Study On Phase Equilibrium Of The Quaternary System Na₂SO₄-MgSO₄-（NH₄）₂SO₄-H₂O And Dehydration Process Of MgSO₄·（NH₄）₂SO₄·6H

Bloedite(Na2SO4·MgSO4·4H2O)resources in China are abundant and have anextensive distribution, especially in a richer brine region. However, due to difficultseparation of bloedite there is no mature industrialization process development at present.Aiming at the development of bloedite, an idea was proposed to separate bloedite by adding(NH4)2SO4as salting-out agent producing N-Mg compound fertilizers and Na2SO4·10H2Oin this paper.The solubilities of the ternary system MgSO4-(NH4)2SO4-H2O and the quaternarysystem Na2SO4-MgSO4-(NH4)2SO4-H2O at0℃were measured and the phase diagrams ofthese two systems were plotted. Based on the phase diagrams obtained, we analysed eachsaturation point and crystalline region. It indicated that the Na2SO4and MgSO4in bloeditecan achieved a even better separation by using (NH4)2SO4salting out method.On the basis of Pitzer model of electrolyte solution theory, ionic strength function wasintroduced to express the interaction parameters between two kinds of electrolyte at0℃.The interaction parameters were regressed from the experimental data of the ternarysystems and the solubilities of Na2SO4-MgSO4-(NH4)2SO4-H2O at0℃were calculated.The results showed that the calculated values were consistent with experimental results well.It indicated that using the Pitzer model to calculate the liquid-solid phase equilibrium ofthis system was feasible.N-Mg compound fertilizers (MgSO4·(NH4)2SO4·6H2O) were produced from thereaction of MgSO4·7H2O, Na2SO4and (NH4)2SO4and the reaction of bloedite with(NH4)2SO4at25℃, respectively. After several times washing by the saturated solution ofmagnesium sulfate the purity of Mg-N compound fertilizers are85.65%and58.25%,respectively. Then water was added into the mother liquid to abstract Na2SO4·10H2O at0℃by the lower temperature crystallization. When sodium sulfate decahydrate was placed indry air, it would gradually lose crystal water and turn to anhydrous sodium sulfate.Therefore, sodium sulfate after drying contained a fraction of anhydrous sodium sulfate.MgSO4·(NH4)2SO4·6H2O is the double salt containing crystal water. If we get rid ofcrystal water, it can effectively avoid deliquescence and conveniently apply in the modernagricultural production. For this reason, thermal stability and decomposition process of MgSO4·(NH4)2SO4·6H2O were studied in the paper by means of TG-DTG and XRDmethods. From the TG-DTG curves we knew that there were four stages in the process ofthermal decomposition and the crystal water can be completely removed between61.2and148.6℃. XRD results showed MgSO4·(NH4)2SO4was a kind of new structure materialwhich was different from MgSO4and (NH4)2SO4mixtures. The most probable mechanismfunctions and kinetics parameters of each stage are calculated by using the non-isothermalisoconversional method. Autocatalytic mechanism was used to describe the dehydrationkinetics of MgSO4·(NH4)2SO4·6H2O with reaction order n=1. The deamination ofMgSO4·(NH4)2SO4was controlled by two-dimension diffusion mechanism fitting Valensiequation. The deamination and dehydration of MgSO4·NH4HSO4and the decomposition ofMgSO4·SO3were controlled by phase boundary reaction model fitting shrinking cylinderequation.