Zone refining and Mond's process.

Zone refining.

Zone refining is used to get metal in the ultrapure state. This method is based on the principle that when impure molten metal is allowed to cool down the pure metal is crystalline out (solidify) first while the impurities present in the metal remains in the liquid phase.

In this method, the impure metal is placed in a pipe and the circular heater is fitted at one end of melt impure metal over that region.Then the heated is move to another portion to cool, the pure metal crystallises out but the impurities remains in a liquid phase and passed into the adjacent molten zone. In this way, the impurities are swept away to another end which is finally discarded.The process is repeated to get metal in the highly pure state.This method is used to purify Ga, Ge and Si.

Or in another language zone refining can be defined as the method of zone refining is used to obtain metals of very high purity. It is based on the principle that the impurity has the greatest solubility in a liquid than in the solid state of the metal.

A circular mobile heater around the rod of an impure metal at one end and is slowly moved along the rod. At the heated zone, the rod melts.As the heated moves forward the pure metal crystallises while the impurities pass into the adjacent molten zone. The process is repeated several times when the impurities are completely swept into the end of the rod which is finally discarded.Semi-conductors, silicon germanium, gallium and indium are purified by this method.

Mond’s process.

Mond’s process is used to extract Ni from (Matte CuS+NiS) and to purify it .Matte is first roasted in the process of acid to get oxides of Cu and Ni.

$$CuS+NiS\xrightarrow{H^+}CuO+NiO+SO_2↑$$

A mixture of CuO and NiO is treated with Conc.H₂SO_4.When only CuO dissolve while NiO remains as such

$$NiO+CuO+H_2SO_4→CuSO_4+H_2O$$

$$NiO=inert$$

$$CuSO_4=soluble$$

Ni is separated and then reduced with water gas to obtain impure Ni.

$$NiO+H_2O+CO→Ni+H_2O+CO_2$$

$$Ni-impure$$

Impure Ni is then treated with CO to obtain Ni(CO)4

$$Ni+4CO→Ni(CO)_4$$

Ni(CO)₄ is volatile and exist as the gas which is circulated into another vessel. Finally Ni(CO)₄ is heated at 160oC when it decomposes to give Ni which is 99.99% purity.

$$NiCO_4 \overset{160^{\circ}{c}}{\rightarrow}Ni+4CO$$

$$Ni=Pure$$

Extraction of Nickel.

This process is useful for the extraction of Nickel in the purest form and is extracted from the natural ore pentlandite (Cus.Nis).

The extraction process involved the following chemical method.

The ore is first treated with the excess of air reroasting of ore which involved the oxidation the FeS into FeO which is removed from the mixture by the formation of slag with SiO₂.

$$[NiS.CuS.FeS+O_2\xrightarrow{Roasting}FeO+(CuS+NiS)$$

$$O_2=excess$$

$$(CuS+NiS)=Residue$$

$$FeO+SiO_2→FeSio_3$$

$$FeSio_3=slag$$

The residue of NiS and CuS is called as the mantle. The pure nickel can be extracted from the mantle by mind's process. The residue of NiS and CuS is now roasted in presence of air with ConC.H₂SO₄ so that NiS and CuS get oxidised

$$[CuS+NiS]+O_2\xrightarrow{Conc\,H_2SO_4+O_2}CuO+NiO+SO_2↑$$

The residue of oxides is now treated with dilute H2So4 where the CuO form CuSO4 and Ni remains unattached.

$$CuO+dil.H_2SO_4→CuSO_4+H_2O$$

$$NiO+dil.H_2SO_4→No \,reaction$$

The residue NiO is treated with the water gas (CO+H₂) where NiO reduced into the crude nickel.

$$2Nio+(CO+H_2)→2Ni+CO_2+H_2O$$

$$(CO+H_2)=water\,gas$$

$$2Ni=Crude\,nickel$$

The crude nickel is then treated with carbon monoxide at 50^oC and forms nickel tetracarbonyl [Ni(CO)₄] which on decomposition at about 180^oC to obtain the pure nickel of about 99.9%.

$$Ni+4CO\overset{50^{\circ}{c}}{\rightarrow}NiCO_4\overset{180^{\circ}{c}}{\rightarrow}Ni+4CO$$

$$Ni=crude$$

$$Ni+4CO=pure\,99.9%$$

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.