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

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Rare Earth Elements Review Section 4 – Resource Processing 4.1 Bevill Amendment The 1980 amendment to the Resource Conservation and Recovery Act (RCRA), known as the Bevill exclusion, was enacted to exclude specific solid wastes from the extraction, beneficiation, and processing of ores and minerals. The EPA established criteria for determining beneficiation and process waste for each mineral production sector and reported this information in the final rule (54 Fed. Reg. 36592, 36616 codified at 261.4(b)(7)). The EPA determined the line between beneficiation and processing of rare earths for one particular rare earth operation is when the ore is digested with concentrated acids or caustics (U.S. EPA, 1991). However, several factors are involved in making Bevill determinations, and official assistance should be sought from the RCRA authorized state or the EPA Regional office. The basic steps in making Bevill determinations are the following: 1. Determine whether the material is considered a solid waste under RCRA. 2. Determine whether the facility is using a primary ore or mineral to produce a final or intermediate product, and also whether less than 50 percent of the feedstock on an annual basis are from secondary sources. 3. Establish whether the material and the operation that generates it are uniquely associated with mineral production. 4. Determine where in the sequence of operations beneficiation ends and mineral processing begins. 5. If the material is a mineral processing waste, determine whether it is one of the 20 special wastes from mineral processing. This process of determination will result in one of the three outcomes: � The material is not a solid waste and therefore not subject to RCRA; � The material is a solid waste, but is exempt from RCRA Subtitle C because of the Mining Waste Exclusion; or � The material is a solid waste that is not exempt from RCRA Subtitle C and is subject to regulation as a hazardous waste if it is listed or characteristic hazardous waste. The EPA offers online compliance assistance on the Bevill Amendment at the following Internet address: http://www.epa.gov/oecaerth/assistance/sectors/minerals/processing/bevillquestions.html. 4.2 Beneficiation Processes As previously described in Section 4.1, the beneficiation process does not alter the chemical composition of the ore; rather, these processes are intended to liberate the mineral ore from the host material. The crushing and grinding steps enable the exploitation of differences in physical properties such as gravimetric, magnetizability, and surface ionization potential to aid in separation (Aplan, 1988). The separation processes typically employed are (1) gravity separators, (2) electrical/magnetic separators, and (3) flotation separators. Gravity concentration methods separate minerals of different specific gravity by exploiting the variance in their gravity-driven movement through a viscous fluid. For successful separation using this technique, there must exist a marked density difference between the mineral and the gangue. Examples of gravity separators include (1) jigs; (2) pinched sluices and cones; (3) spirals; (4) shaking tables; (5) pneumatic tables; (6) duplex concentrators; (7) Mozley laboratory separator; and (8) centrifugal concentrators. 4-4
Rare Earth Elements Review Section 4 – Resource Processing Electrical and magnetic separation methods are considered similar because they exploit some of the same properties of the mineral and thus their application can overlap. Magnetic separators utilize the differences in magnetic properties between the mineral of interest and gangue. There are two primary classifications of materials when applying magnetic separation: (1) diamagnetic and (2) paramagnetic. Diamagnetic materials are repelled along the lines of magnetic force, while paramagnetic materials are attracted along the lines of magnetic force. There are two primary classes of magnetic separators: low- and high-intensity process units. Highly paramagnetic materials (e.g., ferro-magnetic) are typically separated using low-intensity separators, such as drum separators. Specifically for rare earths, roll separators that utilize alternate magnetic and non-magnetic laminations are commonly used. Highintensity separators are employed to separate very weak paramagnetic minerals and include common configurations such as induced roll magnetic separators and Jones separators. Electrical separation exploits electrical conductivity differences among various minerals in the ore. For optimum results, the ore feed needs to completely dry and only one particle deep. This restriction has limited the application primarily to beach and stream placers, such as those where REEs are found. Examples of electrical separators include plate and screen electrostatic separators. In applications where the ore particle size is too small for efficient gravity separation, flotation is typically employed. Flotation exploits the hydrophobicity of the mineral of interest and the hydrophilicity of the gangue. Accordingly, the hydrophobic mineral particles tend to “stick” to the air bubbles that are delivered into the process unit and rise to the surface where they are separated. To aid in this process, a variety of chemicals are added to the ore slurry and include collectors, frothers, and modifiers. 4.2.1 Bastnasite Beneficiation Although there are variations in the beneficiation of bastnasite, the general process involves crushing/grinding and separation by flotation. To provide a relevant example, details of the Molycorp Mountain Pass mine are presented. The ore containing bastnasite (7 percent REO) is crushed, ground, and classified in the milling process to achieve 100 percent passing a 150 mesh sieve prior to separation by hot froth flotation (Gupta and Krishnamurthy, 2004). Prior to flotation, the ore passes through six different conditioning treatments in which steam, soda ash, sodium fluosilicate, sodium lignosulfonate, and steam-distilled tall oil are added to aid the separation of the unwanted materials (often referred to as gangue). This process produces a 60 percent REO bastnasite concentrate. A detailed process flow diagram (PFD) of this process is presented in Figure C-1 in Appendix C. 4.2.2 Monazite/Xenotime Beneficiation Typically associated with dredged mineral sands, the monazite or xenotime ore is separated and concentrated after course grinding via gravimetric, flotation, or magnetic processes. As expected, the complexity of this process is dependent on the specific reserve. A detailed PFD of a conventional monazite extraction and processing operation is presented in Figure C-2 in Appendix C. 4.3 Extraction Processes Hydrometallurgy is the most common chemical extraction method of separating individual REOs from the mineral concentrate. Basicity differences between the various rare earths influence the solubility of their salts, the hydrolysis of ions, and the formation of complex species (Gupta and Krishnamurthy, 2004). The differences in these properties are exploited by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction to separate the individual REOs. Although some of the individual REOs and rare earth chlorides resulting from these processes have market value, further processing and refining are required to produce high-quality pure metal end products to maximize value. These processes are also utilized to recover REEs from recycled materials. Table 4-1 presents a list of rare earth extraction methods and a brief description of each. 4-5
Page 1 and 2: Rare Earth Elements Review Table of
Page 3 and 4: Foreword The U.S. Environmental Pro
Page 5 and 6: Abstract Rare earth elements (REEs)
Page 7 and 8: Table of Contents List of Acronyms
Page 9 and 10: List of Tables 2-1. Abundance of El
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