Synthsis And Thermochemical Properties In The Aqueous System Of Magnesium Borates

Magnesium borates as a kind of important chemical materials are widely used in the fields of alkali-free glass fiber,ceramic,fire retardant,nonlinear optical material,and agriculture and so on.Boron resources are mainly existed either in the form of ascharite,paigeite located in the northeast of China or in the form of liquid ores such as salt lake brines in the west of China.However,there is no magnesium borate resources which can be exploited immediately,so the needs of magnesium borates are mainly depended on importing from other countries.A large number of salt lakes distributed in the west of China with high concentrations of boron and magnesium provides abundant resources for the development and utilization of magnesium borates.Therefore,studies on the synthesis and thermochemical properties of different magnesium borates are essential to promote the comprehensive utilization of the salt lake brine resources.In this paper,three hydrated magnesium borates with high purity:mcallisterite(MgB₆O₁₀·7.5H₂O),hungchaoite?MgB₄O₇·9H₂O?and inderite(Mg₂B₆₀O₁₁·15H₂O)were synthesized by controlling the optimum conditions such as different reaction temperatures,reaction times and style of chemicals with the raw materials of borax,boron acid,magnesium sulfate heptahydrate,basic magnesium carbonate and double deioned water.And then,the re-crystallization high purity products were characterized by chemical analysis,X-ray powder diffraction,FT-IR and TG-DSC.In addition,the possible synthetic mechanism of the reactions in the aqueous were presented.The synthesized product of pinnoite?MgO·B₂O₃·3H₂O?needs further study because of magnesium hydroxide precipitate exiting.Heat capacity is one of the essential properties for a substance,which can reflect the relationship and their interactions between solute and solvent.In this thesis,heat capacities of aqueous systems of MgB₃O₃?OH?₅-H₂O,MgB₄O₅?OH?₄-H₂O and MgB₆O₇?OH?₆-H₂O at 298.15 K were determined by Micro-calorimeter with the type of Setaram BT 2.15 for the purpose of calculating apparent molar heat capacity.The relational expressions of heat capacities and apparent molar heat capacities versus the molality of magnesium borates were fitted.Moreover,the Pitzer single salt parameters of mcallisterite(MgB₆O₁₀·7.5H₂O),hungchaoite?MgB₄O₇·9H₂O?,inderite(Mg₂B₆O₁₁·15H₂O)at 298.15 K were fitted on the basis of the Pizter ion-interaction theory of electrolytes and the series experimental results of the heat capacity data for aqueous systems of MgB₃O₃?OH?₅-H₂O,MgB₄O₅?OH?₄-H₂O,and MgB₆O₇?OH?₆-H₂O at 298.15 K.