Pellet (steel industry)

and pig iron production. The pelletizing of powdered iron ores was first introduced at the end of the nineteenth century, utilizing tar as a binding agent, comprising 1% by weight. This method involved firing the mixture in a rotating drum to create pellets suitable for blast furnaces, while also facilitating the removal of undesirable elements such as sulfur and arsenic through the emitted fumes. During this period, pellet sintering developed alongside grate sintering as an alternative process to address the agglomeration challenges faced by high-quality iron ore products. The resultant product was named "GEROELL", derived from the German word for "rolling." Pellets produced through this method demonstrated faster reduction rates compared to calibrated ores and agglomerates made from the same feedstock. In 1926, an industrial pilot plant was constructed by Krupp in Rheinhausen to explore the potential of this pelletizing technology. However, the plant was later dismantled to make way for the installation of a large-scale grate sintering line, which emerged as a competing process in the industry. Pellet sintering has remained a viable method for processing iron ore. In the United States, this technique was employed to process fine concentrates from the Mesabi Range during World War II. This was necessary as naturally rich iron ores (containing over 50% iron) were being depleted. The development of pelletizing fine magnetite ores, which typically have less than 44 mm in size and are around 85% iron, began around 1943 with support from the University of Minnesota. The process was later adopted in Europe, particularly in Sweden, to facilitate the production of pre-reduced iron ore. Pellet production saw substantial growth between 1960 and 1980 but eventually plateaued at approximately 300 million tons annually. The following data illustrates pellet production over several years: • By 1992, production further rose to 313 million tons. • However, in 2009, production decreased to 215 million tons due to the economic crisis. production rebounded to 388 million tons. The internaltional price for iron pellets hoovered around 45 (±10) US dollars cents per dry long ton unit (DLTU) from 1981 to 1997. == Production ==

in Sweden. Pellets are produced directly at the extraction site by mining companies and are marketed as a distinct product, unlike agglomerates which are typically manufactured at blast furnace sites through the mixing of iron ores from various sources. The pellet production process involves several key stages: The required heat for this process is supplied by burners, which can either add fuel to the ore concentrate or facilitate the oxidation of the ore, depending on the specific type of ore being processed. == Benefits and limitations ==

Benefits Pelletizing ore enhances the efficiency of blast furnaces and direct reduction plants by providing several advantages over raw iron ore: • Handling Resistance: Pellets are more resilient to handling, including in wet conditions, and do not cause clogging in storage hoppers. • Uniform Composition: The consistent and known composition of pellets facilitates a more streamlined process for converting them into iron. • Optimal Porosity: The porosity of pellets enables effective gas-solid chemical reactions within the furnace. This porosity helps maintain the material’s mechanical strength and chemical reactivity, even in the furnace’s highest temperature zones. • Efficient Reduction: The controlled oxidation state of iron oxides in pellets allows carbon monoxide to more effectively reduce Fe2O3 compared to less oxidized compounds like Fe3O4. Pellets generally contain a higher iron content than agglomerated ore, leading to increased plant productivity and reduced fuel consumption. Similar to sinter, the high-temperature roasting and sintering of pellets effectively eliminate undesirable elements such as sulfur. It is also an efficient method for removing zinc, which can otherwise hinder the operation of blast furnaces. With a vaporization temperature of 907°C, zinc is effectively removed during the roasting process, making pelletizing a suitable method for this application. Initially, sulfur accelerates the extraction of oxygen from the iron oxide, but this effect reverses once metallic iron begins to form, significantly slowing the oxygen extraction process. Due to iron's superior absorption characteristics, a substantial portion of gas transport happens at the iron/iron oxide phase boundary. This process depends on the iron's ability to absorb sufficient carbon (carburization). If sulfur obstructs carbon absorption, reduction is limited to the surface of the iron oxide. This restriction results in the formation of elongated, fibrous iron crystals, as iron crystallization can only proceed in the direction of the reducing iron oxide. Consequently, the structure of the granules becomes reinforced and can expand to two or three times their original volume. This expansion, or "swelling," of the granules can lead to blockage or significant damage to the blast furnace, highlighting the challenges associated with using pellets in blast furnace operations. == Composition ==

Pellets, similar to agglomerates, are classified based on their chemical properties as either acidic or basic. To determine the basicity index (ic), the following ratio of mass concentrations is used: i_c = \frac{[CaO]+[MgO]} {[SiO_2]+[Al_2O_3]} This ratio helps in assessing the relative basicity of the pellets, which is important for optimizing their use in blast furnaces and other metallurgical processes. Self-melting pellets Self-melting pellets, also known as basic pellets, are a type of iron ore pellet that was developed in the United States in the 1990s. These pellets are designed for use in blast furnaces and are produced by adding lime (calcium oxide) and magnesia (magnesium oxide) to iron ore concentrate, enhancing their metallurgical properties. Self-melting pellets typically have the following properties: • Iron (Fe) content: 63% • Silicon dioxide (SiO2) content: 4.2% • Magnesium oxide (MgO) content: 1.6% • Calcium oxide to silicon dioxide ratio (CaO/SiO2): 1.10 • Compressive strength: 240 kg per pellet • ISO reducibility: 1.2 • Expansion ratio: 15% • Softening temperature: 1,440°C, with a difference of 80°C between the softening and melting temperatures These pellets are recognized for their high compressive strength and ease of reduction, making them well-suited for blast furnace operations. The production process of self-melting pellets involves incorporating limestone into the iron ore concentrate. This inclusion affects the productivity of pellet plants due to the calcination process, which involves the endothermic process of limestone. As a result, the overall productivity of the pellet plant can decrease by approximately 10 to 15% compared to the production of acid pellets, which do not include lime. Self-melting pellets are appreciated for their enhanced performance in blast furnaces but require consideration of the trade-offs in production efficiency. Pellets with low silica content These pellets are designed for use in direct reduction plants. The typical composition of the pellets includes: 67.8% iron (Fe), 1.7% silicon dioxide (SiO2 ), 0.40% aluminum oxide (Al2O3), 0.50% calcium oxide (CaO), 0.30% magnesium oxide (MgO), and 0.01% phosphorus (P). Low-silica pellets, when doped with lime, can self-fuse. A typical composition for these self-fusing pellets is: 65.1% iron (Fe), 2.5% silicon dioxide (SiO2), 0.45% aluminum oxide (Al2O3 ), 2.25% calcium oxide (CaO), 1.50% magnesium oxide (MgO), and 0.01% phosphorus (P). Other types of pellets To cater to specific customer needs, manufacturers have developed alternative pellet types that offer distinct properties and performance characteristics: • Self-Reducing Pellets: Self-reducing pellets are composed of iron ore and coal, which serve as an internal reducing agent during smelting. This design allows the pellets to undergo reduction without the need for additional reducing materials, enhancing efficiency in certain metallurgical processes. • Magnesian Pellets: Magnesian pellets are created by adding minerals such as olivine or serpentine, which increase the magnesia (MgO) content to approximately 1.5%. These pellets are characterized by their balanced performance in blast furnaces, with an average cold crush resistance of around 180 kg per pellet. The added magnesia helps improve the metallurgical properties of the pellets, making them suitable for specific reduction conditions. These alternative pellet types are designed to address different operational requirements and enhance the flexibility of iron-making processes. == Notes ==

• Agglomerate (steel industry)