Effects of pellets size

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Process Metallurgy

Effect of Iron Ore Pellet Size on its Properties and Microstructure
T. Umadevi, Prasanna Kumar, Naveen F Lobo, P.C. Mahapatra, M. Prabhu and Madhu Ranjan
JSW Steel Limited, Toranagallu – 583119, Bellary, Karnataka, India, uma.devi@jsw.in

The properties of the pellets and their microstructure mainly depend on the raw material mix proportion, raw material chemicalcomposition
and the physicochemical conditions like the temperature and oxygen partial pressure within the induration machine. The pelletising plant
products are in the size range of 8 to 16 mm. With increasing pellet size, the sintering intensity, thermo-chemical conditions and formation
of different phases vary across its cross section. The time required for varies reactions within the pellet isdirectly proportional to the pellet
size. Because of differences in pellet size, the reduction and oxidation process takes place under different conditions resulting in different
phases and microstructures. In this work, detailed studies were carried out on pellets of different size (8 to 16 mm) produced from a
4.2 Mtpa pellet plant for their physical, metallurgical and microstructural properties.It was observed that the pellets in the size range
of +8 to -12 mm showed good strength and lower RDI. It was observed that the amount of hematite, magnetite, porosity, pore density, pore
size and slag phase play a significant role on pellet strength and RDI.
Keywords: iron ore pelletizsation, pellet size, microstructure, physical properties, metallurgical properties
DOI: 10.2374/SRI08SP169;submitted on 4 November 2008, accepted on 26 March 2009

Introduction

Pelletization Process at JSW Steel Limited

During processing of pellets in a blast furnace, their
physical, metallurgical and microstructural properties have
a major influence on their reduction behaviour. The firing
time, temperature, oxidation time and oxygen partial
pressure are decisive for the structural changeswithin the
pellets; the nature of the changes affects, in turn, the
physical and metallurgical properties of the pellets. Based
on the above conditions in the induration machine, the
fired pellets often have non-uniform phases and structures
from the core to the shell of the pellet depending upon
their size. The outer part of the pellet differs distinctly
from the core. The time differencebetween the reduction
and oxidation of the pellet surface and the pellet core
increases with increasing pellet size. For bigger pellets, a
concentric duplex and triplex microstructure has been
observed. Thus, pellet size has a marked effect on the
formation of different phases and microstructure.
Some models were developed for straight grate
induration machines [1-3] and for grate kilnmachines [4].
More recent advances in modelling of straight grate pellet
induration have been achieved by Corem [5]. This
includes the prediction of the pellet metallurgical
properties from green ball composition and thermal history.
All these models assume that for given ore chemistry,
green pellet properties and thermal history, the quality of
the fired pellets is the same. Even keeping theabove
variables constant, the pellet quality varies depending on
the pellet size. This study was carried out to understand
the effect of pellet size on microstructural, physical and
metallurgical properties. The pellets for the present study
were collected from a 4.2Mtpa pellet plant. Pellets of
different size ranging from 8 to 16 mm were tested for
physical and metallurgical properties.The microstructural
and phase distribution studies were carried out by using
optical microscopy and image analyzer technique. From
the above data an attempt was made to correlate the pellet
properties with their microstructures.

Production of iron oxide pellets from iron ore fines
involves different steps from drying to induration. The ore
fines are initially dried to a moisture content...
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