The technology for preparing polysilicon by the zinc reduction method is not the latest technology. In terms of history, it was born before the Siemens method. The zinc reduction method was first tested by DuPont in the United States during World War II, using zinc (Zn) to reduce SiCl4 to produce polysilicon. During the 1950s and 1960s, the global semiconductor industry developed rapidly, and there was an urgent need for high-purity silicon materials. Under this situation, DuPont of the United States developed the zinc reduction method and put it into use in the 1950s. Later, Japan, a major semiconductor country, introduced the technology to produce low-cost and high-purity semiconductor-grade polysilicon. However, after experimental research, It was found that the purity of silicon produced by this technology could only reach 6N~7N, which could not meet the requirements of the semiconductor industry for silicon purity, and at that time solar photovoltaic power generation technology had not attracted people’s attention, so this silicon purification that can meet the purity requirements of solar-grade silicon The technology has not been studied further. With the birth of the Siemens method, the zinc reduction method as a technology was stored in the historical archives of scientific research. Due to the scarcity of technologies dedicated to the production of solar-grade silicon in the world, the production characteristics of high-purity silicon with low cost and low energy consumption by zinc reduction have been re-understood.
The process of producing high-purity polysilicon by zinc reduction method is as follows:
The solar-grade silicon production process of the zinc reduction method (Figure 1) can be roughly divided into: chlorinated distillation, reduction reaction and electrolysis. SiCl4 rectification and purification can remove some impurities: reduce silicon to remove some impurities: electrolyze ZnCl2, recycle Zn and Cl. The difference between the zinc reduction method and the Siemens method is that the reducing agent is zinc, while the µ Siemens method uses hydrogen for reduction; Another big difference is that the zinc reduction method uses silicon tetrachloride distillation, while the Siemens method uses trichlorosilane distillation. The key technological process of zinc reduction method includes zinc reduction reaction process and electrolysis process.
In the process flow, firstly, the metallurgical silicon with a purity of 98% is processed into 100~200μm powder, and then it is reacted with chlorine in a fluidized bed to form silicon tetrachloride, which is then boiled to form steam and some impurities The hydride is also mixed in it. This process is called chlorination; then in the rectification process, under a certain temperature and pressure, iron (Fe), aluminum (AD and other heavy metals, phosphorus (P), boron (B), etc. Impurities are removed in the step-by-step fractionation process. Through this process, high-purity tetrachlorosilane above 4N (99.99%) can be obtained; then the tetrachlorosilane and zinc are mixed in a high-temperature vapor state above 1000 ℃ for reduction Reaction. In order to fully reduce the tetrachlorosilane, zinc vapor is introduced into the quartz tube reaction furnace to form a zinc vapor atmosphere, and then tetrachlorosilane gas is introduced. The two form a reduction reaction in an instant, and the reduction degree is almost reached 100%, the products after the reduction reaction are silicon and ZnCl2 and a very small amount of unreacted substances. Due to the different melting points of the products, the melting point of silicon is 1420℃ and the melting point of zinc chloride is 283℃, so it is reduced near 1000℃ The silicon is transported out of the furnace after being precipitated in the form of solid crystals (needle, flake and granular).
The zinc reduction reaction produces zinc chloride at the same time. The boiling point of zinc chloride is 732°C. In an atmosphere of about 1000°C, the zinc chloride is discharged from the reactor in the form of flue gas and introduced into the electrolysis system. When it is cooled to about 500°C The molten zinc chloride formed in the molten state with good conductivity is introduced into the electrolytic cell, and is electrolyzed into liquid zinc (the melting point of Zn is 419.5C) and chlorine gas through a direct current of 2V, 5000A. Liquid zinc enters the evaporator, and chlorine enters the gas storage tank, and is reused in the reduction process and rectification process, respectively. Since almost no waste is discharged, it is a closed-loop production system, which is different from the problem of coping with by-products. Other chemical purification methods, and the zinc reduction method can use modern control technology to achieve continuous production. It can be said that the zinc reduction method is a low-cost, energy-saving and environmentally friendly high-purity silicon production method. Under different process conditions such as temperature, gas ratio and gas supply rate, different forms of silicon products can be obtained in needle, granular and powder form. Figure 2 shows the needle-shaped solar-grade silicon produced by the zinc reduction method. The zinc reduction method The reaction is completed in an instant, and 70% to 80% of the raw material is formed into solar-grade silicon. This high efficiency cannot be achieved by the traditional Siemens method.
At present, zinc reduction production technology needs to solve several main problems: ①How to form an automated continuous production line. The automated continuous production must ensure that the reaction is as thorough as possible, but also to ensure the continuity of material flow: ②Under what conditions (temperature, Use force, logistics speed) to produce polysilicon in different forms (needle, flake, granular) required by users; ③The evaporation rate of zinc vapor is unstable, how to control the temperature and pressure of the evaporation furnace to make the evaporation rate stable and controllable ④At present, there is no mature ZnCI2 electrolysis technology in the world. The so-called technology mainly refers to the process parameters such as temperature, pressure, current density, voltage, etc., as well as technical secrets or know-how such as the sealing of electrolysis devices and methods to improve electrolysis efficiency. , What type of electrode, etc., need to be researched and developed on the basis of some traditional technology; ⑤The subject of different product properties, such as the electrolysis of MgCl2, the density of magnesium is small, and it can be separated after floating to the upper layer, while zinc The density is large, and it is deposited on the bottom of the tank. How to smoothly separate and discharge the material requires special technology or know-how; ⑥Although it is a closed-loop production process, there are still about 10% of industrial residues that need to be treated. It is necessary to develop efficient treatment and treatment of industrial residues. Recovery method.
Since the material used in the “zinc reduction method” is silicon tetrachloride, which is different from the material used in the traditional “Siemens method”, it has no by-products or three wastes in the production process, short production process, fast reaction speed, stable product quality, It can achieve cost control and other advantages. It is believed that the “zinc reduction method” will definitely attract people’s attention.