Rare Metal and Rare Earths

What are Rare metals?

Rare metals are naturally occurring metallic elements that are considered relatively scarce or challenging to extract. Some rare metals might be abundant in the Earth’s crust but it is difficult to process, and others are genuinely rare in terms of their geological distribution. These metals often exhibit unique chemical and physical properties, making them indispensable in high-tech applications.

Rare metals include Rare Earth Elements (REEs), Platinum Group Metals (PGMs), and other metals like Tungsten, Tantalum, Niobium, Beryllium, Lithium, and Cobalt While some are abundant but difficult to process, others are genuinely rare in geological distribution.

Classification of “Rare Earth Metals”

The classification of “rare metals” is not universally fixed and can depend on the criteria used, such as their crustal abundance, the difficulty of their extraction, their economic significance, and supply risks. These metals are classified as follows:

By Element Group:

• Rare Earth Elements (REEs): This is a distinct group of seventeen metallic elements: the fifteen lanthanides (Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu)), Yttrium (Y), and Scandium (Sc). They share similar chemical properties and often occur together in nature. REEs are sometimes further divided into Light Rare Earth Elements (LREE) (Lanthanum to Samarium) and Heavy Rare Earth Elements (HREE) (Europium to Lutetium, including Yttrium). Scandium is sometimes excluded due to its differing properties.

Platinum Group Metals (PGMs):

• This group comprises six noble metals: Platinum (Pt), Palladium (Pd), Rhodium (Rh), Iridium (Ir), Osmium (Os), and Ruthenium (Ru). They share chemical similarities and often co-exist in geological settings.

Other Rare Metals:

• This is a broader category that includes various metallic elements considered rare based on different criteria. These are: Tungsten (W), Tantalum (Ta), Niobium (Nb), Beryllium (Be), Lithium (Li), Cobalt (Co), Germanium (Ge), Gallium (Ga), Indium (In), Rhenium (Re), Tellurium (Te), Vanadium (V), Zirconium (Zr), Hafnium (Hf), Rubidium (Rb), Caesium (Cs), Thallium (Tl), Cadmium (Cd), Selenium (Se), and Antimony (Sb).

THE SEVEN RAREST METALS

In terms of abundance in the Earth’s crust, the rarest metals are: gold, platinum, osmium, iridium, palladium, ruthenium, rhodium, tellurium and rhenium. These metals are different from Rare Earth Elements, which aren’t actually rare in terms of abundance, but are rarely found in concentrated ore deposits.

Geology of Rare Metals

Rare metals are often associated with specific geological formations controlled by processes viz., tectonic settings, magmatic differentiation, hydrothermal processes, and secondary enrichment. The primary geological environments where rare metals occur include:

Igneous and Magmatic Deposits:

1. Carbonatites and Alkaline Rocks: Carbonatites are igneous rocks containing over 50%, forming from CO₂-rich magmas, are significant hosts for REEs, Niobium, and Tantalum Minerals found in these settings include bastnäsite and monazite. Carbonatites are often enriched in incompatible elements like REEs. They typically form in continental rift zones. Alkaline igneous rocks, formed from cooling magmas derived from partial melting of the Earth’s mantle, are also enriched in elements such as zirconium, niobium, strontium, barium, lithium, and REEs. Examples of REE deposits in carbonatites include Mountain Pass in California and Bayan Obo in Inner Mongolia Peralkaline deposits, oversaturated with Na2O and K2O relative to Al2O3, can also host REE-bearing minerals like apatite, xenotime, and monazite.
2. Layered Mafic-Ultramafic Intrusions: These contain PGMs (Platinum, Palladium, Rhodium, Iridium, Osmium, Ruthenium) and Chromium, forming through fractional crystallization of mafic magmas, concentrating sulfide minerals like sperrylite (PtAs₂) and pentlandite.
3. Pegmatites: These coarse-grained igneous rocks, formed from highly evolved magmas, are rich in Lithium, Beryllium, and Tantalum. Common minerals include spodumene (LiAlSi₂O₆) and beryl [Al₂ (Be₃Si₆O₁₈)]. Pegmatites peripheral to large granitic intrusions can also bear REEs, although they are generally small.
4. Iron-oxide copper-gold deposits: These can contain large amounts of REEs and uranium, like the Olympic Dam deposit in South Australia.
5. Magnetite-apatite replacement deposits: Trace amounts of REEs can be found in these deposits.

Hdrothermal and Metamorphic Deposits:

1. Hydrothermal Veins: These are a source of Tungsten, Molybdenum, and Tin, where hydrothermal solutions precipitate minerals like scheelite (CaWO₄) and cassiterite (SnO₂) in fractures and fault zones. Hydrothermal veins cutting alkaline igneous complexes can also host REE ores. Indium and Gallium can also be found in hydrothermal veins.
2. Skarn Deposits: These host Tungsten and rare earth-bearing minerals, forming through metasomatic reactions between magmatic fluids and carbonate rocks, leading to mineral assemblages including garnet and pyroxene. Skarns associated with alkaline intrusions can also contain economic concentrations of REE-bearing minerals.
3. Greisen Deposits: Greisen is a pneumatolytically altered granitic rock composed largely of quartz, mica, and topaz. The mica is usually muscovite or lepidolite. Tourmaline, fluorite, rutile, cassiterite, and wolframite are common accessory minerals. It forms by hydrothermal alteration of granite, producing minerals like topaz and tourmaline.

Sedimentary and Lateritic Deposits:

1. Placers: These concentrate heavy rare metals like Platinum, Gold, Tantalum, Tin, and Zirconium due to mechanical weathering and erosion, accumulating dense minerals in riverbeds and coastal sands. Monazite and xenotime, which contain REEs, can also be concentrated in placer deposits. The paleoplacer Uranium/Au deposits at Elliot Lake, Ontario, have commercially produced REEs, hosted in pyritic quartz-pebble conglomerates.
2. Lateritic Soils: Formed in tropical environments by intense weathering, these can contain Nickel, Cobalt, and some REEs, with enrichment of iron oxides and secondary minerals like garnierite. Ion-adsorption clay deposits, a specific type of residual deposit formed by leaching REEs from igneous rocks and their adsorption onto clays, are primarily found in China and contain high heavy REE concentrations. Similar deposits might exist in the Eastern US due to similar granitic source rocks and weathering condition.
3. Phosphorite Deposits: Formed as chemical precipitates on continental shelves, these can contain up to 5000 ppm REE, with a mean value of around 1000 ppm, enriched in heavy REEs. REEs substitute for Ca in the mineral francolite in these deposits. Recovery of REEs as a byproduct of phosphate fertilizer manufacture has been investigated.
4. REE-bearing coals and related sedimentary rocks: Overburden and underburden related to coal seams may accumulate REEs from erosion of source material.

Mineralogy of Rare Metals and Rare Earths

The mineralogy of rare metals is complex, as they occur in a variety of mineral forms, often requiring advanced beneficiation techniques for extraction. Understanding these minerals is crucial for exploring new resources and optimizing extraction methods.

Rare Earth Elements (REEs):

Over 270 rare earth minerals have been described.

However, the most important REE minerals in ore deposits are bastnäsite and monazite-(Ce).

Bastnäsite has the idealized chemical formula LnFCO₃. Cerium is present in the largest quantity in bastnäsite-(Ce), which also contains significant amounts of lanthanum, neodymium, and praseodymium. Bastnäsite-(Ce) is the most common, but bastnäsite-(La) and bastnäsite-(Nd) also occur.

Monazite has the idealized chemical formula (Ln,Th)PO₄. Monazite-(Ce) (CePO₄) is a major source of REEs.

Both bastnäsite and monazite contain about 70% rare earth oxides by weight. Monazite can contain 500,000 ppm total REE.

Other significant REE minerals include the carbonates parisite-(Ce) and synchysite-(Ce) (often found intergrown with bastnäsite), and the phosphate xenotime-(Y) (YPO₄). Xenotime can contain 54-65% rare earth oxides.

Complex silicates like allanite-(Ce), eudialyite, and steenstrupine also contain significant amounts of REE, as do the carbonate ancylite-(Ce) and the phosphate churchite-(Y).

Minerals containing smaller amounts of REE can still be important sources. For example, loparite-(Ce), mined in Russia for niobium, contains about 1% rare earth oxide by weight, making REE extraction worthwhile.

Fluorapatite (Ca₅(PO₄)₃F) can sometimes be a source of rare earths as a by-product when mined for phosphorus in fertilizer production. Apatite can contain > 5400 ppm total REE4 and REE-enriched apatite is found.

In placer deposits, monazite and xenotime can be concentrated along with other heavy minerals.

Ion adsorption clay deposits feature REE adsorbed onto clay minerals like kaolinite, halloysite, and illite.

REEs are lithophile elements with a strong affinity for oxygen, often forming oxides, phosphates, and silicates.

Platinum Group Metals (PGMs):

• The main ore mineral for platinum is sperrylite (PtAs₂), associated with nickel ores of the Sudbury Basin, Ontario, Canada.
• Platinum is also found as a native platinum-iridium alloy and as the mineral cooperite (PtS).
• Osmium and iridium occur as an iridium-osmium alloy (iridosmium), with varying proportions.
• Ruthenium is found mostly in ores containing other PGMs. At Sudbury, it is found encapsulated within pentlandite, an iron nickel sulfide mineral.
• Palladium is normally found as a free metal alloyed with platinum and gold.
• PGMs are siderophile elements that readily alloy with iron and are often found in sulfide-rich environments.

Application of Rare Metal and Rare Earths

Applications of different categories of rare metals are:

Rare Earth Elements (REEs):

1. Magnets: Neodymium (Nd) and Dysprosium (Dy) are critical in the production of high-performance permanent magnets used in electric motors in hybrid vehicles, wind power generators, hard disc drives, CD and DVD players, imaging, portable electronics, microphones and speakers, and magnetic refrigeration. For instance, a Toyota Prius uses about 2.2 lbs of Nd in magnets , and large wind turbines can use up to 2 tonnes of magnets containing about 30% REE. Offshore wind turbines can require over a ton of rare earths. Samarium is also used in missile magnets and electric motors.
2. Catalysts: REEs are used in catalytic converters for automobiles, petroleum refining (Fluid Cracking Catalysts – FCC), and diesel additives. Cerium (Ce) is specifically used in catalytic pots. They act as oxygen storage and release agents. Scandium is used in refinery cracking for crude oil.
3. Alloys: REEs are used in metallurgy to increase plasticity and strength of alloys. Yttrium is a stabilizer and mold former for lightweight jet engine turbines and other parts, and a stabilizer in rocket nose cones. They also contribute to efficient hydrogen storage in REE alloys. Niobium and Vanadium, sometimes associated with REEs in applications, are used in low-alloy steels for construction, reducing weight and increasing lifespan.
4. Optics and Electronics: REEs have unique electron configurations leading to applications in LCD and plasma screens (display phosphors), lasers, and fiber optics. Europium (Eu) exhibits red luminescence, and Terbium (Tb) exhibits green luminescence. Praseodymium is used in dyes and magnets. Holmium is used in lasers and superconductors. Erbium is used in nuclear medicine and optics. Thulium is used in X-rays, lasers, and superconductors. Ytterbium is used in stainless steels.
5. Batteries: Lanthanum (La) is used in Nickel-Metal Hydride (NiMH) batteries, including those in hybrid vehicles (22-33 lbs in a Toyota Prius). Lithium, although sometimes classified separately, is essential for lithium-ion batteries in electric vehicles and energy storage.. Yttrium is also used in fuel cells.
6. Glass and Ceramics: REEs like Lanthanum, Cerium, Neodymium, and Praseodymium increase light translucency in glass and are used in special-purpose glass (infrared and ultraviolet absorbing, acid- and heat-resistant). They are also used in ceramics.
7. Lighting: Scandium is used to produce high-intensity lights and in mercury vapor lamps (resembling sunlight. Yttrium, Europium, Terbium, and Cerium are used in luminophores for energy-efficient lamps. Promethium is used in luminescent compounds.
8. Medical Applications: REEs are essential for MRI machines and other diagnostic equipment.. Erbium is used in nuclear medicine. Samarium can be used in medical scanners.
9. Other Uses: REEs are used in water treatment, nuclear fuel rods, pigments, fertilizer, and as medical tracers. Yttrium Iron Garnets (YIG) are used in frequency meters, magnetic field measurement devices, tunable transistors, Gunn oscillators, and cellular communications devices.

Platinum Group Metals (PGMs):

1. Catalytic Converters: Platinum (Pt), Palladium (Pd), and Rhodium (Rh) are primarily known for their use in automotive catalytic converters to reduce emissions. Platinum is also used in fuel cells.
2. Jewelry: Platinum, Palladium, Rhodium, and Iridium are used in jewelry due to their rarity, strength, resistance to tarnishing, and beauty.
3. Electronics: PGMs are increasingly used in advanced electronics. Ruthenium is used to harden platinum and palladium alloys for highly resistant electrical contacts and as a plating metal. Osmium is used in electrical and optical microscopy and in space mirrors and other industrial mirror applications.
4. Medical and Industrial Applications: Platinum is used in medical implants and cancer treatment drugs. Osmium is an important oxidant for industrial applications. Palladium is used in dentistry and surgical instruments.
5. Geochronology: Osmium plays a role in dating rocks. Rhenium’s decay to Osmium is particularly useful for dating ore deposits.

Other Rare Metals:

1. Tantalum (Ta): Primarily used in capacitors for smartphones, computers, and other electronic devices.
2. Niobium (Nb): Enhances the strength of superalloys used in jet engines and spacecraft. It is also used in superconducting materials for magnetic levitation vehicles.
3. Beryllium (Be): Used in X-ray windows and aerospace components. Copper-beryllium alloys are used in cars.
4. Lithium (Li): Essential for lithium-ion batteries used in electric vehicles and energy storage systems.
5. Indium (In): Used in touchscreens and LCD displays.
6. Gallium (Ga): Used in semiconductors, photoluminescent LEDs, advanced radar systems, and satellite communications.
7. Germanium (Ge): Used in photovoltaics and optical fibers.
8. Tungsten (W): Used in precision tools, shielding, and electricity applications.
9. Cobalt (Co): Used in smartphones, computers, magnets, medical implants, and radiation therapy.
10. Rhenium (Re): Important for high-temperature alloys in demand from aerospace and petroleum industries.
11. Tellurium (Te): In demand for fiber optic cables, used in alloys to improve malleability, in the semiconductor industry, and as a key component of cadmium-tellurium photovoltaic power cells.
12. Zirconium (Zr): Found in cars.