Iron ore is a type of minreral and rock from which metallic iron is extracted economically. This ore is normally rich in iron oxides and vary in color from dark grey, bright yellow and deep purple to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), goethite (FeO(OH)), limonite (FeO(OH).n(H2O)) or siderite (FeCO3). Different types of iron ores are shown in Fig 1
Fig 1 Types of Iron ores
The main ores of iron are hematite (70% iron) and magnetite (72% iron). Ores carrying very high quantities of hematite or magnetite (greater than 62% of Fe) are known as “natural ore” or “direct shipping ore”, meaning they can be fed directly into iron making furnaces. Taconite is low grade silica rich iron ore containing up to 30% magnetite and hematite. This deposit can be processed to produce a concentrate of Fe around 65%. This ore is a major source of iron in USA, Canada and China.
Magnetite is also known as magnetic iron ore because it is easily attracted by a magnet. It is a heavy black mineral with metallic luster. Hematite is a steel-gray to iron- black colored mineral which is as hard as magnetite (Moh’s scale hardness is 5.5 to 6.5) but slightly less heavy (usually specific gravity varies from 5.0 to 5.3). It has also a metallic luster.
The three primary sources of iron ore are banded iron formations, magmatic magnetite ore deposits, and hematite ore.
- Banded iron formations (BIF) – These are fine grained metamorphosed sedimentary rocks composed predominantly of magnetite and silica (as quartz). Most of the iron ore is extracted from banded iron formations, the geological structures laid down mostly between 3 and 1.2 billion years ago. Blue green algae released oxygen in the days when the atmosphere and oceans were very poor in oxygen, binding together with dissolved iron in the world’s oceans. These iron fixation events went through cycles as the algae had alternating blooms and busts, leaving the characteristic bands seen in banded iron formations.
- Magmetic magnetite ore deposits – Many a times granite and ultra potassic igneous rocks segregate magnetite crystals and form masses of magnetite suitable for economic concentration.
- Hematite ore – Many large hematite iron ore deposits are sourced from metasomatically altered banded iron formations and rarely igneous accumulations.
The following types of iron ores are economically exploited around the world.
i) Magnetite ore – The mineral magnetite (Fe3O4) is a naturally occurring metallic mineral that is occasionally found in sufficient quantities to be an ore of iron. The key economic parameters for magnetite ore being economic are the crystallinity of the magnetite, the grade of the iron within the BIF host rock, and the contaminant elements which exist within the magnetite concentrate. The size and strip ratio of most magnetite resources is irrelevant as BIF formations can be hundreds of metres thick, with hundreds of kilometres of strike, and can easily come to more than 2,500 million tonnes of contained ore. The typical grade of iron at which a magnetite bearing banded iron formation becomes economic is roughly 25% Fe, which can generally yield a 33% to 40% recovery of magnetite by weight, to produce a concentrate grading in excess of 64% Fe by weight.
ii) Hematite ore – Hematite ore consists of iron oxide (Fe2O3). The amount of hematite needed in any deposit to make it profitable to mine must be in the tens of millions of tons. Hematite deposits are mostly sedimentary in origin, such as the banded iron formations (BIFs). Hematite iron is typically rarer than magnetite bearing BIF or other rocks which form its main source or protolith rock, but it is considerably cheaper and easier to beneficiate the hematite ores and requires considerably less energy to crush and grind. Hematite ores however can contain significantly higher concentrations of penalty elements, typically being higher in phosphorus, water content (especially pisolite sedimentary accumulations) and alumina (clays within pisolites).
iii) Itabirite – It is a laminated, metamorphosed oxide facies iron formation in which (i) the original chert or jasper bands have been recrystallized into megascopically distinguishable grains of quartz and (ii) the iron is present as thin layers of hematite, magnetite, or martite. The term was originally applied in Itabira, Brazil, to a high grade massive specular hematite ore (66% iron) associated with a schistose rock composed of granular quartz and scaly hematite. The term is now widely used outside Brazil.
In India the iron ores occur in different geological formations. The larger concentration of economic deposits is found in sedimentary iron formations of Pre-cambrian age (BIF). The older magnetite-dominant deposits with bands of magnetite are generally of Algoma type associated with banded magnetite quartzites whereas the younger hematite dominant deposits are similar to Lake Superior type, associated with banded hematite quartzite/jasper (BHQ/BHJ) and occur as cappings on hills.
Major iron ore deposits are distributed in five zones designated as Zone – I to Zone-V. These zones have been identified in the country on commercial basis.
- Zone-I – This group of iron ore deposits occur on the Bonai iron ore ranges of Jharkhand and Orissa states and in the adjoining areas in Eastern India.
- Zone-II – This group comprises of the iron ore deposits in the long (225 km.) north-south trend in linear belt in central India which comprises the states of Chhattisgarh and Maharashtra (East).
- Zone – III deposits occur in Bellary-Hospet regions of Karnataka..
- Zone – IV deposits cover the rich magnetitic deposits of Bababudan-Kudremukh area in the state of Karnataka.
- Zone – V deposits cover iron ore of Goa state.
In addition, in south India magnetite rich banded magnetite quartzites occur in parts of Andhra Pradesh near the East Coast while in Tamil Nadu good deposits of magnetite occur in Salem district and in neighboring areas.