The document discusses the Ellingham diagram, which plots the standard free energy of formation (ΔG0) of various oxides and sulfides as a function of temperature. The diagram shows that below a critical temperature, oxides are more stable than metals, while above this temperature metals become more stable. Lines on the diagram represent oxidation/reduction reactions, with their intersection indicating the equilibrium temperature. Additional scales were later added to allow reading the equilibrium oxygen partial pressure directly from the diagram. The Ellingham diagram is a useful tool for predicting spontaneity and equilibrium in metallurgical oxidation/reduction reactions.

1. Technical presentation
The Ellingham Diagram
Bapin Kumar Rout
Research and Development, Steelmaking and Casting Research Group

2. Before we start: Thermodynamic terminologies
 Gibb’s Free Energy ( G ): Energy of the system available to do work
G= H-T*. S
Enthalpy term Entropy term
∆H: Measure of the actual energy that is liberated when the reaction occurs
∆S: Measure of the change in the possibilities for disorder in the products
compared to the reactants
Spontaneity of the Reaction ( G<0)
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3. Free Energy and Equilibrium
Standard state free energy ( G0=):
Under non-standard conditions, we need to use G instead of G°.
1. If G is negative, the forward
reaction is spontaneous.
2. If G is 0, the system is at
equilibrium.
3. If G is positive, the reaction is
spontaneous in the reverse
direction.
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4. The Ellingham Diagram
Ellingham1 plotted experimentally determined standard free energy of formation
(∆G0) of various oxides (and sulfides) using one mole of oxygen with temperature
Ellingham Found that, the standard enthalpy and entropy of formation of a compound
don’t change significantly with temperature as long as there is no change of state
Thus, ∆G0-T relationship is approximated to straight lines:
Y= mx+C Intercept Slope
1Ellingham H. J. T., “Reducibility of Oxides and sulfides in Metallurgical Processes , J Soc Chem Ind (London) 63 125 (1944)
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5. The Ellingham Diagram
Oxide stable Metal stable
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6. The Ellingham Diagram
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7. The Ellingham Diagrams
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8. The Ellingham Diagrams
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9. Two intersecting lines
 T<TE A and BO2 are most stable
 T>TE B and AO are most stable
 At T=TE A,B,AO,BO2 are in equilibrium
A as a reducing agent to reduce BO2 to form B and AO- T> TE
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10. The Ellingham Diagrams
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11. The Ellingham Diagram
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12. The Ellingham Diagrams
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13. The Ellingham Diagrams
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14. Additional scales on Ellingham diagram
Y=-mX
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15. Reading pO2 from Ellingham diagram
• In to avoid calculating the equilibrium
partial pressure for each value of ΔG°,
Richardson2 added a nomographic scale
to the Ellingham diagram
• For a metal oxidation reaction ,
2M (s) + O2 (g) = 2MO (s)
The equilibrium constant has the form
K=1/pO2
∆G= RT ln(pO2)
2 F.D. Richardson and J.H.E. Jeffes, "The Thermodynamics of Substances of Interest in Iron and Steel Making from 0°C to 2400°C: I-Oxides," J. Iron
and Steel Inst. (1948), 160 261.
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16. Reading pO2 from Ellingham diagram
1. Identify a point corresponding to
a selected temperature on the
line for: Fe + O FeO, above M
2. Using this point, and the point O
in the top left corner, draw a line
across the diagram
3. Read the partial pressure of
O2 from the right hand axis
At any oxygen pressure higher than
~10-8.5, the iron will be oxidised at the
temperature of 1600°C
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17. Other gas mixtures
• The oxygen required to cause oxidation in the gas phase need
not to come from oxygen gas. Consider the following reaction:
2CO (g) + O2 (g) = 2CO2 (g)
• For this reaction,
• We see that pO2 is equivalent to a ratio: pco/ pco2
• Similarly for the reaction 2H2+O2=2H2O pO2 is equivalent to a
ratio: pH2O/ pH2
• Thus two nomographic scale may be added to the diagram, with
a new origin, C and H respectively for pco/ pco2 and pH2O/ pH2
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18. CO-CO2 gas mixture as reducing agent (374)
MO2+2CO=M+2CO2
At T>Ts CO-CO2 mixture is reducing
w.r.t MO2 at pCO/pCO2=1
At T<Ts CO-CO2 mixture is oxidising
w.r.t MO2 at pCO/pCO2=1
If CO-CO2 mixture to be made reducing at T<Ts the pCO/pCO2(>1) must be increasing
At T=Tu pCO/pCO2 should be increased from 1to 10 to maintain reaction equillibrium
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19. Limitation
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20. Limitation
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21. References
1. D.R. Gaskell, "Introduction to the Thermodynamics of Materials“
2. http://www.doitpoms.ac.uk/tlplib/ellingham_diagrams/
Thank you
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22. Questions?
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