| The current utilizing method of ascharite resource is producing borax by carbon dioxide-soda process. However, the high content of Mg in ascharite minerals leads to a low recovery rate of borax, which is less than75%. During the process, a great deal of alkaline boron mud is produced, which leads to waste of resources and pollution of environment. Therefore, it is necessary to find a highly efficient and clean utilization method of boron resource. In this study, we extracted magnesium from ascharite by aluminothermic reduction method under vacuum condition. And the residual materials enriching boron can be used to product alkali-free glass fibre. This process completely utilize the resources without any waste residue discharge and is a promising way of ascharite minerals utilization.Ascharite which contains higher ingredient of magnesium and boron was used as the materials to extract magnesium by vacuum aluminothermic reduction method. The present works were divided into three steps, including calcining ascharite mineral, calcining the mixture of ascharite mineral and CaCO3, and aluminothermic reduction process. The main conclusions can be drawn as follows.In the process of calcining ascharite mineral, the results showed that burning loss ratio was improved with increasing temperature and thermostatic time, and the highest burning loss ratio is13.07%. According to the thermal analysis, the decomposition of ascharite was divided into three stages. The first phase is from452℃to668℃and the activation energy is379.6838KJ/mol and pre-exponential factor A is6.361×1021min-1in the process of decomposion; the second phase the temperature is between668～800℃and the activation energy Ea isl282.42KJ/mol and pre-exponential factor is5.47×1065min-1in the process of decomposion; And in the last phase(800～1200℃), the activation energy Ea is1097.69KJ/mol and pre-exponential factor is4.963×1065min-1in the process of decomposion. The main phases produced during the process of calcining ascharite are2MgO·B203and3MgO·B203, and free MgO is very little.In the process of calcining the mixture of ascharite mineral and CaCO3, it is showed that CaO can replace MgO in3MgO·B2O3, the principal phases obtained from calcinations are3CaO·B2O3and MgO, which is conducive to improve the reduction rate of MgO. And the burning loss ratio increases significantly with the increasing calcining time at lower temperature. The burning loss ratio is changed slightly at higher temperature, but larger amounts of free MgO can be produced. The burning loss ratio also can be improved by increasing modestly the pressure of briquetting. Too high pressure in briquetting leads to lower porosity and bigger resistance of gas expellant, as results of incomplete decomposition, reducing both burning loss ratio and reduction rate. The results indicated that the optimal briquetting pressure was90MPa and the pellets were roasted at1100℃for120min.In the part of vacuum aluminothermic reduction, the results showed that the optimum reduction conditions were the reduction temperature1200℃, reduction time120min, excess aluminum powder of15%and CaF2of3%. In this condition, the reduction rate of MgO was87.86%. The principal constituents in the reducing slag were3CaO·B2O3and11CaO-CaF2-7Al2O3. The apparent energies of activation Ea is218.09KJ/mol, and pre-exponential factor is9.18×105min-1in the process of reduction in temperature of1373K～1473K.The results showed that vacuum aluminothermic reduction method can extract magnesium from calcined production of mixture of CaCO3and ascharite mineral which contain higher Mg and B contents and lower Si content. The rich-boron residual materials can be used to product alkali-free glass fibre. This study provides the theoretical basis and technologic support for alkali-free glass fibre industrial. |
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