Mechanism of Dissolution Behavior of the Secondary Alumina

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ALUMINA is the most important raw material for aluminum electrolysis, where it acts as a solute in the aluminum electrolyte. Aluminum smelting cells produce emissions of fluoride consisting of hydrogen fluoride (HF) and sodium aluminum fluorides (Na5Al3F14, Na3AlF6, AlF3). For environmental and economic reasons, these emissions must be captured so that they are not released into the atmosphere.[1] Dry scrubbing systems, which use alumina to capture the fluorides and HF, have been developed, but this also results in changes in alumina properties and chemical composition.[2,3] It has been observed in the laboratory that the secondary alumina (a product from industrial dry scrubbing) behaves differently compared to primary alumina, as it seems to form aggregates to a lesser extent and enjoys a smaller dissolution time. Haverkamp et al. investigated the reason why secondary alumina behaves so differently and found that the presence of NaAlF4 and HF do not appear to enhance the dissolution rate.[4] The primary objective of this investigation conducted by the authors of this article was to study the influence of dry scrubber fluorination on the subsequent dissolution behavior of alumina in molten cryolite electrolytes. The impacts of gaseous HF, particulate fluorides, and carbon dust on the dissolution behavior of alumina were investigated.

YOUJIAN YANG, Ph.D. Candidate, BINGLIANG GAO, ZHAOWEN WANG, and ZHONGNING SHI, Professors, and XIANWEI HU, Associate Professor, are with the School of Materials and Metallurgy, Northeastern University, Shenyang 110004, P.R. China. Contact e-mail: [email protected] Manuscript submitted April 10, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B

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EXPERIMENTAL DETAILS

A. Chemicals The chemicals used in the trials and corresponding treatments are listed in Table I. Tests were conducted in cryolite melt with a molar ratio equal to 2.4 containing 4 wt pct CaF2 and 5 wt pct LiF. The electrolyte has a liquidus point of 1224 K ± 2 K (951 C ± 2 C), which allows the operation temperature of the tests in the range of 1230 K to 1250 K (957 C to 977 C).

B. Apparatus for Observing Dissolution Behavior The experimental apparatus for observing dissolution behavior was a see-through cell, as shown schematically in Figure 1. The cell was made of a fused quartz crucible with a dimension of 55 mm 9 55 mm 9 80 mm. The wall was 3 mm thick. The cell containing electrolyte was located in a resistance-heated furnace with glass windows on its two sides. A sunlamp was used at the rear window to provide back lighting and a video camera (MV-VS078FC, MicroVision Company, Xi’an City, China) was placed close to the front window to record the dynamic process taking place inside the crucible. A temperature controller (DWT-702, Shanghai No.6 Automated Instrumentation Works, Shanghai City, China) with a Pt-PtRh10 type thermocouple was used for measuring and controlling the temperature of the furnace. The weight of electrolyte in each run was 200 g. The electrolyte height was 5.5 cm. The alumina addition to the m