Melting temperature of refining slag containing aluminum ash

Aluminum ash (AA) was attempted to replace Al₂O₃ in LF refining slag, and the melting temperature of refining slag was optimized by adjusting slag system composition to realize resource utilization of aluminum ash and reduce environmental pollution caused by solid waste discharge. Based on the CaO-SiO₂-Al₂O₃ ternary phase diagram, the ratio of basic slag system was designed. When the mass fractions of CaO and Al₂O₃ were controlled within the range of 30%-70% and the mass fraction of SiO₂ was not higher than 15%, the low-melting-point phase 12CaO·7Al₂O₃ was easily formed in the slag. This region belonged to the low-melting-point zone, where the liquidus temperature ranged from 1 350 to 1 450 ℃. Meanwhile, the basicity of the slag system was moderate to meet the requirements of refining. Within this region, six groups of basic slag systems with different basicity were selected. The effects of different aluminum ash addition amounts and basicityon the melting temperature of the slag system were investigated by means of FactSage software and the hemisphere method, and the phase composition was characterized by XRD. Results indicated that the melting temperature of the slag system showed an overall upward trend as the mass fraction of aluminum ash increased from 0 to 20%. In the low-basicity system (C/S<8), the formation of high-melting-point phases was limited when aluminum ash was added in an appropriate amount (AA≤10%, mass fraction same hereinafter). However, excessive addition (AA>10%) might break the original equilibrium, leading to the massive formation of high-melting-point phases (MgAl₂O₄) in the slag and thus an increase in melting temperature. In the high-basicity system (C/S≥8), the introduction of a small amount of aluminum ash (AA≤5%) might promote the formation of more eutectic structures by generating low-melting-point compounds, thereby reducing the melting temperature. On the contrary, excessive addition (AA>5%) would break the optimal eutectic ratio between CaO and Al₂O₃ due to the introduction of excessive Al₂O₃, resulting in a sharp increase in melting temperature. When the aluminum ash addition amount was fixed, the melting temperature of the slag system exhibited a“V”-shaped variation(decreasing first and then increasing) with the basicity increasing from 4 to 11. This variation was related to the formation of the low-melting-point phase 12CaO·7Al₂O₃ and the high-melting-point phase Ca₂SiO₄ in the slag.