Question
Thu September 27, 2012 By: Azer
 

explanation about ellingham diagram

Expert Reply
Wed October 03, 2012
The Ellingham diagram is a tool most often used in extraction metallurgy to find the conditions necessary for the reduction of the ores of important metals. This diagram can be used to quickly and simply find a range of thermodynamic data relating to many metallurgical reactions.
An Ellingham diagram is a plot of DG (change in Gibbs free energy) versus temperature, which, for our purposes, would be a temperature in a continuous furnace by zone or in a batch furnace by time in the cycle. The Ellingham diagram shown is for metals reacting to form oxides. 

Since any explanation of how to calculate and plot an Ellingham diagram is more about the mechanics of the derivation process and less about how to use the tool, which is the intent of this article, the concept is covered very briefly here.

Enthalpy (DH) and entropy (DS) are essentially constant with temperature. Unless a phase change occurs, the free energy (DG) versus temperature plot can be drawn as a series of straight lines, where DS is the slope and DH is the y-intercept.

DG = DH – T D

The slopes of those plotted lines change when any of the materials involved melt (M) or vaporize/boil (B). In a heat-treat system this is (for the most part) irrelevant, except in the brazing and sintering processes where those phase changes (melting) can indeed occur.
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