Separation and recovery of nickel, as a salt, from an EDTA leachate of spent hydrodesulphurization catalyst using precipitation methods

Abstract

The aim of the present work was to recover Ni, as a salt, from an ethylenediaminetetracetic acid (EDTA) solution that also contains Al and Mo, obtained after leaching a spent Ni–Mo hydrodesulphurization catalyst.The pH for EDTA precipitation was optimized in order to maximize further separation of Al and Mo from Ni. At pH=1, 80% of EDTA was precipitated, followed by precipitation of Al and Mo (recovery of 94 and 67%, respectively) at pH 5. From this point, three options for recovering Ni, as a salt, were studied. The first consisted on a second EDTA precipitation at lower pH followed by Ni precipitation, as Ni(OH)2, by increasing the pH; the second and the third ones involved sequential addition of Fe3+ and phosphate ions, with and without the second EDTA precipitation step, respectively. The addition of Fe induces the substitution of Ni in the EDTA complexes, leaving Ni free to precipitate with the phosphate, which maximizes Ni recovery.The highest Ni recovery was 93%, as nickel phosphate, with a purity ≥96% in the sequence involving second EDTA precipitation, where 70% of the remaining EDTA was recovered at pH 0.4, followed by addition of Fe3+ and precipitation as nickel phosphate. However, this option has the highest costs in terms of reagents consumption. By suppressing the second EDTA precipitation step, nickel phosphate was recovered with lower yield and purity (82 and 94%, respectively). In the case, where Ni was precipitated as Ni(OH)2, nickel recovery only reached 70%, with ≥97% purity; still, this possibility has the lowest reagents cost. In the two options that included phosphate addition, the analysis of the obtained solid suggests the Ni3(PO4)2 stoichiometry and the proposed process is clean, with almost no generation of residues, since the precipitated EDTA can be recycled into the leaching stage and the Fe, recovered as a solid, can be reused as an iron source.