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The agglomeration of bastnaesite and fine fluorite severely reduces the recovery of bastnaesite, and a method to address this issue is urgently needed. Using measurement techniques such as micro-flotation, contact angle, zeta potential, in situ attenuated total reflectance-Fourier transform infrared (In situ ATR-FTIR) spectroscopy, and extended theory for calculating the stability of solution colloid (E-DLVO theory), this study investigated the selective adsorption mechanism of sodium hexametaphosphate (SHMP). The flotation results indicated that when sodium oleate (NaOL) was used as the collector, SHMP was 2.5 × 10−6 M, showing an outstanding flotation separation of bastnaesite from fine fluorite. The contact angle and zeta potential results showed that SHMP selectively increased the surface hydrophobicity and decreased the surface potential of fluorite. Additionally, in situ ATR-FTIR spectroscopy measurements revealed that HPO 4 2- and H 2 PO 4 - of SHMP chemically reacted with Ca2+ ions on the surface of fluorite and finally formed CaHPO 4 and Ca(H 2 PO 4) 2 stable complexes. However, SHMP was adsorbed by bastnaesite. Furthermore, EDLVO theoretical calculations confirmed that with the addition of SHMP, the electrostatic attraction of the double electric layer between the particles weakened 10 times, hydration repulsion was intense, and the total force between the particles changed from attractive to repulsive. The results also showed that fluorite and bastnaesite dispersed each other. Thus, SHMP can be used as a high-performance selective depressant for the fluorite flotation to realize bastnaesite flotation. [Display omitted] [ABSTRACT FROM AUTHOR]