IGCC fly ash valorisation. Optimisation of Ge and Ga recovery for an industrial application

Fly ash (FA) disposal in landfills has a significant economic and environmental burden. A promising approach for FA valorisation aside from building purposes is the recovery of valuable elements since FA has a huge potential role as a source of valuable metals. In most cases, reported work in this area has been of theoretical research and further development work is needed. Germanium and gallium occur in relatively high proportions and mainly as water soluble and oxide species, respectively in the Puertollano Integrated Gasification Combined Cycle (IGCC) fly ash. Both the speciation and concentrations of Ge and Ga in this IGCC FA allowed starting research for developing extraction methods. High but not regular extraction yields were achieved in previous investigations. The aim of this research work is focused on the optimisation of Ge and Ga extraction conditions from IGCC FA for its feasible industrial application regardless the operational conditions of the IGCC plant. To this aim, first the daily, monthly, and yearly variability of Ge and Ga concentrations in IGCC fly ashes were determined. Samples were taken during almost 10 years. Then a number of extraction tests were carried out using different extractants to cover a wide range of extraction conditions (acid, alkaline, complexant, reducing and oxidising), on IGCC FA samples produced under different operating conditions. High and regular Ge extraction yields (up to 83% after 2 h leaching period) may be achieved using a weak acid solution. Gallium yielded up to 70 and 64% extraction after a 24 h leaching period using a weak acid and alkaline solution, respectively.