Taconite is the foundation of much of the steel produced in the United States. This low-grade rock is a specific variety of banded iron formation, a sedimentary rock containing iron minerals interlayered with high amounts of quartz. Because its iron content is too low for direct use, taconite must undergo a complex industrial transformation to become an economically viable raw material.
Defining Taconite Ore
Taconite is characterized as a low-grade iron ore because its iron content is typically low, ranging from 25 to 35% iron by weight. The rock is primarily composed of iron oxides, such as magnetite or hematite, embedded in a matrix of silica, which accounts for approximately 45% of its mass. This high silica content makes the crude ore unusable for traditional iron smelting processes, as it would require too much energy to melt and would create excessive waste.
Taconite became a focus of the mining industry in the mid-20th century, particularly in the Lake Superior region’s Mesabi Range in Minnesota. This shift occurred because the region’s initial supply of high-grade, direct-shipping iron ore, which contained 50 to 70% iron, had become largely depleted. Taconite, previously regarded as a waste product, presented a massive reserve that could only be exploited after new processing technology was developed to manage the high proportion of silica.
The Pelletizing Process
The transformation of raw taconite into a usable product requires a multi-step process known as pelletizing, which is designed to concentrate the iron and remove the unwanted silica. The hard, blasted rock is first subjected to a series of crushing steps that reduce the material into smaller fragments. These fragments are then mixed with water and ground in rotating mills until the ore is pulverized into a fine powder, ensuring the iron minerals are fully liberated from the surrounding gangue rock.
The next step involves magnetic separation, where the fine slurry is passed through strong magnetic fields to separate the magnetic iron oxide particles, primarily magnetite, from the non-magnetic silica waste. This magnetic concentration process upgrades the iron content of the powder to approximately 65%. The resulting iron-rich powder, called concentrate, is then dewatered and prepared for agglomeration.
The concentrate is mixed with small amounts of binders, such as bentonite clay and limestone, before being rolled in large, rotating cylinders or drums. This action forms the moist powder into small, uniform balls, typically about 10 to 15 millimeters in diameter, often called “green pellets.” The final and most energy-intensive stage is induration, where these soft green pellets are baked at high temperatures, often exceeding 1,300 degrees Celsius (2,400 degrees Fahrenheit), to harden them.
Primary Industrial Application
The finished, hardened taconite pellets are the primary feed material for ironmaking, serving a direct role in the production of steel. They are shipped to steel mills, typically via the Great Lakes, where they are charged into blast furnaces to be reduced into metallic iron. The uniform size and shape of the pellets are a major advantage for furnace operators because they allow for consistent airflow and gas permeability through the furnace burden.
The high iron concentration of the pellets, which is typically over 65% and low in silica, makes them an energy-efficient raw material. Using high-purity pellets significantly reduces the amount of coke, a carbon fuel source, required in the blast furnace, thereby lowering overall production costs and improving the efficiency of the ironmaking process. Taconite pellets are also used in newer steel production methods, such as the direct reduced iron (DRI) process, which bypasses the traditional blast furnace entirely.