Stabilization of nickel-laden sludge by a high-temperature NiCr2O4 synthesis process

► The nickel-laden sludge was transformed into NiCr2O4 spinel by thermal treatment. ► Leaching concentration of both nickel and chromium decreased because of the formation of NiCr2O4. ► The existence of salt in sludge would disturb the formation of NiCr2O4. ► The Cr/Ni molar >2 was contributive to the stabilization efficiency of heavy metal.