Lecture 26

Lecture 26: Electric Arc Furnace (EAF) Steelmaking

1. Introduction and Global Context

• Global Production Share: EAF steelmaking accounts for 30% to 35% of total global steel production.

• Industrial Applications:

  • Foundries: Used purely as a remelting unit to process liquid metal without refining.

  • Steel Plants: Used for full steelmaking (melting and refining). This lecture focuses exclusively on EAF as a steelmaking unit.

Important Remarks / Instructor Notes

The thermodynamics of steelmaking (e.g., $1600^{\circ }\text{C}$, $\sim 1\text{ atm}$ pressure, chemical reactions between carbon and oxygen) remain identical whether using a Basic Oxygen Furnace (BOF) or an EAF. However, EAF technology differs substantially in terms of vessel geometry, maneuverability, production capacities, and processing rates.

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2. Raw Materials & Hybrid Steelmaking

Feedstock (Board Work & Audio): Scrap + DRI ; HM

• Scrap: The traditional, principal raw material. If high-quality, basic scrap is carefully sorted and selected, little to no refining is required.

• Direct Reduced Iron (DRI) / Sponge Iron:

  • Gas-based DRI: Tends to have a higher degree of metallization.

  • Coal-based DRI: Impregnates carbon into the iron. Adding this to the EAF introduces carbon, which aids in forming “foamy slag.” Since DRI contains gangue (alumina, silica), it inherently increases the slag volume.

• Hot Metal (HM): Modern EAFs increasingly use hot metal.

  • Physical Interpretation: Hot metal contains carbon and silicon. When these elements oxidize, they liberate substantial chemical heat, offsetting electrical energy costs and drastically reducing refining times. EAF traditionally suffered from long refining periods; HM addition makes EAF competitive with BOF.

Hybrid Steelmaking Route

Today, market prices for raw materials (scrap, iron ore) and electricity constantly fluctuate. Modern plants (like ArcelorMittal Nippon Steel in Hazira) employ Hybrid Steelmaking, integrating Midrex (DRI), EAF, BOF, and Blast Furnaces in the same premise. This allows operators to dynamically optimize material flow based on cost.

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3. Mini Mills vs. Integrated Steel Plants

EAFs are the heart of “Mini Mills” or special steel plants.

Board Work: Mini Mill Scale and Capacities

• 1 MTPA : Maximum productivity of a mini mill (1 Million Tons Per Annum).

• 30 : Approximately 30 such plants operate in India.

• ~10 MTPA : Collective production from these Indian mini mills.

• 25 - 60 T : Typical heat capacity (furnace size) is 25 to 60 Tons.

Conceptual Explanation: Why small capacities? Unlike massive 300-ton BOF converters making commodity carbon steel, EAFs often produce high-value, specialized alloy steels (e.g., ball-bearing steel, free-cutting steel, tool steel). Customer demand for these highly specialized grades is much smaller in volume.

EAF vs. BOF Energy Comparison

• BOF Route: Highly energy-intensive globally ($\sim 16.2\text{ GJ/tonne}$) because breaking iron-oxygen bonds in the blast furnace requires massive chemical energy, even though the BOF step itself is autogenous (self-heating).

• EAF Route: Lower overall energy footprint ($\sim 7.6\text{ GJ/tonne}$) but relies heavily on external electrical energy since scrap/DRI lacks the inherent chemical heat of hot metal.

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4. Construction Features and Operations

Vessel Geometry & Components

• Shape: Unlike the pear-shaped BOF converter, the EAF is a shallow, saucer-like vessel.

• Hearth: The bottom part holding the liquid metal. Lined with basic refractories (primarily Magnesite) to withstand the corrosive basic slag.

• Roof: A removable lid mounted on an automated swinging arm to allow top-charging of scrap. Because it faces intense thermal radiation rather than direct slag corrosion, it is lined differently (typically Chromite and Magnesite).

  Refractory Lining (Crucial Exam Point): EAFs use Basic linings (acidic/silica linings would be eaten up by basic refining slags).

  • Hearth (Bottom): Lined with Magnesite (handles chemical corrosion from molten metal/slag).

  • Roof: Lined with Chromite and Magnesite (designed to handle severe thermal radiation).

• Electrodes: Made of graphite, inserted through the roof. The electric arc struck between the electrodes and the metal generates temperatures around $4000^{\circ }\text{C}$.

• Tapping: Uses an Eccentric Bottom Tapping (EBT) facility. Unlike BOF converters, which tilt 70°–75° to pour, the EAF only tilts marginally (8°–15°) to drain the steel through the bottom, preventing slag carryover.

Power Characteristics (Board Work)

• Melting Requirement: 400 - 500 kWh/tonne (Heat required simply to raise scrap from room temperature to melting point).

• Refining & Losses: 150 - 300 kWh/tonne (Additional power needed to refine the melt and compensate for furnace heat losses).

• Ultra-High Power (UHP) Furnaces: Advanced EAFs supply 900 to 1000 kW/tonne, rapidly accelerating melting times.

AC vs. DC Furnaces

• AC Furnaces: Most common. Uses 3 electrodes for the 3 electrical phases.

• DC Furnaces: Uses 1 top electrode (cathode) and a bottom electrode (anode). Current conducts directly through the charge, contributing both arc heating and Joule (resistance) heating. Less noisy and easier to operate, but more expensive to build.

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5. EAF Operational Challenges & Auxiliaries

1. Electrode Wear and Tear: Graphite electrodes are expensive and degrade rapidly due to:

   • Oxidation with atmospheric oxygen.

   • Mechanical wear.

   • Chemical erosion from Foamy Slag Practice. Carbon is injected into the FeO-rich slag, producing CO bubbles that foam the slag to cover the electrode tips. While this protects the furnace walls from arc radiation, the slag chemically eats the graphite.

2. Hydrogen Pickup: Because electrodes get red hot, they are water-cooled. Moisture inevitably enters the furnace atmosphere, leading to very high dissolved hydrogen in the steel (5 to 7 ppm, compared to BOF’s 1-2 ppm). Secondary steelmaking (degassing) is mandatory for critical EAF steel.

3. Bath Agitation: Arcs do not stir the molten bath efficiently. Historically, iron ore was used for stirring (taking up to 10 hours). Modern EAFs utilize Argon bottom-stirring (via porous plugs) and oxygen lancing to agitate the bath and oxidize metalloids.

4. Off-Gas Treatment: EAF off-gas exits at $600^{\circ }\text{C}-700^{\circ }\text{C}$. This sensible heat can be recycled to preheat scrap. However, because market scrap contains heavy and toxic metals, the off-gas must pass through a highly efficient Gas Cleaning Plant (GCP).

Acid lining cheaper Silica than magnesite Basic lining but Basic are used

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6. Scrap Sorting and Chemistry (Critical Step)

Because EAFs utilize mixed market scrap (stainless, machinable, galvanized), sorting is the most crucial preliminary step. Scraps are sorted into four metallurgical categories based on their behavior at $1600^{\circ }\text{C}$:

1. Completely Volatile Material:

   • Elements: Zinc (Zn), Cadmium (Cd), Calcium (Ca), Lead (Pb).

   • Behavior: These vaporize completely at steelmaking temperatures and exit via the off-gas. They require rigorous gas-cleaning protocols but do not contaminate the final liquid steel.

2. Completely Oxidizable Material:

   • Elements: Aluminum (Al), Silicon (Si), Titanium (Ti).

   • Behavior: These elements have a high affinity for oxygen (low on the Ellingham diagram). They will completely oxidize and enter the slag phase.

3. Partially Oxidizable Material:

   • Elements: Chromium (Cr), Phosphorus (P).

   • Behavior: These partition between the liquid metal and the basic slag depending on process parameters.

4. Non-Oxidizable (Noble) Material:

   • Elements: Copper (Cu), Nickel (Ni).

   • Behavior: These elements have very low oxygen affinity (top of the Ellingham diagram) and cannot be removed via oxidation.

   • Physical Interpretation: If you melt stainless steel scrap containing 8% Nickel to produce a plain carbon steel, the Nickel will remain entirely in the bath, permanently contaminating the heat. Scrap containing noble metals must be strictly isolated and only used when those specific alloys are targeted.

Audio V2

Electric Arc Furnace (EAF) Steelmaking: Comprehensive Lecture Notes

1. Global Context & Industry Overview

• Global Production: Electric Arc Furnace (EAF) steelmaking accounts for 30% to 35% of the total steel produced globally.

• Industrial Uses: * Foundries: Used as a pure remelting unit for liquid metal (no refining).

  • Steel Plants: Used for melting and refining steel (the focus of this lecture).

• Historical Context: Gained immense popularity after World War II due to the massive availability of scrap.

• Mini Mills: Special steel plants that rely exclusively on the EAF route (EAF $\to$ Secondary Steelmaking $\to$ Casting).

  • Capacity: In the Indian definition, maximum productivity is about 1 million tons per annum (MTPA).

  • Furnace Size: Typically 25 to 60 tons (much smaller than integrated mill converters which are 200–300 tons).

  • Product: Primarily produce specialized, high-value alloy steels (tool steel, free-cutting steel, ball-bearing steel), rather than plain carbon steel. The high market value of these steels makes mini-mills viable despite electricity costs.

  • Indian Context: India has about 30 such mini-mills producing around 10 million tons of sophisticated steel per year.

• Hybrid Steelmaking: Modern integrated plants (e.g., ArcelorMittal Nippon Steel in Hazira) use a mix of Blast Furnaces (BF), Basic Oxygen Furnaces (BOF), Midrex (DRI), and EAFs to cost-optimize production based on fluctuating raw material and electricity prices.

2. Process Thermodynamics vs. Kinetics

• Thermodynamics are Identical: The physical chemistry of EAF is identical to BOF steelmaking ($1600^{\circ }\text{C}$, $\sim 1\text{ atm}$ pressure). Carbon oxidation happens either by dissolved carbon reacting with dissolved oxygen or with iron oxide ($\text{FeO}$) in the slag.

• Kinetics & Technology are Different: EAF differs from BOF entirely in terms of vessel geometry, maneuverability, production capacities, refining rates, and the absence of a large emulsion phase.

3. Raw Materials & Charge Feed

The EAF is a non-autogenous process (requires external energy), unlike the autogenous BOF process.

• Scrap: The principal raw material. If well-sorted, basic scrap is used, little to no refining is needed.

• Direct Reduced Iron (DRI) / Sponge Iron:

  • Coal-based DRI: Impregnates carbon into the iron. Adding this helps form slag later in the process because of the existing carbon.

  • Gas-based DRI: Has a higher degree of metallization and is slightly more expensive.

  • Gangue in DRI: DRI contains unreduced oxides (alumina, silica) from the original ore. These go directly into the slag. Any $\text{FeO}$ in the DRI also goes to the slag, which lowers the dissolved oxygen level in the bath.

• Hot Metal: Increasingly charged alongside scrap/DRI.

  • Advantage: Contains carbon and silicon. Their oxidation liberates chemical heat, offsetting electrical energy costs and speeding up the process.

4. EAF Construction & Features

• Vessel Shape: EAF is a shallow, saucer-shaped vessel (unlike the pear-shaped BOF).

• Roof: Removable, mounted on an automated robotic arm to allow top-charging of scrap via overhead cranes.

• Refractory Lining (Crucial Exam Point): EAFs use Basic linings (acidic/silica linings would be eaten up by basic refining slags).

  • Hearth (Bottom): Lined with Magnesite (handles chemical corrosion from molten metal/slag).

  • Roof: Lined with Chromite and Magnesite (designed to handle severe thermal radiation).

• Tapping: Uses an EBT (Eccentric Bottom Tapping) facility.

  • Tilting: Unlike BOF which tilts 70 to 75 degrees to tap, an EAF only tilts marginally (8 to 18 degrees max) to drain metal through the EBT.

5. Electrodes & Power Parameters

• Material: Made of highly expensive Graphite due to its electrical conductivity.

• Arc Temperature: Reaches approximately $4000^{\circ }\text{C}$.

• Types of EAFs:

  • AC Furnaces: Have 3 electrodes corresponding to 3 electrical phases.

  • DC Furnaces: Have 1 top electrode (cathode) and 1 bottom anode. Current conducts through the charge causing Joule (resistance) heating. DC furnaces are more expensive to build but less noisy and easier to operate.

• Energy Consumption:

  • EAF Route Total: 7.6 GJ/ton of steel. (Compared to BOF route at 16.2 GJ/ton due to the massive energy required in the blast furnace to break iron-oxygen bonds).

  • Melting requirement: 400 to 500 kWh/ton (just to heat scrap to melting point).

  • Refining requirement: An additional 150 to 300 kWh/ton.

• Power Supply Rates:

  • Small furnaces: $\sim 500\text{ kW/ton}$.

  • Ultra-High Power (UHP) furnaces: 900 to 2000 kW/ton (melts charge extremely fast).

6. Electrode Wear & Foamy Slag Practice

Electrodes undergo severe wear and tear due to mechanical degradation and oxidation.

• Foamy Slag Practice: Coke powder/chunks are injected into the $\text{FeO}$-rich slag. Carbon reacts with $\text{FeO}$ to produce CO gas bubbles, causing the slag to expand (foam) and engulf the electrode tips. This protects the furnace walls from arc radiation but chemically eats away the graphite electrodes.

• Hydrogen Pickup (Major Defect Risk): Electrodes get red hot and require water cooling. This introduces moisture into the furnace atmosphere.

  • EAF steel frequently picks up 5 to 7 ppm of Hydrogen (compared to 1 to 2 ppm in BOF).

  • Since critical applications strictly limit hydrogen to 1 to 1.2 ppm, EAF steel heavily relies on secondary metallurgical degassing.

7. Bath Agitation & Off-Gas Treatment

• Stirring: The electric arc does not stir the liquid bath.

  • Historical method: Iron ore was used as an oxidizer (took 8 to 10 hours for a heat).

  • Modern method: Argon bottom stirring (via porous plugs) and Oxygen lancing (inserted manually or mechanically) are used. Oxygen speeds up melting, oxidizes metalloids, and provides vital physical stirring.

• Off-Gas: Exits at $600^{\circ }\text{C}$ to $700^{\circ }\text{C}$.

  • Scrap preheating: The sensible heat of this off-gas can be used to preheat scrap before charging, drastically saving electrical energy.

  • Toxicity: Because diverse market scrap is used, the off-gas can contain dangerous volatile elements (e.g., Chromium oxidizing into carcinogenic hexavalent chromium). A highly efficient Gas Cleaning Plant (GCP) is mandatory.

8. Scrap Sorting & Chemical Categories (High Exam Priority)

Testing and sorting scrap is the most critical operational step because different elements behave differently at $1600^{\circ }\text{C}$. Elements are categorized based on their position on the Ellingham diagram:

1. Completely Volatile Material:

   • Examples: Zinc (Zn), Calcium (Ca), Cadmium (Cd), Lead (Pb).

   • Behavior: Vaporize completely into the gas phase. They do not contaminate the steel but require a highly efficient gas cleaning apparatus.

2. Completely Oxidizable Material:

   • Examples: Aluminum (Al), Silicon (Si), Titanium (Ti).

   • Behavior: Readily oxidize under steelmaking conditions (positioned low on the Ellingham diagram) and go entirely into the slag phase.

3. Partially Oxidizable Material:

   • Examples: Chromium (Cr), Phosphorus (P).

   • Behavior: Will partially oxidize; some remains in the liquid metal, and some partitions into the slag.

4. Non-Oxidizable Material (Residuals):

   • Examples: Copper (Cu), Nickel (Ni).

   • Behavior: Have very low affinity for oxygen (top of the Ellingham diagram). They cannot be eliminated during EAF refining.

   • Consequence: If you melt stainless steel scrap containing 8% Nickel, all the Nickel remains in the bath. You must strictly avoid this scrap if your final product specification does not permit Nickel.