Pure zeolite synthesis from silica extracted from coal fly ashes

Abstract

Pure zeolites can be synthesised from silica extracted from fly ash by alkaline leaching. If the process is optimised the solid residue arising from this extraction may also contain a relatively high content of zeolitic material mixed with residual fly ash components. Both the pure and the impure zeolitic material have a high potential for application in waste‐water and flue gas‐cleaning technologies. The silica extraction potential of 23 European coal fly ashes covering most of the possible fly ash types is investigated in this study. Optimisation of leaching processes, by varying temperature, time and alkali/fly ash rates, permitted extraction yields up to 140 g of SiO2 per kg using a single step process, but the extraction yields may reach up to 210 g kg−1 by applying thermal pre‐treatments prior to the extraction. The solid residue arising from the silica extraction experiments shows a high NaP1 zeolite content. A high Si/Al ratio of the glass matrix, the occurrence of easily soluble silica phases in the original fly ash and a high reactive surface area were found to be the major parameters influencing silica extraction. High purity 4A and X zeolitic material was obtained by combining the silica extracts from the Meirama fly ash and a waste solution from the Al‐anodising industry. The results allowed conversion of the silica extraction yields to an equivalent 630 g of pure 4A–X zeolite per kg of fly ash with a cation exchange capacity of 4.7 meq g−1.