Determination and Calculation of the Solubility of the Quaternary System NaBr-KBr-NH₄Br–H₂O Containing Solid Solutions at 298.15 and 323.15 K

In order to fully recover and utilize the bromine and potassium resources in seawater bitter brine after magnesium extraction, the solubilities of the quaternary system NaBr–KBr–NH₄Br–H₂O and its subsystem KBr–NH₄Br–H₂O at 298.15 and 323.15 K were investigated by the isothermal dissolution equilibrium method. The equilibrium solid phase composition was determined by the wet residue method and X-ray diffraction. Phase diagrams were drawn by using experimental data. At 298.15 K, K–NH₄ solid solution is formed in the quaternary system NaBr–KBr–NH₄Br–H₂O, and the dry-base phase diagram consists of one invariant point and three crystalline regions (NaBr·2H₂O, (K,NH₄)Br, (NH₄,K)Br). At 323.15 K, the dry basis phase diagram of this quaternary system consists of one invariant point and three crystalline regions (NaBr, (K,NH₄)Br, (NH₄,K)Br). According to a comparison with the quaternary phase diagram at 298.15 K, the crystalline zone of NaBr·2H₂O vanishes, and a single salt crystalline region of NaBr emerges following the temperature increase. Meanwhile, the crystallization zone of (K,NH₄)Br increases and that of (NH₄,K)Br decreases. At 298.15 K, the phase diagram of its ternary subsystem consists of one invariant point and two crystalline regions ((K,NH₄)Br, (NH₄,K)Br). No single salt crystalline region is shown in the phase diagram. By deriving the extended Pitzer model and applying it to the systems NaBr–KBr–NH₄Br–H₂O and KBr–NH₄Br–H₂O, the solubilities of the quaternary and ternary systems with solid solutions at 298.15 K were calculated without taking into account the ionic interaction parameter. The results were in general agreement with the experimental values.