Refractories

CHEM1013

Lesson on

REFRACTORIES

30 Years of Development Chemistry Department

Introduction What is a Furnace • Equipment to melt metals – Casting – Change shape – Change properties

• Type of fuel important – Mostly liquid/gaseous fuel or electricity

• Low efficiencies due to – High operating temperature – Emission of hot exhaust gases

Furnace Components

Chimney: remove combustion gases

Furnace chamber: constructed of insulating materials Hearth: support or carry the steel. Consists of refractory materials

Charging & discharging doors for loading & unloading stock

Burners: raise or maintain chamber temperature

Introduction What are Refractories 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

Refractories Refractory lining of a furnace arc

Refractory walls of a furnace interior with burner blocks (BEE India, 2005)

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

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

• Cold crushing strength – Resistance of refractory to crushing

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

Properties of Refractories • Pyrometric cones – Used in ceramic industries to test ‘refractoriness’ of refractory bricks – Each cone is mix of oxides that melt at specific temperatures

• Pyrometric Cone Equivalent (PCE) • Temperature at which the refractory brick and the cone bend • Refractory cannot be used above this temp

Properties of Refractories • Volume stability, expansion & shrinkage – Permanent changes during refractory service life – Occurs at high temperatures

• Reversible thermal expansion – Phase transformations during heating and cooling

Properties of Refractories • Thermal conductivity – Depends on composition and silica content – Increases with rising temperature

• High thermal conductivity: – Heat transfer through brickwork required – E.g. recuperators, regenerators

• Low thermal conductivity: – Heat conservation required (insulating refractories) – E.g. heat treatment furnaces

Type of Refractories Classification of Refractories Classification method

Examples

Chemical composition ACID, which readily combines with bases

Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallic oxides that resist the action of bases

Magnesite, Chrome-magnesite, Magnesite-chromite, Dolomite

NEUTRAL, which does not combine with acids nor bases

Fireclay bricks, Chrome, Pure Alumina

Special

Carbon, Silicon Carbide, Zirconia

End use

Blast furnace casting pit

Method of manufacture

Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying)

Type of Refractories Fireclay Refractories • Common in industry: materials available and inexpensive • Consist of aluminium silicates • Decreasing melting point (PCE) with increasing impurity and decreasing Al2O3

High Alumina Refractories • 45 - 100% alumina • High alumina % = high refractoriness • Applications: hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks

Type of Refractories Silica Brick • >93% SiO2 made from quality rocks • Iron & steel, glass industry • Advantages: no softening until fusion point is reached; high refractoriness; high resistance to spalling, flux and slag, volume stability

Magnesite • Chemically basic: >85% magnesium oxide • Properties depend on silicate bond concentration

• High slag resistance, especially lime and iron

Type of Refractories Chromite Refractories • Chrome- magnesite – 15-35% Cr2O3 and 42-50% MgO – Used for critical parts of high temp furnaces – Withstand corrosive slags – High refractories

• Magnesite-chromite – >60% MgO and 8-18% Cr2O3 – High temp resistance – Basic slags in steel melting – Better spalling resistance

Type of Refractories Zirconia Refractories • Zirconium dioxide ZrO2 • Stabilized with calcium, magnesium, etc. • High strength, low thermal conductivity, not reactive, low thermal loss • Used in glass furnaces, insulating refractory

Oxide Refractories (Alumina) • Aluminium oxide + alumina impurities • Chemically stable, strong, insoluble, high resistance in oxidizing and reducing atmosphere

• Used in heat processing industry, crucible shaping

Type of Refractories Monolithics • Single piece casts in equipment shape • Replacing conventional refractories • Advantages – Elimination of joints – Faster application – Heat savings – Better spalling resistance – Volume stability – Easy to transport, handle, install – Reduced downtime for repairs

Selecting the Right Refractory Selection criteria • Type of furnace • Type of metal charge • Presence of slag • Area of application • Working temperatures • Extent of abrasion and impact

• Structural load of furnace • Stress due to temp gradient & fluctuations • Chemical compatibility • Heat transfer & fuel conservation • Costs

F U R N A C E

F U R N A C E

OLD FURNACE

Gas fired furnace

Blast furnace

Iridium Crucible

Ceramic_Crucible

Porcelain

Q U A R T Z

Graphite

Crucible heating

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