Metallurgy

METALLURGY

The process of extracting metals from their ores and then refining them for use is called metallurgy. The process of metallurgy depends upon the nature of the ore, nature of the metal and the types of impurities present. Therefore, there is not a single method for the extraction of all metals.  However, most of the metals can be extracted by a general procedure which involves the following steps.

• Crushing and grinding of the ore.
• Concentration of the ore or enrichment of the ore.
• Conversion of ore into metal oxide 
• Extraction of metal from the concentrated ore.
• Refining or purification of the impure metal.

These steps are briefly discussed below-

CRUSHING AND GRINDING OF THE ORE:

Most of the ores occur as big rocks in nature.  They are broken into small pieces with the help of crushers.  These pieces are then reduced to fine powder with the help of a ball mill or a stamp mill.

CONCENTRATION OF THE ORE OR ENRICHMENT OF THE ORE:

The process of removal of unwanted impurities (gangue) from the ore is called ore concentration or ore enrichment. Various methods used for concentration of an ore are-

Hydraulic washing (washing with water):

It is generally used for the enrichment of oxide ores, it is based on the difference in densities of ore and gangue particles. The ore particles are very heavy, they can be separated from lighter gangue particles by washing in stream of water. In this process crushed and finely powdered ore is washed with stream of water. The lighter gangue particles are washed away leaving behind the heavier ore particles. Ore of Sn and Pb are concentrated by this method. Ores of iron, tin and lead are very heavy and, therefore, they are concentrated by this method.

Froth floatation process:

It is used for the concentration of sulphide ores. It is based on the different wetting characteristic of ore and gangue particles with water and oil. Finely powdered ore is mixed with water in a large tank to form a slurry then some pine oil is added to it. The sulphide ores are preferentially wetted by the pine oil, the gangue particles are wetted by the water. When air is blown through the mixture froth carrying metal sulphides rises to the top of the tank and floats as scum, then it is removed and dried. The gangue particles being heavier, sink to the bottom of the tank.

Magnetic separation:

It is based on the difference in the magnetic properties of ores and gangue. It is done by using a magnetic separator it consists of leather belt that moves over two rollers out of which ion is an electromagnetic. The powdered ore is dropped on the moving belt at one end through a hopper. When the ore falls down from the belt at the other end having a magnetic roller, the magnetic ore particles are attracted by the magnet and form and separate heap from the non magnetic impurities.

Chemical Separation:

1. The method is based on the chemical properties of gangue and the ore. One of the best example is purification of bauxite by the Bayer’s Process.

Bauxite is an important form of Al2O3, it consists impurities (i) Fe2O3 (ii) SiO2. Bayer method is used to obtain pure aluminium oxide from bauxite ore. In this method finely powdered  ore is treated with hot NaOH solution, the Al2O3 present in bauxite ore reacts with NaOH to form water soluble sodium meta aluminate.

Fe2O3 does not dissolve in NaOH solution , it is thus separated by filtration. Silica reacts with NaOH to form water soluble sodium silicate

The filtrate containing sodium aluminate/sodium silicate solution is stirred with some freshly prepared Al(OH)3 to induce the precipitation of Al(OH)3. Under these conditions sodium meta aluminate get hydrolysed to form a precipitate of Al(OH)3. 

Al(OH)3 is then filtered, washed, dried and ignited to get pure Al2O3 which is called alumina.

CONVERSION OF METAL ORE INTO METAL OXIDE

Calcination:

This process is carried out specifically for carbonate ores. The main purpose of this process is the removal of CO2 form the ore. Hence the ore is heated to its decomposition and temperature and gets converted to the oxide form. Along with CO2, volatile impurities are also removed in this process. Apart from the carbonate ores, hydrated ores are also subjected to calcinations for the removal of water of crystallization

Example:

ZnCO3(s)   ZnO(S)  + CO2(g)

Zinc carbonate Zinc oxide Carbon dioxide

FeCO3(s)  FeO(s) +     CO2(g)

Siderite Iron (II) Oxide Carbon dioxide

Roasting:

This is generally carried out for sulphide ores. The ore is heated to a temperature below fusion point of the ore  where the ore reacts with the oxygen present in air and forms metal oxide and sulphur dioxide.

Example:

2ZnS           + 3O2   2ZnO(s)     + 2SO2(g)

  Zinc sulphide   Oxygen              Zinc oxide Sulphur dioxide

(Zinc blende ore)  

4FeS2(s)    + 11O2 2Fe2O3(s)   + 8SO2(g)

  Iron pyrites            Oxygen Ferric oxide      Sulphur dioxide

REDUCTION OF METAL FROM METAL OXIDE

Smelting:

The metal oxide formed after calcinations or roasting is converted into metal by reduction.  The method used for reduction of metal oxide depends upon the nature and chemical reactivity of metals.  Smelting is done in blast furnace. During smelting either the metal oxide is smelted or reduced in to metal

Reduction with carbon:  

The oxides of moderately reactive metals (occurring in the middle of reactivity series) like Zinc, copper, nickel, tin, lead etc.  Can be reduced by using carbon as reducing agent.

ZnO(s)    + C(s) Zn(s)         + CO(g)

Zinc oxide    Carbon     Zinc Metal         Carbon Monoxide

Fe2O2(s)  + 3C(s)     →   2Fe(s)        + 3CO(g)

  Ferric oxide     Carbon Iron Metal        Carbon Monoxide

Reduction by heating:

Metals placed low in the reactivity series are very less reactive.  They can be obtained from their oxides by simply heating in air.

2HgS(s)   + 3O2(g)    2HgO (S)      + 2SO2(G)

Mercuric Sulphide     Oxygen Mercuric Oxide   Sulphur

2HgO(s)   2Hg()      + O2(g)

Mercuric Oxide Mercury         Oxygen 

Reduction by Aluminium:

Some metal oxides cannot be reduced by carbon. In such cases as more reactive metal aluminium is used as agent. Aluminium powder reduces metal oxide to metal and itself is oxidized to Al2O3. For example MnO2 is heated with Al powdered, MnO2 is reduced to Mn, Al is oxidized to Al2O3. MnO2 is reduced to Mn, Al is oxidized to Al2O3.

The reduction of metal oxides with Al are highly exothermic, the amount of heat evolved is so large the metals are produced in molten state. The reduction of Fe2O3 with Al is used to join railway tracks or cracked machine parts. The reduction is known as thermal reduction.

Electrolytic reduction:

The oxides of active metals (which are high up in the activity series) are very stable and cannot be reduced by carbon or aluminium.  These metals are commonly extracted by the electrolysis of their fused salts using suitable electrodes.

For example, aluminium oxide is very stable and aluminium cannot be prepared by reduction with carbon.  It is prepared by the electrolysis of molten alumina (Al2O3).

Al3+                         +   3e-    Al

Aluminium ion Electron                Aluminium

(From molten alumina)                                     (At cathode)

(From cathode)

PURIFICATION OR REFINING OF METAL

The metal obtained by any of the above methods is usually impure and is known as crude metal.  The process of purifying the crude metal is called refining. Following methods are used to refine the metal.

Liquation:  

This method is used for refining metals having low melting points such as tin & lead. The metals is placed on the sloping surface in a hearth on heating metals flows down the surface and is collected. Impurities which have high melting points remarks on the Earth.

Distillation:

More volatile metals like Zn, Hg are purified by this method. The impure metal is heated in a retort. The pure metal distils over and is condensed in a reaction. Impurities will be left in the retort

Electrolytic  Refining:

Electrolytic refining means is electolysis. It is the most important, most widely used method by means of which a metal of high purity can be obtained. Many metals like Cu, Zn, Sn, Pb, Cr, Hg, Ag, Au are refined electolytically.

In this process a thick block of impure metal is taken as anode, a strip of pure metal is taken as cathode, water soluble salt (of metal to be refined) is taken as electrolyte. On passing electric current, impure metal dissolves from the anode and goes into the solution, pure metal from the electrolyte deposits on the cathode. Impurities will settle at the bottom as anode mud.