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 t o remove undesirable elements from the leachates. Leach tests without addit ives at several water/solid ratios revealed levels of B, Cr, Mo and SO4 abo ve drinking water standards. A marked reduction in all these elements occur red at 3:1, 6:1 and 20:1 water/solid ratios with a 20 mass% addition of lim e or cement. Reductions were more pronounced for the Class F than for the C lass C fly ash. The mineralogical and chemical composition of fly ash influ ences its behavior with respect to the additive treatment. In this study, a n anionic clay (hydrocalumite) is formed in considerable quantity in the Cl ass F Ay ash treated with lime; and can accommodate anions like berate, chr omate and molybdate into its interlayer region. With Class C fly ash, the l ime-treatment generates hydrogarnet, which has less sites for anion substit ution; and ettringite, which has less preference for chromate and molybdate than hydrocalumite. The overall order of improvement in Ay ash leachate wa ter quality with additive treatment was lime 1 OPC > CKD. (C) 1999 Elsevier Science Ltd. All rights reserved.