Fly ash which has been separated from the flue gas stream as a result of fossil fuels combustion constitutes a huge amount of waste generated worldwide. Due to environmental problems, many directions of their rational use have been developed. Various attempts to convert fly ash into sorption materials, mainly synthetic zeolites, are conducted successfully. In this paper, an attempt was made to convert fly ash from lignite combustion from one of the Polish power plants, using alkaline hydrothermal synthesis. The primary phases in the fly ash were: quartz, gehlenite, mullite, hematite, feldspar, lime, anhydrite, occasionally grains of ZnO phase and pyrrhotite, glass and unburned fuel grains. As a result of hydrothermal synthesis a material containing new phases – pitiglianoite and tobermorite was obtained. Among the primary ash constituents, only gehlenite with an unburned organic substance, on which tobermorite with crystallized pitiglianoite was present. As a result of detailed testing of products after synthesis, it was found that among the tested grains:
• two populations can be distinguished – grains containing MgO and Fe2O3 as well as grains
containing Fe2O3 or MgO or containing none of these components,
• the main quantitative component was pitiglianoite,
• pitiglianoite was present in larger amounts in grains containing Fe2O3 or MgO or in the absence of both components than in grains in which Fe2O3 and MgO were found.
The results of the study indicate that in post-synthesis products, the contribution of components were as follows: pitiglianoite – 39.5% mas., tobermorite – 54% mas., gehlenite – 3% mas. and organic substance – 3.5% mas.
The paper presents the results of hydrothermal zeolitization of fly ash from hard coal combustion in one of the Polish power plants. The synthesis was carried out using various NaOH fly ash mass ratio (3.0, 4.0 and 6.0) and the effect of NaOH concentration in the activating solution on composition of synthesized sample was tested. The process was carried out under the following permanent conditions temperature: 90°C, time – 16 hours, water solution of NaOH (L)/fly ash (g) ratio – 0.025. In the studied fly ash the dominant chemical components were SiO2 and Al2O3, while the main phase components were mullite, quartz and hematite, and a significant share of amorphous substance (glass and unburnt organic substance). After hydrothermal synthesis, the presence of unreacted fly ash phases was found in the products, as well as new phases, the quality and quantity of which depend on the NaOH to fly ash mass ratio used for synthesis:
for ratio 3.0 – Na-LSX type zeolite and hielscherite,
for ratio 4.0 – Na-LSX type zeolite, hielscherite and hydrosodalite,
for ratio 6.0 – hydrosodalite and hielscherite.
The grains in all products of synthesis are poly-mineral. However, it was found that the new phases, overgrowing the unreacted phase components of fly ash, crystallize in a certain order. Hielscherite is the first crystallizing phase, on which the Na-LSX type zeolite crystallizes then, and the whole is covered by hydrosodalite. In the products of synthesis, the share of sodium-containing phases (the Na-LSX type zeolite and hydrosodalite) increases with the increasing concentration of NaOH in the solution used for the process.