Ironmaking chemistry: In a modern blast furnace, which species acts as the principal reducing agent for converting iron oxides to metallic iron?

Introduction / Context:
The blast furnace reduces iron oxides to metallic iron through a counter-current flow of gases generated by coke combustion and reactions in the raceway and stack. Identifying the dominant reducing species is central to understanding furnace thermochemistry.

Given Data / Assumptions:

• Burden: hematite/magnetite, fluxes, coke.
• Hot blast and tuyere reactions generate CO and heat.
• Upper stack temperatures allow gas-solid reduction.

Concept / Approach:
Although solid carbon participates at the lower zones (direct reduction of wüstite), most of the reduction through the stack occurs via the gas phase by carbon monoxide: Fe2O3 → Fe3O4 → FeO → Fe with CO oxidized to CO2. Some H2 participates (from moisture or injected fuels), but CO is the principal agent.

Step-by-Step Solution:
Check gas composition → CO-rich ascending gas.Recall sequence of indirect reductions → dominate in the upper/mid stack.Therefore, select CO as the main reducing agent.

Verification / Alternative check:
Material/heat balance calculations and top gas analysis in furnaces confirm CO's central role; H2 reduction is auxiliary unless specifically enhanced.

Why Other Options Are Wrong:

• Solid C: significant only in direct reduction zone; not overall principal.
• CO2: oxidized form; cannot reduce iron oxide.
• H2 alone: contributes but is not sole/primary in conventional BF practice.
• SO2: irrelevant to iron oxide reduction.

Common Pitfalls:
Overemphasizing direct reduction by solid carbon and ignoring the dominant indirect reduction by CO gas.

Final Answer:
Carbon monoxide (CO)