Digesting high-aluminum coal fly ash with concentrated sulfuric acid at high temperatures

Abstract The decomposition of coal fly ash (CFA) digested with concentrated H 2 SO 4 at a high temperature has been proven to be an efficient way to leach aluminum in view of extracting useful metals from solid wastes. In this research the reaction between the CFA collected from Jungar in Inner Mongolia, China, and concentrated H 2 SO 4 was studied to get a high level of leaching efficiency, which is industrially applicable to extract aluminum. The microstructures, mineralogical transformation and mass transfer were investigated by means of X-ray diffraction, chemical composition analyses, polarization microscopy, electron probe micro-analysis, scanning electron microscopy, and transmission electron microscope observation. The surface of CFA spheres consisting mainly of crystalline phases, e.g., mullite, corundum, was found to be digested by concentrated H 2 SO 4 much easier than the vitreous phases. Most of the CFA spheres can be disrupted at high temperatures (T > 230 °C). The mineralogical transformation in this art follows a route as mullite, corundum + H 2 SO 4  → millosevichite (Al 2 (SO 4 ) 3 ) → leached metal ions + residues rich in SiO 2 . Base on the EPMA analyses the element of chromium that was distributed as substitutions in mullite or corundum was found to be enriched in form of chromium oxide. The maximum leaching efficiency of aluminum reaches 86.0% under the optimized conditions as T = 300 °C, t = 110 min, and molar ratio of H 2 SO 4 :CFA = 1.2. A modified shrinking core model was used to describe the diffusion controlled kinetics, and the apparent rate constant (k d ) were found to be 1.18–1.89 × 10 −3  min −1 at the temperature of 230–300 K. The activation energy of the internal diffusion-controlled reaction in the digesting reaction was evaluated to be 19.23 kJ mol −1 .