Thermodynamic Properties Of Magnesium Borate Aqueous Solution Systems By Calorimetry

Magnesium borates are widely used in many fields,such as enamel,battery manufacturing,medicine,precious metal processing,aerospace,and other life and industrial areas.There are abundant liquid boron mineral resources in the salt lake brine of Qinghai-Tibet Plateau in China.However,the complex and diverse structures of boron and the problems of supersaturation in aqueous solutions hinder the large-scale industrial production of magnesium borate.This work aims to demonstrate the thermodynamic properties of the aqueous solution systems of magnesium borates.According to the Pitzer ion-interaction model,the Pitzer single-salt and mixing parameters were obtained,which supplements the thermodynamic data of boron resources.The minerals of hungchaoite（Mg B₄O₇·9H₂O）,mcallisterite(Mg B₆O₁₀·7.5H₂O),inderite(Mg₂B₆O₁₁·15H₂O),and pinnoite（Mg B₂O₄·3H₂O）as the most common magnesium borates existed in nature were selected as the research objects.The thermodynamic properties of four magnesium borates and their aqueous solutions at 308.15 K were studied.The main conclusions are as follows:（1）The dilution heats of the binary systems（Mg B₄O₅（OH）₄–H₂O）,（Mg B₆O₇（OH）₆–H₂O）,（Mg B₃O₃（OH）₅–H₂O）and（Mg B₂O（OH）₆–H₂O）at 308.15 K have been investigated by the microcalorimetry.On the basis of Debye-Hückel’s limiting law,the relative apparent molar enthalpies of four magnesium boric acid saline solutions were obtained.According to the Pitzer ion-interaction model,the Pitzer single-salt parametersβ_MX^（0）L,β_MX^（1）L,β_MX^（2）L,C_MX^L,of the four binary systems were fitted by the least square method.Those results indicated that the single salt parameters in this work are reliable.（2）The mixing heats of the four ternary systems（Mg B₄O₅（OH）₄–Mg Cl₂–H₂O）,（Mg B₆O₇（OH）₆–Mg Cl₂–H₂O）,（Mg B₃O₃（OH）₅–Mg Cl₂–H₂O）and（Mg B₂O（OH）₆–Mg Cl₂–H₂O）at 308.15 K have been determined by the microcalorimetry.The mixing heat changes regularly with the increase of ionic strength.The correlations between ionic strength and temperature of the four ternary systems are obtained,and the correlation coefficients are higher than 0.99.The ion interaction parameters are obtained by the least-squares fitting mixed ion interaction parameters of the ternary systems.The acquisition of single salt parameters and mixed parameters of magnesium borates solution is of great significance to establishing the thermodynamic model of salt lake brine containing magnesium borate and promoting the comprehensive utilization of salt lake resources.