NEET Chemistry Notes General Principles and Processes of Isolation of Metals -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.
Extraction of a metal from its ores is known as metallurgy. Metallurgical processes may be divided into three processes:
- 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.
- Hydrometallurgical Process
Extraction of metal by the use of their aqueous solution is called hydrometallurgical process.
Ag and Au are extracted by this process.
- 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 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.