According to their mineral composition, various iron-bearing minerals can be divided into four major categories: magnetite, hematite, limonite and siderite. Due to their different chemical composition, crystal structure and geological conditions, various iron ores have different external morphology and physical properties. (1) The main iron-bearing mineral of magnetite is magnetite, and its chemical formula is Fe3O4, in which FeO=31%, Fe2O3=69%, and theoretical iron content is 72.4%. This ore sometimes contains a composite ore of TiO2 and V2O5, which is called titanomagnetite or strontium titanomagnetite. It is rarely encountered in natural pure magnetite ore, and some of the magnetite is often oxidized into semi-artificial hematite and imaginary hematite due to surface oxidation. The so-called imaginary hematite is the oxidation of magnetite (Fe3O4) to hematite (Fe2O3), but it still retains the shape of the original magnetite, so it is called imaginary hematite.
Magnetite has strong magnetism, crystals are often octahedral, and a few are rhombohedral dodecahedrons. The aggregates are often dense blocks, the color streaks are iron black, semi-metallic luster, relative density 4.9 ~ 5.2, hardness 5.5 ~ 6, no cleavage, gangue is mainly quartz and silicate. Poor reduction, generally containing harmful impurities such as sulfur and phosphorus. (2) Hematite Hematite is an anhydrous iron oxide ore with a chemical formula of Fe2O3 and a theoretical iron content of 70%. This ore often forms a huge deposit in nature, and it is the main ore of industrial production in terms of burial and mining.
The iron content of hematite is generally 50% to 60%, and contains less harmful impurities such as sulfur and phosphorus. The reduction is better than that of magnetite. Therefore, hematite is a relatively good ironmaking raw material. Hematite has both native and wild, regenerated hematite magnetite that loses its magnetic properties after oxidation, but still preserves the pseudomorphic hematite of the crystal shape of magnetite, often contained in illusory hematite. Some residual magnetite. Sometimes hematite also contains some weathering products of hematite, such as limonite (2Fe2O3·3H2O). Hematite has a semi-metallic luster, the crystallizer hardness is 5.5-6, the earthy hematite hardness is very low, no cleavage, the relative density is 4.9-5.3, only weak magnetic, the gangue is silicate.
(3) Limonite The limonite is hydrous iron oxide ore, which is formed by weathering of other ores. It is the most widely distributed in nature, but it is rare to have large deposits. Its chemical formula is nFe2O3·mH2O (n=1 to 3, m=1 to 4). Limonite is actually composed of a mixture of goethite (Fe2O3·H2O), hydroarticular iron ore (2Fe2O3·H2O) and iron oxide and argillaceous materials containing different crystal water. Most of the iron-bearing minerals in limonite are in the form of 2Fe2O3·H2O. Generally, the iron ore content of iron ore is 37% to 55%, and sometimes the phosphorus content is high. The limonite has strong water absorption, and generally absorbs a large amount of water. After roasting or heating into the blast furnace, the free water and the crystal water are removed, and the porosity of the ore is increased, which greatly improves the reducibility of the ore. Therefore, limonite is better than hematite and magnetite. At the same time, the iron content of the ore is increased correspondingly due to the removal of moisture.
(4) The siderite iron ore is carbonate iron ore, the chemical formula is FeCO3, and the theoretical iron content is 48.2%. In nature, there is less industrial iron ore than other three ores. Siderite is easily decomposed and oxidized into limonite. Generally, the amount of iron is not high, but after the decomposition of CO2 by heat, not only the iron content is significantly increased but also becomes porous, and the reducing property is good.
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