general principles of isolation of metals a short note

8/17/2019 General Principles of isolation of metals A SHORT NOTE

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General Principles & Processes of Isolation of Metals• Types of Ores:

Ores may be divided into four groups

1. Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are

usually formed in the company of rock or alluvial impurities like clay, sand etc.

2. Oxidised Ores:These ores consist of oxides or oxysalts eg. carbonates, phosphate! and

silicate of metal. "mportant oxide ore includes, #e2O$, %l2O$.2&2O etc. and important

cabonate ores are limestone 'a'O$!, 'alamine (n'O$! etc.

$. Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc.

)xamples are iron pyrites #eS2!. galena *bS!, 'innabar &gS!

+. Halide ores: etallic halides are very fe- in nature. 'hlorides are most common

examples include horn silver %g'l! carnallite 'l. g'l2./&2O and fluorspar 'a#2!

etc.

• Metallurgy:

"t is the process of extracting a metal from its ores. The follo-ing operations are carried out for

obtaining the metal in the pure form.

a! 'rushing of the ore

b! 0ressing or concentration of the ore.

c! eduction of metal.

d! *urification or refining of the metal

• Concentration:

a) Physical Method :

Gravity separation: The po-dered ores is agitated -ith -ater or -ashed -ith a running stream

of -ater. The heavy ore particles of sand, clay etc. are -ashed a-ay.

Froth Floatation Process: The finely divided ore is introduced into -ater containing small

uantity of oil e.g. *ine Oil!. The mixture is agitated violently -ith air a froth is formed -hich


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carries a-ay along -ith it the metallic particles on account of the surface tension forces. The

froth is transferred to another bath -here gangue3free ore settles do-n.

Electro Magnetic Separator:. % magnetic separator consists of a belt moving over t-o rollers,

one of -hich is magnetic. The po-dered ore is dropped on the belt at the other end. agnetic

portion of the ore is attracted by the magnetic roller and falls near to the roller -hile the non3

magnetic impurity falls farther off

) !he"ical Methods

!alcination: 'arbonate or hydrated oxide ores are sub4ected to the action of heat in order of

expel -ater from hydrated oxide and carbon dioxide from a carbonate.

Examples:

(n'O$ 335 (nO 6 'O2


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'a'O$ 335 'aO 6 'O2

%l2O$72&2O 335 %l2O$ 6 2&2O

2#e2O$7$&2O 335 2#e2O$ 6 $&2O

#oasting: Sulphide ores either are sub4ected to the action of heat and air at temperatures belo-

their melting points in order to bring about chemical changes in them.

)xamples:

2*bS 6 $O2 335 2*bO 6 2SO2

*bS 6 2O2 335 *bSO+

2(nS 6 $O2 335 2(nO 6 2SO2

(nS 6 2O2 335 (nSO+

'uS 6 2O2 335 'uSO+

2'u2S 6 $O2 335 2'u2O 6 2SO2

$eaching: "t involves the treatment of the ore -ith a suitable reagent as to make it soluble -hile

impurities remain insoluble. The ore is recovered from the solution by suitable chemical

method.

%l2O$ 6 28aO& 3352 8a%lO2 6 &2O

• Reduction of Free Metal:

a)S"elting: eduction of a metal from its ore by a process involving melting

Several reducing agents such as sodium, magnesium and aluminium are used for reduction.

The calcinated or roasted ore is mixed -ith carbon coal or coke! and heated in a reverberatory

or a blast furnace.

'arbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to

the metal.


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) Flux:

The ores even after concentration contain some earthy matter called gangue -hich is heated

combine -ith this earthy matter to form an easily fusible material. Such a substance is kno-n as

flux and the fusible material formed during reduction process is called slag.

%cidic &luxes like silica, borax etc are used -hen the gangue is basic such as lime or other

metallic oxides like nO, #eO, etc

'asic &luxes like 'aO, lime stone 'a'O$!, magnesite g'O$!, haematite #e2O$! etc are used

-hen the gangue is acidic like silica, *+O19 etc.

• Rening:

The metals obtained by the application of above reduction methods from the concentration ores

are usually impure. The impure metal is thus sub4ected to some purifying process kno-n as

refining in order to remove undesired impurities. arious process for this are

a! ;iuation process b! 0istillation process

c! 'upellation d! *oling

e! )lecrolytic refining f! ?=&3TS

or =>9 ?3T ln


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%n element % can reduce element < if => value for oxidation of % to %O is lo-er than the =>

value for oxidation of < to %@%O! A =>


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) Puri&ication o& 'auxite

c) Electrolysis o& &used pure alu"ina Hall * Her+lt Method)

The addition of cryolite 8a$%l#/! and fluorspar 'a#2! makes alumina a good conductor of

electricity and lo-ers its #usion temperature from 2$2$ to 11+9 . the reaction taking place

during electrolysis.


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) #e&ining o& %lu"iniu":

The graphite rods dipped in pure aluminium and 'uD%l alloy rod at the bottom in the impure

aluminium -ork as conductors. On electrolysis, aluminium is deposited at cathode from the

middle layer and euivalent amount of aluminium is taken up by the middle layer from the

bottom layer impure aluminium!. Therefore, aluminium is transferred from bottom to the top

layer through middle layer -hile the impurities are left behind. %luminium thus obtained is

EE.EBF pure.

• #yrdrometallurgy $sol%ent e!traction

Solvent extraction is the latest separation techniue and has become popular because of its

elegance, simplicity and speed. The method is based on preferential solubility principles.

Solvent or liuid3liuid extraction is based on the principle that a solute can distribute itself in acertain ratio bet-een t-o immiscible solvents, one of -hich is usually -ater and the other an

organic solvent such as benGene, carbon tetrachloride or chloroform. "n certain cases, the solute

can be more or less completely transferred into the organic phase.

• !traction of Iron:

a) ("portant Ores o& (ron:

&aematite #e2O$ red oxide of iron!

;imonite #e2O$7$&2O hydrated oxide of iron!

agnetite #e$O+ magnetic oxide of iron!


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) Extraction o& !ast (ron:

• !traction of Copper:


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a) Ores o& !opper:

'opper glance chalcocite! : 'u2S

'opper pyrites 'halopyrites!: 'u#eS2

alchite : 'uO&!2 7'u'O$

'uprite or uby copper: 'u2O

%Gurite : 'uO&!272'u'O$

• Rening of metals:


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1. ,one re&ining Fractional crystalli-ation) This method is employed for preparing

extremely pure metals. This method is based upon the principle that -hen a molten

solution of the impure metal is allo-ed to cool, the pure metal crystilises out -hile the

impurities remain in the melt.

2. Electrore&ining: "n this method, the impure metal is converted into a block -hich forms

the anode -hile cathode is a rod or plate of pure metal. These electrodes are suspended in

an electrolyte -hich is the solution of a soluble salt of the metal usually a double salt of

the metal. Hhen electric current is passed, metal ions from the electrolyte are deposited at

the cathode in the form of pure metal -hile an euivalent amount of metal dissolves from

the anode and goes into the electrolyte solution as metal ion. The soluble impurities

present in the crude metal anode go into the solution -hile the insoluble impurities settle

do-n belo- the anode as anode mud.

$. /an%r0el Method: "n this method, the metal is converted into it volatile unstable

compound such as iodide leaving behind the impurities. The unstable compound thus

formed is decomposed to get the pure metal.

+. !upellation and Poling are used for refining of metals, cupellation is contain impurities

of other metals -ith traces of lead are removed from silver by heating impure silver -ith

a blast of air in a cupel an oval shaped pan made up of bone ash! in -hich lead is

oxidised to lead oxide *bO! -hich being volatile escapes leaving behind puresilver.*oling is used for refining of such metals -hich contain impurities of its o-n

oxide. "n this process, the molten impure metal is stored -ith green -ooden poles. %t the

high temperature of the molten metal, -ood liberates methane -hich reduces the oxide of

the metal to free metal.


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Electro Magnetic Separator:. % magnetic separator consists of a belt moving over t-o rollers,

one of -hich is magnetic. The po-dered ore is dropped on the belt at the other end. agnetic portion of the ore is attracted by the magnetic roller and falls near to the roller -hile the non3


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magnetic impurity falls farther off

) !he"ical Methods

!alcination: 'arbonate or hydrated oxide ores are sub4ected to the action of heat in order of

expel -ater from hydrated oxide and carbon dioxide from a carbonate.

Examples:

(n'O$ 335 (nO 6 'O2

'a'O$ 335 'aO 6 'O2

%l2O$72&2O 335 %l2O$ 6 2&2O

2#e2O$7$&2O 335 2#e2O$ 6 $&2O

#oasting: Sulphide ores either are sub4ected to the action of heat and air at temperatures belo-

their melting points in order to bring about chemical changes in them.

)xamples:

2*bS 6 $O2 335 2*bO 6 2SO2

*bS 6 2O2 335 *bSO+

2(nS 6 $O2 335 2(nO 6 2SO2

(nS 6 2O2 335 (nSO+

'uS 6 2O2 335 'uSO+

2'u2S 6 $O2 335 2'u2O 6 2SO2


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$eaching: "t involves the treatment of the ore -ith a suitable reagent as to make it soluble -hile

impurities remain insoluble. The ore is recovered from the solution by suitable chemical

method.

%l2O$ 6 28aO& 3352 8a%lO2 6 &2O

#eduction o& Free Metal:

a)S"elting: eduction of a metal from its ore by a process involving melting

Several reducing agents such as sodium, magnesium and aluminium are used for reduction.

The calcinated or roasted ore is mixed -ith carbon coal or coke! and heated in a reverberatory

or a blast furnace.

'arbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to

the metal.

) Flux:

The ores even after concentration contain some earthy matter called gangue -hich is heated

combine -ith this earthy matter to form an easily fusible material. Such a substance is kno-n as

flux and the fusible material formed during reduction process is called slag.

%cidic &luxes like silica, borax etc are used -hen the gangue is basic such as lime or othermetallic oxides like nO, #eO, etc

'asic &luxes like 'aO, lime stone 'a'O$!, magnesite g'O$!, haematite #e2O$! etc are used

-hen the gangue is acidic like silica, *+O19 etc.

#e&ining:

The metals obtained by the application of above reduction methods from the concentration ores

are usually impure. The impure metal is thus sub4ected to some purifying process kno-n asrefining in order to remove undesired impurities. arious process for this are


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a! ;iuation process b! 0istillation process

c! 'upellation d! *oling

e! )lecrolytic refining f! ?=&3TS

or =>9 ?3T ln

%n element % can reduce element < if => value for oxidation of % to %O is lo-er than the =>

value for oxidation of < to %@%O! A =>


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+. 'orundum: %l2O$

C. %lumstone or %lunite: 2SO+7%l2SO+!$7+%lO&!$

) Puri&ication o& 'auxite

c) Electrolysis o& &used pure alu"ina Hall * Her+lt Method)

The addition of cryolite 8a$%l#/! and fluorspar 'a#2! makes alumina a good conductor ofelectricity and lo-ers its #usion temperature from 2$2$ to 11+9 . the reaction taking place

during electrolysis.


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) #e&ining o& %lu"iniu":

The graphite rods dipped in pure aluminium and 'uD%l alloy rod at the bottom in the impure

aluminium -ork as conductors. On electrolysis, aluminium is deposited at cathode from the

middle layer and euivalent amount of aluminium is taken up by the middle layer from the bottom layer impure aluminium!. Therefore, aluminium is transferred from bottom to the top

layer through middle layer -hile the impurities are left behind. %luminium thus obtained isEE.EBF pure.

Hyrdro"etallurgy solvent extraction)

Solvent extraction is the latest separation techniue and has become popular because of its

elegance, simplicity and speed. The method is based on preferential solubility principles.

Solvent or liuid3liuid extraction is based on the principle that a solute can distribute itself in a

certain ratio bet-een t-o immiscible solvents, one of -hich is usually -ater and the other anorganic solvent such as benGene, carbon tetrachloride or chloroform. "n certain cases, the solutecan be more or less completely transferred into the organic phase.

Extraction o& (ron:

a) ("portant Ores o& (ron:

&aematite #e2O$ red oxide of iron!

;imonite #e2O$7$&2O hydrated oxide of iron!

agnetite #e$O+ magnetic oxide of iron!


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) Extraction o& !ast (ron:


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Extraction o& !opper:

a) Ores o& !opper:

'opper glance chalcocite! : 'u2S

'opper pyrites 'halopyrites!: 'u#eS2alchite : 'uO&!2 7'u'O$'uprite or uby copper: 'u2O

%Gurite : 'uO&!272'u'O$

#e&ining o& "etals:


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1. ,one re&ining Fractional crystalli-ation) This method is employed for preparing

extremely pure metals. This method is based upon the principle that -hen a molten

solution of the impure metal is allo-ed to cool, the pure metal crystilises out -hile theimpurities remain in the melt.

2. Electrore&ining: "n this method, the impure metal is converted into a block -hich formsthe anode -hile cathode is a rod or plate of pure metal. These electrodes are suspended in

an electrolyte -hich is the solution of a soluble salt of the metal usually a double salt ofthe metal. Hhen electric current is passed, metal ions from the electrolyte are deposited at

the cathode in the form of pure metal -hile an euivalent amount of metal dissolves from

the anode and goes into the electrolyte solution as metal ion. The soluble impurities present in the crude metal anode go into the solution -hile the insoluble impurities settle

do-n belo- the anode as anode mud.

$. /an%r0el Method: "n this method, the metal is converted into it volatile unstable

compound such as iodide leaving behind the impurities. The unstable compound thus

formed is decomposed to get the pure metal.

+. !upellation and Poling are used for refining of metals, cupellation is contain impuritiesof other metals -ith traces of lead are removed from silver by heating impure silver -ith

a blast of air in a cupel an oval shaped pan made up of bone ash! in -hich lead is

oxidised to lead oxide *bO! -hich being volatile escapes leaving behind pure

silver.*oling is used for refining of such metals -hich contain impurities of its o-noxide. "n this process, the molten impure metal is stored -ith green -ooden poles. %t the

high temperature of the molten metal, -ood liberates methane -hich reduces the oxide of the metal to free metal.

general principles of isolation of metals a short note

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