Metal Production- EAF Steelmaking Flashcards
BOS vs EAF steelmaking for external heating, size, furnace conditions, charge
Ext heat: none in BOS so no lengthy refining. Is in EAF so extended refining possible.
Size: BOS 350 tons (too large for special steels), EAF 150 tons (more suited to special grades).
Furnace conditions: BOS is oxidising, EAF is oxidising or reducing.
Charge: inflexible for BOS as all heat supplied by refining reactions, very flexible for EAF.
Describe the arrangement of an electric arc furnace
Looks like a big teapot. has refractory lined steel shell with walls and a roof. Roof is detachable.Charge of metal scrap at the bottom. Coming out of left side is tapping spout. 3 carbon electrodes in from the roof connected to power supply (500V, 100,000A for 100 tonne furnace). Inspection door on the right. Whole furnace can be tilted to allow ease of hot metal and slag removal.
Capacity of EAF
Capable of melting put to 175 tonnes of cold steel in about 90mins. This is longer time for less steel than oxygen steelmaking
What does normal charge for the EAF consist of?
Mild steel scrap
Alloy scrap if needed
Lime and flux (CaF2) to make basic slag
Fuel if necessary, usually oxygen for refining
Coke to supply carbon
How is scrap chosen?
So that there is a minimum of compositional adjustment needed during the steelmaking. E.g ultra low carbon scrap used for production of ULC steel. Consists of purchased and works arising scrap.
Why is carbon added for the charge?
Usually in the form of coke or in some cases pig iron. Added to give a deliberate excess of carbon so that a carbon boil can occur.
Why is oxygen needed in the charge?
So that carbon can be removed and also that refining of impurities can take place
When are P and S removed?
As is the case for BOS conditions for P removal are opposite to those favourable for S removal. P removal carried out in the early heat (“hot heel”). High basicity of slag aids P removal. S can be removed as sulfides to the slag but normally occurs during tapping or in secondary steelmaking.
When are Mn and Si removed?
Mn and Si combine with oxygen and are removed to the slag?
Advantages of an excess of carbon being added
Produces a carbon boil. C reacting with O can produce up to 40% of the required heat. CO produced causes a bubbling effect known as the carbon boil and causes slag foaming. Buries the arc improving thermal efficiency and prevents nitrogen exposure to the arc. This effect also removes any nitrogen and hydrogen gases.
Standard operation steps from beginning to start of refining
Inspection of furnace to make sure refractory lining is complete.
Roof of furnace and electrodes are removed and the charge is delivered to the furnace via a large basket.
Roof is replaced, electrodes connected to power supply and arc is produced.
Electrodes bore down into the charge and melting begins.
As melting proceeds volume of scrap decreases and more charges of scrap can be introduced into furnace.
Refining can start to begin (before melting is complete).
Standard operation steps from start of refining to tapping
When scrap has completely melted most of Si should have been removed and transfer of P, Mn and S to slag should be underway.
Carbon boil should now be proceeding.
This homogenises temperature, helps the transfer of impurities to slag and sweeps out much of the H2 and N2 gases.
When required carbon level reached oxygen is cut and deoxidisers like Fe/Si and Fe/Mn added to remove excess oxygen in metal and slag.
Furnace is tapped.
Why might alternative sources of iron to scrap be necessary?
No adequate sources of scrap or scrap too expensive.
Levels of impurities like Cu, Ni, Sn, Zn, etc (tramp elements) in scrap are too high (these are very difficult to remove)
How does direct reduced iron work?
It is the most high profile alternative source of iron. Ore is reduced in the solid state (lumps, pellets or powder) to metallic iron. Done at temperature between 700 and 1100C to avoid welding. Exact energy requirements depend on the reductant and the process. Reductant May be CO, H2, C. Reactor may be static bed, fluidised bed, rotary kiln, etc
What is MIDREX process and how does it work?
Ironmaking process for the production of direct reduced iron. Iron oxide in pellet or lump form is introduced through a hopper at the top of the shaft furnace. As the ore descends through the furnace by gravity flow, it is heated and the oxygen is removed from the iron (reduced) by counterflowing gases which have a high H2 and CO content. These gases react with the Fe2O3 in the iron ore and convert it to metallic iron leaving H2O and CO2.
