Electrolytic Treatment of Mercury Electrolytic Treatment of Mercury Bearing Activated Carbon Bearing Activated Carbon

From a Gas Cleaning System: From a Gas Cleaning System: A Pilot Plant Experience.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

IntroductionIntroduction

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 gasescan vary up to 3000 g m-3, and most of them are use for power generation and domestic purposes, implying that the straight combustionof such gas is a considerable source of mercuryemissions.

IntroductionIntroduction

Mercury in Natural GasMercury 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 = 1000 - 2500 g/m3

– Netherlands = 180 g/m3

– Indonesia = 200 g/m3

– United States = 350 g/m3

– South America = 119 g/m3

Cont.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 experimentalprocedures.

ExperimentalExperimental

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.

ExperimentalExperimentalNatural Gas Prospecting

Operation(Deep wells Operation)

Mercury-Bearing Natural Gas

Cleaning OperationSulphur Impregnated

Activated Carbon

Mercury-Free Natural Gas

Mercury-Bearing Activated Carbon

Electrolytic Treatment

Current

Mercury-Free Activated Carbon

Carbon Reactivation

Impure Brine

Chemical Treatment

Brine

ExperimentalExperimentalElectrolytic Process Main Reactions

At the Anode: 2Cl- Cl2 + 2e

6ClO- + 3H2O 2ClO3- + 6H+ + 4Cl- + 3/2O2 + 6e

Hgo + 4Cl- HgCl42-

At the Cathode: H2O + 2e H2 + 2OH-

HgCl42- + 2e Hgo + 4Cl-

In the Bulk: Cl2 + H2O HClO + H+ + Cl-

HClO ClO- + H+

2HClO + ClO- ClO3- + 2H+ + 2Cl-

Hg + 2ClO- + 4Cl- +2H2O Cl2 + HgCl42- + 4OH-

Pilot Scale Pilot Scale Reaction SystemReaction System

ResultsResultsTest Hg Bearing

Carbon (kg)Current

(A)Hg extraction

(%)[Hg] remaining

(ppm)

1 2.5 10.0 96.01 19.67

2 3.5 15.0 98.05 45.60

3 4.5 20.0 99.06 55.7

4 5.0 25.0 99.76 32.0

ConclusionsConclusionsThe experimental results permit to conclude that:

The Hg concentration can be reduced from15g/kg of carbon down to <50ppm, with high extraction efficiencies;

The remaining concentration is not low enoughto be considered as suitable for being recycled,being necessary to extend the electrolysis time;

ConclusionsConclusionsThe final leaching solution should not be released,as it contains high salinity and [Hg+2] not suitablefor being discharged. It should be reused for furthertreatments;

It is recommended to perform the electroleachingprocess in an acidic pH range (between 4 and 6) soas to enhance the mercury dissolution.