The Ames Process for Rare Earth Metals

The Ames Process for the preparation of high purity rare earth metals (REM) is a descendent of the Ames Process for the preparation of uranium for the Manhattan Project - the metallothermic reduction of a metal-salt by an alkaline earth metal:

  2RF3 + 3Ca -> 2R + 3CaF2

In this case the rare earth fluoride is reduced by calcium metal, resulting in the rare earth metal and a calcium fluoride slag. The fluoride input is prepared from high purity oxide via reaction with a fluorine bearing compound:

R2O3 + 6HF ->  2RF3 + 3H2O

Additionally, the Ames Process includes direct reduction of the rare earth oxide for the four REM with very high vapor pressures, Sm, Eu, Tm, and Yb. 

R2O3 + 2La -> La2O3 + 2R(g)

Using lanthanum metal to reduce the oxide, the high vapor pressure species is separated by sublimating it out as a gas which is condensed to a solid in a condenser trap.

This is not where the process ends. Several more steps are used to further refine the metals from their as-reduced state. The selection of the method is determined by the basic properties of the REM we are preparing. Look at the melting-boiling points of the REM on the graph below.

Rare Earth Metals Melting and Boiling Points

There are distinct differences in the melting and boiling behaviors of the REM. The boiling temperatures are an indicator of the vapor pressures relative to each metal. If we rearrange the order of elements from increasing atomic weight to increasing boiling points this graph results:

Rare Earth Metals Melting and Boiling Points, arrange by boiling point

The graph is in order of increasing boiling point (or roughly decreasing vapor pressure). Nd is an exception, however, as you will see it is in the correct grouping. At their melting points, Thulium (Tm) has a vapor pressure of 73.4 mmHg, and Cerium (Ce) has a vapor pressure of 3.6(10)-12 mmHg (1 atmosphere = 760 mmHg). The blue separators define the four processing routes used to prepare the metals:

 

(1) Low melting to high melting, very high vapor pressure metals

  • Direct oxide reduction/sublimation
  • Refined by sublimation to final form

(2) High melting, moderately high vapor pressure metals

  • Calcium reduction of the metal fluoride
  • Vacuum casting to remove volatile impurities
  • Sublimation to remove non-volatile

(3) Moderate to high melting, modest vapor pressure metals

  • Calcium reduction of the metal floride
  • Vacuum casting to remove volatile impurities
  • Distillation to remove non-volatile

(4) Low melting, low vapor pressure metals

  • Calcium reduction of the metal fluoride
  • Vacuum casting to remove volatile impurities
  • Tantalum (Ta) precipitation (see video)

As the vapor pressure decreases to the low levels exhibited by group (4), the ability to refine via sublimation or distillation is lost. Hence the group (4) process does not include such a step. Neodymium (Nd) has a vapor pressure approximating Scandium (Sc) – however, Nd has a much lower solubility of Ta, hence, high purity Nd can be derived following the group (4) route.  The added expense of a distillation or sublimation step is not needed.

If you want to know about the purity of metals resulting from this process please see our discussion on purity page.

The Ames Process if further detailed in the Power Point presentation, "High Purity Rare Earth Metals Preparation."


 

More information on the Ames Process can be found here:

The Rare Earths,  F. H. Spedding & A. H. Daane, eds.

  (1961) John Wiley & Sons.

  Chapter 6 – Preparation of the Rare Earth Fluorides, O. N. Carlson & F. A. Schmidt

  Chapter 8 – Metallothermic Preparation of Rare Earth Metals, A. H. Daane

 

Handbook on the Physics and Chemistry of Rare Earths, Vol 1 – Metals, 

  (1978) K. A. Gschneidner, Jr. & L. R. Eyring, eds.

  Chapter 2 – Preparation and Basic Properties of the Rare Earth metals,

  B. J. Beaudry & K. A. Gschneidner