Resultado final
Around 60Mt of coal fly ash is produced annually within the EU by coal combustion. This fly ash is utilised for about 50% in the cement and concrete industry and in civil construction works. Both applications are of low or even negative economic value. The other 50% of fly ash produced is simply dumped, a practice which is becoming more and more expensive. Actually, this does not comply with the present ideas on sustainable -economically driven management of raw materials and environment.
The main objective of the SILEX project was to indicate whether the fly ashes would serve as sources of silicium to be used for the production of pure (fly ash based) zeolites and if it is so then to indicate whether this or the extraction would be carried out under realistic process conditions and at a competitive price.
Results:
The amount of silicon extracted (as Si free in the reaction solution, thus available for converting into pure zeolites) depends on many factors. The following ones are the most important of them:
- The total amount of Si in a fly ash;
- The amount of Si available in the glass phase + opaline silica;
- The ratio SiO(2)/Al(2)O(3) should be 2,4 or higher (in order to prevent the synthesis of zeolites during the extraction process);
- Extraction conditions in general.
It is known (and indicated in the study carried out by SILEX of 23 fly ashes) the fly ashes can differ considerably in their chemical composition and physical parameters. Their varying technical and environmental quality has as consequence that not all fly ashes are suitable for the conversion into zeolites. With other words the parameters cited here above have consequences for the marketability of fly ashes as raw materials in particular at assessing their suitability for the market segment of fly ash based zeolites (in the future).
Therefore the Checklist on Suitability of Fly Ashes for the Conversion into Zeolites as produced in the SILEX project is of great value for the selection of the suitable ones. This is a remarkable step forward in strengthening the position of fly ashes in the market place for secondary raw materials.
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