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ELLINGHAM DIAGRAM
Ellingham plotted graphs of formation of oxides, sulphides and halides versus temperature. The plots of ΔfG(metal oxide) per mole of oxygen versus temperature are useful in deciding the reducing agent and temperature for reducing a specific metal oxide. All these graphs have positive slopes showing decrease of entropy except for the formation of CO(g) from coke that shows increase in entropy with increase in temperature.
A sudden increase in slope, as in case of Zn or Mg, indicates melting. The temperature at which two graphs intersect give ΔG=0 for the reaction of one oxide with other element. The element of the lower graph works as reducing agent for the oxide of the other, e.g., Below temperature 1623K (approx) Mg metal will reduce Al2O3 (but not economical) and above this temperature it is Al that reduces MgO (but not economical).
The limitations of this method are that it does not discuss the rate of reaction and takes every reaction as an equilibrium process.
APPLICATION OF THE ELLINGHAM DIAGRAMS
Extraction of Iron:-
In Ellingham diagram, the graph of CO à CO2 conversion remains below Fe à Fe2O3 upto 1123K (for Fe à FeO). It is 1073K approximately. So, CO(g) acts as reducing agent upto this temperature.
3Fe2O3 + CO ----> Fe3O4
Fe3O4 + CO ----> 3FeO + CO2
FeO + CO ----> Fe + CO2
Also, graph of C à CO is below the graph of Fe à Fe2O3 after 1123K. So, carbon acts as reducing agent above this temperature.
Fe2O3 + C ----> 3CO + 2Fe
Zones in Blast furnace: Near the bottom coke id filled and in the upper part 8:4:1 proportion of Fe2O3, coke and lime stone. Height of blast furnace is about 150 feet.
(a) Zone of combustion: Near the bottom, hot air is blown that burns coke to produce a temperature of 2100-2200K.
C + O2 ----> CO2 + 393.5KJ
(b) Zone of heat absorbtion: Upgoing CO2 reacts with coke to reduce to temperature to about 1500-1600K.
C + CO2 ----> 2CO – 163KJ
(c) Zone of reduction: Upto 1123K, reduction of Fe2O3 by CO gas gives Fe (in the upper part). Left of Fe2O3 is reduced by coke above 1123K.
(d) Zone of slag formation: Slagging operation takes place at about 1273K.
CaCO3 ----> CaO + CO2
CaO + SiO2 ----> CaSiO3
Impurity
At the base molten iron is collected under the slag. This iron is called pig iron and contains about 4% carbon with many other impurities like, S, P, Si, Mn, etc.
The metal iron is collected from slag by pressing under rollers. The height of the furnace used for the extraction of Fe from Fe2O3 or Fe3O4, is about 150 feet to 200 feet.
Other Forms of Iron
- Cast Iron: It is hard but brittle form of raw iron produced by melting and cooling of pig iron, its colour is gray. Fast cooling keeps the impurity of carbon as graphite. Slow cooling converts most of the carbon into cementite Fe3C and the colour becomes white. The carbon content in cast iron comes down to about 3%. It is of two types:
- White Cast Iron: Carbon is present in the form of cementite (Fe3C). Slow cooling of molten pig iron.
- Grey Cast Iron: Carbon is present in the form of Graphite (Sudden cooling of molten pig iron).
- Wrought iron or malleable iron: It is prepared by heating cast iron in reverberatory furnace lined with haematite which oxides carbon to CO. Lime stone is added to remove Si, S and P as slag. The wrought iron is separated from slag by passing through rollers. It is the purest form of iron and contains carbon only upto 0.5%.
- Steel: It contains 0.1 to 1.7% carbon and remaining iron only. It is commonly prepared in Bessemer converter or open hearth furnace or electric furnace and also by Semen Martin process.
In this process calcium phosphate Ca3(PO4)2 forms as slag which is also known as Thomous slag and is used as fertilizers. Steel
is of three types:
- Mild Steel: Also known as soft steel contains lower percentage of
carbon which is about 0.025 to 0.25% . - Hard Steel: Contains maximum percentage of carbon which is about 2.5 to 5% .
- Alloy Steel: Composition of steel which contain 0.25 to 2.5% of carbon.
