Electro-reductive stripping

Before electro-reductive stripping was investigated, a series of experiments was done to define the ferric reduction power and the kinetics of the above-mentioned unit as a function of temperature and electric power. The catholyte [350 ml] was a 2 M H2SO4 solution with 11.83 g/1 ferric iron. The impeller speed was 330 rpm. The results are shown in Figure 2. 

An option is to integrate an electro-reduction unit into the aqueous flow of an iron strip SX-unit with recycling of the strip solution until a high build-up of iron is obtained. To find out the influence of the initial ferrous concentration in the strip solution on the electro-reductive stripping efficiency, a series of experiments was performed identically with the above-mentioned electro-reductive stripping experiments, but with various initial concentrations of ferrous sulphate. The temperature was 50°C and voltage was set to 20 V. The results are shown in Table I. 

Microwave-activated voltammetry has been applied to the ferrocyanide/ferricyanide redox couple145, reduction of Ru(NH3)g 146, enhanced Pb02 electro-deposition, stripping and electrocatalysis147 and electrodehalogenation in non-aqueous media148. 

Figure 5 shows a comparison of U(IV) concentration profiles realized experimentally for the mixer settler and the pulsed column. The U(IV) profile in the columns shows an inventory of about a tenfold stoichiometric excess for the aqueous phase, relative to the Pu profile to be expected from LWR fuel. The U(IV) production rate in the column can easily be increased to an extent higher than the feed rates of externally produced U(IV) normally required in the conventional reduction column. Therefore one can at least expect equally good results for the U/Pu separation with the electro-reduction column as with the normal procedures. This is also confirmed by experiments in the USA which resulted in the installation of an electro-reduction column in the AGNS Plant at Barnwell. In these experiments even with high acid concentrations (2 M HNO in the aqueous strip, BXS) high plutonium decontamination factors have been achieved (17). 

Electro-reduction deserves further attention as, by applying this technique for iron reduction and stripping, neither chemicals, reductive metals or alloys, nor an autoclave is needed. Because D2EHPA is frequently referenced as the standard for iron extraction from acidic sulphate solutions, especially for Fe/Zn separation, we decided to study some aspects of the electro-reduction of Fe (III)-loaded D2EHPA. 

The equilibrium concentration of iron without electro-reductive [thus after 30 minutes of contact time] is 2.88 and 5.82 g/1 for the 1 M H2SO4 strip at, respectively, 20°C and 50°C. For the 2 M solution, this is 5.48 and 7.26 g/1. It is obvious from these results that the stripping kinetics without any electro-reductive action are considerably faster at 50 °C than at 20 °C. At the end of the experiment, the iron in the strip phase increased to 5.52, 7.84, 7.56 and 10.08 g/1, respectively. Thus, enhanced stripping with electro-reductive action is observed which is clearly more effective at the higher temperature. It can also be seen that, after the current is switched on, it takes some time before the iron concentration in the strip liquor starts to increase. This indicates that, before iron is stripped from the organic phase, the ferric iron in the strip phase has first to be converted to ferrous iron below a critical level. This is also shown in Figure 4 which depicts the total iron, ferric and ferrous concentrations as a ftmction of time for the 2 M, 20 V, 50°C experiment. 

The shaded area in the stripping graph of Figure 4.6(a) stands for the oxidation of Pt-CO, which is produced during the 10 h exposure to 50%C02/50%H2. It is the direct proof for the sentence of RWGS reaction. The Pt-CO formed by electro-reduction of CO2 polarizes HOR in the same manner as Pt-CO formed by direct CO adsorption. Therefore, in the following section, the impact of CO will be focused on the impact of CO. 

Eigure 16a shows CO stripping on Pt(lll), Pt(lll)-Ru (following spontaneous deposition), and Pt(lll)-Ru (where the spontaneously deposited ruthenium has been reduced in hydrogen). Only a very small reduction in overpotential for CO electro-oxidation is observed for Pt(lll)-Ru . The overpotential for CO electro-oxidation on the Pt(l 11) surface has been reduced, however, on the Pt(lll)-Ru surface, and the latter exhibits a doublet structure. This CO stripping result on Pt(l 11)-Ru is nearly identical to that found on the Pt(l 11)-Ru surface where the ruthenium was MVD deposited (Eigure 15) It was concluded that Pt(lll)-Ru ° was decorated islands of Ru . 

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