Dapex process

Dapex process, 6,909,911 uranium ore processing, 6, 903 Data storage... 

Table 12.2 Chemical Basis for the Amex and the Dapex Processes for U(VI) Recovery from H2SO4 Leach Liquors... 

Point of comparison Amex process Dapex process... 

High-grade pitchblende ores are leached with nitric acid to recover uranium. Extraction of uranium from nitrate solutions is usually performed with TBP. TBP-based solvents are used in several areas of the nuclear industry, especially for reprocessing of spent nuclear fuels and for refining the uranium product of the Amex and Dapex processes. Extraction of uranium by TBP solvents is described in sections 12.3.4 and 12.5. 

As noted earlier (see section 12.3.1), the Amex process, which uses long-chain amines, is preferred over the Dapex process, which uses HDEHP, for solvent extraction of uranium from H2SO4 leach solutions. Because the surfactant properties of amine sulfates are conducive to formation of objectionable emulsions, the Amex process is very sensitive to the presence of solids in the H2SO4 leachate. For acceptable phase coalescence in the Amex process, feeds should contain no more than 20 ppm solids. The Dapex process can tolerate feeds containing as much as 100 ppm solids. 

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. 

In the Dapex process, uranium is usually stripped from the loaded organic phase with a solution of sodium carbonate, which converts the extractant to the sodium salt form, while retaining uranium in solution as sodium uranyl tricarbonate ... 

Uranium extracted by HDEHP may be returned to the aqueous phase by contact with a strong acid, which reverses the extraction reaction. Alternatively, contact with sodium carbonate solution may be used to strip the U022+ from the organic phase as carbonate complexes. This approach is used in the DAPEX process and reactions such as that given by equation (60) may be proposed to represent the transfer process. 

The Cleanex process was adapted from the DAPEX process described in Section 65.2.2.1 (iv) and also utilizes HDEHP to coextract the lanthanides and transplutonium elements. Subsequent separation of these two groups may then be carried out using, for example, the Tramex process. ... 

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). 

To maintain solvent capacity for uranium and to prevent contamination of extracted uranium by iron, it is necessary to reduce iron to the unextractable ferrous condition before solvent extraction. This is done by contacting the leach liquor with scrap iron, SO2, or sodium sulfide. Because the iron content of leach liquor is high, reduction is costly, and the Amex process, in which ferric iron does not extract, is preferred for sulfuric acid leach liquors. The high distribution coefficient of other polyvalent cations such as Th and in EHPA makes the Dapex process less selective for uranium than the Amex process. 

In the Dapex process, uranium in the organic phase is usually stripped by contact with an aqueous solution of sodium carbonate, through the reaction... 

Although the Dapex process is no longer being widely used to extract uranium from sulfuric acid leach liquors, organic phosphoric acids are favored for extracting by-product uranium from commercial phosphoric acid. Organic amines are impractical for this application because they are too fully saturated by the strong acid. 

Fig. 4.29.Dapex process for uranium. Owing to transfer of water to the solvent, the aqueous volume decreases during stripping which raises the uranium concentration above that expected from the initial flow volumes (Brown, K. B. et al. Proc. 2nd U.N. Conf. on the Peaceful Uses of Atomic Energy, United Nations, New York, vol. 3, p. 481).

In order to avoid the expensive filtration stage immediately prior to solvent extraction, the extraction is sometimes carried out directly on the leach liquor slurry, where conditions are favourable. The Dapex process is then used, since the amine solvents are sorbed readily on to solid surfaces, and entrainment of amines is excessive. Pulsed columns of the sieve-plate type are considered ideal extraction plant in this case, although mixer-settlers have also been employed. 67, ss. s ... 

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