JP2013144623A - Method for refining silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device for refining silicon, which can easily obtain high purity silicon without forming high melting point insoluble matter such as silicon carbide even when using silicon separated from silicon-containing waste liquid containing carbon as impurities.SOLUTION: A device for refining silicon includes: a crucible for holding molten silicon; a heater for heating the crucible; a silicon charging part for charging silicon separated from silicon-containing waste liquid into the molten silicon; a carbon concentration calculation part for calculating the carbon concentration of the molten silicon in the crucible; and a control part for controlling the addition amount of the silicon to the molten silicon according to the carbon concentration of the molten silicon calculated by a carbon concentration calculation device.

Description

  The present invention relates to an apparatus for purifying silicon and a method for purifying silicon, which are raw materials for solar cells. The present invention particularly relates to a silicon purification apparatus and a silicon purification method for silicon separated from a silicon-containing waste liquid.

  Conventionally, as a silicon material as a raw material for a silicon wafer for solar cells, polysilicon or a cut end material obtained when a cylindrical ingot is formed into a prismatic column is used. These have very few impurities and are sufficiently pure as materials for solar cell grade silicon wafers.

  On the other hand, in recent years, the production amount of solar cells has increased worldwide, and the demand for silicon as a material has also increased. Among them, cutting waste liquid generated by the process of slicing the ingot for manufacturing the wafer contains about half of the silicon as cutting waste in terms of weight ratio and is disposed of as it is. There is a need for a reuse method. However, a lot of foreign matters are mixed in these silicon scraps at the time of cutting, and the silicon itself is oxidized to silicon oxide, so that the purity is remarkably lowered. Therefore, for example, a method as disclosed in Patent Document 1 has been proposed as means for reusing these silicon scraps. The processing method will be described below with reference to the drawings.

  As shown in FIG. 4, a conventional silicon refining device 101 melts a powder 102 containing silicon powder containing silicon oxide, which is separated from waste liquid generated when silicon is machined, to form a molten metal 103. A heating furnace 104, a heating means 105 for heating the heating container 104, and a melting furnace 107 having a hot water discharging means 106 for discharging a silicon melt contained in the molten metal 103, and a low-viscosity silicon melt contained in the molten metal 103 are accommodated and cooled. And a residual container 109 for storing a residual part containing silicon oxide having a high viscosity that adheres to the inner wall or bottom of the heating container 104. In this conventional silicon refining apparatus 101, the silicon powder is heated to a temperature not lower than the melting point of silicon and not higher than 2000 ° C. in a heating container 104 under a reduced pressure or an inert gas atmosphere by separating the silicon powder. The melting step to melt the molten metal 103 and the high viscosity silicon oxide adhering to the inner wall or bottom of the heating vessel 104 contained in the molten metal 103 are left in the heating vessel 104, and the low viscosity silicon melt is discharged into the cooling vessel 108. By passing through the hot water discharge step, high-purity silicon can be obtained by discharging a low-viscosity silicon melt from the heating container 104 to the cooling container 108.

JP 2010-47443 A

  Various impurities are mixed in the silicon-containing waste liquid generated in silicon machining such as an ingot slicing process. In particular, carbon is often mixed, and for example, a processing liquid used as a cooling liquid or a lubricant during processing contains organic carbon. In addition, a carbon plate is mainly used as a base for fixing the ingot, and these carbons are mixed to cut into the base when slicing. Further, diamond and silicon carbide are used for slicing together with metal wires as abrasive grains called fixed abrasive grains or loose abrasive grains, and a lot of these inorganic carbons are also mixed. The mixing amount of these organic carbon and inorganic carbon reaches several atom% to 10 atom% as a solid content in the waste liquid, and occupies most of impurities.

  As seen in Patent Document 1, the melting of silicon is mostly performed in a reduced pressure atmosphere or an inert gas atmosphere. Therefore, carbon (especially inorganic carbon) in silicon remains without being gasified by oxidation, reacts with molten silicon at the time of melting to form insoluble matter as silicon carbide SiC, and remains in the silicon crystal. There was a problem of ending up. In addition, in order to prevent the formation of these insoluble materials, washing with an organic solvent or an acid is performed, or the temperature at the time of melting is set to a temperature much higher than the melting point of silicon, but the process is complicated. Thus, there are problems such as an increase in processing cost.

  The present invention solves the above-described conventional problems, and even when silicon separated from silicon-containing waste liquid containing carbon as an impurity is used as a raw material, high purity without forming insoluble matter such as silicon carbide. An object of the present invention is to provide a silicon purification apparatus and a silicon purification method capable of easily obtaining silicon.

In order to achieve the above object, a silicon purification apparatus according to the present invention comprises a crucible for holding a silicon melt,
A heater for heating the crucible;
A silicon charging unit for charging silicon separated from the silicon-containing waste liquid into the silicon melt;
A carbon concentration calculation unit for calculating the carbon concentration of the silicon melt in the crucible;
A control unit that controls the amount of silicon added to the molten silicon according to the carbon concentration of the molten silicon calculated by the carbon concentration calculating device;
It is characterized by providing. As a result, the intended purpose can be achieved.

Moreover, the silicon purification method according to the present invention is a silicon purification method for purifying silicon in molten silicon,
Melting silicon to form silicon melt;
Intermittently adding silicon separated from the silicon-containing waste liquid to the molten silicon;
Calculating the carbon concentration of the molten silicon;
Including
The amount of silicon added to the silicon melt is controlled in accordance with the carbon concentration of the silicon melt. As a result, the intended purpose can be achieved.

  As described above, according to the silicon purification apparatus and the silicon purification method according to the present invention, even if carbon separated from the silicon-containing waste liquid contains carbon as an impurity, it does not form insoluble matter such as silicon carbide SiC. As a result, high-purity polycrystalline silicon can be obtained at a low cost as compared with conventional purification methods.

It is a schematic sectional drawing which shows the structure of the silicon | silicone purification apparatus in Embodiment 1 of this invention. It is a process flow figure of the silicon purification method in Embodiment 1 of the present invention. It is a schematic sectional drawing which shows the structure of the silicon refinement | purification apparatus in Embodiment 2 of this invention. It is a figure which shows the conventional silicon | silicone purification method described in patent document 1. FIG.

  Hereinafter, a silicon purification apparatus and a silicon purification method according to an embodiment of the present invention will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view showing the configuration of the silicon purification apparatus according to Embodiment 1 of the present invention.
In FIG. 1, the silicon refining apparatus according to Embodiment 1 of the present invention has a configuration in which a crucible 3 is heated by a heater 2 provided in a refining furnace 1. The silicon put in the crucible 3 is melted by being heated by the heater 2 in an inert gas atmosphere such as argon or nitrogen supplied from the gas introduction unit 9 to become a silicon melt 4. In addition, this silicon refining apparatus has a configuration in which silicon particles 5 separated and recovered from silicon-containing waste liquid generated by machining or the like are intermittently added from the inlet 6 into the molten silicon 4, and control linked with the inlet 6. The amount of silicon grains 5 can be controlled by the portion 8. Here, the input amount (also referred to as addition amount) may be the input amount per time. The control unit 8 is interlocked with the gas measuring device 7 and monitors the concentration of carbon monoxide generated from the interface of the molten silicon 4, and the silicon concentration of the silicon particles 5 is determined by the carbon concentration calculated from the value of the carbon monoxide concentration. The input amount is controlled. The gas in the furnace 1 is discharged from the exhaust port 10 to prevent an increase in gas concentration.

  Heater 2 is suitable for furnace material, heat resistance, and furnace gas atmosphere, such as heating using a resistor such as molybdenum disilicide, molybdenum, tungsten, or carbon, or induction heating using a high frequency power source. You can choose.

  As the crucible 3, it is desirable to select a crucible 3 that contains as little impurities as possible from the crucible 3 itself. For example, by selecting the crucible 3 mainly composed of silicon such as silica or quartz, impurities into the molten silicon are selected. Can be prevented.

  The silicon used as the material of the molten silicon 4 is desirably different from the silicon grains 5 to be charged later, but it is desirable to select a high-purity so-called semiconductor or solar cell material. If polysilicon or the like that is often used as a raw material is used, the material cost increases, and the total processing cost increases. In the present invention, it is possible to use a material that is available at a relatively low cost, such as metal silicon that is a raw material of polysilicon, or a scrap material generated by processing an ingot. The initial filling amount of silicon may be increased or decreased in accordance with the size of the crucible 3 or the carbon concentration of the silicon grains 5, but the filling is generally performed so that the volume of the crucible 3 is about 30% to 50% at the time of melting. It is preferable.

  The melting point of silicon is about 1410 ° C., and silicon can be melted by setting the temperature in the crucible 3 to a temperature higher than that, for example, 1450 ° C. to 1600 ° C. If it is 1450 ° C. or lower, there is a high possibility that impurities-containing silicon grains are not sufficiently melted, and if it is 1600 ° C. or higher, the durability of the crucible 3 is hindered and the processing cost is increased. The temperature is preferably within the above temperature range.

  As the silicon particles 5 separated from the silicon-containing waste liquid, for example, those obtained by separating water from the silicon-containing waste liquid by silicon machining or the like are used. Various methods can be used to separate the water. For example, a method such as centrifugal separation or a filter press is relatively easy, and it is preferable because the processing amount can be increased. Moisture still remains in the silicon thus separated, and it is more preferable to dry it to remove the moisture. At this time, it becomes easy to arrange the silicon particles 5 having a size suitable for addition to the crucible 3 by being packed in a mold or formed into a plate shape and dried. In addition, although the silicon | silicone isolate | separated from the silicon-containing waste liquid is set as "silicon grain" here, the shape is not necessarily restricted to a granular form. Needless to say, the silicon 5 may be, for example, a powder or a solid having a larger size.

Next, a method for measuring the carbon concentration in the molten silicon 4 in the crucible 3 will be described. The carbon present in the silicon melt 4 reacts with oxygen present in the silicon melt 4 to vaporize as carbon monoxide and is degassed from the liquid interface of the silicon melt 4. The chemical reaction formula in this case is shown as follows.

At this time, when the concentrations of the carbon C and oxygen O dissolved in the silicon melt 4 and the gasified carbon monoxide CO in the refining furnace 1 are constant, they are said to be in a chemical equilibrium state. An expression representing the equilibrium constant in this case is shown below.

  K on the left side of the above equation 1 represents an equilibrium constant, and takes a certain value peculiar to the chemical reaction at a certain temperature. [C], [O], and [CO] on the right side respectively represent the substance concentrations of carbon, oxygen, and gasified carbon monoxide dissolved in the molten silicon 4. Further, since the equilibrium constant K is expressed as a function of temperature based on thermodynamic data, it can be obtained by calculation if the temperature in the refining furnace 1 is constant. On the other hand, the supply source of oxygen dissolved in the silicon melt 4 is mainly the elution from the oxide film on the silicon surface of the raw material and quartz used as the crucible 3, and in particular, the silicon melt 4 accompanying the dissolution of quartz. There is a lot of oxygen dissolved in. That is, new oxygen is always dissolved in the silicon melt 4, and the amount is close to the upper limit of the solubility of oxygen in the silicon melt 4 (for example, about 40 ppma (parts per million atomic) near the melting point of silicon). Thus, it can be considered to be almost constant depending on the temperature. Therefore, since the oxygen concentration [O] can be set to a constant value depending on the temperature in the above equation 1, the carbon concentration of the silicon melt 4 can be calculated in the above equation 1. As described above, the concentration [CO] of carbon monoxide in the furnace 1 is measured by the gas measuring device 7 to indirectly use the oxygen concentration [O] corresponding to the temperature in the silicon melt 4. The carbon concentration [C] can be grasped.

  The saturation concentration of carbon in the molten silicon 4 is said to be approximately 91 ppma around 1410 ° C., which is the melting point of silicon. When more carbon is present, silicon in the silicon melt 4 reacts with carbon to form silicon carbide SiC. Once silicon carbide is formed, silicon carbide decomposes at a very high temperature (it does not become liquid at atmospheric pressure, but decomposes into Si and graphite), so that it remains insoluble in silicon melt 4. . In order to suppress this, it is important not to raise the carbon concentration in the silicon melt 4 to a saturation concentration or higher, and by controlling the carbon concentration to 91 ppma or less which is the saturation concentration at 1410 ° C. which is the silicon melting temperature, It is possible to reliably prevent the formation of silicon carbide. Therefore, in the silicon purification apparatus and the silicon purification method according to the first embodiment, grasping the carbon concentration in the molten silicon 4 is an effective means for preventing the formation of silicon carbide.

  The carbon concentration in the molten silicon 4 can be reduced by gasification if the holding time in the molten state is lengthened. However, the reaction rate decreases as the concentration decreases, and the efficiency deteriorates. The standard of the carbon concentration of the ingot used for the wafer for solar cells is said to be about 1 ppma, and if the carbon concentration in the silicon melt 4 in the present invention is also reduced to 1 ppma, it is sufficiently applicable as a material for solar cells. it can. In fact, when the molten silicon 4 solidifies, a segregation phenomenon occurs at the interface between the liquid portion and the solid portion, and carbon moves from the solidified silicon to the liquid silicon melt 4 that has not yet solidified. The crystal refining method using this phenomenon is called unidirectional solidification and has been used for a long time as a crystal growth technique for removing metal impurities in the molten silicon 4, but carbon can also be removed by this principle. Therefore, for example, even if the carbon concentration in the molten silicon 4 is about 10 ppma, it is possible to reduce the carbon concentration in the solid silicon to 1 ppma or less by solidifying a part using the segregation phenomenon. That is, if the carbon concentration in the molten silicon 4 is suppressed to 91 ppma or less, there is no problem even if the addition speed of the silicon grains 5 is increased, but it is necessary to lower the carbon concentration to the range of 1 ppma to 10 ppma at the solidifying stage. There is. When the carbon concentration in the molten silicon 4 is about 10 ppma, the carbon concentration of the solidified silicon can be suppressed to 1 ppma or less by solidifying a part as described above. Alternatively, when the carbon concentration in the molten silicon 4 exceeds 10 ppma, carbon monoxide is generated by the reaction between carbon and oxygen in the molten silicon 4, and the degassing causes the carbon concentration to be reduced to 1 ppma to 10 ppma. You have to wait until you reach the range. Therefore, it is only necessary to hold the molten state after the addition is stopped and wait for the carbon concentration in the molten silicon 4 to decrease. Alternatively, solidification can be started immediately by controlling the amount of silicon grains 5 added so that the carbon concentration always falls within the range of 1 to 10 ppma.

  As long as the gas measuring device 7 is a means capable of continuously measuring the gas concentration, there is no difference in the effect no matter what means is used. In addition, when the inside of the refining furnace 1 is a reduced pressure atmosphere, it is necessary to devise gas sampling. In addition, a gas chromatograph, an FT-IR gas analyzer, and the like are more preferable considering that measurement can be performed in a wide concentration range.

Next, the silicon purification method of the present invention will be described using a flowchart. FIG. 2 shows a flowchart.
(1) First, the crucible 3 containing high-purity silicon is set in the furnace (S101).
(2) Next, the heater 2 is operated (S102).
(3) The silicon in the crucible 3 is heated to the melting point or higher and melted to generate the molten silicon 4 (S103). Although the temperature at this time can be increased or decreased depending on the impurity concentration of the silicon grains 5 to be added, it may be maintained within the temperature range of 1450 ° C. to 1600 ° C. Further, depending on the carbon concentration of the molten silicon 4, when it is desired to promote gasification to carbon monoxide, the temperature may be increased, and the temperature may be decreased when the carbon concentration decreases.

  Moreover, what is necessary is just to set as the weight of the silicon melt 4 of an initial stage according to the carbon concentration and addition amount of the silicon grain 5 to process. As the addition amount of the silicon grains 5, for example, the addition amount per hour is determined depending on the amount to be treated per day, but an addition amount of 1 g / second or more is preferable in view of productivity. More preferably, the addition amount is 10 g / sec. However, if the addition amount is excessively increased, the carbon concentration in the silicon melt 4 increases, and silicon carbide is formed when the saturation concentration is 91 ppma or more. Therefore, in consideration of productivity, even if silicon grains 5 are added, the initial concentration of silicon melt 4 is determined from the carbon concentration of silicon grains 5 and the amount of silicon grains 5 added so that the carbon concentration of silicon melt 4 does not exceed 91 ppma. What is necessary is just to set a weight. For example, if the carbon concentration of the silicon grains 5 is 1 atom%, if the addition amount is 1 g / second, 1 kg of molten silicon of 1 kg is used as the initial weight, so that a rapid increase in carbon concentration does not occur. It is unlikely to exceed the saturation concentration of 91 ppma. Considering this dilution factor, if the amount of addition is to be increased to 10 g / second, the 10 kg silicon melt 4 may be used as the initial weight. On the other hand, the carbon concentration of the silicon grains 5 may reach 10 atom% depending on circumstances. In that case, the silicon grain 5 can be added up to 10 g / sec by setting the initial weight of the molten silicon 4 to 100 kg. However, since increasing the initial weight of the molten silicon 4 further adversely affects productivity and processing costs, the amount of silicon particles 5 added is preferably 10 g / second or less. By making the addition amount of the silicon grains 5 below the upper limit (10 g / second) of the above addition quantity, even if the silicon grains 5 containing about 10 atom% of carbon as impurities are added, production is possible. The silicon grains 5 can be continuously added without impairing the properties.

(4) Next, the gas measuring device 7 is operated (S104).
(5) Silicon particles 5 are added to the molten silicon 4 (S105).
(6) Carbon monoxide generated by the reaction between carbon contained in the silicon grains 5 and oxygen in the silicon melt 4 is measured (S106). From the measurement of the carbon concentration, the control period for adding the silicon grains 5 may be, for example, once every 0.5 to 1 minute. As described above, the addition amount of the silicon grains 5 is 1 g to 10 g / second. By setting the amount of addition and the period of concentration control within the above ranges, silicon can be purified efficiently and continuously without forming silicon carbide.
(7) Next, the carbon concentration in the molten silicon 4 is calculated from the measured value of carbon monoxide according to Equation 1 (S107).
(8) It is determined whether or not the carbon concentration of the molten silicon 4 is 1 ppma or less. If the carbon concentration is 1 ppma or less, the addition speed of the silicon grains 5 is increased (S108), and the carbon concentration is calculated again at that time (S106). -S107) If the carbon concentration is still 1 ppma or less, feedback control is performed by the control unit 8 so as to further increase the addition speed.

(9) On the other hand, if it is determined that the carbon concentration exceeds 1 ppma, then the rate of change of the carbon concentration per time is measured (S109). If the change rate of the carbon concentration increases, the addition speed of the silicon grains 5 is decreased (S110). At that time, the change rate of the carbon concentration is measured again (S109). If the change rate still increases, feedback control is performed by the control unit 8 so as to further reduce the addition speed (S110). By this operation, the addition amount of the silicon grains 5 is controlled so that the rate of change of the carbon concentration is substantially constant, and is maintained in that state. The carbon concentration in the molten silicon 4 at this time is preferably in the range of 1 to 10 ppma. By doing so, the increase in the carbon concentration due to the addition of the silicon grains 5 and the decrease in the carbon concentration due to the generation of carbon monoxide balance, and the silicon grains 5 can be continuously added. In addition, while the addition amount of the silicon grains 5 is kept constant (S111), the change rate of the carbon concentration is sequentially measured (S109), and if there is a change, the addition amount of the silicon grains 5 is decreased accordingly. (S110) or hold (S111) is good.

(10) When the addition of the silicon grains 5 is continued (S111), the volume of the molten silicon 4 in the crucible 3 increases. Therefore, the addition of the silicon grains 5 is stopped (S113) when reaching a certain level (S112). At this time, if the carbon concentration is higher than 10 ppma, the carbon concentration can be lowered by maintaining the molten state for a certain period of time.
(11) Finally, the output of the heater 2 is lowered to cool the molten silicon 4 (S114).
Thus, the purification of silicon is completed.

  According to such a configuration, when silicon grains 5 having a high carbon concentration are added and melted, silicon carbide due to the reaction between silicon and carbon, which is expected when it diffuses into the high-purity molten silicon 4 previously melted, is obtained. Formation can be suppressed. Moreover, the carbon concentration in the silicon melt 4 is grasped by calculating the carbon concentration [C] in the silicon melt 4 by using the above equation 1 to calculate the concentration of carbon monoxide generated from the liquid interface of the silicon melt 4. The addition speed of the silicon grains 5 can be controlled so that the formation of silicon carbide does not occur.

  In the first embodiment, the inside of the furnace 1 is an inert gas atmosphere. However, the inert gas may be flowed in a state where the inside of the furnace is decompressed with a vacuum pump or the like.

(Embodiment 2)
FIG. 3 is a schematic cross-sectional view showing the configuration of the silicon purification apparatus in the second embodiment of the present invention. In FIG. 3, the same components as those in FIG.
In the silicon refining device according to the second embodiment, as shown in FIG. 3, a discharge port 11 is provided at the bottom of the crucible 3, and the solidification crucible 12 is refined by a control mechanism (not shown) for controlling the discharge. The structure which moves the made silicon melt 4 is provided. A plurality of solidification crucibles 12 are arranged, and are continuously carried in from the carry-in port 14 by the conveyor 13 and are carried to the carry-out port 15 when the molten silicon 4 is introduced. A shutter 16 is provided inside the furnace at the carry-in port 14 and the carry-out port 15 so that outside air is not mixed into the furnace during carry-in and carry-out.

(1) When transferring the molten silicon 4 from the discharge port 11 to the crucible 12 for solidification, first, the addition of the silicon particles 5 is stopped, the carbon concentration in the molten silicon 4 is confirmed, and the molten state is maintained if necessary. Reduce the carbon concentration sufficiently.
(2) Next, the discharge port 11 is opened, and the silicon melt 4 is transferred to the crucible 12 for solidification. At this time, the entire amount is not transferred, but the initial amount of the molten silicon 4 previously melted is left and only the remaining molten silicon 4 is transferred.

  According to this configuration, the initial amount of the molten silicon 4 remaining in the crucible 3 can be used again as the high-purity molten silicon 4. By repeating this cycle, silicon can be purified continuously without adding high-purity silicon. In the second embodiment, the silicon melt 4 is taken out from the bottom discharge port 11 as means for transferring the molten silicon 4 to the crucible 12 for solidification, but any means can be used as long as the same effect can be obtained. There is no difference in the effect even when using. For example, the crucible 3 may be tilted, or the crucible 3 may be overflowed from the upper part of the crucible 3 and transferred to the solidification crucible 12.

  The silicon purification apparatus and the silicon purification method according to the present invention have the effect of refining high-purity polycrystalline silicon from silicon containing a large amount of impurities such as carbon, and can be used in various processes such as semiconductor wafer manufacturing processes. It can also be applied to the purification of generated silicon-containing waste liquid.

DESCRIPTION OF SYMBOLS 1 Refinement furnace 2 Heating heater 3 Crucible 4 Silicon molten metal 5 Silicon grain 6 Input port 7 Gas measuring device 8 Control part 9 Gas introduction part 10 Exhaust port 11 Discharge port 12 Solidification crucible 13 Conveyor 14 Carry-in port 15 Carry-out port 16 Shutter 101 Silicon Refining device 102 Powder 103 Molten metal 104 Heating vessel 105 Heating means 106 Heating means 107 Melting furnace 108 Cooling vessel 109 Residual portion vessel

Claims (10)

Hold the molten silicon, crucible,
A heater for heating the crucible;
A silicon charging unit for charging silicon separated from the silicon-containing waste liquid into the silicon melt;
A carbon concentration calculation unit for calculating the carbon concentration of the silicon melt in the crucible;
A control unit that controls the amount of silicon added to the molten silicon according to the carbon concentration of the molten silicon calculated by the carbon concentration calculating device;
A silicon purification apparatus comprising:   2. The silicon refining device according to claim 1, wherein the carbon concentration calculation unit measures a concentration of a carbon-containing gas at an interface of the silicon melt in the crucible and calculates the carbon concentration of the silicon melt.   2. The silicon purification apparatus according to claim 1, wherein the control unit controls the amount of silicon added so that the carbon concentration of the molten silicon is maintained within a range of 1 ppma or more and 91 ppma or less. .   The said control part is a silicon | silicone refinement | purification apparatus of Claim 1 which reduces the addition amount of the said silicon to the said molten silicon, when the change rate of the said carbon concentration of the said molten silicon is increasing.   The silicon refining apparatus according to claim 1, wherein the crucible is provided with a discharge port for transferring at least a part of the molten silicon. A silicon purification method for refining silicon in molten silicon,
Melting silicon to form silicon melt;
Intermittently adding silicon separated from the silicon-containing waste liquid to the molten silicon;
Calculating the carbon concentration of the molten silicon;
Including
A silicon refining method, wherein the amount of silicon added to the molten silicon is controlled in accordance with the carbon concentration of the molten silicon.   The silicon refining method according to claim 6, wherein the silicon addition amount is controlled so that the carbon concentration of the molten silicon is maintained within a range of 1 ppma or more and 91 ppma or less.   The step of measuring the carbon concentration of the silicon melt calculates the carbon concentration of the silicon melt by measuring the carbon-containing gas concentration at the silicon melt interface. Method.   The silicon purification apparatus according to claim 6, wherein when the rate of change in the carbon concentration of the molten silicon is increasing, the amount of silicon added to the molten silicon is reduced.   The method for purifying silicon according to claim 6, further comprising a step of leaving an initial amount of the refined molten silicon and transferring the remaining molten silicon to a crucible for solidification.

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