MME331 Fuels, Furnaces and Refractories

FUELS, FURNACES AND REFRACTORIES (MME331)
(CREDITS:3)
Dr. Jayashree Baral
Department of Materials and Metallurgical Engineering
Maulana Azad National Institute of Technology Bhopal
462003

• Conduction
• Convection
• Radiation
• Electrical Heating
Heat Transfer In Furnaces
The primary objective of a furnace is to transfer thermal energy to the product, thus
before we analyze or design any combustion process equipment we must have a
comprehensive knowledge of the fundamentals of heat transfer. If there is a
temperature difference (i.e. a driving force) between two parts of a system, then heat
will be transferred by one or more of three methods
Conduction In a solid, the flow of heat by conduction is the result of the transfer of vibrational
energy from one molecule to the next, and in fluids it occurs in addition as a result of the transfer
of kinetic energy. Conduction may also be created from the movement of free electrons (viz.
metals).

Convection Heat transfer Convective heat transfer is in reality the conduction of
heat through a flowing fluid to a fixed surface, whereby the conductivity is defined by
a convective heat transfer coefficient, h.
Radiation All materials radiate thermal energy in the form of electromagnetic waves.
When radiation falls on a surface it may be reflected, transmitted, or absorbed. The
fraction of energy that is absorbed is manifest as heat.
When considering conduction : the heat transfer rate between two parts of a
system
𝜕𝑄
𝜕𝑡
= −𝐾
𝜕𝑇
𝜕𝑥
Where K is material's conductivity
This is the classical
Fourier equation
This can be used to analyze two scenarios:
• steady-state conduction (-
𝜕𝑇
𝜕𝑥
) is independent of time ‘t’
• transient conduction (-
𝜕𝑇
𝜕𝑥
) depends on time ‘t’

THE ENERGY EQUATION FOR CONDUCTION
If Fourier's equation is applied to a simple,
isotropic solid in Cartesian coordinates and if
the thermal conductivity is assumed to be
constant, the equation for the transient
conservation of thermal energy due to
conduction of heat in a solid with a heat source
(or heat sink) can be derived as follows,
The differential heat conduction equations derive from the application
of Fourier's law of heat conduction, and the basic character of these
equations is dependent upon shape and varies as a function of the
coordinate system chosen to represent the solid
where q″′ is the volumetric heat source and α is the thermal diffusivity, α = k/ρc

THE ENERGY EQUATION FOR CONDUCTION
If the heat source is equal to zero, this reduces to the Fourier
equation, If the temperature in the solid is invariant with respect to
time, this becomes the Poisson equation. if the temperature is time-
invariant and the heat source is zero, this becomes the Laplace equation

STEADY-STATE ONE-DIMENSIONAL SYSTEMS
Infinite flat plate

Series composite wall
Consider a simple series wall made up of two different materials whose
thermal conductivities are k1 and k2. There is a flow of heat from the
gas at temperature Ti through its boundary layer, the composite wall,
and the boundary layer of the gas at T0.
Steady-state temperature
distribution in a composite wall
The unidirectional heat flux through the four
parts of the entire circuit is constant because a
steady-state prevails

Series composite wall
Steady-state temperature
distribution in a composite wall
Thus for the composite wall, the four thermal
resistances are 1/Ahi, L1/k1A, L2/k2A, and 1/Aho.
The total resistance for the whole circuit is
simply their sum, so that the heat flow
is
we only need to know the total temperature
drop across the system to calculate the heat
flux, which we can use to determine the
temperature at any position within the
composite wall.
The flow of heat Q through material, subject to a temperature difference Tj - Tk,
From Ohm's law for electricity, the thermal resistance R, for heat flow is :

TRANSPORT PHENOMENA MATERIALS PROCESSING D.R. Poirier, G.H. Geiger

Furnace: Type and classification
a. What is a furnace?
b. What are the features of a furnace?
c. Furnaces and their applications in high-temperature industries
d. Issues in Furnace design
A furnace is essentially a thermal enclosure and is employed to process raw
materials at high temperatures both in a solid-state and liquid state. Several
industries like iron and steel making, nonferrous metals production, glass making,
manufacturing, ceramic processing, calcination in cement production, etc. employ
furnaces. The principal objectives are
a. To utilize heat efficiently so that losses are minimum, and
b. To handle the different phases (solid, liquid or gaseous) moving at different
velocities for different times and temperatures such that erosion and
corrosion of the refractory are minimum.

What are the features of a furnace?
A furnace will have following essential features:
Furnace name
Furnace purpose:
Furnace temperature:
Energy source
Furnace shape
Furnace material
Furnace charging and discharging
Energy conversion method
Heat transfer mode:
Air supply mode:
Batches or continuous operation:
Furnace atmosphere:
Furnace control:
Furnace flue gas treatment:

A furnace will have the following essential features:
Furnace name: The furnace has a name to identify the features necessary for performing
some processes.
Furnace purpose: The process performed in the furnace has a purpose which could be
physical (heating, melting, etc.), chemical (calcination, roasting, smelting, etc.) or
physicochemical (e.g. sintering) in nature.
Furnace temperature: The furnace should have thermal zone which could be low (< 1000 °C),
high (> 1400 °C) or very high (~ 2000 °C).
Energy source: The furnace uses some energy sources like coal, coke, oil, fuel gas or
electricity.
Furnace shape: The furnace has a typical shape like rectangular chamber, circular tower
(shaft), long chamber (tunnel), rotating drum (rotary kiln), etc.
Furnace material: The furnace has a structure made of refractory material or combination of
refractory materials which would sustain high-temperature working conditions.
Furnace charging and discharging: The furnace structure is designed in such a manner that
it facilitates the charging and discharging of the processed material.
Energy conversion method: The furnace has some means to convert the inherent energy in
the fuel into thermal energy like grate combustion, pulverised coal burner, oil or gas burner,
electrical current flow through resistance or arc gap
Heat transfer mode: The furnace uses some means of heat transfer from source to object like
thermal conduction, convection or radiation.

A furnace will have the following essential features:
Air supply mode: The furnace gas flow could be due to natural draft or forced draft
Batches or continuous operation: The furnace operation could be in batches or made to function
continuously.
Furnace atmosphere: The furnace atmosphere could be made oxidizing, reducing, or inert in nature.
Furnace control: The furnace could be controlled manually or made automated.
Furnace flue gas treatment: The method of discharging flue or waste gases could be after cleaning or
without cleaning.

Furnaces and their applications in high-temperature industries
Furnaces are used for wide variety of processing of raw materials to finished products in
several industries. Broadly they are used either for physical processing or for chemical
processing of raw materials. In the physical processing the state of the reactants remains
unchanged, whereas in the chemical processing state of the reactants changes either to
liquid or gas. In the table given below some applications of furnaces for physical and
chemical processing are given:
P
H
Y
S
I
C
A
L
P
R
O
C
E
S
S
I
N
G

C
H
E
M
I
C
A
L
P
R
O
C
E
S
S
I
N
G

ISSUES IN FURNACE
1. The Source of energy in the processing of raw materials is fossil fuel in most cases. Even
if electric energy is used, it is also derived from fossil fuels. Thus, the energy-efficient
design of thermal exposure is important. particularly heat losses should be as minimum
as possible.
2. In chemical processing, fluid flow is important. Liquid and gases are flowing at high
temperatures so erosion and corrosion of the refractory is important. In addition, fluid flow
also influences the rates of heat and mass transfer. The dead zones should be (dead
zones are those areas in which no movement of solid and liquid takes place) should be
avoided while designing the furnace chamber.
3. Atmosphere in the furnace is also important to avoid oxidation of the material being
heated
4. Control of furnace temperature is also an important issue. Overheating and under‐heating
lead to inefficient utilization of fuel and also overheating or under‐heating of material. The
furnace should be equipped with temperature measurement and control devices.
5. Furnaces are both batch and continuous type. In the continuous type for example in
heating of ferrous material for hot working, the furnace chamber consists of preheating,
heating and soaking zones. The material enters through the preheating zone and exits
the soaking zone for rolling. But the flow of products of combustion is in the reverse
direction. Furnace design is recuperative type in that material exits at the desired
temperature from the soaking zone and the products of combustion

discharge the preheating zone at the lowest possible temperature. Different types of continuous
furnaces are in use, like walking beam type, pusher type, roller hearth type, screw conveyor
type etc.
6. In the batch furnaces, the load is heated for the fixed time and then discharged from the
furnace. There are different types of batch furnaces like box type, integral quench type, pit type
and car bottom type .
7. In many cases the furnace is equipped with either external heat recovery system or internal
heat recovery system. In the external heat recovery system a heat exchanger like recuperator
is installed outside the furnace. Here heat exchanger must be integrated with the furnace
operation. In the internal heat recovery the products of combustion are recirculated in the
furnace itself so that flame temperature is somewhat lowered. The objective is to reduce the
NOx formation.
8. The products of combustion are moving at high speeds in the furnace. The flow of products
of combustion is important to obtain rapid heat transfer and minimum thermal gradient.

Refractory Materials
a. What is a refractory?
b. What are the phases?
c. Properties required in a refractory
d. Selection of refractory
• Refractory is a material that can withstand high temperatures and does not
fuse. Examples are fireclay, alumina, magnesite, chrome magnesite,
dolomite etc.
• Refractory materials are produced to meet the diversified requirements of
high-temperature processes carried out in metal extraction, cement,
glassmaking, manufacturing, ceramic etc. industries.
• Refractories are mainly used in metallurgical industries for the purpose of
providing linings of furnaces, kilns, reactors, boilers and other vessels for
holding and transporting metal and slag.

What are the phases?
The diversified applications of refractory materials in several different types of industries require
diversified properties to meet the physico‐chemical and thermal requirements of different phases.
Slag: It is a mixture mostly molten oxides and sulphides , in some processes phosphate is
also a constituent of slag. Oxides are either acidic such as silica, fireclay or basic like MgO,
MgO‐C, alumina, FeO.
Among sulphides CaS, MnS, FeS, PbS etc, are prominent phases. The slag is molten and its
temperature in different processing lay within the range 1200‐1600°C.
Liquid metal
In metal extraction from ores, metal is extracted in the liquid stage. Composition of metal, and
its temperature are important. For example in iron and steel industry, hot metal is a mixture of
iron, carbon, silicon, manganese and phosphorus. The temperature varies in between 1300
°C to 1600°C. In copper‐making the temperatures are within the range 1100‐1200°C. Molten
aluminum is produced at700‐750°C, and likewise other non ferrous metals.
Matte: it is a high temperatures molten phase and consists of a mixture of molten sulphides
like Cu2 S, Fe S, Ni3 S2 etc. The temperatures vary within the range 1100°C to 1250°C.

What are the phases?
Gases: Several different types of gases like CO, CO2, N2 H2O (vapor), argon,
O2 are used at high temperatures in several unit processes like roasting,
calcination, smelting, refining, converting etc. The temperatures may vary in
between 600OC to1500°C. The gases like CO2, H2O, and O2 are oxidizing,
wheras the gases like CO, and H2 are reducing. N2 and argon are inert.
Speisses are molten solutions of arsenides, or arsenides and antimonides
when the materials being treated contain large quantities of As and Sb.
Drossses are heterogeneous products skimmed or driven form the surface
of molten metal during refining. They are mixtures of precipitated solid and
liquid compounds with substantial proportion of mechanically trapped molten
metal.

In some industrial units more than one phase are present e.g. in steel‐making
vessels slag /metal /gases are simultaneously present in the vessel at high
temperatures. In the heat treating furnaces solid/reducing or oxidizing gases
are simultaneously present

The refractory materials are required to possess many properties. Refractory materials should
have the ability to:
(i) withstand high temperature
(ii) withstand the corrosive action of molten slag and hot gasses
(iii) withstand abrasion and erosion by moving solid charge, flowing liquids
and blowing gases
(iv) withstand working load during service
(v) retain dimensional stability at working temperatures
(vi) sustain repeated thermal cycling
(vii) sustain thermal shock (sudden change in temperature)
(viii) conduct/resist heat flow as needed during use
(ix) store heat in the system
In addition to the above properties, the availability of refractory at suitable
cost would be a desirable factor for its use.

• The refractory materials are required to serve at high temperature, and hence they must
have sufficient strength at working temperature to retain their shape and size.
• This high temperature strength becomes more important when the size of the furnace is
large and load on the hot refractory structure becomes high. It must be noted that strength
measured at room temperature is not the indication for its fitness to use the refractory at
high temperature. As we know that any solid material when heated starts becoming soft at
some temperature due to fusion/melting at grain boundaries, and eventually it becomes
liquid at its melting point. This requires the knowledge of maximum temperature for safer
use of the refractory. This high temperature behaviour of the refractory is tested by
measuring the following properties:
a. PCE (Pyrometric Cone Equivalent) value
b. RUL (Refractoriness Under Load) value
c. Creep at high temperature
d. High Temperature Modulus of Rupture (HMOR)
e. Thermal shock resistance
– Spalling test
– Loss in MOR strength
f. Reversible thermal expansion
g. PLC (Permanent Linear Change) test
High Temperature Behaviour

PCE (Pyrometric Cone Equivalent) value
It is the measure of refractory’s ability to sustain high temperature without fusion or deformation
Significance: This is the most important value for selection of any refractory material for a given application
in the furnace. The maximum working temperature in the furnace is always kept below the PCE value to
avoid refractory failure.
This is measured by heating a standard size cone
made of the material to be tested in a furnace along
with another standard cone having refractoriness
very close to the test material (determined by a pre-
test),

RUL (Refractoriness Under Load) value
It is the capability of a brick to sustain itself without breaking at high temperature under pressure of
overlying load. This working load could be due to burden, liquid metal or its own structural weight. In
simple words, RUL is the crushing strength of a brick at elevated temperature. The crushing strength of
the refractory brick is lowered at elevated temperature due to fusion/melting of grain boundaries.
Significance: The RUL value is a guiding parameter to use the brick at high temperature with safety
against brick failure due to pressure at high temperature
Creep at high temperature
(i) Definition
Creep is a property which indicates deformation of the refractory at high temperature which is subjected
to stress for longer period.
(ii) Significance
This phenomenon is significant for refractories at high temperature. The refractory materials must
maintain dimensional stability under extreme temperatures (including thermal cycling) and constant
corrosion from very hot liquids and gases. The refractory tested for creep under compression
(deformation at a given time and temperature under stress) for normal working conditions of load and
temperature should not exceed 0.3% change in the first 50 hours of the test.

High Temperature Modulus of Rupture (HMOR)
It is the maximum stress that a rectangular test piece of defined size can withstand when it is
bent in a three point bending device. It is expressed as N/mm2 or MPa
HMOR (σF ) is expressed as the ratio of bending moment at the point of failure (M max ) to the
moment of resistance W (the section modulus) at working temperature. It is expressed as
Hooke’s law for elastic materials as follows:
where,
F max is the maximum force exerted
L S is the distance between points of support
b is the breadth of test sample
h is the height of test sample.

Thermal shock resistance
(i) Definition
Thermal shock resistance is a measure of refractory property when it is exposed to alternate
heating and cooling. This thermal shock leads to breaking of refractory particles which is
termed as ‘spalling’ and loss of strength due to micro-cracks and is noted as MOR (Modulus of
rupture strength) value after thermal treatment.
(ii) Significance
It is an important property for a refractory material. Many refractory components in high-
temperature processes undergo heating and cooling. The refractory grains and the grain
bonding material expand while being heated, and contract during cooling. The different
expansion and contraction behaviour of grains and the bond material lead to breaking away
and development of micro cracks. The nature and magnitude of the cracks would decide the
thermal shock resistance of the material.

Thermal expansion
(i) Definition
The increase in volume of the material due to heating is called thermal expansion. This
expansion process is reversible in nature, and material regains its size on cooling, hence, it is
also called reversible thermal expansion . It is the inherent property of all the materials. This
property is measured as linear expansion with heating due to practical reasons.
(ii) Significance
The knowledge of thermal expansion is needed while selecting the refractory for a given
application, designing the furnace.

PLC (Permanent Linear Change) test
(i) Definition
The materials expand on heating, but they regain original shape on cooling
(reversible thermal expansion). The permanent linear/volume change refers to
non-reversible expansion in the refractory materials due to heating process.
This permanent linear/volume changes could be due to the following reasons:
1. Phase changes in the refractory due to allotropic forms having different
specific gravity.
2. Chemical reactions causing formation of new compound having different
specific gravity. This could be due to chemical attack by gas or slag in the
system leading the formation of different compounds with changed
properties.
3. Sintering of the material causing densification and shrinkage.
4. Melting of some phase causing densification and shrinkage.
(ii) Significance
The permanent change in refractory could alter the furnace structure and may
cause its failure. This phenomenon of permanent volume change is significant in
case of silica brick manufacture. The silica undergoes phase changes, and it is
desirable to allow completion of the changes at manufacturing stage such that
their use is made without trouble and is more assured during use. However, this
is not practical due to long time required for phase change, which the
manufacturers are not able to afford for economic reasons. This requires
checking and care during use.

PLC (Permanent Linear Change) test
(i) Definition
The materials expand on heating, but they regain original shape on cooling (reversible thermal
expansion). The permanent linear/volume change refers to non-reversible expansion in the refractory
materials due to heating process. This permanent linear/volume changes could be due to the following
reasons:
▪ Phase changes in the refractory due to allotropic forms having different specific gravity.
▪ Chemical reactions causing formation of new compound having different specific gravity.
This could be due to chemical attack by gas or slag in the system leading the formation
of different compounds with changed properties.
▪ Sintering of the material causing densification and shrinkage.
▪ Melting of some phase causing densification and shrinkage.

(ii) Significance
The permanent change in refractory could alter the furnace structure and may cause its failure. This
phenomenon of permanent volume change is significant in case of silica brick manufacture. The
silica undergoes phase changes, and it is desirable to allow completion of the changes at
manufacturing stage such that their use is made without trouble and is more assured during use.
However, this is not practical due to long time required for phase change, which the manufacturers
are not able to afford for economic reasons. This requires checking and care during use.

Selection of refractory
The selection of refractory is complicated. Among all physical-chemical and thermal properties,
the cost is an important factor. Selection may depend on
1. Furnace design:
• How the furnace is to be heated.
• Whether directly or indirectly. Indirect heating, fuel and air mixture is supplied to
the furnace and here wall of the refractory facing the reaction chamber must
have high refractoriness besides other properties
• Whereas indirect heating walls of the furnace are heated and heat is transferred
from the walls to the charge. Among other properties, the thermal conductivity of
the refractory is important. Whereas indirect heating fuel and air mixture is
supplied to the furnace.
e.g. coke oven, pidgeon’s process for mg production, Kroll Process for Ti extraction
• Condition of heating: There are furnaces which operate continuously and batch type
e.g coke oven is kept continuously at high temperature for months but a cupola operates
intermittently
• Loading and unloading

Operating Factor
Chemistry of phases: The different phases are present at different intervals of time during processing.
The combination of different phases The refractory facing these phases needs careful selection.
Temperature: High temperatures are involved in industrial furnaces. The reaction chamber temperatures
may vary from 1200‐1600°C in liquid state processing and 700‐1200°C in various solid stale processing
operations.
Abrasion due to movement
Molten metal and slag are turbulent in nature. Gases are flowing at high speeds inside the reactors. The
refractory chamber should be able to withstand the erosion and corrosion caused by the movement of the
phases
Lining life
Lining life, i.e. time for complete relining of the furnace is an important consideration and depends on
several factors like, maintenance and repair technologies, condition of the phases, temperature, quality of
the refractory etc.

Manufacture of a refractory : Note that refractory used in high temperature furnaces does not occur as
natural reserves. But refractory is produced by using naturally occurring materials like quartz, magnesite,
dolomite, chromite, bauxite etc.
Figure: Flow sheet illustrating
the manufacture of the
refractory

Application of Refractory Materials
What are the available Refractory materials
The available refractory materials are classified : on the basis of chemical nature
Acid Basic Neutral
(SiO2 is the main constituent) Magnesite Chromite
Example fireclay
Quartz
Silica
MgO – C
Alumina
Dolomite
Alumina – C
Carbon Mullite
Uses Under acidic conditions Under basic conditions Can be either in acidic or in
basic conditions.
• Acidic refractories consist of mostly acidic materials. They are generally affected by basic
materials.
• Basic Refractories are stable to alkaline materials but could not react with acids.
• Neutral Refractories are used in areas where slag and atmosphere are either acidic or basic.
Special Refractory
Special refractory is a kind of refractory material with special properties made of one or more of high melting
point oxides, refractory non-oxides and carbon

Fireclays
• Fireclay refractories are essentially hydrated aluminum silicates with 25% - 45% Al2O3
and 50% - 80% SiO2 and minor other minerals. As fireclay brick is relatively cheap and
its raw materials are widespread,
• it is the most common type of refractory brick and used widely in most furnaces, kilns,
stoves, regenerators, etc.
.

Silica brick
Volume change with temperature
The volume change in silica occurs due to its phase change. When α-Quartz is heated volume
expansion occurs at 575 °C (rhombohedral to hexagonal) due to the formation of β-Quartz. On further
heating (870 °C), very little volume expansion occurs due to the formation of β-tridymite (hexagonal).
The conversion of β-tridymite to cristobalie (at 1470 °C) is associated with shrinkage (hexagonal to
cubic). The total volume expansion from quartz to cristobalie is ~12–13%.
The preparation of silica bricks requires considerable care. This needs understanding the structure of
silica.
The silica has various crystalline forms which changes with heating process. These are as follows:
α-Quartz: It has rhombohedral (trigonal) structure. The helical chains making individual single crystals
optically active.
β-Quartz: It has hexagonal structure. The α-quartz converts to β-quartz at 575 °C.
α-Tridymite: It is a metastable phase under normal pressure having orthorhombic structure.
β-Tridymite: It has hexagonal structure. The β-quartz gets converted to β-tridymite at 867 °C (1140 K).
α-Cristobalite: It is a metastable phase under normal pressure having tetragonal structure.
β-Cristobalite: It has cubic structure. β-tridymite converts to β-cristobalite at 1470 °C. β-cristobalite melts
at 1723 °C to give vitreous silica.

Flow sheet for silica brick
manufacture
Washing : The silica rocks mined from its source are washed to
remove clay impurities adhering on its surface.
Crushing : The rocks are crushed to 50 mm size in jaw crusher
and then further crushed to –20 mm in a cone crusher.
Grinding : The crushed quartz is further ground to fine size
depending upon the requirement.
Mixing : The dry silica powder is mixed with sulphite lye (paper
industry
waste) and water to give green strength to the bricks. The
addition of 0.25% sulphite lye provides good dry strength to the
bricks.
Moulding : The wet silica powder and sulphite lye mix is
moulded in brick shape using suitable machines.
Drying : The moulded bricks are stored in air for some time for
drying
Firing : The dry moulded bricks are fired in kilns. The kiln has
pre-heating, heating and cooling zones. The rate of heating is
done in controlled manner to avoid defects due to expansion. In
the firing process, quartz granules could be noticed up to 750 °C.
The quartz converts to crisobalite when the firing temperature
reaches 1200–1300 °C. The crisobalite changes to tridymite with
soaking time at 1300–1350 °C. The full conversion to tridymite
requires considerable soaking time and energy.

Applications
The silica bricks are used in furnaces requiring the following properties:
High Refractoriness Under Load (RUL) : The silica bricks offer good refractoriness under 0.344
MPa (50 psi) load up-to fusion point 1710–1730 °C temperature.
High resistance to attack by iron oxide and lime : Iron oxide and lime are common as flux and
dust in steel plants. The silica bricks are ideal to resist chemical action. Used in all acid steel-
making furnaces.
High Thermal Shock Resistance: It is resistant to thermal shock in the
Temperature range 600–1700 °C and thus suitable for furnace door and cover refractory (e.g.
EAF swinging cover).
Good thermal conductivity at higher temperature: Its thermal conductivity increases with
temperature which is helpful in using coke ovens.

High alumina refractories
Al2O3 varies from 45 to 95%. Commonly used refractory are sillimanite (Al2O3 61%) and mullite (70 –
85% Al2O3). Some of the properties are
• High refractoriness
• Better resistance to slag and spalling
• Higher load-bearing capacity.
• Fusion point >1850.
Uses: BF stoves, cement and lime rotary kilns, electric are furnace roofs, ladle, glass-making furnaces,
etc. Chromite – Magnesite
The amount of chrome ore is > magnesite. Some properties are
• Used up to 1700 .
• Resistant to thermal shocks
• Basic in nature
Uses in: Inner lining of basic oxygen steelmaking vessel, Side walls of soaking pits etc
Magnesite
These refractory are basic in nature. Some properties are
• High refractoriness and thermal conductivity
• Great resistance to basic slag

Silicon carbide
SiC content exceeds 85% in these type of refractories. Some of the important properties are
Properties:
• High thermal conductivity and high refractoriness
• Resistance to thermal spalling and temperature load-bearing capacity is high
• Inert to acid slags and
• Lightweight
SIALON
This class of refractory is prepared by using alumina and silicon nitride. Powdered mixture of
alumina and silicon nitride is hot pressed at 18 – 30 M Pa and 1700 – 1760 in graphite molds
in order to produce a low porosity dense product. SIALON refractory shows
i. good resistance to oxidation, and action of molten metals like Al, Zn, Cd, Fe and
steel and
ii. resistance to H2SO4, Hcl, borax and alkalis.

Future issues of Refractory technology(14)
Monolithic Refractory
Special Refractory(Kulkarni)

MME331 Fuels, Furnaces and Refractories

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