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Occurrence and extraction of iron

Answer Text:
4. Iron
Occurrence and extraction of iron
- Is the second most abundant metal after aluminium, forming about 7% of the earth’ s crust.
Main ores
- Haematite, #Fe_2O_3#;
- Magnetite, #Fe_3O_4#;
- Siderite, #FeCO_3#;
Extraction from haematite (#Fe_2O_3#).
Summary of the process
- The ore-haematite is crushed and mixed with coke and limestone.
- The mixture is called charge.
- The charge is loaded into the top of a tall furnace called blast furnace.
- Hot air-the blast is pumped into the lower part of the furnace.
- The ore is reduced to iron as the charge falls through the furnace.
- A waste material called slag is formed at the same time.
- The slag floats on the surface of the liquid iron produced.
- Each layer can be tapped off separately.
Details of the extraction process.
(i) The blast furnace Is a tall, somewhat conical furnace usually made of silica and lined on the inside with firebrick Details of the extraction process
Raw materials.
- Iron ore; i.e. haematite
- Coke; C
- Limestone; #CaCO_3#;
- Hot air;
Conditions
- Temperature at the bottom of furnace, #1400^0-1600^oC#
- Temperature at the top of the furnace, #400^oC#
Reactions and processes
Step-1 – crushing and loading
- The ore is crushed into powder form, to increase the surface area for the upcoming reduction/ redox reactions.
- It is then mixed with coke and limestone and then fed at the top of the furnace using the double bell (double-cone devise) changing system
Note:
- The double bell charging system ensures that the furnace can be fed continuously from the top with very little heat loss, by preventing any escape of hot
gases.
- This in turn reduces production costs.
Step 2: -Pre heating of the blast furnace.
- Air that has been preheated to about #700^oC# is blown/ fed into the base of the blast furnace through small pipes called tuyers.
- This provides the required temperatures for the reactions in the blast furnace.
- This results into highest temperatures, about #1600^oC# at the hearth (bottom of the furnace) which then decreases upwards the furnace.
Step 3: -Generation of reducing agents.
- Two reducing agents are used in this process: Coke and carbon (II) oxide; with carbon (II) oxide being the main reducing agent.
(i). Oxidation of coke;
- Coke burns in the blast at the bottom of the furnace.
- The reaction temperatures is about #1600^oC# and the product is Carbon (IV) oxide gas
- This reaction is exothermic, producing a lot of heat in the blast furnace.
Equation:
#C(s) + O_2(g) to CO_2(g)#
(ii). Decomposition of limestone;
- The limestone in the charge decomposes in the blast furnace to calcium oxide (Quicklime) and carbon dioxide.
Equation:
#CaCO_3(s) to CaO(s) + CO_2(g)#
Heat
- The calcium oxide will be used in the removal of the main ore impurity/ silicates/ silica in the form of silicon (IV) oxide.
- The# CO_2# then moves up the blast furnace to regenerate carbon (II) oxide, the chief reducing agent.
(iii). Production of carbon monoxide
- The #CO_2# from oxidation of coke and decomposition of limestone (calcium carbonate) react with (excess) coke, to form carbon (II) oxide
- The reaction occurs higher up in the blast furnace at about #700^oC#;
Equation:
#CO_2(g) + C(s) to 2CO(g)#
Step 4: The actual reduction process
- Reduction of the ore is by either CO or coke, depending on temperatures.
(i). Reduction by coke
- This occurs much lower down the furnace at higher temperatures of about #800^oC# and above.
- This reaction is ordinarily slow and thus serves to only reduce the part of the ore reduced by CO at lower temperatures in the upper parts of the furnace.
Equation:
#2Fe_2O_3(s) + 3C(s) to 4Fe(s)+ CO_2(g)#
Note:
- The resultant #CO_2# is quickly reduced to CO by the white-hot coke to more carbon (II) oxide as per step 3(iii) above.
(ii). Reduction by carbon
(II) oxide
- This is the main reducing agent.
- The reaction between CO and #Fe_2O_3# is relatively faster and occurs at lower temperatures of #500^oC-
700^oC#, higher up the furnace.
Equation:
#Fe_2O_3(s) + 3CO (g) to Fe(s)+ CO_2 (g)#
- The resultant carbon (IV) oxide is also quickly recycled by being reduced to CO by coke to from more reducing agent
(iii). Melting
- The iron produced in both of the reduction processes is in solid state.
- As the iron drops / falls down the furnace, it melts as it passes through the melting zone/ molten zone (#1500^oC-1800^oC#)
- The molten iron runs to the bottom of the furnace.
- Temperatures at the hearth (bottom of the furnace) is maintained at approx. #1400^oC# and yet pure iron melts at about# 1525^oC#.
- Consequently the molten iron would easily solidify at the base (Temp =#1400^oC#)
- However this is not usually the case;
Reason:
-Impurities absorbed by iron during melting (mainly carbon) reducing the melting point to below #1400^oC#.
- The molten iron is then easily tapped off.
Step 5: -Removal of earthy impurities.
- The earthy impurities in the ore (mainly silica) react with calcium oxide from decomposition of limestone to form calcium silicate.
Equation:
#CaO(s) + SiO_2(s) to CaSiO_3(s)#
- These earthy impurities form molten slag whose main component is calcium silicate.
- The slag does not mix with iron but rather floats on top of it, at the base of the furnace. Importance of the slag
- As it floats on top of molten iron it protects it from being re-oxidized by the incoming hot air.
Uses/application of the slag
1. Light-weight building material.
2. Manufacture of cement.
3. Road building material.
Step 6:- Removal of furnace (waste) gases.
- Hot unreacted/waste gases leave at the top of the furnace.
- Main components include Nitrogen, unreacted #CO_2#, unreacted CO, oxygen and Argon (Noble gases)
- Additionally they contain dust particles.
Note:
- Upon removal of dust particles, the furnace gases, being hot can be used to preheat the air blown in at the base.