Ironmaking

There are two process routes for making steel in the UK today: through an Electric Arc Furnace and through the Basic Oxygen Steelmaking (BOS) process.

The BOS process is the major modern process for making bulk steels. Apart from special quality steels (such as stainless steel), all flat products in the UK, and long products over a certain size, are rolled from steel made by the BOS process.

The key component in the BOS is the Basic Oxygen Converter, however before this process can begin a blast furnace is required to create a charge of molten iron.

The Blast Furnace

The raw materials for producing molten iron are iron ore, coking coal and fluxes (materials that help the chemical process) - mainly limestone. The iron ore and coal used in the UK is imported (primarily from the USA, Canada, Brazil, Australia and Scandinavia), because the UK's resources of good quality coking coal and ore are limited and not economically viable.

Unloading iron ore at Redcar's deep water ore terminal

© Tata Steel

The coal and ore arrives by sea in very large ships and is off-loaded at deepwater harbours close to the four steelworks that use it. These steelworks are at Teesside and Scunthorpe on the North East coast, and Port Talbot in South Wales. The iron ores arrive in a number of forms: lumps of ore in the form in which they were mined; fine-sized iron ores; and pellets - fine ores which have been processed to stick together to form hard spheres of iron ore. The coals and ores are transported by conveyor belt or rail to stockyards where they are stored and carefully blended.

Blended coal is first heated in coke ovens to produce coke. This process is known as carbonisation. The gas produced during carbonisation is extracted and used for fuel elsewhere in the steelworks. Other by-products (such as tar and benzole) are also extracted for further refining and sale. Once carbonised, the coke is pushed out of the ovens and allowed to cool.

Fine-sized ore is first mixed with coke and fluxes and heated in a sinter plant. This is a continuous moving belt on which the coke is ignited. The high temperatures generated fuse the ore particles and fluxes together to form a porous clinker called sinter. The use of sinter in the blast furnace helps make the ironmaking process more efficient.

The inside of a blast furnace

© Tata Steel

Iron ore lumps and pellets, coke, sinter and possibly extra flux are carried to the top of the blast furnace on a conveyor or in skips and then tipped, or charged, into the furnace. Hot air (900 degrees C etc) is blasted into the bottom of the furnace through nozzles called tuyeres. The oxygen in the air combusts with the coke to form carbon monoxide gas, and this generates a great deal of heat. Frequently oil or coal is injected with the air, which enables less (relatively expensive) coke to be used. The carbon monoxide flows up through the blast furnace and removes oxygen from the iron ores on their way down, thereby leaving iron. The heat in the furnace melts the iron, and the resulting liquid iron (or hot metal as it is called in the industry) is tapped at regular intervals by opening a hole in the bottom of the furnace and allowing it to flow out. The fluxes combine with the impurities in the coke and ore to form a molten slag, which floats on the iron and is also removed (tapped) at regular intervals.

The hot metal flows into torpedo ladles. These are specially constructed railway containers which transport iron, still in liquid form, to the steel furnace.

The process described above goes on continuously for ten years or more. (This is known as a campaign.) If the furnace were allowed to cool, damage could be caused to its lining of refractory bricks as a result of their contracting as they cooled. Eventually the refractory brick linings are worn away, and at that stage the process is stopped and the furnace relined with new bricks, ready to begin its next campaign.
The iron produced by the blast furnace has a carbon content of 4 to 4.5% as well as a number of other "impurities". This makes it relatively brittle. Steelmaking refines iron, amongst other things by reducing its carbon content, to make it a stronger and more manipulable product.

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