Metallurgical silicon preparation is mainly made by reducing siliceous raw materials with carbon in an electric arc furnace.
When selecting a carbonaceous reducing agent for smelting silicon, the requirements: low ash content, strong reducing ability, high resistance, not easy to graphitize at high temperature, suitable particle size, high mechanical strength, etc. The chemical reaction activity of carbonaceous reducing agent is related to its porosity, density and specific surface area. Generally, carbonaceous reducing agents with high porosity, low density and large specific surface area have good chemical reactivity. Carbonaceous reducing agent is also the main conductor in industrial silicon charge. Only when the resistivity of the reducing agent is high, can the electrode be buried to a certain depth, and it is also possible to use a higher working voltage. And enough electrode burying depth and higher working voltage are necessary to increase output, reduce heat loss, and maintain normal smelting process.
The properties of the carbonaceous reducing agent change during the high-temperature coking process and smelting, that is, different degrees of graphitization are produced. Graphitization starts at 1600°C and ends at 2500°C, which means that the temperature range at which graphitization starts is also the temperature at which the reduction reaction occurs in industrial silicon production. The better the graphitization performance of the reducing agent, the worse its chemical activity and the smaller the specific surface area resistivity. All carbon-containing materials that can be used for electrothermal smelting of alloys essentially contain an allotropic form of carbon-graphite. The graphite crystals in these materials are regular hexagons, with a side length of 1.41Å (1Å=10-10m, the same below), the reference plane spacing is equal to 3.345Å, and the reference planes are arranged symmetrically in even and odd layers.
Certain carbonaceous materials (anthracite, coal, charcoal) are different from graphite. They contain amorphous carbon and consist of small crystals with a graphite lattice. The high dispersibility of amorphous carbon makes its surface energy larger than that of large-grained graphite, and it has higher reaction energy. The straight pores of charcoal facilitate the passage of gas, while the pores of petroleum coke are basically isolated and closed. The reaction capacity depends not only on the total number of pores, but also on the size and nature of the micropores. The reducing agents commonly used in production mainly include charcoal, petroleum coke, coal-based carbon, and low-ash coal.
Charcoal is the dark brown or black porous solid fuel that remains after incomplete combustion of wood or wood materials or pyrolysis in the absence of air. It is an impure amorphous carbon that maintains the original structure of the wood and the residual tar in the pores. In addition to carbon, charcoal also contains hydrogen, oxygen, nitrogen and a small amount of other elements.
Charcoal has a large porosity, a large specific surface area, and has many micropores and transition pores, suitable for gas to pass, and its reducing power is much larger than that of coke. In addition, the ash contained in charcoal, especially alkali metals, alkaline earth metals and their oxides, also have a catalytic effect on the chemical reaction ability of charcoal. Fixed carbon is the most important component in charcoal. According to the regulations of the Ministry of Forestry of the People’s Republic of China, the fixed carbon index of charcoal for industrial use in Northeast China is between 70% and 86% according to different charcoal raw materials. The specific resistance of charcoal is quite large, approximately equal to 1010~1012Ω·cm, and the degree of graphitization of charcoal at high temperature is not obvious, the resistivity is high, and the reaction activity is strong. Although the mechanical strength is slightly lower, it is currently the most suitable for smelting silicon. Good carbon reducing agent. However, due to the lack of domestic technology for the production of charcoal, the environmental pollution during the production process is high, the performance indicators of charcoal products are unstable, and the country advocates the protection of forest resources, the large-scale use of charcoal in industrial silicon production will inevitably be restricted.
(2) Petroleum coke
Petroleum coke has the lowest ash content among all reducing agents used to produce silicon. Petroleum coke is made by coking with heavy thermal cracking residue or thermal decomposition residue (after petroleum refining at 450~700℃). It has irregular shapes and uneven sizes. It is black lumps or granules with metallic luster and granules. It has a porous structure, mainly composed of carbon elements, accounting for more than 80% (mass ratio), and the rest are hydrogen, oxygen, nitrogen, sulfur and metal elements. Petroleum coke has the advantages of high fixed carbon content, low ash content, high mechanical strength, and stable composition. It is one of the carbonaceous reducing agents commonly used by many industrial silicon manufacturers. But petroleum coke is a strong graphitizing reducing agent, resulting in low reducibility and low resistivity. Therefore, it needs to be used in conjunction with charcoal, low-ash bituminous coal, etc., to supplement and fix carbon.
(3) Bituminous coal
The ash content of high-quality bituminous coal is about 3.0%. Domestic manufacturers have used some bituminous coal instead of charcoal, and achieved good technical and economic indicators. Bituminous coal with a low degree of metamorphism has high specific resistance and good reaction performance. It is an ideal reducing agent for silicon smelting. The specific resistance of high-volatile bituminous coal is equivalent to 40 times that of delayed petroleum coke, and it has the characteristics of strong reaction ability and high resistance.
But bituminous coal does not have a porous structure like charcoal and has a small specific surface area. Therefore, when bituminous coal is used as a reducing agent, the air permeability of the charge is not good, and a suitable charge loosening agent must be selected.
(4) Coal-based carbon
Coal-based carbon reducing agent is prepared by adding additives to coal-based processing, and its physical form and performance are similar to charcoal. Coal-based carbon has many pores, thicker pore walls, and higher strength. It has the advantages of high fixed carbon content, low ash content, low sulfur, high chemical activity, and low relative density. It is very suitable as a reducing agent in industrial silicon smelting. In addition, coal-based carbon has a high specific resistance (109Ω·cm at room temperature), which is conducive to the smooth progress of the industrial silicon smelting process. It can be produced with high voltage and ensure the electrode is buried deeply, while reducing power consumption.
Practice has proved that in order to achieve better reduction effects and good economic benefits, several carbonaceous reducing agents are generally used in combination.