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The melting mechanism of lime in the steelmaking process

[12/06/2021]


Research on the process of melting lime into slag shows that the melting of lime is divided into three steps:

(1) Heating of lime blocks and penetration of slag;
(2) FeO, MnO, CaF2, etc. that have penetrated into the pores of the lime and CaO form a melt with a lower melting point to melt the lime;
(3) The melted lime separates from the lime particles and enters the slag;

The lime added into the furnace is heated and penetrated immediately after it comes into contact with the slag. The infiltration process is carried out through the pores of the lime itself, and the outer opening of the pores expands as it melts. The infiltrating slag is saturated with CaO, thus reducing the infiltration rate. The subsequently infiltrated FeO, MnO, CaF2 and other components form a solid solution with CaO to melt the lime. The molten lime decomposes the lime block into multiple small particles, so the contact surface between the slag and the lime is enlarged, and the lime melting is accelerated.

In the process of melting lime into slag, the limiting step is the penetration of slag into the lime pores. The infiltration of slag is related to the size and type of lime pores. The pore radius of hard cast lime increases due to the merger of primary crystals and the porosity decreases, so it is saturated by slag penetration faster. Soft burned or light burned lime is because of the pores The slag absorption rate is high, and the amount of slag absorbed is relatively large. When the slag penetrates into the pores and is not too deep, it will react with CaO to form a low-melting solid solution and leave the body. Although the slag penetration of hard burned lime is deep, the amount of slag infiltrated is relatively small due to low porosity, and because lime has large CaO grains and low activity, it forms a low-melting solid solution with slag slowly, and the quantity is small. Therefore, the melting speed of lime is relatively slow.

According to the kinetic conditions of multiple reactions, the melting process of lime in the slag consists of the following two links:

(1) External mass transfer, which refers to the diffusion and mass transfer of FeO, MaO and other oxides to the surface of the lime block and the mass transfer of molten CaO to the depths of the slag phase. This link is affected by the viscosity, concentration gradient, The influence of the stirring degree of the molten pool and the size of the contact area of the lime slag;
(2) Internal mass transfer, which refers to the mass transfer of slag along the pores, cracks and crystal interfaces of the lime block and the further diffusion of Fe2+ in the lime lattice to form a eutectic, compound or eutectic with a lower melting point than CaO The factors that determine the degree of wettability of the slag on the surface of the lime block and the factors that determine the diffusion of the particles in the CaO crystal affect the second link.

Understanding the dissolution mechanism of lime in the steelmaking process will help us to better produce high-quality lime and provide qualified slagging raw materials for steelmaking.
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