Effect of Circulating Elements on the Dynamic Reduction Swelling Behaviour of Olivine and Acid Iron Ore Pellets under Simulated Blast Furnace Shaft Conditions

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Sulphur and alkalis in the blast furnace gas have been associated affecting the reduction swelling behaviour of iron ore pellets. A tube furnace was used in this study to examine the dynamic reduction swelling behaviour of olivine and acid pellets in CO–CO2–N2 atmosphere with sulphur and potassium in gaseous phases up to 1100°C simulating the conditions in the blast furnace shaft. No abnormal swelling was detected in sulphur or potassium containing CO–CO2–N2 atmospheres during dynamic reduction. Instead, sulphur in the reducing atmosphere was associated with pellet contraction and FeO–FeS melt formation which became more dominant with increasing sulphur partial pressures. In the extreme case, having a maximum of 1.0 vol-% S2 gas in the reducing atmosphere, the reduction reaction of wüstite to metallic iron was hindered. The formation of FeO–FeS liquid phase extends the cohesive zone towards the blast furnace top and lower temperatures and decreases the gas permeability. Furthermore, large amounts of potassium in the reducing atmosphere (max. 0.03 vol-%) led to swelling and cracking in the olivine pellets still remaining in the range of normal swelling.