Reductive stripping

Nitrite ion is often used in plutonium solvent extraction systems to oxidize Pu(III) to Pu(IV) and to reduce Pu(VI) to Pu(IV). But HONO, produced in HN03 media, is extractable into TBP-diluent systems and can interfere with subsequent reductive stripping of plutonium. There is thus a need to find a reagent comparable to nitrite ion in its reactions with Pu(III) and Pu(VI), but which does not extract into TBP solutions. 

The use or safe disposal of the iron residues from zinc production (see Figure 7) presents a major technical problem.204 The use of chelating aminomethylene phosphonic acid extractants such as (28) and (29) to recover iron from these residues has been proposed.205 These give much higher FenI/Znn selectivity than D2EHPA but are more difficult to strip. A reductive-stripping process is proposed.187,205... 

Extraction of gold from acidic chloride media by S-decyldithizone (39) is associated with oxidation of the extractant to the 2H-tetrazolium cation. Reductive stripping results in the regeneration of the extractant and precipitation of Au° as a black powder.3... 

A first atomic layer of Se can be formed by any one of the three methods. In constmction of an ECALE cycle, however, the second and successive atomic layers of Se must be formed by the reductive stripping method (Fig. 56c), because use of a positive deposition or stripping potential would result in loss of the previously deposited group n element, as discussed previously. 

FIG. 68. Schematic illustrating the electrochemical digital etching process on CdTe(lOO). A) initial surface, B) after oxidative stripping of a Cd atomic layer, C) after reductive stripping of the Te atomic layer. 

The sequence of the steps (stripping-oxidation-stripping-reduction), which was changed to reduction-stripping-oxidation-stripping The temperature of the ReDox cycles was increased to 804°C... 

Vijayakumar et al. [69] has described a method for the determination of trace quantities of selenium in plant tissues based on the interaction of selenium(IV)-iodine with an acid medium, leading to the liberation of iodine. This method was utilised for the indirect determination of selenium by AA spectrophotometry. The iodine is extracted into benzene and subsequently reductively stripped into an aqueous solution of ascorbic acid. After the extrac-... 

Adsorptive stripping voltammetry (AdSV) is of increasing importance in trace determinations of pharmaceutical compounds. In this method, the preconcentration step is adsorptive rather than electrolytic, resulting in an adsorbed film of the analate on the electrode surface. The stripping step typically uses LSV or the differential pulse mode in either the cathodic or anodic direction, as required. The HMDE is typically used for cathodic reductive stripping, whereas carbon or noble metal electrodes are used in the adsorptive mode. 

In the reduction stripping process uranium(IV) in the raw wet process acid is oxidized to uranium(Vl) by treatment with sodium chlorate, hydrogen peroxide or air at 60 to 70°C, the uranium(VI) formed being extracted with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate in kerosene and the resulting solution finally reductively stripped repeatedly with aqueous iron(II) solutions. This results in an enrichment by a factor of 40. After oxidation of the stripped solution with sodium chlorate or ambient oxygen and renewed extraction of the uranium(VI) formed with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate, the phosphoric acid is removed from the organic phase by washing. The uranium(Vl) is then stripped with ammonium carbonate and precipitated as the ammonium diuranyl-tricarbonato-complex. This is filtered off, washed and calcined. 

In the oxidation stripping process the uranium(VI) in the raw wet process acid is initially reduced to uranium(IV), which is extracted with a mixture of mono and dioctylphenyl esters of phosphoric acid in kerosene. Oxidation with sodium chlorate in phosphoric acid transfers the uranium to the aqueous phase and it is then extracted with trioctylphosphine oxide/di-(2-ethyIhexyl)-phosphate, as in the reduction stripping process. 

A relatively small amount of energy is required to do the job in a typical reprocessing plant. We estimate, under the most ideal circumstances,that 2000 Watts of absorbed light for a typical 5 ton/day plant would be necessary to achieve the reductive. stripping of plutonium. (This is based on the assumption that the dominant photo-reaction will be UO reduction with a Quantum Yield. 

The exhaustive U, Pu extraction step is best achieved in pulsed columns rather than in mixer settlers in order to keep the contact time lower at the very high radiation level. This operation is expected to produce as its main product a HAW raffinate that is virtually free of Pu (and Np, U) a scrub appears therefore superfluous and would dilute only the HAW. In order to attain still a good extraction efficiency within a few stages, A/O phase ratios < 3 should be avoided. This ratio corresponds to a TBP saturation by heavy metals of about 18%. Considerable amounts of Zr are therefore co-extracted, being however present essentially as inactive isotope. Either a complexing or a reductive stripping is advisable in order to keep the aqueous flow small and the acidity sufficiently high to avoid hydrolysis of Zr. 

Figure 1-7. (Inset) DEA-OPE-SH. CV (black) and background-subtracted voltammogram (gray not to scale) for AGAO on Au-bead electrodes modified with DEA-OPE-SH in 10 mM potassium phosphate, pH 7 (scan rate 100 mV s ). Gold-bead electrodes were soaked in mM solutions of DEA-OPE-SH for 24 h. Reductive stripping analyses of the resulting films indicated -70% coverage. The modified electrodes were then incubated with AGAO for 24 8 h to allow binding to the adsorbed wires. Adapted from ref. 113.

Figure 6 - Plots of the charge involved in the oxidative stripping of Cd () and the reductive stripping of S () as a function of the number of ECALE cycles. 

Several groups have worked on the formation of ZnS using EC-ALE (Table 1) [23, 54, 57, 65, 75, 76, 81,113]. The first study was again by TLEC, where it proved difficult to quantify stripping coulometry for both Zn and S separately for deposits formed with more than 5 cycles. Eoresti and coworkers used a procedure in which S was reductively stripped and Zn oxidatively stripped to accurately determine both Zn and S coverages from coulometry for... 

In this scenario, zinc is selectively extracted from iron, which is left behind and can be re-oxidised/recycled to the leach, or can be bled fh)m the circuit. A typical reagent would be DEHPA, which is relatively selective for zinc ovct iron provided the iron is in the ferrous state. The ferric ion still present in the system will be co-extracted strongly and can only be stripped with a very acidic solution (6N HCl for example). As an alternative to acidic regeneration, iron can be removed from the loaded organic using reductive stripping, with Zn powder as the reductant (16). 

It is clear from the above-mentioned literature that efficient stripping of iron from a loaded solvent, resulting in an iron strip solution with a high iron concentration, is a major problem with most of the proposed extractants or extractant mixtures. Accordingly, to enhance the stripping efficiency "reductive stripping" has been proposed. 

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. 

H. Majima and T. Izaki, "Reductive Stripping of Fe(III)-Loaded D2EHPA with the Aqueous Solutions Containing Sulfur Dioxide", Met. Trans. B. Vol. 16B, 1985, 187-194. 

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