Immobilization of vanadium and nickel in spent fluid catalytic cracking (SFCC) catalysts-based geopolymer

Abstract

In recent years, the use of spent fluid catalytic cracking (SFCC) catalysts to prepare geopolymers has received attention from academic and industrial circles. However, SFCC catalysts have been classified as hazardous waste in China due to their vanadium and nickel content. In this paper, the immobilization rates of vanadium and nickel in geopolymers prepared with SFCC catalysts were studied through a toxicity characteristic leaching procedure (TCLP). The immobilization mechanism of nickel and vanadium during geopolymerization was investigated by XPS, SEM, XRD, and compressive strength measurements. The nickel and vanadium TCLP concentrations of the SFCC catalyst-based geopolymer are 0.032 mg/L and 10.63 mg/L, respectively. Nickel in the SFCC catalyst is partly dissolved and adsorbed by the negatively charged tetrahedral aluminum in the geopolymer. However, because vanadates formed under alkaline conditions are negatively charged and readily soluble in water, vanadium in the SFCC catalyst is neither immobilized by the charge balance nor effectively immobilized by physical encapsulation. Therefore, the use of SFCC catalysts to prepare geopolymers will lead to vanadium contamination in the environment. In order to clarify the maximum vanadium content in SFCC catalysts for preparing geopolymers without environmental risk, SFCC catalysts with different vanadium levels were prepared by leaching. It was concluded that when the vanadium content in the SFCC catalyst is lower than 1861.4 mg/kg, the vanadium content in the leachate is lower than 2 mg/L and the geopolymer can be considered a non-hazardous material.