We get minerals and ores in abundance in the earth’s crust. Some ores have been proven themselves a great resource for mankind. Such as iron obtained from ore of iron (Hematite) built the foundation of industrial revolution. Out of total naturally occurring elements 70% are metals. Metals occur in nature in free as well as in combined states. Generally, reactive metals occur in combined states in the form of oxides, sulfides, carbonates etc. The metals in the middle of the reactivity series such as zinc, iron, lead etc. are moderately reactive and are found in earth’s crust in the form of oxides, sulfides, carbonates etc. Out of these metal compounds, pure metal can be economically extracted from some compounds.
Minerals are naturally occurring, homogeneous inorganic solid substances having a definite chemical composition and characteristic crystalline structure, color and hardness. For example, copper pyrite (CuFeS2), calamine (ZnCO3) etc. are minerals.
A mineral from which metal can be economically extracted in a maximum amount is called an ore. Some examples of ores or metals are listed below in tabular form –
The process of extraction of pure metals from their ores, is called Metallurgy. Methods are used in the extraction of metals from their ores on the basis of their reactivity. For the extraction of highly reactive metals from their ores electrolysis is used and for extraction of metals of medium reactivity calcination, roasting and reduction methods are used. While metals of low reactivity are extracted from their ores by roasting and refining. Although mostly low reactive metals are found in free states such as the gold is generally found in free state or with silver and mercury (As amalgam). Before applying all these methods of extraction of metals according to their reactivity, concentration of ores is done. Ores are found in nature with many impurities such as rock particles, sand and other impurities. These impurities in an ore are called matrix or gauge. The removal of matrix or gauge from the ore is called concentration ore.
Several steps are involved in the extraction of pure metal from ores. Below is given a flow diagram of various steps involved in extraction of metals –
First ores are crushed and grinded and then gauge or matrix is removed from the powdered ore. This is called concentration of ores. Ores of highly, moderately, and low reactive metals are 1st concentrated. This step is common for ores of all types of metals. It is carried out by various methods depending upon the nature of the ore. It is generally carried out by following 4 methods –
Froth floatation method
Gravity separation, froth floatation and electromagnetic separation methods are physical methods while leaching method is a chemical method of concentration of ores.
Gravity separation – It is based on the difference in the weight of metal and matrix. So, due to gravity the heavier one settled down at the bottom. Generally, metals are heavier than matrix, so they get separated at the bottom. This method of separation is also called hydraulic washing as crushed ore particles are mixed with an upward stream of water.
Froth floatation method – It is the method to separate hydrophobic materials from hydrophilic. It is based on the difference in the wetting properties of the ore and the gauge particles. It is generally used for sulfide ores.
Electromagnetic separation – This method is based on the magnetic properties of the metal particles and gauge particles. Powdered ore is placed on the roller belt which is a magnetic belt. So, the magnetic material gets attracted towards the belt and stays on that and thus gets separated. Generally, it is used for iron and manganese ores.
Leaching - It is a chemical method. In this method ores are dissolved in the chemicals so that matrix is left behind and metal is obtained from the chemical solution. Matrix is filtered out.
Then metals are extracted from concentrated ores by various specific procedures based on the reactivity of the metal ore.
Metals which are present at the top of reactivity series are highly reactive metals. As these metals have more affinity towards oxygen than carbon so ores of these metals cannot be reduced to their oxides as extraction of these metals from their oxides will be more difficult and expensive. These metals are obtained from their ores by electrolytic reduction. For example, sodium metal is obtained by the electrolysis of its molten chloride. In this process metals are deposited at cathode (negatively charged electrode) whereas chlorine is liberated at the anode (positively charged electrode).
Metals which are present at the middle of reactivity series are moderately reactive metals such as iron, zinc, lead etc. These metals are generally found in nature as carbonates or sulfides ores. So, these are 1st changed into their oxides as it is easier to extract these metals from their oxides. It is done by following two methods –
Sulfide ores are changed into their oxides by roasting. In this process sulfide ores are heated strongly in the presence of excess of air.
Carbonate ores are changed into their oxides by calcination. In this process carbonate ores are heated strongly in the presence of a limited amount of air.
Metals which are present at the bottom of reactivity series are low reactive metals such as mercury, copper etc. The oxides of these metals can be reduced to metals by heating alone. For example, HgS (Cinnabar) is heated in air, first it converts into its oxide (HgO) and when HgO is further heated at 300℃, it gets reduced to mercury.
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