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Chapter 66.4.6 Scrubbing of tail gases: fluoridesDescriptionGaseous HF is readily absorbed in water or alkaline solutions. The absorption is exothermic.With pure water, an acidic solution is produced which may be re-used on the site. The scrubbingof a vent gas containing HF can be carried out by a once-through water countercurrent contactabsorber. Typically the vent gas will be variable in concentration and flow so the absorptionequipment (frequently a packed tower) has to be designed for maximum gas flow conditionswith a minimum liquid flow, to ensure efficient wetting of the packing. As a consequence, largeliquid effluent flows with no possible valorisation of absorbed HF are obtained.Alternatively the liquid flow can be increased by recirculation of a part of the sprayed liquid,which allows for a higher HF concentration in the liquid and some recovery of the yield. In thiscase, the HF concentration in the gas at the outlet of the equipment is not zero, with the HFconcentration in the exiting gas being driven by a thermodynamic equilibrium, which dependson the liquid composition and the temperature. In addition to the HF concentrationcorresponding to the thermodynamic equilibrium, other sources of acidic emissions can resultfrom gas channelling in the equipment with non absorbed gas leaving the absorption unit orentrainment of liquid acidic droplets with the gas flow. The latter problems normally do notoccur if an alkaline solution is used for absorption but the recovery of the absorbed gas is notfeasible. A demister can also be used after the scrubber. Whenever HF solutions need to berecovered and a very low gas emission needs to be ensured, a two steps absorption system isrecommended.Achieved environmental benefitsFor achievable emission levels for the different scrubber setups, see Table 6.10.Alkaline scrubbing also reduces the emission of SO 2 drastically [6, German UBA, 2000].Scrubber setupSingle-stage, once throughabsorption with copiousamounts of water make up ormultistep water sprayersWater, single-stage, closed loopwith HF solution productionDouble-stage water absorptionLiquid phase alkalineabsorption (single-stage)Combination of water andalkaline absorptionSolid alkaline absorptionFluoride emission level as HFin mg/Nm 35 – 10The HF concentration in theexiting gas depends on the acidconcentration in the loop, so asecond scrubber to clean the offgas is usually necessaryLow emissions valuesTypically 1 – 5Typically 1 – 5