Fundamental Study on Slag Component Effects in the CaO–SiO2–La2O3–CeO2–Al2O3–MgO System and Its Application to the Selective Enrichment of Rare Earth-Rich Phases
摘要
To recover rare earth (RE) resources from the low-grade ores via selective crystallization, the effects of key components, including basicity (the mass ratio of CaO to SiO2), Al2O3, and MgO, on slag viscosity, molten structure, and crystallization behavior were systematically investigated. Designed slags were prepared using chemical reagents, and appropriate compositional ranges were identified to minimize the formation of intrusive minerals. The results indicated that the viscosity of synthetic CaO–SiO2–(Al2O3)–La2O3–CeO2 slags gradually decreased with increasing basicity and MgO content, owing to the concurrent simplification of the molten slag structure. In contrast, as the Al2O3 content rose from 0, 5 to 10, 15 wt. pct, the viscosity exhibited a nonlinear trend, first increasing and then decreasing, which corresponded to a similar pattern of structural change in the molten slag. Crystallization analysis revealed that higher basicity and MgO content enhanced the diversity of crystalline phases and refined grain size, whereas Al2O3 inhibited the formation of Ca3Si3O9 and Ca2SiO2F2, but promoted gehlenite (Ca2Al2SiO7) when the Al2O3 content exceeded 10 wt. pct. The optimal crystallization conditions for obtaining a single RE-rich phase (CaLa4(SiO4)3O-type) were identified as a basicity of ~ 1.2, ~ 5 wt. pct Al2O3, and less than 3 wt. pct MgO. The verification using the Bayan Obo iron ore processing tailings showed that an individual RE-rich phase, (Ca3Ce2[(Si,P)O4]3F), could be precipitated by adjusting the slag composition according to the optimized parameters derived from the synthetic system. This study provides valuable insights into the selective enrichment of RE resources from low-grade ores through a pyrometallurgical process aligned with sustainable development.