Isolation of Elements, Class 12, Chemistry Detailed Chapter Notes PDF Download

Minerals, Ores and Concentration of Ores - Isolation of Elements, Class 12, Chemistry

Minerals : Naturally occuring chemical substance in which metal exist either in its free state or in combined state is called mineral.

Ore : Mineral from which metal can be conventionaly and economically extracted is called ore & impurities associated with it is called gangue or matrix

Types of Ores :

Sulphide Ores : 

Galena : PbS,                               Cinnabar : HgS,

Zinc bllend : Zns,                         Chalcopyrite : CuFeS₂

Copper glance : Cu₂S                    Fool's Gold : FeS₂

Oxide Ores :

Bauxite : Al₂O₃. 2H₂O                   Haematite : Fe₂O₃

Limonite : Fe₂O₃. 3H₂O                Tin stone or Cassiterite : SnO₂

Carbonate Ores : Siderite : FeCO₃  Calamine ZnCO₃

Malacite : Cu(OH)₂CuCO₃ Dolomite CaCO₃. MgCO₃. 2H₂O

lime stone : CaCO₃

 

Sulphate Ores : 

Gypsom : CaSO₄.2H₂O                                           Anylesite PbSO₄

Glauber's salt : Na₂SO₄. 10 H₂O                           Mohr's salt : FeSO₄. (NH₄)₂SO₄. 6H₂O

 

Halide Ores : Rock salt : NaCl                   Cryolite : Na₃AlF₆

Fluorspar : CaF₂                 Carnallite : KCl. MgCl₂. 6H₂O

 

Nitrate Ores : Chiele Saltpeter : NaNO₃                       Indian Salt petre : KNO₃

 

Native Ores : Those metals which are chemically less reactive. They occur in the earth crust in form of free state (lumbs)

e.g : Cu, Ag, Au, Hg, Pd, Pt, Bi

 

General principles and processes involved in the extraction of metal from its ore :

The extraction of metal from its ore is completed in five steps :

Step I : Pulverization : The crushing of ore to powdered state is called pulverisation.

Step II : concentration or Dressing or Beneficiation of ore

Step III : Conversion of Concentrated ore into oxide form

step IV : Reduction of oxide to the metal

Step V: Purification or refining of crude metal :

Step I : Pulverization : The crushing of ore to powdered state is called pulverisation

This process in stamp mill or ball mill

 

Step II : Concentration or Dressing or beneficiation of Ore

(a) By Gravity separation : Ore particles are heavier than the gangue particles. This is used for the separation of most of the gangue particles :

⇒ By Wilfley Table Method

⇒ By Hydraulic Classifier

 

 

(b) By Magnetic separator :

Cassiterite or Tinstone contains impurities of wulframite or wulframates of Fe & Mn.

⇒ Tin stone : SnO₂ → Diamagnetic

⇒ Wulframites or wulframates of

Fe & Mn : FeWO₄, MnWO₄ ⇒ Paramagnetic.

Ulframates of Fe & Mn from Tin stone by magnetic separator.

 

(c) By Froth Floatation Process : This method is used for the concentration of sulphide ores.

It is based on the concept that the sulphide ores are prefrentially wetted by pine oil, camphor oil while gangue particles are prefrentially by water.

This is based on the physical phenomenon of adsorption.

 

Frother : Pine oil, Camphor oil

 

Froth Stabilizers : They reduce surface tension of water e.g. cresols, amines.

Collector : Sodium or Potassium xanthates. It combines with sulphide ore & makes them water replent so that its affinity towards pine oil increases (Adsorption tendency increases)

KOH EtOH → Et O^- K   H₂O

 

Depressant : KCN or NaCN

ZnS is found to be an impurity with the lead sulphide. Therefore to separate out PbS from ZnS depressant KCN or NaCN added.

PbS NaCN → No complex formation due to very law K_sp of PbS.

ZnS 4NaCN → 4Na   [Zn (CN)₄]^2-  S^- -

                                        (water soluble)

                                     

Thus, ZnS becomes water soluble & it remains with gangue while PbS comes out with the froth.

 

Activator : CuSO₄

From galena (PbS.ZnS) ZnS is removed

ZnS 4NaCN  4 Na  +  [Zn(CN)₄]^2-  [Cu(CN)₄]^3-  ZnS ¯

                                   + S^{- -}                              (water soluble)

                                                                           (more stable)

& ZnS is taken out by froth floatation second time.

 

(ii) Chemical Method of Concentration :

Leaching : It involves dissolution of metalic ore in a suitable reagent in which metallic ore is soluble and impurities are insoluble.

Leaching of alumina from bauxite :

Al₂O₃(s) 2NaOH(aq) 3H₂O(I) → 2Na[Al(OH)₄](aq)

Having F₂O₃ as important leachant.

The aluminate in solution is acidified by adding acid and hydrated Al₂O₃ is precipitated.

2Na[Al(OH)₄]_(aq)  H (aq)  Al(OH)₃ ¯ H₂O Na _(aq).

                                                       (white ppt)

Hydrated alumina is filtered, dried and heated to give back pure Al₂O₃ :

Al(OH)₃¯  Al₂O₃(s) 3H₂O(g)

                         white (pure)

 

Other examples :

In the metallurgy of silver and gold, the respective metal/ore is leached with very dilute solution of NaCN or KCN in the presence of air (for O₂) from which the metal is obtained by displacement reaction.

4M(s) 8CN^-(aq) 2H₂O(aq) O₂(g) → 4[M(CN)₂]^- (aq) 4OH^-(aq)

(M = Ag Or Au)

2[M(CN)₂]^- (aq) Zn(s) → [Zn(CN)₄]^2-(aq) 2M(s)¯

 

Step III : Conversion of Concentrated ore into oxide form :

It is done either by calcination or by roasting.

(i) Calcination :

Calcination is carried out for carbonate, hydrated metal oxide & metal hydroxide ores.

It is carried out in the absence of air i.e., heating in absence of air.

Due to calcination ore becomes porous.

Volatile organic impurities get evaporated

 

(ii) Roasting : In the presence of air the sulphide are heated in free supply of air below m.p. Impurities of sulphur, phosphorus, arsenic & antimony are converted into their corresponding volatile oxide & thus get removed.

Moisture & Water of crystallisation are also removed.

 

Step IV : Reduction of oxide to the metal :

(b) Reduction of Metal oxide / conc. ore into free metal .

This can be carried out

(i) chemical reduction

(ii) By self reduction or auto reduction or Air Reduction

(iii) Metal - displacement method

(iv) By electrolytic Reduction

(v) By amulgamation.

(i) & (ii) method are collectively known as Pyrometallurgy

e.g. Sn, Pb, Fe, Hg, Cu, B, Zn, (Based on Ellinghum diagram)

(iii) step is called hydrometallurgy — Cu, Ag, Au are extracted

(iv) step is called Electrometallurgy, Alkali, Alkaline earth metals & Al & base electrolysis

(v) is used for Ag & Au

 

(i) Chemical Reduction :

1. Smelting i.e., carbon Reduction - Reduction of metal oxide by coke, coal & COReduction of the metal oxide usually involves heating it with some other Substance acting as a reducing agent, e.g., C or CO or even another metal. The reducing agent (e.g., carbon) combines with the oxygen of the metal oxide.

M_xO_y + yC → xM y + CO

Some metal oxides get reduced easily while others are very difficult to be reduced. To understand the variation in the temperature requirement for thermal reductions and to predict which element will suit as the reducing agent for a given metal oxide (M_xO_y), Gibbs energy interpretations are done, which is explained by ellingham diagram.

ΔG = ΔH - TΔS

If ΔH is greater than zero then reduction will be feasible on increasing temprature i.e., |TΔS| > |ΔH|

 

Ellingham diagram -

M_(s) +   O_2(g)  M₂O_x(s)

ΔG = ΔH - TΔS

Therefore, For forward rxn ΔS < 0

 

(Ellingham diagram for formation of M₂O_x)

Ellingham diagram is a plot of formation of an element oxide between ΔG & ΔT

 

Ex. Which of the following statements are true :

A → Mg(s)  O₂(g)  MgO(s)

B → Mg(l)  O₂(g)  MgO(s)

C → Mg(g)  O₂(g)  MgO(s)

I : Below 1350° Mg can reduce Al₂O₃

II : Above 1350° C Mg Will reduce Al₂O₃

III : Below 1350° Al can reduce MgO

IV : Above 1350° Al can reduce MgO

V : At 1350° C there is no change in free energy i.e., ΔG = 0

Sol.  ΔG < 0

    (Its ΔG high)                     (Its ΔG less)

Al₂O₃ + 3 Mg  3 MgO + 2Al

At 1350° C both reactions have same G Therefore,ΔG = 0

To carry out smelting below 800°C, CO is used as reducing agent while above 800°C, smelting is carried out by coke.

2C(s) + O₂(g) → 2CO (g) + ΔH = -221.0 kJ/mole

DS = 179.4 J kJ/mol

C(s) + O₂(g) → CO₂(g) + ΔH = - 393.5 kJ/mol

ΔS = 2.89 JK^-1 mole^-1

Aluminium can be extracted from Alumina by carbon reduction but the method is highly uneconomical because -

(i) As the smelting occurs above 200°C hence a part of the aluminium will go into vapour phase (M.P. = 2520°C)

(ii) At this high temperature the liberated Al will combine with the carbon & aluminium carbide will be formed.

(iii) ΔH_for of alumina is high - ve value

Therefore,It is thermodynamically more stable & reduction is more difficult

To extract metal from sulphide ore is carried out by firstly roasting it into metal oxide & followed by its smelting. Metal sulphide or sulphide ore is not directly smelted to metal.

2PbS C  2Pb + CS₂ (Thermodynamically Not feasible)

Pbs  O₂  PbO + SO₂­

PbO + C  Pb CO ­ Thermodynamically feasible

ΔG_f of PbS = -21.9 kcal/mol

ΔG_f of CS₂ = 17.15 kcal/mol

ΔG_f of PbO = -45.1 kcal/mol

ΔG_f of SO₂ = -71.7 kcal/mol

ΔG_f of CO = - 32.8 kcal/mol

 

Flux : Additional substances which are used during metal extration to remove acidic or basic impurity are called flux depending upon nature of impurity flux are of two types.

(i) Basic Flux : It is used to remove acidic impurity eg : CaO, MgO, CaCO₃, MgCO₃ FeCO₃ etc.

(ii)Acidic Flux : It is used to remove basic impurity eg : SiO₂, B₂O₃, P₂O₅, Na₂B₄O₇. (Borax)

Smelting : Phenomenon of slag formation by combining flux with impurity is called smelting.

Flux + Impurity → Slag (Smelting)

        (Basic or acidic)

 

Properties of slag :

(i) Slag has low melting point than metal.

(ii) Slag is lighter than metal therfore it floats over the molten metal and prevents further oxidation of molten metal by air.

(iii) Slag immiscible with molten metal therefore it can be easily separted from molten metal.

 

(b) Gold Schmidt Thermite Reduction :

Thermite : Al powder

Cr₂O₃  + 2Al  2Cr + Al₂O₃

(DG_f = -540 kJ/mole) (DG₃ = -827 kJ/mole)

B₂O₃ + 2Al  2B + Al₂O₃

2Mn₃O₄ + 8Al  9Mn + 4 Al₂O₃

Fe₂O₃ + 2Al  2Fe + Al₂O₃

This method is used for reduction of those metal oxides which are highly stable if they are reduced by coke it will occur at very high temprature & at this high temperature the liberated metal will combine with the coke & carbide will be formed hence Al powder i.e., thermite is used

 

(c) Reduction by Hydrogen :

Because of inflammable nature of hydrogen its use as a reducing agent is very restricted.

Cu₂O + H₂  2Cu + H₂O

M_OO₃ + 3H₂  M₀ + 3H₂O

BCl₃ +  3/2H  B + 3HCl

 

Reduction by other metals :

SiCl₄ + 2Mg  2MgCl₂  Si

Kroll process used for extraction of Ti & Zr

TiCl₄ + 2Mg Ti 2MgCl₂

ZrCl₄  2Mg  Zr 2MgCl₂

I.M.I Process (Imperial Metal Industries)

TiCl₄  4Na  Ti 4NaCl

(ii) By Self reduction or Auto reduction or Air Reduction :

This method is used for extraction of copper, lead, mercury i.e., it is used for the extraction of metal from their sulphide ores.

In this method the sulphide ore is roasted in free supply of air to its metal oxide & then air supply is cut off followed by heating by increasing temprature & metal is extracted by self reduction.

PbS    PbO SO₂ ­

PbS 2O₂  PbSO₄

Now air supply is cut off followed by heating

PbS_(s)  2PbO_(s)  3Pb_(l)  SO₂ ­

Self reduction is responsible for acid rain than roasting because SO₂ dissolves in air, (3927cc CO₂ in 1000cc of H₂O)

 

(iii) By Metal Displacement Method or By Hydrometallurgy :

 

In this method the concentrated ore is treated/ leached with specific chemical reagent that converts the ore into water soluble salt. Now, on adding more electropositive metal into the aqueous salt solution the metal (less electro positive) is displaced

e.g.

Iron is found to be an impurity in the copper ores hence if Zn is added to extract copper, iron will also be displaced along with copper & that is why iron is used.

Both metals which extracted & by which we extracted are water insoluble

 

(iv) Electro Metallurgy : The metal is extracted by passing electricity into its fused salt or in aqueous solution.

Extraction of sodium :

⇒ By electrolysis of Aq. NaCl solution :

NaCl(s) x H₂O  Na (aq) Cl^-(aq)

H₂O  H   OH^-

On passing electricity

At cathode : 2H   2e^-  H₂ ­

ΔG = - n FE°

Therefore,Na  does not discharge at cathode

At anode : 2Cl^-  Cl₂­ 2e^-

In sol : Na   OH^-  NaOH

⇒ By electrolysis of fused NaCl :

 

On Passing electricity

At cathode : 2Na +  2e^-  Na

At Anode : 2Cl^-  Cl₂ ­+   2e^-

In sol. Na + OH^-  NaOH

 

Electrochemical Principles of Metallurgy :

We have seen how principles of thermodynamics are applied to pyrometallurgy. Similar priniciples are effective in the reductions of metal ions in solution or molten state. Here they are reduced by electrolysis or by adding some reducing element.

In the reduction of molten metal salt, electrolysis is done. Such methods are based on electrochemical principles which could be understood through the equation,

ΔG° = -nE°F ..................... (16)

Here n is the number of electrons and E° is the electrode potential of the redox couple formed in the system. More reactive metals have large negative values of the electrode potential. So their reduction is difficult. If the difference of two E° values corresponds to a positive E° and consequently negative DG° in equation (16), then the less reactive metal will come out of the solution and the more reactive metal will go to the solution, e.g.,

Cu² (aq) Fe(s) → Cu(s) Fe² (aq)

In simple electrolysis, the Mⁿ  ions are discharged at negative electrodes (cathodes) and deposited there. Precautions are taken considering the reactivity of the metal produced and suitable materials are used as electrodes. Sometimes a flux is added for making the molten mass more conducting.

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Methods of Refining of Metals - Isolation of Elements, Class 12, Chemistry 

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 skum Oxidation is known by various names : bassemerization (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 colloidial 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) Siliver 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 forrest 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 extrated by parkes process. Commerical 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 Ag₂SO₄and 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 :

(Ag Cu)  Ag (aq) Cu² (aq) 3NO₃^-(aq)  AgCl¯ Cu² (aq) 3NO₃^-

 

Recovergy 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 Furance

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) Bessemerisation : (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 conentration : The ore is crushed, grinded finely and concentrated by froth floatation process.

Roasting : In reverberatory furnance, 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 runing water to remove lighter silicious impurities followed by magnetic separation method to remove the magnetic impurity of Wolframite.

 

(ii) Roasting :

Concentraed 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 strirred 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-magentic) 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 DH = - 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 DH = 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 fromed is called slag formation zone.

(iv) Fusion Zone (1000 - 1500°C)

Since the reduction of CO₂to 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 spngy 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 mannfactured 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 Marthin's process)

(iii) Electrical furnace process

Open hearth process (siemen Mortin'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 requried 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 fumace 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 plundging 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 colling 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 (Balgian 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.

==============================================================

Electrolytic Refining - Isolation of Elements, Class 12, Chemistry

(D) Electrolytic refining :

Anode : Impure Zn

Cathode : Aluminium Electrode

Electrolyte : ZnSO₄(aq) H₂SO₄

* Extraction of Cr From FeCr₂O₄(FeO.Cr₂O ₃) Chromite ORE :

Conc. Step : Chromite ore is concentrated by gravity sep. Method to remove light impurites followed by magnetic sep. method to remove non-magnetic impurities.

Chemical Method to Separate Cr₂O₃ :

4FeO.Cr₂O₃  7O₂ (air) 8 Na₂CO₃/16NaOH  2Fe₂O₃  8Na₂CrO₄  8CO₂/8H₂O

 

2Na  (aq) Cr₂O₇^2-(aq) SO₄^2- (aq)  2Na (aq) CrO₄^2- (aq)

(Orange Solution) yellow solution

Na₂Cr₂O₇.2H₂ O  NaCrO₂  Cr(OH)₃¯

(orange crystal) (green ppt)

Cr₂O₃ (green solid)

Al-reduction method : (Goldsmith alumino thermite process)

 

Impure Cr is purified by electrolytic refining method.

Anode : Impure chromium

Cathode : Hg-electrode

Electrolyte : CrCl₃  HCl

 

* Extraction of Mn from MnO₂ (Pyrolusite) :

Pyrolusite is concentrated by gravity separation method followed by magnetic separation method to remove non-magnetic impurity.

Calcination :

MnO₂ gives explosive rxn. with Al therefore in this step it is converted into mixed oxide Mn₃O₄.

3MnO₂  Mn₃O₄(MnO Mn₂O₃) O₂­

Al-reduction method : (Goldsmith alumino thermite process)

Impure Mn is purified by electrolytic refining method.

Anode : Impure Mn

Cathode : Hg-electrode

Electrolyte : MnSO₄  H₂SO₄

 

8. Aluminium :

Ores :

(i) Bauxite Al₂O₃.2H₂O (main ORE) ; Diaspore Al₂O₃.H₂O ; Corundum Al₂O₃

(ii) Mica K₂O.3Al₂O₃.6SiO ₂.2H₂O ; Kaolin Al₂O₃.2SiO₂.2H ₂O

(iii) Cryolite Na₃AIF₆

Extraction of Al from Bauxite :

Purification of Bauxite : 

 

Electrolysis of pure fused Al₂O₃ (Hall - Heroult method)

Cathode - iron tank lined with graphite

Anode - Carbon rods dipped in molten electrolyte

Electrolyte - molten (20% Al₂O₃  60% Cryolite 20% CaF₂) Temp » 1100°C

Cryolite lowers the melting point of mixture and makes the molten mix conducting.

Na₃AlF₆  3NaF AlF₃

AlF₃ → Al ³  3F^-

At cathode Al ³  3e^- → Al

At anode 3F^- → 3F 3e^-

Al₂O₃  6F → 2AIF₃   O₂­

C   O₂ → CO

CO   O₂ → CO₂

Anodes are periodically changed as they are consumed by oxygen liberated at anode.

Electrolytic refining (Hoppe's Method)

Iron box lined with carbon, contains the three molten layers.

Bottom layer : Impure Al as anode

Middle layer : (Na₃AlF₆  BaF₂) Molten electrolyte

Tope layer : molten pure Al as cathode.

On passing the current aluminium is deposited at cathode from the middle layer and an equivalent amount is taken from andoe (bottom layer) levaing behind the impurites. In this way aluminium obtained is 99.98% pure.

 

9. Magnesium : (Dow's process)

Ores :

Carnallite MgCl₂.KCl 6H₂O (main ore) ; Epsom MgSo₄.7H₂O magnesite MgCO₃ ; Kiesserite MgSO₄. H₂O ; Dolomite MgCO₃. CaCO₃ ; Kainite K₂SO₄. MgSO₄. MgCl₂.6H₂O (Mg is also found in Talc, sabestos and chlorophyll)

Sea water contains 0.13% magnesium as chloride and sulphate. It involves following steps.

(a) Precipitation of magnesium as magnesium hydroxide by slaked lime :

MgCl₂  Ca(OH)₂ → Mg(OH)₂ ¯ CaCl₂ (ppt. reaction)

 

(b) Preparation of hexahydrated magnesium chloride :

Mg(OH)₂  2HCl(aq) → MgCl₂  2H₂O (Neutralisation reaction)

The solution on concentration and crystallisation gives the crystals of MgCl₂.6H₂O

 

(c) Preparation of anhydrous magnesium chloride :

MgCl₂.6H₂O  MgCl₂  6H₂O

* It is not made anhydrous by simple heating because it gets hydrolysed.

MgCl₂.6H₂O  MgO 5H₂O 2HCl

 

(d) Electrolysis of fused anhydrouns MgCl₂ :

Magnesium chloride obtained by any of the above methods is fused and mixed with sodium chloride and calcium chloride in the temperature range of 972 - 1023K. The molten mixture is electrolysed. Magnesium is liberated at the carhode (iron pot) and chlorine is evolved at graphite anode.

MgCl₂  Mg²   2Cl^-

At cathode : Mg²   2e^- → Mg(99% pure) ;

At anode : 2Cl^- → Cl₂  2e^-