Separation of rare earths by ion exchange

Holmium is obtained from monazite, bastnasite and other rare-earth minerals as a by-product during recovery of dysprosium, thulium and other rare-earth metals. The recovery steps in production of all lanthanide elements are very similar. These involve breaking up ores by treatment with hot concentrated sulfuric acid or by caustic fusion separation of rare-earths by ion-exchange processes conversion to halide salts and reduction of the hahde(s) to metal (See Dysprosium, Gadolinium and Erbium). 

Eluants.—Citric acid buffered with ammonium citrate is the first eluant to be used in the separation of rare earths by ion exchange process. It is also the most extensively [85—89] investigated eluant. At low pH the individual rare earths move down the column at different rates. A plot of volume of eluted portion vs. concentration shows typical bell-shaped curves (Fig. 2) with widely spaced maxima characteristic of elution chromatography, although the system makes use of a chelating agent. 

Speddinc, F. H. et al. The separation of rare earths by ion-exchange III, Pilot plant scale separations. J. Amer. Ghent. Soc. 69, 2812 (1947). 

Spedding FH, Fulmer El, Butler TA, Powell JE (1950) The separation of rare earths by ion exchange. IV. Further investigations concerning variables involved in the separation of samarium, neodymium and praseodymium. J Am Chem Soc 92 2349-2354 Lefebvre J (1956) Potentiometric study of complex equilibriums. III. Potentiometric indicators. Compt Rend 242 1729-1732... 

Powell, J.E., 1964, The Separation of Rare Earths by Ion Exchange, in Eyring, L., ed.. Progress in the Science and Technology of the Rare Earths, vol. 1 (Pergamon Press, Oxford) pp. 62-84. 

The lanthanides form many compounds with organic ligands. Some of these compounds are water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating AGENTS). The complex formation is pH-dependent. Oil-soluble compounds are used extensively in the industrial separation of rare earths by liquid—liquid extraction. The preferred extractants are carboxylic acids, organophosphorus acids and esters, and tetraalkylammonium salts. 

The first successful separations of rare earths by this technique was achieved fifty years ago. Two techniques used in the separation of rare earths are (i) displacement chromatography and (ii) elution chromatography. Commercial ion exchangers involving both cation exchangers and anion exchangers are listed in Table 1.18. 

Oct. 18, 1960 Method of Separating Rare Earths by Ion Exchange F.H. Spedding J.E. Powell... 

Hence, once a B ion is freed from the resin, it is immediately complexed and there is much less tendency for it to be resorbed lower down the column as would happen with a stable cationic species. This is an illustration of separation by elution analysis. Its most important application is in the separation of rare earths. When used on a laboratory scale in chemical analysis, this separation technique is known as ion-exchange chromatography. 

The Separation of Rare-Earths, Fission Product, and other Metal Ions and Anions by Adsorption on Ion-Exchange Resins. J. Amer. chem. Soc.- 69, 2769—2881 (1947) (15 papers). 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

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