Investigation of controlling factors on toxic metal leaching behavior in municipal solid wastes incineration fly ash.

Municipal solid wastes incineration (MSWI) fly ash has drawn worldwide attention for its substantial annual generation capacity and high toxic metals leachability. Although many factors have been shown to affect the leachability of metals in fly ash, the controlling factors, which guide the selection of appropriate risk reduction method, remain unclear. The purpose of this study was to evaluate the effects of the two most important factors, total metal content, and remaining alkaline substances of MSWI fly ash, on the leaching behavior of toxic metals. In this work, a series of leaching tests and sequential extraction procedures were performed for seven fly ash samples collected from one MSWI plant. Results show that particulate size distribution, morphology, and mineralogy of all samples are similar, indicating the effects of these properties on metal leaching behavior can be ignored. In leaching tests, although the leaching behavior in terms of metal species and concentration levels vary as expected, only the leachate Pb concentration in four samples (up to 17.32 mg/L) exceeds the threshold in Chinese regulation (0.25 mg/L). The variation of the leachate Pb concentration is not consistent with the change of the total Pb concentration in fly ash. Further correlation analysis evidences that the acid-soluble Pb, which is highly correlated to the calcium content of fly ash, dominates the concentration of leachate Pb. Notably, when the addition of lime is about 1.5 times over the theoretical value, the concentration of leachate Pb would exceed the threshold regardless of the total Pb concentration in fly ash. Overall, this study demonstrates that the remaining alkaline substances (mainly calcium-bearing compounds), rather than the total content of metals, are the controlling factor of metal leaching behavior in fly ash. Thus, strategies to delicately optimize the quantity of lime addition in acid gas purification process should be considered to minimize MSWI fly ash environmental risks in the future.

Environmental risk related to specific processes during scrap computer recycling and disposal.

The purpose of this work was to achieve a better understanding of the generation of toxic chemicals related to specific processes in scrap computer recycling and disposal, such as thermal recycling of printed circuit boards (PCBs) and the landfilling or dumping of cathode ray tubes (CRTs). Tube furnace pyrolysis was carried out to simulate different thermal treatment conditions for the identification of the by-products and potential environmental risk from thermal recycling ofPCBs. The Toxicity Characteristic Leaching Procedure (TCLP) and a column test were used to study the leaching characteristics of lead from waste CRT glass, which is one of the most important environmental concerns arising from the disposal of e-waste. The results indicate that more attention should be paid to the benzene series when recycling PCBs under thermal conditions, especially for workers without any personal protection equipment. The impact of immersion on the leaching of lead from CRT leaded glass was more effective than the impact of washing only by acid rain. Thus when waste leaded glass has to be stored for some reason, the storage facility should be dry.

A combined recovery process of metals in spent lithium-ion batteries.

This work proposes a new process of recovering Co from spent Li-ion batteries (LIBs) by a combination of crushing, ultrasonic washing, acid leaching and precipitation, in which ultrasonic washing was used for the first time as an alternative process to improve the recovery efficiency of Co and reduce energy consumption and pollution. Spent LIBs were crushed with a 12 mm aperture screen, and the undersize products were put into an ultrasonic washing container to separate electrode materials from their support substrate. The washed materials were filtered through a 2mm aperture screen to get underflow products, namely recovered electrodes. Ninety two percent of the Co was transferred to the recovered electrodes where Co accounted for 28% of the mass and impurities, including Al, Fe, and Cu, accounted for 2%. The valuable materials left in 2-12 mm products, including Cu, Al, and Fe, were presented as thin sheets, and could be easily separated. The recovered electrodes were leached with 4.0M HCl for 2.0 h, at 80 degrees C, along with concurrent agitation. Ninety seven percent of the Li and 99% of the Co in recovered electrodes could be dissolved. The impurities could be removed at pH 4.5-6.0 with little loss of Co by chemical precipitation. This process is feasible for recycling spent LIBs in scale-up.