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

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gu e Co e t o a a d oc epos t g ocess a d Rare Earth Elements Review Section astes 3 – Life-Cycle ss o s Stages of Rare Earth Elements Mines Mining Overburden Mine Water Water: Radiologicals Metals AMD Turbidity Air: Dust* Waste Rock Stockpile Sub-ore Ore Stockpile Stockpile Air: Dust* Water: Radiologicals Metals AMD Turbidity Process Product Product management Waste Waste management unit Emissions * In addition to airborne-particulate hazards (e.g., respiratory impacts), a fraction of the dust may include radiological and metals contamination. gu e Co e t o a ace epos t ocess g a d astes ss o s Figure 3-4. Conventional hardrock deposit mining process and wastes emissions. Air: Dust* Dredging Dredge Wastewater Water: Radiologicals Metals AMD Turbidity Waste Rock Stockpile Sub-ore Stockpile Ore Stockpile Air: Dust* Water: Radiologicals Metals AMD Turbidity Process Product Product management Waste Waste management unit Emissions * In addition to airborne-particulate hazards (e.g., respiratory impacts), a fraction of the dust may include radiological and metals contamination. Figure 3-5. Conventional placer deposit processing and wastes emissions. 3-22
Rare Earth Elements Review Section 4 – Resource Processing 4. Resource Processing Rare earth milling and processing is a complex, ore-specific operation that has potential for environmental contamination when not controlled and managed appropriately. Waste streams from REE processing have been identified, and their hazardous waste potential assessed. Although potentially significant, based on case studies and chemical composition, the waste streams with the greatest pollution potential are the tailings and their associated treatment and storage. Heavy metals and radionuclides associated with REE tailings pose the greatest threat to human health and the environment when not controlled. However, adoption of new technologies and management processes show potential to reduce the risk of environmental contamination. The two major mineral sources of REEs are bastnasite and monazite and will be the focus of this section. However, it is important to note that production can come from a variety of minerals, such as xenotime, apatite, yttrofluorite, cerite, and gadolinite. Due to their strong affinity for oxygen, REEs are primarily present as oxidic compounds, and resources are often expressed as REOs. Processing REOs into usable products is a very complex process and often varies significantly between deposits. The major factors affecting the selection of treatment processes are listed below (Ferron et al., 1991): • Type and nature of the deposit (e.g., beach sand, vein type, igneous and complex ores) and its complexity • Type and nature of other valuable minerals present with REOs • Type and nature of gangue minerals present in the deposit (e.g., slimes, clay, soluble gangue) • Type and composition of the individual REO minerals • The social and environmental acceptability of the process. The primary steps involved in processing REOs are separation and concentration from the host material in acidic or alkaline solutions, separation of the REOs using solvent extraction or ion exchange, and reduction of the individual REOs into pure metals (Tran, 1991; Gupta and Krishnamurthy, 2004). The first step typically includes crushing the ore and separating the REO by flotation, magnetic, or gravimetric separation. This separation process dramatically increases the percentage of REOs in the working material. For example, the Mountain Pass mine separates and concentrates REOs in bastnasite from 7 percent to 60 percent. A tremendous amount of discarded waste rock (tailings) is generated in this process and is typically managed onsite or used as backfill material. Chemical changes typically do not occur during the first step, and this process is usually situated near the mine site to reduce transport costs. Subsequent steps in the process aim to change the concentrated mineral into more valuable chemical forms through various thermal and chemical reactions. Typically utilizing hydrometallurgy techniques (e.g., leaching, extraction, precipitation), the mineral concentrates are separated into usable oxides. Further processing by techniques such as metallothermic reduction can refine the oxides or metal mixtures into high-purity REMs. Figures 4-1 and 4-2 present block flow diagrams of typical extraction and processing steps involved in hardrock and placer mining operations that highlight potential environmental emissions. 4-1
Page 1 and 2: EPA 600/R-12/572 | December 2012 |
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
Page 11 and 12: Rare Earth Elements Review List of
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Rare Earth Elements: A Review of Production, Processing ...

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