Bioleaching of rare earth elements from spent fluid catalytic cracking catalyst using Acidothiobacillus ferrooxidans

Abstract

Acidothiobacillus ferrooxidans (A. ferrooxidans), an iron-oxidizing bacteria oxidizes Fe(II) to Fe(III), which is an excellent leaching agent to leach out rare earth elements (REEs) from spent fluid catalytic cracking catalyst (SFCCC). Batch leaching studies were carried out with two different types of A. ferrooxidans (unadapted (uAF) and adapted at 20% pulp density (PD) of SFCCC (aAF20)) and with two different initial concentrations of Fe(II) (2 and 8.84 g/L) in the leaching media. The rapid growth of A. ferrooxidans was observed within 24 h of incubation in the media. An increase in PDs resulted in an increase in pH and decrease in ORP, Fe(III) concentration suggesting inhibition of A. ferrooxidans growth at higher PDs. The amount of Fe(III) formed at any point of time in the leaching media followed the order aAF20 > uAF and 1% > 3%> 5,7%> 10%> 20% PDs. Higher leaching of La was observed at 1% PD while for Ce it was at 5% and 7% PDs. The maximum percentage of La and Ce leaching achieved was 83% and 23% respectively. This corresponds to 11.5 mg of La/gm of SFCCC, 0.211 mg of Ce/gm of SFCCC and these are of high economic importance. The amount of La and Ce bioleached is the highest reported from SFCCC to date. The competitive La leaching efficiency resulted by A. ferrooxidans compared to that of chemical leaching recommend bioleaching for La recovery from SFCCC. While citric acid is preferred for Ce extraction.