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Lime, an essential component in the steel industry
The steel industry cannot be conceived without the use of lime, ever-present during the whole steel-making process. The addition of lime is necessary in electric arc furnaces, AOD converters, in refining ladles… Lime in steel-making can be a purifier to eliminate impurities, to remove sulphur, removing phosphorus -dephosphorization- , it can act as a flux, neutralizing … And all with a specific goal, that of transforming iron ore into steel and other ferrous products. A transformation which needs other products that are also available from Calcinor, such as calcined dolomite and refractories.
Steel-making up to the present today
–La Real Academia de la Lengua Española– The Real Academy of Spanish language defines steel-making as “the metallurgy of iron, steel, of smelting and ferric alloys”, that is, the sector of the metal industry dedicated to iron extraction and processing. The fact is that iron, after oxygen, silicon and aluminium, is one of the most abundant elements on the earth.
The first iron to be used was probably an alloy of Iron (Fe) and Nickel (Ni) which when it was heated, probably by accident, it could be bent and therefore be used to make arrows and tools. It was in the II century when the first furnaces for the treatment of minerals containing iron were constructed, but the installations were primitive and did not produce a high-enough temperature to melt the metal. After the XV century furnaces increased in height and the methods to blow air in were improved, which meant that mould smelting was possible and so the first pig iron, or foundry iron, appeared. And then in 1750 the first homogeneous steel was obtained after heating iron and charcoal in a melting pot.
Although the use of iron goes back to prehistory, advances were not made in the field of steel-making until the XIXth century with the development of the railways. This is when the need to have a forgeable and consistent material first appeared.
At this moment in time, world steel production is 20 times greater than that of all other metals, owing, to a large extent, to its versatility and variety of products that can be made from it.
The UNE 36001 norm includes the following classification of steel products: Irons, steels, foundry or pig-iron, ferro-alloys, ferrous aggregates and special ferric alloys. Especially worth highlighting is steel, a very versatile material that accounts for 80% of metal production and which is obtained by mixing Iron (Fe) and Carbon (C). The carbon content in steel does not exceed 1.5%, whilst foundry iron or pig-iron, which are also alloys of Fe-C, the carbon content is greater (between 2 – 4.5 %), which increases its hardness.
Lime as a necessary element for the purification of metal products
The metal making process cannot be conceived without the use of lime whose mission is to purify the products obtained in the steel mill by modifying their composition. To be specific, calcium oxide or quicklime can execute these three functions in the metal industry:
- Formation of slag: calcium oxide is used in slag formation which, among other things, accumulates the waste materials that are produced in the steel making process. Moreover, it protects the metal from the elements in the atmosphere, such as nitrogen and hydrogen, it is also an insulator allowing the high temperatures reached in the process to be maintained (1,600-1,800ºC).
- Phosphorus removal: Phosphorus, contained in the iron ore and the scrap metal that are used to start the steel-making process, can seriously damage the properties of steel. In large quantities, it lowers the ductility of the steel making it easy to fracture when it is cold-worked. Quicklime added to the metal-making process extracts the phosphorus in the steel, lowering its proportion to levels where its ductility is not affected.
- Sulphur removal: Sulphur can also damage steel by making it more fragile, causing cracks to form. Hence quicklime is used to lower its percentage in the steel compostion and to minimize its negative effects.
So, we are looking at a purification process that adds lime to obtain a metal product with different properties. A process, that can be made in two ways: Primary steel making or secondary steel making.
Types of steel-making processes
1) Integrated or Primary steel-making:
This is carried out in what is called a blast furnace where the fusion of the metal and the first purification takes place. At the top of the furnace the iron ore in pellet form, together with the coal or coke and limestone (CaCO3) or quicklime (CaO), are introduced whilst at the bottom hot air is added with the aim of making the combustion of the coke easier, this interacts to create a reducing atmosphere for the iron.
At the bottom of the furnace two types of molten material accumulate:
- Pig-iron: This is liquid iron which because it is denser, accumulates at the bottom and must be extracted from the furnace and transported to the foundry or the Steel mill (Secondary Steel making). As seen above pig-iron is iron carbon, and is called the first casting or initial smelting. It contains an important proportion of carbon and impurities that are removed in the subsequent steps.
- Slag: Slag is formed by the smelted impurities and because they are less dense, float on top of the pig-iron. They are removed from the metal-making process and used in other industries such as cement-making.
Calcite or lime is used in the steel industry because of its flux action which allows it to lower the concentration of impurities in the steel (above all silica and alumina, but also sulphur, phosphorus…) and so produce slag. To achieve this oxygen is blown through the molten iron, bringing about a fall in the percentage of carbon. In contrast, the silicon and the phosphorus, form oxides which because of their reaction with the added calcium oxide form stable compounds of calcium silicate and calcium phosphate which are eliminated in the slag.
CaO (s) + SiO2 (s) → CaSiO3 (l)
Elimination of phosphorus
3 CaO (s) + P4O10 (s) → Ca23(PO4) 2 (l)
2) Secondary Steel making:
This is the subsequent process that the ferrous material undergoes. In this step the furnace can be fed with pig-iron, scrap or a mix of both. The aims of this secondary treatment are to:
- Carry out an adjustment of the alloying elements: C, Mn, Nb, Ti, etc.
- Lower the concentration of oxygen, hydrogen and nitrogen.
- Adjust the specifications of the non-metallic impurities (phosphorus and sulphur).
- Eliminate non-metallic inclusions, basically oxides (Al2O3)
- Control the morphology of the steel by callibrating the temperature.
To achieve these goals different phases and different additions of products are required, one of which is lime. Secondary Steel making can be divided into 3 main phases:
Fusion
This is normally carried out in electric arc furnace (HAE or EAF). At this stage lime is once again added as fluxing agent to help, on the one hand, with the formation of slag (precipitating silicates) and on the other, to remove phophorus from the steel. In the end the impurities float as slag and when removed a base steel is obtained, generally a carbon steel or low alloy steel.
[C] + [O] = CO
Formation of slag
2(CaO) + [Si] + 2[O] = (2CaO.SiO2)
[Mn] + [O] = (MnO)
Dephosphorization
2[P] + 5[O] + 3(CaO) = (3CaO.P2O5)
Refining
Steel with the final desired composition is obtained in the desulphuring stage, carried out in a ladle furnace (HC or LF). In this secondary metallurgy phase lime is also added, in this case to desulphurize the steel. Moreover, the calcium introduced by the lime has the effect of modifying the morphology and the distribution of the inclusions (oxides and sulphurs generated in this stage), as well as increasing the casting properties of the steel for the subsequent step, continuous casting. The result of this process are steels with a very low content of sulphur and gas, well deoxidized and clean.
In the case that the objective is to obtain stainless steel, the refining process is carried out in a so-called AOD converter and not in a ladle furnace because the decarburization of the steel must be extreme (0.03% C). In this AOD process lime is also added both for the desulphurization as well as for the dephosphorization and slag formation.
Casting
Once the desired composition of the steel is reached, it is the moment to cool, solidify and shape it. This stage is carried out in continuous casting, where the steel is poured into a tundish and then passed through cooled channels which yield solid steel in different formats.
Afterwards, some products are processed at rolling-plants, where a permanent deformation is produced by taking advantage of the steel`s ductility. Rolling can be cold or hot, and the products created are things like sheets, rails and bars, which are used directly. Calcinor products once again play an important role in the processes in this rolling phase.
Calcinor products in the steel industry
All these products which together with lime, quicklime and hydrated lime, are considered as essential in a great industry which is what steel manufacturing is.