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Chapter 44.4.11 Raw gas purificationDescriptionFor production of non-ferrous metals, see also [61, European Commission, 2003].Raw gases from pyrite roasting, non-ferrous metal production and thermal decomposition ofspent acids can contain various contaminants. These are removed prior to H 2 SO 4 production inorder to prevent the contamination of the product H 2 SO 4 or to prevent negative effects on thecatalyst performance and lifetime. Table 4.16 gives an overview of the relevant contaminantsand their origins.ContaminantDustVolatile compounds of As, Se, Cd and HgVapours of volatile metal oxides and chloridesGaseous substances, such as HCl, HF, CO andVOCsDioxins/furans xOriginMainly from roasting of ores, melting andrefining processesFrom roasting of metal oresDecomposition of spent acids, roasting ofpyrite or in non-ferrous metal processingunder reductive conditionsTable 4.16: Overview of contaminants in raw gases[57, Austrian UBA, 2001], x [58, TAK-S, 2003]The following techniques are applied to remove contaminants from the raw gas:• dry coarse and fine particulate removal (cyclones, hot gas ESP)• wet scrubbing (venturi)• cooling/drying• reactive F adsorption (optional)• WESP.The raw gases are cooled to temperatures of about 320 – 400 ºC and about 85 – 90 % of the dustis removed with a cyclone. Dust is further removed with an ESP to levels of about 20 – 200mg/Nm 3 . Depending on the case, the recovered dust is recycled or disposed of. Wet scrubbing iscarried out with 50 % H 2 SO 4 as the scrubbing medium in order to remove HCl and HF, and atthe same time, to condense volatile compounds of Se and As. Settled solids are removed fromthe scrubbing liquid and disposed of and the scrubbing liquid is continuously replaced. Thespent scrubbing liquid is stripped (SO 2 removal), neutralised and discharged, or alternativelyrecycled.After cooling/drying, potential fluoride loads are removed from the gas by reactive adsorbtionin silica beds.Finally, two-stage WESP is applied to reduce dust levels to