LEACHING BEHAVIORS OF NI AND V FROM SPENT CATALYST

The leaching behaviors of Ni and V from a spent hydroprocessing cataly st were assessed in this study. The fractionations of Ni and V on the spent catalyst were determined. An automatic pH/ORP control apparatus was utilized to assess the leaching behaviors of Ni and V. When spent catalyst suspension was continuously aerated with nitrogen for 300 h, the pH did not change, and the redox potential decreased from 521 to 3 40 mV. Leached Ni and V concentrations did not change much during the aeration period, and were about 460 and 70 mg kg(-1), respectively. Th e leaching experiments under equilibrium were first conducted at a pH of 5.0 and various redox potentials (-100, 0, 100, 250, 330, 400 mV). Leached Ni concentrations did not change significantly with redox pote ntial, while leached V concentrations decreased. At pH 8.0 and differe nt redox potentials (-330, -300, -200, -100, 0, 150, 250 mV), leached Ni concentrations were relatively low, and increased with decreasing r edox potential. Leached concentrations of V increased gradually with d ecreasing redox potential, and increased abruptly when the redox poten tial was changed from -300 to -330 mV. It is proposed that the exchang eable fraction of Ni was leached out, while the enhanced leaching of V could be attributed to both changes of fractionation and the reductio n reactions of oxyanions of V. (C) 1997 Elsevier Science B.V.