Hydrogen Fluoride Capture and Mechanisms of Adsorption by Smelter Grade Alumina at Elevated Temperature
Hydrogen Fluoride , Smelter Grade Alumina , Retention Mechanisms , Aluminum Electrolysis , HF Adsorption , Elevated Temperature , Structural Hydroxyls
Aluminum metal is produced industrially by electrolysis of Smelter Grade Alumina (SGA) in a molten electrolyte using the well-known Hall-Héroult process. Water and structural hydroxyls associated with the SGA, and moisture from ambient air drawn into the electrolytic cell, hydrolyze the fluorides present in the electrolysis cell to produce HF gas. HF emissions from electrolytic cells are dry adsorbed (captured) onto the surface of primary alumina via a Dry Scrubbing System (DSS) followed by the subsequent recycle of the alumina, containing the adsorbed fluorides (secondary alumina), back to the electrolytic cell as feed material for the smelting process. Emission compliance is achieved by limiting the off-gas temperature entering the DSS to sustain an acceptable adsorption efficiency. This practice increases the moisture loading to the cell; hence, conventional DSS technology and operating practices promote, not abate, HF formation. This research thesis focused on understanding better the adsorption and retention of HF by alumina at both conventional and elevated temperatures. The goals were to minimize or eliminate the conflict associated with current operating practices, and to increase the alumina quality entering the electrolysis cell. Scientific observations and new discoveries from this investigation provided insight into resolving this conflict, specifically: i. regardless of the capture temperature, HF capture capacity increased directly with the number of accessible SGA pore reaction sites. ii. HF retention capability increased directly with capture temperature, number of accessible hydrate-free reaction sites, and the formation of stable fluoride species. iii. principle mechanisms for HF capture included hydrogen bonding (for initial capture) followed by progressive chemisorption, provided sufficient activation energy was available. iv. principle mechanisms for HF loss included the progressive release of hydrogen bonds above 200 °C in conjunction with hydrolysis of chemisorbed fluorides above 300 °C. v. gibbsite soak calcined at a temperature between 250 °C and 300 °C, before reacting with HF, provided the optimal HF capture capacity and retention capability. HF capture at elevated temperature, when coupled with increased accessible pore specific surface area, has the potential to reduce HF emissions to the environment, and increase the alumina quality entering the electrolysis cell.