Uranium acid solutions, TOPO

Various processes have been used for uranium extraction from phosphoric acid solution their main features are listed in Table 12.4. The HDEHP-TOPO process is increasingly preferred over others because of the stability of the extractant and the well-understood chemistry of the process. 

The mechanism of U02 " extraction by monoalkyl phosphoric acid reagents appears to be a more complex process than for their dialkyl counterparts. This results from the polymerization of the monoalkyl phosphate in the organic phase and the hydration of the extracted uranyl species so that variable stoichiometries arise for the extractant/water/UO complex. The extraction of from sulfuric acid by mono-2,6,8-trimethylnonyl phosphoric acid (H2DDP) and mono-n-butyl phosphoric acid (H2MBP) as 0.05 M solutions in benzene was shown to follow equations (61) and (62) when an excess of extractant was present. When an excess of uranium was present, equations (63) and (64) applied where n, x, y and z were variable numbers which depended upon the extent of extractant polymerization and hydration of the extracted species. Synergistic effects may also be found with the monoalkyl phosphoric acid extractants and in one recent example the use of tri-n-octylphosphine ocxide (TOPO) as a synergist with H2MEHP allowed the extraction of U02 from phosphoric acid solutions. The uranium may be returned to the aqueous phase by contact with concentrated acid, which reverses the extraction process by protonation of the phosphate. 

We have used tri-n-octylphosphine oxide (TOPO) as a solvent extractant of uranium(VI) and thorium(IV) from nitric and hydrochloric acid solutions (1-3). In contrast, the extraction of uranium(VI) and thorium(IV) from nitric and hydrochloric acid solutions has been investigated by tri-n-butylphosphate (TBP) (4, 5). However, since TBP reveals a poor efficiency for the extraction of metals from sulphuric acid solutions, this paper extends the work to the extraction of uranium(VI) from sulphuric acid solutions by TOPO. 

If we assume that the extraction of uranium(VI) from sulphuric acid solutions with TOPO is governed by solvating reaction as in the case of the extraction from nitric and hydrochloric acid solutions ( ,3), viz. 

The kinetics of the DEHPA-TOPO resins tested are slow compared to other conventional anion exchange resins used for uranium sorption, but about 5 to 10 times faster than with an ion-exchange resin containing aminophosphonic groups (DES 467, Dia-Prosin). The rate of uranium adsorption from 6 M phosphoric acid solutions was studied extensively with... 

The extractant is a commercial mixture of mono- and dioctyl phenyl phosphoric acid (OPPA). It is used in conjuction with tributyl phosphate (TBP). Stripping is by ammonium carbonate solution. The mixture shows synergism. Uranium is extracted in the tetravalent state. The process is much less expensive and possesses a higher extracting power than D2EHPA-TOPO combination. 

Subramanian and coworkers developed polymeric sorbents using different support materials (such as Merrifield chloromethylated resin, Amberlite XAD 16) and complexing ligands (amides, phosphonic acids, TTA), and evaluated their binding affinity for U(VI) over other diverse ions, even under high acidities. The practical utility of these sorbents was demonstrated using simulated waste solutions (220-222). Shamsipur et al. reported the solid-phase extraction of ultra trace U(VI) in natural waters using octadecyl silica membrane disks modified by TOPO (223). The method was found satisfactory for the extraction and determination of uranium from different water samples. 

Equal volumes (20 ml) of the TOPO solution in the organic solvent and uranyl sulphate solution containing sulphuric acid were shaken for 10 min in 50 ml stoppered conical flasks in a thermostatic water-bath at the required temperature. Preliminary experiments showed that equilibration is complete in 10 min. The mixture was centrifuged and separated, and uranium was stripped from the organic phase with 0.5 M ammonium carbonate solution, and then the distribution coefficient (the ratio of the equilibrium concentration of uranium in the organic phase to that in the aqueous phase, [U]org/[U]aq) was obtained. 

Furthermore when the effect of diluent on the extraction efficiency of TOPO for uranium(VI) is examined in the extraction of aqueous solutions containing 5 g/1 of uranyl salts in nitric and sulphuric acids, it is found from some representative data (Table 3) that hydrocarbons such as alkane or cycloalkane enhance the extraction efficiencies in all systems, while halogen-substituted hydrocarbon such as chloroform reduces them. 

An especially intriraing use of liquid-membrane technology, reported for the first time recently by Fox and Hayworth et al.," is the recovery of uranium from wet process phosphoric acid (WPPA). In the manufacture of fertilizer fiom phosphate rock, the acid is solubilized by treatment with sulfuric acid. In addition to HjP04, this crude leachate can contain up to 0.18 g/L of uranium. Under oxidizing conditions this occurs as the uranate ion, UO . Solvent extraction (SX) processes have been developed to recover die uranium values. One of the most common of these uses a mixture of di-(2-ethylhexyl) phosphoric acid (D2EHPA) and trioctylpbosphine oxide (TOPO) in kerosene solution. The chemistry of solvent extraction is as follows ... 

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