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50.5 EXTRACTION OF METALS- METALLURGY

As already mentioned only a few most unreactive metals occur in nature in free state. Most of the metals occur in nature in oxidized form as their compounds. Therefore, metals are
generally extracted by subjecting their ores to reduction by chemical methods or by electrolytic methods.

The process of extracting pure metal from its ore is known as metallurgy.       .

Since, the nature of the ore and also the properties of different metals are different, therefore, it is not possible to have the universal scheme which may be applicable to all the metals. However, some common steps involved in the metallurgical operations are:

I     Crushing and grinding of the ore.

II.   Concentration or benefication of the ore.

III. Preliminary treatment of the concentrated ore.

IV.  Reduction.

V.   Purification or refining of crude metal.

I. CRUSHING AND GRINDING OF THE ORE

Most of the ores are obtained from the crust of the earth in the form of huge lumps. These lumps have to be converted. into powdered form so that the chemical changes which have
to take place at the later stages may become convenient. The huge lumps are broken into small pieces in the jaw crushers They are further pulverised in stamp mill or ball mill.

II. CONCENTRATION OR BENEFICATION OF THE ORE

The ore obtained from the earth’s crust is associated with rocky and silicious impurities. It is quite essential to get rid of these impurities so that they may not cause any interference.
in the process of extraction. The removal of unwanted materials such as sand, clays, etc., from the pulverised ore called concentration, dressing or benefication of the ore
The benefication of the ore is carried out by any of the following methods depending upon the nature of the ore an also the impurities present in the ore:

1. Hydraulic Washing, Levigation or Gravit Separation Method. This method is usually applicable to oxide ores in which the ore particles are heavier than the
impurities. The powdered ore is washed with running stream of water. The lighter impurities are washed away leaving behind the heavier ore particles.

Native metals such as gold can be separated from sand and gravel by shaking and washing of earth with water in pan or sieve. The process is known as panning.

2. Froth Floatation Process. This process is generally used for the concentration of sulphide ores. The finely powdered ore and water are taken in a tank. Additional reagents such as pine oils, fatty acids, etc., are added to the mixture. These reagents increase the nonwettability of the mineral particles and are known as collectors. The contents are kept agitated by the blast of air. As a result of agitation
the froth is produced.

The.ore particles are preferentially wetted by the oil and are carried to the surface by the foam. The gangue material, which is preferentially wetted by water sinks to the bottom of the tank. The foam at the surface of the tank is transferred to the other tank where it is washed with water to recover the
ore particles.

3. Magnetic Separation of Impurities: This method is usually employed when either the ore or the gangue is capable of being attracted by the magnetic field. For example, tungstates of iron and manganese from tin stone are separated by this method. The powdered ore is dropped over the belt revolving around the rollers, one of which is magnetic. The magnetic roller attracts the magnetic part of
the ore and they are collected in the form of a heap near it. The non-magnetic part of the ore flies off and forms a heap away from the impurities.

4. Leaching: It is a chemical method for the concentration of the ore. In this process the powdered ore is dissolved selectively in acids, bases or other suitable reagents. The impurities remain undissolvedas sludge. The solution of ore is filtered and the ore is. recovered by precipitation or
crystallisation.

III. PRELIMINARY TREATMENT OF THE CONCENTRATED ORE

The process of extraction of metal from the concentrated ore depends upon the nature of the ore as well as the nature of impurities present in the ore. Before the concentrated ore is subjected to final metallurgical operations in order to get the metal in the free state, the preliminary chemical treatment may be necessary. The objective of this preliminary chemical treatment is:

(a) to get rid of impurities which would cause difficulties in the later stages; and

(b) to convert the ore into oxide of the metal because it is easier to reduce an oxide than the carbonate or sulphide.

The processes employed for preliminary treatment are calcination and roasting.

1. Calcination

It is a process of heating the ore in a limited supply of air below its melting point. The process involves:
-  the removal of volatile impurities,

-  the removal of moisture,

-  the decomposition of any carbonate ore into oxide.

Fe2O3.xH2O        →       Fe2O3 + xH2O (g)

ZnC03             →        ZnO + CO2

     CuC03 .Cu(OH)2        →       2CuO + H2O + CO2

  1. 2.     Roasting

It is the process of heating the ore in the excess supply of air below its melting point. This process is employed when oxidation of the ore is required. As a result of roasting,
-  moisture is driven away,

-  volatile impurities are removed,

-  the impurities like sulphur, phosphorus, arsenic are removed as their oxides,

-  the ore undergoes oxidation to form metal oxide or sulphate.

2PbS + 3O2        →                   2PbO + 2SO2

              2ZnS + 302        →                   2ZnO + S02

It is advantageous to roast a sulphide ore to the oxide before reduction because metal oxides can be reduced to metal by carbon and hydrogen much more easily than sulphides.

IV. REDUCTION

 

After the preliminary treatment, the ore may be subjected to reduction process by one of the following methods depending upon its nature:

 

1. Smelting or Reduction with Carbon. In this process, the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which act as reducing agent) and is heated
to a high temperature above its melting point. During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional
reagent is called flux. Flux combines with the impurities to form a fusible product called slag.

 

Flux + Impurities Slag

 

The selection of flux depends upon the nature of impurities. If purities are acidic in nature, the flux is basic, lime (CaO). On the other hand, for basic impurities, are acidic flux such as silica (SiO2) is used.

CaO    +    SiO2 → CaSiO3

                                                             (Basic           (Acidic      (Slag)

impurity)                  flux)

 

2. Reduction with Aluminium. Certain metal oxides such as CR2O3 and Mn3O4 are not easily reduced with carbon. In such cases is used as reducing agent because it is more electropositive than chromium or manganese. The process of reduction of oxides with aluminium is called aluminothermy. Some examples are:

 

Cr2 + 2A1     →        Al2O3 + 2Cr

3Mn3O4 + 8Al          →        4Al2O3 + 9Mn

 

The chemical methods are suitable for reduction of compounds of metals which are in the middle of the activity series.

 

3. Auto-reduction. Certain metals are obtained from their ores roasting without using any reducing agent. For example, mercury is directly obtained by roasting its ore cinnabar (HgS) in air.

 

HgS + O2                               Hg(l) + SO2(g)

or           2Hgs(s)  + 3O2(g)                  2HgO(s) + 2SO2g)

                                              2HgO(s) + HgS(s)                 3Hg(l) + SO2(g)

 

4. Electrolytic Reduction. The highly electropositive elements such as alkali metals, alkaline earth metals and aluminium cannot be extracted by carbon reduction methods. They are extracted by the electrolysis of their fused salts. The process of extraction of metals by the use of electrolysis phenomenon is called electrometallurgy.

 

For example, sodium metal is extracted by electrolysis of molten sodium chloride containing other salts as impurities.

NaCI(l)                        →                    Na+ + Cl-

At cathode                    Na+ + e-                        →                    Na

At anode                      2Cl‑                    →                    Cl2 + 2e-

 

 

5. Displacement Method (Hydrometallurgy). Some metals like gold and silver are extracted from their concentrated ores by leaching. They are dissolved in suitable reagents like acids or bases leaving behind insoluble impurities. The metal is recovered from the solution by displacement with some more electropositive metal such as zinc. For example, silver ore is leached with dilute solution of sodium cyanide. Silver dissolves forming a complex, sodium dicyanoargentate (I). The solution is further treated with scrap zinc which displaces silver from complex.

 

Ag2S + 4NaCN                       2 Na[Ag(CN)2]             + Na2S

Sod. dicyanoargentate (I)

 

2Na[Ag(CN)2] + Zn         →        Na2 [Zn(CN)4] + 2Ag ↓

Sod. tetracyanozincate (II)

On the basis of reactivity we can group the metals into the following three categories:

(i) Metals of low reactivity.

(ii) Metals of medium reactivity.
(iii) Metals of high reactivity.

Different reduction processes are to be used for obtaining the metals falling in each category.

The relation between the reduction process employed and the position of the metal in the activity series is depicted in Table 50.4.

 

Table 50.4. Position of the Metal in the Activity Series
and the Related Reduction Process

 


V.
REFINING OF METALS

The process of purification of impure metals is known as refining.

Depending upon the nature of the metal and the nature of the impurities present different methods are applied for the refining of metals. Some of the commonly used methods are discussed below:

1. Distillation. Volatile metals like zinc and mercury are purified by this method. The pure metal distils over and is condensed in a receiver. The non-volatile impurities are left behind in the retort.

2. Liquation. The method is used for easily fusible metals like bismuth, tin and lead. The crude metal is placed on the sloping hearth of a furnace and heated gently when the metal melts and flows down, leaving behind the infusible impuritieswhich remain sticking to the floor of the hearth.

3. Electrolytic Refining- or Electro-refining. This method is based on the phenomenon of electrolysis. Many metals like copper, silver, tin, gold, zinc, nickel and chromium are purified by electro-refining.

The impure metal is made anode whereas the thin sheet of pure metal is made cathode. Electrolyte is the solution of some salt of the metal. On passing electricity, the metal from the anode goes into solution as ions due to oxidation while pure metal gets deposited at the cathode due to the reduction
of metal ions. -The-insoluble impurities settle down below the anode as anode mud whereas the soluble impurities go into solution. The reactions taking place at the two electrodes may
be represented as:

 

                                                At cathode:     Mn+ + ne          →        M

At anode:                     M         →        Mn+ + ne-

The various steps involved in the extraction of pure metals from their respective ores are summarized in Fig. 50.2.

 

Fig. 50.2. Steps involved in the extraction of metals from ores.