Treatment of spent pot-lining for recovery of fluoride values

The leachability of NaF, CaF2 and Na3AlF6 from spent pot-lining (SPL) has been studied with the view to dissolve, manipulate solution equilibria, and precipitate fluoride in a form that can be recycled (directly or indirectly) back into the pot. Leaching results have been combined with theoretical stability constant information to understand solution equilibria and identify novel ways that it can be manipulated to produce AlF3 and/or CaF2, to yield a process with the criteria of minimal process reagents, number of steps, energy and waste. While AlF3 is the preferred form for fluoride, its production in acidic media is difficult owing mainly to the unavoidably high concentrations of Na'. So far, we have found a chemical sequence for recovery of all fluoride as NaAlF4. This is presented along with other sequences being pursued. Following removal of fluoride, high degrees of separation (> 95%) of the remaining refractory compounds from the graphite has proved possible using a density separation technique.