Thermodynamic Principles of Metallurgy

Introduction

The process of extracting metals from their ores and transforming them into useful forms is known as metallurgy. Sulphur and silicon are metals stored in the Earth’s crust. Thermodynamic principles enable the extraction of metals. The study of heat, temperature, and other energy forms is called thermodynamics. It studies energy transfer that occurs during chemical and physical changes. It also allows us to anticipate and track these changes.

Thermodynamics principles used in metallurgy

The central concept of thermodynamics that is useful in metallurgy is Gibbs Free Energy. Gibbs Free Energy is the maximum amount of non-expansion work done in an isolated system. It is measured in joule and kilojoule. 

Gibbs Free Energy is the enthalpy minus the product of temperature and entropy. It determines whether the reaction is spontaneous or non-spontaneous. The letter G symbolises free energy. If G is negative, the reaction will take place on its own.

ΔG = ΔH – TΔS.

H stands for the change in enthalpy. A positive value will be assigned to an endothermic reaction, whereas a negative value will be assigned to an exothermic reaction. As a result, G is negative in exothermic reactions. The letter S stands for entropy, or the unpredictability of molecules. This changes with the change in the state of matter.

The other equation, which shows the relation between the equilibrium constant and Gibbs Free energy, is the following: 

ΔG°=- RTlnK_eq

Where, K_eq is the equilibrium constant.

The importance of thermodynamics in metallurgy

• The main importance of thermodynamics in metallurgy is that it helps us predict whether an alloy is in equilibrium.

• The concept of Gibbs free energy and entropy from thermodynamics are used in metallurgy.

• The extraction of metal from their oxides is facilitated by using different reducing agents, such as iron from its oxides, copper from cuprous oxide, zinc from zinc oxide, etc. Thermodynamics helps in finding suitable reducing agents for the metals.

Ellingham Diagram

The Ellingham diagram represents the relationship between temperature and a compound’s stability. The diagram graphically represents the change in Gibbs Energy and its absolute temperature. 

In metallurgy, the Ellingham diagram is used to depict the reduction equation process.

When reducing oxides to produce pure metals, it aids in identifying the optimum reducing agent that must be utilised.

The main features of this diagram are:

• The graph shows ΔG°versus the formation of oxide of elements.

• The entropy change will be negative.

ΔG = ΔH – TΔS

• Now, the slope will be positive and free energy increases due to increasing temperature.

• The lines on the graph will be straight, except for lines with phase changes. 

• The Ellingham diagram shows the plots of G for the oxidation and reduction of comparable common metal species, as well as some reducing agents.

Example: Reduction of iron oxide

The reduction of iron oxides is carried out in a blast furnace at different temperatures. First, haematite is reduced at the top of the furnace, with the temperature ranging from 600 to 700C. the Ellingham diagram suggests that carbon monoxide serves as the stronger reducing agent in the process and has free energy that is more negative than the second equation:

1. 2 CO + O₂ → 2CO₂

2. 2 C + O₂ → 2CO₂

Even in the presence of carbon, haematite is reduced by CO in the top part of the blast furnace – however, this is primarily due to superior kinetics for CO (gaseous) reacting with the ore.

Conclusion

Metallurgy is related to the properties of metals and the process of metals extraction. Different extraction methods are used to extract metals, and the principle of thermodynamics is applied to the process of metallurgy to make it more functional. Gibbs Free Energy is a central principle of thermodynamics that is used in metallurgy. The Ellingham diagram graphically depicts the results, showing the relation between temperature and Gibbs Free Energy. It helps us determine the appropriate reducing agent for a specific metal oxide or sulphide. Metal oxides and sulphides must be removed from ores to obtain pure metals. Furthermore, the Ellingham diagram helps in selecting the best reducing agent.