Actinide metals purification methods

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This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

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. 

In this chapter, preparation and purification methods are reviewed. In view of the expected role of 5 f electrons in the metallic bond of actinides, methods for the preparation of metals have been particularly studied. There has also been important progress in the preparation of simple binary compounds. Special emphasis has been given to the growth of single crystals, particularly needed for the most refined physical techniques. 

The preparation of larger quantities of high purity actinide metals is being based increasingly on separation or purification via evaporation of the actinide metal In these methods, actinide compounds (oxides or carbides) are reduced by metals forming nonvolatile oxides or carbides under conditions where the actinide metals can be volatilized ... 

Efficient purification is achieved by selective evaporation and condensation. This technique is applicable to actinides of medium volatility i.e. Am or Cm The volatile impurities are eUminated by selective condensation of the actinide metal, less volatile impurities are left in the crucible. The efficiency of this refining method is determined by the relative evaporation ratio, which for two elements A and B equals the ratio of their activities at a given temperature. 

In the following, methods for preparation, purification and characterization of actinide metals are reviewed. Properties are presented, the theoretical interpretation of which underlines the special nature of the actinides in comparison with d or 4f (lanthanide) transition metals. 

Preparation Methods. Actinide metal preparation is based on methods known or developed to yield high purity material by metallothermic reduction or thermal dissociation of prepurified compounds. Electrolytic reduction is possible from molten salts, but not from aqueous solutions. Further purification of the metals can be achieved by electrorefining, selective evaporation or chemical vapour transport. 

Purification methods vary in complexity with the chemical and physical characteristics of the actinide metal and with the quantity of material being processed. The principal refining processes, and their applicability, can be summarized as follows ... 

Electrochemical methods. Hie electrolysis of dilute sulfuric acid solutions with a mercury cathode results In the quantitative deposition of Cr, Fe, Co, Nl, Cu, Zn, Qa, Oe, Mo, Rh, Pd, Ag, Cd, In, Sn, Re, Ir, Pt, Au, Hg, and T1 In the cathode. i Arsenic, selenium, tellurium, osmium, and lead are quantitatively separated from the electrolyte, but are not quantitatively deposited In the cathode. Manganese, ruthenium, and antimony are Incompletely separated. Uranium and the remaining actinide elements, rare earth elements, the alkali and alkaline eeu th metals, aluminum, vanadium, zirconium, niobium, etc. remain In solution.Casto and Rodden and Warf— have reviewed the effects of many variables In the electrolytic separation of the above-named elements from uranium. According to Rodden and Warf optimum conditions for the purification of uranium In sulfuric acid solutions with a mercury cathode are electrolyte volume,... 

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