Step V: Purification or refining of crude metal :
(A) By physical Methods :
(c) Zone refining
(B) By Chemical Methods
(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 :
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
2Na[Au(CN)2] + Zn → Na2[Zn(CN)2] + 2Au¯
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
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
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 :
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
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
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)
Cu2S + 2Cu2O 6Cu + SO2 (self reduction)
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 CuSO4, 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
PbS PbSO4 2Pb 2SO2
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.
(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 : SnSO4, 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)
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
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
Si + Fe2O3 → SiO2 + 2 Fe
MnO + SiO2 → MnSiO3 (slag)
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
Si + Fe2O3 → SiO2 + 2 Fe
MnO + SiO2 → MnSiO3 (slag)
S + Fe2O3 → SO2 Fe
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
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.