Unveiling the mechanism of flash evaporation separation in spent carbon cathodes via fast joule heating

Spent carbon cathodes (SCC), a hazardous solid waste from aluminum electrolysis, pose a significant disposal challenge because of the intimate association of valuable graphitized carbon with hazardous fluorides. While fast Joule heating (FJH) has emerged as a powerful technique for material synthesis, its application in flash evaporation separation for complex solid wastes like SCC remains underexplored. This work unveils the pivotal role of the sample heating container (SHC) geometry in determining separation efficacy, a previously overlooked critical factor. We demonstrate that a tubular SHC with lateral vents achieves superior separation by facilitating the efficient escape of volatiles while simultaneously acting as a physical filter to prevent carbon entrainment. The optimal separation conditions were identified as four FJH cycles, a 75% packing density, and a 500 A current, yielding a carbon-rich substrate with 96.0% purity. Comprehensive characterization and thermodynamic analysis revealed two distinct mechanisms for the FJH-treated SCC: flash evaporation of volatile fluorides (NaF, Na 3 AlF 6 ) and carbothermal reduction of refractory oxides (Al 2 O 3 , SiO 2 ). These findings provide valuable insights for FJH application in treating complex multi-component solid wastes.