Ultrasonically enhanced leaching: removal and destruction of cyanide and other ions from used carbon cathodes

The used carbon cathodes (‘Spent Pot Lining’, SPL) from aluminium smelting are typically contaminated with F−, CN− and Na+ which must be removed before disposal to produce material which satisfies environmental guidelines in respect of maximum rates of ion release. Leaching of powdered and crushed carbon samples in aqueous solution were studied in the presence and absence of ultrasound. First, it was demonstrated that the total leaching of F−, CN− and Na+ from the carbon powder could be accomplished after ∼20 min of sonication using ultrasound. Second, carbon particles (<5 mm) showed leaching characteristics under ultrasound which were in good quantitative agreement with the National River Authority (NRA) test under silent conditions; the former being a much more rapid (∼1 h compared to 24 h for the NRA test) but equivalent method to the latter standard test. Third, the removed CN− ions were found to be destroyed in the presence of ultrasound, possibly via the oxidative action of hydrogen peroxide generated by the sonication of water.