Iron Ores


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Iron Ores

 

natural mineral formations that contain iron in amounts and compounds such that the industrial extraction of the metal is feasible. Iron ores vary in their mineral composition, iron content, useful and harmful impurities, conditions of formation, and industrial properties.

The most important iron minerals are magnetite, magnesioferrite, titanomagnetite, hematite, hydrohematite, goethite, hydrogoethite, siderite, and iron chlorites (chamosite, thuringite, and others). The iron content of industrial ores varies within wide limits, from 16 to 70 percent. A distinction is made among rich ores (≥ 50 percent iron), ordinary ores (50–25 percent iron), and poor ores (≤ 25 percent iron).

Depending on their chemical composition, iron ores are used to make pig iron after dressing or in their natural state. Iron ores that contain less than 50 percent iron are usually concentrated (to 60 percent iron) by magnetic separation or gravity dressing. Powdery and sulfurous (> 0.3 percent sulfur) rich ores, as well as beneficiation concentrates, are made into lumps by sintering; so-called pelleted iron ore is made from concentrates. To avoid impairing the quality of the steel or the conditions of smelting, iron ores used in blast furnaces should not contain more than 0.1–0.3 percent sulfur, phosphorus, and copper and 0.05–0.09 percent arsenic, zinc, tin, and lead. Except in certain cases, admixtures of manganese, chromium, nickel, titanium, vanadium, and cobalt are beneficial. The first three elements improve the quality of steel; titanium, vanadium, and cobalt can be extracted during dressing and metallurgical processing.

Types of deposits. Iron ores are divided into three groups (magmatogene, exogenic, and metamorphogenic) according to origin.

MAGMATOGENE. The magmatogene group is subdivided into magmatic, contact-metasomatic (or skarn), and hydro-thermal deposits. Magmatic deposits include dike-shaped, irregular, and sheetlike titanomagnetite deposits associated with gabbro-pyroxenite rock (the Kusa and Kachkanar deposits in the Urals in the USSR, the Bushveld complex in South Africa, and the deposits at Ligang, Tanzania), and apatite-magnetite deposits associated with syenites and syenite-diorites (Lebiazhka in the Urals, USSR; Kiurna and Gallivare, Sweden).

Contact-metasomatic, or skarn, deposits form at contacts or near intrusive masses; surrounding carbonate and other types of rock are changed by high-temperature solutions into skarns, as well as scapolite and pyroxene-albitic rock in which massive and impregnation magnetite ore deposits of complex shape remain separate (in the USSR, the Sokolovka-Sarbai deposits in northwestern Kazakhstan and the Magnitogorsk and Vysokaia Gora deposits in the Urals; a number of deposits in Gornaia Shoriia; and Iron Springs in the USA).

Hydrothermal deposits form upon the action of hot mineralized solutions in cases of the deposition of iron ores in cracks and zones of crumpling, as well as in cases of metasomatic replacement of wall rocks (the Korshunovo and Rudnogorsk magnesioferrite deposits of Eastern Siberia, the hydrogoethite-siderite Abail deposit in Middle Asia, and the siderite deposits of Bilbao in Spain).

EXOGENIC. 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 washdown 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 iron ores and some of the siderites and silicate rocks (in the USSR, the Kerch’ deposit in the Crimea and the Aiat deposit in the Kazakh SSR; and Lahn-Dill in the Federal Republic of Germany).

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, or eluvial, deposits, in which products of weathering that are iron-enriched because of the removal of other components remain in place (bodies of rich hematite-martite ores at Krivoi Rog, the Kursk Magnetic Anomaly, and the Lake Superior region in the USA), and infiltration (cementation) deposits, in which the iron is carried out of the weathering rocks and redeposited in underlying strata (the Alapaevsk deposit in the Urals).

METAMORPHOGENIC. Metamorphogenic (metamorphosed) deposits are preexisting, primarily sedimentary deposits that are transformed under conditions of high temperature and pressure. Under such conditions hydrous ferric oxides and siderite usually become hematite and magnetite. Metamorphic processes are sometimes supplemented by hydro-thermal-metasomatic formation of magnetite ores. This type includes the ferruginous quartzite deposits of Krivoi Rog and the Kursk Magnetic Anomaly and the deposits of the Kola Peninsula, the iron-ore province of Hamersley (Australia), the Labrador Peninsula (Canada), the state of Minas Gerais (Brazil), and Mysore State (India).

Types of iron ores. The main industrial types of iron ores are classified according to the predominant ore mineral.

BOG IRON ORE. In bog iron ore, the ore minerals are represented by hydrous ferric oxides, mainly hydrogoethite. Such ores are common in sedimentary and weathering-crust deposits. The structure is dense or friable; sedimentary ores often have an oolitic texture. The iron content varies from 55 to 30 percent or less. Dressing is usually required. The so-called self-fluxing bog iron ores, for which (SiO2 + Al2O3)/(CaO + MgO) is close to 1, go to smelting with an iron content of up to 30 percent (Lorraine). The bog iron ores of some deposits have a manganese content of 1.0–1.5 percent or more (Bilbao in Spain and Bakal in the USSR). Complex chromium-nickel bog iron ores are important; although they contain 32–48 percent Fe, they also may often have up to 1 percent nickel, up to 2 percent chromium, hundredths of a percent cobalt, and sometimes vanadium. Chromium-nickel cast irons and low-alloyed steels can be smelted directly from such ores without additives.

HEMATITE ORES. The main ore mineral in hematite ores is hematite. They are found mainly in the crust of weathering (oxidation zone) of ferruginous quartzites and skarns of magnetite ores. Such ores are often called martite ores (martite is a pseudomorphosis of hematite after magnetite). The average iron content is 51–60 percent, sometimes higher, with insignificant admixtures of sulfur and phosphorus. Hematite ore deposits containing up to 15–18 percent manganese are known. Hydrothermal deposits of hematite ores are less developed.

MAGNETITE ORES. The ore mineral in magnetite ores is magnetite (sometimes magnesian magnetite), which is frequently martited. Magnetite ores are most characteristic of deposits of the contact-metasomatic type associated with limestone and magnesian skarns. Along with rich massive ores (50–60 percent iron), disseminated ores, which contain less than 50 percent iron, are also widespread. Ore deposits with valuable admixtures, particularly cobalt and manganese, are known. Harmful impurities include sulfide sulfur, phosphorus, and sometimes zinc and arsenic.

Titanomagnetite ores, which are iron-titanium-vanadium complexes, are a special variety of magnetite ores. Disseminated titanomagnetite ores, which are essentially fundamental intrusive rocks with a high content of rock-forming titanomagnetite, are taking on great industrial importance. They usually contain 16–18 percent Fe, but they are easily dressed by magnetic separation (the Kachkanar deposit in the Urals).

SIDERITE ORES. Siderite ores are subdivided into crystalline siderite ores and clay spathic ironstones. The average iron content is 30–35 percent. After roasting, as a result of the removal of CO2, siderite ores become industrially valuable finely porous iron-oxide ores (usually containing up to 1–2 percent manganese; sometimes up to 10 percent). In a zone of oxidation, siderite ores become bog iron ores.

SILICATE IRON ORES. The ore minerals in silicate iron ores are iron chlorites, usually accompanied by hydrous ferric oxides, and sometimes by siderite (25–40 percent iron). There is a negligible content of sulfur and up to 0.9–1.0 percent phosphorus. Silicate ores form beds and lenses in friable sedimentary rocks. They often have an oolitic texture. In the crust of weathering they are converted into bog iron ore and partly into hematites.

FERRUGINOUS QUARTZITES. Ferruginous quartzites (jaspilites and iron hqrnfels) are poor and medium Precambrian metamorphosed iron ores (12–36 percent iron) deposited by thin alternating quartz, magnetite, hematite, and magnetitehematite seams that have silicate and carbonate impurities in places. They contain few admixtures of sulfur and phosphorus. Deposits of iron quartzites usually have large re-serves of metal. Their beneficiation, particularly that of magnetite varieties, yields an entirely economic concentrate with 62–68 percent iron. The quartz is removed from ferruginous quartzites in the crust of weathering, and large de-posits of rich hematite-martite ores are formed.

Most iron ores are used to smelt pig iron, steel, and iron alloys. In relatively small amounts they are used as natural pigments (ochers) and for weighting brown clay solutions. The industrial requirements for iron-ore quality and properties are diverse. Ores rich in phosphorus (up to 0.3–0.4 percent) are used to make some types of foundry cast iron. In smelting open-hearth cast irons (the main product of blast furnaces) using coke, the sulfur content of the ore introduced into the blast furnace should not exceed 0.15 percent. To make cast iron for open-hearth acid conversion, the ores should be particularly low in sulfur and phosphorus; for the basic open-hearth conversion method using tilting open-hearth furnaces, a slightly higher, phosphorus content is permissible, but it should not exceed 1.0–1.5 percent (depending on the iron content). Basic Bessemer cast iron is smelted from phosphorus-containing iron ore with a high iron content. When smelting cast iron of any kind, the zinc content should not exceed 0.05 percent. Ore used in a blast furnace without previous mechanical sintering should have adequate mechanical strength. So-called open-hearth ores used in the charge should be in lumps and should have a high iron content, with no sulfur and phosphorus admixtures. Dense, rich martite ores usually meet these requirements. Magnetite ores containing up to 0.3–0.5 percent copper are used to make steels with improved corrosion resistance.

World mining and processing of iron ores clearly shows a tendency toward a considerable increase in production of poor ores that beneficiate well, particularly magnetitic iron quartzites and to a lesser extent disseminated titanomagnetite ores. The profitability of use of such ores is attained by large-scale mining and beneficiating enterprises and by the perfection of methods of beneficiation and sintering of the concentrates produced, particularly the production of so-called pellets. However, the problem of increasing the re-sources of iron ore that does not require beneficiation is still important.

REFERENCES

Zhelezorudnaia baza chernoi metallurgii SSSR. Moscow, 1957.
Trebovaniia promyshlennosti k kachestvu mineral’nogo syr’ia: Spravochnik dlia geologov, fasc. 59: Zhelezo, 2nd ed. Moscow, 1962.
Obzor mineral’nykh resursov stran kapitalisticheskogo mira. Moscow, 1968. [Yearly survey.]

G. A. SOKOLOV

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