METALLURGY (STEP 3 AND 4)

   Last week we studied first two steps involved in metallurgy.Let's continue that topic and learn about the next two steps.
Starting with -

STEP 3-Conversion of metal oxide into metal

This step is divided into two parts-
(i)Conversion of concentrated ore into metal oxide

It can be done by two methods-
CALCINATION and ROASTING which is explained in the given video.



(ii)Conversion of metal oxide to metal

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

STEP 4-Refining of metal

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 methods are-

(a)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. 
 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. 

(b)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. 

(c)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. 

(d)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.

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. 


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