DEFINITION
The process of the extraction of metals in their pure form is called Metallurgy and the compounds of metals mixed with soil, limestone, sand, and rocks are called as Minerals. Metals are commercially extracted from minerals at low cost and minimum effort. These minerals are known as ores. The flux is a substance which is added to the charge in the furnace to remove the gangue (impurities). Finally, the metallurgy is the process of purification of metals and the formation of alloys.
ADVANTEGES OF METALLURGICAL PROCESSThe metallurgy process is used to
- Eco friendly
- Maximize the energy deficiency
- Minimize the waste
PRINICIPLES OF METALLURGY
The metallurgical process can be containing the following processes:
Crushing and grinding:
The first step of metallurgy process is crushing of ores into a fine powder in a crusher or ball mill. This process is called as pulverization.
The concentration of ores:
The concentration of minerals is the process of removing impurities from the ore and it is also known as ore dressing.
In metallurgy, the concentration of ores can be containing the following methods.
Hydrolytic method:
In this hydrolytic method, the ore is poured over a sloping, vibrating corrugated table with grooves and then the jet of water is allowed to flow over the surface and the denser ore particles settle in the grooves, and the impurities are washed away by water.
Magnetic separation:
In this method, the ore is crushed by placed on a conveyor belt. This belt rotates around two wheels in which one of the wheels is magnetic, and therefore the magnetic particles get attracted to the magnetic wheel and fall apart from the non-magnetic particles.
Froth floatation:
In this froth floatation method, the crushed ore is taken in a large tank which contains oil and water. A current of compressed air is passed through this tank and then the ore gets wet by oil and this ore is separated from the impurities in the form of froth. Ore is lighter, and so it comes on the surface and impurities are left behind.
Roasting and calcination:
In metallurgy, the process of roasting is known as the heating a concentrated ore in the presence of oxygen. This process is applied in the case of sulphide ores and also the ores containing carbonate or hydrated oxides, so we heating the ore is done in the absence of air for melt the ores, and this process is known as calcination.
OCCURRENCE OF METALS
In general, pure metals are shiny and malleable, however, most of them are found in nature as compounds with different properties. Metals having least chemical reactivity such as copper, silver, gold and platinum occur in significant amounts as native elements. Reactive metals such as alkali metals usually occurs in their combined state and are extracted using suitable metallurgical process.
MINERAL AND ORE
A naturally occurring substance obtained by mining which contains the metal in free state or in the form of compounds like oxides, sulphides etc... is called a mineral. In most of the minerals, the metal of interest is present only in small amounts and some of them contains a reasonable percentage of metal.
For example,
Iron is present in around 800 minerals. However, some of them such as hematite magnetite etc., containing high percentage of iron are commonly used for the extraction of iron. Such minerals that contain a high percentage of metal, from which it can be extracted conveniently and economically are called ores. Hence all ores are minerals but all minerals are not ores.
Let us consider another example,
Bauxite and China clay (Al2 O3.SiO2 .2H2 O).
Both are minerals of aluminium. However, aluminium can be commercially extracted from bauxite while extraction from China clay is not a profitable one. Hence the mineral, bauxite is an ore of aluminium while china clay is not. The extraction of a metal of interest from its ore consists of the following metallurgical processes.
- concentration of the ore
- extraction of crude metal
- refining of crude metal
MINERALS AND ORES IN INDIA
| METAL | MINERALS | PLACES OCCURRENCE IN INDIA |
FORMULA |
|
Sodium |
Tincal, Borax |
Ladakh (Kashmir) |
Na2B4O7.10H2O |
|
Magnesium |
Dolomite |
Tamil Nadu |
CaMg (CO 3) 2 |
|
Calcium |
Gypsum |
Rajasthan, Tamilnadu, Jammu and kashmir |
CaSO 4·2H2O |
|
Aluminium |
Bauxite |
UP, Maharashtra, Orissa, MP |
Al2 O3 .nH2 O |
|
Copper |
Chalcopyrite’s or copper pyrites, Malachite |
Jharkhand, Orissa, MP |
CuFeS2, CuCO3 .Cu(OH)2 |
|
Zinc |
Zincblende |
Rajasthan |
ZnS |
|
Manganese |
Pyrolusite |
Maharashtra, Karnataka and Jharkhand |
MnO2 |
|
Iron |
Haematite |
Jharkhand and Karnataka |
Fe2 O3 |
|
Silver |
Silver glance(argentite) |
Mexico |
Ag2 S |
|
Lead |
Galena |
Australia, Columbia |
PbS |
|
Tin |
Cassiterite |
Brazil, England, china |
SnO2 |
Electrochemical principle of metallurgy
Similar to
thermodynamic principles, electrochemical principles also find applications in
metallurgical process. The reduction of oxides of active metals such as sodium,
potassium etc., by carbon is thermodynamically not feasible. Such metals are
extracted from their ores by using electrochemical methods. In this technique,
the metal salts are taken in a fused form or in solution form. The metal ion
present can be reduced by treating it with some suitable reducing agent or by
electrolysis. Gibbs free energy change for the electrolysis process is given by
the following expression
ΔG°
= -nFE°
Where,
n is number of
electrons involved in the reduction process,
F is the Faraday and
E0 is the
electrode potential of the redox couple.
If E0
is positive then the ΔG is negative and the reduction is spontaneous and hence
a redox reaction is planned in such a way that the e.m.f of the net redox
reaction is positive. When a more reactive metal is added to the solution
containing the relatively less reactive metal ions, the more reactive metal
will go into the solution.
For example,
Cu(s) + 2Ag+
(s) → Cu2+ (aq) + 2Ag (s)
Cu2+
(aq) + Zn (s)→ Cu(s) + Zn2+ (aq)
REFINING
PROCESS
Generally,
the metal extracted from its ore contains some impurities such as unreacted
oxide ore, other metals, nonmetals etc... Removal of such impurities associated
with the isolated crude metal is called refining process. In this section, let
us discuss some of the common refining methods.
Distillation
This method is
employed for low boiling volatile metals like zinc (boiling point 1180 K) and
mercury (630 K). In this method, the impure metal is heated to evaporate and
the vapours are condensed to get pure metal.
Liquation
This
method, is employed to remove the impurities with high melting points from
metals having relatively low melting points such as tin (Sb; mp= 904 K), lead
(Pb; mp=600 K), mercury (Hg; mp=234 K), and bismuth (Bi; mp=545 K). In this
process, the crude metal is heated to form fusible liquid and allowed to flow
on a sloping surface. The impure metal is placed on sloping hearth of a
reverberatory furnace and it is heated just above the melting point of the
metal in the absence of air, the molten pure metal flows down and the
impurities are left behind. The molten metal is collected and solidified.
Electrolytic refining
The crude
metal is refined by electrolysis. It is carried out in an electrolytic cell
containing aqueous solution of the salts of the metal of interest. The rods of
impure metal are used as anode and thin strips of pure metal are used as
cathode. The metal of interest dissolves from the anode, pass into the solution
while the same amount of metal ions from the solution will be deposited at the
cathode. During electrolysis, the less electropositive impurities in the anode,
settle down at the bottom and are removed as anode mud.
Let us understand this process by considering
electrolytic refining of silver as an example.
Cathode: Pure
silver
Anode: Impure
silver rods
Electrolyte:
Acidified aqueous solution of silver nitrate.
When a
current is passed through the electrodes the following reactions will take
place
Reaction at anode
Ag (s)→ Ag+ (aq) + 1e-
Reaction at cathode
Ag+ (aq) + 1e→Ag (s)
During electrolysis, at the anode the silver
atoms lose electrons and enter the solution. The positively charged silver
cations migrate towards the cathode and get discharged by gaining electrons and
deposited on the cathode. Other metals such as copper, zinc etc., can also be
refined by this process in a similar manner.
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