Recovery of Mo and Ni from spent acrylonitrile catalysts using an oxidation leaching–chemical precipitation technique

An oxalic acid (H2C2O4) and hydrogen peroxide (H2O2) leaching–chemical precipitation process was developed to recover valuable metals from the spent acrylonitrile catalysts that primarily contain Mo, Ni, Fe and Bi. H2C2O4 and H2O2 leaching studies indicated good leaching efficiencies for Mo and very low efficiencies for Ni because of the formation of soluble molybdenum oxalate complexes and a nickel oxalate (NiC2O4) precipitate, which was separated by filtration. Under the optimal leaching conditions (H2C2O4 concentration of 1.25 mol/L, H2O2 concentration of 0.20 mol/L, leaching temperature of 50 °C, liquid-to-solid (L/S) ratio of 20 mL/g and leaching time of 2.5 h), 99.7% of Mo was dissolved into the liquid phase, and 98.4% of Ni remained in the residues. For purification, the molybdenum oxalate complexes in the leaching liquor were broken down by adding the emulsion of Ca(OH)2 to form calcium molybdate (CaMoO4) and calcium oxalate (CaC2O4) precipitates; molybdenum trioxide (MoO3) was then recycled, followed by processes of acid dissolution, ammonium molybdate deposition, and calcining. The NiC2O4 in the leaching residues was dissolved and converted into soluble nickel-oxalate-ammonium complexes by adding ammonium hydroxide (NH4OH), and pure NiC2O4 powders were obtained from the filtered liquor through an ammonia stilling process after filtration. In this case, 95% of the Mo and 97% of the Ni in the spent catalyst were recovered as MoO3 and NiC2O4 with purities of 97.88% and 99.91%, respectively. Based on the results of the present study, a tentative process flow sheet is proposed.