Carbon was deposited on the surface of an iron powder to study the influence of carbon content and microstructure on defluidization prevention at 823–1173 K. It was found that at each temperature there existed a minimal carbon content (Cmin), below which defluidization occurred and Cmin increases with increasing operation temperature. Microstructure of the deposited carbon, tailored by adjusting deposition conditions, showed significant influence on defluidization. Less carbon would be needed to prevent defluidization under a lower rate of carbon deposition since a more uniformly coating of carbon can be obtainable under such conditions. When the fluidizing gas is nitrogen, defluidization will eventually occur after extended time of fluidization even if the initial carbon content is greater than Cmin, due to attrition that causes the decrease of carbon content with increasing time of fluidization. However, when the fluidizing gas is 70% CO-30% H2, defluidization will not occur if the initial carbon content is greater than Cmin since the carbon content will increase with increasing time of fluidization due to the further deposition of carbon under the CO–H2 atmosphere. Furthermore, a fluidizing phase diagram was established for the carbon-coated iron powder and the mechanism of defluidization prevention was explained using a cohesive force based model.
