Synergistic treatment of blast furnace slag and basic oxygen furnace slag for efficient recovery of iron: Phase transformation and oxidation mechanisms
Lan Huang, Shengli An, Fang Zhang, Jun Peng, Yuxin Chen, Ping Xin, Chunheng Liu
Abstract
The recycling of basic oxygen furnace (BOF) slag is of great importance for the sustainable development of steel industry. In view of the compositional characteristics of BOF slag, a method of separating and recovering iron resources by regulating the composition of BOF slag with blast furnace slag (BFS) as an additive at high temperature was proposed. In this study, FactSage thermodynamic simulation, XRD, SEM-EDS, XPS and EPMA were used to analyze the phase transformation and oxidation mechanism during the modification process, and the effects of addition amount of BOF slag, cooling method and temperature on the modification were studied. The results show that the transformation of Ca2Fe2O5 to (Mn,Mg)yFe3-yO4 can be effectively promoted by the addition of BFS. When the BFS was added at 30 %(R = 1.87), all the calcium ferrate was transformed into spinel, while the content of dicalcium silicate and the stability of the modified slag reached the optimal level. During the cooling process, the slow cooling method was beneficial to the generation and enrichment of (Mn,Mg)yFe3-yO4 particles, which was accomplished by the diffusion of cations. In BFS addition 30 %, temperature 1400 °C and cooling with furnace, the recovery and grade of iron were 65.74 % and 32.07 %, respectively, which had the potential to return to sintering. Meanwhile, tailings can be recovered as a high value-added product. Therefore, the BFS and BOF slag co-processing technology can successfully convert solid waste into useable resources and realize waste valorization. In light of the steel industry's rapid advancements, the availability of high-quality mineral resources is diminishing. Therefore, the recovery of iron from BOF slag is of great significance to the sustainability development. Considering the compositional characteristics of BOF slag, the transformation of the iron-containing phase into (Mn,Mg)yFe3-yO4 is the key step. Thus, a novel process for recovering iron resources by synergistic treatment of blast furnace slag (BFS) and BOF slag was proposed. This research employed FactSage thermodynamic simulation, XRD, SEM-EDS, XPS, and EPMA to analysis the impact of BFS addition (10–50 %), cooling methods (from water-cooling to furnace-cooling), and temperature (1400–1600 °C) on phase transformation and the RO oxidation mechanism, and the conditions of (Mn,Mg)yFe3-yO4 generation and enrichment was obtained. The results show that at BFS addition of 30 %, reaction temperature 1400 °C and furnace-cooling, the iron-containing phase (Ca2Fe2O5 and RO) was almost completely transformed into (Mn,Mg)yFe3-yO4. The oxidation mechanism of RO was formation of (Mn,Mg)yFe3-yO4 by cation diffusion. Under optimal conditions, the iron recovery rate and the grade reached 65.74 % and 32.07 %, respectively, which can be used as raw material for ironmaking. Meanwhile, the main phase of the tailing slag was β-Ca2SiO4, without f-CaO, which has the potential to be used in the cement and concrete industries with the advantages of both low cost and eco-friendly. Therefore, the process with green, efficient and low cost was provided, which is a feasible idea for the comprehensive utilization of industrial solid waste.