2. Metallurgy of Some Important Metals

Metallurgy of Some Important Metals

1.   Extraction of iron (Fe)

Ore: Haematite

Oxide ores of iron, after concentration through calcination / roasting in reverberatory furnace (to remove water, to decompose carbonates and to oxidise sulphides) are mixed with lime stone and coke and fed into a Blast furnace from its top with the help of a cup and cone arrangement. Here, the oxide is reduced to the metal.

Thermodynamics helps us to understand how coke reduces the oxide and why this furnace is chosen. One of the main reduction steps in this process is :

  FeO(s) + C(s)  ® Fe(s/l) + CO (g)     ........ (11)

It can be seen as a couple of two simpler reactions. In one, the reduction of FeO is taking place and in the other, C is being oxidised to CO :   

FeO(s) ®  Fe(s) + O₂ (g)     [DG_{(FeO, Fe)} ]          .............. (12)

 C(s) +  O₂ (g) ®   CO (g)     [DG_{(C, CO)} ]           .............. (13)

 When both the reactions take place to yield the equation (10), the net Gibbs energy change becomes:

D G _(C,CO) + D G _{(FeO, Fe)} = D_rG               .............. (14)

Naturally, the resultant reaction will take place when the right hand side in equation (14)  is negative. In D G⁰ vs T plot representing reaction (12), the plot goes upward and that representing the change C CO (C,CO) goes downward. At temperatures above 1073K (approx.), the C,CO line comes below the Fe,FeO line

 [DG_{(C, CO)}  <  DG_{(Fe, FeO)}]. So in this range, coke will be reducing the FeO and will itself be oxidised to CO. In a similar way the reduction of Fe₃O₄ and Fe₂O₃  at relatively lower temperatures by CO can be explained on the basis of lower lying points of intersection of their curves with the CO, CO₂ curve in the given  figure.

In the Blast furnace, reduction of iron oxides takes place in different temperature ranges. Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about 2200K in the lower portion itself. The burning of coke therefore supplies most of the heat required in the process. The CO and heat moves to upper part of the furnace. In upper part, the temperature is lower and the iron oxides (Fe₂O₃ and Fe₃O₄) coming from the top are reduced in steps to FeO.

Reactions involved : The reactions proceed in several stages at different temperatures. Since the air passes through in a few seconds, the individual reactions does not reach equilibrium.

At 500 – 800 K (lower temperature range in the blast furnace)

3 Fe₂O₃ + CO®2 Fe₃O₄ + CO₂

Fe₃O₄ + CO ® 3Fe + 4 CO₂

Fe₂O₃ + CO ®  2FeO + CO₂ 

 At 900 – 1500 K (higher temperature range in the blast furnace):

C + CO₂ ® 2 CO

FeO + CO ® Fe + CO₂

 Limestone is also decomposed tom CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.

  CaCO3  ® CaO + CO₂

CaO + SiO₂ ® CaSiO₃

Note :

(1) CaSiO₃ is used in cement industry or used in making building material.

(2) Slag CaSiO₃ is ligther than molten Iron, get collected above it and protect iron from oxidation.

Pig Iron : Iron obtained from blast furnance contain 4%C & smaller amount of impurities of S , P , Si , Mn.

Cast Iron : Made by melting pig iron with scarp iron & coke & blast of hot air is passed through the molten mass. Contain about 3% carbon.

Wrought Iron : Purest form of iron prepared from cast iron by oxidising impurities in a reverbertory furnance lined with haematite. It have carbon about 0.1 to 0.21%

Note : (1) In steel making pure O₂ is poured into molten metal where all impurities are converted to oxides then forming slag (basic oxygen) .

(2) Steel making process, from iron is an oxidation process.

(3) In production of steel from haematite ore involve first reduction & oxidation chemical process.

2. Extraction of Copper

(i)  From Cuprous oxide

 Sulphide ore are roasted to give oxide.

 2 Cu₂S + 3O₂ ® 2Cu₂O + 2SO₂ 

 As Cu2O line in D G V/s T graph is almost at top. so it is easily reduce to metal by heating with coke.

Cu₂O + C ® 2 Cu + CO

(ii)   From Copper Glance/ Copper Pyrite [self reduction]

Ore + flux Slag + copper matte

[CuFeS₂]                 FeSiO₃  [Cu₂S + FeS]

(i) 2 CuFeS₂ + 4O₂ ® Cu₂S + 2FeO +  3SO₂.                        

(ii) Cu₂S + FeO + SiO₂ ® FeSiO₃ + Cu₂S.

                                                    Fusible slag       Matte  

* Chemical composition of matte = (Mostly Cu₂|S + some FeS)

2 FeS + 3O₂ ® 2 Fe + 2 SO₂ , 

FeO + SiO₂ ® FeSiO₃ [Slag]

Cu₂S + 3O₂ ® 2 Cu₂O + 2 SO₂

2 Cu₂O + Cu₂S ® 6 Cu + SO₂ [Self Reduction ]   

Note : Solidified copper has blistered appaerance due to evolution of SO₂ so called as blister copper.

(iii)    From Low Grade Ore

Þ  From low grade ore , Cu is extracted by hydrometallurgy. (leaching)

 (a) Low grade ore of Cu

Cu₂O (Cuprite)

Cu₂S [Copper glance] +   CuSO₄

Cu(OH)₂ . CuCO₃ (Malachite)

(b) CuSO₄      Cu

(i) Electrolysing or

(ii) Metal displacement with Fe

Electrolytic Refining [Cu is purified by electro refining]

Anode Þ impure Cu & cathode = pure Cu.

Electrolyte Þ aqueous solution of CuSO₄ :

Anode Þ Cu(s) ® Cu₂+(aq) + 2e–

Cathode Þ Cu₂+(aq) + 2e ®Cu(s)

Anode Mud Þ Ag , Au , Pt.

3. Extraction of lead :

 Ore: PbS (Lead sulphide)

 There are two methods of extracting the element :

(i)  Roast in air to give PbO, and then reduce with coke or CO in a blast furnace.

2PbS(s)  + 3O₂ (g)  2PbO (s)  2Pb(l) + CO₂ (g)

(ii) PbS is partially oxidized by heating and blowing air through it. After some time the air is turned off and heating is continued. The mixture undergoes self reduction as given below. 

3PbS(s)    PbS (s) + 2PbO (s)   3Pb(l) + SO₂ (g)

Chart3:

4.  Extraction of zinc :

Ore: ZnS (Zinc blende)

The ore is roasted in presence of excess of air at temperature  1200 K.

2 ZnS + 3O₂ ® 2 ZnO + 2SO₂ 

The reduction of zinc oxide is done using coke. The temperature in this case is higher than that in case of copper. For the purpose of heating, the oxide is made into brickettes with coke and clay.

  ZnO + C  Zn + CO

The metal is distilled off and collected by rapid chilling.

Note :

ZnO may be reduced by carbon monoxide at 1473 K (i.e. 1200°C) in smelter. The reaction is reversible, and the high temperature is required to move the equilibrium to the right. At this temperature the Zn is gaseous. If the gaseous mixture of Zn and CO₂ was simply removed fom the furnace and cooled, then reoxidation of Zn would occur. Thus the zinc powder obtained would contain large amounts of ZnO.

ZnO + CO  Zn + CO₂

 Chart4 :

5.  Extraction of tin from cassiterite (SnO₂)

(i) Washing with water Þ To remove silicious impurities.

 (ii) Electromagnetic sepration Þ To remove magnetic impurity of wolframite.

(iii) Roasting

(a) To remove volatile impurities S as SO₂ As as AS₂O₃ and Sb as Sb₂O₃.

 (b) ore contain impurites of CuS & FeS which are converted to their sulphate

  CuS + 2O₂ ¾® CuSO₄, FeS + 2O₂ ¾®FeSO₄

(iv) Leaching ® CuSO & FeSO₄ dissolve in water.

(v)  Washing ® Ore is washed with running water. obtained orecontain 60-70% SnO₂ & called as Black Tin.

(vi)  Smelting : SnO₂ is reduced by Carbon.

Product contain traces of Fe which is removed by passing air through molten mixture.

SnO₂ + C  ¾® Sn + CO

2Fe + O₂  ¾®2FeO

Chart5:         

6.  Extraction of Magnesium :

(i)  From Carnallite :

The ore is dehydrated in a current of hydrogen chloride and the mixture of fused chlorides is electrolysed. 

(ii)  From Sea water (Dow’s process) :

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 

(b) Preparation of hexahydrated magnesium chloride :

Mg(OH)₂ + 2HCl(aq) ®  MgCl₂ + 2H₂O

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 anhydrous 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 973 – 1023 K. The molten mixture is electrolysed. Magnesium is liberated at the cathode (iron pot) and chlorine is evolved at graphite anode.

MgCl₂  Mg²⁺ + 2Cl^–         

 

At cathode : Mg2⁺ + 2^e– ®  Mg(99% pure) ;          

At anode  : 2C^l–® Cl₂ + 2e^– 

A stream of coal gas is passed through the pot to prevent oxidation of magnesium metal. The magnesium obtained in liquid state is purified by distillation under reduced pressure.
(1 mm of Hg at 873 K).  

7.  Extraction of gold and silver (MacArthur-Forrest cyanide process) :

Chart6 :

Chart7:  

8. Extraction of Gold & Silver

MacArthur-forrest Cyanide process

(A) From native ore :

(i) Complex formation with CN¯ in presence of air

 Ag + CN¯ + O₂  ¾® [Ag(CN)₂]¯(aq)

In this process, air is used which oxidises metal M(Ag/Au) to M⁺(Ag⁺/Au⁺) which form complex with CN¯ ion.

(ii) Metal displacement with Zn

[Ag(CN)2]¯ + Zn  ¾® [Zn(CN)₄]^2–(aq) + Ag(s)

This is leaching (Hydro Meltallurgy) process.

(B) From Argentite ore

Extraction of Ag from Argentite ore by dissolving in NaCN and then using metal displacement is leaching process,which is an example of hydrometallurgy.

 Ag₂S (conc. ore) + 2 NaCN  2AgCN + Na₂S

Role of air : As Ag₂S & AgCN are in equilibrium , air oxidise

  Na₂S to Na₂SO₄ & shift the equilibrium right.

4 Na₂S + 5O₂ + 2H₂O ® 2Na₂SO₄ + 4 NaOH + 2S

AgCN + NaCN ® Na [Ag(CN)₂] " soluble complex "

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

9. Extraction of Mercury

Chart8: