Oxide reduction process, direct

The following pages will describe several examples of pyrochemical processing as applied to the recycle of plutonium, and will briefly review the fundamental chemistry of these processes. We shall review the conversion of plutonium oxide to plutonium metal by the direct oxide reduction process (DOR),the removal of americium from metallic plutonium by molten salt extraction (MSE), and the purification of metallic... 

The use of this direct oxide reduction process is replacing fluoride reduction as it eliminates neutron exposure to operating personnel (alpha particles from plutonium decay have sufficient energy to eject neutrons from fluorine by the a,n reaction) and eliminates reduction residues which require subsequent recovery. 

It should be noted that all terms concerning the electrons in the metals as well as those connected with the metals not directly participating in the cell reaction (Pt) have disappeared from the final Eq. (3.1.49). This result is of general significance, i.e. the EMFs of cell reactions involving oxidation-reduction processes do not depend on the nature of the metals where those reactions take place. The situation is, of course, different in the case of a metal directly participating in the cell reaction (for example, silver in the above case). 

Because Walton started with pure W and observed no WO3 in the product, he concluded that his method likely follows the direct route. More likely, however, is a two-step oxidation-reduction process that generates volatile W02(0H)2 as an intermediate, such as... 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

Although the law of mass action is equally valid for oxidation-reduction processes, and therefore conclusions as to the direction of reactions may be drawn from the knowledge of equilibrium constants, traditionally a different approach is used for such processes. This has both historical and practical reasons. As pointed out in the previous sections, in oxidation-reduction processes electrons are transferred from one species to another. This transfer may occur directly, i.e. one ion collides with another and during this the electron is passed on from one ion to the other. It is possible, however, to pass these electrons through electrodes and leads from one ion to the other. A suitable device in which this can be achieved is a galvanic cell, one of which is shown in Fig. 1.14. A galvanic cell consists of two half-cells, each made up of an electrode and an electrolyte. The two electrolytes are connected with a salt bridge and, if... 

Calcinm glectrowinning in piutniiium production. A potential application of inorganic membranes in radioactive waste treatment is in the industrially practiced direct oxide reduction process. In this process plutonium oxide is calciothermit y reduced to plutonium in the presence of calcium chloride according to the following reaction ... 

The effluent salt is the radioactively contaminated calcium chloride saturated with calcium oxide which can not be discarded as-is. Thus it is desirable to cathodically reduce calcium oxide to calcium which can then be recycled back to the direct oxide reduction process. The primary difficulty in obtaining a cathodic calcium deposit is due to the evolution of carbonaceous anodic gases which encourage various reverse reactions in the cell leading to loss of deposited calcium. 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

As a result, if PTEFs are calculated for water solutions containing phenol or silver and if one considers that oxidation-reduction processes proceed as the direct addition of individual processes, one should expect, following equation (9-8), the results reported in the last column of Table 9.3. 

Perhaps the most convincing argument in favor of a direct role of copper in the cytochrome oxidase reaction comes from studies done with the electron spin resonance spectrometer. Such studies demonstrate that some of the copper ions undergo valency changes during the oxidation-reduction process. The site of action of the copper in the electron transport chain would then be as follows ... 

Other successful electrochromic devices have been realized by Kelly et al. using polyaniline-impregnated fibres [79]. In situ electrochemical polymerization of polyaniline is used to bind poly aniline to a PET or viscose spacer fabric. The fabric is then impregnated with an electrolyte and sandwiched between two electrodes. For the bottom electrode, carbon black or silver ink can be printed directly on the fabric. Polyaniline colour changes from green to blue through oxidation—reduction processes. However, the lifetime of this structure is also short and does not exceed dozens of oxidation—reduction cycles. 

These and other standard commercial methods of titanium production, such as the sodium-reduction (or Hunter) process, the direct-oxide-reduction process, and the electrolytic process, have been described in detail by MCQUILLAN [Mcq56, Chap. 2], Hoch [Hoc73 ], and ZwiCKER [Zwi74, pp. 21-27], while some new approaches developed in the Soviet Union have been outlined by Reznichenko and coworkers [Rez82, Rez82 J. 

As with electrolytic reductions, the information that has accumulated in the past is now being supplemented by more searching studies of mechanism. The primary step in an oxidation may be the direct transfer of an electron, to yield a carbonium ion or other free radical which then reacts with other species in the adsorption layer or it may be the direct oxidation of a carrier , such as the manganese(II) ion or some other substance that readily enters into a reversible oxidation-reduction process. Also, the solvent may be involved, and in aqueous media the OH ion may lose an electron to become an OH radical, which may then give rise to hydrogen peroxide or to an adsorbed oxygen atom, or may react directly with the organic substrate. Finally, at a lead dioxide electrode, which is often used as anode, the electrode surface itself may be involved in the oxidation. 

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