Solid-Liquid Metastable Equilibria Of Quaternary System K⁺(Mg²⁺),NH₄⁺//Cl^-,SO₄^2--H₂O At 25.0℃ And Application

Chaerhan Salt Lake in Qinghai Province,China,is famous for abundant potassium chloride and magnesium chloride.However,there are many problems in the exploitation of potassium and magnesium resources.Because potassium chloride is not suitable for saline-alkali soil and chloride-sensitive plants,the production of chloride-free potash fertilizers is paid attention.In the production process of potassium sulfate from potassium chloride resources in salt lakes,a large number of magnesium chloride have not been effectively utilized,resulting in local enrichment of magnesium salts.It seriously affects the sustainable development and utilization of potassium and magnesium resources in salt lake.In order to exploit the inorganic salt resources reasonably and economically,it is important to make full use of the local natural resources.The Qaidam area has favorable climatic conditions such as strong wind,drought,less rainfall and large evaporation.Therefore,it is very important to use metastable phase equilibrium to predict the actual evaporation path of salt lake.In order to exploit potassium and magnesium resources efficiently in Chaerhan Salt Lake,the metastable phase equilibrium of K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O and Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O system was studied at 25.0℃,and the process of preparing chlorine-free potassium fertilizer was determined according to the metastable phase diagrams.The metastable solubility of the K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary system and its ternary systems at 25.0oC was measured using the isothermal evaporation method,and the corresponding metastable phase diagrams were plotted.In the metastable phase diagram of the K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O system,there is one invariant point and three crystalline zones that correspond to(K_1-t,NH_{4 t})₂SO₄,(K_1-m,NH_{4 m})Cl,(NH_{4 n},K_1-n)Cl.The crystallization zone of(K_1-t,NH_{4 t})₂SO₄is the largest,which indicates that(K_1-t,NH_{4 t})₂SO₄is easy to crystallize.On the basis of the Pitzer model,the correlation calculation of the solubility of three ternary systems was carried out and the results show that the calculated values are consistent with the experimental data.According to the analysis of the metastable and stable phase diagrams of the K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary system at 25.0oC,a technological process was proposed in this paper.In this process,potassium ammonium sulfate compound fertilizer was prepared via evaporation and crystallization with potassium chloride and ammonium sulfate as raw materials,and the complementary product（ammonium potassium chloride）was obtained via isothermal evaporation.The metastable solubility of the Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary system and its two ternary systems at 25.0oC was measured using the isothermal evaporation method,and the corresponding metastable phase diagrams were plotted.In the metastable phase diagram of the Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O system,there are six invariant points and eight crystalline zones that correspond to Mg SO₄·（NH₄）₂SO₄·6H₂O,（NH₄）₂SO₄,NH₄Cl,Mg Cl₂·NH₄Cl·6H₂O,Mg SO₄·7H₂O,Mg SO₄·6H₂O,Mg SO₄·5H₂O and Mg Cl₂·6H₂O.On the basis of the Pitzer model,the correlation calculation of the solubility was carried out and the results show that the calculated values are consistent with the experimental data.According to the analysis of the metastable and stable phase diagrams of the Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary system at 25.0oC,the route of producing magnesium ammonium sulfate compound fertilizer from magnesium chloride and ammonium sulfate at 25.0oC via evaporation and crystallization was determined and the theoretical calculation of the process was completed according to the material conservation rules,which provided a guiding method for the application of the phase diagrams.