Until fairly recently, the lanthanide - or rare earth - elements (REEs) were a little known group of industrial metals. Locked away at the bottom of the periodic table, REEs were often overlooked, perhaps due in part to their limited uses and difficult to pronounce names.
That was until 2009 when concerns about access to supply of the metal elements attracted global attention to the importance of REEs in green and high-tech applications, including energy storage, hybrid cars and electronic devices.
The lanthanide group includes elements with atomic numbers 57 through 71. Due to their similar properties and tendency to be found with the other lanthanides, scandium (atomic number 21) and yttrium (39) are also normally referred to as rare earth elements.
The Rare Earth Elements (and element symbols):
- 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)
- Scandium (Sc)
- Yttrium (Y)
Characteristics:
Despite their name, 'rare earths' are not uncommon in the earth's crust. In fact, cerium is about three times ore common than lead and all exist in greater quantities than platinum. The problem, however, is that they are widely dispersed and very difficult to separate from one another.
The lanthanide elements have similar atomic structures and are also characterized by complex spectra, which has resulted in their use in pigments, dyes and lighting instruments. Their affinity for non-metallic elements, such as hydrogen, carbon, oxygen, fluoron and nitrogen, forms very stable compounds, but results in difficulties when trying to produce pure metals.
REEs are often categorized into two groups: light-REEs, which include elements numbered 57 through 60 (lanthanum to neodymium), and heavy-REEs, which include elements numbered 62 to 71 (samarium to lutetium, including scandium and yttrium). Due to its instability and radioactivity, promethium (atomic number 61) is much more rare and not produced during commercial rare earth extraction processes.
History:
The names of many REEs refer to the small village of Ytterby, which lies in the Stockholm Archipelago off the east coast of Sweden. Mineral samples from a feldspar mine in Ytterby led to the discovery of many lanthanide elements in the late 18th century. Unique pigments in glazes produced by the feldspar from Ytterby resulted in geochemist Johan Gadolin (from whom gadolinium takes its name) discovering yttrium. Ultimately, four rare earth elements were discovered in the Ytterby ore.
Owing to the difficulty separating REEs, as well as their irregular occurance, most REEs remained laboratory metals for decades. In 1906, however, the first commercial application arose when lighter flints made using the pyrophoric mischmetal (an alloy of light REEs) were designed and produced by Karl Auer von Welsbach.
In the 1970s, the development of high-strength magnets using samarium and neodymium began to increase demand for the metals, while research into the use of REEs as catalysts and phosphors led to more demand for chemical forms of the elements.
By the late 1990s and early 21st century, REEs were being used in a wide variety of industries, including glass polishing (cerium) and colouring (erbium, neodymium), energy storage (lanthanum), electronics (samarium-cobalt and neodymium-iron-boron, or NdFeB, magnets) and x-ray technology (gadolinium).
Production:
As the REEs were beginning to be used in a wider range of industries in the late 1970s, the world's supply of REEs began shifting to China where the existence of large reserves and low production costs resulted in falling international REE prices. Mines in the US, South Africa and India, scaled back and, eventually, stopped production of REEs.
According to US Geological Survey statistics, China accounted for roughly 95% of global REE production in 2010. Chinese production is centred around the Baiyun Obo deposit in the north, as well as the yttrium bearing clay sands in five southern south provinces. The rare earth ion-adsorption clays contain a relatively high content of heavy REEs, whereas the ore body at Baiyun Obo is known for its high content of light REEs.
The Baiyun Obo iron-REE-niobium ore deposit in Inner Mongolia is the world's largest reserve of REEs and the largest source of rare earth concentrate. Rare earth concentrate, which contains 45-60% rare earth content, is a key intermediary in the production process and a by-product of the iron extraction process at Baiyun Obo.
Although occurring together, the 17 REEs differ sufficiently to require a number of different extraction techniques to by employed in order to produce high purity forms of each.
90% of the material produced from Baiyun Obo's rare earth concentrate is first treated with a sulphatizing roast process. In this process, the concentrate is mixed with sulphuric or hydrochloric acid and roasted at 752-932°F (400-500°C) in a rotary kiln. The purified RE sulphate is further treated to produce mixed REE chlorides and individual oxides via solvent extraction.
For the production of independent metals, calcium reduction is often used, which involves loading tantalum crucibles with fluoride forms of each element and calcium. When heated and held at high temperatures for long periods the molten metal separates from the slag, allowing impurities to be removed. The metal is cooled to form ingots and further processed into a high purity product.
To extract metals from the ion-adsorption clays, the ore is first dissolved with hydrochloric acid and then separated by solvent extraction.
In recent years, much investment has been made into potential non-Chinese sources of REEs. Molycorp Minerals has been in the process of restarting its Mountain Pass bastnasite mine in California for a number of years, which between the 1950s and 1970s was the world's largest source of lanthanide metals.
The principal ores in the production of rare earth elements are bastnasite and monazite.
Applications:
As a result of their unique properties, REEs are used in a wide range of industrial and high tech applications.
Mischmetal, as mentioned, has long been used to form flints, but is also used as an alloy to strengthen certain steels.
One of the most important applications for lanthanide metals is in high strength magnets. The development of samarium-cobalt and NdFeB magnets replaced electro magnets by exhibiting previously unachievable magnet energy densities. Rare earth magnets are now used in computer hard disk drives, electric motors and magnetic resonance imaging (MRI).
In chemical forms, the REEs are used in glass production, petroleum refining, water treatment, catalysts, dyes and pigments. Other applications for REEs include:
- In aerospace alloys (scandium)
- In yttrium-aluminum-garnet lasers
- In fluid catalytic cracking catalysts (lanthanum and cerium)
- In high refractive index glass (lanthanum and lutetium)
- As chemical oxidizing agents (cerium)
- In ceramic capacitors (neodymium)
- In phosphors (europium and terbium)
- In mercury-vapor lamps (europium)
- As an MRI contrasting agent (gadolinium)
- In neutron capture (gadolinium)
- In high strength magnets (samarium, dysprosium, neodymium)
- In lasers (various)
- In X-ray machines (thulium)
Sources:
Huang, Xiaowei et al. "Development of Rare Earth Hydrometallurgy Technology in China". Journal of Rare Earths. Vol. 23. No. 1, Feb. 2005 p. 1.
