SINTERING | BURDEN PREPARATION

Sintering

Principle of Sintering :

Sintering is carried out by putting mixture of iron-bearing fines mixed with solid fuel on a permeable great. So the coke breeze is normally available as wasteful product. It is universally incorporated as a solid fuel in the sinter mix.

● It is equally true that is the best choice among the available solid fuels. The top layer of this winter bed is heated to 1200 to 1300℃ by a gas or oil burner and air is drawn downwards through the great, with the help if exhaust blowers connected from underneath to the grate.
Principle of sintering
Principle of sintering

● The narrow combustion zone developed initially at the top layer travels through the bed raising the temperature of the bed, layer by layer, to the sintering level. The cold blast drawn through the bed cools the already sintered layer and therrby gets itself heated. The heat contained in the blast is utilised in drying and preheating the liters in the bed. Then combustion of each layer gets dired and pre-heated by the heat transferred from the upper combustion zones.

●  In the combustion zone, bonding takes place between the grains and a strong and porous aggregate is formed as the process is over when the combustion zone has reached the lowest layer of the bed. It is broken, screened and cooled to produce desired fraction. The undersize is recycled and the oversize is fully cooled and sent to the blast furnace. In the up- draught sintering the initial small layer of the charge on the grate is invited before making up the remaining bed thickness.
Temperature cycle during sintering of iron ore
Temp. Cycle of iron ore fine

● The variable of the sintering process :
 1. Bed permeability as decided by the particle size and shape of the mix.
2. Thickness if the bed.
3. Total volume of air blast drawn through the bed for its sintering.
4. Rate of blast drawn through the bed during sintering.
5. Amount and quality of solid fuel incorporated in the charge.
6. Amount and type of carbonates present in the charge.
7. Amount of moisture in the charge.
8. Nature of ore fines, e.g. it's chemical composition.
9. Any non-unifotmity in the bed composition or in the process of sintering.

●  The heat generated by combustion in the upper layers must travel down the bed at the same rate as the combustion zone and thereby be available at the right time for raising the temperature of the lower layers. The heat wave travels down in the bed with approximately a constant velocity given by an expression.
Sintering equation
Sintering equation

Sinter quality : 

● The process of sintering was originally developed merely to agglomerate the iron ore fines. Once the beneficial effects of sinter as a blast furnace Buren were realised, the physical properties and chemical construction of sinter come to be examined more closely. The objects of sintering therefore enlarged and these are now :
1. To increase the size of ore fines to a level acceptable to the blast furnace.
2. To form a strong agglomerate with high bulk reducibility.
3. To remove volatiles like CO2 from carbonates, H2O from hydroxides or sulphur from sulphides type of ore fines along with their agglomeration.
4. To incorporate flux in the burden.

●  Out of the above four aims first one is a must. The extent to which the second object is met will very much depend on the nature of the ore and other economic considerations of carrying out the process of sintering.

●  There different types of singers are being produced depending upon the extent of addition of flux in the mix and these are :

1. Acid sinter :

The sinter mix dose not contain qflux at all. Flux is added in the furnace separately. The production of this type is fast declining and most modern practices produce self- fluxing or super- fluxed sinter.

2. Fluxed sinter : 

The amount of flux added in the mix in such that the basic it of mix is equal to that of the slag to be produced in the furnace. Separate addition of flux would be required only in proportion to the amount of natural lumpy ore charged in the furnace. This type of sinter may be produced when cent per cent sinter charge is planned. This is also fast declining in preference to super-duper sinter.

3. Super-fluxed sinter : 

The entire amount of flux required to be charged in the furnace. When run on 100% natural ore charge is added in the mix. The basic it of the mix would be obviously more than that of the slag in the furnace, since a part of the charge would either be natural lumpy ore or sinter of lower basicity than that of the normal blast furnace slag, and hence the name. This is aimed in most of the modern practices wherein high percentage of sinter (50-80%) charge in the burden is contemplated.

Mechanism of sintering : 
Sinter bed
Sinter Bed

During sintering on a static bed various zones that are formed. Each layer below the ignited top layer undergoes charges in the order :  wet ore- drying - calcining- prepeating- vombustion- cooling. The some order of  charges take place on a moving bed. The downward movement of each zone and the forward movement of the pallet during something result in the structure which is a vertical section through the bed over the entire effective length of the strand.

●  Two types of bonds may be formed during sintering :

1. Diffusion or recrystallization or solid state bond : 

It is formed as a result of recrystallization of the parent phase at the point of contact of two particles in solid state and hence the name.

2. Slag or glass bond :  

It is formed as a result of formation of low melting slag or glass at the point of contact of two particles, depending upon the mineral constitution, flux addition, etc.

●  As a result the sinter can have three different types of constituents :
1. Original mineral which has not undergone any chemical or physical change during sintering.
2. Original mineral constituents which have undergone changes in their physical structure without any change in their chemistry. Recrystallization is the only change at some of the particle surfaces.
3. Secondary constituents formed due to dissolution or reactions between two or more of the original constituents.e.g. formation of ferrites, spinels, silicates,etc. These new constituents either remain as solution and consolidate as a slag or recrystallize out of solution.

Raw Material Requirements :

Size : 

Sinter is directly related to the size distribution of the mix. These factors decide the contract area particles and which in turn the strength of the sinter. If the contact area will be less and the strength of the sinter will be low and conversely if size is too small, the contact area of the particles will be large and strength will high.

Fuel Content :

Fuel content usually in the range of 6 to 8%.The cost of solid fuel nearly 50% of the total cost of sinter operation. With the progress of sintering the air gets preheated to an increasing higher level so that the fuel required to achieve the same sintering temperature, decrease in the lower region of the bed.

Moisture : 

 The presence of moisture in the sinter mix has several advantages. It maintains proper permeability in the bed during sintering. The rate of rise of temperature at higher temperature levels is more rapid in wet than in dry mix and this is what is desired for better sintering. The presence of moisture increases the specific volume of air required for sintering. This is beneficial from the point for view of heat transfer during sintering.

The level of mositure in the sinter mix is generally around 3.6%.

Circulating load : 

Appropriate crushing and screening of sinter cake leads to the production of a lumpy fraction, as required for the blast furnace and fines are recirculated for sintering. These fines are either mixed with the ore fines in the mixer or a layer of these fines are laid as a separate layer on the grate before charging the ore mix. In this way it helps to improve the permeability of the bed.
Mass balance in sintering - sintering
Mass balance in sintering

● For higher output of the sinter strand the circulating load should be low. A low circulating load,however,reduces the permeability of the bed and consequently the input rate to the strand has to be reduced. These two factors oppose each other and an optimum circulating load is established for maximum output of the acceptable sinter to the blast furnace.

Control of sintering process : 

 The operation of a sintering machine can be controlled by proper adjustments of the following operational variable :
1. Fuel content for heat input.
2. Ignition intensity.
3. Moisture content of mix to control it's permeability.
4. Machine speed to obtain complete 'burn through'.
5. Percent returns.
6. Bed height.

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