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M. Hempel and j. Thoeming
and its dependence on concentrations and temperature. While in hightemperature processes the rates of reaction are generally fast enough to assume
an equilibrium state for equilibrium processes, in hydrometallurgy the reaction
rate is the limitation of thermodynamical or kinetic predictions.
The three fundamental parameters, pH, the redox potential of the solution and
the concentration of complexing agents, determine the speciation of mercury and
the solubility of mercury. Moreover, these three parameters are responsible for
the equilibrium state of all reactions of interest, the reaction rates and also for the
equilibrium between adsorption Idesorption. The parameters can be controlled by
adding appropriate chemicals like acids, oxidizing agents or complexing agents.
4.2.2
Extraction Mechanisms
In soils, mercury is bound or attached to different solid phases. Thus, different
extraction mechanisms have to take place to achieve a selective cleanup. The
chemical mechanisms involved can be classified as:
1. Desorption of adsorbed species.
2. Oxidation of metallic mercury.
3. Complexing extraction of organically complexed mercuric ions by competing
strong complexing agents.
4. Dissolution of precipitated mercury such as mercury oxide (Fig. 4).
It should be noted that the equilibria controlling these mechanisms depend
highly on each other. In particular, readsorption or recomplexation of the
dissolved mercury species on surfaces or organic matter may cause a shift in the
mercury distribution within the different soil phases and lead to a low extraction
efficiency. Besides chemical mechanisms, transport mechanisms also control the
extraction. Since mercury species can be bound to soil phases which are covered
by another phase, a coating of iron oxide for example, diffusion limits the speed
of the extraction process to a great extent. Since soil is a porous system,
micropores account for most of the surface area of the soil particles. Moreover, it
is the inside of these micropores where the adsorbed as well as complexed
mercuric ions are located. Thus, in an agitated leaching module, mass transport
can be enhanced by increasing the turbulence or by diminishing the particle size
by grinding. Another possibility to overcome the problem of mass transport is to
apply an electric field. This results in electro migration of the mercuric ions, the
dissociation of complexes and the electroosmotic flow of the whole pore water.
4.2.3
Technical Approaches
Leaching Techniques. The term leaching is often applied to the process of
extracting a metal from soil. Due to the use of extracting agents, the cleanup of
soils by leaching is also called chemical extraction. Approaches for mercury
M. Hempel and j. Thoeming
and its dependence on concentrations and temperature. While in hightemperature processes the rates of reaction are generally fast enough to assume
an equilibrium state for equilibrium processes, in hydrometallurgy the reaction
rate is the limitation of thermodynamical or kinetic predictions.
The three fundamental parameters, pH, the redox potential of the solution and
the concentration of complexing agents, determine the speciation of mercury and
the solubility of mercury. Moreover, these three parameters are responsible for
the equilibrium state of all reactions of interest, the reaction rates and also for the
equilibrium between adsorption Idesorption. The parameters can be controlled by
adding appropriate chemicals like acids, oxidizing agents or complexing agents.
4.2.2
Extraction Mechanisms
In soils, mercury is bound or attached to different solid phases. Thus, different
extraction mechanisms have to take place to achieve a selective cleanup. The
chemical mechanisms involved can be classified as:
1. Desorption of adsorbed species.
2. Oxidation of metallic mercury.
3. Complexing extraction of organically complexed mercuric ions by competing
strong complexing agents.
4. Dissolution of precipitated mercury such as mercury oxide (Fig. 4).
It should be noted that the equilibria controlling these mechanisms depend
highly on each other. In particular, readsorption or recomplexation of the
dissolved mercury species on surfaces or organic matter may cause a shift in the
mercury distribution within the different soil phases and lead to a low extraction
efficiency. Besides chemical mechanisms, transport mechanisms also control the
extraction. Since mercury species can be bound to soil phases which are covered
by another phase, a coating of iron oxide for example, diffusion limits the speed
of the extraction process to a great extent. Since soil is a porous system,
micropores account for most of the surface area of the soil particles. Moreover, it
is the inside of these micropores where the adsorbed as well as complexed
mercuric ions are located. Thus, in an agitated leaching module, mass transport
can be enhanced by increasing the turbulence or by diminishing the particle size
by grinding. Another possibility to overcome the problem of mass transport is to
apply an electric field. This results in electro migration of the mercuric ions, the
dissociation of complexes and the electroosmotic flow of the whole pore water.
4.2.3
Technical Approaches
Leaching Techniques. The term leaching is often applied to the process of
extracting a metal from soil. Due to the use of extracting agents, the cleanup of
soils by leaching is also called chemical extraction. Approaches for mercury