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Wednesday, April 13, 2022

Vapour phase refining : Mond's process:

Extraction of Nickel (Mond's process) : Nickel is extracted from sulfide ore by roasting followed by reduction with carbon, but the
process is complicated by the fact that nickel is found in association with other metals. The refining is rather unusual, for nickel forms a complex with carbon monoxide
tetracarbonylnickel(O) [Ni(CO)4]. This substance is molecular in molecular in structure and readily volatilized (boiling point 43ºC). It is made by heating nickel powder to 50ºC, in a stream of CO and then decomposed at 200ºC. Any impurity in the nickel sample remains in the solid state and the gas is heated to 230ºC, when it decomposes, giving pure metal and CO, which is recycled. Ni(CO)4 is gaseous and may be produced by warming nickel with
CO at 50ºC.

The sequence of reaction is

 H2O(g) + C +-> CO(g) + H2

Ni(s) + 4 CO(s)-->(50ºC) [Ni(CO4)](g)
[Ni (CO)4](g) (200ºC )--> Ni + 4CO(g)

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Electrolytic refining process : used for Cu , Ni and Al :

Some metals such as Cu, Ni, and AI are refined electrolytically. 

(1) The Hooper process is a process for the electrolytic refining of aluminum. Impure AI forms the anode and pure AI forms the cathode of the Hooper's cell which contains three liquid layers. The bottom layer is molten impure AI, the middle is a fused salt layer containing aluminum fluoride, and the top layer is pure AI. At the anode (bottom layer), AI passes with solution as aluminum ion (AI3+), and at the cathode (top layer), these ions are reduced to the pure metal. In operation, molten metal is added to the bottom of the cell and pure aluminum is drawn off the top.

At anode : AI --> AI3+ + 3e–
At cathode : AI3+ + 3e– --> AI


(2) Copper obtained from the reduction of ores must be purified for use in making electrical wiring
because impurities increase its electrical resistance. The method used is electro-refining. Impure Cu obtained from ores is converted to pure Cu in an electrolyte cell that the impure copper as the anode an pure copper as the cathode. 
The electrolyte is an aqueous solution of CuSO4. At the impure Cu anode, Cu is oxidized along with the more easily oxidized metallic impurities such as Zn and Fe. The less easily oxidized impurities such as Ag, Au, and Pt fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions get reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution.

Anode (oxidation) : 
M (s) --> M2+ (aq) + 2e– (M = Cu, Zn, Fe)
Cathode (reduction) :
 Cu2+ (aq) + 2e– --> Cu(s)

Thus, the net cell reaction simply involves transfer of Cu metal from the impure anode to the pure
cathode, Cu obtained by this process is 99.95% pure.

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Parting process : removal of Zn and Pb from Gold which is obtained by MacArthur-Forrest cyanide process:

Crude gold obtained by MacArthur-Forrest cyanide process and chlorination process contains Ag, Cu, Zn, and sometimes Pb as impurity. 
(A) Zn and Pb are removed by cupellation process. 
(B) Cu and Ag are removed by parting process.

(1) Parting with sulfuric acid or nitric acid:Gold is not attacked by these acids while Cu and Ag dissolve. If, however, the Au content in an impure sample is more than 30%, the Cu and Ag are also not attacked by the acid of any strength. Hence, before the acid treatment, the impure sample is melted with necessary amount of Ag to reduce its gold content to about 25% (quartation). The resulting alloy, after being granulated in water, is boiled with H2SO4 or nitric acid when Cu and Ag pass into solution, leaving Au undissolved . Au is separated and
fused again with borax and nitre when 100% Au is obtained.

(2) Parting with CI2 : Sometimes chlorine is used for the purification of Au. The impure sample of Au is fused with borax and CI2 gas is forced through it. The base metals are converted into chlorides that pass out as fumes at this high temperature, and AgCI forms a separate layer between the fused layer of Au and borax, which is skimmed off and the Au left behind cast into ingots.




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