Leaching of spent pot-lining with aluminium nitrate and nitric acid: Effect of reaction conditions and thermodynamic modelling of solution speciation

A study of the leaching behaviour of spent pot-lining (SPL), a hazardous waste generated at the end-of-life of aluminium smelting cells, has been carried out using Al(NO3)3∙ 9H2O and HNO3 solutions. Following an initial water wash, a single leaching step using 0.5 M HNO3 and 0.36 M Al(NO3)3 at 60 °C extracted a total of 96.3 wt.% of the remaining fluoride, complete extraction of magnesium and 90% of the calcium content, originally in the form of MgF2 and CaF2. In the absence of Al(NO3)3, iron extraction is almost complete when SPL is treated with 0.5 HNO3 M solution at 60 °C. CN− was selectively extracted when 0.2 M HNO3 at 60 °C was used, and could be selectively precipitated from the scrubbing solution in the form of salts of transition metals for catalytic applications. Experimental observations have been correlated with a mathematical model of the solution equilibria, which identified AlF2 + and AlF2+ as the main species responsible for fluoride extraction from Na3AlF6 and CaF2. Selective precipitation by pH adjustment is proposed as a means to recover fluoride and metal values. Aluminium and fluoride can be selectively precipitated as AlF2OH·1.4H2O for conversion into smelter grade (SG) AlF3.