Studies On The Thermodynamics Of The Systems Containing Boron And Lithium Li₂B₄O₇-H₂O, LiCl-Li₂B₄O₇-H₂O By Calorimetry

In the west of China, there are hundreds of salt lakes which contain abundant mineral resources, such as boron, lithium, magnesium and many others. The studies on the thermodynamic properties of the brine systems containing borate, lithium and magnesium ect. and their subsystems are of interest in predicting the thermodynamic behavior of these salt lakes and utilizing the salt-lake-resource comprehensively and reasonably. Calorimetry is widely used to investigate the thermodynamic properties of substances. The research work of this thesis focuses on the studies of thermodynamic properties of the systems LiB₄O₇-H₂O and LiCl- Li₂B₄O₇-H₂O with calorimetry.1) The commercial Li₂B₄O₇ (A.R. grade) was performed using a circulating distillatory. X-ray of the purified product shows that the product is hydrate lithium tetraborate. TG and DTG curves of crystallization product show that the mole ratio of Li₂B₄O₇: H₂O is 1:2.89. The purity of the purified product has been improved by 0.5%-1% higher than Li₂B₄O₇ of A.R. grade. ICP analysis of the purified product shows that the contents of Fe, Ca have come down by one-tenth.2) The enthalpies of dilution have been measured for aqueous Li₂B₄O₇ solutions from 0.0212 mol · kg^-1 to 2.1530 mol · kg^-1 at 298.15K by an RD496-â…¢ microcalorimeter, which have extended into the lower concentration and the supersaturation. The relative apparent molar enthalpies, L_φ, of these solutions have been determined with the aid of the Debye-Huckel limiting law. The relationship between L_φ and m^1/2 was obtained. The relative partial molarenthalpies of the solvent and solute, L\ and L2, were calculated from Z,*. A modified Pitzer ion-interaction model was used to represent the thermodynamic properties of the complex aqueous solutions containing borate and a good fit of the experimental results was obtained with the standard deviation 0.0157.3) The enthalpies of dilution have been measured for LiCl-Li2B4O7 -H2O system in a wide ion-strength range at 298.15K by the RD496- III microcalorimeter. All the 39 sets of data are at different U2B4O7 mole fraction of 0.7, 0.4, 0.2, and distributes uniformly under the saturated molality line. The relative apparent molar enthalpies, L*, of these solutions have been determined with the aid of the Debye-Htlckel limiting law. The relationship between ï¿¡?, and / were obtained. We analyzed the effect of different LJ2B4O7 mole fraction on the Z,*, and fund that under the same condition, the values of L* increased greatly with the adding of the Li2B4O7 mole fraction.4) We reported a detailed investigation of the heat capacities of aqueous Li2B4O7 from 0.0183 mol ? kg' to 1.3016 mol ? kg1 at 298.15K using the RD496-III microcalorimeter. Apparent molar heat capacities were calculated and an empirical equation was derived for the concentration dependence of them. The relationship between Cp>* and m was obtained. For the complex aqueous Li2B4O7 solutions, a large standard deviation raised when we employed the original ion-interaction (Pitzer) equations to fit the experimental data in this work. So we added aparameter of /??ï¿¡ into the equations for the relative apparent heat capacity, and agood fit of the experimental results was obtained with the standard deviation 0.0179.5) Adiabatic calorimetry is frequently applied to measure the heat capacities of substances and thus obtain important information in many aspects such as structures and phase transitions. The molar heat capacities of aqueous Li2B4O7 at the concentration range from 0.0187 mol ? kg"1 to 0.3492 mol ? kg"1 have beenmeasured using an adiabatic calorimetry over the range 80 < (T/K)< 355, and the results are used to derive thermodynamic functions for aqueous U2B4O7 at the smoothed temperatures. The phase transition of the solution is determined based on the curve of the heat capacity with temperature. The temperature of phase transition, the enthalpy and entropy of the phase transition are determined. According to the polynomial equations and thermodynamic relationship, the values of thermodynamic function of the aqueous L12B4O7 are calculated in the temperature range from 80 to 355 K with an interval of 5 K.The research work in this thesis enriched the thermodynamic data of these systems which increased the understanding about the thermodynamic properties for the studied systems and proposed the modified ion-interaction models for the enthalpy and heat capacity. The investigation would be helpful to the design of industrial devices and the control of chemical processes concerned with these systems.