Effect of MgO on Phase Structure and Evolution of Steelmaking Slag During Cooling

Abstract

The underutilization of steelmaking slags is primarily ascribed to instability, predominantly influenced by their intricate phase structure. The presence of MgO, a key constituent in steelmaking slag, contributes to the challenges faced in steelmaking slag recycling and sustainable steel production practices. There are currently uncertainties surrounding the impact of fluctuations in MgO content on the mineral phase of steelmaking slag, as well as ongoing controversy regarding the influence of the RO phase on slag stability. This study investigates the phase evolution and microstructures of steelmaking slag with varying MgO contents during the cooling process through high-temperature experiments and FactSage thermodynamic equilibrium calculations. The principal phases in steelmaking slag, namely α′-Ca₂SiO₄ (C₂S), RO (MgO–FeO solid solution), Ca₂Fe₂O₅ (C₂F) and Ca₃SiO₅ (C₃S), were identified using XRD and SEM-EDS analyses. As the MgO content increases, the MgO content within the RO continued to increase. With a MgO content of 10 mass pct in steelmaking slag, the precipitation of C₃S commences during the cooling of the steelmaking slag. The average size of C₃S reaches up to 400 μm, markedly higher than the 35 μm size of C₂S. This disparity is attributed to the higher nucleation rate of C₂S compared to C₃S, coupled with a lower growth rate. The augmentation of MgO content in steelmaking slag induces changes in the C₃S formation, facilitated by the substitution of Ca²⁺ in C₃S by Fe²⁺ and Mg²⁺.The significance of this work lies in unraveling the intricacies of steelmaking slag behavior, providing crucial insights for optimizing recycling processes. This research sheds light on the factors that influence the phase composition and stability of steelmaking slag, contributing to the utilization of steelmaking slags, promoting environmental sustainability, and bolstering the overall efficiency of metallurgical processes in the current industrial landscape.

Appendix 1

In this model, the steelmaking slag compositions are classified into three categories: glass formers (X_G); modifiers (X_M), and amphoterics (X_A).

$$ X_{G} = X_{{{\text{SiO}}_{2} }} + X_{{{\text{P}}_{2} {\text{O}}_{5} }} $$

(A1)

$$ X_{M} = X_{{{\text{CaO}}}} + X_{{{\text{MgO}}}} + X_{{{\text{FeO}}}} + X_{{{\text{MnO}}}} + 2X_{{{\text{TiO}}_{2} }} $$

(A2)

$$ X_{{\text{A}}} = X_{{{\text{Al}}_{2} {\text{O}}_{3} }} + X_{{{\text{Fe}}_{2} {\text{O}}_{3} }} $$

(A3)

The model assumes Weymann–Frenkel relation

$$ \eta = {\text{AT}}\exp \left( {\frac{{10^{3} B}}{T}} \right) $$

(A4)

$$ - \ln A = 0.29B + 11.57 $$

(A5)

where A and B are composition dependent parameters. The parameter B can be expressed by the third-order polynomial equation (A6), where B₀, B₁, B₂ and B₃ can be obtained by equations (A7) through (A10).

$$ B = B_{0} + B_{1} X_{G} + B_{2} X_{G}^{2} + B_{3} X_{G}^{3} $$

(A6)

$$ B_{0} = 13.8 + 39.93545\alpha_{0} - 44.049\alpha_{0}^{2} $$

(A7)

$$ B_{1} = 30.481 + 117.1505\alpha_{0} - 129.9978\alpha_{0}^{2} $$

(A8)

$$ B_{2} = - 40.9429 + 234.0486\alpha_{0} - 300.04\alpha_{0}^{2} $$

(A9)

$$ B_{3} = 60.7619 - 153.9276\alpha_{0} + 211.161\alpha_{0}^{2} $$

(A10)

$$ \alpha_{0} = \frac{{X_{{\text{M}}} }}{{X_{{\text{M}}} + X_{{\text{A}}} }} $$

(A11)