Partial Reduction of Reactive NiAl₂O₄ Spinel Prepared from a Sulfate Solid Solution

Abstract

Partial reduction of highly reactive NiAl₂O₄ spinel was investigated concerning with the raw spinel powder prepared by thermal decomposition of Al₂(SO₄)₃–NiSO₄ solid solutions. Solid solution of Al₂(SO₄)₃ with NiSO₄ was prepared from mixed aqueous solutions by a rapid dry using micro wave heating. The Ni²⁺ ion is soluble in the anhydrous aluminum sulfate crystal lattice over a wide range up to 65 mol% of (NiSO₄)/(NiSO₄ + Al₂(SO₄)₃). The anhydrous sulfate thermally decomposed to form the crystalline phases of spinel, η-Al₂O₃ and NiO at 900 °C. The lattice parameter of the spinel changed linearly with the Ni content ranging from about 20 mol% to 50 mol% of (NiO)/(NiO + Al₂O₃) without any other crystalline phases coexisted. The compacted NiAl₂O₄ spinel powder buried in a carbon powder was heated to partially reduce. On heating up to 700 °C the NiO segregated from the spinel leading to its enrichment in Al₂O₃. Metal Ni appeared above 800 °C, coexisting with the NiO and the spinel. The lattice parameter of the spinel decreased with decreasing the content of Ni in the spinel. The α-Al₂O₃/Ni composite was formed after heating at 1300 °C, while three phases of the NiO, the metal Ni and the spinel were coexisted until heating at 1200 °C.