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Technical

Iron & steel

Benefits of using lime in iron and steelmaking

Lime is beneficial in iron and steelmaking as it:

  • Is a proven technology
  • Is abundantly available in the UK
  • Acts as a slag forming component to remove impurities
  • Controls the melt chemistry to reduce the damage to furnace linings
  • Helps create a by-product slag that is useful in the construction industry

Introduction to the use of lime in iron and steelmaking

Iron making

Lime is used:

  • As a binder in the production of sinter
  • As a desulfurising agent
Steelmaking

Main functions of lime in the steel industry are:

  • As a slag forming component to remove impurities – using quicklime
  • To control melt chemistry so as to reduce the damage to furnace linings – using dolomitic lime

Iron making

Sintering

Iron ores are usually purchased as fines or as pellets consisting mostly of iron oxide. Iron ore pellets are fed directly into the blast-furnace. Iron ore fines are mixed at the ironworks with limestone and other minerals and pre-fired at low temperatures to form a sinter, i.e. agglomerates that can then be fed to the blast-furnace. Finely divided quicklime is often added to the sinter mixture to assist in binding the material to improve productivity and give a stronger sinter structure.

Use in the blast-furnace

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 iron making process involves the removal of these impurities, usually by forming a slag with quicklime.

Quicklime (CaO) is formed during iron making from added limestone (CaCO3). The heat in the blast-furnace decomposes the limestone to quicklime which then reacts with the impurities and produces the slag. Blast-furnace slag is a useful by-product used widely in the construction industry.

Removing sulfur impurities

Iron from the blast-furnace often contains excessive levels of sulfur. As desulfurisation in the steelmaking process is not compatible with other purification processes, it is often carried out externally to the rest of the process. A simple treatment involves injection of fine quicklime and other minerals into the transfer ladle which moves iron from the blast-furnace to the steelmaking vessel. The slag, containing much of the sulfur from the iron, is removed before the hot metal is conveyed to the steelmaking vessel.

Steelmaking

Steelmaking is the process of further removing impurities and producing a material with carefully controlled levels of carbon, manganese, nickel, chromium etc..

Basic oxygen steelmaking

The major production route for steel is the conversion of molten iron from the blast-furnace into steel using Basic Oxygen Steelmaking (BOS).

In BOS, a vessel, lined with magnesia-based refractories, is charged with molten iron, often with the addition of scrap, iron ore, or waste iron oxide briquettes as supplementary iron and steel sources. Carbon is removed from the melt by injecting oxygen at high speed, which oxidises some of the iron and many of the impurities present (e.g. silica and phosphorous).

Quicklime is added to react with the oxidised impurities and iron oxide to form a slag. Typically quicklime additions of 35-50 kg per tonne of liquid steel are used.

The addition of dolomitic lime reduces the tendency of the slag to dissolve the magnesia refractory lining of the vessel.

Electric arc furnace steelmaking

Electric arc furnace (EAF) steelmaking mainly involves the remelting of scrap steel with less refining. Modern electric arc furnaces use gas and oxygen burners as well as electric power to speed-up production. Quicklime and dolomitic lime are added to the EAF to form a slag and to protect the refractories in a similar way as they do in BOS.

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, the addition of quicklime is an essential component.

Dolomitic lime products are increasingly used to help reduce attack on the refractory lining.

Refractories

A further major use of dolomitic lime products in the iron and steel industry is as a refractory material and a refractory repair material. Dolomitic lime products are used as components in refractory materials and as an ingredient in the melt to reduce the damage to the refractory lining during the steelmaking process.

Safe handling

The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.

However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.

More information

Information on steel making in the UK is available from:

 
 
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