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IntroductionRefractories are materials that canwithstand high temperature withoutsoftening or suffering a deformation inshape.

REFRACTORIES

ACIDIC

REFRACTORIES

Eg: Fire Clay

Silica

BASIC

REFRACTRIES

Eg: Magnesite

Dolomite

NEUTRAL

REFRACTORIES

Eg: Silicon Carbide

Chromite

Acidic contain Al2O3 & SiO2, Basic contain CaO & MgONeutral contain FeO.CrO2 & ZrO2

Dr. Vishnu P. Sondhiya 2

Acidic Refractory

A refractory that is composed principally of silicaand reacts at high temperatures with bases such aslime, alkalies, and basic oxides.

These are used in areas where slag and atmosphereare acidic. They are stable to acids but attacked byalkalis.

The main components of these refractories are silicaalong with alumina (Al2O3).

The steel industries are the largest consumer ofacidic refractories.


Basic Refractories

These are used on areas where slags andatmosphere are basic, stable to alkalinematerials but reacts with acids.

The main raw materials is magnesia (MgO) is avery common example. Other examples includesdolomite (MgCO3 + CaCO3) and chrome-magnesia (Cr2O3 + MgO).


Neutral Refractories

These are used in areas where slags andatmosphere are either acidic or basic and arechemically stable to both acids and bases. Thecommon examples of these materials are alumina(Al2O3), chrome ( Cr2O3) and carbon.

Normally we have to use acidic and basic refractories combined but we use neutral bricks to avoid the reaction.

The neutral bricks are made of graphite and chromites.


CHARACTERISTICS OF GOOD REFRACTORY

Resistance against heat

Resistance against corrosion

Should have high fusing temperature

Ability to withstand high load

Should be chemically inert

Should not undergo deformation

Uniform expansion and contraction


REFREACTORINESS

It is the ability of a refractory material towithstand the heat without appreciablesoftening or deformation under given servicecondition.

Ability to withstand heat without gettingdeformed under operating conditions

It is measured as Softening Temperature

The softening temperature is measured bySeger Cone test

The test is also called Pyrometric Cone test

PROPERTIES OF REFRACTORIES


Pyrometric/Seger Cone Test Refractoriness is usually determined by

comparing the behavior of heat on cone ofmaterial to be tested with series of segercone of standard dimension.

The refractoriness is expressed in terms ofPyrometric Cone (PCE).

Standard Cones are

Pyramidal shaped

have triangular base

38 mm high and 19 mm long side


The standard seger cones are assignednumbers

The test cone is heated uniformly at 20oC/hror 100oC/hr or 150oC/hr or 600oC/hr

When the test cone softens one of the std conealso softens

The serial number of the std cone is noted

This number is PYROMETRIC CONEEQUIVALENT (PCE)


REFRACTORINESS UNDER LOAD (RUL)

Refractories are used in industrial furnaces haveinvariably to withstand varying load of products.

A good refractory should have high temperatureresistance as well as load bearing capacity that isstrength

Strength is calculated by REFRACTORINESS UNDERLOAD (RUL) test.

The test is performed by applying a constant load of3.5 or 1.75 kg/cm2 to the specimen of size 5 cm2 and75 cm high and heating at the rate of 10oC/min in afurnace.

RUL is expressed as temperature at which 10%deformation takes place


OTHER PROPERTIES

Dimension stability

Chemical inertness

Thermal expansion

Thermal conductivity

Porosity

Thermal spalling

Resistant to abrasion or erosion

Electrical conductivity

Heat capacity

Texture

Permeability


Condition leading to failure of refractory material

Using a refractory of less refractoryness that ofthe operating temperature.

Using lower duty refractory bricks in a furnacethan the actual load of raw material andproducts.

Using a brick of higher thermal expansion.

Rapid change in furnace temperature.

Using heavy weight refractory bricks.

Using bricks which undergo considerablevolume change during their use at hightemperature.


COMMON REFRACTORY BRICKS

Silica Bricks

Silica bricks contain 90-95% SiO2 and about 2%lime is added during grinding to furnish thebond.

Crushed Siliceous rock + 2% Lime+ Water

Thick paste

Mixing

Bricks

Burning in kilns at 1500oC for 24 hours

Machine pressing

SILICA REFRACTORY

Preparation(Quartzite)

(Cristobalite)Dr. Vishnu P. Sondhiya 14

Properties

• Color is yellow with brown specks and contain about 25% pores.

• Show acidic character.

• Resistant to thermal spalling (below 800oC)

• High load bearing strength ( 3.5 kg/cm2 up to 1600oC)

• Light rigid and mechanically strong

Uses

•The main use of silica bricks are roof of open hearth furnaces, open hearth steal making furnaces, coke oven walls, roof of electric furnaces, glass furnaces etc.

•Because of their higher thermal conductivity, they are also used in by-products coke-oven and gas retard lining.


Fireclay bricks are made from finely grounded soft plastic material

fireclay (Al2O3.2SiO2.2H2O) with powdered calcined fireclay

(grog). The exact properties of constituents depend on the type of

bricks to be made. Greater is the percentage of grog, the lesser

will be the spalling tendency. General constitution of the fireclay

bricks ranges from 55% SiO2 and 35% Al2O3 to 55% Al2O3 and

40% SiO2. (K2O, FeO, CaO, MnO etc. also used for balancing the

properties).

Properties

• Depending upon iron content they are light yellow to redish brown in color

•Show acidic character

•Resistant to thermal spalling

•High crushing strength (about 200 kg/cm2)

•Properly fired refractory are hard as steel

Uses

Construction of blast furnaces, stoves, oven, crucible furnaces,

flues, charging doors etc.

FIRECLAY REFRACTORY


High-alumina bricksHigh-alumina bricks contain 50% or more ofAl2O3. These bricks are generally made by mixingcalcined bauxite (Al2O3) with clay binder.

Properties: They posses very low coefficient ofexpansion, high porosity, great resistance to slags,very little tendency to spall, high temperature loadbearing capacity, excellent water resistance andstability in oxidizing and reducing conditions, inertto the action of gases. They are good refractory butcostly.

Uses: Lining of Portland cement rotary kiln, soakingpits etc.


BASIC REFRACTORYMAGNESITE REFRACTORIES

PreparationCrushed Magnesite grains (MgO) + caustic magnesia or FeO

Ageing for 10 days

Bricks


MAGNESITE REFRACTORY

Properties•High crushing strength

• Excellent resistant to basic slag but less to acidic slag

•Poor resistance to abrasion and spalling

•Can be used upto 2000oC


DOLOMITE REFRACTORIES

Preparation Calcined Dolomite (CaO + MgO mixture in equimolar ratio)

Silica + water

Bricks


DOLOMITE REFRACTORY

Properties•Compared to Dolomite less strength, more softness and porosity

•Low resistance to thermal shocks

•They can withstand a load of 3.5 kg/cm2 at 1650 °C

Uses: Dolomite bricks are used for basic electric furnace lining, bessemer convertors.


CARBON REFRACTORY

PreparationCoke/Graphite + Clay

Firing at 1300-1400oC

CARBON REFRACTORY

Properties • High thermal conductivity

•Excellent resistant to chemicals, spalling and abrasion

•Practically infusible

•Withstand high temperature fluctuations

During firing contact of the bricks with air is minimized by filling the space in-between the bricks with a mixture of sand and powdered coke.

Uses: Carbon and graphite are widely used as material ofconstruction of electrodes, lining of highly chemically-resistantequipments, atomic reactors, electric furnaces and in copperaluminum, lead smelting furnaces.


Chromite Bricks

Chromite bricks are made by firing at 1500-1700°C crushed chromite (FeO2.Cr2O3) mixed with alittle clay as binding material.

Properties: Chromite bricks are neutral in character.Hey posses high density, resistance to acidic as wellas basic slags and moderate resistance to spalling.They can be used up to 1800 °C and theirrefractoryness under a load of 3.5 kg/cm2 is 1430 °C.


Gypsum Plaster Gypsum is a crystalline mineral of hydrated calcium sulphate

(CaSO4•2H2O). Gypsum is colorless or white, is not highlywater-soluble and is not at all hard. A mixture of gypsum andwater can be poured; the gypsum hardens as the waterevaporates. In art gypsum is mainly used in the partlydehydrated form of plaster of Paris (2CaSO4•H2O) to makecasts of objects.

PREPARATION

plaster of Paris is prepared by heating gypsum(CaSO4.2H2O) at 120°C in rotary kilns, where it getspartially dehydrated.

The temperature should be kept below 140°C otherwisefurther dehydration will take place and the settingproperty of the plaster will be partially reduced.

Gypsum plaster, or plaster of Paris, is produced byheating gypsum to about 120°C.

2CaSO4·4H2O + Heat → 2CaSO4·H2O + 3H2O


PROPERTIES

It is a white powder. When mixed with water (1/3 of itsmass), it evolves heat and quickly sets to a hard porous masswithin 5 to 15 minutes. During setting, a slight expansion(about 1%) in volume occurs so that it fills the mouldcompletely and takes a sharp impression. The process ofsetting occurs as follows:

The first step is called the setting stage, and the second, thehardening stage. The setting of plaster of Paris is catalyzed bysodium chloride, while it is reduced by borax, or alum.

Uses

In surgery for setting broken or fractured bones

For making casts for statues, in dentistry, for surgicalinstruments, and toys etc

In making black board chalks, and statues

In construction industryDr. Vishnu P. Sondhiya 23

A cement is a binder, a substance thatsets and hardens independently, andcan bind other materials together.

Material with adhesive and cohesiveproperties

Any material that binds or unitesessentially like glue

Cement Chemical Composition

Cement CompoundWeight

PercentageChemical Formula

Tricalcium silicate 50 % Ca3SiO5 or 3CaO.SiO2

Dicalcium silicate 25 % Ca2SiO4 or 2CaO.SiO2

Tricalcium aluminate 10 % Ca3Al2O6 or 3CaO .Al2O3

Tetracalcium

aluminoferrite10 %

Ca4Al2Fe2O10 or

4CaO.Al2O3.Fe2O3

Gypsum 5 % CaSO4.2H2O


Raw Materıals Of Portland Cement

1) Calcareous Rocks (CaCO3 > 75%)

Limestone

Marl

Chalk

Marine shell deposits

2) Argillocalcareous Rocks (40%<CaCO3<75%)

Cement rock

Clayey limestone

Clayey marl

Clayey chalkDr. Vishnu P. Sondhiya 26

3) Argillaceous Rocks (CaCO3 < 40%)

Clays

Shales

Slates

Portland cement is made by mixingsubstances containing CaCO3 with substancescontaining SiO2, Al2O3, Fe2O3 and heatingthem to a clinker which is subsequentlyground to powder and mixed with 2-6 %gypsum.


CLINKER GYPSUM


Functions of the ingredients of cements

Lime is the principal constituent of cement. Itsproportion must be properly regulated. Howeverexcess of lime reduce the strength of cement . On theother hand presence of lesser amount of lime thanrequired also reduce the strength of cement and makeit quick setting.

Silica imparts strength to cement.

Alumina makes the cement quick setting. (due toexcess weakness of cement increased)

Gypsum helps to retard the setting action of cement. Itactually enhance the initial setting time of cement.

Iron oxide provides color, strength and hardness to thecement.


PRODUCTION STEPS

1) Raw materials are crushed, screemed & stockpiled.

2) Raw materials are mixed with definite proportionsto obtain “raw mix”. They are mixed either dry (drymixing) or by water (wet mixing).

3) Prepared raw mix is fed into the rotary kiln.

4) As the materials pass through the kiln theirtemperature is rised upto 1300-1600 °C. Theprocess of heating is named as “burning”. Theoutput is known as “clinker” which is 0.15-5 cm indiameter.

5) Clinker is cooled & stored.

6) Clinker is ground with gypsum (3-6%) to adjust setting time.

7) Packing & marketting.Dr. Vishnu P. Sondhiya 30

Manufacturing of Portland Cement

Argillaceous Material

(Clay)

Calcarious Material

(Lime)

Washing GrindingProportioning

DRY PROCESS WET PROCESS WATER

Fine Powder Slurry

ROTARY KILN

Clinker

GYPSUM (2 - 3 %)Cooling Grinding

PORTLAND CEMENT


Raw materials are ground to powder and blended

Burning changes raw mix chemically into cement clinker


REACTIONS IN THE KILN

~100°C→ free water evaporates.

~150-350C°→ loosely bound water is lost from clay.

~350-650°C→decomposition of clay→SiO2 & Al2O3

~600°C→decomposition of MgCO3→MgO & CO2(evaporates)

~900°C→decomposition of CaCO3 → CaO& CO2(evaporates)


~1250-1280°C→liquid formation & start of compound formation.

~1280°C→clinkering begins.

~1400-1500°C→clinkering

~100°C→clinker leaves the kiln & falls into a cooler.

Sometimes the burning process of raw materials is performed in two stages: preheating upto 900°C & rotary kiln


SETTING AND HARDENING OF CEMENT

INITIAL SETTING : Hydration of C3A, C2S, C3S

Gel Formation of C4AF

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

2CaO.SiO2 + 4 H2O 2CaO.SiO2.4H2O

3CaO.SiO2 + 6 H2O 3CaO.SiO2.6H2O

4CaO.Al2O3.Fe2O3 + 7 H2O 3CaO.Al2O3.6H2O + CaO.Fe2O3.H2O

FINAL SETTING AND HARDENING : Hydrolysis of C3S, C2S

2[3CaO.SiO2] + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2

2[2CaO.SiO2] + 4H2O 3CaO.2SiO2.6H2O + Ca(OH)2

When cement is mixed with water to a plastic mass called

cement paste, hydration reaction begin, resulting in the

formation of gel and crystalline products.


ROLE OF CEMENTING MATERIALS

1) GYPSUM : INITIAL SETTING TIME RETARDER

C3A + 6H2O C3A.6H2O + HEAT

C3A + x H2O + y CaSO4. z H2O C3A.yCaSO4.z H2O

Insoluble calcium sulphoaluminate

Quick hardening property

Gypsum

2) Silica / CaO: Imparts srengths

3) Alumina : Imparts strengths, Makes the cement quick setting

4) Fe2O3 : Imparts Grey color, strength and hardness

5) SO3 : Imparts soundness

6) Alkalies (MgO, Na2O, K2O): Lower the clinkering temperature


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