Extraction of Crude Metal from Concentrated Ore (p Nitin)

The process of extraction of metal from the concentrated ore depends upon the nature of the ore as well as the nature of the 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 the preliminary chemical treatment in extraction of crude metal from concentrated ore 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.

The processes of extraction of crude metal from concentrated ore involved for preliminary treatment are Calcination and roasting.

(i)Calcination:It is a process of heating of 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.
ZnCO3 gives ZnO + CO2
PbCO3 gives PbO + CO2
(ii)Roasting:It is the process of heating the ore in excess supply of air below its melting point.The 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.
2PbS + 3O2 gives 2PbO + 2SO2
Both calcination or roasting processes are carried out in a special type of furnace called reverberatory furnace.

extraction of crude metal from concentrated ore :- The reduction of ore

After the preliminary treatment in extraction of crude metal from concentrated ore,the ore may be subjected to reduction process by any one of the following methods depending upon its nature:

1.Smelting: reduction with carbon.In this process,the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which acts 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.The additiobal reagent is called flux.Flux combines with the impurities to form a fusible product called slag.

Flux + Impurities gives Slag
The selection of flux depends upon the nature of impurities.If impurities are acidic in nature,the flux is basic i.e. lime (CaO).On the other hand ,for basic impuritiies,an acidic flux,silica(SiO2 ) is used.
The process of reduction with carbon is carried out in a blast furnace.

2.Reduction with aluminium: Certain metal oxides such as Cr2O3 ,Mn3O4 are not easily reduced with carbon.In such cases,aluminium is used as reducing agent because it is more electropositive than chromium or manganese.The process of reduction of oxides with aluminium is called as aluminothermy.Some examples are:
Cr 2O3 + 2Al gives Al2O3 + 2Cr
3Mn3O4 + 8Al gives 4Al2O3 + 9Mn

3.Reduction with hydrogen or water gas.Hydrogen gas is very effective reducing agent.However its use is limited beacuse of its expensive nature.
WO3 + 3H2 gives W + 3H2O

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.

5.Leaching or hydrometallurgy.Some metals like gold or silver are extracted from their concentrated ores by leaching.They are dissolved in suitable reagents like acids or bases leaving behind soluble impurities.The metal is recovered from the solution by reprecipitation or crystallisation.For example,silver ore is leached with dilute solution of sodium cyanide.

These were the steps in extraction of crude metal from concentrated ore

Processing ...