Defluidization Prevention By Carbon Coating For Direct Reduction Of Fine Iron Ore In A Fluidized Bed Reactor

Fluidized beds are ideal reactors for non-blast furnace ironmaking (pre-reduction of iron ore concentrates) due to their excellent heat and mass transfer efficiency, coke-free ironmaking and direct use of fine iron ore potential. However, owing to the serious sticking and defluidization problem, current state of the art fluidized bed (FB) pre-reduction technologies cannot use directly iron ore concentrates less than 0.15 mm, which are cheap, abundant and easy to be reduced. It is therefore necessary to understand defluidization mechanism and to develop defluidization prevention technologies for the fine iron ore concentrates. Among various defluidization prevention measures, carbon-coating is simple, effective and economical, and therefore shows high potential to be applied in industrial applications. One of the main problems lies in the fact that the direct reduction iron (DRI) obtained contains high carbon content, which severely restricts its application.To solve this problem, high-temperature sticking characteristic and mechanism of fine iron powder/iron ore concentrates were systematically investigated in the present study, where the influence of carbon coating on the fluidization/defluidization characteristics was inspected, with the main focus of reducing the carbon content in DRI through the optimization of the "carbon deposition-reduction" direct reduction process. The main findings of this thesis are summarized as follows:i) High temperature fluidization/defluidization characteristics of iron powder were systematically investigated, and two new fluidization regimes were found for fine iron powder as agglomerating fluidization and gradual defluidization. Moreover, based on the force balance analyses of particles and agglomerates, a theoretical model was proposed to explain and predict the agglomerating fluidization and gradual defluidization, where the calculated average agglomerate sizes showed reasonable agreement with those of experimental. Experimental results showed that the fluidization/defluidization behavior was mainly determined by the formation and growth of agglomerates, where temperature showed the major influence, while the effect of the gas velocity was not significant. Model prediction indicated that when the collision force (Fcoll) between two particles is slightly lower than the cohesion force (Fc), agglomerate will form and continuously grow till the balanced state that Fcoll equals to the Fc, under which agglomerating fluidization will occur if the minimum fluidization velocity of the agglomerates is lower than the operation velocity, otherwise gradual defluidization will occur.ii) The influences of gas type and gas composition on the fine iron ore fluidization/defluidization behavior were investigated in CO-H2 mixture gas. It was found that decomposition of CO could generate carbon-coating layer on particle surfaces and prevent defluidization to some extent. As a result, with the present of CO, the initial defluidization temperature is 50-150℃ higher than that of pure H2. It has been further found that the initial defluidization temperature increases with increasing the H2 mole fraction in the CO-H2 mixture, due to the fact that carbon layer could be deposited more uniformly under high concentration of H2 On the other hand, the defluidization prevention ability decreases with increasing the temperature due to the fact that carbon deposition via the CO decomposition reaction is inhibited at high temperature,iii) The influence of carbon coating on fluidization/defluidization characteristics was clarified through the investigation of particle composition/structure evolvement during the reduction process. Results showed that when the particle carbon content exceeded a critical value (Ccritical), defluidization could be effectively prevented. It has been further found that the Ccritical value could be significantly reduced through measures like increasing the metallization ratio of pre-reduced iron ore, enhancing the carbon coating efficiency, decreasing the temperature, while changing the fluidizing gas type and gas composition showed little influence on the Ccritical.Moreover, stability of the carbon-coating layer also showed remarkable effects on the defluidization, e.g., during the high-temperature reduction, if the deposited carbon was consumed by the particle collision or the chemical reactions with FeO, CO2 or H2, the coating layer might be destroyed, resulting in defluidization, even the initial particle carbon content exceeded the Ccritical value.iiii) Principles for reducing the product carbon content were proposed through two corresponding processes as (1) reducing the Ccritical value, which can be achieved through increasing the reduction rate and reducing the carbon deposition rate during the pre-reduction process (such as by increasing the temperature or the H2 mole fraction in CO-H2 mixture gas) and through decreasing the reduction temperature; (2) balancing the carbon deposition and consumption rates during the high temperature reduction process, preferably, the carbon content of iron ore should be kept constant or slow growth in practice through controlling deposition and reduction process. Consequently, the two-stage direct reduction process of "carbon deposition-reduction" was optimized and DRI with low carbon contents was achieved, e.g., the Ccritical value of iron ore was reduced to 2.98-3.47 wt.%, much lower than the reported value of 13.3 wt.% in literature, and the final DRI carbon content was decreased from 16.5-22.3 wt.% in literature to 5 wt.% in the present study.