Processing of Coal Fly Ash for the Extraction of Alumina Values

Abstract

Coal fly ash is a lightweight incombustible particulate generated during coal combustion. Alumina content in the fly ash is present in the mullite phase, and its high stability calls for alternative processing for conversion to reactive phase before leaching. Mechanical activation with sodium hydroxide was found unsatisfactory for the breakdown of mullite structure, whereas the mullite phase dissociated to sodium silicate and sodium aluminosilicate during the sodium hydroxide thermal treatment. Below 400 °C, hydroxycancrinite is formed, which leads to alumina loss, whereas above 600 °C, it dissociates to sodium aluminate and sodium silicate, facilitating the alumina extraction. Aluminum hydroxide was recovered by acid leaching followed by precipitation and calcination at 1000 °C to obtain gamma-alumina. The aluminum dissolution increased by incorporating thermal treatment from 5 to 80% in muffle and 89.4% in microwave routes. The acid–base precipitation process resulted in the recovery of ~ 98% dissolved aluminum values in the boehmite phase, leaving the solution with less than 50 ppm Al content suitable for leaching application. Microwave processing was found to be a promising and alternative economical route for the alumina extraction from fly ash.