Hydrogen thorium phosphate

Simple thermodynamic calculations based on literature data (5-12) support the choice of phosphates as the optimum mineral phases for actinide immobilization. The calculations considered every relevant species reported (5-72) that contained protons, hydroxide, or the ligand in question for each metal ion. Where necessary, equilibrium constants were corrected to 0.1 M ionic strength using the Davies equation. As an example, the calculated solubility of europium, thorium, and uranium in various media at p[H] 7.0 (p[H] = - log of the hydrogen ion concentration), 0.001 M total ligand concentration, 0.1 M ionic strength, and 25 °C are shown in Table I. Within the constraints of the calculation, the solubility of thorium is limited by Th(OH)4, but the lowest europium and uranyl solubilities are observed for phosphates. 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

Attempts have also been made to solvent-extract thorium directly from the sulphate liquor obtained by the acid breakdown of monazite. Tributyl phosphate can be used as the solvent provided a large concentration of nitric acid is added to the liquor before extraction. In order to make the process economic, a high proportion of the nitric acid must then be recovered by distillation of the raffinate liquor. Processes are also being developed which are based upon the use of higher alkyl phosphate or amine solvents to extract from sulphate solutions without the addition of nitric acid, as in the case of uranium. For example bis(l-isobutyl 1-3-5 dimethyl-hexyl) amine, di-2-ethylhexyl hydrogen phosphate and Primene JM-T have been used. 

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