The aqueous phase, consisting of 16 g.L-1 Zr (0.18 M), 0.36 g.L-1 Hf (0.002 M), and 98 g.L-1 H2SO4
(1.0 M), was contacted with an organic phase of Shellsol 2325 + 1-octanol (5 vol%) and various D2EHPA concentrations between 6.6 and 330 g.L-1. While the work by Wang & Lee (2016) served as the guide for the aqueous phase concentrations used in this study, the low E:M ratio of their study was not used due to the potential of the organic phase quickly becoming saturated in an MBSX column [23]. To avoid such saturation, it was desired to increase the D2EHPA concentration without sacrificing selectivity. Furthermore, the study on the extraction of aroma compounds by Baudot et al. (2001) clearly illustrates the importance of the partition coefficient of a species in the mass transfer of an MBSX process, therefore it was vital to determine the partition coefficients of Zr and Hf. According to Baudot et al. (2001), KOV (as described in Section 2.6.2, Table 2-3) of a species was found to increase with an increase in the partition coefficient, and that for species with a partition coefficient lower than 20, KOV seemed to reach a plateau at shell- side velocities higher than approximately 1.1 cm.s-1 [18]. Therefore, the LLE experiments were designed to answer three questions: Firstly, can the D2EHPA concentration be increased while maintaining selectivity toward Hf? Secondly, what is the highest separation factor that can be achieved at higher D2EHPA concentrations? Thirdly, what are the partition coefficients of Zr and Hf at these higher concentrations?
50 During the LLE work, the D2EHPA concentration was varied between 6.6 and 330 g.L-1 (0.02 to 0.95 M, E:M ranged from 0.11:1 to 5.8:1). At the low end of the concentration range, the D2EHPA concentration corresponded to a severely carrier-limited equilibrium, while at the high end, the concentration corresponded to an almost (5.8:1 instead of 6.0:1) stoichiometric ratio, as given by R 2-3, (the most commonly accepted writing of R 2-3 specifies the dimeric form of D2EHPA as (HA)2, which translates to a stoichiometric ratio of six D2EHPA monomers per molecule of metal).
𝑍𝑟(𝐻𝑓)𝑎𝑞4++ 3(𝐻𝐴)2 𝑜𝑟𝑔 ⇔ 𝑍𝑟(𝐻𝑓)𝐴4(𝐻𝐴)2 𝑜𝑟𝑔+ 4𝐻𝑎𝑞+ R 2-3
The unprocessed LLE data, as well as the necessary background to the calculations is provided in Appendix B1. Figure 4-1 illustrates the extraction of Zr and Hf as a function of the D2EHPA concentration. Throughout the experimental range, the extraction of both species increased relatively uniformly. As the available D2EHPA increased, so too did the effective solubility of the metal in the organic phase. It is also evident that, while Hf was preferentially extracted throughout the entire range, the co-extraction of Zr was significant throughout the range as well. The extraction of Zr and Hf increased, from 4.2% and 4.6% at a D2EHPA concentration of 6.6 g.L-1, to 99.6% and 99.9% at a D2EHPA concentration of 330 g.L-1, respectively. The high co-extraction observed, would pose a significant concern for the efficiency of an industrial-scale separation process, by placing possible stress on downstream units, either resulting in unmet purity specifications, or requiring larger and more expensive units to ensure the required purity.
Figure 4-1: The effect of D2EHPA concentration on the extraction of Zr and Hf. [Zr] = 16 g.L-1, [Hf] = 0.36 g.L-1, [H2SO4] = 98 g.L-1, [D2EHPA] = 6.6 – 330 g.L-1, [1octanol] = 41 g.L-1 in Shellsol 2325. Agitation = 350 RMP, time = 60 minutes.
The separation factor between Hf and Zr (Eq. 2-1), is illustrated in Figure 4-2 as a function of the E:M ratio, where it can be seen that the separation factor increased with increasing E:M ratios up to a ratio of 4.1:1. The lowest separation factor of 1.2 was obtained at the lowest E:M ratio of 0.11:1, which increased to a maximum of 7.5 at an E:M ratio of 4.1:1. At the highest E:M ratio of
51 5.8:1, a significant decline in the separation factor (3.9) was observed. The separation factor of 7.5 at an E:M ratio of 4.1:1 (0.73 M or 235 g.L-1 D2EHPA) was in the same order as the maximum separation factors of 8.8 reported by Wang & Lee (2016) [23] and 7.6 reported by Lee et al. (2015) [16], although the E:M ratio in the present study was significantly higher. The difference could likely be due to the different experimental conditions, specifically the lower metal concentration used in their studies, as the hydrolysis of Zr and Hf has been shown to not only be concentration- dependent but also strongly influence the extraction equilibrium [23]. The decrease in the separation factor at higher E:M ratios have also been observed by Banda et al. (2013) [13], Lee et al. (2015) [16], and Wang & Lee (2016) [23], who have attributed this decrease to the relatively high amount of carrier indiscriminately binding to any metal ions remaining in the aqueous phase, thereby negating any efforts at separation.
Figure 4-2: The effect of D2EHPA concentration on the separation between Zr and Hf. [Zr] = 16 g.L-1, [Hf]=0.36 g.L-1, [H2SO4] = 98 g.L-1, [D2EHPA] = 6.6 – 330 g.L-1, [1-octanol] = 41 g.L-1 in ShellSol 2325. Agitation = 350 RMP, time = 60 minutes.
While the separation factor obtained in this study was similar to other Hf-selective D2EHPA- H2SO4 systems that have been listed in Table 2-1, it was significantly lower than the separation factor (24) of the Cyanex 272-H2SO4 system that was reported by Lee et al. (2015) [16] . While Cyanex 272 would appear superior to D2EHPA in terms of the separation that can be achieved, the high acid concentration required (6.0 M) for the Cyanex 272-H2SO4 system would require specialist materials capable of withstanding such aggressive conditions. According to Banda &
Lee (2015), the separation factors of the commercial MIBK and TBP processes are approximately 7 and 10, respectively [6]. This shows that the separation factor of 7.5 obtained in this study could be sufficient for an industrial-scale SX process, and hence for the purpose of this study.
The effect of the D2EHPA concentration on the partition coefficients (Eq. 2-2) of Zr and Hf is illustrated in Figure 4-3. The partition coefficients of Zr increased from 0.04 at an E:M of 0.11:1
52 to 232 at an E:M of 5.8:1, while that of Hf increased from 0.05 to 745 over the same E:M range.
The partition coefficients of Zr and Hf at the point of best separation at an E:M of 4.1:1 were 21 and 156, respectively.
Figure 4-3: The effect of D2EHPA concentration on the partition coefficients of Zr and Hf. [Zr] = 16 g.L-1, [Hf] = 0.36 g.L-1, [H2SO4] = 98 g.L-1, [D2EHPA] = 6.6 – 330 g.L-1, [1-octanol] = 41g.L-1 in ShellSol 2325. Agitation = 350 RMP, time = 60 minutes.
Log (P) vs. Log [(D2EHPA)2] plots were constructed for Zr and Hf, and the slopes for both Zr and Hf were determined to be approximately 4, indicating that at high metal concentrations or high levels of organic phase saturation the stoichiometric number of D2EHPA dimer molecules required in the extracted complex might be higher (4 vs. 3) than is described by R 2-3. This could result in lower than expected extraction values for a process, due to unexpectedly high usage of the carrier, which could, however, be mitigated by using a higher carrier concentration or larger phase ratio. However, while further studies on both the reaction stoichiometry, and the complexation behaviour at various aqueous concentrations and organic phase saturations are recommended, these aspects fell beyond the scope of this study.
The liquid-liquid extraction experiments posed three questions which can now be answered as follows. Firstly, the selectivity toward Hf can be maintained, even improved, with increasing D2EHPA concentrations. Secondly, the maximum separation factor achieved was 7.5 at a D2EHPA concentration of 235 g.L-1 (0.73 M, E:M 4.1:1). Lastly, the partition coefficients of Zr and Hf at this concentration were 21 and 156, respectively. A D2EHPA concentration 0.72 M was subsequently used for all MBSX experiments.