Iron pellets analysis

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STUDENTS PROGRAMME OF NMD-ATM

2012

PROJECT THEME �

IRON AND STEEL

TITLE �

LOW EXPANSION COEFFICIENT IRON

ORE PELLETS

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CONTENTS

TOPIC

1. ABSTRACT

2. INTRODUCTION

3. OBJECTIVES

4. EXPECTED DELIVERABLES

5. LITERATURE REVIEW

5.1 What are Pellets?

5.2 MECHANISM OF PELLET FORMATION

5.2.1. IRON ORE PREPARATION

5.2.2.PELLETIZATION OR BALLING OF WET ORE FINES

5.2.3. INDURATION (HEAT HARDENING)

5.3 ADVANTAGES OF PELLET FORMATION :

5.4 REDUCTION PROCESS

5.5 RATE OF REDUCTION

5.6 Steel demand in the country

6. EXPERIMENTAL DETAILS

7. DATA ANALYSIS

8. RESULTS AND DISCUSSION

8.1 EFFECT OF REDUCTION TEMPERATURE ON DEGREE OF

REDUCTION OF IRON ORE PELLETS



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8.2 REDUCTION KINETICS OF IRON ORE PELLETS

8.3 CAUSES OF SWELLING OF IRON ORE PELLETS

9. CONCLUSIONS

10. REFRENCES



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1. ABSTRACT:

Steel Industry in India is on an upswing because of the strong global and domestic demand. India�s rapid economic growth and soaring demand by sectors like infrastructure, real estate and automobiles, at home and abroad, has put Indian steel industry on the global map. Iron ore pellets have the potential to contribute a lot to Indian steel and iron production. Pellets are approximately spherical lumps formed by agglomeration of the crushed iron ore fines in presence of moisture and binder, on subsequent induration at 1300°C.A lot of investigations have been carried out on direct reduction process of iron oxides by carbonaceous materials, but little work has been done on the characterization of properties and reduction behaviour of iron ore. In the present project work, an attempt has been made to study the reduction behaviour and kinetics of low expansion coefficient iron ore fired pellets. Work has also been done on the swelling behaviour of low expansion coefficient iron ore pellets.



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2. INTRODUCTION:

From around 400 million tonnes of run-off mines(rom) iron ore every year we get the 270 Mts of lumps and fines. At this rate of mining the proven reserve may last 32-35 years. To meet the increasing demand there are two options: to find new resources or to use the existing resources judiciously. With the existing advanced technology almost whole of the earth is filtered for resources so finding of new resources is less probable which forces us to use the existing resources judiciously. It involves development of agglomeration processes like sintering and pelletisation. This report focuses on the need of development of pelletisation activities. Pellets are approximately spherical lumps formed by agglomeration of the crushed iron ore fines in presence of moisture and binder, on subsequent induration at 1300°C. Low grade iron ore, iron ore fines and iron ore tailings accumulated over the years need to be beneficiated to provide concentrates of required quality to the Indian steel plants. However, these concentrates are too fine in size to be used directly in the existing iron making processes. For utilizing this fine concentrate, pelletization is the only alternative available.

Pelletization is the process of agglomeration of iron ore fines. The particles

smaller than 200m of which nearly 50% with 50m size each are converted

into 12-15mm green pellets with a nodular shape, which on drying and firing

become hard and strong to be used as feed for the blast furnace or DRI units.

The iron ore fines are mainly generated at mines during blasting, handling,

breaking and sizing processes. A large percentage of the total mined ore is

converted into particles below 3mm size fraction which cannot be used by

sinter plants.Such smaller size iron ore is subjected to grinding process and the

fines are used to make pellets. Iron ore fines are also generated at mines, iron

ore beneficiation plants, in sizing process within the steel plants, sludge from

scrubbers, etc.



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3. OBJECTIVES:

To make low expansion coefficient iron ore pellets and encourage the

use of it in steel making processes.

To meet the increasing steel demand in the country by increasing the

usage of low expansion coefficient iron ore pellets.

To analyse the swelling behaviour of pellets, it�s causes and ways to

minimize it.

Study of the effect of temperature on the degree of reduction of iron

ore pellets.

Study of the effect of time on the degree of reduction of iron ore pellets.

4. EXPECTED DELIVERABLES:

Low expansion coefficient pellets.

Analysis of the swelling behaviour of pellets.

Theoretical overview on iron formation with the help of iron ore pellets.

Properties of pellets.



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5. LITERATURE SURVEY:

5.1 What are Pellets?

Pellets are approximately spherical lumps formed by agglomeration of the

crushed iron ore fines in presence of moisture and binder, on subsequent

induration at 1300°C.

5.2 MECHANISM OF PELLET FORMATION:

Steps in pelletizing iron ore:

1. Iron ore preparation

2. Pelletisation or Balling of wet ore fines

3. Pellet hardening process

5.2.1. IRON ORE PREPARATION:

Iron ore preparation can be done by the following steps:

Iron ore is crushed and grinded to fine size.

Gangue is removed by washing of fine ore.

Water is removed from the ore slurry.

Additives are added to the wet ore which is followed by their mixing.

Then it is transported to pelletizing unit.

5.2.2. PELLETIZATION OR BALLING OF WET ORE FINES:

Surface tension of water and gravitational force creates pressure on particles,

so they coalesce together and form nuclei which grow in size into ball. Surface

tension capillary action of water and gravitational forces of particles due to

rotation in the balling unit are responsible for agglomeration of iron ore fines.

The over surface is wetted and coated with water fill when solid particles come

in contact with water. Liquid bridges are formed due to surface tension of

water and as a result of combination of individual water droplets containing

ore grains, first agglomerates, called seeds are formed. The liquid bridges hold

the seeds together as a network. The agglomerates condense and become

denser with water. Capillary forces of liquid bridges are more active in the

stage of ball formation. Surface tension of water droplets becomes fully active



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Sticky Note

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and dominates the capillary forces when the solid particles are fully coated

with water.

Green pellets with a size range of 8-16 mm are prepared in balling drum or discs. Discs are preferred to produce quality green pellets as these are easy to control operation with minimum foot space. The disc is an inclined pan of around 5 to 7.5 meters diameter rotating at around 6 to 8 rpm. The inclination of disc is around 45o and it can be adjusted in the off-line between 45o to 49o. The pre wetted mix is fed into the disc at a controlled rate. Ore fines are lifted upwards until the friction is overcome by gravity and the material rolls down to the bottom of the disc. This rolling action first forms small granules called seeds. Growth occurs in the subsequent revolutions of the disc by the addition of more fresh feeds and by collision between small pellets. As the pellet grows in size, they migrate to the periphery and to the top of the bed in the discs, until they overflow the rim. Pellet growth is controlled by the small amount of water sprayed in the disc and the adjustment in the disc rotational speed. A pellet plant contains a series of balling drums where the iron ore concentrate is formed into soft pellets, in much the same manner that one rolls a snowball, to make a pellet about the size of a marble (between 1/4" and 1/2"). Pellets are screened to meet the size specification, with undersized or oversized pellets crushed and returned to the balling drums.

The prepared wet ore fines are subjected to some kind of motion such that

wet particles agitate and collide to get themselves attached by forces of water

capillary forming between two particles giving nuclei. The nuclei of iron ore,

thus formed, grow to form a pellet of desired size and get discharged from the

pelletizing unit. A rotating inclined disc known as disc pelletizer is the most

commonly used unit by pellet industry.

The soft pellets are then delivered to the roller feeder for final removal of the fines, which are also returned to the balling circuits. The grate is fired by natural gas. From this point, the pellets are charged into the large rotary kiln where they are heat hardened at 2,400 degrees Fahrenheit. The pellets are discharged into the revolving cooler and then moved to the pellet screening plant, onto the pellet load out system. The whole process consumes energy in the form of electricity and natural gas. Efforts had been made to reduce expenditures on energy. The pelletizing process has now been completed. The



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Sticky Note

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pellets are run through a final screening to remove those not meeting size specifications or those that are chipped or broken into fines. Pellets that meet the necessary standards are conveyed to the pellet stockpile. The green pellets made transported to another unit for hardening and strengthening. 5.2.3. INDURATION (HEAT HARDENING):

Pellet induration consists of three main steps: 1. Drying of green pellets. 2. Firing of pellets at around 1300°C to sinter the iron oxide particles. 3. Cooling of hot pellets before discharging.

During drying (180°-350°C), moisture content in the green pellet is evaporated. Surface and interstitial moisture evaporates at lower temperatures where as chemically combined water (as goethite or limonite) or any hydrate or hydroxide combinations lose their water at slightly higher temp. Commencement of solid oxide bonding and grain growth are the important steps in this stage. During firing stage (1200-1300°C), the temperature is below the melting temperature of major oxide phase but within the reactivity range of gangue components and additives. Formation of oxides and slag bonds is decisive in this stage.

5.3 ADVANTAGES OF PELLET FORMATION:

Pellets are highly porous that is (25-30%) so they usually reduced faster

than hard burden sinter or hard natural ores.

Pellets have spherical shapes and they contain open pores, due to which

they have very good bed permeability.

Less heat consumption occurs as compared to sintering.

Pellets are easily reduced.

Pellets have high strength so they can withstand structural breakage

more efficiently.

Pellets provide good resistance to disintegration.

Pellets are easy to handle and transport.

5.4 REDUCTION PROCESS:

Chemical reactions involved

Fe2O3(s) + 3C(s) → 2Fe(s) + 3CO (g)



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Reduction of Hematite

Fe2O3(s) + CO (g) / H2 (g) → 2 Fe3O4(s) + CO2 (g) / H2O (g)�� (1)

Reduction of Magnetite

Fe3O4(s) + CO (g) / H2 (g) → 3FeO(s) + CO2 (g) / H2O (g)�.�� (2)

Reduction of Wustite

FeO(s) + CO (g) / H2 (g) → Fe(s) + CO2 (g) / H2O (g)�� (3)

Other possible reactions

C(s) + CO2 (g) → 2CO (g)�� (4)

C(s) + H2O (g) → H2 (g) + CO (g)��.��.. (5)

The most probable reaction is (1), followed by reactions (2) and (3). The

reactions (4) and (5) are highly endothermic and possible only at high

temperatures. In comparison to CO gas, the reduction reactions with H2 gas

are mostly endothermic and favoured at higher temperatures.

5.5 RATE OF REDUCTION:

The rate of reduction depends upon various factors. Most influencing factors

are:

Particle size of iron ore.

Pellet size.

Temperature of reduction.

Time of reduction.

Presence of catalyst.

Chemical nature of oxide.

With increase in temperature degree of reduction of pellets increases from

850-1000ºC. The swelling behaviour of iron ore pellets increases for

temperatures of 850ºC, 900ºC, while for temperatures of 950ºC and 1000ºC

pellets showed shrinkage behaviour. With increase in time degree of reduction

of pellets increases from 15-120 minutes.



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5.6 STEEL DEMAND IN THE COUNTRY: India has emerged as the fourth largest steel producing nation in the world, as per the recent figures release by World Steel Association in April 2011. In 2010, India was the 5th largest producer, after China, Japan, USA and Russia had recorded a growth of 11.3% in steel production as compared to 2009. Overall domestic crude steel production grew at a compounded annual growth rate of 8.4% during 2005-06 to 2009-10. The Indian steel industry accounted for around 5% of the world�s total production in 2010. Total crude steel production in India for 2010-11 was around 69 million tonnes and it�s expected that the crude steel production in capacity in the country will increase to nearly 110 million tonne by 2012-13. Further, if the proposed expansion plans are implemented as per schedule, India may become the second largest crude steel producer in the world by 2015-16. The demand for steel in the country is currently growing at the rate of over 8% and it is expected that the demand would grow over by 10% in the next five years. So it is necessary to get effective ways of steel production with the help of provided resources and steel production through the pellets path is the most tempting way of steel production presently.

6. EXPERIMENTAL PROCEDURE:

Procedure:

Grinding and Crushing of the iron ore was done.

Proper amount of water was added and pellets were made by hand

rolling.

Pellets were made in size range of about 15 mm.

Air drying of the pellets followed by oven drying at 140ºC to remove

moisture.

To obtain proper strength in the pellet firing was done in a muffle

furnace at 1300ºC for 2 hours. Sufficient time was given to this task and

it made the pellet more useful.

Reduction studies of the fired pellets were carried out in a stainless steel

reactor.

The reactor was made half full with non coking coal of size and the

weighed (fired) pellet was placed centrally on this coal bed and the



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remaining portion of the stainless steel reactor was filled with coal. The

reactor lid had a hole centrally for the escape of the gas.

The reactor was then introduced into the furnace and heated from

room temperature to the temperatures of 850ºC, 900ºC, 950ºC, 1000ºC

and soaked there for 20, 40, 60, 80, 100 and 120 minutes respectively.

7. DATA ANALYSIS:

Percentage reduction is found as R = (Initial oxygen content � Final oxygen

content)*100

At 850ºC :

Time(min) Degree of reduction(%) 20 29.3 40 32.4 60 46.1 80 48.9 100 55.5 120 60.8

At 900ºC :


At 950ºC :

Time(min) Degree of reduction(%) 20 60 40 74.4 60 77.8



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80 80.6 100 80.1 120 82.2

At 1000ºC:




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At 850ºC :

Time(min) Swelling(%) 20 0.3 40 1.4 60 1.6 80 1.7

At 900ºC :

Time(min) Swelling(%) 20 0.6 40 1.5 60 15.6 80 18.8

At 950ºC :

Time(min) Swelling(%) 20 -0.3 40 -0.1 60 -11.6 80 -14.2

At 1000ºC :

Time(min) Swelling(%) 20 -14.3 40 -14.7 60 -12.2 80 -9.8



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The swelling index of the pellets was quite less due to less moisture and proper

hardening. At very high temperature shrinkage occurs.

8. RESULTS AND DISCUSSIONS:

8.1 EFFECT OF REDUCTION TEMPERATURE ON DEGREE OF

REDUCTION OF IRON ORE PELLETS:

We obtain that the reduction was enhanced with increase in temperature.As

the temperature increases, the initially formed iron layer grows through

further reduction which leads to higher degree of reduction at higher

temperatures.

8.2 REDUCTION KINETICS OF IRON ORE PELLETS:

Kinetic studies for estimation of apparent activation energies were carried out

for iron ore pellets at four different temperatures of 850, 900, 950 and 1000ºC

for different time intervals in the range of 15 � 120 minutes. A large number of



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reaction models have been formulated. These kinetic models include

nucleation, chemical control and diffusion control models.

8.3 CAUSES OF SWELLING OF IRON ORE PELLETS:

The main causes of swelling are as follows:

The disruptive stresses set up during the transformation of Fe2O3 to

Fe3O4 .

Formation of iron whiskers during ��FeO��-Fe reduction step.

The presence of components such as Na2O, K2O, CaO, etc.

Temperature of reduction and reducing gas composition. For different

temperatures we get different amounts of swelling.

Disintegration of iron grains during carbon monoxide reduction.

The pellets subjected to 950ºC and 1000ºC have undergone shrinkage because

the heat intensity is high. At such high intensity, nucleation and sintering of

iron is favoured. Due to sintering bonding between particles occur and

particles become compact. This results in shrinkage of pellets at higher

temperatures.

9. CONCLUSIONS:

-At very high temperatures, pellets undergo shrinkage.

-We can decrease the swelling of pellets by increasing their strength.

-Less moisture and firing for strengthening for a greater amount of time -

preserves the disintegration of pellets to a greater extent.

-As the temperature increases more number of iron ore pellets are reduced.

-To meet the increasing demands of steel in the country iron production from

pellets should also be feasibly employed.



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10. REFERENCES:

1) KRC research: Weekender, June 11th, 2007, pg 1-2. 2) KRC research: Weekender, June 11th, 2007, pg 3-4. 3) Dr. R.H. Tupkary and V.R.Tupkary: An Introduction to Modern Iron Making, pg 124-129. 4) Basak Anameric and S.Komar Kawatra: Properties and features of direct reduced iron, 2007, pg 63. 5) Prof. L.N.Upadhyaya and Dr. V.K.Saxena, Reduction of iron ore pellets with non coking coal, IE (I) journal MM, pg 34-36.

6)www.google.com 7)www.wikipedia.com 8) www.indiansteelexpo.in/industry overview/ 9) Theory And Laboratory Experiment In Ferrous Metallurgy by R.C.Gupta, pg 55-85.



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