Ellingham Diagram
Also known as: Gibbs Free Energy Diagram, High-Temperature Reduction Diagram
The Ellingham Diagram, introduced by Harold Ellingham in 1944, is a graphical representation of the Gibbs free energy change for oxide formation and reduction as a function of temperature. It is an essential tool for predicting the thermodynamic feasibility of ore reduction and selecting appropriate reducing agents and temperatures for smelting and roasting operations.
Key highlights
- Visual comparison of multiple reactions and temperatures on a single plot
- Rapidly identifies thermodynamically favorable conditions without calculation
- Guides selection of reducing agent (CO, H2, metal, etc.) and temperature
- Shows where phase changes (melting) occur and their effect on ΔG°
- Widely used in metallurgical education and industry; standard reference tool
Intuition
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How it works
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When to use it
Use Ellingham diagrams when designing new smelting or roasting processes, evaluating ore feasibility, or troubleshooting existing operations. Assume equilibrium and ideal behavior. Kinetics and side reactions must be considered separately. Most valuable for screening alternatives before detailed thermodynamic or experimental studies.
Strengths & limitations
- Visual comparison of multiple reactions and temperatures on a single plot
- Rapidly identifies thermodynamically favorable conditions without calculation
- Guides selection of reducing agent (CO, H2, metal, etc.) and temperature
- Shows where phase changes (melting) occur and their effect on ΔG°
- Widely used in metallurgical education and industry; standard reference tool
- Assumes thermodynamic equilibrium; kinetics (reaction rate) are not shown
- Does not account for side reactions or impurity effects unless explicitly plotted
- Data are often for ideal conditions; real ore and slags deviate
- Extrapolation beyond measured temperature ranges is unreliable
- Multiple simultaneous reactions (e.g., sulfide roasting) require separate analysis
Common pitfalls
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Applications
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Frequently asked
How do I read an Ellingham diagram?
The y-axis is ΔG° (usually in kJ/mol of O2). The x-axis is temperature. Lines represent different oxide-formation reactions. Below a line, the oxide is stable. Above a line, the oxide reduces. Intersecting lines mark transition temperatures where one reducing agent becomes superior to another.
What if thermodynamics says reduction is unfavorable?
The reaction is thermodynamically impossible at that temperature and pressure. Increase temperature or change reducing agent. Alternatively, consider alternative process routes (e.g., sulfate leaching instead of direct reduction).
How do side reactions appear on Ellingham diagrams?
Side reactions are separate lines. If they are thermodynamically more favorable (lower ΔG°) than the desired reaction, they will compete. A good diagram shows all relevant competing reactions.
Do I need to achieve thermodynamic equilibrium?
Rarely. Smelters operate kinetically: they try to drive reactions fast enough, not necessarily to equilibrium. Kinetics (which Ellingham diagrams don't show) often limit conversion. Pilot testing is needed to confirm practical feasibility.
Where do I get accurate thermodynamic data for my ore?
Published databases (NIST, FactSage, HSC Chemistry) contain standard thermodynamic data. For non-standard ores, conduct differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA) experiments to measure ΔH° and ΔS°.
Sources
- 1.Ellingham, H. J. T. (1944). Reducibility of oxides and sulfides. Journal of the Society of Chemical Industry, 63(5), 125-160.
- 2.Richardson, F. D. (2007). Physical chemistry of melts in metallurgy (Vol. 2). Academic Press.
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Cite this page
ScholarGate. (2026, June 3). Ellingham Diagram. ScholarGate. https://scholargate.app/mining-engineering/ellingham-diagram