Solid thorium phosphates

On the basis of X-ray diffraction and thermogravimetric data, Burdese and Borlera [1963BUR/BOR] presented a phase diagram for the system P205-Th02, in which they 

Phase Symmetry Space Group Lattice parameters (A) Reference 

For the metaphosphate Th(P03)4, Burdese and Borlera [1963BUR/B0R] indicate an orthorhombic structure stable up to 1023 K. However, for the same compound, Masse and Grenier [1972MAS/GRE] (see Table X-2) found three structures (ortho- 

More recently, in the framework of broad studies on the potentialities of phosphate matrices for radioactive waste storage, different compounds 

The equation for the fit to in the text of [2002DAC/CHA] seems to be in error, and we have refitted their results for the specific heat to the relation  

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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). 

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. 

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. 

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 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. 

The authors have studied the dissolution of thorium phosphate diphosphate (TPD) with and without trivalent actinides they also studied a solid solution of Th(IV)/Pu(IV) phosphate diphosphate. The experiments consist of leaching studies where the rate of dissolution has been measured as a function of pH. This study does not provide any information on the solubility product of TPD. There is information on estimated equihb-rium constants for MP04(s) phases, where M = Am, Cm, Ce and Pr. These are secondary phases formed in the leach system. 

Th, thorium, was discovered in 1829 by Jons Jakob Berzelius, who isolated a new oxide from a recently discovered mineral which Jens Esmark had sent to him. He called the oxide thoria and the mineral thorite (ThSi04) after the Scandinavian god Thor. Berzelius subsequently made the metal by the reduction of ThF4 with Na. Th now is extracted from monazite, a phosphate of rare earths and Th. The mineral is heated in concentrated NaOH to give hydrous oxides, which are filtered out. HCl is then added to dissolve the solids and when the pH is adjusted to 3.5, Th02 precipitates and the rare earths remain in solution. The Th02 is solubilized and purified by solvent extraction. 

The ni3(P04)4 and U3(P04)4 phosphates were first characterized in 1957-1963 [15-18] as isostructural compounds with monoclinic unit cells, space group P2, or Pm, or P2 m. More recently, the data were critically revised in a number of papers [8,19-24], where the authors used new methods of studying solids, and carried out detailed experiments on the phase formation in the systems Th02 -P2O5 and UO2 - P2O5. In the authors opinion [8,19-24], attempts to synthesize single phase products such as "M3(P04)4" for both thorium and uranium [15-18] failed. This was also the case for the compounds of similar stoichiometry with neptunium (IV) and plutonimn (IV) [19, 24],... 

Parallel experiments with uranyl-phytate mixtures produced a uranyl phosphate solid identified as (U02)3(P04)2 H20 by X-ray powder diffraction (2i), not U02HP04 nH20 as expected. Neither crystalline phosphates nor phytates, were observed in the thorium-phytate mixtures, although amorphous thorium phytates were likely present initially. Hydroxide or oxide species seem to control thorium solubility. 

Monazite concentrate is processed either with sulfuric acid, like bastnasite, to produce a mixture of sulfates but the usual process is an alkaline treatment. The alkali process is preferred since it removes the phosphates more readily [9]. Whichever method is chosen the radioactive thorium must be completely removed. After benefication the monazite concentrate is finely ground and reacted with a hot concentrated sodium hydroxide at 140° to 150°C. Insoluble hydroxides of the rare-earths and thorium are formed while trisodium phosphate and excess sodium hydroxide remain in solution. The next step is hydrochloric acid attack on the solids portion. The thorium remains insoluble and a crude thorium hydroxide can be filtered off Trace contaminants that do carry through into solution, such as uranium and lead, as well as some thorium, are removed by coprecipitation with barium sulphate in a deactivation step. The cerium-containing product will be a rare-earth chloride differing only marginally in the proportions of the various rare- earths present, to the analogous rare-earth chloride produced from bastnasite. 

BRA/DAC2] Brandel, V., Dacheux, N., Genet, M., Hydrothermal synthesis and characterization of the thorium phosphahe hydrogenophosphate, thorium hydroxide phosphate, and dithorium oxide phosphate, J. Solid State Chem., 159, (2001), 139-148. Cited on pages 325, 663. 

Y Lin, CM Wai, FM Jean, RD Braner. Snpercritical flnid extraction of thorium and uranium ions from solid and liqnid materials with flnorinated fS-diketones and tributyl phosphate. Environ Sci Technol 28 1190-1193, 1994. 

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