Assessment of heavy metals mobility and correlative recovery and decontamination from MSWI fly ash: Mechanism and hydrometallurgical process evaluation

Acid controlled washing of municipal solid waste incineration fly ash: Extraction of calcium inhibiting heavy metals and reaction kinetics

Washing method has attracted much attention in the research of municipal solid waste incineration (MSWI) fly ash treatment and resource utilization. However, the controlled leaching of heavy metals and the extraction of recyclable calcium in the washing process are still blank. Acid controlled washing was conducted with different acids, concentrations, times and temperatures to extract calcium while inhibiting heavy metals. The mechanism was investigated by reaction kinetics calculation and washed fly ash characterization. The high Ca concentration of 37,420 mg/L while the low heavy metal concentrations of around or <1 mg/L were achieved at 25 °C for 60 min under a liquid-solid ratio (L/S) of 3/1 in 1.5 M HCl. The reaction kinetics of acid controlled washing conformed the layer diffusion control. The results of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrum (EDS) analysis indicated that the rate-limiting step was the diffusion of ions through the product layer. Simultaneously, the washing solution enriched in Ca, Na and K and the washed fly ash, which met the standard requirements (HJ 1134-2020) for leach toxicity, both had the potential for further resource utilization.

Research on oxygen enrichment for municipal solid waste fly ash melting: A pilot-scale study on natural gas and coal as the melting fuel

High energy consumption is the main obstacle of melting/vitrification technology for the disposal of municipal solid waste incineration fly ash (MSWIFA) for industrial applications. To reduce energy consumption and lower operating costs, oxygen enrichment melting was proposed and studied in this work. This research was conducted in a pilot-scale melting furnace, and three melting conditions were compared and discussed. The results showed that 66% of natural gas was saved and the operating cost was reduced by 55% when oxygen enrichment technology was applied to MSWIFA melting. When coal was used as the fuel with the oxygen enrichment melting technology, the operating cost was even lower at 66.39 dollar/ton of fly ash. Because MSWIFA was a Ca-rich material, the relatively high content of Si and Al in the coal fly ash promoted the formation of vitrificated slag, leading to a reduction in the overall pollution toxicity index (OPTI) of MSWIFA by 99.98%. Meanwhile, SO2, HCl, and secondary fly ash were the main pollutants during MSWIFA melting, and when coal was used as the fuel, the emissions of SO2 and HCl could be reduced and the OPTI of secondary fly ash was suppressed. These results suggested that to obtain the lowest operating cost and reduce secondary pollution during MSWIFA melting, the best option consisted of oxygen enrichment technology with coal as the fuel.

Comprehensive understanding the transition behaviors and mechanisms of chlorine and metal ions in municipal solid waste incineration fly ash during thermal treatment

Municipal solid waste incineration fly ash is classified as the hazardous waste because of its high levels of heavy metals alkali chlorides, and polychlorinated dibenzo-p-dioxins. Thermal treatment is widely used for fly ash treatment because of its advantages of reduction and harmless. The transformation behaviors of chlorine and metal ions during the thermal treatment of fly ash has a significant impact on the harmless and resource of fly ash. At present, the migration behaviors of chlorine and metal ions during thermal treatment of fly ash is not clearly demonstrated. In this manuscript, the phase compositions, transformation behaviors, the variation of mass and content of chlorine and various metal ions were analyzed through diverse characterization methods under different sintering temperatures to understand the migration behaviors of chlorine and metal ions during thermal treatment. Roasting experiments showed that the migration behaviors of heavy metals and chlorides were consistent. The chlorine, sodium, potassium and heavy metal ions can be removed sharply while the calcium, aluminum, magnesium and iron were decreased slightly when the roasting temperature was above 750 °C. The findings also suggested that removed chlorides were soluble chlorides and unstable crystals in municipal solid waste incineration fly ash were inclined to formed steady structure under high temperature. The structure of roasted fly ash became denser and generated ceramic-like particle due to thermal agglomeration and chemical reactions.