Iron making refractories

IRON MAKINGIRON MAKING
REFRACTORY SYSTEMSREFRACTORY SYSTEMS
N. PRAKASAN
ME METALLURGY

IRON MAKING REFRACTORY SYSTEMSIRON MAKING REFRACTORY SYSTEMS
REFRACTORY MATERIAL:
 A material having the ability to retain its physical shape
and chemical identity when subjected to high
temperatures.
 Refractories are inorganic, nonmetallic, porous and
heterogeneous materials composed of thermally stable
mineral aggregates, a binder phase and additives.
 Refractories are heat resistant materials used in all
processes involving high temperatures and/or
corrosive environment.

A refractory material should be able to,
Withstand high temperatures
Withstand action of molten metal, hot gasses
and slag erosion etc;
Withstand load at service conditions;
Resist contamination of the material with which
it comes into contact;
Maintain sufficient dimensional stability at high
temperatures and during repeated thermal
cycling;

Important Properties of Refractories:
Melting point
◦ Melting temperatures specify the ability of materials
to withstand high temperatures without chemical
change and physical destruction.
◦ For Alumina 3720o
F; Graphite C Pure 6300o
F
Size and Dimensional Stability:
◦ Dimensional accuracy and size is extremely
important to enable proper fitting of the refractory
shape and to minimize the thickness and joints in
construction.

Porosity:
◦ Low porosity materials are generally used in hotter
zones, while the highly porous materials (Highly
insulating) are usually used for thermal backup.
◦ Refractory materials with high porosity are usually
NOT chosen when they will be in contact with molten
slag because they cannot be penetrated as easily.
Bulk Density:
◦ An increase in bulk density increases the volume
stability, the heat capacity, as well as the resistance
to abrasion and slag penetration.

Refractoriness under load:
◦ The ability to withstand exposure to elevated
temperatures without undergoing appreciable
deformation is measured in terms of refractoriness.
The refractoriness under load test (RUL test) gives
an indication of the temperature at which the bricks
will collapse, in service conditions with similar load.
Creep at high temperature :
◦ Refractory materials must maintain dimensional
stability under extreme temperatures (including
repeated thermal cycling) and constant corrosion
from very hot liquids and gases.

Thermal Conductivity:
◦ High thermal conductivity refractories are required
for some applications where good heat transfer is
essential such as coke oven walls, regenerators,
muffles and water cooled furnace walls.
◦ However, refractories with lower thermal
conductivity are preferred in industrial applications,
as they help in conserving heat energy.
◦ Porosity is a significant factor in heat flow through
refractories. The thermal conductivity of a refractory
decreases on increasing its porosity.

Classification of Refractories
 Refractories are typically classified on the basis of their
chemical behavior:
 Acid Refractories:
◦ Acid refractories are those which are attacked by
alkalis (basic slags). These are used in areas where
slag and atmosphere are acidic. Examples:
◦ 1) Silica (SiO2), 2) Zirconia (ZrO2),
Neutral Refractories:
◦ Neutral Refractories are chemically stable to both
acids and bases and are used in areas where slag
and atmosphere are either acidic or basic. Example:
◦ 1) Carbon graphite (most inert)
◦ 2) Chromites (Cr2O3); 3) Alumina

Classification of Refractories
Basic Refractories:
◦ Basic refractories are those which are attacked by
acid slags but stable to alkaline slags, dusts and
fumes at elevated temperatures.
◦ Since they do not react with alkaline slags, these
refractories are of considerable importance for
furnace linings where the environment is alkaline;
Example:
◦ Magnesia (MgO) - caustic, sintered and fused
magnesia
◦ Dolomite (CaO*MgO) - sintered and fused dolomite
◦ Chromite -main part of chrome ore

BLAST FURNACE REFRACTORIES:
The life of the lining under the conditions
prevailing inside the furnace decides the
furnace compaign which should not be les than
a few years.
The thickness of the lining depends on the
furnace size.

Blast furnace regionsBlast furnace regions

STACK LINING:
• The lining in stack should have good abrasion
resistance and resistance to carbon monoxide attack.
• In general, Armour plates are used at the throat to
withstand abrading action of the falling burden.
• The entire stock below the top few meters (2-3m) of
height is lined with high duty fire bricks.
• A 35-40% Al2O3 fire bricks with a close texture is
usually preferred.
• These bricks are made by machine molding under high
pressure with absence of air.

HEARTH LINING:
• The lining in hearth should prevent breakouts and have
high refractoriness.
• Carbon blocks are used for lining whole wall
thickness and considerable bottom thickness of the
hearth. The remaining bottom thickness is made by
high duty fire bricks.
• Water cooled copper or steel plates are laid inside the
walls to protect the lining.
• Effective cooling of the hearth walls is improved by
employing graphite as the backing layer with carbon
blocks facing.

BOSH LINING:
• Considering the severity of temperature and chemical
attack in the bosh region the lining should possess,
• Good refractoriness
• Resistance to the action of the molten metal limy and
alkali slags.
• High duty fire bricks with 45-65% Al2O3 are used for
lining of bosh region with copper cooling pates.
• Carbon blocks also used for lining the bosh region
since carbon refractory have better properties than
conventional high duty fire bricks. Carbon lined walls
can be cooled by spray coolers.

Advantages of Carbon lining:
• Increase in overall compaign life of the furnace.
• Minimum breakouts and reduced scaffolds.
• Cooling design becomes more simple.
• More uniform wear of the lining.
• Clean surface in contact with slag and metal
• A relatively thin lining is adequate.
• Disadvantages of Carbon lining
• Higher cost than conventional bricks
• Formation of steam and gases in the metal & slag tap
hole region and faulty coolers can damage the carbon
lining.

The causes of failure of the linings are,
◦ Carbon monoxide attack
◦ Action of Limy and alkali vapours
◦ Abrasion by solids, liquids and gases
◦ Temperature
◦ Action of molten metal
◦ Action of volatile matters

References:
◦ Modern Iron Making _ R H Tupkary

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