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Extraction of Metals

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notes

ENRICHMENT OF ORES:-

Ores mined from the earth are usually contaminated with large amounts of impurities such as soil, sand, etc., called gangue.

The process used for removing the gangue from the ore are based on the differences between the physical or chemical properties of the gangue and the ore.



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Extracting Metals Low in the Activity Series:-

Metals low in the activity series are very unreactive. The oxides of these metals can be reduced to metals by heating alone. For example, cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted into mercuric oxide (HgO). Mercuric oxide is then reduced to mercury on further heating.


copper which is found as Cu2S in nature can be obtained from its ore by just heating in air.

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Extracting Metals towards the Top of the Activity Series:-

The metals high up in the reactivity series are very reactive. They cannot be obtained from their compounds by heating with carbon. For example, carbon cannot reduce the oxides of sodium, magnesium, calcium, aluminium, etc., to the respective metals. This is because these metals have more affinity for oxygen than carbon. These metals are obtained by electrolytic reduction.

Sodium is obtained by electrolysis of molten chloride. Sodium is deposited at cathode, whereas chlorine is liberated at the anode. The reaction is given as

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EXTRACTING METALS IN MIDDLE OF THE ACTIVITY SERIES:-

The metals in the middle of the activity series such as iron, zinc, lead, copper, are moderately reactive. These are usually present as sulphides or carbonates in nature.

It is easier to obtain a metal from its oxide, as compared to its sulphides and carbonates. Therefore, prior to reduction, the metal sulphides and carbonates must be converted into metal oxides.

The sulphide ores are converted into oxides by heating strongly in the presence of excess air. This process is known as roasting.

The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination.

The chemical reaction that takes place during roasting and calcination of zinc ores can be shown as follows –

Roasting:-

Calcination:-

The metal oxides are then reduced to the corresponding metals by using suitable reducing agents such as carbon.

For example, when zinc oxide is heated with carbon, it is reduced to metallic zinc.

ZnO(s) + C(s) → Zn(s) + CO(g)

Obtaining metals from their compounds is also a reduction process.

Reducing agents

Besides using carbon (coke) to reduce metal oxides to metals, sometimes displacement reactions can also be used. The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing agents because they can displace metals of lower reactivity from their compounds. For example, when manganese dioxide is heated with aluminium powder,


In the above reaction manganese dioxide is reduced and aluminium is oxidised to form aluminium oxide.

These displacement reactions are highly exothermic. The amount of heat evolved is so large that the metals are produced in the molten state. In fact, the reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction.

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EXTRACTION OF METALS:-

Some metals are found in the earth’s crust in the free state. Some are found in the form of their compounds. The metals at the bottom of the activity series are the least reactive. They are often found in a free state. For example, gold, silver, platinum and copper are found in the free state. Copper and silver are also found in the combined state as their sulphide or oxide ores. The metals at the top of the activity series (K, Na, Ca, Mg and Al) are so reactive that they are never found in nature as free elements. The metals in the middle of the activity series (Zn, Fe, Pb, etc.) are moderately reactive. They are found in the earth’s crust mainly as oxides, sulphides or carbonates. You will find that the ores of many metals are oxides. This is because oxygen is a very reactive element and is very abundant on the earth.

i) Metal of low reactivity: These metals are mostly found in free states. Eg: Gold, silver, platinum, copper

ii) Metal of medium reactivity: These metals are found mainly as oxides, sulphides or carbonates. Ores of many metals exists in oxides form.

iii) Metal of high reactivity: These metal are highly reactive and thus are never found in nature as free element. Eg: K, Na, Ca, Mg, Al

The diagram below explains the extraction of pure metal from ores in several steps.

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Shaalaa.com | Metal and Non Metals part 14 (Occurence & extraction of metal)

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Metal and Non Metals part 14 (Occurence & extraction of metal) [00:08:16]
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