Extraction of alumina from fly ash by pyro-hydro metallurgical routes: A review

Fly ash is a resulting by-product formed due to the burning of pulverized coal on a steam-generating plant. Fly ash mainly contains alumina and silica, hence it can be an excellent secondary source for alumina. Today only 30% of global fly ash can be reused commercially, the remaining was dumped. This causes waste of resources and environmental pollution too. The alumina in the fly ash was in the form mullite phase which is a stable phase so decomposition of this phase is difficult. The mullite phase can be disilicated by calcination, roasting, etc which were pyro-metallurgical routes. Alumina can be extracted as aluminum hydroxide and other unstable forms from fly ash by hydrometallurgical routes. This process requires a huge amount of energy for reaction which is a major drawback of this process. Till now industrial extraction of alumina from fly ash cannot be feasible. Different researchers and scholars did various works to overcome this difficulty. This paper presents a review for extraction of alumina via these routes. It starts with different concepts used in extraction of alumina from fly ash. The middle portion presents the survey of work done by different scholars. The work finally concludes with future possibilties.