Taconite, a low-grade siliceous iron ore composed of 20–30 percent magnetite that occurs in fine-grained banded iron formations. Taconite is mined primarily in the U.S., in the Mesabi Iron Range in Minnesota and the Marquette Iron Range in Michigan. Recovery of the iron requires fine grinding and concentration of iron-bearing phases, which in turn are formed into pellets suitable for blast furnaces. As high-grade deposits of iron ore have become depleted, taconite deposits have increased in importance as a source of iron ore. Taconite is a very hard rock containing low-grade iron ore used to make iron and steel. Using explosives, taconite is blasted into pieces that are then crushed into smaller pieces at a processing plant. After the iron ore is separated from the taconite, the tailings are the remaining by-product. Other materials generated in the mining process include the “overburden” (material that covers the taconite, comprised primarily of rock, clay and gravel) and low-grade iron ore.
Following development of high-grade direct shipping iron ore deposits on the Mesabi Range, containing up to 65% iron and as little as 1.25% silica, miners termed the unaltered iron-formation wall rock taconite. Iron in ‘taconite’ is common present as magnetite, iron silicates, and iron-bearing carbonates, and locally martite (hematite) formed by oxidation of magnetite. Horizons containing magnetite as the dominant mineral have been extensively mined since 1955 to produce iron ore pellets; the term ‘taconite’ has consequently been colloquially adapted to describe the magnetite iron-formation ores (taconite iron ore), the mining, milling, magnetic separation, and agglomerating process (taconite process), and the product iron ore pellets (taconite pellets).
USES OF TACONITE
Wherever aggregate less than 3/8” is needed, taconite can be substituted. They are especially suitable when high-angularity aggregate is desirable. Taconite tailings can be used in road construction as part of a wear course:
• bituminous pavements (stand-alone aggregate for fine mixes or as fine aggregate supplement)
• thin overlay
• chip seal (seal coat)
or as a base material
• fill (granular borrow, select granular, Class 5 supplement),
• subgrade material,
• backfill in sub-cuts, and
• fine-filter aggregate material in edge drains