Phase Equilibria and Phase Diagrams of Na+//F–, CO32–, PO43–-H2O Quaternary and Subsystems at 313.15 K

Rare-earth ore concentrate is calcined at high temperature by the alkaline method, and the associated elements fluorine and phosphorus are enriched in the form of sodium fluoride and sodium phosphate in water elution. To separate and recover fluorine and phosphorus from the aqueous solution, the solid–liquid equilibrium behaviors of the quaternary systems Na+//F–, CO32–, PO43–-H2O and their two sub-ternary systems NaF-Na3PO4-H2O and Na2CO3-Na3PO4-H2O at 313.15 K were studied via the isothermal solubility phase equilibrium method. Solubility data, liquid density and pH, two complete isothermal phase diagrams of the ternary system, and one complete isothermal phase diagram of the quaternary system were obtained. The results showed that two double salts NaF·2Na3PO4·19H2O and Na3PO4·3Na2CO3·8H2O occurred in the respective ternary systems. Five solid species of one anhydrous salt of NaF, two hydrous salts of Na2CO3·H2O and Na3PO4·12H2O, and two double salts of Na3PO4·3Na2CO3·8H2O and NaF·2Na3PO4·19H2O were found in the quaternary system. The phase diagram of the quaternary system consisted of three invariant points, five crystallization regions, and seven univariate curves. Na3PO4 easily combined with NaF to form double salt NaF·2Na3PO4·19H2O, which occupied a larger phase region in the quaternary phase diagram. The increase in Na2CO3 will lead to the decrease in the solubility of Na3PO4 and NaF in the system.