The Phase Diagram and the Application for the Quaternary System K+, $$ {\text{NH}}{_4^{+}} $$//Cl−, $$ {\text{SO}}{_4^{2 -}} $$SO42-–H2O at 80.0 °C

The solubilities in the quaternary system K+, \( {\text{NH}}{_4^{+}} \)//Cl−, \( {\text{SO}}{_4^{2-}} \)–H2O and its two ternary subsystems NH4Cl–KCl–H2O, (NH4)2SO4–K2SO4–H2O at 80.0 °C were measured using the isothermal dissolution equilibrium method under atmospheric pressure, and the corresponding phase diagrams were plotted. In the phase diagram of the NH4Cl–KCl–H2O system, there are three crystalline zones, which correspond to (K1−m,(NH4)m)Cl, ((NH4)n,K1−n)Cl and the co-existence zone of (K1−m,(NH4)m)Cl and ((NH4)n,K1−n)Cl, respectively. In the phase diagram of the (NH4)2SO4–K2SO4–H2O system, there is only one crystalline zone for (K1−t,(NH4)t)2SO4. In the phase diagram of the K+, \( {\text{NH}}{_4^{+}} \)//Cl−, \( {\text{SO}}{_4^{2-}} \)–H2O system, there are three crystal zones, which correspond to (K1−t,(NH4)t)2SO4, (K1−m,(NH4)m)Cl and ((NH4)n,K1−n)Cl, respectively. According to the analysis and the calculations for the phase diagrams of the K+, \( {\text{NH}}{_4^{+}} \)//Cl−, \( {\text{SO}}{_4^{2 -}} \)–H2O system at 80.0 °C and 50.0 °C, this paper proposes a technological process. In the process, the (K1−t,(NH4)t)2SO4 can be prepared at 80.0 °C and the ((NH4)n,K1−n)Cl can crystallize out at 50.0 °C. The mass fraction of K2SO4 in product L1 (K1−t,(NH4)t)2SO4 (t = 0.1465) is 88.48%. The composition of solid solutions in the K+, \( {\text{NH}}{_4^{+}} \)//Cl−, \( {\text{SO}}{_4^{2 -}} \)–H2O system was experimentally determined and then theoretical calculations about the process can be carried out.