1. How does NaCN acts as a depressant in preventing ZnS from forming the froath ?
2. How will you separate Galena(Pbs) from sphalerite (ZnS) and iron pyrites by froath floatation process ?
3. Which method would you recommend for purification of copper and germanium ?
4. What is the significance of leaching in the extraction of aluminum ?
5. Why is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction ?
6. What is the importance of flux during smelting of iron ore?
7. Why is aluminum is used for electric cables though it is relatively less conducting than copper ?
8.At a site, low grade copper ores are available and zinc and iron scraps are also available. Which of the two scraps would be more suitable for reducing the leached copper ores and why?
9. Why sulphide ores are roasted before reduction?
10. Which of the ores of aluminum, iron, copper can be and zinc concentrated by magnetic separation method?
Solutions:-
Ans1. NaCN forms a layer of zinc complex on the surface of ZnS and thereby prevents it from the formation of froth and thus acts as a depressant.
Ans2. Galena can be separated from a mixture of galena, sphalerite and iron pyrites by using sodium cyanide and alkali as depressants and potassium xanthates as collector. The depressant depress the floatation property of ZnS and iron pyrites thereby making only PbS particles to go into froth. Galena thus can be removed from the froth. Now copper sulphate is added as activator when ZnS floatating character is activated and it comes in froth and get separated. The addition of acid to the remaining part of slurry lead to floatation of iron pyrite.
Ans3. Electrolytic refining process for copper and zone refining method for germanium.
Ans4. Bauxite ore contains mainly alumina along with silica etc as the impurities. These impurities can be removed by the process of leaching which consists in heating the bauxite ore with a concentrated (45%) solution of NaOH at 473-523 K when alumina dissolves in NaOH to produce sodium meta- aluminate and silica dissolves to produce sodium silicate leaving impurities behind.
The impurities are filtered off and sodium meta-aluminate formed is neutrilised by bubbling carbon dioxide through it when hydrated alumina separates out leaving sodium silicate in the solution. The hydrated alumina thus obtained, is filtered, dried and heated to give pure alumina.
Thus, significance of leaching in the process of extracting of aluminum lies in recovering pure alumina from bauxite ore.
Ans5. The entropy of the metal is higher in liquid state than when it is in solid state. The value of entropy change (∆S) of the reduction process is more on positive side when the metal formed in liquid state and the metal oxide which is being reduced is in solid state. Thus, the value of ∆G° becomes more negative and the reduction become easier.
Ans6. Calcium oxide (CaO) used as flux is basic in nature and combines with acidic gangue, silica to form calcium silicate known as slag. Slag forms upper layer over molten iron and does not allow air to come in contact with molten iron and thus molten iron does not get oxidized.
Ans7. It is because aluminum is lighter in weight than copper and thus more wire can be stretched from the same weight of aluminum than copper.
Ans8. Zinc being above iron in the electrochemical series (i.e., iron is more reactive than zinc), the reduction will be faster if zinc scraps are used. But since zinc is more costly than iron, therefore, use of iron scraps will be more advisable and advantageous.
Ans9. It is always advantageous to roast a sulphide ore to oxide before reduction. As, the free energies of formation ( ∆G°f) of most sulphide are greater than that of carbon disulphide and Mercury sulphide. Therefore neither carbon nor hydrogen is a suitable reducing agent for reduction metal sulphide. Moreover, it is also advantageous thermodynamically to roast a sulphide ore to oxide ore. Hence, the common practice is the roast sulphide ore to the oxide prior to reduction.
Ans10. Ores in which one of the component (either the impurity or the actual ore) is magnetic can be concentrated by magnetic separation method. For example, ores of iron like haematite, magnetite, siderite and iron pyrite can be concentrated by magnetic separation method.