Spent Hydrodesulphurization (HDS) catalysts contain rare metals such as molybdenum, vanadium, nickel and cobalt on an alumina carrier. In most cases, spent catalysts are treated with hydrometallurgical leaching processes such as caustic leaching and acid leaching with roasting as a pre-treatment step. In the alkaline leaching processes such as sodium carbonate roasting followed by water leaching, most of the molybdenum and vanadium are selectively leached and most of nickel is left in the residue after the alkaline leaching of spent HDS catalysts. Therefore, it is important to recover nickel from the residue. The main form of nickel in the residue is spinel, therefore, it is difficult to extract nickel from the residue using mineral acids. In the present study, the effect of grinding conditions on the leaching efficiency of nickel from the residue after sodium carbonate roasting followed by water leaching of spent HDS catalysts with sulfuric acid was investigated. The grinding speed, grinding time and the amount of residue in grinding significantly affected the leaching efficiency of nickel and aluminum. In particular, the extraction of aluminum was low in the low amount of the residue fed in grinding, because the particles were aggregated again. However, the extraction of nickel was not affected by the reaggregation of particles. Therefore, the possibility of preferential leaching of nickel from aluminum was suggested.

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Microencapsulation of 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) in the microcapsules with hydrophilic wall were carried out using two-step crosslinking of sodium alginate by Ca²⁺ ion and polyvinyl alcohol by glutaraldehyde. The morphologies of the microcapsules prepared and during the preparation were investigated. The extraction properties of Co(II) in ammonium sulfate solution using the microcapsules were also investigated under various conditions. Spherical and oval microcapsules containing PC88-A droplets with the diameter around 3mm were successfully prepared. The inside of the microcapsules was observed darkly by a lot of small droplets of PC-88A. The extraction of Co(II) using the microcapsules containing PC-88A was sharply increased at pH higher than 4. It was confirmed that Co(II) ion was extracted by two molecules of PC-88A in the condition at high Co(II) concentration range from the analysis of the relationship between the maximum amount of Co(II) extracted and the PC-88A content in the microcapsule. The extraction of Co(II) was analyzed quantitatively by three-step reaction model that is stepwise reaction with the cluster of three PC-88A-dimer, and was able to explain successfully based on the analysis. Co(II) extracted in the microcapsules was successfully back-extracted at pH lower than 2. The microcapsules containing PC-88A could be used repeatedly for several times for the extraction and back-extraction processes of Co(II).

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The objective of this study was to investigate the influence of pulse voltage on acidification and remediation efficiencies of soils contaminated by lead, cadmium, and magnesium. To achieve this objective, a series of bench-scale electrokinetic experiments was performed under the conditions in which the soil pH was gradually decreased from the cathode side. The electrokinetic experimental results demonstrated that the majority of soil pHs were below 4.0 after 36 hours energization and correspondingly, all cationic metals investigated in this study were migrated toward the cathode. Regarding the influence of pulse voltage, the electrokinetic experiments in which a pulse voltage (3 hours ON, 3 hours OFF) was applied demonstrated that applying the pulse voltage did not provide neither positive nor negative influences on the soil acidification and electricity consumption for removing three metals. It was also demonstrated that applying a pulse voltage (2 seconds ON, 2 seconds OFF) negatively affected the remediation performance with respect to the electricity consumption when compared to the case in which constant voltage was applied.

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