Introduction to the process of smelting ferrosilicon in a DC submerged arc furnace
March 09, 2024
Why can silicon iron be smelted using silica, coke, and steel shavings after high-temperature heating in a DC submerged arc furnace? To understand this principle, the first step is to understand the variation patterns of various raw materials used in smelting ferrosilicon under various high temperature conditions.
The main raw material for smelting ferrosilicon is silica, which contains about 98% SiO2. SiO2 is very stable and has a strong affinity between silicon and oxygen, making it difficult to separate. In order to separate and remove oxygen from silica, carbon in coke is used to extract oxygen from SiO2 under high temperature conditions in a DC submerged arc furnace, and the higher the temperature, the stronger the ability of carbon to extract oxygen. At this point, oxygen and carbon in SiO2 react to generate carbon monoxide, which is discharged from the furnace through the material layer. After the oxygen in silicon dioxide is taken away by carbon, the remaining silicon and iron form ferrosilicon.
The reaction between silica and carbon is as follows:
SiO2+2C=Si+2CO ↑
The above equation is an endothermic reaction. From the reaction equation, it can be seen that in order to accelerate the reaction, the electrode should be deeply inserted into the furnace material to increase the furnace temperature, expand the crucible area, and increase the permeability of the material surface to allow carbon monoxide gas to escape as soon as possible.
Due to the presence of steel shavings and iron in the smelting furnace of ferrosilicon, the reduction reaction of silicon dioxide is easier to carry out. This is because the reduced silicon and iron form ferrosilicon, which improves the conditions of the reduction process. Therefore, the more iron there is, the easier the reduction reaction of silicon dioxide is to occur. Production also proves this point. The lower the silicon content in ferrosilicon smelting, the lower the unit power consumption. The electricity consumption for smelting 45 tons of ferrosilicon is approximately 3800-4500 degrees Celsius. The electricity consumption for smelting 75 tons of ferrosilicon is approximately 7000 to 7800 kWh. The electricity consumption per ton of silicon smelting is approximately 10500-12000 kWh.
During the reaction process, the vast majority of silica in silica is reduced by carbon, while other impurities and ash brought in by coke, such as calcium oxide (CaO), phosphorus pentoxide (P2O5), and aluminum trioxide (AI2O3), are also reduced by carbon. The vast majority of phosphorus pentoxide is also reduced. The reactions are as follows:
CaO+C=Ca+CO ↑
P2O5+5C=2P+5CO ↑
AI2O3+3C=2AI+3CO ↑
The carbon monoxide gas generated in each reaction escapes from the DC submerged arc furnace, while other products such as calcium, aluminum, and phosphorus enter the ferrosilicon. Therefore, it is required to minimize impurities in the raw materials to ensure the quality of ferrosilicon.
During the smelting process, a small amount of silica, aluminum oxide, and calcium oxide are not reduced and form slag. The slag composition contains approximately 230-40% SiO; AI2O345~60%; CaO10-20%. The melting point of this type of slag is about 1600-1700 ℃. When the amount of slag is large, the power consumption increases. At the same time, overly sticky slag is difficult to remove from the DC submerged arc furnace, causing deterioration of the furnace condition. Therefore, better raw materials should be used to reduce the amount of slag and unit electricity consumption.
Under normal circumstances, it is advisable to control the slag amount to no more than 5% of the silicon iron content. The above are the basic principles of ferrosilicon smelting, and the basic reactions of ferrosilicon smelting are as follows:
SiO2+2C=Si+2CO ↑
The actual chemical reactions in the DC submerged arc furnace are more complex than this. Experiments have shown that the reduction of oxides is gradually reduced from high valence oxides to low valence oxides. The reduction of silica is first reduced to silicon monoxide (SiO) under high temperature conditions, and then to silicon (Si), in the order of SiO2 → SiO → Si
When smelting ferrosilicon, the following reactions will occur at 1700-1800 ℃:
SiO2+C=SiO+CO ↑
That is to say, carbon dioxide and silicon are first reduced by carbon to carbon monoxide, and then further reduced to silicon. The reaction formula is as follows:
SiO+C=Si+CO ↑
The reduced silicon will partially react with silica to produce silicon monoxide, and the reaction formula is as follows:
SiO2+Si=2SiO ↑
From the above three reaction equations, it can be seen that silicon monoxide is an important link in promoting the smelting reaction. Silicon monoxide exists in a gaseous state at high temperatures and is unstable at low temperatures. Therefore, silicon monoxide is a gas in the crucible inside the furnace. A small amount of silicon monoxide escapes from the furnace mouth and is oxidized by air to become silicon dioxide. After cooling, it turns gray white and partially condenses at electrodes, tube tiles, and other places. At high temperatures above approximately 1700 ℃, most of the silicon monoxide evaporates into the pores of the coke, extensively coming into contact with carbon and reducing to silicon. A small amount of silicon reacts with silicon dioxide in the high temperature region, generating silicon monoxide according to the final reaction formula, and then reacting with carbon, resulting in a continuous reaction. It can be inferred that silicon monoxide is not only an intermediate product of the reaction, but also promotes the acceleration of the reaction.
Due to the fact that silicon monoxide is a gas at high temperatures, it is prone to volatilization and loss, especially when materials collapse or a large fire occurs, white gas that escapes or sprays out is mostly silicon monoxide. Therefore, it is required to handle the phenomenon of material collapse or large fire in a timely manner, otherwise, it will cause a significant loss of silicon monoxide, reduce production, and increase unit power consumption.
