-1-
REFRACTORIES
Refractories: Any substance that is difficult to fuse is a refractory. A refractory is a material which
does not melt easily, because its fusion temperature is very high. Therefore refractories are inorganic
materials which can withstand high temperatures as well as abrasive and corrosive action of molten
metals, slags and gases, without suffering a deformation in shape. The main role of a refractory is to
confine heat in it.
In other words, Furnaces used in extractive metallurgy, cement kilns, glass making etc., have to
be protected with suitable lining materials which should be capable of withstanding the effects of high
temperature, corrosion and erosive action of metals, slag and hot gases. Such lining materials are called
“Refractories”. Eg: SiO2, Al2O3 and MgO.
Their function is not only to confine heat within a furnace but also to transmit heat as in
recuperative and retorts and to store heat in regeneration.
Refractories are available in different shapes and sizes, as bricks, crucibles and tubes, granules,
castables and cements.
Classification of Refractories:
Refractories are classified on the basis of their chemical properties, constituent substances and fusion
temperature into the following categories:

a)
b)
c)
On the basis of fusion temperature ranges, they are classified as:
Normal refractory
:
fusion temperature = 1580 – 17800C
High refractory
:
Fusion temperature = 1780 – 20000C
Super refractory
:
Fusion temperature = > 20000C
Eg:- fire clay
Eg:- chromite
Eg:- Zirconia.

On the basis of chemical composition, they are classified as:
a)
Acidic refractories: These refractories consist of acidic materials like alumina (Al2O3) and
Silica (SiO2). These refractory materials are resistant to acid slags and readily attacked by basic slags
(like CaO, MgO etc.). Hence their contact with these oxide materials should be avoided.
Eg:- alumina, silica and fireclay refractories.
b)
Basic refractories: These refractories consist of basic materials like CaO, MgO etc., and are
resistant to basic slags. The presence of acidic materials like silica is deleterious to their high
temperature performance. Basic refractories find extensive use in some steel making open hearth
furnaces.
Eg:- magnesite and dolomite bricks.
c)
Neutral refractories: These refractories are made from weakly basis or acidic materials like
carbon, zirconia (ZrO2) and chromite (FeO.CrO2). Neutral refractories show resistance to the action of
acidic and basic materials and also show good chemical stability.
Eg:- graphite, zirconia and SiC (Carborandum)
Refractory Type
Neutral
Neutral
Neutral
Acidic
Acidic
Acidic
Basic
Refractory material
Graphite
Zirconia
Silicon Carbide
Alumina
Silica brick
Fire clay brick
Magnesia brick
Fusion Temperature (0C)
3500
2710
2700
2050
1700
1600-1750
2200
-2CRITERIA (CHARACTERISTICS) FOR A GOOD REFRACTORY MATERIAL:
 It must be capable of retaining its original form without cracking or flaking when subjected to
sudden temperature changes. This fractioning or flaking off known as SPALLING, can also be
brought about by compressive stresses imposed by the surrounding furnace structure and the
charge. Thus, a satisfactory refractory should have a high spalling resistance.

It must be capable of retaining its shape and structure at the operating temperature of the furnace.
When a refractoy material is heated it softens and becomes fluid. As the fluidity increases, the
material begins to deform. These changes occur over a range of temperature since they are not
pure compounds. This property of resistance to softening and deforming under its own weight is
called REFRACTORINESS. Thus a refractory material should have high refractoriness.

The refractory material should be such that it does not chemically react with the slag, molten
charge and gases like CO, CO2, SO2, Cl2 etc., Generally these reactions, depend on the
differences in chemical nature. In general, refractory linings that are active in nature like SiO2
should be used for acidic gases, slags; basic refractories like MgO should be used for basic
slags.

Due to impringement of the rapidly moving solid particles of the charge and molten slag,
mechanical removal of the lining occurs. Thus refractories to be used should have resistance to
such abrasive erosion.

A good refractory material should have a softening temperature much higher than the
operating temperature.

They should possess low permeability.

They should be able to withstand the overlying load of structure, at operating temperatures.

They should possess good physical, chemical and mechanical properties.
However no single substance can satisfy all of these requirements and therefore a refractory is usually
formulated from a mixture of compounds. If a given refractory material does not have the above
mentioned characteristics, it will fail in service.
PROPERTIES OF REFRACTORIES:
It is necessary to characterize refractories by a combination of properties since no single property
provides all the necessary information about the performance of refractory. These depends mostly on
their chemical composition, grain size and method of fabrication. These properties are described below:
 Refractoriness: It is the ability of a material to withstand high temperature without appreciable
deformation or softening under given working conditions. It is usually measured as the softening
temperature of the material. As the temperature increases normally a material softens and
deforms, but a refractory should resist such a tendency. Higher the softening temperature, more
valuable is the refractory and since the prime function of a refractory is to withstand high
temperatures, its softening temperature should be well above the operating temperature.
-3Measurement of refractoriness:
This is determined by pyrometric
cone (segar cone) test. The test
refractory in the form of a cone
(38mm height and 19mm base) is
kept alongwith similar sized
standard cones and all are heated
uniformly at 100C per minute.
Each standard cone is made of a
particular refractory with a definite
softening temperature. These standard cones are assigned certain numbers with increasing softening
temperature. When the test cone softens and loses its shape, one of the standard cones whose softening
temperature is close to the test cone will also soften. The serial number of this standard cone will be the
PCE (Pyrometric Cone Equivalent) of the test cone. If the test cone softens earlier than one standard
cone, but latter than the next cone, the PCE value of the test sample is approximately measured as the
average value of the two.
Refractoriness Under Load (RUL) Test:
This property gives an idea of the strength of a refractory. The two essential qualities of refractory are
1) temperature resistance and 2) load bearing capacity. Since very heavy reactants are charged into the
refractory lined furnaces, the refractory should withstand such heavy loads especially at high
temperatures. For example: fireclay refractories collapse at temperature well below their fusion
temperature, when appreciable load is applied. On the otherhand silica refractories withstand load even
at high temperatures. Hence the RUL test is performed to know the safe upper temperature limit up to
which the refractory can be used.
The RUL test is done in a rectangular container by applying a load of 1.75 Kg/cm2 on to the
refractory and heating at a constant rate of 100C per minute. During this process the specimen will
soften and its height will decrease under the load. This decrease in height is measured and when there is
10% decrease to that of the original height, the temperature is noted. The RUL is then expressed as the
temperature at which this 10% deformation occurs.
1.75 Kg/Cm2 load
Hence, RUL is expressed at the temperature where 10%
deformation takes place.
If a refractory brick deforms at
13500C

then it is heavy heat duty brick.
If at 13000C

then medium heat duty brick.
0
and if at 1100 C

then low heat duty brick.
Heat
Chemical inertness:
A refractory material selected for a process should be chemically inert since several chemicals are used
in a furnace, which may react with the refractory material and form fusible products, which may corrode
the furnace and in addition contaminate the furnace product. Since mostly either acidic or basic
environments prevail in the furnace, a simple guideline in the use of refractories is that, an acidic
refractory should not be used in a basic furnace and vice-versa.
eg:- silica bricks being acidic cannot be used in a basic furnace and magnesite bricks being
basic cannot be used in acidic furnace.
Thermal expansion and contraction:
Since all solids expand when heated and contract when cooled, allowance has to be made in the furnace
design for thermal expansion. Expansion of a refractory material decreases the capacity of furnace and
also results in developing internal stresses. Further repeated expansion and contraction contributes to
-4much rapid breakdown of the refractory. Therefore, lower the thermal expansion and contraction, the
better the quality of the refractory.
Thermal conductivity:
Refractories are in general, intermediate as far as thermal conductivity is concerned. It depends among
other things on chemical composition and porosity. As the porosity rises the thermal conductivity
decreases, since the entrapped air in the pores is a poor conductor of heat. This inverse relationship is
used in manufacturing heat insulating refractories. In most applications, refractories should have a low
thermal conductivity to prevent loss of heat. However in some cases, high thermal conductivity is
required as in the case of coke oven walls, muffle furnaces and retorts which are heated from outside.
For these type, densed bricks are used which have the least porosity and have high thermal conductivity.
Porosity:
Refractories usually contain pores which result from their manufacturing methods these pores may be
open or closed. Porosity is an important property of a refractory as it affects many physical and
chemical characteristics of the refractory.
Porosity is the ratio of pore’s volume to the bulk volume. Thus porosity,
P = Porosity
W D
P
X .100
Where
W = Saturated weight of specimen
WA
D = Dry weight of specimen.
A = Saturated weight + moisture content of specimen
Porosity is an important property of refractory bricks, since it affects many other characteristics
i.e., chemical stability, strength, abrasion-resistance and thermal conductivity. Low porosity refractory
material will have greater strength, higher heat capacity and thermal conductivity. On the other hand a
highly porous refractory will allow slags, gases etc., to enter the pores thereby decreasing the strength as
well as resistance to abrasion and corrosion. All these factors ultimately reduce the life of the refractory.
As the porosity rises the thermal conductivity decreases, since the entrapped air in the pores is a poor
conductor of heat. Whereas densed bricks which have the least porosity and have high thermal
conductivity. Therefore in general a good refractory should have low porosity.
Thermal spalling (Spalling Resistance):It is breaking, cracking, peeling off or fracturing of a refractory brick or block, under high temperature.
This can be brought about by thermal, mechanical or structural factors, A desirable refractory should
have appreciable spalling resistance.
Uneven thermal expansion or contraction due to temperature gradients arising out of rapid
heating or cooling, is the main cause of thermal spalling. The spalling tendency is generally directly
proportional to the coefficient of expansion, and inversely to thermal diffusivity. Hence to avoid
spalling, a brick should have a low coefficient of expansion and high thermal diffusivity. Mechanical
spalling may arise as a result of careless removal of slag and clinker from the furnace and also because
of compressive stresses caused by expansion.
Testing of Spalling resistance:
The spalling resistance test can be performed by heating the specimen to a fixed temperature and
then suddenly is heated to the specific temperature for a fixed duration of 8-19 minutes and then
cooling it for the same period. The spalling resistance is measured by observing the number of
cycles of cooling and heating by the same sample without cracking.
-5Resistance to abrasion or erosion :
A good refractory should undergo least abrasion. Resistance to erosion is highly required. Otherwise
the refractory will get spoiled soon.
Electrical conductivity:
Refractories to be used for lining electric furnaces should have low electrical conductivity. Except
graphite, all other refractories are poor conductors of electricity. However, electrical resistance of
refractories decreases rapidly with temperature rise.
Heat capacity:It depends on thermal conductivity, weight of the refractory etc., Dense and heavy refractory bricks
have high heat capacities and useful in regeneration of heat.
Texture:
Coarse or light-textured bricks, because of their large porosity, are light in weight and hence, they are
more resistant to sudden changes in temperatures. However, their crushing strength is low. Such bricks
are more susceptible to the action of abrasion and corrosion. On the other hand, fine or dense-textured
bricks possess low porosity and hence, are heavier in weight. These are not so resistant to sudden
changes in temperature. However, such bricks are less susceptible to action of abrasion and corrosion.
Permeability: It is a measure of rate of diffusion of gases, liquids and molten solids through a
refractory, Permeability depends upon the size and number of connected pores. With the rise of
temperature, the permeability increases, since the viscosity of molten metals decreases with an increase
of temperature. An increase in bulk-density (i.e., ratio of weight to its volume) of a refractory increases
the resistance to slag-penetration and spalling.
CONDITIONS LEADING TO FAILURE OF A REFRACTORY MATERIAL :
 Using a refractory of refractoriness less that of the operating temperature.
 Using lower-duty refractory bricks in a furnace than the actual load of raw materials and
products.
 Using bricks of higher thermal expansion.
 Rapid changes in temperature of the furnace.
 Using heavy-weight refractory bricks.
 Using refractory bricks which are not properly fired.
 Using bricks which undergo considerable volume changes during their use at high temperatures.
 Using acidic/basic refractory in a furnace in which basis/acidic reactants and/or products are
being processed.
MANUFACTURE OF REFRACTORIES:
Manufacture of refractories consists of the following steps:
1. Crushing : The raw materials in the form of big lumps are crushed to about 25 mm size.
2. Grinding : The crushed materials are ground, in suitable grinding machine, down to 200 mesh size.
3. Screening : It is employed to purify the refractory raw materials. For producing better refractories, it
is essential to remove the unwanted materials from the raw materials and this done by: (i) settling, (ii)
magnetic separation, and (iii) chemical methods.
4. Storage : After screening and mineral dressing, the pure materials are stored in storage bins with
bucket elevators.
5. Mixing: It is done so that proper distribution of the plastic materials throughout the mass takes place.
This makes moulding easier.
-66. Moulding : Moulding may be done either manually or mechanically by the application of high
pressure. Hand-moulding produces refractories of low density and low strength ; while mechanicalmoulding produces refractories of high density and strength. In order to increase the density and strength
of refractory by mechanical-moulding, the de-airing of refractory material is essential. De-airing is done:
(i) by applying vacuum through vents in the moulds ; (ii) by allowing air inside the void space in the
refractory to go out by decreasing the rate of pressure application and release of air; (iii) by doublepressing: The material is first pressed and allowed to crack. Then, it is repressed again so as to close the
voids.
7. Drying : Drying is carried out to remove the moisture from refractories, Generally, drying is carried
out very slowly and under well-set conditions of humidity and temperature, depending upon the types of
refractory. Drying is, usually, carried out in tunnel driers. Rack cars of bricks are placed in a long tunnel
and heat is supplied by steam, which passes through the tunnel.
8. Firing : The refractories are fired to stabilize and strengthen their structures. The bricks are,
generally, fired at a temperature as high as or higher than their use temperature. Firing of bricks is done
in tunnel kilns or shaft kilns or rotary kilns. The firing temperature of different bricks are 1,480°C for
high-fired super duty bricks; 1,700°C for kaolin bricks, and 1,870°C for some basic bricks.
FLOW CHART OF MANUFACTURING OF REFRACTORIES
CRUSHING
Crush the raw materials to get 25mm particle size.
GRINDING
Grind the crushed material to get 200 mesh size.
SCREENING
Removal of impurities by
i) Settling, ii) Magnetic screening iii) Chemical method
STORAGE
Stored in storage bins with bucket elevators.
MIXING
Mixing of Plastic to get easy moulding.
MOULDING
Different moulding techniques for specific refractories.
DRYING
It is carried out to remove moisture content.
FIRING
To stabilize and strength firing is done in different kilns.