Abstract
Mg-Al spinel is a superior refractory. Its properties such as slag-resistance, thermal shock-resistance, and elevated-temperature strength are excellent. However, the high melting point (Tm = 2105 oC) has made its firing temperature higher and has cost more in preparation. The aims of this study are thus focussing on lowering the firing temperature, and on promoting the firing yield of spinel by addition of Na3AlF6 (Tm = 1010 oC). Furthermore, a "flame-quench" method for preparing slag-resistance Al2O3-rich spinel is also stressed. The compositions used are 72 Al2O3-28 MgO (wt %), 90 Al2O3-10 MgO (wt %), and 95 Al2O3-5 MgO (wt %) with 1 to 4 wt % Na3AlF6 as additives. Experimental results show a 98 % spinel yield of theoretical values for all these three compositions with 1 wt % Na3AlF6 at 1300 oC for 2 hr under furnace cooling. At temperatures less than 1600 oC, "stoichiometric" composition in spinel is available with Al2O3 content of 72-76 wt % under furnace cooling. Al2O3-rich (81-88 wt %) spinel is obtained by the "flame-quench" method. Furnace cooling gives the 95 Al2O3-5 MgO a spinel with 75 wt % Al2O3, which is quite similar to the level of 79 wt % Al2O3 of commercial Ube product. Microstructural observations show a continuous spinel matrix with a discontinuous phase embedded in for both furnace cooling and quench specimens. The higher firing temperature, the larger and the more amount of precipitated Al2O3 with facets in "square" shape. Higher firing temperature gives higher degree of liquid phase sintering, that makes the diffusion-controlled Jander equation inappropriate to apply for activation energy calculation. However, lower firing temperature experiments clarify that the Jander equation is suitable for activation energy analysis. The activation energy for 72 Al2O3-28 MgO with 1 wt % Na3AlF6 and without Na3AlF6 is 92.3 kJ/mole and 159.5 kJ/mole, respectively. According to experiments, the firing yield of spinel decreases with higher amount of Al2O3 (e.g., 90 wt % and 95 wt %) and Na3AlF6 addition (e.g., 2 wt % and 4 wt %). After firing, fluorine disappears in the specimens.