6 isolation metallurgy

8/9/2019 6 Isolation Metallurgy

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Question 6.1:

Copper can be extracted by hydrometallurgy but not zinc. Explain.

• Answer

The reduction potentials of zinc and iron are lower than that of copper. Inhydrometallurgy, zinc and iron can be used to displace copper from their

solution.

But to displace zinc, more reactive metals i.e., metals having lower reduction

potentials than zinc such as g, Ca, !, etc. are re"uired. But all these metals

react with water with the evolution of #$ gas.

%s a result, these metals cannot be used in hydrometallurgy to extract zinc.

#ence, copper can be extracted by hydrometallurgy but not zinc.

Question 6.2:

&hat is the role of depressant in froth floatation process'

•

Answer

In the froth floatation process, the role of the depressants is to separate two

sulphide ores by selectively preventing one ore from forming froth. (or

example, to separate two sulphide ores )*n+ and bs-, aC is used as a

depressant which selectively allows b+ to come with froth, but prevents *n+from coming to froth. This happens because aC reacts with *n+ to form

a$/*n)C-01.

Question 6.3:

&hy is the extraction of copper from pyrites more difficult than that from its

oxide ore through reduction'

• Answer

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The 2ibbs free energy of formation )3f 2- of Cu$+ is less than that of and

. Therefore, #$ and C cannot reduce Cu$+ to Cu.

4n the other hand, the 2ibbs free energy of formation of is greater

than that of . #ence, C can reduce Cu$4 to Cu.

#ence, the extraction of copper from its pyrite ore is difficult than from its

oxide ore through reduction.

Question 6.4:

Explain5 (i) *one refining (ii) Column chromatography.

• Answer

(i) Zone refining:

This method is based on the principle that impurities are more soluble in the

molten state of metal )the melt- than in the solid state. In the process ofzone refining, a circular mobile heater is fixed at one end of a rod of impure

metal. %s the heater moves, the molten zone of the rod also moves with it.

%s a result, pure metal crystallizes out of the melt and the impurities pass

onto the ad6acent molten zone. This process is repeated several times, which

leads to the segregation of impurities at one end of the rod. Then, the endwith the impurities is cut off. +ilicon, boron, gallium, indium etc. can be

purified by this process.

(ii) Column chromatography:

Column chromatography is a techni"ue used to separate differentcomponents of a mixture. It is a very useful techni"ue used for the

purification of elements available in minute "uantities. It is also used to

remove the impurities that are not very different in chemical properties from

the element to be purified. Chromatography is based on the principle thatdifferent components of a mixture are differently adsorbed on an adsorbent.

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In chromatography, there are two phases5 mobile phase and stationary

phase. The stationary phase is immobile and immiscible. %l$47 column isusually used as the stationary phase in column chromatography. The mobile

phase may be a gas, li"uid, or supercritical fluid in which the sample extract

is dissolved. Then, the mobile phase is forced to move through the stationary

phase. The component that is more strongly adsorbed on the column ta8es alonger time to travel through it than the component that is wea8ly adsorbed.

The adsorbed components are then removed )eluted- using a suitable solvent

)eluant-.

Question 6.5:

4ut of C and C4, which is a better reducing agent at 9:7 !'

• Answer

%t 9:7 !, the value of is less than that of . Therefore, C4 can

be reduced more easily to C4$ than C to C4. #ence, C4 is a better reducing

agent than C at 9:7 !.

Question 6.6:

ame the common elements present in the anode mud in electrolytic refining

of copper. &hy are they so present '

• Answer

In electrolytic refining of copper, the common elements present in anode

mud are selenium, tellurium, silver, gold, platinum, and antimony.

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These elements are very less reactive and are not affected during the

purification process. #ence, they settle down below the anode as anode mud.

Question 6.7:

&rite down the reactions ta8ing place in different zones in the blast furnace

during the extraction of iron.

• Answer

;uring the extraction of iron, the reduction of iron oxides ta8es place in the

blast furnace. In this process, hot air is blown from the bottom of the furnaceand co8e is burnt to raise the temperature up to $$


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Question 6.8:

&rite chemical reactions ta8ing place in the extraction of zinc from zinc

blende.

• Answer

The different steps involved in the extraction of zinc from zinc blende )*n+-

are given below5

(i) Concentration of ore

(irst, the gangue from zinc blende is removed by the froth floatation method.

(ii) Conversion to oxide (Roasting)

+ulphide ore is converted into oxide by the process of roasting. In this

process, *n+ is heated in a regular supply of air in a furnace at a

temperature, which is below the melting point of *n.

(iii) Extraction of zinc from zinc oxide (Reduction)

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*inc is extracted from zinc oxide by the process of reduction. The reduction

of zinc oxide is carried out by mixing it with powdered co8e and then, heatingit at 9:7 !.

(iv) Electrolytic Refining

*inc can be refined by the process of electrolytic refining. In this process,

impure zinc is made the anode while a pure copper strip is made the

cathode. The electrolyte used is an acidified solution of zinc sulphate

)*n+40-. Electrolysis results in the transfer of zinc in pure from the anode tothe cathode.

Question 6.9:

+tate the role of silica in the metallurgy of copper.

• Answer

;uring the roasting of pyrite ore, a mixture of (e4 and Cu$4 is obtained.

The role of silica in the metallurgy of copper is to remove the iron oxide

obtained during the process of roasting as slag. If the sulphide ore of copper

contains iron, then silica )+i4$- is added as flux before roasting. Then, (e4combines with silica to form iron silicate, (e+i47 )slag-.

Question 6.10:

&hat is meant by the term Dchromatography'

• Answer

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Chromatography is a collective term used for a family of laboratory

techni"ues for the separation of mixtures. The term is derived from 2ree8words chroma meaning colour and graphein meaning to write.

Chromatographic techni"ues are based on the principle that different

components are absorbed differently on an absorbent. There are several

chromatographic techni"ues such as paper chromatography, columnchromatography, gas chromatography, etc.

Question 6.11:

&hat criterion is followed for the selection of the stationary phase in

chromatography'

• Answer

The stationary phase is selected in such a way that the components of the

sample have different solubilitys in the phase. #ence, different components

have different rates of movement through the stationary phase and as a

result, can be separated from each other.

Question 6.12:

;escribe a method for refining nic8el.

• Answer

ic8el is refined by onds process. In this process, nic8el is heated in thepresence of carbon monoxide to form nic8el tetracarbonyl, which is a volatile

complex.

Then, the obtained nic8el tetracarbonyl is decomposed by sub6ecting it to a

higher temperature )0=< > 0:< !- to obtain pure nic8el metal.

Question 6.13:

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#ow can you separate alumina from silica in bauxite ore associated with

silica' 2ive e"uations, if any.

• Answer

To separate alumina from silica in bauxite ore associated with silica, first the

powdered ore is digested with a concentrated a4# solution at 0:7 > =$7 !and 7= > 79 bar pressure. This results in the leaching out of alumina )%l$47-

as sodium aluminate and silica )+i4$- as sodium silicate leaving the

impurities behind.

Then, C4$ gas is passed through the resulting solution to neutralize the

aluminate in the solution, which results in the precipitation of hydratedalumina. To induce precipitation, the solution is seeded with freshly prepared

samples of hydrated alumina.

;uring this process, sodium silicate remains in the solution. The obtained

hydrated alumina is filtered, dried, and heated to get bac8 pure alumina.

Question 6.14:

2iving examples, differentiate between roasting and calcination.

• Answer

Foasting is the process of converting sulphide ores to oxides by heating theores in a regular supply of air at a temperature below the melting point of

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the metal. (or example, sulphide ores of *n, b, and Cu are converted to

their respective oxides by this process.

4n the other hand, calcination is the process of converting hydroxide and

carbonate ores to oxides by heating the ores either in the absence or in a

limited supply of air at a temperature below the melting point of the metal.

This process causes the escaping of volatile matter leaving behind the metaloxide. (or example, hydroxide of (e, carbonates of *n, Ca, g are converted

to their respective oxides by this process.

Question 6.15:

#ow is cast iron different from pig iron'

• Answer

The iron obtained from blast furnaces is 8nown as pig iron. It contains

around 0G carbon and many impurities such as +, , +i, n in smaller

amounts.

Cast iron is obtained by melting pig iron and co8e using a hot air blast. It

contains a lower amount of carbon )7G- than pig iron. Hnli8e pig iron, cast

iron is extremely hard and brittle.

Question 6.16:

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;ifferentiate between Dminerals and Dores.

Answer

inerals are naturally occurring chemical substances containing metals. Theyare found in the Earths crust and are obtained by mining.

4res are roc8s and minerals viable to be used as a source of metal.

(or example, there are many minerals containing zinc, but zinc cannot be

extracted profitably )conveniently and economically- from all these minerals.

*inc can be obtained from zinc blende )*n+-, calamine )*nC47-, *incite

)*n4- etc.

Thus, these minerals are called ores of zinc.

Question 6.17:

&hy copper matte is put in silica lined converter'

Answer

Copper matte contains Cu$+ and (e+. Copper matte is put in a silicalined

converter to remove the remaining (e4 and (e+ present in the matte as slag)(e+i47-. %lso, some silica is added to the silicalined converter. Then, a hot

air blast is blown. %s a result, the remaining (e+ and (e4 are converted toiron silicate )(e+i47- and Cu$+ is converted into metallic copper.

Question 6.18:What is the role of cryolite in the metallurgy of aluminium?• Answer

Cryolite )a7%l(9- has two roles in the metallurgy of aluminium5

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1. To decrease the melting point of the mixture from $7$7 ! to AA0< !.

. To increase the electrical conductivity of %l$47.

Question 6.19:

#ow is leaching carried out in case of low grade copper ores'

• Answer In case of low grade copper ores, leaching is carried out using acid or

bacteria in the presence of air. In this process, copper goes into the solution as Cu2+ ions.

The resulting solution is treated with scrap iron or #$ to get metallic copper.

Question 6.20:

&hy is zinc not extracted from zinc oxide through reduction using C4'

• Answer The standard Gibbs free energy of formation of n! from n

is lower than that of C4$ from C4. Therefore, C4 cannot reduce *n4 to *n.

#ence, *n is not extracted from *n4 through reduction using C4.

Question 6.21:

The value of for formation of Cr$47 is > =0< 8Jmol>A and that of %l$ 47 is >

?$: 8Jmol>A. Is the reduction of Cr$47 possible with %l'

• Answer

The value of for the formation of Cr$47 from Cr )>=0< 8Jmol>A- is higher

than that of %l$47 from %l )>?$: 8Jmol>A-. Therefore, %l can reduce Cr$47 to

Cr. #ence, the reduction of Cr$47 with %l is possible.

%lternatively,

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+ubtracting e"uation )ii- from )i-, we have

%s for the reduction reaction of Cr$47 by %l is negative, this reaction ispossible.

Question 6.22:

4ut of C and C4, which is a better reducing agent for *n4 '

• Answer

Feduction of *n4 to *n is usually carried out at A9:7 !. (rom the above

figure, it can be observed that above A


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thermodynamic factor. #ow far do you agree with this statement' +upport

your opinion with two examples.

• Answer

The above figure is a plot of 2ibbs energy vs. T for formation of some

oxides.

It can be observed from the above graph that a metal can reduce the oxide

of other metals, if the standard free energy of formation of the oxide of

the former is more negative than the latter. (or example, since ismore negative than , %l can reduce Cu$4 to Cu, but Cu cannotreduce %l$47. +imilarly, g can reduce *n4 to *n, but *n cannot reduce g4

because is more negative than .

Question 6.24: "ame the processes from which chlorine is obtained as a by#product.

What

will happen if an a"ueous solution of aCl is sub6ected to electrolysis'

• Answer

In the electrolysis of molten aCl, Cl$ is obtained at the anode as a by

product.

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%t cathode5

%t anode5

The overall reaction is as follows5

If an a"ueous solution of aCl is electrolyzed, Cl$ will be obtained at the

anode but at the cathode, #$ will be obtained )instead of a-. This is because

the standard reduction potential of a )EKL > $.:A M- is more negative than

that of #$4 )EK L >


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If a metal is used instead of graphite as the anode, then 4$ will be liberated.

This will not only oxidise the metal of the electrode, but also convert some ofthe %l liberated at the cathode bac8 into %l$47. #ence, graphite is used for

preventing the formation of 4$ at the anode. oreover, graphite is cheaper

than other metals.

Question 6.27:

4utline the principles of refining of metals by the following methods5

(i) *one refining

(ii) Electrolytic refining

(iii) Mapour phase refining

• Answer

(i) Zone refining:

This method is based on the principle that impurities are more soluble in themolten state of metal )the melt- than in the solid state. In the process of

zone refining, a circular mobile heater is fixed at one end of a rod of impure

metal. %s the heater moves, the molten zone of the rod also moves along

with it. %s a result, pure metal crystallizes out of the melt and the impuritiespass to the ad6acent molten zone. This process is repeated several times,

which leads to the segregation of impurities at one end of the rod. Then, the

end with the impurities is cut off. +ilicon, boron, gallium, indium etc. can bepurified by this process.

(ii) Electrolytic refining!

Electrolytic refining is the process of refining impure metals by using

electricity. In this process, impure metal is made the anode and a strip of

pure metal is made the cathode. % solution of a soluble salt of the samemetal is ta8en as the electrolyte. &hen an electric current is passed, metal

ions from the electrolyte are deposited at the cathode as pure metal and the

impure metal from the anode dissolves into the electrolyte in the form of

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ions. The impurities present in the impure metal gets collected below the

anode. This is 8nown as anode mud.

(iii) "apour phase refining

Mapour phase refining is the process of refining metal by converting it into its

volatile compound and then, decomposing it to obtain a pure metal. To carry

out this process,

(i) the metal should form a volatile compound with an available reagent, and

(ii) the volatile compound should be easily decomposable so that the metal

can be easily recovered.

ic8el, zirconium, and titanium are refined using this method.

Question 6.28:

redict conditions under which %l might be expected to reduce g4.

• Answer

%bove A7=

6 isolation metallurgy

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