Uranium milling

In the past, the recovery of uranium from the pregnant liquor solution usually involved precipitation as uranium oxide (UjOg) concentrates (UOC). But as the quality of UOC obtained by precipitation from acid leach is not very good ion exchange and solvent extraction are used in most plants nowadays. Precipitation from carbonate leaching produces a reasonably pure uranium concentrate. A detailed description of these processes and the operational and the economic considerations is beyond the scope of this book and can be found elsewhere (Grenthe 2006). 

There are two other aspects that should be mentioned here that may directly affect the choice of the milling process. First, the uranium ore often contains other metals that have commercial value, like vanadium or niobium, for example, and their recovery may influence the process selected for uranium recuperation. Second, uranium itself may be a by-product of other processes like gold extraction, niobium, and tantalum production or phosphoric acid manufacture. Thus, recovery of low levels of uranium from phosphates, columbite, or gold-bearing minerals may not be economical in itself, but extracting uranium as a by-product from the waste streams of these operations could be commercially sensible. 

The UOC is dried and packed in metal drums and marketed, that is, sent to the UCF. The remainder of the ore, containing most of the radioactivity (from uranium progenies) and nearly all the rock waste material, becomes mine tailings that are placed in storage facilities near the mine. The tailings usually contain low 

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Refining to a High Purity Product. The normal yeUowcake product of uranium milling operations is not generaUy pure enough for use ia most nuclear appHcations. Many additional methods have been used to refine the yeUowcake iato a product of sufficient purity for use ia the nuclear iadustry. The two most common methods for refining uranium to a high purity product are tributyl phosphate (TBP) extraction from HNO solutions, or distiUation of UF, siace this is the feedstock for uranium enrichment plants. 

The licensing process consists of two steps construction and operating license that must be completed before fuel loading. Licensing covers radiological safety, environmental protection, and antitru,st considerations. Activities not defined as production or utilization of special nuclear material (SNM), use simple one-step. Materials Licenses, for the possession of radioactive materials. Examples are uranium mills, solution recovery plants, UO fabrication plants, interim spent fuel storage, and isotopic separation plants. 

Figure 5.38 Routes to uranium mill concentrate processing to power /research reactor fuel products.

Control layers, such as those used to minimize animal intrusion, promote drainage, and control and collect landfill gas, are often included for conventional cover systems and may also be incorporated into ET cover system designs. For example, a proposed monolithic ET cover at Sandia National Laboratories in New Mexico will have a biointrusion fence with 1/4-in. squares between the topsoil layer and the native soil layer to prevent animals from creating preferential pathways, potentially resulting in percolation. The biointrusion layer, however, will not inhibit root growth to allow for transpiration. At another site, Monticello Uranium Mill Tailings Site in Utah, a capillary barrier ET design has a 12-in. soil/rock admixture as an animal intrusion layer located 44 in. below the surface, directly above the capillary barrier layer. 

Uranium Mill Tailings Repository, UT (ACAP project) Hazardous waste landfill Operational July 2000... 

Methods and Results for Properties Contaminated by Uranium Mill Thilings... 

A major objective in developing these risk estimation procedures was to provide a method capable of evaluating hundreds of properties in several communities within the DOE Uranium Mill Tailings Remedial Action Program in a timely manner. Therefore, we chose a calculation scheme that could be performed using commercially available database software (dBASE II, a trademark of Ashton-Tate, Culver City, CA), but that at the same time would be flexible enough that assessments for other contaminants could be readily incorporated. 

We have previously documented the methodology (Marks et al., 1985a) and presented a summary of the technique (Marks et al., 1985b) at the Maastricht, The Netherlands, Seminar on Exposure to Enhanced Natural Radiation and Its Regulatory Implications. This paper represents a synthesis of the work we have conducted to date on risk assessment at uranium mill tailings vicinity properties. 

Table II Range of Observed Radiation Levels and Radionuclide Concentrations at Uranium Mill Tailings Vicinity Properties...

Environmental Protection Agency, Environmental Surveys of the Uranium Mill Tailings Pile and Surrounding Areas, Salt Lake City, Report No. EPA-520/6-74-006, U.S. Environmental Protection Agency, Las Vegas, NV (1974). 

Near Mo mine and mill, Colorado, irrigated with Mo-contaminated effluent from uranium mill 61 (49-72) 4... 

Donahue, R., Hendry, M.J., Landine, P. 2000. Distribution of arsenic and nickel in uranium mill tailings, Rabbit Lake, Saskatchewan, Canada. Applied Geochemistry, 15, 1097-1119. 

Keywords phytoremediation, radionuclides, 137-caesium, 90-strontium, 125-iodine, uranium, radium, uranium mill tailings, biomonitoring... 

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 a group of uranium mill workers, there was an excess of deaths from malignant disease of lymphatic and hematopoietic tissue data from animal experiments suggested that this excess may have resulted from irradiation of lymph nodes by thorium-230, a disintegration product of uranium. Some absorbed uranium is deposited in bone. A potential risk of radiation effects on bone marrow has been postulated, but extensive clinical studies on exposed workers have disclosed no hematologic abnormalities. ... 

Archer VE, Wagoner JK, Lundin FE Jr Cancer mortality among uranium mill workers, y Occup Med 15 11-14, 1973... 

A significantly (p<0.05) increased incidence of malignancies in the lymphatic and hematopoietic tissues of uranium mill workers (cohort of 662 males) was found by Archer et al. (1973). The radioactivity in the tracheobronchial lymph nodes of the workers was found to be primarily the result of alpha emissions from thorium-230 and not from uranium-234 or uranium-238. Consequently, the authors suggested that the increased incidence of malignancies may have been a result of thorium-230 exposure and not uranium exposure. Exposure levels of thorium were not reported therefore, the results of the study are not reported on Table 2-1 or plotted in Figure 2-1. 

After inhalation exposure, the primary route of excretion is in the feces following ciliary clearance from the lungs to the gastrointestinal tract (Wrenn et al. 1981). Fecal excretion may account for as much as 97% of total excretion (Fisher et al. 1983). Higher levels of thorium-230 were excreted in the feces by active crushermen (uranium mill workers exposed to uranium ore dust in the crusher building) compared to retired workers or controls (Fisher et al. 1983). Levels of thorium-230 in the urine were comparable to those of retired workers, and the levels in both were significantly greater than controls. 

EPA (1984) estimated that about 0.2 Ci of thorium-230 is annually emitted into the air from uranium mill facilities, coal-fired utilities and industrial boilers, phosphate rock processing and wet- process fertilizer production facilities, and other mineral extraction and processing facilities. About 0.084 Ci of thorium-234 from uranium fuel cycle facilities and 0.0003 Ci of thorium-232 from underground uranium mines are emitted into the atmosphere annually (EPA 1984). 

Tisher DR, Jackson PO, Brodaczynski GG, et al. 1983. Level of uranium-234 uranium-238 and thorium 230 in excreta of uranium mill crushermen. Health Phys 45 617-630. 

California, agricultural drainwater, 1985-86 Colorado Mining areas Near uranium mill... 

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