Abstract
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.
Highlights
A fluid catalytic cracking (FCC) catalyst is widely used in FCC units in petrochemical industries to convert crude oil into gasoline and other lighter fuel products, and its worldwide supply can reach 840,000 tons every year (Vogt and Weckhuysen, 2015)
The main components of Portland cement are 3CaO · SiO2 (C3S), 2CaO · SiO2 (C2S), and 3CaO · Al2O3 (C3A), which could react with H2O to form C-S-H, C-A-H, ettringite (AFt), and Ca(OH)2 to improve the strength of a brick, and the mechanisms of the reaction are shown in Equations (4)–(7): 3CaO · SiO2 + nH2O → xCaO · SiO2 · yH2O + (3 − x)Ca(OH)2 (4)
This paper focuses on utilizing a spent FCC catalyst as a partial replacement of a fine aggregate in the production of a NS-brick. 20% cement +Na2SO4 is proven to be an optimal binder system, which could improve the compressive strength up to 42.3 MPa
Summary
A fluid catalytic cracking (FCC) catalyst is widely used in FCC units in petrochemical industries to convert crude oil into gasoline and other lighter fuel products, and its worldwide supply can reach 840,000 tons every year (Vogt and Weckhuysen, 2015). A spent FCC catalyst has been identified as a hazardous waste HW50 according to the Chinese National Directory of Hazardous Wastes (HW, 2016), due to excessive contents of heavy metals. A spent FCC catalyst has widely been explored as a replacement of the raw materials in the production (laboratory scale) of a cement mortar (Al-Jabri et al, 2013; Payá et al, 2013), a concrete (Neves et al, 2015), ceramics (Ramezani et al, 2017), a geopolymer (Font et al, 2017), a zeolite (Ferella et al, 2019), an asphalt (Xue et al, 2020), and a brick (Taha et al, 2012)
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