Properties of
Refractories
Refractories are
those materials which have high melting points and have properties which make
them suitable to act as heat resisting barriers between high and low
temperature zones.
The general
requirements of refractories include (i) ability to withstand high temperatures
and trap heat within a limited area such as a furnace, (ii) ability to
withstand action of liquid metal, hot gasses and liquid slag by resisting
erosion and corrosion etc. (iii) ability to withstand load at service
environment, (iv) ability to resist contamination of the material with which it
comes into contact, (v) ability to maintain necessary dimensional stability at
high temperatures and after/during repeated thermal cycling, and (vi) ability
to conserve heat.
Important properties
of refractories include chemical composition, bulk density, apparent porosity,
apparent specific gravity and strength at atmospheric temperatures. These
properties are frequently among those which are used as ‘control points’ in the
manufacturing and quality control process. The chemical composition serves as a
basic for classification of refractories and the density, porosity and strength
are influenced by many other factors. Among these are type and quality of the
raw materials, the size and fit of the particles, moisture content at the time
of pressing, pressure at mould, firing temperature, duration of firing, and the
rate of cooling.
Properties of the refractories can be classified into three main category. namely (i) Physical, (ii) Thermal, and (iii) Chemical
Physical Properties:
Porosity and density – Low porosity of the refractory brick
is desirable since it improves the mechanical strength and other properties of
the refractories. True porosity of a refractory brick is the ratio of the total
pore space (i.e. open and closed pores) of a body to its volume and is
expressed in volume percent. The formula for true porosity is as follows.
True porosity = (S- R)/S X 100 volume %, where S is the density
and R is the bulk density.
The density is the quotient of mass and volume excluding pore
space and is determined on finely crushed material. Frequently, not the values
of the true porosity but those of apparent porosity (open porosity) are used as
the application property. The apparent porosity includes only those holes which
can be infiltrated by water and not the closed holes.
High porosity materials tend to be highly insulating as a result
of high volume of air they trap, since air is a very poor thermal conductor. As
a result, low porosity materials are generally used in hotter zones, while the
more porous materials are usually used for thermal backup. Such materials,
however, do not work with higher temperatures and direct flame impingement, and
are likely to shrink when subjected to such conditions. Refractory materials
with high porosity are usually not chosen when they are to be in contact with
liquid slag since they can be penetrated as easily.
The bulk density is generally considered in conjunction with
apparent porosity. It is a measure of the weight of a given volume of the
refractory. For many refractories, the bulk density provides a general
indication of the product quality. It is considered that the refractory with higher
bulk density (low porosity) is better in quality. This is because an increase
in bulk density increases the volume stability, the heat capacity, as well as
the resistance to abrasion and slag penetration.
Cold crushing strength – The cold crushing strength determines the
ability to withstand the rigorous of transport and handling before the
installation of refractories in the furnace. It can be seen as a useful
indicator to the adequacy of firing and abrasion resistance in consonance with
other properties such as bulk density and porosity. The cold crushing strength
is determined by the methods described in various standards. In order to
evaluate the behaviour at the service temperatures, the hot crushing strength
is also sometimes determined in addition to the cold crushing strength.
Abrasion resistance – The mechanical stress of refractories
is not caused by the pressure alone, but also by the abrasive attack of the
solid charge materials as they slowly pass over the brickwork inside the
furnace. The mechanical stress can also be due to the impingement of the fast
moving gasses loaded with fine solid dust particles. The abrasive stress is
quite well simulated by Bohme’s grinding machine, but the results normally
cannot be applied to the conditions existing in the furnaces being operated at
high temperatures, especially when the strength of the refractory brick is
getting changed due to the chemical influences. There is presently no approved
measuring method available for measuring abrasion resistance, and the abrasion
factor according to Bohme is still being used as a reference value.
Cold modulus of rupture and deformation modulus – Due to the thermal
stress, generally combined with altered physical- chemical conditions because
of infiltrations, strain conditions occur in the refractory brickwork which can
lead to rupture and crack formation. In order to determine the magnitude of the
rupture stress, the resistance to deformation under bending stress, i.e. rupture
strength is measured. Because of their heterogeneous coarse ceramic structure,
refractory bricks are purely elastic within a narrow deformation region.
The ratio of stress of deformation within this purely elastic initial region,
where the deformation is still reversible, is described as the modulus of
elasticity. This is normally measured dynamically by means of ultrasonic or
resonance frequency. Progressive deformation up to rupture is outside the
purely elastic region with the deformations on the less resistant fines content
and for this reason the stress increases more slowly relative to the
deformation. Then the modulus of elasticity is no longer sufficient to
calculate the stress which occurs. Hence, the ratio of rupture stress to
rupture deformation has been introduced as the modulus of deformation. In case
of refractories this modulus is smaller than or at the most equal to the
modulus of elasticity. The determination of the modulus of deformation in cold
condition is carried out, together with the modulus of rupture, on a test bar
resting on two bearing edges and applying a load to the center of the bar.
Thermal Properties:
Pyrometric cone equivalent – Refractories due to their chemical
complexity melt progressively over a range of temperature. The refractoriness
of the refractories is one of the most important properties of the refractory.
As the refractories hardly ever consist of a single compound, reference is
made, not to a specific melting point, but to a softening region. This is
determined with the help of comparative ceramic samples of known softening
behaviour usually known as ‘Seger cones’ (Fig 1). Pyramids are cut from the
refractory bricks which are to be tested. The method of testing is described in
various standards. The equivalent standard cone which melts to the same extent
as the test cone is known as the pyrometric cone equivalent (PCE). The
cone values reported for refractories are based on a defined standard time –
temperature relationship, so different heating rates result in different PCE
values.
Fig 1 Seger cones
Refractoriness under load – Refractoriness points to the
resistance of extreme conditions of heat (temperature more than 1000 deg C) and
corrosion when hot and liquid materials are contained while being transported
and/or processed. The ability to withstand exposure to higher temperatures
without undergoing appreciable deformation is measured in terms of refractoriness.
The tests for refractoriness under load serve to evaluate the softening
behaviour of fired refractory bricks at rising temperature and under constant
load conditions. The softening behaviour under load is not identical to the
melting range of pure raw materials, but is influenced by the content and the
degree of distribution of the low melting point flux agents.
The refractoriness under load test provides an indication of the
temperature at which the refractory bricks collapse, in service conditions with
similar load. However, under actual service where the bricks are heated only on
one face, most of the load is carried by the relatively cooler rigid portion of
the bricks. Hence the refractoriness under load test gives only an index of the
refractory quality, rather than a figure which can be used in a refractory
design. Under service conditions, where the refractory used is heating from all
sides such as checkers, partition walls, etc. the test data is quite
significant.
Refractoriness under load (differential) – In order to
eliminate errors as a result of the inherent expansion of the test equipment
when the refractoriness under load is being tested, and to enable tests to be
carried out in an oxidizing atmosphere, a different method has been developed
to determine the resistance at rising temperature and constant load which is
the refractoriness under load (differential) test. In this method same type of
samples are used as for the refractoriness under load test bur they have an
internal bore to permit rods to be fitted to the upper and the lower sides.
With this method, temperature values are obtained by differential measurements
in an oxidizing atmosphere and these values are considerably lower than the
refractoriness under load values.
Thermal expansion under load (creep) – Refractory
materials must maintain dimensional stability under extreme temperatures
(including repeated thermal cycling) and constant corrosion from very hot
liquids and gases. The thermal expansion under load (creep) of refractory
bricks, which are heated evenly on all sides over a long period during service,
can be tested by a long term test called creep under load. It is a time
dependent property which determines the deformation in a given time and at a
given temperature by a material under stress. The test is conducted as
described in the various standards. For this, the sample of 50 mm diameter and
50 mm height, with an internal bore for the measuring rod, is heated at a
constant speed and under a given load (normally 0.2 N/sq mm) and is normally
held for 10 hours to 50 hours under constant load after the required test
temperature has been reached. The compression of the sample, after
maximum expansion has been attained, is given in relation to the test time as a
measure of creep at a specified test temperature.
Hot modulus of rupture – Hot modulus of rupture is the
resistance to bending stress at high temperature. The resistance to bending
stress of refractory products provides information on their deformation
behaviour at high temperatures. The test samples are bars which are heated in
electric chamber kilns. For the test, the bars are placed on bearing edges of
the kiln and are stressed until fracture occurs at the test temperature by
applying an increasing load to the centre of the bar.
Thermal expansion – All materials experience a change in
volume under the influence of temperature. The contraction or expansion of the
refractories can take place during service. Such permanent changes in
dimensions may be due to (i) the changes in the allotropic forms which cause a
change in specific gravity, (ii) a chemical reaction which produces a new
material of altered specific gravity, (iii) the formation of liquid phase, (iv)
sintering reactions, and (v) may happen on account of fluxing with dust and
slag or by the action of alkalis on fireclay refractories, to form
alkali-alumina silicates, causing expansion and disruption. The reversible
linear expansion curve of most of the refractory bricks is more or less
straight although the absolute amount varies considerably. Silica bricks,
however, have an irregular and strong thermal expansion in the temperature
range of up to 700 deg C. By changes in structure or in firing methods of
refractory bricks, the expansion curve can be influenced within certain
limits. Bricks with high expansion are very susceptible to thermal shock.
Thermal expansion is important in service, as the effects of expansion are to
be taken into account during the installation of refractory lining. If not
done, then edge pressure and premature spalling of the bricks take place.
Reheat change (after shrinkage and after expansion) – After heating to
high temperature and subsequent cooling, a permanent change in the dimension
(permanent linear change) often occurs which is described as after expansion or
after shrinkage. If a refractory brick has very strong after shrinkage then the
joints get enlarged and the brickwork is loosened and no longer tight. In
opposite case, after expansion is also dangerous since this can cause the
destruction of the brickwork through pressure. The permanent linear change
(PLC) of the refractories can be influenced. For obtaining a brick with a
constant volume, the burning of the raw materials and the firing of the bricks
is to be controlled in such a manner so that equilibrium is achieved at the
desired temperature.
Thermal shock resistance – Thermal shock resistance is one of the
most important service properties. It characterizes the behaviour of
refractories to sudden temperature shocks which occurs very often during the
furnace operation. Temperature fluctuations can reduce the strength of the
brick structure to a high degree and can lead to disintegration or spalling in
layers. There are two standard methods for the testing of the thermal shock
resistance. They are (i) water quenching method, and (ii) air quenching method.
In the water quenching method, the test piece is a standard cylinder which is
heated to 950 deg C and then quenched in running cold water. The test is
repeated till the destruction of the sample but not more than 30 times. The
number of quenching till destruction determines the thermal shock resistance.
Air quenching method is used for bricks which are susceptible to hydration. The
process in this method is same except that the quenching is done by blowing
compressed air in place of running water. The thermal shock resistance of
refractories often decreases with increasing firing levels. Those refractories
which retain their quenching resistance even after higher firing temperatures
or service temperatures are the most useful in practice.
Thermal conductivity – Thermal conductivity is defined as the
quantity of heat which will flow through a unit area in direction normal to the
surface area in a defined time with a known temperature gradient under steady
state conditions. It indicates general heat flow characteristics of the
refractory and depends upon the chemical and mineralogical compositions as well
as the application temperature. The unit of the thermal conductivity of
refractories is W/K*m. The determination of thermal conductivity is carried out
by the plate, ball, hollow cylinder or hot wire method. In addition to the
temperature, the co-efficient of thermal conductivity depends also on the
composition of raw materials, the mineralogical structure of the brick mix,
true porosity and pore size, firing temperature and grading. Hence the absolute
values of the thermal conductivity vary widely for the different types of the
refractories. There is a negative temperature gradient of thermal conductivity
in bricks which consist almost exclusively of crystalline components.
Bricks with a high portion of the glassy phase generally have a positive and
small gradient. With rising temperatures the vibration conditions of the
crystalline, non-metallic materials become similar to those of the amorphous
materials and this leads to converging values of the thermal conductivity.
Porosity is a significant factor in heat flow through refractories. The thermal
conductivity of a refractory decreases on increasing its porosity
Chemical Properties:
Chemical composition – The chemical composition of
refractories is of great importance with respect to attack by liquid slags,
flue dusts, glass melts and vapours. Based on the behaviour during contact
reactions, the refractories can be categorized as (i) acid refractories (silica,
silicon carbide, zircon cristobalite, and zircon silicate etc.), (ii) basic
refractories (dolomite, magnesite, chrome magnesite, magnesite chrome, and
forsterite etc.), and (iii) neutral refractories (alumina, carbon, fire clay,
and chromite etc.). Chemical analysis of different refractories can be carried
out by methods described in various standards.
Mineralogical composition and crystal formation – The behaviour of
refractories during their use also depends on the type of raw materials used
and on the reactions achieved during firing of the refractories. A glassy
phase is more susceptible to attack by slag than a tightly interlocked lattice
structure. Aggregate of small crystals can be dissolved and transformed more
rapidly by infiltrating substances than closed crystalline materials. Two
methods are normally used to recognize mineralogical composition. In the first
method a polarizing microscope or a scanning electron microscope is used to
identify the crystal structure while in the second method the crystalline
phases are examined by X-ray examination or by X-ray diffraction analysis.
Gas permeability – Gas permeability describes the property of a solid porous refractory to allow gas to flow through in from one direction under the influence of a pressure gradient. The gas flow can be carried out by means of a pressure or suction pump. The methods of determination of gas permeability are described in various standards. The dimension of gas permeability is sq cm or Perm. As this unit is rather large, the dimensions terms where these units are divided by 10 to the power nine are normally used.
Resistance to slag, glass melts, gasses, and vapours – The slag resistance characterize the resistance of refractories to chemical attack of any type, including that of glass and vapours. The destructive agents coming into contact with refractories are of different composition and the destruction processes are correspondingly many. The method of determination of slag resistance is described in various standards. Refractory brick destructions are not only caused by liquid slags and glass melts but gasses and vapours also influence the service life of the refractory bricks due to several individual reasons
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