Study On Preparation Of High-purity Magnesia Using Bischofite

There are extremely abundant of salt lake resources existing in the western regions of China, such as Xinjiang, Qinghai and Tibet. In recent years, potassium has been large-scale extracted from the salt lake and a large amount of brine, which is rich in magnesium resources (mainly including magnesium chloride and magnesium sulfate, etc.), has been left as the by-product or even waste. For example, There are5.86billion tons of MgCl2in Lop Nur Salt in Xinjiang and16.5billion tons in the Salt Lake of Chaerhan in Qinghai province. However, at present, these magnesium resources have not been effectively exploited, and they are considered as general waste, resulting in serious environmental pollution. High-purity magnesia, as high grade magnesium compounds, is value-added and its demand is large in market. Therefore, the preparation of high-purity magnesia using magnesium resources from salt lake as raw material is the necessary way to the exploitation of salt lake resources. It not only can relieve the domestic demand of magnesia but also has great significance for sustainable development and comprehensive utilization of salt lake resources in China.Using bischofite as raw materials, the preparation of high-purity magnesia was studied through soda ash method in this thesis. Homogeneous precipitant of precursor with various composition and morphologies can be obtained by the reaction of MgCl2and NaCO3, and then sintered magnesia can be prepared by calcining, ball-milling, tabletting and sintering successively. The effects of different precursors and calcining process on the properties of sintered magnesia were investigated. The main research contents include:(1)From the perspective of the composition and morphology of the precursor, the effect of different compositions and morphologies of the precursor on the properties of sintered magnesia were surveyed;(2)Two kinds of magnesium carbonate hydrates (nesquehonite and hydromagnesite) were prepared using homogeneous precipitation process at different reaction conditions. From the viewpoint of energy, the thermal decomposition behavior and decomposition kinetics of them were researched and compared, and then a more appropriate precursor was determined for the preparation of high-purity magnesia;(3)The weight loss and composition changes in the calcining process of the precursor were compared with the analysis results from TG data;(4)The influence of calcining process on the properties of sintered magnesia was also studied.The main results are as follows:(1) The precursors with various composition and morphologies have been synthesized by adjusting the reaction temperature and pH values of the initial reaction solution in the precipitant process. The performance of sintered magnesia from the above different precursors were almost the same:The purity(over99%) of them all achieved the target. The bulk density, apparent porosity, closed porosity and shrinkage ratio were basically the same. The results showed that the performance of sintered magnesia was independent of the composition and morphology of the corresponding precursor.(2) The thermal decomposition processes of nesquehonite (MgCO3·3H2O) and hydromagnesite (4MgCO3·Mg(OH)2·4H2O) were investigated and decomposition kinetics at each stage were studied by TG data. The kinetic parameters of each stage were calculated by means of FWO and KAS equation. The activation energy of nesquehonite are7.58kJ/mol,16.39kJ/mol,37.51kJ/mol and128.74kJ/mol corresponding to the four stages, And hydromagnesite are18.27kJ/mol,22.41kJ/mol and70.79kJ/mol. The combined use of master-plots method proposed by Malek and the integral method made it possible to determine the mechanism of each stage. The mechanism of nesquehonite are R2, F1.6, F2.9and A3, respectively. And hydromagnesite are D1, C1.5and A1. And then the corresponding pre-exponential factors can be obtained. The pre-exponential factors of nesquehonite are3.36×103s-1,4.51×105s-1,2.27×109s-1and6.03×1015s-1, while hydromagnesite are9.78×103s-1,9.45×104s-1and2.59×108s-1. In addition, combined with DSC curves of them, the heat absorption of corresponding stage were compared. The differences of the two kinds of substances were analyzed as the precursor of MgO from the viewpoint of thermaldynamics and kinetics, and the results showed that the heat absorption of nesquehonite was lower than hydromagnesite and its reaction rate constant was larger, thus, nesquehonite was a more appropriate precursor for the preparation of high-purity magnesia.(3) By comparing weight loss and FTIR of the precursors in the calcining process with the calculation results from TG data, it can be found that the FTIR analysis results were consistent with the calculation results from TG data. This will further confirm the composition changes of nesquehonite and hydromagnesite in the thermal decomposition process.(4) The calcining temperature and time had a great influence on the performance of sintered magnesia. Higher temperature and longer time were conductive to sinterability of MgO. The optimum calcining process parameters for nesquehonite were500℃,2h.(5) The agglomeration states of a powder was very important factor that affects the densification kinetics. With the mean size of agglomerates increasing, the property of sintered magnesia got weak, and the bulk density and shrinkage ratio decreased.