1. Basic oxygen furnace
The most commonly applied process for steel-making is the integrated steel-making process via the Blast Furnace – Basic Oxygen Furnace. In the basic oxygen furnace, the iron is combined with varying amounts of steel scrap (less than 30%) and small amounts of flux. A lance is introduced in the vessel and blows 99% pure oxygen causing a temperature rise to 1700°C. The scrap melts, impurities are oxidised, and the carbon content is reduced by 90%, resulting in liquid steel.
Other processes can follow – secondary steel-making processes – where the properties of steel are determined by the addition of other elements, such as boron, chromium and molybdenum, amongst others, ensuring the exact specification can be met. Optimal operation of the blast furnace demands the highest quality of raw materials – the carbon content of coke therefore plays a crucial role in terms of its effect in the furnace and on the hot metal quality. A blast furnace fed with high quality coke requires less coke input, results in higher quality hot metal and better productivity.
Around 0.6 tonnes (600 kg) of coke produces 1 tonne (1000 kg) of steel, which means that around 770 kg of coal are used to produce 1 tonne of steel through this production route. Basic Oxygen Furnaces currently produce about 74% of the world’s steel. A further 25% of steel is produced in Electric Arc Furnaces.
2. Electric arc furnaces
The Electric arc furnace process, or mini-mill, does not involve iron-making. It reuses existing steel, avoiding the need for raw materials and their processing. The furnace is charged with steel scrap, it can also include some direct reduced iron (DRI) or pig iron for chemical balance. The EAF operates on the basis of an electrical charge between two electrodes providing the heat for the process. The power is supplied through the electrodes placed in the furnace, which produce an arc of electricity through the scrap steel (around 35 million watts), which raises the temperature to 1600˚C, melting the scrap. Any impurities may be removed through the use of fluxes and draining off slag through the taphole.
Electric arc furnaces do not use coal as a raw material, but many are reliant on the electricity generated by coal-fired power plant elsewhere in the grid. Around 150 kg of coal are used to produce 1 tonne of steel in electric arc furnaces.
Other steel production methods
Pulverised coal injection
Pulverised Coal Injection (PCI) technology involves injecting coal directly into the blast furnace to provide the carbon for iron-making – displacing some of the coke required for the process. A wider range of coals can be used in PCI, including steam coal which has a lower carbon content than coking coal. This method has a number of advantages, including reducing overall costs and prolonging the life of existing coke batteries.
Steel is 100% recyclable. The BOF process uses up to 30% recycled steel (scrap) and around 90-100% is used in EAF production.