Study On Phase Equilibrium Of Na⁺//F^-,CO₃^2-,PO₄^3--H₂O Multicomponent System

The recovery of fluorine and phosphorus in the alkaline process of Baotou rare earth mine is an industry problem.At present,the recovery rate of sodium salts containing fluorine and phosphorus in wastewater is low.Since the trace amount of Na OH in the waste liquid is placed in the air for a long time,it can form Na ₂CO₃ with CO₂,resulting in the formation of various hydrated salts and double salts in the solution of various salts in the system,which brings difficulties to the extraction and separation of fluorine and phosphorus elements.Therefore,it is necessary to study the phase equilibrium of the Na⁺//F^-,CO₃^2-,PO₄^3--H₂O multi-component system in the waste liquid,obtain the corresponding thermodynamic data,summarize its laws,and use the phase diagram as a theoretical tool to guide practice.In this thesis,the isothermal dissolution equilibrium method was used to study the phase equilibria of Na⁺//F^-,CO₃^2-,PO₄^3--H₂O 298.15K,318.15K and 333.15K in the quaternary system,including the quaternary system Na F-Na₃PO₄-Na₂CO₃-H₂O and its Phase equilibria of ternary subsystems Na₂CO₃-Na F-H₂O,Na₂CO₃-Na₃PO₄-H₂O,Na F-Na₃PO₄-H₂O.The solubility,equilibrium liquid density and p H of the system were measured,and the equilibrium solid phase was measured by X-ray diffraction.The phase diagram,density-composition diagram and p H-composition diagram of each system were drawn.The results show that there is no double salt and solid solution in the ternary system Na₂CO₃-Na F-H₂O,which belongs to the hydrate type I phase diagram.The phase diagram contains 1 full liquid phase zone,2 single salt crystallization zones,1mixed crystallization zone,and 1 Complete solid phase region,1 liquidus line,3univariate solubility curves and 1 common saturation point.The solid-liquid coexistence phase region in the phase diagram includes the Na ₂CO₃·H₂O+L phase region,Na F+Na₂CO₃·H₂O+L Phase area and Na F+L phase area.The total saturation point is Na F+Na₂CO₃·H₂O.In the ternary system Na₂CO₃-Na₃PO₄-H₂O,there is no double salt or solid solution,which belongs to the hydrate type I phase diagram.The phase diagram includes 1 full liquid phase region,3 single salt crystallization regions,1 mixed crystallization region,and 1 full solid phase.Area,4 u nivariate solubility curves and 1 common saturation point.The solid-liquid coexistence phase area in the phase diagram includes Na₃PO₄·12H₂O+L phase area,Na₃PO₄·8H₂O+L phase area,Na₃PO₄·8H₂O+Na₂CO₃·H₂O+L phase zone and Na₂CO₃·H₂O+L phase zone.The total saturation point is Na₃PO₄·8H₂O+Na₂CO₃·H₂O.There is a double salt in the ternary system Na F-Na₃PO₄-H₂O.The double salt is Na₇F（PO₄）₂·19H₂O.The phase diagram includes 1 liquid phase region,3 single salt crystallization regions,2 mixed crystallization regions,and 2 full solid phase regions;1 liquidus,7 univariate curves,solubility curves;2 total saturation points.The solid-liquid coexistence phase area in the phase diagram includes Na F+L phase area,Na F+Na₇F（PO₄）₂·19H₂O+L phase area,Na₇F（PO₄）₂·19H₂O+L phase area,Na₃PO₄·12H₂O+Na₇F（PO₄）₂·19H₂O+L phase area and Na₃PO₄·12H₂O+L phase area.The phase diagram has two common saturation points,the components are Na F+Na₇F（PO₄）₂·19H₂O and Na₃PO₄·12H₂O+Na₇F（PO₄）₂·19H₂O.The dry basis diagram of the quaternary system Na⁺//F^-,CO₃^2-,PO₄^3--H₂O belongs to a simple quaternary phase diagram,and the phase diagram contains 1 co-saturation point,4 univariate curves,and 3 single-salt crystallization regions（Na F,Na₂CO₃·H₂O,Na₃PO₄·8H₂O）and a double salt crystallization zone Na₇F（PO₄）₂·19H₂O;the co-saturation equilibrium solid phase contains Na F,Na₂CO₃·H₂O,Na₇F（PO₄）₂·19H₂O.At 298.15K,the corresponding crystalline regions of each salt are Na₇F（PO₄）₂·19H₂O→Na₃PO₄·8H₂O→Na F→Na₂CO₃·H₂O,and the solubility of the corresponding salt is Na₇F（PO₄）₂·19H₂O→Na₃PO₄·8H₂O→Na F→Na₂CO₃·H₂O;at 318.15K,the corresponding crystalline regions of each salt are Na F→Na₇F（PO₄）₂·19H₂O→Na₃PO₄·8H₂O→Na₂CO₃·H₂O,corresponding to the solubility of the salt The order from small to large is Na F→Na₇F（PO₄）₂·19H₂O→Na₃PO₄·8H₂O→Na₂CO₃·H₂O;at 333.15K,the corresponding crystalline regions of each salt are Na F→Na₇F（PO₄）₂·19H₂O→Na₂CO₃·H₂O→Na₃PO₄·8H₂O,the solubility of the corresponding salts from small to large is Na F→Na₇F（PO₄）₂·19H₂O→Na₂CO₃·H₂O→Na₃PO₄·8H₂O.