WO2010119502A1

WO2010119502A1 - Method for purifying silicon metal

Info

Publication number: WO2010119502A1
Application number: PCT/JP2009/057478
Authority: WO
Inventors: 素行山田
Original assignee: 信越化学工業株式会社
Priority date: 2009-04-14
Filing date: 2009-04-14
Publication date: 2010-10-21

Abstract

Provided is a method for purifying silicon metal that comprises (1) a process for obtaining silicon metal by reducing silicon oxide, (2) a process for removing impurities by gas refining of the silicon metal, (3) a process for dissolving and unidirectionally solidifying the silicon metal from which said impurities have been removed, (4) a process for removing portions of low purity from the unidirectionally solidified silicon metal, and (5) a process for melting the silicon metal, from which said low purity portions have been removed, with an electron beam under reduced pressure to vaporize and remove the impurities. It is possible to obtain highly purified silicon metal from which metallic element impurities such as Fe, certainly, as well as non-metal impurities such as B and P are removed and refined out efficiently and inexpensively with a short process. The silicon metal obtained can be suitably used for solar cells.

Description

Method for purifying metallic silicon

The present invention relates to a method for purifying metal silicon, which removes impurities from metal silicon in a short process and at low cost, thereby purifying the metal silicon.

As fossil energy that emits CO ₂ promotes global warming, various alternatives to fossil energy have been proposed and put into practical use. Among them, because solar power generation is created by energy distributed evenly on the earth, it is possible to extract energy even with relatively small equipment, and because of its long history of practical application, The amount of power generation is increasing year by year.

There are various methods for solar power generation, and among them, a solar battery in which battery cells are formed using a silicon wafer is the most popular solar power generation method. The concentration of the impurity component of silicon used in this solar cell silicon wafer is not required to be as high as that of semiconductor silicon. That is, the required purity of silicon for semiconductors is preferably as low as possible, and the purity of silicon is 99.999999999%, whereas silicon for solar cells is required 99.9999%. The

Conventionally, in order to achieve the impurity level of silicon for solar cells, in addition to a 99.99999999% purity product for semiconductors, the raw material is discarded as impurities concentrated or foreign matter adhered products in the semiconductor silicon manufacturing process. Grades that have been reprocessed and purified have been used.

As described above, since silicon for solar cells is derived from silicon for semiconductors or derivatives thereof, the amount of circulation thereof is affected by the semiconductor industry, and is unable to meet the demand for silicon for solar cells. It was. For this reason, it has been studied to improve the purity of metal silicon having an industrially sufficient production amount and use it as silicon for solar cells.

Furthermore, recently, high-purity silicon is used, and it can be used as a negative electrode active material for Li-ion batteries and high polishing speed can be achieved during polishing. On the basis of this, the development of new applications is also planned by using the various physical, chemical and thermodynamic properties of silicon to form new forms that have never existed before.

By the way, the main impurities of metallic silicon are metallic elements such as iron, aluminum, calcium and titanium, and nonmetallic elements such as boron and phosphorus which act as dopants when silicon is used as a semiconductor material. Among these, the solidification distribution coefficient with silicon is very small, so if you use the solidification segregation phenomenon, you can obtain a portion with a low metal impurity component concentration from the initial solidification phase to the middle phase, and acquire only that portion. Then, silicon with a low metal impurity concentration can be obtained.

As this method, for example, JP-A-1-56312 (Patent Document 1) proposes a method for improving the purity of silicon by solid-directional solidification after filtering solid impurities in molten silicon. Japanese Patent Laid-Open No. 63-45112 (Patent Document 2) improves the purity of silicon by immersing a hollow rotating cooling body in molten silicon and crystallizing silicon on the outer peripheral surface of the cooling body. There is a proposed method. It is clear that this method is a form of unidirectional solidification in that silicon is deposited in the direction centering on the cooling body and the purity of silicon can be improved.

As described above, performing unidirectional solidification in order to improve the purity of silicon is a commonly practiced technique.

However, since boron and phosphorus act as dopant substances in silicon, the concentration should be controlled when silicon is used for solar cells. However, boron and phosphorus have solidification distribution coefficients of 0.8 and 0.35, respectively, which are close to 1 as compared with the distribution coefficients of the above metal elements. For this reason, since impurity concentration using the solidification segregation phenomenon hardly segregates, various methods have been proposed to remove boron and phosphorus by methods other than solidification segregation.

Regarding this, Japanese Patent Application Laid-Open No. 4-193706 (Patent Document 3) contains silicon containing impurity elements such as B, C, P, Fe, Al, etc., and silica having gas blowing tuyere at the bottom as the main components. The amount of impurities can be reduced by blowing Ar or H ₂ or a mixed gas thereof, oxidizing gas or HCl gas from the tuyere and adding SiO ₂ , CaO, CaCl ₂ or CaF _2. A method of reducing is proposed.

In JP-A-5-262512 (Patent Document 4), by irradiating a molten plasma surface with a thermal plasma gas added with water vapor and hydrogen chloride gas, and adding sodium chloride to the plasma gas, boron, A method for reducing the concentration of iron and aluminum is disclosed.

In JP-A-7-309614 (Patent Document 5), the furnace pressure during electron beam melting is maintained at 1 × 10 ⁻⁴ to 5 × 10 ⁻³ Torr, and Ar gas is sprayed onto the surface of molten silicon. Thus, a method for reducing the concentration of phosphorus, aluminum, calcium and the like has been proposed.

Further, Japanese Patent Laid-Open No. 6-345416 (Patent Document 6) discloses that when silicon is dissolved and purified by an electron beam, an oxide such as SiO ₂ or CaO is added as an oxygen source, or H ₂ O, O A method of removing C and B by adding an oxidizing gas such as ₂ to the bath has been proposed.

However, although these methods disclose techniques for removing metal components, boron, and phosphorus, metal silicon cannot be used as silicon for solar cells only by performing each technique separately. By combining a plurality of technical elements, silicon for solar cells can be obtained.

Thus, the method for removing impurities in the metal silicon differs depending on the type of element to be removed. Technology for improving the purity of metallic silicon has been particularly studied for silicon purification for solar cells, and a method for producing silicon for solar cells from metallic silicon has been established by a combination of several technologies.

This is described in Japanese Patent No. 3325900 (Patent Document 7). Metallic silicon is melted under vacuum, and phosphorus contained therein is vaporized and dephosphorized, and then solidified to remove impurity components from the molten metal to obtain an ingot. B. The impurity-enriched portion of the ingot is cut and removed. C. The remaining part after cutting and removing is redissolved, and boron and carbon are oxidized and removed from the molten metal in an oxidizing atmosphere. Subsequently, argon gas or a mixed gas of argon and hydrogen is blown into the molten metal for deoxygenation. D. The molten metal after the deoxidation is cast into a mold and solidified in one direction to obtain an ingot. E. Cut and remove the impurity-enriched part of the ingot obtained by unidirectional solidification. Alternatively, molten metal silicon obtained by refining silicon oxide is transferred to a pan, solidified after oxidizing and removing boron and carbon in an oxidizing atmosphere, and subsequently subjected to the B, C, D, and E steps to obtain metal. A method for obtaining polycrystalline silicon for solar cells from silicon is disclosed.

Japanese Patent Laid-Open No. 2000-327488 (Patent Document 8) discloses that volatile metal is obtained by pulverizing metal silicon and using acid-treated high-purity metal silicon having a purity of 4N or more as a raw material and dissolving it with an electron beam under vacuum. Impurities are evaporated and removed as oxides, and then the crude purified raw material, in which silicon is ingot by solidification rough purification to cut and remove the concentrated portion of heavy metal impurities, is plasma-dissolved in an oxidizing atmosphere to convert nonvolatile impurities into oxides Evaporate and remove as Furthermore, a method for producing silicon for solar cells by ingoting silicon by solidification finishing purification and cutting and removing the heavy metal impurity concentration portion again has been proposed.

However, for example, the method of Japanese Patent No. 3325900 (Patent Document 7) requires many steps and complicated processing. In addition, the method disclosed in Japanese Patent Application Laid-Open No. 2000-327488 (Patent Document 8) requires a long process and requires pulverization of silicon, acid treatment, pH adjustment, etc., and the process is complicated and can be avoided. It includes even the desired process. As described above, the above prior art is not satisfactory, and further improvement has been demanded.

JP-A-1-56312 JP-A-63-45112 Japanese Patent Laid-Open No. 4-193706 JP-A-5-262512 Japanese Patent Laid-Open No. 7-309614 JP-A-6-345416 Japanese Patent No. 3325900 JP 2000-327488 A

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for refining metallic silicon which is a short process and low cost.

As a result of intensive studies to achieve the above object, the present inventor has obtained metal silicon by reducing silicon oxide, and then gas blowing the metal silicon to obtain Fe, Ti, Al, Ca, etc. The impurity metal element and impurities such as B and P are removed. This is further solidified in one direction to remove impurity metal elements such as Fe in particular, and then melted by an electron beam under reduced pressure to vaporize and remove impurities such as B and P in particular, thereby reducing the cost and cost. In this way, it has been found that impurities other than metal components such as B and P as well as metal components such as Fe in metal silicon can be removed to obtain highly purified metal silicon, and the present invention has been achieved. It was.

That is, the present invention provides the following method for purifying metallic silicon.
<1> (1) Step of obtaining metallic silicon by reducing silicon oxide,
(2) A process of removing impurities by gas blowing the metal silicon,
(3) A step of melting and unidirectionally solidifying metallic silicon from which the impurities have been removed
(4) a step of removing a low-purity portion from unidirectionally solidified metal silicon, and (5) a step of vaporizing and removing impurities by melting the metal silicon from which the low-purity portion has been removed under reduced pressure. A method for purifying metallic silicon, wherein
<2> The method for purifying metal silicon according to <1>, wherein the concentration of iron in the metal silicon obtained in the silicon oxide reduction step is 300 to 800 ppm.
<3> The method for purifying metal silicon according to <1> or <2>, wherein the concentration of boron in the metal silicon obtained in the silicon oxide reduction step is 20 ppm or less.
<4> The method for purifying metal silicon according to <1>, <2>, or <3>, wherein the blowing gas used in the gas blowing treatment step includes at least chlorine gas and water vapor.
<5> In the gas blowing treatment step, one or two or more salts selected from CaO, SiO ₂ , CaF ₂ , Al ₂ O ₃ , Na ₂ O and MgO are added as molten flux to the metal silicon. <1> to <4> The method for purifying metal silicon according to any one of <4>.
<6> The method for purifying metal silicon according to any one of <1> to <5>, wherein the concentration of boron in the metal silicon obtained in the gas blowing treatment step is 5 ppm or less.

According to the present invention, it is possible to obtain highly purified metal silicon that is efficiently removed in a short process and purified at low cost, in addition to impurity metal elements such as Fe, as well as impurities other than metal components such as B and P. The metallic silicon obtained by the present invention can be suitably used for solar cells.

It is the schematic which shows an example of the furnace used at the blowing process of this invention. It is the schematic which shows an example of the furnace used with the unidirectional solidification process of this invention. It is the schematic which shows an example of the chamber used at the electron beam irradiation process of this invention.

In the method for purifying metal silicon according to the present invention, metal silicon obtained by reducing silicon oxide is subjected to gas blowing treatment, and further, the metal silicon is unidirectionally solidified and then dissolved by electron beam under reduced pressure to remove impurities. To do.

The production process of metallic silicon, which is the first step of the present invention, involves heating and reacting in a submerged arc furnace a raw material mixture comprising silica (silicon oxide) as a silicon source and carbonaceous material (charcoal) as a reducing agent. In this case, the temperature in the furnace is preferably 1600 ° C. or higher, more preferably 1700 ° C. or higher. This is because the higher the temperature is, the more stable the reduction reaction in the submerged arc furnace is, so that the inclusion of solid inclusions in silicon such as silicon carbide as a reaction material or reaction intermediate decreases. The metallic silicon produced in this process contains impurities such as Fe, Ti, Al, Ca, B, and P.

Among these impurities, metal components such as Fe, Ti, Al, and Ca are impurities contained in a relatively large amount in metallic silicon, but the content concentration of B and P that are dopants in silicon is as follows. Although it is low, when silicon is used as a semiconductor, such as a solar cell, it must be present in silicon at a controlled and appropriate concentration.

As a method for reducing the concentration of impurities present in the metal silicon, the above-described method has already been proposed. However, reducing the impurity concentration of the metal silicon itself discharged from the submerged arc furnace can improve the purity in a later step. This is desirable because it can reduce the load from the beginning.

Specifically, it is effective to reduce the impurity concentration in the raw material of metallic silicon. To explain this, the impurity component in metal silicon, which is usually the highest concentration impurity, is Fe, but it is manufactured by heating and reacting silica as a silicon source and carbonaceous material as a reducing agent in a submerged arc furnace. In general, silicon called metallic silicon contains an iron component of about 1500 ppm to 5000 ppm. The main origin of this iron component is that the iron component present in silica, which is a silicon source, charcoal, which is a charcoal, or the like, is transferred to silicon generated by a reaction in the furnace. Therefore, metal silica with as little impurity metal content as possible can be obtained by selecting raw materials for silica and carbon materials, which are raw materials for metal silicon, while paying attention to the amount of impurities contained therein. For example, high purity can be achieved to some extent by using high-purity quartz instead of silica, chemically synthesized silicon dioxide, or charcoal from which ash has been removed by acid treatment.

However, in such a method, since the raw material cost is increased, the unit cost of the metal silicon itself is increased. Therefore, in the silicon purification method of the present invention, It is preferable not to use metallic silicon using high-purity and expensive raw materials that make the iron concentration less than 300 ppm. The concentration of iron contained in the metal silicon obtained in the process for producing metal silicon of the present invention is preferably 300 to 800 ppm, particularly preferably 300 to 650 ppm, and the concentrations of Ti and Al, which are the main metal impurity components next to iron. However, it is preferable that Ti is 800 ppm or less, particularly 100 to 600 ppm, particularly 100 to 300 ppm, and Al is 100 to 500 ppm, particularly 100 to 300 ppm. The impurity concentration can be measured by the ICP-AES method (manufactured by Perkin Elmer).

Boron and phosphorus, which are dopant components, are also present as components in the raw material of metal silicon, similar to the metal impurity component, and move into the metal silicon during the reduction reaction of silicon dioxide in the submerged arc furnace. Among these dopant substances, boron is particularly a substance that has a high affinity with silicon and is difficult to remove from the silicon. Therefore, it is desirable that the concentration of metal silicon after the manufacturing process is low. Boron in metallic silicon produced in this submerged arc furnace is a metal with a small amount of boron impurities by selecting raw materials while paying attention to the amount of impurities contained in silica and carbonaceous materials, which are raw materials, as in the case of metallic impurity components. Silicon will be obtained. The concentration of boron in the metal silicon is desirably 20 ppm or less, particularly 10 ppm or less, particularly 1 to 5 ppm, in order to reduce the process load of boron removal performed in a subsequent process. Similarly, the phosphorus concentration is preferably 40 ppm or less, particularly 30 ppm or less, particularly 5 to 20 ppm.

In the present invention, the starting silica and charcoal contain a certain amount of metal impurity components such as Fe and impurities such as B and P so that the impurity concentration of the metal silicon obtained in the first step is in the above range. It is preferable to select and use one appropriately.

Metallic silicon is usually produced by flowing out of a submerged arc furnace and solidifying in a vessel. However, molten silicon flowing out of the submerged arc furnace is continuously refined in the second step of the present invention to produce metallic silicon. Impurities such as Al, Ca, B are removed. This process oxidizes the above impurities by blowing an oxidizing gas such as water vapor, oxygen, carbon dioxide, hydrogen chloride, chlorine, or an inert gas such as nitrogen or argon into the molten metal in the state of molten metal silicon. This is a gas blowing process to be removed.

In the removal of impurities by this process, the level of reduction in impurities is insufficient as silicon for solar cells, but it is possible to reduce the load related to high purity during processing in the subsequent process, and a large amount of silicon at a time. In addition, it is possible to treat the process directly to the submerged arc furnace as necessary, and to treat the molten silicon as it is. is there.

The gas blowing process of the present invention uses, for example, an electric furnace 3 having a furnace core tube 2 having a crucible 1 inside as shown in FIG. This can be done by supplying gas to the molten silicon. The gas used in the blowing process is, for example, a gas having a composition such as water vapor 10 to 50 vol%, chlorine 10 to 50 vol% and inert gas 20 to 80 vol%, and a flow rate of 0.4 to 5 m / second, particularly It is preferable to blow into the melt at 0.7 to 3 m / sec for 10 to 200 minutes, particularly 20 to 120 minutes. The treatment temperature is preferably 1450 to 1800 ° C, more preferably 1550 to 1650 ° C. This is because the higher the molten metal temperature is, the more effective the removal of impurities is, but energy is consumed to maintain the molten metal temperature. Also, if the flow rate is too small, the molten metal is not sufficiently stirred, and the contact between the gas and silicon may not be satisfactorily performed. As a result, the removal of impurities may be insufficient. As a result, the molten metal may scatter and the silicon recovery rate may decrease. In order to remove B in silicon quickly and sufficiently, it is preferable that chlorine gas and water vapor are contained in the gas for the blowing process.

In this processing, processing by flux can be performed in parallel. This flux treatment is a method of increasing the purity of silicon by allowing impurities in silicon to move into the flux by allowing molten silicon and molten salt to coexist in the same container. A flux such as CaO, SiO ₂ , CaF ₂ , Al ₂ O ₃ , Na ₂ O, MgO, or a mixture thereof can be selected according to ease of handling, melting point, effect, and the like.

The flux is in a molten state at a temperature equal to or higher than the melting point temperature (1450 ° C.) of the molten silicon. However, it is necessary to prepare for keeping the molten silicon melted even when the flux is brought into contact with silicon, such as by preheating the flux in advance or taking a means capable of electrically heating the ladle. Also in this case, the temperature of the molten silicon is preferably within the above-described range.

The gas blowing process is performed in a state where the melt flux coexists as necessary, and among them, if steam and chlorine are used as the gas and silicon dioxide and calcium oxide are used as the flux, This is a preferable form because boron can be effectively removed. In this case, the use ratio of water vapor and chlorine is preferably 1: 5 to 5: 1 by volume, more preferably 1: 2 to 2: 1. The amount of silicon dioxide and calcium oxide to be added as a flux is preferably 58:42 to 70:30 by mass ratio, and particularly preferably 60:40 to 65:35 by mass ratio. Further, the total amount is preferably 5 to 70% by mass, particularly 10 to 40% by mass, based on the metal silicon. If the amount of flux is too small, the amount of boron in the silicon will not be transferred to the flux, and as a result, boron removal may not be performed sufficiently. In some cases, the initial investment and processing cost may be expensive. Performing this gas blowing process a plurality of times is more effective for removing Al, Ca, B, and the like. However, multiple removals increase the number of processes and increase costs, so it is preferable to perform the removal within three times in order to balance the effect.

Boron is removed in the gas blowing step, but the efficiency of boron removal in this step is not sufficient for the target purity, and it is necessary to further improve the purity of boron in a subsequent step. In order to reduce the load of boron removal in the subsequent process, the boron concentration in the silicon after gas blowing is desirably 5 ppm or less, particularly 1 to 0.1 ppm.

Next, the silicon is subsequently subjected to unidirectional solidification. In general, unidirectional solidification is a method in which molten silicon is solidified by moving in one direction in a furnace, and impurities components are discharged to the liquid phase side to purify the solidified silicon. Although this method requires time to dissolve silicon once, since most metal components have a very small distribution coefficient with silicon, there is an advantage that high-purity silicon can be obtained from the initial stage to the middle stage of solidification of silicon.

To explain this, the solidification segregation phenomenon at the solid phase-liquid phase interface of the impurity metal element component in silicon is determined by the solidification distribution coefficient. The smaller the solidification distribution coefficient, the more the impurity metal component is in the solid phase. More is moved to the liquid phase side without being captured. Due to the behavior of the impurity metal, the impurity concentration of the solid phase is lowered. Many impurity metals in silicon have a very small solidification distribution coefficient. For example, the solidification distribution coefficient of iron which is present in the largest amount as an impurity component in metallic silicon is at most 8 × 10 ⁻⁶ . Therefore, the iron concentration in the solid silicon at the start of solidification is low, and the iron concentration in the solid silicon gradually increases from the middle to the end of solidification.

If a portion having a desired iron concentration is selected from the cast ingot using this solidification segregation phenomenon, silicon having a low iron concentration can be obtained. For impurity metal elements other than iron, impurities are removed by solidification simultaneously with the removal of iron, and as a result, metal impurities are removed in the range from the initial stage of solidification to the middle stage, and a solidified ingot in which impurities are concentrated at the end of solidification. Therefore, high-purity silicon can be obtained by cutting out the impurity-concentrated portion.

For example, an electric furnace 12 having a crucible 11 as shown in FIG. 2 can be used in the unidirectional solidification process of the present invention. The electric furnace 12 is heated by energizing the heater 13, and silicon is melted by raising the temperature to 1450 ° C. or higher of the melting point of silicon. The molten silicon is preferably held in a molten state for a certain time so that the melt has a uniform composition. The higher the temperature at this time, the more homogenized the molten silicon is promoted. Therefore, the temperature is preferably 20 ° C. or more, particularly 50 to 100 ° C. higher than the melting temperature.

The crucible containing the molten silicon having a uniform composition can be moved down the furnace by lowering the pulling shaft 14 and solidified by cooling from the bottom of the crucible. The pulling speed is preferably 1 to 50 mm / hr, more preferably 5 to 20 mm / hr. If it is too slow, it takes too much coagulation treatment time, resulting in a decrease in productivity. If it is too fast, the segregation of impurities is not sufficient, and the amount of highly purified parts obtained may be reduced.

The heat-resistant material such as silicon dioxide, alumina, and graphite is used for the crucible material used in the unidirectional solidification process. Silicon melts and solidifies in this material, but the crucible material and silicon are fixed during solidification, and the crucible material or silicon may be damaged due to the difference in thermal expansion coefficient between the crucible material and silicon when the temperature is lowered. In order to prevent this, a mold release material is applied to the inner wall of the crucible. This release material is required not to adhere to silicon, and silicon nitride is usually selected. As a mold release agent, a silicon nitride powder having a particle size of about 1 μm or less is mixed with a liquid such as water or an additive to make a slurry, which is applied to a crucible and dried several times. .

Since the molten silicon at the time of dissolution and solidification comes into contact with this release agent, if an impurity component is present in the release agent, the component may be transferred to silicon. Therefore, it is desirable to use a silicon nitride powder chemically synthesized from a high-purity raw material, for example, silicon nitride powder synthesized from silicon tetrachloride and ammonia, as the release agent, particularly silicon nitride.

This silicon includes B and P as elements that have not yet been removed as impurities. This element is removed in the next step, an electron beam process.

As the equipment used in the electron beam process, for example, as shown in FIG. 3, a vacuum chamber 22 equipped with a container (water-cooled hearth / crucible) 21 containing silicon from which metal impurity components have been removed by unidirectional solidification, An electron beam gun 23 installed to irradiate an electron beam toward silicon in the container, and a chamber for making the chamber a high vacuum of about 1 × 10 ⁻⁴ to 1 × 10 ⁻² Pa. A vacuum evacuation facility (not shown) is the main facility.

For removing B, P, etc., the chamber is first evacuated to a high vacuum of about 1 × 10 ⁻⁴ to 1 × 10 ⁻² Pa, and then irradiated with an electron beam from an electron beam gun to dissolve silicon. In this case, silicon is accommodated in a container, but a water-cooled copper crucible is preferable as a container to be used from the viewpoint of energization of an electron beam and prevention of contamination from the material.

When melting silicon, in order to dissolve the silicon in the container over a wide range, a beam scan method that irradiates the entire surface of silicon, or a beam scan method that irradiates a part of silicon and raises the silicon temperature of a specific part to a particularly high temperature. Etc. can be appropriately selected. As for the irradiation time, if it is a short time of less than 20 minutes, a sufficient impurity removal effect may not be exhibited, and if it is a long time exceeding 4 hours, the amount of silicon evaporation may increase. The time is preferably 20 minutes to 4 hours, more preferably 30 minutes to 2 hours.

B and P, which are dopant components in silicon dissolved by an electron beam, are removed in the chamber. First, since P has a higher vapor pressure at a higher temperature than silicon, P is removed by positive vaporization in a high-temperature environment (about 1600-2200 ° C) by electron beam irradiation in a high-vacuum environment of the chamber atmosphere. Is done. On the other hand, since B cannot be sufficiently removed only by the P removal atmosphere, B is oxidized or hydrogenated in an electron beam atmosphere by passing a small amount of water vapor, oxygen, etc. in the chamber. Oxides or oxyhydrides are removed by vaporization.

In this case, the electron beam gun is premised on a high vacuum. However, in order to maintain the degree of vacuum in the gun, if the exhaust in the gun and between the gun and the chamber are sufficiently performed, the pressure in the chamber is about 1 Pa. Since it is possible to irradiate an electron beam up to the above, it can be achieved by venting the above gas together with a single substance or a mixed gas, or an inert gas such as nitrogen or argon so as to maintain this degree of vacuum. .

In this way, the removal of B and P can be achieved in one pot with the same equipment. However, in this B removal, if the concentration of B in silicon is high, a long time is required for the removal of B, and the minimum at the time of removal Since the concentration cannot be removed to the concentration required for silicon for semiconductor applications, the B concentration in silicon used for the electron beam treatment is preferably 10 ppm or less, desirably 5 ppm or less, and more desirably 1 ppm or less. Since normal metal silicon has a concentration higher than the concentration of B, it is preferable to adjust and reduce the B concentration in the metal silicon by the gas blowing treatment described above.

The metal silicon is highly purified by performing the above process on the metal silicon. This high-purity silicon is reworked into a form suitable for its purpose. For example, as silicon for solar cells, it is sliced after being ingot by a regular method to become a wafer for solar cells, and if it is used as an active material for batteries, it is appropriately processed for batteries. Then, after being pulverized to an appropriate size, it is mixed with a binder and used.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples.

[Example 1]
1. Manufacturing process of metallic silicon ○ Manufacturing of MG-1 900 ppm of iron in terms of Fe ₂ O ₃ , silica stone containing 510 ppm of titanium in terms of TiO ₂ , 1% by mass of iron in terms of Fe ₂ O _{3 in} ash, and TiO Charcoal containing 0.7% by mass of titanium in terms of ₂ was charged into a submerged arc furnace to produce metallic silicon. The concentrations of iron and titanium in the metal silicon were 800 ppm and 750 ppm, respectively.

2. Blowing process <Preparation of mixed molten salt>
The SiO ₂ powder and CaO powder having a mass ratio of 3: 2 were sufficiently mixed, placed in a graphite crucible having an internal size of 160 mmφ × 250 L, and heated to 1500 ° C. to dissolve. After cooling after dissolution, it was cut into an appropriate size with a diamond cutter.

<Blowing treatment>
Into a graphite crucible 1 having an inner size of 160 mmφ × 250 L as shown in FIG. 1, 2 kg of metal silicon MG-12 and 1330 g of the mixed molten salt were placed. This crucible was placed in a furnace core tube of an electric furnace 3 in which a quartz glass furnace core tube 2 was installed in a vertical shape, and the position was adjusted so that the crucible became the highest temperature part.

The electric furnace was energized, the furnace temperature was set to 1540 ° C., and silicon and flux in the furnace were melted.
The alumina thin tube 4 is installed so that gas can be supplied into the quartz glass furnace core tube 2 from the outside, and gas is supplied from the thin tube 4 to the melt 5 in the crucible. The composition of the gas was Ar 50% by volume, H ₂ O 25 vol%, and ₂ 25% by volume Cl. After this gas aeration was continued for 2 hours and blowing was performed, the temperature of the gas was decreased after the total amount of Ar was reached.

3. Unidirectional solidification step 10 kg of blown silicon after multiple (twice) treatments is placed in a quartz crucible 11 having an inner dimension of 190 squares × 300 D (mm) shown in FIG. Set inside. After the electric furnace 12 was evacuated, Ar was introduced and returned to atmospheric pressure, and Ar was continuously introduced. The electric furnace 12 was energized at a set temperature of 1500 ° C. and left at that temperature for 2 hours after the furnace temperature reached 1500 ° C. Thereafter, the crucible 11 was lowered at 5 mm / hr by lowering the pulling shaft 14 to solidify the entire amount of Si.

When each part of the obtained Si ingot was analyzed by ICP-AES (manufactured by Perkin Elmer Co., Ltd.), metal impurities were concentrated on the top of the ingot as shown in Table 1. The top, middle top, middle, low middle and bottom of the ingot are sampled from the ranges of 5 to 15 mm, 30 to 40 mm, 55 to 65 mm, 80 to 90 mm, and 105 to 115 mm when the top of the ingot is 0 mm. Indicates.

4). Step of removing low-purity portion of metallic silicon It can be seen from Table 1 that the metal impurity element component can be removed by cutting and removing the top portion of the ingot.

5). Electron Beam Process An apparatus used for the electron beam process is shown in FIG. A water-cooled copper hearth 21 is installed in the vacuum chamber 22. The copper hearth has an internal dimension of 400 L × 140 W × 100 D (mm), and silicon after one method solidification step is appropriately reduced in size. In addition, a maximum output 300 kW electron beam gun 23 is installed downward from the upper part of the vacuum chamber.

The vacuum chamber 22 is decompressed to 3 × 10 ⁻³ Pa by an evacuation apparatus, and then irradiated with an electron beam from an electron beam gun 23 at an output of 45 kW to dissolve the silicon, and then continue to be irradiated with the electron beam for 30 minutes. The same state was maintained. After that, by introducing water vapor into the chamber while continuing the electron beam irradiation, the degree of vacuum is set to 5 × 10 ⁻¹ Pa, and after maintaining the molten state while the electron beam is irradiated for 30 minutes, the electron beam irradiation is performed. Was stopped to solidify the silicon.

The silicon sampled in each of the above steps was analyzed with ICP-AES (manufactured by Perkin Elmer). The results are shown in Table 2, and each impurity element was sufficiently reduced.

[Example 2]
1. Manufacturing process of metallic silicon ○ Manufacturing of MG-2 150 ppm of iron in terms of Fe ₂ O ₃ , silica stone containing 140 ppm of titanium in terms of TiO ₂ , 1% by mass of iron in terms of Fe ₂ O _{3 in} ash, and TiO Charcoal containing 0.2% by mass of titanium in terms of ₂ was charged into a submerged arc furnace to produce metallic silicon. The concentrations of iron and titanium in the metal silicon were 700 ppm and 160 ppm, respectively.

Next, impurities in the metal silicon were removed by the same process as in Example 1. In the unidirectional solidification, the pulling-down speed was 10 mm / hr, and in the electron beam treatment, the treatment time in a water vapor atmosphere was 15 minutes. Table 3 shows the analysis results of silicon in each step.

As shown in Table 3, it can be seen that when the concentration of the metallic silicon or the impurity element after the process is low, a sufficient impurity removal effect is exhibited even if the refining conditions are relaxed.

Claims

(1) A step of obtaining metallic silicon by reducing silicon oxide,
(2) A process of removing impurities by gas blowing the metal silicon,
(3) A step of melting and unidirectionally solidifying metallic silicon from which the impurities have been removed
(4) a step of removing a low-purity portion from unidirectionally solidified metal silicon, and (5) a step of vaporizing and removing impurities by melting the metal silicon from which the low-purity portion has been removed under reduced pressure. A method for purifying metallic silicon, wherein
The method for purifying metal silicon according to claim 1, wherein the concentration of iron in the metal silicon obtained in the silicon oxide reduction step is 300 to 800 ppm.
The method for purifying metal silicon according to claim 1 or 2, wherein the concentration of boron in the metal silicon obtained in the silicon oxide reduction step is 20 ppm or less.
4. The method for purifying metal silicon according to claim 1, 2 or 3, wherein the blowing gas used in the gas blowing treatment step contains at least chlorine gas and water vapor.
In the gas blowing process, one or more salts selected from CaO, SiO 2 , CaF 2 , Al 2 O 3 , Na 2 O and MgO are added to metallic silicon as a molten flux. Item 5. The method for purifying metal silicon according to any one of Items 1 to 4.
The method for purifying metal silicon according to any one of claims 1 to 5, wherein the concentration of boron in the metal silicon obtained in the gas blowing treatment step is 5 ppm or less.