8. EAF Mechanical Systems
• An EAF has three primary functions:
1. Containment of steel scrap
2. Heating and melting of steel scrap
3. Transfer of molten steel to the next processing
stage
• Achieved through 3 different systems
9. Mechanical systems of an
EAF
Hydraulic System Cooling water system
• Provides power
for EAF
movements:
electrode
movement
• Ensures good
movements of
EAF
components
Lubrication system
• Provides cooling
of various EAF
elements: shell
cooling
11. Off-Gas Direct Evacuation
System
• Proper ventilation in the furnace is important for:
- Pollution control
- Limitations of excessive emissions
- Limitations of dust build-up
12. Electrical Systems of an
EAF
• Large amount of current supplied to sustain an
electrical arc
• EAF is typically composed of two electrical
systems:
1. Primary system: supplies power from electrical utility
2. Secondary system: steps down voltage from
primary and supplies power to EAF
13. Electrical Systems of an
EAF
Primary System Secondary System
• Vacuum switch
• Motorized Disconnect
Switch
• EAF Transformer
• Tap Changer
• Delta Closure
• Furnace Power Cables
• Bus Bar / Current
Conducting Arm
• Electrode Heads /
Contact Pads
• Electrode Regulation
14. Focus on Electrode
Components
• Electrode heads / Contact Pads:
o Copper plates usually
o Final connection between power supply and graphite
electrode
o Must withstand extreme mechanical and thermal
conditions
16. The Furnace Shell
Hearth Roof
• Contains metal
and slag
• Back lining –
magnesite
bricks
• Working lining
– dolomite or
magnesite
mass
• Exposed to the
most radiant heat
• Roof lining:
alumina,
magnesite bricks
• Water cooled
Sidewalls
• Withstand thermal
shock and
corrosive nature
of slag
• Hot spots on walls
due to arc
radiation
• Same lining as
roof
17. The Carbon Electrodes
• Deliver power to furnace
and form electrical arc
• Graphite electrodes used
in modern steelmaking
o High thermal capacity
• Position: at apexes of
equilateral triangle
• Electrode spacing is crucial
18. The Arc
• Arc discharge between electrodes and furnace
charge
• Arc is plasma of hot ionized gases (thermal
plasma)
• Temperature about 6000°F
• AC current (converted from DC)
19. EAF: Process Overview
• Furnace
Charging
• Melting
• Refining
• De-
slagging
• Tapping
• Furnace
turn-
around
20. Furnace Charging
1. Selection of steel grade
2. Preparation of the charge bucket to ensure good
melting conditions
3. Scrap must be layered according to size and density to
facilitate melting and to protect roof and sidewalls from
electrical discharge
4. Preventative measures to prevent caving-in of material
– this could break electrodes
5. Addition of lime and carbon
21. Melting
• Both electrical and chemical energy supplied to
EAF
• Charge begins at intermediate voltage
• Electrodes bore into the scrap
• High voltage, long arc formed
o Long arc unstable initially: current swings, rapid
vibration of electrodes
o As more steel melts, arc stabilizes and power input is
increased further
22. Refining
• Removal of impurities and undesired components
that effect quality, i.e. P, S, Al, Si...
• Oxygen blown into bath simultaneously with
melting, allowing for refining and melting to be
performed in a side-by-side operation
• All undesired products leave in slag
23. De-slagging
• Furnace is tilted and slag door is opened
• Slag is poured off top of bath
• Slag door is located higher than bath level
24. Tapping
• Once desired steel temperature and composition are
achieved, tap-hole is opened and furnace is tilted
• Steel pours into ladle for transfer
25. Furnace Heat Balance
• 300 kWh/ton minimum required for melting of
steel scrap
• Melting point at 2768°F
• Total theoretical energy requirement: 350 – 370
kWh/ton
• Energy distribution depends mostly on material
being melted
29. Pros & Cons
Method Advantages Disadvantages
Electric Arc Furnace
Uses 100% recycled metal Large capital cost
Flexibility of the process Large amounts of sludge produced
Can use various raw materials Dust and GHG emissions
Location near steel product markets Requires lots of electricity
Basic Oxygen Furnace
Self-sufficient Dependence on blast furnace material
High production rates Emission of contaminants and GHG
Electrolysis
No GHG emissions Still at the testing phase
Purer steel produced
Would not be able to produce large
quantities of steel
Requires less energy
30. Environmental Concerns
• High sound levels
• Dust collection
• Slag production
• Cooling water demand
• Heavy truck traffic for scrap and materials
• Effects of electricity generation
31. Future of EAF
• Sustainability
• Possible replacement by newer technologies
• Future of steel, stainless steel and alloys
32. Future of EAF
• Not a “green” technology – carbon footprint is very large
due to off-gas
33. Future of EAF
• Stainless steel allows for stronger construction than
almost any other material available
• Alternative materials such as carbon nanotubes in
development
34. References
• Jones, Jeremy, A.T.. "Electric Arc Furnace Steelmaking."
Mannesmann Demag Corp. Web. 22 Nov. 2013.
• "Parts of the Electric Arc Furnace." Electric Arc Furnace.
N.p., n.d. Web. 23 Nov. 2013.
http://www.postech.ac.kr/mse/cml/Eng/eaf.htm.
Editor's Notes
There are three primary functions of an electric arc furnace. Steel scrap enters the furnace for processing. Therefore there must be a containment mechanism, which is usually accomplished by a hearth. The second function of an electric arc furnace is the melting of the steel scrap which is achieved through high heating. And lastly, the molten steel and by-products must be removed from the furnace so that the next step in processing may take place.
How are these functions accomplished? There are three main mechanical functions of an EAF that drive the flow of material. First is the hydraulic system, which is responsible for the moving parts of the furnace. This includes the arm that swings the electrode into place and the lift which tips the furnace from side to side for pouring. Next is the heat exchange system, which is driven by temperature gradients between the metals and cooling water. And third is the lubrication system, which ensures that all of the moving parts do not jam or hold up the process.
This is a general overview of the EAF system. Shown here is an schematic of the furnace with the important parts labeled for basic understanding. The design is simple, with only one inlet for steel scrap – and sometimes a second inlet for oxygen – and two outlets: one for steel and one for slag. Steel, as a basic overview, is a high-iron content metal which is used as a strong and resistant material for various construction purposes. Slag is composed of many metal oxides which are created in the melting of steel scrap. Steel is the desired product and slag is an undesired by-product of the steelmaking process. The removal of molten steel from the electric arc furnace is known as tapping. The slag therefore leaves out the slag door when the furnace is tilted in one direction, while the steel leaves through a taphole when the furnace is tilted in the other direction. For a typical EAF operation, tap-to-tap time is between 30 to 40 minutes.
In addition to there being both a molten product and a molten undesired product, there is a large amount of carbonaceous gas produced by the high heating of steel scrap. This gas is mostly composed of carbon monoxide and carbon dioxide – two very dangerous gases if there is an inadequate evacuation system. Also of relative importance is the presence of dust on the steel scrap, which ignites when heated around 1600 degress Celcius and leads to eruptions within the furnace. The removal of this dust is therefore also key to avoid any dangers associated with the release of this energy.
This all creates the need for a vacuum pump to remove these dangerous gases and particulates.
In addition to the mechanical systems of an EAF, there are also two vital electrical systems. The first is the primary electrical system. The primary electrical system is the furnaces indirect connection to the power grid. Steelmaking plants are a primary consumer of electrical energy because of the high demand for it in the EAF process. This electrical energy is delivered to the primary system which in turn connects with the secondary electrical system.
The primary electrical system is composed primarily of a step-down transformer. This is because the power coming in to the primary system is at too high a voltage. For the electrical arc to be sustained within the furnace, a very high current is needed instead.
The primary system then delivers the high current electrical energy to the secondary system, which makes the direct contact with the electrodes, which supply the power to melt the steel.
The advantage to having two electrical systems is safety. In the case of emergency, the electrical current can be cut off by two independent regulatory systems. This helps in preventing damage to the electric arc furnace and the electrodes cause by rapid fluctuations in current and high heating.
The electrodes must supply enough current to the steel scrap to raise the temperature inside the furnace to 1600 C. They must therefore be able to withstand extreme mechanical and thermal conditions. In addition to the heat, there is a large degree of mechanical stress on the electrodes due to rapid current fluctuations and the boring of the electrodes into solid and molten steel. In order to sustain a current, the connection between the electrode and the power supply must be able to withstand these conditions as well. The electrodes are connected to the power supply through copper plates known as contact pads, and couplings known as heads.
The pads must be able to withstand a current of 44000 amperes. They must also maintain a connection in spite of the vibrations and oscillations of the electrodes. The heads must not be conducting. They too must be able to withstand high heating and the rapid vibrations of the electrodes to hold them in place.
In order to sustain a stable arc, the pads and heads must be fully functioning. This is assisted by the use of cooling water, between 2 and 40 gallons per minute, to remove heat which may negatively affect the mechanical properties of the heads and pads. Also of assistance is the impedance control network which regulates the level of the electrodes in the steel bath. If the electrodes are lowered too quickly into the steel bath, the current increases. This subsequently causes a drop in voltage. This drop is measured by the impedance regulator, which is programmed to have a certain set point for the ratio between current and voltage. In order to continue the status quo, the electrode will consequently raise to reduce the current. This results in automatic control over the electrode positioning, which minimizes vibrations in the electrode apparatus and helps to prevent damage to the pads and heads.
Electrode apparatus weighs upwards of 20 tons
Movement due to hydraulic system
Constant motion due to changing scrap/bath level
Necessity for automatic control over electrode position
Influences several characteristic aspects of performance:
Energy input
Mean current
Arc stability
Melting pattern
Energy losses to cooling system
Electrode and sidewall (refractory) consumption
Standard control is “impedance control”
Holding ratio of voltage to electrical current constant
Voltage signal and current signal DC values compared to set point (chosen by steelmaker)
If current exceeds this set point, its signal increases and voltage signal subsequently decreases
Causes imbalance in ratio and an output voltage commands electrode arm to raise in order to reduce current
Next we will focus on the major components of the electric arc furnace. The electric arc furnace is a fairly complicated physical system as it must be built to withstand intense heating. Heating at the levels found inside and EAF often leads to rapid wear and corrosion of the parts exposed.
The largest component of the system is the furnace shell, which contains the steel scrap and molten steel. The second is the steel, which may be of various compositions depending on the temperature of the system. Next are the graphite electrodes, which, for most modern steelmaking processes are intended for alternating current and come in sets of three. Lastly is the arc itself, which is the plasma involved in this system. The arc is responsible for making the connection between the electrodes and the steel bath and directly induces heating in the steel.
The furnace shell comes in three parts. First is the hearth, where the steel scrap is loaded into the EAF and which contains the molten steel. The hearth is made from bricks capable of withstanding the high temperatures reached inside the EAF. Next is the roof, through which the three electrodes are lowered into the steel bath. The roof is also composed of bricks with high heat capacity. It is typically water-cooled by an internally flowing water system. Cooling of the roof is necessary as it is exposed to large amounts of radiant heating from the molten steel and the electrodes. Lastly are the sidewalls, which contain the hearth. Like the hearth, the sidewalls must be durable and heat resistant. This includes resistance to non-uniform heating – as various cold and hot spots will form on the sidewalls as a result of arc radiation. The sidewalls are lined with similar material to the hearth and roof.
Electrodes implicated in the electric arc furnace are always made from carbon. Most, however, use graphite as a conducting material, as opposed to amorphous carbon. The electrodes are made from coke and pitch blended together. Coke provides the main source of carbon while pitch behaves as a binding agent and is also responsible for increasing strength and resistivity. Graphite is the crystallized form of amorphous carbon and is created through heat treating the amorphous carbon rods. The temperature of the rods must be raise to 5000 degrees Fahrenheit or 2760 degrees Celcius to accomplish full crystallization.
As previously mentioned, AC systems utilize three electrodes located at the apexes of an equilateral triangle. The spacing between these electrodes is crucial as it will determine how the steel is heated and how much heat is lost to the sidewalls. Too close a spacing results in heating gradients which are undesirable. Too far a spacing and the arc will damage the sidewalls. Typical spacing for a mid-size electric arc furnace is 1900 mm or about 75 inches. Electrodes are generally between 15 and 30 inches in diameter. They are composed of multiple graphite rods which are fitted together with threaded couplings to allow for easy replacement, which is frequent due to the rapid corrosion of the graphite.
For one ton of steel, it is typical to see about 12-14 pounds of graphite being removed from the electrode. Newer systems only lose about 3.5 to 4.5 pounds per ton of steel. These losses are due to mechanical breakage from steel scrap cave-ins, oxidation of the graphite and the dissolution of graphite into the boiling steel and slag.
The arc is a thermal plasma, meaning the ions and electrons present in the plasma will be approximately the same temperature. The current used in EAF processes for steelmaking generally reaches up to 44000 amperes and 400 to 900 volts. The arc is transferred arc, meaning that it connects directly with the material which provides more efficient heating. The main process by which the arc is generated and maintained is thermal field emission. In thermal field emission, the electrons are supplied by the conducting material, in this case: the graphite electrode and the steel. The movement of these electrons is driven by a very high electric field. This electric field is produced by the immense current flowing through the graphite electrodes.
There are six steps in the EAF melting process. First, is the charging of the furnace, or the loading of steel scrap material into the hearth. Second, is the ignition of the arc and the melting of the charge. Next, it might be desirable to add petroleum coke and oxygen to the system to increase the reduction of iron species such as iron oxides and to form new oxides which are removed in the slag in the following step. After the slag is poured out of the furnace, the molten steel is then tapped and poured into ladles which transport the steel to the next stage, which is either casting or the removal of carbon in the case of stainless steel production. Once the molten steel has been removed, the furnace undergoes a brief inspection to assure that there is no damage that may interfere with the heating of the next charge.
In charging the furnace, the roof and electrodes are first raised and swing to the side to allow the bucket of scrap to dump the charge into the furnace. The charge is sometimes supplemented with lime and additional carbon to increase the recovery of iron from its oxides.
The roof and electrodes are then lowered. Once the electrode tips are in close enough proximity to the steel, the arc forms and melting begins.
The furnace may be charged several times in one heat. Once one batch of steel has melted, another may be added directly to the molten steel. Each time the furnace is charged, there is an associated dead-time between loading periods.
There is also an energy loss caused by charging, as energy is lost each time that roof is opened, on the order of 10-20 kW/ton of steel.
Energy loss is minimized by the addition of 2 or 3 scrap loads per heating cycle.
Once the roof and electrodes have been lowered into position, the transfer of electrical energy begins. The voltage is started at an intermediate level that is maintained until the electrodes have bored into the top layer of scrap. This top layer is generally light scrap that is easily melted in order to expedite boring. About 15% of the scrap is melted in this period. After the electrodes have melted the first layer, the transformer is switched to one that supplies a high voltage, about 800 V. The initial arc is very unstable, as the current changes rapidly due to uneven heating.
As more steel melts, the top of the molten steel charge forms a protective barrier for the roof. Arc stability also increases and power input is further increased. In addition to electrical energy, chemical energy is often added to the furnace to increase melting and to facilitate the melting of large pieces of scrap. Oxygen fuels are typical chemical additives which help speed up melting.
Close attention must be paid to the temperature inside the furnace at this point. Once all of the steel has been melted, the sidewalls begin to absorb heat very quickly, which is damaging to the furnace shell. Therefore, once the charge has been melted completely, which is monitored through a temperature sampling system, the voltage is immediately decreased in the electrodes.
All impurities that reduce the grade of steel are removed in the slag phase. Oxygen aids in the separation of iron from its oxides.
De-slagging is performed before tapping. In de-slagging the top layer of liquid material is poured out from the side of the furnace.
With most of the slag removed, the furnace is then tilted in the opposite direction and the taphole opened. The molten steel then flows from the bottom of the furnace into a ladle and is sent on for further processing.
In theory, to reach the melting point of steel, 300 kWh/ton of energy must be supplied to the electrodes. The electrodes however have only 55 to 65% efficiency: meaning that the final input energy totals around 560 to 680 kWh/ton.
The operation of an electric arc furnace has several environmental considerations. It uses a massive amount of energy to melt the steel and is only economical where there is plentiful electricity and a well-developed grid.
Electric arc furnaces are advantageous to induction furnaces due to their ability to reach very high temperatures within the steel bath. There are, however, newer technologies such as electrolysis and plasma furnaces that may replace the EAF in the long term. Also, as greener and more energy conscious machines are developed, the electric arc furnace may change to meet new standards for electrical consumption.
Iron and steelmaking produces enough carbon dioxide to make it onto the list of the top causes for pollution and the generation of greenhouse gases that provoke global warming scares. The off-gases created by the EAF are allowed to escape into the atmosphere, typically untreated, where they may cause acid rain or the gradual heating of the Earth. For this reason, there is a need to reform the technology to reduce the danger it poses to the climate.
Steel is important to many industries and trades, however and there will always be a need for it or similarly durable and corrosion-resistant materials. Hence the EAF does not have any immediate threat of being replaced.
