Fractionation, crystallization

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. 

Fra.ctiona.1 Crystallization. Fractional crystallization, used until the early part of the twentieth century, is uneconomical for processing large quantities of lanthanides. Many recrystallization steps are required to recover high purity products. Several salts and double salts have been used ... 

Figure 9. A schematic and ideal model showing how the residence time of the magma in a steady-state reservoir of constant mass M, replenished with an influx O of magma and thoroughly mixed, can be calculated from disequilibrium data, in the simplifying case where crystal fractionation is neglected (Pyle 1992). The mass balance equation describing the evolution through time of the concentration [N2] (number of atoms of the daughter nuclide per unit mass of magma) in the reservoir is ...

T = [(N2) - (N2)] / X2 [(N2) - (Ni)] or T = [(N2/Ni) -(N2/Ni)] / X2 [(N2/N1) - 1], where parentheses denote activities or activity ratios (note that (Ni) = (Ni) because of the long half-life of the parent nuclide and the absence of crystal fractionation). If the (N2/N1) ratio is known, then the residence time can be calculated from the measurement of the (N2/N1) ratio in lavas erupted from the central conduit. An eccentric eruption, whose magma has bypassed the reservoir, may provide a value for the (N2/N1) ratio. 

However, the short half-lives of these nuclides make them more useful for constraining the timescales of processes occurring closer to the eruption, such as degassing or crystal fractionation at crustal depths (Condomines et al. 2003). 

In Figure 9.15, the relationship between the fractional change in the elemental ratio and the extent of crystallization F is plotted for different values of AD=Di2—Dn for partition coefficients less than 0.1, several tens of percent fractionation are needed before a change of a few percent in the ratio becomes visible. Crystal fractionation does not change incompatible-element ratios such as La/Yb, Zr/Nb,. .. except in extremely residual melts. 

Ghiorso, M. S. (1985b). Chemical mass transfer in magmatic processes. II. Applications in equilibrium crystallization, fractionation and assimilation. Contrib. Mineral. Petrol., 90, 1021-41. 

Heterogeneities in stable isotopes are difficult to detect, because stable isotope ratios are affected by the various partial melting-crystal fractionation processes that are governed by temperature-dependent fractionation factors between residual crystals and partial melt and between cumulate crystals and residual liquid. Unlike radiogenic isotopes, stable isotopes are also fractionated by low temperature surface processes. Therefore, they offer a potentially important means by which recycled crustal material can be distinguished from intra-mantle fractionation processes. 

Muehlenbachs K, Byerly G (1982) 0 enrichment of silicic magmas caused by crystal fractionation at the Galapagos Spreading Center. Contr Miner Petrol 79 76-79 Muehlenbachs K, Qayton RN (1972) Oxygen isotope studies of fresh and weathered submarine basalts. Can J Earth Sci 9 471 79... 

Sheppard SMF, Harris C (1985) Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites variation with crystal fractionation and interaction with sea water. Contrib Miner Petrol 91 74-81... 

Separation of niobium from tantalum and impurity metals is the most important step in its extraction from the ore. It may be achieved by several methods that include solvent extraction, ion exchange, fractional crystallization, fractional sublimation, and other techniques. Solvent extraction is apphed mostly in several large-scale commercial processes. Although the classical fractional crystalhzation method forms effective separation, it is a tedious... 

In case , a 1 1 double salt of p and n is formed. Pure crystal fractions of p and n may be obtained from p/n-compositions between p and E,or n and E2, respectively. Therefore, in this case, optical resolution is only possible for enantiomerically enriched samples. 

The long story of the methods for the separation of the individual rare earths may broadly be divided into two main parts a) classical methods b) modern methods. Old-fashioned classical techniques like fractional crystallization, fractional precipitation and fractional thermal decomposition were not only used by the early workers in the past, but still remain as very important methods for economical production of rare earths on commercial scales. Modem methods like solvent (liquid-liquid) extraction, ion exchange or chromatographic (paper, thin layer and gas) techniques have both advantages and limitations. 

The main difficulty with the classical fractionation process is that, as the fractionation progresses, the number of fractions increases, and their size becomes smaller. With fractional precipitation processes, the number of operations practicable is much smaller than the fractional crystallization, because of the trouble in redisolving the precipitates and following another reprecipitation. In fractional crystallization schemes sometimes the liquor and crystal fractions can be combined with the help of modem analytical techniques by determining their compositions, thus achieving multiplication of stages. 

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