Alumina fused cast refractory aging monitored by nickel crystal chemistry
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Michel Maquet Saint Gobain Recherche, 93304 Aubervilliers Cedex, France (Received 18 March 1991; accepted 28 May 1991)
Aged bricks of AZS and mixed a-/3-arumina refractories have been sampled in superstructures of glass making furnaces, a- and /3-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, /3-alumina is the only phase present. The primary corundum grains are transformed into secondary /3-alumina under the influence of contaminants from raw materials and oil ashes. The temperature conditions existing in the furnace preclude the formation of j3" alumina. The bright blue color of /3-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 /3-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 /3-alumina, which maintains the refractory performance during furnace operation. The spectroscopic evidence of trace amounts of nickel ( tt-Al2O3 transformation must have been completed within the early months of the furnace operation.
The presence of nickel in /3-alumina is essential in order to understand the kinetics of the transformations observed. It has first to be pointed out that no spectroscopic evidence exists for the location of this element in the conduction plane of /3-alumina. However, the ionic conductivity of Ni in j3- and (3"-aluminas is similar to that observed for the cations usually encountered in this conduction plane (large divalent cations, alkalis). A twostep process may thus be envisaged: (i) the fast diffusion
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V. CONCLUSION Aged high alumina and AZS refractory bricks have been investigated with spectroscopic methods, diffuse reflectance spectroscopy, and electron paramagnetic resonance. At the interface with the furnace atmosphere, they present a continuous layer of Ni-bearing /3-alumina, the formation of which is explained by a high contaminant activity in the furnace atmosphere. The presence of small divalent cations inside the spinel blocks of the /3-alumina structure, illustrated by the bright blue color arising from tetrahedral nickel, explains the stability of this phase at high temperature. This ensures good performance of the refractory bricks during furnace operation, because the presence of /3-alumina hinders the formation of refractory-induced defects in the glasses. High-alumina refractories present, at depth, a corundumrich layer, resulting from a transformation of /3-alumina by sodium loss. The forma
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