Uranium precipitation

Although the process requires the addition of a phosphate donor, such as glycerol-2-phosphate, it may be a valuable tool for cleaning water contaminated with radionuchdes. An alternative mode of uranium precipitation is driven by sulfate-reducing bacteria such as Desulfovibrio desulfuricans which reduce U(VI) to insoluble U(IV). When combined with bicarbonate extraction of contaminated soil, this may provide an effective treatment for removing uranium from contaminated soil (85). 

The extractant is di(2-ethylhexyl) phosphoric acid (DEHPA) in conjuction with trioctyl phosphine oxide (TOPO). Stripping is by ammonium carbonate, and uranium precipitates as ammonium uranyl tricarbonate. The mixture shows a synergistic effect. The mixture is stable, and extracts uranium in hexavalent state. 

Geochemical controls on uranium precipitation in calcrete palaeochannel deposits of Namibia... 

Fuel and Cladding Fabrication — Uranium precipitation in mixed fuels — Acid cleaning of fuel cladding... 

Morrison SJ, Metzler DR, Carpenter CE. Uranium precipitation in a permeable reactive barrier by progressive irreversible dissolution of zerovalent iron. Environ Sci Technol 2001 35 385-390. 

Acid leached ore Alkaline leached ore Uranium precipitate... 

In the second process the alkaline solution is mixed with acid and the carbon dioxide liberated driven of by boiling. The resulting acidic solution is then neutralized with ammonia or magnesium oxide, whereupon uranium precipitates together with molybdenum and vanadium. The process is therefore only used if uranium ores have low concentrations of molybdenum and vanadium. 

Uranium precipitated from acid solutions as diuranates ... 

Independently of their color (calcined uranium oxide is dark-green to black), the resulting uranium concentrate is known as yellow cake . The name refers to the yellow color of the uranium precipitate. 

Recovery of uranium from leach liquors. Uranium may be recovered from leach liquors by precipitation, ion exchange, or solvent extraction. Precipitation with sodium hydroxide was the recovery method used in the first uranium mills. When used on sodium carbonate leach liquors, the uranium precipitate is fairly free of other metallic contaminants, because sodium carbonate dissolves few other metals beside uranium. However, when used in sulfuric acid leach liquors, the uranium precipitate contains other metals, such as iron dissolved from the ore by the add, and is no longer commercially acceptable. Consequently, in the United States, uranium mills emfdoying add leaching now follow it with selective recovery by either solvent extraction or ion exchange. These processes are described in Secs. 8.5 and 8.6, respectively. 

Precipitation. Leach liquor from the first stage filters is clarified in a thickener, mixed with about five times its content of recycled yellow cake, heated to 74°C, and held about 5 h in stirred tanks. Recycle increases the soluble uranium content from about 3.5 to 7.5 g UjOg/liter and was found needed to improve the completeness of uranium precipitation in the next step. Precipitation of uranium by the reaction... 

Thirty percent hydrogen peroxide precipitates thorium peroxide Th207 from solutions of thorium salts. As few cations other than thorium and uranium precipitate under these conditions, this method has been used to purify thorium. 

The steam stripping tower is 7 ft high by 1 ft diameter and contains baffles in the top half. Leach liquor flows down the tower against a counter-current flow of steam. A liquid level is maintained at a height of about 2 ft, and from this reservoir at the base of the tower the liquor and suspended uranium precipitate and pass to a filter. 

The criticality safety of the dissolver was evaluated by establishing the most reactive metal volume fraction (MVF) of the fuel material as a function of boron concentration. UF4—-f- HiO plateout, uranium precipitation, and undissolved heel were evaluated singly and in combination for effect on maximum keff. A maximum k ff of 0.89 was obtained for the design basis conditions. 

Rich and co-workers showed that pitchblende formed before base-metal sulphides in 33 of 37 deposits where both were present. This indicates the possible need for another uranium-precipitating mechanism in addition to the oxidation of sulphides. 

A smaller and simpler kit measures carbonate and bicarbonate that relate to the solubility and mobility of uranium. Hydrogen sulphide, a reliable indicator for the environment of uranium precipitation, may be measured by yet another and even simpler kit. 

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