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THE MILTEC PROCESS

      In addition to the search for new cleaning technologies, the ore supplier has investigated methods of pre-removing mercury from the ore.

      However, these efforts and the existing technology have so far proven to be insufficient in order to meet the levels of emissions set by SFT, of15 kg Hg/year from 2001.

      Thus, Tinfos and the Norwegian Company Miltec have developed a new cleaning technology. Pilot experiments have given an 85 % reduction of emissions. Since the start up of the full-scale operation in April 2000, similar, and even better results have been obtained

      1. PROCESS CHEMISTRY AND TECHNOLOGY

        2. The process gases contain gaseous mercury (Hg0), SO2, H2, CO, CO2, H2O and N2. In addition the gas contain some dust in which mercury-sulphides (HgS) and metal-amalgam (MeHg) are present. As mentioned before the process gases are burned in a steam boiler at the Tinfos thermal power plant. This is done with a surplus of oxygen. The combusted gas contains negligible amounts of CO and H2. Nearly all mercury present follows the gas stream. Some mercury is deposited in the piping, but these amounts are taken out during the yearly cleaning of the piping system.

        By burning the gas with air the amount of gas increases from about 8 000 Nm3 per furnace per hour to about 24 000 Nm3 per furnace per hour. The Hg concentration of the raw gas is about 280 m g/Nm3. Due to the removal of CO from the gas, the following gas treatment is made easier, due to a non-explosion risk gas.

        The combusted gas is scrubbed in a wet gas-scrubber as follows:

         

         

        Fig. The Miltec Process.

        1. The gas is washed with seawater added a 15 % sodium hypochlorite (NaClO) solution. By keeping the red/ox potential and the pH at a

        given level, water-soluble HgCl2 is formed by the following equations:

        Eq. 6 Hgo + NaClO + NaCl = HgCl2 + Na2O

        Other side reactions can be:

        Eq. 7 Na2O + H2O = 2 NaOH

        Eq. 8 HgCl2 + 3 NaCl = Na3HgCl4

        Na3HgCl4 is a stable water-soluble complex, easily absorbed by seawater. If the red/ox potential is too low, the less soluble salt Hg2Cl2 is formed. This product is the end product in other Hg-cleaning operations. The precipitation of this salt often results in clogging in valves and piping. By keeping the red/ox potential and the pH at controlled levels this may avoided.

        In addition, the water collects the dust, and the SO2-gas is oxidized to sulphuric-acid. This prevents the reduction of Hg++ ions to Hg0-gas by the SO2-gas.

        3. The gas and the wash water are let co-currently through a fixed bed of lumpy limestone. The bed acts as a powerful turbulent mixer for the gas and the water. In addition, the sulphuric acid is converted to gypsum (CaSO4); keeping the pH constant around 5-6. By adding enough seawater, the produced gypsum will not attach to the lumpy limestone, but follow the wash water.

        2. The washed gas and wash water hit the surface of the underlying wash water tank. The gas, which is now cleaned, is drawn off, and is continuously let out over the roof.

        The water now containing mercurysalts; dust and gypsum are absorbed in the reservoir tank.

        3. The water is periodically pumped from the reservoir into 2 precipitation tanks, where disodiumsulfide (Na2S) is added. This leads to the formation of mercury-sulphide (HgS) and other metal sulphides precipitate. HgS precipitates after the following equation:

        Eq. 9 HgCl2 + Na2S = HgS + 2 NaCl

        4. The water from the precipitation tanks is filtered, producing a filter cake with the following analysis:

        Hg 1,7 % Mo* 55
        As ,12 % Na 1,1 %
        Ba 242 Ni 381
        Cd* 335 P 508
        Co* 50 Pb* 414
        Cr 671 S 8,5 %
        Cu 95 Sr 193
        K 0,4 % Zn* 0,25 %

        Table 1 Filter-cake analysis. (Analysed by Rødmyr Miljøsenter, Skien, Norway)

        All analysis in ppm unless other stated. *Analysed by ICP-MS, others by ICP-AES.

        Due to the amount of Sulphur, it can be concluded that Hg is in the form of HgS. This is confirmed by SEM-anlysis (Scanning Electron Microscope).

        The filter cake can be deposited on a land fill deposit for special waste, NOAH (Norsk Avfallshåndtering AS), Langøya

        The filtrate, which contains <0,5 mg/l mercury, is returned to the gas-scrubbers in the primary off-gas cleaning system.

         

         

         

     

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