The acid fly ash washing process uses the acid present in the blowdown from wet flue gas cleaning to extract the mobilizable metals in the ashes. In the first step, the mercury contained in the wash water is separated by adsorption on a selective ion exchanger and is simultaneously concentrated (AIK mercury ion process). To achieve the highest possible loading of the ion exchange resin, the wash water is filtered and thus freed from the solids still present.
The extraction of the heavy metals from the fly ash takes place in a stirred tank cascade, whereby the ash is suspended in the wash water. The pH value is adjusted to an optimum, plant-specific value. The heavy metals largely go into solution at this pH, while matrix elements (aluminum oxide, iron oxide, silicon oxide) remain predominantly in the solid. In addition, gypsum is formed and precipitated by the reaction of the sulfate-containing wash water with the calcium of the fly ash and the lime milk added for pH control.
The removed solids are drained on a support belt filter and rinsed by a displacement wash. In Switzerland, this residue complies with the guideline for the disposal of residual substances (Eluate testing) and can be deposited as landfill together with the slag.
The solution containing heavy metals is fed to the wastewater treatment plant (WWT). The hydroxide sludge is drained, rinsed and thermally post-dried in the WWT. Thanks to the high concentration of zinc and the extensive pre-separation of unwanted accompanying substances (calcium sulfate, mercury, chloride), this material is suitable as a secondary raw material for zinc production.
Step 1: Selective mercury separation
The solid matter still contained in the wash water is filtered off via back-flushable cartridge filters in order to achieve the highest possible loading capacity for the mercury in the subsequent ion exchanger. The mercury dissolved in the wash water is completely separated in series-connected selective ion exchangers and fed to the recovery system.
Step 2: Extraction of the fly ash in a stirred tank cascade
The alkaline fly ash is suspended in the acidic wash water. The mobilizable heavy metals of the fly ash are dissolved. By adding a small amount of an oxidizing agent (e.g., H2O2) a high extraction yield. The calcium (Ca2+) contained in the fly ashes reacts with the sulfate (SO42-) contained in the wash water to form poorly soluble gypsum.
Step 3: Drainage and post-treatment of the extracted residues
The suspension from the last extraction vessel flows directly onto a vacuum belt filter. This filter causes a mechanical separation of the liquid from the suspension phase. Under the influence of vacuum, the liquid phase largely separates from the solid phase. A compact ash cake is formed. This contains salts dissolved in the residual moisture and heavy metals, which are washed out by displacement washing. The resulting wash filtrate is collected in the filtrate separator and fed for further treatment to the downstream wastewater treatment plant (WWT) for residual neutralization and heavy metal precipitation.
The fly ash can now be delivered with the slag to a landfill (in Switzerland).
Step 4: Wastewater treatment and separation of the hydroxide sludge
The wastewater containing heavy metals is purified by hydroxide precipitation. This is usually done with lime milk. The hydroxide sludge is thickened by a candle filter and drained by a fully automatic membrane chamber filter press and, if desired, thermally post-dried. Post-drying is economically interesting in order to save transport and energy costs, for example in a zinc recycling process. The last traces of heavy metals are removed from the filtrate by downstream selective ion exchangers.
The FAW process is an effective method for the treatment of residual materials. The application of the technique is particularly recommended for waste incineration plants. The treatment washes the ash with acid and thus extracts the valuable recyclable raw materials. In Switzerland, approx. 4 million tons of waste are recycled each year. Of this, an average of 800,000 tons of slag and 80,000 tons of fly ash are produced.
The mercury is separated in the flue gas cleaning system, from where it enters the wastewater treatment system, where it is removed by the AIK mercury ion process. A considerable amount of zinc is also extracted from the flue dust and recycled. A further proportion is found in the slag. Likewise, due to its volatility, about 75% of the cadmium can be found in the flue dust. Most of this can be recycled together with the zinc.It is precisely these 2 elements that achieve the highest recovery rate of recyclables in a waste incineration plant with an integrated FAW process.
