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REE Mineralogy and Resources Chapter 279 261 hemihydrate (CaSO40.5H2O) and gypsum (CaSO42H2O) are water contents, as is evident from chemical formulae. Zielinski et al. (1993) carried out REE leaching experiments of REE- bearing hemihydrate (0.68 wt.% REEs) produced as a by-product of mineral processing of apatite ore from Kola Peninsula using sulfuric acid on bench scale. This dissolves the hemihydrate and brings the REE into solution. The REE in solution inhibit the reprecipitation of gypsum, allowing for them to be removed through solvent extraction (80–85% REE). Hydrometallurgical processes, including precipitation, solvent extraction, and adsorption, play main roles in the recovery of the dissolved REEs. Precipi- tation and solvent extraction are established techniques for metal recovery and are useful especially for large-scale operations at high metal ion concentrations (Roskill, 2015). For REE recovery, solvent extraction is applied commonly. For example, Zielinski et al. (1993) carried out solvent extraction of REEs for hemihydrate cake produced from Kola apatite. In their experiments, the 40 wt.% solution of ROKANOL PIO in kerosene was used as a solvent for REE extraction. ROKANOL PIO is roughly an equimolar mixture of M2EHPA (mono(2-ethylhexyl)phosphoric acid) and D2EHPA (di(2-ethylhexyl) phosphoric acid). REEs in solvent react with NaSO4, after which REEs are recovered in the form of Na $ Ln double sulfates, which contain 24–25% REEs. The overall REE recovery of the process ranges from 80% to 85%. Advantage of this process does not disturb the phosphoric acid production, and additionally purifies the by-product gypsum so that it can be utilized in the manufacture of building materials. As to other studies, solvent extraction of REEs from apatite has been tested using various solvents, H2SO4 with different concentration and several precipitation methods (eg, Jarosinski et al., 1993; Wang et al., 2010a). However, such solvents are generally used for solutions bearing high REE contents. Adsorption, on the other hand, can be applied to the recovery of metal ions even from low concentration sources with relatively simple processes. Many adsorbents for REEs have been studied, nevertheless, there are no practical adsorbents. Recently, Ogata et al. (2014) synthesized “EDASiDGA” which is a new adsorbent with immobilized diglycol amic acid on the surface of sil- ica gel and easily synthesized at low cost (Fig. 58). EDASiDGA is able to selectively adsorb REEs from a high concentration solution of base metals (eg, Fe, Cu) in a low pH region. It is notable that adsorption ratios of HREEs on EDASiDGA are higher than those of LREEs because HREEs are more valuable (especially Dy) than LREEs. Adsorbed REEs on EDASiDGA are easily leached by 1 M HCl treatment and the EDASiDGA can be used repeat- edly. Apatite ores from the representative apatite deposits contain several thousand ppm of REEs (Table 14), which is not high compared with other REE ores including bastnäsite and monazite from carbonatite and alkaline rocks. Apatite generally contains some minor elements such as Fe, Al, andPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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