Online Notes on "General Principles & Processes Of Isolation of Elements" (Chapter=>6 ) Part 2

These are Online Notes on "General Principles & Processes

Of Isolation of Elements" (Chapter=>6 ) Part 2 for practice of CBSE BOARD, CBSE NEET, CSIR NET Chemical Sciences etc.

1.  Leaching (Chemical separation): It is a process in which ore is treated with suitable solvent which dissolves the ore but not the impurities.

2. Purification of Bauxite by Leaching: Baeyer’s process: Step 1:

Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4](aq)

Step 2:

2Na[Al (OH)4](aq) + CO2 (g) ® Al2O3.xH2O(s) + 2NaHCO3 (aq)

Step 3:

Al₂O₃.xH₂O(s)    ¾¾He¾at¾at1¾47¾0K¾®   Al₂O₃ (s) +   xH2O(g)

3.               Concentration of Gold and Silver Ores by Leaching:

4M(s) + 8CN-  (aq) + 2H O(aq) + O (g)   ®   4[M(CN) ]-(aq) + 4OH-(aq)

2[M(CN) ]-(aq) + Zn(s)   ®   [Zn(CN) ]2-(aq) + 2M(s)

Where :

[M =  Agor Au]

4.       Conversion of ore into oxide: It is easier to reduce oxide than sulphide  or carbonate ore. Therefore, the given ore should be converted into oxide by suitable method:

Roasting	Calcination
It is a process in which ore is heated in a regular supply of air at a temperature below melting point of the metal so as to  convert  the given  ore  into oxide ore.	It is a process of heating ore in limited supply of air so as to convert carbonate ores into  oxides.

Sulphide ores are converted into oxide by roasting	Carbonate ores are converted into oxide by roasting
It    is    also    used   to    remove impurities as volatile oxides	It is also used to remove moisture and volatile impurities
E.g.: 2ZnS+ 3O2  → 2ZnO + 2SO2	E.g.: CaCO3   ¾¾he¾at ¾®CaO + CO 2

5.    Reduction of oxide to metal: The process of converting metal oxide into metal is called reduction. It needs a suitable reducing agent depending upon the reactivity or reducing power of metal. The  common reducing agents used are carbon or carbon monoxide or any other metals like Al, Mg etc.

6. Thermodynamic principles of metallurgy: Some basic concepts of thermodynamics help in understanding the conditions of temperature and selecting suitable reducing agent in metallurgical processes:

a.    Gibbs free energy change at any temperature is given by ΔG = ΔH – TΔS where ΔG is free energy change, ΔH is enthalpy change and ΔS is entropy change.

b.    The relationship between ΔGӨ  and K is ΔGӨ  = –2.303 RT log K

where K is equilibrium constant. R = 8.314 JK-¹ mol-1, T is temperature in Kelvin.

c.  A negative ΔG means +ve value of K i.e., products are formed more than the reactants. The reaction will proceed in forward direction.

d.   If ΔS is +ve, on increasing temperature the value of TΔS increases so that TΔS > ΔH and ΔG will become negative.

7.    Coupled reactions: If reactants and products of two reactions are put together in a system and the net ΔG of two possible reactions  is –ve the overall reaction will take place. These reactions are  called coupled reactions.

8.    Ellingham diagrams: The plots between ΔfGӨ of formation of oxides of elements vs. temperature are called Ellingham diagrams. It provides a sound idea about selecting a reducing agent in reduction of oxides. Such diagrams help in predicting the feasibility of a thermal reduction of an ore. ΔG must be negative at a given temperature for a reaction to be feasible.

9. Limitations of Ellingham Diagrams: It does not take kinetics of reduction into consideration, i.e., how fast reduction will take place cannot be determined.

10. Reduction of iron oxide in blast furnace: Reduction of oxides takes place in different zones.

At 500 – 800 K (lower temperature range in blast furnace) 3Fe2O3 + CO ® 2Fe3O4 + CO2

Fe3O4 + 4CO ® 3Fe + 4CO2

Fe2O3  + CO ® 2FeO + CO2

At 900 – 1500 K (higher temperature range in blast furnace) 

C + CO2  ® 2CO

FeO + CO ® Fe + CO2

Limestone decomposes to CaO and CO2 CaCO3   ¾¾he¾at ¾®CaO + CO

Silica (impurity) reacts with CaO to form calcium silicate which forms slag. It floats over molten iron and prevents oxidation of iron.

CaO + SiO2  ® CaSiO3

                                         Calcium silicate (Slag)
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