Questions - Answers

4 marks questions

1. Write a short note on froth flotation process.

A: Froth- floatation method is used to concentrate sulphide minerals. In this process, finely powdered ore, water, pine oil, sodium ethyl-xanthate (frothing agent) are taken in a tank. The surface of ore particles is wetted by pine oil, sodium ethyl-xanthate forms as a layer on the surface of the ore particles. This suspension is agitated with air. The bubbles of the froth are stabilized by adding "Collectors".

The gangue particles settle down at the bottom of the tank. conditioners like lime, Na₂CO₃ are also added to float the froth along with sulphide ore particles. Copper pyrites, iron pyrites are concentrated by this process.

2. Write short notes on a) Smelting b) Calcination and c) Roasting.

A: Smelting: The process of extraction of a metal in molten state directly from its roasted ore by heating it with coke or CO and flux.
         PbO + C  →  Pb + CO
         Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂
Calcination: The process in which carbonate or bicarbonate ore is heated below its melting point in absence of air to get respective oxides.
         During calcination, ore becomes porous, moisture, volatile impurities P, S, As are removed.
          ZnCO₃      ZnO + CO₂
          2 NaHCO₃     Na₂CO₃ + CO₂ + H₂O
Roasting: The process in which ore is heated strongly in below its melting point in presence of excess of air. Sulphide ores are converted into either oxides (oxidizing roasting), sulphates (sulphatizing roasting) or chlorides (chloridizing roasting).
          During roasting ore becomes porous, moisture, volatile impurities are removed.
          2 ZnS + 3O₂  → 2 ZnO + 2 SO₂

3. Draw a neat sketch of blast furnace & give its working.

A: Blast furnace: It is 100 feet tall cylindrical furnace and has diameter about 25 feet. It is made of wrought iron and inside lined with firebricks. Charge is introduced into furnace through the double cup and cone arrangement present at the top. Hot air is blown through tuyers present at bottom. 400°C to 1500°C temperature is found at different regions of the furnace. An outlet is provided to expel waste gases, near the top of the furnace. hearth is present at the bottom where products are obtained in molten state: Molten metal can be taken out through top hole and slag through slag notch. Cu, Pb, Fe ores are smelted in this furnace.

4. Describe Castner's process for the manufacture of sodium.

A: Castner process: A cylindrical iron tank is provided with a ring of burners at the bottom of the tank is electrolytic cell. Iron rod is introduced into electrolyte from the bottom of the tank. Iron rod acts as cathode. A hollow cylindrical nickel rod surrounds iron acts as anode.

Solid NaOH is heated up to 330° C get fused NaOH (electrolyte). A nickel wire gauze present between anode & cathode prevents passage of Na.
            4 NaOH → 4 Na⁺ + 4 OH^-
At cathode: 4 Na⁺ + 4 e^-→ 4Na
At anode: 4 OH^-  → 2 H₂O + O₂ + 4 e^- 

5. How is Cu extracted from Copper pyrites?
A: Ore is concentrated by froth flotation process. It is roasted on the hearth of the reverberatory furnace.
Cu₂S . Fe₂S₃ + O₂   Cu₂S + 2 FeS + SO₂
2 Cu₂S + 3 O₂    2  Cu₂O + 2 SO₂
2 FeS + 3 O₂  

 2  FeO + 2 SO₂
The roasted ore is mixed with coke and silica and smelted in blast furnace.
2 FeS + 3 O₂   2 FeO + 2 SO₂
FeO + SiO₂   Cu₂S + FeO
Matte formed here is converted into blister copper by blowing a blast of hot air and sand.
2 Cu₂O + Cu₂S    6 Cu + SO₂
Blister copper is refined by electrolysis to get 100% pure copper.

6. Explain the extraction of Zinc from Zinc blende.
A: Zn is mainly extracted from Zinc blende (ZnS). Finely crushed ore is concentrated by gravity separation, electromagnetic separation and froth flotation.
The concentrated ore is roasted to form ZnO.
ZnS + 2 O₂    ZnSO₄
2 ZnSO₄     2 ZnO + 2 SO₂ + O₂
2 ZnS + 3 O₂     2 ZnO + 2 SO₂
ZnO is reduced with coke in retorts by Belgian process at 1673 K.
                 1673 K
ZnO + C       Zn + CO
Zn spelter formed in the retorts consist Cd & Pb impurities in small quantities.
So it is subjected to electrolytic refining to get pure Zn.

7. Explain briefly the extraction of Al from Bauxite.
A:  Al is extracted either from red bauxite (consists iron oxide impurities) or from white bauxite (consists SiO₂). Powdered ore is concentrated by leachal agent NaOH at 35 - 36 bar pressure and 473 − 523 K temperature to get sodium aluminate.

Al₂O₃ + 2 NaOH + 3 H₂O   2Na [Al (OH)₄]
Sodium aluminate is alkaline in nature, neutralised by CO₂ gas.
2 NaAl(OH)₄ + CO₂   Al₂O₃. x H₂O + 2 NaHCO₃
Hydrated alumina is filtered, dried and ignited to give pure Al₂O₃.
                         1470 K
Al₂O₃. x H₂O     Al₂O₃ + x H₂O
Purified Al₂O₃ is added with CaF₂ (Fluorspar) to lower the fusion temperature and Na₃AlF₆ (Cryolite) is added to increase conductivity. This mixture is taken in steel vessel coated with carbon lining acts as cathode and graphite acts as anode and is electrolysed.
2 Al₂O₃ + 3 C   4 Al + 3 CO₂
Reaction at anode: Al⁺³ + 3    Al
at cathode: C + 2 O⁻²   CO₂ + 4 

2 Marks Questions

1. Give the composition of alloys.
     a) Brass      b) German Silver
A: Brass: 60% Cu & 40% Zn
          German Silver: Cu: 25% - 40%
          Zn: 25% - 35%
          Ni: 40% - 50%

2. What is the role of  a) Collector  b) Stabilizer in froth flotation process?
A: a) Collector: The substance that enhances non - wettability of the mineral particles.
           e.g.: Pine oil, Sodium ethyl xanthate.
b) Stabilizer: The substance that stabilize the froth.
      e.g.: Aniline, Cresol.

3. What is the role of depressants in froth flotation process?
A: The substance used to separate 2 different sulphide ores by preventing the formation of froth of one sulphide ore and allowing other ore to form the froth.
 e.g.: NaCN depressant is used to separate ZnS (froth is not formed) and PbS (comes along with froth).

4. What is "matte" and "blister copper"?
A: The molten liquid obtained in reverbaratory furnace contains mainly Cu₂S and little FeS is called "matte". The solidified Cu obtained in Bessemer converter having blisters (due to evolution of SO₂) is called "blister copper".

5. Give 2 uses each of  a) Zinc   b) Copper.
A: Zinc: ¤ Used for galvanisation of iron.
               ¤ Used in alloys brass & german silver.
    Copper: ¤ Used for making electrical wires.
                    ¤ Used in alloys brass & bronze.

6. Give 2 uses each of  a) Iron  b) Aluminium.
A: Iron: * Cast iron used for casting stoves, railway sleepers.
              * In the manufacture of steel & wrought iron.
    Aluminium: *  Al foils used as wrappers for chocolates.
                         *  For making electrical wires and cables.

7. Give 2 uses each of  a) Cast Iron  b) Wrought Iron.
A: Cast Iron: *  For casting stoves, railway sleepers.
                      *  For making toys, gutter pipes.
     Wrought Iron: *  For making wires, chains.
                              *  For making anchors, bolts.

8. Give 2 uses each of  a) Nickel Steel  b) Stainless Steel.
A: Nickel Steel: *  For making cables, automobiles.
                          *  For making pendulum, measuring tapes.
    Stainless Steel: * For making pens, Utensils.
                               * For making cycles, automobiles.

9. What is "poling"?
A: In this method molten metal is covered with carbon powder and is stirred with green wood logs to reduce metal oxide impurities into metal.
e.g.: Sn & Cu metals refining.

10. Explain the terms "Gangue" (matrix) and "Slag".
A: Gangue: Clay, sand like impurities present in the mineral is called Gangue.
     Slag: Easily fusible mass, formed when flux reacts with Gangue.

11. Give 2 limitations of Ellingham diagram.
A: * It does not explain the kinetics of reduction process.
    * Reactants & products are not always in equilibrium.
                           (∵ reactants or produts may be solid).

12. Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
A: Standard Gibbs energy of formation of Cu₂S greater than those of H₂S and CS₂. Where as standard Gibbs energy of formation of Cu₂O is less than of CO₂.
                              Δ
2 Cu₂S + 3 O₂   2 Cu₂O + 2 SO₂
2 Cu₂O + C     4 Cu + CO₂

13. What is Zone refining?
A: Si, Ga, Ge are refined by this method. This method is based on fractional crystallization. When heating coil is moved along the impure metal rod, the pure metal crystallizes out and the impurities remain in the molten metal.

14. What is the role of Graphite rod in the electrometallurgy of Aluminium?
A: In Hall - Heroult process Graphite is taken as anode. O₂ liberated at anode and reacts with Graphite to form CO & CO₂. Half of the carbon anode is burnt during the production of Al. The reactions at anode are
C + O⁻²   CO + 2  
C + 2 O⁻²   CO2 + 4