Spent hydrodesulphurization (HDS) 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 oxidative roasting and grinding conditions on the leaching efficiency of nickel and aluminum from the residue after sodium carbonate roasting followed by water leaching of spent HDS catalysts with sulfuric acid was investigated. The roasting temperature, grinding speed, grinding time and the amount of residue in grinding significantly affected the leaching efficiency of nickel and aluminum. The extraction of aluminum from roasted residue was low compared with unroasted residue. And also, the extraction of aluminum was low in the low amount of the roasted residue fed in grinding, because the particles were aggregated again. Meanwhile, the extraction of nickel was not affected significantly by the oxidative roasting. Then, the extraction of nickel was not affected by the reaggregation of particles under the conditions below 650 rpm of grinding speed and 180 min of grinding time. Therefore, the possibility of preferential leaching of nickel from aluminum was suggested.

Feasibility of a renewable sorbent for phosphorus was determined by testing the anion-exchange ability of hydrotalcite, which holds formate ions in its interlayer. The formate hydrotalcite, successfully prepared by reacting raw hydrotalcite with a formic acid/1-propanol solution, exhibited high performance for phosphorus removal and collection based on a stoichiometric anion-exchange principle. It was estimated that the collected orthophosphate ions existed as poly-anion forms of HPO₄^2– and PO₄^3– with a ratio of 67.6:32.4 in the interlayer after the sorption. The formate hydrotalcite is also able to revert to the starting hydrotalcite after the sorption, when treated with Na₂CO₃ aqueous solution, demonstrating that it can be an attractive alternative to the conventional chloride or nitrate hydrotalcite as an anion exchanger.