Taconite is a variety of low-grade iron ore essential to modern steel production, particularly across North America. The widespread depletion of naturally occurring, high-purity iron deposits required the development of an industrial process to utilize this harder, silica-rich rock. This transformation sequence takes taconite from a dense stone to a concentrated, high-iron feedstock ready for the blast furnace. The process requires mechanical force and chemical separation to unlock the iron content locked within the rock matrix.
Defining Taconite and Its Geological Context
Taconite is a specific type of sedimentary rock classified as a banded iron formation, which represents some of the oldest rock formations on Earth. This ore is characterized by a relatively low iron concentration, typically ranging between 20 and 35 percent by weight. The iron minerals, primarily magnetite and hematite, are finely interlayered with a significant amount of silica, such as quartz and chert, which accounts for about 45 percent of the rock’s mass.
Taconite became important after the exhaustion of naturally high-grade ore, which contained up to 65 percent iron and could be shipped directly for smelting. The process of upgrading taconite was pioneered in the mid-20th century to sustain the domestic steel industry. Today, the largest deposits are mined in the Great Lakes region, notably the Mesabi Range of Minnesota, providing raw material for a significant portion of the continent’s steel production.
Initial Preparation: Mining, Crushing, and Grinding
Processing taconite begins with its extraction from open-pit mines. Because the rock is hard, crews drill into the rock face before using explosives to break the taconite into manageable fragments. The resulting boulders are loaded by electric shovels into haul trucks capable of carrying up to 240 tons of material.
Once transported to the processing plant, the ore undergoes a series of crushing steps. Primary, secondary, and tertiary crushers progressively reduce the taconite from large chunks down to fragments roughly the size of marbles, about three-quarters of an inch in diameter. This initial stage is purely mechanical, focusing only on size reduction before purification begins.
The crushed rock is then mixed with water and subjected to grinding in rotating rod and ball mills. The goal of this wet grinding is to pulverize the material into a fine powder, often finer than 53 micrometers. This size reduction is necessary to physically separate the iron particles from the surrounding silica before purification.
Concentration and Purification (Beneficiation)
The purification stage, known as beneficiation, upgrades the taconite from a low-grade ore to a high-purity concentrate. The finely ground slurry is primarily processed using wet low-intensity magnetic separation, as the iron is often present as the magnetic mineral magnetite. Magnetic drums attract and capture the iron particles while the non-magnetic waste material, or gangue, flows away.
Magnetic separation quickly rejects a large volume of the silica-rich gangue, producing a concentrate of approximately 63 to 64 percent iron. When the iron is present as the non-magnetic hematite, a different approach, such as froth flotation, is used. Flotation involves introducing chemical reagents that attach to the iron particles, allowing them to cling to air bubbles and rise to the surface for collection.
A combination of processes is often used to achieve the highest possible purity for steelmaking. The concentrate from initial magnetic separation may undergo a secondary flotation step to reduce the silica content to as low as four to six percent. Removing silica is important because its presence in the steelmaking furnace would consume excessive energy and require more fluxing agents.
Final Transformation: Pelletizing and Use in Steelmaking
The resulting iron concentrate is a fine powder unsuitable for direct use in a blast furnace, as air currents would blow it out. Therefore, the concentrate must be transformed into a durable, uniform material through pelletizing. The purified powder is dewatered and mixed with a binder, such as bentonite clay, and sometimes limestone as a fluxing agent.
This mixture is introduced into rotating drums, which cause the moist powder to tumble into spherical shapes known as “green balls.” These pellets, typically 10 to 15 millimeters in diameter, are fragile and require a final hardening step. The green pellets are conveyed into kilns and fired at temperatures reaching approximately 3,000 degrees Fahrenheit.
The heat hardens the spheres into durable taconite pellets that can withstand transport and conditions inside a blast furnace. These finished pellets contain over 65 percent iron and are the high-quality feedstock shipped directly to steel mills. In the blast furnace, the pellets are reduced to metallic iron, or pig iron, which is then refined and alloyed into finished steel.