REFRACTORIES 152022 Source Confidential 1 What is a

REFRACTORIES 1/5/2022 Source: Confidential 1

What is a Furnace Introduction • 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

1/5/2022 Source: Confidential 3

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/m 3) – 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 • 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 • 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 Fireclay Refractories • Common in industry: materials available and inexpensive • Consist of aluminium silicates • Decreasing melting point (PCE) with increasing impurity and decreasing Al 2 O 3 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% Si. O 2 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% Cr 2 O 3 and 42 -50% Mg. O – Used for critical parts of high temp furnaces – Withstand corrosive slags – High refractories • Magnesite-chromite – >60% Mg. O and 8 -18% Cr 2 O 3 – High temp resistance – Basic slags in steel melting – Better spalling resistance

Zirconia Refractories Type of Refractories • Zirconium dioxide Zr. O 2 • Stabilized with calcium, magnesium, etc. • High strength, low thermal conductivity, not reactive, low thermal loss • Used in glass furnaces, insulating refractory

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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