Preparation Of High Purity Magnesia From Waste Bischofite By Pyrolysis

Carnallite (KCl·MgCl2·6H2O) resources were abundant in Qinghai Qarhan salt lake, usually which were used as raw materials for the production of potash fertilizer. After potassium chloride was extracted from carnallite, a large amount of bischofite was deposited in Qinghai Qarhan salt lake, which would result in the pollution to environment. In this paper, a high-efficiency process was developed for the preparation of high-purity magnesia from the waste bischofite in Qinghai Qarhan salt lake. Furthermore, the mechanism and kinetics of thermal decomposition of MgCl2·6H2O, preparation process of magnesia from bischofite, and sintering and grain growth kinetics of MgO were investigated in details.Firstly, the mechanism and kinetics of thermal decomposition of MgCl2·6H2O in air were studied. The kinds and morphologies of intermediate products obtained during this thermal decomposition process were investigated using integrated thermal analysis, X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectrum and chemical analysis. It was found that there existed six stages in the thermal decomposition process of MgCl2·6H2O. In the first two stages, four crystallized waters were lost with intermediate products MgCl2·4H2O at342K, MgCl2·2H2O at402K, respectively. The dehydration and hydrolysis coexisted during the third and fourth stages with intermediate products MgCl2·nH2O(1≤n≤2) and MgOHCl at440K, and Mg(OH)C1·0.3H2O and MgOHCl at476K, respectively. In the fifth stage Mg(OH)C1·0.3H2O was dehydrated to produce MgOHCl at508K, and in the last stage MgOHCl was converted into MgO at688K.To restrain the sample hydrolysis, the thermal decomposition of MgCl2·6H2O was carried out under HCl atmosphere until476K when MgCl2H2O was obtained. Then HCl gas was turned off and the decomposing process continued with products Mg3Cl2(OH)4·2H2O at476K, Mg3(OH)4Cl2at493K and MgO at633K. The decomposing temperatures to produce MgO were different between that done under air atmosphere (688K) and that done under HCl atmosphere (633K), because of the crystal structure difference between MgCl2·2H2O and MgCl2·H2O. In microwave field, MgCl2·6H2O was firstly dehydrated to produce MgCl2·2H2O, and then converted into Mg3Cl2(OH)4·2H2O and MgOHCl, last into MgO. Microwave irradiation had high reaction rate, high heating efficiency, but the process was hard to control.Morphology analysis showed that, in air, MgOHCl particle was irregular and had porous structure, Mg(OH)Cl·0.3H2O particle had relatively flat surface, and MgO particle was cylindrical. In HCl atmosphere, Mg3Cl2(OH)4·2H2O particle had irregular shape and tiny needle-like structure, Mg3(OH)4Cl2particle had porous structure and uneven surface, MgO particle had a flake structure. In microwave field, Mg3Cl2(OH)4·2H2O particle had a little tiny needle-like structure, and MgO particle was irregular, without specific structure such as cylindrical or flake. Therefore, the thermal decomposition process of MgCl2·6H2O was very complicated and many kinds of intermediate products would occur with different morphologies, the preparation conditions should be optimized for the production of high-purity magnesia in the further work.Based on the above theoretical research for the thermal decomposition process of MgCl2·6H2O, the preparation process of magnesia from waste bischofite was proposed, that is "calcining-milling-forming-sintering". This proposed process had advantages of lower calcination temperature, high-efficiency and energy-saving. The effects of calcining temperature, milling time, forming pressure and sintering time on the performance of magnesia were studied. The effects of impurities of raw materials and additives on performance of magnesia product were researched. When the contents of NaCl, K2SO4, MgSO4and B impurities were less than0.2wt%,0.15wt%,0.15wt%and0.014wt%in light-burned MgO from raw materials, respectively, there were no adverse effects on bulk density of magnesia. The results showed that TiO2was the best additive for magnesia production, and the optimum amounts were added in the light-burned MgO from raw materials as1wt%in brine,1.5wt%in crystalline bischofite, and0.2wt%in MgCl2·6H2O(AR), respectively. The obtained magnesia purity exceed98wt%, and the bulk density reached3.49g·cm-3Furthermore, the sintering and grain growth kinetics of MgO were studied. Results suggested that the initial stage of sintering was volume diffusion-controlled. The addition of TiO2decreased the activation energy of MgO grain growth, accelerated the growth rate of MgO grain, and markedly promoted the sintering of MgO. Without TiO2addition, MgO grain growth exponent n was3, grain growth activation energy Q was556.9kJ·mol-1, and the process was considered as volume diffusion-controlled. With0.2wt%TiO2addition, MgO grain growth exponent n was2, grain growth activation energy Q was272.8kJ·mol-1, and the process was considered as interface diffusion-controlled. The main mechanism of TiO2promoting the sintering of MgO was that TiO2solubilized in MgO formed unequivalence substitutional solid solutions and cation vacancies which were favorable to cation diffusion.