Metallic iron whisker formation and growth during iron oxide reduction: K₂O effect

Abstract

An attempt has been made to clarify metallic iron whisker growth and swelling behaviour during iron oxide reduction in the presence of K 2 O alkali. Annealed pure and K 2 O doped wü stite compacts were isothermally reduced in a thermogravimetric apparatus with CO gas at 800-1100uC. The reduction rate increased with temperature but decreased with increasing K 2 O content. Morphologically, by increasing reduction temperature, porosity and whisker growth of the metallic iron produced increased, which in turn leads to the apparent swelling and compact disintegration. The reduction of pure wü stite at both the initial and final stages is most likely controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanisms. For K 2 O doped wü stite samples, the reduction at the initial stages is most likely controlled by gaseous diffusion mechanism, while in the final stages, it is most likely controlled by a chemical reaction mechanism.

Dynamic and Isothermal Reduction Swelling Behaviour of Olivine and Acid Iron Ore Pellets under Simulated Blast Furnace Shaft Conditions

“…catastrophic swelling, which may exceed 400% in volume at worst. [4][5][6] This catastrophic swelling is a characteristic of pellets and has been not observed either in sinter or in lump ores.…”

“…Swelling can be categorized into normal swelling, up to 20% in volume, [1][2][3][4][5] and abnormal, i.e. catastrophic swelling, which may exceed 400% in volume at worst.…”

“…2) Various operating parameters such as firing time, rate of reduction, reduction temperature, reduction time, reducing gas atmosphere, partial pressure of reducing gases, total flow rate, impurities in the reducing gas and isothermal/nonisothermal reduction, and pellet properties such as porosity, basicity and gangue content, have been reported to affect the swelling of different pellet types used in blast furnaces. [1][2][3][4][5][6][7] Some authors 10,11) have reported the existence of a critical rate of reduction at which the swelling index of an iron oxide pellet reaches its maximum value. In addition, sulphur 6,22,23,25) and potassium 22) in the reducing atmosphere have been associated with the swelling of pellets.…”

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

“…The reduction swelling results attained in this study indicate that large amounts of potassium in the reducing atmosphere promote pellet swelling, nevertheless including within the range of normal swelling (up to 20% in volume 16) ). The observation is opposite to the study by Bahgat et al,16) in which swelling clearly decreased with increasing the amount of added K2O to wüstite compacts during isothermal reduction. However, it must be taken into account that potassium was added as K2O there, not being in the gaseous form.…”

“…10,15) The effect of potassium on the reduction swelling behaviour has been investigated only in a few studies. Bahgat et al 16) found that swelling and the reduction rate of wüstite compacts both decreased with increasing K2O content. The effect of sodium on the swelling of hematite during reduction has been studied by Lu.…”