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

Abstract

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 °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 ≈ temperature > time > L/S ratio (10−30 mL/g). More zeolites were formed over 20 min, which confirmed the high efficiency of the microwaveassisted 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.

Stabilization of heavy metals in municipal solid waste circulating fluidized bed incineration fly ash by fusion–hydrothermal method

“…When elevating the hydrothermal temperature from 125 to 180 °C and extending the hydrothermal duration from 20 min to 4 h in the only-hydrothermal process, the improvement in the heavy metal immobilization was very limited (see details in Supplementary Sect. 2), which was in accordance with our prior works [16].…”

“…The mixture was heated at 125 °C for 20 min in the microwaveassisted hydrothermal process, and then cooled to ambient temperature. The hydrothermal temperature and duration were optimized based on heavy metal immobilization performance and energy consumption in our prior work [16]. The solid component and the liquid component were separated by centrifugation.…”

“…However, the conventional hydrothermal method often took a long time over several days to obtain a complete reaction. Recently, microwave irradiation was introduced to the hydrothermal process, resulting in the reduction of the reaction time to 10-50 min [15][16][17]. Nevertheless, only by hydrothermal process, it was still unable to reach the standard of Chinese regulation (GB16889-2008) [18] for some heavy metals, such as Cd [16].…”

“…Recently, microwave irradiation was introduced to the hydrothermal process, resulting in the reduction of the reaction time to 10-50 min [15][16][17]. Nevertheless, only by hydrothermal process, it was still unable to reach the standard of Chinese regulation (GB16889-2008) [18] for some heavy metals, such as Cd [16]. On the other hand, a large amount of indissoluble materials, including quartz, remained in the MSWI fly ash even after the microwave-assisted hydrothermal process [16].…”

“…Nevertheless, only by hydrothermal process, it was still unable to reach the standard of Chinese regulation (GB16889-2008) [18] for some heavy metals, such as Cd [16]. On the other hand, a large amount of indissoluble materials, including quartz, remained in the MSWI fly ash even after the microwave-assisted hydrothermal process [16]. These indissoluble materials were unable to be utilized for the zeolite formation when only the hydrothermal process was applied.…”