4. Occurence of Metals

Occurrence of Metals

Most of the elements, especially metals occur in nature in the combined state with other elements. All these compounds of metals are known as minerals. But out of them, only a few are viable sources of that metal. Such sources are called ores.

Au, Pt – exist in the native or free state.

Extraction of Metals

Metals of high reactivity – Na, K, Mg, Al.

Metals of medium reactivity – Fe, Zn, Pb, Sn.

Metals of low reactivity – Cu, Ag, Hg

Roasting

Converts sulphide ores into oxides on heating strongly in the presence of excess air.

It also removes volatile impurities.

2ZnS(s)+3O2(g)+Heat→2ZnO(s)+2SO2(g)

Calcination

Converts carbonate and hydrated ores into oxides on heating strongly in the presence of limited air. It also removes volatile impurities.

ZnCO3(s)+heat→ZnO(s)+CO2(g)

CaCO3(s)+heat→CaO(s)+CO2(g)

2Fe2O3.3H2O(s)+heat→2Fe2O3(s)+3H2O(l)

Extracting Metals Low in Reactivity Series

By self-reduction- when the sulphide ores of less electropositive metals like Hg, Pb, Cu etc., are heated in air, a part of the ore gets converted to oxide which then reacts with the remaining sulphide ore to give the crude metal and sulphur dioxide. In this process, no external reducing agent is used.

1. 2HgS(Cinnabar)+3O2(g)+heat→2HgO(crude metal)+2SO2(g)

2HgO(s)+heat→2Hg(l)+O2(g)

2. Cu2S(Copperpyrite)+3O2(g)+heat→2Cu2O(s)+2SO2(g)

2Cu2O(s)+Cu2S(s)+heat→6Cu(crude metal)+SO2(g)

Extracting Metals in the Middle of Reactivity Series

Smelting – it involves heating the roasted or calcined ore (metal oxide) to a high temperature with a suitable reducing agent. The crude metal is obtained in its molten state.

Fe2O3+3C(coke)→2Fe+3CO2

Aluminothermic reaction – also known as the Goldschmidt reaction is a highly exothermic reaction in which metal oxides usually of Fe and Cr are heated to a high temperature with aluminium.

Fe2O3+2Al→Al2O3+2Fe+heat

Cr2O3+2Al→Al2O3+2Cr+heat

Extraction of Metals Towards the Top of the Reactivity Series

Electrolytic reduction:

1. Down’s process: Molten NaCl is electrolysed in a special apparatus.

At the cathode (reduction):

Na+(molten)+e−→Na(s)

Metal is deposited.

At the anode (oxidation):

2Cl−(molten)→Cl2(g)+2e–

Chlorine gas is liberated.

2. Hall’s process: Mixture of molten alumina and a fluoride solvent usually cryolite, (Na3AlF6) is electrolysed.

At the cathode (reduction):

2Al3++6e–→ 2Al(s)

Metal is deposited.

At the anode (oxidation):

6O2– → 3O2(g)+12e–

Oxygen gas is liberated.

Enrichment of Ores

It means the removal of impurities or gangue from ore, through various physical and chemical processes. The technique used for a particular ore depends on the difference in the properties of the ore and the gangue.

Refining of Metals

Refining of metals – removing impurities or gangue from crude metal. It is the last step in metallurgy and is based on the difference between the properties of metal and the gangue.

Electrolytic Refining

Metals like copper, zinc, nickel, silver, tin, gold etc., are refined electrolytically.

Anode: impure or crude metal

Cathode: a thin strip of pure metal

Electrolyte: aqueous solution of metal salt

From anode (oxidation): metal ions are released into the solution

At cathode (reduction): the equivalent amount of metal from solution is deposited

Impurities deposit at the bottom of the anode.

OCCURANCE OF METALS

MINERALS :- The elements which occur naturally in the earth’s crust are called minerals.

ORES:- Minerals that contain very high percentage of particular metal andthe metal can be profitably extracted from it, such minerals are called ores.

• Metals at the bottom of the activity series are least reactive they are often found in free state . For Example –  Ag,Au, Cu.
• Metals at the ttop of the acitivity series (k,Na, Co, Mg, and Al) are so reactive that they never found in free state.
• Metals in the middle of the activity series ( Zn, Fe,Pb etc are moderately reactive . they occur as sulphates ,oxides or carbonates.
• They ore of many metals are oxide because oxygen is very reactive and is abundant on the earth.
• Steps involved in the extraction of pure metals from ores.

 Step involved in the extraction of metals from ores.

 EXTRACTION OF METALS FROM ORES :-

 1 ENCRICHMENT OF ORES    Ores are usually contaminated with large amounts of impurities such as soil, sand etc called gangue these impuriestes are removed from the ore prior to the extraction of mertal.

 2  Extraction of metals   Metals low in the activity series are very anreactive the oxides of these metals are reduced to metals by heating.

 For example           

 2HgO   →      2Hg  +  O₂

Mercury oxide is reduced to mercury on heating

The metals in the middle of the activity series ( Zn, Fe, Pb, Cu ) are moderately active. The metal sulphides  and carbontes  are converted into metal oxide. The sulphate ores are converted into oxides by heating strongly in the presence of excess air this process is known as roasting .

2 ZnS + 3O₂ →      2ZnO + CO₂

• The carbonate ores are changed into oxides by heating strongly in limited air this process is called calcination .

 Zn CO₃     →     ZnO + CO₂

• Then metal oxides are reduced to corresponding metals by using reducing agent like carbon.

 ZnO  +  C    →   Zn   +  CO

• This reaction of iron (iii) oxide ( Fe2O3) with aluminium is used to join railway tracks or cracked machine parts . this reaction is known as thermit reaction.

 Fe₂O₃    +  2Al    →  2 Fe +  Al₂O₃ + heat

 Metals high up in the reactivity series are very reactive. The metalo are obtained by electrolytic reduction. The metals are deposit at the cathode and chlore is deposited at anode.

At cathode  →   Na⁺    +  e^-    →     Na
At anode   →     2Cl^-   →    Cl₂   +  2e^-

Refining of metals

 The most widely used method for refining impure metals is electronic refining.

Electrolytic refining :-   Metals  ( Cu, Zn , Ag, Au etc ) are refined electrolytically . the impure metal is made the anode and athin strip of pure metal is made the cathode . a solution a metal salt is used as an electrolyte.

Electrolytic refining of copper.

Anode :  Impure copper

Cathode: Strip of pure copper

The insoluble impurities settle at the bottom of the anode and is called anode mud.