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228 Handbook on the Physics and Chemistry of Rare Earths susceptibilities and relative abundances of REE-bearing minerals in parent granites, that is: HREE-rich ion-adsorption ores are derived from HREE-rich granites. A mild and wet climate (temperate or tropical climate of K€oppen–Geiger climate classification), a moderate denudation rate, and a setting of low hills are preferable conditions for both chemical weathering of granite and preser- vation of thick weathering profiles that may host ion-adsorption ore bodies. 4.3 Introduction to Apatite REE Deposit Phosphorous is one of the three major nutrients (N, K, P) required by plants and 80–90% of the phosphate mined in the world is used to produce chemical fer- tilizers. Securing stable supply of phosphate fertilizers is essential to a sustain- able food production considering a continuing rapid growth of world population (Van Kauwenbergh et al., 2013). The world resources of phosphate rocks are more than 300 billion tons (U.S. Geological Survey, 2015). Fig. 46 is a distri- bution map of economic and potentially economic phosphate deposits in the world. “Phosphate rock” is the term generally used in industry to describe min- eral assemblages with a high concentration of phosphate minerals, commonly francolite (Ca5(PO4, CO3)3(OH, F, Cl))—apatite (Ca5(PO4)3(OH, F, Cl)) series. There are two main types of phosphate deposits: sedimentary and igneous. The former deposits sometimes are called phosphorite and contain varieties of fran- colite (CO2-rich fluorapatite), the latter mainly consists of fluorapatite (F-rich apatite) (Ihlen et al., 2014). Later, we use terms sedimentary and igneous apatite deposit, ore, and rock in sense of phosphate deposit, ore, and rock. Apatites contain a variety of REEs in concentrations from several thousands of ppm to several wt.%. They are generally enriched in LREEs (eg, Pan and Fleet, 2002; Roeder et al., 1987), however, some apatites are rich in HREEs: apatites in sedimentary phosphate rocks (Emsbo et al., 2015) and deep-sea mud (Kato et al., 2011; Kon et al., 2014), and igneous phosphate rocks (Hoshino et al., 2015; Roeder et al., 1987: apatites containing over 20 wt.% REE2O3). In addition, both Th and U contents in apatite are very low in contrast to common REE minerals such as monazite and xenotime, and this may be a big advantage over the other types of REE deposits (eg, alkaline REE deposits). Thus, the extraction of REEs from apatite as by-products in the existing plants for phosphate fertilizer would be highly economically efficient and feasible compared with the development of new REE deposits. Phosphoric acid for fertilizer is mostly produced from fluorapatite ore (including phosphorite) using sulfuric acid as follows: Ca5ðPO4Þ3F + 5H2SO4 + xH2O ! 3H3PO4 + 5CaSO4 x=5H2O # + HF " where x1⁄40 for calcium sulfate anhydrate, x1⁄40.5 for hemihydrate, and most commonly x1⁄42 for dihydrate (gypsum) (Peelman et al., 2014; Wang et al., 2010a). This process is also known as the “wet process,” andPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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