Refractories
Refractories are materials that can withstand high temperatures without softening or deformation in shape. Refractories are mainly used for construction of lining in furnaces, kilns, converters, etc. The main function of a refractory is to withstand and maintain high temperatures and to resist the abrasive and corrosive action of molten metals, slags and gases.
Objective of a refractory
The main objective of a refractory is to resist heat losses and also to resist the abrasive and corrosive action of molten metals, slags and gases at higher temperatures, without softening or deformation in shape.
Uses of Refractories
1. Refractories are mostly used for the construction of the lining of the furnaces, tanks, converters, kilns, crucibles, ladles, etc.
2. They are employed for the manufacture of cement, glass, ceramics, paper, metals (both ferrous and non-ferrous ), etc.
Characteristics or requisites of a good refractory
(i) It should be infusible at the operating temperatures.
(ii) It nshould be chemically inert towards the corrosive gases, metallic slags and liquids.
(iii) It should resist the abrading action of flue gases, flames, etc.
(iv) It should not crack and suffer loss in size, at the operating temperatures.
(v) It should expand and contract uniformly, with temperature rise and fall respectively.
(vi) It should be able to withstand overlying load of structures, at operating temperatures.
(vii) It should have high refractoriness.
Classification of refractories
Refractories are classified in the following two ways
According to their chemical properties.
According to their refractoriness.
I. According to their chemical properties
According to the chemical properties, refractories are classified into three main types
Examples (i) Silicon (i) Magnesite (i) Graphite
(ii) Alumina (ii) Dolomite (ii) Carborundum
1. Acidic Refractories
Acidic refractories consists of acidic materials loke alumina(al203) and
Silica (SiO2). They are not attacked by acidic materials, but easily attacked by
basic materials.
Example:
Silica, alumina, fire clay refractories.
2. Basic Refractories
Basic Refractories consist of basic materials like Ca02, Mgo, etc. They are
not attacked by basic materials, but are easily attacked by acidic materials.
Example:
Magnesite, Dolamite refractories.
3. Neutral Refractories
Neutral refractories are made from weakly acidic and basic materials like Carbon, Chromite, Zirconia, etc. They are not attacked by both acidic and basic materials.
Example:
Graphite, Chromite, Zirconia, Carborundum refractories.
II. According to their refractoriness
According to the refractoriness, refractories are classified into four types.
Type of Refractories with PCE value
Example
1. Low heat duty refractories
2. Intermediate heat duty refractories
3. High heat duty refractories
4. Super heat duty refractories
Tuesday, November 17, 2009
Properties of refractories
Properties of refractories
1. Refractoriness
It is the ability of a material to withstand very high temperature without softening or deformation under particular service condition.
How to measure refractoriness
Since most of the refractories are mixtures of several metallic oxides, they do not have a sharp melting points. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed interms of Pyrometric cone Equivalent (PCE).
Pyrometric cone Equivalent (PCE)
Pyrometric cone equivalent is the number which represents the softening temperature of a refractory specimen of standard dimension (38 mm and 19 mm triangular base) and composition.
Objectives of PCE Test
1. To determine the softening temperature of a test refractory material.
2. To classify the refractories.
3. To determine the purity the refractories .
4. To check whether the refractory can be used at the particular servicing temperature.
Measurement
Refractoriness is determined by comparing the softening temperature of a test cone with that of a series of segar cones. Segar cone (also called pyrometric cones) are pyramid shapes.
A test cone is prepared from a refractory, for which the softening temperature to be determined as same dimensions of segar cones. Then test cone is placed in an electric furnace along with segar cones. The furnace is heated at a standard rate of 10° C per minute during which softening of segar cone occur along with test cone. The temperature at which the apex of the cone touches the base is softening temperature.
1. Refractoriness
It is the ability of a material to withstand very high temperature without softening or deformation under particular service condition.
How to measure refractoriness
Since most of the refractories are mixtures of several metallic oxides, they do not have a sharp melting points. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed interms of Pyrometric cone Equivalent (PCE).
Pyrometric cone Equivalent (PCE)
Pyrometric cone equivalent is the number which represents the softening temperature of a refractory specimen of standard dimension (38 mm and 19 mm triangular base) and composition.
Objectives of PCE Test
1. To determine the softening temperature of a test refractory material.
2. To classify the refractories.
3. To determine the purity the refractories .
4. To check whether the refractory can be used at the particular servicing temperature.
Measurement
Refractoriness is determined by comparing the softening temperature of a test cone with that of a series of segar cones. Segar cone (also called pyrometric cones) are pyramid shapes.
A test cone is prepared from a refractory, for which the softening temperature to be determined as same dimensions of segar cones. Then test cone is placed in an electric furnace along with segar cones. The furnace is heated at a standard rate of 10° C per minute during which softening of segar cone occur along with test cone. The temperature at which the apex of the cone touches the base is softening temperature.
characteristics of good refractory
A good refractory should have high refractoriness.
1. Refractoriness under load ( RUL) (or) strength
The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
Refractories used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be measured by RUL test.
RUL Test
RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2 and height 75 cm and heating in a furnace at a standard rate of 10°C per minute. A good refractory should have high RUL value.
2. Porosity
It is defined as the ratio of its pore volume to the bulk volume. Porosity is an important property of refractory bricks, because it affect many other characteristics like chemical stability, strength, abrasion- resistance and thermal conductivity.
Disadvantages of high porosity refractory
(i) It reduces the strength.
(ii) It reduces resistance to abrasion.
(iii) It reduces the resistance to corrosion.
Advantages of high porosity refractory
(i) Highly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it be used for lining in furnaces, ovens, etc.
(ii) Highly porous refractory reduces thermal spalling.
A good refractory should have low porosity.
4. Thermal spalling
Thermal spelling is the property of breaking, cracking or peeling off a refractory material under high temperature.
Thermal spalling is mainly due to
(i) Rapid change in temperature
This causes uneven expansion and contraction within the mass of a refractory, and leads to development of uneven stresses and strains.
(ii) Slag penetration
This causes variation in the co-effcient of expansion and leads to spalling.
Thermal spalling can be decrased by
(i) Using high porosity , low co-effcient of expansion and good thermal conductivity refractory.
(ii) A voiding sudden temperature changes.
(iii) By modifying the furnace design.
A good refractory must show a very good resistance to thermal splling
5. Dimensional stability
It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time.
These dimensional changes are of two types
(i) Reversible or (ii) Irreversible.
(i) Reversible dimensional changes
This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible.
(ii) Irreversible dimensional changes
This may result either in the contraction or expansion of a refractory.
Example-1
Magnesite brick shrink in service
Magnesite is an amorphous material (specific gravity is 3.05). On heating it is gradually converted into more dense crystalline form of periclase (Sp. Gravity = 3.54)
Magnesite Periclase (Amorphous) (Crytalline)
Sp.gr = 3.05 sp.gr = 3.54
Example-2
Silica bricks expand in service.
Silica bricks expand on heating due to the transformation of one from to anther forms. This is accompanied by a considerable increase in volume. A good refractory should have high dimentional stability.
1. Refractoriness under load ( RUL) (or) strength
The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
Refractories used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be measured by RUL test.
RUL Test
RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2 and height 75 cm and heating in a furnace at a standard rate of 10°C per minute. A good refractory should have high RUL value.
2. Porosity
It is defined as the ratio of its pore volume to the bulk volume. Porosity is an important property of refractory bricks, because it affect many other characteristics like chemical stability, strength, abrasion- resistance and thermal conductivity.
Disadvantages of high porosity refractory
(i) It reduces the strength.
(ii) It reduces resistance to abrasion.
(iii) It reduces the resistance to corrosion.
Advantages of high porosity refractory
(i) Highly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it be used for lining in furnaces, ovens, etc.
(ii) Highly porous refractory reduces thermal spalling.
A good refractory should have low porosity.
4. Thermal spalling
Thermal spelling is the property of breaking, cracking or peeling off a refractory material under high temperature.
Thermal spalling is mainly due to
(i) Rapid change in temperature
This causes uneven expansion and contraction within the mass of a refractory, and leads to development of uneven stresses and strains.
(ii) Slag penetration
This causes variation in the co-effcient of expansion and leads to spalling.
Thermal spalling can be decrased by
(i) Using high porosity , low co-effcient of expansion and good thermal conductivity refractory.
(ii) A voiding sudden temperature changes.
(iii) By modifying the furnace design.
A good refractory must show a very good resistance to thermal splling
5. Dimensional stability
It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time.
These dimensional changes are of two types
(i) Reversible or (ii) Irreversible.
(i) Reversible dimensional changes
This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible.
(ii) Irreversible dimensional changes
This may result either in the contraction or expansion of a refractory.
Example-1
Magnesite brick shrink in service
Magnesite is an amorphous material (specific gravity is 3.05). On heating it is gradually converted into more dense crystalline form of periclase (Sp. Gravity = 3.54)
Magnesite Periclase (Amorphous) (Crytalline)
Sp.gr = 3.05 sp.gr = 3.54
Example-2
Silica bricks expand in service.
Silica bricks expand on heating due to the transformation of one from to anther forms. This is accompanied by a considerable increase in volume. A good refractory should have high dimentional stability.
General methods of manufacture of refractories
General methods of manufacture of refractories
The manufacture of refractories involve the following steps.
1. Grinding
Raw materials crushed and ground to fine powder using crushers, pulveriser, ball mills.
2. Mixing
In order to alter the chemical properties of the refractories, two or more powdered raw materials are thoroughly mixed with a suitable binding material, which makes moulding easier.
3. Moulding
Moulding can be done either manually or mechanically by application of high pressure.
Hand-moulding produces refractories of low strength and low density. Mechanical-moulding produces refractories of high strength and high density
4. Drying
Drying is carried out slowly to the moisture from refractories.
5. Firing
It is done at a temperature as high as or higher than their use temperature . Firing is generally carried out in kilns.
The refractories are fired,
(i) to stabilize and strengthen their structures.
(ii) to remove water of hydration.
(iii) to facilitate development of stable mineral to form the finished products.
The manufacture of refractories involve the following steps.
1. Grinding
Raw materials crushed and ground to fine powder using crushers, pulveriser, ball mills.
2. Mixing
In order to alter the chemical properties of the refractories, two or more powdered raw materials are thoroughly mixed with a suitable binding material, which makes moulding easier.
3. Moulding
Moulding can be done either manually or mechanically by application of high pressure.
Hand-moulding produces refractories of low strength and low density. Mechanical-moulding produces refractories of high strength and high density
4. Drying
Drying is carried out slowly to the moisture from refractories.
5. Firing
It is done at a temperature as high as or higher than their use temperature . Firing is generally carried out in kilns.
The refractories are fired,
(i) to stabilize and strengthen their structures.
(ii) to remove water of hydration.
(iii) to facilitate development of stable mineral to form the finished products.
Refractory bricks
Some important refractory bricks
Alumina bricks (or) Fire clay bricks (Acidic refractories)
Alumina bricks contain 50% or more of Al2O3. They are generally manufactured by mixing calcined bauxite (Al2O3 ) with clay binder.
Manufacture
1. Grinding and mixing
The raw materials (calcined bauxite & SiO2 ) and grog (calcined fire clay ) are ground to fine poeder and mixed and mixed with required amount of water to convert it into pasty material.
2. Moulding
The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting.
3.Drying and Firing
The bricks after moulding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to about 1200-1400 for 6-8 days.
Properties
(i) Alumina bricks are acidic refractories.
(ii) They posses very low coefficient of expansion.
(iii) They also posses high porosity, and high temperature load-bearing capacity.
(iv) They are also very stable to both in oxidizing and reducing conditions.
(v) Then posses better resistance to thermal to thermal spalling than silica bricks.
Uses
1. Medium-duty bricks (containing 50 to 60% Al2O3)
It is used in lining of cement rotary kilns soaking pita, reheating furnaces, hearts and walls, etc., which are subjected to high abrasion.
2. High-dury bricts (containing 75% Al2O3)
It is used in hottest zone of cement rotary kilns, lower parts of soaking pits, brass melting reverberatories, aluminium melting furnaces, etc.,
3. Fire clay refractories are larely using in steel industries
Magnesite bricks (Basic refractories )
Mangesite bricks contain mainly MgO. They are generally manufactured by mixing calcined magnesite with caustic magnesia or iron oxide as binding material.
Manufacture
1. Grinding and mixing
The raw materials (calcined magnesite) and binding materials (caustic magnesia) are ground to fine powder and mixed with water to a pasty material.
2. Moulding
Moulding is usually done by machine pressing to a required shape.
3. Drying and firing
Drying is carried out at ordinary temperature to remove the moisture. Firing is done in a kiln at 1500oC for 8 hours and then cooled slowly.
Properties
1. Magnesite bricks are basic refractories.
2. They can be used upto 2000oC without load and upto 1500oC Under a load of 3.5 kg/cm2.
3. They have good resistance to basic slags, but combine with H2O and CO2
4 . They possess good strength, little shrinkage and have lot of spalling.
5. They have poor resistance to abrasions.
Uses
1. They are used where high temperature is required to be maintained, together with great resistance to basic materials.
2. They are used in steel industry for the lining of basic converters and and open –hearth furnaces.
3. They are also used in hot mixer linings, copper converters and reverberatory furnaces.
Zirconia bricks ( Neutral refractories)
Manufacture
They are prepared by mixing zirconite mineral (ZrO2 ) with colloidal zirconia or alumina as binder and finally heated to 1700oC. Small amount of MgO or Cao is added as stabilizer because mineral zirconite undergoes volume changes on hearting and cooling.
Properties
1. Zirconia bricks are neutral refractories.
2. Though zirconia bricks are neutral, they are affected by acidic slags.
3. They can be used upto 2000oC and upto 1500oC under a load of 3.5 kg/cm2 .
4. They are also quie resistant to thermal shocks.
5. Their thermal expansion is low.
Uses
They are used only where very high temperature is maintained, e.g., high – frequency electric furnaces.
Alumina bricks (or) Fire clay bricks (Acidic refractories)
Alumina bricks contain 50% or more of Al2O3. They are generally manufactured by mixing calcined bauxite (Al2O3 ) with clay binder.
Manufacture
1. Grinding and mixing
The raw materials (calcined bauxite & SiO2 ) and grog (calcined fire clay ) are ground to fine poeder and mixed and mixed with required amount of water to convert it into pasty material.
2. Moulding
The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting.
3.Drying and Firing
The bricks after moulding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to about 1200-1400 for 6-8 days.
Properties
(i) Alumina bricks are acidic refractories.
(ii) They posses very low coefficient of expansion.
(iii) They also posses high porosity, and high temperature load-bearing capacity.
(iv) They are also very stable to both in oxidizing and reducing conditions.
(v) Then posses better resistance to thermal to thermal spalling than silica bricks.
Uses
1. Medium-duty bricks (containing 50 to 60% Al2O3)
It is used in lining of cement rotary kilns soaking pita, reheating furnaces, hearts and walls, etc., which are subjected to high abrasion.
2. High-dury bricts (containing 75% Al2O3)
It is used in hottest zone of cement rotary kilns, lower parts of soaking pits, brass melting reverberatories, aluminium melting furnaces, etc.,
3. Fire clay refractories are larely using in steel industries
Magnesite bricks (Basic refractories )
Mangesite bricks contain mainly MgO. They are generally manufactured by mixing calcined magnesite with caustic magnesia or iron oxide as binding material.
Manufacture
1. Grinding and mixing
The raw materials (calcined magnesite) and binding materials (caustic magnesia) are ground to fine powder and mixed with water to a pasty material.
2. Moulding
Moulding is usually done by machine pressing to a required shape.
3. Drying and firing
Drying is carried out at ordinary temperature to remove the moisture. Firing is done in a kiln at 1500oC for 8 hours and then cooled slowly.
Properties
1. Magnesite bricks are basic refractories.
2. They can be used upto 2000oC without load and upto 1500oC Under a load of 3.5 kg/cm2.
3. They have good resistance to basic slags, but combine with H2O and CO2
4 . They possess good strength, little shrinkage and have lot of spalling.
5. They have poor resistance to abrasions.
Uses
1. They are used where high temperature is required to be maintained, together with great resistance to basic materials.
2. They are used in steel industry for the lining of basic converters and and open –hearth furnaces.
3. They are also used in hot mixer linings, copper converters and reverberatory furnaces.
Zirconia bricks ( Neutral refractories)
Manufacture
They are prepared by mixing zirconite mineral (ZrO2 ) with colloidal zirconia or alumina as binder and finally heated to 1700oC. Small amount of MgO or Cao is added as stabilizer because mineral zirconite undergoes volume changes on hearting and cooling.
Properties
1. Zirconia bricks are neutral refractories.
2. Though zirconia bricks are neutral, they are affected by acidic slags.
3. They can be used upto 2000oC and upto 1500oC under a load of 3.5 kg/cm2 .
4. They are also quie resistant to thermal shocks.
5. Their thermal expansion is low.
Uses
They are used only where very high temperature is maintained, e.g., high – frequency electric furnaces.
Abrasives
Abrasives
Definition
Abrasive are hard substances used for polishing shaping operations. They are characterized by high melting point high hardness chemically inactive.
Properties of abrasives
1. Hardness
It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale.
Measurement of hardness using Moh’s scale.
Moh’s scale is a scale, in which common abrasives (natural or artificial) are arranged in the order of their increasing hardness.
Soft abrasives
Abrasives having their hardness 1-4 in Moh’s scale are know as abrasives.
2. Toughness
Abrasive are generally hard and brittle, which is otherwise known as toughness.
3. Abrasive power
It is the strength of an abrasive to grind away another materials. It depends on hardness, toughness and refractoriness.
Characteristics of abrasives
The following are the important characteristics of an abrasive.
(a) It should be very hard.
(b) It should resist the abrading action.
(c) It should be chemically inactive.
(d) It should possess high refractoriness.
(e) It should have high melting point.
(f) It should not be affected by frictional heat.
Classification of abrasives
Abrasive are classified into two types
1. Natural abrasive
(a) Nonsiliceous abrasive (d) Siliceous abrasives
2. Artificial or synthetic abrasive
Definition
Abrasive are hard substances used for polishing shaping operations. They are characterized by high melting point high hardness chemically inactive.
Properties of abrasives
1. Hardness
It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale.
Measurement of hardness using Moh’s scale.
Moh’s scale is a scale, in which common abrasives (natural or artificial) are arranged in the order of their increasing hardness.
Soft abrasives
Abrasives having their hardness 1-4 in Moh’s scale are know as abrasives.
2. Toughness
Abrasive are generally hard and brittle, which is otherwise known as toughness.
3. Abrasive power
It is the strength of an abrasive to grind away another materials. It depends on hardness, toughness and refractoriness.
Characteristics of abrasives
The following are the important characteristics of an abrasive.
(a) It should be very hard.
(b) It should resist the abrading action.
(c) It should be chemically inactive.
(d) It should possess high refractoriness.
(e) It should have high melting point.
(f) It should not be affected by frictional heat.
Classification of abrasives
Abrasive are classified into two types
1. Natural abrasive
(a) Nonsiliceous abrasive (d) Siliceous abrasives
2. Artificial or synthetic abrasive
Natural abrasive
Natural abrasive
(a) Non- Siliceous Abrasives
It is a pure crystalline carbon. It is the hardest known substance. Its hardness is 10 on Moh’s scale. It is chemically inert and not affected by acids or alkalis. The off- colour diamond is called borts and black colour diamond is called carbonado.
Uses : It is used in drill points, cutting rocks, stones and grinding wheels.
2. Corundum
It is a pure crystalline alumina (Al2O3). Its hardness on Moh’s scale is 9.
3. Emery
It is a used for grinding glasses, gems, lenses, metals,etc.
(i) 55-75% crystalline alumina,
(ii) 20-40% magnesite,
(iii) 12% other minerals.
Uses : It is a used in the tip of cutting and drilling tools, and also it is used in making abrasive paper and cloth.
(b) Siliceous abrasive
1. Quartz
It is pure crystalline silica (SiO2). Its hardness is 7 on Moh’s scale.
Uses: It is used for grinding pigments in the paint industry and also as granules in grinding machines.
2. Garnet
It is a mixture of trisilicates of alumina magnesia and ferrous oxide. Its hardness ranges from 6-7.5 on Moh’s scale.
Uses : It is used in making abrasive paper and abrasive cloth and also in glass grinding and polishing metals.
(a) Non- Siliceous Abrasives
It is a pure crystalline carbon. It is the hardest known substance. Its hardness is 10 on Moh’s scale. It is chemically inert and not affected by acids or alkalis. The off- colour diamond is called borts and black colour diamond is called carbonado.
Uses : It is used in drill points, cutting rocks, stones and grinding wheels.
2. Corundum
It is a pure crystalline alumina (Al2O3). Its hardness on Moh’s scale is 9.
3. Emery
It is a used for grinding glasses, gems, lenses, metals,etc.
(i) 55-75% crystalline alumina,
(ii) 20-40% magnesite,
(iii) 12% other minerals.
Uses : It is a used in the tip of cutting and drilling tools, and also it is used in making abrasive paper and cloth.
(b) Siliceous abrasive
1. Quartz
It is pure crystalline silica (SiO2). Its hardness is 7 on Moh’s scale.
Uses: It is used for grinding pigments in the paint industry and also as granules in grinding machines.
2. Garnet
It is a mixture of trisilicates of alumina magnesia and ferrous oxide. Its hardness ranges from 6-7.5 on Moh’s scale.
Uses : It is used in making abrasive paper and abrasive cloth and also in glass grinding and polishing metals.
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