Phase Equilibria and Phase Diagrams of Na⁺//F^–, CO₃^2–, PO₄^3–-H₂O Quaternary and Subsystems at 313.15 K

Abstract

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 − , CO 3 2− , PO 4 3− -H 2 O and their two sub-ternary systems NaF-Na 3 PO 4 -H 2 O and Na 2 CO 3 -Na 3 PO 4 -H 2 O 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 NaFoccurred in the respective ternary systems. Five solid species of one anhydrous salt of NaF, two hydrous salts of Na 2 CO 3 •H 2 O and Na 3 PO 4 •12H 2 O, and two double salts of Na 3 PO 4 •3Na 2 CO 3 •8H 2 O and NaF•2Na 3 PO 4 • 19H 2 O 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. Na 3 PO 4 easily combined with NaF to form double salt NaF•2Na 3 PO 4 •19H 2 O, which occupied a larger phase region in the quaternary phase diagram. The increase in Na 2 CO 3 will lead to the decrease in the solubility of Na 3 PO 4 and NaF in the system.

Solid–Liquid Phase Diagram of the Ternary System NaF–Na3PO4–H2O at 308.15, 323.15, and 348.15 K

“…The comparison of the SL phase behavior data at different temperatures from 293.15 to 348.15 K, including the results measured in this work (at 308.15, 323.15, and 348.15 K) and the data reported in the literature (at 293.15, 298.15, , 313.15, , and 323.15 K), is shown in Figure . At this temperature range, the equilibrium solid phases at the given temperature are NaF (s) , double salt 2Na 3 PO 4 ·NaF·19H 2 O (s) , and Na 3 PO 4 ·12H 2 O (s) and correspondingly two invariant points.…”

“…Nevertheless, when comparing these experimentally measured SL phase behavior data, the reliability of the reported data should be revalidated. First, the SL phase behavior at 298.15 K reported by Roslyakova and Zhikharev do not agree with each other; second, the SL phase behavior at 293.15 K reported by Huang is higher than the result of Zhikharev at 298.15 K, which is unreasonable and one of them is surely inaccurate; and then, Hao reported the SL phase behavior at 313.15 K twice in different studies, , but the two sets of SL phase behavior data do not agree with each other, where one set of SL phase behavior data is too high to be reasonable, even higher than the data at 323.15 K measured by Huang …”

“…In this work, we focus on the SL phase behavior of the NaF–Na 3 PO 4 –H 2 O system, one of the subternary systems of wastewater. Actually, the SL phase behavior of this ternary system at different temperatures, for instance, 293.15 and 323.15, 298.15, , and 313.15 K, , has been reported. Nevertheless, when comparing these experimentally measured SL phase behavior data, the reliability of the reported data should be revalidated.…”

“…The most commonly reported analysis methods for F – are solvent evaporation, ,,, ion chromatograph, , and a fluoride-selective electrode (FSE) method. ,,,, The solvent evaporating method, also called the gravimetric method, measures the mass of the pure solute in a solution, which can be obtained by completely evaporating the rest of the substances in a solution. It is obviously unsuitable for the NaF–Na 3 PO 4 –H 2 O ternary system since Na 3 PO 4 is nonvolatile.…”

“…Inductively coupled plasma (ICP), , ammonium molybdate spectrophotometry, and the gravimetric method , are reported for the determination of the PO 4 3– concentration. The ICP , and ammonium molybdate spectrophotometry methods require dilution of the sample for hundreds or even thousands of times to reach a suitable concentration, during which deviation might be introduced. The gravimetric method was selected to determine the concentration of PO 4 3– .…”