Research On Phase Equilibrium,Structural Properties And Separation Process Of Potassium-Ammonium Based Solid Solution In Aqueous Solid Solution

In the production process of potassium salts such as potassium sulfate and potassium dihydrogen phosphate,the water-salt system containing K+and NH4 is inevitably involved.Because of the same lattice type,similar lattice parameters and similar atomic structure,it is easy to form solid solution.It is difficult to separate and produce high-quality potassium salts in this system.Therefore,in this paper,we take K+,NH4+//SO42-,H2PO4,-H2O as an example,research on phase equilibrium,structural properties and separation process of potassium-ammonium based solid solution in aqueous solid solution.First,solid-liquid equilibrium and physicochemical properties(nD,η,ρ,pH)of the system K+,NH4+//SO42-,H2PO4--H2O and its ternary subsystem was measured at 313.15 K using isothermal method and the phase diagram and physicochemical composition diagram was plotted.Based on the shape of the solubility isotherms and particularly the course of the corresponding tie lines,the resulting phase diagram of the ternary system indicates complete miscibility for K2SO4 and(NH4)2SO4 in the solid state,and it can be clearly verified by XRD studies.Moreover,the relative maximum radius ratio of K to NH4 accords to the formation rule of substituted solid solution,△rmax=(r1-r2)/r2=6.77%<15%,solid solution type can be verified-they were infinite substitutional solid solution.[(NH4)0.104K1.896]SO4 and[(NH4)1.79K0.203]SO4 is solid solution with potassium sulfate and ammonia sulfate as the main phase respectively.As can be seen,from the analysis of FT-IR,wave forms of these two solid solutions are similar,but the absorption position has a slight shift to low wave number direction as the increasing of K2SO4 content;From the analysis of TG-DSC,it can be concluded that the weightlessness decreases with the decrease of NH4 content;From the analysis of mapping,indicating the homogeneous distribution of K+and NH4+.Finally,multistage fractional crystallization to separate K2SO4 from aqueous solid solution has been investigated.A multistage fractional crystallization process will be modeled and explained,based on the phase diagrams and the relevant mass balancing.By multistage fractional crystallization experiment to obtain the purity of the K2SO4 is 95.28%after three times cycle operation and we can get about 99.04%K2SO4 in the fourth operation by theoretical analysis.Based on the shape of the solubility isotherms and particularly the course of the corresponding tie lines,the resulting phase diagram the analysis of XRD of the ternary system indicates the system can be complete solid solution.Moreover,according to the formation rules of substituted solid solution,solid solution type also can be verified-they were infinite substitutional solid solution.What’s more,[K0.78(NH4)0.22]H2PO4 and[K0.11(NH4)0.89]H2PO4 is solid solution with potassium sulfate and ammonia sulfate as the main phase respectively,from the analysis of FT-IR,TG-DSC,mapping,the structural features of solid solution are studied.Finally,based on the phase diagrams and the material balances,multistage fractional crystallization experiment to obtain the purity of the KH2PO4 is close to 97%after five times operation and purity of the NH4H2PO4 reaches about 98%after four time operation.The experiment verified that the multistage fractional purification can improve the purity of the target product for AQ-SS.The research results of this paper have important theoretical value and guiding significnce for producting of high quality potassium sulfate,potassium dihydrogen phosphate and other potassium salt.At the same time,it has reference value for high efficiency and controllable separation of other types solid solution.