Recovery Of Elemental Mercury

An important engineering consideration in the design and operation of the biological process for mercury removal is the fate of the reduced mercury during treatment. A significant portion of the mercury that is reduced in the biological process will exit the bioreactor as a vapor along with other effluent gases (mainly N2. 02 and CO2). When air is used as the oxygen source for the biological process, the partial pressure of the elemental mercury vapor in the effluent gas stream is very low. This makes it difficult to condense the majority of mercury from the effluent gas because condenser temperatures must be very low (theoretically, -40 °C or lower under worst case conditions).

One other method of recovering mercury from the vapor phase is to extract mercury using a suitable solvent (e.g. toluene or chloroform) in a scrubber, e.g. a packed tower. The mercury in the solvent can be reprocessed commercially. But, the poor solubility of mercury in such solvents warrants consumption of huge quantities of solvent thus limiting the use of a packed tower process for mercury recovery. It is therefore apparent that a preconcentration step must be used to facilitate the removal and recovery of mercury from the air phase.

In our laboratory research efforts are directed towards development of a mercury recovery process based on adsorption/desorption mechanisms. The schematic diagram of the process is shown in Figures 3 and 4.

Mercury laden air from the bioreactor will be passed through an adsorbent bed where mercury from the air phase will be extracted and concentrated on the surface of the adsorbent. Two types of adsorbent being investigated are activated carbon type 'AC' and type 'Mersorb 245'. Type 'AC' is an unimpregnated activated carbon which adsorbs by physical forces, whereas type 'Mersorb 245' is impregnated with organic iodide, and chemisorption also takes place. The lean air exiting from the adsorbent bed will be passed through two acidified K2O2O7 baths in series to eliminate residual mercury, if any.

Lean Air

Nutrient

Waste

Nutrient

Waste

Conditioner

Lean Air

Hg Free Sludge

Figure 2. Flow Schematic of the Full Scale Biological Process

Hg Free Liquid

Conditioner

Hg Free Sludge

Figure 2. Flow Schematic of the Full Scale Biological Process

The saturated adsorbent column containing Hg will be desorbed under heat and a purge medium to elute the adsórbate. The eluting gases would be condensed to recover the Hg vapor in the liquid form. The carrier gas from the condenser will also be passed through two acidified K2O2O7 baths in series to eliminate any uncondensed residual mercury.

Preliminary experiments have shown promise for this process. However, research is incomplete at this point to present conclusive data.

ADSORBENT COLUKWI

VENT

FILTER

BGflEACTOR

ADSORBENT COLUKWI

ADSORBENT COLUWJ 2

KiCfiOj TRAPS

RECYCLE AIR

Figure 3. Adsorption of Mercury Vapor Emanating from the Bioreactor

KiCfïO7 VENT TRAPS i

HEAT

HEAT

cooejssi

CONDENSED MERCURY

Hg SATURATED COUJMM

Figure 4. Desorption and Condensation of Elemental Mercury cooejssi

CONDENSED MERCURY

Hg SATURATED COUJMM

Figure 4. Desorption and Condensation of Elemental Mercury

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