The sensitive mobility of Cr in ashes studied by SiO₂-Al₂O₃-Fe₂O₃-CaO system

Since incineration is a feasible method for stabilization/solidification of chromium (Cr)-enriched wastes, the species, distribution, and mobility of Cr in ashes deserve more studies, especially as the function of ash composition. Synthetic Cr-bearing ashes (SAs) were synthesized by SiO₂-Al₂O₃-Fe₂O₃-CaO systems to investigate Cr mobility under 1100 °C. A study from simplicity to complexity. The Cr in SiO₂-CaO is of high mobility with CrO₄^2- formation, in contrary to the moderate mobility in SiO₂-Al₂O₃ and poor mobility in SiO₂-Fe₂O₃. However, species and mobility of Cr are affected by the values of CaO/SiO₂, Al₂O₃/SiO_2, and Fe₂O₃/SiO₂ ratios. When other oxides are added to the two-phase systems above, the fate of Cr is affected more considerably. With the SiO₂ content of 70%, adding a slight amount of CaO (<10%) strengthens the stabilization/solidification of Cr, due to the favorable solid integration under Ca²⁺ fusion. However, the Cr mobility is higher with increasing the CaO content further. The minimum content of CaO is ∼20% to sufficiently decrease the proportion of residual Cr (QCr-S5) in SiO₂-Al₂O₃-CaO, much lower than in SiO₂-Fe₂O₃-CaO, which confirms the easier release of Cr immobilized in Si-Al matrixes. Considering the opposite effects of Fe₂O₃ and CaO on Cr mobility, increasing Fe₂O₃/CaO ratios >3/2 can limit the effect of CaO, leading to the efficient stabilization/solidification of Cr waste. Additionally, the QCr-S5 is 83% with the Fe₂O₃ content of 15% in SiO₂-Al₂O₃-Fe₂O₃, higher than in SiO₂-Al₂O₃ and SiO₂-Fe₂O₃. This suggests the intense stabilization/solidification of Cr, probably due to the formation of amorphous Fe-rich glass. Based on these above, an equation is developed to describe the relationship between ash compositions and QCr-S5 (QCr-S5 = -39.37X₁ + 24.96X₂ + 5.34X₃ - 2.51X₄ + 54.29).