Refractory

REFRACTORY:

    Refractory is a material that can withstand higher temperatures, resistant to heat and pressure without any deformation.

NEED FOR REFRACTORY:

•  Protect the equipment from higher temperature
•   Prolongs the equipment life
•   Protects environment
•   Insulates equipment

CLASSIFICATION OF REFRACTORY BASED ON PROPERTY:

•  Physical classification
•  Chemical classification
•  Thermal classification
•  Chemical behaviour

1. PHYSICAL CLASSIFICATION:

The physical classification of refractory is classified into three types. They are as follows

•         Shaped refractory
•         Unshaped refractory
•         Fibrous refractory

    Shaped refractory: Shaped refractory is a refractory having definite or fixed shaped when dispatched to the user. There are two varieties in shaped refractory which includes dense shaped refractory and porous refractory.

    Unshaped refractory: Unshaped refractory does not have a fixed form. Depends upon application, shape can be designed. Monolithics comes under the unshaped refractory. Based on consistency, unshaped is classified into four varieties which includes castables, ramming mass, gunning and mortars. Ramming mass requires plastic consistency whereas in case of gunning, it requires slip or dry consistency.

Consistency	Percentage (%)
Dry	< 4
Plastic	20-25
Slip	>30

 

    Fibrous refractory: Fibrous refractory contains fibres that can withstand heat and pressure. In fibrous refractory, there are two kinds which includes continuous (long fiber) and discontinuous fiber (short fibers).

2. CHEMICAL CLASSIFICATION:

The chemical classification of refractory is classified into three types based on composition. They are as follows

•             Oxides
•             Non Oxides
•             Composites

 Oxides: Oxide ceramics are chemically inert, oxidation resistant, electrically insulating, lower thermal conductivity. For oxides, the cost of alumina is lower and the cost of zirconia is higher.

 Non Oxides: Non Oxide ceramics are low oxidation resistant, electrically conducting, higher hardness. The cost of this type is higher.

 Composites: Ceramic composite is a combination of both oxide and non oxide ceramics. Composite have good toughness and manufacturing process is complex.

Oxides	Alumina, Silica, Magnesia, Mullite
Non-Oxides	Silicon Nitride, Silicon Carbide. Aluminium Nitride
Composites	Alumina-Silicon Carbide

3. THERMAL CLASSIFICATION:

The thermal classification of refractory is classified into three types based on fusion temperature ranges. They are as follows

•     Normal Refractory: Fusion temperature range lies between 1580^oC-1780^oC
•     High Refractory:  Fusion temperature range lies between 1780^oC-2000^oC
•     Super Refractory: Fusion temperature range lies between >2000^oC

High Refractory and Super Refractory are referred to as High Grade Refractory

Refractory	Temperature range	Example
Normal	1580oC-1780oC	Fireclay
High	1780oC-2000oC	Chrome
Super	>2000oC	Zirconia

4. CHEMICAL BEHAVIOUR:

The chemical behaviour of refractory is classified into three types. They are as follows

•     Acidic refractory: Resistant to acidic slag, but attack by basic slag
•     Basic refractory: Resistant to basic slag, but attack by acidic slag
•     Neutral refractory: Chemically stable to acidic as well as basic slag

Refractory	Example
Acidic	Silica, Fireclay, Mullite
Basic	Magnesia, Dolomite, Forsterite
Neutral	Alumina, Chrome, Carbon

 

    Why Calcium oxide is not used as a refractory material?

    Calcium oxide is hygroscopic in nature which will leads to dimensional changes (there is no stability). Hence Calcium oxide (CaO) is not used as a refractory material but magnesium oxide (MgO) can be used as refractory.