Synthesis of faujasite from coal fly ashes under smooth temperature and pressure conditions: A cost saving process

Two French coal fly ashes (FAs), from Blenod and Bouchain thermal power stations, were studied. In particular, an X-ray diffraction analysis allowed to estimate their approximate mineralogical compositions, i.e. about 80-90, 10 and 5 wt.% of alumino-silica amorphous phase, mullite and quartz, respectively. A preliminary study was carried out in order to specify the best suited molar composition for the transformation of the FA into faujasite at 30 degrees C. This optimised composition, namely 1 SiO2 : 0.37 Al2O3 : 3.8 NaOH : 72.7 H2O, was used to determine the crystallisation kinetics of the Blenod FA at 30 or 50 degrees C. For this purpose, the time dependence of the faujasite (I 11) diffraction peak intensity and of the conversion rate into faujasite deduced from the N-2 adsorption measurements was studied. The highest conversion rates are close to 20-25 wt.% and are reached at 30 or 50 degrees C after about 20 or five days, respectively. Traces of the more stable gismondine zeolite co-crystallises systematically with faujasite. In all performed experiments, a type X faujasite is obtained, the corresponding small octahedral crystals being agglomerated with the residual FA. (c) 2007 Elsevier Inc. All rights reserved.