I. Raw materials
The origin of steel is iron ore, which is the natural form of iron (Fe). Pure iron does not exist in nature. Iron ore is mainly divided into three types: magnetite, hematite and limonite, all iron oxides, differing in their oxidation methods. The higher the iron content in iron ore, the better.
Theoretically, the highest iron content in iron ore is about 72%, and iron ore with iron content above 60% is called rich iron ore. Iron ore is first reduced to iron (pig iron) in a furnace, then sent to a steelmaking furnace for decarburization and refined into steel.
Steel scrap can also be melted and regenerated in a steel furnace. Generally, steel is transformed into products of different properties and shapes according to its use, which are called steel products.
Typically, steel products are made by reducing iron ore, smelting it into pig iron (smelting), refining the pig iron into steel (steel making), and then rolling and processing the steel into various steel products. . Broadly speaking, steel products include cast iron, cast steel, forged steel products and products processed from steel.
Before discussing the raw materials of steel, we need to clarify: what exactly is the difference between steel and iron? Do they have different components? In everyday life, we always refer to steel and iron together as “steel and iron”, which indicates that steel and iron must be the same type of material.
In fact, from a scientific point of view, steel and iron are slightly different. Its main component is the element iron, but they contain different amounts of carbon.
We usually call it “pig iron” when the carbon content is above 2%, and “steel” when it is below this value. Therefore, in the steel and iron smelting process, iron-containing ore is first smelted into molten pig iron in a blast furnace, and then the molten pig iron is refined into steel in a steel furnace.
The raw materials required to produce steel can be divided into four categories for discussion: the first category discusses various iron-containing ore raw materials; the second category is coal and coke; the third category discusses fluxes used in the smelting process to produce slag, such as limestone; The last category is various auxiliary materials such as scrap steel, oxygen, etc.
II. Types and distribution of iron ore
Theoretically, any ore containing iron elements or iron compounds can be called iron ore. However, in industrial or commercial terms, an iron ore must not only contain iron, but also have a utilitarian value.
However, due to the complex nature of determining the value of an ore, it is challenging to establish a definitive standard for iron ore in the industry. For example, iron ore produced in Europe is of poor quality and has a low iron content. They call it iron ore simply because they can't find better ore.
In contrast, Australia has abundant reserves of high-quality, high-iron ore, so the type of ore used in Europe is considered useless in Australia. Thus, what Europe uses as iron ore is not considered as such in the American steel industry.
Furthermore, ores previously considered useless due to their low iron content are now valuable iron ores due to advances in industrial technology and the development of inexpensive methods to enrich the iron content in these ores.
Therefore, we can conclude that the industrial standard of iron ore varies according to regional supply and demand, changes in industrial technology levels and transport conditions. According to current standards, ores with an average iron content of 25% or more are considered valuable iron ores.
Iron exists in nature mainly as compounds, especially in the form of iron oxides, which are particularly abundant. Here are some important types of iron ore (the first three are the main types):
(1) Magnetite is an iron oxide ore with a primary composition of Fe3O4, a compound of Fe2O3 and FeO. It is dark gray, with a specific gravity of about 5.15, containing 72.4% Fe and 27.6% O. It is magnetic and can be conveniently processed by magnetic separation during beneficiation. However, due to its dense structure, it has low reducibility. After prolonged weathering, it transforms into hematite.
(2) Hematite is also an iron oxide ore, mainly composed of Fe2O3. It is dark red, with a specific gravity of about 5.26, containing 70% Fe and 30% O. It is the most important iron ore. Depending on its structure, it can be divided into several types, including red hematite, specular hematite, micaceous hematite and red ochre.
(3) Limonite is an ore containing hydrated iron oxide. It is a generic term for two ores with different structures: goethite (HFeO2) and lepidocrocite (FeO(OH)). Some people write its main chemical formula as m Fe2O3nH2O. It appears earthy yellow or brown, containing about 62% Fe, 27% O, and 11% H2O, with a specific gravity of about 3.6-4.0. It often coexists with other iron ores.
(4) Siderite, an ore containing iron carbonate, consists mainly of FeCO3 and has a bluish-gray color with a specific gravity of around 3.8. This ore often contains a considerable amount of calcium and magnesium salts. Due to the high heat absorption of carbonate ions at temperatures around 800 to 900°C, which releases carbon dioxide, this type of ore is normally roasted before being introduced into the blast furnace.
(5) Ferro Silicate, a complex salt ore, does not have a defined formula and its composition varies greatly. It generally has a deep green color, a specific gravity of about 3.8, and a very low iron content, making it a fairly poor iron ore.
(6) Sulphide Iron ore contains FeS2, with Fe making up only 46.6% and S making up 53.4%. It has a grayish-yellow color and specific gravity of approximately 4.95 to 5.10. Due to the presence of many other valuable metals such as copper, nickel, zinc, gold and silver, this ore is often used as a raw material in other metal smelting industries.
Furthermore, as it contains a large amount of sulfur, it is often used to produce sulfur, making iron a by-product, which, in fact, means that it can no longer be classified as iron ore.
Iron is widely distributed on the Earth's surface and is extremely abundant; Almost all rocks and animal and plant remains contain some iron. According to geological estimates, around 4 to 5% of the Earth's crust is made up of iron.
Although only a small portion of these iron-containing substances are valuable iron ore, the supply is extremely abundant. There is currently no definitive number to estimate Earth's iron ore reserves.
The main iron ore producing countries identified include the United States, Canada, Brazil, Australia, South Africa, India, France, the United Kingdom, Sweden, Spain, the Soviet Union, China and Venezuela.
