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

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.
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