Separation of heavy metals from municipal solid waste incineration fly ash: A review

More than 10 million tons of municipal solid waste incineration (MSWI) is discharged annually on the planet, which endangers ecosystem. Separation of heavy metals is a promising method for disposal of MSWI fly ash, which has been studied for decades. However, there is lack of a systemic review on the research. The current separation method of heavy metals is reviewed, including hydrometallurgy, pyrometallurgy and biohydrometallurgy. Heavy metals migrate from solid phase to liquid phase after washing, acid or complexation leaching, in hydrometallurgy. Heavy metals are volatilized into exhaust gas or reduced into alloy after thermal treatment, in pyrometallurgy. Bacteria and fungus act as a catalyst to enhance the leaching of heavy metals in biohydrometallurgy. Resource consumption and emission is calculated to evaluate the hydrometallurgy and pyrometallurgy process. About 137.5 and 160.0 dollars is earned, after disposal of a ton of MSWI fly ash by hydrometallurgy and pyrometallurgy, respectively. And carbon emission is 729.2 and 947.3 kgCO₂, respectively. The pyrometallurgy process possesses a lower resource consumption and a higher carbon emission than the hydrometallurgy process. In order to reduce carbon emission and recycle the separated heavy metals, we propose an idea of capturing heavy metals with iron into alloy, after thermal reduction with a low carbon reductant of aluminum dross. Carbon emission can be deducted, for the used aluminum dross and the obtained alloy product. We should clarify the reduction reaction mechanism between aluminum dross and heavy metals, and capture mechanism of iron for heavy metals in the future.