005 ± 0.005) MPa. Purification equipment. The silicon purifier according to claim 1, further comprising a buffer tank (260, 261) for suppressing pressure fluctuation of the cooling fluid adjacent to the compressor (270) in the circulation path. The silicon purification apparatus according to claim 1, wherein the circulation path is connected to a supply path for supplying the cooling fluid from outside the circulation path, and a discharge path for discharging the cooling fluid from within the circulation path. Two gate valves (233, 234) are further provided so as to sandwich the cooling body in the circulation path,
The silicon purification apparatus according to claim 6, wherein only a section including the cooling body sandwiched between the two gate valves (233, 234) in the circulation path can be opened to an air atmosphere. In the section of the circulation path, the section opened to the air atmosphere by connecting the supply path upstream from the cooling body and connecting the discharge path downstream from the cooling body. The silicon purifier according to claim 7, wherein the inside of the silicon purifier can be replaced with the cooling fluid. One gate valve (234) of the two gate valves is located at a connection portion (281) between the supply path and the circulation path, and a connection portion (282 between the discharge path and the circulation path). ), The other gate valve (233) of the two gate valves is located, and both the gate valves are three-way valves.

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