Alumina fused cast refractory aging monitored by nickel crystal chemistry

Aged bricks of AZS and mixed α-β-alumina refractories have been sampled in superstructures of glass making furnaces. α- and β-alumina phases contained in these refractories have been investigated by optical absorption spectroscopy, electron paramagnetic resonance, and electron probe microanalysis. On the side of the brick exposed to the tank atmosphere, β-alumina is the only phase present. The primary corundum grains are transformed into secondary β-alumina under the influence of contaminants from raw materials and oil ashes. The temperature conditions existing in the furnace preclude the formation of β” alumina. The bright blue color of β-alumina originates from the presence of tetrahedral Ni²⁺ in Al(2) sites, with no evidence for nickel atoms located in the ionic conduction band. By considering the chemical composition of β-alumina, spectroscopic results are consistent with a mutual interaction between divalent and monovalent species during cation diffusion. Indeed, the small divalent cations such as Ni are located in the spinel block and the larger alkali cations play a charge compensation role in the conduction band. As other divalent cations of small ionic radius, nickel hence helps to stabilize β-alumina, which maintains the refractory performance during furnace operation. The spectroscopic evidence of trace amounts of nickel (<100 ppm) in secondary corundum crystals means that this phase formed at the expense of β-alumina inside the high-alumina refractory brick. By considering the diffusion coefficients of Ni²⁺ in α- and β-alumina, this indicates a fast contamination of the material at an early stage of the furnace history. The formation of a permanent deep layer of primary and secondary corundum has protected the inner part of the refractory brick from further contamination.