Organophosphorus acids uranium

The use of organophosphorus acids, such as di(2-ethylhexyl)phosphoric acid (D2EHPA di(2-ethylhexyl) monohydrogen phosphate 2 R = C4H9CH(Et)CH2), is now well established in the recovery of base metals. This reagent has found commercial application in the separation of cobalt from nickel,67 68 the separation of zinc from impurities such as copper and cadmium,69 the recovery of uranium,68 beryllium70 and vanadium,71 and in separations involving yttrium and the rare-earth metals.72 73... 

Organophosphorus acids were among the first extractants to be used in the commercial recovery of uranium from solutions obtained by the leaching of low-grade ores with sulfuric acid. In the so-called Dapex process,70 114 the leach liquor is extracted with a solution of about 0.1 M D2EHPA in kerosene, and the pH value of the aqueous phase is adjusted to close to 1.0 in order to prevent the coextraction of vanadium impurities. Since iron(III) also extracts under these conditions, the leach liquor is reduced with metallic iron prior to extraction to convert any iron(III) present to the iron(II) state. 

Organophosphorus acid extractants have found considerable use in recent years for the recovery of uranium as a byproduct in the manufacture of wet-process phosphoric acid. This acid is obtained by the digestion of phosphate rock with sulfuric acid, and typically contains 0.1 to 0.2 g of uranium per litre.120 It has been estimated that, in 1976, the wet-process acid produced in the USA alone contained some 2500t of dissolved uranium 121 this therefore represents a valuable potential source of this strategic metal. 

In contrast to the chemistry of extraction of uranium(VI) by organophosphorus acids, that of uranium(IV) has not been widely studied. It is known,135 however, that the extraction of U4+ by mono(2-ethylhexyl)phosphoric acid (2-ethylhexyl dihydrogen phosphate), for example, exceeds that of U022+ by a factor of 105. This clearly provides the basis for the strong extraction of uranium(IV) from phosphoric acid solutions by extractants containing monoalkyl phosphates. The existence of a synergistic effect in the extraction of uranium(IV) by mixtures of mono(2-ethylhexyl)phosphoric acid and neutral organophosphorus compounds was also reported recently.136... 

Organophosphorus acid extractants have also been used commercially to recover thorium from barren solutions obtained from uranium ion-exchange plants. For instance, Rio Tinto Dow Ltd of Canada installed a plant at Elliot Lake in 1959 to extract thorium(IV) from solutions containing only 0.15 g of thorium per litre.137 The loaded organic phase is stripped with 5 M sulfuric acid, from which the product subsequently crystallizes as an acid thorium(IV) sulfate. 

Blake, C.A. Jr. Baes, C.F. Jr. Brown, K.B. Coleman, C.F. White, J.C. Solvent extraction of uranium and other metals by acidic and neutral organophosphorus compounds, In Peaceful Uses of Atomic Energy. Vol. 28, Basic Chemistry in Nuclear Energy, United Nations, Geneva (1958), pp. 289-298. 

Lin, Y. Smart, N.G. Wai, C.M. Supercritical fluid extraction of uranium and thorium from nitric acid solutions with organophosphorus reagents, Environ. Sci. Tech. 29 (1995)... 

Lin, Y., Smart, N. G., Wai, C. M. (1995), Supercritical Fluid Extraction of Uranium and Thorium from Nitric Acid Solutions with Organophosphorus Reagents, Environ. Sci. Technol. 29, 2706-2708. 

The alkyl phosphoric acids were first recognized as excellent actinide extractants because dibutyl phosphoric acid existed as an impurity in TBP (8). They extract well from unsalted solutions and from systems such as sulfate in which TBP is ineffective. Organo-phosphorus acids are thus suitable for uranium hydr metallurgical applications where ore is leached with sulfuric acid. The Dapex process is an example in which bis(2-ethylhexyl) phosphoric acid (HDEHP) is used alone or in synergistic combination with neutral organophosphorus compounds for uranium and vanadium recovery (9,10). 

There are a number of practical synergistic systems that make use of organophosphorus compounds. A classic example is the combination of HDEHP and TOPO. The TOPO is thought to replace water or HDEHP in the coordination sphere of the metal. Some early work in this area included a study of the extraction of uranium in such systems (82). Commercial processes now exist for the recovery of uranium from wet-process phosphoric acid utilizing synergistic systems (83,84). Descriptive studies of such systems have also been made (85,86). 

Processes for recovering uranium from acid leach liquors used in the United States include sdvent extraction with organic amines, sdvent extraction with organophosphorus compounds. 

Metal-complexation/SFE using carbon dioxide has been successfully demonstrated for removal of lanthanides, actinides and various other fission products from solids and liquids (8-18), Direct dissolution of recalcitrant uranium oxides using nitric acid and metal-complexing agents in supercritical fluid carbon dioxide has also been reported (79-25). In this paper we explored supercritical fluid extraction of sorbed plutonium and americium from soil using common organophosphorus and beta-diketone complexants. We also qualitatively characterize actinide sorption to various soil fractions via use of sequential chemical extraction techniques. 

Amine systems achieve higher uranium purity than organophosphorus systems (due to the greater selectivity of amines for uranium) and have lower extractant losses due to their lower aqueous-phase solubility. SX is applied either directly to the weakly acidic leach liquor (also known as the Purlex or Amex process, Vaal River West, South Africa) or to the strongly acidic eluate from an ion-exchange preconcentration treatment of the leach liquor (the Bufflex or Eluex process, Vaal River South, South Africa) (Nicol et al. 1987). A more modem variation (such as at Southern Cross Resources Uranium One, South Africa) is to treat the ore by pressure leaching followed by SX. 

RGANDEHQSFH(ai)3 COMPOlPIDB. Within recent years, a large number of organophosphorous coaq>ounds have been developed and Investigated as extractants for uranium. These compounds have been subdivided In the present paper Into neutral and acidic organophosphorus compounds. 

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