Reduction and Carburization Mechanisms for CO + H2 Reduction in Shaft Furnace Conditions

Direct reduction, with reducing gases containing CO and H₂, is becoming an increasingly important process for reduction of iron ore to iron. There is a need for understanding reduction and carburization mechanisms for CO + H₂ gases in shaft-like conditions. The experimental setup includes a column of pellets, where ingoing gas of 40 pct CO and 60 pct H₂ and known temperature enters at the bottom and exits at the top. Experiments are carried out at 600 °C, 700 °C, 800 °C, and 900 °C for 1, 2.5, or 5 hours with gas flow rate of 4 or 6 nL/min. After reduction, pellets are removed, taking note of vertical position, and analyzed for reduction degree, total carbon, and cementite content. Results show that there is a gradient in reduction and carburization over the column height, which decreases with increasing time. Comparison of thermodynamic calculations with experimental results shows that kinetic factors have a strong influence on reduction and carburization. Consumption of gas by reduction or carburization reactions limits gas suitability at the local reaction sites. It could therefore be of interest to design the shaft process so that reduction and carburization take place in two steps.