Extraction of alumina from coal fly ash using an acid leach-sinter-acid leach technique

• Published in: Hydrometallurgy Vol. 157; pp. 348 - 355
• Main Authors: Shemi, A., Ndlovu, S., Sibanda, V., van Dyk, L.D.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-10-2015
• Subjects: Acid leaching; Aluminium; Coal fly ash; Sintering; Coal fly ash; Acid leaching; Aluminium; Sintering
• ISSN: 0304-386X; 1879-1158
• DOI: 10.1016/j.hydromet.2015.08.023

In this paper, a two-step acid leach process for the extraction of aluminium from coal fly ash has been conducted using an acid leach-sinter-acid leach technique. The technique was formulated based on the dissimilar response characteristics of the amorphous and mullite alumina phases present in coal fly ash. The amorphous phase is reactive and soluble in inorganic acids while the mullite phase is refractory, not amenable to inorganic acids, and requires phase transformation before leaching. Sintering of the coal fly ash residue from the first leaching stage was carried out using a mixture of CFA with calcium carbonate. Results presented in this paper show that, sintering of coal fly ash residue from the first leaching stage successfully transformed most of the mullite phase alumina into a leachable plagioclase phase which completely dissolved in sulphuric acid during the second leaching stage. The results further show that the two dissimilar alumina phases can be processed separately by a novel acid leach-sinter-acid leach technique for alumina extraction from coal fly ash as investigated in this work. This study has also revealed that by employing this technique, coal fly ash can be optimally leached. The implications of the findings are that this could provide an alternative economic process for recovering alumina from readily available secondary resources such as coal fly ash.
•The process of alumina extraction from CFA involved three stages; acid leach, sinter, and acid leach.•The first leaching step extracted up to 89.3% aluminium from the amorphous phase in raw CFA.•The second leaching step extracted up to 84.3% aluminium from the mullite phase in residue-CFA.•A combination of the first and second leaching steps extracted a total of 88.2% aluminium from CFA.•Sintering of coal fly ash residue from the first leaching stage was carried out using mixture of CFA with calcium carbonate.