Home | | Engineering Chemistry II | Important Questions and Answers: Engineering Materials

Chapter: Chemistry - Engineering Materials

| Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail |

Important Questions and Answers: Engineering Materials

Chemistry - Engineering Materials - Important Questions and Answers: Engineering Materials

ENGINEERING MATERIALS

 

01. What are Abrasives? Explain with classification.

 

Abrasives are hard substances used for Grinding, cutting, Shaping, drilling, poslishing andsharpening operations.

 

Ex: Diamond,Talc

 

02. Explain the properties of refractories.

 

Properties : Refractoriness, RUL, Dimensional stability, Thermal spalling,

 

Thermal expansion , porosity.

 

         Refractoriness:

 

                  It is ability of a refractory material to withstand very high temperature without softening or deformation under the working conditions

 

                  It is measured by PCE test.

 

                  PCE= Pyrometric cone equivalent

 

                  P.C.E number and softening temperature of some refractories are as follows a.  Silica

 

bricks

 

           Alumina bricks

 

         Magnesite bricks


          PCE is the number which refer to the softening temperature of a refractory specimen of standard dimension 38mm height and 19mm triangular base.

Measurement:

          This measurement is called segarcone test.

          A test cone is determined by comparing its softening temperature with softening temps.of a series of standard pyrometric cones.

          A test cone is prepared from the sample refactory then it is placed along with the standard segar cone in an electric furnace and heated at a rate of 10oC per minute.

          The temperature at which apex (top) of the cone touches the base is taken as the softening temperature .the PCE number of the standard cone which behaves identically is taken as the PCE of the test sample.


         How Alumina, Magnesite and Silicon carbide are manufactured?

 

Manufacture of Alumina bricks:

 

Alumina bricks is an example for acidic refractories.

 

Alumina bricks are prepared from minerals silimanite and kyanite They are anhydrous aluminosilicate materials Al2O3.SiO2

This mineral is fixed with coal in the ratio 1:1 along with plastic clay as binders.

 

The raw materials are mixed and moulded into bricks The bricks are then dried and fired at 1600oc

 

The final product material contains 63 % alumina and 34% silica approximately. These are used in steel industries.


Manufacture of Magnesite bricks:

 

Magnesite bricks are an example for basic refractories

 

Powdered caleinedmagnoite (Mgo) is mixed with caustic magnesia or iron oxide, as a birder with

 

water mixed and moulded into bricks.

The bricks are then dried and fired at 1500oC

The final product material containsMgO=85%,CaO=2.5% & SiO2=5.5%

There are used in open hearth furnaces, libing converters and reverberatory furnaces.


Manufacture of Silicon carbide (SiC): CARBORUNDUM

 

SiC is an example for very hard synthetic abrasive. It is a mixture of SiO2 and coke.

Its hardness is 9.8 on Moh‟s scale.

 

It is chemically inert and can withstand high temperature.

 

Preparation Or Manufacture:

 

It is prepared by heating a mixture of 60% sand (i.e) SiO2 and 40% coke with a small amount of saw dust and a little salt in an electric furnace to about 1650oC for 36 hours.


2. Burning: Dry raw mix / slurry is carried out in rotary kiln.

 

The Dry raw mix / slurry is fed into the kiln from upper end and the flame is forced into the lower end. Due to slope and slow rotation, the material gradually descends in the klin into different zones of increasing temperatures.

 

 

 

 

(i). Drying Zone: Upper part of the kiln - About 400oc, water in slurry gets evaporated. (ii). Calcination zone: Center part of the kiln - About 1000oc, limestone gets decomposed intoCaO and CO2. CaCO3 ----- CaO + CO2

 

(iii). Clinkering Zone: Lowest part of the kiln - About 1350-1500oc, limestone reacts with clay to form Bogue compounds. C2S,C3S,C3A,C4AF.

 

 The Bogue compounds fuse together to form small, hard, grayish coloured stone like mass called cement clinkers.

 

         Grinding : The hot clinkers are cooled with atmospheric air and then pulverized together with 2-3% gypsum in ball mills. Gypsum act as retarding agent for quick setting cement.

 

         Storage and Packing: The cement coming out of the grinding mill is stored in a concrete storage silos. Then the cement is packed in jute bags by automatic machine.

 

 

 

 

 

07. Explain the properties of Portland cement.

 

Properties: (i). Setting and Hardening of cement. (ii). Heat of Hydration. (i).

 

Setting and Hardening of cement.

 

Setting: It is defined as the stiffening of the original plastic mass, due to the formation of tobermonite gel.

 

Hardening: It is defined as the development of strength due to formation of crystals. When cement is mixed with water, results formation of gel and crystalline products.

 

Chemical reactions:

 

(i). Flash set – When cement is mixed with water, hydration of C3A takes place and the paste becomes quite rigid within a short time.(1 Day)

C3A  + 6H2O -- -- > C3A.6H2O

 

(II). Formation of Tobermonite gel: After the hydration of C3A, C3S begins to hydrate to give Tobermonite gel and crystalline Ca(OH)2. This is responsible for the development of initial strength of cement. The hydration of C3S gets completed within 7 days. It does not contribute much to the strength of cement.

C3S + 6H2O -- -- > C3S2 .3H2O + 3Ca(OH)2

 

(III). Dicalcium silicate (C2S) reacts with water very slowly and gets completed in 7 to 28 days.

2C2S + 4H2O        C3S2 .3H2O +Ca(OH)2

 

The increase in strength between 7 to 28 days is due to the formation of tobermonite gel and crystalline Ca(OH)2.

(iv). The hydration of C4AF takes place initially, the hardening takes place finally through crystallization along with C2S.

C4AF  + 7H2O  -- -- > C3A.6H2O

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.


ii). Heat of Hydration:

When water is mixed with Portland cement some amount of heat is liberated due to hydration and hydrolysis reactions of Bogue compounds. The average quantity of heat is liberated is 500 kJ/kg.

08. Write short notes on special cements like waterproof and white cement.

 

(i). Waterproof cement:

 

It is obtained by adding water- proof materials like calcium stearate, aluminium stearate and gypsum with tannic acid to ordinary Portland cement during grinding.

 

Functions:  (i). To make concrete impervious to water under pressure. (ii).

 

To resist the absorption of water.

 

Properties:

 

(i). It is more expensive than ordinary Portland cement. (ii). It act as pore-blocking and water- repelling agent.

 

Uses:

 

(i). Used in the construction bridges and structures under water.

 

(ii). White Cement:

 

It is white in color due to absence of iron compounds.

 

Such cements are made from raw materials which are free from iron oxide. Properties:

 

(i). It is more expensive than ordinary Portland cement. (ii). It act as pore-blocking and water- repelling agent.

Uses:

 

(i). Repairing and joining marble pillars and blocks, manufacture of tiles and mosaic

 

works.

 

(ii). Used in the construction bridges and structures under water.

 

 

 

09. Explain the manufacture the glass.

 

Raw materials:

 

           Sodium is soda , Na2CO3 – Soft glass.

 

           Potassium is potash, K2CO3 – Hard

 

glass. c) Calcium are limestone, chalk and lime

 

d) Lead are litharge, and red lead – Flint glass e) Silica are quartz, white sand.

 

            Zinc is zinc oxide – Heat and shock proof glass

 

                                       Borate are borax , and boric acid - Heat and shock proof glass h) Cutlets or pieces of broken glass to increase the fusibility.

 

4 steps involved for the manufacturing of glass.

1. Melting: The raw materials in proper proportions are mixed and finely powdered. This mixture called batch is fused with some broken glass, called cullet in the pot of the tank furnace, in which heating is done by burning produces gas and air mixture over the charge.

 

Heating is continued, till the molten mass is free from bubbles and glass-balls, and then cooled to about 800oC.

 

2. Foaming and shaping:

 

Molten glass is then worked into articles of desires shapes by either blowing or moulding or pressing between rollers.

 

3. Annealing:

 

Glass articles are then allowed to cool gradually to room temperature( sudden cooling must be avoided, because cracking occurs ). The longer the annealing period, the better quality of glass.

 

4. Finishing:

 

All glass articles after annealing are subjected to finsish processes such as cleaning, grinding, polishing, cutting etc.

 

 

         Explain the types and properties and uses of glass.

         . Soda lime or soft glass:

 

Raw materials: silica, calcium carbonate and soda ash. Approximate composition: Na2O.CaO.6SiO2 Properties:

 

           They are low cost.

 

           It is resistant to water.

 

           It is attacked by common reagents like acids.

 

Uses:

 

Window glasses, electric bulbs, bottles, jars, cheaper table wares, where high temperature – resistance and chemical stability required.

 

2). Potash lime or Hard glass:

 

Raw materials: silica, calcium carbonate and Potassium carbonate.

 

Approximate composition: K2O.CaO.6SiO2

 

Properties:

 

           Possess high melting point so it will not fuse easily.

 

           Less acted upon by acids, alkali and other solvents than ordinary glasses.

 

Uses:

Chemical apparatus, combustion tubes, which are to be used for heating operations.

 

3). Lead glass or Flint glass:

 

Raw materials: Lead oxide and silica are fused.

 

Approximate composition: K2O.PbO.6SiO2

Properties:

 

a) Lower softening temperature than soda- glass. b) Higher refractive – index.

 

c) Has excellent electrical properties. d) High specific gravity ( 3 to 3.3)

Uses:

 

            High quality table wares, optical purposes(like lenses), neon sign tubing, cathode ray tubes, electrical insulators and in art objects.

 

            High lead content glasses are used for extra-dense optical glasses for windows and shields to protect personnel from x-rays and gamma rays in medical and atomic energy fields respectively.

 

4). Boro silicate glass / Pyrex glass / Jena glass.

 

Raw materials: silica, boron with a small amount of alumina and some oxides. Approximate composition: SiO2(80.5%),B2O3(13%), Al2O3(3%),K2O(3%),Na2O(0.5%)

Properties:

 

           Low thermal efficient of expansion

 

           High chemical resistance.

 

           Very high softening points.

 

           Excellent shock-proof.

 

Uses:

 

Used in industry for pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus, kitchenware, chemical plants, television tubes, electrical insulators.

 

 

 

5). Aluminosillicate glass

 

Raw materials: alumina, silica, boron with a small amount of and some oxides. Approximate composition: SiO2(55%),Al2O3(23%),B2O3(7%), MgO(9%),CaO(5%), Na2O+K2O(1%)

Properties:High softening temperature

 

Uses:

 

High-pressure mercury discharge tubes, chemical combustion tubes, certain domestic equipments, etc,

 

6). Glass wool:

 

It is a fibrous wool-like material, composed of intermingled fine threads of filaments of glass. They are completely alkali free. The glass filaments are obtained by forcing molten glass through small offices. The average diameter of the office is 0.0005 to 0.007mm. Then the filaments of glass so obtained are thrown over a rapidly rotating drum to get wool-like materials

Properties:

 

a). fire- proof and heat proof material.

 

b). low electrical and thermal conductivity.

 

c). resistant to water and most of chemicals. d). high tensile strength – 8 times that of steel.

 

Uses:

 

        Heat insulation purpose- domestic and industrial appliances.

 

        Air filters and dust filtering materials.

 

        Insulation of metal pipelines and walls and roofs of houses.

 

        used in filtration of corrosive liquids like acids.

 

        Manufacture of fibre-glass, by blending with plastic resins.

 

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.