Rare Earth Phosphate Compounds

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

Until 1964, monazite, a thorium-rare-earth phosphate, REPO4TI13 (P04)4, was the main source for the rare-earth elements. Australia, India, Brazil. Malaysia, and the United Slates are active sources. India and Brazil supply a mixed rare-earth chloride compound after thorium is removed chemically from monazite. Bastnasite, a rare-earth fluocarbonate mineral REFCO3. is a primary source for light rare earths. From 1965 to about 1985. an open-pit resource at Mountain Pass, California, has furnished about two-thirds of world requirements for rare-earth oxides. In the early... 

Catalyst preparations have been previously discussed (17-18). Thus, the rare earth phosphates were prepared by conventional methods. They were then modified by introducing an impregnating compound (alkaline earth salt) either directly into the synthesis mixture, or after filtration and drying. After impregnation, this catalyst precursor was c Icined at 500°C for two hours. The product crystals were hexagonal in structuro and the quantity of alkaline earth was measurable by elementary analysis. 

In this section we treat some physical properties of the NaCl-type compounds that reflect the influence of the crystal electric field on the energy of the J ground state of the cationic 4f" levels. Due to the small radius of the 4f orbitals the crystal field of the anion neighbors acts as a small perturbation only. At room temperature no particular f orbitals are favored so that the crystal structure, for instance, is determined by the cation size only. Below say 100 K the influence of the crystal field on the physical properties becomes evident. In contrast to certain rare earth phosphates, arsenates and vanadates, however, crystal-structure distortions never occur due to the Jahn-Teller effect alone but are always coupled with a magnetic transition, that is, the structural changes are due to a magnetostrictive effect. However, the crystal field influences and may even inhibit magnetic transitions as in the case of certain Tm compounds. 

The synergistic corrosion protection reported by Ho, coupled with research indicating that larger size, increased molecular weight and asymmetry of organic inhibitors often improve their effectiveness, " led Markley et al. to investigate other rare earth organophosphate compounds, namely cerium diphenyl phospliate (Ce(dpp)j) and mischmetal diphenyl phosphate (Mm(dpp)3). 

Rare earth phosphates and related phosphor compounds... 

Ternary rare earth phosphates are formed with alkali and alkaline earth elements and in these compounds, too, the composition of the phosphate anion can vary (Hong, 1975a Hong and Chinn, 1976 Koizumi, 1976). From the... 

When the rare earth phosphates are precipitated with sodium phosphate, not only binary and ternary phosphates but also hydroxide phosphates can be obtained (Tananaev and Vasileva, 1963). The formula of these compounds may be as complicated as Prio(OH)3(P04)9 26H2O (Petushkova et al., 1969b). Except for scandium, very few hydrogenphosphates are known for the rare... 

The decomposition reactions of rare earth phosphate minerals promoted by sintering with alkali, alkaline earth, and other compounds have been widely studied (Kizilyalli and Welch, 1980). When the minerals are sintered with alkali compounds the double orthophosphates are formed. With CaO they decompose above 700°C to form calcium phosphate. RPO4 does not react with SiOj, and with AI2O3 the reaction does not begin until 1800°C (Hikichi et al., 1980a). 

Nearly all transition metals are oxidized readily, so most ores are compounds in which the metals have positive oxidation numbers. Examples include oxides (Ti02, mtile Fc2 O3, hematite C112 O, cuprite), sulfides (ZnS, sphalerite M0S2, molybdenite), phosphates (CeP04, monazite YPO4, xenotime both found mixed with other rare earth metal phosphates), and carbonates (FeC03, siderite). Other minerals contain oxoanions (MnW04, wolframite) and even more complex stmctures such as camotite, K2 (002)2 ( 4)2 2 O ... 

In the sequence of presentation originally given, the last compound to be considered was phosphates. An outstanding example in this context is the chlorination of phosphate minerals of rare earths. The chlorination of monazite, for example, may be represented by the reactions ... 

Ninety five percent of the phosphorus on Earth belongs to the minerals of the apatite group. Apatites are inorganic constituents of bones and teeth of vertebrate and man, as well as a basis of many pathologic sohd formations. Minerals of the apatite group are the main raw materials in the production of phosphorus fertilizers, fodder and technical phosphates, elementary phosphorus, and phosphor-organic compounds. The mineral is sometimes substantially enriched in rare-earth elements (REE) making their extraction possible (Altshuller 1980). 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

The subsequent sections are assigned by following the compound sequence of rare earth nanomaterials. Rare earth oxides are often regarded as the most important compoimd class while rare earth hydroxides are frequently used as their synthesis precursors. Therefore, we discuss the nanomaterials of ceria, R2O3 and other rare earth oxides along with the rare earth oxyhydroxides and hydroxides in Section 2. Rare earth oxysalts, such as phosphates, vanadates, and borates are... 

The s)mthesis method in high-boiling solvents has been developed to obtain high quality nanocrystals of a number of metals, metal oxides, semiconductors, and inorganic salts. For rare earth compounds, the s)mthesis of rare earth oxides, phosphates and halides would be discussed in this and latter sections of this chapter. The s)mthesis route is considered as colloidal s)mthesis since the products are usually well dispersed NCs in the reaction solution or certain solvents, and the products could only be separated with particular techniques. 

Nai+x[Zr2-xRx(P04)3] with Xmax < 1, where R is a rare earth cation (Fig. 10) [97-102], One can suppose that a similar situation can be observed for actinide(III) compounds (or solid solutions). However, none of them has been reported to the present time. An overview on known structures of actinide and similar lanthanide phosphates described above demonstrates a wide variety of coordinations for tri- and tetravalent actinides. The ThOn, UOn, and some LnOn polyhedra with n changing from 10 to 6 are shown in Fig. 12. 

The fluorine content of the raw phosphates mentioned in Table 1.5-2 is between 2.3 and 4.8% by weight. The raw phosphate contains numerous other elements particularly aluminum, iron and magnesium, as well as rare earths and sometimes uranium, all in the form of compounds. 

For any one rare-earth ion the SD0 energy level generally decreases from phosphates to arsenates and vanadates. It is well established that this decrease is connected with an increase in covalency. In this way, it will be noted that the arsenates behave as the phosphates. For each type of compounds, we can see that the SD0 energy level decreases from Gd to Y and Lu. 

The rare earth elements are very electropositive, and, as a consequence, they generally form ionic compounds. Mineralogically, the REEs therefore form oxides, halides, carbonates, phosphates and silicates, borates or arsenates, but not sulphides. (Henderson 1996). Their oxidation states are given in Table 3.2. 

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