Extraction and separation of neptunium(IV), plutonium(IV) and americium(III) by bidentate organophosphorus Extractant

Abstract

This paper summarized the extraction and separation of Np(IV), Pu(IV) and Am(III) with dihexyl-N,N-diethyl carbamyl methylene phosphonate (DH-DECMP)-diethyl benzene (DEB) in nitric acid medium. The distribution ration of Np(IV), Pu(IV) and Am(III) was studied as a function of a number of parameters such as concentration of nitric acid, salting-out reagents in the aqueous phase, contact time, and temperature. Stripping and separation of Np(IV), Pu(IV) and Am(III) from pregnant organic phase were studied. The suitable stripping and separation conditions were obtained. The enthalpy changes ΔH Np, ΔH Pu, ΔH Am associated with their extraction process were estimated individually. The composition of extracted complex of Np(IV), Pu(IV) and Am(III) was determined. Effect of Contact Time and Temperature. The experiment shows that equilibrium for all extracted species is reached in approximately 30 seconds of contact. The distribution ratios of Np(IV), Pu(IV) and Am(III) decrease with increase of temperature, while the effect of temperature on distribution ratio (D) of nitric acid is negligible. Consequently, the extraction process of the metallic ions is considered as an exothermic and it is not desired to increase temperature during extraction. In the plot of logD vs. 1/T(K) the straight lines were obtained. According to the slope of line and Van't Hoff equation: Δ lg D/Δ(1/T) = −ΔH/2.303 R, the enthalpy changes ΔH associated with extraction process were estimated as follows: ΔH Np ⋍−3.7 Kcal/Mol ΔH Pu ⋍−7.9 Kcal/Mol ΔH Am ⋍−7.6 Kcal/Mol Effect of Concentration of Nitric Acid. The effect of nitric acid concentration in aqueous phase on extraction of trivalent and tetravalent actinides was investigated. The distribution ratios of Pu(IV), Np(IV) and Am(IV) increase with concentration of nitric acid, so it is capable of extracting these actinides in higher concentration of HNO 3. The data on the distribution ratios of these elements in 3 M HNO 3 medium are as follows: D Np(IV) = 221 D Pu(IV) = 101 D Am(III) = 2.2 Effect of Concentration of the Extractant. The distribution ratios(D) of these elements increase with DHDECMP concentration. The logarithmic plots of D Np, D Pu and D Am versus DHDECMP concentration are straight lines with slopes of 2.0, 1.9 and 2.9 respectively. Therefore, the extraction reaction for Np(IV), Pu(IV) and Am(III) with DHDECMP can be expressed as: Np 4+ aq + 4NO − 3 aq + 2DHDECMP org ⇌ Np(NO 3) 4·2DHDECMP org Pu 4+ aq + 4NO − 3 aq + 2DHDECMP org ⇌ Pu(NO 3) 4·2DHDECMP org Am 3+ aq + 3NO − 3 aq + 3DHDECMP org ⇌ Am(NO 3) 3·3DHDECMP org Effect of Concentration of Salting-Out Reagent. The experiment shows that the distribution ratios(D) of trivalent and tetravalent actinides increase obviously with concentration of Al(NO 3) 3 and NaNO 3 in aqueous phase. When the concentration of Al(NO 3) 3 is equal to 1.5 M in M HNO 3 aqueous phase, the distribution ratios for Np(IV) and Am(III) are 1.15 × 10 3 and 10 respectively. Back-Extraction. The stripping condition of Np, Pu and Am from the organic phase were investigated. It is shown that the Am(III) can recovered quantitatively after twice stripping with 0.01 M HNO 3. With (0.05–0.2 M) HNO 3–0.05 M H 2C 2O 4 for Pu(IV), Np(IV) to be stripped or with (0.05–0.2 M) HNO 3–0.05 M Fe(NH 2SO 3) 2 for Pu(IV) to be stripped, the back-extraction percentage is more than 97 after twice contacts. Separation of Americium from Neptunium or Plutonium by Back-Extraction. Experiments show that the Am(III) was quantitatively stripped from the organic phase with 0.05–0.2 M HNO 3, while most of the Np(IV) and Pu(IV) is remained in the organic phase (Table I). From Table I it can be seen that the separation factor of Am and Np or Am and Pu is more than 50. Consequently, this method may be applied to separating Am from Pu(IV) and Np(IV). t001 Stripping Data for Am, Np and Pu. a Stripping Solution 0.05 M HNO 3 0.2 M HNO 3 Back-Extraction Am 74 71 Percentage Pu 1.0 1.3 Np 1.2 1.2 Separation Factor Am/Pu 74 54 Am/Np 61 59 a Organic Phase: 30 Vol% DHDECMP-DEB pre-equilibrated with 3 M HNO 3 solution containing 239Np, 239Pu and 241Am tracer.: 30 °C ± 1 °C. Contact time: 5 min.