PELLETISATION AGGLOMERATION

Pelletisation

Process :

Pelletisation consists of formation of green ball by rolling of a fine iron bearing with a critical amount of water and external binder or any other additive may be added, if required. These green balls taken of nearly 5 to 20mm size then dried, preheated and fired, under oxidising conditions to a temperature of around 1250 to 1350℃. Bonds of good strength are developed between the particles at such high temperatures.

● This process produces pellets in a highly oxidized state, as against the sintering process in which fuel is Incorporated in the charge and which finally results in a 5 to 20% ferrous iron.
The pelletisation process consist of
1. Feed preparation
2. Green ball production and sizing
3. Green ball induration
     ● Drying
     ● Pre heating
     ● Firing
4. Cooling of hardened pellets.


Bonding Theory :

● The production of green balls is necessary that the green balls possess adequate strength to stand bumps, overlying load and abrasion during screening and handling until finally these are hardened by induration .
So the debedevelop of the theory of balling are

1. Dry material does not pelletise and presence of moisture is essential to roll the power in to balls. Excessive water is also detrimental.

2. Surface tension of water in contact with particles plays a dominant role in binding the particles together.

3. Rolling of moist material leads to the formation of balls of very high densities which otherwise is attainable by compacting powder only under the application of a very high pressure.

4. The ease with which material can be rolled in to balls is almost directly proportional to the surface area of particles that is it's fineness.

● So according to above, there are three different water practicle systems.
1. The pendular state, when water is present just at the point of contact of the particles and surface tension holds the particles together.

2. The funnicular state, when some pores are fully occupied by water in an aggregate system.

3. The capillary state, when all the pores are filled with water but there is no coherent film covering the entire surface of the particles.
Palletization bonding state
Palletization bonding types

● Maximum strength of a green ball produced from a given material will be obtained by compacting the material to the minimum porosity and with just sufficient water to saturate the voids.

● The rolling action during palletization is beneficial in reducing the internal pore space by effecting compaction and mechanical interlooking of the particles. The water contain is therefore quite critical to obtain good quality balls.

Ball Formation Mechanism :

●  The ball formation is a two stage process, that is nucleation or seed formation and their growth. The formation of balls on a pelletizer depends on the moisture content.

● If moisture is less than the critical amount it's distribution tends to be non-uniform, major amount of granulated material leaving the non-granulated material relatively dry, if the moisture level is more than the critical value, growth rate is more but the balls produced are liable to deformation because of their plasticity. Seeds are formed only if critical moisture level is maintained and without which the process cannot proceed properly. The seeds generally have slight excess of water.

●  Ball formation has been observed that the size of the balls produced in a pelletiser from a charge containing right amount of moisture depends on the time and seeds of the pelletiser, that is number of revolution. The three variation is :

1. Nacleii formation region.
2. Transition region.
3. Ball growth region.
Ball formation pelletisation
Ball size as a function of extent of rolling that is number of revolution of the palletiser.

Nucleii Growth Region :

When a wet particle comes in contact with another wet or dry particle a bond is immediately formed between the two. Similarly several such particles initially join during rolling to from a highly porous loosely help aggregate and crumbs which undergo re-arrangement and partial packing in short duration to from small spherical, stable nucleus. This is the nucleation period, a pre-requisite for ball formation since these very nucleii later grow into balls.

Transition period :

After nucleii are formed they pass through a transition period in which the plastic nucleii further re-arrange and get compacted to eliminate the air voids present in them. The system moves from a pendular state through funicular state to the capillary state of bonding.

Ball Growth Region :

In these process some granules may even break because of impacts, abrasion,etc. Growth takes place by two alternative modes.

1. Growth by assimilation is possible when balling process without the addition of fresh feed material.

2. Growth by layering is possible when balling proceeds with the addition of fresh feed material.

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