Lime treatment of fly ash: characterization of leachate composition and solid/water reactions

Standard leach tests of fly ashes often yield solution concentrations of B, Cr, Mo and SO4 above drinking water standards. In this study, the separate addition of lime, Portland cement and cement kiln dust to a Class F and a Class C fly ash has been investigated as a possible remediation treatment to remove undesirable elements from the leachates. Leach tests without additives at several water/solid ratios revealed levels of B, Cr, Mo and SO4 above drinking water standards. A marked reduction in all these elements occurred at 3:1, 6:1 and 20:1 water/solid ratios with a 20 mass% addition of lime or cement. Reductions were more pronounced for the Class F than for the Class C fly ash. The mineralogical and chemical composition of fly ash influences its behavior with respect to the additive treatment. In this study, an anionic clay (hydrocalumite) is formed in considerable quantity in the Class F fly ash treated with lime; and can accommodate anions like borate, chromate and molybdate into its interlayer region. With Class C fly ash, the lime-treatment generates hydrogarnet, which has less sites for anion substitution; and ettringite, which has less preference for chromate and molybdate than hydrocalumite. The overall order of improvement in fly ash leachate water quality with additive treatment was lime >OPC>CKD.