In Situ Monitoring of Pit Gas Composition During Baking of Anodes for Aluminum Electrolysis
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INTRODUCTION
THE most used anode technology in state-of-the-art aluminum electrolysis cells is prebaked carbon anodes.[1,2] Anodes are manufactured in separate anode fabrication plants where the final step is anode baking. The green anodes consist of petroleum coke, anode butts, and coal tar pitch, which are mixed and vibrocompacted to individual anode blocks prior to heat treatment (baking) in the anode baking furnace.[3] Over the course of the lifetime of the furnace, the refractory linings are exposed to both temperature cycling and gaseous reactions, causing material changes in the refractory. Hence, the walls are subjected to stress of a physical, thermal, and chemical nature, reflected in observed degradation of the refractory walls.[3–5] Commonly, the walls are found to bend along the length of the pit walls, which, together with carbon deposition, decreases the pit width and the space in which the
TROND BRANDVIK and TOR GRANDE are with the Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact email: [email protected] HEIKO GAERTNER, ARNE P. RATVIK, and THOR A. AARHAUG are with SINTEF Industry, 7465 Trondheim, Norway. Manuscript submitted October 11, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
anodes are placed during baking. At some point, the pits become too narrow and the walls must be replaced in order to fit the anodes into the pits.[3–5] Investigations of refractory degradation in anode baking furnaces have suggested several possible mechanisms for material degradation.[6–9] The most widely discussed cause of degradation is reactions with sodiumor fluorine-containing compounds. Investigations of spent lining from industrial furnaces have revealed significant levels of sodium in the regions close to the anode pit.[4,5,9,10] The level of amorphous phases is also found to be higher in these regions. Sodium, mostly introduced through frozen bath from the anode butts, forms volatile compounds during heat treatment. Both crystalline and amorphous sodium aluminosilicate phases have been observed in linings due to chemical reactions involving sodium during baking.[4,5,9] However, in later studies, the sodium level in the spent lining is not observed to be equally high, reducing the significance of sodium influence on the refractory stability in some furnaces.[8] In addition to the reactions with volatile sodium fluorides, the effect of reducing atmosphere on refractory stability has been discussed.[9] It is suggested that reducing gaseous compounds (H2, CH4, CO, etc.) could affect the stability of the oxides at elevated temperatures, explaining some of the observations reported.[9] Detailed knowledge on pit gas composition is therefore of high interest and could improve the understanding of the degradation mechanism of the
lining. Investigations of pit gas atmosphere during anode baking have, to the best of the authors’ knowledge, not been reported by others, and the pit gas composition as a function of time and temperature
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