• Thermal decomposition of bastnaesite in an inert atmosphere is strongly temperature-dependent and elevates the REO grade from 72.90 wt% to 86.83 wt% while increasing Ce oxidation.
• Decomposition induces lamellar cracking, increased specific surface area, and particle fragmentation, significantly modifying surface chemistry and raising initial pulp pH to nearly 11.00.
• Flotation of decomposed bastnaesite requires a much higher SHA collector dosage, rising from 20 mg·L−1 for raw ore to 250 mg·L−1, due to enhanced chemical adsorption and internal pore distribution.
• Optimal flotation recovery is achieved at pH 8.00–9.00, but post-decomposition pH adjustment is difficult; these insights guide the development of tailored reagents for bastnaesite roasting products.