Uranium precipitation processes

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). 

Because of the technical importance of solvent extraction, ion-exchange and precipitation processes for the actinides, a major part of their coordination chemistry has been concerned with aqueous solutions, particularly that involving uranium. It is, however, evident that the actinides as a whole have a much stronger tendency to form complexes than the lanthanides and, as a result of the wider range of available oxidation states, their coordination chemistry is more varied. 

Principle of Separation. Thorium co-precipitates with NdF3 while U02+ remains in solution. Filter paper with the neodymium fluoride precipitate and associated thorium is mounted on a planchet and counted for alpha and beta particles. The presence of alpha particles in the NdF3 precipitate indicates the amount of uranium that accompanies the precipitate unless 230Th is present. The ratio of the beta-particle count rate in the precipitate to the count rate in the initial sample of uranium with 234Th and 234mPa indicates the fractional yield of the co-precipitation process. 

Fig. 21.12. Uranium ore mill processes (A) carbonate leach, caustic precipitation process (B) acid leach, ion exchange and acid leach, solvent extraction processes (C) acid leach, resin-in-pulp process.

Generation of liquid waste from the uranium conversion process (see Table 5-1) is generally small and is handled by placing liquid effluent in lined ponds with sealed bottoms. The pond effluent is chemically neutralized to precipitate out uranium and uranium progeny in pond sludge. Water in the ponds is permitted to evaporate and sludge is disposed of as waste under controlled conditions (AEC 1974). 

The ore extracted by open cast or underground mines is coarsely ground in crushers and then finely ground, the thereby released radon being sucked off as a radiation prevention measure. The uranium content is extracted with the help of hydrometallurgical processes, in which the enrichment of uranium via ion exchange or extraction processes and a precipitation process follow the leaching step. 

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. 

The first step is mining of uranium ore, which typically contains only a few pounds of uranium per ton. Uranium values in the ore are concentrated in a uranium mill, which is located near the mine, in order to reduce subsequent shipping charges. Concentration processes frequently used include leaching, precipitation, solvent extraction, and ion exchange. The principles of solvent extraction are described in Chap. 4 applications of solvent extraction and ion exchange to uranium ore processing are taken up in Chap. 5. Uranium concentrates are... 

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. 

The first microgram quantities of plutonium were produced [S6] in 1942 by irradiation of natural uranium with deuterons in the cyclotron of Washington University in St. Louis. This plutonium was separated at the Chicago Metallurgical Laboratory of the Manhattan Project by Seaborg and his collaborators, who employed the method of carrier precipitation frequently used by radiochemists to extract small amounts of radioactive material present at low concentration. As wartime urgency required that a plutonium separation plant be designed and built before macro quantities of plutonium could be available for process development, it was decided to use the same carrier precipitation process that had successfully produced the first small quantities of this element. 

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