Studies On Thermodynamic Properties Of The System Containing Lithium And Boron-Li₂SO₄-Li₂B₄O₇-H₂O With Isopiestic Method At Different Temperatures

The isopiestic molalities and water activities have been determined for the systems Li₂SO₄-H₂O and Li₂B₄O₇-H₂O at 273.15K in the ion strength ranges of 0.6063 1.7640 mol · kg^-1,and 0.1069—0.6478 mol· kg^-1 respectively and for the systems Li₂SO₄-H₂O, Li₂B₄O₇-H₂O and Li₂SO₄-Li₂B₄O₇-H₂O at 298.15K in the ion strength ranges of 0.3577—5.6378 mol·kg^-1, 0.1747—2.4497 mol·kg^-1, 0.2394—5.5399 mol · kg^-1 respectively by the isopiestic method using an improved equipment. Using NaCl(aq) as reference solutions at different temperatures. The osmotic coefficients and saturated vapor pressures have been calculated at the different temperatures .The experimental values of the osmotic coefficients for the Li₂SO₄-H₂O system were consistent with the values in the references. The results revealed the rules of the changing in osmotic coefficients and saturated vapor pressures with ion strengths and the molality fractions of Li₂B₄O₇ at different temperatures. The isopiestic equilibrium molalities were correlated by using the modified and the extended Zdanovskii rules for non-ideal mixtures, and obey well the Zdanovskii rules. The ion interaction parameters of Pitzer model have been determined by using the two methods for Li₂SO₄-Li₂B₄O₇-H₂O at 298.15K system.The parameters of Pitzer model were evaluated from the measured data in this work and the data in the references based on the stoichiometric concentrations of Li₂B₄O₇ in the solutions according to the original or modified Pitzer model by using multiple regression analysis. The standard deviations between the calculated and the experimental osmotic coefficients were reported, which indicated that the Pitzer ion -interaction model can describe the thermodynamic properties for the systems under the experimental conditions. The mean activity coefficients of Li₂SO₄ and Li₂B₄O₇ for Li₂SO₄- Li₂B₄O₇-H₂O system at298.15K and for Li2SO4-H2O and U2B4O7-H2O at 273.15K respectively were calculated using Pitzer model and the results reveal the rules of the mean activity coefficients of U2SO4 and U2B4O7 in the mixing solutions changing with the ion strengths and the molality fractions. This work has offered the lacking thermodynamic data for Li2SO4-H2O and Li2B4O7-H2O at 273.15K and for Li2SO4- U2B4O7-H2O system at 298.15K. This work is useful to improve the thermodynamic model for salt lake brine systems containing Lithium and borates and to make utilization of the resources in salt lake.