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Iron Ores

• satyendra
• March 31, 2013
• 3 Comments
• Banded iron formations, Bog iron, Goethite, Hematite, Iron ore, Limonite, Magneite, Siderite,

Iron Ores

The planet Earth consists of iron (32.1 %), oxygen (30.1 %), silicon (15.1 %), magnesium (13.9 %), sulphur (2.9 %), nickel (1.8 %), calcium (1.5 %), and aluminum (1.4 %) with the remaining 1.2 % being ‘trace amounts’ of the 80 or so remaining elements. Most of the iron constitutes the core of the earth, but iron which is found near the surface of the earth is of importance. These deposits are mostly in the form of iron oxides (mostly hematite and magnetite), though there are iron deposits in smaller percentage in the form of other iron compounds (hydro-oxides, carbonates, and sulphides). Iron is widely distributed in nature (the crust), but since iron is easily combined with other elements to form various iron minerals (compounds), there is little natural pure iron in the earth’s crust.

The iron ore deposits are found in sedimentary rocks. These deposits have been formed by the chemical reaction of iron and oxygen mixed in the marine and fresh water.  The iron ore formation started over 1.8 billion years ago when abundant iron was dissolved in the ocean water which then needed oxygen to make hematite and magnetite. The oxygen was provided when the first organism capable of photo-synthesis began releasing oxygen into the waters. This oxygen combined with the dissolved iron to form hematite and magnetite. These then deposited on the ocean floor abundantly which are now known as banded iron formation.

Banded iron formations (BIFs) are sedimentary rocks containing over 15 % iron predominantly composed of thinly bedded iron minerals and silica (as quartz). Banded iron formations occur exclusively in Precambrian rocks which have been subjected to metamorphism mostly weakly to strongly. Banded iron formations can contain iron in carbonates (siderite or ankerite) or silicates (minnesotaite, greenalite, or grunerite), but in those mined as iron metals, oxides (magnetite or hematite) are the main iron mineral. Banded iron formations are known as taconite inside North America.

Iron ore is an iron mineral which is capable of smelting iron under modern technological conditions and is economically cost-effective. Iron ore is composed of one or more iron containing minerals and gangues, which are also entrained with some impurities. The gangue is also composed of one or several minerals (compounds). Iron-bearing minerals and gangues are called minerals and are compounds with a certain chemical composition and crystal structure.

Iron ore is a type of mineral and rock from which metallic iron is extracted economically. This ore is normally rich in iron oxides and vary in colour from dark grey, bright yellow and deep purple to rusty red. The mineral in the iron ore can vary depending on the deposit. The minerals normally found in the iron ore are magnetite (Fe3O4), hematite (Fe2O3), goethite [FeO(OH)], limonite [FeO(OH).n(H2O)], or siderite (FeCO3). Tab 1 shows some popular minerals of iron. The table includes wustite which is an important mineral of iron but not found in nature and hence it is not an iron ore.

The deposits of iron which are found near the earth surface are known as iron ore deposits. Iron ores are the natural mineral formations which contain iron in amounts and compounds such that the industrial extraction of the metal is feasible. Iron ore vary in their mineral composition, iron content, useful and harmful impurities, conditions of formation, and industrial properties.

The iron content of industrial ores varies within wide limits, from 16 % to 70 %. The iron ore is called rich ore when it contains relatively higher amount of iron normally higher than 50 %. Between 25 % to 50 % iron, the iron ore is called ordinary ore, and with less than 25 % of the iron, the iron ore is called a poor ore. The rich ore has got lower amount of impurity minerals in it. Iron ores containing very high quantities of hematite or magnetite (greater than around 60 % iron) are known as direct shipping ore or natural ore, meaning they can be used directly in ironmaking without any beneficiation.

Types of iron ore deposits

According to their origin, iron ore deposits can be divided into three groups namely (i) magmatogene, (ii) exogenic, and (iii) metamorphogenic.

Magmatogene deposits – The magmatogene deposits are sub-divided into magmatic, contact-metasomatic (or skarn deposit), and hydro-thermal deposits. Magmatic deposits include dike-shaped, irregular, and sheet-like titano-magnetite deposits associated with gabbro-pyroxenite rock, and apatite-magnetite deposits associated with syenites and syenite-diorites.

Contact-metasomatic, or skarn, deposit form at contacts or near intrusive masses, surrounding carbonate and other types of rock which are changed by high temperature solutions into skarns, as well as scapolite and pyroxene-abilitic rock in which massive and impregnation magnetite ore deposits of complex shape remain separate.

Hydro-thermal deposits form upon the action of hot mineralized solutions in cases of the deposition of iron ore in cracks and zones of crumpling, as well as in cases of metasomatic replacement of wall rocks.

Exogenic deposits – Among the exogenic deposits are sedimentary deposits, which are chemical and mechanical sediments of marine and lacustrine basins, less frequently in river valleys and deltas, that form in cases of local enrichment of the waters of a basin with iron compounds and in cases of the wash down of such compounds from the adjacent dry land, they form beds or lenses in sedimentary rock, and sometimes in volcanogenic-sedimentary rock. Exogenic deposits include deposits of bog ore and some of the siderites and silicate rocks.

Deposits of the crust of weathering form as a result of the weathering of rock with iron containing rock forming minerals. A distinction is made among residual, sidual, or eluvial deposits, in which products of weathering are iron enriched because of the removal of other components, remain in place, and infiltration (cementation) deposits, in which the iron is carried out of the weathering rocks and redeposited in underlying strata.

Metamorphogenic (metamorphosed) deposits – These are pre-existing primarily sedimentary deposits which are transformed under conditions of high temperature and pressure. Under such conditions, hydrous ferric oxides and siderite normally become hematite and magnetite. Metamorphic processes are sometimes supplemented by hydro-thermal-metasomatic formation of magnetite ores.

Types of iron ore

There are more than 300 kinds of iron ores which are known. However, there are only 20 types of iron ore raw materials used at this stage, the most important of which is hematite, magnetite, limonite, and siderite. These four main types of iron ores are described below.

Hematite ore – The name hematite is derived from the Greek word for blood ‘haima’, due to the red coloration found in some varieties of hematite. Hematite refers to a ferric oxide containing no crystal water, and its chemical formula is Fe2O3 (iron oxide). The pure hematite theoretical iron content is 69.94 %. Its appearance is from red to light gray, sometimes black, and the stripes are dark red. It is normally known as ‘red mine’. The hematite crystal structure is different, from very dense to very loose and very soft powder, so the hardness is not the same. The former is generally between 5.5 and 6.5 on Mohs scale, while the latter is very low.  The specific gravity is between 4.8 and 5.3. It melts at 1565 deg C. it has metallic to splendent luster. Hematite is abundant in nature, but pure hematite is less, often co-existing with magnetite and limonite.

Hematite ore is a direct-shipping ore with naturally high iron content. Because of its high iron content, hematite ore is to normally undergo only a simple crushing, screening and blending process before it is dispatched from the mines. The amount of hematite needed in any deposit to make it profitable to mine is to be in the quantities of tens of millions of tons.

Hematite deposits are mostly sedimentary in origin, such as the banded iron formations. 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).

Hematite is generally having some amounts of Al2O3, SiO2, and TiO2. Much of the Al2O3 and SiO2 probably arise from contamination with other minerals. Titanium (Ti), however, can replace iron isomorphously in the hematite structure and the mineral ilmenite, FeTiO3, is iso-structural with hematite. The amount of titanium which is taken up depends on the temperature at which the hematite is formed. Around 15 % can enter at 800 deg C and less at lower temperatures.

The structure of hematite (Fig 1) consists of planes of hexagonal, close-packed oxygen atoms perpendicular to the c–axis. Iron atoms are sandwiched between every pair of oxygen planes and occupy two-thirds of the octahedral sites. Neighboring sheets have a common plane of oxygen atoms and are linked by octahedral sharing faces. Iron atoms repel each other across the shared faces and lie in two planes between every pair of oxygen sheets. The resulting octahedra are distorted because the Fe atoms do not lie in the middle of their coordinated octahedral of oxygen atoms.

Hematite is an anti ferromagntic material below the Morin transition at −23 deg C), and a canted anti ferromagnet or weakly ferromagnetic above the Morin transition and below its Neel temperature at 675 deg C, above which it is paramagnetic.

Fig 1 Hematite ore and its crystal structure

Itabirite – It also known as banded-quartz hematite and hematite schist, is a laminated, metamorphosed oxide-facies iron formation in which the original chert or jasper bands have been recrystallized into megascopically distinguishable grains of quartz and the iron is present as thin layers of hematite, magnetite, or martite (pseudomorphs of hematite after magnetite).

The term was originally applied in Itabirito (Pico de Itabirito), in the state of Minas Gerais and southern part of Belo Horizonte, 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.

Magnetite ore – The main iron-bearing mineral of magnetite is tri-iron tetroxide, and its chemical formula is Fe3O4. The theoretical iron content is 72.36 %, the appearance colour is usually carbon black or slightly light blue black, metallic luster, streaks (colour appearing on the board when the surface is uneven on the white porcelain plate) black. The most prominent feature of this ore is its magnetic nature, which is also the origin of its name.

Magnetite contains both ferrous and ferric iron. It differs from most other iron oxides in that it contains both divalent and trivalent iron. It is generally very hard, dense in structure and poor in reducing performance. Generally, the hardness of magnetite is between 5.5 and 6.5 on Mohs scale, and the specific gravity is between 4.6 and 5.2. This ore is widely distributed in nature and has abundant reserves. However, the pure magnetite in the surface of the earth’s crust is rare, because magnetite is a non-high-valence oxide of iron, so oxygen or water continues to oxidize it. Oxidation causes some of the magnetite to be oxidized to hematite, but still retains the crystalline form of magnetite, which is often called imaginary hematite and semi-artificial hematite. The content of oxidation in magnetite is determined by the ratio of total iron (T Fe) to ferrous oxide (FeO) in iron ore. The theoretical value of pure magnetite is 2.34. The larger is the ratio, the more is the oxidation of iron ore.

Magnetite is the most magnetic of all the naturally-occurring minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism.

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 kilometers of strike, and can easily come to more than 2,500 million tons 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, to produce a concentrate grading in excess of 64 % Fe.

The structure of magnetite (Fig 2) is inverse spinel, with ferrous ions forming a face-centered cubic lattice and iron cations occupying interstitial sites. Half of the ferric cations occupy tetrahedral sites while the other half, along with ferrous cations, occupies octahedral sites. Magnetite occurs most commonly as octahedral crystals bounded by {111} planes and as rhombic-dodecahedra. Twinning occurs on the {111} plane.

Fig 2 Magnetite ore and its crystal structure

Limonite Ore – Limonite is one of the principle iron ore (Fig 3) which has been mined from the production of iron since at least 2500 BCE. It is a ferric oxide containing crystal water, and the chemical formula can be expressed by mFe2O3.nH2O. It is actually composed of a mixture of goethite (Fe2O3.H2O),water needle iron ore (2Fe2O3.H2O), iron hydroxide and mud. Most of the limonite in nature exists in the form of 2Fe2O3.3H2O. According to different crystal water content, limonite can be divided into water hematite, needle hematite, limonite, and the like. Limonite is weathered from other iron ore, so its structure is relatively soft, small specific gravity and large water content. The streak is yellowish brown. The crystal water of limonite is easily removed when it is dried. The limonite (the limonite after dehydration) has many pores and is easy to be reduced. However, due to the small hardness of the limonite structure and a large amount of powder, it is generally necessary to pass the agglomeration before it is suitable for ironmaking.

Limonite is relatively dense with a specific gravity varying from 2.7 to 4.3. It varies in colour from a bright lemony yellow to a drab greyish brown. The streak of limonite on an unglazed porcelain plate is always brownish, a character which distinguishes it from hematite with a red streak, or from magnetite with a black streak. The hardness is variable, but generally in the 4 – 5.5 range on the Mohs scale.

Although originally defined as a single mineral, limonite is now recognized as a mixture of related hydrated iron oxide minerals, among them goethite, akaganeite, lepidocrocite, and jarosite. Individual minerals in limonite can form crystals, but limonite does not, although specimens can show a fibrous or micro-crystalline structure, and limonite often occurs in concretionary forms or in compact and earthy masses. Because of its amorphous nature, and occurrence in hydrated areas limonite often presents as a clay or mudstone.

Fig 3 Types of iron ores

Siderite ore – Siderite ore (Fig 3) is an iron carbonate with a chemical formula of FeCO3 (iron carbonate), a theoretical iron content of 48.2 %, a FeO content of 62.1 %, and a CO2 content of 37.9 %. Common in nature is hard and dense siderite, the appearance of the colour is gray and yellow-brown, the specific gravity is 3.8, the hardness is between 3.75 and 4.25 n Mohs scale and it is non-magnetic. The siderite is easily weathered into limonite under the action of oxygen and water. The siderite is often mixed with carbonates such a magnesium, manganese, and calcium. The siderite is generally not high in iron content (30 % to 40 %), but after calcination, the iron content of the CO2 is significantly increased due to the liberation, and the ore becomes porous and becomes an ore with good reduction.

The hydrothermal mineralization tends to form these ores as small ore lenses, often following steeply dipping bedding planes. This makes them not amenable to opencast working, and increases the cost of working them by mining with horizontal stopes. As the individual ore bodies are small, it can also be necessary to duplicate or relocate the pit head machinery. This makes mining of the ore an expensive operation.  The recovered ore also has drawbacks. The carbonate ore is more difficult to smelt than a haematite or other oxide ore. Driving off the CO2 (carbon dioxide) of the ore requires more energy and cannot be used as such in ironmaking. Instead the ore needs a preliminary roasting step.

Iron ore deposits in India

In India the iron ores occur in different geological formations. The larger concentration of economic deposits is found in sedimentary iron formations of Precambrian 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 Odhisha 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 – These deposits occur in Bellary-Hospet regions of Karnataka.

Zone – IV – These deposits cover the rich magnetitic deposits of Bababudan-Kudremukh area in the state of Karnataka.

Zone – V – Zone 5 deposits cover iron ore deposits in the state of Goa. 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.