Recovery of rare earths and aluminum from FCC catalysts manufacturing slag by stepwise leaching and selective precipitation

A stepwise leaching process was developed to recover rare earth elements (REEs) and aluminum (Al) from FCC catalysts manufacturing slag (FCC waste slag), with NaOH and HCl. The FCC waste slag was treated with 2.5 mol/L NaOH at 293 K for 0.5 h to convert 72.4% of Al into NaAlO2. Then, the caustic conversion residue was decompose using 3 mol/L HCl to control leaching pH at 1.5 under 293 K for 3 h. Leaching efficiencies of 94.2%, 98.6% and 98.8% for Al, La and Ce, respectively, were achieved. 98.7% purity of rare earths oxide (REO) product was obtained after further treatments including oxalic acid precipitation and calcination. The filter liquor of oxalic acid precipitation was recycled to acid leaching step to recover the remaining Al. 97.6% REEs and 98.4% Al were recovered in total. Furthermore, the polymerization of metal ions and silicates were found and confirmed by IR and XRD analysis.