Organophosphonic acid nitrilotri: a spatially matched depressant of bastnaesite for separation from fluorite

Nitrilotrimethylene phosphonic acid (nitrilotri, ATMP) enables effective flotation separation of fluorite from bastnaesite through selective surface modification. A multi-technique investigation including micro-flotation tests, zeta potential measurements, contact angle analysis, adsorption studies, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) characterization, and quantum chemical calculations can elucidate the inhibition mechanism. Single-mineral and synthetic mixed-ore flotation experiments demonstrate ATMP's selective inhibition toward bastnaesite, achieving >70% fluorite recovery with <12% bastnaesite recovery at optimal dosage (40 g/t). Mechanistic studies reveal that ATMP preferentially chemisorbs at Ce 3+ active sites on bastnaesite surfaces through dual mechanisms: Firstly, nitrilotri undergoes chemisorption with Ce active sites on the bastnaesite surface, forming Ce-PO(OH) 2 complexes that exhibit strong adsorption on the mineral surface. Secondly, the three phosphonate groups of nitrilotri are arranged in a triangular configuration with an interatomic distance of 0.7512 nm, which spatially aligns with the Ce sites (0.7186 nm interatomic spacing) on bastnaesite. Each nitrilotri molecule chemisorbs with three Ce sites, yielding an adsorption energy of −294.624 kJ/mol, being indicative of a thermodynamically favorable interaction. This site-specific adsorption generates a hydrophilic Ce–phosphonate interface that destabilizes collector (NaOL) adsorption, reducing bastnaesite surface hydrophobicity by 42.78% (contact angle decreases from 65.23° to 22.45°). Conversely, nitrilotri exhibits limited interaction with fluorite due to geometric mismatch at Ca 2+ sites, allowing NaOL-dominated hydrophobization to persist. The fundamental understanding of ligand-receptor complementarity presented here provides a promising strategy for rare earth/fluorite separation in industrial beneficiation circuits.