JPH0476331B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention is aimed at reducing silicon carbide mixed in molten silicon when producing silicon used in solar cells etc. by reducing high purity silica with high purity carbon. The present invention relates to a method for quickly removing silicon carbide from molten silicon. In recent years, a method of reducing high-purity silica with high-purity carbon has been developed as a method for manufacturing silicon for solar cells at low cost, for example, as disclosed in JP-A-61-117110. . In such a silicon manufacturing method using carbon as a reducing agent, a carbon component is inevitably mixed in the obtained silicon. For example, the above-mentioned JP-A-61-
The silicon produced in the reduction furnace of Publication No. 117110 contains a carbon component of 500 to 5000 wt ppm. For this reason, it is necessary to refine the obtained silicon to remove carbon components from the silicon. Among these carbon components, regarding dissolved carbon C ,
By performing depressurization treatment or supplying oxygen to molten silicon, C
An effective method is to promote the reaction of + O → CO↑ and remove it as CO gas. However, with regard to silicon carbide (SiC), which is a carbon component that is combined with carbon and precipitated, even reduced pressure treatment and oxygen supply do not have much effect. This depressurization treatment, oxygen supply treatment temperature 1500-1600
℃, most of the carbon is present in the molten silicon as silicon carbide, so it is necessary to remove the silicon carbide in a pretreatment. (Prior art) In order to separate solid reaction products such as SiO ₂ and SiC from molten silicon, JP-A-60-239317 discloses that molten silicon is separated from a heated SiC/Si composite material. A method has been proposed in which carbon dioxide is filtered through a layer of carbon dioxide, placed in a high-purity graphite mold, and directional solidified. (Problems to be Solved by the Invention) In the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 60-239317, the diameter of the silicon flow path is 3 to 10 μm in the filter.
There are the following layers, which significantly reduces the passage speed of silicon and requires a long processing time, resulting in a high-cost processing method. The present invention advantageously solves these problems, and is capable of efficiently removing silicon carbide from molten silicon and processing a large amount of silicon in a short period of time. The purpose of this study is to propose a method for removing elements. (Means for Solving the Problems) According to Japanese Patent Application Laid-open No. 60-239317, in order to perform effective filtration, the channel diameter must be 3 μm or less. When we conducted an experiment, we found that when removing silicon carbide from molten silicon, a mixed layer (cake layer) of silicon and silicon carbide is formed on the filter at the beginning of the filtration process. Since silicon carbide in the molten silicon that is subsequently processed is trapped on or within the layer, silicon carbide can be effectively removed using a filter with a flow path diameter of 10 μm or more. There was found. It was also found that unlike general surface filtration, clogging due to a cake layer does not occur, and therefore there is almost no change in processing speed depending on the amount of molten silicon processed. This invention is based on the above knowledge. That is, in removing silicon carbide from molten silicon obtained by reducing silica with carbon, this invention passes the molten silicon through a filter with a channel diameter of 10 μm to 20 mm, and deposits silicon carbide on the filter. This method of removing silicon carbide from molten silicon is characterized by forming a cake layer, trapping silicon carbide in this cake layer, and separating silicon carbide from molten silicon. Here, the filter materials include quartz, graphite,
One or more of silicon carbide and silicon nitride are advantageously suitable. (Function) FIG. 1 shows the solid state of the cake when the filtration process is interrupted and the cake is solidified and a cross section is viewed. 1
2 is silicon, and 3 is deposited silicon carbide. Since a cake like the one shown in the figure is formed on the filter when molten silicon is filtered, filtration with a high processing speed can be achieved by using a filter with a flow path diameter of 10 μm or more. be. FIG. 2 shows a filtration device advantageous for the method of the invention. For the filter 7, a packed layer of quartz grains, silicon carbide grains, silicon carbide pellets, etc., a perforated plate of quartz, graphite, etc. are suitable. Furthermore, as shown in FIG. 3, the filter 7 can also be used by providing holes in the bottom wall of the processing container 5. The material for the filter 7 is a substance that does not contaminate silicon (a substance whose main components are C, O, Si, and N).
Any material can be used as long as it does not deform or deteriorate at processing temperatures of 1410°C or higher. The same can be said of the material of the processing container 5. The flow path diameter of the processing container 5 of the filter 7 needs to be small enough to form a cake layer on the filter. Although it varies depending on the carbon concentration of the silicon to be treated, the particle size of the silicon carbide to be removed, the number of channels, etc., cake is more likely to form if the channel diameter is 20 mm or less. Further, in order to facilitate the passage of molten silicon, it is desirable that the diameter of the channel be 10 μm or more. Therefore, in this invention, the flow path diameter is set to 10 μm to 20 μm.
The range is mm. FIG. 4 shows the relationship between filter thickness and carbon concentration after filtration. As is clear from the figure, the filtration efficiency does not change much even if the filter is thickened, so it is advantageous to increase the processing speed by keeping the filter thickness at 100 mm or less. In the case of a small diameter filter, a filter thickness of 5 to 10 mm may be sufficient. When pouring molten silicon into a processing container equipped with a filter and filtering it, the temperature of the molten silicon is 1410 to 1550℃.
It is desirable to maintain the temperature within the range of . If the holding temperature is lower than 1410℃, the molten silicon will solidify and will not pass through the filter, while if it is higher than 1550℃, a large amount of carbon in the container and silicon carbide will dissolve into the molten silicon, reducing purity. It is. This is because it reduces the (Example) As shown in FIGS. 2 and 3, carbon concentration was
Inject 500wt ppm of molten silicon into processing vessel 5.
Filtration operation was carried out while maintaining the temperature of the molten silicon inside at 1450°C. Table 1 shows the amount of treatment in one hour and the carbon concentration after treatment when treated with filters and containers having various flow path diameters.

[Table] Furthermore, Fig. 5 shows the change over time in the throughput when a processing container with holes with a flow path diameter of 10 mm (Example 2) and a filter with a flow path diameter of 3 μm (Comparative Example 1) were used. From Table 1 and FIG. 5, Example 1 according to the present invention
- 6, a sufficient throughput was obtained and was about 1/1 of that of Comparative Example 1.
It can be seen that silicon carbide can be removed in a short time of 100%. Further, even though the carbon concentration after treatment is slightly higher than that of Comparative Example 1 and decarburization in the subsequent process takes time, the time required for the entire process is shortened. (Effects of the Invention) According to the present invention, silicon carbide in molten silicon can be removed in a short time approximately 1/100 of the conventional method, and high-purity silicon used in solar cells etc. can be removed at low cost. can be manufactured.

[Brief explanation of the drawing]

Fig. 1 is an enlarged (2.5x) microphotograph sketch of a cake, Fig. 2 is a cross-sectional view of a filtration device according to the present invention, Fig. 3 is a cross-sectional view of a processing container according to the present invention, and Fig. 4 is a FIG. 5 is a graph showing the relationship between filter thickness and carbon concentration after filtration, and is a graph showing differences in processing speed depending on the channel diameter. 1... = between, 2... silicon, 3... silicon carbide, 4...
Silicon melt, 5...container, 6...cake, 7...filter, 8...heating coil.

Claims (1)

[Claims] 1. In removing silicon carbide from molten silicon obtained by reducing silica with carbon, the molten silicon is passed through a filter with a channel diameter of 10 μm to 20 mm, and silicon carbide is deposited on the filter. A method for removing silicon carbide from molten silicon, which comprises a step of forming a plain cake layer, capturing silicon carbide in this cake layer, and separating the silicon carbide from the molten silicon. 2. The method for removing silicon carbide from molten silicon according to claim 1, wherein the filter is one or more of quartz, graphite, silicon carbide, and silicon nitride.