Industrial silicon and ferrosilicon industries are both high energy consumption industries. They use silica, coke, and (steel scrap), as raw materials to smelt in a submerged arc furnace. China is one of the largest producing countries of industrial silicon and ferrosilicon. Industrial silicon is widely used in the photovoltaic industry and information industry. Ferrosilicon alloy is widely used in the steelmaking industry as a deoxidizing agent and alloying agent, or used in the casting industry as an inoculant or nodulizer.
At present, there are two disadvantages to the production of industrial silicon and ferrosilicon. One is low resource utilization, another is the low thermal efficiency of submerged arc furnaces. Due to the low resource utilization, a large amount of waste residue is generated in industrial silicon and ferrosilicon production, which seriously pollutes the environment.
(Ferro) Silicon Slag
Silicon slag is generated in industrial silicon and ferrosilicon production, 100kg of silicon slag is generated for every ton of ferrosilicon produced. The content of silicon metal in the silicon slag is about 15%. Part of the silicon slag is used to refine silicon metal, a large amount of silicon slag is piled up in the factory, or used for paving, which causes a waste of resources.
Silicon slag contains silicon oxide, calcium oxide, aluminum oxide, and other waste residues, and contains 15%-20% industrial silicon which has high economic values. Recycling the silicon metal in the silicon slag can reduce the pollution of metal silicon producers to the environment, and at the same time bring great economic benefits. Production of silicon briquette from silicon slag is another way to re-use the silicon slag.
Production of Silicon Metal from the Silicon Slag
Due to the high viscosity and poor fluidity of silicon slag, silicon slag is wrapped by waste slag. The silicon is separated from the slag by heating and adding argon with sufficient stirring.
- The silicon slag is crushed by a jaw crusher and beating sander.
- The crushed silicon slag is reselected in the shaker, and smelt the reselected silicon slag in the ladle.
- The process of combining electromagnetic separation and slag treatment is adopted. In the condition of 15kW of heating power and 60 minutes of keeping warm, the recovery rate of silicon metal is up to 96%, and the removal rate of Aluminum, Boron, Iron, Titanium, Phosphorus, respectively up to 98%, 68.5%, 81.5%, 82.3%, 36.8%.
Production of Silicon Carbide Briquette from the Silicon Slag
The silicon slag that contains silicon carbide is used as a high-quality alloying agent additive in the casting industry. Produce the silicon slag into a certain size of briquette for the pig iron smelting. The silicon slag briquette can improve the temperature of the furnace, promote slag discharge, and raise the grade of the products. The silicon slag briquette can replace the ferrosilicon alloy in the small foundry furnace to improve the fluidity of molten iron, and improve the casting resistance and cutting ability. At present, silicon slag briquettes are widely produced and utilized and have formed a certain scale.
Silica fume is a fine particle produced when smelting silicon metal, its main component is SiO2. According to the difference in chemical composition, and particle size, the price of silica fume varies greatly. The high-quality silica fume is produced by developed countries such as Norway, America, etc. The price can reach hundreds or even thousands of dollars.
The silica fume produced by China is usually used as raw materials for concrete and refractories. The price of silica fume ranges from tens to hundreds.
300kg of silica fume is generated for every ton of industrial silicon produced, and 200kg of silica fume is generated for every ton of ferrosilicon produced. In 2019, the total production volume of silicon metal and ferrosilicon in China respectively reach 2200 thousand tons and 5490 thousand tons, the production volume of silica fume should reach 1750 thousand tons. Because many enterprises do not have dust removal equipment, the recovery rate of micro-silica fume is less than one-third. The total value of micro silica fume is more than 80 million per year.
Recycling and Utilization of Silica Fume
The recycling methods of silica fume are bag dust removal method and electric removal method. These two methods can recycle 99% of silica fume. The silica fume can use in the concrete to improve the density of concrete and increase the leak resistance of the concrete.
Silica fume is used as an additive for the refractory to improve its strength and high-temperature resistance.
High Carbon Silicon
In the smelting of silicon metal, due to the insufficient heat of the electrodes in the furnace, the raw materials at the bottom of the furnace are not fully reacted, forming a carbon-containing silicon-containing material, which we call high-carbon silicon.
High-carbon silicon can partially replace ferrosilicon, and its application in steelmaking can reduce the amount of alloy added, reduce the cost of steelmaking and increase economic benefits.
The byproduct of silicon metal (ferrosilicon) production is not only silicon slag, silica fume, and high carbon silicon, but many other products. Rational use of these by-products to protect resources and increase economic benefits.