Elements rare earth

Examples of Rare Earth Elements That Readily Form 

Rare Earth Oxide/Hydroxide Interferes With 

Examples of Rare Earth Elements That Readily Form Doubly Charged Species and the Analyte Masses They Interfere With 

The determination of rare earth elements was one of the very first applications that attracted geochemists to ICP-MS, mainly because of the lengthy sample preparation and analysis times involved with previously used techniques such as ICP-OES and NAA. However, even though ICP-MS offered significant benefits over these techniques, it was not without its problems, because of the potential of spectral interferences from other rare earth elements in rocks or natural water samples. For that reason, instrument parameters have to be optimized, depending on the rare earth elements being determined and the kinds of interferents present in the sample. For example, plasma power and nebulizer gas flows must be adjusted to minimize the formation of oxide species. This is necessary because an oxide or hydroxide species of one rare earth element can spectrally interfere with another rare earth element at 

16 or 17 amu higher. The problem can be alleviated by using a sample desolvation device such as a chilled spray chamber to reduce oxide formation, but unfortunately cannot be completely eliminated. For that reason, to get the best detection capability for rare earth elements in geological matrices, instrument sensitivity must often be sacrificed for low oxide performance, and even then, mathematical correction equations need to be applied. Examples of rare earth elements that readily form oxides/hydroxides, and the elements they interfere with, are shown in Table 19.4.  

It is also worth pointing out that in addition to the formation of oxide species, some rare earth elements can generate high levels of doubly charged ions (ions with two positive charges as opposed to one). This is not so much of a problem with the determination of other rare earth elements, but their spectral impact on other lower-mass analytes. Examples of rare earth elements that easily form doubly charged species include barium, cerium, samarium, and 

The cerium group includes lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, and europium Eu. The yttrium group consists of yttrium Y, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, ytterbium Yb, and lutetium Lu. 

All the rare-earth elements occur in the HI oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable n oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state. 

Rare-earth element hydroxides, M(OH)3, precipitate from nitrate solution at pH values above 6.3-7.8 and reveal no amphoteric properties. Like thorium, the rare-earth elements yield acid-insoluble fluorides and oxalates, and soluble EDTA-, tartrate-, and citrate-complexes. 

Cerium (Ce, at. mass 140.12), which is the most popular rare-earth element, occurs also in the IV oxidation state. In chemical properties, cerium(IV) resembles Th and U(IV). The yellow Ce(OH)4 precipitates at pH 1. Cerium(III) is oxidized to Ce(lV) in acidic media by bismuthate, silver(II) oxide, or persulphate (in the presence of Ag ). 

The most important methods for separating the rare-earth elements are based on ion-exchange. 

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Elements in the s and p blocks of the table are referred to as typical elements whilst those in the d block are called transition elements and those in the/block are called actinides and lanthanides (or rare earth elements). 

Lanthanum was isolated in relatively pure form in 1923. Iron exchange and solvent extraction techniques have led to much easier isolation of the so-called "rare-earth" elements. 

The element occurs along with other rare-earth elements in a variety of minerals. Monazite and bastnasite are the two principal commercial sources of the rare-earth metals. It was prepared in relatively pure form in 1931. 

Terbium has been isolated only in recent years with the development of ion-exchange techniques for separating the rare-earth elements. As with other rare earths, it can be produced by reducing the anhydrous chloride or fluoride with calcium metal in a tantalum crucible. Calcium and tantalum impurities can be removed by vacuum remelting. Other methods of isolation are possible. 

Figure 47.6). By choosing which isotope to mea.sure, all of the rare earth elements can be analyzed accurately and quickly following their ion-exchange separation into just two fractions. 

Ca is replaced by a rare-earth element, resulting in a distorted perovskite stmcture, which is essentially orthorhombic. Orthoferrites, studied extensively in the early 1970s as potential data storage materials based on magnetic bubble domains (10), have been largely replaced by the garnet materials (see... 

D very weak or inactive many metal, alkaline-earth, and rare-earth element haUdes... 

Figure 10 presents the Curie temperature (T ) vs the TM-content (x) for Co- and Fe-based biaary alloys. Alloying rare-earth elements with small amounts of transition metals (x < 0.2) leads to a decrease ia Curie temperature. This is particularly obvious ia the Gd—Co system where it corresponds to a nonmagnetic dilution similar to that of Cu (41,42). This iadicates that TM atoms experience no exchange coupling unless they are surrounded by a minimum number j of other TM atoms. The critical number is j = 5 for Fe and j = 7 for Co. The steep iacrease of for Co-based alloys with x about 0.7 is based on this effect. 

Comparing the relative abundance of the rare earths and the other elements Hsted in Table 1, the rare earths are not so rare. Cerium, the most abundant of the rare-earth elements is roughly as abundant as tin thuHum, the least abundant, is more common than cadmium or silver. Over 200... 

Liquid—Liquid Extraction. The tiquid—tiquid extraction process for the rare-earth separation was discovered by Fischer (14). Extraction of REE using an alcohol, ether, or ketone gives separation factors of up to 1.5. The selectivity of the distribution of two rare-earth elements, REI and RE2, between two nonmiscible tiquid phases is given by the ratio of the distribution coefficients DI and D2 ... 

Industrial Apphcations of Rare Earth Elements," ACS Symp. Ser, 164,1981. 

Laser isotope separation techniques have been demonstrated for many elements, including hydrogen, boron, carbon, nitrogen, oxygen, sHicon, sulfur, chlorine, titanium, selenium, bromine, molybdenum, barium, osmium, mercury, and some of the rare-earth elements. The most significant separation involves uranium, separating uranium-235 [15117-96-1], from uranium-238 [7440-61-1], (see Uranium and uranium compounds). The... 

Some nut trees accumulate mineral elements. Hickory nut is notable as an accumulator of aluminum compounds (30) the ash of its leaves contains up to 37.5% of AI2O2, compared with only 0.032% of aluminum oxide in the ash of the Fnglish walnut s autumn leaves. As an accumulator of rare-earth elements, hickory greatly exceeds all other plants their leaves show up to 2296 ppm of rare earths (scandium, yttrium, lanthanum, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). The amounts of rare-earth elements found in parts of the hickory nut are kernels, at 5 ppm shells, at 7 ppm and shucks, at 17 ppm. The kernel of the Bra2d nut contains large amounts of barium in an insoluble form when the nut is eaten, barium dissolves in the hydrochloric acid of the stomach. 

Oxahc acid is used in various industrial areas, such as textile manufacture and processing, metal surface treatments (qv), leather tanning, cobalt production, and separation and recovery of rare-earth elements. Substantial quantities of oxahc acid are also consumed in the production of agrochemicals, pharmaceuticals, and other chemical derivatives. 

Uses of oxalic acid ia each region are summarized in Table 5 (58). The demand for agrochemical/pharmaceutical production and for separation/recovery of rare-earth elements in each region has been increasing. The use for marble polishing in western Europe is unique to the region. 

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

Anhydrous ammonium oxalate is obtained when the monohydrate is dehydrated at 65°C. The monohydrate is a colorless crystal or white powder, and dissolves in water at 0°C up to 2.17 wt %, and 50°C up to 9.63 wt %. It is slightly soluble in alcohol and insoluble in ether. It is used for textiles, leather tanning, and precipitation of rare-earth elements. 

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