Engineering Chemistry–1–Abrasives

ABRASIVES

Topics discussed

  • Definition
  • Classification with examples
  • Mohs Scale
  • Applications
  • Natural abrasives
  • Synthetic abrasives

Throughout history, humans have used materials ranging from beach sand to walnut shells to paper bags as abrasives . Widely-used naturally occurring abrasives include garnet, cerium oxide, flint, emery, corundum (aluminum oxide), and diamond. These materials may have varying characteristics and chemical compositions depending on the specific geological source. Manufactured versions of these materials are usually more consistent in chemical composition and other characteristics. Abrasives can be distinguished in a variety of ways based on their hardness / color/ chemical composition / crystal shape etc to name a few.

Since the chemical composition- that is, the type of material- determines the abrasiveness, the identification of abrasives based on composition is popular

Definition

Abrasive is a substance used to wear down ( cut / polish / grind /sharp) the surface of the materials with which it is in contact

Abrasion is the ability of a substance to wear or tear the surface of other substance

Abrasives should possess good hardness, toughness and refractoriness

Hardness: Is defined as the ability of a substance to resist scratching

Toughness: Is defined as the ability of a substance to cause indentation or to scratch other material

Refractoriness: Is defined as the ability of a substance to withstand high temperature (Frictional Heat)

Abrasives are classified

Based On Occurrence : Natural and Synthetic

Based on Hardness: Hard ,Moderate and Soft

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Mohs Scale - Mineral Hardness

The Mohs scale of mineral hardness was devised by Frederich Mohs, a German mineralogist in 1812.

To devise the scale, he selected ten minerals as a basis because they were common or readily available, thus the scale is not linear, but is a bit arbitrary.

It is a measure of the relative hardness and resistance to scratching between minerals (tested by Pin on Disc method)

Though there are other scales like Rockwell, Vickers and Brinell, Mohs scale is still the most common scale of mineral hardness.

Moh’s scale

Mineral

Moh’s scale

composition

Talc

1

Mg3Si4O10(OH)2

Gypsum

2

CaSO4.2H2O

Calcite

3

CaCO3

Fluorite

4

CaF2

Apatite

5

CaF2.3Ca3(PO4)2

Feldspar

6

K2O.Al2O3.6H2O

Quartz

7

SiO2

Topaz

8

AlF3.SiO2

Corundum

9

Al2O3

Diamond

10

C

Mnemonics to remember the order of Mhos scale

To Get Candy From Aunt Fanny Quit Teasing Cousin Danny

Mohs scale of common household substances

(Just for information – do not memorise)

Material

Hardness

Plastic

1

Salt

2.3

Fingernail

2.5

Gold , Silver

2.5-3.0

Copper coin

3.5

Platinum

4 – 4.5

Window glass

5.5

Steel File

6.5

Ceramic tile

7.0

Natural Abrasives

Corundum (Al2O3) [Alundum]

Properties :

Crystalline

Very Hard

Moh’s scale-9

brown to grey in colour

Corundum is used in :

In grinding wheels

To grind glass/lens/metals,

Ruby lasers

Diamond (C)

Diamond exist in three major forms

Diamond (gem grade)

Borts – These are diamonds that are off colour or faulty

Carbonado – These are black diamonds mined from Brazil. They have good hardness, but due to lack luster, do not find application as jewelry.

They are commonly used as abrasives

Abrasive grade diamond has the following properties

crystalline

Chemically inactive

Moh’s scale-10

Uses

In bits of drilling points

Saw teeth for cutting rocks

In grinding wheels

In engraving tools

Emery

Composition

Alumina (crystalline) 50 - 75% Magnetite 20 - 40% Other minerals 12 %

Properties

Dark grey to black

Hardness depends upon alumina content

Moh’s scale - 8

Uses

In emery paper and cloth used for polishing.

In bits of cutting and drilling tools

Garnets

Properties

Tri silicates of alumina/ magnesia/ ferrous oxide.

Common garnet- Calcium alminium iron silicate

Moh’s scale 6 -7.5

Uses

To paper/cloth to polish wood /metals,

As bearing pivots in watches

In glass grinding

Quartz (SiO2)

Properties

SiO2 (pure crystalline)

Moh’s scale – 7

Hydrated form of SiO2 is called Flint

Uses

Impure grey quartz used in sand paper

As granules in grinding

Machines used to grind flour, pigments & ores.

Synthetic Abrasives

Silicon Carbide / Carborundum / Crystolon (SiC)

Preparation

Raw materials are

i. Petroleum Coke (source of carbon)

ii. Sand (source of Si)

iii. Saw Dust ( To provide hardness)

All raw materials are sized, dried and mixed along with old charge and fed into the Acheson furnace with little amount of NaCl (flux)

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Properties

M.Pt. is 2700oC

Mohs scale hardness is 9.3

Chemically Inert

High Thermal Stability

Brittle hence strength is less

USES

Cutting tools

Grinding of cast iron, brass, bronze , porcelain marble

Polishing leather, lenses (Abrasive paper and Cloth)

Refractory in furnace

Boron Carbide / Norbide (B4C)

Preparation

i. Petroleum Coke (source of carbon)

ii. Boron Oxide

iii. Saw Dust (To provide hardness)

All raw materials sized, dried and mixed along with old charge and fed into the

Acheson furnace with little amount of NaCl (flux).

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Properties

Moh’s scale hardness - 9 . 7

Chemically inert

Resist oxidation more than diamond.

Uses

Grinding dies

Cutting

Sharpening hard high speed tools

Advantages of Synthetic Abrasives

Unlike natural abrasive synthetic abrasives possess uniform chemical composition

Due to uniformity in the composition the hardness at various points also remain uniform

Important General Applications of Abrasives

In grinding wheels (Bonded abrasives) abrasives are used to grind and polish the surface of metals and composites ,sharpen tools like knife ,saw tooth etc.,

In the form of paper / cloth (abrasive coated on paper / cloth with the help of glue) abrasives are used for cleaning and polishing soft and precious metals (Au, Ag, W etc.,) .They are also used to shine leather, clean lens , windscreen etc.,

In few places abrasives are used in the form of loose powder-for polishing hard metal and wood

Cleaning products also contain abrasives suspended in a paste or cream.

For example tooth paste contains calcium carbonate / silica /mica as a "polishing agent" to remove plaque and other matter from teeth as the hardness of the abrasive used is less than that of tooth enamel but more than that of the contaminating agent(plaque material).

Abrasives may also be used to prepare surfaces for application of paint and varnish

Pumice stone ( abrasive) is used to remove dead cells from skin (usually to keep the feet crack free)

Soft abrasives are used in cosmetic industries in various products to rejunivate skin and maintain a smooth and silky texture