Differential Behavior of Combustion and Gasification Fly Ash from Puertollano Power Plants (Spain) for Zeolite Synthesis and Silica Extraction

Coal gasification (IGCC) and pulverised coal combustion (PCC) fly ashes (FAs), obtained from two power plants fed with the carboniferous bituminous coal from Puertollano (Spain), were characterized. Both FAs were used as raw materials for zeolite synthesis by direct conversion and by alkaline fusion, and SiO2 extraction at laboratory scale. The Puertollano FAs are characterised by a high SiO2 content (59 %) with respect to EU coal FAs. The zeolite synthesis and SiO2 extraction yields obtained for the two types of FAs were compared. High zeolite synthesis yields were obtained from both FAs by using conventional alkaline activation. However, the SiO2 extraction yields were very different. In the IGCC FA the occurrence of Al2O3 only in the highly soluble glass matrix, limits drastically the SiO2 extraction with respect to the very high extraction yields obtained for the PCC FA. In the PCC FA, the occurrence of mullite (a highly insoluble Al-bearing phase) accounts for the trapping of Al2O3 and prevents the fast dissolution of Al2O3 when SiO2 is extracted. If both Al2O3 and SiO2 are extracted simultaneously, the extraction solution is saturated in Al2O3 and the SiO2 extraction is limited by the precipitation of zeolitic material in the extraction residue. The results of the zeolite synthesis from the Si-bearing extracts from both FAs demonstrated that high purity zeolites with high cation exchange capacity (CEC, between 4.3 and 5.3 meq/g) can be produced. Furthermore, the solid residue arising from SiO2 extraction is also a relatively high NaP1 zeolite product (CEC from 2.4 to 2.7 meq/g) equivalent to the direct conversion products. The zeolitic materials synthesised from both FAs by alkaline fusion showed an intermediate (between the high purity zeolites and the direct conversion products) zeolite content with CEC values from 3.4 to 3.7 meq/g. Low leachable metal contents were obtained from high purity A and X zeolites and zeolite material synthesised by alkaline fusion for PCC FA. Conversely, the high metal content leached from zeolites synthesized from IGCC FA limits the possible applications of these products. 2005 World of Coal Ash (WOCA), April 11-15, 2005, Lexington, Kentucky, USA http://www.flyash.info