The raw materials for smelting ferrosilicon include silica, carbonaceous reducing agent and steel shavings. When ferrosilicon is smelted, silicon is reduced from silica containing SiO.
1. Silica
Common silicas are quartz and quartzite. Quartz is a mineral with a dense crystal structure and a density of 2650kg/m3. Pure quartz is colorless or milky white, sometimes gray and reddish due to different impurities, glass luster, brittle, melting point is 1973K. Quartz is more expensive and is only used in the production of crystalline silicon.
Quartzite is made of quartz particles bonded by a binder. The binder is also SO, with high density and compressive strength, yellow, rose and other colors.
Silica for smelting ferrosilicon should meet the following requirements:
The silicon dioxide content in silica should be high, and the impurities should be low, usually requiring SiO2>98%. The reduction of silicon dioxide with carbon requires a relatively high temperature, especially when the content of silicon dioxide is low and combined with calcium oxide to form a stable compound, the reduction temperature is higher, which will increase the power consumption of smelting and the amount of reducing agent.
Impurities Al2O3, CaO, MgO and PzO in silica; the lower the content, the better. It is required that AlOz≤1%, and the silica with high AlzO content is easy to sinter the charge and increase the amount of slag, making it difficult to discharge the slag. When aluminum is reduced it also increases the aluminum content of the alloy. In addition to the silica itself containing Al2O3, the surface of the silica will adhere to soil during transportation, which will also increase the content of Al2O, so the silica should be washed with water several times before smelting. In addition to AlO in silica. In addition to slag-forming oxides, CaO and MgO are also slag-forming oxides, and the reduction of calcium will increase the calcium content in the alloy and affect product quality. In order to ensure product quality and smooth furnace conditions, it is required that CaO+MgO≤1%. The assembly increases the power consumption of smelting.
Phosphorus and sulfur are harmful elements of high-grade high-quality steel. Sulfur can form compounds SiS and SiS with silicon. These compounds are highly volatile at the smelting temperature, so there are generally no special requirements for sulfur. P2O in silica should not exceed 0.02%. This is because 80% of the phosphorus in the charge is reduced into the alloy. High phosphorus will reduce the first-grade rate of ferrosilicon products, and ferrosilicon is easy to pulverize.
FeO in silica is beneficial to furnace conditions, but the reduction of iron consumes a certain amount of energy.
In addition to chemical composition, silica is also required to have high high-temperature strength, that is, anti-explosion. When the silica is heated, it will explode into powder due to the crystal phase transformation, which will reduce the gas permeability of the charge. Due to the low furnace mouth temperature in small electric furnaces, the bursting phenomenon of silica is often not serious, but this property has a significant impact on large furnaces.
In order to make the charge have a certain air permeability and accelerate the melting and reduction of the charge, the charge is required to have a certain particle size. Silica that is too small not only contains more impurities, but also seriously affects the air permeability of the material surface; silica with too large particle size is easy to cause stratification of the charge, delaying the melting and reduction reaction speed of the charge. Usually, the size of silica in large furnaces is 40~120mm, and that in small furnaces (1800kV.A) is 25~80mm, and the size of silica smaller than 20~40mm should not be less than 20.
The selection of furnace material during production is one of the central links of ferrosilicon energy saving, and it is the guarantee for expanding the crucible. The chemical composition of silica should be selected and washed with water to reduce impurities, reduce the amount of slag, reduce the content of impurities in the metal, stabilize the furnace condition, and reduce power consumption and raw material consumption.
2. Reducing agent
The reducing agents for smelting silicon-based alloys include metallurgical coke, charcoal, wood blocks, petroleum coke, pitch coke, gas coal coke, bituminous coal, and none. The main requirements for reducing agents are high fixed carbon content, low ash content, low volatile matter, and porosity. Large, good chemical activity, high resistivity; certain mechanical strength at high temperature; wide distribution, cheap price. From the above requirements, charcoal, petroleum coke, and pitch coke are the most suitable reducing agents, but they are expensive, and they are only used when smelting alloys with high purity and difficult to reduce; bituminous coal, anthracite, and gas coal coke have high resistivity. , low ash content, but brittle after being heated, it can only be used in conjunction with other reducing agents.
The most widely used reducing agent for smelting ferrosilicon is metallurgical coke (broken coke). Metallurgical coke is also the most widely used carbon reducing agent in the production of other ferroalloys. It requires fixed carbon ≥ 84% and ash content ≤ 14%. The ash contains nearly 40% Al20. High ash content will increase the amount of slag and harmful impurities in the alloy. At the same time, the coke ash is the source of phosphorus in the alloy. The water content of coke depends on its storage and transportation conditions, and the water content fluctuates widely (from 4% to 20% or more), and the carbon content has a great relationship with the water content, because the ingredients are calculated according to the weight Therefore, the moisture content in the coke should be checked frequently, and the moisture content is required to be small and stable, and the CaO and PzO contents should also be low.
The particle size of coke has a great influence on smelting. The particle size is large, the resistivity is small, the charge has strong conductivity, the electrode is not easy to be inserted deeply, the temperature of the furnace mouth is high, the heat loss is large, the power consumption is increased, and the reaction area of the large particle size coke is small, and the reduction ability is correspondingly reduced, so the coke particle size is too large. The influence of the environment is very great; the coke with smaller particle size has high resistivity, the electrode is inserted deeply, and the contact surface between the furnace charge and the coke is large, but if the particle size of coke is too small, the gas permeability of the charge will be reduced, the furnace mouth is easy to sting, and the local material collapses Many, so the particle size of coke should be appropriate. Generally, the coke particle size of large furnaces (above 12500kV.A) is 5~18mm, and that of small furnaces (1800kV.A) is 1~8mm, of which 1~3mm is not more than 20%.
ferrosilicon
3. Iron-containing raw materials
The iron-containing raw material is the regulator of the ferrosilicon composition. When ferrosilicon is smelted in an electric furnace, carbon steel turnings with low phosphorus content, no alloying elements and non-ferrous metals are generally used, and the steel turnings should not have foreign inclusions and oil stains. In addition, small scraps and rolled steel skins of other carbon steels can also be used.
During the reduction process of silicon dioxide, steel shavings can promote the reduction, absorb the reduced silicon and destroy silicon carbide. In order to melt the steel shavings as soon as possible and give full play to its effect, it is required that the steel shavings should not be too long. In addition, the long curled steel shavings will make it difficult for the automatic assembly and cannot make it evenly distributed in the furnace, so the curl length of the steel shavings should be less than 100mm.
The use of iron oxide can improve the furnace condition and make the slag flow out easily, but the reduction of iron oxide to iron needs to consume part of coke and electric energy, which increases the unit power consumption and reducing agent consumption.
Iron ore cannot be used as iron-containing raw material. Because the ore will bring a lot of slag, so that a lot of extra electricity will be consumed.