For the production of steel, there is a need for iron raw material in the form of iron ore or scrap metal, together with alloying elements so that the material acquires the desired for properties. In the production of ore-based steel, coal is also needed as a reducing agent and limestone as a slag former.
Iron occurs very rarely as a pure metal in the earth’s crust. On the other hand, there is plenty of iron which is chemically bonded, in association with other elements such as oxygen and sulphur. The most abundant elements in the earth’s crust, in named order, are: oxygen, silicon, aluminium and iron.
For the extraction of metallic iron, the compounds formed between iron and oxygen, so-called oxides, play a wholly dominant role. The iron ore mineral is called magnetite (black ore) or hematite (bloodstone ore), depending on the type of oxide of which it is composed.
Magnetite has acquired its name through its magnetic properties. When it is scratched against an uncoloured surface a black streak is formed. Its chemical formula is often written Fe3O4, but since it is a mixed oxide it is more correct to write FeO • Fe2O3. Hematite or bloodstone ore, Fe2O3, has acquired its name from the blood red streak that is formed when scratched.
In Sweden, iron ore is mined in Kiruna and Malmberget by LKAB, which is Europe’s largest iron ore producer.
After mining, the material is crushed and subsequently the ore is separated from the gangue in a process known as ore dressing. In the case of magnetite ore this takes place through magnetic separation. The processed product is called dressed ore or concentrate. The dressed ore can then be shaped and agglomerated into pellets, small marble-like balls with a specific composition, size and strength. Alternatively, the dressed ore can be used to produce iron-rich sinters i.e. semi-fused and solidified lumps of iron oxide.
Previously, the ore-based steelworks in Sweden had their own sinter plants but nowadays pellets are mainly used as a feedstock in the production of ore-based steel. The last Swedish sinter plant was shut down in 1995.
Swedish iron ore mainly comprises magnetite which has the advantage that, on pellet production, it can utilise the chemical energy that is contained in the magnetite ore. When the pellets are burned, the magnetite is oxidised into hematite which releases energy. Up to 70 per cent of the energy required in the process actually comes from the iron ore. The finished pellets contain about 65 per cent iron.
Iron and steel scrap is used as a secondary raw material, both for scrap-based and ore-based steel production. The scrap is sorted into different classes and the steel plants utilise a mixture of the types of scrap that best suit the steel that it is intended to produce.
Read more about scrap classes in the section on Recycling
Scrap can be divided into three categories depending on origin:
- Internal scrap is scrap that falls to the floor within the plants during steel production and that is directly recovered for the production process. This scrap has the advantage that its precise content is known.
- Engineering workshop scrap is the scrap that arises during the working of steel in workshops, within the construction industry, on bridge building etc.
- Scrap metal collection is the scrap collected from end-of-life products e.g. on demolition of structures and installations and from households. The scrap may include everything from bridge beams to household utensils.
Charcoal was originally used in the production of iron. Charcoal was produced from the combustion of wood in kilns (wood stacks). Today mineral coal is mainly used, but it cannot be utilised directly in the blast furnaces. For this reason the coal is carbonised which implies the removal of water and volatile matter. In this way the coke (carbonised coal) acquires the strength that is required in the blast furnace.
The carbon’s main function in iron production is to act as a reducing agent to convert the iron oxide to iron. But the coke also has further functions: to act as a physical support material for the blast furnace charge, to raise the carbon content in the iron and to supply energy.
Limestone is used as a slag former in the iron and steelmaking process. The slag has several functions, but it is mainly an active component in the metallurgical processes. The slag serves to bind substances which are not desired in the steel being produced. This enables one to control the composition and thereby give the steel improved properties.
Read more about slag in the section Processes.
Steel is an alloy with iron as the base material. All steel includes small quantities of e.g. carbon, silicon and manganese.
During the production process, alloying elements are elements are added e.g. chromium, nickel, molybdenum and vanadium, often in the form of ferrous alloys (ferrosilicon, ferromanganese, ferrochromium and ferrovanadium, etc). This is how the preconditions are created for imparting the desired for properties to the steel: for example, corrosion resistance, hardness, abrasion (wear) resistance and toughness. To find the optimum alloy for each specialised application area of the steel is a science that continuously develops through the research carried on in Sweden and internationally.
The addition of alloying elements is greater for high alloy steels, e.g. stainless steel. A common stainless steel contains 18-20 per cent chromium and 8-10 per cent nickel. Cutlery, saucepans and kitchen sinks/draining boards are frequently produced of this steel grade that is usually called "18/8-steel".
When metal scrap is used in the production of crude steel, so far as possible scrap that contains the alloying elements of which the new steel shall consist is used. During the production process the melted steel is analysed; in order to ensure that it has exactly the right composition alloying elements are also added.