Thermodynamic Simulation and Computational Study of the Carbothermal Reduction of Converter Steel Slag

Abstract

Micropulverization of steel slag is an important way to achieve its efficient utilization. Its purpose is to reduce the particle size of steel slag and improve its iron recovery rate. However, the high hardness and poor grindability of steel slag make it constrained by process and cost. Carbon thermal reduction can reduce the phosphorus dissolved in dicalcium silicate, reduce the influence of phosphorus on the crystal transformation of dicalcium silicate, and facilitate the self-pulverization of steel slag. At the same time, it can reduce the iron oxide in the slag to metallic iron, achieving the goal of recovering iron. To study the changes in phase types and contents of equilibrium products during the carbothermal reduction of converter slag, based on the variables of reduction temperature, alkalinity, and carbon ratio (coke-to-slag ratio), FactSage 7.1 thermodynamic software was used for calculation and analysis. The study reveals under a certain coke-slag ratio, with the increase of reduction temperature, the residual C content in the equilibrium phase composition shows a decreasing trend, while the Fe₃C and P₂ gas contents show an increasing trend, indicating that the high temperature is favorable to the reduction of iron oxides and apatite, especially to the gasification of dephosphorization. With reduction temperature increasing, the contents of Fe₃P/Fe₂P in the equilibrium phase composition decrease, while the contents of Mn₂P and P₂(g) increase. This indicates that the reduction temperature has significant influence on the stability sequence of phosphorus-containing phases, with the stability enhancement order as Fe₃P → Fe₂P → Mn₂P → P₂(g). High temperature favors the gasification and removal of phosphorus. Under constant coke-to-slag ratio and reduction temperature, the increase in the alkalinity of charge leads to elevation of Fe₃C content in the equilibrium phase composition, indicating that higher alkalinity promotes the reduction of iron oxides. As the alkalinity of the mixture increases, the silicate liquid phase content in the equilibrium phase composition shows a decreasing trend, and an alkalinity of 1.8 generates the largest amount of liquid phase. The alkalinity of the mixture in the range of 1.8 to 2.2 is conducive to the generation of α-C₂S and self-pulverization of the product, with the content of α-C₂S being the largest when the alkalinity is 2.0. Changes in the coke-to-slag ratio have minimal impact on the phase types and contents of equilibrium products during the carbothermal reduction of converter slag. Thermodynamic calculations indicate that the temperature range favorable for steel slag micronization is 1450–1500°C, the alkalinity range is 1.8–2.2, and the coke-slag ratio range is 10:90–15:85. This range of conditions facilitates the generation of α-C₂S and the best pulverization of steel slag.