Study on the effect of NH₃ on the solubility of KCl and NH₄Cl: a microscopic view

KCl is an important fertilizer that is used worldwide. NH₄Cl solution is an eluent for potassium extraction from seawater using zeolite. The product solution obtained from the zeolite method for potassium extraction from seawater usually contains both potassium chloride and ammonium chloride. There is no KCl crystallization area in the phase diagram of KCl and NH₄Cl, and it is impossible to obtain pure KCl via simple evaporation method. Based on our former research, KCl crystallization area will appear upon the addition of NH₃ in this system. To investigate the microscopic mechanisms by which NH₃ affects the solubility of KCl and NH₄Cl, a systematic study of single KCl-NH₃-H₂O solution and NH₄Cl-NH₃-H₂O solution systems with different mass fractions at room temperature was carried out using synchrotron X-ray scattering (XRS), Raman spectroscopy and molecular dynamics (MD) simulations. XRS results revealed that with the increase in ammonia concentration in the solution, the peak intensity of the G(r) main peak at 2.85 Å and 3.25 Å in the KCl-NH₃-H₂O system decreased. These conclusions explained the phenomenon of decreasing KCl solubility as the ammonia concentration increased in the KCl-NH₃-H₂O solution at the microscale. The enhanced peak intensity of the main peak at 3.25 Å in the NH₄Cl-NH₃-H₂O system was related to the presence of more N(A)-O(W) and N(N)-N(A) interactions in the solution. The increase in the N(N)-N(A) interaction illustrated that the ammonia molecule was present as solvated ammonium ions, just like the water molecule, revealing the microscopic properties of the increased NH₄Cl solubility with increased ammonia concentration. Raman results showed that compared with the KCl-NH₃-H₂O system, more tetrahedral-type hydrogen bonding networks were disrupted in the NH₄Cl-NH₃-H₂O system with increased ammonia concentration. MD simulation results showed that in the KCl-NH₃-H₂O system, as the ammonia concentration increased, the coordination number of K-Cl increased, and the direct contact between K-Cl and ion pairs increased. The N(N)-N(A) coordination number of the NH₄Cl-NH₃-H₂O system reached its maximum when the NH₃ content increased from 5% to 23%. Based on the above analyses, it can be concluded that ammonia acted as the solvent of NH₄⁺ in the solution system, which induced the micro-mechanism of NH₄Cl solubility increment.