A brief description of pelletizing
Originally the pelletizing process was developed in the United States of America to treat the ultra-fine mineral dressing products obtained from the upgrading of Mesabi ore. The pelletizing process has been developed to provide a blast furnace feed product with iron contents of about 65 percent (Ball et al., 1973).
Iron ore concentrates with very fine particle size can be converted agglomerates by applying sintering or pelletizing techniques. However, sintering cannot be applied efficiently because of too fine particle size of iron ore concentrates. Such materials can lead to problems such as high fuel consumption and low specific output when sintering is applied. Therefore, the most commonly employed agglomeration technique is pelletizing for such a fine sized concentrates. The agglomerates produced by the pelletizing process are called pellets.
Pellets are balls produced from iron ore concentrates and natural high grade iron ores of different mineralogical and chemical composition with some remarkable properties such as;
- Uniform size distribution within a main range of 9-16 mm diameter,
- High and even porosity,
- High iron content more than 63% Fe,
- Practically no loss on ignition or volatiles,
- Uniform mineralogical composition in the form of an easily reducible hematite or hematite-bearing compounds,
- High and uniform mechanical strength,
- Low tendency to abrasion and good behavior during transportation,
- Sufficient mechanical strength even at thermal stress under reducing atmosphere
The pelletizing of iron ore process is essentially based on the formation of wet pellets by rolling a finely ground iron ore or concentrate to which a binder (usually bentonite) is added together with a critical amount of liquid (usually water). The newly formed pellets should have strengths high enough to survive transportation. In iron ore pelletizing, three binders are generally accepted:
- Bentonite, naturally occurring clay found in deposits of varying qualities around the world.
- Hydrated lime produced by the calcination of limestone or inorganic materials.
- Organic binders, either natural or synthetic, such as the cellulose based materials.
Pellets can be produced from magnetite concentrates, hematite concentrates, natural ore fines, artificial magnetite and pyrite cinders. Many pelletizing plants worldwide use magnetite and hematite as iron oxide sources. Use of a binder is necessary to bind the iron ore grains in wet and dry condition. Binders play an important role in the success of pelletizing process.
Bentonite is the most common binder in iron ore pelletization. Bentonite has proven to be the most effective binder owing to its high water adsorption capacity and dry film strength. It is commonly used as a addition level of 0.5-1.5% by weight of iron ore.
Inorganic materials not containing silica, like hydrated lime; have a slight advantage over bentonite when silica levels are the primary concern of steel makers. However, most of them are incapable of controlling water during pelletization, and do little contribution to the cohesive/adhesive forces required to form and maintain green pellet integrity. This results in poor quality green balls that are fragile and easily broken, lowering production rates in iron making facilities. Furthermore, due to the massive amounts required for basicity control and pelletization, some inorganic materials can contribute to spalling of pellets decreasing the quality of the finished pellet product.