Pellets are a processed form of iron ore utilized in the steel industry, specifically designed for direct application in blast furnaces or direct reduction plants. These pellets are spherical in shape, with diameters ranging from 8 to 18 millimeters. The production of iron ore pellets involves several steps, including grinding the ore, mixing it with binders, and then forming and heating the pellets. The iron content of the pellets generally ranges from 62% to 66%. This enrichment process improves the iron concentration and imparts specific chemical and mechanical properties that enhance the efficiency of steel production.
History
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 concept of pellet agglomeration was initially patented by A. Anderson in Sweden in 1912, followed by a similar patent in Germany in 1913. 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. Chile opened its first pelletizing facility in 1978 –Planta de Pellets in the port of Huasco– helping iron mining in Chile to remain competitive in face increased ore production in Australia, Brazil and Liberia. 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:
In 1984, global pellet production reached 189 million tons, with North America producing 90 million tons, the USSR 63 million tons, and other regions 36 million tons. By 1992, production had increased to 264 million tons. In 2008, production further rose to 313 million tons. However, in 2009, production decreased to 215 million tons due to the economic crisis. In 2010, 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
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. Pellets are generally more robust and better suited to handling compared to agglomerates, which are relatively fragile. The production process for pellets can vary significantly depending on the local characteristics of the iron ore, and some facilities may include additional stages, such as arsenic removal. The pellet production process involves several key stages:
Crushing: The iron ore is first finely crushed to separate the valuable iron ore from non-valuable gangue materials. Enrichment: Depending on the ore's characteristics, enrichment is achieved through grinding (which can be conducted in multiple phases and may use either dry or wet methods) and by employing magnetic separation and flotation techniques. Blending: The ore concentrate may be mixed with additives to achieve the desired chemical composition. Common additives include dolomite, olivine, and quartzite, which typically account for 3 to 3.5% of the pellet's weight. Binding: To ensure cohesion during the pelletizing process, an additional binder, usually wet bentonite combined with maize flour or polyacrylamide, is added. These processes ensure that the pellets are produced to meet specific quality standards and can withstand the demands of handling and transportation. The ore concentrate is formed into pellets through a compaction process. This can be performed using various types of mixing equipment, though saucers are the most commonly employed tool. Before being subjected to sintering, the pellets are referred to as "green" or "raw" pellets, and their typical diameter ranges from 5 to 20 mm. Following pellet formation, they are either sent to a consumption plant or directed to a cooking oven. Due to their inherent fragility, which persists despite the binder used, pellets are generally more suitable for processing in a cooking oven rather than a consumption plant. After cooking, the pellets are cooled. The cooking process involves passing the pellets through a chain of contiguous ovens, where they are heated to temperatures of up to 1,200°C. This can be achieved using different methods: a straight grate process for a single, uninterrupted chain or a grate kiln process that includes a rotating cooling tray at the end of the chain. 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:
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