JP2012171858A - Method for melting recovered silicon waste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for melting recovered silicon waste, by which the recovered silicon waste is heated and melted by only an arc furnace without providing a resistance heating furnace or the like together with the arc furnace, an operation is performed on condition that the voltage applied to the recovered silicon waste is 400 V or less, and thereby, large equipment using a high voltage is not required.SOLUTION: In the method for melting recovered silicon waste, the recovered silicon waste is melted by using an arc furnace. The voltage between electrodes of the arc furnace is held at 40-400 V, and at the same time, an electric current is applied in such a state that the electrodes are brought into contact with a conductive material. Thereby, the silicon waste is heated by arc discharge generated in accordance with the rise of temperature of the conductive material. Then, the method is performed by shift to the normal operation of the arc furnace after the arc discharge and the current between electrodes are stabilized.

Description

  The present invention relates to a method for melting recovered silicon scrap discharged and recovered in the manufacture of silicon wafers, IC chips, etc. using an arc furnace, and in particular, the recovered silicon scrap capable of starting the arc furnace efficiently. It relates to the melting method.

  In the manufacturing process of silicon wafers, IC chips and the like, a large amount of silicon waste is discharged, and most of them are discarded. It is said that the total amount discarded as silicon scrap is larger than the total amount of products such as silicon wafers. Further, a great deal of energy is consumed in the production of silicon wafers and the like. In order to improve such a situation, research and development for reusing silicon scraps are underway.

  In order to reuse silicon scraps, the silicon scraps must first be melted. In Patent Document 1, a mixture containing silicon carbide and silicon grains is melted in an inert gas atmosphere, and silicon carbide is first precipitated. And then recovering the molten silicon and then recovering the molten silicon. In the specification, the melting of the mixture containing silicon carbide and silicon grains is not particularly limited as long as it can melt silicon grains and end material silicon under an inert gas atmosphere, and a resistance heater, a heater, etc. It is described that the heating means can be used.

  In Patent Document 2, in the invention of the silicon purification method, the melting of the silicon powder is not particularly limited as long as it can be melted to a predetermined temperature under reduced pressure or an inert atmosphere, but resistance heater, induction heating, arc melting or plasma melting It is described that a heating method can be used. The silicon powder includes silicon powder discharged and collected from a silicon wafer manufacturing plant made of silicon single crystal or polycrystal, or fixed abrasive grains such as diamond or silicon carbide used when cutting the silicon wafer. It is described that free abrasive grains or silicon powder containing a lubricant can be used.

  Patent Document 3 discloses an invention of a method for producing a silicon polycrystal mainly used in the production of solar cells using an arc discharge method in which arc discharge is performed in a gas atmosphere containing at least oxygen and using silicon waste as a raw material. It is disclosed. In this specification, this silicon polycrystal manufacturing method uses an arc discharge method to generate silicon by a reaction between silicon oxide generated from waste silicon and silicon carbide, and thereby impurities contained in waste silicon. It can be used to produce high-purity silicon, and even if the arc discharge method is used, the silicon production efficiency is high compared to the conventional manufacturing method that produces silicon by the reaction of quartz and carbon Has been.

As described in Patent Documents 1 to 3, various heating methods are used for melting the recovered silicon. On the other hand, as described in Patent Document 3, an arc furnace is conventionally used for producing metallic silicon from silica and carbon. Patent Document 4 describes a method of obtaining metallic silicon by filling an arc furnace with SiO ₂ , carbon or a carbon-containing material, silicon carbide, etc., and supplying SiO ₂ to an arc point.

JP 2007-302513 JP JP 2010-47443 A JP 2010-47443 A JP-A-61-117110

  As described in Cited Documents 1 and 2, various melting methods are used for melting the recovered silicon scrap (recovered silicon scrap), but the arc melting method easily reduces the atmosphere during melting. This is a preferable melting method for melting recovered silicon waste and obtaining regenerated silicon. However, in order to melt silicon powder by the arc melting method, since silicon powder has a resistance equivalent to an insulator, it is necessary to heat and melt silicon powder unless the voltage is very high, for example, a voltage exceeding 1000V. There is a problem that it is difficult and takes a long time to melt.

  On the other hand, in the arc melting method described in the cited document 3 or 4, there is no description about melting the recovered silicon scrap or silica, especially about starting the arc furnace and starting the melting thereof. . This is considered to be because there is no such a problem when the recovered silicon waste is heated and melted in a resistance heating furnace using a resistance heater or the like unlike the arc melting method. That is, it is understood that after the recovered silicon waste is first heated by a resistance heater or the like, the recovered silicon waste is melted or melted by an arc furnace after a certain temperature or higher.

  In view of such a conventional problem, the present invention can heat and melt the recovered silicon scrap only in the arc furnace without providing a resistance heating furnace in the arc furnace, and the voltage applied to the recovered silicon scrap is also 400V. An object of the present invention is to provide a method for melting recovered silicon waste that can be operated in the following and does not require large equipment using high voltage.

  The method for melting recovered silicon scraps according to the present invention is a method for melting recovered silicon scraps in which the recovered silicon scraps are melted by an arc furnace, and the voltage between the electrodes of the arc furnace is maintained at 40 to 400 V. The silicon scrap is heated by the arc discharge that occurs as the conductive material is heated, and the arc discharge and the current between the electrodes are stabilized, and then the operation proceeds to normal arc furnace operation. To be implemented.

  In the above invention, the conductive material may be a powder or a solid material made of carbon and / or silicon carbide.

  Moreover, in the said invention, it is good to melt | dissolve a silicon | silicone waste by throwing a slag first into the furnace bottom of an arc furnace, and throwing a silicon | silicone waste on it. Moreover, it is preferable that the electrode interval of the arc furnace can be adjusted.

  The recovered silicon waste is preferably melted in a reducing atmosphere, whereby regenerated silicon can be efficiently obtained from the recovered silicon waste.

  According to the present invention, recovered silicon waste can be efficiently heated and melted only by an arc furnace without using a resistance heating furnace or the like. In addition, the voltage applied to the recovered silicon waste can be operated at 400 V or less, and the recovered silicon waste is heated and melted without requiring a large facility that uses high voltage, and recovered efficiently in a reducing atmosphere. Regenerated silicon can be obtained from silicon waste.

It is a schematic diagram explaining the melting method of the recovery silicon | silicone waste concerning this invention. It is a schematic diagram explaining the change state of the electric current between electrodes until starting an arc furnace and fuse | melting collection | recovery silicon waste.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram for explaining a method for melting recovered silicon waste according to the present invention. In FIG. 1, in the arc furnace 1, slag 15 is first put into the furnace bottom, and recovered silicon waste 13 is put thereon. The conductive material 10 is placed on the upper surface of the recovered silicon waste 13 or in the recovered silicon waste 13. The electrode 5 is in contact with the conductive material 10.

  In the method for melting recovered silicon waste according to the present invention, first, as shown in FIG. 1, electricity is applied between the electrodes while the electrode 5 is in contact with the conductive material 10. When the conductive material 10 is energized, the conductive material 10 is heated, the resistance value thereof decreases, the current increases, and the conductive material 10 rises in temperature. Then, arc discharge occurs toward the conductive material 10. Although the recovered silicon waste 13 is heated by this arc discharge and begins to melt, the arc discharge is unstable and the interelectrode current is also unstable. However, arc discharge and interelectrode current become stable after a while. Therefore, after the arc discharge and the inter-electrode current are stabilized, the operation of the arc furnace by energizing the electrode 5 in contact with the conductive material 10 is shifted to the normal arc furnace operation.

  FIG. 2 shows how the interelectrode current changes when the recovered silicon scrap melting method is performed. The horizontal axis indicates the time after the arc furnace is operated, and the vertical axis indicates the interelectrode current. As shown in FIG. 2, when energization is started between the electrodes, the current between the electrodes gradually increases. When the interelectrode current increases to some extent and reaches the current value indicated by A in FIG. 2, the interelectrode current rapidly increases. When the current value indicated by B in FIG. 2 is reached, the current becomes oscillated and becomes unstable. For a while, the interelectrode current is unstable (C), but after exiting the vibration state (D), the interelectrode current increases and eventually becomes a constant value (E). Note that the time and current values shown in FIG. 2 are values in the case of the embodiment described below.

  In the present invention, the arc furnace 1 is not particularly limited, but is preferably an AC arc furnace. In the case of an AC arc furnace, it is easy to control the inter-electrode resistance value by introducing the conductive material 10 or the like. The working voltage of the arc furnace 1 is 40 to 400V. The working voltage is preferably increased as the processing amount of the recovered silicon waste 13 per hour increases, but in the present invention, a voltage exceeding 400 V is not required. When the operating voltage is less than 40V, arc discharge becomes difficult.

  The electrode 5 is preferably a carbon electrode. As a result, the inside of the arc furnace when the recovered silicon waste 13 is melted can be easily made into a reducing atmosphere, and the recovery silicon waste 13 can be prevented from being oxidized and reduced.

As the conductive material 10, a material having an electrical resistivity sufficient to heat the recovered silicon waste 13 by energizing the conductive material 10 is used. The conductive material 10 may have an electrical resistivity of 10 ⁻³ to 10 ⁻⁶ Ωm, and for example, carbon powder or a solid material can be used. A carbon-based material is preferable because it prevents the recovered silicon waste 13 from being oxidized and can be used as a reducing material. Moreover, a silicon carbide powder or a solid material can be used.

  The conductive material 10 is used in a state where it is embedded in the upper surface of the recovered silicon scrap 13 or in the recovered silicon scrap 13, and the electrode 5 is brought into contact with this to energize and generate heat. The handling is easy, and a predetermined heat generation performance can be exhibited. Further, the conductive material 10 is preferably one that is difficult to combine with Si contained in the recovered silicon waste 13.

  Since the conductive material 10 is used as a heater member for the recovered silicon waste 13 as described above, when the large arc furnace 1 for processing a large amount of recovered silicon waste 13 is used, the voltage between the electrodes is 40 to 400V. In order to keep the range, the resistance value between the electrodes including the conductive material 10 must fall within a predetermined range. For this reason, the shape or the proportion of the conductive material 10 increases. Since the shape or proportion of the conductive material 10 should be as small as possible, it is preferable to use an arc furnace in which the distance between the electrodes can be adjusted. As a result, the arc furnace is first started in a state where the electrode interval is narrow, and after the arc discharge is stabilized (point E in FIG. 2), the normal preferable electrode interval can be obtained.

  The recovered silicon waste 13 is a powdery material that has been discharged and recovered in the manufacture of silicon wafers, IC chips, and the like. The recovered silicon waste 13 may contain free abrasive grains such as silicon carbide, fixed abrasive grains such as diamond, or organic substances such as water and oil, in addition to silicon powder. A moisture content of about 5 to 10% by mass is easy to handle.

As the slag 15, one containing CaO and SiO ₂ as main components can be used. Thereby, it is possible to prevent the recovered silicon waste 13 from being oxidized and promote the reduction. In addition, the slag 15 can function as a filter for filtering the molten silicon obtained by melting or reducing the recovered silicon waste 13.

  The recovered silicon waste 13 is preferably on the upper surface of the slag 15. As a result, when arc discharge is generated toward the conductive material 10, the recovered silicon waste 13 is quickly melted and the formation of the conductor made of molten silicon, which will be described below, is accelerated. ) Can be narrowed. On the other hand, if the slag 15 is on the upper surface of the recovered silicon scrap 13, the slag 15 is melted by arc discharge, and a thick molten slag layer is formed before the molten silicon conductor is formed. Since the electric resistivity is higher than that of molten silicon, there arises a disadvantage that arc discharge becomes difficult. The slag 15 is preferably arranged at the bottom of the arc furnace. Thereby, the damage of the furnace bottom by the arc discharge at the time of starting of an arc furnace can be prevented.

  In normal arc furnace operation, there is a molten metal in the arc furnace, and an electric current flows between the electrode, the molten metal, and the electrode, and arc discharge is performed. In general, the arc furnace is operated with current control or constant impedance control. Refers to normal arc furnace operation performed on the basis of more stable arc discharge.

  The recovered silicon scrap was melted using the AC arc furnace having the configuration shown in FIG. For the three electrodes, graphite electrodes having an outer diameter of 16 mm and a center interval of 100 mm were used. The interelectrode voltage was tested at 70V. Carbon powder was used as the conductive material, and quick lime and quartz sand (mass ratio 0.5: 1 to 1: 5) were used as the slag. Commercially available carbon powder, quicklime and quartz sand were used.

Table 1 shows the result of component analysis (mass%) at the time of recovery of silicon scrap recovered from a printed circuit board manufacturing apparatus manufacturer, a solar cell related manufacturer, or the like. Samples Nos. 1 and 3 contained free abrasive grains, and Sample No. 2 contained fixed abrasive grains. The melting test was performed by drying these recovered silicon wastes and reducing the water content to 5 to 10% by mass and the PG (propylene glycol) content to 10 to 20% by mass. In the component analysis of Table 1, the amount of heat loss of recovered silicon waste at 110 ° C. was defined as the water content, and the amount of decrease when the recovered silicon waste was further heated to 220 ° C. was defined as the PG content. Then, the analysis value of the carbon content is SiC (SiC%), the analysis value of the oxygen content is SiO ₂ (SiO ₂ %), and the silicon content related to the SiC content and the SiO ₂ content is subtracted from the silicon analysis value. The content (Si%) was used.

  The test results were as follows. The time to point A shown in FIG. 2 was 5 to 10 min. The current value at point B was 70-100 A. The oscillation interval (C) of the current was 2 to 3 min. The time to point E was 15-20 min. The current value at that time was 300 to 600A. According to observation in the arc furnace, melting of recovered silicon waste was observed at point B. Then, a hot water pool of molten silicon was observed at point E where the current became stable and took a constant value. According to these observations, stable arc discharge occurs because the recovered silicon scrap is melted and a conductor made of molten silicon is formed, and the electrodes are connected by molten silicon, resulting in a low interelectrode resistance. It is understood that.

1 Arc furnace
5 electrodes
10 Conductive material
13 Recovered silicon scrap
15 Slag

Claims (5)

  A method of melting recovered silicon scraps by melting the recovered silicon scraps in an arc furnace, holding the voltage between the electrodes of the arc furnace at 40 to 400 V and energizing the electrodes in contact with a conductive material, A method for melting recovered silicon waste, wherein the silicon waste is heated by arc discharge generated as the conductive material rises in temperature, and after the arc discharge and the interelectrode current are stabilized, the operation shifts to normal arc furnace operation.   The method for melting recovered silicon waste according to claim 1, wherein the conductive material is a powder or a solid material made of carbon and / or silicon carbide.   The method for melting recovered silicon waste according to claim 1 or 2, wherein the silicon waste is melted by first introducing slag into the furnace bottom of the arc furnace and then introducing silicon waste thereon.   The method for melting recovered silicon waste according to any one of claims 1 to 3, wherein the arc interval of the arc furnace is adjustable.   The method for melting recovered silicon waste according to any one of claims 1 to 4, wherein the silicon waste is melted in a reducing atmosphere.

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