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Crude steel production in the electric arc furnace
Steels based on scrap are melted in electric arc furnaces. The electric arc converts the electrical energy into heat for melting very efficiently and with a high energy density. Apart from scrap, charges can also consist of sponge iron (DRI or HBI) and/or liquid or solid pig iron. Basically, any type of steel can be produced via the electric arc furnace route.
Graphite electrodes conduct the electricity and generate the electric arc.
The main components of the electric arc furnace are the furnace vessel with the tapping bay and work access opening, the removable cover with the graphite electrodes, and the tilting system. The furnace vessel has a refractory lining. The cover is lifted and swivelled to the side to allow filling of the furnace. The scrap is transported over the furnace in scrap baskets that are emptied into the furnace. Then the cover is replaced, the electrodes are lowered and the electric arc ignited on the cold scrap. During the melting process, temperatures of up to 3500°C are created in the arc and up to 1800°C in the steel melt. Additional injection of oxygen or another fuel-gas mixture accelerates the melting process. The furnace is tilted to empty the contents into the steel ladle once the desired chemical composition and steel temperature has been achieved.
We differentiate between DC and AC electric arc furnaces. In general terms, the AC furnace is considerably more efficient than the DC furnace. The latter offers advantages regarding lower circuit feedback, lower noise levels and lower electrode burn-off. The disadvantages lie in the need for operation with a “hot heel” and the more complicated maintenance of the electrodes.
In terms of size and performance, an enormous range of furnaces are used. The smallest electric arc furnaces have tapping weights of 5 tonnes of crude steel and are operated in the foundry industry. The world’s largest electric arc furnaces in the steel industry have tapping weights of 300 tonnes with annual production of 2.6 to 2.8 million tonnes of crude steel. This corresponds to the production capacity of medium-sized basic oxygen steelmaking plants. Depending on the size and configuration of the furnace, energy consumptions of 350 kilowatt-hours per tonne of crude steel with tap-to-tap times of 40 minutes and electrode consumption of 1.1 kg per tonne of crude steel can be achieved nowadays.
Secondary metallurgy: fine-tuning the steel
The high demands made of modern steels necessitate a subsequent treatment of the melt made available in the steel ladle. This applies for both the oxygen converter and the electric arc furnace routes. The main aims of this secondary metallurgical treatment are to adjust the chemical composition of the finished steel, as well as its casting temperature and purity level. Whereby it is necessary to differentiate between the following tasks:
- homogenisation of the melt,
- deep decarbonisation,
- deoxidisation, and
- removal or conditioning of non-metallic inclusions.
Like the range of materials, the metallurgical tasks for the basic oxygen steelmaking plant and the electric steelworks are similarly varied – with comparably complex secondary metallurgical treatments.
Secondary metallurgical process routes: potential treatments
Improved steel qualities with increased flexibility of the metallurgical plants are general trends in secondary metallurgical treatment. For this reason, the further improvement of steelworks logistics remains a permanent task. The number of secondary metallurgical aggregates and plants in the steelworks continues to rise. At the same time, the functionality of the aggregates is increasing – in order to be able to meet the demands for new qualities and works flexibility.