Reduction of fine iron ore via a two-step fluidized bed direct reduction process

• Boudouard reaction is coupled with the direct reduction to avoid defluidization.
• The noncohesive DRIc particles are constructed and reduced at fluidized state.
• The C/Fe ratio has significant influence on stickiness and fluidization behavior.
• A two-step fluidized bed DR process is proposed and proved feasible.
• The operating lines of the fluidization zones are indicated as maps.

The industrial application of fluidized bed direct reduction (DR) process for fine iron ore is hampered by the sticking of direct reduction iron (DRI) particles. In the present study, the carbon precipitation reaction is coupled with the reduction reaction of fine iron ore to modify the cohesive force among DRI particles. The competition between the reduction reaction of fine iron ore and the carbon precipitation reaction leads to three types of fluidization behaviors: fluidization, unstable fluidization and defluidization. The carbon precipitation reaction is dominant at the temperature below 600–675 °C, and the presence of H2 could retard the growth of iron whiskers and promote carbon precipitation. A new type of DRI particle covered with carbon shell is therefore constructed and named as DRIc particle. The growth of iron and carbon gasification can destroy the carbon shell, and lead to the increase of stickiness; however, the presence of CO can retard or prevent the destroying. The C/Fe mass ratio on the surface has significant influence on the stickiness and also the fluidization behavior of DRIc particles. The lower limit of C/Fe mass ratio, below which defluidization occurs, increases sharply with increasing temperature. Based on these findings, a two-step fluidized bed DR process for fine iron ore is proposed and proved feasible, and the operating lines of the fluidization zones are indicated as maps.

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