The solidification and volatilization behavior of heavy metal ions in ceramic proppant made from fly ash

As a type of silica-alumina solid waste, fly ash can be utilized to prepare ceramic proppants by partially substituting bauxite due to a similar chemical composition. However, the heavy metal elements in fly ash including Zn and Cd are subjected to be released during high-temperature calcination, causing harm to some extent. In this study, migration, transformation, solidification, and volatilization of heavy metals in proppant at elevated temperatures are systematically studied by various experimental characterization methods. The results show that the solidification effect of the prepared ceramic proppant on the Zn and Cd elements is favorable, with solidification ratios up to 77.16% and 74.34%. The addition of Cd improves the bulk density and strength of the ceramic proppant, while the addition of Zn significantly reduces the strength and increases the water absorption. Additionally, the majority of doped Zn will combine with Al2O3 to form a new phase, zinc spinel ZnAl2O4, which hinders the formation of the inherent mullite phase. On the contrary, the addition of Cd does not change the phase composition, and the Cd element is mainly solidified in the form of lattice substitution. Finally, the distributions of volatile Zn and Cd in gas- and solid-phase were quantitatively determined by the in situ continuous collection system. In addition, a mathematical model was used to predict the volatilization of heavy metals during the proppant calcination process.