Development And Application Of Comprehensive Thermodynamic Model Of Multitemperature Phase Equilibrium In Salt Lake Brine System

Qaidam basin contains rich potassium lithium magnesite boron resource,which is our country strategic support base of potassium lithium demand.Salt lake brines are of complex composition,mainly sulfate type and chloride type.The production of salt lake is a process of multi-temperature dynamic evaporation and salt analysis.The complicated law of salt analysis results in low yield of potassium and lithium resources produced in salt pans.It is necessary to construct a multi-temperature thermomechanical model which is more suitable for salt lake brine system,express the physical properties and phase equilibrium law of complex brine system,predict the dynamic evaporation and salt evolution process of salt lake,and then optimize the production process and control scheme.Currently,thermodynamic models widely used in water-salt systems,such as Pitzer and e NTRL,are typical physical models.When expressing brine systems containing chemical processes such as ionic hydration,partial ionization of electrolytes,ion association and crystalline precursor,they cannot accurately reflect the physical properties and phase equilibrium laws of solution.Therefore,this study aims to improve the comprehensive thermodynamic model of salt lake brine.Based on the eNRTL model framework,the general model expression of chemical contribution is expanded,and the parameter system of thermodynamic consistency of multi-temperature state in multi-system is constructed.On this basis,the whole process of evaporation and salt evolution of chloride-type salt lake brine is quantified.Specific achievements are as follows:1.Expand the eNRTL model,increase the chemical contribution of solution non-ideality,and form a comprehensive thermodynamic general platform with the improved eNRTL model.By determining the thermodynamic parameters of semi-stable chemical species,the physical properties and phase equilibrium relations of solution containing chemical equilibrium process are accurately expressed.The introduction of hydration ions in Li Cl-H₂O system makes the calculation of saturated vapor pressure and freezing point in the whole temperature range more accurate.[Li Cl·KCl·H₂O],[KCl·Ca Cl₂·3H₂O],[Na Cl·Ca Cl₂·2H₂O],The standard Gibbs free energy of formation,standard enthalpy of formation and isobaric molar heat capacity of[Na Cl·Ca Cl₂·2H₂O]and other associated structures accurately express the unconventional solid-liquid equilibrium of these systems at various temperatures.On this basis,the complete phase diagrams of Li Cl-H₂O binary system,Li Cl-KCl-H₂O,KCl-Ca Cl₂-H₂O and Na Cl-Ca Cl₂-H₂O ternary system are predicted.2.The phase equilibrium of complex quaternary system Li⁺,K⁺,Ca²⁺//Cl^--H₂O constructed by chemically significant Li Cl-KCl-H₂O and KCl-Ca Cl₂-H₂O systems at298.15K and 273.15K was experimentally studied,and the corresponding solubility data were obtained.The solid-liquid equilibria of Li⁺,K⁺,Ca²⁺//Cl^--H₂O quaternion at 298.15K and 273.15K were predicted by chemical contribution model.The results show that the calculated values of the chemical contribution model are consistent with the experimental values,indicating that the model has a good ability to predict the multi-component system with a variety of association structures,which further verifies the model’s ability to express and predict the strong non-ideality of the chloride brine brine system.3.Complete phase diagram fitting,phase diagram structure prediction and consistency verification of parameters（liquid phase characteristic parameters and species parameters）were carried out for two six-member system Li⁺,Na⁺,K⁺,Mg²⁺//Cl^-,SO₄^2--H₂O and Li⁺,Na⁺,K⁺,Mg²⁺,Ca²⁺//Cl^--H₂O of sulfate brine and chloride brine in Qaidam Basin by using a new eNRTL model containing chemical contribution model.The thermodynamic properties of all binary and ternary systems were re-calculated by regression,and all the characteristic parameters were obtained:liquid phase characteristic parameters（9 pairs of salt-water interaction parameters,20 pairs of salt-salt interaction parameters）,88 solid-phase species parameters,1 group of hydration ion parameters（4 pairs）and 3 association structure parameters.The thermodynamic model parameter base of salt lake brine system is unified,and the applicable temperature range is extended to the whole temperature range above-85℃.The prediction of the complete phase diagram structure and the total temperature phase diagram of the salt lake brine system in Qaidam Basin was completed,and the thermodynamic model and database suitable for the salt lake brine system were constructed.4.The new comprehensive thermodynamic model is used to model the evaporation and salting process of salt pans,including:the multi-system saturated vapor pressure model is applied to the large area evaporation rate of salt pans,the prediction of salt lake brine evaporation and salting process,the prediction of the relationship between potassium and lithium loss in salt pans and the structure and operation mode of salt pans.It provides a theoretical tool for quantifying the salt-out state of polythermal process and evaporative salt-out process,analyzing the metallogenic state of salt pans and optimizing the structure of salt pans.The chemical contribution model established in this study has good calculation and prediction ability for electrolyte solution systems containing both ionic hydration and salt pair association,which enhances the calculation ability of the model for complex systems and is more suitable for complex salt lake brine systems,laying a solid model foundation for subsequent research.Based on the thermodynamic model,the dynamic salt evaporation law model reveals the internal relationship between meteorological factors and brine evaporation state,enhances the understanding of the production process of salt lake salt pan,and provides a basic tool for the intelligent production process of salt lake brine.