## 3 Results
### Batch Extraction
#### 3.1.2 Aliquat 336  
Earlier, it was shown that Aliquat 336 has the highest separation factor (the highest selectivity) for Pr/Nd pair among other extractants (_SF_(Pr/Nd) \(\approx\) 2) [27]. However, it should be mentioned that QABs preferentially extract lighter elements, which makes them different compared to organophosphorus extractants (TBP and HDEHP). On the other hand, many aqueous soluble polyaminocarboxylates such as diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) bind \"heavy\" REEs (Tb-Lu) more efficiently than \"light\" REEs (La-Gd) [39], so addition of such acids as a complexing agents can increase the _SF_(Pr/Nd) values. (a) Extraction Isotherms for REEs by Aliquat 336 in The Presence of Ca(NO\({}_{3}\))\({}_{2}\)  
An extraction isotherm was constructed for the system \"0.5 mol\(\cdot\)L\({}^{-1}\) Aliquat 336 in dodecane/10% vol. 1-octanol--Pr(NO\({}_{3}\))\({}_{3}\) and Nd(NO\({}_{3}\))\({}_{3}\) + Ca(NO\({}_{3}\))\({}_{2}\) + 0.3 mol\(\cdot\)L\({}^{-1}\) HNO\({}_{3}\) \" (Figure 3). It is seen that the capacity of the organic phase is relatively low and is 7-10 g\(\cdot\)L\({}^{-1}\), which is consistent with the data obtained in the system containing ammonium nitrate as a salting-out agent [27]. With an increase in the content of the salting-out agent in the aqueous phase, the shape of the isotherms becomes steeper; the saturation of the organic phase is achieved with smaller amounts of REEs in the aqueous phase. Along with this trend, the separation factor of neodymium and praseodymium falls down. The highest value of the separation factor was found at a calcium nitrate concentration of 1 mol\(\cdot\)L\({}^{-1}\) and has a value  
Figure 2: Distribution ratios and separation factors for Nd(III) and Pr(III) in the system “1 mol\(\cdot\)L\({}^{-1}\)HDEHP in dodecane—50 g\(\cdot\)L\({}^{-1}\) REE(NO\({}_{3}\))\({}_{3}\) + HNO\({}_{3}\)”. of 2.0 (in the case of 2,5 mol:L\({}^{-1}\) Ca(NO\({}_{3}\))\({}_{2}\)_SF_(Pr/Nd) \(\approx\) 1.2), which is the same as the maximum value of _SF_(Pr/Nd) published in literature [27]. Further decrease of the salting-out agent content leads to a dramatic decrease in the efficiency of REE extraction (\(D\) values are less than 0.1). As indicated above, in order to achieve higher SF(Pr/Nd)s, additional experiments were performed to determine the effect of the water-soluble complexing agents (EDTA and DTPA). Initial concentration of the REE mixture was 0.01 mol:L\({}^{-1}\), and initial pH of the solution was 2.0-2.5. Addition of calcium nitrate as a salting-out agent resulted in precipitation of calcium polyaminocarboxylates. Thus, calcium nitrate was replaced with sodium nitrate. The obtained data are presented in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Na\({}_{2}\)EDTA, \%** & **20** & **40** & **60** & **80** & **100** \\ \hline \(D\)(Nd) & 0.07 \(\pm\) 0.01 & 0.06 \(\pm\) 0.005 & 0.03 \(\pm\) 0.002 & 0.02 \(\pm\) 0.002 & 0.01 \(\pm\) 0.001 \\ \hline \(D\)(Pr) & 0.15 \(\pm\) 0.02 & 0.14 \(\pm\) 0.005 & 0.08 \(\pm\) 0.005 & 0.04 \(\pm\) 0.002 & 0.016 \(\pm\) 0.002 \\ \hline \(SF\)(Pr/Nd) & 1.8 \(\pm\) 0.3 & 2.3 \(\pm\) 0.2 & 2.6 \(\pm\) 0.2 & 2.1 \(\pm\) 0.2 & 1.6 \(\pm\) 0.2 \\ \hline \(SF_{\rm max}\) & & & 2.6 \(\pm\) 0.2 & & \\ \hline
**Na\({}_{2}\)DTPA, \%** & **20** & **40** & **60** & **80** & **100** \\ \hline \(D\)(Nd) & 0.08 \(\pm\) 0.01 & 0.05 \(\pm\) 0.005 & 0.02 \(\pm\) 0.002 & 0.014 \(\pm\) 0.003 & 0.011 \(\pm\) 0.002 \\ \hline \(D\)(Pr) & 0.16 \(\pm\) 0.01 & 0.14 \(\pm\) 0.01 & 0.07 \(\pm\) 0.005 & 0.040 \(\pm\) 0.006 & 0.019 \(\pm\) 0.005 \\ \hline \(SF\)(Pr/Nd) & 2.1 \(\pm\) 0.2 & 2.7 \(\pm\) 0.2 & 3.6 \(\pm\) 0.3 & 2.9 \(\pm\) 0.3 & 1.8 \(\pm\) 0.2 \\ \hline \(SF_{\rm max}\) & & & 3.6 \(\pm\) 0.3 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distribution ratios and separation factors for the system “0.5 mol:L\({}^{-1}\) Aliquat 336 in dodecane/10% vol. 1-octanol—Pr(NO\({}_{3}\))\({}_{3}\), Nd(NO\({}_{3}\))\({}_{3}\) + 1 mol:L\({}^{-1}\) NaNO\({}_{3}\) + polyaminocarboxylate”.