Ores & Metallurgy

		Extraction of metals from their respective ores is a very scientific process and the theme of this chapter is to explore this process. You will find ores of various metal that is found in the nature and method of extraction of the metals from their respective ores. This chapter also deals with various processes involved in the extraction of metals such as carbon reduction, self reduction, electrolytic reduction, cyanide process and an introduction to various types of furnaces.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1. IIT-JEE Syllabus

Commercially  important ores of iron, copper, lead, magnesium, aluminium , tin and  silver . Carbon reduction process (iron and tin), Self reduction process (copper and lead), Electrolytic reduction process (magnesium and aluminium), Cyanide process  (silver and gold).

2. Introduction

The earth’s crust is the  main source of metals. The occurrence of metal in native or in combined state in the earth’s crust along with  a number of rocky and other impurities depends upon the chemical nature of metals. Metals having less electropositive character have less affinity for oxygen, moisture and occur in free or metallic or native state i.e., in uncombined state e.g. Au, Pt , Ag etc. On the other hand metals with higher electropositive character occurs in combined state i.e., as compounds.

The compound of a metal found in nature is called a mineral. A mineral may be a  single compound or a complex mixture. Those minerals from which metal can be economically extracted are called ores. Thus all ores are minerals but all minerals are not ores. For e.g. copper occurs in nature in the form of several minerals like Cu₂O, Cu₂S, CuFeS₂, but  copper pyrites is considered as the most economical mineral for the extraction of the metal. Hence copper pyrites is the chief ore of copper.

Ores may be divided into  four groups

1. i) Native Ores: These ores contain the metal in free state Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
2. ii) Oxidised Ores: These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe₂O₃, Al₂O₃.2H₂O etc. and important cabonate ores are limestone (CaCO₃), Calamine (ZnCO₃) etc.

iii)  Sulphurised Ores:    These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS₂). galena  (PbS), Cinnabar (HgS)

2. iv) Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl₂.6H₂O and fluorspar (CaF₂) etc.
3. Metallurgy

It is the process of extracting a metal from its ores. The following operations are carried out for obtaining the metal in the pure form.

1. i) Crushing of the ore
2. ii) Dressing or concentration of the ore.

iii)   Reduction of metal.

1. v) Purification or refining of the metal.

3.1       Concentration

Ores usually contain soil, sand, stones and other useless silicates. These undesired impurities present in ores are called Gangue or Matrix. The removal of these impurities from the ores is known as concentration. The concentration process involve physical as well as chemical method.

Physical Method

1. a) Gravity separation: The separation is based on the difference in the specific gravities of the gangue particles and ore particles. The powdered ores is agitated with water or washed with a running stream of water. The heavy ore particles of sand, clay etc. are washed away. For this either wilfley table or hydraulic classifier is used.
2. b) Froth Floatation Process: Metallic sulphides are wetted by certain oil like pine oils and not by water.

The finely divided ore is introduced into water containing small quantity of oil (e.g. Pine Oil). The mixture is agitated violently with air a froth is formed which carries away along with it the metallic particles on account of the surface tension forces. The froth is transferred to another bath where gangue-free ore settles down. As the heavy metallic material is floated upward with the froth, this process is known as froth floatation process. The method is based upon preferential wetting of surfaces by liquids.

1. c) Electro Magnetic Separator: When one component either ore or the impurity is magnetic in nature this method can be used for separation. A magnetic separator consists of a belt moving over two rollers, one of which is magnetic. The powdered ore is dropped on the belt at the other end. Magnetic portion of the ore is attracted by the magnetic roller and falls near to the roller while the non-magnetic impurities fall
   farther off.

Chemical Methods

1. a) Calcination: Calcination is a process in which the ore are usually carbonate or hydrated oxide is subjected to the action of heat in order of expel water from hydrated oxide and carbon dioxide from a carbonate.

     Examples:  ZnCO₃ ¾® ZnO + CO₂

CaCO₃ ¾® CaO +  CO₂

Al₂O₃×2H₂O ¾® Al₂O₃ + 2H₂O

2Fe₂O₃×3H₂O ¾® 2Fe₂O₃ + 3H₂O

Calcination is generally done in a reverberatory furnace.

1. b) Roasting: Roasting is a process in which ores (usually sulphide ores) either alone or along with some other materials are subjected to the action of heat and air at temperatures below their melting points in order to bring about chemical changes in them. Calcination is also roasting but in this case we are concerned mainly with the changes due to the expulsion of some ingredients such as water, carbon dioxide and no other chemical change occurs. But during roasting chemical changes like oxidation, chlorination etc. takes places. The following equations represent roasting:

2PbS + 3O₂ ¾® 2PbO + 2SO₂

PbS + 2O₂ ¾® PbSO₄

2ZnS + 3O₂ ¾® 2ZnO + 2SO₂

ZnS + 2O₂ ¾® ZnSO₄

CuS + 2O₂ ¾® CuSO₄

2Cu₂S + 3O₂ ¾® 2Cu₂O + 2SO­₂

Roasting is generally carried out in a reverberatory furnace or in a blast furnace.

1. c) Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method. e.g. bauxite ore contains ferric oxide, titanium oxide and silica as impurities. When the powdered ore is digested with an aqueous solution of sodium hydroxide at about 150°C under pressure, the alumina dissolves forming soluble sodium meta aluminate while ferric oxide, TiO₂ and silica remains as insoluble part.

Al₂O₃ + 2NaOH  ¾®  + H₂O

     3.2         Reduction  of Free Metal

1. a) Smelting:The reduction of a metal from its ore by a process involving melting is known as smelting. It is generally done in a reverberatory furnace or a blast furnace in a controlled supply of air. Several reducing agents such as sodium, magnesium and aluminium are used for reduction. The calcinated or roasted ore is mixed with carbon (coal or coke) and heated in a reverberatory or a blast furnace. Carbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to the metal.

 

 

1. b) Flux:The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during  reduction process is called slag. Slag is  usually lighter and floats on the surface of the molten metal.

Concentrated  ore + gangue + reduction agent +  flux

 

Metal + slag + gases

            Fluxes are of the following two types:

1. i) Acidic fluxes like silica, borax etc. These are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc. The chemical reaction which takes place in removing a basic gangue is as follows:
2. ii) Basic fluxes like CaO, lime stone (CaCO₃), magnesite (MgCO₃), haematite (Fe₂O₃) etc. These fluxes are used when the gangue is acidic like silica, P_­4O₁₀  For example

The other methods used for the reduction of metals are electrolytic reduction, hydro metallurgy and amalgamation method.

Illustration 1:    Write short notes on

                        (a) Amalgamation               (b) Hydrometallurgy

                        (c) Self reduction               (d) Electrolytic reduction

Solution:        a) This process is used for the extractioin of noble metals like gold and silver from the native ores. The finely crushed ore is brought into contact with mercury which combines with the metal forming its amalgam. The metal is then recovered by distilling the amalgam.

1. b) Hydrometallurgy is the process of bringing the metal into solution by the action of suitable chemical reagents (e..g., sodium cyanide solution or chlorine in presence of water etc.) followed by recovery of the metal by the use of a proper precipitating agent which is a more electropositive metal. For example, poor ores of silver are suspended in a dilute solution of sodium cyanide and air is blown through it when the silver present goes into solution as the argentocyanide complex. From this solution the metal is precipitated by adding zinc turnigs.

4Ag + 8NaCN + O₂ (air) + 2H­₂O ¾® 4Na [Ag(CN)₂] + 4NaOH

or AgCl + 2NaCN ¾® Na[Ag(CN)₂] + NaCl

2Na [Ag(CN)₂] + Zn ¾® Na₂[Zn(CN)₄] + 2Ag¯

1. c) In some cases no extra reducing agent is required. A part of the sulphide ore is changed to oxide or sulphate which then reacts with the remaining part to give the metal and sulphur dioxide. Copper and lead are obtained by this method.

2Cu₂S + 3O₂ ¾® 2Cu₂O + 2SO­₂

Cu₂S + 2Cu₂O ¾® 6Cu + SO₂

2PbS + 3O₂ ¾® 2PbO + 2SO₂

PbS + 2O₂ ¾® PbSO₄

PbSO₄ + PbS ¾® 2Pb + 2SO₂

1. d) The carbon reduction methods are not applicable for the highly electropositive and chemically active metals such as alkali metals, alkaline earth metals, aluminium etc. The oxides of these metals are very stable and have to be heated very strongly with carbon in order to reduce them to metals. But at high temperatures these metals combine with carbon to form carbides, thus creating problem. Hence these metals are extracted by the electrolytic reduction of their fused halides. The metal is liberated at the cathode. Aluminium is however obtained by the electrolytic reduction of alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆).

      3.3       Refining or purification

The metals obtained by the application of above reduction methods from the concentration ores are usually impure. The impure metal is thus subjected to some purifying process known as refining in order to remove undesired impurities. Various process for this are

1. a) Liquation process                   b)   Distillation process
2. c) Cupellation d)   Poling
3. e) Elecrolytic refining f)    Bessemerisation

      3.3.1    Bassemerisation

This is a process in the metallurgy of copper. It involves the reduction of molten matte (mixture of cuprous and ferrous sulphide) obtained after smelting of copper pyrites ore in a Bessemer converter.

The molten matte is taken in Bessemer converter and heated by blowing hot blast of air and sand through tuyeres. The following reactions takes place in the converter.

1. i) Air oxidizes iron sulphide to iron oxide and SO₂

2FeS + 3O₂ ¾® 2FeO + 2SO₂­

1. ii) Sand reacts with iron oxide to form iron silicate

2FeO + SiO₂ ¾® FeSiO₃ (Slag)

iii)   After the iron has been removed, hot blast of air converts a part of cuprous sulphide  to cuprous oxide.

2Cu₂S + 3O₂ ¾® 2Cu­₂O + 2SO₂­

1. iv) The cuprous oxide so formed reacts immediately with remaining cuprous sulphide to form metallic copper.

2Cu₂O + Cu₂S ¾® 6Cu + SO₂­

Since the reaction is exothermic, the heat liberated maintains the crude copper in the molten state. The completion of the reaction is indicated by the appearance of green flame produced by the vaporization of copper.

      3.4       Furnaces

It is a device in which high temperature is produced either by burning a fuel or by using electricity. The important ones are described below.

1. a) Blast Furnace: It is used for smelting iron, copper and lead ores. It has a tall structure made of steel plates rivetted together lined inside with fire bricks. The furnace is provided with an arrangement for blowing air near the base, a slag hole, a tapping hole for removing the molten metal and an exit for waste gases near the top. The charge is introduced at the top. The mouth of the furnace is closed by a special arrangement through which the charge is introduced.
2. b) Reverberatory Furnace: It is used for calcination, roasting or for smelting. The charge in the powdered form along with a flux is placed on the hearth of the furnace. The fuel is placed on grate and the flames are deflected from its low sloping roof. This furnace is used in case of copper, tin, lead and wrought iron.
3. c) Muffle Furnace: A furnace in which the charge container is heated from all sides is called muffle furnace. The burnt materials including the vapours of the metal are allowed to escape through the same opening. The metal is obtained by condensing the vapours. In Belgian process for extracting zinc, the retorts act as muffles and prolongs help in completing the condensation.
4. d) Bassemer Converter: It is a pear-shaped furnace made of steel plates lined inside with lime or magnesium oxide. It is fixed on pivots and can be tilted in any direction. It is provided with pipes through which hot air can be blown for heating purposes.
5. Extraction of Aluminium

     

      4.1       Important ores of aluminium

1. Bauxite Al₂O₃×2H₂O
2. Cryolite Na₃AlF₆
3. Feldspar K₂Oal₂O₃×6SiO₂ or KalSi₃O₈
4. Mica K₂O×3Al₂O₃×6SiO₂×2H₂O
5. Corundum Al₂O₃
6. Alumstone or Alunite K₂SO₄×Al₂(SO₄)₃×4Al(OH)₃

Aluminium is mainly extracted from bauxite ore.

      4.2       Purification of Bauxite

2Al(OH)₃  (calcination of aluminium hydroxide)

      4.3       Electrolysis of fused pure alumina (Hall & Herwlt Method)

The addition of cryolite (Na₃AlF₆) and fluorspar (CaF₂) makes alumina a good conductor of electricity and lowers its Fusion temperature from 2323 to 1140 K. the reaction taking place during electrolysis.

Na₃AlF₆  3NaF + AlF₃

AlF₃  Al³⁺ + 3F^–

At cathode

Al³⁺ + 3e^– ¾® Al

At anode

F^– ¾® F + e^–

2Al₂O₃ + 12 F ¾® 4AlF₃ + 3O₂

2C + O₂ ¾® 2CO

2CO + O₂ ¾® 2CO₂

      4.4       Refining of Aluminium

The aluminium metal obtained by the electrolysis of fused almina is about 99.5% pure. It can be further refined by Hoope’s electrolytic process.

The graphite rods dipped  in pure aluminium and Cu–Al alloy rod at the bottom in the impure aluminium work as conductors. On electrolysis, aluminium is deposited at cathode from the middle layer and equivalent amount of aluminium is taken up by the middle layer from the bottom layer (impure aluminium). Therefore, aluminium is transferred from bottom to the top layer through middle layer while the impurities are left behind. Aluminium thus obtained is 99.98% pure.

Illustration -2:         Aluminium cannot be prepared by the electrolysis of aqueous solution of its salts. Why?

Solution:              Discharge potential of aluminium is higher then the discharge potential of hydrogen. Thus the aqueous solution containing Al⁺³ and H⁺ ion when electrolysed, the H⁺ ions rather than Al⁺³ ions are discharged at cathode and hydrogen is evolved.

5. Extraction of Iron

     

      5.1       Important Ores of Iron

1. i) Haematite Fe₂O₃ (red oxide of iron)
2. ii) Limonite Fe₂O₃×3H₂O (hydrated oxide of iron)

iii)   Magnetite Fe₃O₄ (magnetic oxide of iron)

      5.2       Extraction of Cast Iron

Iron is usually extracted from the haematite. Concentrated ore after calcination is reduced with carbon i.e. smelted in the blast furnace.

Reactions taking place in the blast furnace i)    Zone of combustion   ii)   Zone of reduction Fe2O3 + 3CO ¾® Fe + CO2 iii)   Zone of reduction Fe2O3 + 3CO  ¾® 2Fe + 3CO2 FeO + CO  ¾® Fe + CO2 iv)  Zone of slag formation   ; v)   Zone of fusion lower part of furnace Molten iron is heavier than from molten slag. The two liquids are periodically tapped off. The molten iron tapped off from the furnace is solidifed into blocks called ‘pigs’.

      5.3       Preparation of Wrought Iron

This is done by heating cast iron with haematite (Fe₂O₃) which oxidises C to CO, S to SO₂, Si to SiO₂, P to P₄O₁₀ & Mn to MnO

Fe₂O₃ + 3C ¾® 2Fe + 3CO

Where CO & SO₂ escape, manganous oxide (MnO) and Silica (SiO₂) combine to form slag.

MnO + SiO₂  ¾® MnSiO₃

Similarly phosphorus pentoxide combines with haematite to form ferric phosphate slag.

2Fe₂O₃ + P₄O₁₀ ¾® 4FePO₄

Illustration 3:    Cast iron is hard but pure iron is soft in nature. Explain why?

Solution:        Cast iron contains 2.5 to 5% carbon which makes cast iron hard.

6. Extraction of Copper

 

      6.1       Important Ores of Copper

1. i) Copper glance (chalcocite) Cu₂S
2. ii) Copper pyrites (Chalopyrites) CuFeS₂

iii)   Malchite                                              Cu(OH)₂ ×CuCO₃

1. iv) Cuprite or Ruby copper Cu₂O
2. v) Azurite Cu(OH)₂×2CuCO₃

      6.2       Metallurgy of Copper

Copper is extracted mainly from copper pyrites. The various steps involved are

Copper Pyrites

(CuFeS₂)

 

Concentration by froth floatation

Powdered ore + water  + pine oil + air ¾® sulphide ore in the froth

Rosting in reverberatory furnace in presence of air

S + O₂  ¾® SO₂; 2As + 3O₂ ¾® 2As₂O₃

2CuFeS₂ + O₂ ¾® Cu₂S + 2FeS + SO₂

 

Smelting in blast furnace in presence of air

2FeS + 3O₂ ¾®  2FeO + 2SO₂; FeO + SiO₂ ¾® FeSiO₃ (slag)

 

 

Bessemerisation in bessemer converter in presence of air

2FeS + 3O₂ ¾®  2FeO + 2SO₂; FeO + SiO₂ ¾® FeSiO₃

2Cu₂S + 3O₂ ¾® 2Cu₂O + 2SO₂

2Cu₂O + Cu₂S  6Cu + SO₂

Blister copper (98% Cu + 2% impurities)

*Electrolytic refining

Anode–impure copper plates

Cathode-pure copper plates

Electrolyte -CuSO₄ + H₂SO₄

Pure copper at cathode (99.9% pure)

7. Extraction of Silver

 

      7.1       Important Ores of Silver:

1. i) Argentite or silver glance, Ag₂S
2. ii) Horn Silver AgCl

iii)   Pyragyrite (Ruby Silver),                 Ag₂S, Sb₂S₃

      7.2       Metallurgy of silver:

Silver is mainly extracted from argentite (Ag₂S) by Cyanide Process

silver ore (Argentite)

Concentration by froth floatation

Powdered ore + water  + pine oil + air ¾® sulphide ore in the froth

Cyanidation

Concentrated ore + NaCN solution (0.4 – 0.6%) + Air

Ag₂S + 4NaCN  2Na[Ag(CN)₂] + Na₂S

4Na₂S + 5O₂ + 2H₂O ¾® 2Na₂SO₄ + 4NaOH + 2S

 

Precipitation of silver with zinc

2Na[Ag(CN)₂] + Zn ¾® + Na₂[Zn(CN)₄]

Black ppt. of Ag + KNO₃  compact mass silver metal

Electrolytic refining

Anode–impure silver plates

Cathode-pure silver plates

Electrolyte -AgNO₃ solution + HNO₃

Pure silver at cathode

8. Extraction of Tin

      The chief ore of tin is cassiterite or tin stone, SnO₂. It contains about 10% of tin. The crushed ore is washed with water to remove lighter impurities. The ore is then roasted to remove arsenic and sulphur as volatile oxides. Tin is obtained by reducing SnO₂ with carbon at 1200- 1300⁰C in an electric furnace.

SnO₂ + 2C Sn + 2 CO

The product often contains traces of Fe, which make the metal hard. Fe is removed by blowing air through the molten mixture to oxidise the iron to FeO, which then floats to the surface.

9. Extraction of Lead

      The principal ore of  lead is Galena, PbS. The ore is first concentrated by froth floatation process. The concentrated ore is roasted in air to convert it into lead oxide PbO and lead sulphate. Some galena is also left unchanged. If the air supply is now reduced, the unreacted PbS reacts with PbO and PbSO₄ to produce metal

3PbS  +  5O₂ ¾®  2PbO  +  PbSO₄  +  SO₂

2PbO +  PbS  ¾®  3Pb  +  SO₂

PbSO₄ +  PbS ¾®  2Pb  + 2SO₂

10. Extraction of Magnesium

      Magnesium from sea-water is obtained by the Dow’s process. It consists of electrolysis of molten magnesium chloride using an iron cathode and a graphite anode. Following are the steps that involved during the process.

1. a) Mg⁺² is precipitated as magnesium hydroxide by the addition of slaked lime, Ca(OH)₂ to the sea water.

Mg²⁺ +  Ca(OH)₂  ¾®  Mg(OH)₂  +  Ca²⁺

2. b) Magnesium hydroxide on treatment with hydrochloric acid is converted to chloride which is crystallised as MgCl₂.6H₂O

Mg(OH)₂  + 2HCl ¾®  MgCl₂  +  2H₂O

1. c) Now for electrolysis, magnesium chloride is fused as follows:

On passing a current of dry HCl gas it get partially dehydrated and the chloride thus obtained is added to a molten mix. of sodium chloride and calcium chloride (temp. range 973 – 1023 K).Under this condition MgCl₂ melt with the loss of water.

If magnesium chloride hydrate is heated strongly, it hydrolyses to yield magnesia (magnesium oxide ) which is a refractory.

MgCl₂.6 H₂O ¾®  MgO  +  2HCl + 5H₂O

1. d) Molten mixture of MgCl₂ , NaCl and CaCl₂ is electrolysed. Magnesium is formed at the cathode and chlorine is evolved at the anode. The chlorine is used to make HCl acid which in turn is required for making magnesium chloride.

Cathode  :             Mg²⁺  +  2e^– ¾®  Mg

Anode     :        2Cl^– ¾®  Cl₂  +  2e^–

Illustration 4:    NaCl is added during electrolysis of fused anhydrous MgCl₂. Why?

Solution:        NaCl is added as to lower the fusion temperature of MgCl₂ and to make the mixture a good conductor of electricity.

11. Some Other Important Compounds

 

1.	MgCO3	Magnesite	2.	MgCO3×CaCO3	Dolomite
3.	MgSO4×7H2O	Epsomsalt	4.	KCl×MgCl26H2O	Carnallite
5.	MgAl2O4	Spinal	6.	H2MgSiO3	Talc
7.	Ca2Mg5Si8O22(OH)2	Asbestos	8.	KAlSi3O8	Feldspar
9.	Cu(OH)2CuCO3	Malachite	10.	Cu(OH)22CuCO3	Azurite
11.	PbS	Galena	12.	Fe3O4	Magnetite
13.	3Fe2O33H2O	Cimonite	14.	AgNO3	Lunar Caustic
15.	ZnSO4×7H2O	White Vitriol	16.	SnCl2×5H2O	Butter of Iron
17.	CaO	Quick lime	18.	Na2SO4×10H2O	Glauber’s salt
19.	Pb3O4	Red lead	20.	Pb(OH)22PbCO3	White lead
21.	Na2CO3	Soda ash	22.	NaOH	Caustic Soda
23.	KOH	Caustic potash	24.	Ca(OH)2	Slaked lime
25.	CaSO42H2O	Gypsum	26.	(CaSO4)2H2O	Plaster of Paris
27.	FeSO4×7H2O	Green Vitriol	28.	Hg2Cl2	Calomel
29.	HgCl2	Corrosive Sublimate	30.	PbSO4	Anglesite
31.	PbO	Cittarge	32.	ZnO	Philosopher’s wool
33.	HgS	Cinnabar	34.	BaSO4	Baryte or heavy spar
35.	ZnS	Zinc blende

12. Some important Alloy

 

Sl. No.	Name	Composition
1.	Stainless Steel	Fe, Cr, Ni
2.	Invar	Fe, Ni
3.	Alnico	Fe, Al, Ni, Co
4.	Brass	Cu, Zn
5.	Bronze	Cu, Zn, Sn
6.	Gun Metal	Cu, Sn
7.	Bell Metal	Cu, Sn
8.	German Silver	Cu, Zn, Ni

13. Solved Problems

      13.1     Subjective           

Problem 1:       A student suggested that calcium should be made if CaO is reacted with Aluminium powder. Was student correct?

                        Given         DG_f⁰ (CaO) = – 604.2 kJ/mol

                                          DG_f⁰ (Al₂O₃) = –1582 kJ/mol

Solution:          2Al + 3CaO ¾¾®  Al₂O₃ + 3Ca

DG° = (Al₂O₃) – 3 (CaO)

–1582 – 3 ´ (–604.2)

= –1582.0 + 1812.6 = + 130.6

DG° = +ve Þ reaction is non spontaneous hence not possible. So student was wrong.

Problem 2:       Cinnabar (HgS) and galena (PbS) on roasting often gives their respective metals but zinc blende (ZnS) does not. Explain.

Solution:        On roasting all these sulphides ores are partly converted into their respective oxides. Since the oxides of Hg and Pb are unstable. While that of zinc is stable, therefore, oxides of Hg and Pb bring about the reduction of their sulphides to metals, but zinc oxides does not reduce ZnS to Zn metal

HgS + 3Hg + SO₂

PbS + 3Pb + SO₂

 

Problem 3:       Explain the following

1. a) Although Au is soluble in aqua-regia, Ag is not
2. b) Zinc and hot copper is used for recovery of Ag from the complex [Ag(CN)₂]^–
3. c) Aluminium metal is frequently used as a reducing agent for extraction of metal such as chromium manganese etc.
4. d) Partial roasting of sulphide ore is done in the metallurgy of copper.

Solution:        a)   Au dissolved in aquaregia forming soluble HAuCl₄ but Ag forms insoluble AgCl

Au + 4HCl + 3HNO₃ ¾¾®  + 3NO₂ + 3H₂O

Ag + HCl + HNO₃ ¾¾® + NO₂ + H₂O

1. b) zinc is powerful reducing agent in comparison to copper and zinc is cheaper also than copper.
2. c) Auminium metal is frequently used as a reducing agent for the extraction of metals. Such as Cr and Mn from their respective oxides because aluminium is more electropositive than Cr or Mn. The process of reduction is called alumino thermy.

Cr₂O₃ + 2Al ¾¾®  Al₂O₃ + 2Cr

3Mn₃O₄ + 8Al ¾¾®  4Al₂O₃ + 9 Mn

1. d) Partial roasting of sulphide ore forms some oxide. The oxide then reduces the remaining sulphide ore into metal.

2CuS + 3O₂ ¾¾®  2CuO + 2SO₂

                                                2CuO + CuS ¾¾®  3Cu⁺ + SO₂

Problem 4:       Copper pyrites, CuFeS₂, is the important source of copper. 10g of it was leached with dil. H₂SO₄ and solution diluted to 1 L.

1. a) 10 ml of this solution required 10 ml of 0.02 M KMnO₄ in acidic solution
2. b) 10 ml of this solution was treated with excess of KI and liberated iodine required 10 ml of 0.02 M hypo solution. What is percentage of Cu and Fe in copper pyrites?

Solution:        CuFeS₂ + 2H₂SO₄ ¾¾® CuSO₄ + FeSO₄ + 2H₂S

1. a) Equivalent of Fe⁺² º equivalent of KMnO₄

10 ´ N = 10 ´ 0.02 ´ 5 ´ 10^–3

Þ N = 0.1N

Strength = Normality ´ equivalent wt. = 0.1 ´ 56 = 5.6 gm/ litre

% of Fe⁺² = 56%

1. b) CuSO₄ + 4KI ¾¾® Cu₂I₂ + 2K₂SO₄ + I₂

Equivalent of Cu⁺² = equivalent of I₂ = equivalent of Na₂S₂O₃

Equivalent of Cu⁺²  in 10 ml = 10 ´ 0.02 ´ 1 ´ 10^–3

Equivalent of Cu⁺² in 1000 ml = 100 ´ 10 ´ 0.02 ´ 10^–3 = 0.02

Strength = Normality ´ equivalent weight

= 0.02 ´  = 12.7 g

or percentage of Cu = 12.7%

 

Problem 5:       a)   MgO is used for the lining in steel making furnace.

1. b) Which of the metals Na, Ag and Fe are extracted by
2. i) Complex formation           
3. ii) Reduction with carbon

                              iii)  Electrolysis of fused salts

Solution:        a)   Magnesium oxide acts as a flux to remove impurities of Si, P and S through slag formation

 

1. b) (i) Ag, (ii) Fe, (iii) Na

      13.2     Objective 

 

Problem 1:       The material used in semiconductors

                        (A) Si                                             (B) Sn

                        (C) Ti                                              (D) Cs

Solution:        (A)

Problem 2:       Electrolytic bath electrolytic refining of lead contains

                        (a) H₂SiF₆ only                              

                        (b) PbSiF₆ only

                        (c) H₂SiF₆ in presence gelatin        

                        (d) H₂SiF₆ and PbSiF₆ in presence of gelatin

Solution:        (D)

Problem 3:       Which of the following is manufactured by Siemens Martin process?

                        (A) Pig iron                                    (B) Cast iron

                        (C) wrought iron                             (D) steel

Solution:        (D)

Problem 4:       The process of producing a hard coating of iron nitride on the surface of steel is called

                        (A) Annealing                                 (B) Quenching

                        (C) Tempering                                (D) Nitriding

Solution:        (D)

Problem 5:       Bordeaux mixture  is

                        (A) CuSO₄ +MgSO₄                          (B) FeSO₄ + CuSO₄

                        (C) CuSO₄ and lime                        (D) NiSO₄ + lime

Solution:        (C)

Problem 6:       The main function of roasting is

                              (A) to remove the volatile impurities   

                              (B) oxidation of metal

                              (C) reduction of metal                   

                              (D) to make slag

Solution:        (B)

Problem 7:       Which method of purification is represented  by the following equation

                              Ni + 4CO  Ni(CO)₄  Ni + 4CO

                              (A) van Arkel                                  (B) zone refining

                              (C) mond process                          (D) cupellation

 

Solution:        (C)

Problem 8:       The process of converting hydrated alumina into  anhydrous alumina is called

                              (A) roasting                                    (B) calcination

                              (C) smelting                                   (D) dressing

Solution:        (B)

Problem 9:       Zone refining is a method to obtain

                              (A) Very high temperature              (B) Ultra pure Al

                              (C) Ultra pure metal                        (D) Ultra pure oxides 

Solution:        (C)

Problem 10:     Thomas slag is –

                        (A) Ca₃(PO₄)₂                                   (B) MnSiO₃

                        (C) CaSiO₃                                      (D) FeSiO₃

Solution:        (A)

 

 

 

14. Assignments (Subjective Problems)

 

Level –  I

1. Complete the following reactions:
2. a) Al(OH)₄^– + CO₂ ¾® ? + ? + ?
3. b) Ag⁺  + Na₂S₂O₃ ¾® ? + ?
4. c)  + H₂S  ? + ?
5. A sulfide ore (A) on roasting leaves a residue (B). (B) on heating with chlorine gives (C), soluble in water, addition of excess potassium iodide to a solution of (C) gives a solution (D). A brown precipitate (E)is formed when a solution of ammonium sulfate is added to an alkaline solution of (D). Identify (A) to (E).
6. Write the balanced chemical equations for the following
7. a) Pb₃O₄ is treated with nitric acid
8. b) Zinc oxide is treated with excess of sodium hydroxide solution
9. c) Dilute nitric acid is slowly reacted with metallic tin
10. d) Gold is dissolved in aqua regia
11. e) Iron reacts with cold dilute nitric acid
12. f) Sodium thiosulphate is added slowly to silver nitrate
13. g) Heating of ZnCl₂×2H₂O
14. In moist air copper corrodes to produce a green layer on the surface. Explain.
15. Why does AgNO₃ produce a black stain on the skin.
16. How is ferrochrome produced? What are its application?
17. State the composition of Cu alloys used for coinage.
18. State the composition and use of
19. i) Monel metal
20. ii) German silver
21. Complete the following reactions
22. i) Zn + conc. HNO₃ ¾®
23. ii) Zn + 2NaOH ¾®
24. How does the composition of cast iron differ from wrought iron and steel?

 

 

 

 

 

level – ii

1. What is solvent extraction
2. What is Goldsmidt Thermite Process:?
3. What is the main differences between cupellation and poling?
4. Complete and balance the following equations
5. a) Zn + Fe₂(SO₄)₃ ¾® ? + ?
6. b) CuSO­₄  ? + ?
7. c) Al + conc. H₂SO₄ ¾® ? + ? + ?
8. d) Al₂O₃ + C + N₂  ? + ?
9. e) Pb + O₂ + H₂O ?
10. a) Egg shell is made up of a chemical. In which of the two ores this chemical is present.
11. b) How we can remove impurity of Pb in silver and impurity of Cuprous oxide in copper.
12. Explain the following terms
13. i) Zone refining (Fractional crystallization)
14. ii) Electro-refining:

iii)   Van-Arkel Method:

7. What is meant by passivity of iron.
8. List the reaction which occur at the various part of the blast furnace during manufacture of cast iron.
9. The most stable chlorides of Cu, Ag and Au have the stoichiometric composition CuCl₂ AgCl and AuCl₃ in which the oxidation states are +2, +1, +3 respectively. Justify.
10. Complete the reaction
11. i) 3Cu + conc. 4HNO₃ ¾®
12. ii) Cu + conc. 2H₂SO₄ ¾®

iii)   Cu₂O + 4NH₄OH ¾®

1. iv) 2CuSO₄ + 4KI ¾®

 

 

 

 

 

 

15. Assignments (Objective Problems)

 

Level –  I

1. Cupric oxide is reduced to copper by

(A) H₂                                                        (B) C

(C) CO                                                      (D) All of these

2. Cupric chloride is converted into cuprous chloride by

(A) Zinc                                                     (B) SO₂

(C) copper                                                 (D) All of these

3. White lead is –

(A) Pb(OH)₂.PbCO₃                                 (B) 2PbCO₃.Pb(OH)₂

(C) Pb(OH)₂.Pb(CH₃COO)₂                    (D) PbCO₃.PbO

4. Cerussite is

(A) PbS                                                     (B) PbCO₃

(C) PbSO₄                                                (D) SnO₂

5. Which alloy of aluminium is used in aircraft industry

(a) duralumin                                            (b) Magnalium

(c) Nickeloy                                              (d) alunico

6. The common impurities present in the bauxite ore are

(A) Fe₂O₃ and CuO                                  (B) Fe₂O₃ and PbO

(C) Fe₂O₃ and SiO₂                                  (D) SiO₂ and CuO

7. Froth floatation process is based on

(a) specific gravity of the ore particle

(b) magnetic properties of the ore particle

(c) wetting properties of the ore particle

(d) electrical property o the ore particle

8. The iron manufactured by pudding process is called as

(A) Pig iron                                                (B) cast iron

(C) wrought iron                                        (D) white cast iron

9. Which of the following is a highly deliquescent solid?

(A) FeCl₃                                                                    (B) FeSO₄

(C) CuSO₄                                                (D) All of these

10. The compound undergoing photoreduction is

(A) AgCl                                                    (B) AgF

(C) AgNO₃                                                (D) AgBr

11. In aluminotermic process aluminium acts as

(A) an oxidising agent                               (B) a reducing agent

(C) a flux                                                   (D) a solder

12. Poling process is used

(A) for the removal of Cu₂O from Cu      (B) for the removal of Al₂O₃ from Al

(C) for the removal Fe₂O₃ from Fe          (D) in all the above

13. Calcination is used in metallurgy to remove

(A) H₂O and H₂S                                      (B) H₂O and CO₂

(C) CO₂ and H₂S                                      (D) H₂O and H₂S

14. Mac-Aurthur forrest process is used for the extraction of

(A) Zn                                                        (B) Cu

(C) Fe                                                        (D) Ag

15. Duralumin is an alloy of

(A) Al and Mg                                            (B) Al, Mg, Ni

(C) Al, Mg, Mn, Cu                                   (D) Al and Ni

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Level – II

 

1. Which of the following precipitates Ag from AgNO₃?

(A) Phosphine                                           (B) Arsine

(C) Stibine                                                 (D) All of these

2. Schwitzer’s reagent has the composition of

(A) [Ag(NH₃)₂]Cl                                       (B) Fe₄[Fe(CN)₆]₃

(C) [Cu(NH₃)₄]SO₄                                   (D) K₂[HgI₄]

3. The moleten matte obtained after the treatment of copper pyrites in the blast furnace, has the composition of –

(A) Cu₂S                                                     (B) Cu₂S + FeS

(B) Cu₂S + Fe₂S₃                                       (D) CuS + Fe₂S₃

4. Green coloured ore among the following is

(A) Cu₂O                                                   (B) CuFeS₂

(C) CuCO₃.(Cu(OH)₂                               (D) 2CuCO₃. Cu(OH)₂

5. Bromine is added to cold dilute aqueous solution of sodium hydroxide. The mixture is boiled. Which of the following statements is not true?

(A) During the reaction bromine is present in four different oxidation states.

(B) The greatest difference between the different oxidation states of bromine is 5.

(C) On acidification of the final mixture, bromine is formed

(D) Disproportionation of bromine occurs during the reaction

6. Zeolite, which shows ion exchange ability –

(A) is an ion exchange resin

(B) is a closed packed assemblage of Si and O – atoms

(C) can provide H⁺ ions in place of Na⁺ ions

(D) is a sodium aluminum silicate

7. Stainless steel contains –

(A) Fe, Cr, Cu                                           (B) Fe, Cr, Ni

(C) Fe, Ni, C                                             (D) Fe, Ni, Cu

8. Books, periodicals, magazines and calendars are printed in large numbers. Type metal used in printing presses as alphabet letter printing contains –

(A) sulphur                                                (B) chromium

(C) lead                                                     (D) copper

9. The metallic lustre exhibited by sodium is explained by the presence of –

(A) Na⁺ ions                                              (B) conducting electrons

(C) free protons                                        (D) a body centred cubic lattice

10. During Hoope’s process for electrolytic refining of Al, the middle layer is of –

(A) Pure aluminum                                   (B) impure aluminum

(C) cryolite and BaF₂                                (D) alloys of Al, Ca, Si

11. The silver halide, which is least soluble in NH₄OH is

(A) AgF                                                     (B) AgCl

(C) AgBr                                                    (D) AgI

12. Complex formation method is used for the exraction of

(A) Zn                                                        (B) Ag

(C) Hg                                                       (D) Cu

13. Auto reduction process is used in the extraction of

(A) Cu and Hg                                           (B) Zn and Hg

(C) Cu and Al                                            (D) Fe and Pb

14. Van Arkel method of purification of metals involves converting the metal to a

(A) Volatile stable compound                    (B) Volatile unstable compound

(C) non volatile stable compound             (D) none of the above

15. Which of the following is not a basic flux?

(A) CaCO₃                                                (B) CaO

(C) SiO₂                                                    (D) MgO

 

 

 

 

 

 

 

 

 

 

 

 

 

16. Answers to Objective Assignments

Level – I

1. D                                                               2.         D
2. B                                                               4.         B
3. A                                                               6.         C
4. C                                                               8.         C
5. D                                                               10.       D
6. B                                                               12.       A
7. C                                                               14.       D
8. C

Level – II

1. D                                                               2.         C
2. B                                                               4.         C
3. A                                                               6.         D
4. B                                                               8.         C
5. C                                                               10.       C
6. D                                                               12.       B
7. A                                                               14.       A
8. C

 

6

Ores and Metallurgy

 

   Subjective Problems

LEVEL – I

 

1. a)
2. b) Ag⁺ + 2Na₂S₂O₃ Na₃[Ag(S₂O₃)₂] + Na⁺
3. c)  + H₂S Mn²⁺+ SO₂ + 2OH^–

2.

3. a)   Pb₃O₄ is a mixed oxide, 2PbO, PbO₂, with nitric acid PbO is converted to Pb(NO₃)₂ while PbO₂ remains as such

Pb₃O₄ + 2HNO₃ ¾¾® PbO₂ + Pb(NO₃)₂ + H₂O

1. b) ZnO is dissolved as Zincate ion

ZnO + 2NaOH ¾® Na­₂ZnO₂ + H₂O

1. c) Tin reduces nitric acid (dilute) to ammonia

4Sn +  + 10H⁺ ¾® 4Sn²⁺ +  + 3H₂O

1. d) Gold dissolves in aqua regia as HAuCl₄

Au + 4HCl + 3HNO₃ ¾® HAuCl₄ + 3NO₂ + 3H₂O

1. e) 4Fe + 10HNO₃ ¾® 4Fe (NO₃)₂ + NH₄NO₃ + 3H₂O
2. f) White precipitate changes to yellow, brown and finally becomes black

 

 

1. g) ZnCl₂×2H₂O  ZnCl(OH) + HCl + H₂O
2. In presence of moist air a thin film of green basic copper carbonate is formed on its surface and hence copper corrodes

2Cu + O₂ + H₂O + CO₂ ¾®

5. In presence of organic matter (skin) and light, AgNO₃ decomposes to produce a black stain of metallic silver

2AgNO₃ ¾® 2Ag + 2NO₂ + O₂

6. FeOCr₂O₃ + 4C 2Cr + Fe + HCO­

Ferrochrome (Fe 35%, Cr 65%) is used in steel industry.

7. Ni coins = Cu 75% + Ni 25%

Guinea gold = Au 91%, Cu 9%

8. i) Monel metal: Cu30% + Ni 67% + Fe, Mn 3% used for acid pumps and acid container.
9. ii) German silver: Cu 50% + Zn 25% + Ni 25% used for utensils, resistance coils.
10. i) Zn + conc. HNO₃ ¾® Zn(NO₃)₂ + 2H₂O + 2NO₂­
11. ii) Zn + 2NaOH ¾® Na₂ZnO₂ + H₂­

10.

	C %	Si %	S %	Mn%	P%
Cast iron	2 – 4.5	0.7 – 3.5	0.1 – 0.3	0.5 – 1.0	0 – 0.3
Steel	0.1 – 1.5	0.03 – 2.0	0.02 – 0.2	0.1 – 4.0	0.04 – 0.07
Wrought iron	0.04 – 0.25	0 – 0.14	0.02 – 0.15	Up to 0.25%	0.04 – 0.2

level – ii

1. Solvent extraction is the latest separation technique and has become popular because of its elegance, simplicity and speed. The method is based on preferential solubility principles.

Solvent or liquid-liquid extraction is based on the principle that a solute can distribute itself in a certain ratio between two immiscible solvents, one of which is usually water and the other an organic solvent such as benzene, carbon tetrachloride or chloroform. In certain cases, the solute can be more or less completely transferred into the organic phase. The technique can be used for purpose of preparation, purification, enrichment, separation and analysis.

2. Certain oxides are not satisfactorily reduced by carbon. In such cases aluminium is used as a reducing agent. A mixture of metallic oxide and aluminium powder is ignited in a closed crucible by means of lighted magnesium ribbon. Metals like chromium, manganese etc. are extracted by thermite process as illustrated below.

Cr₂O₃ + 2Al ¾® Al₂O₃ + 2Cr

3Mn₃O₄ ¾® 4Al₂O₃ + 9Mn

The reaction is highly exothermic and the heat produced is sufficient to melt the metal.

3. Both cupellation and poling are used for refining of metals, cupellation is contain impurities of other metals with traces of lead are removed from silver by heating impure silver with a blast of air in a cupel (an oval shaped pan made up of bone ash) in which lead is oxidised to lead oxide (PbO) which being volatile escapes leaving behind pure silver.

Poling is used for refining of such metals which contain impurities of its own oxide. In this process, the molten impure metal is stored with green wooden poles. At the high temperature of the molten metal, wood liberates methane which reduces the oxide of the metal to free metal.

3Cu₂O + CH₄ ¾® 6Cu + 2H₂O + CO

4. a) Zn + Fe₂(SO₄)₃ ZnSO₄ + 2FeSO₄
5. b) CuSO₄ CuO +SO₃
6. c) 2Al + conc. 6H₂SO₄ ¾® Al₂(SO₄)₃ + 3SO₂ + 6H₂O
7. d) Al₂O₃ + 2C + N₂  2AlN + 3CO
8. e) 2Pb + O₂ + 2H₂O  2Pb (OH)₂
9. a) Limestone CaCO₃

Dolomite CaCO₃.MgCO₃

1. b) Impurity of lead in silver can be removed by cupellation method and impurity of cuprous oxide in copper can be removed by poling.
2. i) Zone refining (Fractional crystallization) This method  is employed for preparing extremely pure metals. This method is based upon the principle that when a molten solution of the impure metal is allowed to cool, the pure metal crystallizes out while the impurities remain in the melt.
3. ii) Electro-refining: In this method, the impure metal is converted into a block which forms the anode while cathode is a rod or plate of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal usually a double salt of the metal. When electric current is passed, metal ions from the electrolyte are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as metal ion. The soluble impurities present in the crude metal anode go into the solution while the insoluble impurities settle down below the anode as anode mud.

iii)    Van-Arkel Method: In this method, the metal is converted into it volatile unstable compound such as iodide leaving behind the impurities. The unstable compound thus formed is decomposed to get the pure metal.

Ti(s) + 2I₂(s) + TiI₄(g)  Ti + 2I₂(g)

7. When iron is treated with concentrated HNO₃ it stops showing its normal behaviour such as liberation of H₂ from dil. H­₂SO₄ and precipitation of Cu from CuSO₄ solution. This phenomenon, when iron shows chemical inertness, is called passivity.

If iron is treated with certain oxidizing agents such as H₂CrO₄, KMnO₄ etc., then also it becomes passive. In fact passivity of iron is suggested as formation of thin protective layer of Fe₂O₃ which prevents reaction of iron at room temperature.

8. Zone of fusion (1200°C – 1500°C)

C + O₂ ¾® CO₂

CO₂ +  2CO

            Top section (4000°C – 900°C)

Fe₂O₃ + 3CO  2Fe + 3CO₂­

3Fe₂O₃ + CO  2Fe₃O₄ + CO₂­

Fe₃O₄ + CO  3FeO + CO₂­

FeO + CO  Fe + CO₂­

Fe₂O₃ + 3C  ­

            Middle section (900°C)

CaCO₃ CaO + CO₂­

CaO + SiO₂  CaSiO₃ (slag)

2Ca₃(PO₄)₂ + 6SiO₂  + 10C ¾® 6CaSiO₃ + 10CO­ + 4P

3Fe + P ¾® Fe₃P

9. The (+1) state is expected to be the common and most stable for Cu, Ag and Au as they posses table d¹⁰ configuration. But only Ag⁺ is stable both in solid state and solution, which Cu⁺ and Au⁺ disproportionate very fast in H₂O

2Cu⁺  Cu²⁺ + Cu        K =  = 1.6 ´ 10⁶

3Au⁺  Au³⁺ + 2Au       K =  = 1 ´ 10¹⁰

10. i) 3Cu + conc. 4HNO₃ ¾®Cu(NO₃)₂ + 2NO₂­ + 2H₂O
11. ii) Cu + conc. 2H₂SO₄ ¾® CuSO₄ + SO₂ + 2H₂O

iii)   Cu₂O + 4NH₄OH ¾® 2[Cu(NH₃)₂]OH + 3H₂O

1. iv) 2CuSO₄ + 4KI ¾® 2CuI₂ + 2K₂SO₄

 

6