Abrasives

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

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)

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).

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

Questions & Answers:

What is meant by abrasives?

          Abrasives are hard substances, used for polishing, shaping, grinding cutting operations. They are characterized by high melting point, high hardness and chemically inactive. 

Properties of Abrasives

1. Hardness: It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale. 
2. Toughness: Abrasives are generally hard and brittle, which is otherwise known as toughness.
3. Abrasive power: It is the strength of an abrasive to grind away another material. It depends on hardness, toughness and refractoriness. 

Characteristics of Abrasives 

The followings are the important characteristics of an abrasive,

• It should be very hard. 
• It should resist the abrading action. 
• It should be chemically inactive. I
• It should possess high refractoriness. 
• It should have high melting point. 
• It should not be affected by frictional heat. 

Classification of Abrasives 

Abrasives are classified into two types 

1. Natural abrasives: (a)Non-siliceous abrasives (b)Siliceous abrasives
2. Artificial or synthetic abrasives 

Natural abrasives 

(a) Non-Siliceous Abrasives 

• Diamond 

• Corundum 

• Emery 

1. Diamond 

It is a pure crystalline carbon. It is the hardest known substance. Its hardness is 10 on Mob’s scale. It is chemically inert and not affected by acids or alkalis. The off-color diamond is called borts and black color diamond is called carbonados.

Uses:

It is used in drill points, cutting rocks, stones and grinding wheels. 

2.Corundum 

It is a pure crystalline alumina(Al2 O3)

Its hardness on Mob’s scale is 9. 

Uses:

It is used for grinding glasses, gems, lenses, metals, etc.

3. Emery 

It is a fine-grained, opaque, black coloured mineral. It consists of  (i) 5575% crystalline alumina (ii) 20-40% magnesite (iii) 12% other minerals. Its hardness is 8 on Moh’s scale.

Uses:

It is used in the tip of cutting and drilling tools, and also it is used in making abrasive paper and cloth. 

(b) Siliceous abrasives 

1. Quartz

2. Garnet 

1. Quartz 

It is a pure crystalline silica (Si02). Its hardness is 7 on Mob’s scale. 

Uses:

It is used for grinding pigments in the paint industry and also it is used as granules in grinding machines. 

2. Garnet 

It is a mixture of trisilicates of alumina, magnesia and ferrous oxide. its hardness ranges from 6-7.5 on Moh’s scale.

Uses:

It is used in making abrasive paper and abrasive cloth, and also it is used in glass grinding and polishing metals. 

Synthetic abrasives

I. Silicon carbide or carborundum (SiC)

2. Norbide or Boron carbide (B4C)

 

Silicon carbide or carborundum (SiC)

Manufacture :

Silicon carbide is manufactured by heating sand (60%) and coke (40%) with some saw-dust and a little salt in an electric furnace to about 1800°C.SiO2+ 3C  SiC + 2CO2. Salt reacts with iron and other similar impurities, present in the raw materials, forming volatile chlorides. This also increases the porosity of the final product. The silicon carbide, removed from the furnace, is then mixed with bonding agent (like clay, silicon nitride) and then  shaped, dried and fired. 

► Properties:

1. Silicon carbide possesses a high thermal conductivity, low expansion and high resistance to abrasion and spalling.

2. They are mechanically strong and withstand loads in furnaces upto 1650°C. 

3. Heat conductivity of SiC is intermediate between metals and ceramic materials. 

4. They are electrically intermediate between conductors and insulators. 

5. The strength, density, abrasion-resistance, chemical resistance, softening temperature of the various bonded refractories will be in the following order. 

Self - bonded product > silicon nitride bonded product > clay - bonded product. 

Uses :

1. Silicon carbides are used as heating elements in furnaces in the form of rods and bars. 

2.They are also used for partition walls of chamber kilns, coke ovens, muffle furnaces, and floors of heat-treatment furnaces. 

3. SiC bonded with tar are excellent for making high conductivity crucibles. 

4. For making abrasive papers,cloths and carbon rods in resistance heaters

Norbide or Boron carbide (B4C)

Manufacture :

It is prepared by heating a mixture of boron oxide (B203) and coke in an electric 

furnace to about 2700°C. 2 B203 + 7C  B4C + 6CO

Properties :

1. Its hardness is 9 on moh’s scale. 

2. It is light weight and black colored compound. 

3. It is highly resistant to chemical attack and erosion.

Uses :

1. It is used as hard materials for making grinding dies, and for cutting and sharpening hard high-speed tools. 

2. It is used to prepare scratch and wear resistant t coatings. 

3. It resists oxidation much better than diamond.

4. For cutting and Sharpening of hard high speed tools.

REFRACTORIES 

Introduction:

Refractories are materials that can withstand high temperatures without softening or deformation in shape. Refractories are employed in metallury, engineering and chemical industries like cement, lime, glass, steel, ceramics, etc.The main function of a refractory is to withstand and maintain high temperatures and to resist the abrasive and corrosive action of molten metals, slags and gases. 

Defination of Refractories:

Refractories are materials that can withstand high temperatures without softening or deformatioiz in shape. 

Uses of Refractories: 

1. Refractories are mostly used for the construction of the lining of the furnaces, tanks, converters, kilns, crucibles, ladles, etc. 

2.They are employed for the manufacture of cement, glass, ceramics, paper, metals (both ferrous and non-ferrous), etc. 

Characteristics or Requisites of a Good Refractory

(i). It should be infusible at the operating temperatures. 

(ii). It should be chemically inert towards the corrosive gases, metallic slags and liquids. 

(iii). It should resist the abrading action of flue gases, flames, etc. 

(iv). It should not crack and suffer loss in size, at the operating temperatures. 

(v). It should expand and contract uniformly, with temperature rise and fall respectively.

(vi). It should be able to withstand overlying load of structure, at operating temperatures. 

(vii). It should have high refractoriness. 

Classification of Refractories

Refractories are classified in the following two ways,

(i) According to their chemical properties.

(ii) According to their refractoriness. 

I. According to the chemical properties, refractories are classified into three main types

(i) Acidic Refractories

(ii) Basic Rcfractories

(iii) Neutral Refractories

(i) Acidic Refractories:

Acidic refractories consist of acidic materials like Alumina (Al203) and silica (Si02). They are not 

attacked by acidic materials, but easily attacked by basic materials. (eg.) Silica, Alumina, Fire clay refractories. 

(ii) Basic Rcfractories : 

Basic refractories consist of basic materials like CaO. MgO, etc. They are not attacked by basic 

materials, but are easily attacked by acidic materials. (eg.) Magnesite,Dolomite refractories. 

(iii) Neutral Refractories:

Neutral refractories are made from weakly acidic and basic materials like Carbon, Chromite, Zirconia, etc. They are not attacked by both acidic and basic materials. 

II. According to their refractoriness, refractories are classified into four types.

Type of refractories PCE value refractoriness  examples

1. Low heat duty refractories 

19-28  1520-1630  -

2. Intermediate heat duty refractories

28-30  1630-1670  Fireclay bricks 

3. High heat duty refractories 

30-33  1670-1730  Chromite bricks 

4. Super heat duty refractories

> 33  > 1730  Magnesite bricks 

Properties of Refractories

1. Refractoriness 

It is the ability of a material to withstand very high temperature without softening or deformation under particular service condition. How to measure refractoriness Since most of the refractories are 

mixtures of several metallic oxides,they do not have a sharp melting points. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed interms of Pyrometric 

Cone Equivalent (PCE) Pyrometric Cone Equivalent(PCE) Pyrometric cone equivalent is the number, which represents the softening temperature of a refractory specimen of standard dimension (38 mm hight and 19 mm triangular base) and composition. Some examples are given in the table. 

Some examples of Pyrometric Cone Equivalent (PCE) are given in the table. 

Refractories PCE Softening temp. 

(i) Silica bricks 32  1710° C 

(ii) Alumina bricks 36-38  1800— 1850° C 

(iii) Magnesite bricks 38  1850° C 

Objectives of PCE Test

1. To determine the softening temperature of a test refractory material. 

2. To classify the refractories. 

3. To determine the purity of refractories. 

4. To check whether the refractory can be used at the particular servicing temperature. 

2. Refractoriness under load (RUL) (or) strength 

Definations:

The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL). A good refractory should have high RUL value. RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2and height 75 cm and heating in a furnace at a standard rate of 10° C per minute. Refractories, used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be r1ieasured by RUL test. 

3. Porosity 

It is defined as the ratio of its pore volume to the bulk volume

Disadvantages of high porosity refractory :

 It reduces the strength. 

 It reduces resistance to abrasion. 

 It reduces the resistance to corrosion. 

Advantages of high porosity refractory :

 Lightly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it can be used for lining in furnaces, ovens, etc. 

 highly porous refractory reduces thermal spalling. 

4. Thermal Spalling 

Thermal spalling is the property of breaking, cracking or peeling off a refractory material under high 

temperature. A good refractory must show a very good resistance to spalling. 

5. Dimensional stability 

 It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time. 

 These dimensional changes are of two types, 

(i) Reversible 

(ii) Irreversible. 

(i) Reversible dimensional changes:

This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible. 

(ii) Irreversible dimensional changes:

This may result either in the contraction or expansion of a refractory. Magnesite is an amorphous material (specific gravity is  3.05). On heating it is gradually converted into more dense crystal1ine form of periclase (Sp. gravity = 3.54) 

Alumina bricks (Fire clay bricks)

 Alumina bricks contain 50% or more of Al203. They are generally manufactured by mixing calcined bauxite (A1203) with clay binder.

Manufacture:

1. Grinding and mixing :

The raw materials (calcined bauxite & Si02) and grog (calcined fire clay) are ground to fine powder and are mixed with required amount of water to convert it into pasty material. 

2. Moulding:

The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting. Drying and Firing bricks after molding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to 1200 — 1400°C for 6-8 days. 

Properties of Alumina bricks (Fire clay bricks)

 Alumina bricks are acidic refractory. 

 They posses very low coefficient of expansion. 

 They also posses high porosity, and high temperature load-bearing capacity. 

 They are inert to the action of gases like C 02 H2 and natural gas. 

 They are also very stable to both in oxidizing and reducing conditions. 

 They posses better resistance to thermal spalling than silica bricks. 

Magnesite bricks

Magnesite bricks contain mainly MgO. They are manufactured by mixing calcined magnesite with caustic magnesia.

Zirconia bricks 

Manufacture:

They are prepared by mixing zirconite mineral (Zr02) with colloidal zirconia or alumina as binder and finally heated 700°C. Small amount of MgO or CaO is added as stabilizer because mineral 

zirconite undergoes volume changes on heating /cooling. 

Properties of Zirconia bricks

1. Zirconia bricks are neutral refractories. 

2. Though zirconia bricks are neutral, they are affected by acidic slags. 

3. They can be used upto 2000°C and upto 1500°C under a load of 3.5 kg/cm2. 

4. They are also quite resistant to thermal shocks. 

5. Their thermal expansion is low. 

Uses:

They are used only where very high temperature. e.g., high-frequency electric furnaces.