Extraction of Metals PDF

Occurrence and extraction of metals Metals and their alloys are extensively used in our day- to-day life. They are used for making machines, railways, motor vehicles, bridges, buildings, agricultural tools, aircrafts, ships etc.  Therefore, production of a variety of metals in large qua...

Occurrence and extraction of metals Metals and their alloys are extensively used in our day- to-day life. They are used for making machines, railways, motor vehicles, bridges, buildings, agricultural tools, aircrafts, ships etc.  Therefore, production of a variety of metals in large quantities is necessary for the economic growth of a country. Only a few metals such as gold, silver, mercury etc. occur in free state in nature. Most of the other metals, however, occur in the earth's crust in the combined form, i.e., as compounds with different anions such as oxides, sulphides, halides etc. In view of this, the study of recovery of metals from their ores is very important. Some of the processes of extraction of metals from their ores are called metallurgical processes. Occurrence of Metals Metals occur in nature in free as well as combined form. Metals having low reactivity show little affinity for air, moisture, carbon dioxide or other non-metals present in nature. Such metals may remain in elemental or native (free) state in nature. Such metals are called "noble metals" as they show the least chemical reactivity. For example gold, silver, mercury and platinum occur in free state. On the other hand, most of the metals are active and combine with air, moisture, carbon dioxide and non-metals like oxygen, sulphur, halogens, etc. to form their compounds, like oxides, sulphides, carbonates, halides and silicates. i.e., they occur in nature in a combined state. A naturally occurring material in which a metal or its compound occurs is called a mineral. An ore is a material that contains a sufficiently high concentration  of a mineral to constitute an economically feasible source from which the metal can be recovered. The main active substances present in nature, especially in the atmosphere are oxygen and carbon dioxide. In the earth's crust, sulphur and silicon are found in large quantities. Sea-water contains large quantities of chloride ions (obtained from dissolved sodium chloride). Most active metals are highly electropositive and therefore exist as ions. It is for this reason that most of the important ores of these metals occur as (i) oxides (ii) sulphides (iii) carbonates (iv) halides and (v) silicates. Some sulphide ores undergo oxidation by air to form sulphates. This explains the occurrence of sulphate ores. Ores are invariably found in nature in contact with rocky materials. These rocky or earthy impurities accompanying the ores are termed as gangue or matrix Type of Ore Metals (Common Ores) Native Metals Gold (Au), silver (Ag) Oxide ores Iron (Haematite, Fe2O3 ); Aluminium (Bauxite, Al2O3. 2H2O); Tin (Cassiterite, SnO2); Copper (Cuprite, Cu2O); Zinc (Zincite, ZnO); Titanium (Ilmenite, Type of Ore Metals (Common Ores) Sulphide ores Zinc (Zinc blende, ZnS); Lead (Galena, PbS); Copper (Copper glance, Cu2S); Silver (Silver glance or Argentite, Ag2S); Iron (Iron pyrites, FeS2) Carbonate ores Iron (Siferite, FeCO3); Zinc (Calamine, ZnCO3) , Lead (Cerrusite, PbCO3) Type of Ore Metals (Common Ores) Sulphate ores Lead (Anglesite, PbSO4) Halide ores Silver (Horn silver, AgCl); Sodium (Common salt or Rock salt, NaCl); Aluminium (Cryolite, Na3AlF6) Silicate ores Zinc(Hemimorphite, 2ZnO.SiO2.H2O) The process of extracting the metals from their ores and refining them is called metallurgy. The choice of the process depends upon the nature of the ore and the type of the metal. The metal content in the ore can vary depending upon the impurities present and chemical composition of the ore. Some common steps involved in the extraction of metals from their ores are : (i) Crushing and pulverization (ii) Concentration or dressing of the ore (iii) Calcination or roasting of the ore (iv) Reduction of metal oxides to free metal (v) Purification and refining of metal. Crushing and Pulverization The ore is generally obtained as big rock pieces. These big lumps of the ore are crushed to smaller pieces by using jaw- crushers and grinders. It is easier to work with crushed ore. The big lumps of the ore are brought in between the the plates of a crusher forming a jaw. One of the plates of the crusher is stationary while the other moves to and fro and the crushed pieces are collected below. The crushed pieces of the ore are then pulverized (powdered) in a stamp mill. The heavy stamp rises and falls on a hard die to powder the ore. The powdered ore is then taken out through a screen by a stream of water.