Thermodynamic Properties Of Magnesium Borates And Their Aqueous Solution Systems

Magnesium borates for their superior performances are widely used in chemical industry,alkali-free glass fiber,ceramic,agriculture and other fields.As China is one of the rich-boron resources in the world,the solid boron mineral resources are mainly existed in the northeast whereas the liquid ore resource in the salt lake brines is distributed in the western.It is famous for the rich boron-magnesiurn resources in the Qinghai-Tibet Plateau.There are 14 kinds of natural borate minerals so far and including the nomal five kinds of magnesium borate hydrates:pinnoite?MgB2O4·3H₂O?,hungchaoite?MgB4O7·9H₂O?,mcallisterite?MgB6O10·7.5H₂O?,inderite?Mg2B6011·15H₂O?and kurnakovite?Mg2B6O11·15H₂O?.However,the thermodynamic properties on those nomal magnesium borates are scarce.Therefore,studies on the synthesis and thermodynamic properties of different forms of magnesium borates are essential to provide important basic thermodynamic data for the development and utilization of the salt lake brine resources in the western of China.In this thesis,four kinds of high purity magnesium borate hydrates:pinnoite,inderite,mcallisterite and hungchaoite were synthesized by controlling the optimum conditions,and the high purity products were characterized by X-ray powder diffraction,FT-IR,TG-DSC and chemical analysis.The molar heat capacities of four solid magnesium borates were determined using differential scanning calorimetry?DSC?at the tempreture ranged from 305.15 to 365.15 K,and the molar heat capacities were calculated by least squares method.Polynomial equations of the molar heat capacities versus the temperatures were also fitted.Densities of aqueous systems MgB2O?OH?₆-H₂O,MgB₃O₃?OH?5-H₂O,MgB₄O₅?OH?4-H₂O and MgB₆O₇?OH?₆-H₂O at different concentrations in the tempreture range from 283.15 to 363.15 K were measured by a high-precision densitometer,and the apparent molar volumes were calculated based on experimental densities.The Vogel-Tamman-Fulcher model was used to correlate the densities variation with temperatures and concentrations successfully by least square method.Moreover,the Pitzer single salt parameters of four magnesium borates were obtained for the first time by using the Pizter ion-interaction theory of electrolytes and the series experimental results.The phase transformation behaviors of MgB6O10-H₂O at 288.15 K,298.15 K and 323.15 K were demonstrated with the method of isothermal dissolution equilibrium combined with the concentration of Mg2 +and B₂O₃ in the solution by continuous sampling and analyzing and XRD solid phase identification.