Classification of Refractory Material

Classification of Refractory Materials

Refractory materials are inorganic non-metallic materials which can withstand high temperature without losing their strength and shape. They are widely used in furnaces, kilns, reactors, ladles, and other high-temperature equipment in industries such as steel, cement, glass, foundry, and ceramics.

Refractories can be classified in different ways depending on their chemical nature, temperature resistance, form, function, raw material, and density.

The main classifications of refractory materials are given below.

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1. Classification based on Chemical Nature

This classification depends on the chemical behavior of refractories with slag and furnace atmosphere.

Refractories are divided into three types:

• Acidic refractories

            Acidic refractories resist acidic slag but react with basic slag.
       

        Example products:

• Silica bricks
• Fireclay bricks
• Quartzite bricks
• Acid resistant castable
  
  
• Basic refractories 

             Basic refractories resist basic slag but react with acidic slag.

           Example products:

• Magnesite bricks
• Dolomite bricks
• Magnesia carbon bricks
• Basic ramming mass
  Neutral refractories 

            Neutral refractories are stable against both acidic and basic slag.

           Example products:

• High alumina bricks
• Chromite bricks
• Carbon bricks
• Graphite blocks
• Alumina castable

This classification is important while selecting refractories for steel furnaces, glass furnaces, and kilns.

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2. Classification based on Refractoriness (Temperature Resistance)

Refractories can also be classified according to the maximum temperature they can withstand.

They are divided into:

• Low duty refractories - Refractoriness below 1580°C

         Example products:

• Low duty fireclay bricks 

• Insulating brick 

• Low temperature castable
  
  
• Medium duty refractories - Refractoriness - 1580 to 1750°C

        Example products:

• Medium duty fireclay bricks 

• Boiler refractory bricks 

• Medium grade castable

• High duty refractories - 1750 to 2000°C

         Example products:

• High alumina bricks 

• High duty fireclay bricks 

• High alumina castable 

• Burner blocks

• Super duty refractories - above 2000°C

        Example products:

• Magnesite bricks 

• Magnesia carbon bricks 

• Zircon bricks 

• Chrome magnesite bricks 

• Ultra low cement castable

Low duty refractories are used at lower temperatures, while high duty and super duty refractories are used in very high-temperature furnaces such as steel furnaces, cement kilns, and electric arc furnaces.

This classification helps in selecting the proper refractory according to operating temperature.

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3. Classification based on Form / Physical State

According to physical form, refractories are divided into shaped, monolithic, and fibrous materials.

Shaped refractories

These refractories are manufactured in definite shapes and sizes and remain rigid during use.

Example products:

• Firebricks

• High alumina bricks

• Magnesite bricks

• Precast blocks

• Burner blocks

• Tiles and special shapes

Monolithic Refractories (Unshaped / Loose refractories)

Supplied without fixed shape, form obtained during installation.

These can be further classified as:

a) Powder / Granular / Grain sized refractories

Supplied in dry form.

Example Products:

• Ramming mass

• Dry vibration mass

• Refractory mortar

• Gunning mix

• Sand mix

• Tundish powder

Characteristics:

• Powder or grains

• Need water or compaction

• Shape formed at site
  
  

b) Castable / Plastic / Wet refractories

Supplied as powder but used with liquid or binder.

Example Products:

• Castables

• Low cement castables

• Plastic refractory

• Gunning castable

• Pumpable castable

Characteristics:

• Mixed with water

• Flowable / moldable

• Set after installation
  
  

c) Liquid / Slurry / Coating refractories

Supplied in liquid or semi-liquid form.

Example Products:

• Refractory coating

• Slurry

• Wash / paint

• Zircon coating

• Mold coating (foundry)
  
  

Fibrous insulation materials

These are lightweight and flexible refractory insulation products.

Example Products:

• Ceramic fiber blanket

• Ceramic fiber bulk

• Ceramic fiber board

• Ceramic fiber paper

• Glass wool

• Rock wool

• Mineral wool

Characteristics:

• Flexible or semi rigid

• Low density

• High insulation

• Can be cut / pressed

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4. Classification based on Function (Hot Face and Cold Face Refractories)

Refractories are also classified according to their use in furnace lining.

Hot face refractories

These refractories are in direct contact with flame, molten metal, or hot gases.
They must have high strength, high refractoriness, and good slag resistance.

Example Products:

• Fireclay bricks

• High alumina bricks

• Magnesite bricks

• Dolomite bricks

• Chrome bricks

• Dense castables

Cold face refractories

These refractories are used as insulation behind the hot face lining.
They reduce heat loss and improve energy efficiency.

Example Products:

• Insulating fire brick (IFB)

• Ceramic fiber

• Calcium silicate blocks

• Vermiculite bricks

• Insulating castables

This classification is very important in furnace design because most linings consist of both hot face and insulation layers.

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5. Classification based on Raw Material

Refractories can also be classified according to the main raw material used in their manufacture.

Common types include:

• Clay refractories

        Example products

• Fireclay bricks 

• Fireclay mortar 
  
  

• Silica refractories

        Example products

• Silica bricks 

• Silica checker bricks 

• Silica ramming mass 

• Silica refractory blocks
  
  

• High alumina refractories

        Example products:

• High alumina bricks  

• High alumina mortar 

• High alumina burner blocks 

• High alumina precast shapes
  
  

• Magnesia refractories

        Example products:

• Magnesite bricks 

• Magnesia carbon bricks 

• Magnesia castable 

• Magnesia gunning mix 

• Magnesia spinel bricks
  
  

• Zircon refractories

        Example products:

• Zircon bricks  

• Zircon coating 

• Zircon wash

• Zircon nozzle mix
  
  

• Carbon Refractories

         Example products:

• Carbon bricks 

• Graphite blocks 

• Magnesia carbon bricks 

• Graphite crucibles 

• Carbon lining blocks

Each type has different properties and is used in different industries depending on temperature, slag, and atmosphere.

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Conclusion

Refractory materials can be classified in several ways, and each classification is useful for selecting the correct refractory for a particular application.
In practical furnace construction, selection of refractory depends on temperature, slag, atmosphere, strength, insulation requirement, and cost.

Understanding the classification of refractories is the first step in learning refractory technology.