CLAY :–
Clay is a binding agent mixed to the moulding sand to provide strength. Popular types of clay used are kaolinite or fire clay (Al2O3.2 SiO2.2H2O) and Bentonite (Al2O3.4 SiO2.H2O nH2O). Kaolinite has a melting point from 1750 to 17870C where as Bentonite has a melting temperature range of 1250 to 13000C. Bentonite clay absorbs more water and has increased bonding power. To reduce refractoriness, extra mixtures like lime, alkalis and other oxides are added.
Bentonite is further of two types. (a) Western bentonite and (b) southern bentonite
Western bentonite – It is rich with sodium ion
It has better swelling properties
When it mixes with sand, the volume increases 10 to 20 times.
High dry strength, so lower risk of erosion
Better tolerance of variation in water content
Low green strength
High resistance to burn out
Southern Bentonite - It is rich with calcium ion
It has low dry strength and high green strength
Its properties can be improved by treating it with soda ash (sodium carbonate)
Water:- Used to activate the clay
Generally 2 to 8% of water is required
Other materials added:- Cereal binder – (2%) – to increase the strength
Pitch (by product of coke) – (3%) – to improve hot strength
Saw dust (2% ) – To increase permeability
Clay is a binding agent mixed to the moulding sand to provide strength. Popular types of clay used are kaolinite or fire clay (Al2O3.2 SiO2.2H2O) and Bentonite (Al2O3.4 SiO2.H2O nH2O). Kaolinite has a melting point from 1750 to 17870C where as Bentonite has a melting temperature range of 1250 to 13000C. Bentonite clay absorbs more water and has increased bonding power. To reduce refractoriness, extra mixtures like lime, alkalis and other oxides are added.
Bentonite is further of two types. (a) Western bentonite and (b) southern bentonite
Western bentonite – It is rich with sodium ion
It has better swelling properties
When it mixes with sand, the volume increases 10 to 20 times.
High dry strength, so lower risk of erosion
Better tolerance of variation in water content
Low green strength
High resistance to burn out
Southern Bentonite - It is rich with calcium ion
It has low dry strength and high green strength
Its properties can be improved by treating it with soda ash (sodium carbonate)
Water:- Used to activate the clay
Generally 2 to 8% of water is required
Other materials added:- Cereal binder – (2%) – to increase the strength
Pitch (by product of coke) – (3%) – to improve hot strength
Saw dust (2% ) – To increase permeability
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