Microwave irradiation for the recovery enhancement of fly ash components: Thermodynamic and kinetic aspects

Abstract

This study investigated the use of microwave-assisted leaching to recover the vanadium (V) and nickel (Ni) of fly ash collected from power plants. The fly ash sample was characterized using XRF, ICP-OES, powder X-ray diffraction, FESEM, and a laser particle size analyzer. The sample was found to be valuable as well as hazardous due to the presence of a considerable amount of base and heavy metals with an 80% passing particle size of 70 micrometers. Conventional leaching tests were performed using sulfuric acid (H2SO4), which resulted in 91.2% V, 80.5% Ni, and 57.6% Fe recoveries. Full recovery of the aforementioned components was achieved by adding 10% v/v hydrogen peroxide (H2O2). Microwave-assisted leaching showed promise, with comparable recoveries for Ni and V (complete recovery) at a microwave power of 700 W and a retention time of 30 mins, without the addition of any chemical additive. The study provided thermodynamic and kinetic justification for the efficient dissolution of V and Ni, which occurs via rapid boiling of the sulfuric acid solution (19.5% v/v) overcoming the chemical and liquid diffusion kinetic barriers. This work concludes that microwave-assisted leaching is an effective method for recovering heavy metals from fly ash samples as a complex solid waste.