Thorium phosphate

Thorium metal, 24 759-761 in alloys, 24 760-761 preparation of, 24 759-760 properties of, 24 760-761 reactions of, 24 761 Thorium nitrate, 24 757, 766 Thorium oxalates, 24 768-769 Thorium oxide, 21 491 Thorium oxides, 24 757, 761-762 Thorium oxyhalides, 24 762 Thorium perchlorate, 24 764 Thorium phosphates, 24 765-766 Thorium pnictides, 24 761 Thorium sulfate, 24 764 Thorium-uranium fuel cycle, 24 758-759 Thorocene, 24 772 Thorotrast, 24 775-776 3A zeolite. See Zeolite 3A Three-boiling beet sugar crystallization scheme, 23 463-465 Three-color photography, 19 233-234 3D models, advantages of, 19 520-521 3D physical design software, 19 519-521 3D QSAR models, 10 333. See also QSAR analysis... 

Thorium is a radioactive metal that occurs naturally in several minerals and rocks usually associated with uranium. However, it is approximately three times more abundant in nature than uranium. On average, soil contains 6 to 10 ppm of thorium. Thorium is most commonly found in the rare-earth thorium-phosphate mineral, monazite, which contains 8% 10% thorium. Current production of thorium is, therefore, linked to the production of monazite, which varies between 5500 and 6500 tonnes per year, with approximately 300 to 600 tonnes of thorium recovered (NEA/IAEA, 2006a). 

M. Genet, V. Brandel, M.-P. Lahalle, and E. Simoni, Electronic energy transfer between coumarin 460 and Eu3+in thorium phosphate xerogel, C. R. Acad. Sci. Paris 311 (Series II), 1321-1325 (1990). 

Lanthanum is most commonly obtained from the two naturally occurring rate-earth minerals, monazite and bastnasite. Monazite is a rare earth-thorium phosphate that typically contains lanthanum between 15 to 25%. Bastnasite is a rare earth-fluocarbonate-type mineral in which lanthanum content may vary, usually between 8 to 38%. The recovery of the metal from either of its ores involves three major steps (i) extraction of all rare-earths combined together from the non-rare-earth components of the mineral, (ii) separation or isolation of lanthanum from other lanthanide elements present... 

Ytterbium occurs in minerals euxenite, a complex titanium niobotantalate gadolinite, a rare earth iron beryUium sdicate monazite, a thorium-rare earth phosphate and xenotime, also a rare earth-thorium phosphate. Abundance of ytterbium in the earth s crust is estimated to be 3.2 mg/kg. 

The origin of the anomalous states of Ps is not clear. Theory predicts that r) should be different from q only when Ps is in an excited state [72]. However, it seems unlikely that Ps would be excited in some solids and not in others otherwise very similar (e.g. Ps is normal in Teflon and liquid naphthalene, not in solid naphthalene). Therefore, finding q q cannot be indicative of the presence of an excited state of Ps. Most probably, the use of Eqs. (11-13) is incorrect. The number of systems that have been studied is still too limited to derive firm conclusions. However, it has been proposed that those cases where q q may correspond to some distorted states of Ps (78), which would arise from specific geometric conditions imposed over Ps by the matrices for instance, either when the traps have a shape which is far from spherical (e.g., rod-like in the case of solid naphthalene) or if they are surrounded by ions of quite different charges resulting in interactions of different strength on e+ and e in Ps (e.g., in thorium phosphate). 

In the acid route, the phosphate ore is digested with sulfuric acid at 230 °C for some hours and thorium phosphate is precipitated on adjusting the pH to 1. This is purified by first converting it into Th(OH)4, thence into thorium nitrate, purified as above. 

The principal source of thorium is monazite (p. 425), a phosphate of cerium and lanthanum with up to 15% of thoria. It is dissolved in concentrated sulphuric acid and the thorium phosphate precipitated with magnesium oxide. The washed phosphate heated with sodium carbonate gives crude thoria, ThOg, which is converted to the soluble oxalate and separated from the insoluble oxalates of cerium and lanthanum. After ignition to oxide the nitrate is made, purified by recrystallisation, and again calcined to thoria. 

Figure 18. The thorium phosphate double chain in Na ThP04(P207)2, projected along [010], at left the structure projected along [100] with Na atoms shown as spheres, at right, after [86].

AM2(P04)s. The orthophosphates with tetravalent actinides and monovalent elements with the general composition AM2(P04)3 have been investigated for more than 100 years. The first published work [25] was devoted to the thorium phosphate NaTh2(P04)3. The peculiarities of stmcture formation in the phosphate series AM2(P04)3 are still under study and discussion. 

Several stmctural modifications of the AM2(P04)3 orthophosphates are known at the present time. The realization of one or another modification depends on the kind and combination of the and A cations, on the synthesis conditions, namely temperature, nature of solvent, atmosphere and so on. For a long time, the monoclinic modification, space group C2 tc or Cc, was considered to be the most widely represented among the actinide and monovalent cation phosphates AM2(P04)3. Its formation had been observed for thorium phosphates and arsenates with A = Li, Na, K, Rb, Cs, Cu, Ag, T1 [26-33], for uranium compounds with A = Li, Na, K, Cs [26,29,30,34-38,39,40],... 

Some phosphates of the AM2(P04)3 family form other modifications as well neptunium phosphate NaNp2(P04)3 with monazite type structure, Pl ln (this type was obtained by heating the trigonal form in Ar + 5% H2 at 1100 °C [5,35]) neptunium and plutonium phosphates AgNp2(P04)3 and AgPu2(P04)3 with scheelite type structure, 74i/a [46], thorium phosphate NaTh2(P04)3 with... 

Thorium phosphate-diphosphate Th4(P04)4P207 (TPD, Pcam) is an actinide host phase due to its very high chemical durability and radiation stability [165-167]. TPD is synthesized by drying of thorium nitrate and phosphorus acid or ammonium phosphate solution, cold pressing at 300-800 MPa, and sintering of pellets at 1100-1250 for 10-30 hours. Th" in the TPD structure may be replaced by other tetravalent actinides but its isomorphic capacity is reduced with decreasing cationic radii in the following sequence > Np" > Pu". ... 

Preparation and Characterization of Lanthanide and Actinide Solids. Crystalline / element phosphates were prepared as standards for comparison to the solids produced in the conversion of metal phytates to phosphates. The europium standard prepared was identified by X-ray powder diffiaction as hexagonal EuP04 H20 (JCPDS card number 20-1044), which was dehydrated at 204-234 °C and converted to monoclinic EUPO4 (with the monazite structure) at 500-600 °C. The standard uranyl phosphate solid prepared was the acid phosphate, U02HP04 2H20 (JCPDS card number 13-61). All attempts to prepare a crystalline thorium phosphate failed, though thorium solubility was low. In the latter case the solids were identified as amorphous Th(OH)4 with some minor crystalline inclusions of Th02. 

Althou the principal constituents of monazite are rare earth and thorium phosphates, its composition varies widely within a given deposit and from place to place. Table 6.16 gives the composition of monazite concentrates from different locations. 

Based on preliminary leaching results for TPD (Dacheux et al., in press), normalized dissolution rates are of the order of 10 g/m d for neutral solutions. These leaching data are comparable to those for monazite and better than those for apatite. Thomas et al. (2000, 2001) have completed detailed leaching studies of Th-TPD and Th-U-TPD solid solutions as a function of surface area, flow rate, temperature and pH of the solution. For the Th-TPD, remarkably low leach rates of 10 g/m d were measured. Th-concentrations in solution were controlled by the formation of a thorium phosphate phase, identified by HRTEM, with a solubility <10 M in solutions in contact with the Th-TPD. With the substitution of uranium, the Th-U-TPD shows a slight increase in leach rate, lO " g/m d. The saturation concentrations in solution for U and Th were controlled by the formation of (U02)3(P04)2 5H20 and Th2(P04)2(HP04)H20, respectively. 

Pichot et al. (2001) conducted a preliminary study of irradiation effects on thorium phosphate-diphosphate. Powdered samples were irradiated with 1.5 Gy dose of gamma-rays. The formation of PO b and POO free radicals were detected using electron spin resonance (ESR) and thermoluminescence (TL) methods. These free radicals do not modify the macroscopic properties of the TPD and disappear when the sample is heated at 400°C. The implantation of 1.6 MeV He with a fluence of 10 ions/cm and 5 meV Au with a fluence 4 x 10 ions/cm causes some surface damage to sintered samples. Amorphization and chemical decomposition of the matrix were observed for the dose of 10 ions/cm and higher when irradiated with Pb (200 keV) and Au " (5 MeV). These effects were evidenced by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). 

Benard P, Brandel V, Dacheux N, Jaulmes S, Launay S, Lindecker C, Genet M, Louer D, Quarton M (1996a) Tli4(P04)4P207, a new thorium phosphate Synthesis, characterization, and stmctme determination. Chem Mater 8 181-188... 

Thomas AC, Dacheux N, Le Coustumer P, Brandel V, Genet M (2000) Kinetic and thermodynamic study of the thorium phosphate-diphosphate dissolution J Nucl Mater 281 91-105 Thomas AC, Dacheux N, Le Coustumer P, Brandel V, Genet M (2001) Kinetic and thermodynamic studies of the dissolution of thorium-uranium (IV) phosphate-diphosphate solid solutions. J Nncl Mater 295-... 

The preparation and a number of properties of numerous hydrates of thorium phosphate and of acid and basic phosphates, as well as some of their properties, have been described in the literature -see the review in the Gmelin Handbook [1993BIC], which also includes a survey of ternary phosphates (and their hydrates) with a number of cations. Some of these compounds have been studied in relation with the determination of phosphate complexes in solution and are discussed in the relevant Section X.2.3.2. 

This review paper deals with the syntheses of uranium and thorium phosphates and discusses at length the syntheses of Th4(P04)4P207 for storing radioactive wastes. No thermodynamic data are reported. 

The authors report the syntheses of several different thorium phosphate solids (TI1FPO4 H2O, Th2(P04)2S04-2H20, Th4(P04)4Si04, CsTh2(P04)3, BaTh(P04)2) from aqueous solutions under hydrothermal conditions. The chemical composition of the solids was established by electron probe microanalysis and/or particle induced X-ray emission. The X-ray diffraction patterns and/or infrared spectra were given. No thermodynamic data are reported. 

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