Seed-Assisted Hydrothermal Treatment with Composite Silicon–Aluminum Additive for Solidification of Heavy Metals in Municipal Solid Waste Incineration Fly Ash

Abstract

Owing to the high Ca content but low concentration of Si and Al in fly ash (FA) generated from municipal solid waste incineration (MSWI), hydrothermal treatment (HT) of MSWI FA directly could not result in the synthesis of calcium-containing aluminosilicate minerals zeolites such as tobermorite, an important material for the hydrothermal solidification of heavy metals. According to the research results from the effect of the extra silicon–aluminum source added, with the higher amount (30%) of the composite additive in which the ratio of quality for coal fly ash (CFA) and diatomite is 1:1, tobermorite could be successfully synthesized, which is mainly attributed to the proper element proportion of Ca to Si and Al and Al to Si and Al, adjusted by additives. Furthermore, the tobermorite seed added also could not help the formation of tobermorite, even at a higher reaction temperature of 200 °C, due to the improper element proportion with a 10% mass of the composite additive. With a 30% mass of the composite additive and seed at a higher reaction temperature of 200 °C, the tobermorite seed added could not only promote the formation of tobermorite as early as the first 3 h but also inhibit the hibschite generated from tobermorite. Due to the leaching toxicity of Pb decreasing to the lowest level (0.25 mg/L) being related to the tobermorite largely formed, it can be concluded that the addition of a 30% mass of composite additive (including CFA and diatomite) and a 3% mass of tobermorite seed at a higher reaction temperature of 200 °C would be the recommended technological parameter for seed-assisted hydrothermal solidification of MSWI fly ash.

Effect of Ca–Si–Al Element Proportion on the Formation of Aluminosilicate Minerals and Detoxification of PAHs in Fly Ash from MSW Incineration during the Hydrothermal Process

“…The FA was sieved through a 35-mesh sieve, and the coarse fraction was discarded. 31 The FA was oven-dried at 105 ± 0. a 75-mesh sieve was analyzed to determine its principal properties. 36 SiO 2 and Al 2 O 3 (chemical-purity grade) were selected as the silicon additive and aluminum additive, respectively.…”

“…The low content of Si and Al would constrain zeolite formation during the hydrothermal process. 31 In order to make up the lack of Si and Al in raw FA for HT, coal FA from coal-fired power plants, which contains glassy state Si and Al, was used as silicon−aluminum additive for HT. 29 However, since coal FA is a waste from coal combustion, PAHs have been found in high concentrations in coal FA attributing to the complex composition of coal and its incomplete combustion.…”

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

“…According to previous studies [26][27][28], the molar ratio of Ca, Al, and Si in the hydrothermal solution made a crucial influence on the final product. When the Al contents were relatively higher, the formation of katoite was promoted [26].…”

“…When the Al contents were relatively higher, the formation of katoite was promoted [26]. When the Si contents was relatively higher and the molar ratio of Ca/(Al + Si) was in the range of 0.67-1.2, it was more likely to form tobermorite [27,28]. With fusion pretreatment, the crystal quartz in the raw fly ash was converted to amorphous silicon and silicate salts [29][30][31].…”

Seed-induced Hydrothermal Synthesis of Tobermorite from Municipal Solid Waste Incinerator Fly Ash

“…The suspensions were mixed and transferred to a 300 mL Teflon vessel and heated to T = 150°C (±1°C). After heating for a specific amount time (1~12d), reaction mixtures were quickly quenched within 30 min (Shi et al, 2016). The solid was crushed, filtered and washed with distilled water to remove residual ions and dried at 50°C for 3~5d.…”