NaCI is leached with H2O. Ti is in the form of small granules. These can be fabricated into metal
parts using "powder forming" techniques and sintering in an inert atmosphere. Zr is also produced by Kroll's process.
Wednesday, April 13, 2022
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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