Methods for utilizing drifting pumice

Abstract

To provide a method for producing high-purity metallic silicon using drifting pumice as a raw material that takes energy saving and environmental problems into consideration.SOLUTION: The present invention is characterized by pulverization and reduction treatment in view of a large amount of porous pumice drifted at sea being used as a raw material, and provides a method for producing high-purity metallic silicon using drifting pumice as a raw material.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] It relates to the utilization of floating pumice ejected from submarine volcanoes and the like.

Background Art

[0002] When a volcano at sea erupts, magma is ejected and rapidly cooled to become porous pumice that floats on the sea. In particular, the eruption that occurred at the submarine volcano of Fukutoku-Okanoba at around 6:20 on August 13, 2021, resulted in an unprecedented large amount of pumice floating on the sea. For this reason, the pumice that was pushed towards the coast along with the ocean current entered through the intake ports of ships and the cooling water of boilers, interfering with cooling, and also getting entangled in the rotating parts, making it impossible to function normally. Therefore, it is necessary to remove the pumice floating on the sea and ports, and it is necessary to consider the disposal of the removed pumice. In the utilization of pumice, that is, volcanic ash, Patent Document 1 describes using volcanic ash as a material for a porous ceramic fired body. However, it aims to make a material having water retention and moisture release properties by utilizing the porosity of volcanic ash.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Since around September 2021, production cuts have been implemented at Chinese metallic silicon manufacturing companies that hold a large share of the world market, resulting in shortages and price increases in metallic silicon across the board. This is due to power regulations in China. Despite the fact that the raw materials such as silica for metallic silicon production, which consumes a large amount of electricity, are abundant raw materials on the earth, production has significantly decreased. While arc furnaces are primarily used in China for the production of metallic silicon, they consume a large amount of electricity and, due to the reduction reaction using carbon, emit large amounts of CO and CO2, resulting in environmental damage. Minimizing the massive electricity consumption and CO2 emissions associated with the production of metallic silicon, which is incompatible with energy conservation and environmental destruction, and producing it cheaply is a crucial challenge. Industries that use metallic silicon (silicon with a purity of around 1-2N) as a raw material, namely semiconductor applications (higher purity of 11N or more), automotive applications using semiconductors, solar applications (higher purity of around 5-9N), aluminum alloys, etc., must adopt manufacturing methods that consider energy conservation and environmental issues to prevent chain reactions of supply shortages. For example, 5N means wt%, with five 9s in a row, representing 99.999%.

[0005] Furthermore, when floating pumice is used as a raw material in an arc furnace, the gas escapes easily, and because it is porous, contaminants, gases, and air tend to remain. Also, because it is light, when melted it tends to float upwards or splash, making it difficult to melt and heat uniformly. In addition, electricity costs are higher in Japan compared to China, and the production of metallic silicon using arc furnaces is hardly done domestically. Making it possible to use the abundant volcanic rocks and drifting pumice as raw materials would promote the production of metallic silicon in volcanic Japan, and domestic production would be cost-effective, so establishing a manufacturing method is necessary. [Means for solving the problem]

[0006] This invention provides a method for producing metallic silicon using drifting pumice, characterized in that the raw material is pumice that drifts on the sea. Specifically, this invention utilizes a large quantity of porous pumice that has drifted at sea as a raw material, and features distinctive crushing and reduction processes, as a method for producing high-purity metallic silicon using drifting pumice as the raw material. That is [Effects of the Invention]

[0007] It is possible to manufacture metallic silicon in Japan at low temperatures without using an arc furnace by crushing, decontaminating, and reducing the pumice, which is readily available in large quantities and generated at sea by volcanic eruptions and other events that need to be removed, and then subjecting it to reducing heat treatment. This method also has the effect of reducing material costs by obtaining metallic silicon from volcanic rocks and pumice, which are abundant in Japan. Furthermore, because it directly cleans, removes contaminants and metals using powder, and also reduces CO2 emissions, it can reduce electricity consumption. [Brief explanation of the drawing]

[0008] [Figure 1] Conventional manufacturing process for metallic silicon [Figure 2] First metallic silicon manufacturing process of the present invention [Figure 3] Second metallic silicon manufacturing process of the present invention [Modes for carrying out the invention]

[0009] Figure 1 shows the conventional manufacturing process for metallic silicon. The raw materials used are silica and other minerals containing 99-98% SiO2 (silicon dioxide). These minerals are produced in large quantities in countries like China and are known for their high purity. In arc furnace process 1 (or arc plasma furnace process), silica is melted at a high temperature of over 2000 degrees Celsius. Then, carbon in the electrodes, other coke, and carbon added to wood etc. cause a reduction reaction in which silicon dioxide in the silica is converted into silicon (Si) metal. The metallic silicon is then cooled and extracted as a solid. This yields metallic silicon of a certain purity (approximately 98%). This is so-called 2N purity metallic silicon. In the grinding step 2 prior to the chlorination step, the metallic silicon lumps are ground. To ensure uniform retention during the chlorination process, the mill size is standardized, and the material is sorted using a sieve. In the chlorination step 3, using a known method, the metallic silicon particles (powder) pulverized in the pulverization step 2 are filled into a fluidized bed, heated, and chlorine gas is introduced from the bottom of the reaction vessel. This gas is passed through the metallic silicon particles to convert the metallic silicon into a chloride gas, which is then removed from the top of the reactor and cooled to obtain a liquid silicon chloride. In other words, chlorides of tetrachlorosilane (SiCl4: silicon tetrachloride) and trichlorosilane (SiHCl3: called TCS) are obtained. This metallic silicon chloride is liquid at room temperature. Other metals also exist as chlorides and require separation. In distillation step 4, TCS can be obtained by known methods based on differences in boiling points. The polycrystalline CVD growth step 5 is known as the Siemens method, in which the TCS is reduced in a hydrogen gas atmosphere, and silicon is grown on a silicon seed crystal by CVD at a temperature of about 1000 to 1100 degrees Celsius to obtain high-purity metallic polysilicon (polycrystalline silicon).

[0010] This polycrystalline, high-purity metallic silicon can be purified to a high degree by single crystallization using known methods, becoming a silicon wafer. Patterning is then applied near the surface, followed by oxidation, nitriding, P / N diffusion, and lamination of P / N / insulating layers, which can then be used to create, for example, automotive semiconductor devices.

[0011] As mentioned above, there are challenges and problems with the conventional manufacturing methods. In addition, when using large quantities of porous pumice that have drifted at sea as raw material, the silicon dioxide (SiO2) purity of the pumice is only about 70%, so even if it is reduced in an arc furnace, the purity does not increase, requiring further purification, which is inefficient in terms of energy and cost. Furthermore, porous pumice has a high silicon activation energy, which poses a risk of it becoming SiC in an arc furnace.

[0012] Figure 2 illustrates the first manufacturing method of the present invention. In crushing step 6, the porous pumice that has drifted on the sea is processed for use. To remove impurities, inorganic substances such as Na and Cl from seawater, and water adsorption, acid washing, alkaline washing, and vacuum adsorption / desorption are performed. Furthermore, by utilizing the porous nature and high activation energy of pumice, self-heating is achieved through pulverization, and the atmosphere is subjected to acidic (fluorine, chlorine) atmosphere etching. Additionally, a reducing substance (e.g., aluminum powder) is introduced to allow the reducing substance to adsorb and penetrate into the interior of the powdered pumice. By pulverizing and creating an acidic atmosphere at high temperature, the impurities in the pumice can be reduced. Also, since pumice has a structure close to being hollow inside, it can be easily pulverized, resulting in energy savings. The pulverization size should preferably be finer. If it cannot be properly adsorbed and washed due to floating in powder form, a pulverization degree of about 20 - 100 microns is preferred.

[0013] In the reduction step 7, heating is performed and reduction treatment is carried out using a metal catalyst. A reducing metal (e.g., aluminum, nickel, cobalt, manganese, etc.) is added and mixed. The atmosphere is an inert gas, and heating is performed to carry out a reduction reaction, converting silicon dioxide to metal silicon. The temperature is set to around the melting point of silicon dioxide. As a result, a substance of metal silicon containing removed oxidation, metals (such as iron), calcium, etc. can be obtained. Solidification separation treatment is performed based on the difference in the temperature of metal precipitation during cooling to separate, extract, and solidify the metal silicon. Since all processes are carried out in powder form, less power consumption is required. By introducing a reducing agent at the time of porous pulverization, metal silicon can be efficiently extracted.

[0014] In the pulverization step 8, in order to further increase the purity of the obtained metal silicon, pulverization is performed again, sieving is carried out to adjust the size, and acid washing and alkali washing are performed. For acid washing, hydrochloric acid and hot concentrated sulfuric acid washing are performed to remove other metals. Here, the temperature of the hot concentrated sulfuric acid is 100 degrees or more, and the temperature of the hydrochloric acid washing is 50 degrees or more to remove the metals on the surface and in the pores. At the time of pulverization, a reducing agent introduction treatment is performed prior to the chlorination treatment in the same manner as in the pulverization step 6. The size of the pulverization is set to a size that allows proper fluidization in the fluidized bed chlorination process.

[0015] The chlorination process 3, the distillation process 4, and the polycrystalline CVD growth process 5 are as described above. Because it is made of porous pumice stone, for example, it is possible to adsorb a reducing agent in advance, so production can be carried out efficiently in the chlorination step 3.

[0016] As a result, high-purity polycrystalline metallic silicon can be obtained. The purity of the silicon becomes 11N, and the volcanic pumice becomes high-purity polycrystalline metallic silicon. By using volcanic pumice available in Japan to produce high-purity metallic polycrystalline silicon, we can ensure a stable supply for semiconductor substrate manufacturing, semiconductor device manufacturing, and ultimately for automobiles, PCs, and many other products that utilize semiconductors.

[0017] Figure 3 illustrates a second manufacturing method of the present invention. The grinding step 6 and the reduction step 7 are the same as in the first manufacturing method. The metallic silicon produced in reduction step 7, although not of high purity, can also be used for solar polysilicon (polycrystalline silicon).

[0018] In grinding step 8, (1) When the metallic silicon produced in reduction step 7 is used as is. The material is crushed to a size suitable for use in solar-powered polysilicon. Then, it undergoes alkaline or acid washing. This process produces polysilicon for solar power, resulting in fewer steps and cost advantages. Furthermore, domestic production ensures a stable supply. Equipment space requirements can also be reduced compared to conventional methods. (2) When performing a process to further increase the purity. This is the same as the grinding step 8 of the first manufacturing method described above. To further increase the purity of the obtained metallic silicon through solid-liquid separation in a polycrystalline form, it is crushed again, sieved to match the size, and then subjected to acid washing and alkaline washing. Acid cleaning involves cleaning with hydrochloric acid and hot concentrated sulfuric acid to remove metal and other materials. Here, the temperature is set to 100 degrees Celsius or higher for hot concentrated sulfuric acid washing, and to 50 degrees Celsius or higher for hydrochloric acid washing, to remove metal from the surface and pores. During grinding, a reducing agent is introduced prior to the cast method re-reduction treatment. It is best to use finer grinding particles. If the powder is suspended and cannot be effectively adsorbed and washed, it is preferable to grind it to a size of approximately 20-100 microns. Since the content of boron and phosphorus must be kept to a minimum, boron and phosphorus removal treatment may be performed. In the cast re-reduction step 9, a cast growth method is used, which is known for producing polysilicon for solar applications. A reducing agent is introduced, the material is melted, and a solid-liquid separation process is performed. However, in order to increase the packing density as efficiently as possible, the material is converted into a powder in the pulverization step 8. The resulting metallic silicon can be used as polysilicon for solar power systems, with pumice as the raw material.

[0019] As described above, the second manufacturing method of this application can obtain polysilicon for solar use. This polysilicon is then refined into a circular or rectangular shape, sliced, manufactured into cells (textured, PN junction formed, electrode formed, etc.), and then assembled into modules (plastic laminated, etc.) to form solar products.

[0020] This invention uses pumice as a raw material and eliminates the need for a metallic silicon process using an arc furnace. By directly utilizing the characteristics of pumice to achieve high purity, it enables shortened processes and efficient manufacturing. [Industrial applicability]

[0021] This is extremely useful because it can solve supply problems in industries that use metallic silicon, especially in the automotive semiconductor manufacturing industry. [Explanation of symbols]

[0022] 1. Carbon reduction process using an arc furnace 2. Grinding process of metallic silicon 3. Fluidized Bed Chloride Process 4. Distillation process 5. Polycrystalline CVD (chemical vapor deposition) growth process using the Siemens method 6. Grinding process including washing and reduction treatment when using porous pumice raw material. 7. Reduction process using a reducing agent 8. Grinding process including washing and reduction treatment when using porous pumice raw material. 9. Re-reduction process by casting method for improving metallic silicon purity

Claims

[Claim 1] A method for producing metallic silicon using drifting pumice, characterized in that the raw materials are pumice that has been ejected from a volcano and is floating on the sea, and pumice that has drifted ashore and is floating in a port, the method comprising a step of crushing the pumice to obtain metallic silicon using a metal catalyst reduction reaction step, followed by a step of crushing and purifying the metallic silicon.

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