There are many iron minerals in nature. There are more than 300 types of iron ores that have been known by people, but there are only 20 kinds of iron ore used as raw materials now, the main ones are magnetite, hematite, limonite, and siderite.
The main iron-bearing mineral of magnetite is Fe3O4, and its chemical formula is Fe3O4. The theoretical iron content is 72.4%, and the exterior color is usually carbon black or light blue-black, with metallic luster and black stripe (the color on the board when marking on the uneven white porcelain board). It is commonly known as green mine. The most prominent feature of this ore is its magnetism, which is also the origin of its name. Magnetite is usually very hard, with dense structure and poor reduction performance. Generally, the hardness of magnetite is between 5.5 and 6.5, and the specific gravity is between 4.6 and 5.2. This kind of steel iron ore is widely distributed and abundant in nature.
Hematite refers to ferric oxide without crystal water, and its chemical formula is Fe2O3 (ferric oxide). The theoretical iron content of pure hematite is 70%. Its exterior color is from red to light gray, sometimes black, with dark red stripes. Commonly known as "red ore". Hematite has different crystal structures, from very dense to very dispersed and soft powder, so its hardness is also different. The former is generally between 5.5 and 6.5, while the latter is very low. Generally, it is easier to reduce than magnetite. The proportion is between 4.8 and 5.3.
Limonite is a kind of ferric oxide with crystal water. The chemical formula can be expressed as mfe2o3 · nH2O. It is actually a mixture of goethite (Fe2O3 · H2O), hydrogoethite (2fe2o3 · H2O), iron hydroxide, and slime. Most of the limonite in nature exists in the form of 2fe2o3 · 3H2O. According to the different content of crystal water, limonite can be divided into hematite, goethite, limonite, etc. Limonite is formed by weathering of other types of iron ore, so its structure is relatively soft, with small proportion and large water content. Limonite has different colors due to different crystal water content, from yellow-brown to dark brown or black-gray. Streaks of tan. Limonite crystal water is easy to remove when it is dry, limonite (dehydrated limonite) pores are many, easy to reduce.
Siderite is a kind of iron carbonate. Its chemical formula is FeCO3 (iron carbonate). Its theoretical iron content is 48.2%, FeO content is 62.1%, CO2 content is 37.9%. It is hard and dense siderite commonly seen in nature. Its exterior color is gray and yellow-brown, with a specific gravity of 3.8 and hardness of 3.5-4. It is non-magnetic. Under the action of oxygen and water, siderite is easily weathered into limonite, covering on its surface. Siderite is often mixed with carbonates such as magnesium, manganese, and calcium. Siderite generally has a low iron content (30-40%), but after roasting, due to the significant increase of iron content in CO2 released by decomposition, the ore also becomes porous and becomes an ore with good reducibility.