Recovery of metals from simulated high-level liquid waste with hydrothermal crystallization

Abstract

Separation of metals from high-level radioactive liquid waste (HLLW) streams is a major problem in the nuclear power industry and usually requires additional solvents or adsorption media. In this work, we examined the separation of metals from simulated HLLW mixtures with thermal energy which completely avoids additional components. The 21 element simulated HLLW mixture consisted of nitrates of Cs, Sr, Pd, Na, Ba, Ag, Cd, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Zr, Cr, Mo, Mn, Fe and Ni in 1.6 M nitric acid. Reactive crystallizations were carried out at temperatures up to 723 K and at pressures up to 30 MPa. Remarkably, many of the elements precipitated out over a narrow range of temperatures with high recoveries. In general, most of the precipitates were highly amorphous, stable solids. Elements that could be precipitated in a solid phase with high recoveries (> 95%) were Mo, Zr, Fe, Pd, Cd and Ce. Other metals that could be separated with moderate recoveries (50–91%) were Cr, Pr, Mn and Ni. The elements Sr, Cs and Na were found to remain in the liquid phase. Single-element studies were made on Pd, Ce, Mn, Ru and Rh. It was found that hydrothermal treatment at temperatures higher than 623 K led to recoveries approaching 100% for these elements, except for Mn, in which recoveries were 18–59%. An empirical model provided an excellent fit of the data in terms of the maximum recovery, R MAX, a one-half recovery temperature, T 1 2 , and a sharpness of separation index, S. The model was found to be useful for characterizing the metal separation temperatures. For the simulated stream studied, more than 35 wt.% of the total metals can be separated by elevating the stream to supercritical conditions. Hydrothermal crystallization is an effective technique for recovering metals from HLLW mixtures.