Solubility and Phase Equilibria of Li2CO3–NH4HCO3–H2O and K2CO3–NH4HCO3–H2O Systems under Sealed Equilibrium Conditions at 278.15–298.15 K: Comparison with (NH4)2CO3 Systems

Solid–liquid phase equilibria of the Li2CO3–NH4HCO3–H2O and K2CO3–NH4HCO3–H2O systems were determined under sealed equilibrium conditions at 278.15, 288.15, and 298.15 K using the isothermal dissolution method, with equilibrium pH monitored. The Li2CO3–NH4HCO3–H2O system exhibits two crystallization regions (Li2CO3 and NH4HCO3) and one invariant point over the investigated temperature range. Increasing temperature shifts the invariant point toward the NH4HCO3-rich side, enlarging the Li2CO3 region and reducing the NH4HCO3 region. NH4HCO3 exhibits a salting-in effect on Li2CO3, whereas Li2CO3 changes its effect on NH4HCO3 from salting-in at 278.15 K to slight salting-out at higher temperatures. The K2CO3–NH4HCO3–H2O system forms the double salt K4H2(CO3)3·1.5H2O at 278.15 K, while KHCO3 and NH4HCO3 crystallize in different composition ranges. The double salt was not detected at higher temperatures. NH4HCO3 strongly salts out K2CO3, whereas K2CO3 shows temperature-dependent salting-out/in effects on NH4HCO3. NH4HCO3 addition rapidly lowers equilibrium pH to 8–9 for Li2CO3 systems (lower than in (NH4)2CO3 systems) and 11–13 for K2CO3 systems (more pronounced decrease). These pH differences contribute to distinct phase behaviors: NH4HCO3 promotes Li2CO3 dissolution more effectively than (NH4)2CO3, whereas the opposite occurs for K2CO3. (NH4)2CO3 systems facilitate Li-containing double-salt formation. The results provide fundamental data for ammonia-based carbonation processes and Li/K separation.