Solubility Measurement of the Li ⁺ (K ⁺ ), NH ₄ ⁺ //CO ₃ ^2– -H ₂ O Pseudoternary Systems at 278.15 K, 288.15 K, and 298.15 K

The solubility isotherms of the ternary systems Li+(K+), NH4+//CO32–-H2O are determined at 278.15, 288.15, and 298.15 K using the isothermal dissolution method, and the pH of the equilibrium liquid phase is monitored. The saturated binary Li2CO3 solution shows a pH up to 11.7, while K2CO3 solutions exceeded 14. The addition of (NH4)2CO3 markedly reduces the alkalinity, with the pH ranging from 10.0 to 9.2 and 12.5 to 9.2 for Li- and K-based systems, respectively. Due to the presence of the ionization equilibria NH4+-NH3·H2O and CO32–-HCO3–, both systems are pseudoternary, derived from a more complex Li+(K+), NH4+//CO32–, HCO3–-NH3·H2O equilibrium system. Solubility measurements at 298.15 K reveal that the Li2CO3-(NH4)2CO3–H2O system exhibits two invariant points and three crystallization regions: Li2CO3, Li2CO3·3(NH4)2CO3, and (NH4)2CO3·H2O. The K-based system shows one invariant point and two regions: K2CO3·3H2O and (NH4)2CO3. Lowering the temperature eliminates the Li2CO3·3(NH4)2CO3 crystallization regions, retaining only the Li2CO3 and (NH4)2CO3·H2O regions in the Li-based system. In contrast, in the K-based system, the crystallization region of K2CO3 and the composition of the invariant point remain nearly unchanged with decreasing temperature, while the (NH4)2CO3 crystallization region expands significantly. These results provide theoretical guidance for ammonium carbonate-based ammonia-alkali cyclic carbonation processes in treating sulfate or halide systems.