General process information
High-grade iron ore is crushed for sizing, producing both fine ore as well as lump ore. To beneficiate low-grade ore, lump ore is first grinded into finer particles called grinded ore. Both fine and grinded ores are then subjected to pre-treatments before they can be charged into the blast furnace; that is, the fine ore is processed into sintered ore by sintering, and grinded ore is processed into pellets by pelletizing.
In the sintering process, fine ores 2-3mm in diameter are mixed with coke breeze as a fuel. Burnt limestone powder is used as a flux. These materials are charged in an iron box called a pallet before being ignited. Fine ore particles are partially melted and combined by the combustion heat of the coke to form agglomerate which is then subjected to crushing and screening processes in order to obtain sintered ore 15-30mm in diameter.
Pelletizing is a process that involves mixing very finely ground particles of ore of less than 200 mesh with fluxing materials such as limestone and Bentonite Clay and then shaping them into balls 10-15mm in diameter by a pelletizer, and hardening the balls by firing with heavy oil and/or coal as a fuel resulting in Iron pellets. Cold-bond pellets are also produced by pelletizing, and do not require firing.
Compared with sintered ore, pellets have a higher iron- and a lower gang-content, and pelletizing is suitable for treating the very fine ore that will predominate in the future. However, pellets have the disadvantages that more fossil fuel is consumed during pelletizing and it is difficult to control the radial distribution of the thickness of pellets charged in the blast furnace.