Refractory materials

Rahul MitraIndustrial Chemistry,3rd year

Roll no-316Ramakrishna Mission Vidyamandira

INTRODUCTION

What are are refractory materials?Materials that

Withstand high temperatures and sudden changes Withstand action of molten slag, glass, hot gases etc Withstand load at service conditions Withstand abrasive forces Conserve heat Have low coefficient of thermal expansion Will not contaminate the load

Deffination

Refractory Is a class of materials which are produced from non metallic materials.The are primary materials used inerlinning of industrial; furness and process capability to withstand heat and pressure and are used in steel,aluminium,glass,cement,petrochemicals,non-ferrous metals,thermal power plants and ceramic industries.they are produced in Special shapes and are custom made to suit the requirements of the various industries.

Refractories are classified on the basis of therir chemical composition.end use and manufacturing method.They can be classified acidic basic and nutral refractories.A small range of high melting point material like magnetia,bauxite ,fireclay and silica are used to produce refractories.

Our position in global market

5%

37%

12%

26%

16%4%

Rest of WorldChinaRestOfAsiaEuropeUSI ndia

Global Refractory Production

Industrial uses

Refractories

Refractory lining of a furnace arc

Refractory walls of a furnace interior with burner blocks

Properties of Refractories

Melting point Temperature at which a ‘test pyramid’ (cone) fails to support its own

weight Size

Affects stability of furnace structure Bulk density

Amount of refractory material within a volume (kg/m3) High bulk density = high volume stability, heat capacity and resistance

Porosity Volume of open pores as % of total refractory volume Low porosity = less penetration of molten material

Properties

Cold crushing strength Resistance of refractory to crushing

Creep at high temperature Deformation of refractory material under stress at given time and

temperature Pyrometric cones

Used in ceramic industriesto test ‘refractoriness’ of refractory bricks

Each cone is mix of oxidesthat melt at specific temperatures

Properties

Volume stability, expansion & shrinkage Permanent changes during refractory service life Occurs at high temperatures

Reversible thermal expansion Phase transformations during heating and cooling

Size and dimensional stability The size and shape of the refractories is an important feature in design

since it affects the stability of any structure. Dimensional accuracy and size is extremely important to enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.

Properties

Abrasion resistance  The mechanical stress of refractory bricks is not caused by pressure alone, but

also the abrasive attack of the solid raw materials as it slowly pass over the brickwork and by the impingement of the fast moving gases with fine dust particles. Therefore the cold crushing strength is not alone sufficient to characterize the wear of the refractories. There is no approved method for testing abrasion resistance but there are some methods available to give reference values such as Bohme grinding machine method and sand blast method etc.

Specific heat  The specific heat is a material and temperature related energy factor and is

determined with the help of calorimeters. The factor indicates the amount of energy (calories) needed to raise the temperature of one gram of material by 1 deg C. Compared to water, the specific heats of refractory materials are very low. These values are less than one fourth of value of specific heat of water.

Pyrometric cones

Self supporting pyrometric cones

Pyrometric cones are pyrometric devices that are used to gauge heatwork during the firing of ceramic materials. The pyrometric cone is "A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base at a definitive temperature. Pyrometric cones are made in series, the temperature interval between the successive cones usually being 20 degrees Celsius. The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK).

Pyrometric cones Equivalent(pce)

PCE cones before and after firing

These cones are pyramidal in shape and have a height of 38 mm of a triangular base and 19 mm long sides. They are allowed to heat under 10°C per min as a result of they undergo fusion as a definite temperature.This temperature at which they fusion or softening of the test cones is shown by its apex touching the base.The PCE of the given refractory may be regurded as the no of test cones,which also fuse with the test cone.

Refractory Under Load(RUL)

Refractoriness under load: Refractoriness under load (RUL) evaluates the softening behaviour of fired

refractory bricks at rising temperature and constant load conditions. RUL gives an indication of the temperature at which the brick will collapse in service condition with similar load. However, under actual service conditions the bricks are heated only on one face and most of the load is carried by the relatively cooler rigid portion of the refractory bricks. Hence, the RUL test gives only an index of refractory quality, rather than a figure which can be used in a refractory design. Under service conditions, where the refractory used is heating from all sides such as checkers, partition walls etc. the RUL test data is quite significant.  For RUL, samples in cylindrical shape of 50 mm height and 50 mm diameter are heated at a constant rate under a load of 0.2 N/Sqmm and the change in height includes the thermal expansion and also the expansion of test equipment. The test results are taken from the recording. The initial temperature is taken at 0.6 % compression while the final temperature is taken at 20 % compression or when the specimen has collapsed.

“RUL” curves for different refractory bricks

Types of refractories

3 types- 1)Acid refractories(e.g silica bricks) 2)Basic refractories(e.g magnetia bricks) 3)Nutral refractories(e.g karborundum bricks)

Types

There are three types of refractory material that we discussed in the previous slide- 1)Acid Refractory:This refractories are made up of acidic material like

zircon,fire clay and silica. 2)Basic Refractories:This type is made up of basic material like

dolomite,magnetia. 3)Nutral Refractory:Thus type is made up of alumina ,chromite,silicon

carbide and mulite.we can not use acidic refractory directly in the basic environment and vice versa because they will be corroded.

Classification on basis of temparature range

Refractory:when temperature is between 1580-1780 e.g. fire clay High Refractory:when temperature is between 1780-2000 E.g.

chromites Super refractory:when temperature is between >2000 E.g. zircons

Fire Clay Brics Chromite Bricks Zirconia Brics

Block Diagram of manufacturing refractory

Raw Material Crushing Grinding

ScreeningStorageMixingMolding

Drying Firing Sizing

PackagingTo Despatch

sOme common refractory bricks used in industry

Sillica bricks It contains 90-95 of silica and 2 of cao Cao is added to furnish the bond. Raw material is used quartz ,quartzite,and sand stone Crushed and groud with lime water..and this thik paste is then made into

brick bye hand moulding or machine pressing. then the bricks are dried into the drier and burnt into kilns The temperature is properly maintained

First it is slowly raised in about 24 hours to about 1500˚C and it is maintained for 12 hours so as to allow quaqrtzite to be converted into crystobellite

sOme common refractory bricks used in industry

After it is cooled for one or 2 weeks so thtcrystallobite slowly changes into trydemite.

Sio as we get a mixture of trydemite ande crystallibite in the final brick

Quartzite(Crystalline

)

Tridymite(α form of

silica)

Crystobellite

(β form of sillica)

870˚C

1117˚C

Properties and Uses of Silica Bricks

Properties: i)Yellow in colour,25% porous.They have sp. Gr. Of about 2.3-2.4 ii)They do not contract in use vbut they gtave 50% of permanent

expansion when reheated.and it is reversible and it returns to its original size when cooled.this expansion happens due to allotropic transformation.thuys if quartzite is not fully converted into trydemite and crytallobite it give 17.2% of permanent expansion.so for this the refractory structure may fall.So the heating is necessary.

iii)They have homogenious texture and their refractory under load is 3.5 Kg/sq. cm. upto 1500˚C.

Uses: Used for making the roof of open hearth furness,coke oven walls,roof of

electric furness.They also used as the linning of blast furnest because of their high conductivity.

Magnesia bricks

Magnesia bricks

There are two types of Magnesite bricks:Burned Magnesite brick:

· Material and Technique: magnesia as raw material

· Property: good high-temperature performance, strong slag resistance

· Application: used in permanent linings of EAF and steel Convertors, lime kilns, glass tank regenerators, torpedo cars and non-ferrous furnaces

Fused Magnesite brick: Material and Technique: fused magnesia, shaped under high pressure,

fired under ultra high temperature and finished with post technical treatment

Property: high bulk density, good high-temperature performance and strong hydration resistance

Application: used in the tapping hole of the convertor with severe slag corrosion and wear, the slag line of non-ferrous furnaces, the tapping hole of metal mixer furnaces, and chimney block of glass tank regenerator

Refractory Under Load 3.5 Kg/Sq. cm. upto 1500˚C.But it can withstand upto 2000˚C without any load.

They are used as the basis lnning in the basic converter and open hearth furness.

Zirconia brics

Zirconia bricks

They are obtained by heating zirconite mineral and colloidal zirconia(prepared by drying zirconium nitratesolution and making residue into solution and again drying).Here alumina is used as binding agent.Temparature required is 1700˚C.Since zirconite undergoes volume changes of heating and cooling.So it is stabilized by adding magnesium oxide.Properties:i)They are nutral refractory but they are not quite resistance to acidic slag.Hence they are at the border line of nutral and basic refractory.ii)Used upto 2000˚C.Their rul is 3.5Kg/sq cm upto 1900˚C.They are also resistnce to thermal shock.

Uses

They are very costly,so they are used only where the high temperature is used such as electric furness.

Zircon silicate bricks are used as an inert intermediate layer between high alumina bricks and silica crowns. In addition, they serve as a safety layer in the bottom of the melting tank, as they provide high resistance to metal drop drilling

Beryllia brick

They are made by firing mouled articles from pure beryllia at 1900˚C-2000˚C.

Properties: i) Beryllia bricks possesses high fussion point(2550˚C) ii) Low electrical conductivity ,high thermal conductivity ,good resistance

to thermal shock and inertness to CO and CO2 upto 2000˚C.They also have considerably hot strength.

Uses: i) Having considerable hot strength it is used in jet propulasion fields. ii) They are used in making crucible for melting uranium and thorium in

nuclear engineering.Due to its low nuhtron absorption capacity it is used as moderator in nuclear reactors and also in radiation shields in carbon resistance furnace.

Drawback: Its only drawback is beryllia is capable of volatizing at 1000˚C in

presence of water vapour.Beryllia dust if inhale in small quantity can cause serious health hazard.Since great precaution should be taken.

Carbon brick

Two types of carbon brick: i) CARBON BRICKS PHENOL IMPREGENATED - (AF)

QUALITY. The impervious carbon bricks are  especially

developed for Highly Corrosion Resistance, Excellent Resistance to Thermal Shocks, High Crushing Strength, High Thermal Conductivity & it withstand a temperature upto 180°c. Very minimum Porosity i.e. 2-3 % & Maximum Carbon content i.e.98 % and Low Ash.

ii) NON-IMPRENGNATED CARBON BRICKS ( STANDARD BRICKS “A” QUALITY).

The non-impervious carbon bricks especially designed for Highly Corrosion Resistance including all major Acids and Alkalies. More than 750°C temperature absorbing capacity. Carbon Content is 99% minimum & very low ash i.e. less than 1.0 %

Carbon brick

CARBON BRICKS PHENOL IMPREGENATED

Specifications ResultsSize As per standard; &  thickness 10 To 100 mm  Tol. (+/-)1mm .

Density 1.6 gms./cc Avg.Comp. Strength 600 kg/cm2 Avg.Flexural Strength 300 kg/cm2 Avg.Tensile Strength 125  kg/cm2 Avg.Porosity 4 % Max.Water Absorption 2 % Max.Abradability Index 140 on Morgan ScaleCarbon Content 98 - 99 % Min.Thermal Conductivity 4 – 6 w/mk.Ash Content 1.5 % Max.Temp. Resistivity Upto 180°cChemical Duty Suitable for Phosphoric Acid /

HF / Flurosilicic Acid / H2SO4 & HCL Upto temp. 180°c Suitable for HNO3, max. Con. 30%, Temp. upto 90°c

NON - IMPRENGNATED CARBON BRICKS

Specifications ResultsSize As per standard ; & thickness 10 To 100 mm  Tol. (+/-)1mm .Density 1.5 gms/cc Avg.Comp. Strength 350 kg/cm2 Avg.Flexural Strength 100 - 150 Kg/cm2 Avg.Tensile Strength 60 kg/cm2Porosity 18 – 24 %Water Absorption 12-18 %Carbon Content 99 % Appox.Ash Content 1 % Max.Thermal Conductivity 4 – 6  w/mk.Abradability Index 100 -110 on Morgan ScaleTemp. Resistivity Upto 750°CChemical Duty 

Suitable for Phosphoric Acid /H2SO4/ HF / Flurosilicic Acid / HCL  Upto temp. 750°C. 

Suitable for HNO3 & all Alkalis max. Con. 30%, Temp. upto 120°c

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