Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet A bio-hydrometallurgical approach towards leaching of lanthanum from the spent fluid catalytic cracking catalyst using Aspergillus niger H.M. Mouna, Saroj Sundar Baral ⁎ Department of Chemical Engineering, BITS, K K Birla Goa campus, Pilani 403726, India ARTICLE INFO Keywords: Bioleaching Lanthanum Aspergillus niger Spent fluid catalytic cracking catalysts Organic acids ABSTRACT Spent fluid catalytic cracking catalysts (SFCCC) are the wastes generated from oil refineries, which accounts for 700,000–900,000 metric ton/year worldwide. Lanthanum, a short-term near critical element is the key com- ponent of SFCCC and accounts for 8170 mg/Kg. The current research intends to recover lanthanum from SFCCC by Aspergillus niger and compare the results with strong organic and inorganic acid leaching. Batch studies of one- step bioleaching (OSB) at 1, 3 and 5% pulp densities yielded 63%, 52%, and 33% lanthanum recovery, re- spectively. The decrease in leaching efficiency with an increase in pulp density may be attributed to the in- hibition effect of SFCCC on A. niger. Citric acid was the main lixiviant produced by A. niger, whose production was triggered by SFCCC at ≤1% pulp density. Acidic pH of the leaching medium suggested acidolysis as the key mechanism. Cell-free spent medium bioleaching (cfSMB) resulted in 30.8% leaching recovery, which was sig- nificantly less than the OSB. In chemical leaching, sulfuric and nitric acid showed 38% efficiency whereas oxalic acid showed 5% recovery. Hydrochloric, citric and gluconic acids resulted in 68%, 65%, and 64% leaching recoveries, respectively, which is nearly the same as that offered by OSB. OSB being the greener process, sug- gested for lanthanum leaching from SFCCC over other processes. 1. Introduction The demand for rare earth elements (REE) is increasing significantly as it is a prerequisite for manufacturing electric lights, magnets, bat- teries (Alonso et al., 2012), heavy fluoride glass, scintillators, hot cathode material in vacuum tubes, detectors of neutrons or gamma rays, carbon arc lamps, arc welding electrodes, cracking catalysts, steel alloys and hydrogen sponge alloys (Ferella et al., 2016). Rare earths are relatively abundant in the Earth's crust but discovered minable con- centrations are less common than for most other ores. World resources are contained primarily in bastnaesite, monazite and xenotime ores (Gupta and Krishnamurthy, 1992). Bastnaesite deposits in China and the United States constitute the largest percentage of the world's rare earth economic resources followed by monazite deposits (Hedrick, 2008). China leads the global supply. REE have been labeled as highly critical metals in terms of supply risk and energy economic importance in Europe and in USA (ERECON, 2014). Imports of REE into the USA increased by 6% in 2016 (Lee Bray, 2017). Around 55% of REE con- sumption in the USA as of the year 2016 is by catalysts, creating a scope for recycling spent catalyst for recovering REE. As of now, recycling is limited to batteries (Innocenzi and Vegliò, 2012) permanent magnets, glass craps (Jiang et al., 2005; Kim et al., 2011), lamp phosphors (Tunsu et al., 2014), phosphor powders (Yang et al., 2013) and fluorescent lamps. Lanthanum, a light rare earth metal, is expensive because of its scarce distribution and the difficulties associated with its extraction (Binnemans et al., 2013). Shortage of lanthanum supply due to limited primary sources, demands the need for secondary sources. In this re- gard, recovery of lanthanum and other rare earth elements from wastes would be of great interest (Chu, 2011; Qu et al., 2015). Fluid catalytic cracking catalyst constitute of rare-earth exchanged zeolite in a silica-alumina matrix and is used in petroleum refining in- dustries to convert crude oil into more valuable gasoline blend com- ponents since 1960's (Ferella et al., 2016; Rodríguez et al., 2013). Generation of spent fluid catalytic cracking catalyst (SFCCC) accounts for 700,000–900,000 metric ton/year worldwide (Ferella et al., 2016). Management of spent catalysts spatially and recycling of rare earth metals from spent catalysts have become a great challenge due to en- vironmental issues (Rodríguez et al., 2013). If the concentration of the https://doi.org/10.1016/j.hydromet.2019.01.007 Received 18 July 2018; Received in revised form 4 December 2018; Accepted 17 January 2019 Abbreviation: SFCCC, Spent fluid catalytic cracking catalyst; FFCCC, Fresh fluid catalytic cracking catalyst; OSB, One-step bioleaching; cfSMB, Cell-free spent medium bioleaching; TGA, Thermogravimetric analyzer; ICP-OES, Inductively coupled plasma optical emission spectrometry; EDS, Energy Dispersive X-ray spec- troscopy; XRF, X-ray fluorescence; SEM, Scanning electron microscope; BSFCCC, Bioleached SFCCC; BPLA, Biogenically produced leaching agents ⁎ Corresponding author. E-mail address: ssbaral75@gmail.com (S.S. Baral). Hydrometallurgy 184 (2019) 175–182 Available online 23 January 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved. T