Effect of plasma treatment on Ni molecular environment in a spent catalyst and a plating sludge

High-temperature plasma is used to stabilize the Ni in two wastes: a Ni-plating sludge cake and a spent Ni-containing solid catalyst. Only the latter sample had been thermally treated to recover precious metal by industrial practitioners before we received the wastes. The reaction temperature in the plasma chamber is >1500 °C, the chamber pressure is 0-6000 Pa less than the atmospheric pressure, and the reaction time for stabilizing the wastes is 60 min. The molecular environments of Ni in the stabilized and pre-stabilized sludge cake and spent catalyst samples are investigated with X-ray absorption spectroscopy (XAS) technique. All main peaks in the X-ray absorption near-edge structure (XANES) spectra from both plasma-treated wastes and the as-received spent catalyst are characterized with a shoulder towards the edge jump, suggesting a highly distorted Ni(II) environment in all samples except in the as-received plating sludge cake. Results from one-shell fitting of the Fourier transforms of X-ray absorption fine structure (EXAFS) spectra indicate that treating the spent catalyst with plasma barely increases the first-shell coordination number (from 5.59 to 5.81). For the plating sludge cake treated with plasma, however, Ni first-shell coordination number slightly increases from 5.74 to 6.60, suggesting that treatment at >1500 °C with plasma enhances the crystallite size of nickel compounds due to a greater sintering and/or melting reaction. In addition, the Ni second coordination shell in the sludge cake case before plasma treatment is well resolved; while it mixes with the outer (third and greater) shells after plasma treatment. Because the plating sludge cake (i.e., as-received) has never experienced any high-temperature environment like which the spent catalyst did prior to the plasma treatment, it is concluded that plasma treatment incurs considerable change in Ni molecular environment in the plating sludge cake.