Amex process

Rainey, R. H. Development of the Amex Process for Americium Recovery. 

Point of comparison Amex process Dapex process... 

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. 

Thus there is considerable incentive to find extractants that could tolerate higher quantities of solids in H2SO4 leach liquors. Stripping of uranium from the Amex process extractant and subsequent regeneration of the amine solvent also consume considerable quantities of acid and base. Recovery of uranium from H2SO4 solutions would be simplified if a convenient neutral extractant could be found. An extractant with better selectivity for vanadium and molybdenum than HDEHP and long-chain amines is also desirable. 

Solvent extraction is applied either directly to the weakly acidic leach hquor (the method is known as the Purlex process in South Africa, and the Amex process in the USA) or to the strongly acidic eluate obtained from a preliminary ion-exchange treatment of the leach liquor (known as the Bufflex process in South Africa, and the Eluex process in the USA). In the Purlex process, a 5 /(i solution of a tertiary amine in kerosene is typically used to treat a leach liquor containing up to 1 g of uranium per litre (although concentrations as low as 0.04 g 1 may be encountered) at a pH value of 1 to 2. A modifier such as isodecanol (2-5%) is usually incorporated into the organic phase to prevent the formation of a third phase and to inhibit the formation of an emulsion. The extraction of uranium(VI) is usually considered to proceed according to reactions such as ... 

Figure 13 Outline flowsheets for uranium purification using the DPPA, DAPEX or AMEX processes ...

The alkyl amines offer greater selectivity than organophosphorus compounds in many applications, particularly in uranium hydrometallurgy. Amine extraction is typified by the Amex process, which uses a tertiary or branched secondary amine to extract uranium from sulfate leach liquors (11). A similar process based on the use of a primary or straight-chain secondary amine (sometimes modified with an organic-soluble alcohol) has given good results in thorium recovery (12). 

Leaching operations in the Kerr-McGee mill are described in this section, with reference to Fig. 5.6. Recovery of uranium from leach liquor by solvent extraction with organic amines in the Amex process is to be described in Sec. 8.6. 

Distribution coefficients for uranium and other metals in tnoctylamine are con ared in Table 5.20. Although the conditions are not exactiy those used in the Amex process, they indicate that the only element normally present in leach liquors that extracts readily with uranium is mdybdenum. Ferric iron, whidi is always present in leach liquors and extracts in the Dapex prrocess, is not extracted in the Amex process. Vaiudium, if pentavalent, can be extracted by raising the pH from 1 to 2. 

Figure 5.9 Amex process for recovering uranium from leach liquor. Conditions approximately those of one circuit of Kerr-McGee mill.

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. 

Figure 5.19 is a schematic flow sheet showing the Eluex process as used at the Federal-American Partners uranium miU, and the final steps for precipitating and calcining its uranium product. The Eluex process is a variant of the amine extraction (Amex) process described in Sec. 8.6. 

Because of the complications introduced by ferric iron, Audsley et al. concluded that a solvent that would be selective for thorium in the presence of ferric iron and that would not be inhibited by phosphate would be preferable to the Dapex solvent. They concurred with the conclusion of Crouse and co-workers at Oak Ridge National Laboratory [CS], that long-chain primary amines selectively extract thorium in the presence of uranyl, ferric, and phosphate ions. Compounds of this type are now the preferred extractant for thorium in such systems. Application of this so-called Amex process to thorium extraction from monazite is described in Sec. 8.6. 

Source D. J. Crouse and K. B. Brown, Recovery of Thorium, Uranium, and Rare Earths from Monazite Sulfate Leach Liquors by the Amine Extraction (Amex) Process, Report ORNL-2720, July 16, 1959. 

Figure 6.7 Separation of thorium, uranium, and rare earths from monazite by solvent extraction in Amex process. Circles, relative flow ( ), estimated. Other data from Oak Ridge Laboratory runs fC5].

Fig. 4.28. Amex process for uranium. Stripping (Backwashing) agents (a) NasCOs sol n, e.g. 0-75M NasCOs (b) Chloride sol n, e.g. 1-OM NaCl-0 05M H2SO4 (c) Nitrate sol n, e.g. 0-9M NH4NO3-O IM HNOs (d) MgO slurry, e.g. 20 g MgO/1. (Brown, K.B. et al. Proc. 2nd U. N. Conf. on the Peaceful Uses of Atomic Energy, United Nations, New York, vol. 3.,...

The solubilities of the phosphate solvents in the aqueous phases with which they are in contact are quite low, of the order of a few tens of parts per million, being greatest in the carbonate backwash solutions. Overall solvent losses, including those due to entrainment, are about 0.05 per cent, as for Amex process. 

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