essar steel limited final

SUMMER TRAINING REPORT

(17-05-2010 to 29-06-2010)

ESSAR STEEL LIMITEDPelletization Facility, VIZAG

Submitted to:-

Prof. Shalini Gautam,

Dept. of Fuel and Mineral Engineering

Indian School of Mines Dhanbad

Submitted by:-

Siddharth singh

IIIrd year Undergraduate student,

Dept. of Fuel and Mineral Engineering

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CONTENTS

S.No TITLE Page No

1. Grinding Operations 3

2. Additive Grinding Operations 11

3. Thickening Operations 14

4. Filteration Operations 21

5. Mixing and Balling Operations 27

6. Induration Operations 33

7. E&I Maintenance Operations 41

Grinding Operations

Grinding is the process of size reduction. The process has many advantageous applications. The question “Why do we need grinding?” can be answered in many ways as:

For liberation of ore.

Making the size of the ore appropriate for the subsequent process.

For better handling of ore.

At ESSAR Steel, Vizag we are using semi autogenously grinding in a ball mill where the feed is in the form of slurry (wet grinding). The grinding media is steel balls. The percentage of balls volume is about 33% of the mill volume though for best results of the grinding the mill must be charged with 40%-60% of the ball volume expressed as a percentage of mill volume. Balls of different sizes must be used that is handled here by not removing any balls. The grinding media can be of any shape but generally a balls and rods are used balls provide greater surface area for impact and hence most effective. Sometimes rod slogs are also used for closer size range of grinding but it cannot grind too much finer sizes.

The forces used for grinding can be: Impact, abrasion and chipping in the ball mill.

32 lifters are also provided to lift the balls to make them strike on the ore slurry. Making this arrangement on the walls enable us to run the mill at a speed much lesser then 70% of critical speed which gives the best results for grinding. Grinding in a ball mill depends on:

Mill diameter: Grinding rate is directly proportional to the mill diameter. So to increase the grinding rate the mill diameter can be increased but the energy requirements for the larger mill are also large. So an optimum diameter selection becomes the most important task.

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Feed: for most efficient grinding the feed must be in between 120% to 150% of the balls void volume.

Mill speed: grinding rate generally increases with increasing the speed of the mill. Grinding is maximum at around 70% of the “critical speed”. Critical speed is the rpm at which practically no grinding takes place and all the balls are attached to the walls of the mill. The critical speed for a mill can be calculated by:43.2/√(D-d) where

D: mill diameter

d : ball diameter

Ball diameter: Grinding rate is generally directly proportional to the ball diameter. After a certain maximum ball diameter the grinding rate starts decreasing. The selection of ball diameter is also a function of the feed as well. If the ball diameter is much larger than the feed size then the feed may not even have a chance of hitting the feed particles and the feed just keep fitting into the interspaces of the balls.

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Mill length: Mill length has to be sufficiently long so that the feed particles have sufficient time for grinding. If the mill length is small then the feed may pass the mill without even getting grounded. Such kind of grinding is called short circuiting. But a close circuit mill can be of smaller length then the open circuit mill.

Major equipment involved in grinding operations @ ESSAR Vizag

1. Ball mills: 3 ball mills in all out of which one is single mill and others are primary mill and regrind mill. All running at around 77% of the critical speed which gives the maximum grinding rate. They have a rubber lining so the wear rate is reduced and the mill life can be maximized.

.

Single mill Primary mill RegrindDis. Density 2.8-3.0 2.5-2.6 2.7-2.8Overflow density 1.4-1.5 1.4-1.5Feed rate 180-200 360-400Cy. Feed density 2.15-2.2 2.1-2.2Cy. Pressure 0.6 0.6

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Flowchart of open-circuit grinding

2. Slurry pumps.

3. Hydro cyclones: 6 hydro cyclones for all the three mills each but only for the regrind mill all the cyclones are operational for the rest only 4 cyclones are kept working. The design parameters for all the cyclones are same. Vortex diameter being 73/4” and apex diameter 4”. The optimum cyclone pressure is maintained at 0.5 to 0.7 bar.

Hydro cyclone operation description

Feed enters the cyclone through a tangential entry. It may be involute in some cases but here it is tangential. It has a small inclination of about 5 degree.

As soon as the feed enters it starts a spiral motion due to the pressure that is added on the feed.

Due to the centrifugal force the heavier particles are thrown towards the cyclone boundaries and lighter particles move towards the center because of a drag force.

The drag force is created due to the formation of another spiral of air inside it. For the separation to take place it is necessary to have a central air core which can be confirmed by having sufficiently large.

It can be tested if the discharge at the spigot is of the form of an umbrella.

The fines are recovered from the vortex finder and the coarse particles are recovered from the spigot.

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4. Belt conveyors.

5. Weight feeders.

ADDITIVE GRINDING OPERATIONS

Bentonite grinding description:

Bentonite is grounded in Raymond roller.

Roller mills outer housing is fixed and the rollers rotate touching the periphery of the casing called “die ring”. The grinding occurs in this region.

There are 6 vertical rollers and 6 ploughs that lift the material while mill rotates.

The grounded material is sucked by a fan along with air by a fan which sucks only fines.

Atmospheric air is heated at the hot air generator and is sent to the mill by a blower to the mill. The hot air is used for drying the material to be ground.

A classifier is also provided at the top the roller that rotates in horizontal direction and separates the fines.

The ground product is passed through a cyclone that separates fines and coarse particles which is our desired size.

The cyclone overflow is send to bag filters where dust is separated, fines are exhausted to the atmosphere and the retained particles are send on to the product chain conveyor.

The cyclone underflow is sent on the output chain conveyor which takes it to storage bins from where it can be pumped to the balling area.

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Quality requirements:

Bentonite :80% of -200 mesh or -75microns moisture <12%

Limestone :80% of -200 mesh or -75microns moisture <1%

If wizzer speed is high then material can be grounded finer but the mill throughput decreases and vice versa. Generally wizzer speed for bentonite is 48% and 43% for limestone.

Temperature is maintained by HAG the fuel used is LDO.

Raymond roller mill specifications:

Motor KW : 450

RPM : 600

Current : 56

Coupling type : Tyre coupling

No. of rollers : 6

No. of ploughs : 6

No. of base liners : 4

No of lubrication points: 7

The input size for Bentonite is -10mm and the output from the mill is 200# or -25microns.Swelling index for Bentonite is 33%.


Limestome+ Coke 80% -200 mesh size <1%moisture

Bentonite 80% -200 mesh size <12% moisture

Raw material size fractions must be in:

Limestone/Dolomite -40mm

Bentonite -12mm

Coke -10mm

If the grind is coarse the surface finish will not be fine, if the limestone and coke are not uniform then the induration process is affected.

More moisture is difficult for pneumatic pumping.

So to maintain the size of the finished product can be maintained by adjusting the whizzer speed and the moisture content can be controlled by adjusting the HAG and bag filter temperature.

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THICKENING OPERATIONS

Two primary functions of the thickener are

The production of a thickened underflow of the required concentration by sedimentation and hindered settling ,and

Clarified overflow

Here is a 50 meter diameter thickener, equipped with 2 long and 2 short rake arms, shaped so as to rake the settled solids towards the central outlet. The blades assist the compaction of the settled particles and produce a thicker underflow than cannot be achieved by simple settling.The solids in the thickener move continuously downwards, and then inwards by raking system towards the thickened underflow outlet, while the liquid moves upwards and radially outwards.

The rake drive mechanism is equipped with a torque measuring instrument. If for any reason, there is an increase in driving torque, then the rake mechanism will lift automatically step by step until the normal operating torque is restored. After a preset interval, the rakes will be lowered again step by step.

Equipments involved in thickening operations:

1. Thickener rake mechanism2. Slurry pumps

Thickener rake mechanism:The rake drive mechanism is equipped with a torque measuring instrument. If there is an increase in driving torque, then the rake mechanism will lift automatically step by step until the normal operating torque is restored. After a preset interval, the rakes will be lowered again step by step

Two variable speed slurry pumps of sufficient capacity are installed under the thickener. Under abnormal conditions both pumps may be operated simultaneously,( but as a general rule, one pump is in operation and other serves as a stand by). The under flow pumps variable speed is automatically controlled according to a preset value of slurry density. In automatic operation, the pump speed increases/decreases respectively, if the slurry density increases/ decreases. This means that for a constant feed rate to the thickener, increasing the pump speed and therefore withdrawing more volume from the thickener results in

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decreasing the slurry density.If the slurry density has not reached a preset value, the slurry can be re circulated to the thickener by opening relevant valves.

FILTRATION

Feed to thickener comprises of:

a. Cyclone overflow slurry from iron ore grinding(main feed)b. Slurry from filters overflow or filters dumpc. Slurry from de-dusting (scrubbers) systemd. Filtrate from filter plant

The cyclone overflow contains 25% to 35% solids and this is increased to 70% solids by densification in thickener. The thickener has two products – underflow and overflow. Underflow is the input to filter plant and overflow is clear water after separation that is used as process water.

The key parameters are:

1. Slurry density at the underflow2. Settling rate of the feed material3. Turbidity level in thickener

The density at which slurry is to be pumped to the storage tanks is between 2.0T/m³ and 2.35 T/m³. The slurry density is maintained at the required or set value by varying the speed of the underflow pump. If the density tends to drop below the set value, the pump speed is reduced and if the density is higher than the set value, the pump speed is increased. At any given instant, pump speed is not reduced below a value where the flow of slurry drops down to150 m³/hr.

The settling rate of the feed material is periodically measured. Based on the result as well as the physical level of turbidity in the thickener, the addition of flocculent for enhancing the settling rate is decided.


1. Underflow density: 2.15 to 2.2 tons/m.

2. Underflow out puts should be equal to or more than the total feed to the ball mills at the density mentioned above. For Eg:, If the total feed to the ball mills is 400 tph , then thickener underflow pump discharge should be equal or slightly more than 400 tph.at 2.15 to 2.2 t/m3 To achieve the above mentioned quality requirements pump density set points should be in the range of 2.15 to 2.2t/m3. In addition to this pump speed can be varied or in other terms the lower and higher locks on the pump can be varied to get the density

The density of the slurry at thickener underflow is continuously monitored. When the slurry density is of the required value, it is pumped to slurry storage tanks. If the density is less than the required value, slurry is pumped back to thickener for densification.

From the thickener, the final slurry is pumped to one of the two slurry storage tanks via distributor cone. The slurry tanks are equipped with agitators to maintain the solid particles in suspension and to assist in homogenization. The slurry tanks are interconnected by a cross flow pipes to give increased operational flexibility. From this slurry storage tank slurry is supplied for filtration.

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Activity Description: Thickener

START

FEED TO THICKENER FROM FILTER PLANT AND DEDUSTING SYSTEM

PUMPING OF SETTLED MATERIAL

SETTLEMENT OF FEED MATERIAL

FLOCCULANT ADDITIONFEED TO THICKENER FROM IRON ORE GRINDING

PUMPING TO STORAGE TANKS WITH THE DESIRED DENSITY

STORAGE OF SLURRY IN AGITATOR TANKS

END

SAMPLE TEST BY LAB FOR OVERFLOW SOLIDS

RESULT SENT TO SIC & CCR

A

A

CHECK TURBIDITY

RESULT SENT TO SIC & CCR

A

CHECK DENSITY OF SLURRY WITH

RESPECT TO SET

LEAN DENSITY SLURRY PUMPED BACK TO THICKENER

Technical specifications:

Thickener rake mechanism specifications:

Diameter – 50meters

Slurry level at the rim – 3 meters

No.of long arms – 02

No.of short arms – 02

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Feed rate – Appr.1667 m3/hr slurry with 468t/hr solids

Slurry density at feed – 1.25 ton/m3

Thickener over flow – Appr.1350 m3/hr

Thickener underflow – Appr.308m3/hr

Thickener underflow density – 2.2t/m3

Mode of drive – Chain

Motor power – 5.5kw x 2 no.s

Motor power – 5.5kw x 2 no.s

Rake RPM – 0.05

Reduction details: 1500 to 30rpm to 17rpm to 0.05rpm

Advantages:

It is relatively cheap, high capacity process, and involves very low shear forces, providing good conditions for flocculation of fine particles

Suggestion:

On most modern thickeners these arms rise automatically if the torque exceeds a certain value, thus preventing damage due to overloading.

FILTERING OPERATIONSThe filtering operations at ESSAR VIZAG are of two types(depending on the type of filtering unit employed):-

VACCUUM DISC FILTERATION

Each filter consists of 12 circular discs of filters and which filter cloth is mounted. Each disc has 10 sectors. A total of 10 filters are used here. During the snap blow system of filters blows 6 discs from one side and the other 6 from of the other side. The vacuum pressure inside the filters is kept to be around -1bar to -0.85 bar.

A total of 3 drives are used to run a filter :

1. Main drive: This is connected to the bags and thus run the filters.

2. Agitator: This produces a to and fro motion and prevent the settling of solids in the boot.

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3. Greasing drive: This is the only discontinuous drive. It is used when we want the greasing to be done in the filters. There are 24 greasing points in a filter.

Snap blow arrangement: This is the arrangement which blows the filter bag with high pressure of air blow and thus making the filter cake to break of its surface. This arrangement is acted on each individual sector on six discs from the drive end and on six discs from the non-drive end so as to give time for the filter cake on the conveyor as the breaking event is just differ by few moments.

Filter bags

The selection of proper filter bags is very important because filter bag decide the amount and size of the filter cake.

Filter bags are made up of polypropylene because:

1. It has a light weight.

2. It has a smooth and slippery surface that prevents the bag from tearing.

3. It provides excellent chemical resistance to corrosion.

Selection of a material for the formation for filter bags has to be care full, and the selection depends on:

1. Filter type: vacuum drums, disc filters, belt filters, filter presses.

2. Requirements: low moisture, high productivity, clear filtrate etc.

3. Conditions: particle size, Ph value, temperature, chemicals etc.

4. Operation conditions: speed, agitation, slurry level, cake formation etc.

CERAMIC FILTERATION:

These filters also work like the vacuum filters but here capillarity is also one of the most important factors. In fact here capillarity play as important role as is played by vacuum. It requires 11 drives for its operation. 3 vacuum pipes and one backwash water supply pipe. After every half hour it is given an electric transducer known as slurry ultra. Here two types of filters are used cc45 and cc60 the numbers denote their surface area. It is washed after every 6hours and 30 min. slurry (15 ppm) from expansion tank is used for backwashing. The areas of plate that cannot be used are called dead zones.

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The washing cycle used for the washing of a ceramic filter follows:

Process(wash cycle) Time(min)Acid boosting 8Combined wash 15Break 5Combined wash 10Break 5Oxalic acid wash 5Nitric acid wash 20Flushing 21Total 84

FILTERATION: The process explained

The process of filtration can be modified in many terms and according to the domain of the process it may have different definitions.

What we are doing here is the separation of water from the slurry. The slurry which comes from the cyclones is about 30% solids and 70% liquid but the use of thickener converts it to 30% water and 70% solids. Again in the process of filtration we try to dewater the slurry and bring down the % of water to around 9.5%. This process is carried out by means of vacuum pumps. Vacuum pumps suck a mixture of air and water from the filters via a double stage vacuum receiver system where air and filter water are separated. The pump sucks and creates a vacuum inside the filters so the part of the filters in the slurry forms a layer of slurry over it and the part in the air which already has a layer of the slurry (filter cake) dries it by sucking air from the atmosphere. This process forms a cake of the slurry over the filter bags known as filter cake. This cake is blown off by compressed air by a mechanism called snap blow mechanism. There is just a slight change in case of ceramic filters as there vacuum is not the only force causing the cake formation. Here vacuum and capillarity makes it happen. The process of removal is also a little different as here scrapers are used for scrapping the filter cake instead of air blowers. This cake is made to fall on a conveyor which takes it to the balling area.

The water drawn through the filter cloth that is sucked by the vacuum pump still carries fine particles from the slurry. These fine particles are so fine that they passed the filter cloth. This water is called filtrate. This filtrate is again send to the thickener for again settling of the particles and obtaining the solids out of it.

The moisture content of the filter cake depends on many factors such as:

Fineness of the material, speed of the discs, the feed slurry density, the cake thickness and the suction vacuum.

If we increase the speed of the disc then our production rate will increase but the moisture content of the filter cake also increases. Since now it will have much lesser time for drying in the air. An optimum speed is decided keeping in mind the required moisture content and trying to maximize the rate of production. Also the boot level has to be maintained full always so that we can use the maximum of the filters and we have lesser chance of vacuum leak.

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A comparative analysis of Vacuum disc and Ceramic filters

Vacuum filters Ceramic filters1 In vacuum filters the major force

responsible for the formation of filter cake is vacuum.

In ceramic filters two forces of capillarity and vacuum play equal part in the formation of filter cake.

2 In vacuum filters a snap blow arrangement is required to remove the filter cake from the filters.

In ceramic filters simple scrapers are used to remove the filter cake.

3 The cake thickness is more hence the throughput is high.

The cake thickness is lesser and thus the throughput is lesser.

4 The moisture content is higher. Almost about 11%-12%.

The moisture content is lesser about 9%-10%.

5 The power consumption is higher for the vacuum pumps. It is of order 550W.

The power consumption is lesser. It is about 20W to 25W.

6 Vacuum filters can form a filter cake of any size of solids in the slurry.

Ceramic filters can form filter cake of particles only coarser then 45 microns.

QUALITY REQUIREMENTS:

Thickener output: Underflow density must be around 2.15 to 2.2 tons/cu.m.

Underflow output: It must be equal to or more then the feed to the ball mills. In any conditions it must never be less than the ball mill feed.

FINAL PRODUCT REQUIREMENTS:

Filter cake moisture------------9.5 to 9.8%

Filter cake thickness------------4 to 6 mm

Mixing and Balling Operations

This unit operation involves the addition of the additives to the filter cake, followed by a thorough mixing and subsequent green balling operation.

The purpose of using the various additives has been described below:-

COKE

Coke is formed when coal is heated in the absence of air. It is mixed with lime and grounded in a ball mill. It is used because it helps in the internal cooking of the green balls during firing in the induration unit. It increases the porosity of the green balls which is good for the blast furnace. In blast furnace the carbon from coke is oxidized to carbon monoxide and then finally to carbon dioxide these two reactions are highly exothermic and increase the temperature of the furnace to greater temperature without much use of the fuel. In the extraction of iron in the blast furnace if pellets are not used then coke has to be added separately but here as the pellets are made so the customers of the pellets save their burden and demand for pellets that already have a rich amount of coke. 15 Kg of coke is added per ton of the ore.

3C +3Fe2O3 2CO+2Fe3O4

CO +Fe3O4 CO2 +3FeO+HEAT

LIME

It gives strength to the green balls. It acts as a flux in the blast furnace and forms slag. 20Kg of lime is added per ton of the ore. Again this can be added directly to the blast furnace and no need to add it to the pellets here but to save the trouble at the blast furnace it is added here to the pellets.

CaCO3 +HEAT CaO +CO2

CaO +SiO2 CaSiO3 (slag)

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BENTONITE

Bentonite works as a binder. It is also responsible for the swelling of green balls. 9 kg ofbentonite is added to per ton of ore.

PROCESS DESCRIPTION

The output of the filters that is the filter cake is thoroughly mixed with additives Coke, Limestone and Bentonite. This is mixed in a big mixer. The output of the mixer is a homogenous mixture of all the additives and the ore. This output is used to make green balls on the pelletizing discs.

Variable weigh feeders are used to draw filter cake from its storage. The load cell on the weigh feeder senses the weight on the conveyor. The weigh feeder belt speed is varied to feed the desired quantity of material to the mixture.

Bentonite, Limestone and Coke are drawn by loss-in-weigh feeders. They control the addition of additives and are provided with hoppers mounted on a load cells. The weight loss from the hopper indicated the amount of material discharge for the hopper. A screw located at the bottom of each hopper conveys the additives into the mixture feed chute.

PELLETIZING DISC

It consists of a flat circular base with a wall perpendicular to the plane of the base along the circumference. Scrapers are provided at the base and side wall for deflecting the material. The disc is inclined at an angle greater than the angle of repose of the feed material which is around 440 to 550. The disc rotates in this plane with a variable speed drive. Scrapers don't move with the discs. As the disc rotates the material gets lifted and due to the friction between the ore particles and the disc. The friction between the ore and the disc is much more than the gravitational force at the bottom so the particles gets lifted once the gravitational force becomes greater than the frictional force, the material starts to roll down and they agglomerate. The agglomeration of particles takes place due the

formation of liquid bridge between the particles and continues till the rolling movement is sustained, resulting in the formation of green balls. The scrapers attached to the pelletizing disc provide the upper limit to which particles will be lifted and thus provide the required rolling action. The smaller the particle the longer distance it will be lifted by the rotation of the disc. Once the particles attain a sufficient size they leave the pelletizing disc due the centrifugal force acting on it. These balls thus formed are called green balls. These balls are not green in color but they have a moisture content of around 9.0% to 9.5% that is the reason they are called green balls. The green balls escaped from the discs are guided onto a conveyor which transports the green balls to a reciprocating conveyor.

There are four bed scrapers and one side wall scraper for each disc. The scrapers are supported by a beam and they do not rotate with the disc. The location of scrapers is of great significance they have to be so located that they cover the entire area of the disc. Scrapers help in the formation of material beds so the maximum height of any bed can only be the gap between the bottom of the scraper and the mother plate of the disc. The height of the side walls from the disc bed determines the capacity of the disc. Scrapers also make the material to fall down and thus make it to roll.

All discs are provided with water spraying nozzles. Each disc has 6 nozzles with a ball valve in the water lines. The nozzles are so located that they can cover the entire area of the discs and moisture level can be increased at any point of time if required.

The gap between the scrapers and the disc base is critical for the formation of balls. The gap should be uniform along the length of each scraper and should be same for each disc.

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Major equipments involved in mixing and balling operations

:

Elrich make intensive mixer.

Pelletizing discs.

Belt conveyor.

Loss-in-weigh feeders.

Dosing belt weigh feeders.

Specifications of Pelletizing Disc

Diameter:7500mm

Side wall: 650mm

Speed : 4.8rpm to 7.2rpm

Slope angle:440 to 550

PARAMETERS GOVERNING THE SIZE OF GREEN BALLS

Parameters Trend Size Trend Size

Speed Increases Increases decreases Decreases

Feed Increases decreases decreases Increases

Moisture Increases Increases decreases decreases

Bentonite Increases decreases decreases Increases

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From the disc the green balls land on a conveyor which is 1 m wide. This width is insufficient for the screening of huge amount of green balls thus formed from 5 pelletizing discs so the balls from this conveyor are transferred evenly on to another conveyor which is 4.6m wide. Both of the conveyors are moving a constant speed of 0.35m/s. This wide conveyor is attached with a reciprocating mechanism operating at 50bar pressure that helps in even spreading of the pellets. This conveyor ends on a roller screen. A roller screen is bed of two set of rollers inclined at an angle of 140 to the horizontal. The upper deck of rollers has 18 rollers and the lower deck has 40 rollers. The upper deck lets the below 16mm balls to pass through it and the lower deck lets the below 9mm balls to pass through it. So our required size is obtained from in between these two roller beds.

SIGNIFICANCE OF ROLLER SCREENS

These rollers are rotating at 90rpm. We could have used other screens but then the shape might have deformed and the mobility of the green balls which still have a moisture content of about 9% must have been difficult. The roller here even tries to give them more spherical shape and keep them moving without causing them any damage to their shape.

The undersize and oversize of the roller screens is recirculated for the balling operation.

Induration Operations

The green balls so formed after the balling operations are weak and they have a GCS of only 1.2kg. But even before the green balls goes for the Induration it has to fall 8 times on the conveyors and screen so it has to be tested for its strength.

The ball is made to fall on a test plate repeatedly and the ball must not have any crack at least till the eight drop. For the final product that is to be used in the blast furnace the CCS(cold compressive strength) must not be below 270kg. So it is cooked in the Induration process to give it enough strength.

PELLET CARS

The Induration machine is a chain which has 164 pellet cars, of 4m wide and 120m long. So the total reaction area is 420 m 2. Each pellet car has around 370 grate bars. The grate bars are Cr-Ni alloy (24% Cr, 11% Ni). It has a bed of iron grate bars of life around 4 months on each grate. Both sides of the pellet car can be used. These grates move on a rolling base. This giant moves with a speed of 0.35m/s.

To avoid direct exposure of the green balls to the extreme heat, pellet car and grate bars from extremely high temperature a layer of 80mm of the already cooked pellets is laid on the pellet car. On top of this layer green balls are laid up to a bed height of 430mm. Exposure directly to heat may make the green balls brittle.

The pellet car is driven by wheels (lifting wheels) over the slide rack and rails of the upper strand. At the discharge they are lowered by wheels (lowering wheels) onto the rails of return strand.

Each pellet car also called as “pallet” consists of two outer parts (pallet and casting) and one reversible center parts which are bolted together. The outer parts carry the running and rollers as well as the side walls. The pallet having spring loaded seal bars which are in contact with the lubricating sliding rails on top of the wind box frames to form a tight seal.

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TRAVELLING GRATE

The traveling grate has 19 wind boxes and 2 seal boxes which are connected to the gas/air ducts and which are equipped with double pendulum flaps to catch the tramp pellets. Some wind boxes are equipped with motor driven dampers. This enables us to control the flow of gas in accordance to the process requirements. As the wind boxes have high temperature air/gas in them so naturally the thermal expansion takes place to avoid any leak they are provided with expandable joints. All the fans are provided with inspection doors.

Two sealing wind boxes are provided ahead of the updraft drying zone. They are provided with a throttle valve for optimum condition sucking off excess air, escaping from the updraft drying zone at the entrance of the pallet into the updraft drying zone.

All the wind box frames are sealed to the bottom of the pallet, at the front of the updraft drying zone, at the transition from updraft drying to downdraft drying, at the transition from after firing to cooling at the end of the cooling zone. This restricts the short circuiting of gases.

The traveling gate is covered from all sides by refractory lined hood shaped structure, to avoid the heat exchange and it is divided into several chambers. On both sides of this hood and thus opposite to each other are burning chambers. Taking the extra heat out of these chambers is the “direct recuperation mine”. The mine, the inside lining all are in accordance to the inside temperature so care must be taken for maintaining the temperature around that range. All the zones are separated from one another by refractory lined walls inside the hood. To prevent the sucking of false air or blown off between the hood and the moving pallet special seals are provided on both sides of the machine.

PROCESS DESCRIPTION

Cooking pellets on a moving grate is more beneficial in several ways:

Since the bed thickness of the green balls is not high so excessive pressure is avoided and better permeability is maintained.

In case there are any irregularities in the flow of the gas due to irregularity in the bed, it will influence only a small part as the horizontal movement will eliminate them.

The wind boxes can be sealed. This allows us to eliminate individual zones at different temperature, gas flow rate and flow direction.

Gases from different source at various temperature and pressure can be introduced into the hood directly over the charge.

The burner design permits us to use various types of fuel.

The heat consumption is minimized by extensive using of recuperation of sensible heat of fired pellets.

STAGES OF OPERATION

1. UPDRAFT DRYING:In this process the evaporation of surface moisture takes place. Air moves upwards and heats the green balls from below. The temperature range of 2800C to 3500C is spread in a length of 9m.

2. DOWNDRAFT DRYING:This is same as the updraft drying but here heat is blown above the green balls. Temperature range of 2800C to 3500C is spread in a length of 13.5m.

3. PREHEATING:In this process the moisture is removed to further more extent and the calcination of lime also takes place as the temperature range 3500C to 11000C is spread in a length of 16.5m.

4. FIRING:This zone heats the pellets to high temperature from 11000C to 13000C. This heating is gradual and slow it takes place in a length of 30m.

5. AFTER FIRING:Here the pellets are left alone after being fired to a temperature of 13000C for a length of 3m.

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6. FIRST COOLING ZONE: The pellets are allowed to cool down as they cannot be handled at a temperature of 13000C. For a length of 27m they are allowed to cool of slowly. Their temperature is reduced to around 3000C.

7. SECOND COOLING ZONE: After this process the temperature of the pellets is reduced to 1500C which can be handled easily. This zone extends to 6m. A huge amount of dust is also created here.

Process fans employed in Induration operations:

Fan 2

Fan 12

Fan 22

Fan 32{largest}

Fan 42

Gas flow

To optimize the heat used the heat from the cooling zone is recirculated to the firing zone and the heat from the firing zone is recirculated to the preheating zone as the temperatures are appropriate for these processes. These gases are circulated through steel ducts which are insulated or refractory lined depending on the operating conditions. The duct work is equipped with flexible joints which can expand under high temperature conditions.

There are 13 burner chambers arranged in pairs opposite to each other on the longitudinal side of the hood. This kind of arrangement of burners around the hood enables us to maintain the gas flow and temperature according to our need.

In the preheating zone where only medium temperature is required, for pellet oxidation high and low temperature air streams are mixed. For the purpose of mixing, therefore mixing chambers are arranged on each side of the preheating hood. Throttle valve control the preset value of the designed temperature in this hood. The hot gas temperature above the pellets is measured in each of the heating zone and based on the temperature reading in the thermocouples arranged on both sides of the hood the fuel supply to the burners is adjusted. The intake and circulation of air and gas required for the process is ensured by adjusting the various fan dampers. The efficiency of the Induration process is characterized by the efficient recuperation that is transportation of recovered hot air from the first cooling zone to the preheating and firing zones with fans. The cooling air fan sucks air from the atmosphere through a silencer and forces it to move through Wind boxes of the cooling zone.

Fresh air from the atmosphere is blown over the hot pellets to bring them down to a manageable temperature. This air becomes hot after it had passed over the pellets. This hot air now is collected in the 1st and 2nd cooling hoods. This heat is recycled to the process as a heat carrier. Due to the seal between the traveling grate and the hood a pressure builds up within the hood and this pressure is responsible for the movement of this hot air from the cooling zone to the respective preheating zone through the direct recuperation duct. The hot air collected in the 2nd cooling hood is collected by the up draft drying fan and it is forced to enter the Wind boxes of up draft drying zone.

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The Wind boxes of the downdraft drying zone, preheat zone and the 1st part of the preheat zone are all connected to the Wind box exhaust fan. The waste gases are de-dusted in a multicyclone and discharged into the atmosphere. The waste gases of the firing zone and the after firing zone acts as drying gases in the downdraft and preheating zone. The hood exhaust fan which is installed above the updraft drying hood sucks the humid air out of the hood along with excess air from the wind box recuperation fan via multicyclone to the atmosphere.

The pellets after cooling are transferred into an intermediate bin, from where they are transported to hearth layer separation building (HLSB) with the help of vibro-feeders. A part of the pellets is recycled as hearth layer pellets and are stored in a bin.

E & I Maintenance Operations

For the start of any type of industrial plant the basic requirement is power. This concept is better known as power for production. This power can be given in many forms like heat, coal, fuel oil, and sometimes hydrological power. But the most important source of power that brought the industrial revolution is electric power. It has many advantages over other sources such as it can easily be transported and nowadays most of the machines are made to work on electricity. Again the cost and pollution is also minimized.

Here at ESSAR we use electricity as a major source of power for most of the purposes. Only part of Induration in burners “low stack high Sulphur” is used as fuel oil. Rest all the operations are carried out using electricity so the proper maintenance of this department becomes the most important and 1st part to take care of the pellet plant. To get this power the plant is directly connected to the state power grid. We get here power from main sources:

1. APSEB(Andhra Predesh State Electricity Board)-This lines comes here through navel woff.

2. This is also the state electricity grid that comes through RCL(Rain Calcination Limited) plant.

3. We also have a captive power plant(CPP) of capacity 30 MW. It produces power it is also connected to the state grid so that means when the requirements are high the power is taken from the grid and when the requirements are low the power is send to the grid.

The whole power line is received at the MRSS(Main receiving sub-station) where all the controls are. Here we have 2 main transformers of the same rating 20/25 MVA 20 under normal conditions and it can be increased to 25MVA under forced conditions. The incoming line here is at 132KV which is stepped down to 6.6KV.

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As in case of every industrial plant we have two kinds of loads:

HT (high tension): For all such loads the incoming 132KV line is stepped down to 6.6KV.

LT(low tension): For all such loads the 6.6KV is stepped down to 440V.

The majorequipment in the MRSS are:

C.T( current transformers)

P.T(potential transformers)

A C.T has two ends and a P.T has just one terminal so we can just identify them just by physically looking at them. The basic purpose of using a combination of C.T and P.T is for measuring purposes. The incoming line is 132KV so it can’t be measured by normal equipment like voltmeter or ammeter which is used in normal cases.

CB (circuit breakers):They are used to break a circuit under full load conditions.

Isolators: They are used to break a circuit under no load condition.

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Lightning arrestors: In case of any lightning they come handy as they send that extra electricity to earth and thus the plant is shielded from the huge extra current that comes due to the lightning.

Potential transformer

For the further division of power inside the plant we have 4 LBSS (load based substation):

1. LBSS 1 :It supplies power to pellet plant 1.

2. LBSS 2 :It supplies power to the ball mills as they are the most power consuming motors. The biggest motor in the whole plant is the ball mill motor.

3. LBSS 3 :This substation is for supplying power to the stock pile. In the stock pile we have stackers, reclaimers and conveyors to consume a lot of power.

4. LBSS 4 :It supplies power to pellet plant 2.

CPP (Captive Power Plant ): It has a capacity of 30 MW . It generates by running on a Skoda turbine. It has three supplies:

1. A 10.5 KV to 132 KV line to the main supply. For this we have a 40 mVA transformer.

2. Another line of 10.5KV to 6.6KV. The transformer here has a capacity of 20mVA. This line is connected to the bus coupler and hence to to both of the pellet plants.

3. The third line is connected to all the LT loads through a 10.5KV to 440V line using an auxiliary transformer.

Capacitor banks are also established to maintain a constant power factor and also 55 batteries of 2 volts each to take care of any fault for short time.

BUCHHQ RELAY:It catches fumes from the oil from the transformer and trips.The biggest motor is the ball mill motor. So to start this big motor initially the load of the ball mill is huge so it draws a high amount of current that can be fatal for the wiring and thus the plant as well. So to minimize this current an LRS (liquid resistance starter) is provided. It has two ends a movable end and a non-movable end. Each end has 3 wires for 3 different phases. The container has a solution of Na2CO3 + H2O. This solution is basic is nature and has a pH of 10. Initially the resistance is high so the starting torque is large. Later on when the mill starts moving the movable contacts and the non-movable contacts get attached so

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regular current is drawn from it. The LRS is bypassed from the circuit now. This motor is also provided with “brush lifting mechanism”.