In situ synthesis of Fe-Mo alloys via synergistic smelting of copper slag and spent MoSi2 rods: Phase transformation and decomposition mechanism of MoSi₂         

The large accumulation of copper slag and retired silicon-molybdenum rods leads to the waste of valuable metal elements such as Fe and Mo. To reduce pollution and promote the recovery of metal elements, this paper proposes a process for co-smelting retired silicon-molybdenum rods and copper slag to prepare molybdenum-iron alloy. Through selective oxidation-reduction, the in-situ generation and recovery of Mo-Fe and the removal of Si and O elements from the alloy are achieved, with CaO added to optimize the reaction environment. MoSi₂ is used as the molybdenum source and reducing agent, and the slag is modified during the smelting process. Copper slag serves as a flux and slag-forming agent, providing iron sources to further generate Fe as a Fe sequestrant to capture Mo elements during smelting. By combining thermodynamic calculations and non-isothermal kinetic experiments, the reduction behavior of each component, phase reconstruction, and the decomposition of MoSi₂ in the co-smelting process are systematically studied. The magnetite phase and iron olivine phase gradually decompose under the action of calcium oxide and MoSi₂, Fe₃O₄→FeO→Fe,Fe₂SiO₄→Fe_1.5Ca_0.5(SiO₃)₂→Fe. At around 950-1100°C, MoSi₂ reacts exothermically with copper slag, and Fe3Mo begins to form in-situ. CaO can optimize the thermodynamic conditions of the reaction, promoting its progress. After the system melts, MoSi₂ reacts further with the solid and liquid phases of the slag, causing Mo and Si to diffuse toward the slag, while Fe captures Mo for directional enrichment. Differential scanning calorimetry tests and calculations show that the activation energy for the MoSi₂-driven reduction of copper slag is 113.470kJ/mol, indicating that MoSi₂ can serve as a high-quality reducing agent for copper slag. At optimal conditions, the yields of molybdenum (Mo) and iron (Fe) reached 99.21% and 85.49%, respectively, with the Fe content in secondary slag reduced to 4.04%. The study utilized spent silicon-molybdenum rods as both reducing agents and molybdenum sources, effectively avoiding carbon introduction. This approach provides innovative strategies for the high-value and low-carbon utilization of two solid wastes: spent silicon-molybdenum rods and copper slag. The findings were published in Journal of Materials Research and Technology (JCR Q1) under the title  "In situ synthesis of Fe-Mo alloys via synergistic smelting of copper slag and spent MoSi₂ rods: Phase transformation and decomposition mechanism of MoSi₂".

Figure 1. a) Phase diagram of the Fe-Mo-Si-O system at different temperatures;b) The ΔG_θ–T curves for possible reactions during the MoSi₂ reduction of copper slag;c) Binary phase diagram of MoSi₂ and the main components of copper slag;d) The ternary phase diagram of the main components of copper slag - MoSi₂ - CaO system.

Figure 2. a) DSC curves of samples with different amounts of MoSi₂; b) Effect of temperature on the composition of equilibrium phases during the cooperative melting process in this experiment; c) XRD pattern of the MoSi₂-copper slag sample after heat treatment; d) XRD pattern of the MoSi₂-copper slag-CaO sample after heat treatment.

Figure 3. a) SEM-EDS result diagram of the MoSi₂-copper slag sample after heat treatment; b) SEM-EDS result diagram of the MoSi₂-copper slag-CaO sample after heat treatment.

Figure 4. Decomposition behavior of MoSi₂ during the synergistic smelting process and the phase transformation model in the system

Authors: Zhang Baojing, Liu Zhi, Cheng Junsheng, Yu Le, Zhang Zixin, Li Junxiu, Li Shiheng, Feng Peizhong 

Link: https://www.sciencedirect.com/science/article/pii/S2238785425024135

DOI:https://doi.org/10.1016/j.jmrt.2025.09.158 

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