ppt unit v



Refractories



Glass



Ceramics &

Refractories

with stand high temperature without softening or undergoing any deformation in shape

lining of the furnace, kilns, etc., which are employed for manufacturing of cement, glass, metals etc.,

Characteristics of a good

refractory:-1.

Not to fuse at the operating temperature.

2.

should not undergo any change in physical, chemical and

mechanical properties at high temperatures.

3.

Must bee chemically inert towards the corrosive action of

molten metals, slag’s and gases produced in the furnace.

4.

Be capable of retaining its original form without cracking and

splitting when subjected to sudden temperature changes.

5.

Be strong enough to bear the charge load at the working

temperature.

6.

Resist the abrading action by molten metal or slag.

Properties of

Refractories:-

Refractoriness



Spalling Resistance or Thermal



Spalling Refractoriness under

Load (RUL)



Porosity

Refractoriness

 Ability to with stand high temperatures without softening or deformation under working conditions

 It is a measure of softening or fusion temperature of the material

 Softening temperature must higher than the furnace temperature

 Determined by the standard pyrometric cone equivalent(PCE) test or

Seger cone test

PYROMETRIC CONE EQUIVALENT(PCE) TEST

PCE S. T – o_C

1 1110 2 1120 --

--

Spalling Resistance or Thermal Spalling

 breaking or cracking or peeling of a refractory material

 Uneven thermal expansion causes thermal spalling

 should have a low coefficient of expansion

Refractoriness under Load (RUL)

 withstand varying loads

should have high mechanical strength under operating temperatures

load bearing capacity can be measured RUL test

RUL test

cylinder diameter 50 mm and height 50 mm

heated in a furnace at a rate of 10°C/ minute under a load of 1.75 Kg/cm2

Porosity

porosity reduces the strength and resistance to corrosion ratio of its pores volume to that of its bulk volume

porosity is given as

P = W-D/ W-A x 100

Where W= weight of saturated specimen in air

D = weight of dry specimen in air

A = Weight of saturated specimen submerged in water

Dimensional stability

It is the resistance when exposed to high temperatures dimensional changes may be

reversible or irreversible (permanent) A good refractory must be always reversible

Classification of Refractories

(a) Acid Refractories, (b) Basic Refractories, (c) Neutral Refractories

Acid Refractories

Basic Refractories

Neutral Refractories

Eg. Silica & fire clay

Silica - 50-80% Alumina - > 50%

QUARTZ  TRYDIMITE  CRYSTABOLIE

Uses: internal lining of blast furnaces, open hearths and lime kilns etc.

Eg. Ca & Mg , dolomite and Magnesite greater than 2000o_C

Uses: steel making furnace, rotary kiln,

Metallurgy of copper smelting furnace and antimony ores

E.g. SiC, Carborundum 2500o_C

Glass

Glass is an amorphous, hard, brittle, transparent, super cooled liquid of infinite viscosity obtained by fusing a mixture of a number of metallic silicates, most commonly Na, K, Ca and Pb. It posses no sharp melting point, definite formula or crystalline structure.

Commonly represented as

xR₂O.yMO. 6SiO₂

Where R = Monovalent alkali metal (Na, K, etc.,)

M = Bivalent metal like Ca, Pb, Zn etc.,

Manufacturing of glass

• Shaping • Annealing • Finishing

Soft glass and Soda lime

Materials : Silica, calcium carbonate and soda ash Composition : Na₂O.CaO.6SiO₂

Uses : Window glasses, electric bulbs, plate-glasses, bottles, jars, building blocks and cheaper table ware

Hard glass or potash-lime

Materials : silica, calcium carbonate and potassium carbonates Composition : K₂O.CaO.6SiO₂

Uses : chemical apparatus, combustion tubes, etc., used for heating operation.

Pyrex glass or Jena glass or Boro silicates glass

Materials : Silica, boron, alumina and some alkali oxides Composition : SiO₂ - 80.5% B₂O₃ - 13% Al₂O₃ - 3% K₂O - 3% Na₂O - 0.5%

Ceramics

Greek ceramos means Burnt Stuff

polycrystalline inorganic metallic or non-metallic materials that are

Processed at high temperature

Characteristics of Ceramic

1. Hard, brittle in nature and in the form of amorphous or glassy solids

2. bonding in these materials is of mixed ionic or covalent character

3. good electrical resistance and act as insulators 4. high temperature resistance

5. resistance to chemical attack and weathering 6. high compressive strength and tensile strength

Composition of Ceramic

1. Plastic Portion :Clay

2. A Flux or a glassy materials : feldspar (K₂O. Al₂O₃ 6H₂O) cementing material (KAlSi₃O₈ –

NaAlSi₃O₈ – CaAl₂Si₂O₈)

3. A Non-Plastic Refractory Crystalline Portion :

Ceramics

Classification of Ceramic Materials ( clay, refractory or glassy properties)

1. Functional classification based on industrial applications

Group Example

1. Abrasives Alumina, carborundum. 2. Pure oxide ceramics MgO, Al2O3, SiO2

3. Fire clay products Porcelain, bricks, tiles etc. 4. Inorganic glasses Hard glass, window glass. 5. Cementing materials Lime, Portland cement etc. 6. Rocks Granite, Sand stone etc. 7. Minerals Quartz, calcite etc.

Ceramics

Classification of Ceramic Materials ( clay, refractory or glassy properties)

2. Structural classification based on structural criteria

Group Example

1. Crystalline Ceramics Single phase like MgO or muliphase from MgO to Al₂O₃

2.Non-crystalline Ceramics Natural and synthetic inorganic glasses

3.Glass bonded ceramics Fire – clay products, crystalline phases are held in glassy matrix

Ceramics

Ceramic Materials

Clay Ceramics:(structural): bricks, tiles, blocks and glazed and unglazed decorative products. These are made from relative low-grade clays, shales

White Wares or White Pottery: china clay, feldspar and quartz

Earth wares and Stone wares : Clay 50%, Feldspar 20%, Flint 15%, Kaolin 5% and grog 10%

Glazes: Glassy

Application of Ceramics

Tiles, sanitary ware, insulators and high frequency applications

White wares also used in chemical industries as crucibles, jars and compounds of chemical reactors

Cement

Classification :

Natural cement Artificial cement

Natural Cement: 20 to 40% argillaceous matter (clay) , 80-60% calcareous matter

Example: Roman cement from clay nodules, pozzuolana cement from volcanic powder

They possess hydraulic property. They are quick setting and hence have low strength

Artificial cement :

Composition

Lime (CaO) - 60%

Silica (SiO₂) - 22%

Alumina (Al₂0₃) - 5%

Magnesium (MgO) - 4%

Gypsum (CaSO₄) - 4%

Iron oxide (Fe₂0₃) - 3%

Sulphur trioxide (SO₃) - 1%

Manufacture of Portland Cement

Raw materials required for the manufacture of cement are

1. Calcareous materials (CaO) – lime stone, chalk

2. Argillaceous material (Al

O

.SiO

) – clay, shale, slate

3. Gypsum (CaSO

.2H

O)

Manufacture of cement involves the following steps:

Mixing

Burning

Grinding

Mixing:

Dry process

Burning

(i)Drying Zone:

The temperature prevailing in this zone is about 400°C. Heat the slurry dries up due to evaporation of water.

(ii) Calcining Zone(nodule zone)

The temperature at this zone is around 1000°C. At this temperature limestone decomposes to form small lumps called nodule zone.

CaCO₃ CaO + CO₂

(iii) Clinker Zone

In this zone at 1500°C the lime and clay react to form aluminates and silicates.

These compounds fuse together to form clinkers.

2CaO + SiO₂ Ca₂SiO₄ (C₂S)

3CaO + SiO₂ Ca₃SiO₅ (C₃S)

3CaO + Al₂O₃ Ca₃Al₂O₆ (C₃A)

grinding

Grinding:

The clincker thus produced emerges out of the rotary kiln. It is rapidly cooled by steam of air. The cooled clinker is powdered with 4% gypsum in ball mills. Gypsum is added to retard the early setting of cement.

3CaO.Al₂O₃ + xCaSO₄.7H₂O 3CaO.Al₂O₃ xCaSO₄.7H₂O Tricalcium sulphoaluminate Insoluble