Electrolytic Treatment of Mercury Bearing Activated Carbon From a Gas Cleaning System: A Pilot Plant Experience. Sobral, L.*, Hovart, M.**, Spiric, Z.***, Santos, R.*, Barbosa, L. A. D.* * Centre for Mineral Technology/Brazil ** Josej Stefan Institute/Ljubljana, Slovenia ***INA-Naftaplin, Zagreb, Croatia

Introduction The establishment of emission limit values requires essential improvements on both process technologies itself, and effluent treatment techniques. In particular, the Hg content in natural gases can vary up to 3000  g m -3, and most of them are use for power generation and domestic purposes, implying that the straight combustion of such gas is a considerable source of mercury emissions.

Introduction Mercury in Natural Gas Is naturally occurring in natural gas Has been found in gas processed throughout the world in a wide range of concentrations –Eastern Europe =  g/m 3 –Netherlands = 180  g/m 3 –Indonesia = 200  g/m 3 –United States = 350  g/m 3 –South America = 119  g/m 3

Cont. Conventionally, the natural gas prospecting plants use to pass such gas through a sulphur impregnated activated carbon fixed bed. In this study an electrolytic process was used for removing the elemental mercury from the carbon structure by using it as anode in the reaction system depicted in the experimental procedures.

Experimental Samples of a mercury bearing activated carbon, from a gas cleaning system, were used, as anode, in an electrolytic cell using titanium cathodes. As electrolyte, 1M NaCl solution was used.

Experimental

Experimental Electrolytic Process Main Reactions At the Anode: 2Cl -  Cl 2 + 2e 6ClO - + 3H 2 O  2ClO H + + 4Cl - + 3/2O 2 + 6e Hg o + 4Cl -  HgCl 4 2- At the Cathode: H 2 O + 2e  H 2 + 2OH - HgCl e  Hg o + 4Cl - In the Bulk: Cl 2 + H 2 O  HClO + H + + Cl - HClO  ClO - + H + 2HClO + ClO -  ClO H + + 2Cl - Hg + 2ClO - + 4Cl - +2H 2 O  Cl 2 + HgCl OH -

Pilot Scale Pilot Scale Reaction System

Results TestHg Bearing Carbon (kg) Current (A) Hg extraction (%) [Hg] remaining (ppm)

Conclusions The experimental results permit to conclude that: The Hg concentration can be reduced from 15g/kg of carbon down to <50ppm, with high extraction efficiencies; The remaining concentration is not low enough to be considered as suitable for being recycled, being necessary to extend the electrolysis time;

Conclusions The final leaching solution should not be released, as it contains high salinity and [Hg +2 ] not suitable for being discharged. It should be reused for further treatments; It is recommended to perform the electroleaching process in an acidic pH range (between 4 and 6) so as to enhance the mercury dissolution.