- What is the chemical reaction of the carbon reduction method?
The chemical reaction of the carbon reduction method is usually represented by the following formula:
This is only the expression of the main reaction of the carbon reduction method, and it is also the basis for the general calculation and control of the normal reaction, but the actual chemical reaction of the carbon reduction method is very complicated. The carbon reduction method is realized by the gas phase. After the silica becomes a gas, it is decomposed into silicon monoxide and oxygen:
The following chemical reactions occur between carbon and the above-mentioned decomposition products:
The presence of free carbon leads to the formation of silicon carbide, but at temperatures below 1400 °C, the reaction rate is slow. If the temperature is lower than (1620±30) ℃, and the carbon content is excessive, all the carbon can be converted into silicon carbide.
Next, the reaction product silicon carbide reacts with oxygen as follows to obtain elemental silicon:
In the chemical reaction process of industrial silicon production, there is a clear sequence. That is, in the reaction process, the decomposition of silicon dioxide comes first, followed by the formation of silicon carbide, followed by the destruction of silicon carbide and the reduction of silicon. At the same time, it can be found that this reaction process is a process from high price to low price, that is, SiO2→SiO→Si.
The decomposition of silicon dioxide and the formation of silicon carbide are two important intermediate processes, and the destruction of silicon carbide is the end of industrial silicon production. It can be seen from this that the reduction of silicon is to get rid of the entanglement of oxygen, but without the help of oxygen, the reduction of silicon cannot be completed.
Of course, the chemical reactions of industrial silicon production are complex, and in addition to the above chemical reactions, some other reactions may occur. When the temperature is lower than 1100°C, the gas in the high temperature reaction zone escapes from the material surface, and the residual silicon monoxide in the gas contacts with the oxygen in the air, and the following reactions occur:
It is possible that the following reactions occur on the active surface of the reducing agent:
When the temperature is too high (above 1800°C), the following reactions will occur:
When the furnace material and reducing agent fall, they come into contact with the rising silicon monoxide gas, and the following reactions occur:
The drop of silicon carbide also reacts with silicon dioxide:
In conclusion, in industrial silicon production, the reactions in the electric arc furnace are very complex, and the reactions in each area of the electric arc furnace are also different. But as long as the ratio of raw materials and temperature control are appropriate, the main reaction is still reaction (formula 3) ~ reaction (formula 5). In fact, it is the reaction (Equation 1).
- What are the requirements for silica in industrial silicon production?
The main raw materials for the production of industrial silicon are silica and carbonaceous reducing agents. Due to the strict restrictions on the content of iron, aluminum and calcium in industrial silicon, the requirements for raw materials are also very strict. Generally speaking, the selection of silica is that the higher the content of silica, the better, and the lower the content of harmful impurities such as iron, aluminum, and calcium, the better.
Finding high-quality silica and controlling the impurity content of silica are the basic guarantees of product quality. It can be said that the first key issue in industrial silicon production is to select good raw materials.
The chemical composition requirements for silica in general industrial silicon production are shown in Table 1.
China is rich in silica resources, with origins all over the country, and there are many high-quality silica deposits. The main distribution is shown in Table 2.
Silica particle size requirements: 60~100mm account for 75%, less than 60mm account for 10%~15%, and larger than 100~120mm account for 10%~15%. The particle size of silica is an important technological requirement for smelting. The suitable particle size of silica is affected by various factors such as the type of silica, operating conditions and the type of reducing agent. It depends on the specific smelting conditions. If the particle size is too large, it cannot be adapted to the reaction rate, and it is easy for unreduced silica to enter the liquid silicon, resulting in an increase in the amount of slag. Increase, and affect normal production in severe cases. If the particle size is too small, although the contact surface of the reducing agent can be increased, which is beneficial to the reduction reaction, the gas generated during the reaction cannot be discharged smoothly, and the reaction rate will be slowed down. At the same time, if the particle size is too small, the impurities brought in will also increase, which will affect the product quality. Silica that is generally less than 5mm in production should not be used. In addition, it is necessary to pick out impurities and wash them with water, so that the fine materials are put into the furnace.