Oxidation-reduction reactions molten salt extraction

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

Apart from the most electropositive metals, most other metals extracted through molten salt routes are recovered as solids these include many important refractory and other transition metals, the lanthanides, and some actinides. Particularly interesting problems arise in the electrowinning of the refractory metals. Attempts to deposit these metals in a coherent, massive form of theoretical density usually meet with a number of difficulties. Deposits may be dendritic, for example, if electrodeposition proceeds under mass transfer control, or they may be powdery and nonadherent if secondary reactions, such as alkali metal deposition, followed by backreaction with the solute, occurs. Moreover, powdery deposits may also arise if low oxidation states, formed as intermediates during the reduction process, disproportionate in the metal-melt interphase. Charge-transfer-controlled electrodeposition or coupled chemical steps appear to be a prerequisite for obtaining dense, coherent, and adherent deposits. Such deposits have been obtained... 

The complexing role of fluoride ions in the reduction of refractory metals is now well known for the metal recovery [12]. The technology of extracting tantalum in molten salts is based on the formation of K2TaF7, obtained by reaction of HF on the oxide Ta205 extracted from raw materials [14] before the reduction of this compound by sodium in the liquid phase ... 

The salt purification process is illustrated in Fig. XXIV-9. A fraction of the molten salt is removed from the electrolysis cell and is placed in contact with lithium-rich liquid cadmium. By the exchange reaction between Li and salt-borne TRU and the fission products, the less stable species in the molten salt are transferred to the liquid Cd. Generally, U and TRU are less stable than the rare earth metals and are first transferred to the liquid Cd. The Li concentration in the liquid Cd must be increased to decrease the contamination of the molten salt by TRU. Then, concentration of the fission products is also increased in the liquid Cd. After a forward reductive extraction process, the decontaminated salt with the salt-borne fission products passes through zeolite beds that replace nearly all of the alkali, alkaline earth, and rare earth metals with K and Li by ion exchange. The residual actinides in the molten salt are also adsorbed in the zeolite. The molten salt leaving the zeolite is free of actinides and fission product ions. The purified salt is mixed with an oxidizer such as CdCb and is contacted with liquid Cd that contains U and TRU by the forward reductive extraction process. CdCb will contain U and TRU to be oxidized. U and TRU are transferred to the molten salt from the liquid Cd. The molten salt with U and TRU is recycled to the electrolysis cell. The liquid metal is also recycled to the forward reductive extraction process. 

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