A study on selective leaching of heavy metals vs. iron from fly ash

Incinerator plant waste streams contain underexploited metals originating from households and industrial operators. The objective was to investigate the dissolution behavior of fly ash in: 0.2–7 M HCl, 0.2–7 M H ₂ SO ₄ , 0.5–1 M oxalic acid, 0.5–4 M citric acid, 0.5–7 M acetic acid, water, and ethaline, at 33 °C during 24 h leaching. The capability for high metals’ extraction of Zn, Cu, Ni, Pb, and Fe and management of the heavy metal Pb through leaching or precipitation was investigated. Selective leaching of valuable metals vs. Fe was observed. The target was to find a sustainable leaching method and provide a basis for further development of metal recovery from fly ash. The highest Zn extraction (> 80%) was achieved with 3 M H ₂ SO ₄ and 7 M HCl, which also dissolved Cu (> 87%) and Ni (> 65%). However, more dilute HCl, 0.2 M, was able to provide Zn, Cu, and Ni extractions of 43, 86, and 18%, respectively. Efficient Pb leaching was achieved in HCl—complete extraction was provided by 3 and 5 M HCl, whereas H ₂ SO ₄ did not extract Pb due to PbSO ₄ formation; similarly, in oxalic acid, Pb precipitated as PbC ₂ O ₄ . Ethaline could extract 50% of Pb with high selectivity towards Fe.