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Contents table Refractory Classification Properties of Refractories General Manufacturing (flow sheet) Selection of Refractories Applications -Refractories in Cement Industry. Some Common Refractories
Refractory A refractory material is one that retains its strength and do not fuse at high temperatures. ASTM defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000 °F (811 K; 538 °C)“. Examples Fire clay. Silica. Chromite magnesite.
ClassificationRefractories can be classified -on the basis of chemical composition -method of manufacture -according to their refractoriness.
On the basis of chemical composition Acidic refractories These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. e.g. fire clay, silica, Quartz, Zirconia. Basic refractories These are used on areas where slags and atmosphere are basic, stable to alkaline materials but reacts with acids. e.g. Magnesia , Alumina, Dolomite. Contd…
Contd.. Neutral refractoriesThese are used in areas where the atmosphere is either acidic or basic and are chemically stable to both acids and bases.e.g. Chromite, Carbide, Mullite.
Based on refractoriness Low heat duty refractoriesFor low temperature environment i.e. 1520 —1630 ºC Medium heat duty refractoriesFor temperature ranging from 1630—1670 ºC High heat duty refractoriesFor temperature ranging from 1670—1730 ºC Super duty refractoriesFor temperature above 1730 ºC
On basis of Manufacture Dry pressed Fused Cast Hand molded Formed Unformed
Properties Porosity Greater the porosity of the refractory great is the chance of chemical attack of material Refractoriness or Fusion point The temperature at which the refractory fuses, so it must be higher than the operating temperature of the refractory. Strength It is the resistance of the refractory to loads tension and shear stresses. Contd..
Contd.. Thermal Conductivity For a given composition, a better pressed,less permeable product has higher thermal conductivity. -For use in Furnace and kiln Thermal conductivity should be low. -For use in coke oven and regenerators thermal conductivity should be high. Spalling It is the fracture of refractory due to its uneven expansion on account of heat when they are subjected to rapid heating and cooling. Contd..
Contd.. Slag Resistance It is the resistance to slag action which depends on nature of the slag and refractory. Abrasion resistance It is the resistance to abrasion which can be increased by using dense (low porosity) fine grained and wear resistance refractory. Erosion resistance Resistance to erosion due to mechanical action.
Manufacturing steps Transportation of Raw material Grinding Pre-Treatment Calcination Stabilizer addition Mixing Bonding material Wet Mixing (14-20% water) Semi plastic Dry Mixing ( < 5% water) Moulding Hand Moulding (Wet Mixed) Machine Moulding (Dry and Semi wet Mixed) Contd..
Contd.. Drying Avoids high shrinkage and gives strength. Make refractories safe for handling. Firing Removes water of Hydration, 30% Shrinkage in Volume.
Selection of Refractories Area of application Working temperatures Extent of abrasion and impact Stress due to temperature gradient Heat transfer and fuel conservation Cost consideration
Applications Refractories are meant to sustain at high temperature so the very common applications are Used in furnaces such as blast furnace and coke oven. Used in boilers. Mostly used in cement industry in Preheater Rotary Kiln Burner pipe Clinker cooler
Fire Clay refractories Common in industry: materials available and inexpensive Consist of aluminium silicates Decreasing melting point 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
Silica Bricks >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
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