Explain the purification of sulphide ore by froth floatation method.

Froth floatation is used for separating gangue from sulphide ores. In this method, a suspension of the powdered ore is prepared with water. To it, froth stabilizers and collectors are added. The Froth floatation method can be used to separate two sulphide ores by adjusting the proportion of oil to water or by using ‘depressants’. For instance, in the case of an ore containing zinc sulphide and lead sulphide, the depressant used is sodium cyanide. It selectively prevents zinc sulphide from coming to the froth but allows lead sulphide to come with the froth

Write down the chemical reactions taking place in the extraction of zinc from zinc blende.

The steps involved in the extraction of zinc from zinc blende are: (i)Concentration of ore: Firstly, zinc blende is crushed and the concentration of ore is done by froth floatation method. (ii)Roasting(Conversion to oxide): In this step, ZnS is roasted in the presence of an excess of air at about 1200 K in a furnace. This results in the formation of zinc oxide. 2 ZnS + 3 O 2 → 2 ZnO + 2 SO 2 (iii)Reduction(Extraction of Zn from ZnO ): Zinc oxide obtained after roasting is mixed with powdered coke and then heated to get zinc. ZnO + C → Zn + CO (iv)Electrolytic Refining: In electrolytic refining, impure zinc is taken as the anode while the cathode is a pure copper strip. The electrolyte is an acidified solution of zinc sulphate ZnSO 4 with dilute sulphuric acid. On passing current, electrolysis takes place in which pure zinc is deposited on the copper strip.

Explain zone refining.

The zone refining process is based on the principle that the impurities are more soluble in the molten state than in the solid-state of the metal. A circular mobile heater is fixed at one end of the impure metal rod. The molten zone moves along with the heater which is moved forward as the heater moves forward, the pure metal crystallizes out of the melt and therefore the impurities move into the adjacent molten zone. This process is repeated multiple times and the heater is moved along the same direction. At one end, impurities get concentrated and this end is removed. This technique is extremely useful for producing semiconductors and other metals of very high purity such as silicon, germanium, boron, etc.

Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?

∆ f G ∘ of Cu 2 S is more than that of hydrogen sulphide and carbon disulphide CS 2 . Therefore, coke and H 2 will not be able to reduce Cu 2 S . Whereas ∆ f G ∘ of Cu 2 O is lower than carbon monoxide CO . Therefore, C can reduce copper (I) oxide to copper. C s + Cu 2 O s → 2 Cu s + CO g Therefore, it is more difficult to extract copper from copper pyrites than it is to extract from its oxide ore.