Method for extracting valuable component and method for producing silicon tetrachloride

Abstract

To provide a method for extracting a valuable component capable of increasing a volatilization rate, and a method for producing silicon tetrachloride.SOLUTION: Coal ash is mixed with a carbon source and heat-treated in a chlorine-containing atmosphere to extract valuable components, such as silicon and aluminum, contained in the coal ash as chlorides. The coal ash is used after being pulverized so that the crystallite size including valuable components is equal to or less than 30 nm. By adjusting the crystallite diameter, the reactivity can be improved, and the volatilization rate of the chloride can be increased.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for extracting valuable components from a raw material containing at least one of silicon and aluminum such as coal ash as an oxide, and a method for producing silicon tetrachloride from a raw material containing silicon dioxide such as coal ash.

Background Art

[0002] Coal ash (fly ash) generated during coal combustion can be used as a concrete admixture, but there are many problems in terms of quality stability, and in practice, most of it is disposed of as industrial waste. On the other hand, coal ash contains many components such as silicon dioxide (SiO2) and aluminum oxide (Al2O3) that are valuable in themselves. If these can be extracted with high purity, it is preferable because it is possible to achieve both improvement in economic efficiency and contribution to the realization of a recycling-based society.

[0003] Conventionally, for example, a chlorination volatilization method is known in which a material containing a plurality of compounds is heat-treated in a chlorine atmosphere and volatile separation is performed by utilizing the change in boiling point (see, for example, Patent Document 1). Further, Patent Document 2 describes a method for producing silicon tetrachloride by bringing chlorine into contact with a mixture of a SiO2-containing substance and carbon.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, even when attempting to extract valuable components from coal ash or the like by the chloride volatilization method, the volatilization rates of silicon, aluminum, etc. are as low as about 60% at maximum, the recovery rate of valuable components is low, and there is also a problem that the yield of silicon tetrachloride is low.

[0006] The present invention has been made based on such problems, and an object thereof is to provide a method for extracting valuable components capable of increasing the volatilization rate, and a method for producing silicon tetrachloride.

Means for Solving the Problems

[0007] The method for extracting valuable components of the present invention extracts valuable components from a raw material containing at least one valuable component of silicon (Si) and aluminum (Al) as an oxide, and includes a pulverization step of pulverizing the raw material to make the crystallite diameter containing the valuable component 30 nm or less, a mixing step of mixing a carbon source into the raw material, and a heating step of heat-treating the raw material mixed with the carbon source in a chlorine-containing atmosphere after the pulverization step and the mixing step.

[0008] The method for producing silicon tetrachloride of the present invention produces silicon tetrachloride from a raw material containing silicon dioxide, and includes a pulverization step of pulverizing the raw material to make the crystallite diameter of silicon dioxide 30 nm or less, a mixing step of mixing a carbon source into the raw material, and a heating step of heat-treating the raw material mixed with the carbon source in a chlorine-containing atmosphere after the pulverization step and the mixing step.

Effects of the Invention

[0009] According to the present invention, since the crystallite diameter is set to 30 nm or less, the volatilization rate of the chloride can be increased. Therefore, valuable components can be recovered at a high recovery rate, and silicon tetrachloride can be produced at a high yield.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012] FIG. 1 shows the steps of a method for extracting valuable components and a method for producing silicon tetrachloride according to an embodiment of the present invention. Note that the method for producing silicon tetrachloride according to this embodiment extracts silicon as valuable component as silicon tetrachloride in the method for extracting valuable components according to this embodiment. Therefore, in this embodiment, the method for extracting valuable components will be described.

[0013] The method for extracting valuable components according to this embodiment extracts valuable components from a raw material containing at least one valuable component of silicon and aluminum as an oxide. As the raw material, for example, inorganic materials such as coal ash (fly ash) are preferably mentioned. Coal ash is the ash generated during coal combustion, and its composition varies depending on the coal type, but the main components are silicon dioxide (SiO2) and aluminum oxide (Al2O3). The content of silicon dioxide is, for example, 50% by mass to 65% by mass, and the content of aluminum oxide is, for example, 25% by mass to 30% by mass. The rest contains trace amounts of oxides such as iron oxide (Fe2O3), titanium oxide (TiO2), calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na2O), and potassium oxide (K2O).

[0014] This method for extracting valuable components includes a pulverization step (step S101) of pulverizing raw materials such as coal ash, a mixing step (step S102) of mixing a carbon source into the raw materials such as coal ash, and a heating step (step S103) of heat-treating the raw materials such as coal ash mixed with the carbon source in a chlorine-containing atmosphere after the pulverization step and the mixing step. This is because, in order to efficiently extract and recover only the valuable components from the oxide, a method of extracting in the form of a chloride with a relatively low volatilization temperature is suitable. The chloride volatilized in the heating step is preferably recovered by a recovery step (step S104).

[0015] In the pulverization step (step S101), the raw material is pulverized so that the crystallite diameter (i.e., crystallite size) containing the valuable component is 30 nm or less. This is because the reactivity can be improved and the volatilization rate of the chloride can be increased by adjusting the crystallite diameter. The crystallite diameter can be determined, for example, from the Scherrer's equation shown in Equation (1) based on the width of the diffraction line of X-ray diffraction. In Equation (1), D is the crystallite diameter (nm), K is the Scherrer constant, λ is the wavelength of the X-ray (nm), B is the spread of the diffraction line width (rad), and θ is the Bragg angle (rad). D = Kλ / Bcosθ (1)

[0016] The pulverization is preferably performed, for example, by a planetary ball mill. This is because it is difficult to make the crystallite diameter 30 nm or less with a ball mill, whereas the crystallite diameter can be easily made 30 nm or less by using a planetary ball mill. The pulverization time is preferably, for example, 2 hours or more. This is because the crystallite diameter can be made 30 nm or less.

[0017] In the mixing step (step S102), the carbon source is mixed to promote the reaction between the valuable components in the raw material and chlorine. As the carbon source used in the mixing step, for example, amorphous carbon such as carbon obtained by pyrolyzing an organic compound such as phenolphthalein, coal, charcoal, coke, activated carbon, etc. is desirable. The mixing amount of the carbon source is preferably, for example, an amount in excess of the stoichiometric amount required to chlorinate all the oxides in the coal ash. This is because the raw material after pulverization has a crystallite size of 30 nm or less, and to improve the reactivity between the valuable components in the raw material and chlorine, it is preferable to mix an amount in excess of the stoichiometric amount with respect to the amount of the raw material after pulverization. The carbon source is preferably, for example, in powder form. The mixing of the carbon source may be before or after the pulverization step, but the latter is preferred because there is no need to pulverize the carbon source when using a powdered carbon source.

[0018] In the heating step (step S103), the oxides of the valuable components contained in the raw material are reacted with chlorine and volatilized as chlorides. The heating temperature is preferably, for example, 800 °C or higher and 1000 °C or lower. This is because the temperature at which silicon dioxide and aluminum oxide in the coal ash react with chlorine and rapidly volatilize is 800 °C or higher. The chlorine gas may be supplied only with chlorine gas, or may be supplied after being mixed with an inert gas such as nitrogen gas. The supply amount of the chlorine gas and the reaction time can be adjusted as appropriate. The reaction device may be any device as long as it is provided with a reaction section for the raw material and chlorine, and may be a continuous type or a batch type. An example of the reaction device will be described later.

[0019] The recovery process (step S104) cools and recovers the volatilized chloride, and is not particularly limited as long as it can be recovered. Silicon tetrachloride and aluminum chloride can be separated and recovered by adjusting the temperature during recovery, taking advantage of their different solidification temperatures. When recovering silicon tetrachloride, for example, it is preferable to cool the reaction product to a temperature at which components other than silicon tetrachloride can be condensed, separate the components other than silicon tetrachloride, and then further cool to recover silicon tetrachloride. When recovering aluminum chloride, for example, after recovering reaction products other than aluminum chloride at a temperature of about 300°C, it can be recovered by setting the cooling temperature to 150°C or higher and 200°C or lower.

[0020] This method for extracting valuable components and the method for producing silicon tetrachloride can be carried out, for example, by the reaction apparatus 10 shown in FIG. 2. In the reaction apparatus 10 shown in FIG. 2, the recovery section specifically shows the case of recovering silicon tetrachloride. The reaction apparatus 10 has, for example, a reaction section 11 composed of a reaction tube or the like that houses a mixture of a raw material and a carbon source and reacts with chlorine. On the outer periphery of the reaction section 11, heating means 12 such as a heater are disposed, for example. A chlorine supply source 14 for supplying chlorine gas is connected to the reaction section 11 via a pipe 13, and a valve 13a and a flow meter 13b are disposed, for example, between the chlorine supply source 14 and the reaction section 11. Further, a recovery section 16 for recovering silicon tetrachloride that has passed through the condensation section 15 by a cooling trap is connected to the reaction section 11 via a condensation section 15 that condenses and separates components other than silicon tetrachloride. On the outer periphery of the condensation section 15, temperature adjustment means 17 composed of, for example, a mantle heater for adjusting the temperature of the condensation section 15 are disposed.

[0021] Thus, according to this embodiment, since the crystallite diameter including valuable components is set to 30 nm or less, the volatilization rate of the chloride can be increased. Therefore, valuable components can be recovered at a high recovery rate, and silicon tetrachloride can be produced at a high yield.

Examples

[0022] (Examples 1 to 4) Using the reaction apparatus 10 shown in FIG. 2, silicon tetrachloride was recovered using coal ash as a raw material. Specifically, powdered solid carbon was mixed with coal ash at a ratio of 1 part by mass of solid carbon to 1 part by mass of coal ash, stored in the reaction section 11, supplied with a chlorine-containing gas, and heated to 1000°C. The reaction time was 1 hour. The coal ash was pulverized by a planetary ball mill before mixing with the solid carbon, and the crystallite size of silicon dioxide in the coal ash was changed by varying the pulverization time in Examples 1 to 4. The crystallite size of silicon dioxide was determined from the Scherrer's equation shown in Equation (1) by performing X-ray diffraction on the pulverized coal ash. The pulverization time in Example 1 was 2 hours, in Example 2 was 6 hours, in Example 3 was 12 hours, and in Example 4 was 24 hours. The crystallite size of silicon dioxide in the pulverized coal ash was 28.9 nm in Example 1, 10.0 nm in Example 2, 7.3 nm in Example 3, and 1.5 nm in Example 4. Also, when analyzing the composition of the used coal ash and solid carbon, it was as shown in Tables 1 and 2. The recovery of silicon tetrachloride was carried out with the temperature of the condensation section 15 set at 100°C and recovered by a cooling trap at -40°C.

[0023]

Table 1

[0024]

Table 2

[0025] For Examples 1 to 4, the volatilization amount of silicon was measured. The volatilization amount of silicon was evaluated by fluorescence X-ray analysis of the composition of the samples before and after treatment, and calculated by the following formula (2) based on the weights of the samples before and after chlorination. In formula (2), X0 is the mass% of silicon dioxide (SiO2) in the coal ash before chlorination, X is the mass% of silicon dioxide (SiO2) in the sample after chlorination, and Y is the residue yield after chlorination. The obtained results are shown in Table 3. Silicon volatilization rate [%] = (X0 - YX) / X0 × 100

[0026] As Comparative Example 1 for Examples 1 to 4, silicon tetrachloride was recovered from coal ash in the same manner as in Examples 1 to 4, except that the coal ash was used as it was without being pulverized. Further, as Comparative Example 2 for Examples 1 to 4, silicon tetrachloride was recovered from coal ash in the same manner as in Examples 1 to 4, except that the coal ash was pulverized by a ball mill for 72 hours. Also for Comparative Examples 1 and 2, X-ray diffraction was performed on the coal ash in the same manner as in Examples 1 to 4, and the crystallite size of silicon dioxide was determined. As a result, it was 31.6 nm for Comparative Example 1 and 31.5 nm for Comparative Example 2. That is, even when pulverized by a ball mill, the crystallite size of silicon dioxide hardly changed. Also for Comparative Examples 1 and 2, the volatilization amount of silicon was measured in the same manner as in Examples 1 to 4. The obtained results are shown together with Table 3.

[0027]

Table 3

[0028] As shown in Table 3, according to Examples 1 to 4 in which the crystallite size of silicon dioxide was 30 nm or less, the volatilization rate of silicon could be significantly improved as compared with Comparative Examples 1 and 2 in which the crystallite size of silicon dioxide was larger than 30 nm. That is, it was found that if the crystallite size containing valuable components is 30 nm or less, the valuable components can be recovered at a high recovery rate, and silicon tetrachloride can be produced at a high yield.

[0029] As described above, the present invention has been described with reference to embodiments, but the present invention is not limited to the above embodiments and can be variously modified. For example, in the above embodiments, the case where the raw material is coal ash has been specifically described, but the present invention can be similarly applied to raw materials containing at least one valuable component of silicon and aluminum as an oxide. Also, in the above embodiments, the case of extracting at least one of silicon and aluminum as valuable components from coal ash has been specifically described, but other valuable components contained in coal ash can be extracted in the same manner.

Explanation of Signs

[0030] 10... reaction device, 11... reaction section, 12... heating means, 13... pipe, 13a... valve, 13b... flow meter, 14... chlorine supply source, 15... condensation section, 16... recovery section, 17... temperature adjustment means

Claims

1. A method for extracting valuable components from a raw material containing at least one valuable component of silicon (Si) and aluminum (Al) as an oxide, comprising: A pulverization step of pulverizing the raw material to make the crystallite size containing the valuable component 30 nm or less; A mixing step of mixing a carbon source into the raw material; After the pulverization step and the mixing step, a heating step of heat-treating the raw material mixed with the carbon source in a chlorine-containing atmosphere. The method for extracting valuable components, characterized by comprising the above steps.

2. The method for extracting valuable components according to Claim 1, wherein the raw material contains coal ash.

3. A method for producing silicon tetrachloride from a raw material containing silicon dioxide, comprising: A pulverization step of pulverizing the raw material to make the crystallite size of silicon dioxide 30 nm or less; A mixing step of mixing a carbon source into the raw material; After the pulverization step and the mixing step, a heating step of heat-treating the raw material mixed with the carbon source in a chlorine-containing atmosphere. The method for producing silicon tetrachloride, characterized by comprising the above steps.

4. The method for producing silicon tetrachloride according to Claim 3, wherein the raw material contains coal ash.

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