The name “Rare Earth Metals” has a special ring to it, branding the set of 17 elements on the periodic table as exclusive and elusive. However, as we will soon see, this is misleading, and does not show the whole picture. Much like helium, the rare earth metals are actually quite plentiful in the Earth’s crust. For instance, cerium is the 25th most abundant element in the earth’s crust. But what exactly are the rare earth elements (REE)? REE are the elements in the lanthanides series as well as Yttrium and Scandium that share unique magnetic, phosphorescent and catalytic properties that make them so essential to our modern technologies. However, currently the major supplier of rare earth metals to the rest of the world is China and they have been restricting their trade. Since rare earth metals are used in a myriad of applications ranging from hybrid cars to portable electronics, there is a desperate demand for them that is not being filled.
Rare Earth Metals are produced through several different processes usually involving high temperatures and a series of reactions. In the case of Lanthanum which will be the focus of this post, a derivation of what is called the Ames process is used to purify and create the REE. Lanthanum is generally found in its most stable state:, La2O3 and can be readily found in the earth’s crust at around 32 parts per million (ppm). The shortage, as mentioned before, arises because most of the deposits are not economically viable for extraction. This rare metal oxide then has to be reduced to a pure solid so that it can be used for its various purposes. There are even variations of the Ames process for Lanthanum, but an example will be provided and explained as well as a video with a slightly different reaction.
One reaction is the one below:
La2O3 + 6HF → 2 LaF3 + 3H2O
LaCl3 + 3Li → La + 3LiCl
In the first step of this reaction Lanthanum is fluoridated to become LaF3. However in the second reaction Lanthanum is reduced from an oxidation state of +3 to 0, while Lithium is oxidized from 0 to +1. This reaction does a disservice in understanding just how complicated the process is, which takes many steps, and manipulations of temperature. A very similar Ames process and the steps necessary are further explained in the second half of below video.
One important application of rare earth metals is in hybrid cars. Lanthanum is used to make nickel-metal hydride batteries which are used in most hybrid vehicles. A battery for a Toyota Prius required 10 to 15 kg of Lanthanum; a significant amount of Lanthanum is clearly needed for hybrid cars to expand in use. The electrochemistry of nickel-metal hydride batteries is as follows. The battery contains a positive electrode of nickel oxyhydroxide (NiOOH) and a negative electrode of a hydrogen-absorbing alloy. The discharge reaction on the negative electrode is as follows:
OH− + MH H2O + M + e−
As you can see the M, which stands for an intermetallic compound that will be discussed shortly, is being oxidized into MH. The reaction on the positive electrode is as follows:
NiO(OH) + H2O + e− Ni(OH)2 + OH−
In this half-reaction, the Ni is reduced from an oxidation state of +3 to one of +2. The overall discharge reaction is as follows:
Ni(OH)2 + M NiO(OH) + MH
During the overall discharge, the hydrogen ion moves from the negative electrode to the positive electrode which is the principle behind the reaction. The M in the negative electrode reaction is usually a compound written as AB5 in which A is a mixture of rare earth metals, including lanthanum and B is a mixture of nickel and cobalt, among other metals. This compound of rare earth metals is of vital importance in forming the metal hydride compound necessary for the reaction to take place.
Now that we exploited the useful properties of rare earth metals, there’s no turning back; rare earth metals are too widely used to be abandoned. Although they are non-renewable resources, they are relatively abundant in our earth. Another country needs to step up to take the near monopoly away from China. This is an unnecessary shortage, and certainty a solvable one.