Dephosphorization of hot metal using rare earth oxide-containing slags

3.1 Dephosphorization temperature

Dephosphorization slag No. 6 and metal sample were used to investigate the effect of temperature on the dephosphorization efficiency. The contents of different components in slag and metal sample after dephosphorization experiment are listed in Table 2.

The data in Figure 2 indicate that the degree of dephosphorization initially increased as the temperature increased up to 1,550°C, and then the dephosphorization efficiency decreased as the temperature increased above 1,550°C.

Figure 2

Relationship between dephosphorization efficiency and temperature.

The chemical equation of hot metal dephosphorization using rare earth oxide-containing slags is expressed by the ion theory according to equation (1)

where αi, Pi and K1 are the activity of i, the partial pressure of i and equilibrium constant of equation (1), respectively.

The phosphate capacity CPO43− of the rare earth oxide-containing slags can be defined by equation (3).

where γPO43− is the activity coefficient of PO43−.

According to the equilibrium constant K1 of the dephosphorization reaction and Van’t Hoff’s equation (4), the relation between phosphate capacity of the rare earth oxide-containing slags and temperature can be expressed as equations (5) and (6).

where ΔH1 is the heat change for the dephosphorization reaction using rare earth oxide-containing slags; ΔH1θ is the heat change of dephosphorization reaction (1) when using pure substances; R is constant; γO2− is the activity coefficient of O2−; ΔHPO43− and ΔHO2− are the partial molar heats of solution of PO43− and O2− in molten slags. Neglecting the influence of temperature on the activity coefficients of PO43− and O2−, then equation (5) can be simplified as equation (7).

Ferromanganese dephosphorization is an exothermic reaction, and an increase in the dephosphorization temperature causes a decrease in the phosphate capacity of rare earth oxide-containing slags, as shown in equation (7). The thermodynamic analysis indicates that a lower temperature favours dephosphorization under oxidizing condition. However, dephosphorization process is a liquid phase reaction, controlled by material diffusion in the liquid phase and interface chemical reaction. A lower temperature leads to an uneven melting of the slag, and the transfer of material is rate-limiting, which leads to a poor kinetics of dephosphorization [16]. Therefore, when the temperature is below 1,550°C, the dephosphorization efficiency is mainly influenced by the kinetics, and an increase in temperature up to 1,550°C causes an increase in dephosphorization efficiency. However, when the temperature increase is above 1,550°C, the effect of kinetics on dephosphorization can be neglected, and thermodynamic limitations cause a drop in the dephosphorization efficiency.

3.2 Basicity

Dephosphorization slags Nos. 1, 2, 6 and 9 and metal sample were employed to study the effect of basicity on the dephosphorization efficiency. The contents of different components in slag and metal sample after dephosphorization experiment are listed in Table 3. As shown in Figure 3, the degree of dephosphorization initially increased as the basicity of slag increased up to 3.0, and the degree of dephosphorization has almost no change when the basicity is further increased to 3.5.

Figure 3

Relationship between dephosphorization efficiency and basicity.

With the increase of basicity, the effective solubility of CaO in the slags increases. According to the slag ion structure theory, CaO is alkaline oxide, which can provide free oxygen ions (O²⁻) into the dephosphorization slags [20]. The free oxygen ions (O²⁻) in slags increase as the basicity increases. As shown in equation (1), an increase in the O²⁻ contents facilitates the dephosphorization reaction. In addition, dephosphorization product (P₂O₅) is acid oxide, so an increase in the basicity of dephosphorization slags is beneficial to dilute the concentration of P₂O₅, thus improving the degree of dephosphorization [14].

The dephosphorization efficiency is also affected by the kinetics factor. When the basicity of slags is below 3.0, an increase in the basicity of slags causes an increase in O²⁻. O²⁻ can depolymerize the complicated network structure of the silica complex ions [21]. In addition, an increase in the O²⁻ contents will lead to an increase in the molar ratio between oxygen and silicon (OSI), and the complicated network structure of the silica complex ions is gradually transformed from skeleton (OSI = 2.0), to lamellar (OSI = 2.5), to chain (OSI = 3.0) and finally to Volmer–Weber (OSI = 4.0). Throughout these transformations, the viscous activation energy and viscosity of the slags decrease [22]. A decrease in viscosity of the slags is beneficial to melt evenly, thus improving the dephosphorization efficiency. However, when the basicity of slags is increased to 3.5, the viscosity of the slags increases, thermodynamic and kinetic effects balance out and the degree of dephosphorization has almost no change.

3.3 Rare earth oxide contents in dephosphorization slags

Dephosphorization slags No. 3 to 8 and metal sample were used to study the effect of basicity on the dephosphorization efficiency. The contents of different components in slag and metal sample after dephosphorization experiment are listed in Table 4. For dephosphorization slags with a basicity of 3.0, the degree of dephosphorization increases from 7.57% to 50.45% as the rare earth oxide contents increase from 0 to 10 wt%, and when the rare earth oxide contents increase above 6 wt%, the increasing trend gradually decreases, as shown in Figure 4.

Figure 4

Relationship between dephosphorization efficiency and rare earth oxide content.

Optical basicity is a concept that provides a good foundation for a better understanding of the behaviour of molten slags than the conventional basicity ratio [11,12]. The optical basicity value for the oxide can be calculated by equation (8) [23].

where Λi is the optical basicity of each oxide and X is the Pauling electronegativity of the cation.

The optical basicity of a multicomponent dephosphorization slag can be calculated by equations (9) and (10).

where Ni is the equivalent cation fraction of each oxide; xi is the mole fraction of i component; and nOi is the number of oxygen atoms in the i components of the molten slag.

The optical basicity value of CaO is 1.0, but the optical basicity values of Ce₂O₃ and La₂O₃ reach 1.23 and 1.18. According to equations (9) and (10), the partial substitution of CaO with Ce₂O₃ and La₂O₃ can improve the optical basicity values of dephosphorization slags. Yang et al. [23] reported that an increase in the optical basicity values causes an increase in the phosphate capacity of dephosphorization slags. Therefore, an increase in the rare earth oxide contents in dephosphorization slags causes an increase in the optical basicity values, thus improving the dephosphorization efficiency. In addition, rare earth oxides dissolved in molten slags can ionize a large amount of O²⁻ and depolymerize the complicated network structure of the silica complex ions, thus improving the fluidity of the slags [21]. The slag with good fluidity is beneficial to the dephosphorization material diffusion and improves the interface chemical reaction. Therefore, an increase in the mass fraction of rare earth oxides in the molten slags causes an increase in the dephosphorization efficiency. However, rare earth oxides have a limited solubility in molten slags [15]. When the mass fraction of rare earth oxides increases above 6 wt%, their solubility decreases, leading to a poor fluidity of the slags; thus, the thermodynamic and kinetic effects balance out and the dephosphorization efficiency changes little.

3.4 Quantity of dephosphorization slags

This study used dephosphorization slag No. 6 and metal sample to investigate the effect of the quantity of slags on the dephosphorization efficiency at 1,550°C. The mass fractions of dephosphorization slags were 5, 10 and 15 wt%. The contents of different components in slag and metal after experiment are listed in Table 5. An increase in the amount of dephosphorization slag is beneficial to the dephosphorization reaction, as shown in Figure 5. When the quantity of slags increases from 5 to 15 wt%, the degree of dephosphorization increases from 13.30 to 50.92%.

Figure 5

Relationship between dephosphorization efficiency and quantity of slags.

Due to the pure iron and ferro-phosphorus inevitably containing silicon, the affinity of Si with O is much stronger than that of P. Therefore, Si oxidation consumes a portion of the dephosphorization slag and the product of the Si oxidation reaction decreases the basicity of the dephosphorization slag [14]. However, an increase in the quantity of dephosphorization slags can meet the amount of slag required for dephosphorization, thus increasing the dephosphorization efficiency. In addition, an increase in the amount of dephosphorization slag is beneficial to dilute the concentration of P₂O₅ and decrease the content of phosphate in slag [14]. Therefore, the dephosphorization efficiency increases with increasing the quantity of dephosphorization slags.