REFRACTORIES
Topics
Definition
Classification
Properties
Manufacture of refractory bricks
Properties and applications of Refractory bricks
Definition
Substances which can with stand high temperature without undergoing deformation
are called Refractories.
Examples: Fire clay Bricks, Magnesite bricks, Alumina Bricks etc.,
Classification:
There are many ways in which refractories can be grouped, how ever the most
common methods of classifications are based on
1. Chemical Nature
2. Thermal Conductivity
3. Refractoriness / Thermal Stability / Heat Duty
Chemical Nature
Acid Refractories
Reacts with basic
Slag .
Silica and Alumina
Neutral Refractories
Inert to both acidic
and basic slag
SiC and ZrO2
Basic Refractories
Reacts with acidic
Slag .
Calsite and Magnesite
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
Page 1
3/10/2008
Conductivity
Conducting
Ex.: SiC, ZrC
Non-conducting
Ex.: Silica, Alumina
Low
1520 - 1630
PCE 19 –28
Calcite
Intermediate
1631-1670
PCE 28-30
Fireclay
Heat
Duty
(in oC)
High
1671-1730
PCE 30-33
Chromite
Super
>1730
PCE 33-38
Magnesite
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& Dr. K. Yamuna
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3/10/2008
The following properties of refractory play an important role in selection of a refractory
material for specific application
•
•
•
•
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Refractoriness
Refractoriness Under Load
Thermal Conductivity
Porosity
Thermal Spalling
Refractoriness
Refractoriness is defined as the ability of a material to withstand high temperature.
It is measured in terms of Pyrometric cone equivalence.
This method was first adopted by Seger, hence also called Seger cone equivalence
Pyrometric cones Test Method
This method is used in ceramic industries to test ‘refractoriness’ of
refractory bricks
The test material cone is mix of oxides that melt at specific temperatures
The cone has a equilateral triangular base (19mm)
Height of the cone is 38mm
The cone is heated at standard conditions (ie. at the rate of 10oC/min )
Pyrometric Cone Equivalent (PCE) Measurement
Temperature at which the refractory brick or the cone bend (ie) the
temperature at which apex touches the base is called refractoriness (units
PCE)
Refractory cannot be used above this temp
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Cone # 29: Stiff at test temperature
Cone # 28: Just started Softening
Cone # 27: Apex almost touching base
Refractoriness under Load (RUL)
In actual practice the refractoriness of the material is much less compared to
that found by Serger cones method
This may be due to the load it is exposed during the process of extraction
In order to study the exact serviceable conditions, RUL is adopted
Measure of deformation under load
Test specimen: rectangular in shape with a base area of 5cm2 and height 75cm
Test load:1.75Kg/cm2 (3.75 Kg/cm2)
Heating at a std .rate of 10oC/min.
Temperature at which 10% deformation occurs is RUL
Height in cm
RUL -10%
deformation
Temperature in oC
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Thermal Conductivity
Thermal Conductivity of refractory bricks depends on the composition and
Porosity
High thermal conductivity bricks are used when
Heat transfer through brickwork is required
E.g. recuperators, regenerators
Low thermal conductivity bricks are used when
Heat conservation is required (insulating refractories)
E.g. heat treatment furnaces
Porosity
It is the measure of number of pores incorporated during the manufacture of
refractory bricks
It is expressed as the ratio of volume of open pores to total refractory volume
Low porosity results in less penetration of molten material and high strength of
bricks
Amount of pores present in a refractory material P = W ─ D x 100
W─A
Porosity decreases
Chemical resistance
Strength
Thermal Conductivity
Thermal Spalling
Porosity increases
abrasion
corrosion
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Dimensional Stability
It is the measure of ability of a refractory material to resist any change in
the volume during working conditions.
Dimensional changes could be reversible or irreversible
Irreversible or permanent changes may be due to contraction or expansion of
brick material
Phase transformations may also occur during heating and cooling
Reversible change occur in most bricks at temperature less than 750oC
Irreversible changes occur in the following cases
i. Shrinkage
Magnesite
(amorphous)
Heat
▬▬▬▬▬►
Periclase
(crystalline)
ii. Expansion
∆
∆
Quartz
▬▬▬▬▬▬▬▬▬▬▬►Tridymite ▬▬▬▬▬▬▬▬▬► Crystobalite
(crystalline) 870oC &load
1470oC &Load
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Thermal Spalling
The process of peeling of refractory material under the influence of high
temperature is called thermal spalling. It is due to uneven expansion and
contraction of refractory material
(picture shows the seeping of molten slag ,through a crack formed due to thermal
spalling)
Effects of thermal spalling
Slag penetration
Corrosion
Abrasion
Thermal Spalling can be reduced by
using materials with low coefficient of expansion and avoiding sudden
temperature fluctuation for example addition of alumina in small quantities
decreases expansion to a large extent
Using high porosity bricks
Using good thermal conductivity bricks
Overfiring refractory bricks at high temperature
By designing the furnace such that stress is alineated
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Chemical Activity
A good refractory must be chemically inert
Acidic refractories are affected by basic slag
Basic refractories are affected by acidic slag
Neutral refractories are not affected by both acids and bases
Porosity increases chemical acitivity.
Application of refractory as a lining material in furnace
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
General Method of Manufacture of Refractory Bricks
Crushing
Aim: To crush large lumps of raw material to ~25 mm size.
Method: Hammer mills / Machine crushing
Grinding
Aim: To powder the 25mm size raw material to 200 mesh size
Method: Jaw crushers / Ball mills
Screening
Aim: To remove impurities from raw material
Method: Settling / Magnetic separation / Chemical methods
Mixing
Aim: To mix various raw materials for proper distribution in order to
facilitate moulding
Moulding
Aim: To provide the required shape and structure
Method: Hand moulding / Mechanical moulding followed by deairing
Drying
Aim: To remove moisture and volatile matter
Method: Tunnel driers heated by steam
Firing
Aim: To strengthen and stabilize the refractory
Method: Heating in Tunnels / Shaft / Rotary kiln
Firing Temperature: 1480oC for Super duty bricks, 1700oC for Acidic bricks
and 1870oC for Basic bricks
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& Dr. K. Yamuna
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3/10/2008
The raw materials are crushed, ground, screened and stored separately
Just before moulding they are mixed in the required ratio and processed
Moulding can be done by either one of the following ways
Hand moulding: For low density and low strength bricks
Mechanical Moulding: For high density and high strength bricks
Note:
Deairing is essential for hand pressed refractory
Deairing methods
Vacuum- Sucking air through pores
Double pressing
Deairing increases
Density
strength
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Method of Manufacture of High Alumina Bricks
Crushing
Calcined Bauxite and clay are crushed to ~25 mm size, separately
Method: Hammer mills / Machine crushing
Grinding
The crushed ores are powdered to 200 mesh size separately
Method: Jaw crushers / Ball mills
Screening
Impurities are removed from raw material by adopting one of the methods
depending on the nature of impurity
Method: Settling / Magnetic separation / Chemical methods
Mixing
The above raw materials are mixed for proper distribution and made into
slurry in order to facilitate moulding
Moulding
The plastic (i.e. flexible) slurry is poured into moulds to get
required shape and size by either Hand moulding / Mechanical
moulding followed by deairing
Drying
The moisture and volatile matter are removed by heating the
moulded brick in Tunnel driers heated by steam
Firing
The dried bricks are fired at 1700 – 1800oC in Tunnels / Shaft / Rotary
kiln
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& Dr. K. Yamuna
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3/10/2008
Properties
Acidic and Non conducting (high porosity)
Medium Duty: 50 –60 % alumina
Heavy Duty: 75 % alumina
Low coefficient of expansion hence high resistance to thermal spalling
Appreciable RUL and Abrasion resistance
Applications
Lining of cement rotary kilns, Soaking pits, Hearth furnace
Aluminium and Brass melting furnaces
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& Dr. K. Yamuna
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3/10/2008
Manufacture of Zirconia Bricks
Crushing
Zirconite , CaO or MgO (acts as stabilizer) , and alumina ( acts as
binding material ) are crushed separately to ~25 mm size.
Method: Hammer mills / Machine crushing
Grinding
The crushed ores are powdered to 200 mesh size separately
Method: Jaw crushers / Ball mills
Screening
Impurities are removed from raw materials by adopting one of the
Settling / Magnetic separation / Chemical methods
Mixing
The above concentrated raw materials are mixed for proper distribution
and made into slurry in order to facilitate moulding
Moulding
The plastic (i.e. flexible) slurry is poured into moulds to get
required shape and size by either Hand moulding / Mechanical
moulding followed by deairing
Drying
The moisture and volatile matter are removed by heating the
moulded brick in Tunnel driers heated by steam
Firing
The dried bricks are fired at 1700oC in Tunnels / Shaft / Rotary kiln
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Properties
Zirconia is a Special Neutral refractory
PCE 2000 – 2600oC
RUL- 1900oC at 3.5 Kg/cm2
Good Thermal shock resistance
Good resistance to spalling
Application
Expensive hence application is limited
Used as lining in high frequency electric furnace.
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008
Method of Manufacture of Magnesite Bricks
Crushing
Calcined magnesite, a little amount of caustic magnesia or iron oxide
binding material are crushed into ~25 mm size, separately
Method: Hammer mills / Machine crushing
Grinding
The crushed ores are powdered to 200 mesh size separately
Method: Jaw crushers / Ball mills
Screening
Impurities are removed from raw material by adopting one of the methods
depending on the nature of impurity
Method: Settling / Magnetic separation / Chemical methods
Mixing
The above raw materials are mixed for proper distribution and made into
slurry in order to facilitate moulding
Moulding
The plastic (i.e. flexible) slurry is poured into moulds to get
required shape and size by either Hand moulding / Mechanical
moulding followed by deairing
Drying
The moisture and volatile matter are removed by heating the
moulded brick in Tunnel driers heated by steam
Firing
The dried bricks are fired at 1500oC in Tunnels / Shaft / Rotary kiln
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
Page 15
3/10/2008
Properties:
It is a Basic refractory and exhibit excellent resistance to basic slag
PCE 2000oC
RUL 1500oC at 3.5 Kg/ cm2
very little shrinkage
poor abrasion resistance
Extremely sensitive to temperature hence possess poor spalling resistance
Applications
Lining in open hearth furnace, copper reverbarators and reverbarators used for
smelting lead and antimony.
Lining in furnaces used for refining Au, Ag and Pt.
Hot zones of cement rotary kilns
Prepared by Dr. V.S.Gayathri
& Dr. K. Yamuna
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3/10/2008