General Principles of Extraction of Metals - 1

The earth's crust is the biggest source of metals. Some soluble salts of the metals are also found in sea-water. Metals occur in nature sometimes free but mostly in combined state. The natural mode of occurrence of a metals is largely dependent on its nature. Those metals which are least reactive and have little or no affinity for oxygen, moisture and other chemical reagents occur in free or metallic or native state i.e., in uncombined state. Most of the metals are reactive and hence are found in combined state i.e., as compounds.

The natural substances in which the metals or their compounds occur in the earth are called minerals. The mineral has a definite composition. It may be a single compound or a complex mixture. The minerals from which the metals can be conveniently and economically extracted are known as ores. All the ores are minerals but all minerals cannot be ores. For example, both bauxite (Al₂O₃. 2H₂O) and clay (Al₂O₃.2SiO₂.2H₂O) are minerals of aluminium. It is bauxite which is used for extraction of aluminium and not clay. Thus, bauxite is an ore of aluminium. Ores may be divided into four groups.

1. Native ores : These ores contain metals in free state, e.g., silver, gold, platinum, mercury, copper, etc. These are found usually associated with rock or alluvial materials like clay, sand, etc. Sometimes lumps of pure metals are also found. These are termed nuggets.
2. Oxidised ores : In these ores. Metals re present as their oxides or oxysalts such as carbonates, nitrates, sulphates, phosphates, silicates, etc.
   
3. Halide ores : Metallic halides are very few in nature. Chlorides are most common.
   

 

The whole process of obtaining a pure metal from one of its ores is known as metallurgy.

In order to extract the metal from ores, several physical and chemical methods are used. The method used depends upon the nature of the ore, the properties of the metal and the local conditions, Thus, it is not possible to have a universal method for the extraction of all the metals from their ores. However, the metallurgy of a metal involves three main operations:

1. Concentration or dressing
2. Reduction
3. Purification or refining

1. Concentration or dressing of ores : Ores usually contain soil, sand, stones and other useless silicates. These undesired impurities present in ores are called Gangue or Matrix. The removal of these impurities from the ores is known as concentration. Before the ore is subjected to concentration, it is crushed into small pieces in gyratory crushers. The crushed ore is then grinded with the help of rollers or in the stamp mill to powder form.
   
   Physical Methods
   The following physical methods are generally employed for the concentration of the ores depending upon the nature of the ore.
   1. Gravity separation : The separation is based on the difference in the specific gravities of the gangue particles and the ore particles. The powdered ore is agitated with water or washed with a running stream of water. The heavy ore particles settle down while the lighter particles of sand, clay, etc., are washed away. For this either Wilfley table is used. It a wooden table having slanting floor on which long wooden strips called riffles are fixed.
      
      
      

      The powdered ore is suspended in a stream of water. The heavier ore particles collect behind the riffles and the gangue particles are carried away with the stream of water. Hydraulic classifier is shown in this. Powdered ore is dropped from the top of classifier and strong stream of water is introduced from the bottom. The lighter gangue particles are carried away by the water while the heavier ore particles settle down. Generally, Oxide and carbonate ores are concentrated by this method. For example, tin ore (cassiterite) and iron ore (haematite) are concentrated by gravity method.