TITLE:
A method of improving the quality of iron ore pellets used in blast furnace as
source of iron bearing charge by adding magnesium silicate mineral.
FIELD OF INVENTION:
The present invention relates to a method of improving the quality of iron ore
pellets used in blast furnace as source of iron bearing charge by adding magnesium
silicate mineral.
BACKGROUND OF INVENTION:
Pelletizing is one of the widely used methods of agglomerating iron ore fines to
produce quality feedstock for iron making furnaces. The first large plant (with a
capacity in excess of 6 million tons per annum) in the United States commenced
operation in 1955 and by 1975 iron ore pellet production in the United States was
almost 60 million tons per annum. Pelletizing plants are now operated in many
countries and the world iron ore pelletising capacity is around 400 million tones
per annum that accounts almost 25% of the iron ore processed in the world.
This widely practiced conventional pelletizing process to produce indurated or heat
hardened pellets consists of two distinct operations; preparing pellets at ambient
temperature and then firing them at high temperature in the range of 1250-1350° C
depending upon the material. In the first operation, pellets are formed by rolling

moist fine ore of proper fineness (60% - 80% less than 45 microns) in either a
horizontal drum, or more usually an inclined disk, and are known as "green balls"
or pellets. At this stage the pellets are required to have adequate strength to
withstand handling to the firing stage and sometimes a minimum quantity of an
inorganic or an organic binder is added to achieve adequate strength. In the second
operation the pellets are fired. The resulting fired pellets are typically spherical in
shape, of approximately 8-16 mm diameter, and have cold compression strength of
approximately of 200-250 kg/pellet. "Cold compression strength (CCS)" is a
measure of the load required to be applied to cause crushing of a spherical pellet of
10-12.5 mm diameter placed between two plates of a compression testing machine.
Sufficient CCS is required to withstand the bulk handling for example in loading to
railway cars, unloading of railway cars, at port facilities, at steel works etc.
The quality of iron ore agglomerates plays a vital role in decreasing the reducing
agent consumption and increasing the productivity of blast furnace. In most of the
integrated steel works, the burden mix for blast furnace is decided as per the
availability of the iron ore agglomerates. For blast furnace operating with mixed
burden materials like sinter, pellets and lump ore, where super fluxed sinter is
available with high CaO contents (~9-10%), pellets should of acidic in nature, free
from CaO, to maintain the blast furnace slag chemistry. But acid pellets are known
for their poor high temperature properties like softening-melting characteristics and
reducibility (Ref: US patent 4356027). When acid pellets are charged in to a blast
furnace, a phenomenon called" reduction retardation" occurs under reducing

conditions at temperatures exceeding around 1100 °C. During this phenomenon, no
reduction occurs inside the pellets. In such conditions, the amount of high FeO slag
increases in the blast furnace resulting in slopping, hanging and lowering the gas
permeability thereby causing inefficiency in the furnace operation. If the acid
pellets are tested for softening-melting characteristics to simulate the cohesive
zone in the blast furnace, softening of the pellets occurs at lower temperatures.
This low softening temperature reduces the gaps between the pellet layers thereby
reducing the permeability to the reducing gases. This reduces the efficiency of the
blast furnace. Swelling index of the acid pellets was also found to be high.
Tendency of generating fines, also called Reduction degradation Index (RDI) is
also high for acid pellets that impair the permeability of blast furnace.
• Fujita et al.'s, Patent (US 4367091) dated 4 June. 1983, describes the
addition of CaO+SiO2 in the range of 4-12% to pellets improved the
reducibility by forming precipitated slag phases in the fired pellets that are
supposed to create cracks in the pellets during their reduction.
• McAllister et al.'s, Patent (US 4326887) dated 27 April 1982, describes
addition of steel melt shop slag in the range of 1-20% to pellets improves the
acid pellet properties.
• Taguchi et al.'s, Patent (US 4350523) dated 21 Sep 1982, describes use of
low temperature combustible materials like saw dust or brown coal in the
pellets in the range of 0.5 to 8% to increase the porosity and reduce the FeO
of the pellets thereby improving their properties.

• Sugiyama et al.'s, Patent (US 4356027) dated 25 Oct. 1982, describes use
MgO bearing fluxes like magnesite, brusite, sea water magnesia or magnesia
brick slag in pellets to improve the reduction and load softening properties.
The drawbacks of the earlier processes in the prior art are generation of excess
fines during reduction (limitation in US 4367091); increased sulphur content of
fired pellets (limitation US 4326887); very high grinding energy requirement of
saw dust (saw dust requires 600 kWh/t in contrast to 6-23 kWh/t of iron ore)
(limitation in US 4350523) and high energy requirement during pellet Induration
for dissociation of MgO bearing flux material (limitation in US 4356027). In some
cases, the cost and availability of the additives restrict their use in pelletizing.
Widely available fluxes like limestone and dolomite cannont be added in case of
acid pellets as they increase the CaO content of the pellets. Thus there exists a need
to provide a process or method that overcomes the aforesaid limitations and
improves the acid pellet quality using economically viable methods or processes.
OBJECT OF THE INVENTION:
An object of the invention to provide a method of improving the iron ore pellet
quality without increasing CaO content of the pellets'
Another object of the invention is to improve the softening-melting characteristics
of the pellets;
A still another object of the invention is to reduce the degradation of pellets during
their reduction without affecting the reducibility;

A further object of the invention is to reduce the swelling of the pellets during
reduction;
Yet another object of the invention is to decrease the specific energy consumption
during the pellet induration.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS;
Fig.l: shows cold compression strength of varying MgO pellets.
i
i
Fig 2: shows scanning Electron Microscope image of Pellet A & D with EDS
analysis of all pellets.
Fig. 3: shows high temperature metallurgical properties of acid (pellet A) and
pyroxenite pellet (pellet D).
DETAILED DESCRIPTION OF THE INVENTION;
The present invention relates to improved iron ore pellet quality with excellent
high temperature properties.
According to the present invention, magnesium silicate mineral like pyroxenite is
added to the raw material to the pellet to improve the high temperature properties
of pellets. Unlike limestone or dolomite, magnesium silicate minerals like
pyroxenite do not demand extra energy for calcinations during pellet firing. Acid

pellets without pyroxenite addition generate more amount of high FeO slag in the
pellets. This high FeO slag melts at lower temperature tereby reducing the
softening temperature and reducibility of pellets. Addition of MgO increases the
melting point of slag formed in pellets and melting point of FeO by forming
magnesio-wustite. High melting point slag formed in the pellets improves the
swelling index, softening characteristics and high temperature reducibility.
Production of improved quality pellets by adding pyroxenite involves the
following steps;
1. Raw material preparation to prepare the green pelletizing mix
2. Mixing pyroxenite of desired fineness to the pelletizing mix
3. Preparation of green pellets
4. Firing the green pellets
To prepare green pellets, the iron ore fines should posses required fineness and size
distribution. The ore or mineral fines are ground to the required fineness (60% to
80% less than 45 microns) in the ball mill. If the fines to be pelletized already
posses the required fineness and size distribution, then there is no need of grinding.
Pyroxenite also needs to be ground to the required fineness (60% to 80% less than
45 microns) in the ball mill.
In the present method, ground iron ore fines are mixed with binder, generally
bentonite, and solid fuel like coal fines if required, to prepare the pelletizing mix.
Bentonite is naturally occurring plastic clay of great commercial importance and it

falls under montmorillonite group. Bentonite, (Na Ca) (Al Mg) Si4O10 (OH)2 is
basically an alumino-silicate clay. Pyroxenite of required fineness is added to the
pelletizing mix in the range of 1 to 10% with respect to the weight of the
pelletizing mix so that the MgO content of the pellets is in the range of 0.5 to
3.0%, preferably in the range of 1.0% to 2.0%. Chemical composition of
pyroxenite is shown in Table. 1.
Table. 1 Chemical composition of pyroxenite

Prewetting of the pelletizing mix is done by adding adequate of water, normally in
the range of 4-6% by weight of the mixture. After prewetting the mixture is called
green pelletizing mixture.
Pelletizing is carried out in the normal manner with water being added as
appropriate to the mineral during this operation. The amount of water is of no
special importance to the invention, the amount used being that required for
satisfactory pelletizing, normally in the range of 9-11% by weight of the pelletizing
mix, but changes as per the type of ore or mineral whether they are hydrophilic or
hydrophobic in nature. The green pellets are prepared in the size range of 8-16

mm. After drying the green pellets to remove the moisture to less than 0.5%, they
are fired up to 1300 °C in the normal manner to attain required strength.
Pyroxenite addition resulted in the formation of magnesioferrite in fired iron ore
pellets. Addition up to 4 to 6 % to get 1.0 to 2.0 % MgO in the pellets was found to
be optimum beyond which the amount of relict or partially assimilated magnesium
silicate phase was increased. Acid pellets exhibited higher CCS values compared
to pyroxenite pellets due to the formation of strong hematite bonds. Highest
strength in pyroxenite pellets was observed at 1.5% MgO and thereafter it was
found to decrease due to the inhomogeneous matrix as a result of relict pyroxenite
phase. RDI of pyroxenite pellets was found to be superior, compared to acid
pellets, due to the formation of magnesioferrite and more amount of silicate melt,
which are more stable under the given reducing conditions. Swelling index of acid
pellets was poor due to the formation of low melting point fayalitic slag whereas
pyroxenite pellets exhibited less swelling by forming low FeO high melting slag
that gives sufficient bond strength to withstand the reduction stresses. Pyroxenite
pellets resulted in better softening-melting characteristics compared to acid pellets
due to the formation of high melting point slag and magnesio-wustite phase during
reduction.
It is to be understood that the invention is not limited by the specific example and
embodiment described hereinabove, but includes such changes and modifications
as may be apparent to one skilled in the art.

Example: Improving the quality of acid iron ore pellets by adding pyroxenite.
Ground iron ore fines, solid reductant fines, bentonite and ground pyroxenite as
given in the Table.2 are premixed in the dry state to form a homogeneous mixture.
Prewetting of the dry pelletizing mixture is done by adding 4-6% water. Pelletizing
experiments are conducted on a laboratory balling disc. Diameter of the disc is
600mm and angle of inclination and rpm of the disc were set at 45° and 25
respectively during balling. During pelletizing 1-2% water was added on the disc.
Time taken for pelletizing 2kg batch is approximately 20 minutes.
Table.2 Ingredients of Pellets with varying amount of pyroxenite

Drying of green pellets:
Green pellets are dried in a laboratory hot air dryer at 150°C 5-6 hours to remove
the moisture.

firing of green pellets:
Firing of the pellets was carried out using electrically heated rotary hearth furnace
(RHF). The RHF consists of 5 heating zones and 1 cooling zone. All the zones
were provided with air inlet nozzles to inject ambient air for creating oxidizing
atmosphere during induration.
Physical and metallurgical testing of pellets
Cold compression strength (CCS) of fired pellets was tested as per ISO 4700 and
reduction degradation index (RDI) was tested as per ISO 4696-2. Reducibility
index (RI) and swelling index was tested as per ISO 7215 and ISO 4698
respectively. Softening melting characteristics were tested in the laboratory
softening melting apparatus.

Table 3 Chemical analysis of pellets with varying amounts of pyroxenite addition


Table 4


WE CLAIM:
1. A method for improving the quality of iron ore pellets without adding CaO
bearing fluxes comprising the steps of:
a) Grinding the ore fines into a size of 60-80% less than 45 micron;
b) Mixing the ore fines with an effective amount of pyroxenite
(magnesium silicate) mineral in the range of 0.5 to 10% with respect
to the weight of the mix and adequate amount of binder and water (2-
8%);
c) Subjecting the mixture to the step of palletizing/briquetting to form
green pellets/briquettes;
d) Firing the green pellets in the temperature range of 1280-1320°C;

2. The method as claimed in claim 1, wherein the said pyroxenite is
magnesium silicate.
3. The method as claimed in claim 1, wherein the said magnesium silicate is
dunite or serpentine.
4. The method as claimed in claim 1, wherein the magnesium silicate addition
in such that the MgO content of the pellets is in the range of 0.5 to 3.0%,
preferably in the range of 1.0% to 2.0%.
5. The method as claimed in claim 1, wherein the said fired pellets are superior
to acid pellets in terms of reduction degradation index, swelling index and
softening melting characteristics.

6. The method as claimed in claim 1, wherein the binder is bentonite.

A method for improving the quality of iron ore pellets without adding CaO bearing
fluxes comprising the steps of: Grinding the ore fines into a size of 60-80% less
than 45 micron; Mixing the ore fines with an effective amount of pyroxenite
(magnesium silicate) mineral in the range of 0.5 to 10% with respect to the weight
of the mix and adequate amount of binder and water (2-8%); Subjecting the
mixture to the step of palletizing/briquetting to form green pellets/briquettes;
Firing the green pellets in the temperature range of 1280-1320°C;