444
J. ThOming et al.
Hg + HCIO + 3CI {o} HgCI~ ) + OH
(5)
The dissolved mercury, mainly as HgCI~ , is electrolytically deposited on the
surface of the cathode:
HgCI~ + 2e {o} Hg + 4CI .
(6)
3
Theoretical Considerations
Thermodynamic equilibrium calculations, which provide knowledge of species
stability, are well known for their accuracy. Nevertheless, they can give engineers
only a rough idea about the state of the solidlliquid extraction system. This is due
to the low temperature of these processes - usually 20-80°C. In this temperature
range kinetic factors often cause metastable states differing from predicted
stabilities. This fact can be taken into account by the right choice of species, that
may occur before calculations are done.
The behaviour of mercury during leaching can be controlled by pH, redox
potential and the concentration of complexing agents, and it depends also on the
dilution of mercury. This can be illustrated by thermodynamic stability
diagrams, or more specifically Eh-pH diagrams which show predominant
stability areas of the mercury species. These areas are calculated by solving
equilibrium equations and also constraint equations, in which case the method
proposed by Haung and Cuentas (1988) was used.
In Fig. 3 the mercury/water system is compared for different concentrations of
chloride, the complexing agent. This diagram is calculated for the total amount of
mercury which could occur during leaching of sample G (described below). The
data for calculation were taken from Bailey et al. (1982), Barin (1993) and Knacke
et al. (1991).
For the calculation procedure, the activity coefficients were taken into account
but have only little influence on the distribution of the predominant species. The
activity coefficients can be calculated after DebyeIHiickel only if the ionic
strength (I) is low (I < 0.005 mol/kg). For highly concentrated electrolytes
(I < 6 mol/kg) the equation of Bromley (1973; Rastogi and Tassios 1980) provides
good approximations.
In the pure system, without any complexing agent present, the pH has to be
below 3, at a very high redox potential, to dissolve the main part of the mercury.
Whereas if chloride is added to the system, mercury becomes soluble even at
neutral pH. Nevertheless, a high redox potential is needed for dissolution to
occur which is provided by the hypochlorite. During the electro leaching process,
the solution pH should not be above 8 so as to avoid precipitation of mercury
oxide (HgO), as insoluble as mercurous chloride, which will remain in the
residue as such.
J. ThOming et al.
Hg + HCIO + 3CI {o} HgCI~ ) + OH
(5)
The dissolved mercury, mainly as HgCI~ , is electrolytically deposited on the
surface of the cathode:
HgCI~ + 2e {o} Hg + 4CI .
(6)
3
Theoretical Considerations
Thermodynamic equilibrium calculations, which provide knowledge of species
stability, are well known for their accuracy. Nevertheless, they can give engineers
only a rough idea about the state of the solidlliquid extraction system. This is due
to the low temperature of these processes - usually 20-80°C. In this temperature
range kinetic factors often cause metastable states differing from predicted
stabilities. This fact can be taken into account by the right choice of species, that
may occur before calculations are done.
The behaviour of mercury during leaching can be controlled by pH, redox
potential and the concentration of complexing agents, and it depends also on the
dilution of mercury. This can be illustrated by thermodynamic stability
diagrams, or more specifically Eh-pH diagrams which show predominant
stability areas of the mercury species. These areas are calculated by solving
equilibrium equations and also constraint equations, in which case the method
proposed by Haung and Cuentas (1988) was used.
In Fig. 3 the mercury/water system is compared for different concentrations of
chloride, the complexing agent. This diagram is calculated for the total amount of
mercury which could occur during leaching of sample G (described below). The
data for calculation were taken from Bailey et al. (1982), Barin (1993) and Knacke
et al. (1991).
For the calculation procedure, the activity coefficients were taken into account
but have only little influence on the distribution of the predominant species. The
activity coefficients can be calculated after DebyeIHiickel only if the ionic
strength (I) is low (I < 0.005 mol/kg). For highly concentrated electrolytes
(I < 6 mol/kg) the equation of Bromley (1973; Rastogi and Tassios 1980) provides
good approximations.
In the pure system, without any complexing agent present, the pH has to be
below 3, at a very high redox potential, to dissolve the main part of the mercury.
Whereas if chloride is added to the system, mercury becomes soluble even at
neutral pH. Nevertheless, a high redox potential is needed for dissolution to
occur which is provided by the hypochlorite. During the electro leaching process,
the solution pH should not be above 8 so as to avoid precipitation of mercury
oxide (HgO), as insoluble as mercurous chloride, which will remain in the
residue as such.