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Rare Earth Elements vs. Rare Metals: an overview

In many of the discussion on rare earths there is some confusion between rare earth elements (REEs) and rare metals (RM’s).  Both are strategic minerals, and important in modern society in everything from our cellphones to our cars and even in our houses, but they are different. 

The REEs are the lanthanide series (lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium) plus scandium and yttrium. Usually broken down into the light REEs (LREEs) (lanthanum, cerium, praseodymium, neodymium, samarium, and europium) and the heavy REEs (HREEs) (gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium,) which usually include scandium and yttrium.  REEs (both light and heavy) are among the heaviest naturally occurring non-radioactive elements, and are extensively used in energy and manufacturing technologies.

Contrary to their label, REEs are not necessarily "rare"; the shortage lies in the lack of concentrated deposits that are economically feasible to recover.  Fortunately, REEs are often found together due to similar chemical properties.  REEs form crystalline complexes with nonmetal elements such as in monazite (CeLaPrNd)PO(­4) and can have varying chemical compositions while remaining structurally indistinguishable in nature.  

REEs play a critical role in existing and emerging energy, scientific, and military technologies. For example, dysprosium is used for heat-resistant permanent magnet alloys in wind turbines, and tellurium is used in solar panels. Compact fluorescent bulbs depend heavily on praseodymium as a phosphor material.  And particular as the developed world continues to invest heavily in green energy technologies. In general HREEs are more valuable than LREEs.

The Rare Metals (RMs) (niobium, tantalum, cobalt, indium, zirconium, gallium, and lithium) are a collection of metallic elements used in emerging technologies which are often confused with REEs. The RMs, however, have few common similarities. Unlike REEs they are not found in proximity to one another on the periodic table. Four of these elements are transition metals belonging to three distinct periodic groups and two periods. Two of these elements are metalloids, meaning that they have certain metallic uses and certain non-metallic uses. RMs have very different chemical properties from each other. For instance, cobalt is the only ferromagnetic element of the group. Despite the diversity of these elements, all are critically important for the development of specific technologies such as industrial alloys. Some RMs can be used as substitutes for certain REEs, while other rare metals have completely unique applications.

REEs Geology and Mining

REEs occur in many minerals but typically in concentrations too low to be refined in an economical manner.  While the concentration of REEs in the Earth's crust is estimated to be higher than the concentration of other metals mined for industrial use, such as Cu or Zn, the REEs are not usually concentrated in ore deposits in amounts that can be easily or economically mined.

Economically exploitable REE concentrations are generally found in uncommon types of igneous rocks such as carbonates and alkaline rocks. They can also be found (as parts of mineral compounds) in placer deposits, residual deposits due to weathering, pegmatites, iron-oxides copper-gold deposits, and even in marine phosphates. Combined with the scattered nature of deposits, exploration companies are still searching for suitable locations to mine, and the price volatility of the REE market, especially in the recent years has not helped.

Processing REEs is often a two stage process, with the first stage being a standard mineral processing approach with crushing, scrubbing, and flotation to produce a concentrate.  The second stage is primarily a hydrometallurgical process featuring leaching and precipitation, with each REE being handled by a separate stage. Due to the large number of steps the REEs must go through to be purified requiring many different chemicals and reagents for these processes, there is a potential of creating toxic waste, which must be handled.

RMs Geology and Mining

High-grade ores naturally occur in specific regions within a few countries. Tantalum and niobium are typically found in ores together, primarily in coltan. 80% of coltan is mined in the Congo. Cobalt is typically found in copper or nickel ores. 40% of cobalt also comes from the Congo.

There are no sites specially dedicated to mining indium. It is found in very low quantities in zinc ore.  Like Indium, Gallium is often produced as a byproduct of Zinc and Aluminum mining.

80% of world zirconium comes from igneous rock and gravel mined in South Africa and Australia.  While zirconium is more abundant than copper and lead, most of its sources are not economically viable to mine.

Lithium is extracted from pegmatites, brines, and sedimentary rocks. The highest concentration brines occur in the relatively shallow ocean waters on the coasts of Chile, Argentina, China, and Tibet. Trace amounts of lithium are found in almost all igneous rocks and in the waters of mineral springs, but it is difficult to find economically viable deposits.

The RMs are mined in fairly conventional manners in either surface or a few cases underground operations. The processing is in a conventional manner with crushing, grinding, and flotation being the common processes to produce a concentrate that is then smelted.  Some lithium deposits are from brines where an in-situ method is employed.