Study On Phase Equilibrium To Prepare Chloride-free Potassium Fertilizer And N-Mg Compound Fertilizer With Aslt Lake Carnallite As Raw Material

Qarhan Salt Lake is world-famous for its rich variety of inorganic salt resources,and its inorganic salt reserves rank first in China’s salt lakes.Although its salt lake mineral resources are unique,there are still many problems in the process of resource development,especially the development of potassium resources while neglecting the development of other resources,resulting in weak product competitiveness,serious waste of resources,and the problem of salt lake pollution is becoming increasingly serious.Due to the unique advantages of chloride-free potassium fertilizer,the production of chloride-free potassium fertilizer has been paid more and more attention by salt lake enterprises.Based on the optimization of the process for producing chloride-free potassium fertilizer by reacting salt lake carnallite with industrial by-product ammonium sulfate and the utilization of magnesium chloride by-products after potassium chloride extraction in salt lakes,this paper carried out a phase equilibrium study of the related water-salt system.The isothermal dissolution method was used to determine the stable phase equilibrium data of K⁺,Mg²⁺//Cl^-,SO₄^2--H₂O quaternary interaction system and four ternary sub-systems at 80℃.The corresponding phase diagrams were plotted and analyzed.The quaternary system phase diagram contains nine crystalline regions,which are KCl,K₂SO₄,K₂SO₄·Mg SO₄·4H₂O,Mg SO₄·6H₂O,Mg SO₄·5H₂O,Mg SO₄·4H₂O,KCl·Mg Cl₂·6H₂O,Mg Cl₂·6H₂O and KCl·Mg SO₄·3H₂O respectively.The largest crystalline region is the K₂SO₄crystalline region,which indicates that K₂SO₄is easy to crystallize out under this condition.Based on the analysis and calculation of K⁺,Mg²⁺//Cl^-,SO₄^2--H₂O at 80℃and35℃,potassium chloride and magnesium sulfate were selected as the reaction raw materials,and potassium sulfate was prepared at high temperature,picromerite was prepared at low temperature.A new process for preparing chloride-free potassium fertilizer and by-producing potassium-magnesium compound fertilizer was designed.The isothermal dissolution method was used to determine the stable phase equilibrium data of Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary interaction system and four ternary sub-systems at 90℃.The corresponding phase diagrams were plotted and analyzed.The quaternary system phase diagram contains seven crystalline regions,which are NH₄Cl,（NH₄）₂SO₄,Mg Cl₂·NH₄Cl·6H₂O,Mg SO₄·（NH₄）₂SO₄·6H₂O,Mg SO₄·6H₂O,Mg SO₄·4H₂O and Mg Cl₂·6H₂O respectively.Based on the analysis and calculation of Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O system at 25℃and 90℃,the process of preparing ammonium chloride at high temperature and N-Mg compound fertilizers at low temperature by reacting ammonium sulfate and old brine in salt lake was designed.Determine the appropriate conditions for the reaction.By comparing with the two process routes designed in the literature,the optimal process route was selected.The isothermal evaporation method was used to determine the metastable phase equilibrium data of Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary interaction system and four ternary sub-systems at 0℃.The corresponding phase diagrams were plotted and analyzed.The quaternary system phase diagram contains six crystalline regions,which are（NH₄）₂SO₄,Mg Cl₂·NH₄Cl·6H₂O,Mg SO₄·（NH₄）₂SO₄·6H₂O,Mg SO₄·7H₂O,Mg Cl₂·6H₂O and NH₄Cl respectively.Based on the analysis and calculation of Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O system at25℃and 0℃,the process of preparing ammonium chloride at low temperature and N-Mg compound fertilizers at room temperature by reacting ammonium sulfate and old brine in salt lake was designed.Pitzer electrolyte solution model was used as the theoretical thermodynamic model to calculate the solubility data of the quaternary system studied above.The results show that the experimental values are in good agreement with the theoretical calculated values.