Pelletizing of iron ore

Loose fines of very small particle size (les than 325 mesh) of
iron ore that cannot be sintered are formed into pellets. To the iron ore fines, 0.5-3.0%
bentonite is added as a binder. Coke breeze and some flux (limestone) may also be added.
The mixture is placed in cones, drums or discs, out of which discs are relatively more flexible
with regard to types of ore and they can be controlled better. A disc pelletizer is a rotating
inclined flat plate table. The particles gradually coalesce first, into very small pellets, which
go on taking more and more particles and keep enlarging in size till they attain specified
sizes. Water is sprayed as required. These are called green pellets, and the operation is called
balling. The control is effected by changing the angle (20-800) and the rpm. of the disc (+8)
of the disc. The green pellets are then heat treated at a temperature of above 12000 C –
generally around 13150C – in grate kilns, followed by air cooling with a view to obtaining
necessary strength.
The role of bentonite is as a binder to impart enough bonding strength to the pellets for
withstanding the stress of rotation and heat treatment. The parameters of significance include
swelling capacity, gelling index, gel time, base exchange capacity and green compressive
strength. The values for these parameters as preferred by industries are: gelling Index 20-25;
gel time 7-10 minutes; base exchange capacity 60-65 milli-equivalents per 100 gm; green
compressive strength 7-8 psi. Sodium bentonite which satisfy these specifications, is