Chapter 6 General Principles and Processes of Isolation of Elements

The following reaction takes place during electrolysis of brine

2Cl^– (aq) + 2H₂O(l) → 2OH^– (aq) + H₂(g) + Cl₂(g)

Here, chloride ionsgets oxidised to chlorine gas.

Thus, oxidation occurs and not reduction, hence option (ii) is incorrect. It is a redox reaction and not a displacement reaction,so option (iv) is also incorrect.

The ∆G^° of this reaction is +422kJ which is a positive value so option (iii) is incorrect as well.

Therefore, option (i) is the correct option.

When the ore is heated in a reverberatory furnace after mixing with silica. In the furnace, iron oxide slags’ of as iron silicate and copper is produced in the form of copper matte.

Copper matte consists of Cu₂S and FeS which contains Copper(I) i.e. Cu^- and Iron(II) Fe²⁺.

Auto-reduction is the reduction of less reactive metals like Hg, Cu, Pb etc by itself.

Copper sulphide reduces copper oxide to copper.

This is an exampleofauto-reduction because copper is reduced by itself.

Aluminium is the most abundant metal on earth by weight(approx 8.3%) and iron is the second most abundant metal on earth by weight(approx 5%)

A circular mobile heater is fixed at one end of a rod of the impure metal.Molten zone passes through a rod slowly and the impurities dissolve in the molten metal and move along leaving behind very pure metal which crystallises out. This method of purification is called zone refining and is based on the principle that impurities are more soluble in molten metal than in solid metal.

Extraction of copper happens by auto-reduction.

Here, Cu₂S reduces Cu₂O to Cu and itself gets oxidised to SO₂. The copper thus obtained is called as blister copper which is due to the evolution of SO₂

_{Oxidation takes place at the anode. E°cellby convention represents reduction potential. A higher positive value of E°cellmeans reduction is easier to happen than oxidation. Therefore, for oxidation reaction (ii) is preferred over reaction (i) but this does not happen. Reaction (ii) does not take place because oxidation of water to oxygen is not kinetically favoured and required excess potential(over-potential)}

The following reactions happen during the metallurgy of aluminium at anode where oxidation takes place.

C(s) + O₂^– (melt) → CO(g) + 2e^–

C(s) + 2O₂^– (melt) → CO₂ (g) + 4e^–

At cathode

Al³⁺ (melt)+3e^-→Al(s)

Overall Reaction:

2Al₂O₃ +3C→ 4Al +3CO₂

Solid carbon(graphite) gets oxidised to carbon monoxide and carbon dioxide.

Electrolytic refining is used to purify less reactive metals like Cu, Zn, Au, Ag etc…

In electrolytic refining, the impure metal is made the anode and the pure metal is made the cathode. A suitable electrolyte is chosen and electricity is passed. The impure metal dissolves at the anode and the insoluble impurities are collected as anode mud, while the pure metal gets deposited at the cathode. Metals of very high purity are thus obtained by electrolytic refining.

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)

Here the metal M(M=Ag or Au) is displaced from the complex by the action of zinc as a reducing agent.

FeO(s) → Fe(s) + O₂(g) [ΔG(FeO, Fe)]

C(s) + O₂ (g) → CO (g) [ΔG (C, CO)]

In an Ellingham diagram, the reducing agent reduces the metal oxide only if ∆G is negative. If the difference in ∆G is large and negative then the reduction happens easily. The overall ΔG for the two reactions given above is

ΔG (C, CO) + ΔG (FeO, Fe) = Δ_rG

From the above diagram,at any temperature above point A,

ΔG (C, CO)< ΔG (Fe, FeO)

So, carbon will reduce FeO to Fe and will itself get oxidised to CO because the summation of ΔG is large and negative.

Below point ‘A’, the carbon monoxide line lies below that of oxidation of Fe to FeO. The difference in ∆G is negative in this range and only carbon monoxide can reduce FeO.

The curve for oxidation of Fe to FeO and CO to CO₂ meet at point D and have the same ΔG value.

2Fe+ O₂→ 2FeO ---ΔG₁

2CO + O₂ → 2CO₂ --- ΔG₂

ΔG₁ = ΔG₂ because the curves meet at point D.

For reduction of FeO to Fe the sign of ΔG₁ must be changed. It becomes - ΔG₁. Now the overall ΔG_r= - ΔG₁ + ΔG₂ =0

At point B and E the ∆G values for (a),(b) and (c) is lesser than that for reduction of FeO to Fe which means that the total ∆G for the reaction will be large and negative and the reaction will take place.

Cast iron is made by melting pig iron with scrap iron and coke using hot air blast.

Van Arkel process

Impure Zr or Ti is heated with Iodine in an evacuated vessel and it forms a volatile complex which on decomposition gives the pure metal.

Zr + 2I₂→ ZrI₄

ZrI₄→Zr(pure) + 2I₂ (at 1800K)

Al₂O₃ is a bad conductor of electricity and have very high meting point.

Thus, CaF₂ and Na₃AlF₆ are used in Hall-Heroult’s process to extract Al from Al₂O₃. They lower the melting point of Al₂O₃ and increase the conductivity of the molten mixture.

Collectors like pine oil, xanthenes etc…enhance the non-wettability of sulphide ore because mineral particles become wet by oils while gangue particles are wet by water. So, separation becomes easier.

If there are two sulphides ores present and we want to separate one of them, then depressants are used. Depressants like NaCN selectively allow one sulphide ore to come to the froth.

If the ore contains ZnS and PbS, then the depressant NaCN will let PbS to come to the froth and ZnS will not form the froth.

By using depressants like NaCN, two ores can be separated. NaCN for example selectively prevents ZnS from coming to the froth and instead allow PbSto come with the froth.

The principle ore of aluminium bauxite usually contain Fe₂O₃ and SiO₂ as the common impurities.

Only sulphide ores are concentrated by froth floatation. Galena(PbS) and Copper pyrites(CuFeS₂) are sulphide ores while Haematite(Fe₂O₃) and Magnetite(Fe₃O₄) are oxides and cannot be concentrated by froth floatation.

Calcination involves the heating when the volatile matter escapes leaving behind the metal oxide in the limited supply of air. Ores that contain carbonates, hydroxides or oxides can be calcined.

_Thus,

CaCO₃→CaO + CO₂

_{So, only reaction (i) and (iii) will undergo calcination.}

Haematite and calamine are the only oxides present. Oxides can be reduced easily by carbon

Reduction of haematite:

2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g)

Reduction of calamine:

ZnCO₃(s)→ ZnO(s) + CO₂(g)

Reduction of iron oxides to iron happens at a blast furnace

Fe₂O₃ + CO → 2FeO + CO₂

2FeO + 2CO →2Fe + 2CO₂

-------------------------------

Fe₂O₃ + 3CO → 2Fe + 3CO₂overall reaction

_{----------------------------------------------}

The other reaction that happens is the slag formation,

CaO + SiO₂→ CaSiO₃(slag)

This technique is called vapour phase refining. Mond’s process and Van Arkel process use vapour phase refining.

Mond’s process

Impure Ni is heated in a stream of carbon monoxide and it forms a Nickel tetracarbonyl complex which is volatile and on decomposition gives the pure metal.

Ni + 4CO →Ni(CO)₄

Ni(CO)₄→ Ni(pure) + 4CO (at 460K)

Van Arkel process

Impure Zr or Ti is heated with Iodine in an evacuated vessel and it forms a volatile complex which on decomposition gives the pure metal.

Zr + 2I₂→ ZrI₄

ZrI₄→Zr(pure) + 2I₂ (at 1800K)

A depressant like NaCN prevents one ore (ZnS) to come to the froth and instead lets the ore PbS to come to the froth.

The auto-reduction of copper sulphide and copper oxide gives rise to copper with blistered appearance which is due to the evolution of SO₂ gas.

Thus, option (i) and (iii) are the correct option.

The following reaction takes place during extraction of chlorine from brine

2Cl^– (aq) + 2H₂O(l) → 2OH^– (aq) + H₂(g) + Cl₂(g)

The for this reaction is +422kJmol^-1. Using the formula

, = -nF

Where, is the Gibb’s free energy

n is the number of moles

F is Faraday’s constant

is the electrode potential

Putting the values in the above formula, we get

=- =-2.2V

Therefore, for the overall reaction is positive and for overall reaction has negative value.

Overall reaction:

2Cl^– (aq) + 2H₂O(l) → 2OH^– (aq) + H₂(g) + Cl₂(g)

The for this reaction is +422kJmol^-1.

Using the formula, = -nF

Where, is the Gibb’s free energy

n is the number of moles

F is Faraday’s constant

is the electrode potential

we get =- =-2.2V

For the reaction to take place the external voltage should be greater than 2.2V otherwise the reaction will not happen.

The ∆G for the formation of FeO is less negative than ∆G for the formation of carbon monoxide from carbon. The summation of both the ∆G will be negative about 1073K. Above 1073K the line for formation of FeO lies above the line for the oxidation of C to CO.

So, in this range, coke will be reducing the FeO and will itself be oxidised to CO.

FeO + C → Fe + CO

Wrought iron is the purest form of commercial iron and is obtained from cast iron by oxidising the impuritiesina reverberatory furnace lined with haematite.

Fe₂O₃ + 3 C → 2 Fe + 3CO

Limestone is added as flux to remove the impurities of sulphur, silicon and phosphorous. They form a slag which can be easily removed. The metal is removed from the slag by passing through rollers.

Copper is extracted by hydrometallurgy from low grade copper ores. Cu is treated with scrap iron or H₂. It is leached out with acid.

Cu²⁺(aq) + H₂(g) → Cu(s) + 2H⁺(aq)

In vapour phase refining method, the impure metal is converted to a volatile complex which on decomposition gives the pure metal. The two basic requirements for refining a metal by Mond’s process or Van Arkel method is

(i) The metal should be react easily with the reagent to form the complex.

(ii) The volatile complex should easily decompose and not give any side products and the recovery should also be easy.

Carbon and hydrogen undergo the following reactions during reduction of metal oxides

2Al₂O₃ + 3C → Al₄C₃ + 3O₂

2Al₂O₃ + 6H₂ → 4AlH₃ + 3O₂

Carbon and hydrogen will not reduce oxides to metals but they will instead form hydrides and carbides.Therefore, they are not used as reducing agents.

Two sulphide ores competing to come to the froth can be separated by adjusting the oil to water proportion or by adding a depressant like NaCN. If we have two sulphide ores, ZnS and PbS, NaCN will prevent ZnS from coming to the froth and instead PbS will form the froth. This way the two ores can be separated.

Haematite ore (Fe₂O₃) is used to line the reverberatory furnace.

Fe₂O₃ + 3 C → 2 Fe + 3CO

Haematite is reduced by coke to wrought iron and the coke gets oxidised to CO.

Compound A is eluted in preference to compound B which means that compound A doesn’t adsorb on the column well and moves down the column with alcohol (eluant), while compound B gets adsorbed well on the column and cannot move down.

Sulphide ore of copper is heated in a furnace after mixing with silica because the iron impurities present in the ore can form a slag with silica and will be easily removed. The copper is produced as copper matte.

FeO + SiO₂→ FeSiO₃(slag)

Sulphide ores cannot be easily reduced so they are usually converted to oxides because oxides can be reduced easily.

Van Arkel method is used for refining Zr and Ti. Van Arkel process involves heating the impure metal with Iodine. It forms a volatile complex which on decomposition at high temperatures, gives back the metal but in very pure form.

Refining of Zr and Ti using Van Arkel method

For Zirconium,

Zr(impure) + 2I₂→ ZrI₄

ZrI₄→Zr(pure) + 2I₂ (at 1800K)

For Titanium,

Ti(impure) + 2I₂→ TiI₄

TiI₄ → Ti(pure) + 2I₂ (at 1800K)

The following considerations are to be made during the extraction of metals by electrochemical method:

(i) Reactivity of the metal produced

(ii) Electrodes to be made of suitable material

(iii) Addition of flux for making molten mass conducting

To remove gangue, certain substances are mixed with it. These are called fluxes. A flux can be basic or acidic. Acidic flux removes basic impurity and basic flux removes acidic impurity.

Flux is also used for increasing conductivity.

SiO₂ (Acidic flux)+CaO (Basic impurity) → CaSiO₃(Slag)

MgO(Basic flux) + SiO₂ (Acidic impurity) → MgSiO₃ (Slag)

Semiconductors are refined by zone refining. Zone refining is based on the principle that impurities are more soluble in the molten metal than in the solid metal. A molten zone moves along the impure rod heating it, the impurities dissolve in the molten zone and move along the rod, while the pure metal devoid of impurities gets left behind and crystallises out.

These are the reactions that happen in the blast furnace at 500K-800K range.

3Fe₂O₃ + CO → 2Fe3O4 + CO₂

Fe₃O₄ + CO →3FeO + 2CO₂

Fe₂O₃ + CO → 2FeO + CO₂

In vapour phase refining method, the impure metal is converted to a volatile complex which on decomposition gives the pure metal. The two basic requirements for refining a metal by vapour phase method:

(i) The metal should be react easily with the reagent to form the complex.

(ii) The volatile complex should easily decomposable so that the recovery is easy.

Theextraction of gold and silver involves leaching the metal with CN^–. This is also an oxidation reaction (Ag → Ag⁺ or Au → Au⁺). The metal is later recovered by displacement method.MacArthur-Forrest process of gold cyanidation takes place as follows

4Au(s) + 8CN^– (aq) + 2H₂O(aq) + O₂(g) → 4[Au(CN)₂] ^– (aq) + 4OH^– (aq)

2[Au(CN)₂] ^– (aq) + Zn(s) → 2Au(s) + [Zn(CN)₄] ^2– (aq)

Zinc places the role of a reducing agent. It reduces Au⁺ to pure Au.

A (2) Pendulum rods are made of Nickel steel alloy

B (4) Malachite is an ore of Copper and it is green in colour. It is a copper carbonate hydroxide mineral and the formula for malachite is CuCO₃.Cu(OH)₂

C (5) Calamine is an ore of zinc. It is more appropriately called Smithsonite or zinc spar and is represented by the formula ZnCO₃.

D (3) Cryolite is a mineral of aluminium. It is used in the electrolysis of bauxite as a flux to lower the melting point and to increase the conductivity.

A(4) In column chromatography different compounds get adsorbed at different levels on the column. This gives rise to coloured bands which represent compounds after they get separated.

B(3) Mond’s process is a vapour phase refining technique where an impure metal like Ni forms a volatile complex with carbon monoxide giving rise to the complex Ni(CO)₄. This complex on decomposition gives rise to the pure Ni metal.

C(1) Germanium and Silicon are semiconductors. They are purified using zone refining method where a molten zone passes over the solid rod. The principle behind this method is that impurities are more soluble in the molten metal than in the solid metal. The pure metal stays behind while the impurities move along with the rod to an adjacent molten zone.

D(2) Mercury has a low boiling point so it can be refined using fractional distillation. Mercury with low boiling point distilles out first and is collected as pure mercury distillate in the receiving flask while the impurities are left behind in the boiling flask.

A. (4) For the extraction of Au, cyanide process is used. An anionic complex [Au(CN)₂]^- forms which is reduced by Zn to pure Au.

4Au(s) + 8CN^– (aq) + 2H₂O(aq) + O₂(g) → 4[Au(CN)₂] ^– (aq) + 4OH^– (aq)

2[Au(CN)₂] ^– (aq) + Zn(s) → 2Au(s) + [Zn(CN)₄] ^2– (aq)

B. (2) By adjusting the proportional of oil to water or by adding depressants in Froth floatation method, an ore can be selectively separated as froth from another. If we want PbS to come out as froth but not ZnS then by adding NaCN, ZnS can be separated from PbS. This is called dressing of an ore.

C. (3) Electrolysis of Al₂O₃ is used to extract Aluminium. This process is called as Hall-Heroult’s process. The following reactions take place at the cathode and anode during electrolysis

Cathode: Al³⁺ (melt) + 3e^–→ Al(l)

Anode: C(s) + O₂^– (melt) → CO(g) + 2e^–

C(s) + 2O₂^– (melt) → CO₂ (g) + 4e^–

D. (1) Germanium is purified using zone refining method where a molten zone passes over the solid rod. The principle behind this method is that impurities are more soluble in the molten metal than in the solid metal. The pure metal stays behind while the impurities move along with the rod to an adjacent molten zone which can be eventually cut off. In the end, we get ultrapure Germanium which can be used as a semiconductor.

A(3) Sapphire is a gemstone of corundum which contains trace amounts of Co which gives it the characteristic blue colour.

B(4) Sphalerite is a mineral and chief ore of zinc. It consists of ZnS

C(2) Depressants like NaCN are used to prevent two sulphide ores from forming froth in the Froth floatation method used to extract sulphide ores. NaCN suppresses ZnS and allows PbS to come to the froth.

D(1) Corundum is a crystalline form of alumina (Al₂O₃)

A (2) 2Cu₂O + Cu₂S → 6Cu + SO₂

The copper produced is called Blister Copper because the evolution of SO₂ gives the metal a blister appearance

B(3) A Blast furnace is used to reduce iron oxides to iron in different temperature ranges.

C(4) FeO + SiO₂→ FeSiO₃

Roasting of sulphide ores is done in a reverberatory furnace. The sulphide ores are converted to oxides. If there is any iron present, it is mixed with silica and forms a slag which can be easily removed.

D(1) Hall-Heroult process is used in the extraction of Aluminium

Nickel when heated with carbon monoxide forms a highly volatile complex, Ni(CO)₄ which when heated further decomposes to form pure Ni

Ni + 4CO →Ni(CO)₄

Ni(CO)₄→ Ni(pure) + 4CO (at 460K)

This technique is called vapour phase refining and this reaction is called Mond’s process.

Zirconium also forms a volatile complex with Iodine which when decomposed gives pure Zr. It undergoes vapour phase refining and this process is called Van Arkel method.

Zr + 2I₂→ ZrI₄

ZrI₄→Zr(pure) + 2I₂ (at 1800K)

Sulphide ores are wetted by oil and gangue particles are wetted by water. A paddle agitates the mixture which leads to the formation of froth. Froth carries away the minerals. Cresols are used to stabilise the froth but that is not the reason for concentrating sulphide ores by this method. So, the two statements are individually correct but second statement is not the reason.

The two statements are individually correct. Zone refining is based on the priniciple that impurities are more soluble in molten metal than in solid, so when the molten zone passes over the rod, impurities move along with the melt and the pure metal is left behind. This produces semiconductors of high purity. Semiconductors being of high purity is a consequence of zone refining and not a reason for doing zone refining.

Copper is indeed extracted by hydrometallurgy and hydrometallurgy involves precipitating the ore by a more electropositive metal so the two statements are individually correct but reason does not explain the assertion made. The reason statement is simply a consequence of the assertion statement.

(a) At temperatures below 710K, ∆G°_(C,CO2) <∆G°_(C,CO), so CO₂ is a better reducing agent than CO but for temperatures above 710K, ∆G°_(C,CO) <∆G°_(C,CO2) so CO is a better reducing agent. The lower the value of ∆G°it is better because the difference will then be large and negative which makes the reaction more favourable.

(b) Reduction of sulphide ore is not easy, so it is usually converted into oxide and then reduced. Also, in Ellingham diagram of sulphides, ∆_fG of M_xS is not compensated and reduction becomes tougher.

(c) Silica is added to the sulphide ore of copper to remove iron impurities as slag. Copper is formed as copper matte which contains Cu₂S and FeS.

FeO + SiO₂→ FeSiO₃(slag)

(d) Carbon and hydrogen undergo the following reactions with metal oxides

2Al₂O₃ + 3C → Al₄C₃ + 3O₂

2Al₂O₃ + 6H₂ → 4AlH₃ + 3O₂

Carbon and hydrogen will not reduce oxides to metals but they will instead form hydrides and carbides.Therefore, they are not used as reducing agents.

(e) In vapour phase refining method, the impure metal is converted to a volatile complex which on decomposition gives the pure metal.

Van Arkel’s process is used to purify metals like Zr and Ti. The impure metal is heated with Iodine and it forms a volatile complex TiI₄ which on decomposition gives pure Ti.

Ti(impure) + 2I₂→TiI₄

TiI₄→Ti(pure) + 2I₂ (1800K)