Alumina Structural Hydroxyl as a Continuous Source of HF
The link between moisture on alumina and HF generation in aluminium reduction cells has been long established. The assumption has usually been that the ‘culprit’ is the loosely bound adsorbed water, generating HF via bath hydrolysis as this surface water is flashed off during alumina feeding. Structural water, or more correctly, structural hydroxyl, also makes a significant contribution to HF generation. Laboratory experiments show that hydroxyl can dissolve in molten cryolitic electrolytes and gives rise to electrochemically generated HF. The electrochemically generated HF could be readily distinguished from HF generated via thermal hydrolysis.
Experiments with aluminas of varying combinations of high and low surface adsorbed moisture and structural hydroxyl (as measured by their LOI (20–300) and LOI (300–1000), respectively) confirmed the importance of electrochemically generated HF from structural hydroxyl. While some of the structural hydroxide reacts rapidly at the time of feeding, it also contributes to the steady state HF emission. From plant studies it was estimated that up to 8 kg F/torme Al was generated from structural hydroxyl lor aluminas containing 0.4 wt % LOI (300–1000) and assuming 3wt% alumina in the bath. Structural hydroxyl is found in transition alumina phases in smelter grade aluminas. Their presence ensures that even conservative smelter specifications of surface areas of 60–80 m2/g can be met. Paradoxically, this surface area is specified to ensure that the HF adsorption capacity of the alumina is sufficient for scrubber requirements, but for reasons of both surface and structural water incorporation, having a high surface area also means that the alumina will generate more HF. This reopens the debate on the merits of high surface area aluminas.
KeywordsHF generation fluoride emissions smelter grade alumina structural hydroxy
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