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

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