Statistical Modeling of Zr/Hf Extraction using TBP–D2EHPA Mixtures VAHID REZAEINEJHAD JIRANDEHI, DAVOUD HAGHSHENAS FATMEHSARI, SADEGH FIROOZI, MOHAMMAD TAGHIZADEH, and ESKANDAR KESHAVARZ ALAMDARI In the present work, response surface methodology was employed for the study and prediction of Zr/Hf extraction curves in a solvent extraction system using D2EHPA-TBP mixtures. The effect of change in the levels of temperature, nitric acid concentration, and TBP/D2EHPA ratio (T/D) on the Zr/Hf extraction/separation was studied by the use of central composite design. The results showed a statistically significant effect of T/D, nitric acid concentration, and tem- perature on the extraction percentage of Zr and Hf. In the case of Zr, a statistically significant interaction was found between T/D and nitric acid, whereas for Hf, both interactive terms between temperature and T/D and nitric acid were significant. Additionally, the extraction curves were profitably predicted applying the developed statistical regression equations; this approach is faster and more economical compared with experimentally obtained curves. DOI: 10.1007/s11663-012-9729-0 Ó The Minerals, Metals & Materials Society and ASM International 2012 I. INTRODUCTION THE separation/recovery of zirconium and hafnium is a major hydrometallurgical problem because of the similar physicochemical properties of these metals. Regarding the detrimental effect of hafnium on zirco- nium neutron absorbing properties, [1] many investiga- tions have been focused on Zr/Hf separation by solvent extraction technique. [2–7] Hafnium content of nuclear grade zirconium must be lower than 100 ppm. Thus, identification of the effective parameters in separation processes is a prerequisite for their successful and commercial exploitation. The parameters identified in previous research as significantly influencing the Zr/Hf extraction are acid, extractant and Zr/Hf concentrations, nature of diluents, additives, loading capacity, and temperature. [2,3,6,8] The proper choice of the value of these parameters is necessary if an effective Zr/Hf separation process is to be developed. There is an extensive literature on Zr/Hf, but the optimum level of these parameters cannot be reliably determined by reference to these studies. Hereafter is illustrated by the survey of literature on studies for Zr/Hf extraction/ separation employing different extractants. Taghizadeh et al. [7] studied the effect of acid type/ concentration and type of extractant, Cyanex 272 and D2EHPA, on the selective extraction of Zr over Hf employing the Taguchi method. They showed that Cyanex 272 can successfully separate Zr from Hf with a separation factor of 8.1 in a 2-M nitric acid medium. Moreover, Taghizadeh et al. [9] reported that the stoichi- ometric reaction in a Zr extraction system via Cyanex 272 can be presented as: ZrO NO 3 ð Þ 2 þ2Cyanex272 $ ZrO NO 3 ð Þ 2 :2Cyanex272 ½1 They also found that both Zr and Hf organometallic complexes, in the organic phase, are neutral complexes that formed through a solvation mechanism. [9] The solvation mechanism was also reported by Suryiachat and Distin [10] in the case of Zr chloride extraction by Cyanex 923. They proposed the Zr extraction reaction as: Zr Cl ð Þ 4 þ2Cyanex923 $ Zr Cl ð Þ 4 :2Cyanex923 ½2 Different temperatures have been employed in studies on Zr/Hf extraction and separation such as 297 K and 303 K (30 °C and 24 °C), [6] 293 K to 333 K (20 °C to 60 °C), [2] and 303 K to 333 K (30 °C to 60 °C). [8] However, some works conducted the experiments at room tempera- ture. [3,4,7] Furthermore, the values of nitric acid concen- tration that were employed in previous studies show a wide range, such as 0.5 to 3 M of nitric acid, [7] 3 M of nitric acid, and 3.5 M of NaNO 3 as a salting out agent. [4] The different range of values used or reported as optimum for the above-mentioned parameters cannot be fully explained, and furthermore, the optimum level of these factors might also be influenced by the level of the other important factors. In statistical parlance, there might be an interaction between effective parameters in such a process. A survey of previous literature on Zr/Hf separa- VAHID REZAEINEJHAD JIRANDEHI, M.Sc. Student, DAVOUD HAGHSHENAS FATMEHSARI and SADEGH FIROOZI, Assistant Professors, and ESKANDAR KESHAVARZ ALAMDARI, Associate Professor, are with the Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran. Contact e-mail: s.firoozi@aut.ac.ir MOHAMMAD TAGHIZADEH, Assistant Professor, is with NFCRS, Nuclear Science and Technology Research Institute, Tehran, Iran. Manuscript submitted March 31, 2012. Article published online September 25, 2012. 1262—VOLUME 43B, DECEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B