Extraction of Metals

The process of obtaining metals from its ores is called metallurgy. The naturally occurring compounds of metals are known as minerals and they are usually extracted from the earth`s crust by mining.

 

Key points

  1. Although, seawater contains some soluble salts like sodium chloride, magnesium chloride, the crust of the earth is the major source of metallic ores.

  2. In most of the cases, metals exist in a combined state however some are available in their free state.

  3. A native metal is a natural metal available in the metallic state. Metal is available in its pure form or in the form of an alloy.

  4. Reactive metals share the maximum percentage of metals . They are the least tolerant of the chemical processes such as oxidation, corrosion, etc. 

  5. Non-reactive metals like gold, silver, platinum, etc are found in the native or free state.

  6. Ore is an adulterated form of metal requiring extraction through physical and chemical processing.

 

Formation of Gangue 

Ore -appearance

 

Metals are available in the form of ores, but the ores often found along with guange. The gangue or matrix is an unwanted substance with no commercial significance. It is rocky and sandy material found along with ores. All ores are minerals, but not all minerals are ores. The picture shown above is an iron ore appearing dark with the rough edges . Metallurgy includes series of scientific processes such as smelting, electrolysis, reduction, and oxidation. Different methods are used to extract different metals. Based on their reactivity, metals are classified into high reactivity metals, medium reactivity metals and low reactivity metals. The low reactivity metals are at the bottom of the activity series; they are gold, platinum, silver, and copper occurring in their free State. Most of the metals are in their active form except the copper and silver, as they occur in sulphide and oxide ores. Metals of medium reactivity series are (Zn, Fe, Pb, etc.) These occur as oxides, sulphides or carbonates. Metals of high reactivity (K, Na, Ca, Mg and Al) are very reactive and thus found in combined stat

 

Steps of Metallurgy (metal extraction process)

1. Enrichment of Ores: Enrichment includes concentration of ores, it is otherwise known as the ore dressing. The process of removing gangue (earthly impurities) from its ore is known as the concentration process or ore dressing.

2. Hydraulic washing: It is done based on the principle of density differentiation of materials in the enriched ores. When a gush of water is passed through a chamber, the lighter gangue particles will be washed away and only the heavier minerals will stay at the bottom. Ores of tin and lead are concentrated by this method as they are very heavy.

3. Magnetic separation: Magnetic separation is used for those compounds that get easily attracted by magnets such as ores of iron pyrites, FeS and magnetite, Fe304. After the magnetic separation, the remains are concentrated. The crushed ore is allowed to pass through electromagnetic belts so that only the mineral particles are retained and gangue particles are washed away.

4. Froth flotation: It is most commonly used for sulfide ores. The powdered ore is mixed with water in a tank containing pine or eucalypts oil and then the whole components are agitated through a gush of air. As the ore particles got wet , oil floats to the surface along with froth but, these hydrated impurities get settled to the bottom as they are heavier. Finally, the oil froth with mineral particles which floats on the surface of the water can be skimmed off.

5. Conversion of concentrated ore into metallic oxide: Concentrated ore is converted into metal oxide either by Roasting or Calcination. Roasting is done by heating the concentrated ore to a high temperature in the presence of an ess of air.  Calcination is done by heating the concentrated ore in the absence of air at a temperature not sufficient to melt the ore. The reaction can be written as  2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 Ca

6. Reduction of metallic oxides to metal by smelting: This is done by electrolysis and by reducing agents. By using electrolysis, highly electropositive (reactive) metals like K, Na, Ca, Mg, Al oxides/halides and also fused metallic salts are reduced. For example, Extraction of Aluminium from [Al2O3.2H2O. By using reducing agents, like carbon (coke, charcoal), carbon monoxide. Hydrogen etc., the metallic oxide is reduced to metal. For example, ZnO+CO ------->Zn+Co2.

7. Refining and purification: The metals produced by various reduction processes are not very pure as they still contain a lot of impurities. The most widely used method for refining impure metals is electrolytic refining. 





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