Solvent extraction of tantalum(V) from aqueous sulphate/fluoride solution using trioctyl phosphine oxide in MIBK O. Sanda a, ⁎, E.A. Taiwo b a Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria b Department of Chemical Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria abstract article info Article history: Received 11 April 2012 Received in revised form 11 August 2012 Accepted 12 August 2012 Available online 21 August 2012 Keywords: Solvent extraction Tantalite Synergism PIXE Tantalite–columbite with a combined Ta–Nb content of 33% was leached using 20 M hydrofluoric acid. The acidity of the leach was adjusted with sulphuric acid and the tantalum–niobium content was extracted using methyl isobutyl ketone (MIBK) and trioctyl phosphine oxide (TOPO), with kerosene and xylene as dil- uents. Stripping and precipitation of the extracted species was performed using 30% V/V ammonia solution. Both tantalum and niobium were extracted to different extents, with greater degree of extraction observed for tantalum. The extraction of tantalum was found to be dependent on the concentration of sulphuric acid added. For systems involving TOPO as extractant, kerosene proved to be a better diluent than xylene. Syner- gism was observed with respect to tantalum extraction when 20 g/l solution of TOPO in MIBK was used as the extractant. The number of extraction stages can be greatly reduced using a TOPO — MIBK blend in place of MIBK or TOPO only. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Liquid–liquid extraction is currently the most effective and the only practical way of separating tantalum and niobium to obtain high purity products such as oxides or complex fluorides used as pre- cursors for the production of pure metal powders, carbides, and other products. The industrial process involved in the extraction of tanta- lum uses methyl isobutyl ketone (MIBK) as the extractant, although the use of other solvents such as 2-octanol (Agulyansky et al, 2004; Mayorov and Nikolaev, 2002; Zhu and Cheng, 2011), cyclohexanone (CHN) and tributyl phosphate (TBP) (Zhu and Cheng, 2011), and ter- tiary amines (El Hussaini and Rice, 2004) have been reported. Most of these are however, built around single solvent systems which are usually multistage operations with large solvent inventory. Various investigations on the solvent extraction of metals using a blend of organic solvents have been carried out by different re- searchers. The extraction of zinc and cadmium from chloride solution by mixtures of a primary amine and Bis(2,4,4-trimethylpentyl) phosphinic acid (CYANEX 272) was studied by Jia et al. (2003). The solvent extraction of thorium (IV) and uranium (VI) was investigated by Meera and Reddy (2004) using mixtures of para-substituted 1-phenyl-3-methyl-4-aroyl-5-pyrazolones and crown ethers. Similar work was also done on lanthanoids (Ensor and Reynolds, 1989; Pavithran et al, 2003; Reddy et al, 2006); alkaline earth metals (Chiarizia, et al, 2003; McAlister et al, 2002); vanadium (Remya et al, 2005) and copper (Torkestani et al, 1996). In all the works ex- amined, multiple solvent systems were found to exhibit synergism with respect to the species being extracted. Synergism is the phe- nomenon in which two extractants taken together extract a metal ion species with much higher efficiency as compared to the additive effect of these extractants. The synergistic coefficient (S C ) may be described by: S C ¼ log D 12 D 1 þ D 2   ð1Þ where D 1 , D 2 and D 12 are the distribution coefficients of a metal ion with two extractants taken separately and with the mixture of the two extractants, respectively. The extraction is synergistic when S C is greater than zero and antagonistic when S C is less than zero. So far, most of the documented works on tantalum extraction have been based on single solvent systems (Agulyansky et al, 2004; El Hussaini and Rice, 2004; Mayorov and Nikolaev, 2002). The docu- mentation on multi-solvent systems showed that extraction of the desired substances is greatly enhanced. Since many of the solvents employed for uranium and rare earths also extract tantalum, it would be desirable to investigate the extraction of tantalum in a multi-solvent system. This work examines the possibility of a syner- gistic solvent extraction system development for tantalum/niobium separation using MIBK and trioctyl phosphine oxide (TOPO) as the extractants with the purpose of reducing the number of extraction stages required for this separation. Hydrometallurgy 127–128 (2012) 168–171 ⁎ Corresponding author. Tel.: +234 8027910255. E-mail addresses: osanda@cerd.gov.ng (O. Sanda), etaiwo@oauife.edu.ng (E.A. Taiwo). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.08.006 Contents lists available at SciVerse ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet