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Chapter: Chemistry - Engineering Materials

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Engineering Materials

1 Introduction 2 Abrasives 2.1 Properties of Abrasives 2.2 Classification of Abrasives 2.3 Applications of Abrasives 3 Refractories 3.1 Characteristics of Refractories 3.2 Classification of Refractories 3.3 Properties of Refractories 3.4 Manufacturing of Refractories 4 Portland Cement 4.1 Chemical composition of Portland cement 4.2 Manufacturing methods 4.3 Setting and hardening 4.4 Other types of cements 5 Glass 5.1 Properties of glass 5.2 Manufacturing of glass 5.3 Types and uses of glass 6 Glossary

ENGINEERING MATERIALS

 

1 Introduction

2 Abrasives

2.1 Properties of Abrasives

2.2 Classification of Abrasives

2.3 Applications of Abrasives

3 Refractories

3.1 Characteristics of Refractories

3.2 Classification of Refractories

3.3 Properties of Refractories

3.4 Manufacturing of Refractories

4 Portland Cement

4.1 Chemical composition of Portland cement

4.2 Manufacturing methods

4.3 Setting and hardening

4.4 Other types of cements

5 Glass

5.1 Properties of glass

5.2 Manufacturing of glass

5.3 Types and uses of glass

6 Glossary

 

 

1 INTRODUCTION

 

Materials which are used in manufacturing of wools and equipments and in construction of buildings, where very specific requirements are needed are called engineering materials. These include cement, refractories, abrasives, lubricants, etc,

 

2 ABRASIVES

 

Abrasives are very hard substances used for grinding, shaping and polishing other materials

 

2.1 PROPERTIES

            Have very high melting point

            Chemically inert

 

            High abrasive power (ability to scratch away pr sharp other materials)

            Sometimes hard and brittle or soft and flexible

 

2.2CLASSIFICATION OF ABRASIVES – TYPES &

2.3 APPLICATIONS OF ABRSASIVES

natural abrasives – Eg. Diamond, corundum

 

synthetic abrasives – Eg. carborundum, norbide

 

Hardness is measured in terms of mohs scale.

 

Diamond is taken as the reference and hardness of other materials are determined

 

abrasives with Mohrs scale 1-4 are called soft abrasives

 

 

NATURAL ABRASIVES

Diamond:

            Purest crystalline carbon - Hardest natural substance

 

            Mohrs scale value is 10 -Superior chemical inertness

 

            Used in grinding wheels, drilling tools, cutting glasses, etc

 

Corundum

 

Pure crystalline form of alumina - Mohrs scale value is 9 - Used in grinding glass, gems etc.

 

Emery

55-75% alumina, 20-40% magnetite, 12% others - Black and opaque

 

-Mhos scale value is 8 - Used for making abrasive paper, abrasive cloth, etc.

 

Quartz

 

Pure silicone - Mohrs scale value is 7 - Used in painting industries

 

Garnet

 

Trisilicates of alumina, magnetite and Fe oxide used for the manufacture of abrasive paper and cloth.

 

ARTIFICIAL ABRASIVES

 

Silicon Carbide (SiC)

Manufacture

 

Silicon Carbide is manufactured by heating sand (60%)and coke (40%) with some saw dust and a little salt in an electric furnace to about 1500°C

 

SiO2 + 3C gives SiC + 2CO

The silicon carbide removed from the furnaces, is then mixed with bonding agent(clay, silicon nitride) and than shaped, dried and fired.

 

Properties

 

         Silicon carbide possesses a high thermal conductivity, low expansion and high resistance

 

to abrasion and spalling.

 

        They are mechanically strong. Mohrs scale value is 9.

 

        Bear very high temp. 1650°C

        Has thermal conductivity between metals and ceramics –

They are electrically intermediate between conductors and insulators.

 

Uses

Silicon carbide are used as heating elements in furnaces in the form of rods or bars.

 

         They are also used for partition wall of chamber kilns, coke ovens, muffle furnaces and floors of heat treatment furnaces.

 

Sic bonded with tar are excellent for making high conductivity crucible.

 

Norbide or Boran Carbide (B4C) Manufacture

 

It is prepared by heating a mixture of boran oxide (B2O3) and coke in an electric furnace to about 2700°C

B2O3 +7C give B4C + 6CO

 

Properties

 

        Its hardness is 9 on Mohrs scale.

 

        It is light weight and black colored compound.

        It is highly resistant to chemical attack and erosion.

        It resists oxidation much better than diamond.

 

Uses

 

            It is used as hard materials for making grinding dies, and for cutting and sharpening hard high speed tools.

 

            It is used to prepare scratch and wear resistant coating.

 

 

3 REFRACTORIES

 

Materials that can withstand high temp without softening and deformation in their shape.Used for the construction of furnaces, converters, kilns, crucibles, ladles etc.

 

3.1CHARACTERISTICS

         Infusible at operating temp.

 

         Chemically inert towards corrosive gases, liquids etc. Should not suffer change in size at operating temp. Should have high refractoriness

 

         Should have high load bearing capacity at operating temp.

 

3.2 CLASSIFICATION

Based on chemical nature

· Acidic refractories – Eg. Silica and Alumina

           Basic refractories – Eg. Magnesite and Dolomite

 

         Neutral refractories – Eg. Graphite and Carborundum Based on refractoriness

 

         Low heat duty refractories

         Intermediate heat duty

refractories

 

         High heat duty refractories

         Super heat duty refractories

 

3.3 PROPERTIES of Refractoriness

 

It is the ability to withstand very high temp. without softening or deformation under particular service condition. Since most of the refractories are mixtures of several metallic oxides, they do not have a sharp melting point. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed in terms of pyrometric cone equivalent.(PCE). Pyrometric cone equivalent is the number which represents the softening temperature of a refractory specimen of standard dimension (38mm height and 19mm triangular base) and composition.

 

Objectives of PCE test

 

To determine the softening temperature of a test refractory material.

 

To classify the refractories

 

To determine the purity of the refractoreies

To check whether the refractory can be used at particular servicing temperature.

 

Refractoriness is determined by comparing the softening temperature of a test cone with that of a series of segar cones. Segar cones are pyramid shaped standard refractory of definite composition and dimensions and hence it has a definite softening temperature.

 

A test cone is prepared from a refractory for which the softening temperature to be determined, as the same dimensions of segar cones. Then the test cone is placed in electric furnace. The furnace is heated at a standard rate of100C per minute, during which softening of segar cones occur along with test cone. The temperature at which the apex of the cone touches the base is taken as its softening temperature.

 

RUL – Refractoriness Under Load

 

The temp. at which a std dimensioned specimen of a refractory undergoes 10% deformation with a constant load of 3.5 or 1.75 Kg/cm2 The load bearing capacity of a refractory can be measured by RUL test. A good refractory should have high RUL value

 

Porosity ratio of pore volume to the bulk volume

 

P = (W- D/W- A) X 100

 

W – weight of saturated specimen in air

 

D – weight of dry specimen

A – weight of saturated specimen in water

 

Porosity reduces strength, corrosion resistance thermal conductivity, thermal spalling and abrasion resistance

 

Thermal spalling property of breaking, cracking or peeling of refractory material under high temp. Thermal spalling may be due to rapid change in temp. or slag penetration. A good refractory should show good resistance to thermal spalling

 

Dimensional stability

 

Resistance of refractory to any volume change when exposed to high temp. over a prolonged time. Refractories may undergo reversible or irreversible dimensional changes A good refractory should show minimum level of reversible dimensional changes with temp.

 

3.4 MANUFACTURING OF REFRACTORIES

 

ALUMINA BRICKS

 

Contain 50% of aluminium oxide Manufacture

 

Calcined bauxite, silica and grog (calcined fire clay) are ground well and mixed with water. The pasty mass is converted into bricks by mechanical pressing or slip casting.The bricks are dried and fired at about 1200 to 14000 C for 6-8 days

 

MAGNESITE BRICKS

Contain maximum Magnesium oxide

 

Manufacture

 

Calcined magnesite, magnesia or iron oxide are ground well and mixed with water. The pasty mass is converted into bricks by mechanical pressing or slip casting. The bricks are dried and fired at about 15000 C for 8 hours then cooled slowly

 

ZIRCONIA BRICKS

Contain zirconite

 

Manufacture

 

Zirconite mineral, colloidal zirconia or alumina are ground well and mixed with water and made into bricks. Small amount of MgO or CaO is added as stabilizer. The bricks are dried and fired at about 17000 C

 

 

4 PORTLAND CEMENT

 

It is defined as an extremely finely ground product.

 

It is obtained by heating a mixture of argillaceous (clay containing ) and calcareous (lime containing ) raw materials to about 1500 c. It is then mixed with gypsum to increase the quick setting and hardening property.

 

4.1 CHEMICAL COMPOSITION OF PORTLAND CEMENT

3CaO.SiO2 - Tri calcium Silicate 3CaO.Al2O3 - Tri calcium Aluminate

4CaO.Al2O3.Fe2O3 - Tetra calcium alumino Ferrate

 

4.2 MANUFACTURE OF PORTLAND

CEMENT

 

Raw materials :

 

(i)  Calcareous materials , CaO        Ex: Limestone, chalk.

(ii) Argillaceous materials, Al2O3 and SiO2         Ex: clay, slate etc

 

 

             Powdered coal (or) fuel oil.

             Gypsum (CaSo4.2H2O)

 

Manufacture of Portland cement involves the following steps:

                       Mixing of raw materials

 

                        Burning

 

                        Grinding

 

                        Storage and Packing

 

 

 (I) Mixing of raw materials:

 

(a) Dry Process (b) Wet Process

 

            Dry Process: In dry process, the raw materials like limestone and clay(3:1) are dried, and mixed in definite proportions

 

Wet process : In wet process, the raw materials in definite proportions are finely ground with water and the slurry ( past like) is fed at the top of the rotary kiln.



(II) Burning

 

The burning process is usually done in rotary kiln which is a long horizontal steel cylinder coated with refractory bricks and capable of rotating at 1 rpm 9 Revolution per minute) . The rotary kiln is set at a slight inclination of about 5-60 in order to allow the raw materials fed at one end to travel slowly to the firing and discharge exit end.

 

The slurry of raw materials is allowed to enter from the top end of the rotary kiln. Simultaneously the burning fuel ( like powdered coal or oil) and air are introduced from the lower end of kiln . The slurry gradually comes down in the kiln into the different zones ( Drying Zone at 400o :Calcination zone at 700 -1000 o C and clinkering zone at 1250-1500 o C of increasing temperatures.

 

Drying Zone: The upper part of the rotary kiln is known as drying zone ,where the temperature is about 400 o C . Due to the presence of hot gases in this zone, water is evaporated from the slurry.

 

Calcinations zone: The middle part of the rotary kiln is known as calcining zone where the temperature ranges from 700 -1000 o C. In this zone lime stone is decomposed into CaO and CO2.


(c) Clinkering Zone : The lowest part of the zone is called as clinkering zone, where the temperature is maintained about 1250-1500 o C. In this zone lime reacts with clay ( Containing Al2O3, Fe2O3 and SiO2) and forms aluminates and silicates

 


The mixture is then finely powdered and fed into the top of the rotary kiln.


 

                        Cooling : the hot clinker is cooled with atmospheric air and the hot air thus produced is used for drying the coal before grinding.

 

                        Grinding : The cooled clinker is then finely pulverized with 2-6% gypsum acts as a retarding agent for quick setting of cement.

 

                        Storage and Packing: The cement coming out from the grinding mills is stored in a concrete storage silos. Then the cement is packed in jute bags by automatic machines. Each bag contains 50kgs of cement.

 

PROPERTIES

 

4.3 SETTING AND HARDENING OF CEMENT:

 

When the cement is mixed with water, hydration and hydrolysis of cement begin, resulting in the formation of gel and crystalline products.

 

Setting: It is defined as the stiffening of the original plastic mass, due to initial gel formation. Hardening: It is defined as the development of strength, due to crystallization.

 

Chemical reactions involved in setting and hardening of cement:

 

When water is mixed with cement , hydration of tricalcium aluminate occurs rapidly and the paste becomes quite hard within a short time. This process is known as initial setting of cement.

3CaO.Al2O3  +6H2O-------3CaO.Al2O3.6H2O

 

Role of gypsum in cement:

 

(i) In initial setting process gypsum is added during grinding of cement clinkers to retard the rapid hydration

 

of C3A. Gypsum reacts with C3A to form insoluble calcium sulphoaluminate complex. C3A + 3CaSO4.2H2O--------C3A.3CaSO4.2H2O

 

 After the hydration of C3A,C3S begins to hydrate to give tobermonite gel and crystalline Ca(OH)2. The hydration of C3S takes place within 7days.

 

2(3CaO.SiO2)                                       + 6H2O--------- 3CaO.2SiO2.3H2O         + 3Ca(OH)2  + 500kj/kg

 

 Dicalcium silicate reacts with water slowly and gets finished 7-28days.

 

2(2CaO.SiO2)                   + 4 H2O -------- 3CaO.2SiO2.3H2O       +Ca(OH) 2 +                   250kj/kg

 

 

 Hydration of tetra calcium aluminoferritetakesplace initially, the hardening takes place finally through crystallization along with C2 S.

 

4CaO.Al2O3.Fe2O3   + 7H2O----------3CaO.Al2O3.6H2O(Crystalline) + CaO.Fe2O3.H2O(gel) + 420KJ

 

 

Thus the final setting and hardening of cement is due to the formation of tobermonite gel plus crystallization of Ca(OH)2 and hydrated tricalcium aluminate.

 

4.4 OTHER TYPES OF CEMENT - SPECIAL CEMENT

 

Water Proof Cement :

 

It is obtained by adding water proofing agents like calcium stearate and gypsum with tannic acid to ordinary Portland cement during grinding.

 

Functions of water- Proof cement:

 

Functions of water- proof cement

 

                        To make concrete impervious to water under pressure.

                         To resist the adsorption of water.

 

White cement or White Portland cement

 

It is obtained by heating the raw materials free from iron oxides. It is white in color due to the absence of ferric oxide.

 

It issued for making tiles, mosaic works with some coloring agents like yellow ochre, Venetian red etc. It is used for repairing and joining marble pillars and blocks.

 

 

5 GLASS

 

Glass is an amorphous, hard brittle, transparent, super cooled liquid of infinite viscosity.

Glass may be represented as xR2O.yMO.6SiO2

5.1 GENERAL PROPERTIES OF GLASS:

 

            It is amorphous.

 

            It is very brittle.

 

            It softens on heating.

 

            It has no definite melting point.

 

            It is affected by alkalis.

 

 

            It is a good electrical insulator.

 

            It can absorb, reflect or transmit light.

 

            It is not affected by air water, acids and chemical agents.

 

5.2 MANUFACTURE OF GLASS

 

       Melting :

 

The raw materials in proper proportions are mixed and finely powdered.This homogeneous mixture is known as BATCH is fused with some broken glass called CULLET in the pot of the furnace.The furnace is heated by burning producer gas and air mixture over the charge. The cullet melts at a low temp and assists in melting the rest of the charge.


 

 

Forming and Shaping

 

The molten glass is then worked into articles of desired shapes by either blowing or moulding or pressing between rollers.

 

Annealing:

 

Glass articles are then allowed to cool gradually to room temperature. Suddencooling must be avoided, because cracking occurs.Longer the annealing period, the better is the quality of the glass.

 

Finishing:

 

All glass articles after annealing, are subjected to finishing processes such as

 

(a) Cleaning (b)        grinding     (c) polishing    (d)     cutting        (e) sand blasting

 

5.3 TYPES AND USES OF GLASSES

 

1.Soda-lime or soda glass

 

(i) Raw materials: Silica , calcium carbonate and soda ash (ii) Composition: Na2O. CaO. 6SiO2

 

Properties

           They are low in cost.

 

           They are resistant to water

 

           They are attacked by common reagents like acids.

 

                           They melt easily

 

       Potash lime  or  Hard glass

 

                Raw materials :   Silica, CaCO3, K2CO3

 

                Composition:   K2O.

CaO.6SiO2

 

Properties:

 

(a) They have high melting point.

 

                     They do not fuse easily.

 

                       They are less acted upon acids alkalis, solvents.

 

Uses: Used for manufacturing combustion tubes, chemical apparatus

 

3. Lead glass or Flint glass

 

(i)                   Raw materials:  Lead

oxide, silicva, K2O

(ii)                    Composition:

K2O.PbO.6SiO2

 

Properties:

 

           It is bright and lustrous

 

                  It has high specific gravity. (3 to 3.3)

 

                  It is more expensive to manufacture.

                  It has a lower softening temperature than soda glass.

 

                  It has higher refractive index.

 

Uses: (a) These are used for high quality tablewares.

           They are used in neon sign tubings, optical lenses, electrical insulators, cathode ray tube.

 

 

       Potash lime  or  Hard glass

 

                Raw materials :   Silica, CaCO3, K2CO3

 

                Composition:   K2O.

CaO.6SiO2 (iii)

 

Properties:

 

(a) They have high melting point.

 

                     They do not fuse easily.

 

                       They are less acted upon acids alkalis, solvents.

 

Uses: Used for manufacturing combustion tubes, chemical apparatus

 

       Lead glass or Flint glass

 

             Raw materials:  Lead oxide, silicva, K2O

 

             Composition:  K2O.PbO.6SiO2

 

Properties:

 

                  It is bright and lustrous

 

                  It has high specific gravity. (3 to 3.3)

 

                 It is more expensive to manufacture.

 

                  It has a lower softening temperature than soda glass.

 

                 It has higher refractive index.

Uses: (a) These are used for high quality tablewares.

(b) They are used in neon sign tubings, optical lenses, electrical insulators, cathode ray tube.

 

 

4. Borosilicate glass or Pyrex glass or Jena glass

 

(i)Raw materials: Silica, borax with small amount of alumina and some oxides. (ii) Composition : SiO2 (80.5%); B2O3 (13%)

Al2O3         (3%) K2O (3%)  Na2O (0.5%)

                 Properties:

 

           It possess low thermal coefficient of expansion and high chemical resistance. (2)It possesses very high softening points and excellent resistivity.

 

Uses:  It is used in industry for pipe lines for corrosive liquids, gauge glasses,

 

5. Alumina silicate glass

 

Raw materials:  It has 5% or more alumina

 

(i)Composition:  SiO2  Al2O3, B2O3, MgO, CaO, Na2O K2O

 

Properties:

 

They possess high softening temperature.

Uses:

(a)Used in high pressure mercury discharge tubes

(b)Chemical combustion tubes.

6. Optical or Crookes glass

 

Raw materials: It contains phosphorus, lead silicate with small amount of cerium oxide. Properties:

(a)  Cerium oxide present in the glass absorbs uv light,

(b)  They have low melting point.

 

Uses: optical glasses are used for making lenses.

 

7. Glass wool

 

Glass wool is fibrous wool like material It is composed of intermingled fine threads or filaments of glass.

 

Properties: It is a very good heat and fire proof materials Its electrical conductivity is low.

 

Uses; It is used for heat insulation purposes

 

It is used for electrical and sound insulation.

 

 

 

6 Glossary

 

Abrasives

 

Abrasives are hard substances, used for polishing, shaping, grinding operations. They are characterized by high melting point, high hardness and chemically inactive.

 

Refractories

 

Refractories are materials that can withstand high temperatures without softening or deformation in shape.

 

Refractoriness

 

Refractoriness is the ability of a material to withstand very high temperature without softening or deformation under particular service condition.

 

Pyrometric cone equivalent

 

Pyrometric cone equivalent is a number which represents the softening temperature of a refractory specimen of standard dimension ( 38 mm height and 19 mm triangular base ) and composition.

 

RUL ( Refractoriness Under Load )

The temperature at which the refractory deforms by 10% under a load of 3.5kg/cm2 is called RUL (Refractoriness Under Load)

 

Porosity

Porosity is defined as the ratio of its pore volume to the bulk volume

 

Thermal spalling

 

Thermal spalling is the property of breaking, cracking or peeling off a refractory material under high temperature. A good refractory must show a very good resistance to thermal spalling.

 

Calcination

Heating the ore in absence of air is called Calcination

 

Cement

 

Cement is a material with adhesive and cohesive properties which make it capable of bonding minerals fragments into a compact whole. The name “Portland cement” given originally due to the resemblance of the colour and quality of the hardened cement to Portland sonte ( Portland island is England).


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