Protactinium metal

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Using Pyrex ampoules with resistively heated tungsten wire or strip filaments, protactinium metal has been prepared on the milligram scale (9,13,15). An improved technique is to use a quartz van Arkel-De Boer bulb with an inductively heated W sphere which solves the previous problem of filament breaking and considerably improves the deposition rate of Pa metal (109). 

The efficiency of the van Arkel-De Boer process (Section II,D) for refining thorium and protactinium metals can be increased by repeating the process to achieve higher purity of product metal. 

Protactinium metal was first prepared in 1934 by thermal decomposition of a pentahalide on a hot filament 50). It has since been prepared from PaF4 by metallothermic reduction (Section II,A) with barium 26, 27, 34,102), lithium 40), and calcium 73, 74). However, the highest purity metal is achieved using the iodide transport (van Arkel-De Boer) process (Section II,D). 

Protactinium metal is prepared (1) by reducing the tetrafluoride witli metallic barium at about 1,500 0 (2) by heating the halide, usually the iodide, under a high vacuum and (3) by bombardment of the oxide under high vacuum with 35-keV electrons for hours at a current strength of 0.005 0.010 Amperes. 

Protactinium tetrafluoride, like the other actinide tetrafluorides, possesses the 8-coordinate UF4-type of structure (Table III) but no bond distances are available. It is easily the most stable tetravalent halide of protactinium and can be handled in the atmosphere, at least for a limited period, without hydrolysis or oxidation occurring. As mentioned earlier it is the usual starting material for the preparation of protactinium metal. Tetrafluoride hydrates have not been fully characterized, but a mixed fluorosulfate, PaF2S04 2H20 can be precipitated from aqueous solution (131). Protactinium tetrafluoride is soluble in aqueous ammonium fluoride solutions, for which some spectral properties have been recorded (4, 83). 

Oxide films form readily on protactinium metal and the monoxide PaO has been identified (125) by X-ray powder diffraction. It is cubic, like other actinide monoxides, with = 4.961 A and slowly oxidizes to PaO 2. 

Protactinium metal reacts with hydrogen to form the black hydride, PaHj, which is isostructural with other actinide trihydrides it has only been prepared in submilligram amounts and identified by X-ray powder diffraction analysis (125). 

The intermetallic compounds are synthesized by heating mixtures of actinide oxides or halides with finely divided noble metal powders in pure hydrogen. Protactinium metal was prepared in a modified version of the van Arkel-de Boer procedure protactinium iodide, formed by reaction between iodine and protactinium carbide, was thermally dissociated on a resistance heated tungsten wire (6,7) ... 

To obtain significant quantities of protactinium, a separation procedure was developed for extracting protactinium from the sludge that was left after the ether extraction of uranium at the Springfields refinery. The process yielded 127 g of pure Pa from 60 tons of sludge. Protactinium metal can be obtained by reducing Pap4 with... 

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