Phase equilibrium relations in CaO-SiO2-Nb2O5-5%Na2O-5%Al2O3-3%CaF2 system at 1473 K

Bayan Obo tailings are rich in niobium and other valuable elements; however, the phase evolution of niobium-bearing slag within these tailings remains poorly understood. This knowledge gap significantly impedes comprehensive utilization of these strategic resources. In this study, a CaO-SiO 2 -Nb 2 O 5 -5%Na 2 O-5%Al 2 O 3 -3%CaF 2 slag system was investigated to examine the phase equilibrium types, compositional characteristics, and distributional evolution under varying w (CaO)/ w (SiO 2 ) ratios and w (Nb 2 O 5 ) contents, using high-temperature equilibrium experiments. Following a 24-hour high-temperature equilibrium treatment at 1473 K under an argon atmosphere, a range of compounds including 13CaO·8SiO 2 ·Nb 2 O 5 ·CaF 2 , 2CaO·4SiO 2 ·Na 2 O·Al 2 O 3 , CaO·Nb 2 O 5 ·NaF, 2CaO·Nb 2 O 5 , and CaO·SiO 2 were identified. An increase in Nb 2 O 5 content coupled with a reduction in the w (CaO)/ w (SiO 2 ) ratio favored the formation of CaO·Nb 2 O 5 ·NaF (pyrochlore phase) and 2CaO·Nb 2 O 5 . These two phases exhibited high Nb concentrations, with the CaO·Nb 2 O 5 ·NaF (pyrochlore) phase containing up to approximately 70% Nb. The optimal smelting conditions for niobium enrichment in the pyrochlore phase were identified within the range of w (CaO)/ w (SiO 2 ) = 0.74∼0.80 and w (Nb 2 O 5 ) = 13.61%∼16.11%.