Migration behaviour of heavy metals and vitrification characteristics in melting of municipal solid waste incineration fly ash

Abstract

Municipal solid waste incineration fly ash (MSWI-FA) is an urgently treatable hazardous waste in China. High-temperature melting is a promising resource utilization technology for global solid waste. This study examines physicochemical properties of grate furnace incineration fly ash from northern and southern China, finding Cd, Pb, and Zn leaching toxicity exceeds national standards by 9.14, 8.59, and 1.09 times for northern samples, and 17.58, 7.13, and 1.25 times for southern samples, respectively. Clinker ignition loss during melting reaches approximately 40% for both fly ash types. Heavy metal migration (Pb, Zn, Cd, Cr, Cu, Ni) was analyzed across 200–1600 °C. Cr and Ni, predominantly in the slag phase, exhibited low volatility (~20% at 1600 °C). Fe and Cu showed moderate volatility, exceeding 70% at 1600 °C. Pb, Cd, and Zn demonstrated high volatility, surpassing 90% at 1200 °C. The volatilization sequence is Cd > Pb > Zn > Cu > Ni > Cr. XRD, SEM, TGA analyses revealed melting behavior and vitrification mechanisms. Heavy metals accumulate in secondary fly ash mainly as chlorides (PbCl₂, ZnCl₂, ZnCdCl₄). Slag minerals transition from low-temperature CaCl₂, CaClOH, NaCl, KCl to high-temperature chlorinated (Ca₅Al₂SiO₈Cl₄) and non-chlorinated (Ca₅Al₂SiO₁₀) calcium aluminum silicates. However, no glass peaks were observed in XRD spectra of both fly ashes at 1600 °C. Thermodynamic analysis points to high CaO content as the main cause of elevated melting points. To achieve vitrification below 1500 °C, the optimal SiO₂:CaO:Al₂O₃ ratio should be approximately (3.5–9.0):(0–4.0):(0–3.5).

Graphical abstract

Keywords

Hazardous waste / Heavy metals / MSWI fly ash / Melting / Vitrification

Highlight

	● The thermal ignition loss rate of the FA-B and FA-J fly ashes is about 40%.
	● The order in which the six heavy metals’ volatility drops is Cd > Pb > Zn > Cu > Ni > Cr.
	● CaCl2 in both types of MSWI-FA did not completely evaporate at 1400 °C.
	● FA-B and FA-J have a melting point higher than 1400 °C.
	● High CaO content in MSWI-FA raises its melting point.

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