Preparation of MgO-SnO 2 -TiO 2 Materials and Their Corrosion in Na 3 AlF 6 -AlF 3 -K 3 AlF 6 Bath
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SI3N4 bonded SiC materials have been widely used as sidewall materials for aluminum reduction cell owing to their excellent properties, such as high hardness, good thermal shock resistance, and high thermal conductivity.[1–3] Rapid heat dissipation through the sidewalls because of their high thermal conductivity leads to the formation of a frozen ledge of electrolyte, which can protect the sidewall materials from further attack by the corrosive electrolyte and hence greatly extended the service life of the sidewalls. However, heat dissipation through the sidewalls has to be reduced in order to meet the energy-saving requirements currently faced by aluminum industries.[4,5] This can be achieved by applying a high insulation layer on the outside of the sidewalls; the heat in the cell could then be conserved instead of wasted, leading to potentially 30 to 40 pct energy savings. In this case, however, the frozen ledge of
YIBIAO XU, Ph.D. Student, YAWEI LI and JIANHONG YANG, Professors, SHAOBAI SANG and QINGWEI QIN, Associate Professors, and BO REN, Postgraduate Student, are with the The Key State Laboratory Breeding Base of Ceramics and Refractories, Wuhan University of Science & Technology, Wuhan 430081, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 22, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
electrolyte cannot form on the inside of the sidewall, directly exposing it to the oxidizing gas, corrosive electrolyte, and molten aluminum from the top to bottom and causing a significant reduction in the lifetime of the sidewall if Si3N4 bonded SiC materials are still used.[6] Therefore, new types of sidewall materials are required for withstanding such severe environments within aluminum reduction cells. Several research works have been carried out to develop new kinds of sidewall materials. Mukhlis et al.[6] suggested that a multi-layer strategy can be applied horizontally, using three different materials; each material can withstand the environment of the corresponding zone mentioned above. They suggested that nickel ferrite might be suitable for use in the gas and bath zones due to its high stability in air and electrolyte. Yan et al.[7] studied the dynamic corrosion of nickel ferrite samples in molten bath at a specimen rotation speed of 25 rpm. Specimen with higher porosity showed more severe bath infiltration. It was proposed that the mechanism of corrosion involved grain boundary attack, and therefore specimen with larger grain size should have better corrosion resistance in the bath. This was confirmed by the results of Downie,[8] which showed that nickel ferrite specimen with a larger grain size had better performance than that of the specimen with smaller grains in cryolite-based baths under static conditions. Recently, Nightingale et al.[9] tested the
Table I.
Batch Compositions of the Specimens Compositions (wt pct)
Raw Materials Fused MgO TiO2 SnO2
MTS2
MTS5
MTS10
MTS15
MTS20
93 5 2
90 5 5
85 5 10
80 5 15
75 5 20
corrosion resistance of nickel ferrite in cr
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