Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 234

a buffered acetate solution using sodium amalgam as a reducing agent. Lanthanum,
praseodymium, and neodymium, meanwhile, may be separated by fractional pre-
cipitation using ammonium nitrates. Yttrium, meanwhile, selectively precipitate
with sodium sulphate. Yttrium along with gadolinium, terbium and dysprosium
may also be separated by fractional crystallisation using a sodium rare earth EDTA
(ethylenediaminetetraacetic acid) or RE-bromate, in order to reach a commercial
purity of 99% oxides (Kleinberg et al., 1960; Gupta and Krishnamurthy, 1992).
At Bayan Obo mining complex, the solvent extraction technique (see
Fig. 8.13) is used to successively separate the REE mix. It uses EHEHPA (2-
ethylhexylphosphonic acid mono-2-ethylhexil ester) as a chelating agent in a hy-
drochloric medium. Each rare earth solution is precipitated by ammonium bicar-
bonate or oxalic acid and the salt is further oxidised into the REO (Schüler et al.,
2011). The method is preferably used for separating the light lanthanides because
it is a simple, rapid and selective. Ce(IV ) can e ciently be extracted from the
trivalent lanthanide ions and dissolved in an aqueous nitric acid solution with trib-
utyl phosphate (TBP). The same procedure is also used to separate La;Pr;Sm and
Nd but in this case it requires a series of extraction stages. This latter point is a
drawback in favour of the ion-exchange technique, which can be used to separate
gram or microgram concentrations of lanthanide ions in industrial production.
The ion exchange technique employs resins which consist of organic polymers
with sulphonic or carboxylic groups. The original cations of these groups are re-
placed by ones with a greater a nity for the resin. This a nity is greater, the
smaller the ionic radius of the hydrated substituting ion. Accordingly, the resin is
first flushed with a solution of cupric sulphate to prepare the column. Then, when
passed slowly through a resin, the aqueous trivalent lanthanide ions displace the
copper ones and the lanthanides become fixed in a narrow band at the top. This
exchange process establishes a certain separation between hydrated lanthanide ions
according to their ionic radius with the strongest bond that of the La-ion and the
weakest, Lu-ion 29 . Thereafter the lanthanide ions are eluted with a chelating agent
such as ammonium ethylenediamine tetra acetic acid (NH4 + EDTA) which passes
over the resin and exchanges the ammonium ion for the lanthanide. The stability of
the complexes increase from lanthanum to lutetium, so lutetium is the first eluate
and lanthanum the last. EDTA is therefore an effective means with which to sepa-
rate all rare earths except for the pairs EuGd and Y bLu. Alternatively, HEDTA
(2-hydroxyethylene diamine triacetic acid) is used for separating the TmY bLu
fraction whereby the isolated lanthanide complex easily releases the M +3 ion via a
simple pH shift (Kleinberg et al., 1960; Gupta and Krishnamurthy, 1992; Anderson
and Taylor, 2011).
29 Note that the order of sizes of hydrated ions is the inverse of that corresponding to the simple
ionic radii.