Decomposition behavior and direct reduction kinetics of (Fe/Zn/Ni)O·(Fe/Al)2O3 spinels

(Fe/Zn/Ni)O·(Fe/Al) 2 O 3 spinels are widely used as raw materials for advanced ceramics and refractories and occur as precipitates in slag phases during nonferrous metal pyrometallurgy. The stability and phase-transformation mechanisms of these spinels are important for optimizing material performance and controlling metallurgical processes. The decomposition of Fe 3 O 4 , FeAl 2 O 4 , ZnFe 2 O 4 , ZnAl 2 O 4 , NiFe 2 O 4 , and NiAl 2 O 4 under various atmospheres was experimentally investigated, and phase evolution during reduction was elucidated. Thermogravimetric and differential thermal analyses and reduction experiments showed that Fe(II)-bearing spinels tended to decompose at elevated temperatures in air. ZnFe 2 O 4 and ZnAl 2 O 4 were stable at temperatures up to 1500 °C in air and in an atmosphere with p O2  = 10 −6  atm. At low oxygen partial pressure, aluminate spinels exhibited greater stability. The reduction kinetics of these spinels were also analyzed. The reduction of Fe 3 O 4 , ZnFe 2 O 4 , NiFe 2 O 4 , and NiAl 2 O 4 was described by the Avrami-Erofeev model, indicating that the reduction process was controlled by the nucleation rate. The reduction of ZnAl 2 O 4 was described by a diffusion model.