Keeping heavy metals in the material cycle
In order to keep certain materials in the material cycle, Swiss legislators have stipulated that lead and zinc must be recovered from fly ash – i.e. from the ash residue emitted from filtration plants – from 2026 onwards. EnviroChemie’s Swiss team successfully commissioned an acid fly ash leaching plant with SwissZink, and a further project is in progress. The process for creating recyclable metal hydroxide sludge from fly ash is technically complex. The acidic quench water from the flue gas scrubbing process is used to separate the heavy metals from the ash. The remaining ash that is washed in the acid process is separated from the heavy metal solution using membrane filter presses or rubber belt filters. The concentrated heavy metal solution is then subjected to hydroxide precipitation. The resulting sludge is then separated by means of membrane filter presses. Selective ion exchangers are used to remove the last impurities from the water phase before it is discharged.
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