Hydrothermal formation of ferrite spinels

The possibility of forming spinel-type ferrites MFe 2 O 4 from M(OH) 2 or MO and α-FeO(OH) (goethite) or α-Fe 2 O 3 (hematite) under hydrothermal conditions up to 350° has been investigated experimentally for M = Mg, Ca, Mn, Co, Ni, Cu, and Zn. Formation of MFe 2 O 4 was observed for M = Mn (jacobsite), Co, Ni (trevorite), and Zn (franklinite), and was in general favored by high temperatures, high pH, and the presence of NaNO 3 or, especially, NaCl in solution. α-FeO(OH) is more reactive than α-Fe 2 O 3 , but generally converts to the latter at 220–260°, independently of pH, in neutral or alkaline media. Interpretation of the results is complicated by the ion-exchanging properties and varying degrees of crystallinity of M(OH) 2 , and by the tendency of M(OH) 2 to decompose to MO or to be oxidized at the higher temperatures. Nevertheless, the order of reactivity M = Co ≥ Mn > Ni > Zn > (Mg, Ca, Cu) in MFe 2 O 4 formation is generally valid. The formation of MFe 2 O 4 is limited by kinetic rather than thermodynamic factors for the various M listed (except Cu).