the basic principle of refractories

1. LESSON 2
The basic principle

2. REFRACTORY PRINCIPLES
 Classification of refractories
 The basic principle
 Properties and testing
 Standard properties
 High temperature properties
 Chemical properties

3. THE BASIC PRINCIPLE
 Refractory linings perform 4 primary
functions when material is charged in the
vessel:
 They act as a heat buffer between the walls of
the containing vessel and the hot substance.
 They conserve heat energy,
 reduce energy required for the process, and
 Retain a large proportion of heat in the vessel.

4. THE BASIC PRINCIPLE IS THEREFORE;
 High stability of its oxides and components at
elevated temperatures.
 The measure of its stability is Gibbs standard
free energy (ΔG°). The more negative the value,
the more stable the oxide.
 The value of ΔG° for an oxidation reaction is
thus a measure of chemical affinity of the metal
for oxygen, and the more negative the value of
ΔG° at any temperature, then the more stable
the oxide.

5. THE ELLINGHAM DIAGRAM FOR REFRACTORY
OXIDES
 Ellingham plotted the experimental ΔG°- T
relationships for the oxidation of a series of
metals.
 He found that, the general forms of the
relationships approximated to straight lines
over temperature ranges in which no change
in physical state occurs.

6. ELLINGHAM DIAGRAM READING
 Middle/ intermediate:
 More balanced distribution of metal and oxide
phases, depending on the oxygen potential on the
system.
 Top:
 Elements with strong tendency to remain in the oxide
phases (e.g. Mg, Ca, Al, Zr)
 Common refractory constituents
 Bottom:
 Oxides of elements having a tendency to be
concentrated in the metal phase (e.g. Fe, Ni, Co, Cu)

8. REFRACTORY PRINCIPLES
 Classification of refractories
 The basic principle
 Properties and testing
 Standard properties
 High temperature properties
 Chemical properties

9. PROPERTIES AND TESTING
 A lot of money can be saved by early detection
of specification inconformity.
 It could also potentially generate good profits by
indicating worthwhile new development.
 Testing can be:
 Technical (research & development),Production and
quality assurance department,
 Majority of projects are carried out as customer
service for the marketing and sales department.
 Testing used refractories (Post-mortem), valuable
information on mechanism of refractory wear can be
obtained.

10. REASON FACTORS TO CONSIDER
Product
development
Uncertainly and unpredictability of the outcome of this work
requires a wide range of properties to be tested for.
Quality control/
assurance/ auditing
Big volumes involved require test to be restricted to most
important ones. The selected test must be most indicative to
actual performance for a particular application or with a good
correlation to such properties. The client should be involved to
determine them.
Conforming to
standards (ISO
9002, specification)
Following the procedures required or agreed to.
Suitability for a
specific application
Determine the weir mechanism and use it to establish relevant
properties and tests.
To assess reasons
for anomalous
behavior
A set of reference test done on a previous representative
material is required.
Post mortem
studies aiming at
optimization
As this are used samples, post mortem test will show the
conditions the material was subjected to. Mineralogical and
chemical investigation including the use of microscope
techniques is required.
For design Thermomechanical properties are important test on different

11. REFRACTORY PRINCIPLES
 Classification of refractories
 The basic principle
 Properties and testing
 Standard properties
 High temperature properties
 Chemical properties

12. STANDARD PROPERTIES
 They are properties most commonly
displayed in manufacturer’s data sheets. they
include:
 Chemistry
 mineralogy
 Bulk density and porosity
 Relative density
 Permeability
 Pore size distribution
 Cold crushing strength etc.

13. CHEMICAL ANALYSIS
 It is used to classify the product and to give an
indication of its properties and refractoriness.
 In the aluminosilicate group of refractories, all
oxides other than Al2O3 and SiO2can be
regarded as impurities, of which alkalies are the
least desirable.
 In the basic group, MgO, Cr2O3, Al2O3 and CaO
are considered to be the desirable oxides with
all other as impurities, negatively affecting the
refractories.

14. SOME EXAMPLES OF CHEMICAL COMPOSITIONS OF REFRACTORY BRICKS
BRICK Al2O3 SiO2 MgO CaO Fe2O3 Cr2O3
Silica 0.5 95.5 - 2.5 1.2 -
Fireclay
A4
42.5 53.5 0.6 0.4 2.0 -
Andalusit
e
58.5 37.2 0.3 0.3 0.9 -
Corundu
m
94.7 4.4 0.1 0.1 0.8 -
dolomite 2.0 21.0 35.6 51.7 0.8 -
Magnesite 0.3 1.0 97.0 1.0 0.4 -
Magnesia
chrome
4.0 1.0 70.6 1.9 9.1 13.5

15. MINERALOGICAL ANALYSIS
 It usually involve the study of reactions or
transformations that take place when a
refractory material is subjected to varying
process temperatures and chemical
interactions.
 The mineralogist require a set of high quality
tools, including a high quality polarizing
research microscope, an X-ray
diffractometer, scanning electrode
microscope, etc.

16. BULK DENSITY AND POROSITY
 The two characteristics go hand in hand; a
decrease in porosity leads to an increase in bulk
density.
 Bulk densities of compositionally different
materials cannot be compared since their
relative densities will differ.
 Porosity and bulk density give an indication of
the relative resistance that the material will have
against slag attack.
 There are various methods to determine
porosity and bulk density depending on the
standard used.
 Most methods are based in: a) Evacuation &
b)Mercury volume meter.

17. DENSITIES AND POROSITIES OF REFRACTORY BRICKS
BRICK POROSITY [%] BULK DENSITY
[g/cm3]
RELATIVE
DENSITY [g/cm3]
Fireclay A4 14.3 2.33 2.72
Andalusite 16.8 2.57 3.09
Magnesia-Carbon 6.9 3.02 3.24
Magnesia B1 16.8 2.92 3.51
dolomite 9.0 2.86 3.14

18.  The pressure applied when bricks are
pressed during manufacturing influences
their porosity; the greater the pressure, the
higher the density and the lower the porosity.
 Laminations can occur when too much
pressure is applied. Laminations forms
cracks that adversely affect the porosity and
the permeability of the brick.
 The inherent specific gravity and porosity of
the materials used making the bricks have a
major influence on the porosity of the final
product.
 The lower the inherent porosity of the
particles used, the easier it is to make a low
porosity product.

19. RELATIVE DENSITY (RD)
 The RD of different materials can not be
compared, since the RD depends on the
mineral composition of the specific material.
 But it can be used as the quicker way to tell
whether two materials differ in chemistry.

20. RELATIVE DENSITIES OF REFRACTORY MATERIALS [g/cm3]
Silica 2.32
Alumino -silicates 2.57-3.50
Alumina 3.90
Doloma 3.35
Chrome Magnesia 3.6-3.75
Magnesia Chrome 3.60-3.75
Carbon 1.87-1.97
Magnesia Carbon 3.05- 3.40

21. PERMEABILITY
 Permeability of a material to gas is defined
as the speed at which the gas flows through
the material at a specific pressure difference
and is a directional characteristic.
 It is directional and, although influenced by
porosity, is not always directly proportional.
 Porosity and the ability to let air through are
two important factors to slag attack
resistance.

22.  A porous material allows air to pass through,
slag will also penetrate easily.
 For porosity exceeding 23%, no relation of
porosity and permeability exists, but at lower
levels the permeability drops in relation to
porosity.

23. PORE SIZE DISTRIBUTION
 By forcing mercury into the compact at
different pressures and by determining the
volume absorbed at different pressure, more
information can be obtained concerning the
pore sizes.
 The pore size distribution has an important
function; it net only affects the resistance of a
product to slag attack, but also its resistance
to shock.

24. COLD CRUSHING STRENGTH
 Conventional oxide refractories are strong at
low temperatures and weak at high
temperatures.
 The strength of a material is defined as the
stress under which failure occurs.
 With refractories tensile strength is much
less than compressive strength.
 In fact, if a specimen is loaded in
compression, failure occurs not in
compression but in shear.

25.  This test indicate how hard the brick has been
fired and whether the brick is strong enough to
survive handling.
 The cold crushing strength is calculated as the
force per unit area after fracture has occurred.
 The minimum strength required for handling
bricks is 4MPa.
 Low crushing strengths go hand in hand with
high porosities, poor firing and low resistance to
abrasion.

26.  When a rick has a very high crushing
strength, the high temperature strength
needs to be determined as well, since such
bricks normally form liquids at very high
temperatures, and are therefore not suitable
for use.
 Apart from indicating consistency of bond
formation in a batch of bricks, it is a measure
of resistance to static loading and abrasion at
relatively low temperatures.

27. REFRACTORY PRINCIPLES
 Classification of refractories
 The basic principle
 Properties and testing
 Standard properties
 High temperature properties
 Chemical properties