Separation of metals from simulated mixed waste streams through hydrothermal crystallization in supercritical water

Abstract

Abstract Recovery techniques are needed to separate metals from high level radioactive liquid waste streams. We examined hydrothermal crystallization in supercritical water (SCW) and separation factors for removing metals from simulated radioactive liquid waste streams. Aqueous metal nitrates can react to form metal oxides or hydroxides at these conditions. The 21 element simulated waste stream consisted of nitrates of Cs (6030 ppm), Sr (1220 ppm), Pd (50 ppm), Na (26200 ppm), Ba (3610 ppm), Ag (150 ppm), Cd (250 ppm), Y (1090 ppm), La (2780 ppm), La (2780 ppm), Ce (8530 ppm), Pr (3890 ppm), Nd (9610 ppm), Sm (2000 ppm), Eu (340 ppm), Gd (17840 ppm), Zr (7330 ppm), Cr (1850 ppm), Mo (720 ppm), Mn (880 ppm), Fe (10240 ppm) and Ni (1830 ppm) aqueous solutions. The experimental apparatus consisted of a batch apparatus heated in a salt bath with conditions from 373–723 K, 20–40 MPa and reaction time from 2–30 minutes. After completion of the each experiment, the reactor contents were filtered and analyzed with ICP, X-ray diffraction and SEM, as appropriate. Results are summarized for the 21 element stream at 30 MPa and 30 minutes reaction time. For Mo, Zr, and Fe, a sharp decrease in the liquid phase concentration was observed from the previous stated concentrations to approximately 2, 20, and 40 ppm, respectively as the system entered supercritical conditions. The temperature ranges, where the metals could be recovered, varied. For Mo, a sharp increase in recovery occurred at about 423 K and reached 98%; for Fe, the increase in recovery began at about 473 K amd reached 99%. For Zr, the recovery occurred over a wider range and began from about 373 K and reached 98% at 673 K. For Cr, 75% recovery was approached. More than 99% of Pd could be recovered. Sr, Cs, and Na showed relatively little or no changes in liquid phase concentration. A series of experiments were performed to determine the density effects on metal reactivity. Other metals could be recovered from 4% to 98%. Only Cr, Ce, Pr, Mn, and Ni had recoveries that were affected by density. For these elements, recoveries showed a downward trend with increasing density. An empirical model with a function form for Type V adsorption was found to provide an excellent fit of the data. From the results presented it is possible to selectively separate mixed metals streams with hydrothermal crystallization in supercritical water. For the simulated stream studied, approximately 42 wt% of the metals can be removed by elevating the stream to supercritical conditions. Further experiments using a flow apparatus and modeling of the reactive solubility are in progress.