Binary CaO-P2O5 phosphates: High-temperature phase evolution, acid dissolution behavior, and DFT insights into orthophosphate vs. pyrophosphate reactivity

Phosphorus impurities in vanadium slag can impair leachate quality and reduce the purity of vanadium products during calcification roasting-acid leaching. Their dissolution behavior is closely related to the phase composition and structural characteristics of calcium phosphates. In this study, four representative CaO-P 2 O 5 phosphates, CaP 2 O 6 , Ca 2 P 2 O 7 , Ca 3 P 2 O 8 and Ca 4 P 2 O 9 , were synthesized by solid-state reactions based on FactSage thermodynamic calculations, and their high-temperature phase evolution, acid leaching behavior, and structural origins of dissolution reactivity were investigated. XRD results show that CaP 2 O 6 , Ca 2 P 2 O 7 , Ca 3 P 2 O 8 and Ca 4 P 2 O 9 were obtained as the dominant phases at 800, 900, 900, and 1500 °C, respectively. Leaching experiments in dilute H 2 SO 4 at pH 2–5 reveal that phosphorus leachability follows the order Ca 3 P 2 O 8 > Ca 4 P 2 O 9 > > Ca 2 P 2 O 7 ≈ CaP 2 O 6 . At pH 2 and 50 °C, Ca 3 P 2 O 8 reached a phosphorus leaching fraction of 97.92% after 60 min, whereas Ca 2 P 2 O 7 and CaP 2 O 6 remained below 2%. Residue analysis indicates that Ca 3 P 2 O 8 and Ca 4 P 2 O 9 underwent phase transformation to calcium sulfate-containing products under strongly acidic conditions, while Ca 2 P 2 O 7 and CaP 2 O 6 were largely retained. DFT calculations show that the orthophosphate-like PO 4 3− unit has stronger proton affinity and a lower calculated reaction barrier than the pyrophosphate-like P 2 O 7 4− unit, consistent with the large difference in acid reactivity. This work shows that orthophosphate-like units are highly reactive motifs, whereas pyrophosphate-like units behave as acid-resistant scaffolds under the investigated leaching conditions, providing a structural basis for understanding phosphorus release during calcification roasting–acid leaching.