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SURFACE CHEMISTRY
SURFACE CHEMISTRY
INTRODUCTION It deals with the study of
phenomena that occur at the surfaces or
interfaces of substances, like adsorption,
heterogeneous catalysis, formation of colloids,
corrosion, crystallization, dissolution,
electrode processes, chromatography etc.
Surface chemistry finds its applications in
industry as well as in daily life.
• What is surface?
• The physical boundary of any condensed phase like liquid or solid is
considered as surface. It separates one phase from the other. It can be
considered as series of points which make a plane or layer where one
phase ends and the other begins. The surface may be uni-layered or multilayered.
• The interfaces that exist between two immiscible liquids like oil and water;
between a metal and a gas like platinum and hydrogen; a liquid and a gas
etc., are some examples.
• The interfaces between two phases can be represented as: phase1/phase2
or phase1-phase2.
• Applications of surfaces:
• Most of surface chemistry involves the interaction of surfaces of one
system with the particles of other system. Surfaces play active role in
catalysis, colloid formation, electrode reactions, chromatography etc. In
the current chapter we deal with the surface phenomena and related
topics like:
• 1) Adsorption
• 2) Catalysis
• 3) Colloids
Adsorption
• Process in which a gas, liquid, or solid adheres to the
surface of a solid or (less frequently) a liquid but does not
penetrate it, such as in adsorption of gases by activated
carbon (charcoal). In comparison, a gas or liquid taken-in
during absorption penetrates or mixes with the absorb
Or
• The phenomenon of concentrarion of molecules of a gas or
liquid at the surface of solid.
• The substance that concentrate the surface is adsorbate.
• The solid on whose surface the concentration occurs is
adsorbent.
ADSORPTION
TYPES OF ADSORPTION
• Adsorption is two types. They are
1. Physical adsorption
2. Chemical adsorption
1.Physical adsorption:-The adsorption takes
place due to the adsorption of gas molecules
on the solid surface by Vander Walls force of
attraction. This is also called as Vander walls
Adsorption.
• Chemisorption:- The type of adsorption is due
to the attack by ionic or covalent compounds
• Ex :- hydrogen gas is chemisorbed on nickel.
At first hydrogen molecule is adsorbed by
Vander walls force f attraction then dissociate.
Thus hydrogen atom is adsorbed on nickel
What is Adsorption Isotherm?
The relation between equilibrium pressure of
the gas and the amount of gas adsorbed on
the solid adsorbent at any constant
temperature.
It is given in the form of graph or equation.
It is two types. They are
1.Freundlich Adsorption isotherm
2. Langmuir Adsorption Isotherm
Freundlich Adsorption Isotherm
• In 1909, Freundlich gave an empirical expression
representing the isothermal variation of adsorption of a
quantity of gas adsorbed by unit mass of solid adsorbent
with pressure. This equation is known as Freundlich
Adsorption Isotherm or Freundlich Adsorption equation or
simply Freundlich Isotherm.X/M=KP
• Where x is the mass of the gas adsorbed on mass m of the
adsorbent at pressure p and k, n are constants whose
values depend upon adsorbent and gas at particular
temperature. Though Freundlich Isotherm correctly
established the relationship of adsorption with pressure at
lower values, it failed to predict value of adsorption at
higher pressure.
LANGMUIR ADSORPTION ISOTHERM
Langmuir derived a simple adsorption isotherm
based on theoritical considerations.
In 1916, Irving Langmuir proposed
another Adsorption Isotherm which explained
the variation of Adsorption with pressure. Based
on his theory, he derived Langmuir Equation
which depicted a relationship between the
number of active sites of the surface undergoing
adsorption and pressure
Assumptions of Langmuir Isotherm
•
•
Langmuir proposed his theory by making following assumptions.
1. Fixed number of vacant or adsorption sites are available on the surface of solid.
2. All the vacant sites are of equal size and shape on the surface of adsorbent.
3. Each site can hold maximum of one gaseous molecule and a constant amount of heat
energy is released during this process.
4. Dynamic equilibrium exists between adsorbed gaseous molecules and the free
gaseous molecules.
A(g) + B(s)
ADSORPTION
AB
DESORPTION
• Where A (g) is un adsorbed gaseous molecule, B(s) is unoccupied metal
surface and AB is Adsorbed gaseous molecule.
• 5. Adsorption is monolayer or uni layer.
Derivation
• Langmuir Equation which depicts a relationship between the
number of active sites of the surface undergoing adsorption
(i.e. extent of adsorption) and pressure.
• To derive Langmuir Equation and new parameter ‘ θ ’ is
introduced. Let ‘ θ ’ is the fraction of the covered surface area
(no.of active sites) & (1- θ) is the fraction of Naked surface
area.
• Rate of adsorption of molecules on the surface of the
adsorbent = ka(1 - )
• Rate of desorption = kd
• At equilibrium state the rate of disorption is equal to the
rate of adsorption
•
i.e, kd = ka(1 - )
•
•
•
•
•
•
•
•
•
•
•

= ( KaP/Kd + KaP)
On multiplication of each factor with Kd , then
 = (Ka/Kd)P/1+(Ka/Kd)P
Or
 = Kp /1+ Kp
(since K = Ka/Kd)
K is also known as adsorption co efficient.
The amount of gas adsorbed per gram of the adsorbent is
directly proportional to 
Hence
x α Kp /1+Kp
X = Kl .Kp/1+Kp
→1
Kl is new constant
Eq(1) gives the relation between the amount of gas
adsorbed to the pressure of the gas at any constant temp is
known as Langmuir Adsorption isotherm.
• In order to test the Langmuir Adsorption
isotherm eq(1) is rearranged
• So, that kII = 1/k + p/kII →(2) (kII=kI/k)
• eq(2) is similar to an equation for straight line.
• Thus the plot between p/x and p, straight line is
obtained with intersept 1/kI
•
•
•
•
•
Failures :The isotherm is fails at high pressures.
Langmuir Adsorption isotherm is written as
p/x = 1/kI +p/kII
if the pressure is low the factor p/kII is ignored
& assuming the form x = kIp.
if the pressure is low the factor 1/kI is ignored
& assuming the form x = kII
APPLICATIONS OF ADSORPTION
• Adsorption finds extensive applications both in research
laboratory and in industry. A few applications are discussed
below:
• In preserving vacuum:
In Dewar flasks activated charcoal is placed between the walls
of the flask so that any gas which enters into the annular
space either due to glass imperfection or diffusion though
glass is adsorbed.
• In gass masks:
All gas masks are devices containing suitable adsorbent so
that the poisonous gases present in the atmosphere are
preferentially adsorbed and the air for breathing is purified.
• In clarification of sugar:
Sugar is decolorized by treating sugar solution with charcoal
powder. The latter adsorbs the undesirable colors present.
• In paint industry:
The paint should not contain dissolved gases as otherwise the
paint does not adhere well to the surface to be painted and
thus will have a poor covering power. The dissolved gases are
therefore, removed by suitable adsorbents during
manufacture. Further, all surfaces are covered with layers of
gaseous, liquid or solid films. These have to be removed
before the paint is applied. This is done by suitable liquids
which adsorbs these films. Such liquids are called wetting
agents. The use of spirit as wetting agent in furniture painting
is well known.
• In chromatographic analysis:
The selective adsorbent of certain substances from a solution by a
particular solid adsorbent has helped to develop technique for the
separation of the components of the mixture. This technique is
called chromatographic analysis. For example: in column
chromatography a long and wide vertical tube is filled with a
suitable adsorbent and the solution of the mixture poured from the
top and then collected one by one from the bottom.
• In catalysis:
The action of certain solids as catalysts is best explained in terms of
adsorption. The theory is called adsorption theory. According to this
theory, the gaseous reactants are adsorbed on the surface of the
solid catalyst. As a result, the concentration of the reactants
increases on the surface and hence the rate of reaction increases.
The theory is also able to explain the greater efficiency of the
catalyst in the finely divided state, the action of catalyst promoters
and poisons.
PHASERULE
Introduction
Two or more different phases present in equilibrium with one
another, constitute a heterogenous system. Such
heterogenous system can be conveniently studied with the
help of a generalisation called Gibbs Phase rule. It is
applicable to all heterogenous systems and is also free from
exceptions which are common features of all other
generalisations of physical chemistry. This rule was deduced
on the basis of thermodynamic principles by J. Willard Gibbs .
This rule predicts qualitatively the effect of temperature,
pressure and concentration on a heterogenous equilibrium.
Phase Rule
Gibbs phase rule may be stated as follows :
"In a heterogeneous system in equilibrium, the number of
degrees of freedom plus the number of phases is equal to the
number of components plus two".
Mathematically, F + P = C + 2
where
F = number of degrees of freedom
C = number of components
P = number of phases
Phase
Defination: “ The physically distinct, homogenous and
mechanically separable part of a system are called phases”.
Examples:
(i) A gaseous mixture constitutes a single phase since gases are
completely miscible. Air is a mixture of N2, O2, CO2, water
vapour etc. Which constitute a single phase.
(ii) Two or more liquids which are miscible with one another
constitute a single phase as there is no bounding surfaces
separating the different liquids. e.g., water and alcohol,
benzene & chloroform constitute one phase system.
(iii) A system consisting of a liquid in equilibrium with its vapour
constitute a two phase system
Component
Defination :
The number of component of a system at equilibrium is
defined as the minimum number of independently variable
constituents which are required to express the composition of
each phase in the system.
Component
Examples
(i) Sulphur system: Consists of four phases namely monoclinic
sulphur, rhombic sulphur, liquid sulphur and sulphur vapour.
The composition of each phase of the system can be
expressed in terms of sulphur only, so, it is a one component
system.
(ii) Water system:
It is a one component system because the
composition of each of the three phases present can be
expressed as H20.
(iii) Na2SO4 + water system: Certain salts are capable of existing
as hydrates with different number of water molecules of
crystallization. These hydrates correspond to different solids
and hence to different phases. The system is a two
component , because the composition of each phase of the
hydrates is completely described in terms of the anhydrous
salt and water alone. e.g., Na2S04 + water
Degrees of Freedom :
Defination :
The degree of freedom or variance of a system is defined as
the minimum number of variable factors such as temperature,
pressure and concentration which should be arbitrarily fixed
in order to define the system completely.
Examples
(i) For a given sample of any gas PV = nRT. Any two of the three
variables P, V, T define the system completely. Hence the
system is bivariant or it has two degrees of freedom.
(ii) A gaseous mixture say N2 and O2 gases (mixed 50% each), is
completely defined when three variables temperature,
pressure and concentration are specified. Thus, the degrees of
freedom is three or the system is trivariant.
Phase Rule
Conclusion·
(i) The greater the number of components in a system, the
greater is the degree of freedom for a given number of
phases.
(ii) The greater the number of phases, the smaller is the number
of degrees of freedom.
(iii) The number of phases is maximum when the number of
degrees of freedom = Zero, for a given number of
components. Thus, for
one component system, P max. = 3 .
two component system, P max. = 4
three component system, P max. = 5
Phase Rule
Advantages of Phase' Rule :
(i) It provides a simple method of classifying equilibrium states of
systems.
(ii) The phase rule confirms that the different systems having the
same number of degrees of freedom behave in same manner.
(iii) It is applicable only to macroscopic systems and not
concerned with molecular structure.
(iv) It predicts the behaviour of systems with changes in the
variables that govern the system in equilibrium.
(v) It predicts under given conditions whether a number of
substances taken together would remain in equilibrium as
such or would involve interconversion or elimination of some
of them.
(vi) It takes no account of nature of the reactants or products in
phase reactions.
Phase Rule
(vii) It finds extensive use in the study of many heterogenous
systems. In particular it has been found
extremely useful in the extraction of metals.
Limitations:
(i) The phase rule is applicable to heterogeneous systems in
equilibrium, so, it is therefore of no use for such systems
which are slow in attaining the equilibrium state.
(ii) It is applicable to a single equilibrium state, so it never gives
information about the other possible equilibrium in the
system.
(iii) In Gibbs phase rule, various variables are temperature,
pressure and composition. It does not take in account the
electric and magnetic influences. For consideration of such
variables, the factor 2 of the Phase rule has to be adjusted
accordingly.
Phase Rule
(vi) All the phases in the system must be present under the same
temperature, pressure and gravitational force .
(v) No solid or liquid phases should be finely divided, otherwise
deviation occurs.
Phase diagrams :
The number of phases that exist in equilibrium depends upon
the conditions of temperature and pressure or temperature
and composition, pressure being constant. These conditions
are determined experimentally and interdependence of
values of the variables can be shown graphically using
appropriates coordinates. These diagrams are termed phase
diagram. A phase diagram is the sum total of the description
of the behaviour of the phases under equilibrium. It is very
easy to describe the phase behaviour of a system by such
diagrams and to investigate the conditions in which various
phases will constitute the system .
Phase Rule
Application of Gibbs Phase Rule to One Component System:
From the mathematical expression,
F=C–P+2
When C = 1, P = 1
F = 1-1+2
=2
Hence, all one component systems can be completely
described graphically by stating only two variables, pressure
and temperature on appropriate axis.
Phase Rule
The Water System :
It is a one component system. Water exists in three possible
phases viz. ice (solid) , water (liquid), and vapour (gas). These
three single phases may form four possible equilibria.
(i) Solid
Liquid
(ii) Liquid
Vapour
(iii) Solid
Vapour
(iv) Solid
Liquid
Vapour
The Phase diagram of water system is given as
Phase Rule
Phase Diagram of water system
Phase Rule
The phase diagram consists of :
(i) Stable curves: three OB, OA and OC
(ii) Metastable curve: one OA'
(iii) Areas: three AOB, COB and AOC
(iv) Triple point: One O
(i) Stable curves
OA : It is known as vapour pressure curve of water. The curve
OA starts from point O i.e., freezing point of water, 0.0098°C
under 4.579 mm of Hg pressure and ends at A, the critical
temperature (3740C at 218 atm.). Above critical temp. on the
vapour phase exists whatever may be the value of pressure.
The vapour pressure of water increases with increase in
temperature.
Phase Rule
The rate of increase of its vapour pressure with temperature is
relatively higher at higher temperatures and therefore the
curve OA slants upwards and slopes away from the
temperature axis.
From phase rule,
F=C–P+2
= 1- 2 + 2
=1
The water vapour system is univariant
Phase Rule
OB : It is the sublimation curve of ice. Along this curve, solid ice
is in equilibrium with its vapour.
This curve is not the prolngation of curve A but falls of more
steeply. Curve OB starts From the temperature 0.0098°C
above which solid water i.e., ice cannot exist. The curve
terminates at B i.e., absolute zero (- 273°C). At this
temperature, no vapour can exist and, therefore, only
ice is left. But on other points of the curve OB, ice is in
equilibrium with vapour. Hence, there are two phases.
According to phase rule,
F=C–P+2
=1–2+2
=1
Thus, the system is univariant. This means that for each
temperature; there may be one pressure and for each
pressure there may be one temperature.
Phase Rule
OC : This curve is the melting point curve or fusion curve of ice.
Along this curve two phases, ice and water are in equilibrium.
The inclination of OC line towards the pressure axis indicates
that the melting point of ice is slightly lowered by increase of
pressure. (According to Le Chatelier's principle the increase in
pressure causes the water - ice equilibrium to shift in such a
direction that there is a decrease in volume.) As the melting
point of ice is accompanied by decrease in volume, it should
be lowered by the increase of pressure.
The curve OC starts from point O but there is no limit for this
curve. It goes upto a point corresponding to 2000 atm. and 20°C. According to phase rule,
F=C–P+2
=1-2+2 = 1
Thus, the system is univariant. This means that for any given
pressure, melting point must have one fixed value.
Phase Rule
(ii) Metastable Curve
OA' : It is a metastable curve shown in continuation of AO. When
water is cooled below its freezing point (when it is not
vigorously stirred) without separation of ice, the water is said
to be super cooled. The vapour pressure curve of liquid water
AO extends below O as shown by the dotted curve OA'. Along
curve OA' liquid water coexists with vapour and vapour
pressures are different than over the solid. This equilibrium is
called metastable equilibrium as slight disturbance brings it to
the stable region OB of the phase diagram.
Phase Rule
(iii) Areas
The areas give the conditions of temperature and pressure
under which single phase - ice (solid), water (liquid) and
vapour (gas) can exist. It is necessary to specify both
temperature and pressure to define a system within this area.
In the area BOC, AOC andAOB exists, ice (solid), water (liquid)
and vapour (gas) respectively. In these areas, the degrees of
freedom for the system is two or they are bivariant
(iv) The Triple point O
The point O at which the curves AO, BO and CO meet is called
the triple point. At this point all the three phases viz , ice,
water and vapour co-exist. Thus, P =3. According to phase
rule, at triple point O,
F = C –P + 2,
=1-3+2 ,
F=0
Phase Rule
Thus, the degree of freedom at triple point is zero, which
indicates that there is only one set of variables P, T at which all
the three phases coexist. If any of the variables is changed,
then the number of phases decreases. For example, if the
temperature is raised, heat causes more and more of the solid
(ice) to melt but no change in temperature or pressure of the
system occurs till the whole of the solid has completely
changed into liquid (water) and the system becomes a two
phase system. On applying the pressure to the system,
vapours start condensing to liquid or solid phase. As long as
there are three phases, temperature and pressure remains
same.
The triple point O is a self defined point corresponding to
0.0075°C temperature and 4.579 mm of Hg pressure
(difference from the ordinary freezing point,(0.0000C at
760mm pressure) because freezing temperature is lower than
triple point temperature due to effect of high pressure and
dissolved air).
In some systems, an equilibrium exists between solid liquid phases and gaseous phase is practically absent. Hence
the effect of pressure on such system can be neglected.
Then it is necessary to take into account only two variables
viz. temperature and concentration.
Such system showing solid-liquid equilibrium is called
condensed system and phase rule applied to such systems is
as follows:
F = C – P + 1 … known as condensed phase rule.
It is a two-component system. The phase diagram
of the Pb-Ag system is shown in the fig
Curve AO: Point A is the melting point of pure
Silver. Curve AO shows melting point depression of
silver by the addition of lead. At any point in this
curve there is equilibrium between solid Ag and
liquids part.
According to reduced phase rule equation. The
system is univarient.
F’=C-P+1;
F’=2-2+1; F’=1
Curve BO: Point B is the melting point of pure
lead, (327oC), curve BO shows the melting point
depression of lead on gradual addition of silver to
it. Along this curve solid lead and solution co –
exist and hence the system is univariant.
Point O: The two curves AO & BO meet at point O,
where three phases solid pb, solid Ag and their
solution co-exist, according to condensed phase rule
the system is invariant. F’=C-P+1; F’=2-1+1; F’=2 The
point ‘O’ is known as eutectic point, its composition
(Ag=2.6%; Pb=97.4%) and temperature (3030C) is
known as eutectic composition and eutectic
temperature respectively. Further cooling below the
eutectic temperature respectively. Further cooling
below the eutectic temperature will cause
simultaneous crystallization of a mixture of lead and
silver.
Area AOB: Consists of only one phases namely pb-Ag solution.
According to reduced phase rule equation. F’=C-P+1; F’=2-1+1; F’=2
The system is bivarient i.e., both T & composition has to be
specified to define the system. Let us consider a point p, which
represents a sample of lead containing less than 2.6% silver. On
cooling the temperature falls gradually till point p’. On further
cooling lead begins to separate and the concentration of Ag
increase in the solution till the point O is reached, after that whole
mass solidifies (2.6% Ag; 97.4% pb). This process is utilized in the
pattinson’s process of desilverization of lead. Below the eutectic
point (O), area COEF consists of solid and eutectic compound,
where crystalline silver and eutectic compounds are stable. Similarly
the area ODFG consists of solid Pb and eutectic compound, where
crystalline lead eutectic compounds are stable
ABRASIVES
• ABRASIVES (Def):
• An abrasive is a material, often a mineral, that is used to
shape or finish a workpiece through rubbing which leads to
part of the workpiece being worn away by friction.
Mohs Hardness Scale
Hardness is the imp quality of a abraseive material
Mho scale is used to measure the hardness
It is incresed in the following order(from 1 – 10)
Talc1Gypsum2Calcite3Fluorite4Apatite5Orthoclase6
Quartz7Topaz8Corundum9Diamond10
Types of Abrasives
These are two types 1. Natural Abrasives
2. Artificial or synthetic Abrasives
Natural Abrasives
Diamonds: It is the best hardest material occurs in nature. It is
stand on top in abrasive power. It is used in making drill
points. To drill a rock or to cut, polish these diamonds are
used in the drill tips and polishing wheels. Grinding wheels
are made with diamond tips are used for cutting the rock into
slices.
Corundum: It is made of aluminium oxide with chemical
formula Al2O3 that exist in crystalline form. Heavy abrasive
wheel used in metal industry are made from corundum.
Emery: It has high scratch hardness. It has a character of
consistent break down of it crystal structure under
pressure. It is perfect mixture of black magnetite (Fe2O3)
and corundum.
Granet: It occupy at 6.5 to 7.5 reading on Mohr’s scale of
hardness. It is vastly applied in making of sandpaper used
for surface smoothing and wood polishing. It composition is
mainly of silicate minerals.
Pumice: In lithography, it is widely used in polishing and
cleaning stones. It is a composition made of silicates of
aluminium, sodium and potassium. It has pale grey texture.
Porous blocks are the main source for pumice.
Quartz: It stands seventh position in Mohr’s hardness scale.
It is applied for sand paper making. It is quite brittle in
nature due to the shell like structure of silica crystal.
Sand stone: It is used in making of sharpening
stones, pulp stones and grindstones.
Flint: It is widely used as mill grinding stones. Due
to its conchoidal fracture, it appears in light
colour. It is a modified from silica.
Diatomaceous earth and Tripoli: Diatomite is
obtained from diatoms a unicellular organism,
which have siliceous skeleton. This siliceous
skeleton is deposited over time and form
diatomaceous earth. Similar to this is a Tripoli but
difference in the appearance. It has property that
it sinks in water at once but diatomite floats for
some time and then sinks. These two products
are starting materials for make polishing cream
used for metals.
synthetic types of abrasives
Silicon carbide: It is manufacture in an electrical furnace
with raw materials of coke, sand and sawdust.
Fused alumina or alundum (Al2O3): It is made by arc
resistance furnace from bauxite ore.
Calcium carbide (CaC2): It is manufacture in an electric
furnace at temperature ranging from 2000-2200
degree centigrade. Carbon and quick lime are used as
raw materials.
Boron carbide (B4C): It is also called as norbide. It is
manufactured in resistance furnace at 2600 degree
centigrade. Boric oxide and coke are used as raw
materials.
REFRACTORIES
Refractories
Any material which can withstand high temperature
Importance in construction of
1. Metallurgy
2. Engineering
3. Chemical industries
54
Introduction
Refractory is any material which can withstand high temperature, without
softening or suffering a deformation in shape.
Main Objective
1. To confine heat e.g. to resist loss of heat.
2. To resist abrasive and corrosion action of molten metals, slags and
gases at high temperatures, without undergoing softening or
distortion in shape.
Uses
1. Construction of the linings of the furnaces, tanks, converters, Kilns,
crucibles, ladles etc.
2. Manufacture of metals (Ferrous or non-ferrous), cement, glass,
ceramics, paper, steel etc.
55
Characteristics of Refractories
A good refractory posses following characteristics:
1. Be infusible at the temperature to which it is liable to be exposed.
2. Chemically inerts towards corrosive action of gases, metallic liquids,
and slags.
3. Resist the abrasive action of flue gases, flames, etc.
4. Be able to withstand the overlying load of structures at the operating
temperature.
5. No crack
6. No loss in size.
7. Expand and contract uniformly, with temperature rise and fall
respectively.
56
Classification of Refractories
1. Acid Refractories: important members of this group is Alumina, Silica
gel fireclay refractories.
2. Basic Refractories: Cao, MgO
3. Neutral Refractories: made from weakly acid/basic materials like
Chromite(FeO. CrO2), Zirconia (ZrO2)
Important members of this group are Graphite, Chromite, Zirconia
and carborundum (SiC) refractories.
57
Properties of Refractories
1. Refractoriness
2. Strength or Refractoriness-under load
3. Dimensional Stability
4. Chemical Inertness
5. Thermal Expansion
6. Thermal Conductivity
7. Porosity
8. Thermal Spalling
9. Resistance to abrasion or corrosion
10. Electrical conductivity
11. Heat capacity
12. Texture
13. Permeability
58
Properties of Refractories (contd.)
1. Refractoriness
Ability of a material to withstand the heat, without appreciable
deformation or softening under particular service conditions.
In general, measured as the softening or melting temperature of the
material.
As most of the common refractory materials are mixtures of metallic
oxides, so they do not have a sharp fusion temperature.
Pyrometric Cones Test (Segar Cones Test)
The softening temperature of the refractory material are, generally,
determined by using Pyrometric cones test.
Expressed in terms of Pyrometric cone Equivalents (PCE).
Softening temperature
(Material to be used as refractory)
>> Operating temperature
59
Pyrometric Cones Test (Segar Cones Test)
The refractoriness is, usually, determined by comparing the behaviour
of heat on cone of material to be tested with that of a series of Segar
cones of standard dimensions.
Segar cones melt or fuse at
definite
temperature
when
heated
under
standard
conditions of 10°C / min.
Segar Cone
Pyramid Shaped having triangular base
38 mm high and 19 mm long sides
So the temp. at which the fusion
or softening of the test cones
occurs is indicated by its apex
touching the base.
The PCE value of the given refractory is taken as the no. of the
standard cone, which fuses along with the test cone.
60
Segar Cones Number
Fusion temperature
1
2
3
4
5
6
7
8
9
1110
1120
1140
1160
1180
1200
1230
1250
1280
61
Properties of Refractories (contd.)
2. Strength or Refractories-under load (RUL):
Refractories used in industrial furnaces have invariably to withstand
varying loads of the products, being manufactured at high operating
temperature.
It is, therefore, essential that refractory materials must also possess
high mechanical strength, even at operating temperature, to bear the
maximum possible load, without breaking.
Some refractories like FIRECLAY, High Alumina Bricks softens
gradually over the range of temperature, but under appreciable load,
they collapse, far below their true fusion point, as determined by
segar cones.
On the other hand, other refractories such as Silica Bricks softens
over a relatively narrow range of temperature and exerts good load
bearing characteristics close to their fusion points.
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R.U.L. Test
Refractories-under load Test
R.U.L. test is performed by applying a constant load of 3.5 or 1.75
kg/cm2 to the refractory specimen (of size 5 cm2 and 75 cm high) and
heating in a carbon-resistance furnaces at a standard rate of 10°C /
min.
The record of the height of the specimen vs. temperature is made by
a plot, until the test-piece deforms or collapses by 10%.
The R.U.L. is expressed as the temperature at which 10% deformation
takes place.
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Properties of Refractories (contd.)
3. Dimensional Stability
Resistance of a material to any volume changes, which may occur on
its exposure to high temperature, over a prolonged time.
These dimensional changes may be permanent (irreversible) or
reversible.
Irreversible changes may result either in the contraction or expansion
of a refractory.
The permanent contraction is due to the formation of increasing
amounts of liquid from the low fusible constituents of the refractory
brick, when it is subjected to a long period of soaking at the high
temperature.
The liquid gradually fills the pores of the refractory body, causing a
high degree of vitrification and shrinkage.
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Properties of Refractories (contd.)
4. Chemical Inertness
A refractory should be selected that is chemically inert in use and
does not form fusible products with slags, fuel ashes, furnace gases,
etc.
usually, the environment in most furnaces are either acidic or basic.
It is not recommended to employ Acid refractory in contact with an
alkaline (basic) product or vice-versa.
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Properties of Refractories (contd.)
5. Thermal Expansion
Solid materials, on heating, expands and on cooling it contracts.
So in the designing of the practical furnaces, a refractory material
should have least possible thermal expansion as the expansion affects
all dimensions (e.g. length, area, volume) of the body.
6. Thermal Conductivity
In industrial operations, refractory materials of both high thermal
conductivity and low thermal conductivity are required, depending
upon the type of the furnaces.
In most cases, furnaces is lined with refractories of low heat
conductivities to reduce the heat losses to the outside by radiation;
otherwise maintenance of high temp. inside furnaces will become
difficult.
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Properties of Refractories (contd.)
6. Thermal Conductivity
A good heat conductivity of the refractory material is desirable for
effective heat transmission in furnace construction.
The densest and least porous brick have the highest thermal
conductivity, owing to the absence of air-voids.
On the other hand, in porous bricks, the entrapped air in the pores,
acts as a non-heat conducting material.
For making porous refractory bricks, the refractory material is mixed
with a liberal amount of carbonaceous material, then mould into
bricks and burnt. The carbonaceous material burns off; leaving behind
minute voids, which enhances the insulating quality.
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Properties of Refractories (contd.)
7. Porosity
All refractories contain pores, either due to manufacturing methods or
deliberately made ( by incorporating saw-dust or cork during
manufacture).
Porosity is the ratio of its pore’s volume to the bulk volume.
W- D
X 100
W-A
W = Wt. of saturated specimen.
D = Wt. of Dry specimen.
A = Wt. of saturated specimen submerged in water.
P=
Porosity is an important property of refractory bricks, because it affects
many other characteristics, e.g. chemical stability, strength, abrasionresistance and thermal conductivity.
In a porous refractory, molten charge, slags, gases etc. are likely to
enter more easily to a greater depth and may react and reduces the life
of the refractory material.
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Properties of Refractories (contd.)
7. Porosity
Porosity decreases Strength
resistance to abrasion
resistance to corrosion/ penetration by
slags, gases ec.
Porosity increases
resistance to thermal spalling ( i.e. thermal
shock-resistance
The densest and least porous brick have the highest thermal
conductivity, owing to the absence of air-voids. In porous bricks, the
entrapped air in the pores, acts as a non-heat conducting material.
A good refractory, in general , should have low porosity.
69
Properties of Refractories (contd.)
8. Thermal Spalling
Breaking, cracking, peeling off or fracturing of a refractory brick or
block, under high temperature. So good refractory must show a good
resistance to thermal spalling.
Spalling is caused by rapid changes in temperature, which causes
uneven expansion and contraction within the mass of refractory, thereby
leading to development of internal stresses and strains.
Spalling may also be due to slag penetration into the refractory brick,
thereby causing variation in the coefficient of expansion.
Spalling can be decreased by
• Using high porosity, low coefficient of expansion and good thermal
conductivity refractory bricks.
• Avoiding sudden temp. changes.
• By overfiring the refractories at high temp. for a sufficiently long
time, whereby mineral inversion et. takes place making the
material less susceptible to uneven expansion or contraction, when
heated.
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Properties of Refractories (contd.)
9. Resistance to abrasion or erosion
good refractory must show a good resistance to abrasion or erosion.
10. Electrical conductivity
good refractory must show a low electrical conductivity. Except
graphite, all other refractories are poor conductors of electricity.
11. Heat capacity
Heat capacity of any substance depends on
(a) Thermal conductivity
(b) Specific heat
(c) Specific gravity
71
Properties of Refractories (contd.)
12. Texture
Course or light –textured bricks, because of their large porosity, are
light in weight and hence, they are more resistant to sudden changes
in temperature. 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 light in weight. These are not so resistant to sudden
changes in temp. However, such bricks are less susceptible to action
and corrosion.
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Properties of Refractories (contd.)
13. Permeability
Measure of rate of diffusion of gases, liquids and molten solids
through a refractory.
Permeability depends upon the size and number of connected pores.
Permeability α temperature α
1
--------------------------Viscosity of molten material
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Manufacture of Refractories
Consist of following steps
1. Crushing: Raw material in the form of big lumps are crushed to about
25 mm size.
2. Grinding: The crushed material are grinded in grinding machine down
to 200 mesh size.
3. Screening: Purify the refractory raw materials and remove unwanted
materials from the raw materials and this is done by
(a) settling
(b) magnetic separation
(C) Chemical Methods
4. Storage: After screening and mineral dressing, pure material is 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.
6. 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.
Mechanical- moulding produces refractories of high density and
strength.
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Manufacture of Refractories (contd.)
Consist of following steps
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 by:
(i) 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 double-pressing: the material is first pressed and allowed to
crack. Then, it is pressed again so as to close the voids.
7. Drying: Removal of moisture is done under well set conditions of
humidity and temperature, depending upon the type of refractories.
Drying is usually carried out in tunnel dryers.
8. Firing: To stabilize and strengthen the structure of refractories, Firing
is done. The bricks are , generally, fired at a temperature as high as
or higher than their use temperature.
It is done in tunnel Kilns or Shaft Kilns or rotary kilns.
Firing temperature: 14800C for high-fired super duty bricks
17000C for kaolin bricks
18700C for basic bricks
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