The present overview presents a brief survey on the liquid-liquid extraction mechanism of metal chelates and adducts, the different reported reaction models and the most common methods used to follow the kinetics of extraction process are given. The effect of the different phenomena affecting the extraction rate and the importance of the kinetic information in connection with technical processes and counter-current modelling are also discussed.
Solvent extraction of Pr(III) and Th(IV) has been investigated with commercial extractants of CYANEX 301 (bis(2,4,4-trimethylpentyl) dithiophosphinic acid) and CYANEX 302 (bis(2,4,4-trimethylpentyl) monothiophosphinic acid) in kerosene from nitrate medium. The effects of various parameters affecting the extraction equilibrium of Th(IV) and Pr(III), including temperature, were studied and the stoichiometry of the extracted Th(IV) and Pr(III) species was elucidated. The separation of Th(IV) from Pr(III) depending on the difference in the extraction behavior of the two extractants towards these metals is given and discussed.
Solid-liquid extraction has been used to study the uptake of uranium(VI) and thorium(IV) from sulphuric acid using a TVEX-PHOR resin. The experimental results were found to fit the BET isotherm and show a higher affinity of the TVEX-PHOR resin towards the extraction of uranium than thorium under similar experimental conditions. The best separation of uranium from thorium is obtained from 3M sulphuric acid at V/m ratio of 20 ml/g. Elution of loaded uranium and thorium was carried out with 1M sodium carbonate and 0.075M sulphuric acid, respectively. After the elution of both elements, the regenerated resin could be reused with high efficiency.
The kinetics of extraction of U(IV) by TBP in kerosene was investigated using a stirred Lewis cell. The effect of the different parameters affecting the extraction rate as well as temperature were separately investigated. The rate equation deduced from the experimental results show that the extraction of U(IV) is first order dependent on TBP concentration while it is of zero order with respect to U(IV), H+, NO
and HNO3 concentrations. The data obtained show that the extraction process is governed by chemical reactions taking place at teh interface.
The extraction of Pd(II) by triphenylphosphine sulfide (Ph3PS) in benzene from nitric acid solutions has been investigated. The effect of different parameters on the extraction of palladium was studied. Based on the results, an equilibrium model is proposed. The stoichiometry of the extracted species was elucidated and found to be Pd(NO3)22Ph3PS. Possible interference with selected radionuclides that might exist in high level liquid waste was investigated and discussed.
The extraction of U(IV) by bis(2,4,4-trimethylpentyl) monothiophosphinic acid (CYANEX 302) in kerosene from nitric acid solution has been investigated under equilibrium conditions. The effects of the different parameters affecting the extraction process were studied and the stoichiometry of the extracted species was elucidated. The kinetics of this extraction was also investigated using a stirred Lewis cell. The effects of the different parameters affecting the extraction rate as well as the temperature were separately investigated. The results are interpreted by a reaction mechanism where the extraction process of U(IV) is controlled by a chemical reaction at the interface rather than in the bulk phase.
The kinetics of distribution of Am(III) and Eu(III) between thenoyltrifluoroacetone (HTTA) and triphenylarsine oxide (Ph3AsO) mixture in chloroform and aqueous nitrate medium has been investigated using a stirred Lewis cell at ionic strength of 0.1M. The effect of the concentration of HTTA, Ph3AsO, H+ and NO
on the rate of distribution of Am(III) and Eu(III) was studied. The results were interpreted by reaction mechanisms where the rate-determining steps are the parallel reactions of Am(OH)2+ or Eu(OH)2+ with one HTTA molecule and one Ph3AsO molecule in the aqueous medium. The values at 25 °C of the rate constantkHLL (HL=HTTA andL=Ph3AsO) are 1.6±0.3·106M–2·s–1 and 2.3±±0.3·108M–2·s–1 for Am(III) and Eu(III), respectively.
It is found that from aqueous nitric acid media, Pd is highly extracted by chloroform solutions containing triphenylphosphine (Ph3P) and not extracted by triphenylphosphine oxide (Ph3PO) diluted with chloroform. The effect of diluent, nitric acid molarities, Ph3P concentration and temperature on palladium extraction by Ph3P was investigated. Slope analysis of the experimental results indicated that the main extracted species is in the form of Pd(NO3)2·2Ph3P.IR spectrum analysis of the extracted complex indicated the direct extraction of palladium as nitrate and bonding to phosphorus. Extraction of some fission product radionuclides from 2M HNO3 showed the selective extraction of palladium by Ph3P in chloroform.
The extraction of U(IV) from phosphoric acid by octylphenyl acid phosphate (OPAP) in kerosene was investigated. Parameters affecting the extraction of U(IV) from phosphoric acid were investigated. The effects of H3PO4, H2SO4, H2O2, Na2CO3 and (NH4)2CO3 concentrations, phase ratio and temperature on the stripping of uranium from the organic phase were studied. Based on the bench-scale results, a continuous counter-current extraction flow sheet was developed and tested using a 16-stage horizontal type mixer settler. The continuous extraction scrubbing stripping showed that the extraction efficiency of the developed process is 99%, whereas the stripping efficiency is 97%.