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Wednesday, April 13, 2022

Polling process: This process is used for the purification of copper and tin.


(1) Purification of impure copper : Impure copper is remelted in a reverberatory furnace lined with SiO2 and a blast of O2 is blows into the furnace. O2 oxidises S, Sb and As to their respective oxides which, being, volatile, get volatilised and are thus removed. Fe is oxidised to FeO which forms a slag of FeSiO3
with SiO2 lining of the furnace. Molten copper left behind contains CuO as impurity. This molten copper is treated with powdered anthracite and then stirred with a pole of green wood. Green wood, at high temperature, liberates hydrocarbon gases, which are converted into methane (CH4). Methane thus obtained reduces CuO to free Cu–metal, which is about 99.5% pure and is called tough pitch copper.

Green wood--> Hydrocarbons--> CH4
4CuO + CH4 --> 4Cu (pure metal) + CO2 + 2H2O

(2) Purification of impure tin : Impure tin metal contains the impurities of Cu, Fe, W and
SnO2. The impurity of SnO2 is due to the incomplete reduction of tin stone ore (SnO2) during smelting. In order to remove these impurities, the impure molten tin metal is taken in a big pot and stirred with a pole of green wood. 
Green wood, at high temperature liberates hydrocarbon gases, which are converted into methane CH4. Methane thus obtained reduces SnO2 to pure metal while the impurities of Cu, Fe, W etc. come up to the surface, where they come in contact with air and are oxidised to their respective oxides.
The oxides form a scum on the surface of pure tin metal. This scum is removed from the surface. Tin metal obtained by this method is 99% pure. 
Green wood --> Hydrocarbon-->CH4
2SnO2 + CH4 --> 2Sn + CO2 + 2H2O


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Kroll's Process and Imperial metal industries (IMI) Process of reduction;

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Parke's Process : Desilverisation of lead ;

The removal of the impurities of Ag from the commercial lead is called desilverisation of lead and is done by Parke's process . Thus, Parke's process is the desilverisation of lead.

In Parke's process, the commercial lead, which contains Ag as impurities, is melted in iron pots and 1% of Zn is added to it. The molten mass is thoroughly agitated. Since Ag is about 300 times more soluble in Zn than in Pb, most of the Ag present in the commercial lead as impurity mixes with Zn, to form Zn–Ag alloy. When the whole is cooled, two layers are obtained. The upper layer contains Zn–Ag alloy in the solid state, while the lower layer has lead in the molten state.

This lead containsonly 0.0004% of Ag and hence is almost pure. Lead obtained after removing most of Ag from it (desilverisation of lead) by Parke's process, is called desilverised lead. This lead contains the impurities of metals like Zn, Au, Sb etc. These metal impurities are removed from desilverised lead by Bett's
electrolytic process.

Zn–Ag alloy, formed in the upper layer, is skimmed off from the surface of the molten lead by perforate ladles. This alloy contains lead as impurity. This impurity of Pb is removed from the alloy by liquation process, in which Zn–Ag alloy is heated in a slopping furnace, when the impurity of Pb melts and hence drains away from the solid alloy. Thus purified Zn–Ag is obtained. Now Ag can be obtained from this purified Zn–Ag alloy by distillation process, in which the alloy is heated strongly in presence of little carbon in a fire–clay retort. Zn, being more volatile, distills off while Ag remains in the retort, carbon used in the process reuses the oxide of Zn, if formed. Ag obtained from Zn–Ag alloy is contaminated with a little of Pb as impurity. This impurity of Pb placed in a cupel (cupel is a boat–shaped) dish made of bone ash which is porous in nature) in a reverberatory furnace and heated in the presence of air. By doing so, lead (impurity) is oxidised to PbO(litharge) which volatilises and pure Ag is left behind in the cupel. Last traces of PbO are absorbed by the porous mass of the cupel.

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Tuesday, March 29, 2022

What is thermite Welding or Aluminothermic process and it's application?

Cr2O3 is mixed with requisite amount of Al-powder (this mixture is called thermite mixture) and is placed in a large fire-clay crucible. An intimate mixture of Na2O2 or BaO2 and Mg powder (called ignition mixture or igniter) is placed in a small depression made in the thermite mixture. The crucible is surrounded by sand which prevents the loss of heat by radiation. A piece of Mg ribbon is struck into the ignition mixture and the charge is covered by a layer of fluorspar (CaF2) which acts as a heat insulator. Now Mg-ribbon is ignited so that ignition mixture catches fire and flame is produced, leading to a violent reaction between Mg and BaO2 with the evolution of large amount of heat.
Mg + BaO2 --> BaO + MgO + Heat
Heat produced in the above reaction makes Cr2O3 and AI-powder react together.
Cr2O3 + AI -->2Cr (l) + AI2O3
Molten Cr-metal formed settles down at the bottom of the crucible.
Application:
An application of aluminothermic process has been used for joining the broken pieces of iron (welding). In this process thermite mixture consisting of Fe2O3 and Al-powder in 3 : 1 ratio is placed in a funnel shaped crucible lined internally with magnesite and having a plug hole at its bottom. The thermite mixture is covered with a mixture of BaO2
 plus Mg-powder (ignition mixture) in which a piece of Mg ribbon is inserted. The ends of the iron pieces to be welded are thoroughly cleaned and surrounded by a fire-clay mould. When Mg ribbon
is ignited, ignition mixture catches fire and Fe2O3 gets reduced to Fe by Al-powder.

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Wednesday, March 16, 2022

HgCl2 does not give Chromyl Chloride test Why ?

The chromyl chloride test need heating a sample  containing chloride with potassium dichromate and concentrated sulfuric acid.

 If chloride is present, chromyl chloride is formed and red fumes of CrO2Cl2 are released  If there is no chloride present, no red fumes are produced. Chromyl Chloride not formed with fluorides, bromides, iodides and cyanides, it is only test specific for chlorides

Chromyl chloride test is given by only chloride salts which are infer whether there is halide predominantly  ionic while covalent chlorides such as HgCl2 do not generate Cl- ions thus won't give this test.

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