Geology Reference
In-Depth Information
Finally, hydrometallurgy also benefits from electrochemistry. In fact, a very
helpful tool for understanding the nature of hydrometallurgy is the Pourbaix di-
agram (also named E
H
pH diagram after its axes). It shows the relationship
between the reduction potential and the pH in an aqueous electrochemical sys-
tem and represents the thermodynamic equilibrium lines of a metal and its ions
at a given temperature (commonly room temperature), in a similar fashion to the
conventional phase diagram. As such, it allows for the prediction of the potential
leaching behaviour of different metallic minerals and the possibility of metal ion
precipitation of pregnant solutions. It does not however reflect the kinetics of the
reaction. The diagram can also be used in combination for various metal systems
in such a way that it illustrates at which pH conditions one can selectively precip-
itate a given cation whilst leaving others in the leaching solution. This is the case
for example of the selective precipitation of Fe
2
O
3
at low pH in a mixed pregnant
solution with ZnO.
The Pourbaix diagram results from the plotting of all equilibrium lines corres-
ponding to the various species that a given element may form such as a pure metal,
ion or hydroxide. The lines themselves are drawn according to the Nernst Equation
for the semi-cell system (Eq. (9.22)). A sample line may be obtained as in the case
of iron oxidation in aqueous solution:
Fe
2
O
3
+ 6H
+
! 2Fe
2+
+ 3H
2
O E
=0.658 V
Then
E
H
= E
0
+ 0:0592=2 log([Fe
2+
]
2
)=([H
+
]
6
) = 0:658 0:0592log(Fe
2+
) 0:178pH
If all molar concentrations
3
are equal to ' 10
6 M, E
H
becomes:
E
H
= 1:013 0:178pH
This line represents the stability region boundary of the ion Fe
2+
as a function
of pH as shown in Fig. 9.4, where the dashed lines show the stability limits of water
in the system.
For those readers with a particular interest in the subject, the authors recom-
mend Venkatachalam (1998) and Hayes (2003).
9.5 The exergy of non-fuel mineral resources
The Second Law through entropy and Gibbs free energy explain energy changes be-
tween systems and their direction, the limits and the reaction a
nity of substances
in any given transformation.
Yet both properties are relatively abstract, meaning that they can be di
cult
to appreciate outside the realm of
natural science. A way to bridge across from the
3
Although the Nernst equation rigorously applies to activities, for explanation purposes it is used
for concentrations in aqueous solutions or partial pressures in gases, as they are approximately
equal.
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