NEET Chemistry Notes General Principles and Processes of Isolation of Metals -Earth Crust

NEET Chemistry Notes General Principles and Processes of Isolation of Metals -Earth Crust

Earth Crust

Earth Crust

Earth crust is the source of many elements. Aluminium is the most abundant metal of earth crust and iron comes second. The percentage of different elements in earth crust is 0-49%, Si-26%, Al-7.5%, Fe-4.2%, Ca-3.2%, Na-2.4%, K-2.3%, Mg-2.3%, H-1%. Metals occur in two forms in nature

• in native state and
•  in combined state.

Minerals and Ores

The substance (or compound) in the form of which metal is found in nature is called a mineral and the mineral from which extraction of metal is beneficial and cheap is called an ore. Thus, all ores are minerals but all minerals are not ores.

Types of Ores

Depending upon the nature of associated group or atom, ores are of following types

• Oxide ores, e.g. Haematite (Fe₂0₃), Zincite (ZnO) etc.
•  Sulphide ores, e.g. Galena (PbS), Cinnabar (HgS), Argentite Ag₂S), Ruby silver (Ag₂S • Sb₂S₃).
•  Carbonate ores, e.g. Magnesite(MgC0₃), Siderite (FeC0₃) etc.
• Sulphate ores, e.g. Gypsum (CaS0₄ • 2H₂0), Glauber’s salt (Na₂S0₄ • 10H₂O) etc.
• Silicate ores, eg;Willemite (Zn₂Si0₄), Feldspar (NaAlSi₃O₈)
•  Nitrate ores, are rare because all nitrates are water soluble and at higher temperature, they decompose into oxides or metal.

Gangue or Matrix

The impurities associated with the ore are called gangue or matrix.

Metallurgy

Extraction of a metal from its ores is known as metallurgy. Metallurgical processes may be divided into three processes:

1. Pyrometallurgical Process
   [Extraction through heating]
   Extraction of metals take place at very high temperature. Cu, Fe, Zn, Sn, Pb, Ni, Cr, Hg are extracted by pyrometallurgical process.
2. Hydrometallurgical Process
   Extraction of metal by the use of their aqueous solution is called hydrometallurgical process.
   Ag and Au are extracted by this process.
3. Electrometallurgical Process
   Sodium, potassium, lithium, calcium, magnesium and aluminium are extracted from their molten salt solutions through electrolytic method.

Steps Involved in the Extraction of Metals

Various steps/processes involved in the extraction of a metal are as follows:

• Concentration of Ores
  Removal of unwanted materials from the ores is known as concentration of ores or ore dressing.
  Some of the important procedures are described below
• Hydraulic Washing (Levigation)
  In this process, an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind. The oxides ores of iron (Fe₃0₄ andFe₂0₃) are concentrated by this method.
• Magnetic Separation
  This is based on differences in magnetic properties of the ore components. If either the ore-or the gangue is capable of being attracted by a magnetic field¹,  this process is used.
  In electrostatic separation, electrically charged surfaces are used to separate metallic particles from non-metallic particles of ore.
• Froth Floatation Method
  This method is used for the concentration of sulphide ores. In this process, a suspension of the powdered ore is made with water. To it, collectors and froth stabilisers are added.

Leaching

Leaching is often used if the ore is soluble in some solvent (i.e. acids, bases or other chemicals) but not the impurities, e.g.
(i) Leaching of alumina from bauxite (Baeyer process)

The resulting solution is filtered, cooled and pH is adjusted by neutral with C0₂ causing precipitation of aluminium hydroxide.

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