446
J. ThOming et al.
There is the same effect if we calculate for gold instead of mercury, which means
that traces of gold, in the case of gold-mining residues, can be extracted also:
2Au + 3Cl2 B AuCl 4 + 2CI .
According to Fig. 3, the sodium chloride concentration plays a very important
role in mercury dissolution for producing the most soluble species (HgCI 42 -),
which means that the NaCl concentration of 1.0 molll is more than suitable to
assure the safe production of that species.
The dissolved mercury can be extracted from the solution by cathodic
reduction, Eq. (6) being assumed as the predominant reaction during the
deposition of mercury, from a solution containing more chloride than mercury.
This half-cell reaction has a reversible electrode potential, under standard
conditions, of 0-4033 V(SHE). This potential was calculated from standard Gibbs
energy data (Balej 1985).
Predictions can be made also for electrolytic deposition. This process step is
sensitive to chloride concentration. If the soil is highly contaminated, the
concentration of dissolved mercury is also consequently high. This enhances
calomel formation according to Eq. (8). The equilibrium constant is calculated
after data taken from Lindsay (1979).
HgCI~ + Hgo B Hg2 Cl2 + 2CI ; log K = 5.32.
(8)
This equation describes the disproportion equilibrium between mercuric ions
and elemental mercury on the one hand and calomel on the other. If too much
calomel is present, it forms a non-conducting thin layer on the cathode, which
can bring the process to a stop.
As electrolysis continues, gold is also dissolved and codeposited with mercury,
generating an amalgam. The phase diagram of the system Au-Hg (Hansen 1958)
indicates that at ambient temperature, alloys with more than 5% gold are solid
and single phase.
4
Bench-Scale Experiments
4.1
Materials and Methods
The soil samples from Germany (G) were taken from an area of an inactive chloralkali plant, representing an average of 11 samples described in detail by
Windmoller et al. (1996). The Brazilian sample (B) is a residue from small-scale
gold production of a primary gold ore in Mato Grosso state. In this artisanal
production the fine-grained gold particles are separated by amalgamation from a
gravity concentrate produced by sluice boxes. The residual material from the
amalgamation step, by using pans or rotating drums, is kept in tanks covered
with water (Table 1).
J. ThOming et al.
There is the same effect if we calculate for gold instead of mercury, which means
that traces of gold, in the case of gold-mining residues, can be extracted also:
2Au + 3Cl2 B AuCl 4 + 2CI .
According to Fig. 3, the sodium chloride concentration plays a very important
role in mercury dissolution for producing the most soluble species (HgCI 42 -),
which means that the NaCl concentration of 1.0 molll is more than suitable to
assure the safe production of that species.
The dissolved mercury can be extracted from the solution by cathodic
reduction, Eq. (6) being assumed as the predominant reaction during the
deposition of mercury, from a solution containing more chloride than mercury.
This half-cell reaction has a reversible electrode potential, under standard
conditions, of 0-4033 V(SHE). This potential was calculated from standard Gibbs
energy data (Balej 1985).
Predictions can be made also for electrolytic deposition. This process step is
sensitive to chloride concentration. If the soil is highly contaminated, the
concentration of dissolved mercury is also consequently high. This enhances
calomel formation according to Eq. (8). The equilibrium constant is calculated
after data taken from Lindsay (1979).
HgCI~ + Hgo B Hg2 Cl2 + 2CI ; log K = 5.32.
(8)
This equation describes the disproportion equilibrium between mercuric ions
and elemental mercury on the one hand and calomel on the other. If too much
calomel is present, it forms a non-conducting thin layer on the cathode, which
can bring the process to a stop.
As electrolysis continues, gold is also dissolved and codeposited with mercury,
generating an amalgam. The phase diagram of the system Au-Hg (Hansen 1958)
indicates that at ambient temperature, alloys with more than 5% gold are solid
and single phase.
4
Bench-Scale Experiments
4.1
Materials and Methods
The soil samples from Germany (G) were taken from an area of an inactive chloralkali plant, representing an average of 11 samples described in detail by
Windmoller et al. (1996). The Brazilian sample (B) is a residue from small-scale
gold production of a primary gold ore in Mato Grosso state. In this artisanal
production the fine-grained gold particles are separated by amalgamation from a
gravity concentrate produced by sluice boxes. The residual material from the
amalgamation step, by using pans or rotating drums, is kept in tanks covered
with water (Table 1).