Draft:Pellet (steel industry)

Iron ore pellets produced by LKAB, from ore mined in northern Sweden.

In the steel industry, pellets are a form of iron ore conditioning produced by mines for direct use in blast furnaces or direct reduction plants. Pellets are spheres measuring between 8 and 18 mm in diameter.

They are manufactured using a combination of chemical and thermal processes to enrich the ore with iron up to 67% to 72% and give it the desired chemical and mechanical characteristics.

History

Expansion of pellet production, compared with agglomerate and pig iron production.

The pelletizing of powdered iron ores was first introduced at the end of the nineteenth century. However, the binding agent was tar (1% by weight), fired in a rotating drum to produce pellets suitable for blast furnaces while removing undesirable elements such as sulfur and arsenic carried away by the fumes.

Pellet sintering emerged concurrently with grate sintering. It was then proposed as an alternative for treating high-quality products with significant agglomeration issues. Pellet agglomeration was first developed in Sweden (patented by A. Anderson in 1912) and subsequently in Germany (patented in 1913). The new product was named "GEROELL," derived from the word "rolling." It was observed that the process reduced faster than calibrated ores and agglomerates made from the same material. In 1926, Krupp constructed an industrial pilot plant in Rheinhausen. However, it was subsequently dismantled to accommodate the installation of a large-scale grate sintering line, which was a competing process.

However, pellet sintering remained a viable process. In the United States, the process was used to process fine concentrates from the Mesabi Range deposit, produced during World War II to replace ores naturally rich in iron (over 50%), which were being depleted. The intensive development of pelletizing these new, very fine magnetite ores (85% < 44 mm) began around 1943 under the impetus of the University of Minnesota. The process was subsequently reinitiated in Europe, particularly in Sweden, as a prerequisite for the manufacture of pre-reduced iron ore.

Despite significant growth between 1960 and 1980, pellet production reached a plateau at approximately 300 Mt annually. Pellet production can be broken down as follows:

• In 1984, 189 Mt of pellets were produced, with 90 Mt produced in North America, 63 Mt produced in the USSR, and 36 Mt produced elsewhere in the world by mining companies.
• In 1992, 264 Mt of pellets were produced.
• In 2008, 313 Mt of pellets were produced.
• In 2009, 215 Mt of pellets were produced due to the economic crisis.
• In 2010, 388 Mt of pellets were produced.

Production

Enrichment and pelletizing plant at the Kiruna mine in Sweden.

Pellets are manufactured on the extraction site and marketed by mining companies, in contrast to agglomerates, which are generally produced at the foot of blast furnaces by mixing iron ores of various origins. Pellets are more resilient to the rigors of handling than agglomerates, which are relatively fragile. The production process at each mine varies significantly depending on the characteristics of the local iron ore. Some plants are equipped with units to remove arsenic from the ore. The primary stages remain consistent regardless of location.

The first step is to finely crush the ore to remove as much non-valuable iron ore gangue as possible. The enrichment stages are dependent on the ore's nature. Grinding can be conducted in multiple phases, either dry or with water. Enrichment is achieved through the use of magnetic separation and flotation techniques. Subsequently, the ore concentrate may be blended with additives (such as dolomite, olivine, and quartzite) to achieve the desired chemical composition. These elements typically correspond to 3 to 3.5% of the pellet weight in this case. Another additive, typically wet bentonite (combined with maize flour or polyacrylamide), is used to ensure the material's cohesion during pelletizing.

A saucer used to make pellets from iron ore concentrate.

The ore concentrate is then compacted into pellets. While pellets can be manufactured in various mixing drums, saucers are the most commonly used tool. Once removed from the saucer, the pellets are designated as "green" or "raw" pellets before sintering by firing. The typical grain size is between 5 and 20 mm.

Subsequently, the pellets can be directed to either the consumption plant or a cooking oven. Despite the binder used, their fragility makes them somewhat crumbly, so they are better suited to the cooking oven. After the process, the material is cooled.

The cooking process can be carried out on a chain that passes through contiguous ovens and is heated up to 1,200°C. This can be done using a straight grate process if there is a single straight chain or a grate kiln when the chain opens onto a rotating cooling tray. The burners provide the necessary heat to either add fuel to the ore concentrate or oxidize the ore in the case of certain ores.

Benefits and limitations

Benefits

Pelletizing ore optimizes the performance of blast furnaces and direct reduction plants. In comparison to raw iron ore, the following advantages can be noted:

• The raw material is more resistant to handling, even in wet conditions, and does not clog storage hoppers.
• The composition of the pellets is known and homogeneous, streamlining the process of transforming them into iron.

• Pellet porosity allows for optimal gas-solid chemical reactions within the furnace. The material's hot mechanical strength and chemical reactivity are maintained even in the stove's hottest areas.
• Iron oxides are overoxidized, as the carbon monoxide more effectively reduces Fe₂O₃ in the blast furnace than less oxidized compounds, particularly Fe₃O₄.

Pellets have a higher iron content than agglomerated ore, which boosts plant productivity and reduces fuel consumption. Furthermore, they are significantly more durable and can withstand repeated handling. The slightly higher price (pellets cost, on average, 70% more than raw ore) is commensurate with their value in use, and the steel industry mixes them with sinter in highly variable proportions.

As with sinter, the high-temperature roasting and sintering stage eliminates undesirable elements, notably sulfur. Additionally, it is an effective method for removing zinc, which can otherwise impede the optimal functioning of blast furnaces. Its vaporization temperature of 907°C aligns with the optimal range for a well-executed roasting process, making it an ideal choice for this application.

Limitations

It should be noted that pellets are susceptible to damage from sulfur during the reduction process. Even low proportions of sulfur dioxide (SO₂) can disrupt the operation of a blast furnace fed with pellets, with effects observed at concentrations as low as 5 to 50 ppm in the reduction gas. The intricate mechanism was only fully elucidated towards the end of the 20th century. Initially, sulfur speeds up oxygen extraction, but this process is reversed as soon as the first metallic iron appears, significantly slowing down oxygen extraction. The reason for this unusual reaction is the strong affinity of sulfur for the metallic iron that forms on the surface of the pellets. This prevents carbon from penetrating.

Additionally, the reaction of wustite (FeO) with carbon monoxide (CO) occurs not only on the surface of the FeO but also beneath the surface of the already reduced iron. Due to the superior absorption characteristics of iron, a significant portion of the gas transport occurs at this point, along with the iron/iron oxide phase boundary. However, this only happens when the iron can absorb sufficient carbon (carburization). If carbon absorption is obstructed by sulfur, reduction can only occur at the surface of the iron oxide. As iron crystallization can only occur in the direction of the reducing iron oxide, the result is an elongated, fibrous configuration of iron crystals. The structure of the granules, which the first phase of reduction has already loosened, is reinforced, and the volume of the granules can increase to two or three times their initial value. The simultaneous exaggerated increase in granule volume ("swelling") has the potential to block or even cause significant damage to the blast furnace.

Composition

As with agglomerates, pellets are typically classified as either acidic or basic. The following ratio of mass concentrations is used to calculate the basicity index i_c:

$i_{c}={\frac {[CaO]+[MgO]}{[SiO_{2}]+[Al_{2}O_{3}]}}$

It is common practice to simplify matters by calculating a simplified basicity index i, equal to the CaO/SiO₂ ratio. Pellets with an index i less than 1 are classified as "acidic," while those greater than 1 are typically designated as "basic." Pellets with an index i equal to 1 are identified as "self-melting."

Pellets can be rich in hematite, but the proportion of hematite must remain limited. Otherwise, the pellet structure would loosen too much during reduction, resulting in the pellets collapsing into concentrated dust under the weight of the stacked charges.

Acid pellets

Acid pellets are typically produced without the addition of any additives. The chemical composition of these pellets, expressed as a percentage by mass, is 2.2% SiO₂ and 0.2% CaO. In the USA during the 1990s, the typical acid pellet had the following characteristics:

• 66% Fe, 4.8% SiO₂, 0.2% MgO, and a CaO/SiO₂ ratio of 0.04.

• The compressive strength is 250 kg.

• The ISO reducibility is 1.0.

• The material exhibits a swelling ratio of 16%.

• The softening temperature is 1290°C, with a difference of 230°C between the softening and melting temperatures.

Unlike agglomerated ores, pellets are often acidified by various silicates from the binder required for pelletizing. This is due to their solid spherical shape, which makes them less prone to disintegration and weakening of mechanical properties, allowing them to maintain an acidic composition.

Acid pellets have excellent mechanical strength, with a crush resistance of over 250 kg/pellet. However, their reducibility could be better. Additionally, they are likely to swell in lime, typically for i = CaO / SiO₂ > 0.25, which could potentially damage a blast furnace.

Self-melting pellets

Self-melting, or basic pellets, was first developed in the United States in the 1990s. These pellets are manufactured from an iron ore concentrate to which lime and magnesia have been added. In the United States, the typical self-fusing pellet has the following characteristics:

• Fe = 63%; SiO₂ = 4.2%; MgO = 1.6%; CaO/SiO₂= 1.10.
• The compressive strength is 240 kg.
• The ISO reducibility is 1.2.
• The expansion ratio is 15%.
• The softening temperature is 1,440°C, with a difference of 80°C between the softening and melting temperatures.

These pellets are compress-resistant (approximately 240 kg/pellet) and easy to reduce, making them an ideal choice for blast furnace operation. However, adding limestone to the ore concentrate reduces the productivity of the pellet plant since calcination leads to decarbonization, which is an endothermic process. Consequently, the plant's productivity is reduced by 10 to 15% compared to the production of acid pellets.

Pellets with low silica content

This pellet is designed for use in direct reduction plants. The typical composition is as follows: The composition of the pellet is as follows: Fe = 67,8 %; SiO₂ = 1,7 %; Al₂O₃ = 0,40 %; CaO = 0,50 %; MgO = 0,30 %; P = 0,01 %.

It should be noted that low-silica pellets will self-fuse when doped with lime. A typical composition would be as follows: The composition of the pellet is as follows: Fe = 65,1%; SiO₂ = 2,5 %; Al₂O₃ = 0,45 %; CaO = 2,25 %; MgO = 1,50%; P = 0,01 %.

Other types of pellets

To meet specific customer requirements, manufacturers have developed alternative pellet types. These include:

• Self-reducing pellets: Made from iron ore and coal.
• Magnesian pellets: Produced with the addition of olivine or serpentine, resulting in a magnesia content of approximately 1.5%. Their cold crush resistance is average, at approximately 180 kg/pellet.

Bibliography

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Related articles

• Agglomerate (steel industry)