Thermodynamic Properties And Thermodynamic Model Of Sodium Borate Brine System At 323.15 K

Due to the decreasing of solid mineral resources,thel liquid mineral resources such as salt lake brine have become the hot spot of development.Salt lake brine is mainly distributed in Qinghai,Xinjiang,Tibet,Sichuan and so on,which contains rich boron resources.Thermodynamic properties and thermodynamics model in sodium borate brine systems at 323.15 K were studied in this paper.The research provides important basic data and theoretical guidance on development and utilization of superior brine boron resources,promote the high value of salt chemical products and upgrade of industrial chain,and is of great significance for rational development of salt lake brine.In this paper,the phase equilibria in ternary systems NaBO₂-Na₂B₄O₇-H₂O and Na₂B₄O₇-NaB₅O₈-H₂O at 323.15 K were investigated with the isothermal dissolution method.Both ternary systems belong to simple satiation system.There is only one invariant point saturated with NaBO₂·4H₂O+Na₂B₄O₇·10H₂O in system NaBO₂-Na₂B₄O₇-H₂O and Na₂B₄O₇·10H₂O+NaB₅O₈·5H₂O in system Na₂B₄O₇-NaB₅O₈-H₂O,two solubility curves and two single salt crystallization zones.No double salt and solid solution forms in the two systems.The physicochemical properties（refractive index and p H）of the prepared solution were determined.The physicochemical properties change regularly with the concentration changing.The calculated refractive indices with empirical formulas agree with the experimental data.The Pitzer single salt and mixing parameters were fitted with the solubilities in the ternary systems.The calculated solubilities are in good agreement with the experimental values,which shows the Pitzer model can be used in the solubility calculation.The distribution of boron species in the ternary systems NaBO₂-Na₂B₄O₇-H₂O and Na₂B₄O₇-Na₂B₅O₈-H₂O at 298.15 K and 323.15 K were calculated with the equilibrium constants for different boron species and p H values.The main boron species are B（OH）₃,B₃O₃（OH）₄^-,B₃O₃（OH）₅^2-and B₄O₅（OH）₄^2-in the ternary system NaBO₂-Na₂B₄O₇-H₂O,the other boron species can be neglected.In the ternary system Na₂B₄O₇-Na₂B₅O₈-H₂O,the main boron species are B（OH）₃,B₃O₃（OH）₄^-,B₄O₅（OH）₄^2-and B₅O₆（OH）₄^-.By comparing distribution of boron species at 298.15 K and 323.15 K,it was found that the changing trends for boron species in the systems are similar at the two temperatures.The thermodynamic properties of ternary systems NaB（OH）₄-Na₂B₄O₅（OH）₄-H₂O and Na₂B₄O₅（OH）₄-NaB₅O₆（OH）₄-H₂O 323.15 K were studied by isopiestic method.The water activity was calculated by using the isopiestic equilibrium concentration.Two models were applied to fit the parameters.The first model（model I）assumes that there are only two boron ions B（OH）₄^-（B₅O₆（OH）₄^-）和B₄O₅（OH）₄^2-in the solution.The second model（model II）assumes that there are four forms for boron corresponding to B（OH）₄^-,B₄O₅（OH）₄^2-,B（OH）₃ and B₃O₃（OH）₄^-in the solution.The fitted parameters with two models were then used for water activity calculation.The calculated data with two models agree with the experimental values.The calculated data with model II are more reliable than those with model I.The results show that model II for four boron species are more reliable in thermodynamic property calculation in the systems containing sodium borate.