Study on the effect of NH3 on the solubility of KCl and NH4Cl – from microscopic view

Abstract

KCl is a kind of quite important fertilizer in the world. NH4Cl solution is the eluent for potassium extraction from seawater by zeolite. Normally, there is no KCl crystallization area in the phase diagram of KCl and NH4Cl, and it’s impossible to get pure KCl by simple evaporation method. Base on our former research the KCl crystallization area will appear with the addition of NH3 in this system. But the Microscopic mechanisms how does NH3 influence the solubility of KCl and NH4Cl has not been discovered. 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 in this paper. This work was accomplished through Synchrotron X-ray scattering (XRS), Raman spectroscopy and molecular dynamics (MD) simulations. The XRS results indicated that with the increase of ammonia concentration in the solution, the peak intensity of the G(r) main peak at 2.85Å and 3.25 Å in the KCl-NH3-H2O system decreased. These conclusions explained microscopically the phenomenon of decreasing solubility of KCl as the concentration of ammonia increased in the KCl-NH3-H2O solution. 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 acted as a solvation of ammonium ions, just like the water molecule, revealing the microscopic properties of increased NH4Cl solubility due to the increased ammonia concentration. Raman results showed that more tetrahedral-type hydrogen bonding networks were disrupted in the NH4Cl-NH3-H2O system with increased ammonia concentration. From MD simulation, it 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 content of NH3 increased from 5% to 23%. Based on the above analysis, 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.