COMPUTER EXERCISE
KGP002
E - pH chart
Theory
The computer programme used in the exercise is called FactSage 5.2. This programme is
better adjusted for pyrometallurgical calculations and the only hydrometallurgical charts that
can be performed with the programme are E-pH charts. The database for species in solution is
limited but will probably be improved. It is also possible to create your own database file
where the thermodynamic data of interest can be stored.
In hydrometallurgical applications the interest often lies in the compound that initially
precipitates from a solution even if that specie not is the most thermodynamically stable
compound. Because of this it is in some cases necessary to reduce some of the solid phases
from the calculations. For instance, if ferric ions are precipitated from a leaching solution it is
Fe(OH)3(s) that initially is formed. Ferric hydroxide is not the most thermodynamically stable
compound and with time it will transform into hematite (Fe2O3), according to:
2Fe(OH)3(s) => Fe2O3(s) + 3H2O
Tasks
1. Concentration dependence in E-pH charts:
Make charts of the Cu-H2O system (25C) with the following concentrations of dissolved
species: 1, 10-3, 10-6 and 10-9 M. Describe the changes that occurs at the boarder between
different phases as the concentration of dissolved species decreases. Some new phases are
added at lower concentrations. What is this due to?
2. Temperature dependence in E-pH charts:
Make a chart over the Fe- H2O system with the concentration 10-3 M, including only the
following solid species: Fe, Fe(OH)2 and Fe(OH)3. Make charts at the following temperatures;
25C, 60C, 95C and 130C. Describe what happens with the phase boarders when the
temperature increases.
3. Using E-pH charts as a tool to predict how metal ions in solution can be separated:
Assume that you have a leachate with pH=0.5 and a redox potential of 600 mV relative SHE
(standard hydrogen electrode) and dissolved species at a concentration of 10-3 M regarding
copper and iron. At first construct a 2 metal chart at 25C using the same solid phases as in
task 1 and 2.
a) Suggest a method (addition of some reagent) for selective removal of iron from the solution
by making changes in pH and/or redox potential.
b) As above (3.a) but in this case make a selective removal of the copper from the solution.