Separation of rare earths

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

The liquid-liquid extraction (solvent extraction) process was developed about 50 years ago and has found wide application in the hydrometallurgy of rare refractory and rare earth metals. Liquid-liquid extraction is used successfully for the separation of problematic pairs of metals such as niobium and tantalum, zirconium and hafnium, cobalt and nickel etc. Moreover, liquid-liquid extraction is the only method available for the separation of rare earth group elements to obtain individual metals. 

Solvent extraction in the separation of rare earths and trivalent actinides. B. Weaver, Ion Exch. Solvent Extr., 1974, 6,190-277 (538). 

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. 

Weaver, L. (1956). Separation of rare earths by liquid-liquid extraction, page 50 in Rare Earths in Biochemical and Medical Research A Conference Sponsored by the Medical Division, Oak Ridge Institute of Nuclear Studies, October 1955, Report No. ORINS-12, Kyker, G. C. and Anderson, E. B., Eds. (Office of Technical Services, Washington). 

Rapid Separation of Rare-Earth Fission Products by Cation-Exchange, Using Lactic Acid Eluant. J. inorg. and nuclear Chem. I, 163 (1955)-... 

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). 

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). 

Praesodymium may be recovered from its minerals monazite and bastana-site. The didymia extract of rare earth minerals is a mixture of praesodymia and neodymia, primarily oxides of praesodymium and neodymium. Several methods are known for isolation of rare earths. These are applicable to all rare earths including praesodymium. They include solvent extractions, ion-exchange, and fractional crystallization. While the first two methods form easy and rapid separation of rare earth metals, fractional crystaUization is more tedious. Extractions and separations of rare earths have been discussed in detail earlier (see Neodymium and Cerium). 

Samarium ore usually is digested with concentrated sulfuric or hydrochloric acid. The extraction process is similar to other lanthanide elements. Recovery of the metal generally consists of three basic steps. These are (1) opening the ore, (2) separation of rare earths first to various fractions and finally to their individual compounds, usually oxides or halides, and (3) reduc-... 

Yttrium oxide is produced as an intermediate in recovery of yttrium from xenotime and monazite (See Yttrium, Recovery). The oxide is produced after separation of rare earth sulfates obtained from digesting the mineral with sulfuric acid on a cation exchange bed, precipitating yttrium fraction as oxalate, and igniting the oxalate at 750°C. 

The separation of rare earth oxides at 2500° C in a Solar furnace has been attempted [48], and Ce4+-oxide was obtained in a pure state from its mixture with lanthanum oxide. 

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. 

Although all factors involved in the TBP extraction process have not been fully evaluated, a successfull separation of rare earths on a large scale has been achieved by this method. 

The separation of rare earths by anion exchange resins depends on somewhat different factors for a given chelating agent than to the cation exchange resins. For EDTA in an anion exchange process the order of elution is not the inverse order of the stability constants as in cation exchange process, rather [79] 4... 

The application of cellulosic anion exchanger in the separation of trace amounts of rare earths has also been investigated. Diethylaminoe-thyl cellulose paper and 0.026M citric acid were found to be the most satisfactory. A separation factor of 2.6 between Eu and Ce was obtained [123]. It has been found [124] that a mixture of HC1 and various aliphatic alcohols can be successfully used as eluant for the separation of rare earths by paper chromatography (Whatman No. 1). 

Extraction of the rare earths with acetylacetone has been investigated [418, 419] and is found to be enhanced by the decreasing basicity of the rare earth ions. The gas chromatographic separation of rare earth complexes with 2,2,6,6-tetramethyl-3,5-heptanedione has already been mentioned. The acetylacetonate complexes of the rare earths are reported to exist as either anhydrous [420, 421], mono- [422], di- [422] or trihy-drates [422, 423], Stites et al. [424] have studied the pH of the precipitation of several rare earth acetylacetonates and reported the melting points of the complexes. The europium acetylacetonate precipitated at pH 6.5, and melted at 144—45° C. The existence of monomers and dimers for these complexes in nonaqueous solvents has been proposed [421, 425-427],... 

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. 

Zvarova, T. S., Zvara, I. Separation of rare-earth elements by gas chromatography of their chlorides. I. Chromatog. 44, 604 (1969). 

Molybdenyl acetylacetonate, 6 147 12-Molybdosilicic acid, 1 127 analysis of, 1 128 Monazite, extraction of, 2 38 separation of rare earths from, 2 56... 

Crown ethers in the separation of rare-earth elements 92CLY77. 

Separation of rare earths by dynamic coating ion-interaction chromatography. 68... 

Homogeneous precipitation may be more useful in precipitation separation of rare earths. This involves addition of reagents which release the precipitating agent slowly in solution. Addition of trichloroacetic acid and dimethyl oxalate in place of Na2CC>3 and... 

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. 

Quantitative separations of rare earths have been achieved on a micro scale with different complexing agents as elements and some of the data are summarized in Table 1.23. 

Large scale separation of rare earths involve use of EDTA and NTA with the resin in cupric state. Zinc form resin has also been used with NTA. In some cases HEDTA is useful and in some cases superior to EDTA and NTA since resins in hydrogen form can be used. EDTA is probably the best reagent for the separation of the entire series of rare earths. 

An example of the application of dynamic ion-exchange chromatography for the direct separation of rare earths is shown in Fig. 1.22. The sample was a sodium hydroxide leach solution from an aluminium processing operation and contained high concentrations of sodium, iron and aluminium. Due to matrix interference, these solutions could not be accurately analysed by inductively coupled plasma emission spectroscopy. Fig. 1.22 shows the chromatogram when the sample was separated by dynamic ion-exchange... 

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