Bentonite is commonly used in pelleting of iron ore.
The use of bentonite as binding agent is quite common in the world due to its availability and its being cheap. Montmorillonite montmorillonite (Al, Mg)2 (OH)2 (Si, Al)4 O10 n. H2O, which the main mineral of bentonite, is fine grained. Specific surface area reaches up to 10.000 cm2/gr.
Natural bentonite is comprised of 15-20 layers, each one of these has the thicknes of 20*A. These thin layers can slide on each other and get water between them. The water between the layers causes the bentonite to swell. The distance between the layers getting water reaches 9.9’, and this shows the swelling capacity of bentonite.
Due to the high swelling rate with water and being more viscous, natural sodium bentonites are more suitable to be used in pelletizing than calcium bentonites.
The calcium bentonites to be used in pelletizing are generally activated with carbonated water. In the mean time the Ca++ or MG++ ions present on the main bentonite layers can exchange their place with each other.
As a result of such activation, the swelling capacity of the bentonite can be significantly improved. The natural calcium bentonites swell in 200% of their volume, whereas this swelling rate of activated bentonites can reach up to 600% according to their activation rate.
The usability of bentonite generally originates from its water absorbing capacity. The water, which is received between each layer of montmorillonite and which can be bonded with Van Der Walls forces, slides the ore particles on each other in a better way and thus raw ore dropping number is increased. Surface tension forces decrease during drying, and only adhesion forces have role in pellet strength. Upon addition of bentonite, the stay time of the water is elongated, as well as additional binding forces are acquired.
The humidity of filter cake, the specific surface area of the ore determined according to the pellet type, the distribution bentonite particles in filter cake and the quality of bentonite are amongst the effective factors in using bentonite successfully in pelletization.
Bentonite plays an important role in using degradable iron ores with low grades upon pelleting. The features desired in iron ore pellets acquired with the use of 0.8% bentonite are as follows:
- Dry compressive strength at 100 °C – min 5.5kg/cm2
- Dry compressive strength at 400 °C – min 20kg/cm2
- Burning compressive strength at 1000 °C – min 75kg/cm2
- Burning compressive strength at 1200 °C – min 200kg/cm2
- Wet compressive strength at 1-1,5 kg/cm2