Calcific stabilization and clay solidification of fluorides in graphitized sidewall blocks from aluminum electrolysis

Abstract

Few studies have reported the hazard-free treatment of graphitized sidewall block (GSB), which is a numerous-fluoride-containing hazardous waste from aluminum electrolysis. The performance, parameters, and mechanism of the hazard-free treatment of GSB via calcific stabilization (Calci-S), clay solidification (Clay-S), and their co-processing (Co–S/S) were investigated. The single-variable experimental results showed that Calci-S and Clay-S were both favorable hazard-free choices for treating GSB, whereas Co–S/S was not a reasonable choice. Using Calci-S, the leached fluoride concentrations met the standards of China, Brazil, and Singapore at temperatures above 700 °C for over 30 min with more than 3 g CaCO3, and using Clay-S, leached fluoride concentrations met the standards at temperatures above 850 °C for over 90 min with more than 30 g clay. Response surface methodology results indicated that the decreasing order of impact on the fluoride immobilization percentages of Calci-S and Clay-S was doses, heating temperatures, and heating time. The optimum parameters of Calci-S were at 582 °C for 94 min with 4 g CaCO3, and those of Clay-S were at 838 °C for 175 min with 61 g clay. The mechanism analysis of fluoride immobilization using x-rays diffraction and scanning electron microscope showed that the soluble fluorides, NaF and AlF3, in GSB was converted into water-insoluble CaF2 and cuspidine in Calci-S, whereas SiO2 in clay reacted with sodium in GSB and formed a glass-like viscous diffusion barrier (Na23.55Al24.00Si24.00O96) encapsulating the fluoride ions in Clay-S. To summarize, this study presents the two innovative and effective hazard-free approaches of Calci-S and Clay-S for GSB and provides valuable insights into the fluoride immobilization mechanism, thereby providing more options for GSB disposal.