Separation and enrichment of valuable elements from slag containing rare earth, niobium and titanium via Fe–Si bath smelting reduction

To recover the valuable elements in Bayan Obo tailinðøgs, Fe–Si bath smelting reduction was adopted to separate and enrich rare earth elements (REE), niobium and titanium from the REE–Nb–Ti-containing slag. The reduction reaction process of the Fe–Si bath and the migration behavior of valuable elements in the solidification and crystallization process of silicothermic reduction tailings were investigated, and a treatment method for efficiently separating and enriching REE, Nb and Ti was explored. Thermodynamic analysis indicated that at 1600 °C, with a 6 wt.% addition of Si as the reducing agent, the niobium oxide in the REE–Nb–Ti-containing slag could be selectively reduced to metallic Nb. In the Fe–Si bath reduction process, the Nb mass fraction in the metal phase increased with prolonged reaction time, peaking at 2.77%, while the Ti mass fraction consistently stayed below 0.12%. Lowering the w(CaO)/w(SiO₂) enhanced the migration of Nb from slag to metal phase and reduced the Ti impurities. During solidification and crystallization, a significant quantity of perovskite precipitated from reduction tailings, with the REE dissolving into this perovskite. By adjusting the w(CaO)/w(SiO₂) in tailings to 1.2–1.9 and maintaining a temperature of 1100 °C for 4 h, the perovskite area fraction in the final slag could exceed 37%. Finally, a method was proposed to separate and enrich valuable elements in REE–Nb–Ti-containing slags via Fe–Si bath smelting reduction and crystallization control.