What are Abrasive? How are they Classified? Give their uses.

ABRASIVES

Abrasives play a commendable role in our daily life and industry. For example, a house wife uses an abrasive stone to keep her kitchen knives sharp, a dentist uses an abrasive powder when he cleans his teeth and tiny abrasive wheels to smooth down fillings.

Abrasives are hard substances used for shaping, grinding and polishing the surfaces.

Characteristics of abrasives

Abrasives have the following characteristics:

(i) They are very hard.

(ii) They are chemically inert.

(iii) They have high melting point.

(iv) They possess high refractoriness.

(v) They resist the abrading action.

(vi) They are prone to mechanical shock.

(vii) They are affected by frictional heat.

Properties of abrasives

Hardness of an abrasive is a most important quality. It is measured by Mohs scale.
They also posses high melting point and chemical inertness.
They are widely used in the form of granules or powder.
Some examples of abrasives are diamond, corundum, boron carbide $(B 4 C)$ , carborundum $(SiC)$ etc.

Hardness of an abrasive

The capacity of an abrasive to grind another surface is known as ‘Hardness of an abrasive’. Hard abrasive has high scratch power.

Mohs, a mineralogist, has suggested a scale for determining the hardness of abrasive substances known as Mohs scale. The hardness of hardest material diamond is taken as 10 in the scale.

Mohs scale of hardness

Hardness is the most important property of an abrasive. It is the ability or capacity of an abrasive to grind away another substance.

Scale and Sludge in boilers

Fig (1) Scale and Sludge in boilers

The Mohs scale is given as follows:

(1) Talc (2) Gypsum (3) Calcite (4) Fluorite (5) Apatite

(6) Feldspar (7) Quartz (8) Topaz (9) Corundum (10) Diamond

Name of the abrasives	Chemical formula	Mohs number
Talc	$3MgO 4SiO 2 H 2 O$	1
Gypsum	$CaSO 4 \cdot2H 2 O$	2
Calcite	$CaCO 3$	3
Fluorite	$CaF 2$	4
Apatite	$CaF 2 3Ca 3 (PO 4) 2$	5
Feldspar	$K 2 O Al 2 O 3 6H 2 O$	6
Quartz	$SiO 2$	7
Topaz	$AlF 3 SiO 2$	8
Corundum	$Al 2 O 3$	9
Diamond	$C$	10

The hardness of any material on Mohs scale therefore falls between 1 and 10 as shown in Table.(1) Abrasives having their hardness in the range 1-4 in Mohs scale are called soft abrasives.

Abrasive power: The strength of an abrasive to grind away a surface is known as abrasive power. It depends upon the hardness, toughness and refractoriness of an abrasive.

Classification of Abrasives

Abrasives are broadly classified into two types. They are:

(i) Natural abrasives.

(ii) Artificial abrasives.

Difference between natural and artificial abrasives

Natural abrasives	Artificial abrasives
They occur in nature.	They are synthesised on a commercial scale. Hence, they are also called synthetic abrasives.
Some of the example of natural abrasives are diamond, emery, corundum, quartz etc.	Some of the example of this type are carborundum or silicon carbide $(SiC)$ and Boron carbide (B₄C ) etc.
They do not contain silica and hence called as non-siliceous abrasives.	They contain silica and hence called as siliceous abrasives.

Natural abrasives

Natural abrasives are further classified as:

(i) Non-siliceous natural abrasives

(1) Diamond

Diamond occurs in nature.
It consists of crystalline carbon.
It is the hardest among abrasives.

Uses

It is used for cutting, grinding and polishing the surfaces. Diamond is also used as a rock driller and as a rock-cutting saw.

(2) Corundum

Corundum is crystallised aluminium oxide $(Al 2 O 3)$ .
It comes next to diamond in hardness.

Uses

It is used as an abrasive for grinding glass, gems, metals etc.

(3) Emery

It is a mixture of crystalline alumina and magnetite $(Fe 3 O 4)$ with a little amount of other minerals.
Its hardness is about 8 on Mohs scale.
The grinding power of emery entirely depends on the amount of alumina present in it. Its true hardness depends on the nature of other ingredients present in it.