Effect of Reduction Conditions on the Swelling Behaviour of Cement-bonded Briquettes

Abstract

Cement-bonded agglomerates of by-products generated in iron and steel plants are commonly used as burden material for blast furnaces. It has been observed that under certain conditions the briquettes containing pellet-fines show a tendency to swell catastrophically when reduced at 900-1 000°C using carbon monoxide as reducing agent. This swelling is dependent upon a number of factors like: reducing temperature, reducing conditions and composition of reducing gas. The optical micrographs do not show the formation of iron whiskers as the cause of swelling; instead, the reduced iron particles seem to move apart, thereby causing swelling. This paper describes the effect of various parameters that cause the abnormal swelling of briquettes.

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

“…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.…”

“…Normal swelling up to 20 vol-% occurs during the transformation of the pellets from hematite to magnetite in the initial stages of reduction, in which the crystal structure changes from hexagonal to cubic. 2,7,8) Singh and Björkman 2) state that conversion from magnetite to wüstite can also lead to swelling to some extent. Abnormal swelling occurs during the transformation of wüstite to metallic iron in the latter stages of reduction and is generally believed to be caused by the growth of iron whiskers in the reduction of wüstite.…”

“…5,7,[9][10][11][12][13][14][15] However, the exact mechanism of swelling is not fully known. 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.…”

“…Due to the increased porosity of pellets, normal swelling can be considered advantageous for the reduction process, 2) while abnormal swelling detracts from the mechanical properties of the pellets, thus increasing the generation of dust and fines, which lowers the permeability of the pellet bed and makes operating problems more likely to occur. 4) The ISIJ International, Vol.…”

Reduction Kinetics and Catastrophic Swelling of MnO2-doped Fe2O3 Compacts with CO at 1073-1373 K

“…4) Maneesh et al 5,6) studied the effect of reduction conditions on the swelling behavior of cement bonded briquettes. They found that the swelling of briquettes increases with the increase in average particle size of the raw material also the briquettes containing a larger amount of slag-forming oxides, CaO, SiO 2 , MgO and Al 2 O 3 may swell more.…”