Characterisation studies on swelling behaviour of iron ore pellets

Abstract

At JSW Steel Limited (JSWSL), pellets form the major part of the iron-bearing feed to corex and blast furnace. JSWSL produces low-basicity pellets ((CaO/SiO 2 ) -0.40 to 0.50). The quality of the pellet is affected by the raw material chemistry (gangue content), flux proportion and their subsequent heat treatment to produce the fired pellets. The raw material silica, limestone addition, i.e. basicity -CaO/SiO 2 of pellet decides the mode, temperature and the amount of melt formed. The properties of the pellets are, therefore, largely governed by the form and degree of bonding achieved between ore particles and also by the stability of these bonding phases during the reduction of iron oxides. In the present study, laboratory pelletisation experiments have been carried out to know the effects of silica and basicity on the microstructure and swelling behaviour of pellets during reduction. Phase analysis was carried out using image analyser, and chemical analysis of oxide and slag phases was carried out using SEM-EDS. From the laboratory studies, it was observed that the swelling index of the pellets decreased with an increase in silica content due to the decrease in porosity. The presence of higher silica in pellet hinders the reduction step of haematite to magnetite at lower temperatures. Pellets with basicity range 0 to 0.1 exhibited lower swelling index due to the formation of high melting point fayalite phase and also at this basicity range the structure is held together by the seam-like compounds between Fe 2 O 3 and SiO 2 primarily at high silica content. Higher swelling index was observed at the basicity range 0.3 to 0.7 due to the presence of low melting point calcium olivines (1115°C) between fayalite (FeSiO 4 ) and dicalcium silicate (Ca 2 SiO 4 ). Low melting point slag phase enhances the swelling index of the pellets. Swelling index of the pellets considerably dropped between the basicity range 0.9 to 1.1 due to the formation of calcium ferrite phases with a close pore structure.

Mechanism on reduction swelling of pellets prepared from Bayan Obo iron ore concentrate

“…If the reduction swelling rate of pellets is higher than 20%, the gas properties of the blast furnace will decrease, and the air permeability will be unevenly distributed, which will affect the normal production of the blast furnace. It is generally believed that the reduction swelling rate of pellets higher than 20% is abnormal swelling [13,[16][17][18][19][20]. If a large proportion of Bayan Obo iron ore concentrate is added in the production of pellets, it will cause abnormal swelling of the pellets during the reduction process.…”

“…Based on the iron oxide reduction theory of pellets [14,19,25], this paper divides the reduction and swelling process of pellets prepared from Bayan Obo iron ore concentrate into three stages [14,17,26–29]: …”

Swelling Behavior of Iron Ore Pellets during Reduction in H2 and N2/H2 Atmospheres at Different Temperatures

“…This can be attributed to an increase in the reduction rate of the iron oxide pellets through their pores at the higher temperatures. [26,34] Hence, the role of the pores formed in the pellets is an important one, as the pores affect the rate of reduction of hematite to metallic iron. The samples with a higher iron oxide content but lower-gangue content had a higher RSI.…”

Experimental Investigation of Different Fineness and Firing Temperatures on Pellets Properties of Different Iron Ore fines from Indian Mines

“…[41]. The formation of the melting slag phase disturbs the structure of the pellet and increases the R.D.I., as the Iron Ore No.2 pellets slag phase is higher than Iron Ore No.1 pellets, corresponding to all firing temperatures in increasing order [40,42]. Iron Ore No.2 has a higher R.D.I.…”