Modelling of ammoniacal oxygen leaching of metallic iron in a stirred slurry reactor

Abstract

A new model for iron dissolution kinetics by oxidative ammoniacal leaching similar to that encountered in the Becher process for synthetic rutile manufacture is proposed. The model takes into account solid-liquid and gas-liquid oxygen mass transfer as well as oxidation of Fe 2+ to Fe 3+ ions as the controlling steps in the initial linear phase of the overall process. The model has been validated for the system containing metallic iron particles in a multiphase, sparged, agitated reactor under isothermal conditions. Along with kinetic measurements, the variation in pH and oxidation reduction potential were also monitored continuously. Both disc type turbines and axial flow turbines were used as the agitators. The model was found to be applicable over a modest range of process parameters (8 ≤ specific energy dissipation rate ≤ 15 kW/m 3, 8 × 10 −5 ≤ air flow rate ≤ 2 × 10 −4 m 3/s, iron particle size 230 μm, partial pressure of oxygen ≤2.03 × 10 4 Pa and 5 ≤ weight percentage of iron particles ≤ 40). The results indicate that Sano's correlation for solid-liquid mass transfer and Linek's correlation for gas-liquid mass transfer are adequate for describing the data. The rate of iron dissolution was insensitive to the loading of iron particles, which is explained as being due to the solid particles acting as oxygen sinks inside an encapsulated shell around the bubble. Variations in pH, oxidation reduction potential and Fe 2+ ion concentration with solid loading and oxygen partial pressure are consistent with the overall reaction mechanism proposed in this work.