Iron, along with its generic products, is
currently the most widely utilised metal in the various sectors of the world’s
economy. Many factors contribute to this, ranging from the good mechanical
properties it possesses to the low cost associated with its production. Iron is
mainly produced through two methods; the blast furnace, BF, route (pig iron),
and the direct reduction, DR, route (sponge iron).
Iron ore can be used directly in its natural form as a raw
material for processing iron or it can be upgraded through beneficiation before
it’s charged into the BF or DR furnaces. The feedstock is evaluated for
physical and metallurgical properties. Physical properties give an indication
of the material behaviour during handling and descent in the furnace.
Metallurgical properties on the other hand indicate the materials’ behaviour
during the reduction process. In selecting iron ore for iron and steel
industries, some of the properties which need to be considered include (i) tumbler,
abrasion and shatter indices, (ii) porosity, (iii) chemical composition, (iv)
loss on ignition, (v) reduction behavior, and (vi) thermal degradation.
The
strength and resistance of the iron ore to degradation (Physical Properties) is
represented by Tumbler Index, Abrasion Index, Shatter Index and Thermal
Degradation Index as well
as their apparent porosity.
(i) Tumbler Index
(i) Tumbler Index
The Tumbler Index is a relative measure of the resistance of
the material to breakage or degradation by impact.
(ii) Abrasion Index:
(ii) Abrasion Index:
It is a relative measure the degradation of iron ore by
abrasion. The Tumbler Index and Abrasion Index are determined in a tumbler test
apparatus as per the Bureau of Indian Standard No. IS:6495 .
A tumble strength test
measures two mechanisms of feedstock degradation, that is, the Tumble Index
(TI) and the Abrasion Index (AI). It was carried out following the
International Standard ISO 3271:1995(E) for determination of Tumble Strength
for iron ore . Precisely, a 15 kg test block sample was tumbled in a circular
drum rotating at 25 rpm for 200 revolutions. Subsequently, the ore was screened
and fractions +6.3 mm and −0.5 mm were obtained. The percentage of the
fractions in proportion to the feed weight is the value of the TI (+6.3 mm) and
AI (−0.5 mm). The test was repeated four times and the average values for these
tests represent the final TI and AI data.
(iii) Shatter Index
It is a measure of resistance to free fall of the material .
it is determined by the quantity of material which after being tested , retains
a size over specified dimension. This test can be carried out as per Bureau Of
Indian Standard No.IS:9963.
For estimation of a Shatter
Index, a dried lump iron ore sample (10 kg) of size −40 + 10 mm was dropped 4
times from a height of 2 m onto a cast iron floor (0.5 × 0.5 × 0.03 m).
Thereafter, the iron ore was screened and the shatter index expressed as the
wt% passing through a 5 mm sized screen (i.e., −5 mm fraction).
(iv) Thermal Degradation Index:
It is a measure of the tendency of the iron ore bearing
materials to undergo size degradation as a result of either thermal shock or
reduction or both .The term decrepitation or low temperature breakdown (LTB)
are also used in place of degradation in technical literature.
(v)
The apparent
porosity
It was determined using the GeoPyc 1360
pycnometer. A quantity of helium was placed in the sample chamber and its
volume was measured. Thereafter, a 2.0 g iron ore piece was placed in the
chamber together with the helium gas and the equipment registered the new
volume values. The difference in the new and original helium volume gave the
sample’s envelope and skeletal volumes. The difference in the envelope and
skeletal volumes indicates the percentage of porosity of the sample.
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