| The aqueous salt solution is widely distributed in nature and is an important ore-forming fluid for the hydrothermal metal deposits,and it plays an indispensable geological role during the formation of the deposits.The pressure-volume-temperature-composition（PVTx）and phase-equilibrium properties of saline solution have been widely used in the geochemical research,such as fluid inclusion studies,fluid flow simulation,fluid-rock interaction,etc.Previous researchers have successfully predicted the volume（density）properties of multi-component electrolyte solutions(Na+,Ca2+,K+,Mg2+,Cl-,SO42-)at low and medium temperatures and pressures,but the studies of the PVTx properties of aqueous sodium nitrate solutions are of insufficient,and the model of sodium nitrate solubility in water is not accurate enough.This work mainly involves calculation and simulation for above system,and the results are as follows:1.Based on the Pitzer electrolyte theory,the method of the Mao and Duan（2008）density model was used by the relationship between the apparent molar volume of the electrolyte and the infinite dilute molar volume.An accurate PVTx model（density model）for the binary sodium nitrate-water system has been developed.First,the experimental PVTx data of the system were collected and compared with each other to obtain reliable data;Then,on the basis of the reliable experimental data,the linear least square method was used to regress model parameters.After many tries,19 parameters were finally obtained.The results show that the average density error of the model is about 0.15%,which is close to the experimental accuracy.The valid temperature-pressure-salinity range of the density model is 278.15-587.38 K,1-300 bar and0-10.83 mol·kg-1.With the model,the density,apparent molar volume and infinite dilute apparent molar volume of sodium nitrate in water can be calculated.2.In terms of the concentration formula of the saturated solution of a single solid phase,the phase equilibrium study of the sodium nitrate-water system was carried out.The solubility model for the sodium nitrate in pure water has been improved,and the corresponding parameters were obtained by the least square method.Compared with the previous models,this model dramatically reduces the calculation error,and the average error between the model and the experimental data is only 1.27%under conditions of high temperatures.The solubility calculated by the model is within the experimental error range,which shows the reliability of the model.The effective temperature range of the model is 255.45-568.5 K.The PVTx model and the solubility model developed in this work for the sodium nitrate-water system,can be extended to study the corresponding thermodynamic properties of multi-component mixed salt solutions provided that certain mixing rule is used. |
