Wednesday, February 19, 2014

History of The Periodic Table Part 5: Rare Earths - A Story of Discovery, Demand and International Intrigue

The discovery of the rare earth elements is an especially interesting story. These elements make up half of an assemblage of thirty elements that are found in Group 3 on the periodic table, shown below. These thirty elements are the lanthanide and actinide series, the two groups that don't fit in and are located in the separate f-block (the single and double asterisked series).

Rare earth elements include all of the lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium), as well as two Group 3 non-lanthanides (scandium (Z=21) and yttrium (Z=39)) because they are found in the same ores as the lanthanides and share similar chemical properties.

Rare earths routinely make it into the international news because they are at the heart of the burgeoning global high-tech economy. They are used in everything from cellphones to missile systems to hybrid cars.

Examples of How Rare Earths are Used

Neodymium (below right) is used in laser technology but it is better known for its use as a rare earth magnet.;Wikipedia
Neodymium magnets are the most powerful permanent magnets known, Developed in 1982, these magnets, made from an alloy of neodymium, iron and boron (Nd2Fe14B), have replaced many other types of magnet in applications ranging from cordless tools to hard disk drives to missiles. The unique tetragonal crystal structure of Nd2Fe14B gives it very high resistance to being demagnetized and very high saturation magnetization, which means its maximum magnetic energy density makes it a very powerful magnet.

The power of these magnets depends on their alloy composition, their microstructure and how they are made. There are two basic manufacturing processes for neodymium magnets. The most powerful neodymium magnets are sintered. The raw materials are melted and cast into ingot molds. The ingots are then pulverized into tiny particles, which are sintered. The powder is magnetically aligned into blocks and then the blocks are heat treated, cut to shape and magnetized.

Yttrium (below) is used in lasers and superconductors.

Europium is never found in nature as the pure metal shown below because it almost instantly oxidizes in air.

Europium is one of several rare earth metals that are very useful as phosphors in lights and in TV screens. A phosphor is a substance that emits visible light when it is energized, usually by a beam of electrons (in the case of LCD lights and flat panel TV screens) or by ultraviolet light (in the case of fluorescent lights). In a TV, a pattern of red, blue and green phosphors create all the colours we see. In a fluorescent light, a mixture of the three phosphors combine into what we see as white light. Most of europium's uses exploit its fluorescence in one of its two possible (+2 or +3) oxidation states. The fluorescent nature of europium was known for many centuries but people called the substance fluorite (CaF2), shown below left, not knowing that its blue fluorescence came from Europium (Eu3+) impurities within the crystal structure of the mineral rather than from CaF2 itself.

A sample of fluorite from a mine in Durham, England in daylight (A) and under ultraviolet light (B) displays its natural bright blue fluorescence, below left.

Didier Descouens;Wikipedia
This is a natural example of doping - adding an impurity to a crystal structure in order to change its properties. In the 1960's, europium doping was put to its first commercial use as a bright red phosphor as the substance yttrium orthovanadate (YVO4:Eu3+). Red europium phosphors are shown below the mineral sample as part of a CRT (cathode ray tube) screen used in older (fat) TV sets and computer screens.

Until the 1960's, colour TV was impossible because three basic phosphors - red, blue and green - were needed to make all colours and no bright red phosphors were known. It is still one of very few ions known to emit bright red when it is excited.

Light of a specific wavelength is emitted when an electron in an atom drops back down from a higher energy state to a lower energy state. The wavelength of this emission depends on the symmetry of the site of the europium ion inside the crystal matrix it's embedded in, and this depends on different factors including what kind of mineral is doped with europium ions and whether the europium ion is in its +3 oxidation state or +2 oxidation state. For this reason, europium can be used in many different host materials to attain a wide range of phosphor colours. And it is why the Eu3+ ion glows blue in fluorite (CaF2:Eu3+) but bright red in oxides such as YVO4:Eu3+ or Eu2O3.

Terbium (below) is used in lasers, as a bright green phosphor in projection TV's and as an X-ray phosphor, and it is used as well in fuel cells and sonar systems.;Wikipedia
What Makes Rare Earth Elements So Useful and Valuable?

The world mines almost 150,000 metric tons of rare earth elements a year, with China producing 97% of the total. Although rare earth elements have a vast array of very specific as well as more general uses, it is their unique electron configurations, explored in detail in the previous article, History of the Periodic Table Part 4: Lanthanides and Actinides - Elemental Misfits? which gives them their particular value. Electron configuration plus atomic size (most exhibit lanthanide contraction) makes these elements unique physically and chemically. The practical uses of rare earth elements depends on their physical and chemical properties, some of which are shared among all of them, and this makes them difficult to separate from each other in the ore. Other qualities - optical, electric, magnetic, thermal, etc. - are specific to each particular element.

Thanks to their useful and unique properties rare earth elements are increasingly used in electronics, as powerful magnets (neodymium), catalysts (cerium and lanthanum), and in ceramics (all rare earths) and alloys (cerium, lanthanum, neodymium). Thermal properties of some rare earth elements help them stabilize alloys under great stress and varying temperature, such as parts inside jet engines.

Rare earth elements are especially and increasingly important in new low-carbon technologies (dysprosium, neodymium, terbium, etc.), such as electric vehicles, solar energy, fuel cells, wind energy technology and low-energy lighting. Global demand for them is already very high and is set to grow quickly. If you Google "rare earth shortage" or "rare earth demand" you will get many hits, all describing an increasingly critical global situation.

The commerce of these useful elements is as mysterious as the elements themselves, one reason being they are not traded on the global stock exchange like precious metals are. Instead, they are traded on the private market. They are also not usually sold in pure form but in mixtures (usually as very stable oxides) of varying purity. This makes procurement, buying and selling complicated for the burgeoning high-tech industry. There is the perception that these elements are rare (hence the name rare earth) and that their sources are rapidly being depleted. Like the gold rush, prices are pushed up and politics comes into play. They are now so valuable that the recycling of electronics and recovery from mining tailings have become economically viable sources of various rare earth elements.

Surprisingly, rare earths are actually quite abundant in the Earth's crust. Often many rare earth elements are found together in the same deposit. The problem is that they are also found in a dispersed state rather than in pure deposits like gold, copper and silver are, and they tend to be chemically very similar to each other, making separation difficult. These two considerations, along with high demand, are what make the rare earths so expensive. If you are interested in following their prices try It follows, along with other metals, one of the largest global suppliers of rare earths, HEFA Rare Earth, with a parent company in China and a Canadian subdivision. You might be surprised by their incredible price volatility.

Discovery of the Rare Earths

Chemical and physical similarity among the rare earths is what made their discoveries, beginning in 1794, such a long and harrowing process. Most of them were found in black mineral deposits in a Swedish mine. When the first of these elements were discovered, only two methods for extraction and separation were available - repeated precipitation and/or crystallization. These are two processes that you may have performed yourself in either high school or first-year university. Carrying them out divides the black ore into two different precipitates, suggesting the presence of two elements. At the time, the researchers had no idea there were actually far more elements present. It took 30 years to figure out that they could separate each of the two precipitates into different salts (elements) by heating them and dissolving the product in nitric acid.

However, unknown to them at the time, one element they called didymium was especially stubborn. It was actually still a mixture of elements. Not until optical flame spectroscopy was developed in the late 1800's did researchers find several unknown spectral lines in the didymium sample, indicating that a second element must be present. It was found to be a mixture of two elements - praseodymium and neodymium - that could be separated, thanks to a new procedure called fractional crystallization. Didymium is dissolved in nitric acid over heat and then the solution is cooled. The elements will crystallize out and settle but the precipitate will contain more of one element than the other one due to a subtle difference in solubility. This is repeated over and over in a cascade process until the two elements are finally separated.

Rare Earth Supply and Demand

Deposits of rare earth-rich elements are found all over the world - India, Brazil, South Africa and the United States, but production in these places dwindled over the years. In fact, China, which contains only 23% of the world's rare earth supply most of which is in a deposit in inner Mongolia (shown below), supplies almost all of the world's rare earths.

A NASA satellite view of Bayan Obo mine in Mongolia (false colour) is shown below - vegetation appears red, grassland is light brown, rocks are black, and water is green. Two circular open-pit mines are visible, as well as a number of tailings ponds and tailings piles.

Increasing demand is straining this supply and, as you can imagine, China is in an enviable position to restrict and manipulate this supply, an uncomfortable situation for other countries. Interestingly as of 2010, the USGS (U.S. Geological Survey) indicated that the U.S. actually has as much as 13 million metric tons of rare earth elements. Between 1965 and 1995, the Mountain Pass Mine in California supplied most of the world with rare earths, most of the demand at the time being for europium (for TV's). It closed in 2002 because China developed its own supply and undercut US pricing, making mining and purification uneconomical in the US. Since then, China has been flexing its muscle. It banned export of rare earths altogether to Japan in 2010, making the world very nervous. In 2012, the Mountain Pass Mine reopened under new environmental restrictions to resume production of rare earths and secure a supply for the large US market. This will be a story to follow in the next few years as other countries, such as Brazil in particular, begin to take advantage of their own large rare earth deposits and perhaps jump into the lucrative high-tech race themselves.

In the final article in this series we explore the future of the periodic table.

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