Solidification/stabilization of spent cathode carbon from aluminum electrolysis by vitric, kaolin and calcification agent: fluorides immobilization and cyanides decomposition

Abstract

Spent cathode carbon (SCC) is a hazardous waste containing fluorides and cyanides from aluminum electrolysis. Many literatures have focused on SCC leaching; however, SCC hazard-free treatment remains understudied. This article used 10.0 g raw SCC sample to explore the vitric/kaolin solidification and calcium stabilization of SCC, and analyze their hazard-free mechanisms by the methods of XRD and SEM. The leached fluorides were all below the Chinese identification standard for hazardous wastes (GB5085.3-2007), whether at 750/950 °C for 60 min above 8.0 g vitric, or at 1200 °C for 120 min with above 8.0 g kaolin, or above 700 °C for more than 30 min with above 0.5 g CaCO₃. Kaolin/vitric solidification relied on the massive addition of vitric and kaolin to produce glassy or glass-like material (K₂O·Al₂O₃·6SiO₂) which may retain fluoride. Calcium stabilization converted soluble fluoride NaF in raw SCC sample into insoluble CaF₂. Heating 60 min at 500-1200 °C at oxygen atmosphere decomposed almost of cyanides, with leached cyanides meeting Chinese standard GB5085.3-2007. Mass-loss rates of kaolin addition came from a large amount of adsorbed water and structural water in kaolinite and illite wai lost, and that of CaCO₃/CaSO₄ addition was attributed to their decomposition into volatile CO₂/SO₂, while that of CaO was a little negative due to its absorption of water vapor and CO₂. In brief, as the effective hazard-free manner of SCC, both kaolin/vitric solidification and calcium stabilization successfully have achieved fluoride immobilization and cyanide decomposition.

Keywords: Calcium stabilization; Cyanides decomposition; Fluorides immobilization; Spent cathode carbon; Vitric/kaolin solidification.