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Metals and non-metals/General Metallurgy - The Process of Extraction of Metals from their Ores and Refining them
 
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 General Metallurgy - The Process of Extraction of Metals from their  Ores and Refining them


Metals are one of the two major classifications of elements (non-metals being the other). Almost all metals are found in the combined state as compounds on the crust of the earth. This is due to their high reactivity. However, there are few exceptions. Such as gold, silver, copper, platinum and bismuth, which are found in the free state, due to their low reactivity. 
Aluminium is the most abundant metal in the earth's crust, followed by iron and calcium. 
 
Metals % (weight)
Aluminium - Al 7.0
Iron - Fe 4.0
Calcium - Ca 3.0
Sodium - Na 2.5 
Potassium - K 2.5
Magnesium - Mg 2.0
Titanium - Ti 0.6

Metallurgy
The various processes involved in the extraction of metals from their ores and refining them are known as metallurgy.

Minerals
The natural materials in which the metals or their compounds occur in the earth are called minerals.

Ores
Those minerals from which the metals can be extracted profitably are called ores.

Gangue or Matrix
The rocky impurities, including silica and earthly particles, present in an ore are called gangue or matrix.

Types of Ores

Ores occur in the form of compounds. These compounds are generally oxides, sulphides, carbonates or halides. 

Classification of ores
 
Type of ore Metals Compound (in the ore)
Oxide Ores Aluminium Bauxite - Al2O3, 2H2O
  Copper Cuprite - Cu2O
  Iron Haematite - Fe2O3
    Magnetite - Fe3O4
Sulphide Ores Iron Iron Pyrite - FeS
  Copper Copper Glance - Cu2S
Copper Pyrites - CuFeS2
  Zinc Zinc Blende - ZnS 
Carbonate Ores Calcium Limestone - CaCO3
Zinc Calamine - ZnCO3
Halide Ores Sodium Rock Salt - NaCl
Calcium Fluorspar - CaF2
Silver Horn Silver - AgCl
Aluminium Cryolite - Na3AlF6

Processes involved in metallurgy
 
Main process Sub-process
1. Concentration of the ore     (removal of unwanted      metals and gangue to      purify the ore). a. Hydraulic washing. 
b. Gravity separation. 
c. Froth floatation. 
d. Magnetic separation. 
e. Chemical separation.
2. Conversion into metal     oxide.  a. Calcination for carbonate ore. 
b. Roasting for sulphide ore.
3. Reduction of metal oxide to     metal. a. Roasting - mercury (Hg) is produced by roasting     Cinnabar (HgS) in air. 
b. Reduction - using highly reactive         elements. Example: aluminium reduces manganese     dioxide to manganese. 
c.  Electrolytic reduction - highly reactive elements,     such as sodium and mercury, are obtained by     electrolytic reduction when the metal is deposited     at cathode.
4. Refining of impure metal     into pure metals. a. Electrolytic refining - of copper, gold, tin, lead,     chromium, nickel, etc. 
b. Liquation process - for tin, lead and bismuth.
c. Distillation process - for zinc, mercury. 
d. Oxidation process - for iron.

Each of the main processes mentioned above are discussed below in detail. 
 
 

Concentration of the ore
1. Hydraulic washing process (Gravity separation) 
This process separates the heavier ore particles from the lighter gangue particles. This is done by washing them in a stream (jet) of water over a vibrating, sloped table with grooves. Denser ore particles settle in grooves. Lighter gangue particles are washed away. 
 
  Hydraulic Washing (Gravity Separation) 

2. Froth floatation process
In this process, separation of the ore and gangue particles is done by preferential wetting. This process is generally used for sulphide ores of copper, lead and zinc. The finely powdered ore is mixed with water and a suitable oil in a large tank. A current of compressed air agitates the mixture. 
The ore particles are wetted by oil and forms a froth at the top, which is removed. The gangue particles wetted by water settle down. Ore preferentially wetted by oil is removed as froth. Gangue wetted by water is removed after it settles down. 
 
  Froth Floatation 

3. Magnetic separation process
This process is used in the extraction of metals which exhibit magnetic properties. For example, in the extraction of iron, crushed magnetite ore (iron) particles are separated using their magnetic property. The pulverized ore is moved on a conveyor belt. Electro-magnetic wheel of the conveyor attracts only the magnetic particles into a separate heap. 
 
  Magnetic Separation 

Only the magnetic particles are attracted by the magnetic wheel. These particles fall separately into a different heap as shown in the diagram. 

4. Chemical separation
This process utilises the difference in some chemical properties of the metal and gangue particles for their separation. For example, in the Bayer's process of aluminium extraction, the bauxite ore is treated with hot sodium hydroxide solution. Water soluble sodium aluminate formed is filtered to separate the undissolved gangue particles. Sodium aluminate (NaAlO2) is further processed to get aluminium oxide (Al2O3). 
 
 

Conversion of concentrated ore into metal oxide
It is easier to obtain metals from their oxides rather than from carbonates or sulphides. So the ore is first converted into an oxide. A carbonate ore is first converted into oxide ore by calcination. A sulphide ore is converted into oxide ore by roasting. 

1. Calcination process
It is the process of heating the concentrated ore at a temperature not sufficient to melt the ore. The water content and the volatile impurities also get expelled. Zinc carbonate in its ore calamine (ZnCO3) is calcinated to obtain zinc oxide (ZnO). 
 
 
ZnCO3 -------------> ZnO + CO2
calcination

2. Roasting process
It is the process of heating the concentrated ore in the presence of excess air. Volatile impurities also get expelled. 
Zinc sulphide in its ore zinc blende ( ZnS) is roasted to obtain zinc oxide (ZnO). 
 
 
2ZnS + 3O2 -----> 2ZnO + 2SO2
(from air)

Mercuric oxide is obtained by roasting cinnabar (HgS) in air. 
 
 
2ZnS + 3O2 -----> 2ZnO + 2SO2

Calcination Roasting
1. Converts carbonate ores into oxides. 
2. Ore is heated in the absence of air.
1. Converts sulphide ores into oxides. 
2. Ore is heated in the presence of air.

Calcination and roasting processes are selective processes. These processes cannot be used for oxide and chloride ores. 
 
 

Reduction of metallic oxide to metal 
 
Reduction 
The conversion of metal oxide into metal (by removal of oxygen) is called reduction. 

Generally the 3 methods used are:
  • Reduction by heating the oxide 
  • Chemical reduction 
  • Electrolytic reduction. 

Reduction by heating alone (Heating process) 
The oxides of metals that are low in the reactivity series can be reduced to obtain the metals by heating their ore. 
For example, mercuric oxide (HgO), obtained from its ore mercuric sulphide (HgS), when heated to about 3000 C forms mercury metal. 
 
2HgS + 3O2 Roasting 
------->
2HgO + 2SO2
Mercuric sulphide (from air) Mercuric 
oxide

 
 
 
2HgO Heat 
--------->
2Hg + O2
(Reduction) (Mercury)

Roasting and reduction processes are carried out simultaneously. 

Chemical reduction process
Under this process the oxides of metals that are in the middle of the reactivity series are reduced to free metals using chemical reducing agents such as carbon, aluminium, sodium or calcium. 

1. Reduction by carbon process 
Oxides of metals like zinc, iron, copper, nickel, tin and lead are reduced using carbon as the reducing agent. Carbon can be used only if it has greater affinity for oxygen than the metal. For example, carbon can reduce copper oxide to copper, but it cannot reduce calcium oxide. It can reduce zinc oxide. 
 
 
ZnO + C -----> Zn + CO
Zinc metal Carbon 
monooxide

2. Reduction with aluminium by thermite process
Metals which are too active to be obtained by reduction of their oxides with carbon are reduced using aluminium, which is a more powerful reducing agent. Chromium and manganese oxides are reduced using aluminum. This reaction is highly exothermic. 
 
 
Cr2O3 + 2Al -----> 2Cr + Al2O3
Chromium metal

3MnO2 + 4Al -----> 3Mn + 2Al2O3
Manganese metal

Electrolytic reduction process
The oxides (or chlorides) of highly reactive metals like sodium, magnesium, aluminium and calcium cannot be reduced by using carbon or aluminium. 
Electrolytic reduction is the process used to extract the above metals. Molten oxides (or chlorides) are electrolysed . The cathode acts as a powerful reducing agent by supplying electrons to reduce the metal ions into metal. 
Fused alumina (molten aluminium oxide) is electolysed in a carbon lined iron box. The box itself is the cathode. The aluminium ions are reduced by the cathode. 

At the cathode 
 
Al3+ + 3e-  electrolysis
------------------>
Al
Aluminium Ion Aluminium 
Atom

MgCl2 electrolysis 
---------------->
Mg + Cl2

Refining of metals
This process ensures the separation of even the residual impurities from the extracted metals. Refining methods are different for different metals. The methods depend upon the purpose for which the metal is to be used. Refining can also be used to recover some valuable by-products such as silver or gold. 

The two main categories of refining are:
 
Physical methods
  • Liquation 
  • Distillation 
  • Zone refining (Fractional refining) 

Chemical methods
  • Oxidative refining 
  • Electrolytic refining 
  • Van Arkel method (Vapour phase refining) 

Liquation method
Readily fusible metals (low melting points) like tin, lead and bismuth are purified by liquation. 
The impurities do not fuse and are left behind. 
 
  Liquation Method 

In this process, the block of impure metal is kept on the sloping floor of a hearth and heated slowly. The pure metal liquifies (melts) and flows down the furnace. The non-volatile impurities are infusible and remain behind. 

Distillation method
In this process, metals with low boiling point, such as zinc,calcium and mercury are vaporized in a vessel. The pure vapours are condensed into pure metal in a different vessel. The non-volatile impurities are not vaporised and so are left behind. 

Oxidation method
In this process, the impurities are oxidised instead of the metal itself. Air is passed through the molten metal. The impurities like phosphorus, sulphur, silicon and manganese get oxidised and rise to the surface of the molten metal, which are then removed. 

Electrolytic refining method
The process of electrolysis is used to obtain very highly purified metals. It is very widely used to obtain refined copper, zinc, tin, lead, chromium, nickel, silver and gold metals. 
 
In this process,
  • The impure slab of the metals is made the anode 
  • A pure thin sheet of metal is made the cathode 
  • A salt solution of the metal is used as the electrolyte. 

  Electrolytic Refining 

On passing current, pure metal from the electrolyte is deposited on the cathode. 
The impure metal dissolves from the anode and goes into the electrolyte. The impurities collect as the anode mud below the anode. 

Methods for obtaining metals of very high purity
 
The requirement of very high purity metals has increased due to the advancement in space technology, atomic energy and semi-conductor devices. 
For example:
  • Uranium used in nuclear plants should not have more than one part per million (1 ppm) of boron. Boron in excess of this limit, will stop the chain reaction initiated, by absorbing the neutrons released. 
  • Germanium used in semi-conductor devices should not have more than one part of copper in ten million parts. 

Zone refining (Fractional crystallization)
This process is used in the refining of Germanium. It is based on the property that when an impure molten metal is cooled gradually, only the pure crystals of the metals are formed. The impurities are left in the remaining part of the molten metal. 
A circular heater fitted around an impure germanium rod is slowly moved from one end to the other. The heater melts a band of metal and as the heater moves ahead, the pure metal crystallizes out of the melt. As the heater moves ahead, the impurities are swept forward in the molten zone. Finally, all impurities reach the other end of the rod. This end is cut and discarded. Silicon and gallium used as semi-conductors are also zone refined. 
 
  Zone Refining 

Van Arkel method (Vapour Phase Refining )
This method is based on the thermal decomposition of metal components. The process of decompositon of a compound into different substances due to the supply of heat is called thermal decomposition. 
Titanium metal is purified by this method. When the impure titanium metal is heated with iodine at a temperature of 2500 C, volatile titanium tetra-iodide is formed (TiI4). The impurities are left behind, as they do not react with iodine. 
The titanium tetra-iodide vapour is passed over a hot tungsten filament at 14000 C. The vapour is decomposed and pure titanium is deposited on the filament and is removed. The iodine is reused. 
 
 
Ti + 2I2 2500
------->
TiI4
Titanium tetra-iodide

TiI4 14000
----------->
Ti + 2I2
Tungsten 
filament 
metal