Electroleaching: a Mobile Cleanup Process
for Mercury Contaminated Materials
J. THOMING, 1. SOBRAL, and R.D. WILKEN
To date, there is no satisfactory technique known for simultaneous remediation of materials
contaminated with mercury and other metals. In the case of mercury-polluted tailings from Brazilian
gold-mining areas, there is the opportunity to extract mercury and traces of gold hydrometallurgically, thus making the remediation process cost-effective. Hydrometallurgical techniques are widely
used in ore treatment, whereas soil remediation applications are rare. This chapter outlines two
variants of an electroleaching process and gives an overview of its thermodynamic fundamentals.
Model calculations are presented as well as results from laboratory experiments.
The leaching agent, hypochlorous acid, can be produced by electrolytic oxidation of a sodium
chloride solution. After leaching, the leachate is treated electrolytically to deposit the metals in the
elemental form and to regenerate the leaching agent for recirculation. The leaching mechanism is a
mercury oxidation and a complexation of mercuric ions by chloride ions, forming, for example, a
tetra-chloro-mercurate-complex. The main process parameters, such as pH, Eh and element
concentrations, control the speciation of mercury. It is illustrated how leaching efficiency and
electrolytical mercury deposition are both dominated by the mercury species in solution.
The electroleaching could be conducted in two different reaction systems: (1) as a i-reactor process
with an in-suspension production of the leaching agent (EL-IS) and (2) as a 2-reactor process with an
in-line production (EL-IL). While the more costly EL-IL is capable of recovering mercury sufficiently
(>99% removal rate) even from highly contaminated German industrial samples, the EL-IS is the best
solution for less contaminated Brazilian gold-mining residues. After treatment, the mercury
concentration is lower than I mg/kg and more than 82% of the residual gold can be recovered.
1
Introduction
A major source of mercury issues to the environment has traditionally been the
chemical industry such as chlor-alkali plants. Another source is the gold mine
industry that uses elemental mercury to amalgamate gold in gravity concentrates.
Amalgamation has been used as an effective means of separating gold from host
rock. Amalgamation of run-of-mine ores has gradually diminished for health and
environmental reasons and the process is now virtually obsolete, but it is still
commonly employed in the treatment of gravity concentrates and has the
advantages of being a simple and cost-effective process.
The conventional technique to treat mercury-bearing residues deals with the
thermal decomposition. During this process, the mercury is volatilized by
increasing the temperature and immediately condensates. The use of this route
entails some drawbacks, such as: the need of using large-capacity equipment for
treating great amounts of residues, and the difficult task of dealing with the large
Environmental Science
Mercury Contaminated Sites (ed. by R. Ebinghaus et al.l
© Springer-Verlag Berlin Heidelberg 1999
for Mercury Contaminated Materials
J. THOMING, 1. SOBRAL, and R.D. WILKEN
To date, there is no satisfactory technique known for simultaneous remediation of materials
contaminated with mercury and other metals. In the case of mercury-polluted tailings from Brazilian
gold-mining areas, there is the opportunity to extract mercury and traces of gold hydrometallurgically, thus making the remediation process cost-effective. Hydrometallurgical techniques are widely
used in ore treatment, whereas soil remediation applications are rare. This chapter outlines two
variants of an electroleaching process and gives an overview of its thermodynamic fundamentals.
Model calculations are presented as well as results from laboratory experiments.
The leaching agent, hypochlorous acid, can be produced by electrolytic oxidation of a sodium
chloride solution. After leaching, the leachate is treated electrolytically to deposit the metals in the
elemental form and to regenerate the leaching agent for recirculation. The leaching mechanism is a
mercury oxidation and a complexation of mercuric ions by chloride ions, forming, for example, a
tetra-chloro-mercurate-complex. The main process parameters, such as pH, Eh and element
concentrations, control the speciation of mercury. It is illustrated how leaching efficiency and
electrolytical mercury deposition are both dominated by the mercury species in solution.
The electroleaching could be conducted in two different reaction systems: (1) as a i-reactor process
with an in-suspension production of the leaching agent (EL-IS) and (2) as a 2-reactor process with an
in-line production (EL-IL). While the more costly EL-IL is capable of recovering mercury sufficiently
(>99% removal rate) even from highly contaminated German industrial samples, the EL-IS is the best
solution for less contaminated Brazilian gold-mining residues. After treatment, the mercury
concentration is lower than I mg/kg and more than 82% of the residual gold can be recovered.
1
Introduction
A major source of mercury issues to the environment has traditionally been the
chemical industry such as chlor-alkali plants. Another source is the gold mine
industry that uses elemental mercury to amalgamate gold in gravity concentrates.
Amalgamation has been used as an effective means of separating gold from host
rock. Amalgamation of run-of-mine ores has gradually diminished for health and
environmental reasons and the process is now virtually obsolete, but it is still
commonly employed in the treatment of gravity concentrates and has the
advantages of being a simple and cost-effective process.
The conventional technique to treat mercury-bearing residues deals with the
thermal decomposition. During this process, the mercury is volatilized by
increasing the temperature and immediately condensates. The use of this route
entails some drawbacks, such as: the need of using large-capacity equipment for
treating great amounts of residues, and the difficult task of dealing with the large
Environmental Science
Mercury Contaminated Sites (ed. by R. Ebinghaus et al.l
© Springer-Verlag Berlin Heidelberg 1999