PELLETISATION | BURDEN PREPARATION

Pelletisation

The pelletisation process consist of the following :
1. Feed preparation
2. Green ball production and sizing.
3. Green ball induration
   ● Drying
   ● Pre heating
   ● Firing.
4. Cooling of hardened pellets.

Feed Preparation :

● The hardened pellets should be strong and adequately porous to possess good reducibility. So the green balls must possess good strength and porosity. The green ball feed should be 100 mesh (200um) in size, with more than 60% as 350 mesh(nearer to 40um). A right combination of size and shape of feed particles led to maximum mechanical interlocking and surface tension to obtain strong green pellets.

● The finer size and the larger is the specific surface area, which indicates that a surface area of 0.2 m.m/g (blaine) is likely to be a minium value for successful balling and the grinding should be directed to this end. Over grinding is detrimental since it reduces the porosity of the final product.

Green Ball Production :

● Green ball are economically produced in balling drums or disc are rarely in other specialized equipments. The rolling action of the equipment is chiefly responsible in presence of critical amount of water develops the nessary strength in the balls. The balling equipment should produce maximum if rolling motion of particle, particularly as they grow. The balling equipment would require a rotating surface where the pellet mix is fed.

● The formation of balls on rotating surface is a two stage process. The nuclei or seeds are first formed and there growth then takes place later. It has been observed that these two stages of ball formation take place in to two different zones of the pelletizer, that is the nucleation zone and the growth zone.

Disc Pelletiser

● A disc sloping peripheral wall which is rotated around it's own centre, in an inclined position to horizontal. These discs resemble flying scrapper and are normally 3.6 to 5.6m in diameter and are inclined at about 45° to the horizontal.

● Scraper is provided to prevent build up of moist material on the disc. It can also control the material flow pattern on the disc. Due to water is added seeds are easily formed, by which frictional drag against the disc decrease and the centrifugal force acquired by them increase, and consequently they move out of nucleation zone.

● The height and width of trajectory of the ball movement increase with the sizes of the ball unit eventually the balls are deflected downward by the scraper. During this movements the balls encounter fresh feed and growth takes place more by layering while compaction assimilation plays a relatively minor role.
Disc pelletiser
Disc pelletiser

● In contineouc operation the discharge of desired size balls is balanced by an equivalent addition of feed. The concept of formation of balls in two stages implies that there is a specific residence time for any given mix to produce balls of the required size.

● For steady state conditions the residence time required for balling may be ascertained by diving the weight of material being actively balled on the disc by the feed rate.

● The rate of production of balls in disc pelletiser are :
1. Diameter of the disc
2. Height of the peripheral wall
3. Angle of inclination of the disc with horizehori.
4. Speed of rotation.
5. Place of the disc where mix is fed.
6. Place where water is sprayed on the disc
7. Rate of feed.
8. Rate of moisture addition.
9. Rate of withdrawal if the product.
10. Nature and size of the feed.
11. Desired size range of the pellets and percentage recycled load.
12. Any other addition like binder, flux, etc.


Drum Pelletiser

● The drum pelletizer is a simple drum, open at both ends with a length to diameter ratio 2.5 to 3.5, rotating around it's own axis in a slight inclined position to the horizehori. The drums are 2 to 3m dia and 6 to 9m length and rotate at 10 to 15rpm depends upon the diameter.

● The angle of inclination in working position is about 2 to 10°. The charge is fed through that end which is at higher level, water sprays are also located there. The material rolls over the surface of the rotating drum and slides Downwards due to inclination of the drum. This motion of the balled is called cascading which takes place until the balls emerge out of lower end.
Drum pelletiser
Drum pelletiser

● Some fundamental difference between the behavior of drum and disc pelletizer. The drum doesn't act as a classifier but the disc have classified, and hence the time of ball growth is not equal to the residence time of the feed.

● The size range of the output is large and as a result it must be operated in closed circuit with the screen. The ball growth usually effected in more than one pass through the drum.

● The rate of production of desire size of balls is a function of the following:
1. Speed of rotation that is peripheral speed.
2. Angle of the inclination.
3. Diameter of the drum.
4. Rate of feed.
5. Depth of material in residence.
6. Moisture content
7. Nature and sciences of the feed
8. Any other addition such as binder, flux etc.

● The effect of speed of rotation is similar to that in disc pelletisation. The correct speed of rotation is 25 to 35% of the critical speed at which material will centrifuge. The optimum loading of the drum is equivalent to about 5% of the drum volume. Optimum loading and correct speed of rotation leads to a correct rolling and tumbling eventually resulting in a continuous steady state operation.

● A high recycle rate favours growth by layering and produces a product of close size range. For a continuous stabilised steady state operation the net rate of production of seeds must equal the rate of production of green balls and the rate of incremental growth during the passage through the drum should give rise to the correct size of the balls. Incremental growth can be increased by decreasing the slope of the drum which increasing the rolling time per pass and vise versa.

Induration of Pellets :

● The green balls from the palletization plant are sent to the insurance plant where these are subjected to drying, pre heating and firing, cooling. For this purpose the pellets are subjected to a continuous heating and cooling cycle depending upon the variable of balling process,

● The different types of heat hardening equipment are in commercial use at the present time
1. The vertical shaft furnace
2. The traveling grate
3. The grate kiln.

● In general green pellets are fed at one end of these modern machines and harden and cooled pellets are withdrawn from the other.

Drying :

● The green balls are dried by passing hot air through the bed of the balls laid on a grate. The temperature of the hit blast and volume required for drying are largely a function of the moisture content of green ball and the nature of the ore from which the balls are made.

● Drying may be carried out by using either down draught or an up draught. Down draught drying is believed to be safe for green balls containing less than 9% moisture. Up draught drying followed by down draught drying should be employed, using drying gas, at the high temperature and flow rates.

Pre heating and firing :

● In the pre heating stage partial oxidation of magnetite and sulphides if present, along with the calcination of carbonates and hydroides.

● The solid bonding and grain growth commences in the pre heating periods. These are completed in the firing stage. Liquid formation by chemical combination of various acid and basic constituent and consequently the development of slag bonds occurs predominantly during firing.

● The bond strength is essentially developed at the firing temperature bug the heating cycle should be decided in relation to the chemistry of the pellets so as to finally develop optimum quality of hardened pellets.

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