Tadao Fujimoto*, Mitsuyuki Nishihara*, Katsuya Kawamoto* and Masakatsu Hiraoka**

(*KUBOTA Corporation, **Kyoto University)

Abstract

Most of the mercury contained in the Municipal Solid Waste (MSW) is emitted into the flue gas by incineration. When a wet-scrubber is employed for the removal of acid gases, water soluble mercury (HgCl2) is simultaneously removed from the flue gas. In this study, the removal and recovery of mercury from the scrubber wastewater is investigated. Mercury compounds are first converted to metal mercury using reducing agents, vaporized by high temperature and then condensed and separated out by a cooling unit. Emphasis is placed on the closed system where the air, as a carrier gas for gaseous metal mercury, is continuously circulated from a condenser to the inlet of a reduction tank. Stannous chloride is found to be the most suitable reductant through laboratory scale experiments. From the pilot plant experiment, the desirable pH in the reducing tank is found to be basic. Relatively short residence time (0.5-1h) of the wastewater is needed in order to avoid re-dissolution of the circulating gas mercury into the wastewater. The system is applied to a full scale MSW incineration plant where more than 98.5% of the mercury in the scrubber wastewater has been recovered.

Key words: mercury, MSW incineration, reduction-vaporization, recovery, closed system