Nickel aluminate spinel formation during sintering of simulated Ni-laden sludge and kaolinite

Incorporating Ni-laden waste sludge into kaolinite-based construction ceramic materials appears promising based on the identified nickel bearing phases, evaluated incorporation efficiency and nickel leachability of the products. Nickel aluminate spinel (NiAl2O4) results from sintering kaolinite and nickel oxide between 990 and 1480 °C, with more than 90% incorporation efficiency achieved at 1250 °C and 3 h sintering. At lower temperature (990 °C), NiAl2O4 formed from the reaction between nickel oxide and the defect spinel generated from the kaolinite–mullite reaction series. In addition to sintering temperature and time, four raw material mixing procedures were employed, and the ball-milled slurry samples had the highest nickel incorporation efficiency. Prolonged leach testing of NiO, NiAl2O4 and the product from sintered kaolinite + NiO mixtures was carried out using the TCLP extraction fluid #1 (pH 4.9) to evaluate the product stability, and the results revealed the superiority of spinel products over NiO in stabilizing nickel.