The separation of samarium, europium and neodymium from other lanthanides by distillation.

[NNadir]

Going through some old unsorted files among the papers I've collected over the years, I came across an oldie but goody, this paper:

Technique for Enhanced Rare Earth Separation. (Tetsuya Uda,1* K. Thomas Jacob,2 Masahiro Hirasawa1, Science 289 2326-2329 2000) The term "Rare Earths" is a colloquial term for "lanthanides" and as generally ill advised since the lanthanides are not really rare although they, like many other elements in the periodic table are subject to depletion, and within one century, reserves of them are expected to be depleted.

If we cared about future generations, this would upset us, but we don't care about future generations so I guess it's OK with us.

The extraction and separation of lanthanides as a class - they always occur in combination with one another albeit in varying proportions along with the radioactive potential nuclear fuel, the element thorium - is very dirty and energy intensive chemistry, particularly when such separations are exempt from environmental laws or where environmental laws are not enforced. (Many of the lanthanides themselves, lanthanum, neodymium, samarium among them have long lived naturally occurring radioactive isotopes.)

The world sources of the lanthanides are dominated by Chinese production.

These elements are very important in any devices involving magnetism, which is most of our electronic stuff like hard disks and similar devices and, on a macroscopic scale things like generators, most notably in wind turbines and electric cars, neither of which are as "green" as advertised, and neither of which represent efficient utilization of materials, since both have low capacity utilization.

Since the chemistry of all of the lanthanides are very close the separation of them from one another is an industrially challenging procedure, generally involving industrial scale chromatography or solvent extraction.

This paper is interesting because it describes a process for separating the lanthanides from molten salts by distillation by exploiting the differing stability of their +2 oxidation state, which are, apparently appreciably volatile. Distillation in general is a cleaner process than either solvent extraction or chromatography, at least where clean energy is available. (Distillation is also utilized to make some very dirty fuels; probably the largest use of distillation in the world is to refine petroleum, a very dangerous and unsustainable fuel.)

Some text from the paper, beginning with the introductory paragraph which succinctly says (without sarcasm) what I touched on above:

Rare earth elements and compounds find application in many advanced materials of current interest such as high-performance magnets, fluorescent materials, chemical sensors, high-temperature superconductors, magnetooptical disks, and nickel–metal hydride batteries. Powerful rare earth permanent magnets such as Nd2Fe14B and SmCo5/Sm2Co17have revolutionized technology, allowing miniaturization of devices such as the hard disk drive and compact disc player. However, the production cost of rare earth permanent magnets is very high, because of the high cost of extracting pure Sm or Nd metal used in their manufacture. The separation of individual rare earth elements is a difficult process involving solvent extraction or ion exchange...

Some process description:

A molybdenum boat, containing a mixture of trichlorides and a reductant, was placed at the middle of the graphite ring adjacent to the closed end of the stainless steel tube. Trichlorides (purity: 99.9%) used as the starting material were distilled once in a quartz reaction tube before use. The trichlorides in the Sm-Nd system were mixed in an equimolar ratio. In the Pr-Nd system, NdCl3, PrCl3, and Nd metal were mixed such that the rare earth elements were in an equimolar ratio. These mixtures were then transferred into the stainless steel tube in a glove box filled with Ar gas. Selective reduction was carried out in vacuum under conditions shown in Figs. 3 and 4. The temperature of the Mo boat was then raised to 1173 or1273 K for the vacuum distillation, after which the stainless steel tube was cooled to room temperature and the changes in mass of the inner graphite rings were measured in the glovebox. The chlorides deposited on the rings and the residue in the Mo boat were collected.

Some notes on material efficiency:

By using the technique we developed, separation can be achieved not only from mixtures of rare earths, but also from mixtures of rare earths and other transition metals. Many of the transition metal chlorides such as those of Fe and Co have higher vapor pressures than rare earth trichlorides. A large amount of scrap containing rare earths, Fe, and Co is produced during the manufacture of the rare earth magnets. The process outlined here is suitable for the recovery of rare earths from this scrap. (1–3).

Separation factors such as 570 for neodymium to samarium were obtained.

It's been 17 years since this paper was published, and I have no idea whether the techniques described herein have ever found industrial application, many ideas - even exceptionally good ideas - do not go commercial. The paper has, however, been cited 89 times according to Google Scholar.

I find it interesting for its utility for the recovery of lanthanides from used nuclear fuels, where they occur as potentially valuable fission products.

Enjoy the holiday weekend.

[NRaleighLiberal]

1. I love your chemistry posts!

I have to think back a bit - got my degree in 1982 and though I apply a scientific method to what I do in gardening (such as tomato breeding - and even my annual gardening projects), it is amazing how I still ponder molecular structures when looking at what our commonly used items are composed of.

The area of my thesis was using novel ways to synthesize polycyclic aromatic hydrocarbons...aryne chemistry. It was fun, that's for sure!

[NNadir]

2. Thanks. Knowing chemistry certainly changes one's life and the way that one looks at everything.

Polycyclics are a huge field right now, in particular the grandest polycyclic of them all, graphene.

Thanks again for your kind words. I'm glad someone enjoys these.