ICP-MS Determination of Uranium and Thorium in Ilmenite with Separation by Anion-Exchange

Ilmenite was crushed to pass through sample sieve with boring of 0.074 mm. 0.10 to 0.50 g of the ilmenite sample was taken and placed into a porcelain crucible in which 2 g of powdered K₂S₂O₇ had been bedded at the bottom of the crucible. Another portion of 3 g of K₂S₂O₇ was added to the crucible and well mixed with the sample. After putting on the crucible cover, the crucible was placed into a muffle furnace and the temperature was elevated. At 350℃, the crucible was shaken carefully by swirling to make the sample dispersed homogeneously in the melted flux. Then the temperature was raised to 600℃, and the sample with the flux in the crucible was fused for 1 h, and after removing the crucible cover, the sample was fused for 30 min in addition to have the residual SO₃ volatized. The crucible was then taken out from the muffle furnace, and after cooling, water was added to dissolved the melt and the volume of the sample solution was made up to 100.0 mL. On the other hand, a definite amount of D201 anion exchange resin was taken and pretreated as prescribed. And the resin was packed into an exchange column. It was found that sulfate anions of uranium and thorium can be satisfactorily separated from the matrix of ilmenite by exchange adsorption on the D201 anion exchange resin in a acidic sulfate solution with H₂SO₄ of 0.01 mol·L^-1 and K₂SO₄ of 0.5 mol·L^-1, giving rates of exchange adsorption over 98%. Based on this finding, 5.0 mL of the sample solution together with 20 mL of the acidic sulfate solution were added into the reservoir at the top of the exchange column, and the sulfate solution containing the sample was then passed through the column by dropping, after which the column was rinsed twice with 0.01 mol·L^-1 surfuric acid solution (5 mL for each rinse), and finally U and Th were eluted from the exchange column with 14 mL of 0.5 mol·L^-1 HCl solution and 6 mL of water successively. The eluates were collected and diluted to 50.0 mL, which was used for the ICP-MS analysis under the working conditions of the instrument. 10 μg·L^-1 rhodium standard solution was added as internal standard online through the tee of the instrument. Isotopes of ²³⁸U, ²³²Th and ¹⁰³Rh with relatively higher atomic abundance were selected in the determination. Linearity ranges of the standard curves of U and Th were found same within 100 μg·L^-1, and values of detection limits (3s) found were 0.029 μg·L^-1 and 0.036 μg·L^-1 respectively. Values of RSDs (n=5) found for the 2 elements were less than 10%. The proposed method was used in analyzing 4 national standards for the contents of U and Th, giving results consistent with the certified values.