Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet Recovery of germanium from leach solutions of fly ash using solvent extraction with various extractants Hossein Kamran Haghighi a , Mehdi Irannajad a, ⁎ , Agustin Fortuny b , Ana Maria Sastre c a Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran b Department of Chemical Engineering, Universitat Politècnica de Catalunya, EPSEVG, Av. Víctor Balaguer s/n, 08800 Vilanova i la Geltrú, Spain c Department of Chemical Engineering, Universitat Politècnica de Catalunya, ESTEIB, Av. Diagonal 647, 08028 Barcelona, Spain ARTICLE INFO Keywords: Germanium Alamine 336 Aliquat 336 Cyanex 923 Fly ashes ABSTRACT The solvent extraction of germanium and some heavy metals by commercial tri-octyl/decyl amine (Alamine 336), N-methyl-N, N-dioctyl chloride (Aliquat 336) and phosphine oxide (Cyanex 923) has been studied. In each extraction system, germanium was only extracted from a solution containing nickel, cadmium, cobalt, and zinc, which had a composition similar to gasification coal fly ash aqueous leach solutions. Under a comparable condition, the germanium extraction efficiency by the aforementioned extractants was in the order Aliquat 336 > Alamine 336 > Cyanex 923. The slope analysis method showed that 2 moles of Alamine 336 and Aliquat 336, as well as 4 moles of Cyanex 923, participated in the extraction of germanium. In amine extraction systems, tartaric acid was required as a complexant used to convert germanium to anionic species. As a result, the ratio of 2 (mole ration of tartaric acid to Ge) was required to complete anionic complexation. On the other hand, oxalic acid with a concentration of 0.1 M was chosen as a proper solvated complexant in the Cyanex 923 system. HCl solutions with concentrations of 1 and 2 M can properly strip germanium from the loaded Alamine 336 and Aliquat 336, respectively. In addition, 0.1 M NaOH was sufficient for germanium stripping from Cyanex 923. Consequently, it can be concluded that Aliquat 336 can be an economical and industry-friendly extractant for germanium solvent extraction from a mixed solution. 1. Introduction Germanium is a rare element, which does not occur in a specific ore body (Liu et al., 2017). It can be found as a sub-element in zinc, copper, aluminum, and coal fly ash (FA) resources (Arroyo et al., 2009; Harbuck et al., 1991). However, it has been reported that the main economic resources of germanium are zinc ores and coals (Depuydt et al., 2007; Nusen et al., 2015). Coal and its related products can be an important resource of germanium. The content of germanium in coal may be enriched several times in gasification coal fly ashes (GCFA). Among the elements existing in GCFA, germanium in the form of GeO 2 and GeS 2 can usually be leached with water (Arroyo and Fernández-Pereira, 2008; Font et al., 2005). The obtained aqueous leach solution from the leaching step is usually treated in a hydrometallurgical process to se- parate germanium from other impurities such as molybdenum, nickel, zinc and other heavy metals (Arroyo and Fernández-Pereira, 2008). Among hydrometallurgical processes, solvent extraction (SX) is a pro- mising separation technique due to its high efficiency and flexibility. Anion exchange and solvation extractants such as amines and phosphine oxides can be useful to separate anionic/neutral species from impurities in mixed solutions. In the current study, Alamine 336, Aliquat 336, and Cyanex 923 were used to separate germanium from aqueous solutions containing heavy metals. Alamine 336 (tri-octyl/ decyl amine) is an extractant with the anionic exchange mechanism, which can form organic complexes with anionic species. Anionic spe- cies such as molybdenum (MoO 2 -4 , HMoO -4 , and Mo 7 O 24 -6 )(Morı ́ s et al., 1999), cobalt (CoCl 4 2 - and CoCl 3 - )(Filiz et al., 2006), chro- mium(VI) (CrO 3 SO 4 2 - and HCrO 4 - )(Bachmann et al., 2010) and platinum (PtCl 4 -2 )(Reddy et al., 2010) have been effectively extracted by Alamine 336 at acidic pHs. Some extractants with a similar com- position to Alamine 336 such as N235 (tri(octyl/decyl) amine) and TOA (tri(octyl) amine) have been used to extract germanium from aqueous solutions. A SX process was developed to extract germanium and gal- lium anionic species from aqueous solutions of the zinc refinery residue leaching step using N235 mixed with TBP (Liu et al., 2017). The mentioned process was based on complexing germanium species with oxalates to form anionic oxalate‑germanium complexes ([Ge (C 2 O 4 ) 3 ] 2 - ) which could be extracted by N235, i.e. an anion exchange extractant. Furthermore, Arroyo and Fernández-Pereira (2008) ex- tracted germanium-catechol anionic species ([Ge(C 6 H 4 O 2 ) 3 ] 2 - ) from https://doi.org/10.1016/j.hydromet.2017.11.006 Received 15 August 2017; Received in revised form 7 November 2017; Accepted 15 November 2017 ⁎ Corresponding author. E-mail address: iranajad@aut.ac.ir (M. Irannajad). Hydrometallurgy 175 (2018) 164–169 Available online 17 November 2017 0304-386X/ © 2017 Elsevier B.V. All rights reserved. T