Sintering of (Ni,Mg)(Al,Fe) 2 O 4 Materials and their Corrosion Process in Na 3 AlF 6 -AlF 3 -K 3 AlF 6 Electrolyte

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SI3N4-BONDED SiC refractories have been currently widely used as sidewall materials in aluminum reduction cell because of their excellent properties, such as high corrosion resistance to electrolyte, good thermal shock resistance, reasonable thermal expansion, and high thermal conductivity.[1–3] In practice, due to rapid heat dissipation through the sidewalls, a layer of solidified electrolyte forms on the sidewalls. This layer is normally termed side-ledge, which protects the sidewalls from a corrosive electrolyte melt and hence greatly extends their service life.[4,5] For this ledge to form and be stable, a large amount of heat accounting for approximately 35 pct of the total energy consumption has to be transferred.[6] Thus, reducing the heat dissipation through sidewalls could greatly increase energy efficiency and indirectly reduce environmental impact. To conserve the heat in the cell, high insulation layers could be applied outside the sidewalls.[7,8] In this case,

YIBIAO XU, YAWEI LI, JIANHONG YANG, SHAOBAI SANG, and QINGHU WANG are with The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, P. R. China. Contact e-mail: [email protected] Manuscript submitted November 29, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B

however, the sidewalls could be in direct contact with oxidizing gas and corrosive electrolyte since the frozen ledge would no longer be existent. Additionally, at the bath/gas interface, the Si3N4-bonded SiC sidewall materials would be significantly corroded owing to the combination attack of both oxidizing gas and corrosive electrolyte.[1] Therefore, new types of sidewall materials with higher resistance to oxidation and electrolyte are required for gas and bath zones within ledge-free cell. It is well known that using carbon anodes for aluminum electrolysis has many disadvantages, such as the consumption of anode carbon and the emission of carbon dioxide and fluorocarbon.[9] The implementation of inert anode for the production of aluminum has attracted a lot of attention in recent years, because such anode is non-consumable and only O2 generates during electrolysis process, which is apparently environment-friendly.[10,11] NiFe2O4 spinel ceramic has been considered as the most promising inert anode due to its high stability in air and molten electrolyte.[12–14] Since the desirable properties of sidewall materials are quite similar to those of the proposed inert anodes, NiFe2O4 is of interest as a novel sideling material.[15,16] However, the possibility of molten aluminum contamination because of preferential dissolution of Fe during service and the high manufacturing costs due to the scarcity of Ni resources eliminate the possibility of the wide use of the pure NiFe2O4 material as sidewalls.[4,17,18]

Some other spinels, such as FeAl2O4, MgAl2O4, and NiAl2O4, also possess low solubility in the electrolyte although their chemical stability in the electrolyte is lower than that of NiFe2O4.[5,19,20] Also, the incorporation of other