Techniques electrorefining

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

A schematic representation of an electrorefining cell is shown in Figure 11. The basic chemistry of the electrorefining technique is as follows ... 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

The above techniques that involve high-temperature processes are known as pyromet-allurgy. Another common technique involves the electrolytic reduction of metal compounds, often referred to as hydrometallurgy or electrorefining, depending on whether the procedure is carried out before or after the metal has already been separated from its ore, respectively. 

In electrorefining, the metal to be refined is used as the anode whic dissolves in the electrolyte and is deposited as electrolytic-grade mett at the cathode. The impurities present in the anode remain on it, fall oi to the bottom of the cell as slime, or go into solution but are prevente. from moving toward the cathode by precipitation with some chemics reagent such as another metal added to the electrolyte. The buildup c metallic impurities that are dissolved but not deposited at the cathode i reduced by circulation of fresh electrolyte through the cells. Electrorefinin techniques are used in producing gold, silver, copper, nickel, cobalt, lead tin, antimony, bismuth, indium, and mercury. 

In contrast to the silicon process, durable electrorefining of Mb and Ta was more successful. The processes were performed in the molten mixture KCl-NaCl-K2NbF7 (K2Tap7) at 720 10 °C in course of 1 or 2 weeks. Two different types of electrolysers were used, i.e., laboratory type and pilot plant cells, enabling operating currents up to 50 and 300 A, respectively. The techniques, other conditions and results of the experiments were described in detail in [12]. 

Under a formal cooperation agreement with IPEN, uranium recovery by electrorefining techniques is being investigated, also to understand the basic processes involved. 

Also with the same Institute, metal fuel recycling is being researched using electrorefining techniques. We expect to have a first sample of actinide electrodeposited by the end of 1995 ... 

Alternatives to aqueous processing or dry or nonaqueous processing techniques have heen tried hut none, until recently, have been used on a true industrial scale for fuel reprocessing. Examples of these include separations based on (1) differences in volatility of the hahdes, especially fluorine compounds (2) molten salt (liquid-liquid) extraction where fuel dissolved in a molten salt is then contacted with a heat resistant low volatility second phase such as 100% TBP or a liquid metal and, (3) electrorefining, where controlling the cell potential results in removal of a metal from a molten salt by the selective deposition (reduction) on a cathode. 

M. Sheedy, P. Pajunen and B. Wesstrom, Control of copper electrolyte impurities -overview of the short bed ion exchange technique and Phelps Dodge El Paso case study, Proceedings of the Sixth International Copper-Cobre Conference Volume V Copper Electrorefining and Electrowinning, eds. G. Houlachi, J. Edwards and T. Robinson (Montreal, Quebec CIM, 2007), 345-357. 

Anode quality is vital in particular, insoluble particulate matter (from soluble Cu anodes) in copper electrorefining or PbSO (from inert Pb- Ag or Pb-Sb) in copper or zinc elect rowinning may cause deleterious deposits. Metallurgically homogeneous, fine-grained soluble anodes are usually preferred via, for example, rolled fabrication techniques. 

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