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260 Handbook on the Physics and Chemistry of Rare Earths The main reaction called “wet process” in the phosphoric acid production is as follows (also see Section 4.3): Ca5ðPO4Þ3F + 5H2SO4 + xH2O ! 3H3PO4 + 5CaSO4 x=5H2O # + HF " As shown in this chemical equation, (hydrous) calcium sulfate is produced from the reaction and this by-product is called “phosphogypsum” (Zielinski et al., 1993). Phosphogypsum is divided into the following three types according to its hydrous contents: (1) anhydrite, which is corresponding to anhydrous calcium sulfate, CaSO4 (x1⁄40 in the above equation); (2) basanite, calcium sulfate hemihydrate, CaSO40.5H2O (x 1⁄4 0.5); and (3) gypsum, calcium sulfate dihydrate, CaSO42H2O (x1⁄42). Among them, gypsum is the most common by-product of wet process: for example, 5 tons of gypsum formed per ton of P2O5 production under typical processing conditions in the current plants. Many treatments for production of high-purity phosphoric acid were proposed. The following five processes are well known: (1) Dihydrate process, (2) Hemihydrate process, (3) Di-Hemihydrate process (double-stage), (4) Hemi-Dihydrate process (single-stage), and (5) Hemi-Dihydrate process (double-stage). Each method has both advantages and disadvantages (see EFMA, 2000 for detail). Dihydrate process, which is (1) in the earlier mentioned treatments, is most common in the above five processes of phosphorous acid production because of its low cost and simple flow. In this process, gypsum (calcium sul- fate dihydrate) is produced as by-product of phosphorous acid and incorpo- rates 80% of REEs into the crystal structure (Zielinski et al., 1993). In some countries, eg, Poland, the dumped gypsum represents the largest national REE resource and thus there have been some attempts to process the copious amount of REE containing gypsum already produced by the industry world- wide (Peelman et al., 2014). Lokshin et al. (2002), for example, carried out REE leaching experiments of REE-bearing gypsum produced by dihydrate process of apatite ores from Khibiny deposits (REE content: 0.30–0.37 wt.%) using HNO3. As the result, they succeeded to leach 90% of REE by 10–20% HNO3. There is also another process that uses NH4CO3 for REE-bearing gypsum. In this process, all REEs are incorporated in secondary precipitated CaCO3 and easily leached with HNO3, in addition, useful Ca(NO3)2 is produced as a by-product (Habashi, 1985; Peelman et al., 2014). Alternatively, the REE-bearing CaCO3 can be calcined to CaO and leached with NH4Cl leaving an REE-rich residue (Peelman et al., 2014). In the view of REE recovery, Di-hemihydrate process, which is (3) in the earlier mentioned treatments, is effective because this selectively produces REE-bearing hemihydrate in the decomposition process of REE-bearing apa- tite ore and it is easy to leach REEs from hemihydrate using dilute sulfuric acid (Peelman et al., 2014; Zielinski et al., 1993). The differences betweenPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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