Study On The Phase Equilibrium Of Chlorine-free Potash Fertilizer Prepared Via Thereaction Between Carnallite And Ammonium Sulfate

Qarhan Salt Lake in Qinghai province,China,is the largest soluble potassium and magnesium salt lake in the world.The carnallite?KCl·MgCl₂·6H₂O?was produced through the process of solar evaporation,then potassium chloride was prepared by the traditional method of decomposing carnallite with water.Although potassium chloride can meet the need of most plants for potash fertilizer,it still cannot be suitable for the crops in the saline-alkali fileds and some fear-chlorion crops,such as tobacco,sweet potato,beet,and so on.Accordingly,this article proposed a new technology to produce potassium sulfate and ammonium chloride from carnallite and the industrial by-product ammonium sulfate.The solubilities of the system K₂SO₄-MgSO₄-?NH₄?₂SO₄-H₂O and K⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O were measured by isothermal method at 25°C and their dry salt phase diagrams were plotted.By analysing the saturation points and the crystalline regions in these phase diagrams,we find that the quaternary system K₂SO₄-MgSO₄-?NH₄?₂SO₄-H₂O at 25?formed a new double salt:(K_1-n,NH_4n)₂SO₄·MgSO₄·6H₂O?n:0.552-0.986?.The double salt has a largest crystalline zone in the quinary system K⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O at 25?.The solubilities of the system Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O were measured by isothermal method at 60°C and their dry salt phase diagrams were plotted.And then the saturation points and the crystalline regions in these phase were analyzed.By contrasting the phase diagrams of the system Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O at 25 and 0?,we found that this system has a largest crystalline zone of MgSO₄·?NH₄?₂SO₄·6H₂O,and NH₄Cl has a respectable crystalline zone too.Based on the extended Pitzer model,ionic strength function was introduced to express the interaction parameters between two kinds of electrolyte.Then the Pitzer model was applied into the systems which were studied in the paper and its sub-systems from the literature data,and the Pitzer interaction parameters was calculated.The interaction parameters were plug into the system K₂SO₄-MgSO₄-?NH₄?₂SO₄-H₂O at 25?and Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O at 60?,and the solubilities of systems were calculated.The results show that the calculated values were consistent with experimental results well.