This article will introduce the upstream raw materials of industrial silicon, the output of industrial silicon, downstream demand and application from multiple dimensions, so as to further popularize the basic knowledge of industrial silicon.
1. Introduction of industrial silicon industry chain
Industrial silicon, also known as metalloid silicon, is a trade name that appeared in the mid-1960s. Silicon metal is a product smelted by silica and carbonaceous reducing agent in a submerged arc furnace. The content of the main component silicon is about 98% (in recent years, the silicon content of 99.99% is also listed in the metal silicon), and the rest of impurities For iron, aluminum, calcium and so on.
Industrial silicon production is smelted in an electric furnace container with silicon ore as raw material and carbon material as reducing agent. The main cost components consist of raw materials, auxiliary materials and electric energy. The downstream products are mainly organic silicon, silicon aluminum alloy, polysilicon, silicon carbide, etc.
The midstream of industrial silicon is mainly used for applications in four directions, (1) aluminum alloy, adding silicon in the aluminum alloy casting process can improve the high temperature fluidity and wear resistance of the alloy, while reducing shrinkage and hot cracking tendency; (2) Organosilicon, that is, an organosilicon compound, has the properties of both inorganic materials and organic materials. It has the basic properties of low surface tension, low viscosity-temperature coefficient, high compressibility, high gas permeability, and has high and low temperature resistance, electrical insulation, oxidation resistance and stability. (3) Polysilicon, metal silicon can be prepared into polysilicon after further purification. Polysilicon is an extremely important photovoltaic and semiconductor material; (4) refractory materials, etc.
2. Industrial silicon cost structure
Each ton of industrial silicon production raw materials requires 2.7-3 tons of silica, 0.6 tons of petroleum coke, 1.5 tons of clean coal, 0.3 tons of wood chips, and 0.08 tons of graphite electrodes. At the same time, it needs to consume about 13000kWh of electricity. On the cost side, the electricity cost is the highest, reaching 5,200 yuan/ton in early 2022, accounting for 36%; the total cost of carbonaceous reducing agents (petroleum coke, silicon coal, charcoal, wood chips) is 3,700 yuan/ton, accounting for 25%; The cost of raw material silica is only 8%. Under different processes (such as different types and ratios of reducing agents added), the cost will also vary accordingly. In the past year, the smelting cost of industrial silicon has been continuously rising due to the influence of raw materials.
3. Introduction of industrial silicon raw materials
Industrial silicon raw materials are divided into silica, carbon reducing agent, and graphite electrode, wherein the carbon reducing agent is mixed with petroleum coke, charcoal, bituminous coal, wood chips, etc. in different proportions.
Silica: abundant resources, but high-quality ore (silicon 98% + purity) is scarce
Silicon is extremely rich in resources in the earth’s crust, second only to oxygen, accounting for more than a quarter of the total weight of the earth’s crust, and exists in rocks, gravels and dust in the form of silica or silicate. The purest silicon mineral is quartz or silica. my country is rich in quartz ore resources, there are many kinds of quartz sand deposits, and the retained ore reserves exceed 4 billion tons, but the quality is not high. High-quality vein quartz only accounts for 0.93% of my country’s quartz ore resources. The quality of silica directly determines the quality of industrial silicon. Yunnan Province, the main producing area, has issued a policy requiring small-scale smelting furnaces to be phased out within a time limit. More complex process requirements and higher ore quality requirements will reduce the output of high-grade industrial silicon.
Silica quality requirements
For the silica of high-grade industrial silicon, its oxide impurity content should reach: iron oxide (Fe2O3) <0.15%; alumina (Al2O3) <0.20%; calcium oxide (CaO) <0.15%.
Ultra-high-quality industrial silicon, stricter control of oxide impurities, and strict requirements for boron (B) phosphorus (P) in silica, iron oxide (Fe2O3) <0.1%; alumina (Al2O3) <0.15%; oxidation Calcium (CaO) <0.10%.
Silica particle size is another important process condition for smelting production. my country’s current small industrial silicon electric furnace silica enters the furnace with a grain size of 20-60mm, medium-sized electric furnaces with a silica grain size of 30-80mm, and large-scale electric furnaces with a silica grain size of 40-100mm.
Carbon reducing agent (petroleum coke, bituminous coal, charcoal, wood chips)
The basic requirements for reducing agents in industrial silicon production: certain particle size and mechanical strength, low ash content, strong chemical reactivity, high fixed carbon, high specific resistance, and moderate volatility. Commonly used reducing agents for industrial silicon are: blue charcoal, charcoal, bituminous coal, and petroleum coke. In the production of industrial silicon, different types of reducing agents are selected and the ratio between them is adjusted to control the quality and furnace conditions of industrial silicon.
The main target of petroleum coke is the petrochemical plant 4B coke, and an enterprise in Xinjiang consumes 0.6 tons of petroleum coke to produce a single ton of industrial silicon. The quality indicators of silicon coal are very strict, mainly containing less than 0.2% of iron, less than 0.8% of aluminum, and less than 0.5% of calcium. Some manufacturers use charcoal or wood chips instead. A single ton of wood chips consumes 0.3 tons of industrial silicon and 0.6-0.8 tons of charcoal.
When producing industrial silicon, the lower part of the conductive electrode is buried in the charge, an arc is formed in the charge layer, and the heat energy generated by the resistance of the charge itself is used to heat the charge. Among them, the submerged electric furnace with high current density requires graphite electrodes.