Magnesia Carbon Bricks for BOF & EAF: Selection & Performance Guide

Why MgO-C Bricks Dominate Steelmaking Vessels

Magnesia-carbon (MgO-C) bricks revolutionized steelmaking refractories in the 1980s and remain the material of choice for the working lining of BOFs (Basic Oxygen Furnaces), EAFs (Electric Arc Furnaces), and steel ladle slag lines worldwide. Their success comes from the unique combination of magnesia's resistance to basic slags and carbon's non-wetting behavior, high thermal conductivity, and resistance to thermal shock.

Composition and Raw Materials

An MgO-C brick is a composite material consisting of:

• Magnesia aggregate (75–92%): The primary component. Available as fused magnesia (highest purity, MgO 96–99%), sintered (dead-burnt) magnesia (MgO 90–97%), or electro-fused magnesia.
• Carbon source (5–20%): Primarily flake graphite. The flake size, carbon content, and ash content all affect performance.
• Binder (3–5%): Resin (phenolic) is the standard binder. Some specialty bricks use pitch or a combination.
• Antioxidants (1–5%): Metallic powders added to protect the carbon from oxidation at high temperature.

Carbon Content Selection

The carbon content is one of the most important design parameters:

Carbon Content	Properties	Typical Application
18–20%	Best thermal shock resistance, highest thermal conductivity, best slag resistance. Higher porosity, more oxidation-prone.	BOF trunnion, impact pad, EAF hot spots
14–16%	Good balance of properties. Standard for high-wear zones.	BOF barrel, EAF sidewall, ladle slag line
10–12%	Lower thermal conductivity (less heat loss), better oxidation resistance. Adequate slag resistance.	BOF upper cone, ladle barrel, moderate-wear zones
5–8%	Lowest heat loss, highest oxidation resistance. Lower thermal shock and slag resistance.	Back-up applications, ladle permanent lining backing

The trend in the industry is toward lower carbon content to reduce heat losses (and therefore energy costs) while maintaining performance through improved magnesia quality and antioxidant technology. Many BOFs now use 12–14% carbon where they previously used 16–18%.

Magnesia Quality: The Foundation of Performance

The quality of the magnesia aggregate is the single most important factor in MgO-C brick performance:

• MgO purity: Higher is better. Fused magnesia with MgO > 97% gives the best slag resistance.
• CaO/SiO₂ ratio: A high C/S ratio (> 2.0) in the magnesia ensures that the silicate impurity phases are high-melting dicalcium silicate (C2S, melting at 2,130 degC) rather than low-melting monticellite (CaO.MgO.SiO₂, melting at 1,490 degC). This dramatically improves hot strength.
• Crystal size: Larger periclase (MgO) crystals in fused magnesia provide fewer grain boundaries for slag to attack. Fused magnesia with crystal size > 1,000 microns is preferred for the most demanding applications.
• Bulk density of grain: > 3.50 g/cm³ for fused, > 3.40 g/cm³ for sintered.

Antioxidant Selection

The carbon in MgO-C bricks is continuously attacked by oxygen (from the atmosphere and from FeO in the slag) and CO₂ at high temperatures. Antioxidants are metallic powders added to the brick to protect the carbon by preferentially reacting with oxygen and forming protective oxide or carbide phases.

Antioxidant	Mechanism	Advantages	Limitations
Metallic Al	Forms Al4C3 and Al2O3, sealing pores	Very effective carbon protection; forms dense Al2O3 layer	Al4C3 hydrates in presence of moisture (storage issue)
Metallic Si	Forms SiC and SiO2	Good carbon protection; SiC improves hot strength	Less effective than Al alone
Al-Mg alloy	Forms MgAl2O4 (spinel) in situ	Spinel has excellent slag resistance; volume expansion fills pores	More expensive
B4C	Oxidizes preferentially to protect carbon	Effective at lower temperatures	Can increase low-melting phases if over-used

Most modern MgO-C bricks use a combination of 2–4% metallic Al and/or Al-Mg alloy. The specific selection depends on the operating conditions and the target properties.

BOF Lining Design with MgO-C Bricks

A BOF typically has 5–7 distinct zones with different MgO-C specifications:

• Trunnion area: 18–20% C, fused MgO, highest grade. This sees the most severe conditions (impact from scrap charging, slag erosion, thermal cycling).
• Charge pad/impact zone: 16–18% C, fused MgO, thicker bricks for mechanical protection.
• Barrel (knuckle to cone): 14–16% C, fused + sintered MgO blend.
• Upper cone: 10–14% C, sintered MgO. Lower temperatures but oxidizing conditions.
• Bottom: 12–16% C with emphasis on impact resistance and slag penetration resistance.
• Tap hole: Special tar-impregnated or resin-bonded MgO-C shapes.

EAF Lining Design with MgO-C Bricks

EAF lining zones include:

• Hot spots (near electrodes): 16–18% C, highest quality fused MgO. These areas see direct electric arc radiation at temperatures exceeding 3,000 degC locally.
• Sidewall: 12–16% C, fused or fused+sintered MgO.
• Slag line: 14–16% C, critical zone where the highly basic, FeO-rich EAF slag attacks the lining most aggressively.
• Bottom (hearth): Rammed magnesia or MgO-C bricks, depending on design.

Campaign Life Expectations

Vessel	Typical Campaign (heats)	Best Practice (heats)	Key Driver
BOF (composite blown)	2,000–5,000	6,000–10,000+	Slag splashing, maintenance practice
EAF (AC)	500–1,500	1,500–3,000	Hot spot management, gunning repair
EAF (DC)	300–800	800–1,500	Bottom electrode area management
Steel ladle slag line	60–120	120–200	Slag composition, temperature control

Performance Optimization Tips

1. Slag splashing (BOF): Blowing nitrogen through the lance after steel tapping coats the lining with a protective slag layer. This single practice has more than doubled BOF campaigns worldwide.
2. Gunning maintenance: Regular gunning of worn areas between campaigns extends life significantly. Use MgO-based gunning compound that is compatible with the MgO-C brickwork.
3. Control FeO in slag: FeO is the primary corrosive agent for MgO-C bricks. Every percentage point reduction in slag FeO improves lining life.
4. Proper brick installation: Tight brickwork with proper expansion allowances prevents brick movement and mechanical failure.

SAPL: MgO-C Bricks for Steelmaking

Shanker Agencies supplies MgO-C bricks across the full range of carbon contents and magnesia qualities for BOFs, EAFs, ladles, and converters. Our technical team works closely with steel plants to optimize zonal lining designs and improve campaign life. We supply from established manufacturers with full test certification and provide ongoing technical support throughout the campaign. Contact us to discuss your steelmaking refractory requirements.