Initial non-radioactive tests to evaluate performance of the spectroscopic equipment under the flow conditions employed 30% TBP/n-docecane and five aqueous solutions containing variable nitric acid and neodymium nitrate concentrations were introduced into the extractor bank in a counter­current manner. The spectroscopic probes were positioned to measure the effluent from the extractor banks of the raffinate (aqueous) stream. Starting with a water wash and then proceeding with solutions 1-5 (compositions are given in Fig. 4.13) in sequential order, approximately 200 mL of each aqueous phase solution was delivered through the contactors, while the organic phase delivery was constant during the experiment. The Raman spectra collected during the experiment are shown in Fig. 4.13.

The nitrate and neodymium concentrations of the solutions were deter­mined using PLS analysis of the spectroscopic data. Figure 4.14 shows the plots of both the nitrate and neodymium concentrations as a function of experimental time from the Raman and vis-NIR data, respectively. The initial feed concentration is outlined in light grey, and the measured con­centration in the raffinate is indicated as dark grey symbols. As expected, the nitrate concentration in the raffinate (Fig. 4.14 a) nearly mirrors the

4.13 Raman spectra of aqueous raffinate solutions collected on-line during counter current flow extraction experiment showing variable nitrate response at 1050 cm-1.

concentration in the feed, attributed to the insignificant extraction of total nitrate into the organic phase. In contrast, the neodymium shows appre­ciable extraction when the total nitrate concentration exceeds approxi­mately 1 M (Fig. 4.14 b). This figure also depicts the noticeable mixing regions reflecting the consequent switching of the feed solutions; the mea­sured concentration of the analyte lags in time because of the time needed for the new solution to be integrated into the counter-current contactor bank.

The results of a batch contact distribution experiment for Nd(NO3)3 in variable nitrate with 30% TBP/n-dodecane is shown in Fig. 4.15. The measured concentrations for Nd3+ in the feed and raffinate are plotted in the figure at left, and the resulting distribution values for Nd (DNd) as a function of total nitrate concentration are shown in the figure on the right. Distribution values for Nd3+ depend linearly on the nitrate concentration

4.15

Batch contact distribution experiment for Nd(NO3)3 in variable nitrate (composition of solutions 1-5 are shown in Fig. 4.13) with TBP/ dodecane.

in the aqueous phase with the corresponding slope of nearly 3 (3.22) con­firming the extraction of Nd(NO3)3 by TBP. This verifies that the decrease of Nd optical signal observed in the raffinate stream (for solutions 4 and 5) is due to the enhanced Nd extraction leading to the depletion of the Np in aqueous phase and not due to the instrumental failure.