Solid–liquid phase equilibria in the aqueous system containing the chlorides of potassium, ammonium, and calcium at 298.2, 323.2, and 348.2 K

• The quaternary system K+, NH4+, Ca2+//Cl−-H2O exhibits complex phase equilibria with formation of solid solutions and double salts at all studied temperatures. • At 298.2 K, the phase diagram shows three invariant points and five crystalline regions, while at 323.2 and 348.2 K, it expands to five invariant points and seven regions, indicating increased complexity with temperature. • The solid solution (KCl)x(NH4Cl)1−x has the largest crystallization region at all temperatures, making it the most readily crystallized phase in the system. • The hydration state of CaCl2 changes with temperature: CaCl2·6H2O at 298.2 K and CaCl2·2H2O at 323.2 and 348.2 K, affecting the overall phase behavior. • The findings provide essential data for optimizing extraction and separation processes of potassium, ammonium, and calcium from deep brine resources.