Refractories are materials that are able to withstand higher temperature,thermal load and resistance to abrasion and chemical attacks.
All industries whether directly or indirectly depend on this refractory materials for their manufacturing system. Some of them are:
· cement industries
· lime,ceramic and glass industries
· iron and steel industiries
· fertiliser,urea & petrochemical industries
· refinineries & oil industries
· metal foundries
· power industries
· boilers
· sugar & chemical industries
· metal industries
· engineering industries
· all kinds of furnaces
· paper & baking industries
· food industries
· organic and inorganic industires
· acid industries
· electrical industries like insulators
· nuclear industries
· waste heat recovery and waste burning industries
· aeronautical industries etc.,
Refractories protect the metal shell of any equipment like kiln etc., from heat,high temperature and abrasion.
History
There is no clear idea when the refractories are invented, but it was assumed that the requirement to withstand higher temperatures for the production of metals and glasses would be their cause for invention.
Initially only ceramic clay was used for making refractory materials which had poor performance.This led to innovation and development of the refractory materials to improve its performance.But this innovation works started during the 19th century only.
Initially only ceramic clay was used for making refractory materials which had poor performance.This led to innovation and development of the refractory materials to improve its performance.But this innovation works started during the 19th century only.
Innovation
One of the methods that the refractory manufacturers applied to increase the work performance of their refractory product was they increased the purity of the main refractory component by decreasing the other impurities.This was done by physical and chemical treatment methods like floatation, precipitation etc.,
Let me brief you with some examples:
· Natural alumina has a melting point of around 2000 deg Celsius, but a refined and fused alumina has a melting point of 2500 deg Celsius.
· 60% zircon material can withstand between 1800 to 2000 deg Celsius, but above 96% zircon material can withstand up to 2500 deg Celsius.
By refining and enriching the purity of the refractory material, its performance can be enhanced.
Classification
According to raw materials used, refractories are classified into three types:
1. Acid refractory (alumina, fireclay types)
2. Basic refractory (magnesia,dolomite types)
3. Neutral refractory (carbon, chromite types)
But industrially, they classify refractory as:
1. Alumina/high alumina refractory
2. Basic or magnesite refractory
3. Fire clay type refractory
4. Silicon refractory
5. Special purpose refractory
6. Unshaped refractory materials like castables, mortars etc.,
7. Insulating materials
Some persons further sub-divide the refractory according to installation method:
1. Casting type refractory
2. Ramming type refractory
3. Vibrating type refractory
Let us see about each refractory type briefly.
High alumina refractory
These come under alumina type refractory. Any refractory product having more than 45% alumina is called High alumina refractory.
Basic raw materials are bauxite,mullite,shale clay,corundum etc.,
Any raw material having more than 80% alumina is fit for refractory production.
High alumina bricks/castables are more resistant to chemical attack and abrasion. High alumina bricks can withstand upto 1800 degree Celsius.
(courtesy:www.msmemart.com)
A simple note: More the alumina content = more the withstanding temperature.
HA70-25 alumina brick means: that the alumina brick has 70% alumina content and 2.5% content iron content.Lesser the iron content, the more the withstanding temperature as iron is an impurity for alumina brick.
Fire clay bricks
Basic raw material required for fire clay brick manufacture was clay at fired condition which is called charmotte. Fire clay bricks contains 45% alumina (maximum) and upto 80% silica.
Fire clay bricks was suspected to be first invented refractory brick. It can withstand upto 1200 to1500 deg Celcius.Both fire clay and high alumina bricks manufacturing process are similar, but the quantity of alumina content may differ.
Fire clay bricks has good thermal shock and abrasion resistance.It has good resistance to alkali attack also.
Also note that both high alumina and fire clay bricks appear similarly.
Basic bricks
They are of two types:
· Magnesia bricks which was produced from fused magnesia or periclase (MgO)
· Dolomite bricks which was produced from fused dolomite. Generally dolomite bricks/refractories has calcium oxides and magnesium oxides.
Magnesia bricks are more resistant to slag attack and liquid content in kiln and also they have good compressive strength even at higher temperatures.
Normally basic bricks can withstand upto 1800 deg Celcius.
Normally basic bricks can withstand upto 1800 deg Celcius.
Why chromite added in some basic bricks?
To improve the thermal shock withstanding capacity, chromite is added. So if a brick is named as magnesia-chromite, then it means that the brick contains chromite content. If a brick is named as magnesia brick, then it means that the brick has only magnesia content and no chromite content.
Do you know what is spinel and why it was added?
Spinel is material which as 70% alumina and 30% magnesia content. It can withstand upto 2100 deg Celcius.It was added to some basic bricks upto 24% (max).
Upto its withstanding temperature,spinel will not react with other chemical compounds. It will not dissolve in most of the acids and has good resistant to metals. But chances of spinel getting destroyed at higher temperatures due to contact with liquid phase in kiln are also to be kept in mind.
Upto its withstanding temperature,spinel will not react with other chemical compounds. It will not dissolve in most of the acids and has good resistant to metals. But chances of spinel getting destroyed at higher temperatures due to contact with liquid phase in kiln are also to be kept in mind.
Compared to magnesite-chromite bricks, magnesia-spinel bricks/refractories performs well even at low oxygen levels.
Silica bricks
(fused silica brick)
Mostly used in glass industries,coke ovens, blast furnaces etc., these bricks are made from silicon-di-oxide ie., silica. Various forms of silica like quartz, cristobalite etc., are used as raw material to manufacture silica bricks.These bricks have good resistance to alkali attack.
Different grades of these silica bricks are available but those bricks which contains more than 90% silica are worth when compared.. Ensure care that the iron content in the brick is less than 2% as its a impurity here which fuses at lower temperatures compared to silica.
These bricks can withstand upto 1650 deg Celcius.
Different grades of these silica bricks are available but those bricks which contains more than 90% silica are worth when compared.. Ensure care that the iron content in the brick is less than 2% as its a impurity here which fuses at lower temperatures compared to silica.
These bricks can withstand upto 1650 deg Celcius.
Insulating bricks/material
Mostly used for insulating purpose at the back of the refractory bricks.Their properties are:
1. Light weight
2. Good resistance to thermal conductivity
3. Good resistance to chemical and alkali attacks
4. High resistance to dust attacks.
These bricks can withstand upto 1200 deg Celcius. They are made from high silica content.
Do you know how insulating bricks work as a bad conductor of heat?
Its simple. The insulating material/brick contains more voids. Air gets filled in these voids due to which is heat conduction reduces.
Note: There are some insulating bricks which can be used as front end refractory bricks directly.
