Phase Equilibrium Research and Application of Na ₂ SO ₄ –K ₂ SO ₄ –(NH ₄ ) ₂ SO ₄ –H ₂ O System

The extraction of potassium from seawater is a strategic issue to address the shortage of soluble potassium salts in China. The ion-exchange selective adsorption method for extracting potassium from seawater has attracted much attention due to its high selectivity and significant economic and technical advantages. The potassium-rich eluent belongs to the Na2SO4–K2SO4–(NH4)2SO4–H2O quaternary water-salt system, and the efficient separation and recovery of the three salts is a core challenge for industrial application. To solve this problem, this study systematically determined the solubility data of the Na2SO4–K2SO4–(NH4)2SO4–H2O quaternary system and its ternary subsystems at 298.15, 323.15, and 353.15 K, and drew the corresponding phase diagrams. Meanwhile, the Pitzer model was adopted for solubility calculation, and the theoretical values were in good agreement with the experimental values, verifying the accuracy of the model. Based on the multitemperature phase diagrams, this study designed a two-step cyclic crystallization process, successfully separating the double salts and solid solutions in the system through stepwise crystallization. Verification experiments indicated that the process is feasible and can produce K2SO4 with 99.56% purity and Na2SO4·10H2O with 99.72% purity, achieving the resourceful and high-value utilization of each component.