Cerium Extraction

L. H. Delmau, P. V. Bonnesen, G. J. Van Berkel and B. A. Moyer, Improved performance of the alkaline-side CSEX process for cerium extraction from alkaline high-level waste obtained by characterization of the effect of surfactant impurities, ORNL/TM-1999/209, ORNL, USA, 1999. 

REEs (from Ge to Gd) and what was held to he yttrium was a mixture of yttrium REEs (from Tb to Lu). Thus, in 1794 and 1803, respectively, no real yttrium and cerium were discovered. In 1826 G. Mosander, a disciple of Rerzel-ius, suspected that cerium extracted from cerite contained an impurity. Thirteen years needed the scientist to turn his conjecture into confidence. 

An alternative American process uses a feed produced by oxalate precipitation of thorium and rare earths. This precipitate is calcined to the oxides and dissolved in nitric acid for extraction with undiluted TBP. After stripping with 8N nitric acid, a high proportion of cerium extracts with the thorium, but the other rare earths are eliminated. The cerium is then back-washed in a separate extractor by means of 0 1 N sodium nitrite solution, which reduces it to the solvent-insoluble cerous condition. Thorium is then backwashed in the last extractor with either water or 2 per cent sulphuric acid. In order to make this process economic it was necessary to devise an efficient system of oxalic acid recovery. This was based upon treatment of the thorium and rare earth oxalates with sodium hydroxide and recycling the resulting sodium oxalate to the precipitation stage. 

The cerium extraction reaction into SME 529 solution in n-heptane is... 

Greek lanthanein, to lie hidden) Mosander in 1839 extracted a new earth lanthana, from impure cerium nitrate and recognized the new element. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Ce(IV) extracts more readily iato organic solvents than do the trivalent Ln(III) ions providing a route to 99% and higher purity cerium compounds. Any Ce(III) content of mixed lanthanide aqueous systems can be oxidi2ed to Ce(IV) and the resultiag solutioa, eg, of nitrates, contacted with an organic extractant such as tributyl phosphate dissolved in kerosene. The Ce(IV) preferentially transfers into the organic phase. In a separate step the cerium can be recovered by reduction to Ce(III) followed by extraction back into the aqueous phase. Cerium is then precipitated and calcined to produce the oxide. 

Four years before isolating yttria, Mosander extracted lanthanum oxide as an impurity from cerium nitrate (hence the name from Greek XavOaveiv, to hide), but it was not until 1923 that metallic lanthanum in a relatively pure form was obtained, by electrolysis of fused halides. 

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

Tri-n-butyl phosphate, ( -C4H9)3P04. This solvent is useful for the extraction of metal thiocyanate complexes, of nitrates from nitric acid solution (e.g. cerium, thallium, and uranium), of chloride complexes, and of acetic acid from aqueous solution. In the analysis of steel, iron(III) may be removed as the soluble iron(III) thiocyanate . The solvent is non-volatile, non-flammable, and rapid in its action. 

Discussion. Iron(III) (50-200 fig) can be extracted from aqueous solution with a 1 per cent solution of 8-hydroxyquinoline in chloroform by double extraction when the pH of the aqueous solution is between 2 and 10. At a pH of 2-2.5 nickel, cobalt, cerium(III), and aluminium do not interfere. Iron(III) oxinate is dark-coloured in chloroform and absorbs at 470 nm. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Ethereal extracts of pulp exploded during or after concentration by evaporation. Although the ether used for the extraction previously had been freed from peroxides by treatment with cerium(III) hydroxide, the ethereal extracts had been stored for 3 weeks before concentration was effected. (During this time the ether and/or extracted terpenes would be expected to again form peroxides, but no attempt seems to have been made to test for, or to remove them before distillation was begun). 

The transition-metal and rare-earth core-line XPS spectra show little, if any, BE shifts at all. Nevertheless, information about atomic charge and valence states can be extracted by examining other features in the spectra. The plasmon loss satellite intensity found in the spectra of Co-containing compounds provides a particularly useful handle on the Co charge. The lineshapes of RE spectra are characteristic of their valence state, as seen in the distinction between trivalent and tetravalent cerium in CeFe4Pni2 compounds. 

Flynn [72] has described a solvent extraction procedure for the determination of 54manganese in seawater in which the sample with bismuth, cerium, and chromium carriers, is extracted with a heptane solution of bis(2-ethylhexyl) phosphate and the manganese back-extracted with 1M hydrochloric acid. After... 

In this method lg of soil is refluxed with 2M sodium hydroxide after centrifuging to remove solids. The clean extract is digested with perchloric acid-nitric acid at 265 °C. The iodine content of the extract is determined by the catalytic action of iodine on the oxidation of arsenic III ions with cerium IV ions. Between 96 and 97% recovery of added iodine spikes to soil were obtained by this method. 

In 1942, the Mallinckrodt Chemical Company adapted a diethylether extraction process to purify tons of uranium for the U.S. Manhattan Project [2] later, after an explosion, the process was switched to less volatile extractants. For simultaneous large-scale recovery of the plutonium in the spent fuel elements from the production reactors at Hanford, United States, methyl isobutyl ketone (MIBK) was originally chosen as extractant/solvent in the so-called Redox solvent extraction process. In the British Windscale plant, now Sellafield, another extractant/solvent, dibutylcarbitol (DBC or Butex), was preferred for reprocessing spent nuclear reactor fuels. These early extractants have now been replaced by tributylphosphate [TBP], diluted in an aliphatic hydrocarbon or mixture of such hydrocarbons, following the discovery of Warf [9] in 1945 that TBP separates tetravalent cerium from... 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

Cerium is obtained from its ores by chemical processing and separation. The process involves separation of cerium from other rare-eartb metals present in the ore. Tbe ore is crushed, ground, and treated with acid. Tbe extract solution is buffered to pH 3 and tbe element is precipitated selectively as Ce4+ salt. Cerium also may be separated from other metals by an ion-exchange process. 

Elemental composition Ce 81.41%, 0 18.59%. The oxide can be determined by x-ray techniques. The compound may be digested with HNO3—HCl mixture, the acid extract diluted appropriately and analyzed by AA or ICP spectrophotometry (see Cerium). 

Acid soluble rare earth salt solution after the removal of cerium may be subjected to ion exchange, fractional crystalhzation or solvent extraction processes to separate individual rare earths. Europium is obtained commercially from rare earths mixture by the McCoy process. Solution containing Eu3+ is treated with Zn in the presence of barium and sulfate ions. The triva-lent europium is reduced to divalent state whereby it coprecipitates as europium sulfate, EuS04 with isomorphous barium sulfate, BaS04. Mixed europium(ll) barium sulfate is treated with nitric acid or hydrogen peroxide to oxidize Eu(ll) to Eu(lll) salt which is soluble. This separates Eu3+ from barium. The process is repeated several times to concentrate and upgrade europium content to about 50% of the total rare earth oxides in the mixture. Treatment with concentrated hydrochloric acid precipitates europium(ll) chloride dihydrate, EuCb 2H2O with a yield over 99%. 

Lutetium is produced commercially from monazite. The metal is recovered as a by-product during large-scale extraction of other heavy rare earths (See Cerium, Erbium, Holmium). The pure metal is obtained by reduction of lutetium chloride or lutetium fluoride by a alkali or alkaline earth metal at... 

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). 

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