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Technical

Iron & steel

Lime and dolomitic lime in the iron and steel industry

The principal uses of lime and dolomitic lime in this industry are as follows:

SmeltingIron making
  • as a binder in the production of sinter
  • As a desulphurising agent
Steelmaking
  • as a slag forming component to remove impurities in
  • BOS steelmaking
  • Electric Arc steelmaking    
  • Secondary steelmaking

Dolomitic refractories are also used in specific applications in the steel industry.

There are other general uses for lime products in the iron and steel industry, such as water treatment, neutralisation of acid liquor etc. which are discussed in detail elsewhere.

Iron making

The process of iron making is the reduction of iron oxide to iron. Most of the world’s iron is made in blast furnaces, which produce liquid iron. Iron oxides normally occur as iron ores which also contain quantities of impurities particularly silica. Thus the commercial ironmaking process also involves the removal of these impurities, usually by forming a slag with  lime or dolime.

Lime is normally added as limestone. The heat in the blast furnace decomposes the limestone to lime, which then reacts with the impurities.

Other reduction techniques reduce the iron oxide to iron in a solid state. These processes such as DRI (directly reduced iron) HBI (hot briquetted iron) use higher quality ores with lower levels of impurities. The impurities are removed when the iron is eventually melted and purified to produce steel, normally in an electric arc furnace.

Thus the main slag former in iron making is limestone, or dolomite rather than lime/dolomitic lime.

There are however some smaller uses of lime in ironmaking:

Iron ores are usually purchased as fines or as pellets. Iron ore pellets contain slagforming components, and are fed directly to the blast furnace. Iron ore fines are mixed at the ironworks with limestone, olivine, dolomite etc. and pre-fired at low temperature to form sinter, i.e. agglomerates which can then be fed to the blast furnace. Finely divided lime/dolime is often added to the iron ore mixture to assist in binding the material to improve productivity and give a stronger sinter.           

Iron from the blast furnace often contains excessive levels of sulphur. As desulphurisation in the steelmaking process is not compatible with other purification processes, it is often carried out externally to the BOS vessel. A simple treatment involves injection of fine lime into the transfer ladle which moves iron from the torpedo ladle to the steelmaking vessel. The lime may be mixed with calcium carbide or with magnesium or aluminium metal. The slag, containing much of the sulphur from the iron is removed before the hot metal is charged to the steelmaking vessel to prevent the sulphur partitioning back into the metal.

SteelSteelmaking

Steelmaking is the process of  removing impurities from iron to give a material with carefully controlled levels of carbon, manganese, silica etc.

The major production route for steel has been the conversion of molten iron from the blast furnace using a basic oxygen furnace/converter. This is known variously  as BOS (basic oxygen steelmaking) BOP (basic oxygen process)  LD (after Linz Donawitz, the mostly widely used manufacturer of converters).

In BOS a furnace, lined with magnesia, or magnesia-carbon refractories is charged with molten iron, often with the addition of scrap, iron ore, or waste oxide briquettes as supplementary iron sources and temperature regulators. Carbon is removed from the melt by injecting oxygen at high speed. This also oxidises some of the iron and many of the impurities present (e.g. Silica, Phosphorous)

Near the start of the blow quicklime and dolomitic lime are added, although sometimes these are added gradually throughout the heat. The lime and magnesia react with the oxidised impurities and iron oxide to form a slag. The slag is thus composed of complex calcium and magnesium silicates, aluminosilicates, ferrites phosphates etc. The formation of a ‘good’ basic slag is essential to ensure that a steel with the correct levels of impurities. Basicity levels (normally CaO/SiO2, or [CaO+MgO]/[SiO2+Al2O3]) are normally kept in the range 2.5 - 3.5. Improvements in technology have give rise to more pure iron, allowing lime additions to be reduced. Typically lime additions of 35-50kg per tonne of liquid steel are used,

The dolomitic lime, containing lime and magnesia, serves 2 purposes. Firstly it acts as source of some of the CaO required in the process. The magnesia forms complexes with other slag components. These allow  the viscosity of slag, to be modified. The viscosity affects the ability of the slag to mix with the metal and to adsorb the impurities, and also its physical impact on the refractory lining. The cooled or modified slag is often used to protect the refractory lining  by coating (washing or splashing) the furnace with slag after the steel has been tapped out. The addition of dolomitic lime (to around 8%  or less commonly 14% MgO) also reduces the tendency of the slag to dissolve the magnesia refractory lining of the vessel.

The addition of the correct levels of lime and dolomitic lime are now considered essential at most modern basic oxygen steelmaking plants.

Electic arc steelmaking has until relatively recently involved the remelting of scrap steel, with relatively little refining. This has now changed and large, high productivity units produce steel from scrap of varying quality, together with DRI, HBI etc. Modern furnaces melt using gas and oxygen burners as well as electric power to reduce tap-to-tap times. Lime and dolomitic lime are added to the EAF to form a slag and to protect the refractories in the same way as they do in BOS.

The smaller size of the furnace, less extreme agitation and short power-on  times usually mean that the lime/dolomitic lime is added as a smaller size (20 -5 mm) than in the BOS (50-20 mm). Where high strength steels (for automotive, construction tools, wires etc.) are produced, an increasing trend in both types of steelmaking, H and/or C pickup are critical and lime/dolomitic lime with low residual CO2 and no hydration are required.

Secondary steelmaking

Secondary steelmaking is the name for a range of processes which can be used to modify the properties of the steel from the BOS and EAF just prior to casting. These include further purification, gas removal and temperature adjustment. In many of these processes another slag is created in the ladle or vessel. This may be simply lime or  may be a more complex mixture, often a proprietary formulation including lime, dolomitic lime, alumina, fluorspar, etc. depending on the job the slag is required to do. Components in synthetic slags for secondary steelmaking are normally finely divided to give good mixing, and to speed reaction times. Such slags are often added in bags, giving the flexibility to change the slag depending upon the requirement of the particular steel grade being manufactured. Dolomitic lime is increasingly used to help reduce attack on the refractory lining, particularly at the slag line.

Refractories

A further major use of lime products in the iron and steel industry is not as a consumable, but as a refractory material used to contain the molten metal. Within the BOS vessel, refractories are usually based on fairly pure magnesia because of its greater resilience. The same also applies to high wear areas in EAF and in ladles, particularly the slag lines. However in the main body of  ladles, AOD, and to a lesser extent EAF, dolomite bricks are often used, made by firing or resin bonding high density dolomite, fired to high temperature.

 Monolithic dolomite based refractories are also used to form the  hearth and banks in EAF, and to maintain them on a regular basis. The maintenance of these areas is often carried out using a high iron, sintered dolomite which is applied on a frequent basis between heats using a spinning machine. This is lowered into the furnace, and applies several tonnes of fettling material in a couple of minutes, between heats. This fast, low cost solution is used particularly in those parts of the world where dolomitic lime is not readily available, and refractory attack is consequently high. In these plants the dolomitic material is used partly as a sacrificial refractory, dissolving in the slag to increase MgO levels, and reduce further attack.
 
 
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