Solidification and stabilization of spent catalyst from catalytic cracking in fludized bed-fly ash with portland cement

Abstract

The spent catalyst from fluidized-bed catalytic cracking unit (FCC) is waste material since it is a waste from oil refinery. The use of spent FCC catalyst in construction ash been proposed recently as it possesses pozzolanic activity and its heavy metal can be stabilized in the structure. The research is aimed to investigate the mechanism that stabilizes the heavy metal from FCC paste (CSP), fly ash paste (CFA) and FCC and fly ash paste (CSFA). Their microstructure developments were examined by X-Ray diffraction and scanning electron microscope (SEM). The pozzolanic reaction was monitored through calcium hydroxide consumption using XRD. The porosity and the leaching of heavy metals from the samples were measured in parallel with other tests. The experiment used sand to binder ratio of 2.75:1 and water to binder ratio of 0.55:1. Phase analysis by XRD reveals that no heavy metal phase exists (except Fe), suggesting that heavy metals from CSP, CFA, and CSFA do not form a new phase. This is because the heavy metals exist inside C-S-H structure of cement paste which can not be detected by XRD or all of metals can be dissolved out since pH in cement paste can reach to 12 within a few hours of mixing. The leaching test shows that the leaching rate at later age is much lower than at early age. As shown by the consumption of calcium hydroxide, the presence of FCC and fly ash prolongs the hydration process. As a result, there is less hydration product at early age to stabilize heavy metals. The hydration rate increases significantly at later age due to pozzolanic action of FCC and fly ash. Consequently, the heavy metals can be encapsulated inside these products. The permeability result also confirms that dense structure as the total porosity reduces significantly. It is a greater decrease for CSP 15 paste and CFA 20 paste but only 5-10% of CSFA pastes.