Mechanisms and kinetics of zinc and iron separation enhanced by calcified carbothermal reduction for electric arc furnace dust

A high basicity charge prepared with electric arc furnace dust (EAFD), carbonaceous reducing agent and CaO is proposed. The mechanisms of enhancing separation of zinc and iron by calcified carbothermic reduction of the high basicity charge were analyzed by combining thermal analysis kinetics and experiment. The influences of roasting temperature, carbon ratio (n(c)/n(o), molar ratio of carbon in graphitic carbon powder to oxygen in EAFD), and CaO dosage on phase transition and dezincification ratio in EAFD were investigated. The results show that the intermediates Ca2Fe2O5 and Fe0.85-xZnxO can be produced from the zinc-iron separation of zinc ferrate during the process of calcified carbothermic reduction of EAFD. Addition of CaO and C results in the following transition pathways: ZnFe2O4+ CaO -> Ca2Fe2O5+ZnO -> Ca2Fe2O5+Zn(g)-> CaO+Fe; Fe0.85-xZnxO+CaO -> Ca2Fe2O5+FeO+ZnO -> CaO+Fe+Zn(g). In the range of n(c)/n(o) of 0.4-1.2 and roasting temperature of 1,000-1,100 degrees C, the addition of CaO can promote reduction and dezincification. Based on the Kissinger-Akahira-Sunose (KAS) and Coats-Redfern methods, the kinetic results show that the calcified carbothermic reduction process can be divided into three stages: initial stage (alpha=0-0.3), middle stage (alpha=0.3-0.45), and final stage (alpha=0.45-1.0). The average activation energy of the initial stage is 305.01 kJ center dot mol(-1), and the reaction mechanism is one-dimensional diffusion. The average activation energy is 315.67 kJ center dot mol(-1) for the middle stage and 288.22 kJ center dot mol(-1) for the final stage. The chemical reaction equation is found to be the most suitable mechanism in the medium and final stages. It is also found that the addition of CaO can reduce the average activation energy by about 32 kJ center dot mol(-1) and shorten the intermediate stage of the reaction.