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230 Handbook on the Physics and Chemistry of Rare Earths gypsum (CaSO42H2O) is precipitated as solid residue of this reaction. Gypsum is highly stable and has high partition coefficients for REEs, and thus it is extremely important to study effective methods of REE recovery (eg, solvent extraction, adsorption) in the process of phosphoric acid produc- tion from apatite. Many economic igneous phosphate deposits show relatively high REE contents compared to sedimentary phosphate deposits, for example, apatite ores from Khibiny (Kola Peninsula, Russia) and Phalaborwa (South Africa) contains from several thousands to 8000 ppm REEs (Koopman and Witkamp, 2000; Table 14). Although the REE grades are lower than REE ore from Mountain Pass in the United States that contains abundant bastnäsite and monazite (8.9 wt.% of REE2O3: Castor, 2008a,b) and than REE-rich alka- line syenite from Nechalacho in Canada that contains zircon and fergusonite as REE host minerals (1.5 wt.% of REE2O3: Sheard et al., 2012), REE pro- ductions as by-product from apatite ores should be considered due to the ear- lier mentioned characteristics of apatite. Apatite crystals show various chemical compositions due to high structural tolerance (Pan and Fleet, 2002), and apatite ore for phosphoric acid production also contains impurities such as Fe, Mg, Si, and Al to some extend (EFMA, 2000). Therefore, studies concerning effective methods for REE recovery form phosphoric acid containing a variety of impurities and low REE contents are extremely important. Although, at the present time, no phosphate deposit pro- ducing REEs as by-products of fertilizer production is officially known, recent years have brought some significant progress in this field as mentioned later. In addition, about 80% of the REE in apatite rocks is incorporated into gypsum (CaSO42H2O) in the current process for phosphoric acid production from the apatite rocks (Peelman et al., 2014; Zielinski et al., 1993). If REE recovery from the apatite rocks are improved or REEs are recoverable from the residual gypsum, a significant amount of REEs can be produced from the apatite rocks as a by-product of phosphoric acid production. In Section 4.3.1, we firstly discuss REEs and actinide substitution mecha- nism of apatite and apatite-group minerals, secondary introduce the classifica- tion and variety of apatite deposit types by focusing on REE resource (especially HREEs). We finally present the studies concerning precipitation, solvent extraction and adsorption for REE recovery in the process of phospho- ric acid production. 4.3.1 REE and Actinide Crystal Chemistry of Apatite-Group Minerals The apatite-group minerals of the general formula, M10(ZO4)6X2 (M1⁄4Ca, Sr, Pb, Na, ..., Z1⁄4P, As, Si, V, ..., and X1⁄4F, OH, Cl, ...), are remarkably tol- erant to structural distortions and chemical substitutions, and consequently, are extremely diverse in chemical composition (eg, Elliott, 1994; Kreidler and Hummel, 1970; McConnell, 1973; Roy et al., 1978). Therefore, many apatite-group minerals occur in nature (Table 15) and more than 100 syntheticPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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