REACTION CHEMISTRY IN DISSOLUTION OF POLYCRYSTALLINE ALUMINA IN LIME-ALUMINA-SILICA SLAG

The reaction chemistry involved in the dissolution of polycrystalline alumina in a lime-alumina-silica slag was studied at temperatures betw een 1400 and 1550 degrees C, and the phases formed at different stages of the dissolution process were characterised by X-ray powder diffrac tion and scanning electron microscopy. At the initial stages of the di ssolution process, interdiffusion and reaction between the various com ponents present in the slag and alumina allow the formation of a gehle nite rich boundary layer adjacent to the Al2O3 surface, and a reaction zone consisting of a coherent layer of calcium dialuminate, CaO . 2Al (2)O(3), at the melt/alumina interface. With increasing temperature an d time, CaO . 2Al(2)O(3) reacts with Al2O3 to form calcium hexaluminat e, CaO . 6Al(2)O(3), at the interface. Because of large differences in the molar volume between CaO . 6Al(2)O(3) and Al2O3, the interface di sintegrates ates and allows molten slag to penetrate into the bulk of the Al2O3 specimen. The liquid becomes saturated with Al2O3 at the gra in boundaries and leads to the crystallisation of anorthite, CnO.Al2O3 .2SiO(2), from the melt. Since anorthite is compatible with CaO . 6Al( 2)O(3) and Al2O3, no further reaction occurs and the dissolution proce ss stops in the bulk of the alumina specimen. (C) 1997 The Institute o f Materials.