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b) Replacement of Ca2+ and B4+ in homilite (Fe21 Â»Cat4") B2Si20io by Y3+ and Be3+, respectively, giving (Fe2+, Y| ) Be2Si20io does not change the crystal structure. Here the charge balance is quite effectively maintained.
d) Because of the similarity in ionic size, lanthanides and yttrium can replace Ca2+ (0.83) from scheelite (calcium wolframate), whereas scandium (0.83) can only be incorporated in (Fe, Mn) WO4 in place of either Fe2f (0.83) or Mn2+ (0.91). Simultaneous exchange of WO* by NbOÂ® or TaOf- takes place.
The same type of ionic size relationship is observed in uranates (U4+ = 1.05), where Gd, Tb, Dy, Ho and Er are more abundant than other rare earths.
e) Goldschmidt observed segregation of heavy rare earths, especially ytterbium in zirconates (Zr4+ = 0.87). Of all rare earths the ionic size of ytterbium is closer to Zr4+ than any other. Similarly in granites Y3+ and Sc3+ concentrate more than other rare earths.
Methods of Separation of Individual Rare Earth Elements
In the previous chapter we mentioned that the number of rare earth-containing minerals is amazingly high. However, very few of them besides monazite have been processed on an industrial scale. At the beginning of this century monazite was used almost exclusively in thorium production for the incandescent gas mantle industry. With the advent of atomic energy the interest in monazite became greater, thus reviving the market for thorium, and increasing the production of rare earths. Monazite, as well as bastnaesite, are the sources of light rare earths. In the USA euxenite is processed for the recovery of uranium, niobium and tantalum, but no data on the recovery of rare earths has been disclosed.
At the end of this chapter we will treat the working up of monazite as a representative for the mode of recovering rare earths from these minerals, but at first we will consider the different techniques for the separation of the individual components from a rare earth concentrate.
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.
(i) Fractional crystallization. â€”La, Pr andNd can be effectively separated from the more soluble rare earths as their double magnesium nitrate. According to Kbemers [SO] most of the commercially produced La is obtained by fractional crystallization of double ammonium nitrates. Recent critical reviews on this process are available [31, 32].
Sm, Eu and Gd can be concentrated by crystallization through double magnesium nitrates, followed by crystallization of bismuth magnesium nitrates . Sm and Eu are then removed by a proceedure based on valence change, and Gd is recovered by bromate crystallizations. Yttrium group earths may be conveniently separated by bromate crystallization.
Brunisholz et al. [33â€”36] have very carefully and extensively studied the rare earth-EDTA systems following the suggestion of Marsh  and Moeller , and they were able to fractionate various rare earth mixture [39â€”42].
(ii) Fractional precipitation. â€” Differences in basicity are often used in preliminary separation processes. Separation of La from other rare earths, concentration of less soluble yttrium group rare earths and the purification of Y from yttrium concentrate are a few examples of fractional precipita tion as hydroxides or basic salts. Aqueous, or better gaseous, ammonia is most convenient for large scale operations. The precipitated hydroxides are usually coagulated by heating to improve filterability, although a better yield is obtained at low temperature.
Double sulphate precipitation is one of the most common methods used in industry for the separation of cerium group from yttrium group rare earths. Various other prĂ©cipitants such as chromĂ˘tes, double chromĂ˘tes, ferrocyanides, phosphates etc. have been tried.
Homogeneous precipitation. â€”A modification of the fractional crystallization process in which the precipitating agent can be introduced at a controlled rate is effected by homogeneous precipitation. The efficiency of the separation is greatly improved in this method. Hydrolysis of urea in a mixture can generate hydroxyl ion at a controlled rate, and provides a means for basicity separation of yttrium group rare earths . In a homogeneous medium oxalate ions may be conveniently generated by the hydrolysis of dimethyl oxalate, and carbonate ions by the hydrolysis of trichloroacetic acid.