State the principle involved in the following metallurgical processes :

(a) Froth Floatation

(b) Zone Refining

(c) Vapour Phase Refining

OR

Give one point of difference between the following :

(a) Cast Iron and Pig Iron

(b) Hydraulic Washing and Liquation

(c) Leaching and Roasting

(a) Froth floatation method of concentration of a sulphide ore.

Sulphide ores are preferentially wetted by oils while the gangue is preferentially wetted by water .The ore particles get adsorbed on oil droplets and come to the surface where they can be collected as froth gangue is wetted by water and gets settled down. Eg. It is done for PbS ores.

Depressants help to separate two sulphide ores by selective prevention of froth formation by one ore and allowing the other to come into froth. NaCN is used as a depressantin order to separate two sulphide ores (ZnS and PbS).

(b) Metals like Germanium are purified using Zone refining. It is based on the principle that the impurities are more soluble in the molten state than in the solid state of metal.

In the zone refining process, a circular movable heater is fixed at one end of a metal rod which is made of the impure metal. Now, this heater is moved slowly across the metal rod,due to which, the metallic impurities melt temporarilyon this heater. The melt containing the impurities moves forward along with the heater as the metal rod progresses further. As the heater moves along the rod, the pure metal is left to solidify. As discussed, the impurities are more soluble in their melt state, so the concentration of the impurities in the melt increases as the heater moves forward. Finally, the impurities are accumulated at one end of the metal rod. This process is done several times. The accumulated impurities are removed, leaving behind the pure metal.

(c) The basic principle behind vapour phase refining of metal ores is that first the impure metal forms a volatile compound with a suitable reagent. Next, the volatile compound is easily decomposed on heating giving the pure metal.

Some examples involving vapour phase refining are:

• Nickel is refined by Mond’s process. Here, in this process, nickel is heated in a steam of carbon monoxide (CO) to form a volatile compound, nickel carbonyl Ni(CO)₄. Then, nickel carbonyl vapours on further heating, decomposes thereby giving pure nickel.

Mond’s process

• Vapour phase refining of titanium metal.

In this method, first, the metal titanium is converted into its volatile compound (like its iodide) and collected elsewhere. Next, it is then decomposed to give pure metal.

OR

(a) Cast Iron and Pig Iron

(b) Hydraulic Washing and Liquation

(c) Leaching and Roasting

Reaction involving roasting: