Crystallization Thermodynamics Of Potassium Hexafluorosilicate In HCl-H₃PO₄-NaCl-MgCl₂-CaCl₂-AlCl₃-FeCl₃-H₂O System

The soluble hoevellite resources are severely scarce in China,however,the insoluble potash ores,such as phosphorus-potassium associated ore are very abundant.Therefore,the economical and effective extraction of soluble potassium from phosphorus-potassium associated ore has become an urgent matter.The potassium hexafluorosilicate?K₂SiF₆?product can be obtained from acid-leaching solution of phosphorus-potassium associated ore by using crystallization method.In order to obtain high quality potassium hexafluorosilicate,the crystallization thermodynamics such as solubility and metastable zone width for potassium hexafluorosilicate of acid-leaching solution are urgently needed.Because the acid-leaching solution has complicated compostions,such as hydrogen,chlorine,fluorine,potassium,sodium,aluminium,iron,calcium,magnesium,phosphorus,and each compostion may have various species,the current relevant research reports are extremely limited.In this thesis,the solubility of K₂SiF₆ in HCl-H₃PO₄-NaCl-MgCl₂-CaCl₂-AlCl₃-FeCl₃-H₂O system was determined by equilibrium method considering the ion species and concentrations of the acid-leaching solution of phosphorus-potassium associated ore.At the same time,the stability of the equilibrium solid phase in the corresponding system was also analyzed.In addition,the metastable zone width of K₂SiF₆ in the HCl-H₃PO₄-H₂O system was measured by the non-isothermal method and the apparent nucleation progression m in each system was calculated by the classical nucleation theory equation.The main research contents and conclusions are as follows:?1?The equilibrium temperature plays an important role in the course of dissolution,and the solubility values of K₂SiF₆ in each system always increase with increasing temperature.The solubility of K₂SiF₆ in HCl solutions increases slowly when the concentration of hydrochloric acid is less than 3 mol/L,on the contrary,the solubility decrease slowly when hydrochloric acid is higher than 3 mol/L.The solubility of K₂SiF₆in H₃PO₄ solutions increases first and then decreases with the increase of the concentration of H₃PO₄ solution.The solubility reaches a maximum when the concentration of phosphoric acid is 1.5-2.0 mol/L within 50°C,yet the solubility reaches a maximum when the concentration of phosphoric acid is 2.5 mol/L with the dissolution equilibrium temperature higher than 60°C.In NaCl,MgCl₂,CaCl₂,FeCl₃ solutions,the solubility of K₂SiF₆ increases with the increase of component concentration.The solubility of K₂SiF₆ in AlCl₃ solution gradually decreases with the increase of AlCl₃concentration.?2?The equilibrium solid phase is K₂SiF₆ in H₂O,HCl and H₃PO₄ solutions.The morphologies of equilibrium phase in pure water are cubic crystals or cubic crystal cluster.The morphologies of equilibrium solid phase in HCl solutions are the massive cluster crystal accumulation with some agglomeration.The morphologies of equilibrium solid phase in H₃PO₄ system are relatively uniform with the irregular prismatic particles.?3?The hydrolysis is more obvious in the solutions of NaCl,MgCl₂,CaCl₂,AlCl₃and FeCl₃ system and the stability of the equilibrium solid phase has been greatly changed.The new phase KNaSiF₆ was detected in the equilibrium solid phase of NaCl solutions,and the edges of cubic crystalline crystals were disappeared completely with no homogeneous crystalline form.The edges of cubic crystalline crystals were vanished and changed into approximately polyprismatic crystals in MgCl₂ solutions.In CaCl₂ solutions,a new phase of CaF₂ in the equilibrium solid phase was appeared and the equilibrium solid phase obtained has no fixed crystal form covered with some small crystals.There was no fixed crystalline form of the equilibrium solid phase in AlCl₃ solutions,and a part of the solid began to cake and carry more powder crystals.Most of the morphologies of equilibrium solid phase in FeCl₃ solutions are completely cubic crystals or cuboid clusters formed,and few of them are amorphous crystalline powders.?4?The metastable zone width of K₂SiF₆ in the HCl-H₃PO₄-H₂O system was narrowed with the increase of saturation temperature and broadened with the increase of cooling rate.There is no significant change of the apparent nucleation progression m in each system and the value of m increases slightly with increasing temperature.The apparent nucleation progression m in the H₃PO₄ solutions is slightly larger than the value of m in the HCl solutions under the same experimental conditions.