Rare Earth Elements: A Review of Production, Processing ...

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Rare Earth Elements Review Section 6 – Human Health and Environmental Risks were associated with the discharge of metals and other constituents (i.e., oil and grease, copper, chromium, cadmium, iron, lead, and total suspended solids) in concentrations that exceeded permitted levels. Earth-moving equipment was also used to dredge iron ore tailings from settling ponds and to dispose of the materials in a nearby creek without a permit that affected nearby wetlands. 6.1.1.2 Air Pathway Fugitive Dust Direct exposure of humans can occur from inhalation of fine dusts (i.e., particulates) or by ingestion or dermal contact of contaminated dusts. Particulates or fugitive dust from storage piles, conveyor systems, site roads, or other areas can be transported by wind and may be deposited and accumulated in downwind areas, including surface soils and surface water bodies (e.g., ponds, pit lake), or be inhaled by site workers and nearby residents. Dust can be an irritant, a toxicant, or a carcinogen, depending on the particles’ physicochemistry, and can be composed of inorganic and organic chemicals. However, the presence of physical barriers, such as vegetation or structural foundations, may dampen or reduce the transport of particles as wind-blown dust. Accumulated mine sediments or dust may become secondary sources of chemicals transported to groundwater via leaching and percolation. Aerosols and Chemical Vapors Mine workers can be exposed to aerosols from numerous processes, including comminution (i.e., the process in which solid materials are reduced in size by crushing, grinding, and other techniques), reentrainment (i.e., air being exhausted is immediately brought back into the system through the air intake and/or other openings), and combustion sources. Aerosols are dispersed mixtures of dust and/or chemicalcontaining water vapor. Cutting, drilling, and blasting of the parent rock, especially in underground mines, creates aerosols with a composition similar to the parent rock. As previously discussed, comminuting the ore underground is sometime practiced for efficient transport out of the mine area, and aerosols can accumulate in these areas. The dusts from ore mineral liberation and separation steps settle to other areas such as ventilation systems, roadways, work areas, and nearby surrounding areas. Aeration ponds are sometimes used to treat waste waters on a mine site, and the aerators used to disturb the surface of the water can create aerosols; the problem can become worse if surfactants are used and not managed properly. Aerosols can potentially accumulate along perimeter areas of ponds and lagoons and contaminate soils, sediments, surface water, and shallow groundwater through deposition and transport. Radioactivity Lanthanides and yttrium are recovered primarily from ores and minerals that naturally contain uranium and thorium; however, some level of radioactive materials is found in association with many REE deposits. As a result, the waste rock and sludges from the extraction of rare earths also contain these radionuclides and are considered Technologically-Enhanced, Naturally-Occurring Radioactive Materials (TENORM). TENORM wastes contain radionuclides in concentrations that could give rise to unacceptable radiation levels. The EPA has estimated that levels of naturally occurring radioactivity contained in common rare earth (e.g., monazite, xenotime, bastnasite) deposits in the United States range from 5.7 to 3,224 picoCuries per gram (pCi/g). The USGS has estimated uranium and thorium content of minerals found in REE deposits. Additional information can be found in Long et al., 2010. Radioactive elements, such as thorium and uranium, can concentrate in mining dusts and sediments that must be managed. Radon gas can also be emitted from these sources. Transport of particulates containing uranium and thorium may occur by any of the transport pathways described above. Acidic groundwater and surface water and low concentrations of organic material in soils can contribute to the mobility and transport of radioactive materials. Accumulations of sediments deposited by runoff and dusts also can concentrate radioactive materials. External exposure to naturally occurring radiation and radon gas is often limited to soil or waste materials that are within several inches of the ground or pile surface; 6-8
Rare Earth Elements Review Section 6 – Human Health and Environmental Risks radioactive materials found deeper in the soil column or accumulated sediments are generally shielded by the top layer of soil. Geometric attenuation generally limits the external radiation from naturally radioactive materials with no interposed shielding materials to within a few meters (i.e., less than 5 meters and often less than 1 to 2 meters from the source). Radioactivity can become concentrated in mineral scales that develop in groundwater recovery wells, holding tanks, aeration ponds, and milling processes areas. Inhalation of contaminated dusts is generally of greatest concern for naturally occurring radioactive materials. Tailings storage facilities typically receive the bulk of processing water, which is normally recycled, but some of it could be released into the environment through seepage or overflows due to unusually high rainfall. The tailings storage facilities (TSF), when dry, usually represent the main source of radon/thorium and dust emissions to the environment. Special containment arrangements are required for the disposal of tailings generated during chemical and/or thermal processing of uranium and thoriumbearing minerals to ensure that environmentally mobile radionuclides are not released into the surrounding environment. The disposal of contaminated equipment and materials also needs to be controlled. In addition, the possibility of off-site contamination from trucks and equipment moving offsite needs to be considered. After milling and concentrating, the mineral is sometimes stockpiled at the mine site prior to transport to the processing sites, and these stockpiles may contain radioactive minerals in concentrations sufficient to produce elevated radiation levels and radon. The stockpiles, therefore, need to be protected against unauthorized access and also against the possibility of the material spreading through wind saltation. Ideally, stockpiles containing radionuclides in concentrations that require signposting of areas as “supervised” and/or “controlled” should be located on a concrete slab to simplify the management and clean-up operations. The tailings from the separation and downstream processing of minerals may contain radionuclides in concentrations that could give rise to unacceptable levels of radiation and radon. Appropriate management is typically required, and the disposal of the waste will depend on the method used to process the mineral and on the respective levels of radiation and the concentration of radon gas emitted. The possibility of ground water contamination increases if chemical and/or thermal treatment of the mineral occurs (e.g., in the case of separating the heavy mineral sands). Otherwise, the radionuclides in the tailings could be considered as remaining bound in the individual mineral grains; therefore, the possibility of the contamination of groundwater by radionuclides from tailings is not considered to be less significant. Radium may be present in the tailings water, which would require removal before being disposed. Where any grinding, chemical, and/or thermal treatment of minerals containing radionuclides takes place, additional safeguards must be implemented due to the fact that secular equilibrium in both uranium and thorium decay chains may be disrupted. This could result in an increased environmental mobility of radionuclides, such as radium and radon. Cleaning of certain minerals prior to processing (such as, for example, the cleaning of the heavy mineral sand grains) may produce finely powdered waste (slimes). Slime wastes may have significant uranium or thorium content, and the disposal as radioactive waste may be required. The equipment used in downstream processing of minerals often becomes contaminated by NORM or TENORM. Contaminated equipment must either be disposed of properly, or thoroughly decontaminated prior to any re-use. Scales and sludge build on the inside surfaces of pipes and vessels used in chemical and thermal processing, and these materials often have elevated levels of radionuclides. 6-9
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Rare Earth Elements Review Table of
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Foreword The U.S. Environmental Pro
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Abstract Rare earth elements (REEs)
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Table of Contents List of Acronyms
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List of Tables 2-1. Abundance of El
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Rare Earth Elements Review List of
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Rare Earth Elements Review Section
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