Refractories act as a thermal barrier between a hot medium and the wall of the containing vessel, represent a chemical protective barrier against corrosion, ensure a physical protection, prevent the erosion of walls by the circulating hot medium and act as thermal insulation for heat retention.
classification of refractories and commonly used refractory bricks
Classification of Refractories And Commonly Used Refractory Bricks
Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of
thermally stable mineral aggregates, a binder phase and additives. These materials have ability to
retain its physical shape and chemical identity when subjected to high temperatures.
Refractories can be classified in the different following ways.
Classification based on chemical composition
1.Acid refractories –These are used in areas where slag and atmosphere are acidic. They are stable
to acids but attacked by alkalis. Example of these refractories are silica (SiO2), zirconia
(ZrO2),alumina, fire-clay refractories etc.
2.Basic refractories –These refractories are used on areas where slags and atmosphere are basic,
they are stable to alkaline slag, dust and fumes at the elevated temperatures. Example of these
refractories are magnesia, dolomite and chromite.
3.Neutral refractories – These refractories are chemically stable to both acids and bases and used
in the areas where slag and environment are either acidic or basic. Examples are carbon graphite,
chromites and alumina. Grphite is the least reactive and is extensively used in the furnaces where
the process of oxidation can be controlled.
Classification based on physical form
1. Shaped refractories – These are commonly known as refractory bricks and are those which have
fixed shape. The shapes can be standard shape or special shape. Standard shaped bricks have
dimensions that are conformed to by most refractory manufacturers and are generally applicable to
kilns and furnaces of the same type. On the other hand, special shaped refractories are specifically
made for particular furnace. These refractory shapes may not be applicable to another furnace of
the same type. Standard shaped refractories are always machine pressed and thus have uniformity
of properties. Special shapes are usually hand molded and are normally associated with slight
variation in properties.
2. Unshaped refractories – These refractories are without definite form and are only given shape
after application. It forms joint less lining and are also known as monolithic refractories. These
refractories are further categorized as plastic refractories, ramming mixes, castables, gunning
mixes, fettling mixes and mortars.
Classification based on method of manufacture
1. dry press process,
2. fused cast,
3. hand molded,
4. formed (normal, fired or chemical bonded)
5.unformed (monolithic, plastics, ramming masses, gunning, castables and spray masses).
6. Un-formed Dry Vibratable refractories
Classification based on the oxide content
1. single oxide refractories such as alumina, magnesia, and zirconia,
2. mixed oxide refractories such as spinel and mullite
3. non oxide refractories such as borides, carbides and silicates.
Classification according to refractoriness
1. Low heat duty refractories – These refractories have refractoriness in the range of 1520 deg C to
1630 deg C and have pyrometric cone equivalent (PCE) value in the range of 19 to 28. Example of
these refractories is silica bricks.
2. Intermediate heat duty refractories – These refractories have refractoriness in the range of 1630
deg C to 1670 deg C and have pyrometric cone equivalent (PCE) value in the range of 28 to 30.
Example of these refractories is fire clay bricks.
3. High heat duty refractories – These refractories have refractoriness in the range of 1670 deg C
to 1730 deg C and have pyrometric cone equivalent (PCE) value in the range of 30 to 33. Example
of these refractories is chromite bricks.
4. Super heat duty refractories – These refractories have refractoriness greater than 1730 deg C
and have pyrometric cone equivalent (PCE) value greater than 33. Example of these refractories is
Refractories act as a thermal barrier between a hot medium and the wall of the containing vessel,
represent a chemical protective barrier against corrosion, ensure a physical protection, prevent the
erosion of walls by the circulating hot medium and act as thermal insulation for heat retention.
Below are some commonly used refractory materials
1.Zirconia refractories- Zirconia refractories contain Zirconium dioxide (ZrO2) which is a
polymorphic material. It has certain difficuties in its usage and fabrication as a refractoy
material ,hence it is stablized by incorporating small quantities of calcium, magnesium and cerium
oxides. The properties of zirconia refractories are dependent on the degree of stabiliation and
quantity of stabilizer as well as the quantity of original raw material. Zirconia refractories have a
very high strength at room temperature which is maintained up to temperatures as high as 1480
deg C. They are used for high temperature application. Since the thermal conductivity of
zirconium dioxide is much lower than other refractory materials, therefore, it is used as a high
temperature insulating refractory. Zirconia refractories have very low thermal losses and does not
reacts with liquid metals and hence it is useful making refractory crucibles.
2.Alumina refractories – Alumina refractories contain alumina (Al2O3) which is one of the most
chemically stable oxides. Alumina offers excellent hardness, strength and spalling resistance. It is
insoluble in water, superheated steam and in most inorganic acids and alkalis. Alumina refractories
carry all purpose characteristics of fireclay refractories into higher temperature ranges which make
these refractories suitable for lining furnaces up to 1850 deg C. Alumina refractories have high
resistance in oxidizing and reducing atmospheres. With increase in alumina content, the
refractoriness of the high alumina refractories increases. These refractories are specified by the
amount of alumina in it. The 50 %, 60 %, 70 % and 80 % classes contain their respective alumina
content with an allowable range of +/- 2.5 %.
3.Silica refractories – Silica refractories are those refractories which contain at least 93 % silica
(SiO2). Silica refractories have the outstanding property of excellent mechanical strength at
temperatures approaching their actual fusion point. This property of silica refractories contrast that
of many other refractories which begins to fuse and creep at temperatures considerably lower than
their fusion points. The major drawback of silica refractories is that they are susceptible to spalling
at temperatures below 650 deg C. Temperature fluctuations above 650 deg C do not affect silica
refractory adversely and in this range it is classed as a good spalling resistant refractory. Silica
refractories need special precaution during heating and during cooling since it undergoes phase
changes. Silica refractories are not of practical use if the furnace is to cool down to room
4.Magnesite refractories – Magnesite refractories are chemically basic refractories containing at
least 85 % magnesium oxide. They are manufactured either from natural occurring magnesite or
sea water magnesia. Raw magnesite is dead burnt to produce magnesia (MgO) for making these
refractories. These refractories have excellent resistance to basic slags specially lime and iron rich
slags but their physical properties are relatively poor. For steel making furnaces especially BOF
(basic oxygen furnace) normally carbon is added to magnesia to produce magnesia carbon
refractories. Magnesia carbon refractories have better resistance to highly basic slags at high
5.Chromite refractories –Chromite refractories is along with chromite, magnesite at present. There
is diffrerence between chrome magnesite and magnesite chrome refractories. While chrome-
magnesite refractories usually contain 15 % to 35 % Cr2O3 an 42 % to 50 % MgO, magnesite-
chrome refractories contain at least 60 % MgO and 8 % to 18 % Cr2O3. Chrome magnesite
refractories are used for building the critical paths of the high temperature furnaces. These
refractories can withstand corrosive slags and gases and have high refractoriness. Magnesite
refractories are suitable for service at the highest temperatures an in contact with more basic slags.
These refractories has better spalling resistance than chrome magnesite refractories.
6.Carbon refractories – In carbon refractories the principle component is carbon. These
refractories are characterized of a high refractoriness, high thermal conductivity and high chemical
resistance but are highly susceptible to oxidation. Beccause of low interfacial tension between
carbon and slag melts, there is little slag infiltration. Carbon refractories are extremely resistant to
thermal shock because of high thermal conductivity and low thermal expansion. carbon
refractories are susceptible to attack by oxygen, steam and CO2 in an oxidizing atmosphere above
400 deg C.
7.Fireclay refractories – Fireclay refractories are essentially hydrated aluminum silicates with
minor proportion of other minerals. The composition of fireclay refractories consists of SiO2 less
than 78 % and Al2O3 less than 44 %. Four standard classes of fireclay refractories are super duty,
high duty, medium duty and low duty. These classes cover the range from around 18 % alumina to
44 % alumina and from about 50 % silica to 80 % silica. Characteristically, fireclay bricks begin
to soften far below their fusion temperature and under load actual deformation take place. The
amount of deformation depends upon the load and once started this deformation is slow but
continuous process unless the load or the temperature is reduced. Due to this reason fire clay
bricks are not being used in wide sprung arches in furnaces operating continuously at high
8.Insulating refractories –Insulating refractories are high porosity refractories with low thermal
conductivity used to reduce the rate of heat flow and thus reduce heat losses by maximizing heat
conservation within a furnace. Insulating refractories are lighter with low densities.