Extraction of iron
Manufacture of steel Rusting of iron
Some important question.
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Points to remember
At the end of the topic, you will know about:
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IRON(फलाम)
Occurrence (Ore) In Nepal
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Copper pyrite
Magnetite
Haematite
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major ore
Steps involved in the metallurgy
Iron is generally extracted from its major ore Haematite. It involves the following steps:
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Contd.
2Fe2o3.3H20 2Fe2o3 + 3H20
P4 + 5O2 2P2O5
4As + 3O2 2As2O3
S + O2 SO2
C +O2 CO2
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Contd.
c) If iron carbonate is present in the ore, it decompose to form ferrous oxide. The ferrous oxide is converted into ferric oxide.
FeCO3 FeO + CO2
4FeO + O2 2Fe2O3( Cant form slag/stable)
There is also probability of combination of ferrous oxide with silica to form a slag of ferrous silicate and loss of iron takes place.
FeO + SiO2 FeSiO3
Slag (Iron Silicate)
In order to avoid any loss of iron in the form of slag, ferrous oxide should be oxidised to ferric oxide.
2FeO +2O2 2Fe2O3
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Contd..
4.Smelting or reduction: The calcined or roasted ore is reduced to iron in a blast furnace by smelting or carbon reduction process.
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Contd..
Fig Blast furnace for extraction of iron
Description about furnace:
The blast furnace is tall cylindrical furnace made up of steel and is internally lined with fire proof bricks. At the top the furnace, it is provided with double cup and cone arrangement which helps to introduce charge from top without letting any gas to escape out. The furnace is also provided with small pipes called tuyeres to pass the hot air inside furnace.
The charge consisting of limestone, coke and ore(1:4:8) is passed through the mouth of the furnace .At the same time preheated air at about 700°c is sent upward from the tuyere. The hot air get in contact with the charge falling downward inside the furnace and following reaction occurs at different zones.
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Contd..
a. Zone of combustion (1500 - 1700oC)
This region lies near the hearth of furnace. Combustion of carbon takes place in this zone and being exothermic, large amount of heat is produced.
C + O2 CO2 +97kcal
As Co2 moves up, combine with layer of hot coke and gets reduced to CO. this reaction is endothermic which causes the gradual decrease in temperature as we go from bottom to top.
CO2 + C 2CO-heat
b. Zone of fusion (1200-1300oC)
This region lies above the zone of combustion. Carbon dioxide gas produced in the zone of combustion rises up meets with hot coke and carbon dioxide is reduced to carbon monoxide as:
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Contd..
CO2+C. 2CO-heat (endoth)
The spongy iron formed in the zone of reduction slide down to this zone and fuses (melts) to form molten iron. The molten iron then absorbs impurity of S,P, As and Si. It absorbs carbon in the form of cementite.
3Fe+C Fe3C
cementite
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c. Zone of slag formation (800-1000oC)
This region lies almost in the middle part of furnace. Limestone present here is decomposed according to following reaction.
CaCO3 CaO + CO2
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Contd…
The temperature of this zone is about 800oC to 1000oC. The lime produced by the decomposition of CaCO3 combines with silica (which acts as impurities in the ore) to form slag.
CaO + SiO2 CaSiO3
d. Zone of reduction (500 - 700oC)
It is the uppermost part of the furnace where ferric oxide is reduced to iron by carbon monoxide. The iron formed will be spongy in nature because the temperature is not sufficient to melt the iron.
Fe2O3 + 3CO 2Fe3O4 + 3CO2
Fe3O4 +CO 3FeO + CO2
FeO +CO Fe +CO2
Spongy iron
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impurities Slag
Contd
At the end of smelting, the spongy iron melts and two separate layer are formed at the bottom of the furnace. The molten metal is removed from metal outlet and is allowed to solidify. The iron obtained is called pig iron. It is remelted in a vertical furnace and remolded to get cast iron.
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End of extraction
DIFFERENT FORM OF IRON
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Cast/pig Iron | Steel | Wrought Iron |
1. Contains 2-5% carbon | 1. 0.2- 2% carbon | 1. May contain upto 0.25% carbon |
2. Is hard and brittle | 2. Hard and tensile | 2. Soft and ductile |
3. M.p. is about 1200°c | 3. M.p. is about 13- 1400°c | 3. M.p. is about 1500°c |
4. Doesn’t rust easily | 4. Depend upon quality of steel | 4. Rust easily |
5. Can’t be welded | 5. Can be welded | 5. Can be welded |
6. Are used to make pipes, heavy machines etc. | 6. Are used to make Knife,Sword, Utensils etc | 6. Are used to make chains , core of magnet etc. |
STEEL
Steel is an alloy of iron containing 0.2 to 2% carbon as one of the constituents and traces of sulphur, phosphorous along with some metals.
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Manufacture of steel by Basic oxygen method:
Principle and process: Basic oxygen method is the refined version of Bessemer process where blowing of air is replaced with blowing of pure oxygen.The blowing of oxygen lowers the carbon content of molten cast iron.This metod is named basic method as basic flux obtained from dolomite is used to remove the acidic impurities and for lining the converter(Linz-Donawitz converter).
The basic oxygen converter is a cylyndrical vessel with an open at the top. The furnace is lined with basic flux(dolomite,CaCO3.MgCO3).The mixture of 70-80% hot Cast iron and 20-30% of molten Scrap iron and little heamatite is fed into LD converter.The pure oxygen is blown through the nozzles water cooled lance inserted at supersonic velocity on to the charge while inert gas is blown from the bottom.
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Contd..
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Contd..
Fig: Basic oxygen process for the manufacture of steel
During the process follwing reaction takes place.
C +O2 CO2
CO2 +C 2CO
2CO +O2 2CO2
After passing the oxygen for few minutes, the supply of oxygen is paused and suitable flux containing calcium oxide is added to removes the impurities in the form of fusible slag.
CaCO3 CaO +CO2
S +O2 SO2
Si +O2 SiO2
P4 +5O2 2P2O5
3CaO + P2O5 Ca3(PO4)2 (slag)
CaO +SiO2 CaSiO3 (slag)
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Contd..
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After this,the blowing of oxygen is continued through the lance till the carbon content get adjusted.During this period, samples are drawn from time to time and subjected to the chemical test whether the content of carbon is maintained.After that converter is tilted and molten steel is tapped off from the furnace below the slag level.The whole process completes in 45 minutes.
Advantages of Basic oxygen process:
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Manufacturing of steel by open hearth process
Open Hearth Process
This process was developed by Siemen and Martin. This is the modern process for the manufacture of steel. For the manufacture of steel by this process, 70-80% cast iron, 20-30% scrap iron and little haematite is used. The charge is heated by passing producer gas (CO + N2 in 1:2) at about1500°C temperature where impurities are oxidised by heamatite.
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contd
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During the process samples are drawn from to time and subjected to the chemical test whether impurities are removed or not.when test shows that the impurities are removed completely and metals becomes pure,then calculated amount of carbon, manganese and silicon are added to the pure metal. After that it is stirred thoroughly to get molten steel. The mixture of carbon, manganese and silicon is called spiegeleisen.
Fe2O3 +3C 2Fe +3CO
2Fe2O3 +3S 4Fe +3SO2
5Fe2O3 +6P 10Fe +3P2O5
2Fe2O3 +3Si 4Fe +3SiO2
3CaO +P2O5 Ca3(PO4)2 (Slag)
CaO +SiO2 CaSiO3 (Slag)
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A. Acidic Process
If impurities present in the iron are basic in nature(presence of Mn) or the phosphorus content is low, acidic process is applied. In acidic process, the hearth of the furnace is lined with acidic material such as SiO2. The impurities are oxidized by haematite ore.
contd
Flue gas
Fig: open hearth furnace
Fig: Open hearth furnace
Finally molten steel is taken out and allowed to solidify in the air to get solid steel.
Advantages of open hearth process:
a. This is slow process and can be controlled accurately.
b. Steel of high quality can be obtained.
c. Iron is not lost in the form slag.
d. Impurities are oxidised by heamatite and it self gets converted into steel.
e. Scrap iron of very low grade can be also employed directly.
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contd
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Contd..
Varieties of iron: There are three variety of iron depending on the contents of carbon.
A. Cast iron: It is the impure form of iron and contains highest percentage of carbon. It contains 5%carbon along with little amount of phosphorus, manganese, silicon, Sulphur,arsenic etc. When cast iron is heated strongly up to red hot and then allowed to solidify suddenly by plunging into cold water,it becomes hard and brittle due to the presence of carbide (cementite). Such type of iron is called White cast iron.
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On the other hand,when cast iron heated strongly up to red hot and then allowed to solidify in the air, it becomes soft and less brittle in nature due to the arrangement of carbon in the form of graphite. This type of iron is called gray cast iron.
It is mainly used in making heavy machinery parts,pipes, steel and wrought iron.
B.Wrought iron.
It is the purest form of iron and contains lowest percentage of carbon. It contains only0.12-0.25% carbon along with traces of phosphorus ,manganese and silicon. It is manufactured by heating cast iron with heamatite to remove it's impurities.
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Wrought iron is mainly used in making chains,bolts, electrical wire.piano wire,tools etc.
It is also used in making railway tracks,bridge etc
C.Steel iron. It is an intermediate product of iron regarding the contents of carbon.It contains 0.1-1.5%carbon along with Sulphur, phosphorus, manganese and silicon. It is also manufactured from cast iron by removing its impurities and adding some special impurities.It is mainly used in making household utensils
Fe2O3 +3C 2Fe +3CO
2Fe2O3 +3S 4Fe +3SO2
5Fe2O3 +6P 10Fe +3P2O5
2Fe2O3 +3Si 4Fe +3SiO2
3CaO +P2O5 Ca3(PO4)2 (Slag)
Differentiate between Open hearth and bessemer process
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| Open Hearth furnace | Basic Oxygen method |
1. | Impurities are oxidised by haematite Fe2O3 | Impurities are oxidised by blowing pure oxygen through nozzle of steel lance. |
2. | Percentage of carbon is adjusted by adding spiegeleisen | Percentage of carbon is adjusted by oxidising carbon in the form of CO. |
3. | The furnace is heated by burning producer gas. | No such arrangement is possible.However,molten iron from blast furnace is provided. |
4. | It is slow process and takes about 8-9 hour. | It is slow process and within 45 minutes steel is obtained. |
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�� Heat Treatment of Steel
The phenomenon of heating and cooling solid form of steel under controlled conditions so as to modify certain physical properties without changing chemical composition is called heat treatment of steel. Particularly, heat treatment of steel develops following changes in the steel.
1. Refinement of structure
2. Removal of imprisoned gases
3. Change in internal stresses.
Some heat treatment processes are given below:
a. Hardening (Quenching): The process of heating steel below its melting point (up to red hot ) cooling it suddenly (quickly) either plunging in oil or in water is called hardening or quenching. The steel thus obtained is called hardened or quenched steel.
Hardening increases the resistance to tear, ability to cut other metals and brittleness of steel. Such types of steels are used for cutting tools.
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Contd..
b. Annealing (softening): The process of heating steel to bright redness(below M.p.) followed by slow cooling is called annealing. The steel thus obtained is soft, weak and ductile. It removes strain in the steel.
This type of steel is soft and ductile and in suitable for fabrication process.
c. Tempering of steel: If hardened or quenched steel is heated to a temperature much below redness (200-350oC) and then cooled slowly, it neither becomes too hard nor too brittle. The process is called tempering of steel and steel thus obtained is called tempered steel. Tempering is quenching followed by annealing of steel.
The tempered steel is used for making razor blades, knifes, axe etc.
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Theory of rusting
When iron is exposed to air for certain time, Iron is oxidised , resulting in the formation of loosely bonded reddish brown layer called rust. This process of formation of rust is called rusting. Commonly rust is represented as, Fe203.XH2O.
The theory of rusting is best explained by electrochemical theory. Electrochemical theory suggest that rusting doesn’t occurs by direct contact of Iron and moist oxygen. Instead it is due to tiny electric cell set up between irregular surface of iron and the impurities( Sn, Au- less electropositive metal), where gases such as CO2,H2S,SO2 etc act as an electrolytic solution.
Following electrode reaction takes place at Iron and produce rust.
At Anode
Iron is oxidised to Fe++
Fe Fe++ + 2e-
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At cathode
Oxygen is reduced to hydroxyl ion(OH-) and H+ ion into hydrogen. The hydrogen atom on iron surface reduces dissolved oxygen into H2O.
O2 + 2H2O + 4e- 4OH-
2H+ + 2e- H2
( H+ is obtained from dissociation of H2O and H2Co3 present in atmosphere)
Further, The ferrous and Hydroxyl ion combine to form ferrous hydroxide. On atmospheric oxidation, Fe(OH)2 is converted to ferric hydroxide.
Fe++ + 2OH- 4Fe(OH)2
4Fe(OH)2 + 2H2O +O2 4Fe(OH)3
Finally, Ferric hydroxide on exposure to open air dissociate to form hydrated ferric oxide which is called rust.
2Fe(OH)3 Fe2O3 +.3H2O
(Rust)
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Fig: process of rusting
Contd.
Fe2O3+xH2O Fe2O3.xH2O
Rust
a. Cathodic protection or sacrificial protection:
Zn Zn+ + 2e- (oxidation of Zn takes place due to lower reduction potential and protec iron sheet from rusting.)
i) Zinc layer forms a barrier between iron surface and atmosphere.
ii) If the rusting environment is set up, it is not the iron but zinc which undergoes corrosion. This is due to lower reduction potential of zinc. Galvanization can also be done.
b. By the formation of protective barrier:
i) Coating of oil and grease may be used on tools and machinery parts to prevent from rusting.
ii) Thin coating of paint, enamel, lacquer, varnishes may be used to prevent from rusting. (Barrier protection)
iii) By passing steam on the red hot iron (Barf's protection)
c. By the formation of alloys
d. Uses of corrosion inhibitors: using alkali
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Prevention of Rusting
Questions
Ans: Spiegelisen is a alloy of containing certain amount of carbon ,Si and Manganese. The function of spiegeleisen is to get the desired quality of steel. The nature of steel depend upon the amount of spiegeleisen added.
2. What is mohr’s salt?
Ans: It is a double salt of ferrous sulphate and ammonium sulphate and is written in formula as FeSO4.(NH4)2SO4.6H2O
3. What happens when iron is passed through steam?
Ans: Iron is not attacked by water at ordinary temperature. But when steam is passed over heated iron, ferrosoferric oxide and hydrogen gas is formed.
3Fe + 4H2O Fe3O4 + H2
steam 850° (red hot)
Ans: Concentrated nitric acid makes Iron passive due to formation of thin layer of ferrosoferric oxide( Fe3O4) on its layer. This activity of making the iron passive using oxidising agents is called passivity of Iron.
3Fe + 8HNO3 Fe3O4 + 8NO2 + 4H2O
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4. Write about biological importance of iron
Ans: Iron plays an important role in the transfer of oxygen by the cytochrome, molecule involved in energy production. Iron is involved in the formation of connective tissues of several neurotransmitters in the brain. The important role of iron is to strengthen the immune system. Due to presence of iron (haemoglobin) human blood is red.
5. Draw the structure of ferric cholride and write its use.
Ans: Cl Cl Cl
Fe Fe
Cl Cl Cl
6. Write the use of mohr’s salt.( FeSO4.(NH4)2SO4.6H2O)
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