Successive selective leaching procedures for valorization of spent pot lining carbon

Spent pot lining (SPL) is a hazardous by-product obtained during the decomposition of carbon cathodes in the electrolytic cells used in aluminium smelting. The carbon cathode lining materials degrade over time thereby affecting the performance of the electrolytic cell. Fluoride contents of up to 20 wt% and cyanides up to 1 wt% have been reported as the main environmental concerns in SPL. This study investigated the effect of successive selective leaching (SSL) of pollutants from the SPL in obtaining suitable carbon for use in energy storage applications. Compositional and microstructural analyses were performed on the leached and as-received SPL samples using X-ray fluorescence spectroscopy (XRF), transmission electronic microscopy – energy dispersive X-ray (TEM-EDX), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. In this study, the XRD analysis revealed that fluoride ions in the SPL exist as sodium fluoride (NaF) and calcium fluoride (CaF2). The results indicated that the SSL of SPL is dependent on pH, temperature, types of lixiviant used for the selective leaching and number of washings. It was established that increasing the number of washings with steady stirring rate reduced electrical conductivity and fluoride concentration in the filtrate. Furthermore, high ion removal was achieved at 100 ℃. The current study therefore demonstrated successful removal of pollutants such as CaF2, SiO2, NaF, Al2O3 and Na3Al11O17 while providing insight into the chemical structure and morphology of the treated and untreated SPL.