Effects of Microwave-Assisted Hydrothermal Treatment on the Major Heavy Metals of Municipal Solid Waste Incineration Fly Ash in a Circulating Fluidized Bed

In this paper, microwave-assisted hydrothermal treatment was performed to stabilize the heavy metals of municipal solid waste incineration (MSWI) fly ashes in a circulating fluidized bed (CFB). Influences of the types of chemical additives, reagent concentration, liquid/solid ratio, reaction temperature, and reaction time were investigated by single and orthogonal experiments. A solid waste extraction procedure for leaching toxicity-acetic acid buffer solution method (HJ/T 300-2007) was adopted to detect the toxicity of raw fly ash and the hydrothermal products. The effect of pH on the leaching test of raw and treated fly ash was also carried out. Characteristics of fly ash were determined by XRF and XRD, and the leaching concentration was determined by ICP-MS. Experimental results revealed that the stabilization of heavy metals in fly ash was facilitated by a microwave-assisted hydrothermal process and, except for Cd, the regulatory limits were achieved when fly ash was treated with 1 mol/L NaOH and a liquid-to-solid ratio of 3.5 mL/g at 125 degrees C for 20 min of microwave-assisted hydrothermal heating. It was further concluded that the significance of factors was in the order of reagent > concentration approximate to temperature > time > L/S ratio (10-30 mL/g). More zeolites were formed over 20 min, which confirmed the high efficiency of the microwave assisted hydrothermal treatment. In pH experiments, it was found that the safe pH range of treated fly ash was broadened from 7.5 to 11 to 6.3-13, which led to better environmental adaptability. The microwave-assisted hydrothermal treatment may be applied to harmlessly manage the MSWI fly ash or to recover and utilize MSWI fly ash in a high efficiency, energy saving way compared to traditional hydrothermal treatment.