JPH0274513A - Method for removing silicon carbide in molten silicon - Google Patents

Abstract

PURPOSE:To produce high-purity silicon in a short time at a low cost by reducing silica with carbon, passing the resultant molten silicon through a filter having a specific passage diameter and capturing SiC with a cake layer of SiC formed on the filter. CONSTITUTION:Silica is reduced with carbon to provide molten silica 4, which is then injected into a treating vessel 5 having a filter 7, consisting of one or two or more of quartz, graphite, SiC and Si3N4. The passage diameter thereof is 10mum-20mm. A layer of a SiC cake 6 is formed on the filter 7 by passing the molten silica 4 therethrough and the resultant SiC is captured by the cake layer, separated and removed from the molten silica. The SiC can be removed in a short period of about 1/500 of that in a conventional method according to the above-mentioned method. Thereby, high-purity silicon used for solar cells, etc., can be produced at a low cost.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention provides a method for manufacturing silicon used in solar cells by reducing high-purity silica with high-purity carbon.
The present invention relates to a method for removing silicon carbide from molten silicon, which can quickly remove silicon carbide mixed in molten silicon.

In recent years, as a method for manufacturing silicon for solar cells at low cost, a method has been developed in which high-purity silica is reduced with high-purity carbon, as disclosed in, for example, Japanese Patent Application Laid-open No. 117110/1983.

In such a silicon manufacturing method using carbon as a reducing agent, a carbon component is inevitably mixed in the obtained silicon. For example, silicon produced in the reduction furnace of JP-A No. 61-117110 mentioned above contains 1000 to 5000
Contains wt ppm carbon content.

For this reason, it is necessary to refine the obtained silicon to remove carbon components from the silicon.

Among these carbon components, dissolved carbon -C- is treated by reducing pressure or supplying oxygen to the molten silicon to reduce C+0→CO.
An effective method is to accelerate the reaction described above and remove it as CO gas. However, carbon components that combine with carbon and precipitate:
Regarding silicon carbide (SiC), even if reduced pressure treatment and oxygen supply are performed, there is no significant effect.

This depressurization treatment and oxygen supply treatment temperature range from 1500 to 1600.
At °C, most of the carbon components appear as silicon carbide, so it is necessary to remove silicon carbide from the molten silicon in a pretreatment.

(Prior Art) In order to separate solid reaction products such as SiO Filtered through a layer and received into a high purity graphite mold,
A method of performing directional solidification has been proposed.

(Problems to be Solved by the Invention) In the method disclosed in the above-mentioned Japanese Patent Application Laid-open No. 60-239317, there is a layer in the filter with a silicon flow path diameter of 3 to 10 μm or less, and therefore the passage speed of silicon is reduced. There has been a problem that the processing method is expensive because it significantly reduces the processing time and takes processing time.

This invention advantageously solves these problems,
The purpose of the present invention is to propose a method for removing silicon carbide from molten silicon that can efficiently remove silicon carbide from molten silicon and process a large amount of silicon in a short time.

(Means for Solving the Problems) According to Japanese Patent Application Laid-Open No. 60-239317, in order to perform effective filtration, the flow path diameter must be 3 μm or less, but the inventors have We conducted an experiment and 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, and then a cake layer is formed on the filter. It has been found that silicon carbide in the molten silicon to be treated is trapped on or within the layer, so silicon carbide can be effectively removed using a filter with a flow path diameter of 10 μm or more. .

It has also been found that, unlike general surface filtration, clogging due to a residual layer does not occur, and the processing speed hardly changes depending on the amount of molten silicon processed.

This invention is based on the above knowledge.

In other words, this invention is capable of removing silicon carbide from molten silicon obtained by reducing silica with carbon.
After passing the molten silicon through a filter of 0 um to 20 mm to form a cake layer of silicon carbide on the filter, silicon carbide is captured in this cake layer and silicon carbide is separated from the molten silicon. This is a method for removing silicon carbide from molten silicon.

As the material for the filter, one or more of quartz, graphite, silicon carbide, and silicon nitride are advantageously suitable.

(Function) Figure 1 shows the solid state of the cake when the filtration process is interrupted and the cake is coagulated and viewed from above.

1 is a gap, 2 is silicon, and 3 is silicon carbide that is being deposited. Since a cake like the one shown in the figure is formed on the filter during filtration of fused silica, the flow path diameter is 10 μm.
By processing using the above filter, filtration with high processing speed can be realized.

FIG. 2 shows a filtration device advantageous for the method of the invention.

As the filter 7, a packed layer of quartz grains, silicon carbide powder, silicon carbide pellets, etc., a perforated plate of quartz, graphite, etc. are suitable. Furthermore, as shown in FIG. 3, holes can be provided in the bottom wall of the processing container 5 to be used instead of the filter.

The filter material is made of substances that do not contaminate silicon (
a substance whose main components are C, O, Si, and N), and 141
Any material that does not deform or deteriorate at a processing temperature of 0° C. or higher can be used. 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 be formed when 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
The range is from μm to 20 mm. Note that in order to make the flow path diameter 10 μm with a packed bed of quartz grains or the like, it is desirable that the particle size of these particles is about 50 μm.

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 the filter thickness is 10
It is advantageous to increase the processing speed by reducing the thickness to 0 mm or less; depending on the type of filter, a thickness of about 40 mm is sufficient, and for small diameter filters a filter thickness of 5 to 10 mm may be sufficient.

When molten silicon is poured into a processing container equipped with a filter and filtered, it is desirable to maintain the molten silicon within a temperature range of 1410 to 1550°C. Holding temperature is 1410
This is because if the temperature is lower than 1550°C, the molten silicon will solidify and will not pass through the filter, whereas if it is higher than 1550°C, carbon in the impurities will dissolve in the molten silicon, reducing its purity.

(Example) A carbon concentration of 500 wt ppm was installed in a filtration device having filters with various flow path diameters as shown in FIGS. 2 and 3.
of molten silicon was injected, and the filtration operation was performed while maintaining the temperature of the molten silicon in the processing container 5 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.

Further, FIG. 5 shows the change over time in the throughput when using 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).

From Table 1 and FIG. 5, Examples 1 to 6 according to the present invention are as follows:
It can be seen that a sufficient treatment amount can be obtained and silicon carbide can be removed in a short time of about 11,500 in Comparative Example 1. 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 of about 11,500 ml compared to conventional methods, and high purity silicon used in solar cells etc. can be produced at low cost. can do.

[Brief explanation of the drawing]

Fig. 1 is an enlarged (2.5x) microscopic 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; Fig. 4; FIG. 5 is a graph showing the relationship between filter thickness and carbon concentration after filtration, and FIG. 5 is a graph showing the difference in processing speed depending on the flow path diameter. l...Gap 2...Silicon 3...Silicon carbide 4...Silicon melt 5...Container
6...Cake 7...Filter
8...Heating coil patent applicant Kawasaki Steel Co., Ltd.

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 carbonized onto the filter. A method for removing silicon carbide from molten silicon, which comprises the steps of forming a silicon 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.