Hydroxylation and sulfidation for nickel recovery from a spent catalyst containing the chelating agent, 2,2'-bipyridine.

Techniques for recovering nickel (Ni) from various Ni-containing products are needed for resource circulation. In this study, Ni recovery from a spent catalyst containing 2,2'-bipyridine (bpy) was conducted by precipitation using hydroxylation and sulfidation. In the absence of bpy, both methods completely precipitated Ni as estimated in chemical equilibrium calculations. For an actual spent catalyst with a bpy/Ni molar ratio of one, the recovery rates were reduced to approximately 70% and 90% for the hydroxylation and sulfidation methods, respectively. Similar values were obtained for a simulated spent catalyst with a bpy/Ni molar ratio of one. Precipitation was inhibited in both methods for simulated spent catalyst with an initial bpy/Ni molar ratio of three. Ultraviolet-visible spectroscopy revealed that the bpy/Ni molar ratio increased with Ni precipitation, and Ni that remained in the solution was converted from Ni(bpy) to Ni(bpy). Fourier transform infrared spectra showed that the precipitates obtained by the sulfidation method contained bpy in a complex with Ni, and thermogravimetry-differential thermal analysis curves showed different proportions from those of the simulated spent catalysts. The precipitates formed in the presence of bpy were thin film fragments. It is known that S forms an ion bridge with the Ni(bpy) complex, and in the sulfidation method, Ni precipitated as a S-Ni-bpy cluster. These findings establish the chemical composition of Ni recovered from spent catalysts and show that the Ni recoverability depends on the bpy/Ni molar ratio.