Ellingham diagram
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Transcript of Ellingham diagram
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The Ellingham Diagram
Bapin Kumar Rout
Technical presentation
Research and Development, Steelmaking and Casting Research Group
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Before we start: Thermodynamic terminologies
Gibb’s Free Energy ( G ): Energy of the system available to do work
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G= H-T*. S
∆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
Enthalpy term Entropy term
Spontaneity of the Reaction ( G<0)
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Free Energy and Equilibrium
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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.
Standard state free energy ( G0=):
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The Ellingham Diagram
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1Ellingham H. J. T., “Reducibility of Oxides and sulfides in Metallurgical Processes , J Soc Chem Ind (London) 63 125 (1944)
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
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The Ellingham Diagram
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Oxide stable Metal stable
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The Ellingham Diagram
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The Ellingham Diagrams
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The Ellingham Diagrams
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Two intersecting lines
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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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The Ellingham Diagrams
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The Ellingham Diagram
Research and Development, Steelmaking and Casting Research Group
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The Ellingham Diagrams
Research and Development, Steelmaking and Casting Research Group
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The Ellingham Diagrams
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Additional scales on Ellingham diagram
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Y=-mX
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Reading pO2 from Ellingham diagram
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• 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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Reading pO2 from Ellingham diagram
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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 thetemperature of 1600°C
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Other gas mixtures
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• 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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CO-CO2 gas mixture as reducing agent (374)
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MO2+2CO=M+2CO2
At T>Ts CO-CO2 mixture is reducing w.r.t MO2 at pCO/pCO2=1At T<Ts CO-CO2 mixture is oxidisingw.r.t MO2 at pCO/pCO2=1
If CO-CO2 mixture to be made reducing at T<Ts the pCO/pCO2(>1) must be increasingAt T=Tu pCO/pCO2 should be increased from 1to 10 to maintain reaction equillibrium
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Limitation
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Limitation
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References
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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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Questions?
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