Study On Hydrothermal Synthesis Process Of Magnesium Borate For Special Glass

Alkaline earth metal borates are important boron compounds which are mainly used in the alkali-free glass fiber and ceramic industries.The synthesis of magnesium borates with particular specification requirements are demanded with the development of special galss and ceramic industries.The requirements of magnesium borates for special galss are as follows:the mass fractions of B₂O₃ and MgO in theses magnesium borates should be within a limited range and without free MgO,the product must be crystalline hydrated magnesium borates with a strict limitation of impurity.This project is to develop a feasible process for manufacturing magnesium borate hydrates to meet the requirements of special glass industries.In this paper,the reaction of boric acid with light burned magnesia in water affords magnesium borates by a hydrothermal method.In order to gain an insight into the formation of magnesium borate crystals,the effects of B₂O₃/MgO molar ratio of raw materials,reaction time,temperature and liquid to solid ratio on the synthetic product are investigated, respectively.XRD and TG-DTG analyses of the products show that they consist of magnesium hexaborate hydrate(MgO·3B₂O₃·7H₂O) and ascharite(2MgO·B₂O₃·H₂O).The results indicate that high B₂O₃/MgO molar ratio of raw materials and low reaction liquid-solid ratio favor the products with a high B₂O₃/MgO molar ratio and vice versa.There exists free MgO in the product when the temperature is below 140℃or the reaction time is not enough because the reaction of magnesium oxide with boric acid is not completed.In addition,the formation of the mixed crystals of magnesium hexaborate hydrate and ascharite was also discussed by means of chemical analysis,XRD and TG-DTG characterizations.Ascharite is mainly formed at high temperature while magnesium hexaborate hydrate was precipitated at low temperature.According to the above mentioned investigation,the circulation experiments of manufacturing magnesium borate were carried out in order to probe into industry operation. The experiment results on the synthesis of magnesium borate in the returned mother liquor and wash liquid demonstrate that the contents of B₂O₃ and MgO in the obtained mother liquor and wash liquid can be maintained constant to a certain extent.The results of circulation experiments show that magnesium borates with B₂O₃ 50-55%and MgO 25-30%can be obtained at 150℃under the conditions of B₂O₃/MgO molar ratio of raw materials 1.82, liquid to solid ratio 12-14ml/g and reaction time 10 h.In order to investigate the crystallization behavior of different magnesium borates,the fraction crystallization and its circulation experiment were performed,the results indicate that circulation operations were stable and more complicated.The products of fraction crystallization can be divided into two types,one mainly consists of ascharite with high magnesium content and the other contains a lot of magnesium hexaborate hydrate with high boron content,whose B₂O₃/MgO molar ratio range from 0.7 to 3.0.Therefore,the B₂O₃/MgO molar ratio of magnesium borate can be controlled by changing the mixing proportion of two kinds of magnesium borate products to meet the requirements for different industries.