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The aim of this study is to examine the effectiveness of low temperature alkaline hydrothermal treatment methods of Greek lignite fly ashes for the synthesis of specific zeolites suitable for environmental and agricultural applications. Relatively high silica fly ash samples from Meliti and Megalopoli Electric Power Stations, were tested as raw materials that underwent alkaline hydrothermal treatments by NaOH and KOH solutions, at about 100oC, with a constant ratio of raw material/alkaline solution 100gr/L. The applied techniques mainly aimed at the dissolution of Al-Si bearing phases of the fly ash and the subsequent precipitation of the zeolitic minerals. Both the initial materials and the final solid products were subjected to XRD and SEM study along with EPMA for the identification of their mineral phases. XRF was used for the determination of the materials’ chemical changes after the thermal alkali digestion. Furthermore, the CEC values of the original and the treated materials were determined. An 1M NaOH activation solution was used to produce Na-P1 zeolite (Na6Al6Si10O32•12H2O), a synthetic mineral with high ion exchange capacity, due to the substitution of Si(IV) by Al(III) in its structure, which results in an overall negative charge, leading to applications as ion exchange or molecular sieve. This zeolite has an affinity with some metal ions, generally found in acid mine drainage effluents, and may be used for remediation. As KOH solutions present lower conversion efficiency than the respective NaOH under the same temperature, the experimental conditions were modified and a 6N KOH solution as activation agent was used and the treatment period was extending to 72h and the K-bearing zeolite-F (KAlSiO41,5H2O) was synthesized. The Zeolite-F rich has a potential in soil amendments and slow release fertilizers. Moreover, those treated fly ashes exhibit more than ten times higher CEC values than the initial materials due to the presence of the zeolites. |
