BANSAL CLASSES METALLURGY

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ORES, MINERALS AND EXTRACTIVE METALLURGY

Ores and minerals Commonly occurring ores of iron, copper, lead, magnesium and aluminium.

(Mains Only : Tin and silver)

Extractive metallurgy (Chemical principles and reactions only; industrial details excluded): Carbon reduction method (iron)(Mains Only : Tin also),

Self reduction method (copper and lead),

Electrolytic reduction method (magnesium and aluminium),

(Mains Only : cyanide process :Gold and silver)

ORES AND MINERALS

Metals occur in nature in combined form as minerals.

Minerals from which a metal can be profitably extracted is termed as ore. e.g. FeS₂ is a mineral of iron, not anore.

IMPORTANT ORES OF SOME METALS (Principal ore is given in bold letters) Iron: In the combined state, iron occurs in the following minerals.

Haematite, Fe₂O₃ Magnetite, Fe₃O₄ Limonite, 3Fe₂O₃ · 3H₂O Spathic iron ore, FeCO₃ Tin: Cassiterite or tin stone, SnO₂.

Copper occurs in the native state as well as in the compounds form. The natural ores of copper are

Copper pyrites, CuFeS₂ Malachite, Cu(OH)₂ · CuCO₃(green) Cuprite or ruby copper, Cu₂O Azurite, Cu(OH)₂·2CuCO₃

Copper glance, Cu₂S Cu₅FeS₄(peacock ore) Lead:

Galena, PbS Cerussite, PbCO₃ Anglesite, PbSO₄ Wulfenite, PbMnO₄ Stolzite, PbWO₄

Magnesium :

Dolomite, MgCO₃ · CaCO₃ Carnallite, MgCl₂·KCl·6H₂O

Magnesite, MgCO₃ Epsomite(epsom salt), MgCO₃·7H₂O Kiesserite, MgSO₄·H₂O Kainite, MgSO₄·KCl·3H₂O

Schonite, MgSO₄·K₂SO₄·6H₂O

Magnesium is widely distributed in nature in rocks, spring and seawater. In rocks and silicates it occurs in mineral like olivine (Mg₂SiO₄), spinel (MgAl₂O₄),

talc (Mg₃H₂(SiO₃)₄), asbestos (CaMg₃(SiO₃)₄), etc.

Aluminium: Aluminium is the third most abundant element of earth’s crust.

Oxides: Corundum, Al₂O₃; diaspore, Al₂O₃·H₂O and bauxite, Al₂O₃·2H₂O.

Fluorides: Cryolite, Na₃AlF₆

Silicates: Feldspar, KAlSiO₃O₈, mica(KAlSi₃O₁₀(OH)₂) and kaolinite (Al(OH)₄, Si₂O₅)

Basic Sulphates: Alunite or alumstone, K₂SO₄·Al₂(SO₄)₃·2Al(OH)₃

Basic Phosphates: Turquoise, AlPO₄·Al(OH)₃·H₂O

Aluminates: Aluminates of Mg, Fe and Mn.

Silver in the native form is associated with copper and gold. The main ores of silver are

Argentite or silver glance, Ag₂S Horn silver, AgCl

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TYPES OF METALLURGY

(a) pyro metallurgy : Extraction of metal from ore by using heat energy. Seteps involved are : Calcination, roasting, reduction etc.

Ex. Less reactive metals : Cu, Fe, CO, Ni, Zn, Snm Pb etc. (b) Hydro metallurgy : (Ag, Au, Cu) - This is wet metallurgy process

Cu  Pyro + Hydro

Ag and Au  By cynide process.

Steps are : (i) Complex formation (ii) Metal displacement (i) AgCl or AuCl _NaCN _ _Na[Ag(CN)

2]

(Sodium argento cynide) (ii) 2 Na [Ag(CN)₂] _Zn _Ag__{+ Na}

2 [Zn(CN)4] (Impure)

(c) Electrocal metallurgy : This process used for highly electro positive metal (s-block and A) metal obtained by electrolysis of fused salt/anhydrous medium.

(d) Ion exchange metallurgy : Transuranic element are obtained by this method. STEPS INVOLVED IN THE EXTRACTION OF METALS The extraction of metal from its ore is completed in the following four steps.

(a) Crushing and grinding (b) Pulverisation (c) Concentration of the ore (d) Reduction to the metal (e) Refining of the metal.

CONCENTRATION OF THE ORE

The removal of impurities from the ore is called its concentration. It is carried out in one or more of the following steps. Thes undesired impurity are gengue or matrix.

By Physicals seperation

(a) Gravity separation (Lavigation) : The method of concentration of the ore is based on the difference in the specific lavigation gravities of the ore and the gangue particles.

Powdered ore is agitated with a running stream of water. The lighter gangue particles are taken away by water while heavier ore particles settle down. Ex. Oxygenated ores.

(b) Froth Floatation Method : This method is mainly employed for the concentration of sulphide ores.

The method is based on the difference wetting characteristics of the gangue and the sulplide ore with water and oil. The gangue preferrentially watted by water and the ore by oil.

The crushed ore along with water is taken in a floatation cell. Various substances are added depending on the nature of the ore and a current of air is blown in. The substances added are usually of three types.

(i) Frothers : The generate a stable froth which rises to the top of the tank. Example of frother is pine oil, Eucalyptus oil etc.

(ii) Collectors or floating agents: These attach themselves by polar ground to the grain of the ores which then become water repellant and pass on into the froth. Example : sodiumethyl / ethyly xanthate. (iii) Activators or Depressants : These reagents activate or depress the flotation properly and help

in the separation of different sulphide ores present in a mixture. An example of depressant is NaCN, An activator is CuSO₄.

(c) Magnetic Separation: If the ore and not the gangue or the gangue and not the ore is attracted by a magnet, the two can be separated by this method.

FROTH FLOATATION COMPRESSED AIR OIL WATER MIXTURE     

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FROTH SKIMMER CONCENTRATED ORE GANGUE Fig.2

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Magnetite (Fe₃O₄) is concentrated by this method, (FeWO₄) wolframite removed from SnO₂, FeO removed from chromite (Cr₂O₃FeO).

By Chemical Separation:

Some of the ores are concentrated by means of chemical treatment.

Fig.1 MAGNETIC SEPARATION ORE NON-MAGNETIC PARTICLES MAGNETIC PARTICLES

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ELECTRO-MAGNETIC WHEEL

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Leaching: It involues the treatment of the ore with a suitable reagent. as to makee it soluble while impurity remain insoluble. The ore is recoverd from the solution by suitable chemical method. (i) Beyer’s Process : Ex. Bauxite ore contain impurity Fe₂O₃, TiO₂, SiO₂ when it dissolve in aq.

NaOH/pressure + 150°C bauxite is dissolve but other are not dissolve. Al₂O₃ + 2 NaOH  2 NaAlO₂ + H₂O

NaAlO₂ + 2H₂O  Al(OH)₃ + NaOH

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Al₂O₃ + 3H₂O

(ii) Cyanide process: Their process used for Au, Ag by (Mac-Arthur forrest cyanide process). CALCINATION

Calcination is a process in which ore is heated, generally in the absence of air, to expel water from a hydrated oxide or carbon dioxide from a carbonate at temperature below their melting points. For Example :

(i) Al₂O₃. 2H₂O Al₂O₃ + 2H₂O, 2 Al(OH)₃  Al₂O₃ + 3 H₂O (ii) 2Fe₂O₃. 3H₂O  2Fe₂O₃ + 3H₂O

(iii) CaCO₃  CaO + CO₂

(iv) CaCO₃. MgCO₃  CaO + MgO + 2CO₂_

(v) MgCO₃  MgO + CO₂_

(vi) ZnCO₃  ZnO + CO₂_

(vii) FeCO₃  FeO + CO₂_

Advantanges of Calcination:

(i) Moisture is removed. (ii) Organic matter is destroyed (iii) The hydroxide and carbonates ores are convered into their oxides. (iv) The metal become porous and easily workable.

ROASTING (Metal sulphides  Metal oxide + SO₂)

The removal of excess sulphur contained in sulphide ores by heating in an excess of air is called roasting.

The cencentrated sulphide ore is heated in reverberatory furnace, below its melting poing of fusion temperature in the presence of an excess of air with or without the addition of an external substance. In roasting desinite chemical like oxidation, chlorination etc., take place but in calcination does not

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occur any major chemical changes. (i) 2 ZnS + 3O₂ 2 ZnO + 2SO₂ (ii) ZnS + 2O₂  ZnSO₄

(iii) 2 Cu₄s + 3O₂  2 Cu₂O + SO₂_ (iv) 4 FeS₂ + 11O₂  2Fe₂O₃ + 8SO₂ (v) HgS + O₂  HgO + SO₂ (vi) 2 As₂S₃ + 9O₂  2 As₂O₃ + 6 SO₂ Advantages of Roasting:

(i) Excess of sulphur is removed as volattile oxide. S + O₂  SO₂ _

(air)

(ii) The metal sulphide is converted into metal oxide.

(iii) Impurities of arsenic and antinony are removed as their volatile oxides. Sb₄ + 3O₂  2Sb₂O₃

As₄ + 3O₂  2As₂O₃ Types of Roasting :

(a) oxidising S  SO₂

(b) Partical oxidising or sulphating

2 SO 2 4 SO S

(c) Chlorinating S  Cl By substitution reaction In partical roasting process : In Pbs or ZnS

2PbS + 3 O₂  2PbO + 2 So₂ PbS + 2 O₂  PbSO₄

REDUCTION OF ORE OF THE METAL

The calcined roasted ore in then reduced to the metallic state in either of the following ways. (a) Reduction by carbon (Smelting) : (This is common method of reduction)

“Reduction of the oxide with carbon at high temperature is known as smalting”.

The oxides of less electropositive metals like Pb, Zn, Fem Sn,Cu, etc,are reduced by strongly heating them with coal or cock, in the blast furnace.

Slag : Fusible metarial during reduction process. Slag : Gangue + substance (for remove gangue). Fluxes acidic : Borax, SiO₂ (Remove basic impurity)

Fluxes basic : MgO, MgCO₃,CaCO₃ (Remove acidic impurity) Smelting:

* Concentrate ore (ore + gangue) + RA (carbon) + Flux (RA Reducing agent)

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Metal + Slag + gases

* Cr₂O₃ Carbon is not used for reduction

Mn₃O₄ Carbon is not used for reduction MnO₂ Carbon is not used for reduction

* Coke is not used for reduction of s-block oxide Al₂O₃ (due to formation of metalcarbides) CaO + 2 C  CaC₂ + CO Some reactions: (i) CuO + CO  CO₂+ Cu PbO + C  Pb + CO Fe₂O₃ + 3C  Fe + 3CO ZnO + C  Zn + CO ZnO + CO  Zn + CO₂

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SiO₂ + CaCO₃  CaSiO₃ + CO₂ _ P₂O₅ + 3 CaO  Ca₃ (PO₄)₂ (iii) Basic impurity + Flux  slag

MgCO₃ + SiO₂  MgSiO₃ + CO₂ _ FeO + SiO₂  FeSiO₃ Note: Blue flame is obtained by burning of CO in smelting process.

(b) Self Reduction: Compounds of certain metals are reduced to metal without using any additional reducing agent.ores of Cu, Pb, Hg etc.

Their sulphide ores are partially roasted to give some oxide. This oxide is now reduced to the metal by the remaning sulphide ore at elevated temperatures in the absence of air. The process is known as self reduction.

Self Reduction for Pb:

(i) 2PbS + 3O₂ _{}Roasting _ _{2 PbO + 2SO}

2 (Galena) (Air) PbS + 2PbO High temperature Absence of air  3Pb + 2SO₂

(unroasted ore) (roasted ore) (Self reduction) (ii) 2 Cu₂O + Cu₂S _ _ _{6 Cu + SO}

2

(c) Metal Displacement method: In this method a water soluble compound is obtained from the ore. The aqueous solution of the compound is reacted with a more electropositive metal which displaces, the metal from the solution.

(i) Zairvogel process for silver. Ag₂S + 2O₂ _{85 C}0

 Ag₂SO₄

Argentite

Ag₂SO₄ (aq) + Cu  CuSO₄ (aq) + 2 Ag (S) (Scrap copper)

(ii) Seperation Ag by complex formation (Cynide process)

Silver and gold are extracted by a method involving complex formation. Ag₂S + 4 NaCN _Air _{2 Na [Ag(CN)}

2] + Na2SO4 (powdered argentite)

2 Na [Ag(CN)₂] + Zn Na₂ [Zn (CN)₄] (aq) + 2 Ag _ Black ptt. (d) Electrolytic reduction

This process is mainly used for the extraction of highly electropositive metals.

Electrolysis is caried out in a large cells and a small amount of another suitable electrolyte is added which:

(i) Lower the melting point of the main electrolyte (ii) Enhances its conductivity

(iii) Reduces corrosion troubles Ex.: Na, K, Mg, Ca, Al, etc.,

e.g., Manufacture of metallic sodium (Down’s process)

Molten NaCl containing a little CaCl₂ is electrolyzed between graphite anode and iron cathode. The various reactions that take place are

On Fusion : NaCl __ Na+_{+ Cl}–_{(Ions become mobile)} On Electrolysis : At Cathode : Na+_{+ e}– _ _{Na (reduction)}

(Metallic sodium) At Anode : 2 Cl– __Cl

2 (g) + 2e–

(e) Reduction by Al: This process is employed in the case of those metals which have very high melting points and are to be extracted fromtheir oxides.

Cr₂O₃ + 2 Al  2Cr + Al₂ O₃ 3 Mn₃O₄ + 8 Al  9 Mn + 4 Al₂O₃

REFINING OF METALS

Metals obtained by the reduction of its compound still contains some objectionable substance and have to be refined.

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Depending upon the nature of the metal and impurties, the following methods are used for purification of the metals.

(a) Liquation: This method is used for the refining of metals having low melting point and are associated with high metal uimpurities.

Ex. Pb, Sn, Sb, Bi and Hg.

The impure metal is heatd on the sloping hearth of a furnace.

The pure metal flows down leaving behind the non-fusible material on the hearth.

(b) Distillation : metals having low boiling point are refined by this method, for example, zinc, cadmium and mercury

(c) Distribution method : The parke’s process, is used for the disilverization lead (PbS + 01 to 1% Ag) is based on distrubution priciple. The principle follows as

* Molten zinc and molten lean form a two phase system. * Silver is more siluble in molten zinc then the molten lead. * The Zn- Ag alloys is lighter than Pb and freezes fast.

* The floating solidified Zn- Ag Alloy can be easily removed from molten Pb. * From Zn- Ag alloy, zinc is seperated by distillation.

(d) Zone refining or fractional crystallisation: Metals of very high purity are obtained by zone refining.

This refining method is based on the fact that impurities tend to remain dissolved in molten metal. Ge, Si and Ga used as semiconductors are refined in this manner.

(e) Oxidation process (Pyrometallergical oxidation process) : These processes are used for refining of metals associated with impurities having high affinity for oxygen than the metal itself. Cupellation, pudding and bessemerization are important oxidation processes employed for refining different metals.

(f) Electro-refinig of Metals : Metals such as Cu, Zn, Ag, Sn, Al, Ni, Cr are refining by this method. The impure metal is mede the anode of a electrolytic cell, while cathode is thin plate of pure metal. Electrolyte is the solution of a double salt of the metal.

On passing the electric current pure metal from the anode dissolves and gets deposited at the cathode.

The soluble impurities go into the solution while insoluble or less electropositive impurities settle down below the anode as anode mud or sludge . Ex.

Electrorefining of copper

Anode : Blister copper (90%) Cathode : Pure copper

Electrolyte : An aqueous solution of CuSO₄ (15% + 5 % dill H₂SO₄)

Electrorefining of Pb (Bett’s Process)

Anode : Impure lead., Cathode : Pure lead. Electrolyte : A mixture of PbSiF₆ and H₂SiF₆

(g) Vapour phase refining : (i) Van-Arkel Process :

Employed to get metal in very pure from of small quantities.

In this method, the metal is converted into a volatile unsteadble compound. (e.g., ionide), and impurities are not affected during compound formation. The compound thus obtained is decomposed toget the pure metal. Employed for purification of metals like titanium and zirconium.

Ti (s) + 2 I₂ _{}523 K _{Ti l}

4(g) Impure

Til₄(g) _{}1700 K _{Ti(s) + 2I}

2(g)

(ii) Mond’s process : Nickel is purified by using CO gas. This involves the formation of nickel tetracarbonyl.

Ni_(impure) + 4 CO _ _ _[Ni(CO)

4]   Ni(pure) + 4CO 

(h) Some other methods :

(i) Kroll collen process

TiCl₄ + 2Mg _ _ _2MgCl

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(ii) IMI method

TiCl₄ + 4 Na _ _ _{4 NaCl + Ti}

(iii) Amalgamation process

for nobel metal Au, Ag from the native ore.

or powder + Hg _{}distilled _{Almalgam (i) Hg(vapour)} _{(ii) Metal}

FURNACES. Furnaces are either powered by fuel (mostly coal, as in blast furnace or reverberatory furnace) or by electricity. Fuel fired furnaces generator maximum temperature of 1400-1500°C while electric furnaces can supply as high as 3000° temperature.

Kilns. When fuel and ore are mixed and heated. No reduction occurs. e.g. Lime Kilns.

Blast Furnace.Fuel and ore are mixed and charged from the top of furnace and hot air is blown from the holes (tuyers) near the bottom. The ore is reduced as it descends down.

Reverberatory Furnace.Material to be heated is placed on the hearth of a reverberatory furnace. As hot air is blown in (see fig.) flames rise and hit the concave top of the furnace, thereby turning in and heating the hearth. fuel and ore are separate in this case.

Fig. 3

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GENERAL CLASSIFICATION OF EXTRACTION PROCESSES

An ore to be treated must first be examined. Following methods should be tried (in this sequence).

Mechanical Separation If metal occurs in its native state(e.g. gold), it is simply separated mechanically by crushing the nuggets or rocks and separating it.

Thermal Decomposition : If a particular compound can simply decompose on heating then the metal is recovered by simple heating. e.g. Hg from HgO. Two interesting processes are described below.

(i) Mond’s Process. When nickel oxide is heated with water gas (CO + H₂), H₂ reduces nickel oxide to nickel, which readily combines with CO to form Ni(CO)₄ , a highly inflammable, volatile gas. This gas, when separated out and heated to 180°C, decomposes to give pure nickel.

(ii) van Arkel’s Process. Many metals (e.g. zirconium) form volatile iodides which, when contacted with hot

tungsten wire, release I₂ and the metal is deposited on the wire. When sufficient quantity of metal has been deposited, tungsten core is bored out. This method is used for obtaining small quantities of highly pure metal.

Displacement Method : In this method a cheaper metal, which occupies a higher place in electrochemical series, displaces a costlier metal from its salt solution. e.g. displacement of gold and silver from their solution by scrap zinc in cyanide process. Another example is treatment of lean ore of copper (containing very small amount of copper). Such ores are dug out and dumped in trenches in open. The rainwater collects in trenches and dissolves the sulphides when they are oxidised to sulphates by atmospheric oxygen. CuS + 2O₂ ¾® CuSO₄(aq)

After a year or two, dilute solution of copper sulphate is simply pumped out leaving behind all the mud and other impurities. This solution, when treated with scrap iron, precipitates copper.

Cu2+_{(aq) + Fe ¾® Fe}2+_{(aq) + Cu}

High Temperature Chemical Reduction.

(i) By Carbon. e.g. in metallurgy of iron and tin (discussed later).

(ii) Thermite Process : Reduction by aluminium is highly exothermic, so much so that the products are formed

in molten state. This is thermite process. e.g. Cr₂O₃ + 2Al ¾® Al₂O₃ + 2Cr + heat

(iii) Self Reduction as in the case od copper and lead (discussed later). ELECTROLYTIC REDUCTION

(i) In aqueous solution and

(ii) In fused melts, if the metal is too reactive. It is costly, hence it is resorted to only when no other method is available. e.g. for Na, Mg, Al etc. (discussed later).

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EXTRACTIVE METALLURGY IRON AND TIN

Both iron and tin are extracted by the carbon reduction method. Extraction of Iron

Iron is extracted from its principal ore, haematite. After the preliminary washing, concentration and roasting, the ore is smelted in the presence of coke and limestone in a blast furnace (fig.1).

Roasted ore (8 parts) with desulphurized coke (4 parts) and limestone pieces (1 part) is fed into the blast furnace from the top. Preheated air is blown in through water-jacketed pipes called tuyeres fixed in the lower part of the furnace. There is a temperature gradient as we move from the bottom (temperature about 2000K) to the top (temperature about 500K) of the blast furnace. The blast furnace may be broadly divided into three main parts as described in the following.

1. Zinc of fusion The lower portion where coke burns and produced carbon dioxide and a lot of heating is known as zone of fusion:

C + O₂ ® CO₂ DH = -406 kJ mol-1

Here the temperature is about 1775 K. A little above this, where temperature is above this, where temperature is about 1475 K - 1575 K, iron coming from above melts.

2. Zone of heat absorption The middle portion (temperature 1075 K - 1275 K), CO₂ rising up is reduced to CO with the absorption of heat:

CO₂ + C ® 2CO DH = 163 kJ mol-1

In this portion, limestone coming from above is decomposed and the resultant lime (CaO), which acts as flux, combines with silica (present as impurity-gangue) to form calcium silicate (fusible slag):

CaCO₃ ® CaO + CO₂

CaO + SiO₂ ® CaSiO₃

3. Zone of reduction The upper portion (675K -975K) where iron oxide is reduced to spongy iron by carbon monoxide rising up the furnace:

Fe₂O₃ + 3CO ® 2Fe + 3CO₂ The reduction is believed to take place in stages:

3Fe₂O₃ + CO ® 2Fe₃O₄ + CO₂ Fe₃O₄ + CO ® 3FeO + CO₂ FeO + CO ® Fe + CO₂

Fig.4

BLAST FURNACE WASTE GASES WASTE GASES WASTE GASES WASTE GASES WASTE GASES FIRE BRICKS FIRE BRICKS FIRE BRICKS FIRE BRICKS FIRE BRICKS

HOT AIR BLAST HOT AIR BLASTHOT AIR BLAST HOT AIR BLASTHOT AIR BLAST

SLAG SLAGSLAG SLAG SLAG 4444K 4444K4444K 4444K 4444K 4444K 4444K4444K 4444K4444K 4444K 4444K4444K 4444K4444K IRON IRON IRON IRON IRON HEARTH HEARTHHEARTH HEARTHHEARTH 444 444 444 444 444KKKKK 4444 4444 4444 4444 4444KKKKK

CHARGE (ORE, LIMESTONE AND COKE)

GASES RISEGASES RISEGASES RISEGASES RISEGASES RISE

C4 O C4 O C4 O C4 O C4 O 4 4 4 4 4  CO CO CO CO CO44444

SOLID CHARGE DESCENDSSOLID CHARGE DESCENDSSOLID CHARGE DESCENDSSOLID CHARGE DESCENDSSOLID CHARGE DESCENDS

4Fe 4Fe 4Fe 4Fe

4Fe₄₄₄₄₄OOOOO₄₄₄₄₄ 4 CO 4 CO 4 CO 4 CO 4 CO  4Fe 4Fe 4Fe 4Fe 4Fe₄₄₄₄₄OOOOO₄₄₄₄₄ - CO - CO - CO - CO - CO₄₄₄₄₄ CaCO

CaCO CaCO CaCO

CaCO₄₄₄₄₄  CaO 4 CO CaO 4 CO CaO 4 CO CaO 4 CO CaO 4 CO₄₄₄₄₄ Fe

Fe Fe Fe

Fe₄₄₄₄₄OOOOO₄₄₄₄₄ 4 CO 4 CO 4 CO 4 CO 4 CO  4FeO 4 CO 4FeO 4 CO 4FeO 4 CO 4FeO 4 CO 4FeO 4 CO₄₄₄₄₄

PhosphatesandSilicatesreduced, PhosphatesandSilicatesreduced, PhosphatesandSilicatesreduced, PhosphatesandSilicatesreduced, PhosphatesandSilicatesreduced,

Impure iron melts Impure iron meltsImpure iron melts Impure iron meltsImpure iron melts C4CO C4COC4CO C4COC4CO 4 4 4 4

4  4CO 4CO 4CO 4CO 4CO

FeO 4 CO FeO 4 CO FeO 4 CO

FeO 4 CO FeO 4 CO  Fe(S) 4 CO Fe(S) 4 CO Fe(S) 4 CO Fe(S) 4 CO Fe(S) 4 CO

4 4 4 4 4 PandSpassintomolteniron PandSpassintomolteniron PandSpassintomolteniron PandSpassintomolteniron PandSpassintomolteniron

Molten slag forms Molten slag forms Molten slag forms Molten slag forms Molten slag forms

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At the bottom of the furnace the molten iron sinks down while above this floats the fusible slag which protects the molten iron form oxidation. These two can be removed from different holes (Fig. 4). Waste gases escaping at the top consists of about 30% CO, 10% CO₂ and the rest nitrogen. Iron obtained from the blast furnace is known as pig iron.

Pig iron contains about 2-5% carbon as well as other impurities (usually Si, Mn, S and P). Pig iron is converted into cast iron by remelting in a vertical furnace heated by coke. Cast iron expands on solidification and is used for casting various articles. Wrought iron, which is the purest form of iron, can be obtained by heating cast iron in a reverberatory furnace lined with iron oxide. Wrought iron contains about 0.2% carbon.

Extraction of Tin.

Metallic tin is extracted from tin stone which contains about 10% of the metal as SnO₂ , the rest being siliceous matter, tungstates of Fe, Cu and As.

After crushing, the ore is concentrated by washing in a current of water (Gravity process to remove lighter gangue particles) and by magnetic separator to remove tungstates of Fe and Mn.

The ore is roasted to remove A and As their oxides. The ore then may be washed to remove sulphates of Cu and Fe. This gives black tin.

Finally, the ore is smelted in a reverberatory furnace or in a blast furnace at 1475-1575K. The ore is mixed with one-fifth of its mass of powdered anthracite (coal) and little of lime or fluorspar which is used as flux. Tin oxide is reduced to tin:

SnO₂ + 2C ® Sn + 2CO

The molten metal collected from the bottom of furnace contains impurities such as Fe, Pb, S and As. The metal may be purified electrolytically

REFINING OF TIN

(i) Liquation or sweating- When the block of impure tin is heated on the sloping hearth of reverberatory

furnace tin, alongwith lead and bismuth (all having a much lower melting points than other metals), run off leaving dross, an alloy of Sn, Fe, Cu, W, As.

(ii) Poling (stirring with logs of green wood) of this sweated tin is done. Impurities get oxidised & form

scum which is skimmed off. 99% Sn is obtained.

Scum & dross are repurified. Slag contains 10-25% Sn as SnSiO₃ because of amphoteric nature of tin. This is recovered by smelting with carbon and CaO flux at a much higher temperature.

SnSiO₃ + CaO + C ® Sn + CaSiO₃ + CO

Electrolytic refining : Cathode-pure metal, Anode -pure tin, Electrolyte SnSO₄(aq) with sulphuric acid and hydrofluosilicic acid.

COPPER AND LEAD

Both copper and lead may be extracted by self-reduction method.

Extraction of Copper :Copper is mainly extracted from copper pyrites. After the concentration of its ore by froth flotation process, the ore is roasted in a current of air to remove arsenic, antimony and much of sulphur. The reactions occurring are

(i) 2CuFeS₂ + O₂ ® Cu₂S + 2FeS + SO₂ - (major reaction) (ii) 2Cu₂S + 3O₂ ® 2Cu₂O + 2SO₂

(iii) 2FeS + 3O₂ ® 2FeO + 2SO₂ (minor reactions)

The ore is then mixed with a little coke and sand and smelted in a water-jacketed blast furnace. The minor reactions that occured during roasting continue here. Ferrous oxide combines with sand to form a fusible slag. Cuprous oxide formed combines with ferrous sulphide to give ferrous oxide and cuprous sulphide. This is because iron has more affinity for oxygen than copper.

(iv) FeO + SiO₂ ® FeSiO₃ (v) Cu₂ O + FeS ® Cu₂S + FeO

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Molten mass collected from the bottom of furnace contains largely cuprous sulphide and a little ferrous sulphide. This molten mass is known as matte.

The molten matte is finally transferred to Bessemer converter (Fig. 5). A blast of sand and air is blown in the converter through tuyeres which are situated a little above the bottom. This causes removal of S and As oxides and ferrous oxide as slag (reaction iv). At the same time Cu₂S is oxidized mostly into Cu₂O (reaction ii) and partly into CuO and CuSO₄. All these react with Cu₂S giving copper. The reactions are

(ii) 2Cu₂S + 3O₂ ® 2Cu₂O + 2SO₂ -2Cu₂S + 5O₂ ® 2CuSO₄ + 2CuO 2Cu₂O + Cu₂S ® 6Cu + SO₂ -CuSO₄ + Cu₂S ® 3Cu + 2SO₂ -Cu₂S + 2 CuO ® 4Cu + SO₂

-Finally, copper may be refined electrolytically (electrolyte; copper sulphate: anode; impure copper and cathode; pure copper).

Extraction of lead: Lead is mainly extracted from galena. After the concentration of the ore by froth flotation process, the ore is roasted in a reverberatory furnace for about six hours at a moderate temperature in a current of air. Part of galena is converted into lead oxide and lead sulphate. After this, the supply of air is stopped and small quantities of carbon, quicklime and cheap iron ore are added along with increase of temperature. At this stage, unreacted sulphide reacts with the lead oxide and sulphate giving metallic lead:

PbS + 2PbO ® 3Pb + 2SO₂ PbS + PbSO₄ ® 2Pb + 2SO₂

The obtained lead contains impurities such as Cu, Ag, Bi, Sb and Sn. Silver is removed by Parke’s process where molten zinc is added to molten impure lead. The former is immiscible with the latter. Silver is more soluble in molten zinc than in molten lead. Zinc-silver alloy solidifies earlier then molten lead and thus can be separated. After this, crude lead is refined electrolytically (Electrolyte; lead silicofluoride, PbSiF₆ and hydrofluosilicic acid, H₂SiF₆ with a little gelatin, anode; crude lead and cathode; pure lead).

MAGNESIUM AND ALUMINIUM

Extraction of magnesium Magnesium is commonly obtained by the electrolysis of fused magnesium chloride containing a little (25%) sodium chloride and sodium fluoride at 7000_{C in an}

air-tight iron pot which itself serves as the cathode, the anode being a graphite rod which dips into the electrolyte. The anode is surrounded by a perforated porcelain tube for the exit of chlorine. The electrolysis is carried out in the atmosphere of coal gas so as to prevent the attack of atmospheric oxygen and nitrogen on magnesium. Molten magnesium being lighter then the electrolyte, it floats over

C

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

SiO

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M

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FIG.

4 FIG.

4 BESSEMER

CONVERTER

BESSEMER

CONVERTER

BESSEMER

CONVERTER

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the fused electrolyte and is withdrawn (Fig. 6). Voltage ~ 6V.

In Dow process, magnesium is recovered from seawater as magnesium chloride which is then electrolysed using cell described above (Fig. 6).

Dow’s Sea Water Process. Sea water contains 0.13% Mg ions. Mg2+_{(seawater) + Ca(OH)}

2 (from oyster shells) ® Mg(OH)2 + CaCl2¾¾¾¾¾¾® MgCl2.2H2O

MgCl₂._2H

2O ¾¾¾¾¾¾® MgCl2.1.5H2O ¾¾¾¾® MgCl2

Dow’s Natural Brine Process.

MgCO₃.CaCO₃ ¾¾® MgO.CaO ¾¾¾® CaCl₂ (aq)+ MgCl₂(aq) ¾¾¾® MgCl₂(aq) + CaCO₃ (dolomite) (calcined dolomite)

The reaction is : CaCl₂._MgCl

2(aq) + MgO.CaO + 2CO2 ¾® MgCl2(aq) + 2CaCO3 3

Electrolysis. Anhydrous carnallite (KCl·MgCl₂·6H₂O) may also be employed as the starting material of magnesium chloride. The cathode may be a layer of molten lead on the floor of the cell and anode may be graphite rods which are suspended above the molten lead. Magnesium liberated at the cathode dissolves in molten lead. The alloy of lead-magnesium is subjected to electrolysis to obtain pure magnesium (electrolyte: fused carnallite, anode-lead-magnesium alloy and cathode-steel rods.)

Extraction of Aluminium Aluminium is isolated from the electrolysis of bauxite, Al₂O₃· 2H₂O. Since it is difficult to purify aluminium, bauxite ore is purified either by Baeyer's process (or Hall's process) or Serpek's process depending upon the impurity present in the ore.

If the bauxite contains iron oxide as the impurity, one can use Baeyer's or Hall's process as described below.

Baeyer's Process: Finally ground ore is roasted to convert ferrous oxide to ferric oxide and then digested with

concentrated caustic soda solution at 423K. Al₂O₃ dissolves while Fe₂O₃ remains undissolved. The latter is filtered off and from the solution Al(OH)₃ gives Al₂O₃.

Al₂O₃ + 2OH-_{+ 3H}

2O ® 2Al(OH)-4

Aluminate ion dissolves Al(OH)

-4 + H+ ® Al(OH)3 + H2O

precipitates 2Al(OH)₃

 

heat



_Al

2O3 + 3H2O

Hall's Process: In this process the ore is fused with sodium carbonate when soluble meta-aluminate (NaAlO₂) is produced. This is extracted with water leaving behind iron oxide. Carbon dioxide at 323-333 K is passed through water extract to get Al(OH)₃ which on heating gives Al₂O₃.

Al₂O₃ + Na₂CO₃



fused





_2NaAlO

2 + CO2

extracted with water

FIG. 4

ELECTROLYTIC CELL FOR THE

PRODUCTION OF MAGNESIUM

Graphiteanode GGrraapphhiitteeaannooddee Graphiteanode Graphiteanode Porcelain PPoorrcceellaaiinn PPoorrcceellaaiinn hood hhoooodd hhoooodd Inertgas IInneerrttggaass Inertgas Inertgas (coal gas) (coal (coal gas)gas) (coal gas) (coal gas) Ironcathode Ironcathode Ironcathode Ironcathode Ironcathode Mg MMgg MMgg Molten Molten Molten Molten Molten electrolyte electrolyte electrolyte eelleeccttrroollyyttee Cl ClCl ClCl₄₄₄₄₄ Inertgas Inertgas Inertgas Inertgas Inertgas Iron Iron Iron Iron Iron cell cell cell cell cell

(14)

2NaAlO₂ + 3H₂O + CO₂ ® 2Al(OH)₃ + Na₂CO₃ 2Al(OH)₃

 

heat



_Al

2 O3 + 3H2O

If the impurity is silica, the Serpek's process is used to purify bauxite.

Serpek's Process : The powdered ore is mixed with coke and heated to 2075K in a current of nitrogen. Silica

present is reduced to silicon which volatilizes off and alumina gives aluminium nitride. The hydrolysis of the latter gives Al(OH)₃, heating of which gives Al₂O₃.

SiO₂ + 2C ® Si - + 2CO2-Al₂O₃ + 3C + N₂ ® 2AlN + 3CO AlN + 3H₂O ® Al(OH)₃ + NH₃ 2Al(OH)₃

 

_heat



Al₂O₃ + 3H₂O

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINIUM

After obtaining pure Al₂O₃, it is dissolved in fused cryollite, Na₃AlF₆, with a little fluorspar, CaF₂ and is electrolysed in an iron tank lined with blocks of carbon which serve as the cathode. The anode consists of a number of graphite rods suspended vertically inside the tank (Fig. 7)

Aluminium gets settled at the bottom of the tank and can be removed. The reactions occurring at the electrodes are

Cathode Al3+_{+ 3e}-_{® Al}

Anode 2O

2-2 ® O2 + 4e

-C + O₂ ® CO₂

Anode is replaced periodically because of its consumption. SILVER AND GOLD

Cyanide Process: Silver and gold are extracted by the cyanide process (MacArthur-Forrest process). After the preliminary crushing and concentration by froth floatation process, the ore (crushed auriferous rocks in the case of gold)is leached with dilute (0.4-7%) solution of sodium cyanide made alkaline by adding lime kept agitated by a current of air. Silver (or gold) pass into solution as argentocyanide (or aurocyanide) :

Ag₂S + 4NaCN l 2Na[Ag(CN)₂] + Na₂S

The air blown in remove Na₂S as Na₂S₂O₃ and Na₂SO₄ causing the above reaction to proceed to completion. 2Na₂S + 2O₂ + H₂O ® Na₂S₂O₃ + 2NaOH

Na₂S₂O₃ + 2NaOH + 2O₂ ® 2Na₂SO₄ + H₂O

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

The solution obtained above is filtered and treated with scrap iron or zinc when silver (or gold) get precipitated: 2Ag(CN)

-2 + Zn ® Zn(CN)2-4 + 2Ag

2Na[Au(CN)₂] + Zn ® Na₂[Zn(CN)₄] + 2Au

The obtained silver is purified electrolytically (electrolyte:silver nitrate solution containing 1% nitric acid, anode: impure silver and cathode: pure silver). The impurities like zinc and copper pass into the solution while gold falls down as anode mud.

Gold thus obtained is contaminated by zinc which is dissolved out by sulphuric acid. The dried residue of gold is then fused under borax (flux) in graphite crucible and the melted down gold (bullion) which invariably contains silver, is sent for refining.