Molten Salt Synthesis Of Magnesium Aluminate Spinel

MgAl₂O₄ powders had been prepared by molten salt method (MSS) using these two groups of industrial raw materials: :(1). Bauxite and light burning magnesia. (2). Industrial alumina and light burning magnesia.MgAl₂O₄ powders had been prepared at low temperature using the first group materials. The influence of reaction temperature, kinds of salts, size of raw materials and holding time on the phase composition and crystal growth of the products were investigated. The results showed that MgAl₂O₄ produced by MSS in LiCl salt melt appeared at 900℃, and the content of MgAl₂O₄, increased as the reaction temperature increased. LiAlSiO₄ remained in the powders synthesized in LiCl salt melt for 2h or 3h at 1150℃while disappeared for 4h. MgAl₂O₄ prepared in LiCl salt melt were octahedral crystal and had single phase, while in NaCl or KCl melt, unfortunately, a small amounts of unreacted Al₂O₃ with undeveloped crystallization and a large number of agglomerations were observed. There were three important factors influenced on the phase composition and crystal growth of spinel:(1)Melting points of salts. The lower the melting point, the earlier the formation of MgAl₂O₄ began. (2). Solubility of raw materials in salts. The higher solubility of in salts, the quicker the reaction between MgO and Al₂O₃ was. (3) Viscosity of salts at high temperature. The higher viscosity of the salt melt, the slower the reaction was. It also found that the particles. size of raw materials had an important influence on the phase composition of final products. LiAlSiO₄ decreased and even disappeared when the size of raw materials decreased. The MgAl₂O₄ crystal size became larger and more uniform with the reaction time increasing. The grain size and morphology of MgAl₂O₄ powders basically retained those of bauxite, indicating that the"template formation"acted a leading role in this process.Pure MgAl₂O₄ powders were synthesized by molten salt method in chloride salts melt at different temperatures using the second group of raw materials, and the results were compared with the conventional mixed oxide synthesis (CMOS). MgAl₂O₄ powders with single phase had been prepared in NaCl or KCl melt at 950℃and in LiCl melt at 800℃. Prolonging reaction time up to 5h, the performing temperature of MgAl₂O₄ decreased to 700℃in LiCl melt. However, there was a small amounts of Al₂O₃ and MgO in the final product using CMOS at 1400℃for 3h, suggested that the synthesis temperature of pure MgAl₂O₄ could be remarkably lower by MSS comparing with CMOS. In NaCl or KCl melt systems, squama-like MgAl₂O₄ crystals formed on the surface of the grains. The MgAl₂O₄ crystals on the large grains seemed much bigger than those on the small grains, while in LiCl melt, small uniform sized MgAl₂O₄ crystals formed on the grains had an elliptical shape with 20nm thick and 0.5-1um long. It seemed like that even smaller nano-sized MgAl₂O₄ crystals grew on surface of the flaky crystals. All these results showed that the size of raw materials and the viscosity of salts melt played a real important part in MgAl₂O₄ formation temperature and the crystal growth, indicating that "template formation"mechanism more or less promoted the process although"dissolution-precipitation"acted a leading role in the MSS process.