Utilization of Spent FCC Catalyst as Fine Aggregate in Non-sintered Brick: Alkali Activation and Environmental Risk Assessment

This study focuses on the recycling of a spent fluid catalytic cracking (FCC) catalyst to produce catalyst-based non-sintered bricks (CN-bricks) for the recovery of its aluminosilicate components and the solidification of heavy metals. The effects of the content of cement (10-20%), the proportion of FCC (10-40%), and the type of an activator (NaOH/Na2SiO3/Na2SO4) on the performance of a CN-brick were investigated in terms of the mechanical strength and leaching behavior. The results show that an optimal binder system of 20% cement + Na2SO4 could promote the compressive strength up to 42.3 MPa; the proportion of an optimal spent FCC catalyst of 20% could achieve the lowest porosity and water absorption. The microscopic mechanism of a cementitious process was analyzed by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), proving that C-S-H and ettringite (AFt) are the two main hydration products of a CN-brick. Na2SO4 is superior to NaOH or Na2SiO3 as an activator since Na2SO4 takes advantage of the aluminum-rich property of a spent FCC catalyst and specifically promote the formation of a needle-like AFt. Quantitative environmental risk assessment for the utilization of a CN-brick on roads was carried out based on the leaching test of a toxicity characteristic leaching procedure (TCLP), NEN 7371 maximum availability test, and the hazard Index (HI) identification, and a final HI 0.0045 (<1.0) indicates an acceptable risk for environment and nearby residents as CN-bricks are utilized on roads for 30 years.

Automated summary

This study focuses on recycling spent FCC catalyst to produce CN-bricks for recovering aluminosilicate and solidifying heavy metals. The optimal binder system and catalyst proportion were determined to improve mechanical strength and reduce porosity. Na2SO4 was found to be superior to NaOH and Na2SiO3 as an activator, promoting the formation of needle-like AFt. Environmental risk assessment showed that utilizing CN-bricks on roads for 30 years poses an acceptable risk to the environment and nearby residents.