Uranium, recovery

The development of the novel Davy-McKee combined mixer—settler (CMS) has been described (121). It consists of a single vessel (Fig. 13d) in which three 2ones coexist under operating conditions. A detailed description of units used for uranium recovery has been reported (122), and the units have also been studied at the laboratory scale (123). AppHcation of the Davy combined mixer electrostatically assisted settler (CMAS) to copper stripping from an organic solvent extraction solution has been reported (124). 

Some North Dakota lignite ashes have also been observed to have above-average concentrations of uranium (21,22), lea ding to interest in processing the ash for uranium recovery. However, this ash may be classified as hazardous. 

Beginning in approximately 1975, both IMG and Ereeport Minerals operated large uranium recovery plants in the United States using this technology. Several plants continue to mn but a number have been closed because of the depressed uranium prices that resulted when uranium from the former Soviet Union flooded Western markets. A relatively small plant is operated by Prayon in Belgium (40). TOPO is available from Cytec Industries Inc. as CYANEX 921 extractant. D2EHPA is available from Albright Wilson Ltd. and is also sold by Daihachi as DP-8R. 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Carbonates. Actinide carbonate complexes are of interest not only because of their fundamental chemistry and environmental behavior (150), but also because of extensive industrial appHcations, primarily in uranium recovery from ores and nuclear fuel reprocessing. 

Table 5.10 Salient features of uranium recovery processes as a byproduct of phosphatic fertilizer industry.

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri-n-butyl phosphate (TBP) in... 

Uranium production, 17 525-526 by country, 25 400t Uranium radioisotopes, 21 319 Uranium reactor fuel manufacture, hydrogen fluoride in, 14 19 Uranium recovery, ion-exchange resins in, 14 421-422... 

The concentration of uranium contained in phosphate rocks (50 200 ppm) is higher than that in seawater (see section 12.3.5). Even though economic recovery of uranium from phosphate rock is difficult, several phosphoric acid plants include operation of uranium recovery facilities. 

Radium, thorium, and other radionuclides accumulate in uranium mill tailings. The potential environmental effects of these radionuclides has become of increasing concern to the public. In the future, it may be necessary to modify existing uranium recovery processes to accommodate removal of radium and perhaps other radioactive decay products of uranium. 

In response to the success of microbially mediated cleanup of metal-contaminated waters, some commercial bioremediation products such as BIOCLAIM, AlgaSORB, and BIO-FIX are available. More detailed descriptions are provided by Brierley (1990). BIOCLAIM and BIO-FIX use immobilized bacterial preparations while AlgaSORB utilizes a nonviable algae matrix for metal removal. In addition to these products, there are several proposed proprietary processes including the use of immobilized Rhizopus arrhizus biomass for uranium recovery (Tsezos, McCready Bell, 1989). A froth flotation method for enhanced contact between biomass and contaminated water has been proposed by Smith, Yang Wharton (1988). 

Burger, L.L. 1958. The decomposition reactions of tributyl phsophate and its diluents and their effect on uranium recovery processes. Progress in Nuclear Energy Series HI Pergamon Press London, Process Chemistry, Vol. 2, 7-5, 307-319. 

W.C. Babcock, R.W. Baker, D.J. Kelly and E.D. LaChapelle, Coupled Transport Membranes for Uranium Recovery, in Proceedings of ISEC 80, University of Liege, Liege, Belgium (1980). 

Uranium Recovery from Wet Process Phosphoric Acid... 

Uranium recovery was briefly described in the ninth edition. Since 1990, all uranium recovery contracts have expired in the United States and the recovery facilities moth-balled or scrapped. There is no indication the situation will change in the near future, because the reduced price of uranium no longer makes its recovery from phosphoric acid economical. 

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri- -butyl phosphate (TBP) in kerosene. First operated by the U.S. Atomic Energy Commission at its Savannah River plant, SC, in 1954 and at Hanford, WA, in 1956. Now in operation, with modifications, in several countries. Sites include Savannah River (SC), Cap de la Hague (France), Marcoule (France), Sellafield (England), Karlsruhe (Germany), and Trombay (India). See also Recuplex. 

Ion-exchange resins arc employed in water-desalting membranes, - in the depuration from fluoride ions," in uranium recovery (Table 39), etc. (see also Ion exchangers, Sec. A.2). 

If all of these elements are incorporated into a basic hydrologic flow model, then detailed modelling of the behavior of the leaching system will be possible. The development of such comprehensive models will not only aid in the optimization of solution compositions for the most effective uranium recovery, but will also allow a more realistic environmental impact assessment and corrective measures if required. 

The ICP-MS method was developed for measuring total uranium in water and wastes. The sample preparation is minimal—filtration for dissolved uranium, acid digestion for total recoverable uranium. Recovery is quantitative (near 100%) for a variety of aqueous and solid matrices and detection limits are low, 0.1 pg/L for aqueous samples and 0.05 mg/kg for solid samples (Long and Martin 1991). 

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