Long-term material balance of iron in aluminum reduction cells
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INTRODUCTION
IRON is introduced into aluminum cells in different ways. The main sources of iron in an electrolyte are the raw materials, e.g., alumina, anode materials and AlF3; the secondary sources are comprised mostly of step tools, e.g., corroded nipples of anode butts, steel rods in the cathode supply, and other technological operations in which steel tools are used. The entire amount of iron is distributed between the aluminum and the electrolyte. Most of the iron escapes from the cells with the anode gases, which are emitted from the reduction cells. These effluents consist principally of carbon and alumina particles, gaseous fluorides from the bath, and other gaseous compounds generated during electrolysis. The effluents are adsorbed on alumina in dry scrubbers, in which alumina is used as a medium for sorbing the effluents. The advantages of using alumina as a sorbent include its well-known ability to remove fluoride and also its ability to be returned directly back into the cell as secondary alumina. Iron adsorbed on secondary alumina returns back into the electrolyte, also.[1–4] The presence of iron affects the electrolysis in different ways. It decreases the current efficiency and the metal purity, and it also has a negative influence on the cell operation.[5,6] In the present work, the complete mass balance of iron in the prebaked anode cells for the period 1998 through 2002 was calculated. II. RESULTS OF STATISTICAL ANALYSIS The analyzed materials (flows) are listed below, with their frequently used notations: FRANTISˇEK SˇIMKO, Scientist, is with the Institute of Inorganic Chemistry SAS, 845 36 Bratislava, Slovakia. Contact e-mail: [email protected] ˇ is retired. MILAN STASˇ, Metallurgical Engineer, is VLADIMÍR DAN EK ˇ with Slovalco Inc., 965 48 Ziar and Hronom, Slovakia. Manuscript submitted January 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
(a) primary alumina (alumina entering the dry scrubber); (b) secondary alumina (alumina entering the cells from the dry scrubber); (c) AlF3; (d) prebaked anodes; (e) anode butts; (f) aluminum; (g) molten bath; and (h) crushed bath (material consisting of the anode covering together with the covering remains, and some parts of the solid bath that solidify during the exchange operations of the anodes). The data on the content of iron, with respect to the iron oxide in all input and output materials, were supplied by ˇ Slovalco, Inc. (Ziar nad Hronom, Slovakia). The analytical methods used were as follow: X-ray fluorescence spectroscopy (for the baths), optical emissions spectroscopy (for the primary and secondary alumina, AlF3, and aluminum), and atomic-absorption spectroscopy (for the anodes and anode butts). The results of the iron oxide analysis in the primary and secondary alumina, the AlF3, and the molten and crushed baths, and the results of the iron analysis in the prebaked anodes, the anode butts, and the aluminum, in the period 1998 through 2002, were examined by means of statistical analysis. The samples for analysis were taken in three differe
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