Studies on the solidification mechanisms of Ni and Cd in cement clinker during cement kiln co-processing of hazardous wastes

• Cd concentrated in Cao phases but distributed mainly in C3S phases.
• Ni was mainly formed MgNiO2, also present in C4AF and C3S.
• Cd and Ni solidified by isomorphous replacement and formation of a new or interstitial solid solution.
• Ca–Cd–O sosoloid and Ni–Mg compound were formed in solidification.
• Cd and Ni incorporated in C3S by substituting Ca and in C4AF by inter lattice space.

To clarify the solidification mechanisms of heavy metals in cement clinker during the cement kiln co-processing of hazardous wastes, cement clinker samples were produced. EPMA1 and XANES2 spectroscopy were employed to acquire information on the microstructure and the Cd and Ni species formed in the cement system. The average Cd concentrations in CaO, C4AF3 and C3S4 were 3.92%, 1.21% and 0.75%, respectively, but Cd was mostly present in C3S in clinker (71.0%). The solidification methods of Cd during the calcination process included the formation of a Ca–Cd–O sosoloid, substitution for Ca in C3S and C4AF (isomorphous replacement) and inter the space of the lattice (interstitial solid solution). Ni was combined mainly with Mg in the form of a new Ni–Mg compound (distribution ratio of 61.2%), which was MgNiO2, as confirmed by XANES analysis. Ni was also present in C3S and C4AF (distribution ratios of 24.9% and 10.3%, respectively) by replacement of Ca2+ in C3S and substituting Fe in C4AF (isomorphous replacement), and also by inter the spaces of the C4AF lattice (interstitial solid solution).