Phosphorus speciation and leaching behavior in nitric acid leaching residue of rare earth phosphate ore

This study addresses the valorization of phosphorus from nitric-acid leach residues of Zhijin (Guizhou) rare-earth phosphate ores through multi-scale characterization and leaching kinetics. These refractory solids exhibit total phosphorus of 13.97% (as P₂O₅), predominantly as labile exchangeable phosphorus (64.78%) with free phosphate constituting 28.16%. Exchangeable phosphorus resides principally in 3CaO·Al₂O₃ (32.97%), CaSiO₃ (16.71%), and Ca-bearing Fe-silicates (15.70%), anchored via surface adsorption, lattice substitution, and interfacial enrichment. Tri-modal speciation encompasses free orthophosphate, metal phosphates, and phosphorus oxides. A two-stage water-acid leaching cascade was accordingly established, achieving 98.60% extraction efficiency. Water leaching selectively stripped exchangeable phosphorus, while dilute HNO₃ leaching (1.17 mol/L) targeted bound phases, surpassing single-stage water or acid leaching at optimal concentration (4.86 mol/L) by approximately 33 and 4 percentage points, respectively. Kinetic analysis revealed water leaching transitions progressively from intra-particle diffusion control through diffusion-reaction synergy to chemical reaction dominance with increasing liquid-solid ratio. Acid leaching shifts between mixed control (0.5-2 M HNO₃) and diffusion-only control (critical concentration 4 M) via concentration modulation, governed by interfacial reaction-mass transfer-concentration gradient synergy. This investigation addresses a critical data gap in phosphorus deportment within rare-earth-bearing acid leach residues, establishing an integrated phase-characterization/kinetics-prediction framework that furnishes theoretical rationale for phosphorus valorization, thereby catalyzing advancements in circular phosphorus economy and sustainable chemical engineering.