Electric Arc Furnaces in Steelmaking

Michael GaribaldiRose-Lyne McCall

Many applications of steel

Piping

Construction

Automobiles

Steelmaking Processes

Steel

Siemens

Process

Bessemer

Process

Basic Oxygen Furnace

Electric Arc

Furnace

Crucible

Process

Main ways to produce steel

Basic Oxygen Furnace Electric Arc Furnace

Electric Arc Furnace Products

Key parameters for steelmaking

Composition

TemperatureTime

EAF Mechanical Systems

• An EAF has three primary functions:1. Containment of steel scrap2. Heating and melting of steel scrap3. Transfer of molten steel to the next

processing stage

• Achieved through 3 different systems

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

System Overview

Off-Gas Direct Evacuation System

• Proper ventilation in the furnace is important for:

- Pollution control- Limitations of excessive emissions- Limitations of dust build-up

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 utility2. Secondary system: steps down voltage from

primary and supplies power to EAF

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

Focus on Electrode Components

• Electrode heads / Contact Pads:o Copper plates usuallyo Final connection between power supply and

graphite electrodeo Must withstand extreme mechanical and

thermal conditions

Major Components of an EAF

Arc

The Furnace ShellHearth 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

The Carbon Electrodes• Deliver power to furnace

and form electrical arc• Graphite electrodes used

in modern steelmakingo High thermal capacity

• Position: at apexes of equilateral triangle

• Electrode spacing is crucial

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)

EAF: Process Overview

• Furnace Charging

• Melting• Refining• De-

slagging• Tapping• Furnace

turn-around

Furnace Charging1. Selection of steel grade2. Preparation of the charge bucket to ensure good

melting conditions3. 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

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

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

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

Tapping• Once desired steel temperature and composition

are achieved, tap-hole is opened and furnace is tilted

• Steel pours into ladle for transfer

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

Electric Arc Furnaces• High electricity

demand• Uses only recycled

metal

Basic Oxygen Furnaces

• Uses recycled steel as well as new molten iron

• Depends on the blast furnace operation step before it

Electrolysis for steelmaking

Pros & ConsMethod 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 emissionsLocation 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 producedWould not be able to produce large quantities of steel

Requires less energy     

Environmental Concerns

• High sound levels• Dust collection• Slag production• Cooling water demand• Heavy truck traffic for scrap and materials• Effects of electricity generation

Future of EAF• Sustainability • Possible replacement by newer

technologies• Future of steel, stainless steel and alloys

Future of EAF• Not a “green” technology – carbon footprint is

very large due to off-gas

Future of EAF• Stainless steel allows for stronger construction

than almost any other material available• Alternative materials such as carbon nanotubes in

development

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.