Uranium black

Uranium is present in small (50—200 ppm) amounts in phosphate rock and it can be economically feasible to separate the uranium as a by-product from the cmde black acid (30% phosphoric acid) obtained from the leaching of phosphate for fertilizers (qv). The development and design of processes to produce 500 t U Og per year at Ereeport, Louisiana have been detailed (272). 

Uranium [7440-61-17 is a naturally occurring radioactive element with atomic number 92 and atomic mass 238.03. Uranium was discovered in a pitchblende [1317-75-5] specimen ia 1789 by M. H. Klaproth (1) who named the element uranit after the planet Uranus, which had been recendy discovered. For 50 years the material discovered by Klaproth was thought to be metallic uranium. Pnligot showed that the uranit discovered by Klaproth was really uranium dioxide [1344-57-6] UO2, and obtained the tme elemental uranium as a black powder in 1841 by reduction of UCl [10026-10-5] with potassium (2). 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Bromides and Iodides. The red-brown tribromide, UBr [13470-19-4], and the black tniodide, Ul [13775-18-3], may both be prepared by direct interaction of the elements, ie, uranium metal with X2 (X = Br, I). The tribromide has also been prepared by interaction of UH and HBr, producing H2 as a reaction product. The tribromide and tniodide complexes are both polymeric soflds with a local bicapped trigonal prismatic coordination geometry. The tribromide is soluble in H2O and decomposes in alcohols. 

Anderson RF (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3 145-164 Anderson RF, Fleisher MQ, LeHuray AP (1989) Concentration, oxidation state, and particulate flux of uranium in the Black Sea. Geochim Cosmochim Acta 53 2215-2224 Back W, Hanshaw BB, Pyler TE, Plummer LN, Weiede AE (1979) Geochemical significance of groundwater discharge in Caleta Xel Ha, Quintana Roo, Mexico. Water Res 15 1521-1535 Barnes CE, Cochran JK (1990) Uranium removal in oceanic sediments and the oceanic U balance. Earth. Planet. Sci. Lett 97 94-101... 

Besides the conventional uranium resources, there are also the so-called unconventional uranium resources , which are defined as deposits with very low uranium content, from which uranium is typically only recoverable as a minor by-product. These unconventional uranium resources are obtained from the extraction of phosphates, non-ferrous ores and carbonatites, as well as black schist and lignite. It has to be noted that the distinction between conventional and unconventional resources is not entirely clear cut, but is, instead, somewhat transitional. 

Uranium also combines with oxygen in various ratios. For instance, uranium dioxide (UO ) is a brownish-black powder that was once thought to be pure uranium. Uranium trioxide (UOj), a heavy orangish-powder, was once referred to as uranyl oxide. 

Uranium forms several oxides. The main oxides are brown-black UO2, orange yellow UO3, and nonstoichiometric greenish black U3O8. The most stable oxide is dioxide, UO2. Heating the metal in air or oxygen at 150 to 350°C forms UO2 and UsOs. A trihydride, UH3, is obtained when metal is heated in hydrogen at 250°C. 

Uranium dissolves rapidly in nitric and hydrochloric acids. With hydrochloric acid, a black residue often remains. In sulfuric, hydrofluoric, and phosphoric acids, the metal dissolves slowly. Uranium is not affected by alkalies. 

Pig. 12. The experimental (full black circles) and calculated lattice parameters of the uranium chalcogenides and pnictides... 

When Klaproth dissolved some pitchblende in nitric acid and neutralized the acid with potash, he obtained a yellow precipitate which dissolved in excess potash. Klaproth concluded correctly that the mineral must contain a new element, which he named in honor of the new planet, Uranus, which Herschelhad recently discovered (12). He then attempted to obtain metallic uranium just as Hjelm had prepared metallic molybdenum. By strongly heating an oil paste of the yellow oxide in a charcoal crucible, he obtained a black powder with a metallic luster, and thought he had succeeded in isolating metallic uranium (29). For over fifty years the elementary nature of his product was accepted by chemists, but in 1841 Peligot showed that this supposed uranium metal was really an oxide. 

Chromite. In a letter to Scherers Journal, dated St. Petersburg, November 12, 1798, Count Musin-Pushkin wrote You already know that Mr. Lowitz and Mr. Klaproth have independently discovered chromium combined with iron in a fossil I sent them. This ore looks like the black uranium ore (pitchblende), but has a more metallic luster ... 

Fig. 12. Transect of the ore deposit with the surrounding mineralogy and sampling points (black circles) and flow paths tested in the calculations at the Cigar Lake uranium deposit (arrows).

Euxenite (Loranskite). (Y, Ca, Ce, U, Th)-(Nb, Ta, Ti)20. A rare-earth mineral, occurring in Norway, Madagascar, Canada Pennsylvania. It is brownish-black, brilliant to vitreous sp gr 5.0—5.9 and hardness 5— 6. Used as a source of uranium, niobium and tantalum... 

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