Rare earth metals cation exchange resins

The ratio of the distribution coefficients of pertechnetate and perrhenate is about 1.6 to 2, comparable to adjacent rare earth metals. Technetium and rhenium may be separated by ion-exchange chromatography. However, efficient separations require some care and tend to be slow. On the other hand, cation exchange resins adsorb technetiiun only to a negligible extent so that pertechnetate can be rapidly separated from cationic elements . 

An alternative approach which allows the separation of an excess of alkali metal ions from other cations uses a chelating ion-exchange resin. This type of resin forms chelates with the metal ions. The most common of these is Chelex-100 . This resin contains iminodiacetic acid functional groups which behave in a similar way to ethylenediaminetetraacetic acid (EDTA). It has been found that Chelex-100 , in acetate buffer at pH 5-6, can retain Al, Bi, Cd, Co, Cu, Fe, Ni, Pb, Mn, Mo, Sc, Sn, Th, U, V, W, Zn and Y, plus various rare-earth metals, while at the same time it does not retain alkali metals (e.g. Li, Na, Rb and Cs), alkali-earth metals (Be, Ca, Mg, Sr and Ba) and anions (F-, Cl-, Br- and I-). 

Besides the treatment of raw water there is a big variety of applications of ion exchangers. Wastewater of the metal industiy is often contaminated with small amounts of metal ions. These problematic ions can be replaced by ions which are present in natural water. A subsequent treatment with anion and cation exchange resins results in an elimination of metal ions to a high degree. Rare-earth ions in water can be successfully separated by ion exchange. Organic ionic substances can also be treated by ion exchange. 

A process has been developed by Ayres for the purification of zirconium, in which the various impurities are absorbed upon a cation-exchange resin. The zirconium itself is not absorbed as it is in the colloidal condition. This state is not difficult to achieve with, for example, zirconyl nitrate ZrO(NOs)2, since it is normally hydrolysed to the highly insoluble hydrated oxide in a neutral or near neutral solution. A zirconium ore is therefore broken in concentrated sulphuric acid and the soluble zirconium sulphate converted to the nitrate by suitable means and passed through a column of resin in the usual manner. Amberlite I.R.-100 has been used, in the hydrogen form. Impurities such as iron, beryllium and rare earth elements are absorbed completely, together with about 80 per cent of the titanium. The resin capacity for zirconium, however, is as low as 0-84 mmoles/100 cm of resin, and it is therefore recovered virtually completely in the pure column effluent. The very small amount of zirconium taken up by the resin is probably retained by a surface absorption process rather than true ion-exchange. The zirconium can be precipitated by alkah from the effluent as the hydrated oxide, in massive form, for conversion to other compounds and finally to metal. The resin is regenerated for further use by elution of the cation impurities with, for example, dilute sulphuric acid. 

Several methods have been used to separate the lanthanides chemically solvent extraction, ion exchange chromatography, HPLC using Q-hydroxyisobutyric acid and, in limited cases, selective reduction of a particular metal cation.40-43 The use of di(2-ethylhexyl)orthophosphoric acid (HDEHP) for the separation of various rare-earth elements via solvent extraction has also been reported.44 16 This separation method is based on the strong tendency of Ln3+ ions to form complexes with various anions (i.e., Cl- or N03 ) and their wide range of affinities for com-plexation to dialkyl orthophosphoric acid. When the HDEHP is attached to a solid phase resin, the lanthanides can be selected with various concentrations of acid in order of size, with the smallest ion being the most highly retained. 

---