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

Two French coal fly ashes (FAs), from Blénod 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 °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 Blénod FA at 30 or 50 °C. For this purpose, the time dependence of the faujasite (1 1 1) diffraction peak intensity and of the conversion rate into faujasite deduced from the N2 adsorption measurements was studied. The highest conversion rates are close to 20–25 wt.% and are reached at 30 or 50 °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.