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ODUCTION

IN the Hall-Heroult aluminum electrolysis process, primary alumina is used as a raw material as well as the absorbent in the dry scrubbing system.[1] The scrubbing system captures the solid and gaseous fluoride emissions and other particulates generated during electrolysis for economic and environmental reasons. The byproduct, secondary alumina, is then added to the smelter to reduce fluoride loss. In commercial aluminum production, some carbon dust is usually introduced into the cell from the anodes. Consequently, industrial secondary alumina generally not only contains 0.2 to 1.2 wt pct fluorine, but also 0.3 to 1.0 wt pct carbon which is entrained in the rising draft due to the oxidation of carbon anodes and then adsorbed in the secondary alumina.[2,3] Both plant experience and laboratory studies show that secondary alumina usually has faster dissolution rate than that of primary alumina. Haverkamp et al. investigated the influence of sodium aluminum fluoride, HF. and moisture on secondary alumina dissolution and found that fluoride and moisture assist dissolution, while HF appeared not to have an effect.[4] The present authors do not deny that fluoride in secondary alumina contributes to its dissolution, but suggest that the effect of carbon component has seldom been considered. In our earlier work, we found the dissolution rate of secondary alumina was significantly faster than that of the primary alumina. By comparing the YOUJIAN YANG, Ph.D. Candidate, BINGLIANG GAO, ZHAOWEN WANG, and ZHONGNING SHI, Professors, XIANWEI HU, Associate Professor, and JIANGYU YU, Assistant Engineer, are with the Department of Nonferrous Metallurgy in School of Materials and Metallurgy, Northeastern University, Shenyang 110004, P.R. China. Contact e-mail: [email protected] Manuscript submitted September 25, 2013. Article published online April 19, 2014. 1150—VOLUME 45B, JUNE 2014

dissolution rate of secondary alumina samples before and after preheating in a glove box, the positive effect of carbon dust on dissolution was proved.[5] Carbon particles in secondary alumina can burn immediately with moisture or entrained air in the alumina after being added into the high temperature molten electrolyte. The combustion heat would heat the alumina particles, and the flash-off carbon dioxide may further contribute to their dispersion, the so-called gas-stirring. This study assesses the effect of carbon on secondary alumina dissolution.

II.

EXPERIMENTAL

A. Chemicals Analytical reagents calcium fluoride, lithium fluoride, sodium carbonate, calcium carbonate, and chemically pure cryolite were dried for 2 hours at 673 K (400 C). Chemically pure aluminum fluoride trihydrate mixed with ammonium hydrogen fluoride at mass ratio of 7:3 was dried for 2 hours at 423 K and 523 K (150 C and 250 C), respectively, and then kept for at least 3 hours at 773 K (500 C).[6] All of the chemicals were kept in a dry box before use. Primary alumina and secondary alumina (containing 0.55 wt pct fluorine and 0.52 wt pct carbon) used in the experiments were taken fr