Claims:WE CLAIM:

1. System for drying of green pellets in an induration furnace comprising :

recuperation means carrying flow of hot gases in a temperature of 800-950 0C;
a bed of pellets carried on a travelling grate which passes through a drying zone;
said drying zone comprising of an updraft drying zone (UDZ) wherein hot air is forced to travel up through the pellets involving hot air supply duct , and a following downdraft drying zone (DDZ) wherein hot air is forced downwards through the moving pellets involving hot air duct line;
a secondary downdraft duct as additional source of heat for controlled drying in said downdraft drying zone operatively connected to said recuperation means for supplying hot gases into said downdraft drying zone providing for controlling heat transfer for uniform drying and controlling moisture gradient across the pellet bed .

2. System as claimed in claim 1 wherein said secondary downdraft duct is adapted for reducing the thermal shocks on pellets and control the temperature difference in up-draft and down-draft zones of induration furnace.

3. System as claimed in anyone of claims 1 or 2 wherein said secondary downdraft duct as a source of heat for controlled drying in said downdraft drying zone comprise tapping from direct recuperation and diverting flow of gases to said down draft zone.

4. System as claimed in anyone of claims 1 to 3 comprising said secondary downdraft duct having dampers for controlling the flow of hot gases into said downdraft zone.

5. System as claimed in claim 4 comprising means for controlling said damper opening to control the temperature in down draft zone of induration furnace.

6. System as claimed in anyone of claims 1 to 5 comprising source of heat in down draft zone by creating the tapping in direct recuperation and diverting the flow of gas to down draft zone by means of two separate ducts preferably through two numbers of secondary hoods.

7. System as claimed in anyone of claims 1 to 6 wherein said Secondary down draft duct is connected to the downdraft hood preferably via two separates pipes incorporated with actuator activated movable dampers, preferably the total diameter of secondary duct is in the range of 1400 mm to 2000mm preferably 1500 mm with 280 to 320 mm preferably 300 mm thickness of refractory lining such as to have a working diameter in the range of 1000 to 1300 preferably about 1200 mm and wherein the hot air flows from secondary down draft through dampers and then enters the DD zone of refractory.

8. Method of drying green pellets involving induration furnace including an updraft drying zone and a down draft drying zone comprising :
controlling the temperature in the downdraft drying zone for drying in said downdraft drying zone involving a secondary downdraft duct as additional controlled source of heat operatively connected to recuperation means and by supplying hot gases into said downdraft drying zone to thereby favour controlling heat transfer for uniform drying and controlling moisture gradient across the pellet bed.

9. Method of drying green pellets as claimed in claim 8 wherein the hot gases from said recuperation means used is in the temperature range of 800-9500C.

10. Method of drying green pellets as claimed anyone of claims 8 to 9 wherein said controlling the temperature in the downdraft drying zone for drying is carried out by controlling of magnitude of opening of cooperative dampers in said secondary downdraft duct.

11. Method of drying green pellets as claimed anyone of claims 8 to 10 wherein high temperature flow of gas from DR to DD zones is involved for desired controlling the temperature of the DD zone as compared to up-draft zone for uniformity and pellet structural stability and moisture control.

12. Method of drying green pellets as claimed anyone of claims 8 to 11 wherein the UDD and DDD temperature is maintained same or DDD temperature is maintained less than UDD temperature to avoid thermal and stress gradient of pellets by removing the retain moisture from the pellets.

13. Method of drying green pellets as claimed anyone of claims 8 to 12 wherein the magnitude of opening of the dampers is controlled to control the flow rate of hot gases into the secondary hood such that when the damper is 100% opened the actuator plate is kept parallel to the cross section of pipes and when the opening is required to be 100% closed it is maintained perpendicular to cross section of duct/pipe.

14. Method of drying green pellets as claimed anyone of claims 8 to 13 wherein the vacuum pressure in secondary downdraft duct is maintained in ranges from 8-10 mm of water column at 100% damper opening to 3-5 mm water column at 30% of water column.

15. Method of drying green pellets as claimed anyone of claims 8 to 14 wherein by supplying said hot air through said secondary duct into the down draft drying zone Temp distribution in DD zone is increased to 2000C from 60 0C and similarly the temperature in preheating zone is increased to 550 0C from 450 0C;
the temperature in DD zone in increased to 210-230 0C at lower feed input to induration furnace ranging from 400-450 tph and wherein the heating caused using Secondary duct downdraft energy is instantaneous, greatly reducing processing time and operating costs ;
improves the burn through temperature to 230-250 0C from 180-200 0C at higher input to induration ranges from 500-520 tph.

16. Method of drying green pellets as claimed anyone of claims 8 to 15 wherein the magnitude of damper opening is adjusted based on green Pellet moisture varying from 9-10.5% and magnitude of damper opening for temperature adjustment is inversely proportional to green pellet moisture.

17. Method of drying green pellets as claimed anyone of claims 8 to 16 wherein :
at less than or equal to 9% moisture in green pellets, weight % reduction in crack pellets and weight % reduction in unfired pellet is controlled such that at 100% damper opening is 50-55% and 75-80% respectively with improved Reduction Degradation Index and tumbler index of pellets;at 9-10% moisture in green pellets, weight % reduction in crack pellets and weight % reduction unfired pellet is 58-61% and 79-82% respectively. The optimized % damper opening is 100% and 70%; and at 10-10.5% moisture in green pellet, maximum weight % reduction in crack pellets and weight % reduction in unfired pellet is attainable at 30% damper opening of downdraft zone.

Dated this the 10th day of July, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent) , Description:FIELD OF THE INVENTION

The present invention relates to the system and method of Drying iron ore pellets in Down Draft (DD) zone of induration furnace and improvement in fired pellet properties. More particularly, the present invention is directed to a method of drying of green iron ore pellets in straight grate induration furnace for improving heating efficiency of downdraft zone and preheating zone resulting in minimizing the thermal gradient on green pellets, reduced spalling of dried pellets and improved bed permeability.Importantly, the present invention providesthe source of heat in down draft zone by creating the tapping in direct recuperation (DR) and diverted the flow of gas to down draft zone by means of two separate ducts. The high temperature flow of gas from DR to DD zones helps in increasing the temperature of the DD zone as compared to up-draft zone.Typically, flow of hot air having temperature of 800-900 oC is diverted from direct recirculation pipes to down draft zone through two numbers of secondary hood to improve the process of drying by downwardly sucking the air through the semi dried layers of iron ore pellets. Dampers control the flow of air in DD zone. The magnitude of the opening of dampers controls the temperature in down draft zone of induration furnace.The improvementin DD zoneutilizes the high temperature of gas flow and subsequently improves the temperature of DD zone and off-gases which in turn improve the drying efficiency of DD zone and very little amount of wet pellets enters into preheating zone of induration furnace.Typically, the improvement in drying cycle of green pellet in DD zone helps in reducing the unfired and cracked pellets which helps in improving the tumbler index, abrasion index and Reduction Degradation Index of fired pellets.

BACKGROUND OF THE INVENTION

Several processes have been in use over the years for drying of green iron ore pellets by removing residual moisture and to produce stronger pellets with better abrasion resistance as adjudged by crushing tests and optimum porosity. The feed (concentrate) having more than 62% Fe of suitable size fraction is used to produce the green pellets. The size requirement of iron concentrate in the present pellet making process is -45 micron more than 65% and -10 micron in the range of 30-35%.

The additives are added in concentrates to improve the balling, chemistry and properties of the product. Some of the common additives are bentonite clay, limestone and dolomite if fluxed pellets are produced, and sometimes organic binder too. The pelletization of concentrate is accomplished through balling of feed materials in stages (i.e., nucleation and growth) that increase the size of the pellet by forming layers of concentrate upon smaller pelletsuntil the pellets reach the desired size. The product from a balling drum is screened to selectively size the product. The undersized material is circulated back into the balling drum. The circulated material is called seed pellets. The mixture of particle sizes makes a pellet of maximum density. The additives also fill the interstitial spaces and often provide a pathway for the gradual removal of water from the inside of the pellet. Pathways are also provided for oxygen to enter the inside of the pellet during the firing of the pellet. Knowledge of the removal of water from the inside of pellets is necessary to appreciate the contributions that the present invention provides towards the firing ofhematite pellets.

The prior art of the green pellet drying and the limitations of the system which negatively impact the firing of the pellets are now summarized. It should be noted, however, that even a detailed explanation of the physical changes of the product is an over simplification of a complex process.

US patent 2011/0068521 A1 titled “Method and apparatus for drying iron ore pellets” stated that a current of drying gas is forced upwardly through the bed of pellets to at least partially dry some of the pellets. Radiant heat reflectors above the bed reject heat back onto the pellets with air jets simultaneously directing air downwardly onto the top surface of the bed. Air is forced between the reflectors are placed and onto the bed to, especially below the firebrick wall between the end of the updraft drying zone and the downdraft drying zone. Water is sprayed over the hot pellets to produce steam which is added to a current of air to produce a stream of hot, humid air that is passed through thebed to improve pellet characteristics and recover waste heat.

WO 03/102250 describes the use of microwave energy to treat ores to facilitate subsequent processing of the ores to recover valuable components such as metals from the ores. The microwave energy caused some form of structural alteration of the ore particles without significantly altering the mineralogy, i.e. composition, of the ore. The structural alteration occurred as the result of differences in thermal expansion of minerals within ore particles, as a consequence of exposure to microwave energy, resulting in regions of high stress/ strain within the ore particles and leading to micro cracking or other physical changes within the ore particles.

US 6226891 B1 in the present method of drying iron ore pellets, e.g., magnetite pellets, moisture containing iron ore pellets are formed into a bed comprising of a layer of pellets. A current of drying gas is forced upwardly through the bed of pellets to at least partially dry some of the pellets. Radiant heat reflectors above the bed reflect heat back onto thepellets with air jets simultaneously directing air downwardly onto the top surface of the bed. Air is forced between the reflectors are placed and onto the bed to, especially below the firebrick wall between the end of the updraft drying Zone and the downdraft drying Zone. Water is applied to the hot pellets to produce steam as well as water vapour which is added to a current of air to produce a stream of hot, humid air that is passed through the bed to improve pellet characteristics and recover waste heat. Air supplied at a selected point is cut off responsive to the presence of a blow hole in the bed so as to improve drying efficiency and improveuniformity of drying.

The most commonly employed method of heat hardening, as described in US Patent No. 3947001 issued to Leighton, is of disposing the iron ore pellets on a travelling grate to carry the pellets through a tunnel-like enclosure where they are progressively subjected to drying, preheating, firing, after-firing and cooling. An alternative technology, as described for example in US Patent No. 4168951 issued to Drugge or as proposed by Metso, is to use a rotary kiln for the firing zone of the process instead of a moving grate. In US Patent No. 3947001, that direct exposure of the radiant heat from the burning fuel flame has to be avoided as it could cause overheating of the top layer of pellets. To overcome this possibility, it was proposed to install each burner in a small port extension located on the furnace tunnel, thus shielding the pellets from the radiant heat from the burners.

As stated in US Patent No. 2014/0311286 A1, the induration of iron ore concentrate pellets is achieved in a tunnel furnace heated by plasma torches, wherein the generation of the conventional iron ore pelletizing processes is reduced using electricity powered plasma torches instead of burning of pulverized coal, thereby reducing the industrial pollution of the atmosphere considerably.

Thus, there have been an important need of developing system and method for an improved iron ore pellet drying process to reduce the thermal gradient for drying the pellet having lower Blaine number and high pellet moisture, i.e., more than 10.5%, to obtain improved final pellet properties at the output of induration furnace.

OBJECTS OF THE INVENTION

The basic object of the present invention is to provide a system and method of Drying iron ore pellets in Down Draft zone of induration furnace and improvement in fired pellet properties.

A further object of the present invention is directed to a method of drying iron ore pellets wherein increase of temperature and drying efficiency of down draft (DD) zone of induration furnace is implemented by means of high temperature gas flow through design modification wherebythe high temperature gas flow is diverted from direct recirculation (DR) pipes to down draft zone through two numbers of secondary hoods.

The still furtherobject of the present invention is directed to improve the process of drying of pellets by controlling the flow of gas through damper wherein the magnitude of opening of dampers controls the temperature in down draft zone of induration furnace which in turn helps in reducing the thermal shocks on pellets and reduces the temperature difference in up draft and down draft zones of induration furnace and thus favouring increasing the temperature of preheating zone.

Another objective of the present invention is directed to provide an improved drying process suited for drying of green iron ore pellets so as to produce pellets with improved strength and abrasion resistance.

Yet another objective of the present invention is directed to produce fired pellets with less cracks which help in increasing the tumbler index and reduces the reduction degradation index (RDI) of fired pellets, by way of maintaining a uniform drying to reduce the moisture gradient substantially across the pellet bed.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a system for drying of green pellets in an induration furnace comprising:

Recuperation means carrying flow of hot gases in a temperature range of 800-950 0C;
A bed of pellets carried on a travelling grate which passes through a drying zone;
said drying zone comprising of an updraft drying zone (UDZ) wherein hot air is forced to travel up through the pellets involving hot air supply duct , and a following downdraft drying zone (DDZ) wherein hot air is forced downwards through the moving pellets involving hot air duct line;
a secondary downdraft duct as additional source of heat for controlled drying in said downdraft drying zone operatively connected to said recuperation means for supplying hot gases into said downdraft drying zone providing for controlling heat transfer for uniform drying and controlling moisture gradient across the pellet bed .

A further aspect of the present invention is directed to said system wherein said secondary downdraft duct is adapted for reducing the thermal shocks on pellets and contro the temperature difference in up-draft and down-draft zones of induration furnace.

A still further aspect of the present invention is directed to said system wherein said secondary downdraft duct as a source of heat for controlled drying in said downdraft drying zone comprise tapping from direct recuperation and diverting flow of gases to said down draft zone.

A still further aspect of the present invention is directed to said system comprising said secondary downdraft duct having dampers for controlling the flow of hot gases into said downdraft zone.

Another aspect of the present invention is directed to said system comprising means for controlling said damper opening to control the temperature in down draft zone of induration furnace.

Yet another aspect of the present invention is directed to said System as claimed in anyone of claims 1 to 5 comprising source of heat in down draft zone by creating the tapping in direct recuperation and diverting the flow of gas to down draft zone by means of two separate ducts preferably through two numbers of secondary hoods.

A still further aspect of the present invention is directed to said system wherein said Secondary down draft duct is connected to the downdraft hood preferably via two separates pipes incorporated with actuator activated movable dampers, preferably the total diameter of secondary duct is in the range of 1400 mm to 2000 mm preferably 1500 mm with 280 to 320 mm preferably 300 mm thickness of refractory lining such as to have a working diameter in the range of 1000 mm to 1300 mm preferably about 1200 mm and wherein the hot air flows from secondary down draft through dampers and then enters the DD zone of refractory.

A further aspect of the present invention is directed to a method of drying green pellets involving induration furnace including an updraft drying zone and a down draft drying zone comprising :
controlling the temperature in the downdraft drying zone for drying in said downdraft drying zone involving a secondary downdraft duct as additional controlled source of heat operatively connected to recuperation means and by supplying hot gases into said downdraft drying zone to thereby favour controlling heat transfer for uniform drying and controlling moisture gradient across the pellet bed.

A still further aspect of the present invention is directed to said method of drying green pellets wherein the hot gases from said recuperation means used is in the temperature range of 800-950 0C.

Another aspect of the present invention is directed to saidmethod of drying green pellets wherein said controlling the temperature in the downdraft drying zone for drying is carried out by controlling of magnitude of opening of cooperative dampers in said secondary downdraft duct.

Yet another aspect of the present invention is directed to said method of drying green pellets wherein high temperature flow of gas from DR to DD zones is involved for desired controlling the temperature of the DD zone as compared to up-draft zone for uniformity and pellet structural stability and moisture control.

A still further aspect of the present invention is directed to said method of drying green pellets wherein the UDD and DDD temperature is maintained same or DDD temperature is maintained less than UDD temperature to avoid thermal and stress gradient of pellets by removing the retain moisture from the pellets.

A still further aspect of the present invention is directed to said method of drying green pellets wherein the magnitude of opening of the dampers is controlled to control the flow rate of hot gases into the secondary hood such that when the damper is 100% opened the actuator plate is kept parallel to the cross section of pipes and when the opening is required to be 100% closed it is maintained perpendicular to cross section of duct/pipe.

Another aspect of the present invention is directed to said method of drying green pellets wherein the vacuum pressure in secondary downdraft duct is maintained in ranges from 8-10 mm of water column at 100% damper opening to 3-5 mm water column at 30% of water column.

Yet another aspect of the present invention is directed to said method of drying green pellets wherein by supplying said hot air through said secondary duct into the down draft drying zone Temp distribution in DD zone is increased to 200 0C from 60 0C and similarly the temperature in preheating zone is increased to 550 0C from 450 0C;
the temperature in DD zone in increased to 210-230 0C at lower feed input to induration furnace ranging from 400-450 tph and wherein the heating caused using Secondary duct downdraft energy is instantaneous, greatly reducing processing time and operating costs ;
improves the burn through temperature to 230-250 0C from 180-200 0C at higher input to induration ranges from 500-520 tph.

A further aspect of the present invention is directed to said method of drying green pellets wherein the magnitude of damper opening is adjusted based on green Pellet moisture varying from 9-10.5% and magnitude of damper opening for temperature adjustment is inversely proportional to green pellet moisture.

A still further aspect of the present invention is directed to said method of drying green pellets wherein :
at less than or equal to 9% moisture in green pellets, weight % reduction in crack pellets and weight % reduction in unfired pellet is controlled such that at 100% damper opening is 50-55% and 75-80% respectively with improved Reduction Degradation Index and tumbler index of pellets;at 9-10% moisture in green pellets, weight % reduction in crack pellets and weight % reduction unfired pellet is 58-61% and 79-82% respectively. The optimized % damper opening is 100% and 70%; and at 10-10.5% moisture in green pellet, maximum weight % reduction in crack pellets and weight % reduction in unfired pellet is attainable at 30% damper opening of downdraft zone.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non-limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows a schematic diagram of process flow for iron ore pelletization, indicating the sequence of raw material additions and induration of pelletsillustrate a first embodiment of the present invention.
Figure 2: shows an isometric view of the modified system according to present invention comprising of the modification in gasflow and improvement in temperature in down draft zone through secondary draftduct.
Figure 3: A diagrammatic view on a larger scale showing the secondary duct along with two numbers of down draft hood and position of dampers for temperature control.
Figure 4: Graphical representation of effect of magnitude of damper opening on temperature distribution in DDD zone and firing zone of induration furnace.
Figure 5: Photograph of the actual and modified down draft zone of induration furnace wherein:
A-Thermogram image of DD duct without modification,
B- Thermogram image with modification,
C-Digital image with design modification.

Figure- 6a,b,c: shows the effect of modified drying cycle on % cracking of pellet , CCS of pellets and Reduction Degradation Index of pellets.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention relates to system and method of drying iron ore pellets in Down Draft zone of induration furnace and in particular a method of drying of green iron ore pellets in straight grate induration furnace for improving heating efficiency of downdraft zone and preheating zone resulting in minimizing the thermal gradient on green pellets, reduces spalling of dried pellets and improves bed permeability directed to improving pellet properties.

There are different zones or sections of the induration furnace named to describe the process that occurs in each zone of the furnace. Generally, the first zone of a travelling grate furnace is the updraft drying zone (UDZ), the next zone is called the downdraft drying zone (DDZ). The present invention is made in this section of the furnace.

For example, consider that a hearth layer of fired pellets of 70 mm height is placed upon the pallet grate bars. A layer of green pellets of 400-430 mm height is then placed upon the hearthlayer, making a total depth of 470-500 mm. The hearth layer is dry and the pellets in the finished pellet layer contain 8.5 -10.5% water. The grate bars are aligned on the pallet to provide openings about ¼ inch wide to permit hot air to flow through the openings.

The updraft drying zone of the furnace consists of wind boxes beneath the travelling grates. Each wind box is designed to provide a reasonably airtight seal to force air under pressure up through the bed of pellets that is on the travelling grate. A large quantity of air is directed upward through both the hearth layer and the layer of finished pellets. The air temperature is generally 600 °C to 850°C. This description applies to a continuously travelling grate machine that is in equilibrium for temperature and air flow.

Hot air is forced up through the pellet bed by a forced draft fan. Sufficient upward velocity and static pressure is maintained to establish an upwardair flow. The hot air blowing by the finished pellets evaporates surface water while water inside the pellets slowly evaporates. Some of the heat energy warms the pellets, but most of the heat is used to evaporate water on and within the pellets. The heating and evaporation proceed from the bottom up through the pellet bed. The transfer of heat travels slowly up through the pellet bed. The evaporation of Water cools the air by an amount of energy called the heat of vaporization. The heat transferred to solid masses such as the pallet frames and the hearth layer is called sensible heat transfer.

Moist air travelling up through a bed of cold pellets is eventually cooled to the dew point temperature so that water vapour condenses on the cool pellets, thereby increasing the water content of the pellets. Air travelling up through the pellet bed also carries moisture entirely through the pellet bed. The amount of water removed is consistent with themoisture carrying capacity of the air. The amount of water vapour present in 100% relative humidity value for the temperature that the air leaves the pellet bed. Some of the water evaporated from the lower half of the pellet bed is, however, merely transferred by the condensing action to the cooler pellets in the upper portion of the pellet bed. The pellets on the top of the pellet bed increase in Water content by the condensing of water vapour upon their surface so that pellets that originally had less than 10%, now will contain over 12% water, mainly on the surface of each pellet.

At the end of the UDZ, the pellets at the bottom of the pellet bed are at the temperature and water content correct for the next stage of the firing process prior to the actual firing process. However, in the sequence being described they will not be fired until the end of the firing sequence. At the end of the UDZ the pellets in the top 4 inches of the pellet bed still are wet (over 10% water) and these are the pellets that are to be fired in the final zone, the downdraftfiring Zone (DFZ) because the DFZ fires the top of the pellet bed first. Following the UDZ is the DDZ in which the air direction is down onto the pellet bed.

The top pellets entering this zone are wet with a water content exceeding 6-8 %. For a depth of 30 or 40 mm the pellets are wetter than when they were initially placed on the pallets. The thrust of air directed upon the pellet bed and the suction of the waste gas fan in the DDZ provide energy to draw air down through the bed of pellets. The pellets are in the downdraft drying zone of the furnace for only about 5-6 minutes.

Very little drying takes place in the DDZ of the furnace. This becomes clear when one considers how hard it is to suck air downwardly through 450-500mm of pellets bed height, especially when the top 10-50 mm are wet. Any water that is evaporated expands to steam and artificially increases the volume of gas travelling through the bed of pellets. This is an important factor upon which the present invention is based. The present invention will effectively minimize the problemcaused by inadequate drying that occurs in both the downdraft drying Zones of pelletizing furnaces.

Following the DDZ, the pellets enter the preheating zone (PZ) with no delay. The temperature in the PZ is typically 600°C to 800°C. A waste gas fan draws the heated air and combustion gasses through the pellet bed. Pellets that are wet to a depth of about 10-15 mm from the top of the bed with about 10% water are exposed to hot air which Flows downwardly through that mass of pellets.

The balling drum additives such as bentonite clay, organic binder, limestone or a similar basic oxide present in the pellets, provide pathways for water vapour to escape. The limestone is added when fluxed pellets are desired. If adequate amounts of additives are not present to provide a pathway for steam to escape the pellets’ interior, the pellets may explode and break off part of the outside of the pellet. This unfavourable characteristic is called spalling.

The present method of drying iron ore pellets, moisture containing iron ore pellets are formed into a bed comprising a multiplicity of the pellets. The heating efficiency of DDZ is increased or temperature of zone is increased by secondary duct of down draft. The Flow of gases from Direct recuperation system is diverted to DDZ by tapping the DR zone into two separate duct which insure the flow of hot gases from top portion in DDZ of induration furnace which further flow downward. A current of drying gas is forced downwardly through the bed of pellets for uniformly drying of semi dried and partially dried pellets.

Among the other aspects of the invention is a provision for the improvement of heat and moisture transfer within the pellet bed, and the reduction of losses due to the presence of blowholes in the pellet bed.Moreover, the heating caused using Secondary duct downdraft energy is essentially instantaneous, greatly reducing processing time and operating costs when compared with the use of conventional rotary kilns, shaft furnaces and grate kilns.A still further aspect of the present invention is directed to control the damper position of secondary down draft duct to control the temperature inside the furnace based on the rate of feed input to the induration furnace.

The heat transfer rates experienced during induration influences the final pellet quality and strength. Too rapid a pre-heating rate in the drying/oxidation Zone can result in an inferior pellet due to sintering of the outer surface of the pellet resulting in an outer shell or semi-impermeable layer (the so-called “core and shell effect”) which severely restricts oxygen diffusion into the centre of the pellets. Pellets produced in this way exhibit strong shells but weak core structures, culminating in a poor overall physical strength. In the Down draft zone, the pellets develop sufficient strength to resist breakage and crumbling which occurs as a result of the tumbling action within the curing Zone.

Accompanying Figure 1 shows a schematic diagram of material flow and various equipments required for iron ore pelletization. The circuit for raw material addition consists of a bunker containing bentonite 201 which is feed to belt conveyor with screw dozer 211. Other raw material like iron ore fine, Coke+Limestone, Dolomite, and ESP dust fed from bunker 103,104,105, and 106 respectively. The overall un-homogenize material fed to high speed mixer 401. During whole process of pelletization the moisture added not only in mixture but also in disc pelletizer so to control the fines discharge and shape factor of pellets. Addition of moisture in Disc pelletizer results in bigger size of ball and difficulty in maintaining the final moisture of green pellet. Addition of water in mixer activated the additives and helps in early formation of ball. This helps in generating nuclei for pellet formation even at higher disc speed.

Drying of the screened green pellets in the drying zone is conducted at moderate temperatures, ranging from ambient to 3000 C. to facilitate in moisture removal. Drying is best conducted using a gradual increase in temperature so as to obviate the risk of pellet cracking, “core and shell” phenomena (excessively rapid drying) or general weakening of pellet structure. The present invention is based on the realisation that the heat transfer rates experienced during drying and induration influences the final pellet quality and strength. It is important to control the heat transfer rate to ensure that the pellets are not weakened by structural cracking.

Accompanying Figure 2 shows a modification in present induration furnace drying zone. The design reveals the increase in heating efficiency of Down Draft Zone or temperature of DD zone by secondary duct of down draft 123. The Flow of gases from Direct recuperation system 132 is diverted to DDZ by tapping the DR zone having temperature ranges from 800-950 0 C into secondary down draft-duct. Further the secondary down draft-duct 123 is tapped in to two separate down draft hood 121 and 122 which insure the flow of hot gases from top portion in DDZ of induration furnace.

The hot gases from recuperation discharge duct 131 goes to downdraft draft hood for partial or secondary drying of green pellets. Pressure gauge 151 and thermocouple 141 & 142 incorporated in secondary downdraft from flow and temperature measurement. The diagrammatic view on a larger scale in accompanying Figure 3 shows the modification and exact position of present modification in induration furnace.Secondary down draft duct is connected to the downdraft hood via two separates pipes incorporated with actuator activated movable dampers. The total diameter of secondary duct is 1500 mm with 300 mm thickness of refractory lining made working diameter is 1200 mm. The hot air flows from secondary down draft through dampers and then enters the DD zone of refractory.

The modified design helps in improving or increasing the heating efficiency of down draft zone as shown in Figure 4. The modification shows in increasing the DD done at different magnitude of damper opening. The design further helps not only improving the heating efficiency of DD zone & Preheating zone but also improves the burn through temperature to 230-2500 C from 180-2000 C at higher input to induration ranges from 500-520 tph. The burn through temperature also improves in all cases damper openings.

Figure 5 shows a photograph of the Photograph of the actual and modified down draft zone of induration furnacein accordance to the present invention. Figure 6 shows the laboratory results of effect of various combination of updraft and downdraft temperature on pellets CCS, Reduction Degradation Index and cracking tendency. Its conclude that the Downdraft temperature should always be greater or equal to the updraft temperature. This helps in reducing the thermal gradient inside the pellets and allows maximum removal of moisture before the pellets reach to the preheating zone.

Experimental Results:
Green pellets were prepared using 0.5 % activated bentonite binder having pellet basicity of 0.34. The moisture in green pellets was maintained in the range of 9 - 10.55%. The green pellets were subjected to firing in laboratory Raising Hearth Furnace (RHF) to study the physical and metallurgical properties. The heating rate was maintained at 20oC/min and the temperature of drying, firing and cooling was kept as per design of experiments (DOE). The total induration cycle for the pellets was 40 minutes (as per plant speed 3m/min).

Expt. No. Zone Temperature, oC
UDD DDD Preheating Firing After firing First
cooling Second cooling
1 200 130 650-700 1300 1200 295 165
2 200 180
3 200 240
4 300 240
5 300 280
6 300 160
7 300 130
Time (min) 7 5 3 10 3 8 4

It is found that, the lower the difference of UDD and DDD temperature lesser will be the crack generation in the pellets and better will be the Reduction Degradation Index of pellets. Pellets drying at 200oC UDD and 130oC DDD result in excessive generation of open mouth crack pellets and it also contains lots of hair line crack.

In UDD, hot air will make the lower part of the bed dry quickly but the air will cool from its inlet temperature to its evaporation temperature which may lead to condensation in the upper part of the bed. The rate of water evaporation should be equal to the rate of vapor diffusion from core to the surface of the pellets. When pellets reach DDD zone and if the temperature is lower than UDD zone the removal of moisture is hindered this phenomenon is also applicable if the pellet having higher mean particle size (MPS). The heat supplied in DDD zone is such that the remaining moisture in the pellets pores can escape through capillary action (movement of water molecule from core to case) without putting extra pressure inside the pellets which may lead to weaker core and hard surface.This is the reason that either the UDD and DDD temperature should be same or DDD temperature should be little less than UDD temperature to avoid thermal and Stress gradient of pellets by removing the retain moisture from the pellets. The temperature combination of 200-180oC,200-240oC, 300-240oC and 300-280oC gave satisfactorily results.

Pellet Plant Basket trials results
The green pellets from disc pelletizer were collected having varying moisture (9-10.5%) from pellet plant.The green pellets were screened for fineness removal and sized by hand (>15 mm pellets returned as feed material). Selected pellets were subjected to drop tests with the averagenumber of drops before pellet fracture averaging an acceptable 8 to 10 drops.

The green pellets are then placed in cage type stainless steel basket of around 20 kg. The height of pellets maintained in such a way that in should match the bed height of pellet plant. The basket is then placed in the induration furnace, from where the green pellets entering in the furnace. The induration of pellets started and the pellets are indurated for around 40-45 min (based on vertical speed of machine). The fired pellets basket was then removed/ pulled from the exit zone of induration furnace (after cooling). The results of basket trials were stated in Table 1, 2 and 3with respect to the fired pellet properties.

Table 1: Green pellet moisture (= 9%)

% Damper opening % crack % unfired Tumbler index RDI of pellets (-6.3mm)
0 % 15.54 5.45 90.12 14.45
30 14.32 4.14 90.45 12.11
50 12.48 3.62 91.68 11.56
70 9.12 3.17 91.75 10.35
100 6.54 1.1 92.42 9.56

Table 2: Green pellet moisture (9-10%)

% Damper opening % crack % unfired Tumbler index RDI of pellets (-6.3mm)
0 % 17.47 7.45 90.72 14.86
30 14.65 4.76 90.94 13.14
50 13.11 4.22 91.98 12.25
70 10.20 3.84 92.03 11.18
100 7.23 1.56 92.23 10.25

Table 3: Green pellet moisture (10-10.5%)

% Damper opening % crack % unfired Tumbler index RDI of pellets (-6.3mm)
0 % 21.25 9.24 89.58 15.11
30 7.18 1.25 91.03 10.56
50 5.45 1.03 91.98 10.46
70 17.85 5.11 90.02 14.94
100 19.45 5.63 89.95 15.68

It is found from above plant trials optimization that the pellet moisture is a controlling factor for Crack generation in pellets. For example, the pellets at moisture level of 9% required full opening of damper to get desired pellet properties. As per full opening of damper results in increased in DD zone temperature (180-2000C) which is actually required to take the retained moisture from the pellet bed. However at lower damper opening the temperature keep going down and it’s not sufficient for removal of moisture from the pellets. The same pellets once reach to preheating zone senses the shock of high temperature and pellets having more susceptibility to cracking at that time.

Similarly, the pellet with higher moisture content more than 10% required 50% of opening as such opening give rise to temperature of DD zone at the range of (120-1400C) as higher moisture pellets requires the gradual removal of moisture from the pellets and 50% opening of damper reduce down the temperature gradient from up draft zone and Down draft zone which helps in complete drying of pellets before entering in preheating zone.

In the same way the pellets with intermediate moisture range of moisture (9-10%) the damper opening of 70 - 100% proved to be beneficial to get the desired fired pellet properties.

It is thus possible by way of the present invention to provide a system and method ofdrying iron ore pellets in Down Draft Zone (DDZ) of induration furnace comprising of an increased in heating efficiency of Down Draft Zone or temperature of DD zone by secondary duct of down draft. The Flow of gases from Direct recuperation system is diverted to DDZ by tapping the DR zone having temperature ranges from 800-950 0 C into secondary down draft-duct, whereby the heating caused using Secondary duct downdraft energy is essentially instantaneous, greatly reducing processing time and operating costs when compared with the use of conventional rotary kilns, shaft furnaces and grate kilns, resulting in reduction of unfired and cracked pellets which helps in improving pellet properties such as the tumbler index, abrasion index and Reduction Degradation Index of fired pellets.