Rare Earth Elements: A Review of Production, Processing ...
Rare Earth Elements: A Review of Production, Processing ...
Rare Earth Elements: A Review of Production, Processing ...
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<strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> <strong>Review</strong> Section 4 – Resource <strong>Processing</strong><br />
Table 4-1. <strong>Rare</strong> earth extraction methods (adapted from Meyer and Bras, 2011)<br />
Method Type Extraction Trait Process<br />
Liquid-Liquid<br />
Extraction<br />
Solid-Liquid<br />
Extraction<br />
Hydrometallurgy Solubility The liquid containing the desired element is mixed with<br />
an immiscible solvent, which preferentially dissolves the<br />
desired element. When the liquids separate, the<br />
desired element separates with the solvent.<br />
Hydrometallurgy Solubility The solid is placed into a solvent, which dissolves the<br />
desired soluble component.<br />
Solid Phase Hydrometallurgy Solubility The fluid containing the desired element is poured<br />
through a sorbent bed, which forms equilibrium by the<br />
liquid adsorption to the solid surface or penetration <strong>of</strong><br />
the outer layer <strong>of</strong> molecules on that source. Either<br />
undesired components can be washed-out or elutriants<br />
can be used to selectively extract the desired elements.<br />
Ion Exchange Hydrometallurgy Chemical Affinity The fluid containing the desired elements are mixed<br />
with an elutriant and poured through a resin. The<br />
molecules are separated based on their affinity split<br />
between the elutriant and the resin.<br />
Super Critical<br />
Extraction<br />
Hydrometallurgy Variety The fluid containing the desired element undergoes a<br />
reaction with CO2 at or over the critical temperature <strong>of</strong><br />
31 ◦ C and critical pressure <strong>of</strong> 72.9 atm. This amplifies<br />
minute differences between elements to allow<br />
separation.<br />
Electrowinning Electrometallurgy Electronegativity A current is passed from an inert anode through a liquid<br />
leach solution containing the metal. The metal is<br />
extracted by an electroplating process, which deposits<br />
the rare earths onto the cathode.<br />
Electrorefining Electrometallurgy Electronegativity The anode is composed <strong>of</strong> the recycled material. When<br />
the current passes from the anode to the cathode<br />
through the acidic electrolyte, the anode corrodes,<br />
releasing the rare earth solution into the solution, then<br />
electrowinning occurs.<br />
Electro Slag<br />
Refining<br />
Pyrometallurgy Density Electricity melts the metal. The molten metal is<br />
combined with a reactive flux, which causes the<br />
impurities to float <strong>of</strong>f the molten metal into the slag.<br />
4.3.1 Bastnasite Extraction<br />
The desired end product(s) dictates the subsequent processing steps <strong>of</strong> the bastnasite concentrate. Typical<br />
processes include leaching, washing, filtering, and drying or calcining to increase the percent REO from<br />
approximately 60% up to as much as 90%. In previous operation, the Mountain Pass Mine produced three<br />
commercial grades <strong>of</strong> bastnasite, with end uses such as glass polishing powders and master alloys for iron<br />
and steel production. To produce individual lanthanides, the concentrate was first calcined to convert the<br />
contained cerium to plus four valency while leaving the other lanthanides in the plus 3 valency. Acid<br />
digestion followed and resulted in the dissolution <strong>of</strong> most <strong>of</strong> the non-cerium lanthanides. The resulting<br />
solution was then processed using multistage solvent extraction to produce high-purity rare earth<br />
compounds, such as (1) neodymium-praseodymium carbonate, (2) lanthanum hydrate, (3) cerium<br />
concentrate, (4) samarium oxide, (5) gadolinium oxide, (6) terbium oxide, and (7) europium oxide. A<br />
detailed PFD <strong>of</strong> this process in presented in Figure C-1 in Appendix C.<br />
It is important to note that the Mountain Pass Mine (Molycorp) extraction processes and associated<br />
equipment are currently being upgraded and the previously described process may not accurately reflect<br />
planned or current practice.<br />
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