The periodic table incorporates more than 100 elemental varieties that make up the entire universe. There are common elements and other not so common elements. Contrary to their descriptive name, the rare earth elements, or metals, are not so very uncommon. But, they are relatively unknown.
The lanthanides constitute fifteen of the seventeen rare earth elements. The remaining two are nearly identical, both physically and chemically, to the lanthanides and occur with them in nature. The fifteen are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The two non-lanthanides are scandium and yttrium.
Why are these elements deemed rare, and why are they called earths? Since they are so similar, how were they first separated, purified, and identified? Finally, what is special about them, since nations are anxious about maintaining control over the supply?
Are Rare Earths Really Rare? or ‘Earth?’
To answer the first question: the rare earth elements are rare in a sense. They occur in rich deposits – but only in limited areas. They also occur in lesser quantities over widespread areas. They frequently occur together and are difficult to separate. These elements must be highly purified to be useful. In other words, for practical purposes, they might as well be rare.
As to their being rare earths… The word earths is an old term meaning oxides. This term was used, because early on in the processing of the rare earth metals, the substances formed were the oxides, rather than the metals. These elements underwent initial investigation during the late 1700s and early 1800s.
Separating Rare Earth Elements
As alluded to earlier, the rare earth elements are so similar in physical and chemical properties that it is extremely difficult to isolate and purify them. Historically, chemists have chosen the difficult procedure of fractional crystallization for the task. It sounds like it should be easy enough. However, scientists didn’t even know the number of the rare earth elements. And even if there were only two rare earth elements, being so similar, how could they know when separation was complete?
In order to successfully employ fractional crystallization, scientists needed to determine the best choice of starting materials. One might think it would be an easy matter to separate the chlorides, or perhaps the nitrates – but such simple compounds don’t vary enough to allow satisfactory separation. For such reasons, it took many decades to achieve successful isolation. More modern and successful techniques, such as ion exchange chromatography, were not available until the 20th century.
Ion Exchange Chromatography
One 1966 ion exchange patent discloses an EDTA-ion-exchange process. EDTA is a chelating agent, ethylene diamine tetracetic acid. This is a relatively complex material that constitutes the anionic (with negatively-charged ions) portion of the rare earth compounds. Rare earth EDTA complexes offer suitable properties that enable separation. It should be pointed out that an anionic resin might be used instead.
The mixture of rare earth compounds dissolved in a mobile liquid phase is introduced into a column containing a stationary ion exchange polymer resin. The resin may consist of positively-charged cationic polymer, or negatively-charged anionic polymer.
Once the mixed rare earth compounds are introduced into the column, the polymer forms weak bonds with the rare earth or the EDTA ions. The exact strength of the bonds is related to each of the rare earth metals.
A mobile phase or solvent is introduced that competes with the polymer for the attached ions. The compounds exchange back-and-forth between the resin and the solvent.
The relative bond strengths cause the metal salts to travel along the polymer, each at a different rate. Eventually the distance traveled is great enough to accomplish complete fractionation. At this point, the fractions are collected in vessels. Separation is complete. If it is not, the process can be repeated.
What Are They Good For?
There is a song by Edwin Starr with lyrics that run, “War, huh! What is it good for? Absolutely nothing.” That is not the case for the rare earth elements. At first, they were under-appreciated. But due to technology, they are now in high demand. So much so that governments are keeping strict watch on the available supply. Yes, these ‘rare’ elements are useful in such devices as lasers, superconductors, ceramic glazes, lighting, powerful magnets, catalytic converters, and x-ray and MRI scanning systems.