Methods of Refining of Metals | Chemistry for JEE Main & Advanced PDF Download

Step V: Purification or refining of crude metal :

(A) By physical Methods :

(a) liquation

(b) Distillation

(c) Zone refining

(B) By Chemical Methods

(a) Oxidation

(b) Poling

(c) Vapour Phase Refining

(C) By Electrolytic Refining

(A) By Physical Method :

(a) Liquation Method :

This method is used for refining of those metal which have very low m.p. in comparison to impurity present in them.

e.g. → Sn, Pb, Hg, Bi, Zn

(b) Distillation Method : It is used for refining of those metals which are volatile & hence it is used for refining of Zn, Cd, Hg (i.e., is of filled d orbital metal)

(c) Zone Refining : (Si, Ge, Pb, B, Ga, In)

Concept : Impurities are more soluble in the melt than in the solid state. (Fractional crystallization)

(B) By Chemical Methods :

(a) Oxidation : (Fe, Sn, Pb)

eg. Pig iron : M.P. 1080

Impurities : C, P, Mn, Si

This method is used for the refining of metals in which impurtities are more oxidisable than metal itself.

When impurities oxidise they are converted into either volatile oxides or non volatile oxides Non-volatile oxides are removed either by slag formation or by removing their scum Oxidation is known by various names : bessemerisation (Fe), Cupellation (Ag), Softening (Pb) or Puddling (for iron), tossing (for iron)

(b) Polling : When along with impurities the metal to be refine is also oxidised part then this method is used. In this method the molten impure metal is steared with green wood log, The hydrocarbons released from the log reduce metal oxide into metal while impurity oxides are not reduced. This method is used for refining of Cu & Tin. In both metals during poling iron get oxidised into FeO which in turn is oxidised into Fe₂O₃ while in case of Sn, SnO₂ is reduced to tin (Sn) & in case of Cu copper (I) oxide i.e., cuprous oxide is reduced to Cu,

Cu₂O → Cu

(c) Vapour Phase Refining : Impure metal is allowed to react with a suitable reagent such that a volatile unstable compound is formed & then the compound is decomposed to pure free metal when it is subjected to heat.

Mond's Process : used for refining of Ni 

(C) By Electrolytic Refining :

Anode : Impure metal

cathode : Pure metal

Electrolyte : Aq. salt sol. of metal/fused metal salt Acid

Anode Mud : Metals which are less electropositive than the metal to be refined.

In electrolyte : More electropositive metals are found.

eg. in Cu : Ag, Au, Fe, Zn

1. Gold :

Occurrence :

Found mostly in free state in quartz veins called auriferous quartz.

Extraction of gold from native ore :

(A) Crushing and concentration :

The gold ore is crushed, powdered finely and concentrated by washing with water.

(B) (Treatment with 0.25-1% sodium cyanide or potassium cyanide solution) Extraction of Au, from Native ore by Mac-Arthur forest cyanide process.

4Au + 8NaCN + 2H₂O O₂ → 4Na[Au(CN)₂] + 4NaOH

                                                      soluble

2Na[Au(CN)₂] + Zn → Na₂[Zn(CN)₂] + 2Au¯

                                         (Impure)

Impure Au is purified by Electrolytic refining method or by amalgamation.

 (C) Electrolytic refining method :

Anode : Impure Au

Cathode : Pure Au

Electrolyte : 4% AuCl₃ solution acidified with 10% HCl

Purple of Cassius : It is of colloidal gold solution :

2AuCl₃ + 3SnCl₂ → 2Au + 3SnCl₄

                                            (Very dil.)

The gold thus precipitated is absorbed by Sn (OH)₄ formed by hydrolysis of SnCl₄

SnCl₄ + 4H₂O → Sn(OH)₄ + 4HCl

This form of gold is purple in colour named after its discoverer, Cassius.

2. Silver :

Ores : (i) Silver glance or argentite Ag₂S (main ore)

(ii) Ruby silver or pyrargyrite 3Ag₂S. Sb₂S₃

(iii) Horn silver or chlorargyrite AgCl.

Extraction of silver from silver glance :

(A) Crushing and concentration : The ore is crushed, powdered and concentrated by froth floatation process.

(B) Extraction of Ag by Mac-Arthur forest cyanide process : (Treatment with 0.4-0.7% sodium cyanide solution)

Ag₂S + 4NaCN  2Na [Ag(CN)₂] + Na₂S (soluble)

The role of air is to oxidise Na₂S so that reaction proceed in the forward direction.

 Impure Ag is purified by Electolytic refining method or by amalgamation.

(C) Electrolytic refining : Anode : Impure Ag

Cathode : Pure Ag

Electrolyte : AgNO₃(aq) HNO₃.

Silver from (commercial lead) argentiferous lead by Parke's process :

Desilverisation of Lead :

Lead extracted from galena (PbS) contains impurities of Cu, Ag, Bi, As, Fe Zn, Sn, etc. and is called commercial lead or argentiferous lead. This contains Ag upto 2% which is extracted by parkes process. Commercial lead is mixed with a large quantity of Zn and the mixture is melted, where Zn-Ag alloy is formed and Pb remains in the molten state. The alloy is strongly heated where Zn is distilled of leaving Ag. This silver contains some Pb impurity which is removed by cupellation process. Impure Ag is melted in a cupel (a boat shaped dish made of bone ash) by passing Hot blast of air. Pb is oxidised to PbO (litherge) which is either absorbed by cupel or carried away by blast of air leaving pure Ag.

Separation of silver from gold (Parting with conc. H₂SO₄)

Alloy (Au < 20%) is boiled with conc. H₂SO₄ where Ag is dissolved as Ag₂SO₄ and Au remains as spongy mass.

Ag₂SO₄  + Zn → 2Ag + ZnSO₄ (Metal displacement reaction)

(sparingly soluble solution)

If alloy contains Au > 20%, then some Ag is added to it so as to reduce the % Au below 20.

Silver from silver coin or silver ornaments :

(AgCu)  Ag (aq) + Cu²⁺ (aq) + 3NO₃^-(aq)  AgCl¯ + Cu²⁺ (aq) + 3NO₃^-

Recovery of Ag from AgCl :

(i) By treating with KCN solution :

2AgCl + 2NaCN → 2Na[Ag(CN)₂] + 2NaCl

soluble complex

2Na[Ag(CN)₂] + Zn(dust) → 2Ag + Na₂[Zn(CN)₄]

(ii) Boiling with caustic soda and glucose.

2AgCl + 2NaOH → Ag₂O + 2NaCl + H₂O  2Ag + C₆H₁₂ + O₇ (Gluconic Acid)

(iii) 2AgCl + Na₂CO₃ 2Ag + CO₂  + O₂  +2NaCl

3. Copper :

Ores :

Copper pyrites or Chalcopyrites CuFeS₂ (main ore) ; Cuprite or ruby copper Cu₂O ; Malachite CuCO₃Cu(OH)₂(green) ; Azurite 2CuCO₃.Cu(OH)₂(Blue) ; Copper glance Cu₂S, bornite (peacock ore) Cu₅FeS₄.

Extraction of copper from copper pyrites :

(A) Crushing and concentration : Ore is first crushed and then powedered finely and powdered ore is concentrated by froth floatation process.

(B) Roasting : Concentrated ore along with SiO₂ is heated in excess of air in a reverberatory furnace.

(Cu₂S FeS FeS₂) + 2CuFeS₂  + O₂ → Cu₂S + 2FeS + SO₂­

2FeS + 3O₂ → 2FeO + 2SO₂­ (Major oxidation)

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂­ (Minor oxidation)

Cu₂O + FeS → Cu₂S + FeO

Reverberatory Furnace

S + O₂ → SO₂ ­ 

 4As SO₂ → 2As₂O₃ 

4Sd 3O₂ → 2Sb₂O₃­

Volatile impurities are removed in this step.

(C) Slag formation :

Roasted ore mixed with sand and strongly heated in furnace.

FeO + SiO₂ → FeSiO₃

flux slag

Upper layer containing slag is removed and lower layer contains mostly Cu₂S (98%) with little amount of FeS(2%) is called matte.

(D) Bessemerization : (Self - reduction)

2FeS + 3O₂ 2FeO + 2SO₂

FeO + SiO₂ FeSiO₃(slag)

2Cu₂S + 3O₂  2Cu₂O 2SO₂ (partial roasting)

                             (limited air)

Cu₂S + 2Cu₂O  6Cu + SO₂ (self reduction)

                                           (R.A.) (impure)

Impure copper obtained has blister appearances and therefore called blister copper.

Poling : Molten Cu is stirred with poles of green wood to reduce any copper oxide impurity into Cu.

Electrolytic refining :

Anode - impure Cu ;

Cathode - Pure Cu ;

Electrolyte CuSO_4, H₂SO₄.

The more electropositive impurities like Zn, Fe, Ni etc. get dissolved in solution and less positive impurities like Ag, Au collect below anode as anode mud.

4. Lead :

Ores : Galena PbS (Main ore) ; Anglesite PbSO₄ ; Cerussite PbCO₃

Extraction of lead from galena :

Crushing and concentration : The ore is crushed, grinded finely and concentrated by froth floatation process.

Roasting : In reverberatory furnace, limited supply of air is passed at moderate temperature.

PbS + 2O₂  PbSO₄ ;

2PbS + 3O₂  2PbO + 2SO₂

Self reduction : Air supply is cut off and the temperature is increased to melt the change in reverberatory furnace.

PbS + 2PbO 3Pb SO₂

           (R.A.) impure

PbS PbSO₄ 2Pb 2SO₂

(R.A.) impure

SiO₂ + CaO (flux)  CaSiO₃ (slag)

PbSiO₃ + CaO (lime)  PbO+ CaSiO₃ (slag)

In this way, lime (CaO) prevents formation of PbSiO₃.

Impure Pb is purified by electrolytic refining method or by liquation and poling.

Electrolytic refining :

Anode - Impure Pb

Cathode - Pure Pb

Electrolyte - PbSiF₆  H₂SiF₆  gelatine

5. Tin :

Ores : (i) Cassiterite or Tin stone SnO₂ (Main ore) (It contains impurities of pyrites of Cu and Fe and magnetic impurity of wolframite which is a mixture of FeWO₄ , MnWO₄).

This mineral is also called black tin to distinguish it from the metal Sn which is also called white tin.

Extraction of Sn from cassiterite :

(i) Crushing and concentration :

The ore is crushed and washed with a stream of running water to remove lighter silicious impurities followed by magnetic separation method to remove the magnetic impurity of Wolframite.

(ii) Roasting :

Concentrated ore is heated in pressence of air, and volatile impurities (S as SO₂, As as As₂O₃ and Sb as Sb₂O₃) are removed. The impurities of pyrites of Cu and Fe are converted into their respective oxides and sulphates.

(iii) Washing :

Sulphates of copper and iron are dissolved in water. The ore thus obtained contains 60-70% SnO₂ and is called as black tin.

Carbon reduction :

The black tin is mixed with anthracite coal and heated to about 1300°C. If SiO₂ is present as impurity then CaO is added as flux.

Refining :

(i) Liquation and poling :

Impure Sn is melted on the sloping hearth where Sn(m.pt. 232°C) is first melt and flows out leaving behind the less fusible impurities of Cu, Fe, W etc. The liquid Sn is then stirred with poles of green wood to reduce SnO₂ (Impurity) to Sn.

(ii) Electrolytic refining :

Anode : Impure Sn

Cathode : Pure Sn

Electrolyte : SnSO_4, H₂SO₄

6. Iron :

Ores : Haematite Fe₂O₃(Main ORE) ; Limonite Fe₂O₃.3H₂O ; Magnetite Fe₃O₄ ; Siderite FeCO₃ ; Iron pyrites FeS₂

Extraction of Iron from ore haematite :

Crushing and concentration : The oxide ore is first crushed in jaw crushers and then is broken in small pieces. Haematite (non-magnetic) is washed with running water to remove earthy and siliceous impurities by levigation.

Calcination following by roasting :

The concentrated ore is roasted with excess air in a reverberatory furnace. During roasting step, the following changes occur :

(a) If FeCO₃ is present as impurity, it gets decomposed into FeO which is oxidised by air to Fe₂O₃.

FeCO₃  FeO + CO₂ (calcination)

siderite

4FeO + O₂ (air)  2Fe₂O₃ (Roasting)

In this way, formation of FeSiO₃ slag is prevented during melting, and following reaction does not occur.

SiO₂  + FeO → FeSiO₃ (slag)

(b) The impurities of S, As are also removed as their volatile oxides

S + O₂ → SO₂­

4As + 3O₂ → 2As₂O₃­,

The entire mass becomes porous and hence the reduction of Fe₂O₃ to spongy iron becomes easy at later stage.

Reduction in blast furnace.

(Fe₂O₃ ore lime stone coke) is smelted in blast furnace and following changes take place.

(i) Combustion Zone (155 - 1700°C)

(a) (Combustion zone) a blast of dry preheated air is blown into the furnace from near the bottom of the furnace. Near the bottom, the preheated air comes in contact with the falling coke and combustion of coke into CO₂ takes place.

C + O → CO₂  ΔH = - 393.5 kJ

CO₂ produced in the combustion zone rises up and meets with more coke in fusion zone and gets reduced to CO.

CO₂  + C → 2CO ΔH = 163.0 kJ

(ii) Reduction zone (250 - 700°C)

Near the top of the furnace (reduction zone), the temperature varies from 250-700°C. Here the oxide ore (Fe₂O₃) is reduced to spongy iron with the help of uprising vapours of CO produced in the zone of fusion.

Fe₂O₃ + 3CO2Fe +3CO₂

Actually above Reduction, takes in 3 steps :

3Fe₂O₃  +CO → 2Fe₃O₄ + CO₂ ­

Fe₃O₄  +CO → 3FeO +CO₂ ­

FeO+ CO → Fe + CO₂ ­

(Spongy iron)

Any Fe₂O₃ which escapes from reduction in reduction zone is reduced in fusion zone.

(iii) Slag formation zone (700-1000°C)

In the middle of the furnace (slag formation zone) where the temperature varies from 700-1000°C, lime stone (CaCO₃) present in the change decomposes into CaO and CO₂.

CaCO₃  CaO CO₂

CaO combines with the impurity of SiO₂ and forms a fusible slag of CaSiO₃. Thus CaO acts as a basic flux.

CaO(basic flux) + SiO₂ (acidic impurity) → CaSiO₃(slag)

Slag is lighter than the molten iron. It moves down and floats over molten iron. This region where slag is formed is called slag formation zone.

(iv) Fusion Zone (1000 - 1500°C)

Since the reduction of CO₂ to CO is an endothermic reaction (Heat is required), temperature is decreased to about 1500°C. Fe₂O₃ is reduced to Fe which might not have been reduced in the reduction zone.

Fe₂O₃ + 3C → 2Fe + 3CO

Impurities are also reduced and get mixed up with spongy Iron.

MnO₂  + 2C → Mn + 2CO

2P₂O₅ + 10C → P₄  +10CO

SO₂ + C → S + 2CO

SiO₂ + 2C → Si + 2CO

Spongy iron produced in the reduction zone melts here and gets impured in called pig iron, while slag being lighter floats over and thus prevents oxidation of Fe by blast of hot air.

Types of Iron :

1. Cast iron (2% to 5% carbon & other impurity)

2. Steel (0.5% to 2% carbon & other impurity)

3. Wrought iron (< 0.5% carbon & other impurity)

Manufacturing of wrought from cast Iron :

Wrought iron is manufactured from puddling furnace having inner lining of haematite (Fe₂O₃) oxidant for impurities present in cast iron.

Mn + Fe₂O₃ → MnO + 2 Fe

(O.A.) (basic)

Si + Fe₂O₃ → SiO₂  + 2 Fe

(O.A.) (basic)

MnO + SiO₂ → MnSiO₃  (slag)

(O.A.) (basic)

S + Fe₂O₃ → SO₂­ 2 Fe

C + Fe₂O₃ → 3CO 2 Fe

3P₄  +10Fe₂O₃ → 6P₂O₅  +20Fe

P₂O₅ + Fe₂O₃ → 2FePO₄(slag)

 Manufacturing of steel from cast from :

(i) Bassemerisation (already discussed)

(ii) Open-Hearth process (Siemen Martin's process)

(iii) Electrical furnace process

Open hearth process (siemen Martin's process)

Mn + Fe₂O₃ → MnO 2 Fe

(O.A.) (basic)

Si + Fe₂O₃ → SiO₂  + 2 Fe

(O.A.) (Acidic)

MnO + SiO₂ → MnSiO₃  (slag)

(O.A.) (basic)

S + Fe₂O₃ → SO₂­ Fe

(O.A.)

C + Fe₂O₃ → 3CO ­+ 2Fe

3P₄ + 10Fe₂O₃ → 6P₂O₅  + 20Fe

P₂O₅  + 3CaO → Ca₃(PO₄)₂ (Thomas slag)

Open hearth furnace for the manufacture steel from cast iron

After adding required amount of spiegeleisen steel is formed.

In this method 2 - 5% iron is also oxidised by air because hearth is open therefore this method is discarded is modern age.

In modern age steel is manufactured by electrical furnace process or by L.D. process. In electrical furnace process heating effect is produced by passing electricity and all chemical reaction are similar to open-hearth process.

Heating Treatment of STEEL

(i) Annealing : Process of heating steel upto redness and then cooling it slowly is called annealing, In this way steel becomes soft, malleable and elastic.

(ii) Quenching : Process of heating steel upto redness and then cooling it suddenly by plunging in into oil or water is called quenching. In this way steel become hard and brittle.

(iii) Tempering : Process of heating quenched steel much below redness and then coiling it slowly is called tempering. In this steel becomes neither so hard nor so brittle.

Surface Treatment of steel :

(i) Case - Hardening : Process of forming hard coating of iron carbide over mild steel by heating it with charcoal is called case - hardening.

(ii) Nitriding : Process of forming hard coating of iron nitride by heating steel with ammonia gas is called nitriding.

 7. Zinc :

Ores : Zinc blende ZnS (main ore), Zincite (ZnO), Calamine, ZnCO₃.

 Extraction of zinc from zinc blende :

(A) Crushing and concentration :

The ore is crushed and concentrated by froth floatation process.

(B) Roasting :

The concentrated ore is roasted in presence of excess of air 

2ZnS + 3O₂ 2ZnO + 2SO₂­

ZnS + 2O₂  ZnSO₄

2ZnSO₄  2ZnO­ + 2SO₂ + O₂

ZnSO₄ decomposed at higher temperature

(C) Carbon Reduction (Belgian process) :

Roasted ore is heated with coke in a vertical fire clay retort.

ZnO­ + C  Zn ­+ CO

(R.A.) vapour

Vapours of zinc are collected by rapid cooling to get zinc spelter (Impure Zn).

Impure Zn is purified either by electrolytic refining method or by distillation.