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Metallurgy

All elements are found in nature either in the Free State like gold, or in the combined state in the form of different compounds.

Metallic compounds can be in the form of oxides, carbonates or sulphides. These metallic compounds are also called minerals. For the extraction of metals, we require some procedure which is economically cheap and practically easy and must give maximum yield of metal.

 

What is Metallurgy?

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Most of the metals occur in the crust of the earth in the combined state, in the formed of compounds known as minerals. These minerals have lots of impurities. The abundance of different metals in the earth crust varies from metal to metal, like oxygen (45.5%) is the most abundant element in the earth’s crust followed by silicon (27.7%) and aluminum with 8.3%. Since the different metals are present in varying levels and have different physical and chemical properties, the extraction of different metals from their metallic compounds requires different methods. The process of extraction of metals from their respective metallic compounds is termed as metallurgy.

Metallurgy Definition

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The branch of chemistry which deals with the properties of matter and their application in various areas of science and engineering is called as materials science. The domain of materials science in which we are interested in, is related to the studies of the physical and chemical behavior of metallic elements and their mixtures, and is known as metallurgy.

Any metallurgy process involves some common steps.

  1. Crushing and grinding of a suitable metallic compound
  2. Concentration of the metallic compound
  3. Extraction of metal from the metallic compound
  4. Refining of the crude metal

History of Metallurgy

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The process of metallurgy is one of the oldest branches of science; it can be traced back to 6000 BC. In the 17th century, only 12 elements were known and this number increased to 24 by the 19th century. Gold and copper are the first metals to be discovered and from that time to 770 years back, only 12 elements were discovered. Out of these elements some were discovered in the 13th and 14th centuries like arsenic, antimony, zinc and bismuth while platinum was discovered in the 16th century. Rest of the seven metals are called Metals of Antiquity as they were the metals upon which civilization was based.

These seven Metals of Antiquity were

  1. Gold (ca) 6000 BC
  2. Copper,(ca) 4200 BC
  3. Silver,(ca) 4000 BC
  4. Lead, (ca) 3500 BC
  5. Tin, (ca) 1750 BC
  6. Iron,smelted, (ca) 1500 BC
  7. Mercury, (ca) 750 BC

Out of the Metals of Antiquity, five metals, gold, silver, copper, iron and mercury can be found in their native states, but the occurrence of these metals was not abundant. After these seven metals of antiquity Arsenic was the next element discovered in the 13th century by Albertus Magnus through the heating of arsenious oxide.

The next metal to be discovered was antimony which was extracted from antimony sulphide after roasting it in an iron pot in 1560. In 1595; bismuth was extracted by the reduction of bismuth oxide in the presence of carbon. No doubt zinc was a well known element that time but it was first extracted by the distillation process and later, a mixture of calamine (zinc oxide) and charcoal was heated in an earthenware pot to produce zinc metal. After that, several other metals were extracted from their respective ores during the 1700's like Nickel, Manganese, Molybdenum, Tungsten, Beryllium, Chromium, Cobalt, Uranium, Zirconium Tellurium and Yttrium.

Metallurgy Terms

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There are so many terms which are specifically used in the different metrical steps of different metals.

  1. Mineral: The naturally occurring chemical substance in the form of which the metal occurs in the earth with various impurities.
  2. Ore: The mineral form from which a metal can be extracted conveniently and economically.
  3. Pulverization: The grinding and crushing of the ore into small pieces by using ball mill or stamp mill.
  4. Benefaction: The process of removal of unwanted impurities from the ore.
  5. Ore-dressing: The removal of unwanted silicon, and other impurities from the ore, to increase the concentration of metallic compound in the given amount of ore.
  6. Levigation: The process of removal of heavy ore particles from lighter particles by washing with water.
  7. Gangue or matrix: The unwanted impurities present in the ore.
  8. Electromagnetic separation: Separation of magnetic or non-magnetic unwanted impurities from the ore by using an electromagnet.
  9. Froth-flotation method: Removal of impurities from sulphide ores by wetting the surface of sulphide ore with oils while the gangue is wetted with water.
  10. Collector: Chemical substance which increases the non-wettability of ore particles in the froth-flotation method.
  11. Froth-stabilizer: Chemical substances which stabilize the froth formed during the froth-flotation method.
  12. Leaching: The process of treatment of the ore with suitable reagent which can selectively dissolve the ore but not it's impurities.
  13. Calcination: The process of conversion of the ore into it's oxide by heating it below its melting point in the absence of oxygen.
  14. Roasting: The process of conversion of the ore into it's oxide by heating it below its melting point in the presence of oxygen.
  15. Flux: Chemical substance which can combine with gangue for separating it from the crude metal.
  16. Slag: the fusible material formed by combination of flux and gangue.
  17. Pyrometallurgy: Conversion of the metallic oxide to metal in the presence of a suitable reducing agent at high temperature.
  18. Hydrometallurgy: Conversion of the ore to crude metal in the presence of a suitable reducing agent in a solvent.
  19. Electrometallurgy: Electrolysis of a suitable metallic compound to get pure metal.
  20. Ageing (Age Hardening): The process of changing the structure which may occur gradually in certain metals and alloys at atmospheric temperature or more rapidly at higher temperatures.
  21. Annealing: The heating process followed by cooling to make an object soft and for removing stresses.
  22. Anodizing: The process of coating metallic alloy like aluminum or aluminum alloys with a layer consisting essentially of aluminum oxide.
  23. Poling: A refining process of crude metal for removing metal oxide impurities by stirring molten metal in a ladle with a pole of green wood.

Extractive Metallurgy

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The study of the processes used in the separation and concentration of a metallic compound to get pure metal is called extractive metallurgy. It is a sequence of processes from mining the ore from the earth's curst and removing the impurities, followed by conversion of the ore into crude metal which is further purified to get maximum yield of metal. The different steps are as follows.

1. Crushing and grinding of ore

In this step, the big lumps of ore are crushed by using a ball mill or stamp mill into small pieces so they can react quickly. This step is also termed as pulverization.

2. Ore-dressing or benefaction of ore

This step involves the removal of gangue or matrix by using a suitable method which depends on the type of ore. For example; if the ore is heavier than gangue, simple washing by water can be used, which is termed as hydraulic washing or levigation or gravity separation method.

Ore Dressing

For sulphide ores froth –flotation method is used in which the impure ore is mixed with water and pine oil to make a suspension. The suspension is violently agitated by the rotating paddle which draws in air, causing frothing. Due to this the ore particles which are wetted by oil come to the surface with the froth, which can easily be separated to get pure ore.


Froth Floatation Process

If any one part of the impure ore is magnetic in nature (either ore or gangue) electromagnetic method can be used in which the impure ore is dropped over a conveyer belt moving around two rollers, one of which is an electromagnet. The magnetic parts will be attracted by the electromagnet and collect as a separate heap from the non-magnetic part.
Magnetic Seperation


3. Conversion of ore to their oxide

It involves two methods.

  • Calcination: This method is used for carbonate and hydrated ores which are heated strongly below the melting point in a limited supply of oxygen. The carbonate ores release carbon dioxide and form metal oxide while hydrated ores become dehydrated. For example;
CaCO3 → CaO + CO2
Fe2O3.3H2O → Fe2O3 + 3H2O

  • Roasting: This method is used for sulphide ores in which the ore is heated below its melting point in the presence of excess air. This step removes moisture and organic matter associated with the ore as well as non-metallic impurities like phosphorus, arsenic etc.


4. Conversion of metallic oxide in to metal

There are three different methods for the conversion of metallic oxides into metal.

  • Pyro metallurgy: It is a high temperature process in which ore particles undergo reactions to form intermediate compounds for further processing or to get converted into their elemental or metallic state. It involves the heating of the material obtained from roasting or calcination, in the presence of a suitable reducing agent. During the heating process some chemical substance is added which further reacts with gangue at high temperature and is known as flux.
  • Hydro metallurgy: The process of dissolving an ore in a suitable reagent followed by the extraction of the metal either by electrolysis or displacement of the metal by a more electropositive metal is called hydro metallurgy. This process uses aqueous solutions to extract metals from their ores. The most common hydro metallurgical process is the extraction of gold and silver.
  • Electro metallurgy: Electro metallurgy is the metallurgical processes in which the extraction of metal takes place in an electrolytic cell. The molten metallic salt is taken as the electrolyte in the cell, with suitable electrodes. The molten metallic salt dissociates in pure metal and is collected at the cathode. For example, Dow process is used for the extraction of magnesium from its salt, magnesium chloride, which is the main component of sea water.
MgCl2 → Mg2+ + 2Cl-

Hence magnesium metal is collected at the cathode and chlorine gas is released as a side product.


5. Refining of metal

The process of purifying the crude metal is called as refining. There are various methods for the refining process which depends upon the physical as well as chemical properties of metal. For example; distillation, liquation, electrolytic refining, zone refining, vapor-phase refining and chromatography.

Iron metallurgy

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Iron is usually extracted by using oxide ores like

  • Haematite (Fe2O3)
  • Limonite ( Fe2O3 .3H2O)
  • Magnetite (Fe3O4)
  • Siderite (FeCO3)
  • Iron pyrites (FeS)

Out of these ores, haematite is the best ore of iron. Iron metallurgy involves the following steps.

  1. Concentration: Since iron ores are heavier than their impurities, hydraulic washing is used for the concentration of the ore. For iron sulphide, the froth-flotation method is a good method for concentration or ore-dressing.
  2. Calcination: The oxide ores of iron are heated in the presence of limited oxygen in a reverberatory furnace. This process is used to remove moisture and non-metallic impurities associated with the ore. The ferrous oxide present in the ore is oxidized to ferric oxide.
  3. Smelting: The calcined ore is reduced with carbon in a blast furnace at high temperature. The blast furnace is a tall cylindrical furnace made up of steel and lined with fire bricks. It is slightly narrow at the top and wide at center with a narrow bottom. There is a cup and cone arrangement in the furnace which helps to feed the charge from the top without letting any gas from inside escape. The charge consists of ore, coke and limestone in a 8:4:1 ratio. The temperature of the blast furnace is maintained at 1000 K by blowing hot air in the furnace. Different reactions take place at different temperatures.


Zone of combustion:

Temperature is around 1500-2170K

C+ O2
$\rightarrow$ CO2
CO2 + C
$\rightarrow$ 2CO


Zone of slag formation:

Temperature -1123K

CaCO3 $\rightarrow$ CaO + CO2

CaO + SiO2 $\rightarrow$ CaSiO3


Zone of reduction:

Temperature-823K

Fe2O3 + CO $\rightarrow$ 2FeO +CO2

Fe3O4 + CO $\rightarrow$ 3FeO + CO2

FeO + CO $\rightarrow$ Fe + CO2

Iron metallurgy

The iron obtained from the blast furnace is called pig iron and contains 4% of carbon with lots of other impurities. This pig iron is further converted in to cast iron which has less content of carbon and is brittle in nature. The purest form of iron is called wrought iron, having only 0.2-0.5% of carbon with traces of phosphorus and silicon. Wrought iron is ductile, soft and malleable in nature.

Powder Metallurgy Process

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The process of formation of precision metal component from metal powder is called Powder metallurgy (PM). For making the required shape of the metal component, the metal powder is first pressed into the product shape at room temperature and then heated, so that the metal particles are fused together without melting.

The metallic components produced by powder metallurgy have adequate physical and mechanical properties. Powder metallurgy is a cost effective process to produce a component of a given shape and the required dimensional tolerances compared to the cost of casting or making it as a wrought iron product. The main industrial applications of PM parts include self – lubricating bearings, porous metal filters and many engineered shapes, like gears, cams, brackets, sprockets, etc. In powder metallurgy there are three main steps; mixing (or blending), compacting, and sintering.

  • Mixing: This step involves the homogeneous mixing of powdered metal or alloy powders which depends on the need.
  • Compacting: The controlled amount of the mixed metal powder is subjected to high pressure in the range of 100 MPa to 1000 MPa in a precision die. The pressure required depends upon the nature and shape of the particles as well as the method of mixing. The compacting process is generally done at room temperature so that the metallic powder is consolidated and densified into a shaped model. These models are known as “green compact.” The compacting process involves certain steps.
The empty die cavity is filled with mixed metallic powder.
The metal powder in the die is pressed by the simultaneous movement of upper and lower punches.
The upper punch is withdrawn, and the green compact is ejected from the die by the lower punch.
The green compact is pushed out of the pressing area so that the next operating cycle can start.
  • Sintering: The heating process of green compact is known as sintering. The compact metal is heated in a protective atmosphere furnace to a suitable temperature, below the melting point of the metal. This process produces the physical and mechanical properties in the powder metal part by developing metallurgical bonds between the powder particles. The structure and porosity of sintered compact depends on the temperature, time, and processing method.
 Powder Metallurgy Process

Bronze Metallurgy

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Bronze is a metal alloy. It is made by blending copper and tin in different amounts. The amount of copper, tin, and other elements depends on the use of bronze. Some other elements like manganese, lead, and phosphorous are added in the making of bronze, which adds the specific properties in this alloy. For example, the addition of phosphorus increases the alloy hardness while the use of lead makes it easier for casting. The lead bronze contains 10 to 29% of lead.

The aluminum bronze is good in strength and are resistant to corrosion. Bronze is used in bells, gears, valves, pipes, and plumbing fittings material etc. It is very useful in industries as it contains some specific properties. It is highly useful in machine parts due to minimal friction.

Due to non sparking property makes it useful in combustible environments. The property of natural patina of bronze makes the alloy dark, dull in color. It forms a protective layer which prevents the alloy from oxidation below the surface of alloy.

Physical Metallurgy

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It is the science related to the physical and mechanical character of the metal and alloy. These properties are affected by chemical, mechanical and thermal treatment of metal and alloy. Some of the important concepts of physical metallurgy are given below.
  • Recrystallization and recovery
  • Grain growth
  • Nucleation and growth
  • Carburizing
  • Hardenability of steel
  • Eutactoid transformation in steel
  • Tempering of mertencite
  • Electrical resistivity of alloy
  • Electrical conductivity
  • Dielectric behavior


  1. Recrystallization and recovery - When the metals are cold worked; some amount of energy is stored in its deformed structure which acts as a thermodynamic driving force. This stored energy induced the metal to get un deformed state. Thus the formation of the defect Free State is known as Recrystallization. It is done at particular temperature which is known as recrystallization temperature. For pure metal its equal to 0.3 to 0.4 T and for alloys its 0.5Twhere t is melting point.
  2. Grain growth - In the grain growth, the effect of time and temperature is studied. The grain growth depends on the following factors like the annealing temperature, the duration of annealing process, the degree of cold work and the use of additives in metal and alloy. The annealing process is the process of grain growth. If the temperature is higher than the Recrystallization temperature then the grain are continuously grow and thus the large crystal are formed by absorbing the small ones.
  3. Nucleation and growth - The main objective is to measure the nucleation and growth rate of metal and alloy. There are two phase of transformation one is homogeneous and the other is heterogeneous. In the most of the alloys, heterogeneous transformation is observed. The rate of heterogeneous is determined by the rate of the nucleation and growth. The rate of nuclei is expressed in terms of number of nuclei formed per unit time and per unit volume and the growth rate is expressed in terms of cm per second.
  4. Carburizing - This is the process of measuring the diffusion coefficient of carbon in steel.
  5. Hardenability of steel - The susceptibility to hardness of steel is measured by quenching. The susceptibility of steel shows the depth of hardening of steel under given cooling condition. The hardenability is related to the isothermal transformation. Steel will harden up to high degree with low critical cooling rate than steel with high critical cooling rate.
  6. Eutactoid transformation in steel -This involves the measurement of inter-lamellar spacing and volume fraction of pearlite. When the high temperature of austenite transforms into the aggregate of two layers ferrite and cementite is known as pearlite. This transformation is affected by time, temperature and pearlier morphology factors.
  7. Tempering of mertencite - It involves the study of decomposition of mercentite in the process of tempering. Tempering is the simple heat treatment which is used to increase the physical properties of quenched steel.
  8. Electrical resistivity of alloy - It includes the calculation of band gap energy in the semiconductors like germanium etc.
  9. Electrical conductivity of ionic solid - The electrical conductivity of ionic solids is due to the migration of ions in the influence of electric field. The migration is because of presence of some lattice defects in crystal.
  10. Dielectric behavior of barium titanate - The dielectric behavior of barium titanate is measured by calculate the dielectric constant and Curei-weiss temperature. The polymorphic transformation occurs in barium titanate. It is from Rhombohedral to Orthorhombic to Tetragonal to Cubic. All these are interconvertible to each other at particular temperature.
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