Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng A simple, efficient and selective process for recycling La (and Al) from fluid cracking catalysts using an environmentally friendly strategy S. Maryam Sadeghi a , João Jesus a , Edgar Pinto b,c , Agostinho A. Almeida b , Helena M.V.M. Soares a, ⁎ a REQUIMTE/LAQV, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal b REQUIMTE/LAQV, Departamento de Ciências Químicas, Faculdade de Farmácia, Universidade do Porto, Portugal c Departamento de Saúde Ambiental, Escola Superior de Saúde, P.Porto. CISA/Centro de Investigação em Saúde e Ambiente, Rua Dr. António Bernardino de Almeida, 400, 4200-072 Porto, Portugal ARTICLE INFO Keywords: Fluid cracking catalysts Flexible and universal process Nearly closed process Recovery of La Recovery of Al ABSTRACT Spent fluid cracking catalyst (FCC) is an abundant waste material derived from oil refining processes and notably characterized by its content in rare earth metals, such as cerium (Ce) and lanthanum (La). In this work, it was our main aim to develop a simple but effective flowsheet, based on a single acid assisted leaching step followed by two consecutive precipitation steps, for recovering La with high purity from the FCCs. Firstly, three (conven- tional, ultrasound- and microwave-assisted) leaching strategies were tested using two acids (sulphuric acid, H 2 SO 4 , and hydrochloric acid, HCl). Microwave-assisted leaching was revealed to be the most efficient [(99.4 ± 0.9)% of La] and fastest leached (1 cycle of 90 s) strategy using a lower concentration of acid (1 M HCl) and low liquid-solid ratio (L/S = 5). Subsequently, a sequential selective alkaline and oxalate precipitation was capable of producing an aluminium hydroxide with 88.7% of purity by increasing the pH up to 6 and a highly pure (99.7%) salt of lanthanum oxalate (which can be calcinated into a reusable lanthanum oxide) using a reduced oxalate concentration. The proposed process is independent of the initial Al concentration present in the hydrochloric acid FCCs leachates and widely applicable (for [La] > 0.04 M, complete La precipitation can be achieved regardless of its initial concentration using a molar [oxalate]/[La] ratio of 2). Moreover, it is sig- nificantly simpler and faster than existing methods and minimizes the consumption of energy and reagents to a bare minimum, with accompanying cost reduction and environmental benefits. 1. Introduction Fluid cracking catalysts (FCCs) are used in the oil industry to break down, i.e., to crack large molecules into smaller hydrocarbons of in- terest, to produce gasoline and other products. Since 2014, the world supply accounted for 840 thousand metric tons of FCC (Ferella et al., 2019) with an estimated annual increase of 5%. As a result of this an- nual production, about 80 thousand tons of waste slag in China (Wang et al., 2017a) were produced every year while the use of FCC resulted in an estimated 20 thousand tons of spent FCCs worldwide (Zhao et al., 2017). In terms of metal(loid)s composition of the fresh FCCs, the following average values can be considered: 141 g/kg of aluminum (Al), 260 g/kg of silicon (Si), 2.7 g/kg of lanthanum (La) and 3.4 g/kg of cerium (Ce), among other minor components, such as vanadium (V < 33 mg/ kg) and nickel (Ni; 31 mg/kg) (Ferella et al., 2016). Spent FCC, by com- parison with fresh FCC, has increased Ni (3930 mg/kg) and V (1455 mg/kg) content (Ferella et al., 2016) as a result of the impurities accumulated during the catalytic reactions. Nowadays, the main options for handling spent FCC is their disposal in landfill or reuse as a cement additive or as catalyst in other reactions while the recovery of the rare earth metals (REEs) contained within is less frequent (Ferella et al., 2016). It is estimated that 17,800 tons of lanthanum oxide (La 2 O 3 ) were used for FCC production in 2008 alone, which represents 46% of its worldwide use (Akah, 2017). Considering the large volume of FCC waste generated on a yearly basis as well as its content of REEs, which are critical raw materials of strategic im- portance for the EU (EU Commission, 2017), recycling this material may be a relevant secondary source of La and Ce. However, these metals should be recovered efficiently (with high yield) at high purity in order that they can be truly returned to the economy and, thus, meet the requirements of the circular economy. The recycling of REEs from FCCs requires a leaching step, where metals are transferred into the liquid phase and a subsequent step (or https://doi.org/10.1016/j.mineng.2020.106375 Received 20 November 2019; Received in revised form 7 April 2020; Accepted 8 April 2020 ⁎ Corresponding author at: Departamento de Engenharia Química, Faculdade de Engenharia do Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal. E-mail address: hsoares@fe.up.pt (H.M.V.M. Soares). Minerals Engineering 156 (2020) 106375 0892-6875/ © 2020 Elsevier Ltd. All rights reserved. T