Kinetic Studies on the Precipitation of Hematite from Iron‐Rich Spinel Solid Solutions

The kinetics of hematite precipitation from iron‐rich spinel solid solutions has been studied with special emphasis on the effect of divalent cations and iron‐oxide concentration. The precipitation kinetics is expressed by the Avrami equation α= 1 – exp [– k ( t –t 0 ) n ] with n = 1, where α, k , and t are fractional precipitation, proportionality constant, and time, respectively. The process is characterized by an activation energy of 30·0.5 kcal/mol, independent of iron‐oxide concentration. Stabilization of spinel solid solutions by the cation is most pronounced for Ni followed by Mg and Zn in the order stated, and the effect of the cation is interpreted in terms of the cation distribution in the spinel lattice.