Study on the effect of NH on the solubility of KCl and NHCl: a microscopic view.

KCl is an important fertilizer that is used worldwide. NHCl 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 NHCl, and it is impossible to obtain pure KCl simple evaporation method. Based on our former research, KCl crystallization area will appear upon the addition of NH in this system. To investigate the microscopic mechanisms by which NH affects the solubility of KCl and NHCl, a systematic study of single KCl-NH-HO solution and NHCl-NH-HO 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 () main peak at 2.85 Å and 3.25 Å in the KCl-NH-HO system decreased. These conclusions explained the phenomenon of decreasing KCl solubility as the ammonia concentration increased in the KCl-NH-HO solution at the microscale. The enhanced peak intensity of the main peak at 3.25 Å in the NHCl-NH-HO 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 NHCl solubility with increased ammonia concentration. Raman results showed that compared with the KCl-NH-HO system, more tetrahedral-type hydrogen bonding networks were disrupted in the NHCl-NH-HO system with increased ammonia concentration. MD simulation results showed that in the KCl-NH-HO 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 NHCl-NH-HO system reached its maximum when the NH content increased from 5% to 23%. Based on the above analyses, it can be concluded that ammonia acted as the solvent of NH in the solution system, which induced the micro-mechanism of NHCl solubility increment.