The influencing factors of silicon thermal reduction method for smelting rare earth ferrosilicon alloy in DC electric arc furnace

March 13, 2024

For the production of rare earth ferrosilicon alloy by DC Electric Arc Furnace silicothermal method, years of scientific research and production practice have proven that the alkalinity of ingredients, slag to agent ratio, temperature, and stirring strength have a decisive impact on the grade and yield of rare earth alloy. The alkalinity of ingredients, slag to agent ratio, reduction temperature, and stirring strength are commonly referred to as the four elements in the smelting process of rare earth ferrosilicon alloys.
1. Alkalinity of ingredients
Rare earth minerals such as fluorocarbon cerium and monazite are transformed into spinel and cerium calcium silicate during the production of rare earth rich slag. Only by reacting with lime at high temperatures (1240-1300 ℃) can rare earth oxides be free in the form of cerium spinel and reduced by silicon or calcium silicate. So lime is the key to promoting the decomposition of rare earth minerals. After adding the reducing agent ferrosilicon, cerium spinel is reduced to rare earth silicides, and lime reacts with the reaction product silica to form dicalcium silicate, tricalcium silicate, etc., reducing the activity of silica and promoting rare earth reduction. However, excessive alkalinity reduces the concentration of rare earth elements in the slag, while also making the slag sticky, affecting the diffusion of reactants and hindering the progress of reduction reactions.
2. Slag to agent ratio
The ratio of rare earth rich slag in a DC Electric Arc Furnace to the mass of silicon iron used is called the slag agent ratio. The slag to agent ratio is an important data in ingredient calculation. The experiment conducted by the Shanghai Institute of Metallurgy shows that under the same operating conditions with a selected ingredient alkalinity of 4.0 and a temperature of 1200-1300 ℃, smelting low-grade alloys yields high rare earth elements, while smelting high-grade alloys yields low rare earth elements.
3. Reduction temperature
Silicon thermal reduction method is used to prepare rare earth silicon iron alloy. When the reduction temperature is high, the alkalinity of the slag is low, which is conducive to the diffusion of reaction ions, so the rare earth content in the alloy reaches its peak quickly. However, if the temperature is too high, the oxidation rate of the alloy accelerates, making it difficult to control the alloy composition during the smelting process. The oxidized rare earths in the alloy return to the slag, causing a decrease in the alloy grade. It is generally believed that the most suitable reduction temperature is 1300~1350 ℃. If the reduction temperature is raised to 1400-1450 ℃, there is a tendency for the rare earth content in the secondary slag to increase and the rare earth recovery rate to decrease.
4. Mixing intensity
The reaction between rare earth slag and liquid ferrosilicon belongs to liquid-liquid reaction, and the diffusion of reactants is a limiting factor in the reaction rate. The liquid-liquid reaction can only be carried out at the interface, and stirring can make the reducing agent ferrosilicon fully contact with the reduced material rare earth slag, expand the contact surface, increase the collision of reactants and the opportunity for products to leave, strengthen the reaction, and reduce smelting time.

Author:

Mr. Ziyu Song

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13949514533@163.com

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8618790865262